Due to a growing global population and increasing pollution of our fresh water by industrial activities, the supply of clean drinking water will be one of the main challenges for upcoming generations. New water purification technologies are required to solve this problem. Currently, evaporation and reverse osmosis are the two main strategies used to purify and desalinate water. Both processes are highly energy intensive and thus less economical. At this point it might be helpful to ask ourselves if we could learn from nature by applying water management strategies of biological organisms to our technical systems.
Various plants and animals live in highly saline environments. For instance mangroves are plants that grow in pure sea water, filtering out all salts by a process called ultrafiltration. Also, marine birds, are able to excrete a highly concentrated salt solution via specific salt glands. All of these organisms make use of the same underlying strategy called forward osmosis. The excretory systems of biological organisms use osmotic gradients to transport water through membranes. Since membranes are moderately permeable to water this is quite a slow process, thus nature came up with specific protein channels which increase the transport rate of the water molecules. These protein channels are called aquaporins and are ubiquitous among all living organisms, from bacteria and plants to humans.
Aquaporins are trans-membrane protein channels with a transport rate of approximately 3 x 109 water molecules per channel per second. The highly selective nature of transporting only water molecules, excluding all other solutes, is realized by the proteins three-dimensional structure. Six trans-membrane domains and two short pore forming loop domains form a constriction of around 30 pico-meters, which equals the width of a single water molecule. Additionally, positively charged amino acids form the channel’s inner part which attracts water molecules due to hydrogen bonding.
The Danish company Aquaporin makes use of nature’s strategies by using a forward osmotic system incorporating aquaporins to increase the water transport rate. Aquaporins are embedded into artificial membranes simulating the natural behavior of biological membranes. Since aquaporins are ubiquitous among all living organisms they can easily be produced using bacteria or algae for instance. One approach is to use aquaporin-membranes to produce gel-filled vesicles, which are introduced into a saline water phase that has to be purified. The gel inside the vesicles has to have a higher osmotic potential than the saline water phase for the water molecules to enter the vesicles. This leads to a swelling of the vesicles. The swollen vesicles can then easily be filtered out of the saline phase. The second step is to introduce the swollen vesicles into a secondary draw solution, which has to have a higher osmotic potential than the vesicles inner gel-filled matrix. Thus, water inside the vesicles will exit and enter the secondary draw solution which leads to a shrinking of the vesicles. It is important that the secondary draw solution is separable from the purified water. Sugar or CO2/ammonia draw solutions could be used, for example. This approach might have a huge potential in revolutionizing water purification technology. Further information are available on the official website of Aquaporin.