Synergistic dual-modification of polyurethane sponge with MIL-53(Al) and dimethylsilicone oil for efficient oil–water separation of oily sludge thermal washing wastewater

Oil and water are natural enemies. When they mix, as they often do in the aftermath of an industrial accident or during the processing of crude oil, they form a stubborn, messy emulsion that is incredibly difficult to separate. This is a persistent problem for industries that deal with oily sludge, a hazardous waste product created when oil is extracted and refined. To clean this sludge, companies often use heat and water to wash out the oil, but this process generates vast amounts of wastewater filled with tiny, suspended oil droplets and chemical contaminants. Traditional methods to clean this water, such as burning it or using harsh chemicals, are either wasteful or create new environmental hazards. The most promising solution has long been to use a material that acts like a sponge, but one that is smart enough to drink up the oil while letting the water pass right through. However, ordinary sponges are not selective; they soak up both, or they become clogged and useless after a single use.

Researchers at Zhejiang Sci-Tech University have developed a new type of sponge that solves these problems by combining two distinct materials into a single, highly efficient tool. They started with a standard piece of polyurethane foam, the kind of soft, porous material found in household sponges, and gave it a dual-layer makeover. First, they coated the sponge with a metal-organic framework, a crystalline material made of aluminum and organic molecules that creates a vast network of tiny, microscopic tunnels. This layer adds immense surface area and roughness, providing countless nooks and crannies for oil molecules to get trapped. Second, they treated the entire structure with dimethylsilicone oil, a substance known for its ability to repel water and attract oil. This combination creates a surface that is so water-repellent that water beads up and rolls off, while oil is drawn in instantly.

The team tested this new composite sponge, which they named M/Si-6@PU, on real wastewater generated from washing oily sludge. The results were striking. When a drop of oil touched the surface of the modified sponge, it vanished into the material in just 2.6 seconds, a speed that is nearly half the time it took for the same drop to be absorbed by an unmodified sponge. The sponge proved capable of holding a massive amount of oil relative to its own weight. It could absorb up to 43 grams of toluene for every gram of sponge, and it performed similarly well with other common oils like pump oil, diesel, and vegetable oil. Even when faced with the complex, dirty reality of actual industrial wastewater, the sponge managed to hold 31 grams of oil per gram of material.

Beyond its ability to grab oil quickly, the sponge demonstrated remarkable durability. In tests where the material was used to filter wastewater repeatedly, it maintained a separation efficiency of over 99 percent for the first eight cycles. Even after thirty cycles of use, it still removed more than 93 percent of the oil, showing that it does not easily wear out or lose its effectiveness. The researchers also tested its resilience in extreme conditions, submerging it in highly acidic and highly alkaline liquids for an hour. While the sponge began to show signs of wetting after that time, it held its hydrophobic properties long enough to be useful in harsh industrial environments where other materials might fail immediately.

The mechanism behind this success relies on the unique structure created by the two coatings. The metal-organic framework provides a rough, hierarchical texture that traps tiny pockets of air, creating a barrier that water cannot easily cross. Meanwhile, the silicone oil layer lowers the surface energy of the sponge, making it naturally attracted to oil and repelled by water. When the wastewater touches the sponge, the sharp, microscopic structures physically break apart the stable emulsion, allowing the oil to be pulled deep into the sponge's pores by capillary forces. At the same time, the chemical nature of the coating ensures that the oil sticks firmly to the sponge while the water flows through. This dual action allows the material to not only separate free-floating oil but also to capture dissolved organic pollutants and emulsified droplets that usually slip through other filters.

The study confirms that this approach is a viable path forward for treating difficult industrial waste. By using a simple dipping process to apply these two materials, the researchers created a scalable solution that does not require complex machinery or expensive chemicals. The sponge's ability to handle real-world wastewater, resist chemical corrosion, and maintain high performance over many uses suggests it could be a practical tool for cleaning up oil spills and treating industrial runoff. The work highlights how combining a rigid, porous structure with a flexible, water-repelling coating can create a material that is far more effective than the sum of its parts, offering a new way to tackle one of the most persistent challenges in environmental cleanup.