Adsorption-desorption characteristic of thermo-magneto-responsive poly(N-isopropylacrylamide)-co-acrylic acid composite hydrogel towards chromium (III) ions

The thermo-responsive swelling-shrinking behaviour of the poly(N-isopropylacrylamide) (PNIPAM) hydrogels under the effect of temperature change have mechanistic function upon controlling the uptake and release of Cr³⁺ ions by the hydrogel networks. In this study, acrylic acid (AA) was co-polymerized with PNIPAM onto the silica-coated magnetic nanoparticles (MNPs) to form thermo-magneto-responsive (PNIPAM-co-AA)-silica-PVP-MNPs composite hydrogels with increased density of Cr³⁺ binding groups. Its temperature-manipulated adsorption-desorption efficiency as well as their magnetic phase separability were investigated. Chromium-loaded hydrogels shown apparent phase separation under influence of low gradient magnetic field. Desorption of Cr³⁺ gradually predominated as temperature increased from 298 K to 323 K. Under high initial Cr³⁺ concentration whereby physical adsorption is prevailing, the re-uptake of Cr³⁺ occurred when the solution being cooled down to 298 K. Furthermore, the adsorption kinetic analysis revealed that the adsorption of Cr³⁺ by (PNIPAM-co-AA)-silica-PVP-MNPs hydrogels was governed by surface precipitation of Cr³⁺.