Lignin separation from black liquor by mixed matrix polysulfone nanofiltration membrane filled with multiwalled carbon nanotubes

Abstract

Carbon nanotubes (CNT)/polysulfone (PSf) mixed matrix membranes (MMM) were prepared by the non-solvent induced phase inversion method and applied in the separation of lignin from black liquor (BL). Three different type of membranes were developed to minimise the fouling phenomenon, which is one of the major concerns regarding the application of polymeric membranes for BL fractionation. The effect of CNT on the filtration and morphological properties were investigated by analyzing pure water flux, porosity, mean pore size, scanning electron microscopy, atomic force microscopy, Fourier transform infrared spectroscopy and water contact angle. These analyses indicated that with increasing the nanotubes content (0–0.5%) resulted in an increase of ≈ 3 times in the mean pores size, water flux (14.8–132.5 Lm−2 h−1 at 15 bar), porosity (74.4–79.7%) and reduction of surface roughness of the prepared membranes. The performance of these membranes was analyzed in terms of lignin rejection and fouling resistance using standard lignoboost lignin solution at various pressures and compared with a commercial polyethersulfone membrane. It was observed that reduction in the permeate flux was lowest (~10% at 12 bar) in the 0.5% CNT-based membranes. However, in terms of lignin rejection, the highest (80–98%) and lowest (52–76%) values were achieved with the simple PSf membrane (0% CNT) and 0.1% CNT membrane, respectively. Further, fouling studies showed that through chemical cleaning, the initial flux was completely recovered for CNT-based membranes without disturbing their flux and rejection behavior. Among the 3 types of prepared membranes, 0.5% CNT membranes showed a good balance between permeate flux and lignin rejection. The application of these membranes for nanofiltration of industrial BL, pre-treated by ultrafiltration, allowed to reach 82–84% rejection for lignin and 37–42% for hemicelluloses. This suggests a significant degree of selective separation of lignin from BL using CNT-based MMM.