The cross-linked polyaminodiphosphonate for the removal of metal ions from wastewater is a cross-linked anionic polyelectrolyte synthesized via cyclocopolymerization of diallylaminomethyldiphosphonic acid and 1,1,4,4-tetraallylpiperazinium dichloride (10 mol %), a cross-linker, in the presence of tert-butylhydroperoxide in aqueous solution at 85° C., followed by treatment with NaOH. The cross-linked polyaminodiphosphonate may be used to remove copper and cadmium ions from wastewater. The adsorption process is spontaneous and endothermic in nature, with negative and positive values for ΔG and ΔH, respectively. The efficiency of Cu2+ and Cd2+ removal by the cross-linked polyaminodiphosphonate was found to be 96.8% and 93.8%, respectively.