Oxidative conversion of HEDP to phosphate in charged water microdroplets

Organophosphonates are persistent anthropogenic phosphorus compounds because of their stable carbon-phosphorus bonds. Whether such compounds can be oxidatively converted to inorganic phosphate at dynamic air-water interfaces remains poorly understood. Here, using 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) as a model organophosphonate, we show its stepwise oxidation in charged water microdroplets under mild conditions. Online mass spectrometry revealed a marked decrease in the deprotonated HEDP signal and the appearance of H₂PO₄^- at m/z 96.9685, while ion chromatography and CO₂ analysis independently confirmed phosphate formation and carbon oxidation. Charge measurements and high-speed imaging showed atomization-induced charge separation, and simulations indicated possible local electric-field enhancement between oppositely charged droplets. Electron paramagnetic resonance, melatonin oxidation, H₂¹⁸O labeling, and tert-butyl alcohol scavenging experiments further showed that water-derived hydroxyl radicals are generated during atomization and participate in HEDP oxidation. Time-resolved product profiles identified transient phosphorus-containing intermediates and small organic acids, followed by the accumulation of phosphate and CO₂. Detected carbon and phosphorus recoveries further constrained the elemental redistribution during the reaction. These results establish charged microdroplets as a model interfacial system for HEDP-to-phosphate conversion and support a mechanism involving water-derived hydroxyl radicals, with possible contributions from localized electric fields and oxygen-involved radical chemistry.