Physical modification of hydroxyapatite: the drastic enhancement of both cation (Cd2+) and anion (F−) adsorption and recycling efficiency

Abstract

Hydroxyapatite was physically modified using a combination of ultrasonication and freeze-drying techniques (FD-HAp). This amalgamation has considerably reduced the particle size (24 × 10 ± 3 nm) and has enhanced the surface activation energy, activation of surface functional groups (PO₄³⁻, OH⁻, and CO₃²⁻), and specific surface area (150 m² g⁻¹). Consequently, within 5 min at pH 7, FD-HAp attained 208% Cd²⁺ (298 mg g⁻¹) and 488% F⁻ (200 mg g⁻¹) higher adsorption capacities compared to previously reported HAp nanoparticles. In addition, FD-HAp exhibited high selectivity for Cd²⁺ (excluding Pb²⁺) and F⁻ in the presence of other competing ions, and it has a rapid (5 min) recycling efficiency up to 7 cycles for Cd²⁺ (85%) and F⁻ (98%). The Cd²⁺ and F⁻ adsorption behavior and their capacity varied corresponding to the pH values due to protonation and deprotonation of the surface functional groups. The Cd²⁺ and F⁻ adsorption was monolayered chemisorption, as confirmed by the pseudo-second-order kinetics and Langmuir isotherm. In addition, the Gaussian energy distribution of Cd²⁺ (21.61 kJ mol⁻¹) and F⁻ (9.89 kJ mol⁻¹) adsorption on the FD-HAp surface confirms the strong chemisorption and chemical ion exchange processes, respectively. In adsorption thermodynamics, the negative and positive values of Gibbs free energy and enthalpy suggest that Cd²⁺ and F⁻ adsorption was spontaneous and endothermic in nature. The removal performance of both cationic and anionic contaminants reveals that FD-HAp is an ideal adsorbent for the removal of toxic ions. Hence, the present study emphasizes a rational design for HAp modification, providing an alternative approach to nanoparticle synthesis that is highly efficient and cost-effective.