By mimicking the way some fishes eat, a new membrane easily separates and collects spilled oil on water without getting clogged (ACS Nano 2017, DOI: 10.1021/acsnano.6b07918). It could be an efficient and cost-effective way to clean up large oil spills, its developers say.
Disaster responders typically clean up large oil spills by containing the slick with floating booms and using skimmers to remove it. Many researchers are developing separation membranes that could potentially be faster and cheaper. These are designed to repel water or attract oil, which helps them separate the two liquids. But the membranes’ pores tend to get clogged with oil, which makes them ineffective after a while.
Dongliang Tian of Beihang University, Ziqi Sun of the University of Wollongong, and their colleagues designed a new filter inspired by the throat structures of filter-feeding fish. To filter out tiny prey suspended in water, these fish have bony arches in their throats that get narrower and more closely spaced deeper into the throat. Water flows into the throat, gradually seeping out from the spaces between the arches and out through the gills, while food particles collect at the back.
To mimic that process, the researchers made a 3-cm-long stainless steel membrane containing five mesh sections with gradually decreasing pore size—from 150 nm to 30 nm—from one end to the other. They coated the membrane with nanosheets of cobalt oxide which intertwine with each other, forming tiny pockets that lock in water, making the membrane water-attracting, or hydrophilic. Then they tilted the membrane so that the large pores were at the bottom and pushed it with a controller attached to the top, emulating how a ship might push the angled membrane, bottom edge first, through the water.
When the driving system moves the membrane through an oil-water mixture, the liquids stream up the membrane. Water permeates the membrane and forms a layer along its hydrophilic surface, preventing oil from clogging the pores. The large pores at the bottom, which encounter the highest water flux, allow water to flow through faster, Tian explains, while the water-logged small pores at the top repel oil, allowing it to easily flow over the top of the membrane into a container. Though oil may pass through the large pores on the first pass, the system makes multiple sweeps through a contaminated area, capturing more oil with each pass.
The researchers tested the system in a 100-cm-long, 10-cm-wide sink filled with a mixture of water and one of a variety of oils: crude oil, diesel fuel, corn oil, or hydraulic fluid. They were able to continually collect oil at a calculated rate of 50 liters per minute for each meter length of the membrane, pushing the membrane along the sink more than 2,000 times over 100 minutes. The efficiency drops by less than 3% after those uses, and Tian says cleaning the membrane would restore its efficiency.
By comparison, when the team tested a conventional setup by pouring an oil-water mixture through a 90-µm pore size membrane, the membrane became clogged and unusable after 50 uses.
This novel filtration technique could enable “one-step, fast, continuous, and high-throughput spilled oil collection,” says Lin Feng of Tsinghua University. The technique has promise for use in large-scale oil spills, especially on lakes and quiet seas, she says. Waves could be a challenge since the water could spill over the top of the membrane into the oil-collection vessel and reduce the membrane’s efficiency.
The researchers suggest that adjusting variables including pore sizes, the ship’s speed, and the membrane length and tilt could help withstand choppy water.