The coprecipitation of heavy metals (HMs) with Fe­(III) in the presence of dissolved organic matter (DOM) is a crucial process to control the mobility of HMs in the environment, but its underlying immobilization mechanisms are unclear. In this study, Cr­(III) immobilization by coprecipitation with Fe­(III) in the presence of straw-derived DOMs under different Fe/C molar ratios, pHs, and ionic strengths was investigated using scanning transmission X-ray microscopy (STXM) and ptychography and X-ray absorption near-edge structure (XANES) spectroscopy. The results showed that Cr­(III) retention was enhanced in the presence of DOM, a maximum of which was achieved at an Fe/C molar ratio of 0.5. The increase of pH and ionic strength could also promote Cr­(III) immobilization. Cr K-edge XANES results indicated that Fe (oxy)­hydroxide fractions, instead of organics, provided the predominant binding sites for Cr­(III), which was directly confirmed by high spatial resolution STXM-ptychography analysis at the sub-micron- and nanoscales. Moreover, organics could indirectly facilitate Cr immobilization by improving the aggregation and deposition of coprecipitate particles through DOM bridging or electrostatic interactions. Additionally, C K-edge XANES analysis further indicated that the carboxylic groups of DOM were complexed with Fe (oxy)­hydroxides, which probably contributed to DOM bridging. This study provides a new insight into Cr­(III) immobilization mechanisms in its coprecipitation with Fe­(III) and DOM, which could have important implications on the management of Cr­(III)-enriched soils, particularly with crop straw returning.

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