Synthesis and characterisation of a polyacrylamide–polyacrylic acid copolymer hydrogel for environmental analysis of Cu and Cd

Abstract

A polyacrylamide–polyacrylic acid copolymer hydrogel was prepared by the controlled hydrolysis of polyacrylamide in an alkaline solution of 10% sodium hydroxide. The structure and composition of the resulting copolymer hydrogel was approximately two acrylic acid units for every acrylamide unit. The capacity of the copolymer hydrogel to bind various metal ions was tested under a range of uptake conditions, with varying uptake time, pH and ionic strength. Ions such as Cu 2+ and Cd 2+ were bound more strongly to the copolymer hydrogel than the competing ions of Na +, K +, Ca 2+ and Mg 2+, particularly at pH>5, largely due to the increased acidity of these transition metal ions. Metals bound to the copolymer hydrogel were efficiently (>94%) eluted in 2 M HNO 3 solution. The copolymer was found to have a p K a of ∼4.5 and had an equilibrium swelling ratio of 120 at pH>6 with an ionic strength equivalent to 0.01 M NaCl, and a swelling ratio of 265 at low ionic strengths. This material should be of use for the recovery and separation of Cu 2+ and Cd 2+ ions, and trace environmental analysis applications such as the diffusive gradients in thin films (DGT) technique.