Soaking up oil spills — with cotton
This low-grade natural material may become a super picker-upper for petroleum
Crude oil is still washing ashore more than four years after the BP Deepwater Horizon accident spilled more than 200 million gallons of this petroleum into the Gulf of Mexico. Fisheries, wildlife and ecosystems could suffer for decades. Now help for cleaning up such disasters comes from a crop people have grown for thousands of years: cotton. But this material is a lot different from the fabric in your favorite tee shirt.
To work well on oil spills, the substance used to pick up the mess — a sorbent — should sop up oil but not water. Cotton in its natural form has a waxy coating. As such, it will “absorb oil and repel water,” explains Seshadri Ramkumar. He’s a materials scientist at Texas Tech University in Lubbock.
Just throwing a huge wad of cotton onto a spill isn’t enough, however. Cotton soaks up oil best when it can use three processes at once. In the first — adsorption — oil clings to the surface of the cotton fibers. The fibers may also absorb oil, bringing it inside the fibers. (That’s the same process by which plant roots take up water from the soil.)
Finally, cotton can soak up oil by letting it flow into channel-like spaces that form between its fibers. This last process is known as capillary action. It’s the process by which blood flows into a narrow tube when a nurse pricks your finger for a sample. The tiny spaces between cotton fibers can act like those blood tubes. But in natural cotton, oil can’t get far because the fibers are tangled.To untangle them, the researchers card — or comb — the cotton. A carding machine has a cylinder with rows and rows of tiny prongs. The machine pulls the fibers straight so that they all go in the same direction. “It’s just like you’re combing your hair,” explains Vinitkumar Singh. A graduate student at Texas Tech, he also worked on this project.
The researchers stacked up layer after layer of carded cotton. “Everything is in the same direction,” Ramkumar explains. Together, these layers make a batting. It’s similar to the batting used to fill the inside of a quilt. But instead of being stitched or pressed tightly down, the batting for cleaning up oil must stay loose.
Friction between the layers makes them cling loosely together. “It is not a very strong bond,” says Singh. That looseness creates lots of channels into which oil can flow and collect.
When combined, the three sopping processes let cotton soak up oil quite well. And low-grade cotton that’s not mature works about 7 percent better than high-quality mature cotton. The reason: Immature cotton has more wax. Thus, it repels water better. Those young fibers also are finer. That gives them a relatively bigger surface area for adsorption and to form channels for capillary action.
In lab tests, the low-grade cotton batting absorbed 50 times its weight in oil. That’s better than what many plastic materials do. And unlike plastics, cotton decomposes naturally when it can’t be used any more. Ramkumar and his colleagues at Texas Tech and Cotton Incorporated in Cary, N.C., reported their findings in the July 30 Industrial & Engineering Chemistry Research.
Other advantages — and questions
“Cotton is also easy to remove once it’s done its job,” Ramkumar told Science News for Students. Oil-soaked batting will float on water. That’s because it has a lower specific density than water. With less mass than the same volume of water, this oil helps keep the cotton batting afloat.
Using low-grade cotton for oil clean-ups also could bring farmers more money when crops don’t mature due to drought or other problems. Roughly one-fifth of the cotton grown in Texas, for instance, falls into the low-grade category, Ramkumar says. It usually sells for less money because immature cotton has less cellulose. Fabric mills that make clothing don’t want it because this kind of cotton doesn’t handle dyes well. But what makes a poor cotton for clothing may prove a superior type for oil clean-ups.
The novel structure of the batting might help it sop up oil better, says Paul Sawhney. He’s a textile scientist with the U.S. Agricultural Research Service in New Orleans, La.
But as a cleanup tool, what also will matter is how the batting holds up, Sawhney notes. “Once the oil is in there, you’re talking about 50 times more weight,” he points out. The batting needs to hold that liquid in. And the batting should stay intact when it’s moved and eventually lifted up for removal.
Field tests can explore different ways to ensure that. Lightly needlepunching or stitching the batting’s layers together might help, Sawhney says. Encasing the batting in an expandable web is another idea.
But that’s how science works. Each advance suggests more questions to explore.
Sadly, spills happen. Indeed, hundreds of gallons of motor oil and hydraulic fluid spilled into the Grand River in Michigan earlier this year. A ship collision spilled oil into the Mississippi River last month. And some 9,000 gallons of diesel fuel spilled into the Ohio River from a power plant near Cincinnati. Accidents can be limited — but never completely prevented. That’s why having cleanup tools at hand is important — especially simple, inexpensive and high-performing options, such as raw-cotton batting might offer.
absorption The process by which a fluid penetrates another material, such as crystals or fibers. Liquids and nutrients enter plant roots through absorption.
adsorption The process by which a substance sticks to, or adheres, to the outer surface of another material. Sunscreen stays on your skin because of adsorption.
batting In textile science, a lofty material, usually nonwoven, such as the filling between quilt layers. In baseball, the act of swinging a machine-tooled stick with hopes of hitting a ball.
capillary action The force that governs the movement of a liquid along the surface of a solid. Because molecules of the liquid are attracted to the surface and to each other, they can pull each other along. Capillary action explains how sponges wick up liquids.
cellulose A type of fiber found in plant cell walls. It is formed by chains of glucose molecules.
chemistry The field of science that deals with the composition, structure and properties of substances and how they interact with one another. Chemists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances.
crude oil Petroleum in the form that it comes out of the ground.
ecosystem A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.
engineering The field of research that uses math and science to solve practical problems.
friction The resistance that one surface or object encounters when moving over or through another.
materials science The study of how the atomic and molecular structure of a material is related to its overall properties. Materials scientists can design new materials or analyze existing ones. Their analyses of a material’s overall properties (such as density, strength and melting point) can help engineers and other researchers select materials that best suited to a new application.
needlepunching Perforating layers of fibers to hold them together or make a design.
plastic Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.
specific density A measure of mass per unit of volume. The specific density of water is usually 1 gram per cubic centimeter.
textile Cloth or fabric, which can be woven or nonwoven.