Today, more than 1 billion people are suffering from the lack of potable water. About 2.3 billion people (41 percent of the earth’s population) live in regions with water scarcity; this number is estimated to be 3.5 billion by 2025.1

96.5 percent of the world’s water is found in seas and oceans, and the remainder is found as ice caps, brackish water, and fresh water sources (e.g. lakes, rivers, and ground waters). To overcome water shortage problems, methods such as water conservation and dam construction have been applied for several years, but they are not enough against increasing water demand and decreasing fresh water sources.2

Water is also very important for generating energy, and vice versa. The largest portion of U.S. electric production is provided by thermoelectric power generation, where steam-driven turbine generators are used to generate electricity. In 2000, thermoelectric power plants used 39 percent of all fresh water sources in the United States.3 All these reasons make the production of drinking water a worldwide issue.

Desalination

Since most of world’s water supply is found in oceans and seas, desalination is the process of removing salts and minerals from either ocean or brackish water to make it safe for human consumption and use. The most widely applied desalination processes are divided into two main categories, thermal distillation processes and membrane processes.

Desalination via thermal distillation methods, which separate liquid mixtures based on their boiling points, mainly fall into three categories: multi-stage flash (MSF), multi-effect distillation (MED), and mechanical vapor compression (MVC). Thermal distillation processes require the evaporation of water while leaving the salt in a concentrated brine. Middle Eastern countries mainly use thermal-based desalination plants to produce fresh water because of their easily accessible fossil fuel sources.2, 4

Membrane-based separations are the main choice of producing potable water in countries outside the Middle East. More than 50 percent of the newly installed desalination plants have been using reverse osmosis (RO) membrane technology (since 2001).2

Membrane separations

A membrane is an interphase between two adjacent phases acting as a selective barrier, regulating the transport of substances between the two compartments. It is a very thin film that allows passage of some types of substances while preventing the passage of other substances, depending on their sizes. Membranes used for separation technology gave rise to an interdisciplinary area including many fields of science and engineering such as chemistry, chemical engineering, material science, process engineering, environmental science, ecology, and economics.5, 6 Today, the membrane industry is impressively large. The membrane separation technology market is quite diverse and ranges from medicine to the chemical industry, and the most important markets are medical devices and water treatment. There was a $2 billion sale of synthetic membranes worldwide in 2003.6

Water purification membranes

Water treatment processes employ several types of membranes. They include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes. They are designed to remove materials of increasing sizes. MF membranes have the largest pore size and typically reject large particles and various microorganisms. UF membranes have smaller pores than MF membranes and, therefore, in addition to large particles and microorganisms, they can reject bacteria and soluble macromolecules such as proteins. RO membranes are effectively nonporous and therefore exclude particles and even many low molar mass species such as salt ions, organic substances, etc.7 NF membranes are relatively new and are sometimes called “loose” RO membranes. They are porous membranes, but since the pores are ten of angstroms or less, they exhibit performance between that of RO and UF membranes.8 Of these membranes, NF and RO membranes constitute the dominant technology for desalination of water.9

2.1 Nanofiltration Membranes

Membranes for nanofiltration (NF) are usually comprised of cellulose acetate or aromatic polyamides. NF allows diffusion of organic compounds, and rejects some salts with low pressures being applied. NF itself cannot purify seawater to drinking water standards, but it is a process that can be used to produce mildly salty water, or as a water-softening technique.2, 4 When NF is coupled with RO, then it can be used to turn seawater into drinking water.10

Nanofiltration membranes usually have negative charges (e.g., carboxylate groups, sulfonate groups, etc.), and as a result, ion repulsion is a major factor in determining salt rejection. More highly charged ions, such as sulfate, are more highly rejected than monovalent ions, such as chloride, by a negatively charged nanofiltration membrane. In particular, NF membranes are used to remove divalent ions such as calcium and magnesium, which are mainly responsible for water hardness. These membranes also usually display good rejection of organic compounds with molecular weights above 200 to 500 grams.2,11,12

2.2 Reverse osmosis membranes

Osmosis is a natural process in which water molecules move across a semipermeable membrane from a lower solute concentration area to the higher solute concentration area. Water flows until a chemical potential equilibrium of water is established. When equilibrium is reached, the pressure difference between the two sides of the membrane is equal to the osmotic pressure of the solution.12

Reverse osmosis (RO) is the process of forcing water from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure that is greater than the osmotic pressure. As a result, separation of water from the solution occurs as pure water from the high concentration side to the low concentration side. The RO process includes a feed water source, feed pre-treatment, a high-pressure pump, RO membrane modules and post-treatment steps.

