Decomposition of PFOA in IEX regeneration wastewater: Comparison of UV/sulfur-based processes, key parameters and submicellar aggregates on degradation kinetics

The ultraviolet/sulfite (UV/S) destruction of perfluorooctanoic acid (PFOA) in seven solutions, representing ion exchange (IEX) regeneration wastes, was investigated, where Na2SO4, Na2SO3 and NaCl were the most promising regenerants in degrading PFOA. A complete degradation and 94.7 % defluorination of 10 ppm PFOA were achieved in 10 % (1.7 M) NaCl regenerant containing waste within fluence of 248 J.cm(-2). Among the four investigated key parameters (pH, sulfite dose, temperature, and initial PFOA concentration), an unexpected trend was observed for the effect of PFOA initial concentration on degradation efficiency. At outset of process, higher initial PFOA concentrations led to substantial decrease in PFOA concentration, but this trend reversed as the process continued. This drop in concentration is explained based on the fast aggregation of PFOA as revealed in molecular dynamic simulations and thermodynamic calculation. The reactivity of aggregates toward e(aq)(-) was three orders of magnitude less than that of monomers. Incorporating aggregation into the kinetic model led to a noteworthy improvement in the model prediction of PFOA concentration profiles, closely aligned with the experimental data. Neglecting aggregation kinetics in models can lead to an overestimation of the required fluence for PFOA degradation, with significant cost implications for large-scale treatment system design. This study underscores the critical and often underestimated issue of PFAS aggregation and its profound implications for the effectiveness of UV/S treatment in addressing IEX regeneration wastes.