Cadmium and lead ion capture with three dimensionally ordered macroporous hydroxyapatite

The capability of three dimensionally ordered macroporous (3DOM) hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂ (HAp), to capture cadmium and lead ions from their respective salt solutions was studied as a function of temperature. Synthesis of 3DOM material was achieved by colloidal crystal templating of polystyrene spheres (1 μm diameter) using calcium nitrate (Ca(NO₃)₂) and orthophosphoric acid (H₃PO₄) as precursors. The macroporous product consisted primarily of HAp (>80% depending on the sintering temperature) together with amorphous calcium phosphate. The sorption ability of 3DOM material to Cd/Pb ion was benchmarked against HAp powder prepared via the same route without the template. On the basis of quantitative X-ray diffraction (XRD) and analytical transmission electron microscopy (ATEM) 3DOM HAp demonstrated a higher uptake of cadmium, viz. x = 0.71 in Ca_10-xCd_x(PO₄) ₆(OH)₂ than nonporous HAp (x = 0.42). The incorporation of Cd was homogeneous in the 3DOM HAp crystals (as compared to the powder) leading to a decrease in lattice parameters as Cd²⁺ has a smaller ionic radius compared to Ca²⁺. A preference for Cd to enter the Ca ^II tunnel site of HAp was consistent with this being the readily exchangeable site. The lead-bearing solution acted to collapse the macropores through the rapid crystallization of pyromorphite (Pb₁₀(PO ₄)₆(OH)₂) via a dissolution-precipitation mechanism, possibly promoted by the amorphous component, that overwhelmed HAp ion exchange. The rapid crystallochemical incorporation of Cd and fixation of Pb by 3DOM HAp demonstrates the potential of thin-walled porous structures for the treatment of contaminated waters.