For sustainable society


Sponge that adsorbs oil from wastewater

Many people may have heard of the term the “shale revolution,” the boom in shale gas and shale oil development in the United States since 2006. Shale gas is natural gas that exists in gaps between rocks known as “shale,” and shale oil is generated from the shale. The United States was the country with excess oil imports before the time of the shale revolution, but as the production of shale gas and oil has increased, the gap between imports and exports has gradually been reduced. Last September, export volume exceeded import volume for the first time in 70 years.

There is a concern that shale gas and oil will cause environmental destruction during oil-drilling, and oil-polluted water is discharged by the hydraulic fracturing (*1) technology used when extracting them. The water is clarified; however, only the oil film floating on the surface is removed, which means it is not completely removed.

Research teams at the University of Toronto in Canada and Imperial College London have worked hard to clean the oil-polluted water and have developed a special sponge. The sponge removes more than 90% of tiny oil droplets from wastewater in just 10 minutes. This revolutionary sponge was completed by improving a prototype reported two years ago. The prototype had two challenges. The first was that it took three hours to remove 95% or more of the oil droplets, making it difficult to be used for an industrial-scale process. Second, the optimal pH for the prototype sponge was 5.6, but real-life wastewater ranged in pH from around four to 10, which showed the removal rate dropped off. To solve these two challenges, researchers used polyurethane foams, such as those used in couch cushions, and found that tiny droplets of oil could be separated from wastewater. In addition, they controlled the surface area, surface chemical properties, and pore size and designed the newest sponge that collects and adsorbs oil. This allows the newest sponge to work over a wider pH range than the prototype and remove more than 92% of the oil in just 10 minutes at an optimal pH 5.6.

You may be wondering how to treat the sponge after absorbing the oil. Actually, oil can be discharged by being processed with a solvent, and both sponge and oil can be reused.  According to tests by the research team, there was no performance degradation with 10 attempts.

“I want to use the sponge to treat discharged water from gas, mining and textile industries, rid contaminated rivers of organics and pathogens, and make it affordable cost for use in developing countries,” says Pavani Cherukupally, a graduate student at the University of Toronto. I hope that the use of the sponge will spread in the future.

*1 hydraulic fracturing: a technology with digging a deep hole into the ground, pouring a large amount of water containing chemicals at high pressure, and artificially creating cracks to extract oil.

Photo: University of Toronto

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