Highly Effective Polyelectrolyte Complex of Chitosan–Pectin/Hydroxyapatite Nanocomposites: A Drastic Enhancement in Fluoride Adsorption

A novel, eco-friendly, cross-linker-free polyelectrolyte complex (chitosan–pectin (CP)) integrated with hydroxyapatite (HAp) was successfully developed using a freeze-drying technique. The incorporation of CP significantly enhances the specific surface area (100 to 276 m²/g) and colloidal stability of HAp due to the presence of abundant surface functional groups (−NH₂, COO^–, OH^–, and COOCH₃). Consequently, the HAp/CP3 nanocomposite (chitosan-to-pectin ratio of 1:4) exhibits an exceptional fluoride (F^–) adsorption capacity of 195 mg/g, reaching equilibrium within 10 min (pH 7). The adsorption of F^– is strongly influenced by the solution pH. As the pH increases from 3 to 11, the adsorption capacity decreases from 345 to 110 mg/g, attributed to the protonation/deprotonation process. Notably, the presence of competing anions does not affect the F^– adsorption efficiency. Furthermore, the HAp/CP3 nanocomposite demonstrates excellent recyclability, retaining 96% of its adsorption efficiency over seven consecutive cycles. The adsorption mechanism follows monolayer chemisorption, as described by Langmuir and the second-order kinetics. A Gaussian energy distribution value of 35 kJ/mol confirms that strong chemisorption governs the adsorption process. The negative Gibbs free energy (ΔG°) and enthalpy (ΔH°) values indicate that the adsorption is spontaneous and exothermic. The crystalline phase and surface characteristics before and after adsorption confirm that adsorption is primarily driven by electrostatic interactions, followed by surface complexation and ion exchange. Overall, this study highlights the nontoxic synthesis of the HAp/CP3 nanocomposite as a highly efficient, environmentally friendly, and robust adsorbent for F^– removal, offering a promising solution for water purification applications.