Mineralogical and geochemical constraints on chromium oxidation induced by birnessite

We have explored redox reactions between dissolved Cr and the phyllomanganate birnessite with high resolution through simultaneous synchrotron X-ray diffraction, X-ray spectroscopy, and fluid analysis at different concentrations of solution pH. Specifically, we collected time-resolved synchrotron X-ray diffraction patterns and X-ray absorption near edge structure (XANES) spectra from triclinic Na-birnessite every 15 min while passing pH controlled 1.0 mM Cr(III) nitrate solutions through a capillary cell. In addition, we quantified Cr(VI) concentrations of the eluate solution every 15 min using spectrophotometry. Consistent with previous studies, we observed an increased rate of Cr(VI) production with decreasing pH. We attribute the comparatively slow kinetics of Cr(III) oxidation at pH 5.0 and 4.0 to a transformation from triclinic to hexagonal birnessite. This solid-state transition reproducibly coincided with a ∼10-fold decline in the extent of oxidation of aqueous Cr(III). Control experiments without Cr(III) revealed no evidence for birnessite transformation within the same time frame, and experiments with hexagonal birnessite as the starting material generated solutions with low fractions (∼3 mol%) of dissolved Cr(VI) from start to finish. At pH 3.0 and 2.0, however, production of Cr(VI) was consistently higher than was observed at pH 5.0 and 4.0. Specifically, at pH 2.0, 80 mol% of the influent Cr(III) was oxidized to Cr(VI) during the experiment compared to 20 mol% at pH 5.0. XANES analyses showed evidence for both Cr(III) and Cr(VI) adsorbing onto the surface of birnessite at all pH values. We propose that Cr(III) is oxidized to Cr(VI) by an electron exchange that reduces Mn(III) in birnessite to Mn(II). At pH 3.5 and higher, the structure of birnessite consequently transforms to hexagonal birnessite. By this pathway, the birnessite crystal structure critically controls the oxidation of dissolved Cr(III) due to the accessibility of reactive Mn(III) in triclinic birnessite relative to hexagonal birnessite. Below pH 3.5, however, birnessite dissolution systematically exposes reactive sites that enable the continuous oxidation of Cr(III) to Cr(VI).