Effect of bacterial cellulose crystal form on its oil-water separation

Cellulose hydrogels have demonstrated outstanding performance in separating oil-in-water emulsions, particularly notable for efficient "water-removing" behavior. However, the strong intrinsic hydration ability of cellulose often limits separation flux, and the influence of cellulose crystalline forms on separation performance remains largely unexplored. In this study, bacterial cellulose (BC) hydrogel was used as the starting material. The crystal structure was converted to cellulose II via alkali treatment and to cellulose III through ethylenediamine treatment. The structure, wettability, and separation performance of the three crystalline cellulose hydrogels (BC-I, BC-II, and BC-III) were systematically investigated for various oil-in-water emulsions. The results showed that all three hydrogels exhibit superhydrophilicity and underwater superoleophobicity, achieving separation efficiencies exceeding 98.1% for all emulsions. However, a significant difference in separation flux was observed, in the order: BC-III > BC-I > BC-II. Notably, the BC-III hydrogel attained a maximum flux of 2806.5 L m^-2 h^-1 MPa^-1 for a cyclohexane-in-water emulsion. The performance differences are mainly attributed to the microstructural and hydration state changes induced by crystalline transformation: BC-II exhibited the lowest flux due to its dense fibrous network and high bound water content, whereas BC-III, while retaining a porous network, optimized water transport channels through its specific crystalline arrangement, resulting in the highest separation flux. This work reveals that the crystalline form of cellulose is a critical factor in regulating its oil-water separation performance, providing a novel strategy for designing high-flux cellulose-based separation membranes.