Structural Determinants of Organic Acids Governing Cr(VI) Reduction by Sulfidated Nano Zero-Valent Iron

Sulfidated nano zerovalent iron (S-nZVI) shows promise for Cr(VI) remediation, yet the influence of coexisting organic acid (OAs) structures on Cr(VI) reduction by S-nZVI with different sulfur contents remains unclear. Herein, polysulfide (S x 2– ) precursors induced Fe 0 oxidation to produce high-sulfur S x 2– @nZVI with 9-fold higher sulfidation efficiency than monosulfide-derived S 2– @nZVI. S x 2– @nZVI exhibited superior Cr(VI) reduction due to the reduced interfacial resistance and increased Fe x S y serving as secondary electron donors. OAs enhanced Cr(VI) reduction via proton supply and complexation-induced depassivation. Elongated alkyl chains suppressed OA ionization, increasing H + availability and promoting direct electron donation from iron (poly)sulfides. In contrast, numerous hydroxyl (−OH) groups reduced H + availability but strengthened complexation, mitigating the surface passivation. However, strong complexation of –OH increased the negative surface potential of Fe(III/II)-OA complexes, intensifying electrostatic repulsion with S 2– /S x 2– and HCrO 4 – /CrO 4 2–, which suppressed Fe(II) regeneration and Fe(II)-OA electron transfer to Cr(VI). OAs with weaker-complexing alkyl chains exhibited the opposite effect and effectively suppressed Fe/Cr leaching. Consequently, long alkyl chains with high H + capacity and low complexation favor high-sulfur S x 2– @nZVI for Cr(VI) removal, while α-OH groups with strong complexation and extra electron-donating function govern low-sulfur S 2– @nZVI performance. These findings provide mechanistic guidance for S-nZVI design and OA selection in Cr(VI) remediation.