Enhanced reductive degradation of chloramphenicol by sulfidated microscale zero-valent iron: Sulfur-induced mechanism, competitive kinetics, and new transformation pathway

• Published in: Water research (Oxford) Vol. 233; p. 119743
• Main Authors: Dai, Yinshun, Du, Weiyu, Jiang, Chao, Wu, Wanqi, Dong, Yamin, Duan, Liangfeng, Sun, Shiwen, Zhang, Baiyu, Zhao, Shan
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 15.04.2023
• Subjects: Amines; Chloramphenicol - chemistry; Competitive dynamics model; Correlation analysis; DFT; Iron - chemistry; Iron sulfides; Kinetics; Sulfur; Toxicity assessment; Water Pollutants, Chemical - chemistry; Competitive dynamics model; DFT; Iron sulfides; Correlation analysis; Toxicity assessment
• ISSN: 0043-1354; 1879-2448
• DOI: 10.1016/j.watres.2023.119743

•CAP degradation and electron utilization of (FeSx+ZVI)bm and FeSx@ZVI are distinct.•The roles of sulfidation and FeSx speciation in CAP degradation were clarified.•The transformation of nitroso-CAP to amine-CAP is mediated by azoxy products.•Competitive reactions of CAP with degradation products and O2 are related to ELUMO.•H* and DET have different contributions to nitro reduction and dechlorination. Crystalline iron sulfide (FeSx, i.e., FeS or FeS2) minerals as sulfur sources were used to prepare the mechanochemically sulfidated microscale zero-valent iron ((FeSx+ZVI)bm). Metastable FeS and FeS2 precursors were generated via aqueous coprecipitation and applied to fabricate FeSx@ZVI samples. (FeSx+ZVI)bm and FeSx@ZVI exhibited better chloramphenicol (CAP) degradation than ZVI due to the increase in specific surface areas, the decrease of electrochemical impedance, the formation of galvanic cells, and sulfur-induced pitting and local acidity. (FeSx+ZVI)bm had better CAP removal capacity than FeSx@ZVI under different S/Fe molar ratios, initial pH, and oxygen conditions. At the same time, FeSx@ZVI showed better electron utilization under oxic conditions, related to their Fe0 and sulfur spatial distribution. Nitro reduction and dechlorination of CAP by (FeSx+ZVI)bm produced nitroso, azoxy, amine, and monodechlorination products, while dechlorination was not involved in the degradation process of CAP by FeSx@ZVI. A new transformation pathway of nitroso-CAP to amine-CAP mediated by azoxy products is proposed via coupling a chain decay multispecies model and DFT calculations. The larger competitive reaction rates among O2, CAP, and its degradation products was determined by their lower LUMO energy. The contribution of direct electron transfer to nitro reduction was greater than that of atomic hydrogen, but the opposite was true for dechlorination. FeSx@ZVI had a larger DET contribution than (FeSx+ZVI)bm, and FeS2 promoted the DET contribution better than FeS. Toxicity assessment indicated that the rapid transformation of nitroso and azoxy products was crucial for eliminating the biotoxicity of CAP. [Display omitted]