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.