Chromium(VI) removal by siderite (FeCO3) in anoxic aqueous solutions: An X-ray absorption spectroscopy investigation.

Chromium(VI) is an environmental contaminant of priority concern, which can be treated by reduction of toxic Cr(VI) to non-toxic Cr(III). Siderite (FeCO3), an Fe(II)-containing mineral, occurs in many anaerobic sediments and groundwater systems and is extremely reactive, thus making it a potentially important host-phase in governing the fate and transport of Cr(VI) in a range of anoxic aqueous environments. Here, we investigate the fate of Cr(VI) during sorption and reduction by siderite, as well as speciation of Cr(VI)-reacted siderite as a function of varying pH (4-10) and initial Cr(VI) concentrations (0.5-10 mM) under strictly oxygen-free conditions. Notably, up to 97% and 91% of initial added aqueous Cr(VI) (0.5-10 mM) was reduced to Cr(III) at pH 4 and 5, respectively. However, the reduction efficiency of Cr(VI) by siderite slightly decreased in the pH range from 6 to 10 (~90% Cr(III)), only at 0.5-1.5 mM initial Cr(VI) concentrations. Chromium K-edge XANES spectroscopy confirmed the complete reduction of sorbed Cr(VI) to Cr(III) after equilibration of siderite with low (2 mM) and high (10 mM) initial Cr(VI) concentrations at pH 5, 7 and 9. In addition, Fe K-edge XANES and EXAFS spectra of solid-phase samples showed that 74-78% and 78-89% of Fe remained as siderite, respectively, after equilibration with Cr(VI) at pH 7 and 9, whereas only 24% remained for the low Cr(VI) treatment at pH 5. The results imply that under anoxic aqueous conditions, siderite can help immobilize and detoxify Cr(VI) with the extent of these coupled sorption and redox reactions being controlled by initial Cr(VI) concentrations and pH.