Tailoring the structural and morphological properties of hydroxyapatite materials to enhance the capture efficiency towards copper(ii) and lead(ii) ions

Abstract

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 < 1.67 (d-HAP), and hydroxyapatite precipitated from hydro-alcoholic solution (a-HAP) were prepared and used as adsorbents for copper and lead ions. Tests were carried out with simulated polluted waters containing single- and binary-metal concentrations of the two ions, with investigation on the selectivity of the metal capture process. The prepared materials were characterized using a suite of methods including FT-IR spectroscopy, X-ray powder diffraction (XRPD), thermogravimetric (TG) analysis, and N₂-adsorption/desorption analyses. The results showed that for Pb²⁺, the removal efficiency maximum (>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.