Fractionation of black liquor using ZnO nanoparticles/PES ultrafiltration membranes: Effect of operating variables

Abstract

Lignin and hemicelluloses are considered potential raw materials for biorefineries but, their availability from waste stream, such as black liquor (BL) is still a challenge. In this study, four types of ZnO-based polyethersulfone membranes (with 0, 0.2, 0.5 and 1% ZnO concentration) were prepared and studied for the fractionation of industrial BL into hemicellulose and lignin rich streams. The membranes were characterized in terms of pure water flux, permeability, hydrophilicity, morphology by scanning electron microscopy and atomic force microscopy, BL filtration tests and fouling. The membranes with 0.5% ZnO concentration, offered best performance and suffered with lowest flux reduction due to fouling, 11.2% after 75 min of BL filtration at 4 bar. Therefore, these membranes were used to optimize the filtration process variables namely, temperature, pressure and feed concentration, aiming to maximize their fractionation efficiency. Increasing temperature and pressure favored the BL permeate flux and membranes showed a good stability up to 80 °C. However, at high temperature, higher fouling and lower separation efficiency was observed. In this context, 60 °C was suggested for lowest fouling (5.88% flux reduction) and offered a higher separation efficiency with hemicellulose and lignin rejection in the range 65–80% and 37–61%, respectively. Further, the feed concentration effect showed that by diluting the feed, the rejection efficiency did not improve, and it only increased the filtration load by increasing feed volume. These findings imply that, under optimum conditions, 0.5% ZnO-based mixed matrix membranes can provide efficient BL fractionation, due to differences in lignin and hemicelluloses rejection, with minimal membrane fouling.