Novel cross-linked polyphosphonate for the removal of Pb ²⁺ and Cu ²⁺ from aqueous solution

A novel cross-linked polyzwitterionic acid (CPZA) was synthesized via cyclopolymerization of diallylaminomethylphosphonic acid (90 mol %) and 1,1,4,4-tetraallylpiperazinium dichloride (10 mol %), a cross-linker, with sulfur dioxide in the presence of azoisobutyronitrile (AIBN) in dimethylsulfoxide (DMSO) at 65 °C. CPZA, upon treatment with NaOH, was converted into a cross-linked anionic polyelectrolyte (CAPE). The experimental data for the adsorption of Pb ²⁺ and Cu ²⁺ on CAPE fitted the Lagergren second-order kinetic model as well as Langmuir and Freundlich isotherm models. The adsorption capacity of Pb ²⁺ was higher than that of Cu ²⁺, and the rate of adsorption of both metals was found to be almost equal. The maximum adsorption capacity of Pb ²⁺ and Cu ²⁺ were determined to be 2.76 and 2.22 mmol g ^-1, respectively. The adsorption process was spontaneous and endothermic in nature with negative and positive values for ΔG and ΔH, respectively. The relatively low activation energies of 25.5 and 27.1 kJ/mol for Pb ²⁺ and Cu ²⁺, respectively, indicated the adsorption as a favorable process. The excellent adsorption and very good desorption efficiency implied the efficacy of the resin in removing as well as recovering the metal ions from aqueous solution. An efficient synthetic access to the resin would enable its use in the treatment of contaminated wastewater.