Copper doping improves hydroxyapatite sorption for arsenate in simulated groundwaters

Hydroxyapatite (HAP) has been widely used to immobilize many cationic heavy metals in water and soils. Compared with its strong sorption for metal cations, the abilities of HAP to sorb metal anions, such as arsenic, are less significant. Improving HAP sorption for anionic arsenic species is important for expanding its application potential because the presence of arsenic in the environment has raised serious health concerns and there is need for cost-effective remediation methods. In this work, we report an innovative method of copper doping to improve a synthetic HAP sorption for arsenate, which is a primary aqueous arsenic species, in simulated groundwaters. The undoped HAP and copper doped HAP (CuHAP) were characterized with XRD, FTIR, N₂ adsorption, and SEM, and then evaluated as sorbents for arsenate removal tests. The experimental results suggest that copper doping changed the morphology and increased the surface area of HAP. The CuHAP sorbed 1.6-9.1x more arsenate than the undoped HAP did in a simulated groundwater at pH of 7.7-8.0. The improved arsenate sorption is presumably due to the increase in surface area of HAP as a result of copper doping. In addition to the copper doping level, the arsenate sorption to HAP and CuHAP can also be increased with increasing water pH and calcium concentration. The experimental data indicate that sorbent dissolution is an important factor governing arsenate sorption to HAP and CuHAP.