Novel polymer sponge likes to soak up oil

Drilling and fracking for oil under the seabed produces 100 billion barrels of oil-contaminated wastewater every 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 the same approaches 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) in Canada and Imperial College London in the UK have developed a sponge that removes over 90% of oil microdroplets from wastewater within 10 minutes. After capturing oil from wastewater, the sponge can be treated with a solvent, which releases the oil from the sponge. This allows the oil to be recycled, while the sponge is ready to be used again.

The new sponge improves upon a previous version, which lead author Pavani Cherukupally, now at Imperial's Department of Chemical Engineering, developed during her PhD at the U of T. Although the previous sponge removed more than 95% of the oil in the samples tested, it took three hours to do so – far longer than would be useful for industry.

Acidity and alkalinity also presented an issue, as the pH of contaminated wastewater dictated how well the sponge worked. "The optimal pH for our system was 5.6, but real-life wastewater can range in pH from four to 10," said Cherukupally. "As we got toward the top of that scale, we saw oil removal drop off significantly, down to just six or seven per cent."

Now, Cherukupally, together with colleagues at U of T and Imperial, has chemically modified the sponge to be of potential use to industry. As the researchers report in a paper in Nature Sustainability, the new sponge works faster, and over a much wider pH range, than the previous version.

To create the original sponge, Cherukupally used ordinary polyurethane foams – similar to those found in couch cushions – to separate tiny droplets of oil from wastewater. The team carefully tweaked the pore size, surface chemistry and surface area of these foams to create a sponge that attracts and captures oil droplets – a process known as 'adsorption' – while letting water flow through.

To improve the sponge's properties in the new study, 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 gave better control over the sponge's surface area and surface chemistry, improving its ability to capture and retain oil droplets – a concept known as critical surface energy. After use, the sponge could be removed from the water and treated with a solvent, releasing the oil from its surface.

"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.

"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."

"Current strategies for oil spill cleanup are focused on the floating oil slick, but they miss the microdroplets that form in the water," said co-author Amy Bilton, a professor at U of T. "Though our sponge was designed for industrial wastewater, adapting it for freshwater or marine conditions could help reduce environmental contamination from future spills."

Cherukupally will continue to improve the sponge's performance for oil applications, but she has also teamed up with Huw Williams at Imperial's Department of Life Sciences to investigate how the sponges could be used to remove bacteria from saltwater. In addition, she wants to use the sponges to treat contamination from gas, mining and textile industries, and intends to make the technology affordable for use in developing countries – mainly for ridding contaminated rivers of organics, heavy metals and pathogens.

This story is adapted from material from Imperial College London, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.