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.