Through a New Lens: A New Method for Removing Microplastics from Salt and Freshwater

BY TALIA DAGHIGHIAN

The issue of plastic pollution has become a prevalent one, so much so that it’s predicted that by 2050, there will be more plastic than fish in the ocean by weight. Though larger plastic particles break down into smaller particles over time, small nanoparticles and microparticles – particles in the nanometer and micrometer size range – remain in the environment, unable to be seen by the human eye.² An increased exposure to nanoplastic particles negatively impacts marine life by causing a blockage in the digestive tract, suffocation, or drowning.² Furthermore, as these particles are swept up in the water cycle, they can also return to soil through precipitation, being ingested by wildlife and also impacting human food supply.² 

In response to this issue, scientists have been working to develop methods to remove micro and nanoplastics from fresh and salt water. Previous methods have included filtering the water through a membrane, which was a slow process that would often result in clogging.² Another method involved adding salt to solutions to have the micro and nanoplastics float to the very top, but that was not widely used due to the difficulty of collecting small particles at the water surface.² With these methods having fallen short, scientists have taken to finding a new method to filter water particles.

One of these new methods involves an iron oxide compound with a hydrophobic coating, which repels water while attracting nonpolar such as plastic.² The combination of iron oxide and the hydrophobic coating is formed into a cubic nanoparticle of about 60-120 nanometers.² These particles are then put in the water to attach to the plastic nanoplastics, after which they are removed by a magnet.

The Iron Oxide Nanoparticles, or IONPs, were made out of iron chloride, oleic acid, and varying hydrophobic coatings.² The first component, iron, is a useful metal to use because of its oxidation and reduction abilities with multiple pollutants.¹ The second component, oleic acid, forms protective layers that unify the shape of the particles, making it a good coating.¹ Lastly, hydrophobic coatings of various hydrophobicities were used to coat the iron nanoparticles.²  Since a hydrophobic particle is not charged, varying hydrophobicities entails different levels of polarity – or distribution of charge in the molecules. Furthermore, plastic nanoparticles are nonpolar, because they do not dissolve in water, and therefore the IONPs can attach to them and prove successful in removing them from water. Because of the limitations of the coating materials, half of the particles were synthesized in argon gas – a previous method – and the other half were synthesized in air.

Following the creation of the particles, scientists tested the arrangement of atoms in the samples, which plays a large part in the magnetic forces of the particle and can thus be used to determine the superior sample. To do this, X-ray diffraction was used, and it was determined that 72% of the particles made in air and only 40% of those made in argon had the optimal structure for the best magnetic properties.² The scientists then tested the ability of the different coatings to repel water by measuring the contact angle of a droplet of water on a glass wafer, to which the IONP was attached. A higher contact angle of the water droplet indicated a more hydrophobic coating.² Of the hydrophobic coatings, the most effective was the Siliclad coated, with an angle greater than 150°. Other coatings with a high angle included oleate, with 115°, and 4% amine, with 106.1°.³ 

Finally, a water sample with small nurdles – or plastic fibers – was used to determine which iron oxide particles were the most successful at picking up nanoplastics. The best were those made with Monocarboxydecyl, 4% amine, and 2% amine as their hydrophobic coatings. As part of the process, the nurdles were then removed magnetically with a 2-inch magnet, which separated the particles from the iron oxide with 100% recovery, meaning they were all able to pick up all of the nanoplastics.³

Though this method proved highly useful, there’s still a limitation: it was tested on a small amount of water with a small magnet being used to remove the iron oxide compounds.² It is not yet clear how the method will perform in a larger body of water with a larger, multi-step magnet system.^2,3 However, since the iron-oxide particles are cost-efficient, there shouldn’t be financial difficulties to carrying these processes out.³ If successful on a larger scale, the iron nanoparticles have the potential to be a permanent solution to microplastics and nanoplastics in water. 

The continued work towards finding a way to remove dangerous nanoplastics from water is sure to make positive impacts not only for marine life that are near the water these nanoplastics are in but for humans as well. With the excessive pollution in all areas of the environment, the work towards removing nanoplastics in fresh and saltwater is one step in the right direction of reversing the harm done to the Earth and helping all species that inhabit it.

References

1. Xu, W.; Yang, T.; Liu, S.; Du, L.; Chen, Q.; Li, X.; Dong, J.; Tan, X.; Liu, Y.; Zhou, L.; Gong,    Y.; Lu, S.; Zhang, Z. Insights into the synthesis, types and application of iron nanoparticles: The overlooked significance of environmental effects. https://www.sciencedirect.com/science/article/pii/S016041202100605X#:~:text=Compared%20with%20traditional%20absorbents%2C%20iron,polluted%20water%2C%20soil%20and%20sediments (accessed May 3, 2023). 
2. Martin, L. M. A.; Sheng, J.; Zimba, P. V.; Zhu, L.; Fadare, O. O.; Haley, C.; Wang, M.; Phillips, T. D.; Conkle, J.; Xu, W. Testing an iron oxide nanoparticle-based method for magnetic separation of Nanoplastics and microplastics from water. https://www.mdpi.com/2079-4991/12/14/2348/htm (accessed May 3, 2023). 
3. Silva, L. G.; Solis-Pomar, F.; Gutiérrez-Lazos, C. D.; Meléndrez, M. F.; Martinez, E.; Fundora, A.; Pérez-Tijerina, E. Synthesis of Fe nanoparticles functionalized with oleic acid synthesized by inert gas condensation. https://www.hindawi.com/journals/jnm/2014/643967/ (accessed May 3, 2023). 
4. Water contact angles on a silicon wafer after deposition of: (a) pfe … https://www.researchgate.net/figure/Water-contact-angles-on-a-silicon-wafer-after-deposition-of-a-PFE-for-2-min-thickness_fig4_283030274 (accessed May 4, 2023).