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 m(2)/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 degrees C than that at 25 degrees 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.

Automated summary

Hydroxyapatite-based adsorbent (HAP(PT)) was prepared from phosphate tailings and showed efficient removal of Pb, Cd, and Zn from solution. The adsorption capacity of HAP(PT) was influenced by temperature and pH, and showed high adsorption capacity in mono-metal solutions. Selective adsorption for Pb was observed in the presence of Cd and Zn, while competitive adsorption reduced the adsorption quantity for all metals. The results provided insights into the immobilization mechanisms and potential applications of HAP(PT) in heavy metal removal and phosphate tailings recycling.