Structural and chromatographic characterization of cation-exchange membranes based on carboxymethyl/nanofibrillated cellulose using lysozyme

Bio-based membranes are becoming highly-desired low-cost, environmentally friendly, and readily available supports for the separation and purification of biomacromolecules. In this work, weak cation-exchange and highly (> 95%) microporous (> 80 mu m) cellulose-based membranes were prepared from different weight ratios of carboxymethyl cellulose (CMC) as anionic polymer and cellulose nanofibrils (CNFs) as a stabilizing and structural filler, by the freeze-casting process and citric-acid (CA) mediated in situ cross-linking (esterification). It was ascertained that mono-esterified/grafted CA also contributes to the total carboxylic groups (1.7-2.6 mmol/g), while the CMC-induced CNF orientation affected the membrane's morphology and lysozyme (Lys) binding capacity. A static binding capacity (SBC) between 370 and 1080 mg/g, and equilibrium within 3.3 h for 1 g/mL Lys was thus achieved with increasing the total solid and CMC content by forming more isotropic microporous structures. The selected membranes were then packed in a chromatographic housing, analyzed for pressure drop, and evaluated for dynamic binding capacity (DBC), depending on the process performance (flow rates, Lys concentration). A DBC in the 165-417 mg/g range was determined at a throughput of 0.5 mL/min, and elution yield of 78-99% with > 95% recovery. The Lys adsorption and transfer were reduced by the increasing flow rate and membrane density due to compressibility issues, resulting in smaller and irregularly distributed pores and the unavailability of carboxylic groups. Although the DBC was still comparable with the commercial CIM (R) monoliths, the convection-based transport of molecules inside the membrane and the membrane stiffness needs to be improved in further research.