Thermal stimulation unlocks ligand-dependent selective pathways in Cu(II)/PMS oxidation of recalcitrant phosphonates

This study investigated the degradation performance of two representative chelating phosphonates, nitrilotris(methylenephosphonic acid) (NTMP) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), in a homogeneous thermal-Cu(II) co-activated peroxymonosulfate (PMS) system at 60 °C. With trace Cu(II) (5 μM), 92.12 % of NTMP and 65.53 % of PBTC were converted to phosphate within 30 min, but exhibited obvious different degradation process. Mechanistic analysis revealed that the electron-donating NTMP, upon complexation with Cu(II), directly transferred electrons to Cu(II) under thermal stimulation, reducing it to Cu(I) without PMS involvement and thereby alleviating steric hindrance. The generated Cu(I) was subsequently oxidized by PMS through a two-electron process to form Cu(III). By contrast, the electron-withdrawing PBTC facilitated Cu(II) oxidation to Cu(III) under thermal stimulation, accompanied by PMS O-O bond cleavage and the generation of Cu(III)-bound •OH. Cu(III) dominated NTMP oxidation, whereas Cu(III)-bound •OH governed PBTC degradation, and both pathways exhibited relatively high selectivity. This work demonstrates that thermal input tailors catalytic pathways in Cu(II)/PMS systems via ligand electronic effects, overcoming structural constraints of pollutants and broadening the applicability of Cu-based AOPs for water treatment.