Temperature swing adsorption of gold(III) ions on poly(N-isopropylacrylamide) gel

The possibility of temperature swing adsorption (TSA) of heavy metals on thermosensitive N-isopropylacrylamide (NIPA) gel has been examined. The NIPA gel has a high degree of affinity for Au(III) ions. The amount of Au(III) ions adsorbed on the NIPA gel is low at 10 °C and high at 50 °C, and the amount adsorbed increases in a stepwise manner with temperature around 32 °C, the lower critical solution temperature (LCST) of poly(NIPA). Au(III) ions tend to be well adsorbed on shrunken gel, but are adsorbed poorly on swollen gel. The adsorption capacity of 0.637 mmol-Au/g-dry gel at 50 °C was obtained from a Langmuir-type isotherm. The NIPA gel adsorbs and desorbs Au(III) ions reversibly by TSA between 50 °C and 10 °C. In addition, the enrichment of Au(III) ions in the diluted solution through TSA was successfully conducted. To discuss the mechanism of adsorption of Au(III) ions on the NIPA gel, the adsorption of Au(III) ions on non-thermosensitive N,N-dimethylacrylamide and thermosensitive N,N-diethylacrylamide gels, which have a structure similar to that of the NIPA gel, was also investigated. These gels adsorb Au(III) ions strongly; however, the amount of Au(III) ions adsorbed is independent of the temperature.