FeN X in Fe single-atom catalysts can be the active site for adsorption and activation of reactants. In addition, FeN X species have been shown to facilitate electron transfer between Fe and the carbon supports used in newly developed metal−air batteries. We hypothesized that the combination of FeN X species with granular zero-valent iron (ZVI) might result in catalyzed reductive decontamination of groundwater contaminants such as trichloroethylene (TCE). Here, such materials synthesized by ball milling microscale ZVI with melamine and the resulting N species were mainly in the form of pyridinic, pyrrolic, and graphitic N. This new material (abbreviated as N−C-mZVI bm ) dechlorinated TCE at higher rates than bare mZVI bm (about 3.5-fold) due to facilitated electron transfer through (or around) the surface layer of iron oxides by the newly formed Fe−N X (C). N−C-mZVI bm gave higher k TCE (0.4−1.14 day −1 ) than mZVI bm (0−0.4 day −1 ) over a wide range of pH values (4− 11). Unlike most ZVI systems, k TCE for N−C-mZVI bm increased with increasing pH values. This is because the oxide layer that passivates Fe 0 at a high pH is disrupted by Fe−N X (C) formed on N−C-mZVI bm , thereby allowing TCE dechlorination and HER under basic conditions. Serial respike experiments gave no evidence of decreased performance of N−C-mZVI bm , showing that the advantages of this material might remain under field applications.

Sulfidated zero-valent iron (S-ZVI) enhances the degradation of chlorinated hydrocarbon (CHC) in contaminated groundwater. Despite numerous studies of S-ZVI, a versatile strategy to improve its dechlorination kinetics, electron efficiency (ε e ), and dechlorination capacity is still needed. Here, we used heteroatom incorporation of N(C) and S by ball-milling of microscale ZVI with melamine and sulfur via nitridation and sulfidation to synthesize S-N(C)-mZVI bm particles that contain reactive Fe-N X (C) and FeS species. Sulfidation and nitridation synergistically increased the trichloroethene (TCE) dechlorination rate, with reaction constants k SA of 2.98 × 10 −2 L•h −1 •m −2 by S-N(C)-mZVI bm , compared to 1.77 × 10 −3 and 8.15 × 10 −5 L•h −1 •m −2 by S-mZVI bm and N(C)-mZVI bm , respectively. Data show that sulfidation suppressed the reductive dissociation of N(C) from S-N(C)-mZVI bm , which stabilized the reactive Fe-N X (C) and reserved electrons for TCE dechlorination. In addition to lowering H 2 production, S-N(C)-mZVI bm dechlorinated TCE to less reduced products (e.g., acetylene), contributing to the material's higher ε e and dechlorination capacity. This synergistic effect on TCE degradation can be extended to other recalcitrant CHCs (e.g., chloroform) in both deionized and groundwater. This multiheteroatom incorporation approach to optimize ZVI for groundwater remediation provides a basis for further advances in reactive material synthesis.

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.

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