Sulfidation of Zero-Valent Iron by Direct Reaction with Elemental Sulfur in Water: Efficiencies, Mechanism, and Dechlorination of Trichloroethylene

Abstract

Sulfidation can enhance both the reactivity and selectivity (i.e., electron efficiency, εe) of zero-valent iron (ZVI) in contaminant removal, which may make this technology cost-effective for a wider range of water treatment applications. However, current sulfidation methods involve either hazardous or unstable sulfidation agents (e.g., Na2S, Na2S2O3, and Na2S2O4) or energy-intensive preparations (e.g., mechanochemical sulfidation with elemental sulfur). In this study, we demonstrate that very efficient sulfidation of microscale ZVI (mZVI) can be achieved at all S/Fe molar ratios (∼100% sulfidation efficiency, εs) simply by direct reaction between elemental sulfur (S0) and ZVI in an aqueous suspension at ambient temperature. In comparison, the εs values obtained using Na2S, Na2S2O3, or Na2S2O4 as the sulfidation agents were only ∼23, ∼75, and ∼38%, respectively. The sulfidated mZVI produced using the new method reacts with trichloroethylene (TCE) with very high rates and electron efficiencies: rate constants and electron efficiencies were 800- and 79-fold higher than those of the unsulfidated mZVI. The enhanced performance of this material, together with the operational advantages of S0 for sulfidation (including safety, stability, and cost), may make it a desirable product for full-scale engineering applications.

Recent Advances in Sulfidated Zerovalent Iron for Contaminant Transformation

“…The shell was comprised predominantly of iron sulfides (FeS or FeS 2 ) and iron oxides (Fe 2+ -O). Subsequent studies adopted or modified the method by Xu et al, making it the most common approach for the synthesis of S-mZVI two‑step , and Na 2 S the preferred sulfidation agent, though other sulfur sources are gaining popularity (e.g., S 0 and Na 2 S 2 O 3 ). ,,,− ,− …”

“…Xu et al found that the iron (Fe 2+ , Fe 3+ ) and sulfur precursors (Na 2 S 2 O 4 , Na 2 S, and K 2 S 6 ) used for synthesis significantly affect the performance of nZVI and S-nZVI for the degradation of TCE, which may contribute to the large variability in reactivity observed among different studies. In addition, ball milling ,− or post synthesis with elemental sulfur seems to be superior sulfidation methods for the degradation of TCE with reported R values larger than 100, albeit with limited data available.…”

“…This may explain the lack of correlation of R with the optimal [S/Fe] dosed of S-(n)­ZVI obtained from the two-step method where relatively limited sulfur loading on the particle is achievable, but significant correlation of R with the optimal [S/Fe] dosed is found for those from one-step synthesis using Na 2 S 2 O 4 ( p < 0.05) where higher extent of sulfur loading is observed (SI Figure S2). Complete transformation of S 0 to FeS was observed during sulfidation of mZVI by ball milling and two-step sulfidation of mZVI with S 0 (i.e., [S/Fe] dosed = [S/Fe] particle = 0.1), ,,− which could partially explain its superior performance for the degradation of TCE (Figure A), because the reported optimal [S/Fe] dosed values from aqueous sulfidation are typically less than 0.1, resulting in even smaller [S/Fe] particle (SI Table S1). Thus, further optimization of S-(n)­ZVI should aim to incorporate higher amount of sulfur or increase the sulfidation ratio in the particle phase to maximize its reactivity for contaminant transformation.…”

Strained Zero‐Valent Iron for Highly Efficient Heavy Metal Removal

“…One compelling solution to tackle these problems is through the deployment of microscale zero‐valent iron (mZVI), which is one of the most important multifunctional environmental remediation materials owing to its low cost, high abundance, and environmental benignity. [ 2 ] mZVI bears a captivating core‐shell structure, featured by an iron oxide layer containing abundant rich Lewis acid or base sites for heavy metal ion complexation, and an iron core acting as an electron reservoir (Fe(II)/Fe(0), −0.44 V versus SHE) towards reductive heavy metal immobilization. [ 3 ] Moreover, the solid‐liquid separation property of mZVI when subject to magnetic fields makes it reliable for in situ remediation, as well as effortless recycling without producing secondary pollutants.…”

Response surface design for removal of Cr(VI) by hydrogel-supported sulfidated nano zero-valent iron (S-nZVI@H)

“…Nevertheless, nZVI particles have a number of limitations that affect their reaction with contaminants: (1) nZVI easily aggregate based on the action of van der Waals force and magnetic force, and eventually reduce the migration in aquifer (Phenrat et al 2007;Israelachvili et al 2008;Li et al 2017). (2) nZVI particles with extremely high reactivity can react with other substances in the water besides the target contaminants, resulting in poor selectivity for pollutants (Cai et al 2020;He et al 2020). (3) When using nZVI in practice, a passivation layer can be easily to form on the surface of nZVI prohibiting Fe 0 core to react with pollutants, thus lower the reactivity (Sarathy et al 2008).…”