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 [MnCO3(s)], have
shown low U (–1) concentrations.
Given the limited understanding of U and MnCO3(s) interaction,
this study investigates the impact of rhodochrosite precipitation
on U(VI) fate in simple solutions supersaturated with respect to MnCO3(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 MnCO3(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
MnCO3(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.