Study On Preparation And Properties Of High Flux, Anti-Fouling Nanofiltration Membranes And Their Application In Separation Of Antibiotics

Separation technique plays an important role in modern industry, especially in chemical engineering. As one of the most attractive separation technique, membrane separation technique has been believed as an alternative to traditional separation technique such as extraction, distillation in recent years for their energy-conservation, low emssion, high automation and low occupation area. Among all of membrane separation process, nanofiltration（NF） is generally regarded as a molecular scale separation technique that is suitable for small organic molecules and inorganic salts since the nanofiltration membranes own unique pore structures and charged properties. As a pressure driven membrane separation process without phase transition during the separaion process, nanofiltration can be empolyed for a number of active molecules separation including antibiotics, pharmaceutical molecules and catalysis. In nowadays, most of commercial nanofiltration membranes show the low permeability, poor anti-fouling and bad anti-cholorine properties limiting their application. The design of novel composite NF membranes with high permeability, good anti-fouling according to the size of targeted molecules becomes the focus in this filed. Herein, novel hydrophilic composite nanofitlration membranes have been developed through different membrane manufacture method for antibiotics separation in diferernt scale. The structure, properties and separation performance of the novel composite membranes have been investigated in detail. In addition, the effects of the operation conditions on the separation performance to the antibiotics of the novel composite NF membranes were also studied for enriching the application of NF membranes.At first, the novel polyamide composite NF membranes have been developed by interfacial polymerization of amino-terminated PEG and trimesoyl chloride（TMC）. The optimum separation performance has been obtained via changing the conditions of interfacial polymerization and tuning the molecular weight and molecular strucutre of amino terminated PEG. The chemical composition and surface structure of the novel polyamide composite NF membranes were characterized by FT-IR, XPS, SEM, zeta potential and water contact angel analysis. The pore size and pore size distribution have been investigated by solute transport method, and the anti-fouling and anti-chlorine of composite membranes have been studied in detail as well. The membranes fabricated by amino terminated PEG with molecualr weight of 600 g mol^-1 and by 4 arm amino terminated PEG with molecular weight of 2000 g mol^-1 exhibit high hydrophilicity with water contact angle lower than 36 o, and high permeabilities with 13.2 and 9.4 L m^-2 h^-1 bar^-1, respectively, which are two times higher than that of traditional polyamide membranes prepared from m-phenylenediamine（MPDA） and TMC. The mean effective pore radius of the novel composite membranes is 0.42 and 0.38 nm, respectively, and the MWCO of the membranes are about 677.8 g mol^-1 and 496.2 g mol^-1, respectively. The membranes show positive charge with Mg Cl2 rejection of 95.4% and 98.1%, respectively. Compared with traditional polyamide composite NF membranes fabricated by MPDA which is marked as MPDA/TMC, the novel polyamide composite NF membranes exhibit much better anti-fouling and antichlorine performance due to the saturated CH2-O-CH2 in the main chain of polyamide selective layer.Secondly, the novel polyamide composite NF membranes is developed for antibiotics sepatation.When applied to separate positive antibiotics, the solution flux of the novel polyamide composite NF membranes was 3.5 times higher than that of MPDA/TMC composite nanofiltration membrane under 5 bar(7.8 L m^-2 h^-1) with similar rejection above 92.4 %, exhibiting potential application for positive antibiotic separation. The pore size of the novel polyamide NF membranes can be finely tuned further to enhance the rejections to negative or amphipathic antibiotics with smaller molecular weight, maintaining the solution flux three times higher than that of traditional polyamide membranes. The separation performance of antibiotics of the composite NF membranes can be optimized by changing the operation conditions since the mass transfer process and concentration polarization of the membranes can be adjusted by changing operation conditions including feed temperature, operation pressure and flowrate of feedstock. The modified composite NF membranes show solution flux as 61.5 L m^-2 h^-1 with cefalexin（CA） rejection as high as 93.1 % when the temperature of feed solution is 30 oC under flowrate of 70 L h^-1 and 8 bar. The rejection to antibiotics of the modified composite membranes can even maintain above 90% when the feed concentration is as high as 800 ppm. The flux is very stable and esay to recover to initial value once the membranes were washed by distill water when the membranes were applied in long term separation testment.At last, we developed a novel loose composite NF membranes by co-coating of gallic acid（GA） and polyethyleneimine（PEI）. The reaction mechanism between gallic acid and polyethyleneimine was explored by studying the membrane forming process of the GA/PEI freestanding membranes, the self-healing behavior of GA/PEI freestanding membranes, chemical composition and surfance struture of the GA/PEI freestanding membranes. Then, the novel GA/PEI composite NF membranes was developed and the effects of coating conditions on the separation performance of GA/PEI composite NF membranes were investigated in detail. Furthermore, the physical, chemical and surface strucutre of the GA/PEI composite membranes were systematically investigated as well. The mean effective pore radius of the novel loose GA/PEI composite membranes is about 0.53 nm with molecuar weight cut off of 950.0 g mol^-1. The membranes exhibit high permeablity above 18.0 L m^-2 h^-1 bar^-1 and excellent hydrophilicity with water contact angle below 40 o. The membranes show excellent anti-fouling performance as well due to their excellent hydrophilicity. Moreover, the membranes were stable in organic solvents like alcohol and THF, exhibiting excellent solvent resistant properties. The novel loose membranes show excellent comprehensive separation performance to organic molecular with molecualr weight ranging from 700 to 1000 g mol^-1 like azithromycin（AT） and rose bengal（RB）. When used for AT separation, the membranes shows solution flux as high as 41.3 L m^-2 h^-1 and rejections as high as 96.3% with flowrate of feedstock of 70 L h^-1 at 25 oC under 5.0 bar.