Preparation and characterisation of a poly(acrylamidoglycolic acid-co-acrylamide) hydrogel for selective binding of Cu and application to diffusive gradients in thin films measurements

A poly(acrylamidoglycolic acid-co-acrylamide) poly(AAGA-co-AAm)] hydrogel was prepared by copolymerising 2-acrylamidoglycolic acid (AAGA) with acrylamide (AAm). The copolymer hydrogel composition and structure was characterised by FTIR spectroscopy and elemental microanalysis and found to contain 3.5 AAGA monomer units for each AAm monomer unit. This was similar to the monomer ratios used in the synthesis. The metal ion binding properties of the hydrogel were characterised for a range of metal ions (Cu²⁺, Cd²⁺, K⁺, Na⁺, Mg²⁺ and Ca²⁺) under varying conditions of pH, ionic strength, metal concentration and time. The hydrogel was shown to bind Cu²⁺ and Cd²⁺ strongly under non-competitive binding conditions, with binding capacities of 5.3 and 5.1 μmol cm⁻², respectively. The binding capacity of each metal decreased, under competitive binding conditions (with a range of metal ions present at 17.8 μN), to 1.3 and 0.17 μmol cm⁻², respectively, indicating stronger selectivity for Cu²⁺. The metal ions were readily recovered (>94%) by eluting with 2 M nitric acid solution for 24 h. The binding capacities for Cu²⁺ and Cd²⁺ were also found to decrease with increasing ionic strength and at pH values <5. The copolymer was found to have an equilibrium swelling ratio (q_w) of over 500 at a maxima of pH 5.4 and at low ionic strengths. Finally, the copolymer hydrogel was tested as a binding phase with the diffusive gradients in thin films technique. A linear mass vs. time relationship was observed for Cu²⁺ in synthetic Windermere water with a recovery of approximately 100%.