Apr 8 2021
A research team, headed by Nanyang Technological University, Singapore (NTU Singapore) has produced a biodegradable, reusable sponge that can instantly absorb various organic solvents, including oil, from polluted water sources. This feature makes it a potential alternative for dealing with marine oil spills.
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The sponge is composed of sunflower pollen and is hydrophobic, which means it repels water and this feature can be attributed to a layer of natural fatty acid on the sponge.
In laboratory experiments, the team demonstrated the ability of the sponge to soak up oil contaminants of numerous densities, like motor oil and gasoline, at a speed similar to that of other oil absorbents available in the market.
It is difficut to clean up oil spills, which cause significant long-lasting damage to the marine ecosystem. Traditional clean-up techniques, such as using chemical dispersants to break down oil into tiny droplets, or soaking it up with costly, non-recyclable materials, may lead to more damage.
To date, the investigators have designed sponges measuring 5 cm in diameter. According to the team, which included researchers from NTU Singapore and Sungkyunkwan University in South Korea, when these sponges are upgraded, they could provide an eco-friendly alternative to deal with marine oil spills.
By finetuning the material properties of pollen, our team successfully developed a sponge that can selectively target oil in contaminated water sources and absorb it. Using a material that is found abundantly in nature also makes the sponge affordable, biodegradable, and eco-friendly.
Cho Nam-Joon, Study Lead and Professor, School of Materials Science and Engineering, Nanyang Technological University, Singapore
The new study is based on NTU Singapore’s body of work on identifying new applications for pollen, called the diamond of the plant kingdom for its tough exterior, by converting its hard shell into microgel particles. This gel-like, soft material is subsequently utilized as a building block for a new class of materials that are environmentally sustainable.
In 2020, Professor Cho, along with President and Professor Subra Suresh from NTU Singapore, had headed a research group to produce a paper-like material from pollen as a greener substitute to paper produced from trees.
The new 'pollen paper' curls and bends in response to varying levels of environmental humidity—a feature that may prove handy for artificial muscles, soft robots and sensors.
Pollen that is not used for plant pollination is often considered biological waste. Through our work, we try to find new uses for this ‘waste’ and turn it into a natural resource that is renewable, affordable, and biodegradable. Pollen is also biocompatible. It does not cause an immunological, allergic or toxic reaction when exposed to body tissues, making it potentially suitable for applications such as wound dressing, prosthetics, and implantable electronics.
Cho Nam-Joon, Study Lead and Professor, School of Materials Science and Engineering, Nanyang Technological University, Singapore
Professor Cho also holds the Materials Research Society of Singapore Chair in Materials Science and Engineering. The results of the study were published in Advanced Functional Materials—a scientific journal—in March 2021.
Building a Sponge from Pollen
To create the sponge, the researchers from NTU Singapore initially converted the ultra-strong pollen grains, which were extracted from sunflowers, into a gel-like, flexible material via a chemical process similar to conventional soap-making.
In this process, the sticky oil-based pollen cement that coats the surface of the grain is removed and the pollen is then incubated in alkaline conditions for a period of three days. The gel-like material, thus obtained, was subsequently freeze-dried.
Such processes led to the formation of pollen sponges that have 3D porous designs. The sponges were fleetingly heated to a temperature of 200 °C; this process stabilizes their structure and form after constantly absorbing and discharging liquids. The team observed that this heating also improved the resistance of the sponge to deformation by two-fold.
To ensure that the sponge selectively focuses on oil and does not soak up water, the team applied a layer of stearic acid—a form of fatty acid often found in vegetable and animal fats. This makes the sponge hydrophobic and at the same time maintains its structural integrity.
To conduct oil-absorption tests on the pollen sponge, the researchers used organic solvents and oils of different densities, like pump oil, gasoline, and n-hexane (a chemical present in crude oil).
The team observed that the sponge had an absorption capacity ranging from 9.7 to more than 29.3 g/g. This is similar to commercial polypropylene absorbents, which are essentially derivatives of petroleum and have an absorption capacity that ranges between 8.1 and 24.6 g/g.
The researchers also tested the sponge for its reusability and durability by continuously soaking it in silicone oil and then removing the oil. They discovered that this process could continue for a minimum of 10 cycles.
In an ultimate proof-of-concept experiment, the researchers tested the potential of a certain sponge, measuring 5 mm in height and 1.5 cm in diameter, to soak up motor oil from a contaminated water specimen. The sponge instantly soaked up the motor oil within 2 minutes.
“Collectively, these results demonstrate that the pollen sponge can selectively absorb and release oil contaminants and has similar performance levels to commercial oil absorbents while demonstrating compelling properties such as low cost, biocompatibility, and sustainable production,” added Professor Cho, who is also the study corresponding author.
Looking ahead, the team has planned to upgrade the size of pollen sponges to fulfill industry requirements. They are also exploring ways to team up with international partners and non-governmental organizations to perform pilot tests with pollen sponges in actual settings.
We hope our innovative pollen materials can one day replace widely-used plastics and help to curb the global issue of plastic pollution.
Cho Nam-Joon, Study Lead and Professor, School of Materials Science and Engineering, Nanyang Technological University, Singapore
Journal Reference:
Hwang, Y., et al. (2021) Colloid-Mediated Fabrication of a 3D Pollen Sponge for Oil Remediation Applications. Advanced Functional Materials. doi.org/10.1002/adfm.202101091.
Source: https://www.ntu.edu.sg/