Dual Activation of Ferrous Iron via Surface Engineering of Zero-Valent Iron with Low-Molecular-Weight Organic Acids

Sustainable generation of ferrous iron (Fe(II)) through engineered zero-valent iron (ZVI, Fe(0)) offers an innovative strategy for efficient pollutant reduction. This study presents a wet ball milling-surface engineering strategy for synthesizing low-molecular-weight organic acid (LMWOA)-modified ZVI, which enables a novel dual redox activation mechanism for Fe(II)─a functionality that cannot be achieved by conventional wet ball milling or external LMWOA addition. X-ray absorption fine structure spectroscopy and density functional theory calculations demonstrate that α-hydroxy carboxylic acids (e.g., citric, tartaric, and malic acids) form five-membered chelate rings via hydrogen bonding, thereby significantly enhancing electron transfer kinetics from Fe(0) to Fe(II). Meanwhile, non-α-hydroxy carboxylic acids (e.g., succinic/acetic/formic acids) promote Fe(III) adsorption-reduction cycles, thus sustaining Fe(II) regeneration. Notably, ZVI modified with oxalic acid or ascorbic acid exhibited a synergistic effect of both pathways, resulting in the highest Cr(VI) removal capacities, with 19.7- and 22.6-fold increases in Cr(VI) removal and Fe(III) recovery rates of 81.7% and 108.9% relative to unmodified ZVI, respectively. This enhanced performance can be attributed to the improved dissolution of Fe(II) and the elevated levels of structurally bound Fe(II), which collectively promote sustained electron generation and effective transfer to Cr(VI). These findings indicate that LMWOA-modified ZVI establishes a tunable Fe(II) activation system, thereby positioning LMWOA as a promising strategic platform for groundwater remediation.