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 Mnbased amendments for in situ remediation.

Automated summary

Groundwater contamination with fluoride and uranium is a common issue in many parts of India, with limited understanding of their sources and mobilization mechanisms. The present study identified calcite precipitation-induced fluorite dissolution as a key factor contributing to the coexistence of elevated F and U in shallow groundwater, while elevated Mn in deeper groundwater may play a critical role in controlling F and U concentrations in the shallow aquifer.