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DANYELLE MEDEIROS DE ARAÚJO
Estudo da participação das Espécies Fortemente Oxidantes produzidas durante a
oxidação eletroquímica de corantes usando eletrodo de diamante dopado com boro:
Função das propriedades eletroquímicas do eletrodo e condições experimentais
Tese submetida ao Programa de Pós-
Graduação em Química da Universidade
Federal do Rio Grande do Norte, em
cumprimento às exigências para obtenção
do título de Doutor em Química.
Natal - RN
2014
Catalogação da Publicação na Fonte. UFRN / SISBI / Biblioteca Setorial
Centro de Ciências Exatas e da Terra – CCET.
Araújo, Danyelle Medeiros de.
Estudo da participação das espécies fortemente oxidantes produzidas durante a
oxidação eletroquímica de corantes usando eletrodo de diamante dopado com boro:
função das propriedades eletroquímicas do eletrodo e condições experimentais /
Danyelle Medeiros de Araújo. - Natal, 2014.
124 f. il.:
Orientador: Prof. Dr. Carlos Alberto Martinez-Huitle.
Coorientador: Prof. Dr. Djalma Ribeiro da Silva.
Tese (Doutorado) – Universidade Federal do Rio Grande do Norte. Centro de
Ciências Exatas e da Terra. Programa de Pós-Graduação em Química.
1. Eletroquímica – Tese. 2. Oxidação – Tese. 3. Radicais hidroxilas – Tese. 4.
Ativação por luz – Tese. 5. Efluentes reais e sintéticos – Tese. I. Martinez-Huitle,
Carlos Alberto. II. Silva, Djalma Ribeiro da. III. Título.
RN/UF/BSE-CCET CDU: 544.6
3
Esta tese é dedicada a
Ailton e Zefinha, meus pais,
por preencherem minha vida
com muito amor e constante inspiração;
e a Gabriel, meu adorável filho,
razão sobre a qual construo minha vida.
4
AGRADECIMENTOS
Minha eterna gratidão!
A Deus, que através da sua presença me abençoa e capacita em tudo que me destina.
A meu filho, Gabriel, que nunca me questionou o porquê de tanto tempo na
universidade e pela força que sempre me tem dado para seguir em frente.
À minha mãe, Zefinha, pelas orações, pelo zelo; pelo amor; pelos cuidados, bem
como, pela atenção que ela e seu esposo deram ao meu rapaz, no período em que me ausentei
para Europa em função do Doutorado.
A meu pai, Ailton, do qual herdei o gosto pela honestidade, dignidade e justiça. É
minha referência de vida, ensina-me a não esperar a oportunidade para vencer, mas a vencer a
mim mesma para concretizar os meus sonhos. Nessa caminhada, sempre falava o quão
importante é a carreira de doutor. À sua esposa, Vera, que exige que eu escreva cada vez
melhor, ajudando-me a trocar as reticências por pontos finais, além de todo o seu carinho.
Ao meu irmão, Tanny, a sua esposa e ao meu sobrinho pelo apoio e carinho.
A meu namorado, Kleber, pela paciência, força e carinho durante essa caminhada.
Ao Professor Dr. Carlos Alberto Martinez, por ter me aceito como sua orientanda
nessa tese, pela dedicação incansável, estimulando-me a evidenciar um profundo senso de
propósito, inspirando-me a buscar o meu desenvolvimento e capacidade profissional.
Ao Professor Dr. Manuel Andrés Rodrigo, por ter me aceitado no período do
doutorado sanduíche na Espanha, por sua disponibilidade e por todas as oportunidades que me
foram proporcionadas.
Ao professor Dr. Djalma Ribeiro da Silva, que, desde o início da minha graduação me
incentivou a seguir a carreira acadêmica e oportunizou-me desenvolver essa tese.
À professora Dra. Nedja Suely Fernandes, pelos ensinamentos, por saber que podia
contar com ela quando necessitasse.
A todo o grupo do LEAA da UFRN, em especial às minhas amigas: Aline, Elisama,
Eliane e Carol, pela aprendizagem, amizade e carinho.
Ao grupo do laboratório de Castilla de La Mancha, em especial a: Salva, Conchi,
Sondos, Sara, Maria José, pelos ensinamentos.
Aos amigos que conquistei na Espanha e que fizeram os meus dias mais felizes, Diana,
Alexandra, Jorge, Gabriel, Vero e Carmen.
5
AGRADECIMENTOS ESPECIAIS
6
RESUMO
ARAÚJO, D. M. Estudo da participação das Espécies Fortemente Oxidantes produzidas
durante a oxidação eletroquímica de corantes usando eletrodo de diamante dopado com
boro: Função das propriedades eletroquímicas do eletrodo e condições experimentais.
2014, setembro. Tese de Doutorado – Universidade Federal do Rio Grande do Norte.
A indústria têxtil é uma das que mais polui no mundo (AHMEDCHEKKAT et al. 2011),
gerando efluentes com altos índices de carga orgânica. Entre os poluentes presentes nesses
efluentes encontram-se os corantes, substâncias muitas vezes com estruturas complexas, de
características tóxicas e carcinogênicas, além de possuir uma forte coloração. O descarte
incorreto dessas substâncias ao meio ambiente, sem realizar um pré-tratamento pode
ocasionar grandes impactos ambientais. Esta tese tem como objetivos utilizar uma técnica de
oxidação eletroquímica com ânodo de diamante dopado com boro, BDD (sigla em inglês),
para o tratamento de um corante sintético e de um efluente têxtil real. Além de estudar o
comportamento de diferentes eletrólitos (HClO4, H3PO4, NaCl e Na2SO4) e densidades de
correntes (15; 60; 90 e 120 mA.cm-2
), com a Rodamina B (RhB) e comparar os métodos de
fotólise, eletrólise e fotoeletrólise utilizando o H3PO4 e o Na2SO4. Estudos de oxidação
eletroquímica foram efetuados em diferentes relações de BDD sp3/sp
2 com solução de RhB.
Para alcançar esses objetivos, análises de pH, condutividade, UV-visível, COT, HPLC e CG-
MS foram desenvolvidas. Através dos resultados com a Rodamina B, observou-se que em
todos os casos ocorreu a sua mineralização, independente do eletrólito e da densidade de
corrente, porém esses parâmetros afetam na velocidade e a eficiência de mineralização. A
radiação com luz foi favorável durante a eletrólise da RhB com fosfato e sulfato. Em relação à
oxidação em ânodo de BDD com diferentes configurações sp3/sp
2 (165, 176, 206, 220, 262 e
329), o com menor teor de carbono-sp3 teve um maior tempo favorecendo a conversão
eletroquímica da RhB, ao invés da combustão. O com maior teor de carbono sobre os anodos
de BDD levou o maior favorecimento da oxidação eletroquímica direta.
Palavras chave: eletroquímica, oxidação, radicais hidroxilas, ativação por luz, efluentes reais
e sintéticos.
7
ABSTRACT
ARAÚJO, D. M. Study of the involvement of strong oxidizing species produced during
the electrochemical oxidation of dye using boron-doped diamond electrode: Role of the
electrochemical properties of the electrode and experimental conditions. 2014,
september. PhD thesis - Universidade Federal do Rio Grande do Norte.
The textile industry is one of the most polluting in the world (AHMEDCHEKKAT et al.
2011), generating wastewater with high organic loading. Among the pollutants present in
these effluents are dyes, substances with complex structures, toxic and carcinogenic
characteristics, besides having a strong staining. Improper disposal of these substances to the
environment, without performing a pre-treatment can cause major environmental impacts.
The objective this thesis to use a technique of electrochemical oxidation of boron doped
diamond anode, BDD, for the treatment of a synthetic dye and a textile real effluent. In
addition to studying the behavior of different electrolytes (HClO4, H3PO4, NaCl and Na2SO4)
and current densities (15, 60, 90 and 120 mA.cm-2
), and compare the methods with
Rhodamine B (RhB) photolysis, electrolysis and photoelectrocatalytic using H3PO4 and
Na2SO4. Electrochemical oxidation studies were performed in different ratio sp3/sp
2 of BDD
with solution of RhB. To achieve these objectives, analysis of pH, conductivity, UV-visible,
TOC, HPLC and GC-MS were developed. Based on the results with the Rhodamine B, it was
observed that in all cases occurred at mineralization, independent of electrolyte and current
density, but these parameters affect the speed and efficiency of mineralization. The radiation
of light was favorable during the electrolysis of RhB with phosphate and sulfate. Regarding
the oxidation in BDD anode with different ratio sp3/sp
2 (165, 176, 206, 220, 262 e 329), with
lower carbon-sp3 had a longer favoring the electrochemical conversion of RhB, instead of
combustion. The greater the carbon content on the anodes BDD took the biggest favor of
direct electrochemical oxidation.
Keywords: electrochemical, oxidation, hydroxyl radicals, activation by light, real and
synthetic effluents.
8
LISTA DE FIGURAS
CAPÍTULO 3
Figura 1 Cadeia têxtil simplificada................................................................................... 26
Figura 2 Rio Jianhe em 2011, Luoyang/China................................................................. 27
Figura 3 Estrutura molecular da Rodamina B.................................................................. 29
Figura 4 Degradação fotocatalítica por micro-ondas da RhB com TiO2 suportado em
carvão ativado....................................................................................................................
30
Figura 5 Esquema de oxidação eletroquímica de compostos orgânicos em ânodos
ativos e não-ativos.............................................................................................................
39
CAPÍTULO 4
Figure 1 BDD anode (left) and stainless steel cathode (right)…………………………. 53
Figure 2 Mineralization of Rhodamine B, as a function of Q, during electrolysis in
different supporting electrolytes at two current densities (a) 60 mA cm-2
and (b) 120
mA cm-2
: Na2SO4; HClO4; H3PO4; NaCl........................................................
54
Figure 3 Changes in the COD, as a function of Q, during electrolysis of Rhodamine
solutions containing different supporting electrolytes at two current densities (a) 60
mA cm-2
and (b) 120 mA cm-2
: Na2SO4; HClO4; H3PO4; NaCl....................
55
Figure 4 Influence of the current density on the first order kinetic constants for the
CDEO of Rhodamine B: Na2SO4; HClO4; H3PO4; NaCl................................
56
Figure 5 Instantanteous current efficiency constants for the electrolysis of Rhodamine
B solutions in different supporting electrolyte media: Na2SO4; HClO4; H3PO4;
NaCl…………………………………………………………………………………..
57
Figure 6 Main intermediates detected, as a function of time, during the electrolysis of
Rhodamine B solutions containing different supporting electrolytes ((a) HClO4, (b)
NaCl, (c) H3PO4 and (d) Na2SO4 at 60 mA cm-2
: phthalic acid (); 2,5-
hydroxybenzoic acid (); benzoic acid (); 3-dinitrobenzoic acid (♦); α-
hidroxyglutaric acid (); intermediate 6 () and intermediate 7 ()……………………
58
Figure 7 Influence of the current density on the maximum concentration of
9
intermediates detected during the electrolysis of Rhodamine B solutions containing
different supporting electrolytes, (a) HClO4, (b) NaCl, (c) H3PO4 and (d) Na2SO4:
phthalic acid (); 2,5-hydroxybenzoic acid (); benzoic acid (); 3-dinitrobenzoic
acid (♦); α- hidroxyglutaric acid (); intermediate 6 () and intermediate 7 ()……….
60
Figure 8 Electrochemical pathways degradation for RhB as a function of supporting
electrolyte: blue pathway is according to the intermediates produced when HClO4,
H3PO4 and Na2SO4 were used while red pathway is followed when NaCl was used.
Intermediates detected: phthalic acid (I1); benzoic acid (I2); 3-dinitrobenzoic acid (I3);
2.5-hydroxybenzoic acid (I4); α- hidroxyglutaric acid (I5); oxalic acid (I6), intermediate
7 (I7) and chloroform (I8)………………………………………………………………
61
CAPÍTULO 5
Figure 1 Removal of color during the photolysis, electrolysis and photoelectrolysis of
solutions containing Rhodamine b in a) Na2SO4: 0 mA cm-2 no UV irradiation UV
irradiation; 15 mA cm-2 no UV irradiation UV irradiation; 90 mA cm
-2 no UV
irradiation UV irradiation b) H3PO4:0 mA cm-2 no UV irradiation UV
irradiation; 15 mA cm-2 no UV irradiation UV irradiation; 90 mA cm
-2 no UV
irradiation UV irradiation……………………………………………………………
75
Figure 2 Oxidation and mineralization during the electrolysis and photoelectrolysis of
solutions containing Rhodamine b in Na2SO4: 15 mA cm-2 no UV irradiation UV
irradiation; 90 mA cm-2 no UV irradiation UV irradiation…………………………
76
Figure 3 Oxidation and mineralization during the electrolysis and photoelectrolysis of
solutions containing Rhodamine b in H3PO4: 15 mA cm-2 no UV irradiation UV
irradiation; 90 mA cm-2 no UV irradiation UV irradiation………………………...
76
Figure 4 Production of oxidants by electrolyses with conductive-diamond of solutions
containing Na2SO4 no UV irradiation UV irradiation; H3PO4 no UV irradiation
UV irradiation…………………………………………………………………………
78
Figure 5 Oxidant consumed (initial – final) in chemical oxidation tests of Rhodamine
with the oxidant produced electrolytically from Na2SO4 no UV irradiation UV
irradiation; H3PO4 no UV irradiation UV Irradiation…………………………….
80
Figure 6 Changes in the COD during the chemical oxidation of Rhodamine B
10
solutions with the oxidant solution produced by the electrolyses of Na2SO4 no UV
irradiation UV irradiation; H3PO4 no UV irradiation UV Irradiation…………….
81
Figure 7 Mechanisms of the main processes occurring during photo-electrolysis of
Rhodamine B……………………………………………………………………………
83
CAPÍTULO 6
Figure 1 Effect of the sp3/sp
2 ratio of the BDD anodes on the absorbance (a), TOC (b)
and COD (c) removal, as a function of Q, during electrolysis of RhB solutions………
98
Figure 2 Chromatographic areas of aromatic intermediates produced, as a function of
time, during RhB electro-oxidation using BDD6 (a) and BDD1 (b). I1, no identified
intermediate………………………………………………………………………………
99
Figure 3 Influence of sp3/sp
2 ratio on the production of aliphatic acids, as a function of
chromatographic areas and time, during RhB electro-oxidation using BDD6 (a) and
BDD1 (b). I1 and I2, no identified intermediates…………………………………………
100
CAPÍTULO 7
Figure 1 Electrochemical decolourisation process of a real textile effluent (effluent as
obtained), as a function of time, by applying different current densities (20, 40 and 60
mA cm-2
) at 25° C and flow rate of 250 dm3 h
-1. Inset: Decrease of HU, as a function
of time, at different applied current densities. Dashed line indicates the Brazilian limit
regulation in HU to discharge the effluent……………………………………………….
111
Figure 2 Influence of applied current on the evolution of COD and TCE (inset), as a
function of time, during electrochemical treatment of actual textile effluent (as
discharged) on BDD anode at different current densities. Conditions: T = 25 °C and
flow rate of 250 dm3 h
-1………………………………………………………………….
112
Figure 3 Colour removal of a real effluent, as a function of time, applying different
current densities using BDD anode. Experimental conditions: T = 25° C, 5 g dm-3
of
Na2SO4 and flow rate = 250 dm3 h
-1……………………………………………………
113
Figure 4 Influence of applied current on the evolution of COD and TCE (inset), as a
function of time, during electrochemical treatment of actual textile using BDD anode.
11
Experimental conditions: T = 25°C, 5 g dm-3
of Na2SO4 and flow rate = 250 dm3 h
-1…. 114
Figure 5 Influence of Na2SO4 concentration on the evolution of COD and %TCE
(inset), as a function of time, during electrochemical treatment of actual textile effluent
on BDD anode by applying 20 mA cm−2
of applied current density. Conditions: T = 25
°C, flow rate = 250 dm3 h
-1………………………………………………………………
115
Figure 6 Comparison of the influence of temperature on the evolution of COD, as a
function of time and %TCE (inset) during oxidation of actual textile effluent on BDD
anode. Conditions: 40 mA cm-2
of current density; 5 g of Na2SO4 and flow rate = 250
dm3 h
-1……………………………………………………………………………………
116
Figure 7 Influence of flow rate on the evolution of COD, as a function of time and
%TCE (inset) during oxidation of actual textile effluent on BDD anode. Conditions: 40
mA cm-2
of current density; 5 g of Na2SO4 and T = 25 °C……………………………...
118
Figure 8 Energy consumption of the electrochemical process, as a function of COD
removal and effluent conditions, during oxidation of actual textile effluent on BDD
anode by applying different current densities……………………………………………
118
12
LISTA DE TABELAS
CAPÍTULO 3
Tabela 1 Classificação segundo as classes químicas e por substrato............................. 28
Tabela 2 Produtos da degradação fotocatalítica da RhB por CG-MS............................ 31
Tabela 3 Exemplos de processos oxidativos avançados e as reações envolvidas na
produção de radicais hidroxilas.......................................................................................
33
Tabela 4 Poder de oxidação do material anódico em meio ácido.................................. 41
13
LISTA DE SIGLAS
ABIQUIM Associação brasileira da indústria química
BDD Boron doped diamond anode
COT Carbono orgânico total
CG-MS Cromatografia gasosa acoplada à espectroscopia de massa
CL-MS Cromatografia líquida acoplada à espectroscopia de massa
CONAMA Conselho nacional do meio ambiente
DQO Demanda química de oxigênio
PEOA Processo eletroquímico oxidativo avançado
ELISA Ensaio imunoabsorvente ligado à enzima
HPLC High-performance liquid chromatography
OEA Oxidação eletroquímica avançada
OEDC Oxidação eletroquímica diamante condutor
ONU Organização das nações unidas
POA Processo oxidativo avançado
RhB Rodamina B
SISNAMA Sistema nacional do meio ambiente
UV-Vis. Ultravioleta na região do visível
14
SUMÁRIO
CAPÍTULO 1
1.1 INTRODUÇÃO.................................................................................................. 19
CAPÍTULO 2
2.1 OBJETIVOS........................................................................................................ 23
2.1.1 Objetivos Gerais.............................................................................................. 23
2.1.1 Objetivos Específicos...................................................................................... 23
CAPÍTULO 3
3.1 REVISÃO BIBLIOGRÁFICA............................................................................ 25
3.1.1 A Água.............................................................................................................. 25
3.1.2 Indústria Têxtil................................................................................................ 25
3.1.3 Corantes........................................................................................................... 27
3.1.3.1 Rodamina B e sua Degradação...................................................................... 28
3.1.4 Resolução Ambiental...................................................................................... 31
3.1.5 Tratamentos Convencionais de Efluentes..................................................... 32
3.1.6 Processos Oxidativos Avançados................................................................ 33
3.1.6.1 Processo de Oxidação Eletroquímica Avançada............................................ 34
3.1.6.1.1 Influência da produção de oxidantes por eletrólise na oxidação
anódica mediada......................................................................................................
35
3.1.6.1.2 Produção de oxidantes por processos de oxidação direta..................... 36
3.1.7 Ativação de Oxidantes.................................................................................... 38
3.1.8 Influência do Material Anódico – Eletrodos Ativos e Não-ativos.............. 39
3.1.9 Eletrodo de Diamante Dopado com Boro ................................................... 41
3.1.9.1 Configuração sp3/sp
2..................................................................................... 42
3.1.10 Referências.................................................................................................... 42
15
CAPÍTULO 4
INFLUENCE OF MEDIATED PROCESSES ON THE REMOVAL OF
RHODAMINE WITH CONDUCTIVE-DIAMOND ELECTROCHEMICAL
OXIDATION………………………………………………………………………
49
4.1 Abstract……………………………………………………………………..... 49
4.2 Introduction…………………………………………………………………... 51
4.3 Materials and Methods………………………………………………………. 51
4.3.1 Chemicals……………………………………………………………………. 52
4.3.2 Analytical procedures……………………………………………………….. 52
4.3.3 Electrochemical cells………………………………………………………… 52
4.3.4 Experimental procedures…………………………………………………….. 53
4.4 Results and Discussion………………………………………………………... 53
4.5 Conclusions………………………………………………………………..…... 62
4.6 Acknowledgements………………………………………………………….... 62
4.7 References…………..……………………………………………………….... 62
CAPÍTULO 5
PEROXO- AIDED PHOTO-ELECTROLYSIS OF RHODAMINE B………….. 69
5.1 Abstract……………………………………………………………………….. 69
5.2 Introduction…………………………………………………………………... 69
5.3 Materials and Methods………………………………………………………. 72
5.3.1 Chemicals…………………………………………………………………….. 72
5.3.2 Analytical procedures………………………………………………………... 72
5.3.3 Bulk electrolysis……………………………………………………………… 72
5.3.4 Chemical oxidation tests …………………………………………………….. 73
5.3.5 Light irradiation……………………………………………………………… 73
5.4 Results and Discussion………………………………………………………... 73
5.5 Conclusions……………………………………………………………………. 83
5.6 Acknowledgements…………………………………………………………..... 84
5.7 References…………………………………………………………………....... 84
16
CAPÍTULO 6
ELECTROCHEMICAL CONVERSION/COMBUSTION OF A MODEL
ORGANIC POLLUTANT ON BDD ANODE: ROLE OF sp3/sp
2
RATIO……………………………………………………………………………..
