Microplastics – small pieces of plastic less than five millimeters in length – are becoming a ubiquitous environmental pollutant. Studies suggest that these tiny bits are potentially harmful on their own, and it’s unclear what impact they might have on pollutants that attach themselves to them. Now researchers report in ACS’ Letters on Environmental Science and Technology show that UV filters used in products like sunscreen can make chromium metal more toxic when attached to microplastics.
Since microplastics can accumulate other environmental pollutants such as heavy metals or organic molecules on their surfaces, they could pose an even bigger problem for wildlife, plants or humans than originally thought. Previous research has shown that heavy metals can easily attach to microplastics and that this combination could potentially harm aquatic life. But beyond sticking to other pollutants, microplastics and the cocktail of substances on them could interact with each other and change their chemical properties. For example, certain metals such as chromium (Cr) could adopt different oxidation states on the surfaces of microplastics. And although Cr(III) is relatively safe, Cr(VI) is toxic. Therefore, Kelvin Sze-Yin Leung and colleagues wanted to study for the first time how the oxidation state of Cr might change when bound to microplastics and how this might be influenced by a common organic contaminant: UV filter molecules.
Researchers prepared mixtures of Cr and polystyrene microplastic particles both with and without benzophenone-type UV filters. The team found that microplastics can aggregate even more Cr in the presence of a UV filter. In addition, the oxidation state of Cr was higher in the mixtures containing the filters. Finally, the team tested whether this increased oxidation state leads to environmental toxicity for a microalgae population. The growth of the microalgae was inhibited when exposed to the mixture containing the filter molecule, suggesting that Cr was now in its more toxic form. According to the researchers, this means that microplastics can help convert pollutants into a more harmful form – a previously unproven interaction.
The authors acknowledge funding from the Hong Kong Research Grants Council and the Hong Kong Baptist University Seed Fund.
Materials provided by American Chemical Society. Note: Content can be edited for style and length.