Integrated system of electrocoagulation and electrooxidation for industrial indigo wastewater: Performance improvement and catalytic mechanism

To address the technical bottleneck of limited oxidative capacity in conventional electrocoagulation (EC) for treating indigo dyeing wastewater—a representative recalcitrant effluent in the textile industry—this study proposes a synergistic electrocoagulation-electrooxidation (EC-EO) system driven by a nitrogen-doped carbon (N–C) cathode. The N–C cathode was fabricated via pyrolysis, with pyridinic-N and graphitic-N collaboratively modulating the oxygen reduction reaction (ORR) pathway. Under near-neutral pH conditions (pH = 6), the cathode exhibited an efficient ORR performance with a hydrogen peroxide (H 2 O 2 ) yield of 15.67 mg L -1 . The in-situ generated reactive oxygen species selectively oxidized S 2 O 4 2- to SO 4 2- , thereby disrupting the reducing nature of the pollutants and significantly lowering the chemical oxygen demand (COD). Nitrogen doping enhanced the electronic conductivity of the cathode, markedly reducing the interfacial resistance at the aluminum anode and promoting continuous Al 3+ release, which facilitated the formation of flocs. During a 10 V/50 min reaction, simultaneous electrocatalytic oxidation and EC sedimentation were achieved, resulting in a decolorization efficiency of 99.9% and a COD removal rate that was 64.3% higher than that of standalone EC. After three treatment cycles, the system exhibited less than 18% performance decay, and the energy consumption per unit COD removed decreased by 41.45%. This work demonstrates a cathode-engineering-based approach to simultaneously enhance oxidation and coagulation efficacy, offering a novel and efficient strategy for the integrated treatment of industrial wastewater containing reductive contaminants.