Unveiling chromium dynamics: XANES spectroscopy insights in two Fe-Mn nodule-rich red soil profiles

Understanding Cr, Fe, and Mn speciation is essential to evaluate Cr stability and transformation in highly weathered red soils, which are vital for agriculture and elemental cycling in southern China. Here, XANES spectroscopy was applied to investigate Cr, Fe, and Mn speciation in two Fe-Mn nodule (FMN)-rich red soil profiles. Cr primarily exists as Cr(III)-Fe(III) co-precipitates, accounting for ∼90 % of total Cr in FMNs and 60-70 % in surrounding soils, with increasing proportions of Cr-bearing silicates in the latter. Goethite and magnetite are the main Fe species in FMNs, while lepidocrocite dominates in surrounding soils. Mn(III) hydroxides and organically complexed Mn dominate in FMNs, contrasting with tectomanganate and physisorbed Mn in surrounding soils. The Mn average oxidation state is higher in non-paddy soil than in paddy soil, but increases in the bottom horizons of the paddy profile. Pre-edge analysis indicates negligible Cr(VI) in FMNs; however, its presence in surrounding soils is not removable by a KH₂PO₄ solution, suggesting limited mobility. Integrated with previously published Cr isotope data, the results suggest that Cr(VI) released through oxidative weathering is largely reduced and incorporated into Fe (hydr)oxides within FMNs, whereas in surrounding soils, it undergoes incomplete local reduction with isotopic fractionation, migrates downward, and is captured within Fe (hydr)oxides. In deeper horizons, the coexistence of amorphous Fe (hydr)oxides, elevated Cr concentrations, and high-valent Mn (hydr)oxides implies that Cr(III) may be reoxidized under redox fluctuations. The combined XANES and Cr isotope approach clarifies vertical Cr transformation mechanisms and informs environmental risk assessment in red soils.