Development of aquaporin based biomimetic membranes for desalination applications
Date of Issue2014
School of Civil and Environmental Engineering
Singapore Membrane Technology Centre
Nature has contributed the best desalination material, aquaporin (AQP), for us. Attributed to its extremely high permeability for water molecules as well as high rejection to small solutes, in recent years there is a surge to mimic the natural cellular membranes by incorporating AQPs into various ultrathin films for water filtration applications. In this work, efforts are made to explore an effective way to design, prepare and evaluate highly permeable and selective AQP-based biomimetic membranes for water purification applications. As a start, this thesis presents a comprehensive literature review on the AQPs and related AQP-based biomimetic membranes research. Basic information about AQPs, lipid and block copolymer systems is reviewed. Strategies for membrane protein reconstitution into lipid vesicles (liposomes) and polymer vesicles (polymersomes) are described. In addition, different types of AQP-based biomimetic membranes are reviewed and discussed in detail. Moreover, a series of characterization techniques commonly applied in protein/biomimetic membrane research are reviewed. Equipped with fundamental knowledge and understanding of the state-of -the-art technology, a preliminary study was carried out to develop AQP-based biomimetic membranes. To make biomimetic membranes for water purification applications, preparing a defect-free platform for AQP incorporation on a suitable substrate is one of the critical steps. Two methods were used to prepare supported lipid membranes on a nanofiltration membrane surface under a benign pH condition of 7.8. One was direct vesicle fusion onto a hydrophilic membrane NF-270; the other was vesicle fusion facilitated by hydraulic pressure on a modified hydrophilic membrane NF-270, whose surface was spin-coated with positively charged lipids. Experiments revealed that the supported lipid membrane prepared by spin coating with vesicle fusion had a much lower defect density than that prepared by vesicle fusion only. It seems that the surface roughness and charge are main factors determining the quality of a supported lipid membrane. AQPs were successfully incorporated into 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes and its water permeability was examined by stopped-flow experiments. However, after the proteoliposomes were fused onto the modified substrate, the AQPs function in the resultant membrane was not observed. Atomic force microscopy (AFM) images showed the presence of unfused proteoliposomes on the substrate. This may suggest that the presence of AQPs in the liposomes can significantly change the way that these vesicles interact with a surface. In order to understand these problems that were encountered, different characterization techniques were employed to investigate the vesicle characteristics and the fusion behaviour of liposomes and proteoliposomes on solid surfaces. The results showed that after incorporation of aquaporin Z (AqpZ), the size and surface charge density of the proteoliposomes changed significantly compared with those of liposomes. Although the liposome could easily form a supported lipid bilayer on silica via vesicle rupture, it was much more difficult for proteoliposomes to fuse completely into bilayers on the same substrate. In addition, the fusion of proteoliposomes was further hindered after the density of incorporated AqpZ was increased, suggesting that the proteoliposomes with more proteins become more robust. However, both the liposome and proteoliposome had difficulty in fusing to supported lipid bilayers on the surface of a polyelectrolyte layer even though they carried an opposite charge, indicating that the polymer might play an important role in stabilizing the vesicles. It was also observed that a high concentration of AqpZ could be incorporated into the liposome even though its nominal permeability decreased. Since it was difficult for proteoliposomes to fuse to bilayers completely because of the incorporated AQPs, we proposed a novel protocol to design an AQP-based membrane: the AqpZ incorporated proteoliposomes were immobilized in the inner surface of a hollow fibre membrane and subsequently encapsulated by a polyamide layer formed by interfacial polymerization, instead of being fused to bilayers. Different hollow fibre composite membranes with various lipid-to-AQP ratios were prepared and explored. The AQP-based hollow fibre composite membrane exhibited an increased water flux and salt rejection with an increase in the loading ratio of AQPs. The membrane with a high AQP-to-lipid ratio exhibited a 40 L•m-2•h-1•bar-1 flux at 5 bar, almost 200% more than the flux of a commercial reverse osmosis membrane. This membrane also exhibited 500 ppm NaCl rejection above 97.5%, which was also superior to the commercial membrane tested under the same conditions. The forward osmosis test of the AqpZ-based hollow fibre membrane also yielded a super high water flux. It showed that the most of introduced proteoliposomes were totally embedded in the polyamide layer, maintaining an intact spherical shape even after long-term pressure-driven filtration test. Series of experiments proved that it was the embedded AqpZ that enhanced the water flux and the integration between the embedded proteoliposomes and polyamide layer was entirely defect-free. It was evaluated that the AqpZ-incorporated hollow fibre composite membrane could cut the power consumption by up to 40% in a Newater plant for wastewater treatment, if these hollow fibres were assembled in a standard 8-inch commercial membrane module (assuming a 80% packing density, giving a 59 m2 membrane area). The use of AQP within membranes has been taken beyond the concept stage and offers a novel means for tailoring the membrane materials and improving the membrane performance. Based on the previous work, a modified method was proposed to prepare a nanofiltration (NF) membrane based on AQPs. The proteoliposome, incorporated with AqpZ, was embedded into the selective layer through crosslinking of a polyelectrolyte with the membrane substrate made by poly(amide-imide) (PAI). The proteoliposome with mutants of AqpZ, which had a low water permeability, was selected as a control group. The water flux of the AqpZ-based membrane was around 50% higher than the mutant one. At the optimal preparation conditions, the AqpZ-based membrane displayed a water flux of 36.6 L•m-2•h-1 with a MgCl2 rejection of 95% at 1 bar. Results from our studies indicated that AqpZ could maintain a high water transport ability (about 250 µm/s for proteoliposomes with LPR 400) even at high temperature (70 °C for 2 hours) and keep its activity in the presence of highly charged polyelectrolytes (polyethyleneimine). These results and findings may provide useful insights for developing the next generation of biomimetic membranes.
DRNTU::Engineering::Environmental engineering::Water treatment