A Novel Thermosensitive Poly(N-isopropylacrylamide)-Based Hydrogel Crosslinked with Cellulose Acetate, Exhibiting an Increased LCST, as a Removal Agent for Nickel(II) in Aqueous Solutions

The poly(N-isopropylacrylamide) (PNIPAM) and cellulose acetate (CA) hydrogels were synthesized both in the presence and absence of N,N\(^\prime\)-methylenebis(acrylamide) (NMBA) as a crosslinker. In the absence of NMBA, chemical crosslinking between PNIPAM and CA was demonstrated through free radical polymerization in acetone as the solvent, which has not been reported previously. These hydrogels exhibit smaller swelling ratios (2 to 22 water grams per xerogel gram) and larger compression moduli (from 0.39 to 2.62 MPa) than homo NIPA hydrogels. The lower critical solution temperature (LCST) values for these hydrogels increased from a range of 38 to beyond 50\(^\circ\)C, depending on the CA concentration, and were higher than those of homo PNIPAM hydrogels. The formulations with 50 wt.% solids and 10 and 15 wt.% CA were barely affected in their swelling capacity when heated to 50\(\,^\circ\)C. These hydrogels were used to remove \(\hbox {Ni}^{2+}\) from aqueous solutions. The adsorption capacity of these hydrogels ranged from 2 to 38 mg of \(\hbox {Ni}^{2+}\) per gram of xerogel. The hydrogels synthesized without NMBA, exhibited typical PNIPAM LCST values, so they were used to adsorb \(\hbox {Ni}^{2+}\) in solution and release it through the shrinkage process. When these hydrogels were reused four times in a row, the removal efficiency averaged 80% for each use and the overall remotion of \(\textrm{Ni}^{2+}\) ranged from 97 to 151 mg per gram of xerogel. A potential application for cleaning polluted waters with \(\hbox {Ni}^{2+}\) using PNIPAM-CA hydrogels is proposed herein, the cost of producing 1 g of these hydrogels in laboratory conditions is approximately 3 USD.