Cost-Effective Cathode Materials To Electrochemically Tackle Aquatic Selenite Pollution

Abstract

Direct electrochemical reduction (DER) of selenite has been extensively explored for industrial electroplating, and its high selectivity toward aqueous selenite offers new insight into treating complex Se-laden wastewater. While the benchmark study confirms the feasibility of selenite DER with a gold cathode, the high material cost burdens its industrial applications. In this paper, we evaluate six cost-effective cathode materials on their ability to remove aqueous selenite through DER, including nickel, graphite, copper, iron, stainless steel, and titanium. We focus on their removal efficiency, removal kinetics, Faradaic efficiency, and underlying electroreduction mechanisms. Under a chronoamperometry mode, nickel and graphite exhibit 6 h linear removal kinetics of 134.7 and 186.0 mg Se(IV) m–2 h–1 and 24 h removal efficiencies of 67 and 94%, respectively. Graphite’s initial 6 h Faradaic efficiency (28.3%) is much higher than nickel’s (15.9%) due to fewer side reactions. When switching to the chronopotentiometry mode, both cathode materials experience increases in energy consumption, and a notable drop in Se removal is observed using a graphite cathode (77%). We further confirm Se insertion in graphite is possible, owing to graphite’s porous and layered structure. Compared with other metal cathodes, the corrosion-free and cost-effective graphite does not release metal ions into the water matrix and offers excellent Se(IV) removal on par with the gold electrode. Our results suggest value in future work to decipher the Se insertion mechanism in carbon-based electrodes and evaluate the performance of insertion cathodes when treating complex Se-laden wastewaters.