Recycling of phosphate tailings for an efficient hydroxyapatite-based adsorbent to immobilize heavy metal cations

Hydroxyapatite (HAP) is a promising adsorbent for immobilizing heavy metals in soil and water. However, the preparation and modification of HAP from pure chemicals increases its cost and limits its large-scale practical application. In this study, a hydroxyapatite-based adsorbent (HAP_PT) was prepared from phosphate tailing produced in the phosphorus industry to sequester Pb, Cd and Zn from solution. The results showed that HAP_PT was composed of HAP and MgO, with a surface area of 27.74 m²/g. The kinetics studies showed that most Pb and Cd were removed from the initial solution in 4 h and the adsorption of Zn increased with increasing contact time. Metals presented higher adsorption capacities at 35 °C than that at 25 °C. The adsorption isotherms showed that HAP_PT presented high adsorption capacities for Pb, Cd and Zn in mono-metal solutions. The adsorption capacity of Cd at pH 6 was higher than that at pH 3, but the adsorption for Pb and Zn was similar at both pHs. HAP_PT has selectivity for Pb in Pb-Cd-Zn multi-metals solution, and competitive adsorption reduced the adsorption quantity by 53%, 93% and 79% for Pb, Cd and Zn, respectively. The combined results of TEM-EDS, XRD and XPS showed that Pb was immobilized by forming phosphates due to the dissolution of HAP, whereas Cd and Zn were immobilized by forming hydroxide precipitates resulting from the function of MgO in HAP_PT. The results of this study provided an efficient adsorbent for the removal of heavy metals in solution and provided a new perspective on the recycling of phosphate tailings in the phosphorus industry.