Adsorption and desorption of calcium ions by temperature swing with copolymer of thermosensitive and chelating components grafted on porous ethylene vinyl acetate disk

A novel adsorbent composed of a copolymer of a thermosensitive component, N-isopropylacrylamide (NIPAM), and a chelating component, N-(4-vinyl)benzylethylenediamine (VBEDA), was developed, and the adsorption and desorption behaviors of Ca2+ ions were investigated. The copolymer of NIPAM and VBEDA was grafted onto the pore surfaces of a porous ethylene vinyl acetate (EVA) disk by a plasma-initiated polymerization method. The copolymerization ratios of VBEDA were 3 and 6 mol%. Although a hydrophilic component, VBEDA, was copolymerized with NIPAM, the transition temperature of the copolymer was almost the same as that of poly(NIPAM). The permeability through the polymer-grafted EVA disk strongly depended on the temperature. It was quite low below the transition temperature of the grafted polymer, and increased drastically when the temperature exceeded the transition temperature. The polymer-grafted EVA disk adsorbed Ca2+ ions at low temperatures and the adsorption amount decreased significantly when the temperature exceeded the transition temperature of the grafted polymer, and repeated adsorption and desorption by temperature swings were possible. Furthermore, it was suggested that one Ca2+ ion was captured by three VBEDA groups, and this multi-point coordination bond was destroyed by heating as a result of shrinkage of the grafted poly(NIPAM-co-VBEDA) chains, leading to desorption of Ca2+ ion.