Treatment of industrial etching terminal wastewater using ZnFe₂O₄/g-C₃N₄ heterojunctions photo-assisted cathodes in single-chamber microbial electrolysis cells

Cost-effective, semiconductor ZnFe₂O₄/g-C₃N₄ heterojunction cathodes were investigated to achieve efficient treatment of industrial etching terminal wastewater in photo-assisted, single-chamber, microbial electrolysis cells (PS-MECs). The PS-MECs performance progressively increased over time, reaching significant Ni(II) removal (4.4 mg/L/h), recalcitrant organics mineralization (11.3 mg/L/h), hydrogen production (0.55 m³/m³/d) and solar-to-hydrogen conversion efficiency (6.7%) after 12 days fed-batch operation. The progressive deposition of Ni over the cathodes and the physiological release of extracellular polymeric substances (EPS) dynamically influenced the proportions of reactive oxidative species. Triplet ³EPS* (78%) on both electrodes and cathodic holes (22%) contributed to recalcitrant organics mineralization during the 12th fed-batch operational cycle, while cathodic holes (65%) exceeded anodic ³EPS* (35%) during the 1st-cycle. Significantly different bacterial communities were observed over the cathodes (Acinetobacter (17.7%) and Staphylococcus (16.8%) and anodes (Novosphingobium (42.9%)) after the 12th-cycle, as confirmed by KEGG PICRUSt analysis. This study broadens the application of cost-effective PS-MECs for industrial wastewater treatment.