Enhanced removal of sulfadiazine by sulfidated ZVI activated persulfate process: Performance, mechanisms and degradation pathways

Sulfidation treatment has been considered as an effective method to improve the activity of zero valent iron (ZVI) for reductive removal of contaminants. However, the role and mechanism of sulfide-modified zero valent iron (S-ZVI) in advanced oxidation reaction were worth further exploration. In this study, influences of different reaction conditions, including initial pH, iron dosage, persulfate (PDS) dosage and inorganic ions on sulfadiazine (SDZ) removal by mechanical ball-milling sulfidated microscale zero valent iron (S-ZVI(bm))/PDS process and mechanical ball-milling microscale zero valent iron (ZVI(bm))/PDS were comparatively investigated. By contrastive analyses, S-ZVI(bm) exhibited more excellent performance on activation of PDS. Meanwhile, SO42-, Cl-, and NO3- showed different effects on SDZ degradation by ZVI(bm)/PDS and S-ZVI(bm)/PDS. The potential mechanisms that S-ZVIbm exceeding ZVIbm in activating PDS were clarified from two aspects, which were better iron dissolution and iron circulation caused by iron sulfides generated during sulfidation treatment. Besides, chemical quenching experiments and electron paramagnetic resonance (EPR) studies identified that both SO4 center dot- and center dot OH were generated in ZVI(bm)/PDS and S-ZVI(bm)/PDS, and SO4 center dot- was the dominant one. Two different degradation pathways of SDZ in the S-ZVI(bm)/PDS process were proposed based on the experimental intermediates identification and density functional theory (DFT) calculation. According to the results obtained in this study, the SZVI(bm)/PDS process was recommended as an effective method to degrade contaminants.