RO membranes are capable of rejecting monovalent ions such as sodium and chloride, which makes the RO process a valuable method for desalination. Membranes used for RO processes have salt rejections of more than 99 percent. RO membranes do not have distinct pores, but rather rely on free volume within the polymer film.

RO membrane separations depend highly on the properties of the polymer film such as the chemical and physical structure of the membrane material. Desired RO membranes should be resistant to chemical substances and microbial organisms, stable over a long time both mechanically and structurally, and have ideal separation properties such as high water flux, high salt rejection, chlorine, and fouling (clogging of membrane pores) resistance.

Approximately one billion of six billion people in the world live in water-stressed areas, and RO membrane technology is the leading desalination technology to overcome the problem of insufficient clean water and estimated to continue its leadership in the near future.13 Scientists and engineers are extensively investigating the development of the most efficient membrane desalination technology to produce the cheapest potable water.

On the other hand, cells use membranes, though scientists do not try to further develop them, since they were already designed in a perfect manner. Cellular membranes have a phospholipid structure with embedded proteins. They control many different kinds of transportations of substances in and out of cells (e.g. sugar, drugs, ions). They are so well designed that they know which substances are helpful or harmful for the cell, and decide on the passage of substances based on that. Many researchers have tried countless times for many years to produce an equally wonderful membrane technology for making clean water. But cellular membranes, consisting of hundreds of functions in living organisms, do not form spontaneously.

REFERENCES

1) R.F. Service, Freshwater resources, desalination freshens up. Science, (2006). 313, 1088- 1090.

2) L.F. Greenlee, D.F.Lawler, B.D. Freeman, B. Marrot, P. Moulin, Reverse osmosis desalination: Water sources, technology and today’s challenges. Water Research (2009), 43, 2317-2348.

3) T.J. Feeley, T.J. Skone, G.J.Stiegel, A. McNemar, M.Nemeth, B. Schimmoller, J.T. Murphy, L. Manfredo, Water: A critical resource in the thermoelectric power industry.Energy (2008), 33, 1-11.

4) G. A. Tularam, M. Ilahee, Environmental concerns of desalinating seawater using reverse osmosis. J. Environ. Monit.(2007), 9, 805–813.

5) P. Vandezande, L. E. M. Gevers, I. F. J. Vankelecom, Solvent resistant nanofiltration: separating on a molecular level. Chem. Soc. Rev.(2008), 37, 365–405.

6) M. Ulbricht, Advanced functional polymer membranes. Polymer (2006), 47, 2217–2262.

7) R.H. Perry, D.W.Green, Eds., Perry’s Chemical Engineers’ Handbook, 7th ed., McGraw-Hill: New York, 1997.

8) Sagle, A., and B. Freeman, “Fundamentals of Membranes for Water Treatment,” in The Future of Desalination in Texas: Volume 2, Report Number 363, Texas Water Development Board, Austin, TX, pp. 137-154 (2004).

9) H.B.Park, B.D.Freeman, Z.Zhang, M.Sankir, J.E.McGrath, Highly Chlorine-Tolerant Polymers for Desalination, Angew. Chem. Int. Ed. (2008), 47, 6019-6024.

10) N. Hilal, H. Al-Zoubi, N. A. Darwish, A. W. Mohammad, M. Abu Arabi, A comprehensive review of nanofiltration membranes: Treatment, pretreatment, modelling, and atomic force microscopy, Desalination (2004), 170, 281-308.

11) A. Gorenflo, D. Velazquez-Padron, F.H. Frimmel, Nanofiltration of a German groundwater of high hardness and NOM content: performance and costs. Desalination (2002), 151, 253-265.

