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

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.

Morphology and structure of in situ FeS affect Cr(VI) removal by sulfidated microscale zero-valent iron with short-term ultrasonication.

The properties of sulfidated zero-valent iron (S-ZVI) are considered to be determined by the entire structure of Fe0 and FexSy as a whole, but few studies focus on the influence of the morphology and structure of the external FexSy layer on the performance of S-ZVI. In this study, after the sulfidation of microscale ZVI in acetate (HAc-NaAc) and 2-(N-morpholino) ethanesulfonic acid (MES) buffer solution, the S-mZVIHAc-NaAc surface presented the in situ growth of the FeS nanosheet, while the S-mZVIMES surface was dominated by agglomerated FeS sub-micron particles. Under short-term ultrasonication, S-mZVIHAc-NaAc was superior to removing Cr(VI) than S-mZVIMES, and the clearance of the passivation layer by ultrasound maximized the conductivity of the FeS nanosheet to strengthen the sulfidation contribution. However, agglomerated FeS particles were easily separated from S-mZVIMES by ultrasonication, resulting in the suppression of its sulfidation contribution. The removal of Cr(VI) by S-ZVI increased linearly with FeS content, and the chemical combination of FeS with ZVI had more significant synergy than their physical mixture. The FeS nanosheet with excellent conductivity and large vertical space benefited the generation of dissolved and surface-associated Fe(II) as electron donors and structural Fe(II) as the electron shuttle. Understanding the relationship between FeS structure and S-ZVI performance will pave a way for optimizing the synthesis of S-ZVI.