Preparation And Antibacterial Property Of Polyethersulfone Hybrid Ultrafiltration Membrane Containing Immobilized Lysozyme

One of the most serious operational problems in membrane application isbiofouling. Biofouling is defined as undesirable adsorption or precipitation ofmicroorganisms especially bacteria on or in the membrane to form biofilm, whichmay lead to the membrane permeability and flux loss, energy consumption increasing,as well as lower membrane life. So it is necessary to modify PES membrane toimprove its antibacterial properties, prevent the formation of biofilm, and finallyinhibit membrane biofouling. In this thesis, lysozyme, as a natural antibacterialprotein, was employed as a representative enzyme to improve membrane antibacterialabilities. The immobilization of lysozyme is important, since it can markedly improvethe stability of enzymes under extreme conditions. Halloysite nanotubes (HNTs) andGraphene oxide (GO) were used as raw materials to immobilize lysozyme.Immobilized lysozyme was then blended with polyethersulfone (PES) to preparehybrid antibacterial ultrafiltration membranes, and the performance evaluation ofwhich were studied. The main contents include:HNTs, as a raw materials, was modified with isocyano group (NCO), and thenreact with succinic anhydride (SAA) to graft into carboxyl group, HNTs-COOH. Next,lysozyme was covalently immobilized onto the modified HNTs (HNTs-COOH) using1-ethyl-s-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide(NHS). Fluorescent observation showed that the lysozyme was covalentlyimmobilized onto HNTs-COOH, which was HNTs-COOH-Ly. The hybridultrafiltration membranes were prepared with HNTs-COOH-Ly and then tested forcharacterizing their performance. The results show that the surface hydrophilicity ofhybrid membrane was significantly improved and the microscopic structure had slightchanges. The immobilized lysozyme in the membranes resulted in an increase inwater flux and a decrease in the rejection ratio for PEG20000. However the rejectionto PVA was kept at a high level. When the HNTs-COOH-Ly content in cast solutionof membrane was3%, the water flux of resulted membrane reached400L/(m2·h), and its rejections to PEG20000and PVA30000-70000were69%and99.9%, respectively.The mechanical strength of hybrid membranes was increased. Moreover, the hybridmembrane shows a good antibacterial property against Gram-negative bacteria(E.coli), and its antibacterial rate was63%.AlOOH was deposited on the surface of HNTs by the layer-by-layer (LBL)method, and designated as HNTs@AlOOH. Then the growth of LDH nanoplateletson the surface of HNTs@AlOOH was carried out by in-situ growth technique throughtransformation of AlOOH into LDH, which was denoted as HNTs@LDH. Next,lysozyme was immobilized on the HNTs@LDH, confirmed by Fluorescent images.The prepared hybrid ultrafiltration membrane with HNTs@LDH-Ly wascharacterized for its performance. The results indicated that mechanical strength ofhybrid membranes was increased since the addition of HNTs@LDH-Lynanocomposites. The water flux of hybrid membrane increased significantly with anincrease in the amount of HNTs@LDH-Ly, especially when the HNTs@LDH-Lycontent was3%, the water flux of the membrane reached528L/(m2·h), and therejections to PEG20000and PVA30000-70000were64%and99.9%, respectively.The antibacterial rates of the hybrid membrane against E. coli was reached87%.GO, having a large specific surface area and abundant functional groups,provides an ideal substrate for lysozyme immobilization. The lysozymeimmobilization on the GO sheets could take place readily without any cross-linkingreagents, which is mainly determined by electrostatic interaction of GO-lysozyme(denoted as GO-Ly(EI)). And when using coupling reagents (EDC and NHS), theamine groups of lysozyme can form amide bonds with the carboxylic groups of GOthrough this covalent interaction (designated as GO-Ly(CI)). The immobilizedlysozyme of GO-Ly(EI) and GO-Ly(CI) were blended with PES to prepare hybridultrafiltration membrane, respectively. With the increase of the immobilized lysozymecontent, mechanical strength of hybrid membrane was increased, the surfacehydrophilicity was significantly improved, and the hybrid membrane with GO-Ly(EI)has a higher hydrophilicity. When the GO-Ly(EI) and GO-Ly(CI) content was1.5%,the water flux of the membrane reached318L/(m2·h) and308L/(m2·h), and theantibacterial rates of the hybrid membrane against E. coli were reached68%and63%, respectively. The rejection to PEG20000decreased to about73%, but maintainedgood separation performance for PVA30000-70000.GO sheets can be readily reduced under a mild condition using L-ascorbic acid(L-AA). Chemically reduced graphene oxide (CRGO) is lack of surface oxygenfunctional groups and the electrostatic interaction between CRGO sheets is weakerthan that between GO sheets. CRGO has a hydrophobic surface and the adsorption oflysozyme on CRGO (CRGO-Ly) was governed by a hydrophobic interaction. Withthe addition of CRGO-Ly, the microscopic structure had some slight changes, but thewater flux and the rejection varied greatly. When the CRGO-Ly content was1.5%,the water flux of the membrane reached372L/(m2·h), and the membrane rejection toPEG20000decreased to63%. However, the membrane rejection to PVA30000-70000could reach and keep at99%. CRGO may impart less perturbation to the lysozymeand thus the antibacterial rates of the hybrid membrane against E. coli was reached71%.The above research shows that the hybrid membranes improve the antibacterialproperty. It is expected to be applied in the field of water treatment.