Calcium Phosphate Nanoadsorbents for Water Defluorination

The resource–structure–function relationship of calcium phosphate (CaP) materials for water defluorination remains poorly understood. In this study, various CaP samples were prepared by a wet precipitation method with varying solution pH values and drying temperatures. The main phases of the acidic and alkaline CaP nanomaterials were dicalcium phosphate dihydrate and hydroxyapatite, respectively. The increase of drying temperature resulted in phase transformations, decreased active site number, and enhanced crystallinity. The superior fluoride (F–) removal performance of the CaP samples prepared under acidic and low-temperature drying conditions compared to the others was attributed to their differences in components, crystallinity, and reactivity. The synthesis conditions, with a solution pH of 6.5 and an air-drying temperature of 40 °C, yielded the optimal CaP nanomaterial. The F– sorption ratio on CaP-6.5–40 increased rapidly from ∼7% to ∼74% within 0.5–2 h, eventually rising to ∼93% after 48 h. The F– removal efficiency of ∼91% at 19.2 h and 0.4 g/L for a multiple-time addition group was comparable to that at 24 h and 0.5 g/L for a single-time addition group. The mixed inorganic cations and anions in solution had an ignorable effect on F– removal. The primary removal mechanism of F– by CaP-6.5–40 was the dissolution of dicalcium phosphate dihydrate and the subsequent precipitation of a more stable fluorapatite phase. Overall, the prepared CaP-6.5–40 nanomaterial holds promise for a highly efficient purification of fluoride-contaminated water bodies.