Efficient phosphonate degradation by Ti4O7 electrode under realistic conditions: Key contribution of HOCl and long-term stability

Using phosphorus-containing scale inhibitors (i.e., phosphonates) in cooling water poses a significant challenge to water pollution and eutrophication during discharge. Here we report an efficient electrochemical oxidation (EO) process based on a low-cost titanium suboxide (Ti₄O₇) inert anode. We found that ubiquitous chloride ions (Cl⁻) in cooling water significantly promoted the transformation of methylene phosphonic acid (NTMP, a representative phosphonate) into orthophosphate. In the Cl⁻-Ti₄O₇-AO system, hypochlorous acid (HOCl) is the primary reactive species responsible for the sequential cleavage of C–P and C–C bonds, converting organic phosphorus into inorganic phosphate. Furthermore, the presence of HCO₃⁻ or humic acid (HA) under realistic conditions showed negligible effects on the degradation of NTMP. In long-term operation, the Ti₄O₇-AO system sustained 100% conversion efficiency of NTMP over continuous-flow operation mode for more than 720 h, even though the cathode was almost entirely covered with CaCO₃ deposits. The results revealed that cathode scaling does not profoundly affect the conversion of organic phosphorus to phosphate in the Ti₄O₇-AO system. Overall, our study elucidates the mechanism and robust efficiency of the Cl⁻-Ti₄O₇-AO system in treating phosphonate-laden wastewater and offers a new solution for dealing with cooling water by coupling electrochemical oxidation with ubiquitous chloride ions.