Stabilized green rusts for aqueous Cr(VI) removal: Fast kinetics, high iron utilization rate and anti-acidification

Green rusts (GRs) are redox active towards contaminants but they are not stable for long distance transport during the soil and groundwater remediation. In this study, green rust chloride (GR) was stabilized by selected regents, including silicate (Si), phosphate (P), fulvic acid (FA), carboxymethyl cellulose (CMC) and bone char (BC), then these stabilized GR, collectively named GR-X, would be further applied for Cr(VI) removal from aqueous solution. The stabilization experiment demonstrated that the release of Fe(II) from GR was effectively suppressed by above reagents, enabling at least 50% lower Fe(II) leaching from the stabilized GR-X than that from the pristine GR. The intact hexagonal GR plates and crystallinity were also confirmed by the SEM images and XRD patterns after storage for 7 days, indicating the stable structure of GR-X was remained. In the Cr(VI) removal tests, Cr(VI) was eliminated by GR-X in seconds with a Fe(II) utilization efficiency over 90%. The Cr species examination demonstrated that the GR-X was able to transfer Cr(VI) into stable Cr(III)-Fe(III) precipitates (Fe-Mn oxides fraction). After Cr(VI) removal tests, all reactors were exposed to the air for 1 week to monitor pH fluctuation and evaluated the risk of acidification. The results indicate that, except for GR-Si system, the other post-remediation systems are stable and the pH buffering ability of GR-X could avoid acidification and lower the Cr leaching risk.