Effect of different CO32-to PO43- molar ratios on the properties, morphology, and Pb(II) removal performance of carbonated hydroxyapatite

Hydroxyapatite (HAP) and a series of carbonate-substituted hydroxyapatite (CHAP) materials were prepared using a facile hydrothermal method and characterized by powder X-ray diffractometer (XRD), nitrogen adsorption-desorption analyzer, transmission electron microscope (TEM), Fourier transform infrared spectrometer (FT-IR), and elemental analyzer. The effect of different CO32- to PO43- molar ratios on the properties, morphology, and Pb(II) removal performance of the synthesized materials was investigated. Varying the molar ratios of CO32- to PO43-, from 0:19 (HAP) to 5:14 (CHAP5), 10:9 (CHAP10), and 15:4 (CHAP15), resulted in smaller particle size, lower crystallinity, and higher surface area of materials with higher CO32- concentrations. Consequently, CHAP15 demonstrated significantly superior maximum adsorption capacity (1697 mg g(-1)) among all the synthesized materials. The isotherm and kinetics of Pb(II) adsorption by CHAP15 were satisfactorily described by the Langmuir isotherm model and the pseudo-first-order kinetic model, respectively. The dominant mechanism for Pb(II) removal by CHAP15 was the dissolution of adsorbent followed by the reaction of Pb(II) ions with phosphate ions and with carbonate ions to form poorly-soluble compounds: Pb-10(PO4)(6)(OH)(2) and Pb-3(CO3)(2)(OH)(2).

Automated summary

Hydroxyapatite and carbonate-substituted hydroxyapatite materials were prepared using a hydrothermal method, with higher CO32- concentrations resulting in smaller particle size, lower crystallinity, and higher surface area. CHAP15 exhibited superior Pb(II) adsorption performance, with the adsorption mechanism involving the formation of poorly-soluble compounds through the reaction of Pb(II) ions with phosphate and carbonate ions.