The Role of Manganese Carbonate Precipitation in Controlling Fluoride and Uranium Mobilization in Groundwater

Groundwater contamination with fluoride
(F) and uranium (U) has
been reported in many parts of India. However, the sources and mobilization
mechanisms of these contaminants remain poorly understood. The present
study aimed to identify the processes governing the coexistence of
elevated F and U in groundwater at a typical site in India’s
middle Gangetic plain. Sustained groundwater sampling at 21 locations
over two years indicated persistence of high F and U in a shallow
aquifer (12 m depth), but absence of these pollutants in a deeper
aquifer (30 m depth). For both the aquifers, Mn exhibited strong inverse
correlations with F (−0.587; p < 0.01)
and U (−0.581; p < 0.01). X-ray diffraction
analysis of representative sediment cores indicated few differences
in the mineralogy of the two aquifers, which consisted of fluorite
(CaF2(s)) and calcite (CaCO3(s)), among others.
Analysis of groundwater speciation and saturation state and sequential
extraction on aquifer sediments suggested that elevated F in shallow
groundwater occurred due to calcite precipitation-induced fluorite
dissolution. The conditions in both the aquifers were oxidizing with
respect to U, but reducing with respect to Mn. Elevated U was attributed
to carbonate-promoted mobilization from iron–manganese (Mn)
and residual sediment fractions. In the deeper groundwater, elevated
Mn and lower pH levels persisted with conditions at saturation with
respect to rhodochrosite (MnCO3(s)). Furthermore, medium
(30 d) to long-term (300 d) batch experiments were performed to systematically
evaluate the role of variable Mn on calcite precipitation under approximate
in situ conditions. Precipitation of rhodochrosite outcompeted calcite
precipitation and resulted in lower pH compared to pH of Mn-free systems,
which (a) inhibited calcite precipitation and associated fluorite
dissolution and (b) constrained pH and alkalinity in the deeper groundwater.
These findings have implications for understanding F and U mobilization
in comparable Mn-deficient sites and development of appropriate Mn-based
amendments for in situ remediation.