Modifying hydroxyapatite nanoparticles with humic acid for highly efficient removal of Cu(II) from aqueous solution

• HAP surface was modified successfully with HA.
• HAP/HA were used for Cu(II) adsorption firstly and showed high adsorption capacity.
• The maximum adsorption capacity was determined to be 58.42 mg/g at 313 K.
• All experimental data was analyzed using isotherm and kinetic models.
• Adsorption mechanism involved ion exchange and surface complexation.

Hydroxyapatite (HAP) could be a kind of potential low cost and biocompatible adsorbent for potentially toxic metals due to its special chemical composition and crystal structure. Humic acid (HA) modified HAP nanoparticles (HAP/HA) was prepared to adsorb Cu(II) ions from aqueous solution. The obtained materials were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscope (SEM). Then, batch experiments were carried out to investigate the possible adsorption mechanisms of Cu(II) ions onto HAP/HA nanoparticles. Results illustrated that the adsorption capacity of the modified HAP for Cu(II) was improved significantly as compared to that of the bare HAP. Kinetic experiments revealed that the pseudo-second-order model exhibited the best correlation with the adsorption data. Equilibrium data were best described by Sips model, and the estimated maximum adsorption capacity of HAP/HA was 58.42 mg/g at 313 K, displaying higher efficiency for Cu(II) removal than previously reported adsorbents. Cu(II) removal could be decreased in the presence of coexisting cations due to the competition of adsorption sites. The mechanisms for the adsorption of Cu(II) on HAP/HA nanoparticles might include ion exchange with Ca2+ of HAP, and surface complexation of Cu(II) on the POH groups of HAP and/or carboxyl and/or phenolic groups in HA. Findings of the present work highlight the potential for using HA modified HAP nanoparticles as a biocompatible and environmental-friendly adsorbent for Cu(II) removal from aqueous solution.

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