Challenges and contribution of electrochemical driven by-products in tannery wastewater treatment: Optimization, detection and distribution of reactive oxidation species

Electrochemical oxidation (EO) is an excellent approach for the treatment of persistent pollutant from synthesistic and real wastewater than conventional wastewater treatment processes. Chloride and sulfate salts generally used and present in natural wastewater that affect the EO process. In this research, the effect of electrolyte concentration on active sulfate (SO₄^2⁻) species (HSO₄^⁻, SO₄^•⁻ and S₂O₈²⁻) formation, chlorinated by-products distribution (ClO₄^-, ClO₃^-, Cl₂), and tannery effluent degradation have been examined while using graphite electrodes. A full factorial design was used to optimize the three independent factors, namely: initial pH (pH_o): 3-11, current (I): 1-3 A, and electrolysis time (t): 20-110 min for the responses of chemical oxygen demand (COD) and chromium (Cr) removal. Under the optimum treatment conditions of 3 A current, 90 min electrolysis time, 600 mg L^-1 Na₂SO₄ concentration and pH_o of 7, more than 88% COD and 90% Cr removal were achieved under optimal conditions. Qualitative and quantitative analysis confirmed the formation and distribution of various reactive oxidation species and a plausible mechanism was discussed. EO processes yielded almost total mineralization due to the synergistic action of generated active chlorine, sulfate species and hydroxyl radicals. A relatively higher amount of ClO₃⁻ was occurred that sign the efficient ^•OH generation in sulfate mediated EO because the ClO₃⁻ formation is certainly associated to ^•OH concentration. Overall results demonstrate that sulfate enriched electrolyte systems are helpful for EO of hazardous organic pollutants.