Oxidative conversion of HEDP to phosphate in charged water microdroplets

Abstract

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 H 2 PO 4 - at m/z 96.9685, while ion chromatography and CO 2 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, H 2 18 O 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 CO 2 . 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.