Impact of Phosphate on Fluoride Removal by Calcite

Fluoride can be removed from wastewater by addition of calcite. However, fluoride is often found in complex mixtures where other contaminants can affect the removal process. Batch experiments were used to determine the impact of phosphate (PO43−) on the removal of fluoride from solution by calcite. In the experiments, up to 2,000 mg/L F− and 500 mg/L PO43− were used with 150 μm and 1.18 mm calcite fractions at solid-to-solution ratios of 1.00 and 33.33 g/L. Experimental results were analyzed with the geochemical model PHREEQ, which assumes fluoride and PO43− removal by precipitation as fluorite, fluorapatite, and hydroxyapatite. The model predicted that PO43− had no influence on fluoride removal below pH 11.6. This was inconsistent with the experimental data showing that the presence of a relatively small amount (17 mg/L) of PO43− resulted in a 20% reduction in F− removal, whereas 500 mg/L PO43− resulted in complete (100%) inhibition of fluoride removal. Results indicated that the removal of fluoride via precipitation is inhibited by PO43− adsorption at the calcite surface, and that there was competition for available surface adsorption sites with PO43− preferentially removed over F−. The amount of PO43− adsorbed was in very good agreement with the number of available adsorption sites calculated from Kr-BET surface area measurements. To date no studies have examined the impact of phosphate on the removal of fluoride from water using calcite, a cost-effective, readily available material commonly used as a water filter substrate. Importantly, this study provides quantitative information for environmental engineers and decision makers on the impact of phosphate on the ability of calcite to remove fluoride from contaminated water.