Temperature- and pH-Responsive Super-Absorbent Hydrogel Based on Grafted Cellulose and Capable of Heavy Metal Removal from Aqueous Solutions

In this work, we prepared highly swelling, stimuli-responsive hydrogels capable of the highly efficient adsorption of inorganic pollutants. The hydrogels were based on hydroxypropyl methyl cellulose (HPMC) grafted with acrylamide (AM) and 3-sulfopropyl acrylate (SPA) and were synthesized via the growth (radical polymerization) of the grafted copolymer chains on HPMC, which was activated by radical oxidation. These grafted structures were crosslinked to an infinite network by a small amount of di-vinyl comonomer. HPMC was chosen as a cheap hydrophilic and naturally sourced polymer backbone, while AM and SPA were employed to preferentially bond coordinating and cationic inorganic pollutants, respectively. All the gels displayed a pronounced elastic character, as well as considerably high values of stress at break (several hundred %). The gel with the highest fraction of the ionic comonomer SPA (with an AM/SPA ratio = 0.5) displayed the highest equilibrium swelling ratio (12,100%), the highest volume response to temperature and pH, and the fastest swelling kinetics, but also the lowest modulus. The other gels (with AM/SPA = 1 and 2) displayed several times higher moduli but more modest pH responses and only very modest temperature sensitivity. Cr(VI) adsorption tests indicated that the prepared hydrogels removed this species from water very efficiently: between 90 and 96% in one step. The hydrogels with AM/SPA ratios of 0.5 and 1 appeared to be promising regenerable (via pH) materials for repeated Cr(VI) adsorption.