Marine oil spills can have disastrous consequences for ocean ecosystems. Existing methods for cleaning up oil spills are costly and inefficient and underscore the need for environmentally friendly remediation strategies. A research collaboration between Central South University, Huazhong University of Science and Technology, and Ben-Gurion University of the Negev demonstrated that laser-treated cork can offer a sustainable solution for the cleanup of oil-contaminated seawater. The researchers used a femtosecond laser processing technique to transform ordinary cork into a solar-driven tool for treating oil spills. The laser-modified cork, which the researchers call FSLA-cork, exhibited a high rate of light absorption and efficient photothermal conversion. Like many scientific discoveries, FSLA-cork was the byproduct of unrelated research. “In a different laser experiment, we accidentally found that the wettability of the cork processed using a laser changed significantly, gaining superhydrophobic (water-repelling) and superoleophilic (oil-attracting) properties,” researcher Yuchun He said. “After appropriately adjusting the processing parameters, the surface of the cork became very dark, which made us realize that it might be an excellent material for photothermal conversion.” When the researchers treated the cork with a femtosecond laser, carbonization occurred, creating regular micro- and nanostructures on the surface of the material. They examined the nanoscopic structural changes in the cork and measured the ratio of oxygen and carbon in the material. They measured changes in the angle at which water and oil contacted the surface of the cork, and measured the material’s light wave absorption, reflection, and emission across the spectrum to determine its durability after multiple cycles of warming and cooling. The optimized FSLA-cork demonstrated excellent durability, maintaining a water droplet contact angle of about 160° after 100 days of storage. “Combining these results with the ecofriendly, recyclable advantages of cork, we thought of using it for marine oil spill cleanup,” professor Kai Yin said. “To our knowledge, no one else has tried using cork for cleaning up marine oil spills.” To remove oil from water, the authors used cork treated by a fast-pulsing laser method that removes some oxygen from the material, increasing the relative carbon and making it more water-repelling and oil-attracting. Laser treatment also alters the structure of the cork. When viewed at the nanoscopic level, the material has deep grooves, which increase the total surface area of the cork and allow it to trap sunlight and warm the oil, making the oil easier to collect. Courtesy of Yuchun He. The carbonized surface of the cork had a uniform microgroove structure that gave it a solar absorption rate of greater than 90%. The material’s high light absorption rate enabled it to reduce the viscosity of adsorbed oil through solar energy irradiation, without the need for additional energy. The photothermal properties of the material caused it to warm quickly in the sun and the material’s deep grooves increased the amount of surface area exposed to sunlight. The researchers found that the material could be rapidly heated from ambient conditions to 58 °C in 10 seconds under 1.8 solar irradiation. Heating the spilled oil made it easier to collect. At one-sun intensity, the researchers observed an oil absorption rate of 4.02 grams per square centimeter (4.02 g cm−2) within 200 seconds. To further enhance the efficiency of the oil collection process, the researchers developed a viscous oil recovery system with a centrifugal pump. The femtosecond laser treatments not only helped the material absorb oil, but also helped it keep out water. “When the cork undergoes a fast-pulsing laser treatment, its surface microstructure becomes rougher,” Yin said. “This micro- to nano-level roughness enhances hydrophobicity.” As a result, the cork collects the oil without absorbing water, so the oil can be extracted from the cork and possibly reused. In contrast to chemical-based remediation methods, no harmful by-products are produced with the use of FSLA-cork. The bark of the cork oak can self-renew, and harvesting it does not harm the tree. When the bark is removed, the trees amplify their biological activity to replace the bark and increase their carbon storage, so harvesting cork helps mitigate carbon emissions. The femtosecond laser processing technique is also an environmentally friendly processing method. The properties of the FSLA-cork material, which include hydrophobicity, stability, high photothermal conversion efficiency, low raw material cost, and availability, in combination with a clean, efficient preparation process, make it a viable method for the large-scale cleanup of marine oil spills. “Oil recovery is a complex and systematic task, and participating in oil recovery throughout its entire life cycle is our goal,” He said. “The next step is to prepare electrothermal materials using polyurethane foam as the skeleton for oil adsorption, combining photothermal and electrothermal techniques to form an all-weather oil recovery system.” The research was published in Applied Physics Letters (www.doi.org/10.1063/5.0199291).
