Leveraging an optimized dosing strategy for enhanced degradation of perfluorooctanoic acid by UV/sulfite/iodide: Efficacy and mechanism

Perfluorooctanoic acid (PFOA) is a persistent organic pollutant that poses serious environmental concerns. In this study, we explored its reductive degradation via a UV/sulfite/iodide system, with a focus on optimizing the dosing strategy. The results showed that the degradation efficiency increased significantly with pH, from 28 % at pH 4.0–92 % at pH 11.0. Increasing the iodide concentration improved the performance, with 97.0 % degradation and 40.0 % defluorination achieved at a PFOA-to-iodide molar ratio of 1:100. The sulfite dosage played a key role: degradation reached 80 % at a 1:1000 PFOA-to-sulfite ratio, but excessive sulfite (1:2000) slightly reduced the efficiency to 75 %, indicating an inhibitory effect. To overcome this, a continuous sulfite dosing strategy was developed, which increased degradation from 82.5 % to 98 % and defluorination from 22 % to 34 % compared with direct dosing. Natural organic matter (humic acid) and common anions (SO₄²⁻, HCO₃⁻, HPO₄²⁻) were found to suppress PFOA degradation, whereas Cl⁻ had a minimal effect. SO₄²⁻ reduced the observed rate constant from 0.0228 to 0.0102 min⁻¹ at 50 mM. Quenching experiments and probe tests identified hydrated electrons (eaq⁻) as the main reactive species. Three possible degradation pathways were proposed based on intermediate analysis. In addition, probe tests confirmed that more hydrated electrons can be retained in the continuous dosing system for PFOA degradation. These findings highlight the importance of an optimized dosing strategy and provide new insights into the mechanism of PFOA degradation in the UV/sulfite/iodide system.