TIME-RESOLVED SYNCHROTRON X-RAY DIFFRACTION AND CHEMICAL ANALYSIS OF REDOX REACTIONS BETWEEN AQUEOUS CR(III) AND TRICLINIC NA-BIRNESSITE

Manganese (Mn) oxides are highly redox active minerals that are pervasive in soils and sediments as aggregates and coatings. Mn oxides are the only minerals known to oxidize Cr(III) to Cr(VI) in both experimental and natural systems. Cr(VI) is a mobile contaminant in groundwater and can promote carcinogenesis in humans. 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 1 mM Cr(III) nitrate solutions through a capillary cell, and we 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 4 and 5 within 6 hr of the reaction. Control experiments without Cr(III) revealed no evidence for birnessite transformation. 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) hr and then sharply decreased to ~0.03 mM Cr(VI) upon initiation of the transformation from triclinic to hexagonal birnessite. These low levels of Cr(VI) persisted for the next 5-10 hr. We observed no Cr (hydr)oxide precipitates during these reactions. Our research suggests that Cr(III) induces the transition from triclinic to hexagonal birnessite at pH 4 and 5, and the phase of birnessite in turn controls the oxidation of Cr(III).