Fluoride removal by calcite and hydroxyapatite

Abstract

Elevated fluoride concentrations in drinking water cause dental and skeletal disease and pose a major human health challenge in many parts of the world. Precipitation of fluoride in mineral matrices is an attractive method of remediation due to its low cost and ease of use. Here we demonstrate that fluoride can be effectively removed from groundwater by adding a mixture of hydroxyapatite and calcite and cycling the pH. Below 20 ppm fluoride, fluoride precipitates as fluoroapatite on the hydroxyapatite surface; at higher fluoride concentrations, fluoride precipitates as fluorite. The efficiency of fluoride removal can be controlled by changing the hydroxyapatite : calcite ratio. Experiments with simulated groundwater from areas with high geogenic fluoride show that common groundwater ions have minimal effect on fluoride sorption or precipitation. This technique thus has widespread applicability to remediation of fluoride-contaminated groundwater around the world as well as ancillary waste streams where fluoride is a byproduct of industrial processes.