Electrochemical Oxidation of Reactive Black 5 and Blue 19 Dyes Using a Non Commercial Boron-Doped Diamond Electrode

Abstract

respectively azo and anthraquinone textile dyes, by electrochemical oxidation using a non commercial boron doped diamond electrode supported on titanium metallic with a relation between B and C equal to 15,000 ppm. Pt was used as counter electrode and Ag/AgCl (sat) was the reference electrode. The variables investigated to optimize the degradation were support electrolyte concentration (0.05, 0.1 and 0.2 mol L -1 of K 2 SO 4 ), temperature (25, 35 and 45 ºC) and pH (2.5 and 10) by applying a current density of 75 mA cm -2. Total decolorization was observed in all electrochemical conditions studied. The highest rates of mineralization and chromatographic area removal were attained at high support electrolyte concentration due to the increase of conductivity, which caused more generation of hydroxyl radicals. RP5 was degraded at lower electrolysis time than RA19 with smaller requirement of electric charge. The mineralization rate increased in acid medium and at higher temperature since the increase in temperature favors the generation reactions of persulfate. These conditions led to the highest levels of current efficiencies and lower energy consumed.Keywords: boron-doped diamond electrode; Reactive Black 5; Reactive Blue 19; decolorization; hydroxyl radical; persulfate. INTRODUÇÃOOs corantes sintéticos são compostos de extrema importância para o setor industrial, principalmente, para as indústrias têxteis que são responsáveis pela enorme descarga de efluentes contendo corantes e outros resíduos indesejáveis, sendo potenciais poluidoras ambientais. 1-4Os efluentes têxteis apresentam alta demanda química e biológica de oxigênio, elevada salinidade e intensa coloração devido à presença de corantes, os quais são fotolítica e quimicamente estáveis, podem exibir toxicidade aguda e serem carcinogênicos e/ou mutagênicos. Sendo assim, quando os corantes são descartados incorretamente no meio ambiente podem ocasionar problemas ambientais e, sobretudo, ser nocivos à saúde humana e dos animais. 5,6 Os corantes reativos, por exemplo, são solúveis em água, aniôni-cos e ligam-se às fibras do tecido por meio de ligações covalentes, pela reação de adição nucleofílica do grupo vinilsulfona ao grupo hidroxila da celulose, o que confere ao tecido tingido alta estabilidade da cor adquirida. 7,8 Entretanto, esses corantes apresentam baixa eficiência de fixação às fibras devido à competitividade de duas reações, a primeira sendo do grupo reativo vinilsulfona com as fibras e a segunda a reação de hidrólise do grupo vinilsulfona produzindo o 2-hidroxietilsulfona, que não se fixa a fibra e é perdido no efluente.9 As Figuras 1a e 1b ilustram as estruturas moleculares de dois corantes reativos, o Reativo Preto 5 (RP5) e o Azul (RA19), respectivamente, que apresentam como grupo reativo a sulfatoetilsulfonila.O corante RP5, com grupo cromóforo azo, é um dos corantes mais utilizados no tingimento de algodão, no qual emprega-se, aproximadamente, 40 g do corante dissolvidos em 70 a 150 L de água para cada kg de algodão tingido. 10 No entanto, ...

Interactive Lecture in Redox Chemistry: Analysis of the Impact of the Dissemination of University Scientific Research among High School Students

“…In the experiment, the Blue 19 (RA19) reactive dye was used. It is a synthetic dye, extremely important in the textile industry, soluble in water with a bright blue tint, and resistant to light. − A piece of steel wool and 0.5% hydrogen peroxide was also used. Figure represents the structural formula of the dye RA19 and the photos of the reaction vessel showing what happens when the various equivalents of hydrogen peroxide are added to the dye solution.…”

“…The reaction observed in the experiment is the degradation of the dye. This degradation is done through the advanced oxidative process (POA). − This process is characterized by the formation of • OH hydroxyl radicals. This is the main step due to the fact that hydroxyl radicals have a high oxidizing power and short life span, which makes oxidation of organic compounds possible. − The generation of hydroxyl radicals may come from oxidants such as hydrogen peroxide (H 2 O 2 ). , When metallic iron Fe 0 comes into contact with reactive dye blue 19, Fe 0 oxidizes to form Fe 2+ and consequently the organic compound is reduced, gaining these two electrons.…”

“…In an acidic environment, the primary (−NH 2 −) and secondary (−NH−) amines present in the compound’s molecular structure have a tendency to assume a positive charge resulting in the protonation of these groups. The presence of positively or negatively charged groups can cause a displacement of the electronic density and may alter the stability of these molecules in addition to favoring the electrochemical oxidation process of the NH 2 to NO 2 group (eq ): From the addition of hydrogen peroxide, the Fe 2+ formed previously reacts with the peroxide generating the hydroxyl radical (eq ). This hydroxyl radical reacts with the dye RA 19 decomposing it into another less toxic organic substance. , The chemical equations for this reaction step are A reaction scheme can be seen in Figure .…”