Effect of Specific Active Chlorine Species and Temperature on the Electrochemical Degradation of the Reactive Blue 19 Dye Using a Boron-Doped Diamond or DSA Anode in a Flow Reactor

Abstract

Galvanostatic electrolyses were carried out to investigate the effect of specific active chlorine species (Cl2, HClO, or ClO–) and temperature on the degradation of reactive blue 19 dye (RB-19) solutions (500 mg L–1 in 0.1 mol L–1 Na2SO4), in the presence or absence of chloride ions, using two anodes of markedly different oxidation power: dimensionally stable anode (DSA — Ti/TiO2RuO2) or boron-doped diamond (Nb/BDD). In the absence of chloride ions in solution (at pH 6.0 and 25 °C), the Nb/BDD anode, due to its higher oxidation power, was notably more effective in decolorizing the RB-19 dye than the DSA anode. Yet, when this decolorization was carried out by chloride-mediated electrooxidation (2 g L–1 NaCl added to the dye solution), both anodes presented significantly enhanced performances, but now greater decolorization rates were attained with the DSA anode, especially at pH 1.0, when Cl2 is the predominant active chlorine species in solution; actually, the highest decolorization rates are attained at 10 °C, when the Cl2 solubility in solution increases. Clearly, Cl2 is the active chlorine species that most readily oxidizes the anthraquinone chromophore center of the RB-19 molecule. The rate of chloride-mediated COD abatement of the RB-19 dye solution was also analyzed (at pH 6.0 and 25 °C); now the oxidation power of the anode is again paramount and Nb/BDD presents a significantly superior performance, because hydroxyl radicals play a crucial role in the oxidation of organics, beyond that possible for the active chlorine species.