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Toxins attached to microplastics threaten the environment, and us

By Lirong Shi

The special exhibit Plastic Entanglements: Ecology, Aesthetics, Materials at the Chazen Museum of Art in Madison, Sept 23, 2019. Credit: Dean Knetter/WPR

After squeezing out that last usable drop of hand cream and forcing that last bit of color onto our lips, resigned, we throw the product container away having finished it. However, while the product is “finished,” there is always some residual product left behind consisting of organic particles called cosmetic particles. These cosmetic particles, along with plastic products such as plastic bags and Styrofoam cups that we use daily, all end up in landfill piles. As these plastics decompose into the environment, they can break down into very tiny fragments—smaller than 5mm in size, or smaller than a pencil eraser—known as microplastics.

At such small sizes, microplastics can be blown away by wind and carried by storm drains, canals, or rivers, where they eventually can end up in the ocean. Along this journey, they can pick up organic particles and other toxins. Once attached, these toxic and harmful organic particles can easily be inhaled or ingested by an array of living beings in the ocean, such as fish and shrimp. Not only does this impact the aquatic organisms that directly come into contact with the microplastics, but also impacts those creatures—larger animals, including humans—who eat those smaller aquatic species. Once people hunt those creatures and put them in the dining room, that’s when the harmful toxins that we produced come full circle—not only harming the ecosystem, but also harming us.

Illustration of the process of how contaminants attach to microplastics and are ingested by living beings in the sea. Credit: Lirong Shi

But what harm do these microplastics do to us once they are on our plate? Plastics are biochemically inert materials that generally do not interact with the endocrine system in our body—the system of hormones that control brain and nervous function, thyroid health, blood sugar levels, and reproduction. Because of their large molecular size, plastics are prohibited from penetrating the cell membrane. However, the toxins, which have a much smaller molecular size, can detach from microplastics, penetrate into the cells, and interact with—and ultimately disrupt—the endocrine system of our body. In turn, they can impact mobility, reproduction, development, and induce cancer.

But how could this happen? Why can those toxins attach to microplastics and eventually get digested by us? Organic and metallic contaminants have a high affinity for plastics, so there is a high potential for plastics to transfer those toxins. And absorption and/or adsorption of contaminants to plastics may also inhibit contaminant biodegradation, thus increasing their environmental persistence. But during digestion, the transfer of contaminants from plastics to organisms is very likely. Studies shows that the quantity of contaminants desorbed (or released) from plastics was greatly enhanced by the presence of surfactants (a primary part of most cleaning detergents) and organic matters, which suggests the acidic gastric conditions in our stomach favor this detachment process. This also indicates the detrimental consequences of plastic ingestion on organisms, including gastrointestinal blockage, ulceration, internal perforation and even death.

The good news is that microplastics can be modified by changing their properties and their environmental factors to impact the different adhering abilities they have for toxins. By understanding this process, scientists will be able to design better materials that can either stop toxins from attaching to the plastics in the first place or stop them from leaching into organisms during digestion. To obtain this knowledge, it is essential to study the interactions that occur at the boundaries between toxins and plastic materials, known as interfacial interactions. Studying various toxins and plastics under different environmental conditions can help us understand what plastic materials are more promising than others to reduce transmission of toxins, and what kind of environment can help that process.

To study the relationship between microplastics and toxics, we do interfacial study. This type of inquiry looks at the structural changes of different materials happening at the boundary between those materials (aka interface) due to their interfacial interactions. To perform the interfacial study in the lab, we use an instrument called sum frequency generation (SFG) vibrational spectroscopy. SFG probes the interface in a nondestructive manner, enabling us to study the interfacial interaction of plastics and toxins at a molecular level. Unlike other spectrometers, the SFG system has two laser beams overlapped temporally and spatially at the sample interface. According to its special selection rule, only asymmetric molecules ordered at the interface can generate signals, making it an ideal surface-specific technique.

Illustration of ordering molecules at the interface that can generate SFG signal and those cannot. Credit: Lirong Shi.

For microplastics and toxins specifically, SFG can be used to study the interfacial structure of adsorbed organic contaminants (like certain toxins) on different plastic materials under different conditions. Researchers note that along with buoyant plastic debris, these toxins are at highest concentrations in the water-air interfacial microlayer, where they can be found at concentrations 500 times greater than in the underlying bulk water. That means toxins are more likely to attach to the plastics that are floating on the water surface than those that are sunk in the water bottom. In addition, a rate-modeling study found toxins transferred off plastics to be 30 times greater under body conditions (a simulated gut environment, in this study) than in seawater. This finding demonstrates those toxins attached to microplastics can be more harmful because they do not go away easily in seawater, but they will readily get off in the fish body and disrupt normal biological activities.

Based on these and other studies, our group members designed relevant experimental models to simulate humid and dry conditions to study the interactions between one type of organic particle and different plastics under dynamic processes. The SFG results showed the interactions between the organic particles and plastics varied a lot, with different plastic materials and the desorption behavior seeming to favor an environment similar to biological conditions, just as described in the literature.

Plastic pollution has been a long-term issue for decades. The global release of primary plastics into oceans is estimated at 8 million tons per year and this number will keep increasing. Accumulation of improperly disposed plastics can impact wildlife through ingestion or entanglement, and the chemicals used to produce plastics can be transferred from wildlife to humans by food. In addition to the harm and toxicity of disposed plastics themselves, toxins that attach to them can pose great threats to the health of wildlife and human. These findings urge for relevant studies and SFG technique is proved to have the potential to advance interfacial understanding of organic contaminants and microplastics.

Global release of plastics (blue) and microplastics (orange) originated from mismanagement of wastes into the world oceans. Credit: IUCN

The science behind these interfacial studies and adsorption behavior of contaminants on plastics can be popularized with the intention of raising people’s awareness of waste sorting and recycling. It is important for people to separate different kinds of waste and for regulators to properly handle the separate waste. The most important things we can do in our daily life to mitigate plastic pollution are listed as follows:

Credit: Lirong Shi

When I was visiting the Chazen Museum of Art at the University of Wisconsin-Madison, I was shocked by the power of the art and the scenery they showed about plastic pollution. If no action is taken, the world may be surrounded by garbage and the scenery in the artwork would be our foreseeable future.

LIRONG SHI is a third-year graduate student from the Chemistry Department at the University of Michigan. She is also pursuing a certificate in science, technology and public policy. Lirong is passionate about policy issues around science and technology and dedicated to promoting environmental justice and equity. Lirong is currently serving as public engagement committee in the Engaging Scientists in Policy and Advocacy (ESPA) organization and Mandarin Team Lead at Respond Crisis Translation.

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