Wooden sponges can separate water and oil

Once fuel oils, industrial solvents, and other organic liquids enter a body of water, they are difficult to remove. The materials commonly used to extract such contaminants often require complex assembly procedures and are not always mechanically robust or environmentally friendly. Now Xiaoqing Wang and colleagues at the Research Institute of Wood Industry in Beijing have developed a relatively simple treatment process that turns balsa wood into a mechanically robust sponge that can selectively remove organics from water.

Natural wood lacks the compressibility and absorbency usually associated with sponges. To give the balsa wood those properties, the researchers removed two of its primary cell-wall components, lignin and hemicellulose, by using aqueous chemical treatments. That changed the wood’s cell structure from a rigid honeycomb to a compressible, lamellar morphology. Even after 100 compressions, the sponge sprang back to its initial size, which indicated that it kept its mechanical robustness despite the removed material.

To prevent the sponge from absorbing water along with organic liquids, the researchers made the sponge hydrophobic by using a silane vapor deposition treatment. It readily absorbed silicone oil (top row of the figure), dichloromethane (bottom row), and numerous other oils and organic liquids. The sponge held 16–41 times its own weight in liquid, which filled 70% of its volume.

The balsa-wood sponges float on water, so they are easy to retrieve. They can then be squeezed out and reused because they keep most of their absorbency. The researchers demonstrated that the sponges can also be used for the continuous removal of contaminants from water by inserting a glass tube into the sponge and pumping the liquid out as it is absorbed. Because the silane-treated wood sponges perform well and their production is scalable, they show great promise as candidates for large-scale removal of oil and other organic pollutants from water. (H. Guan, Z. Cheng, X. Wang, ACS Nano 12, 10365, 2018 .)