Researchers from the Hebrew University of Jerusalem deposited polyethylene particles in three agricultural soils in Israel and examined their interactions with 48 organic pollutants. In about 90% of the materials, environmental aging of the plastic had little effect on adsorption and release – but the researchers emphasize that this does not remove the concern about the accumulation of microplastics in the soil.
Are particles microplastic Do the pollutants that accumulate in the soil become more dangerous as they age? One hypothesis that has been put forward in recent years is that the changes that occur on the surface of the particles gradually turn them into a kind of "chemical sponges," capable of binding to them pesticides, drugs, andOrganic pollutants others and influence their movement in the environment.
A new study by researchers fromThe Hebrew University in Jerusalem points to a more moderate picture – at least when it comes to a certain and common type of PolyethyleneThe plastic particles did change after a prolonged stay in the soil, but in most cases the changes had little effect on their ability to adsorb and release organic pollutants.
The study, published in the journal Soil & Environmental Health, was conducted under the leadership of research student Yuval Shahar, Dr. Aviatar ben Mordechai and Prof. Benny Hefetz From the faculty toagricultureRobert H. Smith School of Food and Environment at the Hebrew University. The team also included Raz Lev and Ward Mordechai, in collaboration with the Agricultural Research Administration – Volcanic Institute.
20 months on three farmlands
The researchers examined linear low-density polyethylene, LLDPE, a common material in agricultural tarpaulins used to cover greenhouses and ground covers. They used pieces of used tarpaulins and buried them for 20 months in three different agricultural soils in Israel.
The goal was to test the process under conditions closer to the real agricultural environment, rather than just a short experiment in which plastic is exposed to radiation or oxidizing agents in the laboratory. While in the soil, the particles were exposed to the organic matter, minerals, moisture, and biological activity typical of the soil.
After the particles were removed, it was discovered that their surface had indeed changed. Natural organic matter had accumulated on the plastic, forming a layer that changed the chemical composition of its surface. Most of the changes occurred within the first year, after which they largely stabilized.
Such a change might seemingly increase the number of sites to which pollutants can bind. However, when the researchers examined the practical significance of the change, the results differed from the conventional hypothesis.
48 pollutants originating fromWastewater
The team examined the interactions of the particles with 48 organic pollutants found in wastewater used for agricultural irrigation around the world. The substances included pharmaceuticals, pesticides and other organic compounds originating from wastewater.
The researchers examined both adsorption – the transfer of the material from solution to the plastic surface – and its re-release. In about 90% of the materials tested, aging in soil caused only a small or no significant change in the interactions with microplastics.
Most of the contaminants were relatively weakly bound to polyethylene and also easily released from it. This means that the bonds were mostly weak and reversible, rather than a process in which the substances were permanently trapped within the particle.
“Our findings challenge the assumption that aging sharply increases the environmental risk posed by polyethylene microplastics,” said Dr. Ben-Mordechai. He said that while the particles did change after months in the soil, the changes in the surface had only a minimal effect on the interaction with most pollutants.
The land still plays the central role
The study suggests that in an agricultural environment, the minerals and organic matter in the soil are much more important than the polyethylene in determining the fate of pollutants. The soil itself can adsorb compounds, release them, influence their degradation, and determine their availability to plants and microorganisms.
The differences between the three soils – including varying levels of organic matter and clay – had almost no effect on how the polyethylene particles aged or on the direct interactions between them and the pollutants. The organic layer that formed on the surface of the particles was the main factor in the changes measured.
“Microplastics are still an environmental problem, but the results help clarify where the main risks lie,” said Prof. Hefetz. “When it comes to the behavior of pollutants in agricultural soil, the soil itself – not the plastic – does most of the work.”
Not a conclusion about all types of plastic
The findings are important for risk assessment, but it should not be concluded that all microplastic particles behave similarly. The study focused on a specific type of polyethylene, while microplastic is an umbrella term for a variety of polymers with different chemical structures and properties.
The size of the particles, their shape, their origin, their level of wear, and the types of additives added during manufacturing can also affect how they react with contaminants. Tire wear particles, for example, contain a complex mixture of rubber and chemical additives and may behave differently than polyethylene sheeting.
The study also does not state that the accumulation of polyethylene in the soil is not harmful. The particles break down very slowly, continue to accumulate in fields and may affect the structure of the soil, the organisms living in it and biological processes. The limited conclusion is that the aging of LLDPE particles did not make them, in most of the cases examined, much more efficient carriers of organic pollutants.
This distinction avoids both over-reducing and over-representing risk. It also highlights the need to test different types of plastics and pollutants separately, under realistic environmental conditions and over time.
Questions and Answers
What did the researchers test?
The researchers buried polyethylene particles from three experimental groups in agricultural soils in Israel for 20 months, and then tested the adsorption and release of 48 organic pollutants.
Have the particles changed in the soil?
Yes. Natural organic matter coated the surface of the particles and changed their chemistry. Most of the changes occurred in the first year, after which the particles remained relatively stable.
Did aging increase the adsorption of pollutants?
For about 90% of the pollutants tested, only a small change or no significant change was found. In most cases, the associations with plastic were weak and reversible.
Does the study prove that microplastics are not dangerous?
No. The study looked at a specific type of polyethylene and one aspect of the risk – interactions with organic pollutants. Microplastics continue to accumulate in soil and may cause other effects that were not examined in this study.
More on the subject on the science website
- Tire dust can also get on your plate: abrasion particles release pollutants into the soil and plants
- Microplastics: A Global Crisis
- Israeli study: Seasonal streams release microplastics into the Mediterranean Sea in waves
- The Israeli coastline is polluted with more than two tons of microplastics
- Light and non-biodegradable: the plastic particles that are carried by the wind and we breathe
For the scientific article: Opening the scientific article