93
6.1 Abstract……………………………………………………………………….. 93
6.2 Introduction…………………………………………………………………… 93
6.3 Methodology…………………………………………………………………... 95
6.3.1 Chemicals……………………………………………………………………. 95
6.3.2 Analytical procedures………………………………………………………... 95
6.3.3 Electrochemical cell and bulk electrolysis…………………………………... 96
6.4 Results and Discussion………………………………………………………... 96
6.5 Conclusions……………………………………………………………………. 101
6.6 Acknowledgements…………………………………………………………… 101
6.7 References…………………………………………………………………….. 101
CAPÍTULO 7
APPLICABILITY OF DIAMOND ELECTRODE/ANODE TO THE
ELECTROCHEMICAL TREATMENT OF A REAL TEXTILE EFFLUENT…..
105
7.1 Abstract……………………………………………………………………….. 105
7.2 Introduction…………………………………………………………………… 105
7.3 Materials and Methods……………………………………………………….. 107
7.3.1 Textile dye effluent characteristics…………………………………………... 107
7.3.2 Anodic oxidation experiments……………………………………………….. 108
7.3.3 Depuration monitoring methods……………………………………………... 108
7.4 Results and discussion…………………………….………………………….. 110
7.4.1 Preliminary electrochemical experiments of a real textile effluent………….. 110
7.4.2 Electrochemical decolourisation of a real textile effluent adding Na2SO4….. 113
7.4.3 COD removal by electrochemical treatment…………………………………. 114
7.4.4 Effect of Na2SO4 dissolved in the effluent…………………………………… 115
7.4.5 Influence of temperature……………………………………………………... 115
17
7.4.6 Effect of the flow rate……………………………………………………….. 116
7.4.7 Energy consumption and cost estimation…………………………………….. 117
7.5 Concluding Remarks…………………………………………………………. 117
7.6 References……………………………………………………………………... 118
CAPÍTULO 8
8.0 CONCLUSÃO E CONSIDERAÇÕES FINAIS……………………………… 124
18
CAPÍTULO 1
INTRODUÇÃO
19
1.1 INTRODUÇÃO
Nas últimas décadas, o rápido crescimento da consciência pública sobre os problemas
ambientais tem obrigado a muitos governos introduzir legislações que prescreva e limita a
emissão de poluentes no meio ambiente.
Dentro deste contexto, a indústria têxtil é considerada a indústria que mais polui o
meio ambiente. Os corantes, por exemplo, são um dos contaminantes mais amplamente
encontrados em ambientes aquáticos, devido ao seu enorme volume de produção nas
indústrias, biodegradação lenta, descoloração e toxicidade. Cerca de 800.000 toneladas de
corantes são produzidas anualmente no mundo, e cerca de 10-15% dos corantes têxteis
sintéticos usados se perdem nos fluxos de resíduos durante as operações de fabricação ou
processamento (AHMEDCHEKKAT et al. 2011).
A Rodamina, corante sintético estudado nesta tese, é geralmente utilizada como um
modelo de substância para degradação por se tratar de um importante representante dos
corantes xanteno, também conhecido como violeta básica 10. É altamente solúvel em água,
não volátil e possui coloração violeta avermelhado.
O lançamento deste corante em ambientes naturais não é apenas perigoso para a vida
aquática, mas também para os seres humanos, por se tratar de uma substância mutagênica
(MISHRA e GOGATE, 2011; DUA et al. 2012).
A fim de reduzir o impacto ambiental destes poluentes, diversas tecnologias
convencionais e não convencionais têm sido utilizadas, porém, as convencionais não
conseguem degradar todas as substâncias orgânicas presentes nos efluentes têxteis,
necessitando de tratamentos complementares.
A oxidação eletroquímica avançada, OEA, tem sido utilizada como um tratamento
eletroquímico não convencional para a degradação de poluentes orgânicos. Ela é considerada
como uma alternativa eficiente devido à sua facilidade de operação, ampla gama de condições
de tratamento e respeito ao meio ambiente (DUA et al. 2012).
A presente tese está dividida em oito capítulos e será apresentada em forma de quatro
artigos. A nosso ver, essa modalidade é muito mais objetiva que o modelo de tese tradicional,
uma vez que propicia uma divulgação mais prática e rápida dos resultados obtidos. No
primeiro Capítulo será abordada toda a parte introdutória do trabalho, justificando a
importância de ter estudado o tema abordado por esta tese; No segundo Capítulo estão
20
presentes os objetivos gerais e específicos que foram cumpridos no decorrer da tese; O
terceiro Capítulo, a revisão bibliográfica onde foram abordados temas como: oxidação
eletroquímica, tipos de ânodos, corantes, entre outros, tomando como base em artigos
renomados publicados desde 1994 até os dias atuais. Nos Capítulos 4, 5, 6 e 7 estão presentes
os artigos publicados e que foram enviados para publicação, como detalhados a seguir:
O Capítulo 4 teve como objetivo estudar um corante sintético, denominado por
Rodamina B (RhB), em diferentes eletrólitos (NaCl, HClO4, H3PO4 e Na2SO4) e densidades
de corrente (15, 60, 90 e 120 mA cm-2
), através da Oxidação Eletroquímica com Diamante
Condutor, a fim de compreender o processo mediado para a sua degradação. Para isso fez-se
necessário realizar análises de Demanda Química de Oxigênio, Cromatografia Gasosa
acoplada a Espectrometria de Massa e Cromatografia Líquida de Alta Eficiência durante todo
o processo de degradação da RhB, bem como, o estudo cinético para observar para melhor
visualização da influência dos eletrólitos e das densidades de correntes estudadas. Este
Capítulo gerou o terceiro artigo desta tese e foi aceito recentemente no jornal científico
Applied Catalysis B: Environmental.
No Capítulo 5 estudou-se a oxidação da RhB em dois meios: fosfato e sulfato, através
do efeito do grupo peróxido por ativação de luz utilizando ânodo de Diamante Dopado com
Boro, onde comparou-se o processo dessa degradação por eletrólise, fotólise e fotoeletrólise,
sendo este o objetivo do Capítulo. Este estudo se fez necessário após ter sido realizado o
trabalho presente no Capítulo 4, onde se pode observar além da influência das diferentes
densidades de corrente a influência da ativação de luz em dois meios anteriormente estudados.
Análises de UV-Visível, Demanda Química de Oxigênio, Carbono Orgânico Total,
Cromatografia Líquida de Alta Eficiência foram efetuadas no decorrer da degradação da RhB,
além das análises de pH e Condutividade. Os resultados deste Capítulo foram submetidos no
jornal científico Environmental Science & Technology.
Todos os experimentos desenvolvidos nesta tese foram realizados com o ânodo de
Diamante Dopado com Boro por apresentar inúmeras vantagens, tais como: superfície inerte,
baixas propriedades de adsorção, estabilidade à corrosão mesmo em meio bastante ácido e
elevado sobrepotencial de oxigênio (PANIZZA; CERISOLA, 2007). Com o intuito de realizar
uma maior quantidade de experimentos em um menor tempo, se realizou experimentos em
paralelo utilizando diferentes ânodos de BDD nas mesmas condições experimentas para
observar se iriam ocorrer processos semelhantes de degradação, porém observou-se que não e
21
assim, se fez necessário realizar um estudo com diferentes ânodos de BDD a fim de
apresentar evidências críticas sobre a influência da configuração sp3/sp
2 do ânodo de BDD
através da oxidação eletroquímica da Rodamina B, um poluente orgânico modelo. Através
deste trabalho, surgiu o Capítulo 6 desta tese. Esse Capítulo gerou um artigo no jornal
científico Electrochemistry Communications publicado recentemente.
A aplicação de um método eletroquímico para um efluente real da indústria têxtil local
foi estudado e teve por objetivo a avaliação da aplicabilidade do processo de oxidação
eletroquímica através dos parâmetros: densidade de corrente, temperatura, adição de sais e
condições de fluxo, usando BDD como uma alternativa para tratamento de efluentes reais.
Análises de DQO, UV- Visível e de Custo Energético foram realizadas. Este trabalho
representou o Capítulo 7 desta tese. Os resultados deste Capítulo foram publicados no Journal
of Electroanalytical Chemistry.
Por fim, o Capítulo 8 apresenta as conclusões e as considerações finais de forma
resumida dos Capítulos 4 a 7.
22
CAPÍTULO 2
OBJETIVOS
23
2.1 OBJETIVOS
2.1.1 Objetivos Gerais
Avaliar a produção de espécies fortemente oxidantes e condições experimentais
durante a OE de um corante sintético e um efluente têxtil real com ânodo de diamante
dopado com boro;
Estudar a influência das propriedades do material de BDD (composição do eletrodo
(diamante (sp3)/(grafite (sp
2)) durante a OE.
2.1.2 Objetivos Específicos
Avaliar o efeito das variáveis experimentais: densidade de corrente elétrica e eletrólito
usados para cada experimento;
Aplicar o processo de OE na eliminação da matéria orgânica em corantes sintéticos e
reais;
Monitorar e comparar a eliminação dos compostos estudados mediante o uso de
técnicas analíticas (DQO, COT, UV-Visível e HPLC);
Estudar o comportamento da ativação das espécies oxidantes com e sem ativação de
luz UV para a degradação da RhB;
Analisar, através da ativação química, o comportamento dos agentes oxidantes;
Estudar um eletrólito e uma densidade de corrente aplicada a diferentes ânodos de
BDD;
Identificar intermediários gerados durante a degradação da RhB, através das técnicas
de HPLC e CG-MS;
Calcular o consumo energético quando aplicado a um efluente têxtil real.
24
CAPÍTULO 3
REVISÃO BIBLIOGRÁFICA
25
3.1 REVISÃO BIBIOGRÁFICA
3.1.1 A Água
A água, em termos químicos, é uma substância que possui em sua estrutura dois
átomos de hidrogênio e um átomo de oxigênio, apresentando uma massa molar de 18 g mol-1
.
É uma substância indispensável ao planeta, responsável pelo funcionamento e manutenção do
corpo humano, irrigação na agricultura, geração de energia nas usinas hidrelétricas, entre
outras.
Cerca de 70% da superfície terrestre é composta por água, porém apenas 2,5% são de
água doce, onde 99,7% desse volume estão presentes na forma de geleiras, cobertas por neve
e em águas subterrâneas. Ou seja, apenas 0,3% desse recurso estão próprios para o consumo e
disponíveis em rios e lagos. Mesmo em pequena porcentagem seria suficiente para abastecer a
vida na terra se não fosse à ação predatória do homem, segundo a Organização das Nações
Unidas (ONU).
Pesquisas publicadas pela ONU mostram que todos os dias 2 milhões de toneladas de
dejetos humanos são eliminados nos cursos de água em todo o mundo, e que mais de 80% das
águas residuais do planeta não são coletadas, nem tratadas, além da poluição existente em rios
e lagos proveniente das indústrias.
3.1.2 Indústria Têxtil
A indústria têxtil é responsável pela transformação de fibra em fios, de fios em tecidos e
de tecidos em roupas. Esse tipo de processo industrial inicia-se a partir da divisão das fibras, a
fiação, tecelagem e ou/ malharia, beneficiamento e enobrecimento dos fios e tecidos, até o
processo de confecções, como presentes de forma simplificada na Figura 1 (BASTIAN et al.
2009).
No processo de fiação ocorre a obtenção do fio a partir das fibras têxteis, que pode ser
enviado para o beneficiamento ou diretamente para tecelagens e malharias. No
beneficiamento é realizada a preparação dos fios para seu uso final ou não, envolvendo
tingimento, engomagem, retorção (linhas, fios especiais, etc.) e tratamento especiais. Na
tecelagem e/ou malharia ocorre à elaboração de tecido plano, tecidos de malha circular ou
26
retilínea, a partir dos fios têxteis. Na etapa de enobrecimento segue a preparação, o
tingimento, a estamparia e o acabamento de tecidos, malhas ou artigos confeccionados. E no
setor de confecções ocorre a aplicação diversificada de tecnologias para os produtos têxteis,
acrescida de acessórios incorporados nas peças (BASTIAN et al. 2009). Em ambas as etapas
de beneficiamento e enobrecimento ocorrem o processo de tingimento, ou seja, necessitam da
aplicação de corantes nas fibras/ou tecidos.
Figura 1 – Cadeia têxtil simplificada.
Fonte: BASTIAN et al. (2009).
Ultimamente, existe uma grande preocupação com o descarte de efluentes produzidos
durante qualquer processo industrial, devido à escassez de recursos híbridos e os impactos
ambientais gerados pelos mesmos (DEGAKI et al. 2014).
A indústria têxtil é um dos exemplos de indústria que mais polui entre todos os setores
industriais, tanto em termos de volume, quanto em relação à composição.
Segundo Asgheret et al. (2009), até 2009 mais de 10.000 diferentes corantes e
pigmentos foram usados em indústrias de tingimento e impressão de todo o mundo. A
produção total de corante mundial está estimada em 800.000 toneladas por ano e, vale
ressaltar que pelo menos entre 10-15% do corante usado por essas indústrias entra no
ambiente através dos efluentes.
Os corantes são os compostos mais problemáticos nos efluentes têxteis devido a sua
alta solubilidade em água, baixa biodegradabilidade, além de possuir estruturas complexas,
muito delas com cadeias aromáticas, apresentando comportamentos tóxicos, cancerígenos e
mutagênicos. Esses corantes quando descartados em ambientes aquáticos sem um tratamento
27
adequado, além de provocar poluição visual, Figura 2, alteram os ciclos biológicos, mesmo
em baixas concentrações, 1 mg. L-1
, impedindo a passagem da radiação solar e, assim,
afetando os seres vivos que habitam nestes ecossistemas (MARTÍNEZ-HUITLE et al. 2012;
DEGAKI et al. 2014).
Figura 2 - Rio Jianhe em 2011, Luoyang/China.
Fonte: Reuters/China Daily (2011).
Diante desse contexto, o tratamento de efluentes têxteis antes do descarte ao meio
ambiente é de extrema importância, uma vez que pode diminuir a sua coloração e reduzir o
índice de toxicidade.
3.1.3 Corantes
Segundo Guaratin; Zanoni (2000), a arte da tinturaria de tecidos iniciou-se há milhares
de anos. Hoje, a indústria de corantes desenvolve um importante papel na economia do
mundo, nos quais são bastantes aplicados em várias atividades fabris (PEIXOTO et al. 2013).
Corantes podem ser definidos como substâncias intensamente coloridas que lhes
conferem cor, quando aplicadas a um material. De acordo com a Associação Brasileira da
Indústria Química (ABIQUIM, 2014), eles podem ser fixados nos materiais por quatro vias:
adsorção, solução, retenção mecânica ou por ligações químicas covalentes ou iônicas.
Segundo ABIQUIM (2014), os corantes podem ser classificados de acordo com sua
classe química e também em relação à utilização por substrato, como presente na Tabela 1.
28
Tabela 1 - Classificação segundo as classes químicas e por substrato.
CLASSE QUÍMICA POR SUBSTRATO
Acridina À Cuba Sulfurados
Azo À Tina
Azóico Ácidos
Bases de oxidação Ao Enxofre
Difenilmetano Azóicos
Ftalocianina Básicos
Nitroso Diretos
Oxazina Dispersos
Tiazol Reativos
Xanteno Solventes
Fonte: ABIQUIM (2014).
Entre as classes de corantes citadas anteriormente, o corante presente nessa tese está
classificado como: xanteno.
3.1.3.1 Rodamina B e sua Degradação
Dentre inúmeros corantes utilizados pela indústria têxtil, a Rodamina B, RhB, Figura
3, também conhecida por violeta básico 10, pertence à classe dos xantenos por ter como base
na sua estrutura o próprio xanteno. É um corante altamente solúvel em água, metanol e etanol
(XIAO et al. 2014).
A RhB é aplicada na biotecnologia, como por exemplo: em Ensaio Imunoadsorvente
Ligado à Enzima (ELISA) por apresentar características fluorescentes. Este ensaio permite a
detecção de anticorpos específicos, como por exemplo, no plasma sanguíneo (MISHRA et al.
2011). Além da biotecnologia, é utilizada na indústria têxtil (ALHAMEDI et al. 2009) e de
papel por apresentar boa solubilidade em água sendo assim, adsorvida sobre todos os tipos de
fibras naturais e sintéticas.
Apesar do seu vasto poder de aplicação, a RhB pode causar graves impactos ao meu
ambiente após o seu descarte por ser de difícil degradação.
29
Figura 3 - Estrutura molecular da Rodamina B.
Fonte: Autor (2014).
Segundo Mallah et al. (2013), alguns derivados dos xantenos possuem características
tóxicas, afetando a parte neuronal, fazendo-se necessário a realização de eficientes
tratamentos dos efluentes que contêm esse tipo de corante para posteriores descartes.
A degradação da Rodamina B tem sido estudada utilizando vários métodos, como por
exemplo: foto-oxidação na presença de peróxido de hidrogênio (ALHAMEDI et al. 2009);
oxidação eletroquímica com ânodo de dióxido de chumbo dopado com cério em diferentes
densidades de corrente (LI et al. 2013); processo fotocatalítico por micro-ondas, usando o
TiO2 suportado em carvão ativado, entre outros (ZHONG et al. 2009b).
Zhong et al. (2009a; 2009b), diferentemente dos outros autores citados, propuseram
um mecanismo de reação durante a degradação da Rodamina B, como presente na Figura 4.
Esse mecanismo foi sugerido através de análises dos intermediários por cromatografia líquida
acoplada à espectroscopia de massa, bem como por cromatografia gasosa acoplada à
espectroscopia de massa; já a amostra bruta de RhB foi detectada por HPLC.
30
Figura 4 - Degradação fotocatalítica por micro-ondas da Rodamina B com TiO2 suportado em carvão ativado.
Fonte: ZHONG et al. (2009b).
De acordo com o mecanismo apresentado na Figura 4, a degradação da Rodamina B
ocorreu por meio de dois processos competitivos: o da N-etilação e outro referente à
destruição da estrutura do xanteno conjugado (LI et al. 2007).
Intermediários por meio da reação de N-etilação foram observados em pequena
proporção, como é o caso do N, N-dietil-N’-etil-rodamina por cromatografia líquida acoplada
à espectroscopia de massa, onde resultou da perda de um grupo etil do anel xanteno da
estrutura da Rodamina B bruta. Segundo Zhong et al. (2009a; 2009b) esta provável N-etilação
da Rodamina B pode estar presente no sistema, mas não é o processo dominante. Além disso,
foi destacada a presença de dois isômeros gerados a partir dos intermediários da N-etilação,
como mostra a Figura 4 (ZHONG et al. 2009a; 2009b).
31
Após a destruição da estrutura do conjugado da Rodamina B, outros dois processos
foram observados: a abertura do anel e em seguida a mineralização. Algumas moléculas de
ácidos orgânicos foram produzidas e detectadas através da técnica de CG-MS, como
apresentadas na Tabela 2, que foram mineralizados a água e a gás carbônico.
Tabela 2 - Produtos da degradação fotocatalítica da Rodamina B através da técnica de CG-MS.
m/z Tempo de retenção (min) Intermediários
90 6.737 Ácido oxálico
104 7.106 Ácido malônico
122 9.140 Ácido benzóico
118 9.720 Ácido succínico
167 9.812 Ácido 3- nitrobenzóico
148 9.862 Anidrido ftálico
116 10.024 Ácido maléico
148 10.544 Ácido 2-hidroxipentanedióico
146 11.454 Ácido adípico
138 11.927 Ácido 3-hidroxibenzóico
166 12.915 Ácido ftálico
166 13.686 Ácido tereftálico
154 14.480 Ácido 2,5-dihidroxibenzóico
Fonte: ZHONG et al. (2009a; 2009b).
