UV-driven trace Cu(II)/peroxymonosulfate for efficient degradation of phosphonate: Mechanism and broad pH adaptability

Organic phosphonates detected frequently in water bodies pose severely environmental risks, and the cleavage of their C-P bonds to converse orthophosphate (PO₄^3-) serves as a prerequisite step for achieving deep phosphorus elimination. Although the in situ generation of Cu(III) via Cu(II)/oxidant system for selective phosphonates degradation has been studied, the oxidation efficiency is constrained by sluggish Cu(II)/Cu(I) reduction, especially under acidic conditions. In this study, the introduction of UV irradiation accelerated the Cu(I)/Cu(II)/Cu(III) cycle in Cu(II)/peroxymonosulfate (PMS) process, enabling efficient and selective oxidation of 1,1-diphosphonic acid (HEDP, a typical phosphonate) into PO₄^3- across a wide pH range. UV-driven trace Cu(II)/PMS system can convert 90 % of HEDP into PO₄^3- within 10 min at the pH range of 4–10, which was significantly higher than the conversion efficiency of HEDP by UV/PMS, Cu(II)/PMS and UV/Cu(II)/H₂O₂ processes. The decomposition of HEDP was enhanced with increasing Cu(II) and PMS concentrations. Notably, mechanistic investigation revealed that Cu(III)-induced intramolecular electron transfer was the key contributor during the UV/Cu(II)/PMS-driven decomposition of HEDP into PO₄^3-. The experimental results of competitive ligands clearly suggested that the high selectivity of HEDP oxidation by UV/Cu(II)/PMS was closely related to the complexation of Cu(II) with HEDP. Additionally, although natural organic matter and inorganic anions to some extent affected HEDP degradation, UV-driven trace Cu(II)/PMS system still exhibited satisfactory results in treating HEDP in actual wastewater. This study proposes a strategy for efficient phosphonate removal under varying pH conditions, which provides new insights for practical wastewater treatment applications.