Manganese (Mn) oxides are highly redox active minerals pervasive in soils and sediments on the surface of the Earth. Mn oxides are the only minerals known to oxidize Cr(III) to Cr(VI) in both experimental and natural systems. Cr(VI), a mobile contaminant in groundwater, can promote carcinogenesis. Cr(III), however, is a nonreactive trace metal essential for glucose uptake by mammals. Among all Mn oxide polymorphs, birnessite [(Na, Ca)0.5 (Mn4+, Mn3+)2 O4 . 1.5 H2O] induces the fastest oxidation kinetics and the highest reactivity with Cr(III). However, the mechanism of electron transfer between dissolved Cr and Mn oxide particles, particularly with respect to birnessite, is poorly understood. Mitigation and remediation of Cr(VI) requires a quantitative knowledge of this mechanism. Here we present a novel method for simultaneous measurement of the birnessite structure and the extent of Cr oxidation at different solution pH conditions. We collected time-resolved synchrotron X-ray diffraction (TR-XRD) patterns of birnessite every 15s while passing pH controlled 1 mM Cr(III) nitrate solutions through a capillary cell, and quantified Cr(VI) concentrations of the eluate solution every 20 min using spectrophotometry. Consistent with Fischer (2011), our TR-XRD results revealed that dissolved Cr(III) will induce a transformation of triclinic to hexagonal birnessite at pH 2,3,4, and 5 within 6 hrs of reaction. Control experiments without Cr(III) revealed no evidence for birnessite transformation. Consistent with Fendorf and Zasoski (1992), we observed different behaviors with respect to pH. At pH 4 and 5, our spectrophotometric measurements of the eluate showed that initial concentrations of ~0.2 mM Cr(VI) persisted for 1 (pH 5) to 2 (pH 4) hours and then sharply decreased to ~0.03 mM Cr(VI) upon transformation from triclinic to hexagonal birnessite. These low levels of Cr(VI) persisted for the next 5-10 hours. At pH 2 and 3, analyses indicated concentrations of ~0.8 mM Cr(VI) and ~0.35 mM Cr(VI), respectively. However, we did not observe any corresponding decrease in the oxidation of Cr(III) when triclinic birnessite transformed to hexagonal birnessite at pH 2 and 3. We detected no Cr (hydr)oxide precipitates during any reactions. Our research suggests that the variety of birnessite critically controls the oxidation of Cr(III) at pH 4 and 5 due to the accessibility of Mn(III) in triclinic birnessite, but this factor is less significant at pH 2 and 3.

Time-Resolved synchotron x-ray scattering and chemical analysis of redox reactions between aqueous Cr(III) and triclinic Na-birnessite

253rd National Meeting (2017)

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