EFFECT OF MANGANESE ON FLUORIDE MOBILIZATION IN HIGH CARBONATED GROUNDWATER

Elevated fluoride (F) contamination in groundwater is a major water quality problem in low-to-middle income countries, causing fluorosis and stiffness of gait in affected populations. To understand the role of geological constituents on processes governing F mobilization in groundwater, a coupled field and bench-scale study of a contaminated site in the middle Gangetic plains of India was performed over 2 y. Groundwater from twenty-one locations and aquifer sediments from two locations were collected and characterized. The sampled groundwater represented two aquifers at 12 m (shallow) and 30 m (deep) below the surface. Elevated dissolved inorganic carbon (DIC) concentration (139 ± 38 mg L^-1) in all the groundwater samples was ~70 times higher than the predicted DIC concentration at atmospheric pCO2 and measured pH. Fluoride concentration (2.8 ± 1.3 mg L^-1) in the shallow aquifer was above the WHO permissible limit (1.5 mg L^-1), whereas the concentration in the deep aquifer was 0.5 ± 0.4 mg L^-1. Statistical and geochemical analyses of groundwater data along with solid phase characterization results from sequential extraction and XRD indicated calcite precipitation-driven fluorite dissolution as the process responsible for higher F in the shallow aquifer. Mn concentration (259 ± 173 µg L^-1) in the deep aquifer was higher than the concentration (21 ± 21 µg L^-1) in shallow aquifer. In deep aquifer, groundwater was at saturation with respect to rhodochrosite (MnCO₃), with a strong negative correlation between F and Mn. To comprehend possible effect of Mn on the calcite precipitation process, a batch experiment under aquifer-relevant conditions was performed at initial supersaturation with respect to calcite and rhodochrosite in ultrapure water, synthetic groundwater, and uncontaminated groundwater. More Mn precipitated when compared to Ca after 30 d of reaction. XRD and SEM-EDS analyses on solids collected from batch reactors confirmed precipitation of calcite and rhodochrosite. The thermodynamically feasible rhodochrosite precipitation hindered calcite precipitation in deep aquifer resulting in reduced fluorite dissolution. Findings from this study could help in better understanding of F mobilization mechanisms in other contaminated aquifers and aid in the development of F mitigation techniques.