Crude oil is sticky stuff and often clogs filters and other equipment used in the oil and gas industry. To address this problem, scientists at the U.S. Department of Energy’s (DOE) Argonne National Laboratory developed a new coating which should prolong the lifetime of key industrial equipment.
The coating creates thin films of water-loving, oil-repelling metal-oxide molecules on the surface of filter membranes. These molecules grab onto any loose water atoms while avoiding oil. To scientists, these properties are known as hydrophilicity and oleophobicity, respectively.
“One of the best ways to clean oily water is with membranes,” says Seth Darling, director of the Institute for Molecular Engineering at Argonne. “But oil sticks on the membrane, clogs the holes, and the membrane soon stops working. Today, if people have an oil-fouled membrane, either they replace it or try to clean it with harsh chemicals to wash away the oil.”
The scientists used atomic layer deposition to deposit a thin coating of the metal oxide on the filter’s membrane. Atomic layer deposition isn’t new, but according to Darling, it’s never been used to modify filter membranes before. The coating is just a few nanometers thick; if it were any thicker, it would close off the tiny pores. The goal was to create only a minimal change of the pore structure while completely altering the chemistry of the pores’ lining. The team experimented with different metal oxides on off-the-shelf commercial polymer membranes to find which ones worked the best.
To create this coating in the past, researchers tried attaching nanoparticles to a membrane by flowing them through or growing them on it. But particles tend to get ripped off as water flows through the filters. Atomic layer deposition is different because the metal oxide film forms strong chemical bonds with the filter.
The team found that tin oxide and titanium oxide formed the tightest bonds with water molecules, capturing them and layering them across the surface. But when oil contacts the membrane, it stays separate because it flows over the water layer.
This could be a boon for the oil and gas industry, where fouled membranes are a costly hassle. For instance, when oil companies replace clogged filters when hydraulic fracturing, they must shut down the equipment to make the change. Oil-resistant membranes could significantly reduce the need both for filter replacement and the downtime it creates. Another problem in the industry involves the water used in fracking, which is often returned to the ground with oil, salt, and other contaminants. Contaminated water cannot be returned to the ground if it poses a threat to aquifers, so the industry must find another way to dispose of it.
The membranes used now can remove the other contaminants, but they cannot handle oil. The new coating would let them reuse the oily water.
The coating could also help in oil spill cleanups. In an oil spill response, diesel fuel serves as a cleaning agent on pipes and containers, which leaves them coated with diesel mixed with oil and dirt. But pipe and container surfaces treated with the oxides could just be rinsed clean.