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

Widespread\ngeogenic uranium (U) contamination of Indian groundwaters\nis of serious concern; yet little is known of the dominant forms and\nrelease mechanisms of U in these aquifers. Interestingly, manganese\n(Mn)-rich aquifers, highly buffered by dissolved inorganic carbon\n(DIC) and saturated with rhodochrosite [MnCO<sub>3(s)</sub>], have\nshown low U (–1) concentrations.\nGiven the limited understanding of U and MnCO<sub>3(s)</sub> interaction,\nthis study investigates the impact of rhodochrosite precipitation\non U(VI) fate in simple solutions supersaturated with respect to MnCO<sub>3(s)</sub> (Saturation Index ∼ 2.7) under conditions relevant\nto carbonate-buffered mixed oxic aquifers. Year-long batch experiments\nwere performed over variable initial U concentrations (0–500\nμM). While uranium uptake was ∼50% within 30 days, the\nuptake was near-complete after 1 year for all conditions. No known\nU-bearing solid phases were detected, except for the highest initial\nU concentrations (500 μM), consistent with solution saturation\nstates. Characterization of solids collected after 30 days and 1 year\nwith X-ray diffraction, electron-based imaging, X-ray absorption near-edge,\nX-ray photoelectron, and Raman spectroscopies confirmed rhodochrosite\nas the only major precipitated phase, with slightly altered crystal\nstructure from uranyl incorporation. While these incorporations seemed\nto initially (30 days) reduce the crystallinity of rhodochrosite,\nall U-incorporated rhodochrosite turned crystalline in the long term\n(1 year). Also, the incorporated U did not undergo a redox change.\nInstead, the U(VI) coordination environment changed as U(VI) uptake\nincreased. Reactive modeling of Mn and DIC data exhibited a decrease\nin the surface area normalized rate constants of MnCO<sub>3(s)</sub> precipitation from 5.9 to 3.3 × 10<sup>–8</sup> mol\nm<sup>–2</sup> h<sup>–1</sup> with an increase in initial\nU(VI) from 0 to 500 μM. The notable incorporation of U(VI) by\nMnCO<sub>3(s)</sub> in just 30 days, a phenomenon that is rapid on\na geological time scale, has implications on U(VI) fate in high carbonated\noxic-anoxic aquifers with high Mn concentrations.