Microplastics, i.e., plastic particles less than five millimeters in diameter, are increasingly found in the environment, particularly in the oceans. Microplastics might be harmful to the environment, e.g., for marine organisms. In addition, other pollutants, such as heavy metals or organic molecules, can adsorb to microplastics and accumulate. The microplastics and the mixture of substances on them could also interact with each other, changing the chemical composition of the components. For example, chromium could take on different oxidation states while on the surfaces of microplastics, forming toxic Cr(VI) from the less harmful Cr(III).
Kelvin Sze-Yin Leung, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, China, and Shenzhen Virtual University Park, China, and colleagues have found that, when attached to microplastics, UV filters used in products such as sunscreens can enhance chromium uptake and lead to a higher Cr(VI) abundance. The team prepared mixtures of Cr and polystyrene microplastic particles both with and without benzophenone-type UV filters. The mixtures were then studied using X-ray photoelectron spectroscopy (XPS) and tested in toxicity experiments.
The researchers found that microplastics can aggregate more Cr in the presence of a UV filter than without it. They attribute this to the formation of Cr–UV filter complexes. In addition, the average oxidation state of Cr was higher in the mixtures containing the UV filters, possibly due to the oxidation of Cr(III)–UV filter complexes. Finally, the team tested whether this increased oxidation state led to environmental toxicity, using microalgae as a model. The microalgae’s growth was inhibited when exposed to the mixture containing the filter molecule. Overall, the work indicates that coexisting pollutants can affect the oxidation states and toxicities of heavy metals bound to microplastics.
- Sorption Behavior, Speciation, and Toxicity of Microplastic-Bound Chromium in Multisolute Systems,
Wai-Kit Ho, Japhet Cheuk-Fung Law, Jason Cho-Wai Lo, Isaac Kai-Xin Chng, Catherine Hong-Huan Hor, Kelvin Sze-Yin Leung,
Environ. Sci. Technol. Lett. 2022.