Electrochemical Phosphite Removal and Recovery via the Ti₄O₇ Monolithic Electrode: Reaction Mechanism and Kinetic Modeling

Phosphite is present in certain industrial wastewaters; yet, its treatment has long been overlooked. Inadequate phosphite removal exacerbates water eutrophication, disrupts ecological balance, and poses risks to public health. Herein, we prepared titanium suboxide (Ti4O7) monolithic electrodes through a pressing–sintering method with the fabricated electrodes exhibiting abundant reactive sites, excellent conductivity, and remarkable stability. Subsequently, an electrochemical system was constructed using a Ti4O7 electrode as the anode and titanium (Ti) plate as the cathode for phosphite oxidation. This system effectively activates chloride ions (Cl–) and water molecules (H2O) to generate the reactive chlorine species (RCS) and hydroxyl radicals (HO•), respectively. When operated at a current density of 10 mA/cm2, the electrochemical system achieved a 95% oxidation of phosphite to phosphate within 2 h. Both inherent direct electron transfer (DET) capability of the Ti4O7 anode (∼40%) and reactive oxidants (RCS and HO•; ∼60%) contributed to phosphite oxidation. A mechanistically based kinetic model was developed to describe the rate and extent of phosphite oxidation under a range of operating conditions. The system maintained over 90% phosphite oxidation efficiency over 10 consecutive cycles, with no observable variation in the electrode's morphology, crystalline structure, and chemical composition. Furthermore, phosphate (formed via phosphite oxidation) was successfully recovered as a hydroxyapatite product via precipitation with calcium ion (Ca2+). Overall, this work highlights the potential of Ti4O7-based electrochemical systems for the integrated removal and recovery of phosphite from industrial wastewater.