12) M.E.Williams, A Brief Review of Reverse Osmosis Membrane Technology,EET Corporation and Williams Engineering Services Company, Inc., Harriman, TN, 2003.

13) K. P. Lee, T. C. Arnot, D. Mattia, A Review of Reverse Osmosis Membrane Materials for Desalination – Development to Date and Future Potential. J. Membr. Sci. 370 (2011) 1-22.

I am placed inside your chest cavity as two air sacks-or bellows-surrounded by your muscles. I took my first breath right after birth and I still keep working non-stop. Even while you are sleeping, I fulfill my function with the automatic command I receive from the respiratory center at the back of your brain. My close friend Heart started working even before me while you were in the womb. I was resting then; actually I hadn’t even formed fully. As all your needs like food and oxygen were met in the body of your mother-whose heart you occasionally break-I didn’t have to make extra effort to get air, being filled and emptied. Even if I had attempted to do so, I would have had no chance of succeeding; since you were contained in the amniotic fluid, an attempt to breathe could have caused you to drown.

The first breath I take after birth is critical and rather difficult, since the windpipe is still much narrower than normal. On the other hand, the number of my alveoli where oxygen exchange with the capillaries is realized is so high in relation to body size that it balances the situation. When I make my first move and fill with air, I put pressure on the arteries and veins. Then the vessel directly connecting my artery to my mother’s aorta is dismissed, the curtain between the valves is closed and the blood circulations are separated. If this curtain is not properly closed and a gap remains in between, the oxygen-rich blood and the used-up blood mix and result in the disease known as cyanosis-or “blue baby” syndrome. As these two kinds of blood mix, the tissues are not supplied with sufficient oxygen and the white parts of the skin and eyes assume a bluish appearance.

Turning blue-purple due to lack of oxygen in the tissues is the same for smokers. Cigarettes-my archenemy-contain hundreds of toxic substances, such as carbon monoxide, which combine with the hemoglobin in blood and prevent oxygen transfer. Therefore, the lips of smokers turn slightly purple. You need to be careful with the air you inhale. The windpipe which brings air into me is covered with a ciliated epithelial tissue which catches the dust brought along and sweeps it outside. While you are asleep, the vibrating cilia of this sweeper work throughout the night and in the morning you get rid of the outcome of their propulsion by clearing your throat. However, every draw of a smoker kills 800–1,000 of our ciliated epithelial cells. After some time, they become unable to sweep the toxins (carbon, sulfur, lead, etc) inhaled with the air. I can’t stand it anymore! The increased air pollution is already putting enough strain on us… this habit is just too much for a lung to handle! It is just… an open invitation for cancer! Sorry, Peter, I didn’t mean to be rude. I appreciate that you don’t smoke, but I wish those who do would realize how splendid a mechanism they are destroying.

Now let me tell you about what a work of art I am. As you also know, art in a structure becomes more meaningful with functionality. As is the case with my other friends with which I work in your body, I am perfectly made to fulfill my duty. In other words, never mind forming an organ like me as a consequence of molecules and cells accidentally coming together, even a single protein molecule in my structure does not come to existence through unconscious causes.

With every breath you take, the pressure of the oxygen within the air inhaled rises, so it passes through my membranes by diffusion and into the adjacent capillaries; there it combines with hemoglobin molecules. At the same time, the carbon dioxide passes through the same membranes into me, and I dispose of it. Both of these are easier said than done! You breathe 13–14 times a minute and the whole thing is repeated over and over. As I keep expanding and contracting during breathing, which you are unaware of most of the time, first of all I need to be very flexible. Together with this flexibility, my most important quality is having the largest possible surface area within the smallest volume. My surface area of around 100m2 (as large as a tennis court) is made to fit into your chest cavity in the form of thin membranes so that my large surface allows gas diffusion. These membranes need to be kept wet; a special fluid is secreted as a precaution and respiration is realized smoothly. Without this fluid, my membranes would just stick together, unable to carry out their duty.

