An X-ray absorption spectroscopic study of the Fe(II)-induced transformation of Cr(VI)-substituted schwertmannite.

The environmental chemistry of Cr is of widespread interest due to the hazardous nature of Cr(VI). Because of similar atomic size and charge, CrVIO42- can substitute for SO42- within schwertmannite - an Fe(III) oxyhydroxysulfate mineral that occurs widely in acidic and sulfate-rich systems. The presence of aqueous Fe(II) can induce transformation of schwertmannite to more stable Fe(III) phases (e.g. goethite) which may potentially impact the behaviour of co-associated Cr(VI). Here, we investigate the Fe(II)-induced transformation of Cr(VI)-substituted schwertmannite as a function of pH (4-8) and the degree of Cr(VI) substitution (0.16-13 mol% CrVIO42--for-SO42- substitution). Iron K-edge EXAFS spectroscopy revealed that higher levels of Cr(VI) substitution inhibited Fe(II)-induced schwertmannite transformation. Chromium K-edge XANES spectroscopy indicated that this outcome could be partly attributed to consumption of Fe(II) by reaction with Cr(VI), and the resulting formation of a passivating Cr(III)-Fe(III) hydroxide phase which stabilizes schwertmannite at greater levels of Cr(VI) substitution and at higher pH while also decreasing further reduction of structural Cr(VI). Overall, this study enriches our understanding of interactions between hazardous Cr(VI) and schwertmannite in environmental and engineered systems.