Mechanistic insights and environmental ramifications of Cr(III) oxidation to Cr(VI) in soil and groundwater systems: bridging geochemical mechanisms and emerging remediation strategies

The oxidation of trivalent chromium [Cr(III)] to hexavalent chromium [Cr(VI)] is a significant environmental process that threatens soil and groundwater quality and poses serious ecological and human health risks. This review provides a critical synthesis of current knowledge about the abiotic, biotic, and thermally induced mechanisms that drive Cr(III) oxidation, including reactions with Mn(IV) oxides, hydrogen peroxide, photochemically generated oxidants, and wildfire-related thermal transformations. Environmental factors such as pH, redox potential, mineralogy, and organic matter content are revealed to play a key role in regulating Cr redox dynamics. Cr(VI) is highly soluble, mobile, and bioavailable, exerting toxic effects on microbial communities, plant growth, and human health even at trace levels. Key attenuation pathways, including chemical and microbial reduction, biosorption, and co-precipitation with Fe-bearing minerals, are critically discussed, with particular attention to re-oxidation, which undermines long-term remediation. This review further evaluates emerging remediation strategies, such as redox manipulation, biochar-assisted immobilization, and coupled plant-microbe systems, emphasizing the need for site-specific, adaptive approaches that account for spatial and temporal environmental variability. Future research should focus on Cr(VI) generation under post-fire scenarios, isotopic tracing, and field-scale testing of sustainable immobilization materials to improve predictive models and ensure long-term mitigation of Cr(VI) contamination in soil-groundwater systems.