3.1.4 Resolução Ambiental
O Conselho Nacional do Meio Ambiente, CONAMA, é o órgão consultivo e
deliberativo do Sistema Nacional do Meio Ambiente, SISNAMA, onde foi instituído pela Lei
6.938/81, que dispõe sobre a Política Nacional do Meio Ambiente, regulamentado pelo
Decreto 99.274/90. No Brasil, o CONAMA é o órgão responsável pela fiscalização do
descarte de efluentes no meio ambiente.
De acordo com a resolução nº 430, de 13 de maio de 2011, do CONAMA, o
lançamento indireto de efluentes no corpo receptor deverá observar o disposto nesta
32
resolução, quando verificada a inexistência de legislação ou normas específicas, disposições
do órgão ambiental competente, bem como diretrizes da operadora dos sistemas de coleta e
tratamento de esgoto sanitário.
Como citado no Art. 3º, “os efluentes de qualquer fonte poluidora poderão ser
lançados diretamente nos corpos receptores depois de um devido tratamento e desde que
obedeçam às condições padrões e exigências dispostas nesta Resolução”. Parâmetros como
pH, temperatura e compostos inorgânicos são limitados por esta Resolução para lançamento
dos efluentes no meio ambiente. No caso dos efluentes têxteis não existe uma norma
específica para o descarte dos mesmos no meio ambiente, se fazendo necessário seguir a
Resolução citada anteriormente.
3.1.5 Tratamentos Convencionais de Efluentes
Os tratamentos convencionais de efluentes estão classificados em três grupos, são eles:
físicos, biológicos e químicos.
Os tratamentos físicos são processos de separação, onde não ocorre nenhum tipo de
reação. Nesses métodos, os contaminantes são separados do meio aquoso, sem degradá-los,
por precipitação, floculação, coagulação, sedimentação, adsorção sobre carvão ativo ou
filtração com membranas.
No método químico, faz-se necessário utilizar reagentes para que ocorra a oxidação do
material orgânico presente nos efluentes, eliminando ou modificando a sua estrutura química.
Nesse método, alguns oxidantes fortes são utilizados como, por exemplo: o cloro,
permanganato de potássio e o ozônio, porém, algumas desvantagens são observadas em
relação à escolha desses oxidantes. No caso do cloro, podem gerar intermediários mais
prejudiciais do que os contidos nos efluentes iniciais (SEGURA, 2014; SIRÉS et al. 2014). O
permanganato gera resíduos de dióxido de manganês, além de ser mais caro que o cloro. Já o
ozônio é pouco solúvel em água, instável, tóxico e de difícil manuseio.
A aplicação dos tratamentos físico-químicos em águas residuais da indústria têxtil
possui algumas limitações devido à formação de lodo durante o processo, a necessidade
periódica de regenerar os materiais adsorventes, baixa eficiência de remoção e a não
eliminação do material orgânico em baixas concentrações (PANIZZA; CERISOLA, 2007;
PEKEL et al. 2013).
33
Já os métodos biológicos são os mais utilizados para o tratamento de efluentes devido
aos seus baixos custos, porém, quando se trata de efluentes contaminados com substâncias
orgânicas, a oxidação biológica pode ser dificultada devido à presença de compostos tóxicos e
moléculas refratárias ao tratamento biológico, além de requerer muito tempo e necessidade de
uma grande superfície de contato (SIRÉS et al. 2014).
3.1.6 Processos Oxidativos Avançados
Durante a última década, uma nova classe de tratamentos está sendo bastante estudada
por inúmeros grupos de pesquisas, que torna possível eliminar os poluentes presentes em
efluentes, usando técnicas de oxidação conhecidas como processos oxidativos avançados
(POA). Exemplos desse tipo de processo estão presentes na Tabela 3.
Tabela 3 – Exemplos de processos oxidativos avançados e as reações envolvidas na produção de radicais
hidroxila.
Processos Reações
Sem
luz
Ozônio com pH elevado
Ozônio + H2O2
Ozônio + catalisador
Fenton
Com
luz
Ozônio + UV +
Ozônio + H2O2+ UV
A adição de H2O2 acelera a decomposição do ozônio,
favorecendo no aumento da geração de radicais
hidroxilas.
H2O2 + UV
Foto-Fenton
Fonte: SIRÉS et al. (2014).
34
Os POA podem ser definidos como métodos de oxidação da fase aquosa, baseados na
intermediação de espécies altamente reativas em mecanismos que levam a destruição dos
poluentes (BRILLAS et al. 2009; SIRÉS et al. 2014), onde o radical hidroxila, considerado
um oxidante forte, é capaz de destruir a maior parte dos contaminantes orgânicos e
organometálicos até a sua completa mineralização em CO2, água e íons inorgânicos.
Esses radicais reagem rapidamente com os compostos orgânicos, representados pela
letra R, principalmente pela abstração de um átomo de hidrogênio (alifáticos) ou através da
adição de uma ligação insaturada (aromáticos) para iniciar uma oxidação radical em cadeia,
como presentes nas Equações de 1-6.
(1)
(2)
(3)
(4)
(5)
(6)
3.1.6.1 Processo de Oxidação Eletroquímica Avançada
As tecnologias eletroquímicas têm sido uma excelente opção para a remoção de cor e
para a redução da toxicidade de efluentes têxtil, oferecendo meios eficazes para resolver os
problemas ambientais relacionados aos efluentes gerados pelos processos industriais
(MARTINEZ-HUITLE et al. 2012; DIAGNE et al. 2014).
Dentre as tecnologias eletroquímicas, o processo de oxidação eletroquímica avançada
(POEA) apresenta características atraentes tais como: eficiência energética, fácil manuseio,
segurança (por operar em condições brandas) versatilidade (podendo ser aplicado aos
efluentes com demanda química de oxigênio no intervalo entre 0,1 a 100 g L-1
) e
compatibilidade ambiental, tendo como principal reagente o elétron, um reagente limpo
(PANIZZA; CERISOLA, 2007; SIRÉS et al. 2014).
Os processos de oxidação eletroquímica avançada incluem métodos que geram
hidroxilas heterogêneas na superfície do ânodo, como é o caso da oxidação anódica e os que
geram hidroxilas homogêneas produzidas na superfície da solução, como exemplos podem
35
citar os métodos de: eletron-Fenton, fotoeletron-Fenton (BRILLAS et al. 2009) e sono
eletroquímica (OTURON; BRILLAS, 2007; SIRÉS et al. 2014). Essas técnicas têm sido
bastante reportadas na literatura devido a sua eficiência e flexibilidade para tratamentos de
efluentes sintéticos (ANDRADE et al. 2009), descoloração de corantes (GONZÁLEZ et al.
2012; GARCIA et al. 2011, 2012; PANIZZA et al. 2011), degradação de herbicidas
(BRILLAS et al. 2007), entre outros.
As principais vantagens dos EAOPs para o tratamento de efluentes industriais estão
em relação à possibilidade de degradar a demanda química de oxigênio e o carbono orgânico
total a partir de um valor de várias centenas de gramas de O2 por litro para valores de
miligramas de O2 por litro ou de microgramas de O2 por litro, chegando à redução de 99%.
(BRILLAS et al. 2009; SIRÉS et al. 2014).
3.1.6.1.1 Influência da produção de oxidantes por eletrólise na oxidação anódica
mediada
A oxidação anódica mediada é a oxidação dos poluentes contidos nos efluentes,
através da reação química entre estes compostos e os oxidantes produzidos na superfície do
eletrodo. Os oxidantes mais comuns são: os radicais hidroxilas, cloro, (per) bromato,
persulfato, peróxido de hidrogênio, percarbonato, entre outros (PANIZZA; CERISOLA 2009;
SIRÉS et al. 2014).
Dessa forma, a oxidação anódica, além de oxidar diretamente os poluentes orgânicos
na superfície do ânodo, também gera a formação de oxidantes que podem atuar tanto sobre a
superfície dos eletrodos, como também no processo de oxidação da solução em estudo
(PANIZZA; CERISOLA 2009).
Segundo SIRÉS et al. (2014), um dos exemplos mais apresentados na literatura em
relação a oxidação eletroquímica mediada está no efeito do cloreto na oxidação de compostos
orgânicos, como apresentados nas Equações de 7 a 10.
(7)
(meio ácido) (8)
(meio alcalino) (9)
36
(10)
O cloreto normalmente está presente em águas residuais, onde são facilmente oxidados
a cloro em diversos tipos de ânodos, Equação 7. Esse oxidante gasoso difunde-se em dois
meios, ácido e alcalino, nas águas residuais, formando ácido hipocloroso e íons cloreto, bem
como hipoclorito e íons cloreto respectivamente, Equações 8 e 9, no meio da reação em
grandes quantidades. A desprotonação do ácido hipocloroso produzindo hipoclorito está
presente na Equação 10. A mistura resultante de cloro, hipoclorito e ácido hipocloroso
tornam-se totalmente reativos com muitos compostos orgânicos levando a uma completa
mineralização (SIRÉS et al. 2014).
Embora a oxidação mediada por cloreto seja bastante conhecida, não é o único caso de
processo de oxidação mediada, três importantes aspectos devem ser levados em conta com
relação à oxidação mediada, são eles (SIRÉS et al. 2014):
(a) A produção eletroquímica direta de oxidantes na superfície do ânodo por espécies não
oxidante contida no resíduo e o transporte destas espécies em direção as águas residuais;
(b) O efeito das matérias oxidantes produzidas eletrodicamente sobre os poluentes orgânicos;
(c) A ativação dos oxidantes em grandes quantidades, isto é, a formação das espécies
altamente reativas a partir de oxidantes pouco reativos.
3.1.6.1.2 Produção de oxidantes por processos de oxidação direta
Três pontos principais devem ser considerados em relação à produção de oxidantes por
processos de oxidação direta, durante o tratamento eletroquímico de efluentes (SIRÉS et al.
2014):
a) A oxidação direta de espécies na superfície do ânodo, envolvendo a formação de espécies
de radicais que se combinam para produzir oxidantes estáveis;
b) A oxidação da água para radicais hidroxila e outros ataques deste poderoso oxidante para
espécies que promovem a formação de radicais, seguindo da combinação de radicais levando
a produção de oxidantes estáveis;
c) E a redução de oxigênio produzindo peróxido de hidrogênio na superfície do cátodo.
Na primeira maneira de produzir oxidantes, a oxidação direta ocorre em muitas
espécies presentes nos efluentes, utilizando ânodos de BDD, PbO2 revestido e platina
37
promovendo a formação de radicais (ânions) como sulfatos, fosfatos e cloretos. Esses radicais
podem se combinar e gerar oxidantes estáveis no meio da reação, incluindo peroxossulfatos,
peroxofosfatos e cloro, como presentes nas Equações de 11 a 13.
(11)
(12)
(13)
Referente à segunda forma de produzir oxidantes, os radicais hidroxilas são os
intermediários gerados através da oxidação anódica da água para oxigênio. Essa oxidação
pode ser vista de forma positiva ou negativa dependendo do ânodo a ser utilizado, ativo ou
não-ativo, classificação dada por Comninellis (1994).
Nos ânodos ativos, a oxidação da água em oxigênio ocorre através de uma reação
indesejável no tratamento eletroquímico dos poluentes, afetando seriamente a eficiência do
processo, gerando um aumento significativo nos custos de operação. Já nos ânodos não-
ativos, os radicais hidroxila ajudam o processo de mineralização dos compostos orgânicos,
contribuindo para explicar as suas altas eficiências nas eletrólises (SIRÉS et al. 2014).
Alguns exemplos de reações promovidas pelos radicais hidroxilas estão presentes nas
Equações 14 a 16, como também os novos oxidantes gerados a partir do produto dessas
reações com os radicais hidroxila, Equações de 17 e 18 (SIRÉS et al. 2014).
(14)
(15)
(16)
(17)
(18)
O terceiro processo de produção de oxidantes consiste na produção de peróxido de
hidrogênio por redução de oxigênio na superfície do cátodo. Nesse caso, é utilizado um tipo
especial de cátodo, conhecido por cátodo de difusão a gás (SIRÉS et al. 2014).
38
3.1.7 Ativação de Oxidantes
Os métodos de ativação de oxidantes muitas vezes se faz necessário para aumentar a
reatividade das matérias oxidantes produzidas pelos processos de oxidação eletroquímica
avançada em compostos orgânicos. Três métodos são utilizados para ativar esses oxidantes: a
ativação química, a ativação por radiação de luz e a ativação por radiação ultrason. Dentre
esses métodos, serão abordados os dois primeiros nessa tese.
A ativação química é uma das maneiras mais importante para aumentar a eficiência de
um oxidante. Essa ativação envolve a combinação do oxidante produzido eletroquimicamente
com outra espécie, o que leva a produção de outra espécie bem mais reativa. Um exemplo
desse tipo de ativação é a combinação sinérgica de oxidantes que resulta quando o ozônio e o
peróxido de hidrogênio são combinados. Essa mistura resulta também no aumento da
produção de radicais hidroxila, ao qual se explica a melhor eficiência dos processos em que a
formação de ambos os oxidantes é promovida (SIRÉS et al. 2014).
A ativação por radiação de luz é um processo onde ocorre a formação de espécies
altamente ativas pela irradiação das mesmas. Esse tipo de radiação pode ser aplicado de forma
natural ou artificial. No processo natural utiliza-se luz solar (SALAZAR et al. 2011), já no
processo artificial lâmpadas do tipo UV. Essa foto ativação eletroquimicamente gera espécies
reativas, como é o caso do peróxido de hidrogênio e do ozônio, como presentes nas Equações
19 e 20, ajudando na degradação dos compostos orgânicos presentes em efluentes (SIRÉS et
al. 2014).
(19)
(20)
Além dos processos citados nas Equações 19 e 20, a ativação por luz aliada aos
processos de oxidação eletroquímica avançada podem decompor compostos peróxidos, como
peroxofosfatos, peroxossulfatos e peroxocarbonatos (SIRÉS et al. 2014).
3.1.8 Influência do Material Anódico – Eletrodos Ativos e Não-ativos
Inúmeros tipos de eletrodos têm sido utilizados como ânodos nos últimos anos em
conjunto com as tecnologias eletroquímicas, dentre eles, o grafite, a platina, IrO2, RuO2,
39
SnO2, PbO2, Ti/Pt, Ti/Pt-Ir, Ti/PbO2, Ti revestido com óxido de Ru/Ir/Ta e os filmes de BDD
(MARTINEZ-HUITLE et al. 2012).
A natureza desses materiais influencia fortemente na eficiência de oxidação, onde
existem ânodos que favorecem a oxidação parcial e seletiva de poluentes, ocorrendo o
processo de conversão, conhecidos por ânodos ativos; Outros que favorecem a combustão
completa para o CO2, chamados de ânodos não-ativos (COMNINELLIS, 1994; PANIZZA e
CERISOLA, 2007). De acordo com o esquema de oxidação eletroquímica presente na Figura
5 pode-se entender o comportamento desses dois tipos de ânodos na oxidação de compostos
orgânicos em eletrodo de óxido metálico com evolução simultânea de oxigênio.
Figura 5 - Esquema de oxidação eletroquímica de compostos orgânicos em ânodos ativos e não-ativos.
Fonte: COMNINELLIS (1994).
Segundo Comninellis (1994), a primeira etapa que ocorre no esquema da Figura 5 é a
reação de transferência de oxigênio, ao qual, ocorre a descarga de moléculas de água para
formar radicais hidroxilas adsorvidos, conforme a Equação 21.
(21)
As seguintes etapas dependem da natureza do material de cada eletrodo, onde podem
ser ativos ou não-ativos:
Nos eletrodos "ativos", os estados de oxidação são mais elevados estando disponíveis
na sua superfície. Nesse caso os radicais hidroxilas adsorvidos podem interagir com o ânodo,
formando o chamado óxido superior, representados na Equação 22.
40
(22)
Nesse caso, o par redox presente na superfície MOx +1 / MOx, as vezes chamado de "oxigênio
ativo" quimissorvido, pode atuar como um mediador na conversão ou na oxidação seletiva de
orgânicos em eletrodos "ativos", Equação 23.
(23)
Em relação ao comportamento dos eletrodos “não-ativos”, diferente dos ativos, a formação de
um óxido superior é excluído. Esse processo é conhecido por adsorção do" oxigênio ativo",
podendo ajudar a oxidação não seletiva de produtos orgânicos, resultando na combustão
completa para CO2, presente na Equação 24.
(24)
Em ambos os casos, eletrodos ativos e não-ativos, ocorre paralelamente uma reação de
competição de evolução de oxigênio durante a oxidação eletroquímica de compostos
orgânicos, diminuindo a eficiência do processo anódico (PANIZZA; CERISOLA, 2009).
De acordo com os mecanismos de reação presentes nas Equações de 21 a 24, ânodos
com baixo sobrepotencial de oxigênio são considerados ativos. Como exemplos desses
ânodos pode-se citar os de grafite e platina, entre outros.
Já os ânodos que possuem um sobrepotencial elevado de oxigênio são considerados
não-ativos, como é o caso dos ânodos de PbO2 e BDD (CAÑIZARES et al. 2008;
MARTINEZ-HUITLE et al. 2012).
3.1.9 Eletrodo de Diamante Dopado com Boro (BDD)
O BDD é um eletrodo sintético de diamante dopado com boro, sendo considerado um
ânodo com alto poder de oxidação, como apresentado na Tabela 4, ao qual tem despertado
41
grande interesse de vários grupos de pesquisa nos últimos anos (COMNINELLIS; CHEN,
2010).
Tabela 4 - Poder de oxidação do material anódico em meio ácido.
Eletrodos
Potencial de
oxidação (V)
Sobre potencial de
evolução de O2 (V)
Poder de oxidação
do ânodo
RuO2 – TiO2 1,4 – 1,7 0,18
IrO2 – Ta2O5 1,5 – 1,8 0,25
Ti/Pt 1,7 – 1,9 0,3
Ti/`PbO2 1,8 – 2,0 0,5
Ti/SnO2-Sb2O5 1,9 – 2,2 0,7
p-Si/BDD 2,2 – 2,6 1,3
Fonte: COMNINELLIS; CHEN (2010).
O ânodo de BDD combinado a técnicas eletroquímicas possui uma alta eficiência para
tratamentos de efluentes reais (PANIZZA; CERISOLA, 2010; AQUINO et al. 2011; SILVA
et al. 2013) e sintéticos (JUANG et al. 2013), bem como corantes sintéticos (AQUINO et al.
2013), entre outros, por apresentar várias características tecnologicamente importantes, tais
como: possuir uma superfície inerte, apresentar baixas propriedades de adsorção e ter
estabilidade à corrosão mesmo em meio bastante ácido e em casos de elevado sobrepotencial
de oxigênio (PANIZZA; CERISOLA, 2007).
Muitos trabalhos estão sendo reportados pela literatura onde se tem demonstrado que
ânodos de BDD permitem uma mineralização completa, com alta eficiência de corrente em
vários tipos de compostos orgânicos, como por exemplo: em fenóis, naftol, ácidos
carboxílicos, corantes sintéticos e efluentes reais (PANIZZA; CERISOLA, 2007).
3.1.9.1 Configuração sp3/sp
2
O eletrodo de BDD está sendo bastante utilizado juntamente com as técnicas
eletroquímicas para tratamento de efluente devido a inúmeras vantagens em relação aos
42
outros tipos de ânodos presentes na literatura. Porém estudos detalhados em relação ao efeito
de suas características, como por exemplo, da configuração sp3/sp
2 em diferentes ânodos de
BDD não tem sido ainda bastante discutido (CAÑIZARES et al. 2008 a, 2008 b).
De acordo com Cañizares et al. (2008 a, 2008 b), o processo eletroquímico pode ser
bastante influenciado quando comparados as diferentes naturezas dos ânodos de BDD, onde
as diferentes características nesses ânodos afetam no processo de degradação dos efluentes em
estudo.
