Electrochemical degradation of indigo carmine by low voltage pulse electrolysis

Degradation of indigo carmine (IC) in aqueous solution via electrochemical oxidation (EO) was investigated by low voltage pulse electrolysis. The effects of experimental operating variables on total organic carbon(TOC) removal efficiency were studied, including initial pH, NaCl addition, charging voltage/frequency, and initial concentration of IC. The IC removal efficiency (in TOC) was higher under highly acidic or alkaline conditions, increased with the NaCl addition and an increase in charging voltage or frequency, but decreased with an increase in the initial concentration of IC. Intermediates formed during the degradation such as isatin-5-sulfonic acid (m/z = 226), 2-amino-α-oxo-5-sulfo-benzeneacetic acid (m/z = 242), 2-amino-5-sulfobenzoic acid (m/z = 198), and 2-amino-5-hydroxybenzoic acid (m/z = 152) were identified, on the basis of which a degradation pathway was proposed for the electrochemical degradation of IC in aqueous solution. The electrical energy consumption (Ec) in electrocatalytic oxidation was 159 kWh/kgTOC and the current efficiency is 15.3%. The EO technique on the basis of low voltage pulse electrolysis may be a promising approach to treatment of textile wastewater.