Post-sulfidation of biochar supported nanoscale zero-valent iron with different sulfur precursors: Reactivity and selectivity on tetrabromobisphenol A reduction

Herein, post-sulfidation method is applied for the first time to synthesize biochar supported sulfidated nanoscale zero valent iron (S-nZVI/BC), and the impact of sulfur (S) precursors (Na2S, Na2S2O3, and Na2S2O4) on the physicochemical properties, reactivity, selectivity, pH resistance, and long-term performance of S-nZVI/BC was investigated for tetrabromobisphenol A (TBBPA) degradation. Detailed characterizations indicated that S-nZVI/BC with different S precursors altered distribution/content of S species (i.e., S2−, Sn2−, and SO42−) in shell and core (>10 nm). The BC largely contributed to increase the sulfidation efficiency and dispersion of S-nZVI. The reactivity of S2−-nZVI/BC was ∼ 9.6 and ∼ 3.0–6.7 times higher than that of nZVI/BC and other S-nZVI/BCs, respectively, deriving from the higher Fe0 crystallinity, more hydrophobicity and proportion of reduced S species. Remarkably, the adverse effect of surface oxidized S species (i.e., SO42−) was more significant than the promoting effect induced by the surface reduced S species (i.e., S2− and Sn2−) on reactivity when there were excessive SO42− species. The selectivity of S2O42−-nZVI/BC was ∼ 203 and ∼ 1.5–3.5 times higher than that of nZVI/BC and other S-nZVI/BCs at same dosed S/Fe molar ratio (0.3), respectively, which was well correlated with the [S/Fe]particle molar ratio. Besides, increased electron efficiency with pH rise was because the passivation of surface FexOy layer (hydrogen evolution reaction sites) enabled electrons more selectively transported to FeSx surface (debromination sites). S2−-nZVI/BC remained more reactive after aging in water over 30 d than nZVI/BC and other S-nZVI/BCs. This work suggested that S precursors had crucial effects on the property and performance of S-nZVI/BC, which can be designed for specific application scenarios.