Lignin-based hydrogels with “super-swelling” capacities for dye removal

Lignin is a complex natural polymer and it is one of the main constituent of the lignocellulosic biomass. Moreover, it is a bio-renewable material and it is available in large amounts as by-product of residues from the wood and paper industries. Lignin-based hydrogels with high swelling capabilities were prepared by crosslinking poly (methyl vinyl ether co‑maleic acid) and different technical lignins in ammonium and sodium hydroxide solutions. The produced hydrogels showed a wide range of water absorption capacities varying from 13 to 130 g of water per 1 g of sample. It was observed that materials with higher water uptake poorer mechanical performance, as evaluated in terms of storage and loss modulus (G′ and G″, respectively). Methylene blue (MB) was used as a model dye to evaluate the adsorption and release capabilities of the lignin hydrogels. Results suggested that these hydrogels showed a high MB load efficiency, which ranged from 440 mg to 840 mg/g. On the contrary, the percentages of MB released depended on the negative surface charge of the hydrogels, showing values which ranged from 0.06 to 0.35%. Thus, these materials have potential to be used as adsorbents for the removal of organic dyes from waste water.