Solvation structure of hydrated electrons overlooked in the degradation of PFAS by UV/sulfite

The UV/sulfite system is currently regarded as the most effective method for degrading and defluorinating per- and poly-fluoroalkyl substances (PFAS), operating via hydrated electrons (e_aq^-). Previous studies largely concentrate on PFAS molecular structure, overlooking the critical influence of e_aq^- microstructure. By integrating ab initio molecular dynamics simulations with experiments, we reveal strong correlations between cavity radius and PFAS removal efficiency (R² = 0.99) and defluorination reaction (R² = 0.89). Expanding the cavity radius enhances spin density at its centre (R² = 0.83), increasing the availability of solvated electrons for interaction with PFAS active sites and facilitating diffusion-controlled electron transfer that drives defluorination. Under alkaline conditions, an increase in cavity radius from 1.52 to 1.82 Å enhances spin-density delocalisation within the e_aq^- cavity, improving degradation efficiency. These findings are supported by experimental results revealing >97% degradation of hexafluoropropylene oxide dimer acid (or its ammonium salt 'GenX') at pH 12. Concentrated e_aq^- enables cooperative multi-site attack on PFAS molecules, accounting for the enhanced defluorination. Overall, this study reveals an atomic-scale correlation between e_aq^- microstructure and defluorination dynamics. Beyond conventional approaches of modifying PFAS structures, we propose that regulating the solvation structure of e_aq^- offers a strategy for efficient degradation.