It is now possible to continuously remove oil and other pollutants from water, thanks to a new sponge that absorbs oil, but is water-repellant.
The development greatly simplifies the process of separating oil from water, and also has applications in environmental cleanup.
Researchers at Purdue University in Indiana, USA, modified melamine sponges by dipping them into a solution containing a small amount of silicone rubber called PDMS, and the solvent hexane. The thin coating from this process repels water, but allows oil to be absorbed into the sponge, making the material both superhydrophobic and superoleophilic.
The base material, melamine sponge, is an ultra-low-weight, porous material found in various various products, including household cleaning pads and in insulation. According to Suresh V. Garimella, Purdue University's executive vice president for research and partnerships and the Goodson Distinguished Professor of Mechanical Engineering, the wide availability of the material and the simple manufacturing process means the technology can be quickly scaled up.
This is unlike other similar technologies being developed, which tend to be expensive, difficult to scale up, or require the use of exotic materials like carbon nanotubes and graphene.
"It is manufactured using a very inexpensive one-step process to coat the melamine sponges, and the material can be reused many times," said Garimella, "We believe this can be readily adopted for the cleanup of oil spills and industrial chemical leaks.”
The researchers have demonstrated that the new material can remove oils and organic chemical pollutants that are immiscible (does not readily mix in) with water such as hydrocarbons, coolants and insulating fluid from electrical transformers, carcinogenic compounds called PCBs and certain pesticides.
For maximum efficacy, the pollutant should have a low surface tension. The difference in surface tension is key for the separation mechanism to work. The relatively high surface tension of water causes it to be repelled by the coating on the sponge.
In demonstrations, the researchers contaminated a container of water with red-coloured silicone oil. They then swept the sponge material over the surface of the liquids, absorbing the oil layer but not the water. The oil-saturated sponge then floats in the water for easy retrieval.
By allowing the sponge to “self-select” for oils, the researchers say the technology could make chemical and oil spill cleanups much quicker and efficient. While the sponge material can absorb approximately 45 to 75 times is own weight, it could also be used continuously by applying suction, so the sponge continuously draws out the oil, but leaves the water behind.