An oil-catching sponge, developed at the University of Toronto and Imperial, could help thwart water contamination from offshore oil drilling.
Drilling and fracking for oil under the seabed produces 100 billion barrels of oil-contaminated wastewater each year by releasing tiny oil droplets into surrounding water.
Most efforts to remove oil from water focus on removing large oil slicks from industrial spills but these aren’t suitable for removing tiny droplets. Instead, scientists are looking for new ways to clean the water.
Now, researchers at the University of Toronto (U of T) and Imperial College London have developed a sponge that removes over 90 per cent of oil microdroplets from wastewater within ten minutes.
When the sponge is used, the oil coats its surfaces like a thin film, via a process called adsorption. After use, the sponge can be treated with a solvent, which releases the oil from the sponge. The oil can then be recycled; the sponge, ready to be used again.
The sponge improves upon a previous concept: lead author Dr Pavani Cherukupally, now of Imperial’s Department of Chemical Engineering, had developed an early version of the sponge during her PhD at the U of T.
Although the previous sponge removed more than 95 per cent of the oil in the samples tested, it took three hours to do so – far longer than would be useful in industry.
Additionally, the sponge worked well only within a specific pH range.
Now, Dr Cherukupally, together with U of T and Imperial academics, has chemically modified the sponge to be of potential use to industry. The new sponge works faster, and over a much wider pH range than the previous version.
The results are published in Nature Sustainability.
"We manipulated (the sponge) to make droplets cling on tight. It’s all about strategically selecting the characteristics of the pores and their surfaces." said Dr Pavani Cherukupally, Department of Chemical Engineering.
To create the original sponge, Dr Cherukupally used ordinary polyurethane foams — similar to those found in couch cushions — to separate tiny droplets of oil from wastewater. The team carefully tweaked pore size, surface chemistry, and surface area, to create a sponge that attracts and captures oil droplets while letting water flow through.
To improve the sponge’s properties in the new study, Dr Cherukupally’s team worked with U of T chemists to add tiny particles of a material known as nanocrystalline silicon to the foam surfaces. This meant they could better control the sponge’s surface area and surface chemistry, improving its ability to capture and retain oil droplets as a coating on the pores' surfaces – a concept known as critical surface energy.
After use, the sponge can be removed from the water and re-used.
Dr Cherukupally said: “The critical surface energy concept comes from biofouling research, which tries to prevent microorganisms and creatures like barnacles from attaching to surfaces like ship hulls.
“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.
“It’s all about strategically selecting the characteristics of the pores and their surfaces. Commercial sponges already have tiny pores to capture tiny droplets: Polyurethane sponges are made from petrochemicals, so they have already had chemical groups which make them good at capturing droplets.
“The problem was that we had fewer chemical groups than what was needed to capture all the droplets. I therefore worked with U of T chemists to increase the number of chemical groups, and with Imperial’s Professor Daryl Williams to get the right amount of coating.”
Co-author Professor Amy Bilton from U of T said: “Current strategies for oil spill cleanup are focused on the floating oil slick, but they miss the microdroplets that form in the water.
“Though our sponge was designed for industrial wastewater, adapting it for freshwater or marine conditions could help reduce environmental contamination from future spills.”
Dr Cherukupally will continue to improve the sponge’s performance for oil applications and has teamed up with Dr Huw Williams at Imperial’s Department of Life Sciences to investigate how the sponges could remove bacteria from saltwater.
She also wants to use the sponges to treat contamination from gas, mining, and textile industries, and wants to make the technology affordable for use in developing countries – mainly for ridding contaminated rivers of organics, heavy metals, and pathogens.
Surface-engineered sponges for recovery of crude oil microdroplets from wastewater. Pavani Cherukupally, Wei Sun, Annabelle P. Y. Wong, Daryl R. Williams, Geoffrey A. Ozin, Amy M. Bilton, and Chul B. Park. 16 December 2019 in Nature Sustainability. https://doi.org/10.1038/s41893-019-0446-4.
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