Chromium Immobilization as Cr-Spinel by Regulation of Fe(II) and Fe(III) Concentrations

The complex environmental conditions at Cr-contaminated sites, characterized by uneven ion distribution, oxidants competition, and limited solid-phase mobility, lead to inadequate mixing of Fe-based reducing agents with Cr, posing significant challenges to the effectiveness of Cr remediation through Cr-spinel precipitation. This study investigates the distinct roles of Fe(II), Fe(III), and Cr(III) in Cr-spinel crystallization under ambient temperature and pressure. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray absorption near-edge structure spectroscopy, and Mössbauer spectroscopy were employed to elucidate the phase composition, microstructure, and ion coordination within the precipitates. Our findings indicate that Fe(II) acts as a catalyst in the formation of the spinel phase, occupying octahedral sites within the spinel structure. Under the catalytic influence of Fe(II), Fe(III) transitions into the spinel phase, occupying both the tetrahedral and the remaining octahedral sites. Meanwhile, Cr(III), due to its high octahedral site preference energy, preferentially occupies the octahedral sites. When Fe(II) or Fe(III) is present but does not meet the ideal stoichiometric ratio, a deficiency in Fe(II) leads to low yield and poor crystallinity of Cr-spinel, whereas a deficiency in Fe(III) can completely inhibit its formation. Conversely, when either Fe(II) or Fe(III) is in excess, the formation of Cr-spinel remains feasible. Furthermore, metastable Cr phases can be transformed into stable Cr-spinel by adjusting the Fe(II)/Fe(III)/Cr(III) ratio. These results highlight the broad range of conditions under which Cr-spinel mineralization can occur in environmental settings, enhancing our understanding of the mechanisms driving Cr-spinel formation in Cr-contaminated sites treated with Fe-based reducing agents. This research provides critical insights for optimizing Cr remediation strategies.