Effect of low-crystallinity Fe (Oxyhydr)oxides on dissolved organic matter-mediated Cr(VI) reductive immobilization and concurrent carbon sequestration

Abstract Minerals such as Fe (oxyhydr)oxides control the transport of chromium (Cr). However, the influence of dissolved organic matter (DOM) on the transformation and fate of highly toxic hexavalent Cr (Cr(VI)) mediated by various Fe (oxyhydr)oxides remains unclear. In this study, the FT-ICR MS test-derived DOM molecular properties and results of a mass difference network analysis revealed that the transfer of electrons between DOM and Cr(VI) preferentially occurred on the surface of ferrihydrite rather than in solution. In contrast, goethite and hematite showed the opposite phenomenon. The spatial distribution of different organic carbon (OC) and Cr species, as determined by transmission electron microscopy- electron energy loss spectroscopy tests, revealed the bonding styles of OC and Cr on the surface of Fe (oxyhydr)oxides. Owing to the differences in adsorption sites and electron transfer pathways, C and Cr were stabilized on the ferrihydrite surface by electrostatic adsorption, ligand exchange, cation exchange, cation bridging, and lattice doping, whereas goethite and hematite surfaces were more inclined to undergo electrostatic adsorption and ligand exchange. These results reveal that low crystallinity Fe (oxyhydr)oxides play a dominant role in the Fe (oxyhydr)oxides-mediated reduction and sequestration of Cr(VI) by DOM in water-soil environments. Leaching experiments with actual stabilized mine soils also confirmed the effectiveness of the strategy that employs organic matter and in situ Fe (oxyhydr)oxides for the synergistic remediation of Cr-contaminated mine soils. This knowledge will improve our comprehension of the geochemical cycling of Fe, Cr, and C, facilitating the development of technologies for the simultaneous sequestration of Cr and C. Graphical Abstract