Synergetic Effect of Hydroxyapatite/Chitosan-Alginate Polyelectrolyte Nanocomposite for Effective Fluoride (F ^– ) Adsorption

A hydroxyapatite/chitosan–alginate (HAp/CA) nanocomposite was synthesized via coprecipitation followed by lyophilization for efficient fluoride (F – ) removal from water. The incorporation of the chitosan–alginate polyelectrolyte complex improved the specific surface area (159 m 2 /g), colloidal stability, and surface functionality of HAp, thereby enhancing its adsorption performance. Among the synthesized materials, the HAp/CA3 nanocomposite (chitosan/alginate = 4:1) exhibited the highest F – adsorption capacity of 98 mg/g (at pH 7) within 30 min, which was approximately ten times greater than that of pristine HAp. Adsorption equilibrium and kinetic studies revealed that F – uptake follows the Langmuir isotherm and pseudo-second-order kinetic models, indicating monolayer chemisorption. Thermodynamic parameters confirmed that the adsorption process is spontaneous and endothermic. Mechanistic investigations using XRD, FTIR, and XPS analyses confirmed that F – adsorption is governed predominantly by surface complexation, with additional contributions from electrostatic attraction and ion exchange with surface hydroxyl groups, while protonated amine and carboxylate functionalities provide complementary adsorption sites. The HAp/CA3 nanocomposite showed excellent selectivity toward F – in the presence of competing anions and retained 94% of its adsorption efficiency after 10 adsorption–desorption cycles. Economic analysis further demonstrated the cost-effectiveness of the synthesized adsorbent. Overall, the innovation of this work lies in the development of a nontoxic, cross-linker-free, lyophilized HAp/chitosan–alginate nanocomposite with a highly porous architecture, exhibiting enhanced F – adsorption performance, reusability, and cost-effectiveness for practical water treatment applications. Furthermore, comprehensive mechanistic investigations provide a clear understanding of the F – removal process, distinguishing this material from previously reported HAp-, chitosan-, and alginate-based adsorbents.