Cañizares et al. (2008a, 2008b) estudaram onze ânodos de BDD variando três
característica do diamante: a sua dopagem com boro (entre 100 a 8000 ppm), espessura da
camada (1,05 a 2,33 µm) e a relação sp3/sp
2 (43 a 105), bem como outras duas características
referentes ao substrato p-Si, como rugosidade (< 0,1 e entre 0,3-0,5 µm) e resistividade (10 e
100 mΩ cm), utilizando a oxidação eletroquímica para o tratamento de efluentes sintéticos
contendo fenol. Entre os parâmetros estudados, os que se destacaram através de um
tratamento estatístico para a oxidação do fenol foram à rugosidade e a relação sp3/sp
2.
Observou-se, através da rugosidade, que quanto mais áspera à superfície do ânodo maior era a
eficiência do processo, favorecendo assim, a oxidação direta. Em relação à configuração
sp3/sp
2, o aumento do valor dessa interação favoreceu numa maior eficiência no processo de
oxidação do fenol.
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48
CAPÍTULO 4
INFLUENCE OF MEDIATED PROCESSES ON THE REMOVAL OF RHODAMINE
WITH CONDUCTIVE-DIAMOND ELECTROCHEMICAL OXIDATION
49
INFLUENCE OF MEDIATED PROCESSES ON THE REMOVAL OF RHODAMINE
WITH CONDUCTIVE-DIAMOND ELECTROCHEMICAL OXIDATION
Danyelle Medeiros de Araújo1, Cristina Sáez
2, Carlos A. Martínez-Huitle
1,*,
Pablo Cañizares2, M. A. Rodrigo
2
1 Institute of Chemistry, Federal University of Rio Grande do Norte, Lagoa Nova CEP 59078-
970, Natal, RN, Brazil
2 Department of Chemical Engineering, Enrique Costa Building, Campus Universitario s/n
13071 Ciudad Real, Spain
4.1 Abstract
The influence of the mediated oxidation on the removal of Rhodamine B (xanthene dye)
solutions with conductive-diamond electrochemical oxidation (CDEO) is studied. To do this,
four different supporting electrolytes have been used: Na2SO4, HClO4, H3PO4 and NaCl.
Total removal of organic pollutants is attained with CDEO regardless of the supporting
electrolyte media used, although media clearly influences on efficiency and rate of the
processes. Sulfate and phosphate media show a similar behavior, whereas electrolysis in
perchlorate media behaves surprisingly better than chloride media. Current density is playing
an important role. In all cases, CDEO follows a first order kinetic (linear trend in semi
logarithmic plot) and kinetic constants are generally much greater than expected according to
a single mass transfer electrolytic model. This is not the expected result for a direct
electrochemical oxidation process and it indicates the importance of mediated electrochemical
processes in the removal of Rhodamine B. The harsh oxidation conditions of CDEO leads to
the formation of less reaction intermediates than other technologies. The presence of short
chain aliphatic acids is discharted, and the intermediates (aromatic acids) formed during the
initial stages of the process are rapidly mineralized to carbon dioxide. In chloride media,
chlorinated intermediates are also formed by the action of hypochlorite.
Keywords: Electrochemical oxidation, diamond, dyes, mediated oxidation, supporting
electrolyte.
50
4.2 Introduction
The treatment of textile effluents has given much attention in the last years because the
discharge of highly pigmented synthetic dyes to the ecosystem causes significant damage to
aquatic and human lives (CARNEIRO et al. 2010). Conventional technologies are inefficient
to remove these organics due to the high water solubility of dyes (in the case of using
coagulation and precipitation) (MUTHUKUMAR et al. 2005), and to the typical pH and salt
concentration of dye effluents (in the case of using biological methods) (PANDEY et al.
2007). In this point, Advanced Oxidation Processes appear as a good alternative for the
treatment of synthetic dyes effluents. Among them, Fenton oxidation (CRUZ-GONZALEZ et
al. 2012; PANIZZA et al. 2009b), photocatalysis (HE et al. 2009; MILLER et al. 2013),
sonochemical degradation (GUZMAN-DUQUE et al. 2011; MEROUANI et al. 2010) and
electrochemical technologies (MARTÍNEZ-HUITLE; FERRO, 2006; SAEZ et al. 2007;
BRILLAS et al. 2009; MARTÍNEZ-HUITLE; BRILLAS, 2009; PANIZZA; CERISOLA,
2009; ROCHA et al. 2012; MARTINEZ-HUITLE et al. 2012; TAVARES et al. 2012) have
been widely studied in literature for the treatment of a great variety of dyes. In general, these
technologies are able to attain very good results in terms of decolorization due to the rapid
cleavage of chromophore group of the dye molecule (MARTÍNEZ-HUITLE; BRILLAS,
2009). However, mineralization efficiency shows a marked influence of the oxidative capacity
of each degradation technology. Generally, aromatic and aliphatic acids are the main
intermediates detected during dye degradation process, although their maximum
concentration depends on the conditions used in each case. In this point, recent works have
shown that electrochemical oxidation, and in particular conductive-diamond electrochemical
oxidation (CDEO), can be successfully applied with high organic removal rates and without
important operational limitations (FAOUZI et al. 2006; SAEZ et al. 2007; AQUINO et al.
2012; AQUINO et al. 2013; RAMIREZ et al. 2013; SIRÉS et al. 2014; YU et al. 2014). The
harsh oxidation conditions attained with CDEO are explained in terms of the oxidation
mechanisms involved in the process. In this way, it is well documented that during CDEO a
noteworthy production of hydroxyl radicals in the nearness of diamond surfaces takes place.
These radicals are fully available for oxidizing reactions but their lifetime is very short and
not enough to let them diffuse to the bulk solution (SIRÉS et al. 2014). Thus, the action of
51
these radicals is limited to the short region close to the electrode surface where they are
produced, and the kinetic of these processes is usually controlled by mass transport.
Additionally, it is well documented the availability of CDEO to produce inorganic
oxidants, which are difficult or even impossible to be produced with other different anodic
materials, from the oxidation of supporting electrolyte (CAÑIZARES et al., 2005a;
CAÑIZARES et al. 2009; SÁEZ et al. 2008). In fact, some works have also been focused on
the electrochemical synthesis with diamond anodes of powerful oxidants (SANCHEZ-
CARRETERO et al. 2011) such as persulfates (SERRANO et al. 2002), perphosphates
(CAÑIZARES et al. 2005a), perchlorates (BERGMANN et al. 2014; SÁNCHEZ-
CARRETERO et al. 2011) and hypochlorite (CAÑIZARES et al. 2009; VACCA et al. 2011).
Thus, besides direct electrooxidation on the surface and oxidation by means of hydroxyl
radicals in a region close to the electrode surface, the oxidation mediated by other oxidants
electrogenerated on the diamond surface from the electrolyte salts should be taken into
account, as it seems to increase the global oxidation efficiency (CAÑIZARES et al. 2005b;
PANIZZA; CERISOLA, 2009a; RODRIGO et al. 2010; AQUINO et al. 2012).
The dye under consideration is RhB. It is widely used in textiles, leathers and food
stuffs with high water solubility (HOU et al. 2011; MILLER et al. 2013). Thus, this work
focuses on the CDEO of synthetic Rhodamine B (selected as model of xanthene dyes with
very good stability) solutions in different supporting media in order to increase the
understanding of the role of mediated oxidation on the degradation process.
4.3 Materials and Methods
4.3.1 Chemicals
Rhodamine was supplied by Sigma-Aldrich Laborchemikalien GmbH (Steinheim,
Germany). Anhydrous sodium sulphate, sodium chloride, perchloric acid and phosphoric acid,
used as supporting electrolytes, were analytical grade purchased from Fluka. All solutions
were prepared with high-purity water obtained from a Millipore Milli-Q system, with
resistivity > 18 M cm at 25ºC. Sulphuric acid and sodium hydroxide used to adjust the
solution pH were analytical grade and supplied by Panreac Química S.A. (Barcelona, Spain).
52
4.3.2 Analytical procedures
The Chemical Oxygen Demand (COD) concentration was monitored using
spectrophotometer Hach Modelo DR/ 2000. Measurements of pH and conductivity were
carried out with an InoLab WTW pH-meter and a GLP 31 Crison conductimeter, respectively.
The concentrations of the compounds were quantified by HPLC (Agilent 1100 series). The
detection wavelength used to detect dye was 254 nm. The column temperature was 25ºC.
Volume injection was set to 50 μL. The analytical column used was Phenomenex Gemini 5
μm C18. The mobile phases were 0.9 ml of 98% formic acid in milli-Q water for analyzing
intermediates. Samples extracted from electrolyzed solutions were filtered with 0.20 μm
Nylon filters before analysis. Moreover, the acids intermediates formed during the
experiments were detected with a detection wavelength of 254 nm.
Samples of analyte were extracted into non-aqueous medium (2 mL of acetonitrile
HPLC grade with 20 µL of electrolysis sample) and were subjected to GC-MS analysis using
GC-FOCUS and MS-ISQ Thermo Scientific to identify the intermediates following the
conditions: GC: Varian column VF5 ms with a composition of 5% de fenil-arylene and 95%
de dimetilpolisiloxane. Temperature program: 40ºC – 5 min; 12°C/min – 100ºC; 10ºC/min –
200 ºC and 10ºC/min - 270 ºC – 5 min. Injector: 220ºC. Mode: Splitless. Gas flow: 0.8
mL/min. MS: Transfer line: 270ºC; ions source temperature: 220ºC, Mass range: 40-500 m/z.
Injection: 1µL.
4.3.3 Electrochemical cells
Electrolyses were carried out in a single compartment electrochemical flow cell
working under a batch-operation mode [CAÑIZARES et al. 2005b]. Conductive –Diamond
Electrodes (p-Si–boron-doped diamond) were used as anode and a stainless steel (AISI 304)
as cathode (Figure 1). Both electrodes were circular (100 mm diameter) with a geometric area
of 78 cm2
and an electrode gap of 9 mm. Boron-doped diamond films were provided by
Adamant Technologies (Neuchatel, Switzerland) and synthesized by the hot filament
chemical vapor deposition technique (HF CVD) on single-crystal p-type Si <100> wafers (0.1
Ωcm, Siltronix).
53
Figure 1. BDD anode (left) and stainless steel cathode (right).
Fonte: Autor (2014).
4.3.4 Experimental procedures
Bench-scale electrolyses of 1000 cm3 of wastewater were carried out under
galvanostatic conditions. The current density employed ranged from 15-120 mA cm-2
. The
cell voltage did not vary during electrolysis, indicating that conductive-diamond layers did
not undergo appreciable deterioration or passivation phenomena. Prior to use in galvanostatic
electrolysis assays, the electrode was polarized during 10 min in a 3.5 x 10-3
M Na2SO4
solution at 15 mA cm-2
to remove any kind of impurity from its surface.
The wastewater consisting of 2.09 x 10-4
M of Rhodamine B and 7.04 x 10-3
M of
different electrolytes (Na2SO4, HClO4, H3PO4 and NaCl) was stored in a glass tank and
circulated through the electrolytic cell by means of a centrifugal pump (flow rate 21.4 dm3 h
-
1). A heat exchanger coupled with a controlled thermostatic bath (Digiterm 100, JP Selecta,
Barcelona, Spain) was used to maintain the temperature at the desired set point (25ºC).
4.4 Results and discussion
Figure 2 shows the progress of the mineralization during electrolyses at two large
current densities (60 and 120 mA cm-2
) of synthetic wastewater consisting of aqueous
solutions containing 100 mg dm-3
of Rhodamine B (RhB) and different electrolytes (Na2SO4,
HClO4, H3PO4 and NaCl). As it can be observed, total removal of organic pollutants can be
attained with CDEO regardless of the supporting electrolyte media used, although media
clearly influences on efficiency and rate of the processes, and current density is playing an
54
important role. This is interesting because it confirms that CDEO is robust enough to deplete
pollution but it also informs about the occurrence of different mechanisms of oxidation that
should be related with the supporting media. Thus, although chloride media could be expected
to show the best results because of the well-known production of chlorine (and then of
hypochlorite and hypochloric acid), it attains the worst results and, in fact, it is the only case
in which the complete mineralization is not attained for a current charge applied of 100 mA h
dm-3
. This behavior can be attributed to an interaction between hydroxyl radicals and Cl−
(chloride acts as scavenger of OH) to form different active chlorine species on BDD surface
(CAÑIZARES et al. 2009; SÁNCHEZ-CARRETERO et al. 2011; BERGMANN et al. 2009)
with this non-active material and to the formation of refractory species by chlorination. At the
same time, the chloride concentration in the solution can also promote the importance of Cl2
production at BDD surface, decreasing the Cl−, and consequently, the production of active
chlorine species.
Figure 2. Mineralization of Rhodamine B, as a function of Q, during electrolysis in different supporting
electrolytes at two current densities (a) 60 mA cm-2
and (b) 120 mA cm-2
: Na2SO4; HClO4; H3PO4;
NaCl.
0 50 100 1500.0
0.5
1.0
a)
Q/ Ah dm-3
Min
era
lization/
0/ 1
0 50 100 1500.0
0.5
1.0
b)
Q/ Ah dm-3
Min
era
lization/
0/ 1
A very interesting observation is the effect of current density in the perchlorate test
that shifts towards higher efficiencies at higher current densities. Initially, it is strange because
in that media no oxidants production is expected and perchlorate is not a good oxidant at
room temperature so, it is not expected to participate in the oxidation. However, higher
concentration of hydroxyl radicals are produced under these experimental conditions as well
55
as it can be related to the nature of the hydroxyl radicals produced at different non-active
anodes, as already proposed by Bejan and co-workers (BEJAN et al. 2012). On the other
hand, sulfate and phosphate media show a similar behavior in both current densities tested.
Figure 3 shows a semi-logarithmic plot of the COD decay with the current charge for
the same current densities shown in Figure 2.
Figure 3.Changes in the COD, as a function of Q, during electrolysis of Rhodamine solutions containing
different supporting electrolytes at two current densities (a) 60 mA cm-2
and (b) 120 mA cm-2
: Na2SO4;
HClO4; H3PO4; NaCl.
0 50 100 1500.01
0.1
1
a)
Q/ Ah dm-3
CO
D/C
OD
0/ m
g d
m-3
0 50 100 1500.01
0.1
1
b)
Q/ Ah dm-3
CO
D/C
OD
0/ m
g d
m-3
As it can be observed, except for an abrupt decrease during the very first stages of the
electrolysis of the synthetic wastewater with chloride media, all tests follow a clear linear
trend suggesting that they can be modeled with a first order kinetic, as they were carried out
under galvanostatic conditions. This divergence can be explained in terms of the masking
effects of chlorides on the measurement of COD. Not great differences are obtained if results
of the electrolyses of sulfate and phosphate media are compared. In fact, results with these
two supporting electrolytes do not seem to depend on the current density and on the applied
charge. However, as it was forwarded by the changes in the mineralization, perchlorate
removal rate increases very importantly when current density increases. The same can be
observed in the electrolysis in chloride media.
To better observe this influence of the supporting media and current density, and
taking into account the linear trend observed in COD decay vs. Q, experimental results of
each test were fitted to a first order kinetic decay. Figure 4 shows the influence of the
supporting media and applied current density on the value of these first order kinetic constants
56
for COD removal. These constants were calculated from the slope of semi-logarithmic vs.
time plots.
Figure 4. Influence of the current density on the first order kinetic constants for the CDEO of Rhodamine B:
Na2SO4; HClO4; H3PO4; NaCl.
0 50 100 1500.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
j / mA cm-2
k / m
in-1
For the case of NaCl, the points corresponding to the first oxidation stage (which fit
well to this trend) were used to fit the constant, whereas for the rest of supporting electrolytes
the complete set of data was used. The increase of kinetic constants with the applied current
density is more important from current densities above 100 mA cm-2
. This is the expected
behavior of a process with a significant contribution of mediated electro-oxidation: the higher
the current density, the higher the production of mediated electro-reagents and, hence, the
higher the kinetic constant. The lower values obtained in the electrolysis carried out with
sodium chlorine as supporting electrolyte can be related to the importance of Cl2 production at
BDD surface, decreasing the production of active chlorine species, and consequently,
disfavoring the oxidation of organic matter. In addition, it is important to notice that
calculated kinetic constants are much greater than the value expected for a purely mass
transfer controlled process (around 0.01 min-1
), mainly in the case of working at 120 mA cm-
2. This value was calculated from a ferro-ferricyanide standard test carried out with the same
experimental setup and under the same flow conditions (AQUINO et al. 2012). This certifies
that mediated oxidation carried out by electrogenerated oxidants in bulk region plays an
important role in the degradation of RhB and mineralization process.
Efficiency of electrolytic processes can be discussed in terms of different parameters
such as Total Current efficiency (TCE), Alternating Current efficiency (ACE), etc. However,
57
in case of a good fitting of results in galvanostatic experiments to a first order kinetics (as it is
the case of the results obtained in this work), it can be demonstrated that efficiency depends
linearly on the COD concentration and the constant can be easily calculated from the kinetic
constant as reported in literature (AQUINO et al. 2014). This constant is of a great importance
because it gives directly the value of the efficiency (% of the applied current used in oxidizing
organics) if multiplied by the COD (mg dm-3
) and in the authors’ opinion is the most
interesting efficiency parameter to characterize CDEO processes. Figure 5 shows the changes
in this parameter for the four supporting electrolytes and current densities assessed. As it can
be observed, efficiency decreases as the current density increases, although it increases
slightly at high current density (above 100 mAcm-2
).
Figure 5. Instantanteous current efficiency constants for the electrolysis of Rhodamine B solutions in different
supporting electrolyte media: Na2SO4; HClO4; H3PO4; NaCl.
0 50 100 1500.00
0.02
0.04
0.06
0.08
0.10
0.12
tCODη.tICE
j / mA cm-2
Other important point to be studied is the production of intermediates, because some
of them can be even more hazardous than the initial pollutant. Their relative concentration and
trend can help to increase the understanding of the process. Figure 6 shows the
58
Figure 6. Main intermediates detected and RhB, as a function of time, during the electrolysis of Rhodamine B solutions containing different supporting electrolytes ((a)
HClO4, (b) NaCl, (c) H3PO4 and (d) Na2SO4) at 60 mA cm-2
: (*) RhB; phthalic acid (); 2,5-hydroxybenzoic acid (); benzoic acid (); 3-dinitrobenzoic acid (♦); α-
hidroxyglutaric acid (); intermediate 6 () and intermediate 7 ().
0 100 200 300 400 5000
20000
40000
60000
80000
100000
120000
140000
160000
180000
a)
t / min.
Chro
mato
gra
phic
are
a
0 100 200 300 400 5000
20000
40000
60000
80000
100000
120000
140000
160000
180000
b)
t / min.
Chro
mato
gra
phic
are
a
0 100 200 300 400 5000
1000
2000
3000
4000
5000
t / min.
Chro
mato
gra
phic
are
a
0 100 200 300 400 5000
20000
40000
60000
80000
100000
120000
140000
160000
180000
c)
t / min.
Chro
ma
tog
rap
hic
are
a
0 100 200 300 400 5000
20000
40000
60000
80000
100000
120000
140000
160000
180000
d)
t / min.
Chro
ma
tog
rap
hic
are
a
59
Restricting now our analysis to the intermediates produced and identified, a lower
amount of intermediates (phthalic acid (); 2.5-hydroxybenzoic acid () and benzoic acid
(), with low-units of chromatographic area) is produced when HClO4 is used as supporting
electrolyte, being completely oxidized after 300 min of electrolysis (Fig. 6a). For the case of
H3PO4 and Na2SO4, the same intermediates were identified (Figs. 6c and 6d); however, the
concentration of them is very different than those obtained to HClO4 (Fig. 6a). An important
observation is the significant concentration produced of 2.5-hydroxybenzoic acid when
HClO4, H3PO4 and Na2SO4 are used as supporting electrolytes (Fig. 6a, 6c and 6d) while the
concentration of the other intermediates is minor (phthalic acid () and benzoic acid (),
respectively). However, the production of 2.5-hydroxybenzoic acid indicates that RhB is
fragmented to form benzoic acid (without formation of N-de-ethylated intermediates), which
is successively oxidized to 2.5-hydroxybenzoic acid, justifying its higher concentration. Then,
RhB is preferentially oxidized by strong oxidants (hydroxyl radicals and persulfates/
peroxodiphosphates) produced at BDD surface, depending on the supporting electrolyte used.
These intermediates are completely eliminated after 300 min, in all cases, evidencing the
higher electrocatalytic efficiency of CDEO process to remove dyes. It is important to remark
that, traces of oxalic acid were detected in the end of electrolyses when H3PO4 and Na2SO4
are used.
