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.
Selenium (Se) is a naturally occurring metalloid that has been widely sourced for health care, clean energy technology development, and agricultural activities. These activities have accelerated Se release into the aquatic environment, urging engineered solutions to mitigate aquatic Se pollution and their ecological impact. Our lab previously demonstrated a direct electrochemical reduction approach to convert 97% of soluble selenite into elemental Se(0) on a gold cathode. This study builds upon our prior success in selenite separation to further explore alternative cathode materials to gold. Six economically competitive materials (Ni, graphite, Cu, Fe, stainless steel, and Ti) were evaluated for key performance parameters, including Se removal efficiency, Faradaic efficiency, energy consumption, and electrode durability. Preliminary cyclic voltammetry scans revealed that Ni and graphite could sustain Se(IV)/Se(0) reduction in diluted water matrices within their electrochemical window. The subsequent 24-h chronoamperometry (CA) test demonstrated 67% selenite removal using a Ni cathode, while graphite offered a better removal efficiency (92%). Separated selenite ions were primarily plated as elemental Se(0) on the cathode surface (Ni & graphite) or within the electrode structure (graphite). While graphite has better selenite removal than Ni, it demands more energy input and has lower Faradaic efficiency (C-3.7% vs. Ni-12.7%). Switching from a CA mode to a chronopotentiometry (CP) mode did not significantly impact selenite removal performance, and the energy consumption was increased by 15%. Continued exfoliation is observed on the graphite electrode surface, potentially due to the expansion of inner gas bubbles and the lattice destruction caused by Se(0) insertion. Our results suggest graphite offers comparable selenite separation performance to a gold cathode. Still, electrode exfoliation and higher energy input due to parasitic reactions must be appropriately addressed in future research efforts.
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