Purification process coupling membrane filtration and adsorption for deep eutectic solvent (DES) recycling after lignin extraction

Deep eutectic solvents (DES) are increasingly used for lignocellulose fractionation due to their low toxicity, biodegradability, and high lignin solvatation capacity. However, the industrial deployment of DES-based lignocellulose fractionation remains strongly limited by large solvent requirements and solvent recyclability issues arising from progressive accumulation of lignin degradation products and hemicellulose fragments during repeated extraction cycles. This study develops an integrated purification strategy based on membrane filtration and adsorption for impurity removal from DES liquors. Selected ultrafiltration and nanofiltration membranes, and adsorption resin, showed relevant DES purification performances, with lignin degradation products and hemicellulose removal. Subsequently, an integrated process was applied, consisting of ultrafiltration, followed by continuous adsorption and nanofiltration, forming a robust train capable of recovering purified DES after evaporation. Continuous adsorption removed approximately 50% of lignin degradation products, while nanofiltration concentrated nearly 70% of total hemicelluloses in the retentate stream. This process not only limits impurity accumulation in recycled DES but also facilitates the selective compounds recovery for further valorization. Compared with untreated recycled DES, the purified recycled DES showed lignin extraction yield (∼90%) close to that of fresh DES (∼70%), demonstrating the restoration of solvent performance. In contrast, untreated recycled DES led to significant modification of fractionation performances (∼150%), highlighting the strong impact of impurity buildup on apparent extraction yields. This work demonstrates the potential of hybrid membrane-adsorption processing for DES purification and reuse in lignocellulosic biorefineries. The approach provides a practical route toward cleaner solvent cycles, reduced DES consumption, and improved environmental and economic performance, with clear opportunities for further optimization and scale-up.