You can compare the course of the air inhaled to that of a car passing from a highway onto increasingly smaller roads and in the end reaching a dead end in the small sacks named alveoli. The air coming in through the mouth and nose unites at the expressway named the trachea, or the windpipe, which is 15cm long and 2–3cm in diameter. Incidentally, I have a couple of things to tell you about the way you breathe. As a matter of fact, inhaling is the duty of the nose. I’m sure it also has a lot to say as well, but let me just mention a simple fact about it. Now, you should inhale through your nose, so that the air you take in gets warm, wet, and is cleaned from dust. If you try to breathe this way, you do not trouble me much, and reduce the risk of catching a cold or an infection of upper respiratory system. Inhaling through the mouth helps dust and germs get into me and you might contract various illnesses from bronchitis to pneumonia. Now you know why kids who have adenoids who sleep with their mouth open get ill so easily. Sorry, I couldn’t help speaking on behalf on the nose.

Well, what were we talking about before that? Oh yes! The ways through which the inhaled air passes. As the name suggests, the windpipe which makes the air reach me is a cylindrical tube surrounded by 16–20 cartilaginous rings. As it is placed beside the esophagus, one side of the rings is made of soft cartilaginous tissue instead of hard, so that they don’t hinder swallowing. The muscular tissue near these rings helps them widen and narrow during respiration or coughing. I sometimes warn you by making you cough. Maybe it seems to be a disturbance, but if I don’t push out air by coughing through the contracted windpipe, contaminants can clog me up and cause you to suffocate. Therefore, the burst of air-what you call a cough-is a great blessing to you.

The sound system at the tip of the windpipe is another wonder. The used air I send out vibrates the cords in that voice-box and produces such melodies, gives voice to such speech! The air divides into the two lungs. My two sides are not symmetrical; the one on the right is divided into three, and the one on the left into two. I think this was meant to make room for the neighbor on the left, the heart. In addition, if there’s any cancer growth in me, the diseased part can be taken out by an operation and I can keep on functioning. God knows the wisdom behind this form. After that, these main bronchi separate into 8–10 thinner branches, like highways connecting to narrower roads. This branching resembles a tree turned upside down. At the tips of these thin branches are the respiratory bronchioles resembling clusters of grapes. The small spheres which make up the cluster are the end of the road and are the most vital parts. These spheres named alveoli are made of very thin membrane and they are surrounded by a net of capillaries (picture 5). These are the functional spots where gas exchange is realized.

I hang in the thorax with veins and arteries all around. There are two layers of protective membrane over me. One of them is stuck on me, whereas the other is stuck on the ribs which form the chest cavity. There is a fine and slippery fluid in between these two layers and it neutralizes the friction every time I inflate and deflate. If it hadn’t been placed there, I would wear out and be damaged. As I inflate during inhalation, the chest cavity should expand simultaneously to make space for me. If it weren’t given a flexible form, I would fail to breathe and you would eventually die. Fortunately, the protective set of ribs and their connection with the spine are flexible enough to make me work comfortably. In addition, the dome-shaped muscular partition (diaphragm) separating the thorax from the abdomen contracts and pushes down the organs in the abdomen. Thanks to the simultaneously programmed movement of both the ribs and the diaphragm I inflate with air and expand.

Being in constant contact with the outer environment makes me susceptible to various diseases. Coughing is among the foremost signals I give in the case of disease, and sometimes-excuse me-I produce a mixture of blood and phlegm. Also, I may have difficulty in breathing and warn you with chest pain. You should be alert to my signals. If bacteria and viruses infect me, they might reproduce inside my air sacs, and cause stiffening and suppuration.

I am particularly sensitive to allergic disorders. When the straight muscles on the walls of my bronchi contact an alien substance, pollens for instance, the consequent histamine secretion makes my muscles contract. In addition, allergic diseases, which can affect blood vessels, affect me a lot since I happen to be one of the major organs contributing to blood circulation. As a result of the contraction of my bronchial muscles and difficulty in disposing of the mucus I secrete to defend myself, I have trouble with breathing-you call it asthma.

In addition to this, we can mention diseases like emphysema, acute or chronic bronchitis as problems I frequently face. Even your anger has a great impact on me. Breathing becomes more difficult immediately.

Peter, I’m sorry, it is not possible to summarize a work of art like me within a few pages, but I need to stop now… but please, keep away from polluted areas and cigarette smoke! Send me as much fresh air as you can. And even though you mostly take me for granted, like my other teammates, please reflect upon what a blessing I am.

