The influence of equilibration conditions and hydroxyapatite physico-chemical properties onto retention of Cu2+ ions

The influence of equilibration conditions and hydroxyapatite (HAP) physico-chemical properties onto retention of Cu2+ ions was studied in batch conditions. The amount of cation removed from the solution increased with increasing pH, reaching almost 100% at pH 3, 4 and 7 for 5 × 10−4, 10−3 and 5 × 10−3 mol/dm3 solutions, respectively. Contact time necessary for reaching equilibrium was found to increase with the increase of Cu2+ concentration. Kinetic and equilibrium data were best described by pseudo-second-order kinetic model and Langmuir theoretical model. The calculated values of separation factors and Gibbs free energy change confirmed that the sorption was spontaneous and thermodynamically feasible at room temperature. The experiments conducted using HAP samples with different physico-chemical characteristics have revealed that the amounts of sorbed Cu2+ depended mainly on the specific surface area and crystallinity of the applied powders. Desorption of Cu2+ was more efficient in acidic conditions than in the solution of competing cation—Ca2+. The samples with higher sorption capacities also demonstrated higher stability; consequently, from the aspects of both higher sorption and lower desorption, utilization of low-crystalline HAP samples with high specific surface area was superior for immobilization of Cu2+ ions. Taking into account molar Cu/Ca ratios, observed final pH changes, copper speciation in the function of pH and the results of X-ray diffraction analyses, conclusions about sorption mechanisms at different experimental conditions were derived.