Sustainable fabrication of chitosan membranes with optimized performance for ultrafiltration

A key to the sustainable development of membrane technology is the green fabrication of biopolymer membranes, whose filtration performance is comparable to that made from petroleum-based polymers. In this study, chitosan membranes were fabricated via wet precipitation of the alkaline dope; both the composition and temperature of the coagulation bath were regulated to optimize the filtration performance for ultrafiltration. It is revealed by comparing the morphological characterization with the filtration-performance evaluation that the use of pure ethanol as the coagulant at a subzero temperature would be in favor of the formation of a laminated substructure, which can enhance the water permeance not at the cost of decreasing the rejection of micron-sized particles and macromolecules. Optical coherence tomography (OCT) was employed to in-situ characterize the film formation in an effort to understand how the variation in coagulating conditions could change the geometrical and topological characteristics of the resulting membrane. The OCT-based characterization highlights the key role of the Liesegang phenomenon in forming the hierarchical substructures, while resolving the synergistic effects emanating from the ethanol-induced film shrinkage and the structural preservation in the subzero environment.