On contrary, for NaCl, seven sub-products are produced (phthalic acid (); 2.5-
hydroxybenzoic acid (); benzoic acid (); 3-dinitrobenzoic acid (♦); α- hidroxyglutaric acid
(); chloroform () and intermediate 7 ()) when RhB was electrochemically oxidized. No
complete elimination of all intermediates formed was accomplished after 500 min of
electrolysis, but lower concentration of them remains in solution as showed by minor
chromatographic areas recorded. This fact may be explained by the effective attack of
hydroxyl radicals and active chlorine species produced on BDD surface (ROCHA et al. 2014)
at NaCl media to RhB. It is important to remark that, the presence of chloroform in the
absence of other chlorinated precursors, seems to suggest that, it might be related with some
gas-phase reactivity along the complex analytical path itself, rather than to reactions at/near
electrode (anode) surface in aqueous media, opening the possibility of the use of BDD anodes
for Cl-mediated oxidation under specific conditions of NaCl concentration. The
electrochemical pathway for RhB as a function of supporting electrolyte is present in Figure
7.
60
Figure 7. Electrochemical pathways degradation for RhB as a function of supporting electrolyte: blue pathway is
according to the intermediates produced when HClO4, H3PO4 and Na2SO4 were used while red pathway is
followed when NaCl was used. Intermediates detected: phthalic acid (I1); benzoic acid (I2); 3-dinitrobenzoic acid
(I3); 2.5-hydroxybenzoic acid (I4); α- hidroxyglutaric acid (I5); oxalic acid (I6), intermediate 7 (I7) and
chloroform (I8).
Figure 8 shows the maximum relative chromatographic area of the intermediates
generated at different applied current density and supporting media. Comparing results,
neither current density nor supporting media influence significantly in the maximum
concentration of each intermediate, except in the case of chlorine media in which chlorinated
intermediates in lower relative concentration seem to be produced (chloroform () and
intermediate 7 ()). At the light of these results, CDEO leads to the formation of less reaction
intermediates than other technologies such as photocatalysis degradation (HE et al. 2009,
ZHONG et al. 2009) and Fenton oxidation (HOU et al. 2011) in which benzoic and phtalic
acids, aliphatic acids (mainly adipic and glutaric acids) and alcohols of short chain are formed
in relevant concentrations. In this case, short chain intermediates are not detected (at least
with the analytical method used). The strong oxidation conditions of this technology favor the
mineralization of principal intermediates into carbon dioxide and refractory species are not
formed in any case.
61
Figure 8. Influence of the current density on the maximum concentration of intermediates detected during the electrolysis of Rhodamine B solutions containing different
supporting electrolytes, (a) HClO4, (b) NaCl, (c) H3PO4 and (d) Na2SO4: phthalic acid (); 2.5-hydroxybenzoic acid (); benzoic acid (); 3-dinitrobenzoic acid (♦); α-
hidroxyglutaric acid (); intermediate 6 () and intermediate 7 ().
0 40 80 1200
4
8
12
16a)
j/ mA cm-2
Inte
rmed
iate
s (
rela
tive
ch
rom
ato
gra
ph
ic a
rea)
0 40 80 1200
1
2
3b)
j/ mA cm-2
Inte
rmed
iate
s (
rela
tive
ch
rom
ato
gra
ph
ic a
rea)
0 40 80 1200
2
4
6
8
10c)
j/ mA cm-2
Inte
rmed
iate
s (
rela
tive
ch
rom
ato
gra
ph
ic a
rea)
0 40 80 1200
4
8
12d)
j/ mA cm-2
Inte
rmed
iate
s (
rela
tive
ch
rom
ato
gra
ph
ic a
rea)
62
4.5 Conclusions
From this work the following conclusions can be drawn:
RhB can be successfully removed using conductive-diamond electrochemical oxidation
regardless of the supporting electrolyte media, although media clearly influences on
efficiency, rate and the degradation pathway of the processes, and current density is playing
an important role.
Kinetic constants are much greater than the value expected for a purely mass transfer
controlled process, indicating that there is a significant contribution of mediated oxidation
processes.
The harsh oxidation conditions of CDEO favor the rapid mineralization of RhB into
carbon dioxide. Aromatics intermediates (mainly phthalic acid and benzoic acids) formed by
the cleavage of RhB molecule are the only intermediates detected in the first stages of the
degradation process. Neither short chain intermediates nor refractory species are formed in
any case. In case of chloride media, chlorinated intermediates can be formed by the attack of
hypochlorite to organic compounds (in this case, chloroform and intermediate 7). Considering
that 50 ppm is the permissible exposure limit of chloroform [MARTÍNEZ-HUITLE;
BRILLAS, 2008], only lower concentrations of it were detected when NaCl was used as
supporting electrolyte, being feasible the application of CDEO.
4.6 Acknowledgements
D.M.A. acknowledges CAPES for PhD fellowship and CNPq for the scholarship given
for “doutorado sanduíche” under “Ciências sem Fronteiras” program to develop the
experimental research at the UCLM-Spain. This work has been supported by the Spanish
Government through the project CTM2013-45612-R and EU through project FEDER 2007-
2013 PP201010 (Planta Piloto de Estación de Regeneración de Aguas Depuradas).
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CAPÍTULO 5
PEROXO- AIDED PHOTO-ELECTROLYSIS OF RHODAMINE B
69
PEROXO- AIDED PHOTO-ELECTROLYSIS OF RHODAMINE B
Danyelle Medeiros de Araújo1, Cristina Sáez
2, Pablo Cañizares
2, Carlos A. Martínez-Huitle
1,*,
Manuel Andrés Rodrigo2
1 Federal University of Rio Grande do Norte, Instituto de Química, Lagoa Nova CEP 59078-
970 - Natal, RN, Brazil
2 Department of Chemical Engineering. Universidad de Castilla-La Mancha, Campus
Universitario s/n, 13071 Ciudad Real, Spain
5.1 Abstract
This work aims at giving light about the mechanisms of the electrolysis of organic pollutants
during the treatment of wastewater containing sulfate and phosphate salts as supporting
electrolytes. To do this, the treatment of synthetic wastewater containing Rhodamine B is
assessed by electrolysis, photoelectrolysis and chemical oxidation with the oxidants produced
electrolytically from a sulfate or phosphate solution. Results show that Rhodamine B was
effectively oxidized by electrolysis and by chemical oxidation with the oxidants produced
during the electrolysis of sulfate or phosphate solutions (peroxodisulfate and
peroxodiphosphate, respectively). Light irradiation has a very positive effect during the
electrolysis of RhB being this effect is greater at high current densities. However, this effect
was much lower in the case of the chemical oxidation with peroxosulfates and
peroxophosphates produced electrolytically. This behavior is explained in terms of the effect
of the direct electrochemical oxidation processes and by the activation of oxidants in the bulk
of the electrolytic treatment by production of highly efficient radicals.
Keywords: electrolysis, photoelectrolysis, peroxosalts, rhodamine B, boron doped diamond
5.2 Introduction
Electrochemical oxidation of organics has been a key topic in environmental
electrochemistry for the last two decades with two main research lines: development and
testing of new electrode materials and combination of electrolysis with other technologies
(BEBELIS et al. 2013;SIRÉS et al. 2014).
70
Novel anode materials were hot topic at the turn of the 21st century with the
development of novel conductive diamond coatings (RODRIGO et al. 2001; POLCARO et al.
2004; 2005; PANIZZA; CERISOLA, 2005; MARTINEZ-HUITLE; FERRO, 2006; 2009;
BRILLAS; MARTINEZ-HUITLE, 2011; MARTINEZ-HUITLE; BRILLAS, 2009;
SCIALDONE et al. 2012; SIRES; BRILLAS, 2012; PANIZZA; MARTINEZ-HUITLE,
2013), robust in terms of chemical and electrochemical stability and efficient in terms of the
use of electricity.
This made this technology become a promising alternative to other Advanced
Oxidation Processes (AOP) like Fenton Oxidation and Ozonation (CANIZARES et al. 2009b;
ELAHMADI et al. 2009; MICO et al. 2010), especially for the coarse removal of COD during
the treatment of highly loaded aqueous wastes (up to 20000 g m-3
) down to the typical
discharge limits to municipal sewer system (approx. 1500 g m-3
) (RODRIGO et al., 2010b;
SOLANO et al. 2013; SANTOS et al. 2014).
Nowadays, combination of anodic oxidation with other processes such as oxidants
production, ultraviolet light (UV) irradiation or ultrasound (US) irradiation is becoming the
new technological challenge (POLCARO et al. 2009; SCIALDONE et al. 2009; VACCA et
al. 2011; WOLS; HOFMAN-CARIS, 2012; SHIH et al. 2014) with highly efficient
technologies as relevant as the electro-Fenton or the photo-electrolysis which are enlarging
the range of applicability of electrolysis in the lower limit of concentration from the 1500 mg
dm-3
of the single electrolysis to very much lower concentrations of pollutants (OTURAN,
2000; 2008; BRILLAS et al. 2009; SOUZA et al. 2013a; PANIZZA et al. 2013;
SCIALDONE et al. 2013; BORRAS et al. 2013) and hence, they are expanding the range of
applicability from the treatment of industrial wastes to other very interesting problems, such
as reclaiming of urban wastewater and removal of persistent pollutants in diluted flows
(RODRIGO et al. 2010a).
Combination of anodes based on conductive anode coatings and light irradiation has
been studied in several recent manuscripts (SOUZA et al. 2013b). As stated in them,
combination of light irradiation and electrolysis is not looking always for heterogeneous
photocatalytic processes occurring on the surface of the anode, but in many cases is looking
for the homogeneous activation of oxidants occurring in the bulk (ALVES et al. 2010;
MALPASS et al. 2010). At this point, it is interesting to take in mind that during anodic
oxidation of wastewater with diamond anodes, it is known the production of many types of
71
oxidants (CANIZARES et al. 2005c; 2009c) including peroxo-species when carbonates,
sulfates and phosphates are contained as salt in wastewater (Eqs. 1 to 3).
2 SO42-
→ S2O82-
+ 2 e- (1)
2 PO43-
→ P2O84-
+ 2 e- (2)
2 CO32-
→ C2O62-
+ 2 e- (3)
Production of these peroxosalts has been studied in other research works using a
suitable raw matter to maximize its efficiency (SERRANO et al. 2002; CANIZARES et al.
2005b; VELAZQUEZ-PENA et al. 2013). In addition, their occurrence during the oxidation
of wastewater has been explained for the remediation of wastewater (PANIZZA; CERISOLA,
2009). Peroxo group –O=O- is known to conform a very interesting group of oxidants
(SANCHEZ et al. 2013). They are powerful oxidants from the viewpoint of reduction
potential, but presence of oxidants in the wastewater during electrolysis clearly suggests that
they are kinetically slow and need presence of activation agents to be transformed into more
active species from the kinetic point of view (particularly radicals) for peroxoanions. This
production of radicals can be promoted using light irradiation (as it can be seen in reactions 4
to 6) but they also can be formed by the synergistic interactions of oxidants in the bulk
(chemical activation).
S2O82
+ hν→ 2(SO4) (4)
P2O84
+ hν→ 2(PO42-
) (5)
C2O62
+ hν→ 2(CO3) (6)
The aim of this work is to study, for a synthetic wastewater containing a large
molecule (the dye Rhodamine B) and sodium phosphate or sulfate as main salt, the effect of
this peroxo group on the oxidation trying to give light on the mechanisms that are occurring
during the electrolysis of wastewater with diamond anodes, trying to determine if the results
of the electrolysis are a consequence of the addition of oxidation processes or if there are any
synergistic process (SCIALDONE, 2009; 2010; MARTINEZ-HUITLE; ANDRADE, 2011;
OLIVEIRA et al. 2012) and trying to determine the effect on results of light irradiation.
72
5.3 Material and Methods
5.3.1 Chemicals
The chemical reagents used in this study were of high purity. Solution of 7.04×10-3
M
Na2SO4 or H3PO4 were prepared and after that, this electrolyte was used for preparing a
solution of RhB with concentration of 2.09×10-4
M, for each experiment. All aqueous
solutions were prepared using Milli-Q water.
5.3.2 Analytical procedures
Color removal was monitored by UV-visible technique using a UV-1603
spectrophotometer Model Shimadzu, at a wavelength of 550 nm. Chemical oxygen demand
(COD) was performed by using pre-dosage vials with 2 mL of sample. For the analysis of
total organic carbon (TOC), it was necessary minimum volume of 10 mL of each sample
which were placed in a glass all of the equipment and analyzed. These analyzes were carried
out in a TOC analysator Multi N/C and the results obtained through the Analytikjena
program. RhB samples before and after the electro-oxidation were collected and transferred to
vials own high-resolution liquid chromatography, 50 µL of each sample was injected for
analysis in Agilent 1100 HPLC (flow of 0.200 mL/min). Two columns were used (aromatics
and aliphatic acids) for the analysis of intermediates generated by the electrochemical
oxidation of Rhodamine B. The mobile phases were 0.9 ml of 98% formic acid and 1:1
acetonitrile in milli-Q water for analyzing aromatics and acids, respectively.
5.3.3 Bulk electrolysis
Electro-oxidation experiments were carried out in a flow electrochemical cell
(undivided) with a BDD disc electrode and stainless steel as anode and cathode, respectively
(each with 78 cm2 of geometrical area). The anodic oxidation experiments of RhB solutions (1
L) were performed under galvanostatic conditions using a power supply. Experiments were
carried out at 25°C by applying a current density (j) of 15 or 90 mA cm-2
. Each one of the
73
experiments was stopped when a value of zero was attained at TOC measurements. The
chemical analysis of UV-visible, TOC and HPLC were performed for all samples.
5.3.4 Chemical oxidation tests
Solutions of oxidants were prepared by electrolyzing a solution of sodium sulfate or
phosphoric acid at 90 mA cm-2
for 240 minutes. Then the resulting solutions were dosed to
synthetic wastewater containing RhB to assess the chemical oxidation of this organic by the
oxidants produced electrolytically. The chemical analysis of UV-visible, TOC and HPLC
were performed for all samples.
5.3.5 Light irradiation
A UV lamp VL-215MC (Vilber Lourmat), λ = 254 nm, intensity of 930 µW/cm2 and
energy 4.43-6.20 eV irradiating 15 watts directly to the quartz cover of the electrochemical
cell (in photoelectrolysis tests) or to the beakers in which the chemical oxidation was carried
out (in the photochemical oxidation test).
5.4 Results and Discussion
Figure 1 compares the removal of color from wastewater containing RhB by the
different technologies studied in this work. As it can be seen, single photolysis only attains a
very slight change in the color that seems to be more important because of the strange
behavior of the raw solution of RhB when no current and no light irradiation is applied, but
that it is negligible if compared to the electrolytic experiments. Thus, in experiments with not
UV irradiation and no electrolysis (nil current application), absorbance fluctuates around a
value slightly over the initial value and this could be explained by the disaggregation of small
colloids of RhB due to the shear rate produced during the circulation of the wastewater
through the experimental setup. When these results are compared to those obtained irradiating
UV light (and no current), it can be observed a small improvement when UV light is
irradiated under the same flow conditions suggesting that irradiation is affecting to the
chromophere group although total removal is small and it is under 10% in both cases.
74
When current is applied, removal of color becomes faster and so does the differences
between single electrolysis and light irradiation. This faster decrease can be explained in
terms of the direct and mediated processes that attack the chromophere group of RhB during
electrolysis, including the formation on the anode surface, and the subsequent participation in
the oxidation reactions, of peroxosulphates and peroxophosphates (eqs 1 to 3). The
occurrence of both oxidants has been demonstrated in previous works (Serrano et al., 2002;
Weiss et al., 2008; Canizares et al., 2008), and their role has been pointed out in the
comparison of the results obtained during the treatment of different types of wastewater
containing different supporting electrolyte salts (CANIZARES et al. 2005d; 2009a; AQUINO
et al. 2012). Differences observed with light irradiation are greater than in the corresponding
non-electrolytic system and this improvement could be related with the activation of the
peroxo oxidants by light irradiation, forming sulfate and phosphate radicals that contribute to
the faster depletion of color (eqs 4 to 6). Differences seem to be greater at larger current
densities for the phosphate containing solution and at lower in the case of the sulfate
containing solution (but still significant).
To produce efficiently the peroxo-salts oxidants, large current densities are to be
applied and in the literature it is found that change between low and high current densities is
of a paramount significance in this production (CANIZARES et al. 2007). For a current
density of 15 mA cm-2
production of peroxo-salts is not favoured (because of the resulting
low anodic potential) while for 90 mA cm-2
this production is expected to be carried out with
a great efficiency. This suggests that, from the view point of the oxidation of the cromophere
group, perphosphate is taking more advantage from light irradiation that persulfates because
at low current densities the role of these radicals is expected to be much smaller because of
their small production.
75
Figure 1. Removal of color during the photolysis, electrolysis and photoelectrolysis of solutions containing
Rhodamine b in a) Na2SO4: 0 mA cm-2 no UV irradiation UV irradiation; 15 mA cm
-2 no UV irradiation
UV irradiation; 90 mA cm-2 no UV irradiation UV irradiation b) H3PO4: 0 mA cm
-2 no UV irradiation
UV irradiation; 15 mA cm-2 no UV irradiation UV irradiation; 90 mA cm
-2 no UV irradiation UV
irradiation.
0 60 120 180 2400.0
0.2
0.4
0.6
0.8
1.0
1.2(a
t / min
Abs / A
bs
0
0 60 120 180 2400.0
0.2
0.4
0.6
0.8
1.0
1.2 (b
t / min
Abs / A
bs
0
The activation produced by light irradiation is more clearly seem when the effect of
irradiation is compared not for the oxidation of a particular chromophere group but for the
complete oxidation of the pollutants contained in wastewater. This is what it can be observed
when changes in the chemical oxygen demand (COD) and the total organic carbon (TOC) are
assessed. In Figure 2, these changes are represented versus current charge passed for synthetic
solutions of RhB containing sulfate as electrolyte and they clearly show that operating at low
current densities is much more efficient than operating at large current densities (because a
lower charge is required to remove the RhB) and also that effect of light irradiation is higher
at high current densities. As in most electrochemical wastewater treatment processes,
concentration of pollutant in the raw wastewater is low and the electrolytic process is under
mass transport control during the complete electrolytic process. At low current densities
limitations are smaller and production of oxidants is not promoted and this explains the
almost nil effect of light irradiation because activation of persulfate cannot produce a great
change on results. On the contrary, when current density is high, production of oxidants is
promoted and at the same time mass transfer limitations becomes more important explaining
the large differences observed in the oxidation progress (changes in the COD) and in the
mineralization (changes in the TOC).
76
Figure 2. Oxidation and mineralization during the electrolysis and photoelectrolysis of solutions containing
Rhodamine b in Na2SO4: 15 mA cm-2 no UV irradiation UV irradiation; 90 mA cm
-2 no UV irradiation
UV irradiation.
0 5 10 15 20 25 300.0
0.2
0.4
0.6
0.8
1.0
Q/ Ah dm-3
CO
D/C
OD
0/ m
g d
m-3
0 5 10 15 20 25 300.0
0.2
0.4
0.6
0.8
1.0
1.2
Q/ Ah dm-3
TO
C/T
OC
0/ m
g d
m-3
Figure 3 shows the changes observed in the treatment of RhB in phosphate media. As
it can be seen, exactly the same qualitative behavior. In this case, it can be explained by the
production of peroxophosphates instead of peroxosulfates. Production of peroxophosphates is
favored at large current densities, just in the conditions in which mass transfer limitations
becomes more significant. As a consequence the process at 90 mA cm-2
is clearly less
efficient that at 15 mA cm-2
, but the effect of light irradiation (and hence of the activation of
oxidants) is more clearly observed.
Figure 3. Oxidation and mineralization during the electrolysis and photoelectrolysis of solutions containing
Rhodamine b in H3PO4: 15 mA cm-2 no UV irradiation UV irradiation; 90 mA cm
-2 no UV irradiation
UV irradiation.
0 5 10 15 20 25 300.0
0.2
0.4
0.6
0.8
1.0
1.2
Q/ Ah dm-3
CO
D/C
OD
0/ m
g d
m-3
0 5 10 15 20 25 300.0
0.2
0.4
0.6
0.8
1.0
1.2
Q/ Ah dm-3
TO
C/T
OC
0/ m
g d
m-3
77
Production of persulfates and perphosphates is known for a long time. Effects of these
oxidants have been claimed to explain the electrolysis of many electrochemical oxidation
processes described in the literature (MARTINEZ-HUITLE; FERRO, 2006; PANIZZA;
CERISOLA, 2009). To check if this effect is additive to the other electrolytic processes
(meaning that it could be explained directly by the chemical oxidation of RhB by the
oxidants), electrolytic production of oxidants and the use of the resulting solution for
chemical tests was carried out.
