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A first-of-its-kind study rates the degree of environmental damage of the types of microplastics* found in shampoo bottles

The plastic products made of polypropylene, such as shampoo bottles, are the most dangerous for the marine environment

Microplastics on the beach. Illustration: depositphotos.com
Microplastics on the beach. Illustration: depositphotos.com

A new study from Tel Aviv University took an innovative approach to assessing the environmental damage of microplastics. Microplastic is actually a general name for plastic particles that are formed with the breakdown of plastic products from different polymer bases, and cause a lot of damage to the environment in general and the marine environment in particular. As part of this, the researchers rated the degree of environmental damage of six types of microplastics and found that the most dangerous polymer base is polypropylene, which is used, among other things, to make lunch boxes and shampoo bottles. Following it in the ranking: polyethylene (super bags), polystyrene (styrofoam and HDP), polyethylene terephthalate (beverage bottles), and polylactic acid.

The researchers: "Our research is intended to signal: Pay attention! Today it is not practical to completely stop the use of plastic, so it is important that we know what the most dangerous types are, and turn the spotlight on them. Such information is intended to support decision-making processes, for the purpose of establishing an environmental policy and enforcing it."

The research was led by Dr. Ines Zucker, head of the Laboratory for Environmental Nanotechnology at Tel Aviv University, and PhD student Andrey Eitan Rubin from the Porter School of Environmental Studies. Doctoral student Rima Ganaim from the Porter School and research student Shiri Levy from the Department of Materials Science and Engineering also participated in the research. The article was published in the journal Science of the Total Environment.

"We have been engaged in research on microplastics and nanoplastics since the establishment of the laboratory," says Dr. Zucker. "Assessing the presence of plastic particles causes damage to the environment, the climate, animals, and sometimes also to the health of humans, and the issue is at the forefront of global research today. However, most studies estimate the risk associated with the presence of microplastics in a narrow prism - according to a single estimate chosen by the researchers. In our research, we sought to reach a more accurate assessment by combining 5 different indicators: the frequency of the presence of the plastic particles, their tendency to break down (as a result of oxidation or mechanical breakage), the water's oxidation potential, and toxicity."

What is the most common plastic?

In the first phase, the researchers reviewed 50 scientific articles that determined which types of microplastics are found in seawater samples from all over the world. Based on the review, they chose to focus on 6 types of polymers - the basic component of plastic. First, 3 polymers were selected that are produced from polluting fossil fuels: polyethylene - the most common, found for example in plastic bags; Polypropylene - from which, among other things, shampoo bottles and lunch boxes are made; and polystyrene - the basis of Styrofoam and disposable utensils. Against them, 3 types of relatively 'green' polymers were tested: PET - the plastic from which drinking bottles are made, which includes 25% or 50% recycled component; And PLA - the only polymer based on corn starch, which should be a natural and renewable alternative to fossil fuels. Based on the literature review, the researchers examined the first parameter in the study - the distribution of the different types of microplastics in the marine environment. They found that polyethylene particles are the most common.

Later, plastic samples made of the 6 types of polymers were placed on the roof of a building in the faculty of engineering at the university, in containers of a solution simulating sea water, for a period of one year. The researchers explain that in this way a prolonged and double exposure was achieved - both to the conditions of the sea and to the elements of the weather - which simulates the exposure of plastic waste in the oceans.

Small particles - more dangerous

During and at the end of the year, the plastic samples were tested to measure 2 additional parameters: the level of oxidation, which makes the plastic more fragile, and the potential for mechanical breakage. The researchers explain: "Both phenomena cause plastic to break down into smaller and smaller particles - micro-plastic and even nano-plastic. The smaller the particles, the greater their toxicity potential for animals as well as humans, because they are able to pass more barriers inside the body, penetrate the bloodstream and even reach the brain." The study revealed that polypropylene oxidizes more than the other polymers, while polystyrene breaks more than the others, into the smallest particles, following the application of mechanical forces.

In addition, the researchers took samples from the solutions in which the various polymers were soaked, to examine changes in the acidity of the water due to the prolonged weathering. Andrey Rubin explains: "The increase in the acidity of sea water has great significance for climate change. Naturally, carbon dioxide from the atmosphere dissolves in seawater, and is deposited in the water in the form of a salt known as calcium carbonate (calcium carbonate). An increase in the acidity of seawater damages the ability of the oceans to absorb and fix CO2, so that a larger amount remains in the atmosphere and negatively affects climate change." The study found that the polypropylene polymer increases the acidity of the water by three orders of magnitude - more than any other polymer tested in the experiment.

The researchers have now filtered the solutions to separate them into two: the solid microplastic particles that remain in them, and the liquid extract - the solution into which the additives that were incorporated into the plastic during its production to give it desirable properties such as color, flexibility or strength have flowed. In the microbiology laboratory, marine bacteria were exposed to the two filter products - the solid and the liquid, in order to test their degree of toxicity. According to Robin "Bacteria are the primary creatures in the marine food chain, and damage to them could undermine the entire food web. That is why it is so important to understand if, how, and to what extent they are harmed by different types of plastic."

The study found that the solid plastic particles caused little if any damage to the bacteria, but the extract - those chemicals that leak into the environment as a result of the plastic's wear and tear - turned out to be extremely toxic, and caused a significant slowdown in the bacteria's natural rate of division. The researchers were surprised to discover that the most toxic substance for bacteria is precisely the extract of PLA produced from corn starch. This finding indicates, according to them, that it is not enough to develop natural polymers that will replace fossil fuels in plastic production. It is also important to emphasize the reduction or replacement of the chemical additives incorporated in all plastic products.

Focus the spotlight in the right direction

After the data was received, each of the 6 polymers was given a score for each of the 5 parameters - distribution, oxidation, mechanical fragility, seawater acidification, and toxicity to marine bacteria. All scores were weighted to rank the degree of environmental damage of the different types of microplastics. The weighing revealed that the most dangerous polymer for the environment is polypropylene, which as mentioned is used to produce lunch boxes, shampoo bottles, etc. It was also found that PLA, the polymer made from corn starch, is the safest from an environmental point of view.

Dr. Zucker concludes: "Plastic waste is currently a huge environmental threat, and 5 plastic islands have already formed in the oceans, the total area of ​​which probably reaches millions of square kilometers. Since it is not practical today to completely stop the production and use of plastic, it is important to know what to focus on as long as a set of risk parameters is taken into account - and this is the main goal of this study. Our findings are intended to help decision-makers and policy-makers struggling with the phenomenon, as well as researchers looking for solutions, to focus the spotlight in the right direction. Specifically, we found that polypropylene is the most dangerous polymer for the environment, and we recommend putting emphasis on plastic products of this type."

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