Excellent defluoridation performance of tricalcium phosphate as seed crystal for high fluoride groundwater

The control and removal of fluoride from groundwater is critical for the safety of drinking water and human health. In this study, a method of using tricalcium phosphate (TCP) as a seed crystal to induce fluorapatite (FAP) crystallization for fluoride removal was proposed, and its performance and mechanism were studied. With TCP dosage of 2 g/L and Ca/P/F molar ratio of 4:3:1, the F⁻ concentration decreased from 9.5 to below 1 mg/L within 20 min and remained stable within a pH range of 3–10, superior to FAP/calcite seed crystal. However, the increased solubility of TCP at low pH leads to an increase in the proportion of homogeneous nucleation of FAP. Furthermore, the presence of Mg²⁺, SiO₃²⁻, HCO₃⁻, and citric acid inhibited the crystallization of FAP, leading to the reduction of defluoridation performance. Adjusting the pH of fluoridated groundwater from 8 to 6 effectively mitigated the adverse effects of high HCO₃⁻ levels in groundwater. During the defluoridation in a fluidized bed reactor (FBR) for fluoridated groundwater, a stable effluent F⁻ concentration of below 1 mg/L was demonstrated within the operation of 9 days. The potential defluoridation mechanism involves the initial binding of Ca²⁺ and phosphate ions to form various intermediate species, the interaction of intermediates with F⁻ and Ca²⁺ to generate FAP, and the dissolution or aggregation with other nuclei on the seed crystal surface. This study could provide references on the possible application of TCP in groundwater defluoridation.