Cellulose Acetate Hollow Fiber Nanofiltration Membrane Separation Performance

Cellulose acetate hollow fiber membranes now are commonly used as reverse osmosis membranes for its relatively low permeability and solute separation capability. In conderration of the fact that cellulose acetate hollow fiber membrane possesses the advantages of large specific surface area, low cost, excellent chlorine resistance and self reinforcement, it is also of great interest to develop cellulose acetate hollow fiber membranes for nanofiltration, through which the CA hollow fiber membranes could be widely used in the fields of water treatment and solute separation and concentration of the industries of medicine, biology, food and chemistry. Therefore, in this paper, cellulose acetate hollow fiber NF membranes were manufactured from cellulose acetate hollow fiber RO membranes by inversion method, through which the water flux and solute separation capability of the original membrane were significantly improved with no sacrifice of mechcanical strength.Cellulose acetate hollow fiber nanofiltration membranes were fabricated through surface modification of the CA hollow fiber reverse osmosis membranes through hydrolysis using NaOH solution followed by carboxymethylation using chloroacetic acid solution. The effects of the parameters of hydrolysis process including the NaOH concentration, hydrolysis time and hydrolysis temperature, and the conditions of carboxymethylation process such as the chloroacetic acid concentration, reaction time, reaction temperature and solvent of chloroacetic were systematically studied.The surface properties of the resulting membranes were characterized by using Attenuated total reflectance infrared (ATR-IR), Scanning electron microscopy (SEM), Zeta potential Analyzer and contact angle measurement instrument. The separation properties of the obtained membranes in terms of solute rejection rate, water permeability and molecular weight cut-off (MWCO) were studied through pressurized cross-flow permeation test with aqueous solution containing different modle solutes including NaCl, Na2SO4, and PEG of different molecular weights under different operating conditions. The modified cellulose acetate hollow fiber membranes were used for the treatment of dyeing wastewater through pressurized cross-flow permeation tests and submerged filtrations, respectively, under different conditions.The study of the surface properties of the modified membrane showed that modification could be limited only on the surface of the original membrane and the inner structure of original membrane could be maintained through regulating the parameters of the processes of hydrolysis and carboxymethylation. The tendece of the membrane surface to be electric neutrality indicated that plenty of hydroxyl groups were formed on the surface of the hydrolyzed membrane after hydrolysis. While, in the process of carboxymethylation using chloroacetic acid made, the hydroxyl groups on the surface of the hydrolyzed membrane were partilly replaced by carboxymethyl groups, which made the surface of the final resulting membrane be more negetivly. Additionally, contact angle measurements also revealed that the membrane surface became more hydrophilic after hydrolysis and carboxymethylation.The study of the separation properties of the resultant membranes indicates that the pore size and the solute separation capability of the modified membrane were largely affected by parameters of the hydrolysis and carboxymethylation processes including NaOH concentration, hydrolysis time, hydrolysis temperature, chloroacetic acid concentration, reaction time, reaction temperature as well as the solvent of chloroacetic acid solution. The optimum conditions for the modification of cellulose acetate reverse osmosis for the preparation of nanofiltration membrane were as follows:NaOH concentration=0.1M, hydrolysis time=10min, hydrolysis temperature=20℃, chloroacetic acid concentration=3%wt/v, reaction time=1h, reaction temperature=20℃, solvent of chloroacetic acid solution=mixture of ethanol and water (v/v=1/1). The modified membrane prepared under the optimum conditions exhibited improved solute selectivity and pure water flux of up to21.0l/m2·h at0.5Mpa and20℃, which was much more than that of the oringinal membrane (about12.5l/m2·h under the same test ing condition).The study of the separation performance of the resulting modified membrane under different oprating conditions indicated that the water flux and rejection rate to electrolyte solution were influenced by the feed concentration, operating pressure and operating time. With increasing the feeding concentration or operating pressure, the water flux ascended dramatically but the rejection rates to NaCl and Na2SO4changed slightly. Additionally, long-term tests also revealed that the modified membrane possessed good performance stability.The application of the resultant membrane to the treatment of dye aqueous solution was conducted through cross-flow permeation tests with aqueous solutions containing NaCl and different types of dyes such as Methyl red, neutral red, Methylene blue, Rhodamine B, Methyl blue and Congo red. The dye removal rate and permeate flux of the modified membrane to anionic dye aqueous solutions were better than those of the original membranes. It was found that both the Donnan and sieving effects influenced the separation performace of modified membrane to dye aqueous, and the dyes with higher molecular weight and/or more negative charges would be more effectively rejected by the modified membrane. The rejection rates to Methyl red, Methyl blue and Congo red were relatively higher and were82.1%,99.9%and99.2%, respectively, but the removal rates to Methylene blue and Rhodamine B were relatively lower. The operating pressure, NaCl concentration, operating time as well as the feed dye concentration affected the separation performance of modified membrane to dye aqueous solution. Furthuremore, the modified membrane could also be used to treat dye aqueous solution through submerged nanofiltration. The removal rates of the modified membrane to Methyl blue and Congo red were99.4%and99.8%, respectively, under the trans-membrane pressure of80kPa and20℃. Dye removal tests demonstrated that carboxymethylated cellulose acetate hollow fiber NF membrane possesses good prospects in the treatment of dyeing wastewater.