Adsorptive features of polyacrylamide-apatite composite for Pb2+, UO22+ and Th4+

Micro-composite of polyacrylamide (PAA) and apatite (Apt) was prepared by direct polymerization of acrylamide in a suspension of Apt and characterized by means of FT-IR, XRD, SEM and BET analysis. The adsorptive features of PAA-Apt and Apt were then investigated for Pb2+, UO22+ and Th4+ in view of dependency on ion concentration, temperature, kinetics, ion selectivity and reusability. Experimentally obtained isotherms were evaluated with reference to Langmuir, Freundlich and Dubinin-Radushkevich (DR) models. Apt in PAA-Apt had higher adsorption capacity (0.81, 1.27 and 0.69 mol kg−1) than bare Apt (0.28, 0.41 and 1.33 mol kg−1) for Pb2+ and Th4+, but not for UO22+. The affinity to PAA-Apt increased for Pb2+ and UO22+ but not changed for Th4+. The values of enthalpy and entropy changed were positive for all ions for both Apt and PAA-Apt. Free enthalpy change was ΔG < 0. Well compatibility of adsorption kinetics to the pseudo-second-order model predicated that the rate-controlling step was a chemical sorption. This was consistent with the free energy values derived from DR model. The reusability tests for Pb2+ for five uses proved that the composite was reusable to provide a mean adsorption of 53.2 ± 0.7% from 4 × 10−3 M Pb2+ solution and complete recovery of the adsorbed ion was possible (98 ± 1%). The results of this investigation suggested that the use of Apt in the micro-composite form with PAA significantly enhanced the adsorptive features of Apt.