Thus, in order to know the maximum concentration of peroxo-compounds that could
be formed in the electrolytic systems and the influence of light irradiation on this production,
four electrolyses were performed. The liquid electrolysed was a synthetic solution containing
the same concentration of sulfate or phosphate than the synthetic wastewaters discussed in
figures 1 to 3 but in this case no RhB was added in order to avoid reactions between the
oxidants and the organic. Production of oxidants in those solutions with no organic matter is
shown in Figure 4 when light is irradiated and in single electrolyses.
As it can be observed not great differences are obtained. Production of oxidants is very
rapid during the firsts stages of the electrolysis but it meets a plateau zone, for a concentration
around 2-4 mmol dm-3
of oxidants (for both sulfate and phosphate solutions although results
seems to be slightly better for phosphate). This plateau is the consequence of the equilibrium
between the electrolytic production and the decomposition of the oxidant. The production is
carried out in a non-divided cell and the cathodic decomposition of the oxidant is favored
when concentration increases as it was shown in a previous manuscript focused on the
optimized production of peroxophosphate (CANIZARES et al. 2005a). Light irradiation only
seems to have a small influence on the production of peroxosulfates decreasing it slightly.
This could be explained by the recombination of the radicals produced by light
irradiation in a system in which no other reaction could develop except for water oxidation in
the case of peroxophosphate. Transformation of persulfate into hydrogen peroxides (well-
known process) can be responsible for this difference in the case of sulfate. This production of
hydrogen peroxide is a clear difference between the peroxophosphate and the peroxosulfate
anions and it can be promoted chemically.
78
Figure 4. Production of oxidants by electrolyses with conductive-diamond of solutions containing Na2SO4 no
UV irradiation UV irradiation; H3PO4 .
0 30 60 90 120 150 180 210 2400
1.010 -3
2.010 -3
3.010 -3
4.010 -3
t / min.
Oxi
dants
/ m
ol dm
-3
In addition to peroxosulphate and peroxophosphate, other oxidants may be forming
during the electrolyses and hence their role should be accounted. Thus, production of
hydroxyl radicals was demonstrated in electrolysis with diamond coating anodes by Marselli
and coworkers (MARSELLI et al. 2003) as shown in equation (7).
H2O (OH)· + H
+ + e- (7)
However, lifetime of this hydroxyl radical is extremely short (GROENEN-SERRANO
et al. 2013) and it is known to be transformed into more stable oxidants by combining with
other hydroxyl radicals (equation 8) or oxygen (equation 9) producing hydrogen peroxide and
ozone.
2 OH H2O2 (8)
O2+ 2 OH O3 +H2O (9)
In addition hydroxyl radicals can combine with sulfates and phosphates contained in
the electrolyte (eqs. 10 and 12). In this later case, production of peroxo-salts is the final step
(eqs. 11 and 13) so, qualitatively, there is no way to discriminate between peroxo-salts
produced by direct electrolysis or by hydroxyl-radical mediated electrolysis.
79
HSO4– + OH (SO4
–)
+ H2O (10)
(SO4–) + SO4
– S2O8
2– (11)
HPO42–
+ OH ( PO42–
) + H2O (12)
(PO42–
) + PO42– P2O8
4– (13)
As for peroxophosphate and peroxosulfate (eqs.5 and 6), ozone and hydrogen
pexoxide are affected by UV light irradiation (Eq. 7 and 8) and they can regenerate the
hydroxyl radical in the bulk under light irradiation as shown in equation 14 and 15 (BRILLAS
et al. 2009; OTURAN et al. 2001).
H2O2+ hν→ 2OH (14)
H2O2 +O3+ hν→ 2OH+O2 (15)
Again, and due to its very high reactivity, this hydroxyl radical is expected to combine
with other species or to react with organics.
When the solutions of oxidants produced electrolytically are added to a wastewater
containing RhB, oxidants are consumed and RhB is oxidized although, as it is going to be
shown, results are different of that expected, meaning that electrochemical process could have
been having synergistic processes. Thus, as it can be seen in Figure 5, the higher the
concentration of oxidant added the higher is the concentration of oxidant removed meaning
that it could be oxidizing organics. No great differences are observed when light is irradiated
and they seem to be random rather than showing a marked trend. This means that peroxo-
compounds oxidize organic matter but light irradiation has not a clear effect out of an
electrolytic environment.
80
Figure 5. Oxidant consumed (initial – final) in chemical oxidation tests of rhodamine with the oxidant produced
electrolytically from Na2SO4 no UV irradiation UV irradiation; H3PO4 no UV irradiation UV
Irradiation.
0 10 20 30 40 50 600
2.010 -4
4.010 -4
6.010 -4
8.010 -4
1.010 -3
Oxidant dose / % oxidant solution
Oxi
dant re
move
d/ m
ol dm
-3
This is also observed in the changes in the COD (Figure 6) obtained with the addition
of the solutions of oxidants, in which it can be clearly seem that oxidants produced
electrolytically interacts chemically with RhB but not a clear influence of light irradiation can
be drawn. Peroxophosphate seems to be much more effective and this can be related because
of a better interaction with the chromophere group of the RhB molecules as it could be seen in
part b of the Figure. This better performance of peroxophosphate in the oxidation of the
chromophere group was pointed out in the electrolysis of the synthetic solutions of
Rhodamine B and confirms that, in spite of their similar chemical structure, reactivity of
peroxosulfates and peroxophosphates is somewhat different.
81
Figure 6. Changes in the COD (a) during the chemical oxidation of Rhodamine B (b) solutions with the oxidant
solution produced by the electrolyses of Na2SO4 no UV irradiation UV irradiation; H3PO4 no UV
irradiation UV Irradiation.
0 10 20 30 40 50 60-10
-5
0
5
10
15
20
25 a)
Oxidant dose / % oxidant solution
C
OD
/CO
D0 (
%)
0 10 20 30 40 50 60-10
-5
0
5
10
15
20
25
30
35b)
Oxidant dose / % oxidant solution
R
hB
/ R
hB
0 (
%)
Anyhow, results of the chemical oxidation test are positive, because they show that the
oxidant solution produced during the electrochemical process can interact with the organic
pollutant although they are clearly worse than expected according to electrolysis in terms of
the catalytic effect of light irradiation. Hence, they clearly suggest that peroxo-species play a
different role in the electrolysis of organics than that expected according to their chemical
reactivity. This more important role could be associated with synergistic interactions with
products formed during the processes and also with the intermediates formed during the direct
processes. It is important also to show the large differences observed between
peroxophosphate and peroxosulfate solutions. Both are capable to oxidize the dye but, in spite
the oxidant group is the same, much better behavior is obtained with peroxophosphate. This
effect should be explained by surplus formation of radicals. The huge concentration of
oxidants could result in a less efficient process, in which these radicals instead of attacking
organics combine among them to form more stable and less aggressive oxidants which have
an smaller oxidation capability and then, can decompose forming oxygen (SOUZA et al.
2013b). Eqs 15 and 16 summarize some of the reactions that could explain this behavior in
our experimental system for sulfate.
OH + (SO4-) HSO5
- (15)
82
HSO5- HSO4
- + 0.5 O2 (16)
This results also shows that in spite electrolysis of lowly loaded wastewater is mass
transport controlled, separated production of oxidants (which can be very efficient because of
the use of optimized conditions) and application to the wastewater in a later chemical
oxidation (two-stage electrolysis) does not seem to be a very interesting choice because some
important processes could be missed as compared with the results obtained in the one-stage
electrolytic processes. At this point, it is worth taking into account that the two-stage
electrolysis lacks the effect of the direct electrolysis, oxygen and pH. As it is known, oxygen
is contained in large quantities in the electrolytic media because of water electrolysis. This
oxygen can combine the hydroxyl radicals forming ozone (eq.9) and helps in the oxidation of
organics. Likewise, it may be reduced on the cathode surface to hydrogen peroxide giving
chance to many other reactions in the bulk (ISARAIN-CHAVEZ et al. 2013). Regarding the
pH, water oxidation and reduction processes generate profiles of pH between the anode and
the cathode, which can stand for these large differences. Effect of oxidants is known to be
very dependent on the pH. This effect is particularly studied for the peroxophosphates in
which speciation and hence reactivity is strongly improved at the acidic conditions produced
on the nearness of the anode surface because of the production of monoperoxophosphoric acid
(Weiss et al. 2008), which is known to be much more efficient in the oxidation of organics
that the peroxodiphosphate species. A last explanation that could help to interpret results is
that interactions between oxidants are known to produce extremely powerful radical oxidants.
In particular those of hydrogen peroxide with persulfate and perphosphate and ozone
should be accounted (equations 17 to 19), and their effect could be magnified by the light
irradiation. This means that in the two- stage oxidation some of the oxidants produced could
have been lost (because they do not find any species to oxidize) while in the one-stage
electrolysis, these synergistic reactions that develops in the bulk could improve the efficiency
in the oxidation of organics.
H2O2 +2O32OH +3O2 (17)
P2O84-
+ H2O2 2 (PO42-
) + 2 OH (18)
S2O82-
+ H2O22 (SO4-) + 2 OH (19)
83
Hence, production of peroxo-salts is a very important process happening during the
electrolysis of wastewater containing sulfate and phosphate as supporting electrolyte and its
effect is very significant and should be taken into account in order to explain the oxidation of
organics. However, results are not only a consequence of these oxidants and direct and also
synergistic oxidants interaction processes are occurring during the electrolysis that could not
be reproduced if the process is split into two separated stages. Most relevant processes are
summarized graphically in the mechanistic scheme shown in Figure 7.
Figure 7. Mechanisms of the main processes occurring during photo-electrolysis of
Rhodamine B.
5.5 Conclusions
From this work the following conclusions can be drawn:
1. Light irradiation has a positive effect during the electrolysis of RhB with phosphate or
sulfate as supporting electrolyte. This effect is greater at high current densities
2. RhB is oxidized chemically by the oxidants produced electrolytically during the
electrolysis of a phosphate or sulfate solution in the same conditions used for the
Cathode
Ox
Red
e-
e-
H2 + OH-
H2O
Anode
RB
Products
Direct
oxidatione-
H2O
H+ + O2
e-
e-
H2O2
O2
RB
Products
SO42-
e-
S2O82-
H2O
Mediated
oxidation
RB
Products
SO42-
e-
S2O82-
RB
Products
H2O2
H2O
OH·
e-
O2+ 2 OH· O3 +H2O
H2O2 +2O32OH· +3O2
P2O84- + H2O2 2 PO4
2-· + 2 OH·
S2O82- + H2O22 SO4
-· + 2 OH·
H2O2 + hν → 2 OH
H2O2 +O3 + hν → 2 OH+O2
P2O84 + hν → 2 (PO4
2-)
S2O82 + hν → 2 (SO4
)
RB
Products
Photolytic
activation of
oxidants
Chemical activation
of oxidants
RB
84
electrolysis of wastewater. In spite of having the same group, perphosphate solution
seems to be much more aggressive than persulphate solution with RhB in particular
with the cromophere group
3. Results of the electrolysis and in particular those of light irradiation are not reproduced
by the chemical interactions between the oxidant solution produced during electrolysis
and the RhB. Changes in the pH, effect of cathodically produced hydrogen peroxide
are speculated to be the cause of the synergistic effect observed.
5.6 Acknowledgements
D.M.A. acknowledges the CAPES for PhD fellowship and CNPq for the fellowship
given for “doutorado sanduíche” under “Ciências sem Fronteiras” program to develop the
experimental research at the UCLM-Spain. Financial support of the Spanish government and
EU through project FEDER 2007-2013 PP201010 (Planta Piloto de Estación de Estación de
Regeneración de Aguas Depuradas) is gratefully acknowledged.
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92
CAPÍTULO 6
ELECTROCHEMICAL CONVERSION/COMBUSTION OF A MODEL ORGANIC
POLLUTANT ON BDD ANODE: ROLE OF sp3/sp
2RATIO
93
ELECTROCHEMICAL CONVERSION/COMBUSTION OF A MODEL ORGANIC
POLLUTANT ON BDD ANODE: ROLE OF sp3/sp
2RATIO
Danyelle Medeiros de Araújo1, Pablo Cañizares
2, Carlos A. Martínez-Huitle
1,*,
Manuel Andrés Rodrigo2
1 Institute of Chemistry, Federal University of Rio Grande do Norte, Lagoa Nova CEP 59078-
970 - Natal, RN, Brazil
2 Department of Chemical Engineering, Universidad de Castilla-La Mancha, Campus
Universitario s/n, 13071 Ciudad Real, Spain
6.1 Abstract
In this work, it is presented critical evidence about the influence of sp3/sp
2 ratio on the
performance of electrochemical oxidation (combustion or conversion) of Rhodamine B
(RhB), used as a model organic pollutant. Results demonstrate that the higher the content in
diamond-carbon, the greater are the TOC and COD decay rates and hence the oxidation of
organic to CO2.The evidence of chromatographic analysis also indicates that the oxidation
carried out by the diamonds with lower content of sp3-carbon is softer, favoring
electrochemical conversion of RhB instead of mineralization. This degradation pathway is
followed because higher graphite content on BDD anode leads to a higher direct
electrochemical activity. These results are of a paramount significance for the choice of
electrodes that could guarantee high efficiencies in wastewater remediation processes;
because they clearly indicate that the sp3-sp
2 carbon ratio should be kept as higher as possible
in order to deplete completely pollutants and intermediates from the waste.
Keywords: diamond electrode; conversion; combustion; organic matter, hydroxyl radicals,
active and non-active anodes.
6.2 Introduction
In the last three decades, BDD anode has been defined as non-active electrode, since it
is expected that it does not provide any catalytically active site for the adsorption of reactants
94
and/or products in aqueous media (MARSELLI, et al. 2003; BRILLAS; MARTINEZ-
HUITLE, 2011). Hydroxyl radical (BDD(•OH)) formed from water discharge on its surface
from reaction (physically adsorbed on the anode surface): H2O OH + e
− + H
+, is then
considered the responsible species for the electrochemical combustion of organic pollutants,
although slower reactions with other reactive oxygen species (H2O2 and O3) and
electrogenerated oxidants (active chlorine, peroxodisulphate, peroxodicarbonate or
peroxodiphosphate) are also feasible (MARTINEZ-HUITLE; FERRO, 2006; MARTINEZ-
HUITLE; BRILLAS, 2009; PANIZZA; CERISOLA, 2009; BRILLAS et al. 2009).
Many research groups have demonstrated that BDD anodes allow complete
mineralization up to near 100% of current efficiency of a large number of organic pollutants
(MARTINEZ-HUITLE; BRILLAS, 2009; PANIZZA; CERISOLA, 2009; BRILLAS et al.
2009). Unfortunately, most of these studies have been focused on the study of the feasibility
of the use of this technology to treat different types of pollutants and on the selection of the
experimental conditions to improve the performance of this technique (MARTINEZ-
HUITLE; FERRO, 2006; MARTINEZ-HUITLE; BRILLAS, 2009; PANIZZA; CERISOLA,
2009).
Nevertheless, some authors have questioned the non-active nature of BDD material as
well as the reactivity of hydroxyl radicals (VATISTAS, 2010; BEJAN et al. 2012) because
some results have demonstrated the occurrence of partial oxidation or complete mineralization
of organic pollutants when BDD is used as anode. In this point, it seems reasonable to think
that the electrochemical process can be greatly affected by the characteristic of the BDD
electrodes used. Therefore, few works have showed that the conductive layer characteristics
(sp3/sp
2 ratio, boron content, BDD layer-thickness) and the substrate properties (resistivity
and roughness of the surface) have an important influence in the bulk electrolysis results
(CANIZARES et al. 2008a,b; GUINEA et al. 2009).
The most relevant result supposes that the electro-oxidation mechanism is strongly
influenced by the BDD characteristics; particularly the ratio diamond/graphite carbon
(GUINEA et al. 2009). High graphite content favors direct oxidation of the pollutant on the
electrode surface and it leads to the formation of many intermediates. On contrary, high
diamond content seems to favor the complete oxidation of the organic to CO2, thanks to the
contribution of the oxidants (hydroxyl radicals and electrogenerated reagents) present in a
region close to the electrode surface. However, these assumptions were not completely
95
verified because no quantification of intermediates was performed as well as no complete
mineralization was attained, for enrofloxacin oxidation (GUINEA et al. 2009). Also, a limited
range of the sp3/sp
2 ratio was considered (ranging from 45 to 105).
Thus, the goal of this work has been to present critical evidence about the influence of
sp3/sp
2 ratio on the performance of electrochemical oxidation (EO) of a model organic
pollutant, RhB. To do this, it has been used BDD samples by one of the more important
diamond-electrodes manufacturers with different sp3/sp
2 ratio, ranging from 165 to 329.
Results obtained will help to understand the electrocatalytic properties of BDD in the
oxidation of a model pollutant and to elucidate the mechanism involved (electrochemical
combustion or conversion).
6.3 Methodology
6.3.1 Chemicals
Chemical reagents used in this study were of high purity. RhB solutions with
concentration of 2.09×10−4
M, for each experiment, were prepared by using Na2SO4 as
supporting electrolyte (using Milli-Q water).
6.3.2 Analytical procedures
Color removal was monitored by UV-visible technique using a UV-1603
spectrophotometer Model Shimadizu, at a wavelength of 554 nm. Chemical oxygen demand
(COD) was performed by using pre-dosage vials with 2 mL of sample. Samples of 10 mL
were collected to determine total organic carbon (TOC) content, using a TOC analysator
Multi N/C and the results were obtained through the Analytikjena program. Electrolysis
samples before and after the EO of RhB at different diamond anodes were collected and
analyzed by HPLC in Agilent 1100 HPLC (Two columns were used (aromatics and aliphatic
acids) to determine the intermediates generated). Also, few samples of anolyte were extracted
into non-aqueous medium and were subjected to GC-MS analysis using GC-FOCUS and MS-
ISQ Thermo Scientific to identify the intermediates (column VF5 ms with a composition of
5% de fenil-arylene and 95% de dimetilpolisiloxane. Program: 40ºC–5 min; 12°C/min–100ºC;
96
10ºC/min–200ºC and 10ºC/min–270ºC–5 min. Injector: 220ºC. Mode: Splitless. Gas flow: 0.8
mL/min. Transfer line: 270ºC; ions source temperature: 220ºC, Mass range: 40-500 m/z).
6.3.3 Electrochemical cell and bulk electrolysis
Bulk oxidations were carried out in an undivided electrochemical cell by using
different BDD anodes (provided by Adamant Technologies (Neuchatel, Switzerland) and
synthesized by the hot filament chemical vapor deposition technique (HF CVD)
monocrystalline p-type Si substrate (thickness 2 mm, resistivity 100 mΩcm), diamond coating
of 2-3 μm, boron concentration 500 ppm, 78 cm2 of geometrical area and with distinct sp
3/sp
2
ratio (estimated by Raman spectrometry): BDD1 = 165, BDD2 = 176, BDD3 = 206, BDD4 =
220, BDD5 = 262 and BDD6 = 329. Stainless steel was used as cathode. EO experiments of
RhB solutions (1 L) were performed under galvanostatic conditions using a power supply.
Experiments were performed at 25°C by applying a current density of 90 mA cm−2
. Each one
of the experiments was stopped when a value of zero was attained at TOC measurements.
Analysis of UV-visible, TOC and HPLC were performed for all samples in order to
understand the influence of sp3/sp
2 ratio on the EO performance (the trueness of the results
was evaluated by three independent analyses).
6.4 Results and discussion
Figure 1 shows the changes in absorbance, TOC and COD, as a function of electric
charge passed (Q), during galvanostatic electrolysis of RhB solution (1 L) using different
BDD anodes by applying 90 mA cm−2
at 25°C. It can be observed that all parameters decrease
with the current charge passed in a similar way for all essays performed. For all type of BDD
anodes, color decayed continuously until it disappeared after about 25 Ah dm−3
, leading to
complete discoloration; independent on the sp3/sp
2 ratio (Fig. 1a). Conversely, discoloration
rate seems depends on sp3/sp
2 ratio, being significant at BDD6. The absorbance changes were
reasonably rapid, indicating that during the first treatment stages there are mechanisms
involving dye oxidation to other more simple organics. Oxidation of this complex molecule
can result in formation of many intermediates by chromophore group cleavage by hydroxyl
97
radicals attack prior to production of aliphatic carboxylic acids and CO2 (MARTINEZ-
HUITLE; FERRO, 2006).
