Fluoride removal by induced crystallization using fluorapatite/calcite seed crystals

Abstract

The removal of fluoride from water by induced crystallization using fluorapatite (FAP)/calcite seed crystals was investigated. Batch experiments were conducted to determine optimal operating parameters. The effect of FAP/calcite mass ratio of the seed crystals, initial seed crystal concentration, molar ratios of Ca2+:PO43−:F− and contact time were examined. FAP/calcite with any mass ratio reduced fluoride concentrations from 9.5mg/L (0.5mM) to less than 1mg/L. The use of FAP alone and calcite alone was not effective. Use of initial seed crystal concentrations greater than 4.0g/L were correlated to residual fluoride concentrations less than 1.0mg/L. The optimal parameters for fluoride removal were an initial seed crystal concentration of 6g/L (FAP 4.8g/L, calcite 1.2g/L), Ca/P/F molar ratio of 8:4:1, and contact time of 1h. The final pH was always close to neutral. The presence of natural organic matter (NOM) in the raw water was found to not affect fluoride removal. SEM–EDS and XRD analysis showed that fluoride removal was due to crystallization of Ca5(PO4)3F on the surface of FAP. Phosphate, but not fluoride, accumulated on calcite by adsorption. The seed crystal could be regenerated easily, and used repeatedly, and used seed crystals could also be used as resources in phosphorus fertilizer production. Induced crystallization using FAP/calcite as a seed crystal is an economically and environmentally friendly process, and has the potential to remove fluoride from groundwater.