Dual pathways of Mn oxide-driven Cr(III) release and transport in serpentine topsoil: OM-Cr(III) colloids and Cr(VI).

Geogenic Cr(III) oxidative release in serpentine soils is a continuous source of Cr contamination in surface and groundwater. Although manganese (Mn) oxides have been shown to be a key factor driving Cr(III) oxidative release, their oxidative mechanism and release fluxes in serpentine soils remain unclear. In this study, the dual mechanism of δ-MnO2 regulating Cr release and migration from surface soil was revealed through column leaching experiments. The results showed that δ-MnO2 significantly promoted soil Cr release, and most of the Cr(VI) generated by oxidation went into the aqueous phase. δ-MnO2 addition slowed down the loss of mobile fractions Cr (F1, F3, and F4), 78-94 % of Cr in the effluent originated from organic fraction Cr (F4), while F6 and F7 were less affected, demonstrating chemical speciation selectivity. Validation experiments confirmed that δ-MnO2 primarily drives Cr release by oxidizing and desorbing OM-Cr(III), with minimal contribution from inorganic Cr-bearing minerals. Cr(III) accounted for >70 % of the Cr in the effluent, mostly migrating in the form of OM-Cr(III) colloids, while Cr(VI) accounted for only 30 %. This indicates that in OM-rich surface soils, the oxidation and decomposition of OM by natural oxidants (such as biogenic Mn oxides) and the release of bound Cr(III) are the core processes of Cr loss. Cr(III) is the main speciation entering the aqueous system. Therefore, in Cr-contaminated soils rich in OM, Mn input (such as the application of Mn-containing fertilizers or the use of Mn-containing irrigation water) should be avoided to inhibit the promotion of the Mn(II)-Mn(IV) cycle driven by Mn(II) oxidizing microorganisms on the release of OM-Cr(III) oxidation, thereby reducing the risk of Cr pollution in the water environment at the source.