Impact of Manganese Carbonate Precipitation on Uranium(VI) Fate in Conditions Relevant to Carbonate-Buffered Aquifers

Widespread geogenic uranium (U) contamination of Indian groundwaters is of serious concern; yet little is known of the dominant forms and release mechanisms of U in these aquifers. Interestingly, manganese (Mn)-rich aquifers, highly buffered by dissolved inorganic carbon (DIC) and saturated with rhodochrosite [MnCO_3(s)], have shown low U (<WHO limit of 30 μg L^-1) concentrations. Given the limited understanding of U and MnCO_3(s) interaction, this study investigates the impact of rhodochrosite precipitation on U(VI) fate in simple solutions supersaturated with respect to MnCO_3(s) (Saturation Index ∼ 2.7) under conditions relevant to carbonate-buffered mixed oxic aquifers. Year-long batch experiments were performed over variable initial U concentrations (0-500 μM). While uranium uptake was ∼50% within 30 days, the uptake was near-complete after 1 year for all conditions. No known U-bearing solid phases were detected, except for the highest initial U concentrations (500 μM), consistent with solution saturation states. Characterization of solids collected after 30 days and 1 year with X-ray diffraction, electron-based imaging, X-ray absorption near-edge, X-ray photoelectron, and Raman spectroscopies confirmed rhodochrosite as the only major precipitated phase, with slightly altered crystal structure from uranyl incorporation. While these incorporations seemed to initially (30 days) reduce the crystallinity of rhodochrosite, all U-incorporated rhodochrosite turned crystalline in the long term (1 year). Also, the incorporated U did not undergo a redox change. Instead, the U(VI) coordination environment changed as U(VI) uptake increased. Reactive modeling of Mn and DIC data exhibited a decrease in the surface area normalized rate constants of MnCO_3(s) precipitation from 5.9 to 3.3 × 10^-8 mol m^-2 h^-1 with an increase in initial U(VI) from 0 to 500 μM. The notable incorporation of U(VI) by MnCO_3(s) in just 30 days, a phenomenon that is rapid on a geological time scale, has implications on U(VI) fate in high carbonated oxic-anoxic aquifers with high Mn concentrations.