LAUSR

This study deals with the use of calcium hydroxyapatite (HAP) materials as adsorbents for removing heavy metal ions from polluted waters. The synthesis conditions (i.e. P/Ca stoichiometry and nature of the solvent) of HAP samples were properly varied in order to prepare three different samples with tuned properties. Stoichiometric hydroxyapatite, with Ca/P = 1.67 (s-HAP), Ca-deficient hydroxyapatite, with Ca/P 99%) was independent of the intrinsic properties of the adsorbents, the initial concentration of Pb2+, and the co-presence of Cu2+. Conversely, the removal of Cu2+ was affected by the structural properties of the HAP adsorbents, d-HAP being the most efficient adsorbent for Cu2+ (93%). Immobilization of Pb2+ and Cu2+ onto HAP occurred by two different mechanisms: a dissolution–precipitation mechanism with formation of a Pb10(PO4)6(OH)2 phase was responsible for the Pb2+ uptake, whereas Cu2+ was immobilised by surface complexations involving Ca2+ species with carbonate and hydroxyl groups of HAP. In addition, leaching tests revealed that the confinement of the two metal species on HAP samples was permanent.