In the United States alone, 15 billion barrels of oil-contaminated wastewater are produced annually from seabed drilling and fracking. While large oil spills are the most visible repercussion, scientists say that oil micro-pollutants are just as prevalent but even harder to clean-up — until now. New research has designed a super-absorbent sponge that can collect up to 99 percent of these pollutants and be reused up to ten times.
The study was published Monday in the journal Nature Sustainability and explores how the roughness, pore size and surface chemistry of a sponge affects its ability to absorb these tiny oil droplets. In particular, a barrier to overcome for this team was to be able to maintain absorption effectiveness at different pH levels, which are dictated by the pollutants. By physically and chemically manipulating the composition of their sponge, the team was able to modify its total surface energy such that it could effectively collect oil across a wide range of pH environments.
Pavani Cherukupally, the study’s lead author, said in a statement that this approach is borrowed from another kind of nautical research on deep-sea barnacles.
“The critical surface energy concept comes from the world of biofouling research — trying to prevent microorganisms and creatures like barnacles from attaching to surfaces like ship hulls,” said Cherukupally. “Normally, you want to keep critical surface energy in a certain range to prevent attachment, but in our case, we manipulated it to get droplets to cling on tight.”
Crucially, the sponge itself was also hydrophobic, meaning that it was able to collect only the pollutant materials without becoming waterlogged itself.
After the oil droplets have been initially collected by the sponge, they can then be extracted using a special solvent. This solvent completely releases the droplets from the sponge’s surface and leaves it clean and ready to use again. The authors found that the sponge could be reused up to ten times. The study authors also write that the collected oil can be reused to create extended economic value as well.
While this sponge is not the first micro-pollutant collection approach ever taken — even by Cherukupally herself! —, this design does stand apart from its predecessors in a few key ways. Namely, some of the sponge’s characteristics — like pore size — were designed not only to affect its surface energy but to solve flaws from previous approaches as well.
For example, while activated carbon is a material that is also used for pollutant absorption, it suffers from quickly clogging pores. This sponge’s pores were relatively larger (though still minuscule at just 200 - 1,000 micrometers) and thus allowed the micropollutants to better adhere to internal surfaces without creating a roadblock.
While the sponge in this study was specifically designed to capture pollutants in a salt-water environment, the authors hope that future iterations of this research could work in freshwater as well.
Part of this interest, said Cherukupally, is to explore how such a sponge might be used to collect industrial pollutants and even pathogens from rivers in developing countries. As river water is still a primary source of drinking water in so many parts of the world, having ways to effectively and cheaply clean it is essential to global public health and well being.
In the United States, the oil industry produces over 15 billion barrels of wastewater contaminated with crude oil microdroplets annually. Current methods are ineffective for the removal of these microdroplets at the variable pH conditions commonly found in wastewater. Here, an innovative surface-engineered sponge (SEnS) that synergistically combines surface chemistry, charge and roughness, provides a solution to this problem. Over broad pH conditions, the SEnS rapidly adsorbed oil microdroplets with 95–99% removal efficiency, predominantly facilitated by Lifshitz–van der Waals forces. At the optimum pH, 92% of the oil was adsorbed within 10 min. The oil was subsequently recovered by solvent extraction under ambient conditions, and the cleaned SEnS was reused for oil microdroplets adsorption ten times. The combined efficacy and reusability can enable largescale removal and recovery of crude oil microdroplets from wastewater.
