Activation of hydrogen peroxide, persulfate, and free chlorine by steel anode for treatment of municipal and livestock wastewater: Unravelling the role of oxidants speciation

Abstract

Despite the extensive application of electrochemical advanced oxidation processes (EAOPs) in wastewater treatment, the exact speciation of oxidants and their effects on pollutants removal efficiency, by-products formation, and effluent toxicity are largely unknown. In this study, galvanostatic steel anodes were used to drive the electrochemical activation of hydrogen peroxide (EAHP), persulfate (EAP), and free chlorine (EAFC), for industrial-scale treatment of municipal and livestock wastewater with a focus on micropollutants and transformation products (MTPs) and effluent toxicity. Response surface methodology determined the optimized conditions for each treatment towards total organic carbon ([TOC]0 = 180 mg/L) removal at pH 3.0: persulfate dose = 0.12 mmol/min, 26.5 mA/cm2; free chlorine dose = 0.29 mmol/min, 37.4 mA/cm2; H2O2 dose = 0.20 mmol/min, 45 mA/cm2. Probe-compound degradation revealed that HO•, SO4•− and FeIVO2+ species were simultaneously generated in EAP, whereas HO• and FeIVO2+ were the principal oxidants in EAHP and EAFC, respectively. Samples were analyzed via liquid and gas chromatography in non-target screening (NTS) mode to monitor the generation or removal of MTPs and by-products including compounds that have not been reported previously. The speciation of oxidants, shifted in presence of halide ions (Cl−, Br−) in real wastewater samples, significantly affected the mineralization efficiency and by-product formation. The production of halogenated by-products in EAFC and EAP substantially increased the effluent toxicity, whereas EAHP provided non-toxic effluent and the highest mineralization efficiency (75 − 80%) to be nominated as the best strategy.