Electrochemical degradation of azo dye C.I. Reactive Red 195 by anodic oxidation on Ti/SnO2–Sb/PbO2 electrodes

This study investigated the electrochemical degradation of C.I. Reactive Red 195 (RR195) in aqueous solution on a Ti/SnO2–Sb/PbO2 electrode. The influence of operating variables on the mineralization efficiency was studied as a function of the current density, the initial pH, the initial concentration of the dye and the addition of NaCl. The efficiency of RR195 mineralization decreased with increased initial concentration, from 100 mg L−1 to 400 mg L−1. The current density had both a positive and a negative effect on degradation rates, and no significant effect of initial pH on RR195 mineralization was observed. Measurement of absorbance was used to discriminate the effect of NaCl in the electro-oxidation process. We found that the decolouring efficiency increased whereas the mineralization efficiency decreased with the increasing concentration of NaCl. The intermediates formed during the degradation were detected by gas chromatography–mass spectrometry, and the major aromatic intermediates identified were 1-(3,6,8-trihydroxy-1-naphthyl)urea, nitrobenzene, benzo-1,4-quinone, (3,6,8-trihydroxy-1-naphthyl)carbamic acid and phthalic acid. Quantitative measurement of organic and inorganic ions was done by ion chromatography. On the basis of the reaction products identified, a possible degradation pathway for the anodic oxidation of RR195 in aqueous solution is proposed.