Self-Enhanced Selective Oxidation of Phosphonate into Phosphate by Cu(II)/H₂O₂: Performance, Mechanism, and...

Phosphonate is an important category of highly soluble organophosphorus in contaminated waters, and its oxidative transformation into phosphate is usually a prerequisite step to achieve the in-depth removal of the total phosphorus. Currently, selective oxidation of phosphonate into phosphate is urgently desired as conventional advanced oxidation processes suffer from severe matrix interferences. Herein, we employed 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) as a model phosphonate and demonstrated its efficient and selective oxidation by the Cu(II)/H₂O₂ process at alkaline pH. In the presence of trace Cu(II) (0.020 mM), 90.8% of HEDP (0.10 mM) was converted to phosphate by H₂O₂ in 30 min at pH 9.5, whereas negligible conversion was observed by UV/H₂O₂ or a Fenton reaction (pH = 3.0). The oxidation of HEDP by Cu(II)/H₂O₂ was insignificantly affected by natural organic matters (10.0 mg TOC/L) and various anions including chloride, sulfate, and nitrate (10.0 mM). The complexation of Cu(II) with HEDP coupling Cu(III) produced in situ enabled an intramolecular electron transfer process, which features high selective oxidation. Selective degradation of HEDP was further validated by adding stoichiometric H₂O₂ into an industrial effluent, where the existing Cu(II) could serve as the catalyst. This study also provides a successful case to trigger selective oxidation of trace pollutants of concern upon synergizing with the nature of the contaminated water.