Biomimetic approaches for the fabrication of hollow fiber nanofiltration membranes

Membrane technologies have become pioneer technology for the water treatment. However, the operation cost of the membrane is dependent on fouling which is "the Achilles heel of the membranes" and consumed energy. The high energy requirements in commercial nanofiltration (NF), reverse osmosis (RO) and forward osmosis (FO) technologies lead researchers to develop new membrane designs having high flux values with high salt rejection values. Biomimetic approaches have gained an important place in the scientific and the technical fields. For the research and development of the membranes, the terms "learning from nature" and "innovation by imitating" become significant. In this respect, Aquaporin Z (AqpZ) protein became popular for the last eleven years. These membranes promise extremely high performance enhancements and offer a decrease in energy consumption. This thesis aims to conduct a very comprehensive study on the development of hollow fiber nanofiltration membranes with incorporated AqpZ. For this purpose, AqpZ protein was cloned from E.Coli bacteria by using many molecular methods such as PCR, ligation, chemical transformation etc. After AqpZ gene is confirmed by sequencing, purification of AqpZ is optimized. Functionality of AqpZ is obtained by reconstituting AqpZ into liposomes. Film rehydration method is used for liposome preparation. In the thesis, the effect of Aquaporin Z (AqpZ) reconstitution on liposome characteristics and behaviors was investigated by evaluating the adsorption performance of proteoliposomes onto gold, polysulfone (PS) and polyacrylonitrile (PAN) surfaces. Commercial AqpZ were used as positive control for cloned AqpZ. AqpZ incorporation into liposomes did not significantly change mean liposome diameter and surface charge, but the water permeability was increased up to almost six fold. Adsorption kinetics was monitored on gold, as the reference substrate, on PS and on PAN spin-coated sensor surfaces, as common nanofiltration membrane materials, via Quartz Crystal Microbalance with dissipation (QCM-D). The results of viscoelastic modelling showed stiffer membrane structure for the lipid bilayer of proteoliposomes compared to that of liposomes. Although liposomes and proteoliposomes were found to be successfully adsorbed on negatively charged PS and neutral charged PAN surfaces, the interaction strength and adsorption rate was found to be different for polymers having different surface properties indicating the importance of polymer for the stabilization of proteoliposomes. According to the findings of this study, PS was chosen as polymer material for membrane fabrication. Interfacial polymerization method was used to fabricate hollow fiber nanofiltration membranes. Proteoliposomes were added to the aqueous phase of interfacial polymerization. Proteoliposome concentration and interfacial polymerization conditions were optimized with flat sheet membranes. Two different proteoliposome concentrations, two different amine monomer reaction times were used and the effects of thermal treatment was investigated for the optimization of flat sheet membranes. Membrane performances were compared in terms of specific permeate flux, MgSO4 and humic rejection; flux recovery after humic acid filtration, the performance of cloned AqpZ incorporated membranes improved 1.7 times regarding TFC membranes. According to the results, increasing liposome concentration did not improved nanofiltration membrane performance. On the contrary, salt rejection performance decreased dramatically. 0.1 mg/mL proteoliposome concentration, 2 min piperazine monomer reaction time and thermal treatment at 50°C for 5 min was found to be the optimized condition. Hollow fiber ultrafiltration membranes were fabricated by phase inversion method. Ultrafiltration (UF) membrane morphology and performance was confirmed by characterization methods. UF membranes were used as support layer for nanofiltration membrane fabrication. For this purposes, in the thesis the combined effects of fabrication conditions and casting mixture composition on the properties of hollow fiber (HF) membranes were investigated. Three variables evaluated are air gap (dgap= 0 cm and 2.5 cm); coagulation bath temperature (Tc= 25°C; 35°C; 45°C) and molecular weight of polyvinylpyrrolidone (MWPVP = 10 kDa; 40 kDa; 65 kDa; 360 kDa; 1000 kDa). Prepared HF membranes were characterized in terms of specific permeate flux; surface morphology, charge and hydrophilicity; bovine serum albumin (BSA) rejection, Young modulus, and resistance to fouling by BSA. Optimal membrane morphology and performance was achieved at higher Tc and higher MWPVP. Optimized conditions were used for the preparation of hollow fiber nanofiltration membranes with Aquaporin Z (AqpZ) immobilized on the outer surface of the separation layer. Three ultrafiltration supports – polymeric, nanocomposite and reinforced were evaluated in terms of their effects on the separation properties of membranes with AqpZ. Surface characterization showed that AqpZ-reconstituted proteoliposomes were successfully integrated with the polyamide matrix leading to permeate flux enhancement of up to 2.5 times. The membranes were also evaluated in terms of salt and organic matter removal. Reinforced UF support allowed for the most effective integration between the polyamide layer and proteoliposomes giving the highest figure of merit – an aggregate metric of membrane performance that includes water permeability, selectivity and resistance to fouling.