Organic acids promote the generation of reactive oxygen species by Fe(III)-modified montmorillonite: Unraveling the Cr(VI) reduction mechanism

Widespread iron-bearing clay minerals exhibit excellent performance in remediating Cr(VI) contamination in aqueous and soil environments. However, the environmental factors and mechanisms underpinning Cr(VI) reduction via Fe(III)-exchanged clay surfaces remain insufficiently explored. In this study, we delve into the synergistic effects of Fe(III)-exchanged montmorillonite (MMT) and organic acids on Cr(VI) reduction. The results showed that Cr(VI) removal efficiency was significantly influenced by the type and concentration of organic acid, as well as the initial pH of the solution. Notably, the presence of ascorbic acid (H2A) significantly promoted Cr(VI) reduction by Fe(III)-exchanged MMT at pH 3.0, achieving rates 4-7 times higher than those observed with other organic acids. Spectral analyses identified Cr(OH)3, Fe(III)-Cr(III) complexes and Cr2O3 as the primary reduction products of Cr(VI). Further investigations through chemical probe experiments and radical quenching tests demonstrated that exchangeable Fe(III) on the surface of MMT was reduced to Fe(II) by H2A. The resulting Fe(II) participates in Fenton reaction, generating superoxide radical (O2 center dot-) and hydroxyl radical (center dot OH), which act as potent electron donors to facilitate Cr(VI) reduction. Additionally, both H2A and Fe(II) directly contribute to the partial reduction of Cr(VI). These findings expand the potential applications of Fe(III)-exchanged MMT in treating Cr(VI)-contaminated wastewater, providing a promising strategy for environmental remediation and pollution control.