Nano-Hydroxyapatite Encapsulated inside an Anion Exchanger for Efficient Defluoridation of Neutral and Weakly Alkaline Water

The efficient removal of fluoride from weakly alkaline groundwater still remains a great challenge, owing to the high hydration energy of fluoride. Herein, we conducted in situ precipitation of nanosized hydroxyapatite (nano-HAP) inside a polystyrene anion exchanger D201. The resultant HAP@D201 exhibited almost constant defluoridation efficiency in a wide pH range, especially much higher F– removal efficiency under neutral and weakly alkaline conditions (pH 7–10) than currently available materials. XPS, XRD, and 19F MAS NMR analysis demonstrated that the pH-insensitive defluoridation performance of HAP@D201 was mainly attributed to lattice replacement between the hydroxide ions inside nano-HAP and fluoride in solution, owing to the similar ionic radius of fluoride (1.33 Å) with the hydroxide ion (1.37 Å). The ubiquitous anions like sulfate, chloride, nitrate, and bicarbonate posed negligible effects on the defluoridation performance of HAP@D201, because of the larger ionic radius than for hydroxide ion. Moreover, the HAP@D201 column was capable of successively producing 160, 108, and 92 bed volume (BV) clean water ([F] < 1.5 mg/L) from synthetic water at pH 5.0, 8.0, and 9.0, respectively, 2.5 times more than commercial defluorination material. After adsorption, the exhausted HAP@D201 could be fully refreshed by alkaline solution and retained constant defluoridation efficiency in 10 cyclic column adsorption-regeneration runs.