Strontium‐Substituted Hydroxyapatite for Fluoride Removal From Water: Adsorption Equilibrium, Kinetics, and Thermodynamic Studies

ABSTRACT Strontium‐substituted calcium hydroxyapatite (Sr–CaHAp) was synthesized via a coprecipitation method and evaluated for fluoride removal and antibacterial activity. The materials were characterized using XRD, FTIR, TEM, BET, XPS, and pH‐drift analyses. Rietveld refinement confirmed successful substitution of Ca 2+ with larger Sr 2+ ions, leading to increased lattice parameters and unit cell volume. TEM revealed rod‐like particles (40–55 nm), and BET measurements showed enhanced surface area upon Sr incorporation, with Sr(2)CaHAp exhibiting the highest value. XPS further validated Sr 2+ integration into the CaHAp lattice. Optimal adsorption occurred at pH 6.5, 6.0 g L −1 dosage, and 1.5 h contact time. Sr(2)CaHAp achieved the highest Langmuir monolayer capacity of 625 mg g −1 , maintaining 98.9%–95% removal up to 100 ppm fluoride. Efficiency declined from 87.9% to 35% between 200–2000 ppm. Isotherm analysis showed Freundlich behavior for Sr(2)CaHAp and Langmuir behavior for pristine CaHAp. Kinetic data fitted the pseudo‐second‐order model, and thermodynamic analyses indicated a spontaneous, endothermic process. Mechanistic studies revealed that fluoride adsorption proceeds via ion exchange with CaOH/SrOH surface groups and electrostatic attraction at low pH, while complexation with deprotonated sites dominates at higher pH. Fixed‐bed data fit the Clark model ( R 2 = 0.989). Sr(2)CaHAp also exhibited strong reusability and reduced fluoride in real groundwater to below WHO limits, along with notable antibacterial activity, highlighting its promise as a bifunctional water‐purification material.