Contamination of the Earth’s natural waters is a common malady stemming from human activities. The sources of aquatic and marine contamination are diverse, yet they all ultimately lead to the same final consequences; the death of aquatic species and the inhibition of aquatic habitat resiliency. Oil-spills are arguably the most widely known examples of water contamination, and can severely affect the well-being of aquatic, marine, terrestrial, and avian species. Despite the hazardous effects of oil-spills, the solutions for cleaning them, such as flocculation, oil skimming, and combustion, remain very costly and/or highly inefficient.
Hope is not yet lost however, as there appears to be a new oil-separation technology that most might not predict. Foam! Yes, foam, that light-as-a-feather-stiff-as-a-board material we all know and love. However, the foam that is the subject of this post is no ordinary insulating or packaging foam. The foam I speak of is developed through several chemical processes that ensure it is both superhydrophobic (water rejecting) and superoleophilic (oil accepting), which is a crucial property in water-separating materials.
First, the surface of a polyurethane foam sheet is coated with a graphite colloid, on which copper is then electroplated. The foam sheet is then placed into an aqueous solution of sodium hydroxide and potassium persulfate. The reaction between the copper and the aqueous solution results in the formation of complex nanostructures of copper hydroxide (Cu(OH)2) on the foam surface.
The foam is then treated with n-dodecanethiol dissolved in ethanol, which reacts with the copper hydroxide to form an organic copper compound (Cu(SC12H25)2).
The formation of the organic copper compound is important for three reasons: (1) the organic copper compound replaces the metal oxide which prevents the metal oxide from reacting and degrading from exposure to water, (2) the formation of an organic surface substrate allows for other organic substances to pass into the foam very easily, and (3) the organic copper compound retains the nanostructure network formed by the copper hydroxide.
The third reason mentioned above is the most important in ensuring that the copper foam does not allow water past the surface. Nano structures such as these form a rough surface for water molecules. This rough surface does not allow water to pass through it because water has very strong cohesive properties, meaning that it would rather stay as a droplet than form smaller individual micro-droplets that can pass through the pore size of the foam. Additionally, once the foam is wetted with oil, the water will simply not pass because oil and water do not mix.
All in all, treated copper foam presents tremendous possibilities for more efficient oil-spill cleanup, yet extensive field testing is still required to determine the practicality of superhydrophobic/superoleophilic foams. I for one have concerns relating to large scale (coastal/oceanic-scale) implementation of treated copper foam and similar materials. However, I am optimistic that such materials could be effectively used in smaller scale cleanup operations, where the actual volume of oil in water is limited to more manageable quantities. Additionally, the economic, energetic, and environmental feasibility associated with producing the metallic foam is uncertain. Foam is relatively cheap, but what about the energy demands of electroplating or the toxicity of the waste produced from creating the chemicals required to treat the copper foam? What are some thoughts you have? Please share any comments, questions, or concerns below.
Zhang, J., Ji, K., Chen, J., Ding, Y., & Dai, Z. (2015). A three-dimensional porous metal foam with selective-wettability for oil–water separation. Journal of Materials Science, 50(16), 5371-5377. doi:10.1007/s10853-015-9057-2