One-step phosphite removal by an electroactive CNT filter functionalized with TiO₂/CeO_x nanocomposites

Compared with phosphate (+5 valence), phosphite (HPO₃^2-/H₂PO₃^-, +3 valence) possesses higher solubility, and is more resistant to biotransformation. Herein, we designed a one-step electroactive filter technology for rapid and efficient phosphite removal. The filter consists of carbon nanotubes (CNT) and functionalized with nanoscale TiCe binary oxides. The phosphite removal kinetics and capacity increased with electric field (e.g., from 54.5% at 0 V to 75.6% at 2 V) and flow rate (e.g., from 63.1% at 1.5 mL/min to 81.2% at 6 mL/min). This can be attributed to synergistic effects of the filter's electrochemical reactivity, limited pore size, more exposed active sites and flow-through design. Meanwhile, phosphite can be converted to phosphate once adsorbed under electric field. The TiO₂/CeO_x-CNT filter could work effectively across a wide pH range, and the presence of various coexisting anions posed negligible impact on phosphite removal. Electrochemical characterizations verified the essential role of CeO_x and applied electric field, which synergistically accelerated electron transfer rate and increased charge capacity. The TiO₂/CeO_x-CNT filter can be regenerated effectively by chemical washing. The system efficacy was further supported by a comparable phosphite removal efficiency of 72.8% in actual lake water conditions. Therefore, this TiO₂/CeO_x-CNT filter technology is promising for mitigating the challenging issue of phosphite contamination from water bodies.