On the other hand, TOC results reveal that complete elimination of organic matter is
achieved at all BDD anodes. However, highest removal rate is achieved at BDD6 (sp3/sp
2 =
329), see Fig. 1b, than the rate observed at BDD1 with lower sp3/sp
2 ratio. In fact, TOC was
completely removed after passing 44 Ah dm−3
with BDD6, while BDD1 needed 74 Ah dm−3
to
complete the elimination of TOC. Despite the complete TOC decay seems to indicate that
BDD anodic oxidation is a good choice for the removal of this complex pollutant, and it
depends on diamond/graphite relation; differences between the results obtained in the
electrolysis with the different diamonds are more evident in terms of the COD than in TOC
changes. This can be observed in Figure 1c where BDD6 with high diamond content leads to
fast rate and complete organic removal, while diamond with high graphite content favors
lower rate degradation. Based on these results, we can suggest that the EO of RhB leads to
CO2 when a diamond anode with high diamond content was used, favoring the
electrochemical combustion pathway. It is due to the contribution of the oxidants (OH and
electrogenerated reagents) present on the anode surface and the reaction cage (MARSELLI, et
al. 2003; GUINEA et al. 2009).
Conversely, high graphite content favors electrochemical conversion of the pollutant,
producing many intermediates. These figures are in convergence with the assumptions
suggested by Guinea et al. (GUINEA et al. 2009) for the oxidation of enrofloxacin.
98
Figure 1. Effect of the sp3/sp
2 ratio of the BDD anodes on the absorbance (a), TOC (b) and COD (c) removal, as a function of Q, during electrolysis of RhB solutions.
0 20 40 60 80 1000.0
0.2
0.4
0.6
0.8
1.0
BDD6 (sp3/sp2 329)
BDD2 (sp3/sp2 176)
BDD1 (sp3/sp2 165)
BDD5 (sp3/sp2 262)
BDD3 (sp3/sp2 206)
BDD4 (sp3/sp2 220)
(a
Q / A h dm-3
Ab
s /
Ab
s0
0 20 40 60 80 1000
20
40
60
80
BDD4 (sp3/sp2 220)
BDD1 (sp3/sp2 165)
BDD3 (sp3/sp2 206)
BDD6 (sp3/sp2 329)
BDD2 (sp3/sp2 176)
BDD5 (sp3/sp2 262)
(b
Q / A h dm-3
TO
C /
mg
dm
-3
0 20 40 60 80 1000
50
100
150
200
BDD4 (sp3/sp2 220)
BDD1 (sp3/sp2 165)
BDD3 (sp3/sp2 206)
BDD6 (sp3/sp2 329)
BDD2 (sp3/sp2 176)
BDD5 (sp3/sp2 262)
(c
Q / A h dm-3
CO
D /
mg
dm
-3
99
Although these affirmations seem feasible, TOC and COD removals are not enough
information to confirm these approaches; therefore, qualitative/quantitative analyses would be
required to verify these hypotheses. Then, to address these questions, we analyzed the
generation of by-products, in terms of chromatographic area of each one of signals obtained
(HPLC) and identification of the intermediates (GC/MS), during the EO of RhB at BDD6 and
BDD1. Fig. 2 and 3 compare the number of intermediates produced (aromatics and aliphatic
acids respectively), as a function of chromatographic area, during EO of RhB by using high
diamond (sp3) and high graphite (sp
2) anodes.
Figure 2. Chromatographic areas of aromatic intermediates produced, as a function of time, during RhB electro-
oxidation using BDD6 (a) and BDD1 (b). I1, no identified intermediate.
From these results, it is apparent that more aromatic intermediates are formed at BDD6
(see Fig. 2a), while a minor number of them are produced when BDD1 was used (Fig. 2b).
However, an important observation is the significant chromatographic area achieved by
intermediates produced by BDD1 (Fig. 2b), indicating that high concentration of N-ethyl-N-
ethylrhodamine (~27000 units) is attained after 10 min of electrolysis, and it decays slower
until 180 min. Other sub-products are produced (intermediates identified as isomer of phthalic
acid, benzoic and phthalic acids) but with minor areas. On contrary, BDD6 produces a large
number of by-products (Fig. 2a) including N-ethyl-N-ethylrhodamine, but these intermediates
are quickly degraded. This fact can be explained by the effective attack of OH to N-
terminated groups of RhB and to the nature of the OH produced at different non-active
anodes, as already proposed by Bejan and co-workers (BEJAN et al. 2012).
100
Figure 3. Influence of sp3/sp
2 ratio on the production of aliphatic acids, as a function of chromatographic areas
and time, during RhB electro-oxidation using BDD6 (a) and BDD1 (b). I1 and I2, no identified intermediates.
Restricting now our analysis to the number of aliphatic acids produced (Figure 3), a
lower amount of intermediates (oxalic, formic, acetic and α- hidroxyglutaric acids plus I2),
with low-units of chromatographic area, is produced on BDD6, being quasi completely
oxidized after 60 min of electrolysis (Figure 3a). For the case of BDD1, a large number of
aliphatic acids are generated (malonic, oxalic, formic, succinic, acetic, 2-
hydroxypentanedioic, adipic, α- hidroxyglutaric acids and I1) with a relevant predominance of
adipic acid, even after 60 min of electrooxidation (Fig. 3b).
The evidence of chromatographic analysis suggests that the oxidation carried out by
the diamond with lower content of sp3-carbon is softer, favoring electrochemical conversion
of RhB. This degradation pathway is followed because higher graphite content on BDD anode
leads to a higher direct electrochemical activity (GUINEA et al. 2011) that is related to better
adsorption of reactants on sp2 carbon. On the contrary, the high level of electrochemical
mineralization obtained by higher diamond content in BDD6 is caused by the oxidants
generated from water electrolysis and supporting electrolyte. Even when the number of
aromatic intermediates is formed at this BDD anode, these are oxidized to CO2. In fact,
inorganic carbon measurements showed clearly that higher concentrations of CO2 are
produced at BDD6 after 15 min of electrolysis (4.9 mg dm-3
), while at BDD1, lower
concentrations were detected (1.7 mg dm-3
).
101
6.5 Conclusions
Theoretically, based on the existing literature (BRILLAS; MARTINEZ-HUITLE,
2011), diamond anode is predominantly considered as an ideal non-active anode. However,
our results evidenced that the non-active nature of diamond electrode is strongly influenced
by the ratio diamond(sp3)/graphite(sp
2) carbon content. Higher content in diamond-carbon the
greater the TOC and COD decays in the bulk electrolysis by electrochemical combustion
(oxidation of organic to CO2). This may be confirmed by HPLC and GC/MS analyses
performed for the intermediates formed.
Conversely, high graphite content favors the electrochemical conversion (formation of
many intermediates) due to the adsorption of reactants on sp2 carbon. The sp
2 carbon species
can be considered as the primary pathway for the charge transfer process (direct oxidation)
(DUO et al. 2003). All by-products identified are in accordance with (HE et al. 2009).
We cannot be certain about the reactivity of OH (BEJAN, et al. 2012), but it could be
related with the stability of N-ethyl-N-ethylrhodamine formed at both BDD anodes. Further
experiments with BDD6 and BDD1 are in progress to clarify this BDD property.
6.6 Acknowledgements
D.M.A. acknowledges CAPES for her PhD fellowship and CNPq for the scholarship
given for “doutorado sanduiche” under “Ciências sem Fronteiras” program to develop the
experimental research at the UCLM-Spain. Financial support of the Spanish government and
EU through project FEDER 2007-2013 PP201010 (Planta Piloto de Estación de Regeneración
de Águas Depuradas) is gratefully acknowledged.
6.7 References
BEJAN, D.; GUINEA, E.; BUNCE, N. J. On the nature of the hydroxyl radicals produced at
boron-doped diamond and Ebonex® anodes. Electrochimica Acta, v. 69, p. 275–281,
2012.Disponível em:
<http://www.sciencedirect.com/science/article/pii/S001346861200312X> Acesso em: 19 de
setembro de 2013.
BRILLAS, E.; SIRES, I.; OTURAN, M. A. Electro-fenton process and related
electrochemical technologies based on fenton’s reaction chemistry. Chemical Reviews, v.
102
109, p. 6570-6631, 2009. Disponível em:
<http://pubs.acs.org/doi/pdf/10.1021/cr900136g>Acesso em: 17 de maio de 2013.
BRILLAS, E.; MARTINEZ-HUITLE, C. A. (Eds); Synthetic Diamond Films: Preparation,
Electrochemistry, Characterization and Applications, Wiley, 2011.
CANIZARES, P.; SAEZ, C.; MARTINEZ, F.; RODRIGO, M. A. The role of the
characteristics of p-Si BDD anodes on the efficiency of wastewater electro-oxidation
processes. Electrochemical Solid-State Letters, v. 11, p. E15–E19, 2008a. Disponível em:
<DOI: 10.1149/1.2916436> Acesso em: 20 de fevereiro de 2014.
CANIZARES, P.; SAEZ, C.; SANCHEZ-CARRETERO, A.; RODRIGO, M. A. Influence of
the characteristics of p-Si BDD anodes on the efficiency of peroxodiphosphate
electrosynthesis process. Electrochemistry Communications, v. 10, p. 602–606, 2008b.
Disponível em:
<http://www.sciencedirect.com/science/article/pii/S1388248108000489> Acesso em: 17 de
janeiro de 2014.
DUO, I.; FUJISHIMA, A.; COMNINELLIS, C. Electron transfer kinetics on composite
diamond (sp3)-graphite (sp
2) electrodes. Electrochemistry Communications, v. 5, p. 695–
700, 2003. Disponível em:
<http://www.sciencedirect.com/science/article/pii/S1388248103001693>Acesso em: 12 de
dezembro de 2013.
GUINEA, E.; CENTELLAS, F.; BRILLAS, E.; CANIZARES, P.; SAEZ, C.; RODRIGO, M.
A. Oxidation of enrofloxacin with condutive-diamond electrochemical oxidation, ozonation
and fenton oxidation. A comparison. Water Research, v. 43, p. 2131-2138, 2009. Disponível
em: <http://www.sciencedirect.com/science/article/pii/S0043135409000918> Acesso em: 17
de janeiro de 2014.
HE, Z.; SUN, C.; YANG, S.; DING, Y.; HE, H.; WANG, Z. Photocatalytic degradation of
rhodamine B by Bi2WO6 with electron accepting agent under microwave irradiation:
mechanism and pathway. Journal of Hazardous Materials, v. 162, p. 1477–1486, 2009.
Disponível em: <http://www.sciencedirect.com/science/article/pii/S0304389408009217>
Acesso em: 17 de maio de 2013.
MARSELLI, B.; GARCIA-GOMEZ, J.; MICHAUD, P.A.; RODRIGO, M. A.;
COMNINELLIS, C. Electrogeneration of hydroxyl radicals on boron-doped diamond
electrodes. Journal Electrochemical Society, v. 150, p. D79–D83, 2003. Disponível em:
<http://jes.ecsdl.org/content/150/3/D79> Acesso em: 10 de fevereiro de 2013.
MARTINEZ-HUITLE, C. A.; FERRO, S. Electrochemical oxidation of organic pollutants for
the wastewater treatment: direct and indirect processes. Chemical Society Reviews, v. 35, p
1324-1340, 2006. Disponível em:
<http://pubs.rsc.org/en/content/articlepdf/2006/cs/b517632h> Acesso em: 17 de maio de
2013.
103
MARTINEZ-HUITLE, C. A.; BRILLAS, E. Decontamination of wastewaters containing
synthetic organic dyes by electrochemical methods: a general review
Applied Catalysis B: Environmental, v. 87, p.105-145. 2009. Disponível em:
<http://www.sciencedirect.com/science/article/pii/S0926337308003718>. Acesso: 08 de
fevereiro de 2013.
PANIZZA, M.; CERISOLA, G. Direct and mediated anodic oxidation of organic pollutants.
Chemical Reviews, v. 109, p. 6541-6569, 2009. Disponível em:
<http://pubs.acs.org/doi/pdf/10.1021/cr9001319>Acesso: 08 de fevereiro de 2013.
VATISTAS, N. Adsorption layer and its characteristic to modulate the electro-oxidation
runway of organic species. Journal of Applied Electrochemistry, v. 40, p. 1743-1750, 2010.
Disponível em: <http://link.springer.com/article/10.1007%2Fs10800-010-0134-6#page-1>
Acesso em: 17 de maio de 2013.
104
CAPÍTULO 7
APPLICABILITY OF DIAMOND ANODE TO THE ELECTROCHEMICAL
TREATMENT OF A REAL TEXTILE EFFLUENT
105
APPLICABILITY OF DIAMOND ANODE TO THE ELECTROCHEMICAL
TREATMENT OF A REAL TEXTILE EFFLUENT
Carlos A. Martínez-Huitle1,*
, Elisama Vieira dos Santos1,
Danyelle Medeiros de Araújo1, Marco Panizza
2,*
1 Institute of Chemistry,Federal University of Rio Grande do Norte,
Lagoa Nova CEP 59078-970 - Natal, RN, Brazil.
2 Department of Chemical and Process Engineering, University of Genoa,
P.le J.F. Kennedy 1 – 16129 Genova, Italy.
7.1 Abstract
In this work, a real effluent discharged by Brazilian textile industry, has been
electrochemically treated using boron doped diamond (BDD) anode for removing chemical
oxygen demand (COD) and colour. Preliminary experiments were performed under real
discharged effluent conditions (pH and conductivity) in order to verify the applicability of this
treatment process. However, a partial elimination of COD and colour effluent were achieved,
depending on applied current density (20, 40 and 60 mA cm-2
), respectively. Therefore,
different set of experiments were carried out where an amount Na2SO4 was added in the real
effluent; then, COD of textile effluent was satisfactorily reduced by employing different
operational conditions (current density, temperature, Na2SO4 concentration, flow rate),
reducing the time of depuration and consequently, the costs, confirming the potential
efficiency of this textile effluent treatment.
Keywords: electrochemical treatment; diamond electrode; real effluent; textile dyes; pre-
treatment.
7.2 Introduction
Textile industry produces large quantities of wastewater during the washing and dyeing
process, which contain large quantities of dye, and are disposed together with the textile
106
effluent. The pollution potential of textile dyes and intermediates compounds was first raised
due to its toxicity and carcinogenicity that can cause damage to human health and
environment. It should be mentioned that coloration in water courses affects water
transparency and gas solubility (FORGACS, et al. 2004; MARTINEZ-HUITLE; BRILLAS,
2009).
Thus, the development of treatment technologies suitable for the removal of colour and
reduction of toxicity of textile effluents is important. For the removal of dyes from wastewater
a wide range of techniques have been developed and proposed (SHAUL et al. 1991; GUPTA
et al. 1992; SHUKLA; GUPTA, 1992; KABADASIL et al. 1999; TUNAY, et al. 1999; HAO
et al. 2000; BOOPATHY, 2000; ROBINSON et al. 2001; FORGACS et al. 2004;
MARTINEZ-HUITLE; BRILLAS, 2009; NASCIMENTO et al. 2011). However, the most
useful industrial treatment is bioremediation technology because it can be performed on site,
at lower cost, with limited inconveniences, minimal environmental impact, it eliminates the
waste permanently and it can be used in conjunction with methods of physical and chemical
treatments (NASCIMENTO et al. 2011). But, this process requires long times and specific
treatment conditions and in addition, heavy metals, radio nucleotides, complex molecules,
biorefractary and some chlorinated compounds are not suitable for bioremediation
(BOOPATHY, 2000).
In this context, electrochemical technologies have caused great interest because they offer
effective means to solve environmental problems related to industrial processes (CHEN,
2004; MARTÍNEZ-HUITLE; FERRO, 2006; PANIZZA; CERISOLA, 2009; BRILLAS, et al.
2009). These techniques have been investigated for decolourisationand degrading dyes from
aqueous solutions (dyes solutions, synthetic and actual wastewaters) by several scientific
groups and a wide variety of electrode materials have been suggested, such as graphite,
platinum, IrO2, RuO2, SnO2, PbO2, Ti/Pt, Ti/Pt–Ir, Ti/PbO2, Ti/PdO–Co3O4 and Ti/RhOx–
TiO2, Ti coated with oxides of Ru/Ir/Ta and BDD thin films (MARTINEZ-HUITLE;
BRILLAS, 2009). However, electrochemical oxidation of organics to CO2 occurs with a
significant rate and efficiency only using anodes with high oxygen evolution overpotential
such as PbO2 and BDD (PANIZZA; CERISOLA, 2009).
Using these anodes, at high potential, highly reactive OH are generated on their surface
by water discharge, thus leading to overall combustion of organic compounds (CHEN, 2004;
MARTÍNEZ-HUITLE; FERRO, 2006; PANIZZA; CERISOLA, 2009):
107
H2O OH + e
- + H
+ (1)
Organics + OH CO2+ H2O (2)
Furthermore, this technology would has generally public acceptance, which makes the
treatment using anodes with high oxygen evolution overpotential promising alternative to
replace or complement the conventional treatments.
Although more reports involving electrochemical treatment of dye wastewaters have been
published in the last five years, there are few reports concerning to the use of this process
using BDD anodes to degrade real textile effluents (CHATZISYMEON et al. 2006;
SAKALIS et al. 2006; KOPARAL et al. 2007; MALPASS et al. 2007, 2008; AQUINO et al.
2011; PERALTA-HERNÁNDEZ et al. 2012). For this reason, the purpose of the present
study was to evaluate the applicability of electrochemical oxidation process using diamond
electrode as alternative depuration (decolourisation and detoxification) treatment of textile
effluent obtained from a Brazilian textile industry. The influence of the main operating
parameters, such as current density, temperature, flow conditions and addition salts, on the
COD and colour removal were investigated, in order to identify the electrochemical
conditions which give high current efficiency with low energy requirements for attaining
Brazilian legal requirements (IDEMA, 2004).
7.3 Materials and Methods
7.3.1 Textile dye effluent characteristics
The effluent sample was collected from outlet discharged effluent in a Brazilian textile
industry situated in Natal (Northeast of Brazil). It was mainly composed of dyes, amylum and
different additives. In order to remove the suspended solids that influence the electrochemical
process, the effluent was subjected to a filtration pre-treatment using a 170 mesh. After
filtration, the effluent contained a high concentration of COD (650 mg dm−3
) and Hazen Units
(1204 HU) [ISO 2211:1973]. Its conductivity was 2.70 mS cm−1
and the pH was around 10.2.
It is worth noting that these conditions were determined from the effluent, as discharged after
textile colouration process; without any physical-chemical treatment.
108
7.3.2 Anodic oxidation experiments
Bulk electrochemical oxidations in batch under steady conditions were conducted using an
undivided electrolytic flow cell under galvanostatic conditions, similar to electrochemical
system already reported in other works (PANIZZA; CERISOLA, 2009; MARTÍNEZ-
HUITLE et al. 2004). The textile effluent was stored in a thermostated glass reservoir of 1
dm3 and it was recirculated through the electrolytic cell by means of a centrifugal pump
working in the flow range 200–400 dm3 h
−1. The cell contained a BDD electrode as anode and
a 54.7 cm2 Ti plate as cathode. BDD anode was circular with a diameter of 10 cm, but with an
effective area of 54.7 cm2 and with one side only exposed to the solution. The inter electrode
distance was 1 cm. BDD electrode was supplied by Adamant Technologies (Switzerland) and
it was synthesized as described in previous works (MARTÍNEZ-HUITLE et al. 2004)
maintaining the quality parameters (single-crystal with a thickness of 1 μm (±5%) and a
resistivity of 15 mΩ cm (±30%) with a boron concentration of 5000 ppm, p-silicon wafers (1–
3 mΩ cm and 1 mm thick)). In order to stabilize its surface (hydrophilic nature) and to obtain
reproducible results, the BDD electrode was pre-treated at 25 °C by anodic polarisation in 1
M HClO4 at 10 mA cm−2
for 30 min (PANIZZA; CERISOLA, 2009).
