Just how risky are plastic additives that dilute in water?

It is well-known that discarded plastic can break down into microplastics and cause harm to humans and the environment. Now, scientists have improved the method to estimate just how risky hazardous micropollutants from plastic waste are to the public.

Rivers filled wih plastic will spill the waste near and into the sea. Photo by Martine Perret/UNMIT 3 December 2008

Plastics are almost always enriched with additives, which makes them easier to process, more resistant, or more performant. But this poses a problem: when the polymer material is left in the environment for long durations, these additives can easily leach out and contaminate the environment.

This is the case with styrene oligomers (SOs), for example. SOs are a type of plastic additive commonly found in polystyrene, which have been causing growing concern due to their effects on hormonal disruption and thyroid function, explains a statement.

Game-changing risk assessment

Authorities usually rely on scientists’ risk assessments to evaluate such public hazards and determine the appropriate action to minimize their impact. But scientists struggle to accurately measure the proportion of leachable plastic additives, as it is difficult to discriminate between leached compounds and those still bound to the source plastic material.

Adding to the problem is the fact that these additives can diffuse into the environment at different rates. Now, in a new study, Prof. Seung-Kyu Kim from Incheon National University, Korea, and his team have come up with an assessment method that could change the game.

Prof. Kim and his team collected surface sediments from an artificial lake connected to the Yellow Sea, with several potential sources of SO pollution from the surrounding land area and from marine buoys.

Different dilution rates

“We were hoping that the distribution of SO contaminants in the lake’s sediments would help identify their most likely source and measure the leachable amount from the source material,” Prof. Kim explains.

The scientists also examined one of these potential sources by dissecting a locally used polystyrene buoy, measuring the concentration of SOs in it and how much leached out of it.

A key finding from their investigation was that SO dimers (SDs) and trimers (STs) dilute in water at different rates, so their composition in coastal sediments is very different from what can be observed in the buoys and other potential sources.

This was especially true for STs: heavy, hydrophobic molecules that tended to remain in the source microplastics and moved at a slower rate in the lake. The lighter SD molecules leached out much more readily and traveled further. This meant that the SD to ST ratio would increase further away from the source of the contaminant.

A reference index

Based on this dynamic, the researchers suggest using this ratio as a “reference index” to identify the source of SOs and to estimate the bioavailable fraction of SOs in a given sample.

In Prof. Kim’s words, this would “be critically important to the assessment of ecological and human risk caused by plastic additives”, enabling more accurate risk assessments for potential exposure, and perhaps, formulating policies for disallowing certain more leachable, and therefore more hazardous, additives.

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