Removal of Zinc and Cadmium Ions Using a Cross-linked Polyaminophosphonate

A cross-linked polyzwitterionic acid (CPZA) containing aminophosphonic acid motifs was synthesized via cyclocopolymerization of diallylaminomethylphosphonic acid and cross-linker 1,1,4,4-tetraallylpiperazinium dichloride (10mol%) using tert-butylhydroperoxide as the initiator. The CPZA upon treatment with NaOH was converted to a cross-linked anionic polyelectrolyte (CAPE 5). The adsorption of Zn2+ and Cd2+ on CAPE 5 showed that the experimental data fit the Lagergren second-order kinetic model and Langmuir, as well as Freundlich isotherm models. The adsorption capacity of Zn2+ was found to be higher than that of Cd2+, and the adsorption process was spontaneous and endothermic in nature. The low activation energy of 11.3 and 9.3 kJ/mol for Zn2+ and Cd2+, respectively indicated the adsorptions as a favorable process. The resin demonstrated excellent adsorption efficiency in removing the metal ions from aqueous solution.