Nature has a remedy for oil spills, and it’s all over the place

Certain aquatic plants that are unusually water-repellent and oil-attracting may represent an opportunity for eco-friendly cleanup of oil spills—and removal of invasive weeds in the process, according to research published last week in the journal Bioinspiration and Biomimetics.

Existing methods of dealing with oil spills all have significant drawbacks. Chemical dispersants and burning can spread toxins around, while environmentally friendly materials like sawdust and wheat straw absorb water in addition to oil, making cleanup messy and inefficient.

To improve the situation, researchers from the Karlsruhe Institute of Technology and the Nees Institute for Biodiversity of Plants in Germany turned to Salvinia molesta, an aquatic fern, and Pistia stratiotes, a type of water lettuce.

The leaves of these plants are covered with tiny, hair-like structures called trichomes, which in turn are coated with water-resistant wax. This specialized leaf anatomy repels water and traps air, helping the plant float on the surface of the water. As it turns out, it can also trap a lot of oil.

The researchers showed that the dried leaves of both plants absorb more oil than two commercial oil sorbents used for oil spill cleanup, Deurex Pure and Öl-Ex. They are also superior to an experimental product known as nanofur developed by scientists at the Karlsruhe Institute of Technology. S. molesta leaves can sop up a puddle of oil in less than 30 seconds.

The leaves of S. molesta and P. stratiotes absorb more oil than lotus (Nelumbo nucifera) leaves, which have a waxy coating but no hairs. In addition, the synthetic oil sorbents with a hairy texture, nanofur and Deurex Pure, soak up more oil than the non-hairy option, Öl-Ex. These results suggest that trichomes are crucial to oil absorption.

To find out more about how these hairs work, the researchers tested the leaves of four different Salvinia species, each with differently shaped and sized trichomes: short and slightly bent; in groups of four; short and bent, with pairs connected at the tips; and tall, with groups of four connected at the tips. This last type of trichome belongs to S. molesta and is the most efficient at absorbing oil. The connected ends, which give the trichome a shape reminiscent of an eggbeater, play a key role in trapping oil, the researchers say.

These insights could give researchers clues to improve nanofur or create other synthetic substances for oil cleanup. Specifically, instead of designing materials with any old hairy surface, they should aim for long, interconnected hairs that will form pockets to trap oil.

The study also suggests an even simpler potential solution to oil spill cleanup: just use the leaves of S. molesta or P. stratiotes themselves to mop up spills. In fact, harvesting large quantities of these plants would help solve a second problem, since the two species are considered invasive weeds choking subtropical waterways all over the world.

Past studies have suggested that these plants could help remove contaminants like heavy metals, nitrate, and phosphate from water. Now researchers have added oil to that list, which just goes to show that every once in a while, you even have to give weeds their due. —Sarah DeWeerdt | 23 August 2016

Source: Zeiger C et al. Microstructures of superhydrophobic plant leaves – inspiration for efficient oil spill cleanup materials. Bioinspiration and Biomimetics. 2016.

Header image: Water fern (Salvinia spp.) at the University of Oxford Botanic Garden. Credit: Tim Waters via Flickr.