Ultrafiltration-based diafiltration for post-delignification fractionation of lignin from a deep eutectic solvent comprised of lactic acid and choline chloride

Lignocellulosic biomass is a valuable renewable resource that has the potential to be refined into multiple fractions of valuable biomaterials. Extracting and purifying lignin without compromising cellulose quality is a challenging goal. Deep Eutectic Solvents (DESs) have been reported as suitable delignification solvents, and for process development for cellulose extraction, lignin removal is essential. This work describes an investigation on continuous ultrafiltration (UF)-based diafiltration (DF) for recovering and purifying lignin dissolved in a DES-pulping stream comprising lactic acid and choline chloride following lignocellulose delignification. Experiments have been carried out in a stirred dead-end setup. The results indicate that lignin with a molecular weight of 200 g/mol or higher can be rejected by a polyethersulfone UF membrane with a molecular weight cutoff of 5000, reaching rejection values close to 0.85 for the feed solution with lignin concentration ranging from 4.5 to 30 g/L. It was discovered that membrane fouling and flux decline are DES concentration-dependent. A modeling approach was developed to predict the system size and operating conditions necessary to conduct the diafiltration process on a large scale, for which experimental data on lignin fouling at various feed concentrations and hydrodynamic flow characteristics were used. Based on the modeling results, lignin dissolved in DES-dark liquor (lignin-loaded DES) may well be separated and purified by a continuous UF-DF system.