Sponge speeds up removal of oil from wastewater

Drilling for oil under the seabed is said to produce 100 billion barrels of oil-contaminated wastewater every year by releasing tiny oil droplets into surrounding water.

To mitigate against this, researchers at the University of Toronto (U of T) and Imperial College London have developed a sponge that is claimed to remove over 90 per cent of oil microdroplets from wastewater within ten minutes.

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After capturing oil from wastewater, the sponge can be treated with a solvent, which releases the oil from the sponge. The oil can be recycled, and the sponge can be used again.

Imperial’s Dr Pavani Cherukupally developed an early version of the sponge during her PhD at the U of T. The sponge is claimed to have removed over 95 per cent of the oil in the samples tested, but it took three hours to do so. Acidity and alkalinity also presented an issue, as the pH of contaminated wastewater dictated how well the sponge worked.

In a statement, Dr Cherukupally said: “The optimal pH for our system was 5.6, but real-life wastewater can range in pH from four to ten. As we got toward the top of that scale, we saw oil removal drop off significantly, down to just six or seven per cent.”

Dr Cherukupally, along with colleagues from U of T and Imperial, has now chemically modified the sponge to work faster and over a wider pH range than the previous version. The results are published in Nature Sustainability.

To create the original sponge, Dr Cherukupally used polyurethane foams to separate tiny droplets of oil from wastewater. The team carefully adjusted pore size, surface chemistry, and surface area, to create a sponge that adsorbs oil droplets while letting water flow through.

To improve the sponge’s properties in the new study, Dr Cherukupally’s team added nanocrystalline silicon to the foam surfaces. They could then better control the sponge’s surface area and surface chemistry, improving its ability to capture and retain oil droplets, a concept called critical surface energy.

After use, the sponge could be removed from the water and treated with a solvent, releasing the oil from its surface.

Dr Cherukupally said: “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.

“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 Prof Daryl Williams to get the right amount of coating.”

Co-author Prof Amy Bilton from U of T said: “Current strategies for oil spill clean-up 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.”