Removal of Fluoride from Geothermal Water by Waste-Synthesized Al(OH)3-HAP@ZMS Composite Adsorbent: Sorption Capability and Mechanism

The use of industrial and biological waste to synthesize adsorbent materials for the treatment of high fluorinated geothermal water is a cost-effective way to address the fluorine pollution problem while also reusing waste resources. In this study, modified hydroxyapatite-loaded zeolite molecular sieve composites (Al(OH)3-HAP@ZMS) were synthesized using fly ash and shell as precursors, which were then cation doped and compound modified to ensure adsorption while remaining cost-effective. When the material ratio of ZMS to Al(OH)3-HAP was 1:6, the best adsorption material was obtained. Under the optimum adsorption conditions (dosage = 6 g/L, T = 50 °C), Al(OH)3-HAP@ZMS achieved a removal rate of 92.5% and an adsorption capacity of 1.56 mg/g for 10 mg/L F− solution and can reduce the F− concentration to below the national standard concentration of 1 mg/L. The fitting of kinetic, isothermal, and thermodynamic data revealed that F− adsorption by Al(OH)3-HAP@ZMS followed the pseudo-second-order kinetics and Freundlich model, confirming that the adsorption process was primarily chemisorption and multilayer adsorption. The adsorption processes included electrostatic attraction, ion exchange between -OH and F−, complexation of fluoro-aluminum, and co-precipitation of Ca2+ with F−. At the same time, Al(OH)3-HAP@ZMS demonstrated good anti-interference performance in natural waters against co-existing ions and some heat resistance. Therefore, the composite adsorbent Al(OH)3-HAP@ZMS, as an effective and environmentally friendly low-cost adsorbent, is expected to achieve the practical engineering application of geothermal water defluoridation in Guanzhong, maximizing the dual benefits of “fluoride remove” and “waste resource reuse.”