Facile synthesis and characterization of thermo-magneto-responsive poly(N-isopropylacrylamide)-magnetite composite hydrogel and its adsorption-desorption study on chromium (III)

Poly(N-isopropylacrylamide)-magnetite composite hydrogels with thermo-magneto-responsiveness were synthesized via layer-by-layer coatings with and without using silanization precursor, namely 3-(trimethoxysilyl)propyl methacrylate (MPS) as bridging agents for polymerizing N-isopropylacrylamide (NIPAM). Dynamic light scattering (DLS) was employed to monitor the temperature dependent change in hydrodynamic diameters, D_h of the hydrogels. The morphological transition from the stage of bare magnetite nanoparticles (MNPs) until PNIPAM-magnetite composite hydrogels was inspected by using transmission electron microscope (TEM). MPS-modified silica-PVP-MNPs showed the lowest suspension stability whereas, PVP-functionalized MNPs showed the sluggish response towards low gradient magnetic field. PNIPAM-silica-PVP-MNPs was selected as adsorbent for the Cr³⁺ ions the adsorption test. In the temperature manipulated adsorption-desorption test, the adsorption of Cr³⁺ prevailed as temperature increased from 25 °C to 50 °C and the release of Cr³⁺ into bulk phase took place when the reaction solution was cooled back to 25 °C as depicted by the drop in normalized adsorption capacity, q_e/C_e from 36.22, 59.65 and 54.76 to 17.87, 36.21 and 41.67 for C₀ of 20, 60 and 100 mg L⁻¹.