Over the years, a wide range of solutions have been deployed to tackle such spills, including membrane filtration, floatation, and in-situ combustion, all of which can be costly and inefficient. Another option is to use oil-absorbing materials, and for this, many people look to aerogels. Lightweight yet robust, these porous structures can absorb liquids many times their own weight. In a recent issue of Carbon [DOI: 10.1016/j.carbon.2022.03.015], researchers at Nanjing Forestry University report on an aerogel made from modified lignin, which they say exhibits excellent oil and water separation performance in harsh environments.
Lignocellulose is one of the most abundant raw materials on Earth. Comprised of three polymers – cellulose, hemicellulose and lignin – it is found in all plant matter, and as such, is the feedstock for the pulp and paper industry. Of the 70 million tons of lignin produced by that industry every year, 98 % of it goes to waste, which prompted this group of researchers to investigate its potential for use as a reusable oil/water separation medium.
They started by combining graphene oxide and aminated lignin, before adding borax to form additional cross-links between the two. After heating and freeze-drying, the composite aerogel was formed. It was further modified with methyltrimethoxysilane to make the structure more thermally stable. This step also transformed the surface from hydrophilic to superhydrophobic. The resulting MTMS-modified lignin-graphene oxide aerogel (MLGA) – a cylinder measuring 18 mm in height – was highly porous, with a rough, textured surface.
The aerogel was put through a series of tests for oil absorbency and reusability.
First, the aerogel was weighed before and after being placed in one of a series of light and heavy oils; n-hexane, acetone, toluene, dichloromethane, and chloroform. The researchers found that the higher the oil density, the stronger the adsorption capacity of MLGA. When the oils were combined with water, the aerogel absorbed the oil while repelling the water. They also showed that MLGA could work very efficiently as a continuous oil/water separation medium, when connected to a peristaltic pump and plastic tubing.
Next, the aerogel underwent combustion experiments, to see how feasible it would be as a ‘regeneration’ step. Lignin’s high pyrolysis temperature means that it can be directly ignited, and the addition of Si-O bonds further improve its flame retardancy. So in these experiments, once fully saturated with oil, the aerogel was lit using a lighter until the flame burned out. Combustion complete, the aerogel could then be reused, often within 1 min of initial ignition. This was repeated for 20 more cycles, during which the aerogel retained its mechanical stability and superhydrophobicity.
The acid/alkali resistance and salt/temperature tolerance of the aerogel was also tested by immersing it in a series of different solutions. The authors say that even in a hot, saline solution, the MLGA was found to “maintain excellent reusability and stability, which suggests broad application prospects in the marine environment.”
They conclude, “We believe that this low-cost, environmentally friendly oil-absorbing material has great potential for the treatment of oil spill pollution.”
This story was written for Materials Today, and was published on their site on 7th March 2022: https://www.materialstoday.com/carbon/news/aerogel-clean-up-oil-spills/
Original research paper: Yiying Yue, Yu Wang, Junyao Li, Wanli Cheng, Guangping Han, Tao Lu, Chaobo Huang, Qinglin Wu, Jianchun Jiang. “High strength and ultralight lignin-mediated fire-resistant aerogel for repeated oil/water separation,” Carbon 193 (2022) 285-297. DOI: 10.1016/j.carbon.2022.03.015