Removal of Cu(II) from wastewater using doped HAP-coated-limestone

Reducing copper content in copper-containing water is a worldwide environmental problem. It has been shown that hydroxyapatite (HAP) has a significant adsorption effect on heavy metals (including Cu2+). In this study, doped HAP-coated-limestone was synthesized using waste limestone, diammonium hydrogen phosphate and two kinds of chloride (Mg2+, Sr2+). The materials were characterized by X-ray diffraction (XRD), Fourier transform infrared spectrophotometry (FTIR), thermogravimetric analysis (TGA), Brunner-Emmet-Teller (BET) measurements and scanning electron microscope (SEM). It was found that the structure of Mg2+ and Sr2+ doped HAP-coated-limestone was similar to that of HAP-coated-limestone. The surface of doped HAP-coated-limestone is rougher, and the specific surface area is larger than that of HAP-coated-limestone. Some factors have been investigated affecting the uptake behavior, such as doped element, contact time, temperature and Cu2+ concentration to optimize the conditions. The adsorption isotherms of Langmuir and Freundlich were studied, and it was proved that the adsorption isotherm of Langmuir was more consistent with the adsorption process of doped HAP-coated-limestone. The quasi-second-order dynamics can be better used to describe the experimental results compared with quasi-first-order dynamics. At the temperature of 30 °C, the adsorption capacity of HAP-coated-limestone for Cu (II) is 90.90 mg·g−1, while the adsorption capacity is 214.20 and 121.61 mg·g−1 for Mg2+ and Sr2+ doped HAP-coated-limestone, respectively. Through the calculation of thermodynamic parameters, the results show that the adsorption process of Cu (II) on HAP-coated-limestone and doped HAP-coated-limestone becomes easier with the increase of temperature in a certain temperature range. The adsorption process on HAP-coated-limestone and doped HAP-coated-limestone is spontaneous and endothermic.