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 (eaq−). Previous studies largely concentrate on PFAS molecular structure, overlooking the critical influence of eaq− 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 eaq− 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 eaq− enables cooperative multi-site attack on PFAS molecules, accounting for the enhanced defluorination. Overall, this study reveals an atomic-scale correlation between eaq− microstructure and defluorination dynamics. Beyond conventional approaches of modifying PFAS structures, we propose that regulating the solvation structure of eaq− offers a strategy for efficient degradation. This work establishes a relationship between the cavity radius of hydrated electrons and PFAS degradation, which shifts focus from PFAS structure to reactive species structures, thus opening an overlooked way to tune advanced reduction processes.