Poly(N-ethylacrylamide) Hydrogels for Lignin Separation

Poly( N -ethylacrylamide) hydrogel (PEAM) and N -ethylacrylamide bulk hydrogel with microstructure (EAM-BHM) are prepared. The lower critical solution temperature (LCST) of PEAM is determined to be 73 °C by DSC, and the microstructures of the hydrogels are characterized by SEM. When the microgel particles are incorporated into the matrix of EAM hydrogel, the formed bulk hydrogels have a faster swelling rate and a higher swelling ratio compared to those of the conventional PEAM hydrogels. The swelling diffusion mechanism in PEAM hydrogel changes from Fickian to non-Fickian when the cross-linker content is increased to 8 wt %. Both EAM-BHMs and conventional PEAM can be used to separate lignin at a high temperature (∼55 °C, typical in a pulp mill). Due to the microgel particles, the EAM-BHMs have a higher swelling ratio in lignin solution while the large surface area of the microgel also increases the adsorption of lignin on the hydrogel, thus decreasing the separation efficiency. The effects of lignin concentration, separation temperature, and salt on the separation efficiency are also studied.