Metal Hydroxide Formation in DMAPAA Hydrogel and Novel Metal Ion Removal Method

Hydrogels based on copolymers of N-[3-(dimethylamino) propyl] acrylamide (DMAPAA) and N,N'-methylenebis(acryl amide) (MBAA) were prepared as media for selective metal ion removal. Polymerization mixtures were polymerized to create cylindrical shapes of 6 mm in diameter. The hydrogels were protonated when they were swollen in water and exhibited higher pH internally than the surrounding water because of ionic interaction. The pH of the hydrogel could be controlled by the adjusting the preparation conditions such as the centration of monomer or crosslinker. Metal ion removal experiments were carried out by swelling the hydrogels in aqueous solutions of metal ions. The metal ions diffused into the hydrogels to produce metal hydroxide in the hydrogel. The hydrogels containing metal hydroxides were easily separated from the solution without controlling the pH of solution or the sedimentation process. The amount of metal removed in the hydrogel increased with the concentration of the metal ions in solution. A target metal ion could be selectively removed from mixed metal ion solutions by controlling the pH of the hydrogel. The metal hydroxide with lower solubility product was preferentially removed by the hydrogel. Hydrogel containing metal hydroxide could be regenerated repeatedly by immersion in aqueous sodium hydroxide solution. These results suggest the possibility of replacing the coagulation and/or adsorption methods of metal ion removal with a novel method with a simple and low emission process.