Synthesis and characterization of poly(pentamethylene-2,6-pyridinedicarboxy-late) and the study of its ability to absorb Pb(II), Cd(II) and Zn(II) ions

Poly(pentamethylenepyridine-2,6-dicarboxylate) is prepared by the stepwise reaction of equal amounts of 2,6-pyridinedicarbonyl dichloride and 1,5-pentanediol using 1,2-dichlorobenzene as an inactive solvent. The resulting homopolymer is characterized using Fourier-transform infrared spectroscopy, 1H and 13C-NMR, inherent viscosity, elemental analysis, thermal analysis, and scanning electron microscopy. Atomic absorption spectroscopy is used to determine metal ion concentrations. Metal uptake of the polymer towards three toxic divalent ions: Pb(II), Zn(II), and Cd(II) in aqueous solutions is tested by batch and column methods. The influence of different variables such as pH, temperature, concentration, and contact time on the sorption process is also taken into consideration. Pseudo-second-order kinetic models fit the metal adsorption process better than pseudo- first-order models. Langmuir, Freundlich, and Dubinin–Radushkevich adsorption isotherms are applied, establishing that the Langmuir isotherm is the best to represent it. The sequence for increasing adsorption capacity is: Pb(II) > Cd(II) > Zn(II). The sorption process of these ions on the polymer is favorable at high temperatures; the enthalpy values show an endothermic process. Furthermore, positive entropy values mean that randomness increases during the adsorption process. Column experiments are used by treating the loaded polymer with 0.1 M HNO3 and 0.1 M ethylenediaminetetraacetic acid, leading to a good percent recovery for HNO3.