Separation of lignin from aqueous mixtures by ionic and nonionic temperature-sensitive hydrogels

Three ionic and nonionic temperature-sensitive hydrogels, poly(N-isopropyla crylamide) (PNIPAAm), poly(N-isopropylacrylamide-co-acrylic acid) (NIPAAm-A A), and poly(N-isopropylacrylamide -co-hydroxypropyl methacrylate) (NIPAAm- HPMA), are synthesized and used to concentrate the lignin solutions of inte rest in the pulp and paper industry. The interactions between hydrogels and solutes play an important role in the separation. Because of the absorptio n of lignins in the hydrogels, the separation efficiency of the PNIPAAm hyd rogel is low. By introducing the hydrophilic monomers AA and HPMA into the NIPAAm hydrogel, the separation efficiencies are increased because of the i onic repulsion between lignin and the incorporated hydrophilic groups. In a ddition, the separation efficiencies are affected by the extent of cross-li nking and the solute concentration. The time dependence of the separation e fficiency suggests that the separation is diffusion-controlled.