Accelerated Degradation of Perfluorosulfonates and Perfluorocarboxylates by UV/Sulfite + Iodide: Reaction Mechanisms and System Efficiencies

Abstract

The addition of iodide (I^-) in the UV/sulfite system (UV/S) significantly accelerated the reductive degradation of perfluorosulfonates (PFSAs, C_<i>n</i>F_2<i>n</i>+1SO₃^-) and perfluorocarboxylates (PFCAs, C_<i>n</i>F_2<i>n</i>+1COO^-). Using the highly recalcitrant perfluorobutane sulfonate (C₄F₉SO₃^-) as a probe, we optimized the UV/sulfite + iodide system (UV/S + I) to degrade <i>n</i> = 1-7 PFCAs and <i>n</i> = 4, 6, 8 PFSAs. In general, the kinetics of per- and polyfluoroalkyl substance (PFAS) decay, defluorination, and transformation product formations in UV/S + I were up to three times faster than those in UV/S. Both systems achieve a similar maximum defluorination. The enhanced reaction rates and optimized photoreactor settings lowered the EE/O for PFCA degradation below 1.5 kW h m^-3. The relatively high quantum yield of e_aq^- from I^- made the availability of hydrated electrons (e_aq^-) in UV/S + I and UV/I two times greater than that in UV/S. Meanwhile, the rapid scavenging of reactive iodine species by SO₃^2- made the lifetime of e_aq^- in UV/S + I eight times longer than that in UV/I. The addition of I^- also substantially enhanced SO₃^2- utilization in treating concentrated PFAS. The optimized UV/S + I system achieved >99.7% removal of most PFSAs and PFCAs and >90% overall defluorination in a synthetic solution of concentrated PFAS mixtures and NaCl. We extended the discussion over molecular transformation mechanisms, development of PFAS degradation technologies, and the fate of iodine species.