Removal of nickel ions from aqueous solution by low energy-consuming sorption process involving thermosensitive copolymers with phosphonic acid groups

Abstract

In order to remove metal ions from wastewaters, thermosensitive copolymers bearing sorption properties toward metal cations were prepared by free radical copolymerization between the N-n-propylacrylamide (NnPAAm) and the (dimethoxyphosphoryl)methyl 2-methylacrylate (MAPC1), followed by a hydrolysis of the phosphonated esters into phosphonic diacid groups (hMAPC1). The thermosensitivity and the sorption abilities of the resulting poly(NnPAAm-stat-hMAPC1) copolymers were studied. Lower Critical Solution Temperatures (LCST) of these copolymers ranged from 22°C to 26°C, depending on the molar ratio of phosphonated monomers and were lower than those obtained with usual poly(N-isopropylacrylamide)-based polymers. The influence of both the temperature and the pH on the sorption properties of the copolymers was evaluated for Ni2+ cations. The most interesting results were obtained for temperatures around the LCST, i.e. when the proximity of the complexing groups favored the sorption of metallic cations. Concerning the pH effect, the maximum sorption capacity was obtained at pH 7, i.e. in the absence of competition between the sorption of H+ and Ni2+ ions on the phosphonic acid groups. The influence of the molar ratio of metal ions and phosphonate moieties was also studied and different sorption mechanisms were proposed.