YES, WE CAN!
Manfred Wilhelm, a scientist at the Karlsruhe Institute of Technology, has a dream: going where no one has been before. Today he’s using the superabsorbers that are normally inside baby diapers to make polluted water drinkable—and potentially change the world
When Manfred Wilhelm needs time to himself, he goes down to the basement. He has three teenage daughters and holds a professorship at an internationally renowned university, so he rarely has uninterrupted time for his work. In his house near Karlsruhe, he writes expert opinions, edits scientific articles, and thinks about his specialist field, polymer chemistry. He jots down his best ideas for research projects on a whiteboard behind his desk at the Karlsruhe Institute of Technology (KIT), where he has taught and researched polymer materials for the past 11 years. Laypeople would simply call them plastics.
»Science is the power of anticipation. I want to create something that still hasn’t been created«
teaches and researches at the Institute for Chemical Technology and Polymer Chemistry at the Karlsruhe Institute of Technology (KIT)
Students looking for topics for their bachelor’s, master’s, or doctoral theses often find what they’re looking for on Wilhelm’s whiteboard. Three theses are currently devoted to the topic of water. Depending on their conclusions, they might be able to change the world—with a type of plastic that so far has mainly been used as a toilet.
Superabsorbers are plastics that can absorb huge volumes of liquid. Industrial companies primarily use this white granulate to make baby diapers. The first Pampers were lined with superabsorbers in 1987. Since then, practically all modern baby diapers use superabsorbers, which can easily soak up 500 times their own volume of water and retain it securely. Drier baby bottoms, happier infants, and free-spending parents are the three pillars of the diaper business.
“I actually had a brainstorm while changing a diaper,” says Wilhelm, who is now 51. During a family vacation on the Baltic Sea he noticed how fast his young daughter’s diapers also swelled up as she played in the salt water. He had previously been reading up on polyelectrolytes for his inaugural lecture at the university, and a few years before that he had worked at the renowned Weitzman Institute in Israel, commuting between the Max Planck Institute in Mainz and the Rehovot research Institute south of Tel Aviv. “I still remembered Israel’s difficulty with securing freshwater,” he says. “Every day there was something in The Jerusalem Post about the current level of water in the Sea of Galilee.” All of these thoughts and memories coalesced one day at the changing table. Wouldn’t it be possible to use superabsorbers to desalinate seawater? The answer is yes, as Wilhelm’s student Lukas Arens is demonstrating in his doctoral thesis. All the same, it’s not very easy.
A question of energy
To understand why not, we first have to know how superabsorbers soak up water. Wilhelm explains the process by having visitors stir a teaspoon of grainy white superabsorber powder into a beaker of water. Within seconds, it forms a tough gel that can’t be removed by tipping or shaking the beaker. The secret lies in the powder’s molecular structure. “This is a polymer—a long molecular chain of acrylic acids that forms a three-dimensional network that acts like a molecular sponge,” Wilhelm explains. He jumps up and grabs from a shelf a brightly colored plastic ball that could have come from a toy store. It’s a Hoberman sphere, which can be pulled in every direction via clever hinges to form ever larger spheres. “The superabsorber polymer can do this even better. In contact with water, it grows a thousandfold,” says Wilhelm. “It packs the water molecules into the interior of its web of cross-links and holds them there by means of ionic interactions.” The tighter the meshes of the cross-links—that is, the greater the degree of cross-linking—the more firmly the superabsorber holds the water, and the less it swells up.
»The problem of desalination has basically been solved, but we want to prove that there’s another way to do it«
a doctoral candidate at KIT, is developing systems to make desalination by means of superabsorbers measurable
For his thesis, Arens is now exploiting the fact that freshwater is quickly soaked up by the superabsorber, whereas salt and salt water take longer. “And vice versa,” he says, meaning that when he presses a superabsorber that is full of salt water, the salt water comes out first, followed by the freshwater. “By applying the necessary pressure, we can separate freshwater from salt water,” he concludes. In a long series of trials at the laboratory in Karlsruhe, he is using a specially made press to find out how effective and efficient this method is, how much energy it requires, and how strongly cross-linked it has to be.
It’s ultimately a question of energy. “Theoretically, the smallest amount of energy needed for desalination is one kilowatt-hour per cubic meter of the resulting freshwater,” says Wilhelm. “Present-day facilities need around ten times that much energy. We’re already in the same order of magnitude.” He’s thinking of the huge desalination plants in Saudi Arabia and the United Arab Emirates, where gigantic power plants produce hundreds of millions of liters of drinking water every day, partly by turning the seawater to steam and partly through reverse osmosis, in which water is pressed through special membranes. “Just the fact that we’re in effect using a three-dimensional membrane in our superabsorber offers this process advantages,” says Wilhelm.