Kinetic and isotherm adsorption studies of Zn (II) ions on hydroxyapatite nanoparticles: Linear and nonlinear analyses

Abstract Rapid industrialization has led to significant environmental challenges, including the disposal of effluents with high zinc ion concentrations. This study investigates the use of hydroxyapatite nanoparticles as an adsorbent for Zn (II) ions from aqueous solutions at neutral pH. These nanoparticles are characterized by their high purity, mesoporous structure, and a specific surface area of 60.97 ± .40 m 2 g −1 . Their thermal stability was confirmed by thermogravimetric analysis, and zeta potential measurements indicated a surface charge close to the isoelectric point. Adsorption experiments showed that the kinetics fit well with the pseudo‐second‐order model, with a rate constant of 2.58 ± .49 min −1 mg −1 and a determination coefficient ( R 2 ) of 1.00. Isotherm analyses using Redlich–Peterson, Langmuir, and Freundlich models revealed the highest adsorption capacity and best fitting with the Redlich–Peterson model, showing maximum capacities of 30.36 mg g −1 (linear fitting) and 32.11 mg g −1 (nonlinear fitting). These fits achieved R 2 values of .9949 and .9923, respectively, suggesting efficient and reliable adsorption profiles. This research highlights the potential of hydroxyapatite nanoparticles for effective zinc ion removal, supporting their application in sustainable industrial practices and environmental remediation.