Effect of Surface Modification of Polyamide-Based Reverse Osmosis Membranes by Glycerol Monoacrylate–Butyl Acrylate Copolymers on Antifouling

Suppression of membrane fouling is essential for making reverse osmosis (RO) membrane systems more economical. In the present study, we synthesized polymers bearing a glycerol monoacrylate moiety as an antifouling unit and a butyl acrylate moiety as a membrane-adsorbing unit. We modified RO membranes by immersion in solutions of the synthesized copolymers as a simple antifouling method. We evaluated the membrane antifouling performance by assessing its permeability to bovine serum albumin as a foulant. Compared with the pristine membrane, the copolymer-modified RO membrane had a higher normalized water permeability and longer water retention (24 h). This enhancement was attributed to the hydrophilicity of the glycerol monoacrylate moiety, membrane modification by the butyl acrylate moiety, and the formation of intermediate water with a small quantity of nonfreezing water in the polymer, as determined by differential scanning calorimetry

Preparation of Microfiltration Hollow Fiber Membranes from Cellulose Triacetate by Thermally Induced Phase Separation

For the first time, self-standing microfiltration (MF) hollow fiber membranes were prepared from cellulose triacetate (CTA) via the thermally induced phase separation (TIPS) method. The resultant membranes were compared with counterparts prepared from cellulose diacetate (CDA) and cellulose acetate propionate (CAP). Extensive solvent screening by considering the Hansen solubility parameters of the polymer and solvent, the polymer’s solubility at high temperature, solidification of the polymer solution at low temperature, viscosity, and processability of the polymeric solution, is the most challenging issue for cellulose membrane preparation. Different phase separation mechanisms were identified for CTA, CDA, and CAP polymer solutions prepared using the screened solvents for membrane preparation. CTA solutions in binary organic solvents possessed the appropriate properties for membrane preparation via liquid–liquid phase separation, followed by a solid–liquid phase separation (polymer crystallization) mechanism. For the prepared CTA hollow fiber membranes, the maximum stress was 3–5 times higher than those of the CDA and CAP membranes. The temperature gap between the cloud point and crystallization onset in the polymer solution plays a crucial role in membrane formation. All of the CTA, CDA, and CAP membranes had a very porous bulk structure with a pore size of ∼100 nm or larger, as well as pores several hundred nanometers in size at the inner surface. Using an air gap distance of 0 mm, the appropriate organic solvents mixed in an optimized ratio, and a solvent for cellulose derivatives as the quench bath media, it was possible to obtain a CTA MF hollow fiber membrane with high pure water permeance and notably high rejection of 100 nm silica nanoparticles. It is expected that these membranes can play a great role in pharmaceutical separation

Comparison of Fouling Behavior in Cellulose Triacetate Membranes Applied in Forward and Reverse Osmosis

Membrane fouling is inevitable during the membrane separation process. The difference in the driving force of reverse osmosis (RO) and forward osmosis (FO) affects the behavior of foulants. Thus, in this work, we examined the behavior of different foulants during the FO or RO process, including before and after physical cleaning of the membrane. The foulants used were alginate (Alg-Na), humic acid (HA), bovine serum albumin (BSA), and colloidal silica. The commercial cellulose triacetate membrane was used for both FO and RO processes to investigate the behavior of foulants fairly. During the RO process, the formation of the gel network between alginate and calcium ions tends to accumulate on the surface of the membrane, leading to the formation of a dense layer of the foulant, consequently decreasing the flux. Having HA in the feed, RO and FO processes had a similar flux decline, whereas having alginate and BSA, the flux decline during the RO process was higher than the FO process. When colloidal silica was presented in the feed, the membrane in the RO process had constant flux throughout the testing, whereas the membrane in the FO process had a remarkable decrease in flux. Silicas were adhered more on the membrane tested in FO. It was presumed that the reverse salt diffusion facilitates the aggregation of the silica on the membrane surface, leading to a reduction of flux by cake-enhanced concentration polarization in the foulant layer of silica. Therefore, the foulant properties, type of draw solution, the structure of the foulant layer, and the interaction between the foulant and membrane are important to consider in the fouling behavior in RO and FO processes. This understanding of the fouling behavior in the FO process will lead to the development of the optimum FO process

Structural Studies of Bulk to Nanosize Niobium Oxides with Correlation to Their Acidity

Hydrated niobium oxides are used as strong solid acids with a wide variety of catalytic applications, yet the correlations between structure and acidity remain unclear. New insights into the structural features giving rise to Lewis and Brønsted acid sites are presently achieved. It appears that Lewis acid sites can arise from lower coordinate NbO₅ and in some cases NbO₄ sites, which are due to the formation of oxygen vacancies in thin and flexible NbO₆ systems. Such structural flexibility of Nb–O systems is particularly pronounced in high surface area nanostructured materials, including few-layer to monolayer or mesoporous Nb₂O₅·<i>n</i>H₂O synthesized in the presence of stabilizers. Bulk materials on the other hand only possess a few acid sites due to lower surface areas and structural rigidity: small numbers of Brønsted acid sites on HNb₃O₈ arise from a protonic structure due to the water content, whereas no acid sites are detected for anhydrous crystalline H-Nb₂O₅