Molecular Mechanisms of Chromium(III) Immobilization by Organo–Ferrihydrite Co-precipitates: The Significant Roles of Ferrihydrite and Carboxyl

The interaction mechanisms of heavy metals with organo–Fe hydroxides co-precipitates (OFC) remain unclear due to the structural complexity of the OFC. In this study, batch experiments were conducted to investigate the immobilization mechanisms of Cr­(III) by the OFC, which was prepared by co-precipitating Fe³⁺ with rice/rape straw-derived dissolved organic carbon, through sorption and co-precipitation using synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy and scanning transmission X-ray microscopy (STXM). At an Fe/C molar ratio ≥ 0.3, both the sorption and co-precipitation immobilized the majority of Cr­(III), but the co-precipitation desorbed less Cr­(III) than the sorption regardless of DOC loadings and sources. In contrast, Cr­(III) immobilization was significantly reduced at an Fe/C molar ratio of 0.1 for both reactions. Linear combination fitting of Cr K-edge XANES spectra revealed the predominance of ferrihydrite-bound Cr­(III), but enhanced organic Cr­(III) occurred with increased organic carbon (OC) loading for both the sorption and co-precipitation. STXM coupled with multi-edge XANES analysis confirmed the primary association of Cr­(III) with ferrihydrite and directly probed carboxyl as the binding site for Cr­(III) retention on the OC constituents of the OFC. These results provided new molecular-level insights into the Cr­(III) retention mechanisms on the OFC, particularly for the interactions of Cr­(III) and OC constituents of the OFC, which could benefit the management of Cr-contaminated soils with straw returning.