Microplastics can make other pollutants more toxic

Microplastics are tiny plastic pieces measuring less than five millimeters that are becoming a ubiquitous ecological contaminant. While many previous studies have argued that, on their own, these plastic particles are potentially harmful, until recently it has been unclear what effect they could have on pollutants that latch onto them. 

Now, a team of scientists led by the Hong Kong Polytechnic University and the Hong Kong Baptist University have found that microplastics could make other pollutants significantly more harmful. For instance, when attached to microplastics, UV filters used in products such as sunscreens can make chromium metal more toxic.

Since microplastics can accumulate other environmental pollutants on their surfaces – such as heavy metals or organic molecules – they could pose an even greater threat to plants, animals, and humans than previously thought. Besides just sticking to other contaminants, microplastics and the cocktail of substances on them could also combine with each other, altering their chemical properties. For example, metals such as chromium (Cr) could take on different oxidation states while attached to the surfaces of microplastics, and become more toxic.

To test how to oxidation state of Cr could change when bound to microplastics, and how this could be affected by UV filter molecules (a widespread organic contaminant), the researchers created combinations of Cr and polystyrene microplastic particles both with and without benzophenone-type UV filters. The analysis revealed not only that, in the presence of UV filters, microplastics could aggregate even more Cr, but also that the oxidation state of Cr was higher in the mixtures containing the UV filters.

In a second step, the experts investigated whether this increased oxidation state translated to environmental toxicity for microalgae, and discovered that the microalgae’s growth was inhibited when exposed to the mixture, suggesting that Cr was now indeed in a more toxic form.

“Our results shed light on the complex role of microplastics ─ not just accumulating, but also transforming pollutants ─ in a true cocktail of chemicals,” the authors wrote. “More comprehensive assessments of how microplastic-bound metal complexes affect human health should be conducted, particularly as microplastics continue to accumulate in our drinking water sources and tap water.”  

The study is published in the journal Environmental Science & Technology Letters.

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By Andrei Ionescu, Earth.com Staff Writer

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