Scientists from Korean Than Long University and Vietnamese Penikaa Institute of Science and Technology created a sponge for gathering oil spills, which operates on solar energy. It is based on loofah – a natural botanical pouf widely used in households and in agriculture in tropical countries. As it comes, it is not fit for working with petroleum products because it absorbs water very well, but the researchers successfully modified it turning into efficient sorbent capable of not only absorbing oil, but of partial releasing it back without mechanical squeezing.
At the first stage, lignin was chemically removed from loofah maintaining the 3D network of cellulose fibers with big capillary tubes. Then carbon quantum dots – the carbon nano particles with the size of less than 10 nanometers efficiently absorbing light in ultra-violet and visible range were applied to the surface of the fibers. The final stage was non-equilibrium plasma processing in argon, which allowed for removing a part of hydrophilic functional groups and for making the sponge surface water-repellent and oleophilic. Eventually, the water contact angle increased from 76.9° of the initial loofah up to 129.6° of the plasma-processed sample, and made 112.3° for the option with carbon dots, which proves fundamentally different behavior of the material at the water-oil interface.
Sunlight plays the key role in the sponge operation. Due to carbon quantum dots, the material absorbs circa 80% of radiation within 200-800 nm range, while as the non-modified loofah absorbs only about 20%. In case the illumination can be compared with normal sunlight during a clear day, the surface of the dry material is warmed up to 48-52 °C just within several minutes, and in case oil is present – up to 42-44 °C. Such temperature rise decreases the viscosity of petroleum products and speeds up their flow in the capillary tubes. During the experiments, the speed of oil moving inside the material reached 0.14 cm per second when illuminated and 0.086 cm per second in the dark.
As for its absorbing capability, the material demonstrated the parameters compatible with the best bio-mass and synthetic sorbents. In the dark the plasma-processed sponge with carbon dots absorbed up to 6.6 grams of oil per each gram of its own weight, and in the light – circa 6 g/g. To compare: the initial loofah captures approximately 4.4 g/g. At the same time, when warmed by the sunlight, the effect is dual – oil penetrates inside the material quicker, but simultaneously it gets out of big pores quicker as well.
This feature actually was the main result of the study. Different from the majority of sorbents requiring squeezing or pressing, the modified loofah is capable of releasing part of the collected oil by action of gravity. During the experiments, the efficiency of such self-release reached 41.7%, which is approximately 2.5 grams of oil per one gram of the material in the light. Even in the dark the sponge released circa 1.3 g/g. The maximum draining intensity was observed during the first 40 seconds, and after that the process stabilized without any deterioration of the structure.
In case the stated parameters are confirmed during the pilot run, the development of the Asian researchers may be a real breakthrough in removing oil spills in coastal and marine eco-systems.
