Hydrophobic Sulfur Core–shell Layered Metallic Iron for Nitrate Reduction with Nearly 100% Dinitrogen Selectivity: Mechanism and Field Studies

We prepared hydrophobic sulfur (S) core-shell-layered nano-zero-valent iron (Fe) (S-nZVI) via a post-sulfidation method with varying Fe/S mass ratios for NO3- reduction. Notably, S(0.125)ZVI (Fe/S = 0.125) showed good N-O cleavage properties owing to its high electron (e(-)) transfer efficiency and low surface passivation. As a result, the S(0.125)ZVI exhibited higher selectivity of NO3- reduction toward N-2 than sole nZVI in synthetic and actual NO3- groundwater in batch experiments. Density functional theory (DFT) calculations showed that H-2 evolution over S-nZVI was suppressed by the S atom in the hollow site of the Fe(1 1 0) surface, resulting in nearly 100 % denitrification selectivity. Quenching tests revealed that e(-) transfer through the S atom toward the surface bounded by NOx species is the dominant denitrification mechanism of S-nZVI. Up-flow column tests using actual groundwater were conducted for 127 d, and S(0.125)ZVI demonstrated a removal capacity of up to 1907 mg-N/g NO3-. Field experiments using S0.125ZVI for NO3- -contaminated groundwater remediation were conducted over four months, confirming that S-nZVI may be an alternative to nZVI for in situ groundwater remediation.