Mechanistic insights into hexamethylphosphoramide degradation and phosphorus recovery via mZVI/O₃: Beyond conventional oxidation through iron-phosphorus speciation

In this study, ferrous ions (Fe²⁺), ferric ions (Fe³⁺), and micron-scale zero-valent iron (mZVI) were employed to activate O₃ for the degradation of hexamethylphosphoramide (HMPA) with simultaneous phosphorus recovery. Among the tested systems, mZVI/O₃ exhibited the highest performance, achieving 84.8% mineralization (i.e., conversion of organophosphorus into inorganic phosphate, PO₄³⁻) of 50 μM HMPA and 88.9% phosphorus recovery under optimal conditions (pH = 3, mZVI dosage = 5 g/L). Organophosphorus compounds promoted continuous iron release from mZVI surfaces, mitigating passivation. Concurrently, competitive O₃ consumption between HMPA degradation and mZVI oxidation minimized unproductive iron dissolution. The continuous release of nascent ferric hydroxides from mZVI facilitated efficient coagulation, enhancing organophosphorus species transfer to the solid phase, with recovery efficiencies 3.4–7.6 times higher than those of homogeneous iron systems. Hydroxyl radicals were identified as the dominant reactive oxygen species, driving HMPA mineralization and phosphorus conversion to PO₄³⁻, followed by its incorporation into iron-based precipitates. Reusability tests demonstrated stable catalytic activity, with HMPA removal efficiencies remaining above 80% over five cycles. A stepwise degradation pathway involving phosphoramide oxidation and P-N bond cleavage was proposed. This study provides an effective strategy for coupling organophosphorus removal with phosphorus recovery, highlighting the potential of the mZVI/O₃ system for treating organophosphorus-containing wastewater.