Mechanism of mineral adsorption enhancing the reduction of hexavalent chromium by natural organic matter

Abstract

Natural organic matter (NOM) serves a crucial electron reservoir for the reduction of hexavalent chromium (Cr(VI)) in subsurface environments. However, the influence of mineral adsorption on Cr(VI) reduction by NOM remains poorly understood, despite the widespread interaction among NOM, mineral matrices and Cr(VI) in natural environments. In this study, aluminum oxides (Al₂O₃) and Fe oxyhydroxides (such as ferrihydrite) were chosen as representative minerals to investigate how mineral adsorption influences on the reduction of Cr(VI) by both native and reduced NOM across a pH range of 5–9. Results of this study showed that the extent of Cr(VI) reduction (10 μM) was 1.5–11.1 % in the native NOM (12 mg/L) system, while it increased to 10.6–19.4 % in the native NOM-Al₂O₃ systems and to 19.4–25.4 % in the native NOM-ferrihydrite systems. Similarly, the extent of Cr(VI) reduction was 16.7–23.1 % in the reduced NOM system, while it increased to 17.5–38.3 % in the reduced NOM-Al₂O₃ systems and to 30–54.9 % in the reduced NOM-ferrihydrite systems. This enhancement effect increased with higher amounts of Al₂O₃ and ferrihydrite but diminished as the solution pH increased from 5 to 9. The enhancement of Cr(VI) reduction by Fe oxyhydroxides at the same dosage followed by the sequence: ferrihydrite > lepidocrocite > goethite > hematite. In NOM system alone, phenolic moieties in NOM mainly contributed to Cr(VI) reduction. In NOM-Al₂O₃/Fe oxyhydroxide systems, mineral adsorption shifted the reaction site from solution phase to mineral surface, subsequently inducing oxidative polymerization of polyphenolic compounds in NOM, which generated more phenolic moieties that further facilitated the reduction of Cr(VI). Additionally, Fe oxyhydroxides served as electron shuttles, facilitating electron transfer from NOM to Cr(VI). This study highlights a previously unrecognized catalytic role of mineral adsorption in enhancing Cr(VI) reduction by NOM in subsurface environments.