Enhanced removal of perfluorooctanoic acid by VUV/sulfite/iodide: efficiencies, influencing factors, and decomposition mechanism†

Perfluoroalkyl acids (PFAAs) such as perfluorooctanoic acid (PFOA) have been referred to as “forever chemicals” and are toxic and bioaccumulative. Previous studies have indicated that the defluorination of PFOA is incomplete by various advanced reductive processes. In this study, we proposed combining sulfite (SO₃²⁻) with iodide (I⁻) for the advanced reduction of PFOA under vacuum ultraviolet (VUV) radiation. The degradation and defluorination ratios of PFOA reached 100% within 30 min and 99.2% within 6 h, respectively. Hydrated electrons (e_aq⁻) and VUV photolysis are major contributors to PFOA removal. The VUV/SO₃²⁻/I⁻ system was superior to UV/SO₃²⁻/I⁻ for PFOA decomposition with a synergistic factor of 1.36 and a higher e_aq⁻ yield. The optimal dosage of I⁻ could be reduced by half owing to the stronger absorption coefficient under VUV radiation. HCO₃⁻, Cu²⁺, and humic acid could inhibit the decomposition of PFOA. Fe³⁺ and SO₄²⁻ had slight and negligible effects on the performance of the VUV/SO₃²⁻/I⁻ process, respectively. We determined the most active sites for nucleophilic attack by utilizing the Fukui function indices of PFOA anions using density functional theory (DFT) calculations. C7 polyfluorinated carboxylate esters, short-chain hydrogen-containing and sulfonated intermediates were identified in PFOA degradation for the first time in the study. This study provides a feasible approach for the environmental remediation of PFOA.