Hemicellulose-based hydrogels promising for metal ion removal

Polysaccharides are the main constituent of plants and the most abundant natural material in the world. In wood, cellulose represents approximately 40% and hemicelluloses 20-30% of the dry mass. Hemicelluloses can nowadays be isolated in large quantity by different means. Spruce O-acetyl galactoglucomannan (GGM), which is the major hemicellulose type in softwood, can be recovered from the waste stream of mechanical pulping processes using spruce wood as the raw material or isolated directly from wood by pressurized hot-water extraction. In this work, functional hydrogels using GGM as a crosslinker was synthesized for the removal of toxic metals and metalloid ions from aqueous solutions. In a first reaction step, GGM was converted in a transesterification using glycidyl methacrylate (GMA) to produce GGM macromonomer which was ready to be applied as a crosslinker in a free-radical polymerization. With the GGM macromonomer as a crosslinker, synthesis was carried out using [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MeDMA) as a positively charged monomer. The polymerization resulted in hydrogels with different swelling rates, depending on the chain length of the GGM crosslinker and the density of the functionalization of the GGM chain. The hydrogels revealed a high surface area and showed high ability to remove arsenate and chromate ions from aqueous solutions with an ion exchange mechanism. Moreover, to improve the mechanical strength of the hydrogels, nanocellulose as a reinforcement matrix was successfully incorporated. Thus, the high potential of using these structured composites in water purification has been underlined.