The anodic oxidation experiments of real textile effluent were performed under
galvanostatic conditions using a MINIPA 3015 power supply. Experiments were performed at
25°C for studying the role of applied current density (j = 20, 40 and 60 mA cm-2
), while the
temperature effect (25 and 60°C) was studied under a current density of 40 mA cm-2
. The
temperature of the electrolyte was controlled using a jacket- thermostat.
7.3.3 Depuration monitoring methods
Colour removal was monitored by measuring absorbance decrease; using a UV 1800
Shimadzu spectrophotometer. Experimentally, decolourization process was determined by the
expression (MARTINEZ-HUITLE; BRILLAS, 2009):
0
relative absorbancetABS
ABS
(3)
109
where ABSt correspond to the absorbance, at the maximum visible wavelength of the
wastewater, at time t and ABS0 is its initial absorbance. Hazen Units (UH) is a industrial
parameter to determine the effluent colour conditions for discharging procedure after
treatment and these were determined using a Hach Model DR/2500 spectrophotometer
calibrated with a method 8025 (Pt-Co units). pH variation was measured using a Methrom pH
meter. Decontamination of real effluent was monitored from the abatement of their COD.
Values were obtained, using a HANNA HI 83099 spectrophotometer after digestion of
samples in a HANNA thermo-reactor, in order to estimate the Total Current efficiency (TCE,
in %) for anodic oxidation of textile effluent, using the following relationship:
0% 100
8
tCOD CODTCE FV
I t
(4)
where COD0 and CODt are chemical oxygen demands at times t=0 (initial) and t (time t) in g
O2 dm−3
, respectively; I the current (A), F the Faraday constant (96,487 C mol−1
), V the
electrolyte volume (dm3), 8 is the oxygen equivalent mass (g eq.
−1) and Δt is the electrolysis
time, allowing for a global determination of the overall efficiency of the process.
Additionally, the limiting current can be estimated from the value of COD using the
equation 3 for anodic oxidation of a real wastewater, as indicated by Panizza and Cerisola
(PANIZZA; CERISOLA, 2009).
lim( ) 4 ( )mI t FAk COD t (5)
where Ilim(t) is the limiting current (A) at a given time t, 4 the number of exchanged electrons,
A the electrode area (m2), F the Faraday’s constant, km the average mass transport coefficient
in the electrochemical reactor (m s−1
) and COD(t) the chemical oxygen demand (mol O2 m−3
)
at a given time t.
The energy consumption per volume of treated effluent was estimated and expressed in
kWh m-3
. The average cell voltage during the electrolysis (cell voltage is reasonably constant
with just some minor oscillations, for this reason is calculated the average cell voltage), is
taken for calculating the energy consumption by expression (MARTINEZ-HUITLE;
BRILLAS, 2009):
110
1000
cE I tEnergy consumption
V
(6)
where t is the time of electrolysis (h); ΔEc (V) and I (A) are the average cell voltage and the
electrolysis current, respectively; and V is the sample volume (m3).
7.4 Results and discussion
7.4.1 Preliminary electrochemical experiments of a real textile effluent
As it was evidenced by other authors (MARTÍNEZ-HUITLE et al. 2004; MARTÍNEZ-
HUITLE; FERRO, 2006; CHATZISYMEON et al. 2006; SAKALIS et al. 2006; KOPARAL
et al. 2007; MALPASS et al. 2007, 2008; PANIZZA; CERISOLA, 2009; BRILLAS, et al.
2009; BENSALAH et al. 2009; PANIZZA; CERISOLA, 2010; AQUINO et al. 2011;
PERALTA-HERNÁNDEZet al. 2012), BDD has great oxidation ability to remove organic
pollutants, requiring shorter electrolysis time to reach overall mineralization, thus leading to
remarkably higher current efficiency and relative energy consumptions. For this reason, the
good removal efficiencies suggest the possibility of using electrochemical oxidation as
treatment technology for treating real textile effluents (MARTÍNEZ-HUITLE; FERRO, 2006;
CHATZISYMEON et al. 2006; SAKALIS et al. 2006; KOPARAL et al. 2007; MALPASS et
al. 2007, 2008; PANIZZA; CERISOLA, 2009; BRILLAS, et al. 2009; PANIZZA;
CERISOLA, 2010 ; AQUINO et al. 2011; PERALTA-HERNÁNDEZ et al. 2012).
In this context, an effluent of an actual Brazilian textile industry was employed to assess
the efficiency on electrochemical treatment using diamond electrodes as an alternative for
removing organic matter and colour. As shown in Figure 1, the decrease of the absorbance, as
a function of time, was achieved during galvanostatic electrolysis of real textile effluent (1
dm3) after 12 h by applying different current densities under real discharged effluent
conditions (see section 2). However, the Brazilian regulations for colour removal (IDEMA,
2004), i.e. 300 HU, were completely attained at 40 and 60 mA cm-2
, after 6 and 4 h,
respectively; as showed in inset of Fig. 1.
111
Figure 1. Electrochemical decolourisation process of a real textile effluent (effluent as obtained), as a function
of time, by applying different current densities (20, 40 and 60 mA cm-2
) at 25°C and flow rate of 250 dm3 h
-1.
Inset: Decrease of HU, as a function of time, at different applied current densities. Dashed line indicates the
Brazilian limit regulation in HU to discharge the effluent.
0 5 10 150.0
0.5
1.0
1.5
60 mA cm-2
40 mA cm-2
20 mA cm-2
Time / h
Ab
st / A
bs
0
0 5 100
300
600
900
Time / h
Un
it H
azen
/ (
Pt-
Co
co
lor
un
its)
Whereas at 20 mA cm-2
of current density, no more than partial colour removal was achieved
(i.e. ≈50%, 400 HU) after 12 h of electrolysis.
Furthermore, the COD results clearly indicate that the highest removal rate is achieved at
40 and 60 mAcm-2
, see Figure 2, because of there is a greater charge passing into the cell that
favours the electrogeneration of more hydroxyl radicals produced on BDD surface (Eq. 1).
Despite the complete COD decay occurs under last conditions, long times are required for
complete removal, 19 and 17 h, respectively. In contrast, incomplete COD removal was
achieved by applying 20 mA cm-2
after 23 h (Figure 2). These results clearly indicate that, as
observed during colour removal, the low conductivity and organic matter dissolved in the
effluent complicate the depuration treatment. Consequently, increasing current density and
time treatment, a higher charge consumed for complete mineralization is needed because
during the electrochemical process a relative greater amount of OH wasted in parasite non-
oxidizing reactions such as oxygen evolution. It is confirmed from the current efficiencies
(TCE, in %) obtained for each current density applied (inset Figure 2) under these conditions,
ranging from 23 to 14%.
112
Figure 2. Influence of applied current on the evolution of COD and TCE (inset), as a function of time, during
electrochemical treatment of actual textile effluent (as discharged) on BDD anode at different current densities.
Conditions: T = 25 °C and flow rate of 250 dm3 h
-1.
0 5 10 15 20 250
200
400
600
800
60 mA cm-2
40 mA cm-2
20 mA cm-2
Time / h
CO
D / p
pm
0 5 10 15 20 250
5
10
15
20
25
Time / h
TC
E / %
This behaviour is frequently characteristic of electrolysis under mass transport control
when the electrolysis is performed applying a current higher than the limiting one, as already
indicated by other authors (PANIZZA; CERISOLA, 2010; BENSALAH et al. 2009). For a
recirculation rate of 250 dm3 h
−1 the mass transfer coefficient was 2.5 × 10
−5 m s
−1 and the
limiting current results in a value of 1.07 A, according Eq. 5. This current is relatively low
than all the currents applied in this work (1.1–3.3A), suggesting that the oxidation in these
conditions could be occurring under mass transport control. These assumptions, treating a real
effluent, are in agreement with the studies recently published by Panizza and Cerisola
(PANIZZA; CERISOLA, 2010) during the anodic oxidation of a real carwash wastewater.
Although the applicability of this treatment seems feasible, long times would be required
to complete decolourisation and organic matter removal (more than 18 h, see Figure 2).
However, as confirmed by other authors (PANIZZA; CERISOLA, 2010; BENSALAH et al.
2009; PANIZZA; CERISOLA, 2008), the colour and COD can be completely removed by
electrochemical oxidation with BDD anode generating effective oxidant species (chlorine,
hydrogen peroxide, perphosphates, and peroxodisulphates) on its surface. For this reason, new
set of experiments was performed to favour the production of reactive oxidant species, plus
hydroxyl radicals.
113
7.4.2 Electrochemical decolourisation of a real textile effluent adding Na2SO4
Electrooxidation experiments were performed in order to further achieve complete colour
elimination. Firstly, an amount of Na2SO4 (5 g dm-3
) was dissolved in the real effluent. After
that, a number of electrolysis-experiments were carried out by applying different values of
current density (20, 40 and 60 mA cm−2
at 25°C), as investigated under original discharged
effluent conditions. It is important to remark that after addition of Na2SO4, pH value
decreases from 10.2 to 9.1.
As it can be observed from Figure 3, when Na2SO4 was added in the effluent, colour
removal rate was significantly increased, achieving different values of efficiency (95%, 100%
and 100% of colour removal after 12 h, 8 h and 4 h of electrolysis, for 20, 40 and 60 mAcm−2
,
respectively). Under these experimental conditions, the Brazilian regulations concerning to
the colour removal (low than 300 HU) were attained, in all cases (data not showed). These
findings were achieved due to the increase in the conductivity of real effluent when Na2SO4
was added, favouring the electrogeneration of peroxodisulphates on BDD surface. In fact,
electrolysis BDD anodes in aqueous media, containing sulphate ions, generates
peroxodisulphate (equation 7) (PANIZZA; CERISOLA, 2010; BENSALAH et al. 2009;
PANIZZA; CERISOLA, 2008).
2- 2- -
4 2 82SO S O +2e (7)
Figure 3. Colour removal of a real effluent, as a function of time, applying different current densities using BDD
anode. Experimental conditions: T=25°C, 5 g dm-3
of Na2SO4 and flow rate=250 dm3 h
-1.
0 5 100.0
0.2
0.4
0.6
0.8
1.0
60 mA cm-2
40 mA cm-2
20 mA cm-2
Time / h
Ab
st / A
bs
0
114
These powerful oxidizing agents can oxidize organic materials by a chemical reaction
whose rate increases with the amount of sulphate ions in solution or/and temperature
(PANIZZA; CERISOLA, 2008; MARSELLI et al. 2003).
7.4.3 COD removal by electrochemical treatment
COD decay was also monitored by applying 20, 40 and 60 mAcm−2
of current density
when an amount of 5 g of Na2SO4 were dissolved in a 1 dm3 of the effluent. Figure 4 shows
the influence of the current density on the COD decay during the electrochemical oxidation of
the real textile wastewater on BDD anode, as a function of time, at 25°C.
Results clearly indicate that the complete COD removal is achieved, in all cases; because
these conditions favour the electrogeneration of more hydroxyl radicals (MARSELLI et al.
2003) and peroxodisulphates (PANIZZA; CERISOLA, 2008; CAÑIZARES et al. 2006;
FAOUZI et al. 2006) on BDD surface. It is important to mention that, treatment time, in the
presence of 1 g dm-3
Na2SO4 (Figure 4), was reduced respect to the time employed to
eliminate COD completely at real discharged conditions (Figure 2). On the other hand, an
increase on current efficiency was observed (inset on Figure 4) because the electrogeneration
of peroxodisulphates avoid mass transport limitations and also the secondary reaction of
oxygen evolution.
Figure 4. Influence of applied current on the evolution of COD and TCE (inset), as a function of time, during
electrochemical treatment of actual textile using BDD anode. Experimental conditions: T = 25°C, 5 g dm-3
of
Na2SO4 and flow rate = 250 dm3 h
-1.
0 5 10 150
200
400
600
800
60 mA cm-2
40 mA cm-2
20 mA cm-2
Time / h
CO
D / p
pm
0 5 10 15 200
20
40
60
80
Time / h
TC
E / %
115
7.4.4 Effect of Na2SO4 dissolved in the effluent
Figure 5 shows the influence of Na2SO4 concentration (in g dm-3
) as a function of the time
and current efficiency (inset) values during galvanostatic electrolysis of real textile
wastewaters by applying 20 mA cm−2
of applied current density.
As can be observed, the COD removal was poor in absence of Na2SO4 in solution (Figure
5). However, COD removal rate increases significantly when the amount of Na2SO4 was
increased in solution. But, no relevant efficiencies were achieved when the Na2SO4
concentration was increased from 5 to 20 g (inset in Figure 5). It can also be seen, that
maximum efficiencies were obtained for the initial stages of the process (high COD
concentrations), and after given a decrease in the current efficiencies, continuously down to
very low COD values; employing similar electrolysis-times to achieve complete COD
abatement. Then, from industrial point of view, only an amount of 5 g of Na2SO4 can be
added to the effluent to favor the complete removal of dissolved organic matter.
Figure 5. Influence of Na2SO4 concentration on the evolution of COD and %TCE (inset), as a function of time,
during electrochemical treatment of actual textile effluent on BDD anode by applying 20 mA cm−2
of applied
current density. Conditions: T = 25 °C, flow rate=250 dm3 h
-1.
0 5 10 15 20 250
200
400
600
800
20 g
15 g
10 g
5 g
0 g
Time / h
CO
D / p
pm
0 5 10 15 200
10
20
30
40
50
Na2SO4 / g
TC
E / %
7.4.5 Influence of temperature
The treatment of the actual textile effluent was also carried out at 60 °C by applying
current densities of 20, 40 and 60 mA cm-2
. The latter temperature was selected, because it
mimics the real temperature of the effluent discharged by the textile industry (60°C). It was
observed (Figure 6) that changes in temperature have a strong influence on oxidation rate at
116
all current densities and varying the temperature from 25 to 60°C; since the COD removal
after 9 or 10 h of treatment was 100% at 60°C. As reported in literature (PANIZZA;
CERISOLA, 2010; BENSALAH et al. 2009), this behaviour was principally attributed not to
an increase of the activity of the anodes (due to an increase of temperature) but to an increase
of the indirect reaction of organics with electrogenerated oxidizing agents from electrolyte
oxidation.
In fact, as mentioned above, peroxodisulphates can be formed in solutions containing
sulphates, especially at higher temperatures (60°C), as already demonstrated by other authors
(MARTINEZ-HUITLE; BRILLAS, 2009; PANIZZA; CERISOLA, 2010; PANIZZA;
CERISOLA, 2008), during electrolysis with BDD electrodes. These reagents are very
powerful oxidants and can oxidize organic matter leading to an increase in colour removal
rates and COD decay. Complete decolourisation (data not showed) and COD removal rates
(Figure 6) noticeably increased when the temperature was increased. Under these conditions,
a modest increase on TCE was also achieved respect to the temperature of 25°C.
Figure 6. Comparison of the influence of temperature on the evolution of COD, as a function of time and %TCE
(inset) during oxidation of actual textile effluent on BDD anode. Conditions: 40 mA cm-2
of current density; 5 g
of Na2SO4 and flow rate = 250 dm3 h
-1.
0 5 10 15 200
200
400
600
800
60 mA cm-2
40 mA cm-2
20 mA cm-2
60 mA cm-2
40 mA cm-2
20 mA cm-2
60 °C
25 °C
Time / h
CO
D / p
pm
0 5 10 15 200
20
40
60
80
100
Time / h
TC
E / %
7.4.6 Effect of the flow rate
In order to verify the important role of mass transfer on the electrochemical treatment of
real textile effluent, the influence of flow rate during BDD-anodic oxidation was studied
varying the flow rate. Experiments were performed at four rates, in the range of 200–400 dm3
h-1
. Figure 7 shows the flow rate effect on the COD removal as a function of the time, during
117
galvanostatic electrolysis by applying 40 mA cm−2
of current density. As can be seen, the total
COD removal was achieved under different flow rates. It appears that the hydrodynamic
conditions affect the rate of COD removal because this behavior is due to that sulfates present
in solution produce a chemical oxidant and therefore these avoid the problems due to mass
transport.
7.4.7 Energy consumption and cost estimation
Comparison between the trend of energy consumption (kWh dm-3
) as a function of COD
removal, at different applied current densities (20, 40 and 60 mA cm-2
) and experimental
conditions (real discharged conditions and Na2SO4 dissolved in the effluent) is presented in
Fig. 8. As can be observed, BDD consumed less energy when an amount of Na2SO4 was
added in the effluent than that consumed under real discharged effluent conditions. In fact, in
the presence of Na2SO4 less time is necessary for COD removal and also the cell potential is
low (4.1 V). These results point out the high performance BDD-anodic oxidation for treating
textile wastewaters. However, this electrochemical process can be a feasible pre-treatment
method as a previous step to biological depuration or it could be coupled with other
wastewater treatments (e. g.: UV irradiation, Fenton, adsorption) reducing significantly the
cost and time treatment.
7.5 Concluding remarks
On basis of the results obtained for anodic oxidation of a dyestuff effluent, the
electrochemical technology can be suitable as an alternative for pre-treatment of textile real
effluents under the real discharge conditions employed by the Brazilian textile industry (COD,
pH = 10 and temp = 60°C) for complete COD and colour removal.
However, if an amount of Na2SO4 is added, the efficiency of the process can be strongly
improved. It is important to mention that in the Brazilian textile industry, complete COD and
colour removal are attained after 5 or 6 days of biological depuration together with a
subsequent physical-chemical treatment (under specific pH and temperature conditions).
118
Figure 7. Influence of flow rate on the evolution of COD, as a function of time and %TCE (inset) during
oxidation of actual textile effluent on BDD anode. Conditions: 40 mA cm-2
of current density; 5 g of Na2SO4 and
T = 25 °C.
0 5 10 150
200
400
600
800
300 dm3/h
400 dm3/h
200 dm3/h
250 dm3/h
Time / h
CO
D / p
pm
0 5 10 15 200
10
20
30
40
50
Time / h
TC
E / %
Figure 8. Energy consumption of the electrochemical process, as a function of COD removal and effluent
conditions, during oxidation of actual textile effluent on BDD anode by applying different current densities.
0 25 50 75 1000
100
200
300
400
500
60 mA cm-2
40 mA cm-2
20 mA cm-2
60 mA cm-2
40 mA cm-2
20 mA cm-2
under dischargedconditions
5 g of Na2SO4
dissolved in theeffluent
COD removal / %
En
erg
y c
on
su
mp
tio
n / k
W h
m-3
Conversely, COD and colour were efficiently reduced after 15 h of electrochemical process,
attaining Brazilian legal requirements and maintaining the same discharged conditions with
the addition of a small amount of Na2SO4; in consequence, the time treatment was reduced
and costs, confirming the potential efficiency of this dyestuff treatment.
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123
CAPÍTULO 8
CONCLUSÃO E CONSIDERAÇÕES FINAIS
124
8.0 CONCLUSÃO E CONSIDERAÇÕES FINAIS
A Rodamina B pode ser eliminada com sucesso através da oxidação eletroquímica
utilizando ânodo de BDD, favorecendo uma rápida mineralização, independente do eletrólito
utilizado, mas vale ressaltar que os mesmos podem influenciar na taxa, eficiência e via de
degradação do processo. Valores referentes à cinética foram maiores do que o esperado, isso
indica que houve um favorecimento dos processos de oxidação mediada.
Ao realizar o estudo da eletrólise da RhB com irradiação de luz em meio fosfato e
sulfato, observou-se que houve um efeito positivo, onde essa substância é oxidada
quimicamente pelos oxidantes produzidos. Entre os dois meios, a solução de perfosfato é mais
agressiva que a de persulfato.
Através do estudo da degradação da RhB em diferentes ânodos de BDD observa-se
que a configuração sp3/sp
2 influencia no comportamento desse eletrodo, apresentando dois
tipos de comportamentos: ânodo não-ativo (oxidação da matéria orgânica em CO2) e o ativo
(conversão eletroquímica). Outras experiências com BDD6 e BDD1 estão em andamento para
esclarecer esta propriedade BDD.
Em relação aos dados obtidos para a aplicação da oxidação eletroquímica com ânodo
de BDD em um efluente real da indústria têxtil observou-se que a uma temperatura de 60 ºC e
um pH 10, ocorreu a degradação desse corante em 15 horas de processo, bem diferente
quando comparado aos métodos convencionais, 5 a 6 dias.
De acordo com os dados apresentados nessa tese, a oxidação eletroquímica é um
método rápido, limpo e eficaz para o tratamento de efluentes sintéticos e efluentes reais da
indústria têxtil.
