Washington, DC - In early 2018, Cape Town, South Africa, became the first major city to come face-to-face with something unthinkable in the modern world: It nearly ran out of fresh, drinkable water. Around the world, aquifers that took millions of years to fill are being emptied as populations increase and new cities emerge. Changing weather patterns are drying out regions that have never had to struggle with water security. While heavy rains temporarily delayed the emergency in South Africa, the time when many parts of the world will no longer have adequate stores of drinkable water to sustain their populations, ominously referred to as “Day Zero,” may be coming sooner than we think.
This is not to mention that there are already nearly 1 billion people in the developing world who don’t have regular access to drinkable water.
The good news is that we can do something about it, but it will take a little effort to tap into the supply. As you may have noticed, 71 percent of Earth’s surface is covered with water. However, 96 percent of that water is held in Earth’s oceans. Therein lies the challenge.
Growing up on the Texas Gulf Coast, my family often went to the beach for some fun in the sun. If you ever enjoyed the ocean waves, you know just how salty seawater is. You can often see the salt on your skin after the water has evaporated in the hot sun. In fact, there is enough salt in seawater (roughly 35 grams per liter) to change its density from 1.000 kg/L to 1.025 kg/L. That difference is enough that eggs, which sink in tap water, will float in seawater. And if you have ever accidentally gotten a mouthful of it, you know how unpleasant it tastes. Even if it didn’t taste bad, drinking seawater will only make you more dehydrated, which is not good. So how do we get all that salt out?
That is where desalination, or de-salting, comes into play. Water-starved countries like Saudi Arabia use a variety of desalination techniques to supply their populations with freshwater, but most of it comes from heating the water to very high temperatures to create steam. The heating process decouples the salt and water molecules, and the desalinated steam is collected and condensed into water you can drink. Distillation, however, is an expensive, energy intensive and time-consuming process. Ideally, we would have a desalination process that doesn’t require all that energy and that can produce clean water essentially on demand without having to wait for it to cool down.
One energy efficient and relatively fast way of desalinating water is by using a semipermeable membrane to do “reverse osmosis.”
But before we get to what reverse osmosis is, perhaps we should talk a little about osmosis first. As you may remember from high school biology class, osmosis is the process by which water (or other liquids) spontaneously moves from an area of low concentration to an area of high concentration, typically across a semipermeable barrier, until the concentration on both sides is equal. For instance, if the water in a cell has a higher salt concentration than the water outside of it, the cell will naturally “pull” water from its surroundings in through its membrane to dilute the salt. Another prime example is the flow of water and nutrients into the roots of plants—if the plant is dry, the concentration of water outside the plant roots triggers the spontaneous movement of water through the cell walls, providing nourishment to the plant.
Conversely, in reverse osmosis, we “push” water from a region of high salt concentration, such as seawater, to a region of low salt concentration to make drinkable water. What makes this possible is an extremely thin (about 100 nanometers, or 100 times thinner than the average human hair), dense type of plastic membrane. The key to reverse osmosis desalination is to make the membrane love water and hate salt. At the same time, the membrane must be thin enough to make it easy to get water to go through and strong enough to withstand the necessary pressure without breaking.
Finding that balance is tricky, but here at NIST, we take to difficult tasks like a duck takes to, well, water! We use our advanced fabrication and measurement techniques to tease out how changes in the chemistry of the membranes affect their properties, for better or for worse. One of the things we do is measure the swelling, or dimensional changes, of these ultrathin membranes. These measurements give us a sense of its solubility, or how much water likes to go into the membrane. And once water is inside the membrane, we measure its diffusivity, which is how fast the water moves through it. Then, as we change the chemistry of the membrane, we can probe how the solubility and diffusivity of both water and salt are changed as a result and draw conclusions about the relationships between the membrane’s structure and its properties. And these techniques are not just limited to water and salt—we can probe the solubility and diffusivity of a whole host of penetrants of interest to industry. One example that has been gaining interest is organic solvent nanofiltration: Membranes can be used to recover high-value products like pharmaceutical compounds or drugs from unwanted manufacturing precursors or byproducts. Traditional distillation is energy intensive and the elevated temperatures can damage these sometimes-sensitive materials.
We also develop ways to measure the strength of these ultrathin membranes. One leverages a technique I developed based on surface wrinkling. Here, we carefully layer the membrane we’re interested in over different rubber and rubber-like materials, such as polydimethylsiloxane, or PDMS for short. By gently tugging on the PDMS, we can stretch the various thin, water-filtering membrane formulations we want to test. When we let go, the membranes spring back and wrinkle up. The pattern of these wrinkles provides a measure of the membranes’ stiffness or modulus. If we tug a little harder, we can induce cracking or fracture the membrane altogether. By measuring the spacing between the cracks, we get a sense of how tough or resilient the membrane is. As we tinker with the membrane chemistry, we can follow how the mechanical robustness of the membranes change for better or worse to find membranes with the qualities we want.
Those formulations in hand, we can pass them off to industry to be improved and implemented in the real world.
Water is a precious resource; one we often take for granted … until the well runs dry. My colleagues and I are working to make sure that Day Zero never arrives by expanding access to clean water in places where they need it most. And when my mom asks me what I do at work, I can honestly say that I am trying to save the world—one drop of water at a time.
