Mechanisms of generation and accumulation of geogenic Cr(VI) in serpentinite-weathered soils

• Cr(VI) mainly originates from contact oxidation of sub-stable Cr(OH) 3 by Mn oxides. • Reducing agents inhibit the generation and accumulation of Cr(VI). • Adsorption trapping by Fe/Mn oxides dominates Cr(VI) accumulation. Natural oxidation of geogenic Cr(III) to carcinogenic Cr(VI) is a major source of Cr contamination in soil and water. However, the mechanisms of Cr(VI) generation and occurrence under anoxic conditions remain unclear. Although Mn(III/IV) oxides are considered key oxidants, their interactions with stable Cr(III) minerals and the subsequent fate of Cr(VI) represent critical knowledge gaps. This study combines field profile analysis of serpentinite weathering in southwest China with laboratory experiments to clarify these processes. We introduce a new mechanism termed “in situ oxidation-surface adsorption” mechanism: the stability of secondary Cr(III) minerals governs oxidation sensitivity, Mn(III/IV) oxides drive Cr(VI) generation via direct surface contact, while Fe(III)/Mn(III/IV) oxides act as effective adsorbents retaining Cr(VI) in the solid-phase. Experimental results indicate that sub-stable Cr(OH) 3 formed during serpentinite weathering is the primary contributor to Cr(VI) generation, while Cr 2 O 3 is negligible. In conditions with low reductants, more than 99.8% of the Cr(VI) generated is adsorbed onto the surfaces of Fe(III)/Mn(III/IV) oxides, creating a stable “Cr(VI) reservoir” with limited release into the aqueous phase. These findings challenge conventional dissolution-migration-oxidation models and enhance our understanding of Cr(VI) generation and accumulation in anoxic soils, providing crucial insights for assessing and managing geological Cr risks.