Temperature and pH-dependent swelling and copper(ii) adsorption of poly(N-isopropylacrylamide) copolymer hydrogel

Abstract

Poly(N-isopropylacrylamide-co-acrylamide-co-maleic acid) (P(NIPAM-AM-MA)) hydrogel has been synthesized by free radical polymerization. The incorporation of functional monomer in the hydrogel was confirmed by Fourier transform infrared spectrometer (FTIR). Swelling measurements and differential scanning calorimeter (DSC) were employed to investigate the volume phase transition of P(NIPAM-AM-MA) hydrogel. P(NIPAM-AM-MA) shows higher swelling ratio and LCST than poly(N-isopropylacrylamide) (PNIPAM) and poly(N-isopropylacrylamide-co-acrylamide) (P(NIPAM-AM)). The adsorption behavior of copper(II) (Cu²⁺) ions on P(NIPAM-AM-MA) hydrogel is temperature and pH-dependent. The adsorption isotherm is well fitted by the Freundlich model and the adsorption kinetics can be described by the pseudo-second order equation. In 20 mL of CuSO₄ solution containing 4 mg Cu²⁺, the adsorption capacity reaches 24.4 mg g⁻¹ dry hydrogel at 30 °C and pH = 5. After the volume phase transition, the Cu²⁺-loaded P(NIPAM-AM-MA) hydrogel can release almost 90% of absorbed water containing few heavy metal ion. Synchrotron radiation small-angle X-ray scattering (SAXS) was used to study the effect of temperature and Cu²⁺ ions on the microstructure of P(NIPAM-AM-MA) hydrogel. The occurrence of volume phase transition increases the size of cross-linked domains and mass fractal dimension, while the presence of Cu²⁺ ions has an opposite effect. The adsorbed hydrogel can be easily regenerated by hydrochloric acid and reused in the following adsorption process. This pH and temperature sensitive hydrogel may be used for water purification and enrichment of heavy metal ions.