Experimental and theoretical study on the adsorption mechanism of Amino trimethylphosphate (ATMP) functionalized hydroxyapatite on Pb (II) and Cd (II)

Amino trimethylposphonate(ATMP) functionalized HAP were synthesized with different ATMP addition (0, 5%, 10%, 15%) and the adsorption performance towards Pb 2+ and Cd 2+ were studied in an aqueous solution. Under the optimum condition, HAP with ATMP addition of 10% (10AT-HAP) displayed the most efficient adsorption performance with the adsorption capacity of 1458.79 mg/g and 321.26 mg/g towards Pb 2+ and Cd 2+ , respectively, which improved 76% and 92% than that of the artificially prepared HAP in the laboratory, respectively. For both heavy metal ions , the adsorption mechanism also included ion exchange and surface complexation, in which the surface complexation played a predominant role. In addition, XRD showed that a new crystal phase was formed after Pb 2+ adsorption, while no new phase appeared after Cd 2+ adsorption, indicating that the adsorption mechanism of Pb 2+ involved dissolution-precipitation. The fitting results of adsorption isotherm and kinetics showed that the adsorption of Pb 2+ and Cd 2+ on HAP and 10AT-HAP were well fitted by the Langmuir isotherm model and Pseudo-second model. Moreover, Multiwfn wavefunction program based on density functional theory (DFT) was applied to deeply probe and vividly visualize the interaction sites, interaction types, interaction strength and the essence of interactions between HAP and ATMP or ATMP-HAP composite and heavy metal ions.