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_0.125ZVI (Fe/S = 0.125) showed good N[sbnd]O cleavage properties owing to its high electron (e⁻) transfer efficiency and low surface passivation. As a result, the S_0.125ZVI exhibited higher selectivity of NO₃⁻ reduction toward N₂ than sole nZVI in synthetic and actual NO₃⁻ groundwater in batch experiments. Density functional theory (DFT) calculations showed that H₂ 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 NO_x species is the dominant denitrification mechanism of S-nZVI. Up-flow column tests using actual groundwater were conducted for 127 d, and S_0.125ZVI demonstrated a removal capacity of up to 1907 mg-N/g NO₃⁻. Field experiments using S_0.125ZVI 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.