A novel cross-linked polyzwitterion/anion having pH-responsive carboxylate and sulfonate groups for the removal of Sr²⁺ from aqueous solution at low concentrations

A novel cross-linked polyzwitterion (CPZ) was synthesized via cycloterpolymerization of N,N-diallyl-N-sulfopropylammonioethanoic acid (92.5 mol%), a cross-linker 1,1,4,4-tetraallylpiperazinium dichloride (7.5 mol%), and sulfur dioxide (100 mol%) in the presence of azoisobutyronitrile in dimethylsulfoxide at 60 C. CPZ, upon treatment with NaOH, was converted into a cross-linked polyzwitterion/anion (CPZA). The experimental data for the adsorption of Sr²⁺ on CPZA fitted the pseudo-second-order kinetic model and Freundlich as well as Temkin isotherm models. The adsorption process was spontaneous and exothermic in nature with negative values for both ΔG and ΔH. The low activation energy of 7.18 kJ/mol indicated the adsorption as a favorable process. The removal for the initial concentrations of 200 ppb and 1000 ppb (i.e., 1 ppm) of Sr²⁺ ions was observed to be 87% and 92%, respectively. An efficient synthetic access to the resin and excellent adsorption capacity and desorption would enable its use in the treatment of radioactive nuclear waste containing Sr²⁺ ions. The CPZA provided an opportunity to test the efficacy of a zwiiterionic/anionic group in the removal of Sr²⁺ ions in low concentrations.