Irfan Yilmaz is a professor of biology at Dokuz Eylül University, Izmir, Turkey.

In May 2003, Roderick Mackinnon’s research group at Rockefeller University has published the three-dimensional atomic structure of a voltage-gated potassium channel. MacKinnon also received the Nobel Prize in 2003 for the first potassium channel structure

How electrical signals are conducted through nerve cells has been understood with some detail in the last fifty years, after the seminal work of Hodgkin and Huxley, which earned them a Nobel Prize in 1963. After 40 years, another study is appreciated in the scientific community as the end of one era and the beginning of another one.

Cells, body’s building blocks, are bound by membranes that are impermeable to ions, atoms that carry positive or negative charge. Nerve cells are specialized for having very long arms that can carry electric potentials on their membranes for very long distances. The electric potential (voltage difference) is the result of unequal number of charges across the inside and outside of the cell membrane. Hodgkin and Huxley explained the movement of the electric potential on the membranes by the selective opening of ion channels, proteins embedded in membranes to allow the passage of certain ions. As channels are opened in one part of the nerve cell to allow an electrical potential change, the neighboring stretches somehow sense the change in voltage and open their channels as well. The propagation of the electric potential depends on the fast sensing, opening and closing of these channels. The molecular details of these voltage-gated channels have been somewhat obscure; however important properties have been determined using methods pioneered by Hodgkin and Huxley, generally dubbed electrophysiology, the study of biological material using electrical methods. Scientists

have imagined channel proteins as many helices running through the membrane, some of which could be pulled to the inside or outside of the cell depending on the voltage across the

membrane. Movement of these helices across the membrane could open channels for potassium or sodium. Finally in May 2003, Roderick Mackinnon’s research group at Rockefeller University has published the three-dimensional atomic structure of a voltage-gated potassium channel. MacKinnon also received the Nobel Prize in 2003 for the first potassium channel structure, which explained the selectivity of a potassium channel for allowing only potassium ions to pass, but nothing else.

MacKinnon is a mixed structural biologist and electrophysiologist. His group has isolated potassium channel molecules from membranes, and tested them for strict functionality as it has been observed that protein molecules isolated out of their natural environments may lose their activities and chemical properties. The next step of their study was to crystallize the channel molecules-much like forming salt crystals-and diffract x-rays through the aligned molecules in the lattice of their crystals. These diffraction patterns are mathematically solved to give atomic positions of electrons, hence atoms, in the protein crystal. Several research groups failed at this step, since purified potassium channels never formed crystals, as flexible protein molecules are unlikely to form crystals. Here, MacKinnon cleverly used antibodies they developed specifically against the channel. These antibodies, used radically differently than they are in nature, overcame the flexibility of the voltage-sensing regions of the channel, by binding to the channel and fixing it to a specific conformation. The ordered channel crystallized and gave the structure of the potassium channel and the antibody bound to it.

The structure of the channel consists mostly of helices, which in the center surround a potassium-conduction pore, and “voltage-sensing paddles” in the periphery. When the gate is closed, the helical paddles lay parallel at the cellular side of the membrane. Gate opening only happens when the paddles are pulled through the membrane to the outer side as voltage pulls the positive charge of the paddles to the outside (see figure). Now, it looks like 50 years of data collected by electrophysiologists and biochemists can be explained coherently (such as the observed 14 positive charges residing on the paddles moving through the membrane). As is usual with such discoveries, many new questions are asked, such as how the charged paddles can go through the oily, highly uncharged membranes. There are also valid scientific objections, such as the antibody distorting the potassium channel structure. However, this is one of those bright moments in any field of research, when researchers go over the threshold of collection of enough observations to discovery and understanding.

References

• Jiang, Y., Lee, A., Chen, J., Ruta, V., Cadene, M., Chait, B.T., MacKinnon, R., Nature 423, 33-41 (2003).
• Jiang, Y., Ruta, V., Chen, J., Lee, A., MacKinnon, R., Nature 423, 42-48 (2003).
• Sigworth, F.J., Nature 423, 21-22 (2003).