Molecular mechanisms of Chromium(III) sorption by organo-ferrihydrite coprecipitates induced by crop straws

Agricultural organo-ferrihydrite (Fh) coprecipitates (OFCs), resulting from the coprecipitation of Fe(III) and dissolved organic carbon (DOC) from returned straws, significantly affect the bioavailability of heavy metals in farmland. However, the molecular sorption mechanisms of Cr(III) by the OFCs remain unclear. Here, we explored the sorption behaviors of Cr(III) by the OFCs formed with wheat or maize straws derived-DOC (wheat-DOC or maize-DOC) under various environmental conditions, and further underlying molecular sorption mechanisms using Cr K-edge X-ray absorption near edge structure (XANES) spectroscopy. Results showed that high C loadings reduced the specific surface areas (SSAs) and Cr(III) sorption capacities of the OFCs, implying the blockage of binding sites by C loading. Additionally, although the wheat-DOC induced OFC had a smaller SSA than the maize-DOC induced OFC, their Cr(III) sorption were comparable, which was likely to be compensated by the more carboxyl in the wheat-DOC. Moreover, at a higher ionic strength, the increased or slightly decreased Cr(III) sorption indicated that the inner-sphere sorption was dominant regardless of high or low C loadings, which was also supported by the extremely low Cr(III) extraction percentage. The Cr K-edge XANES spectroscopy suggested that Cr(III) could be immobilized by both the Fh and organic fractions, with the Fh fractions playing a significant role. These findings contribute to a molecular-level mechanistic understanding of Cr(III) sorption by the OFC, which will aid in the prevention and control of Cr-contaminated agricultural soils.