Aquaporin-based biomimetic reverse osmosis membranes: Stability and long term performance
Introduction
Water resource recovery is an important strategy to meet the ever-increasing demand for water in highly urbanized societies. The reverse osmosis (RO) membrane is a state-of-the-art technology for water treatment, such as desalination and reclamation, due to its highly efficient removal of salts and small molecular impurities [1], [2], [3], [4], [5], [6]. In a typical RO process, water flows from the high salinity side to the low salinity side across the membrane under hydraulic pressure, while various ions and pollutants are rejected by the membrane. Although high quality water can be generated by RO membranes, the energy consumption is significant and influenced by the relatively low water permeability of the membrane [5]. It has been estimated that a tripling of RO permeability could reduce energy from seawater RO by 15% or for brackish water (similar to reclamation feed water) by up to 46% [7]. Alternatively the membrane area could be reduced by 40–60%, respectively. This provides a strong incentive to develop RO membranes with significantly higher water permeabilities.
Aquaporins (AQPs) are biological proteins that form selective natural water channels. They have received increasing attention because of their high water permeability (each water channel can pass ~109 water molecules per second) and superior selectivity (i.e., the water channel only allows water passage while fully rejecting solutes) [8], [9], [10], [11], [12]. In theory, proper integration of AQPs into a membrane could make the membrane highly water permeable [8], [13]. In practice, there are two generic types of biomimetic aquaporin membrane based on different structure designs; (i) AQP incorporated into a bilayer on a membrane support and (ii) AQP immobilized into a dense polymer layer [2]. Representatives of the former type include free standing lipid membranes [14], [15], lipids on polymeric materials [16], [17] and lipids on a porous support or membrane surface [18], [19], [20], [21], [22], [23], [24], [25]. However, this type of composite structure is unable to produce a large defect-free area and is hard to scale up. In addition, the mechanical strength is another concern when the membrane is used at high hydraulic pressure in desalination or water reuse processes. On the other hand, the membranes with AQPs immobilized in a dense polymer layer can be made using layer by layer assembly [20], [22] or chemical cross-linking [26] to encapsulate proteoliposomes, which contain AQPs, into the membrane selective layer. The resultant membranes have very high water flux and good rejection against multivalent ions. However, these membranes normally present low rejection of monovalent ions and are not suitable for in the desalination process.
Alternatively, thin film composite AQP-based biomimetic (ABM) membranes with a relatively large area can be made via the interfacial polymerization (IP) method [12], [27]. Briefly, a m-phenylene-diamine (MPD) solution containing a certain amount of the AQP-containing proteoliposomes (vesicles) is used to react with a tri-mesoyl chloride (TMC) solution to form a thin and dense selective layer, where the proteolipsomes are embedded into the selective layer. It has been shown that the embedded AQP loaded proteoliposomes can enhance the water permeability while not adversely affecting the solute permeability of the selective layer. Excellent water permeabilities of ABM membranes (i.e., around 4 L/m2 h bar for flat sheet and around 8 L/m2 h bar for hollow fiber) have been reported [12], [27]. However, the mechanical strength of the flat-sheet ABM membranes tested was relatively weak due to lack of a support layer. In addition, the stability of these ABM membranes was only tested for a few tens of hours with single salt or single foulant containing solutions. Given the biological origin of the embedded AQPs this raises the issue of longer term stability with real world feed and cleaning regimes.
The objectives of the current study were to fabricate ABM membranes with high mechanical strength and to test their stability. The stability of the ABM membranes was evaluated in terms of the chemical stability (response to chemical cleaning agents), temperature stability and pressure stability. In addition, the long-term performance (i.e., around 100 days of operation) of the ABM membrane processing a real RO feed from water reclamation process, with periodic cleaning (five times), was evaluated. To the best of our knowledge, this is the first study to systematically investigate the stability of the ABM membrane and to evaluate its long-term performance using a real RO feed water. This study should provide confidence in the potential and applicability of AQP-based biomimetic membranes for practical applications.
Section snippets
Chemicals and materials
Escherichia coli lipids (20 mg/ml, Avanti Polar Lipids, Alabama, USA), were dissolved into chloroform and used to incorporate the native (wild) AQPs and mutant AQPs according to previous protocols [27]. n-Octyl-b-D-glucopyranoside (OG, ultrapure grade, Merck, Germany) was used as a detergent during proteoliposome reconstitution. Phosphate buffered saline (PBS) solution (pH around 7.4) was used to rehydrate the lipid. The bio-beads were purchased from Bio-rad laboratories in United States.
The size and activity of liposomes and proteoliposomes
The size and water permeability of liposome, proteoliposome and mutant proteoliposome are shown in Table 1. It can be seen that all the vesicles have sizes within the range of 80–120 nm, and the size distributions are quite narrow indicated by their small polydispersity index (PDI) (i.e., less than 0.2). The proteoliposomes showed much higher water permeability due to the high water permeability of the active AQPs, which is consistent with our previous study [27]. In contrast, liposomes and
Conclusions
Robust and high performance aquaporin-based biomimetic (ABM) membranes were successfully fabricated and their stability and long-term RO performance were symmetrically evaluated in the current study. Several findings can be concluded as follows:
- (1)
The incorporation of aquaporin-based proteolipsomes into the selective layer helps to increase the water permeability of the RO membrane due to the highly permeable nature of proteoliposomes for water while maintaining similar solute permeability
Acknowledgments
The research grant is supported by the Singapore National Research Foundation under its Environmental & Water Technologies Strategic Research Programme and administered by the Environment & Water Industry Programme Office (EWI) of the PUB (1301-IRIS-44). The research is also funded by PUB, Singapore's National Water Agency. Funding support from the Singapore Economic Development Board to the Singapore Membrane Technology Centre is also gratefully acknowledged.
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