Supported Biomimetic Membranes for Pressure-Driven Water Purification

Abstract

The desalination technology of choice today is reverse osmosis (RO). In this process saline water is forced using mechanical pressure across a membrane that can selectively pass water and reject almost all ions and most neutral molecules. Reverse osmosis proved to be robust and most energy efficient technology that can be exploited for a wide range of water sources. Nevertheless water desalination is still an energy intensive process. Theoretical calculations show that the minimum required energy for seawater desalination is about 1 kW hr m-3, whereas the most energy optimized plants require 2-4 kWh m-3, suggesting that there is much room for improving the membranes performance and energy efficiency (Shannon et al. 2008). Despite the numerous improvements the RO membranes have gone through over the last 5 decades, they are still inferior to cell membranes both in terms of permeability and, especially, selectivity. The fast and selective water transport in biological membranes is achieved by means of aquaporins, specialized trance-membrane proteins. The osmotic water permeability of aquaporins was shown to be in the range of 6×10-14 11×10-14 cm3 sec-1 channel-1 for AQP1 (Saparov et al. 2001) and their ion rejection exceeds by far the ion rejection of the most advanced commercial membranes. Estimates show that a biomimetic lipid bilayer with incorporated aquaporins with a lipids to protein ratio (LPR) of 50, would yield a membrane with a hydraulic permeability of ~9 – 16.5 L m-2 hr-1 bar-1 whereas the permeability of seawater RO membrane does not exceed 2 L m-2 hr-1 bar-1 (Kaufman, Berman & Freger 2010). Biological membranes are also known to reject small molecules, such as urea or boric acid, which are poorly removed by commercial membranes (Borgnia et al. 1999). The combination of ultra-selectivity with extremely high water permeability makes biomimetic membranes highly attractive for water purification applications. It is important to emphasize that water transport through biological membranes, usually existing as microscopic free-standing self-supporting structures, is usually driven by electrochemical potential gradients, i.e. osmotic pressure or electric field (Tanaka, Sackmann 2005). However, for practical membrane applications, such as water purification, the use of hydraulic pressure as a driving force and planar membranes of macroscopic dimensions, the common configuration in membrane technology, will be far more preferable. However, the preparation of such biomimetic membranes poses a number of challenging questions, such as: how can one prepare a large and defect-free biomimetic planar membrane? can they be made to withstand hydraulic pressures? how can aquaporins be incorporated in such membranes? will aquaporins keep their activity under a hydraulic pressure gradient?