Polyacrylic acid-mediated sulfamethoxazole degradation by hydroxyl radicals during FeS oxidation under aerobic conditions

Iron sulfide (FeS) nanoparticles demonstrate great potential for degrading organic contaminants under aerobic conditions through the production of hydroxyl radicals (•OH). However, its oxidative efficacy is often constrained by low electron utilization efficiency. In this study, we developed a polyacrylic acid (PAA)-mediated FeS oxidation system and investigated its effectiveness in sulfamethoxazole (SMX) degradation. The results indicate that PAA substantially enhanced the degradation of SMX by FeS at oxic conditions. As PAA dosage increased from 0.1 to 0.4 g/L, the observed degradation rate constants ( k obs ) of SMX were enhanced by 3- to 10-fold. Quenching experiments identified •OH as the primary reactive species responsible for SMX degradation. The electron utilization efficiency of FeS for •OH production was significantly increased from 0.3 % in FeS-only system to 2.1 % upon the addition of 0.4 g/L PAA. The enhancement was primarily attributed to the formation of dissolved Fe(II)-PAA complexes and the promotion of Fe redox cycling by PAA. By combining intermediate identification with density functional theory calculations, the degradation pathway of SMX was elucidated thoroughly. Additionally, the FeS/PAA/O 2 system exhibited exceptional degradation efficiency toward other common organic pollutants. X-ray diffraction and transmission electron microscopy revealed that PAA inhibited the formation of crystalline γ-FeOOH during FeS oxidation, which instead leads to the generation of amorphous structures. Collectively, this study provides fundamental insights into the oxidizing performance of FeS/PAA system and proposes a promising remediation technique for removing sulfonamide antibiotics.