Ubiquitous organo-ferrihydrite coprecipitates (OFC) significantly affect the mobility and availability of Cr in soil through sorption, but the underlying sorption mechanisms remain unclear at the molecular level. Due to the potential formation of OFC in agricultural soils with returned crop straws, we synthesized OFC with rice/rape straw-derived carbon (C) sources and different loadings. The molecular sorption mechanisms of Cr(III) to the synthesized OFC under different conditions were investigated by Cr K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and scanning transmission X-ray microscopy (STXM). Cr(III) sorption by OFC decreased with increasing C loading and decreasing pH, regardless of C sources. Moreover, inhibition of Cr(III) sorption to OFC with high C loading occurred when ionic strength (IS) increased, suggesting the presence of outer-sphere complexed Cr(III). EXAFS analysis revealed that more Cr(III) were bound to ferrihydrite of the OFC at a relatively high pH, and organically bound Cr(III) enhanced when increasing C loading and decreasing IS. STXM analysis strongly suggested that C loading reduced Cr(III) sorption through blocking the binding sites on the ferrihydrite, which overwhelmed Cr(III) retention by the direct binding of Cr(III) to carboxyl of the particulate organic matter (OM) and OM coated on the Fh fractions of the OFC. These findings facilitated the comprehensive understanding of the sorption mechanisms of Cr(III) to OFC at the molecular level, which will assist the prediction of Cr(III) mobility in soils, particularly for Cr(III)-contaminated agricultural soils with the application of crop straws.

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