Microplastics—small pieces of plastic less than five millimeters in length—are becoming a ubiquitous ecological pollutant. Research has shown that, on their own, these tiny fragments are potentially harmful, but it's unclear what effect they might have on the pollutants that cling to them.

A team of scientists led by the Hong Kong Polytechnic University and Hong Kong Baptist University has discovered that microplastics can make other pollutants more harmful. Because microplastics can accumulate other environmental pollutants on their surfaces -- such as heavy metals or organic molecules -- they may pose a greater threat to plants, animals and humans than previously thought. In addition to sticking to other pollutants, microplastics and the mixture of substances on them can also bind to each other, changing their chemical properties.

Related research was published in the journal Environmental Science & Technology Letters.

This study aimed to investigate the role of organic chemicals in the adsorption and oxidation states of microplastics (MPs) bound metals, and ultimately their ecotoxicological effects. Polystyrene (PS) was used as a model MP because it is one of the most widely identified polymers in studies of microplastic occurrence. Chromium (Cr) was chosen as the model metal because it has two stable oxidation states, Cr(III) and Cr(VI), which exhibit distinctly different chemical and toxicological profiles, Cr(III) is relatively safe, and Cr(VI) (VI) Toxic. For the organic chemical model, benzophenone (BP) type UV filter molecules were chosen because of their ubiquity in aquatic environments. And, more importantly, because they are able to coordinate with transition metals to form complexes. We hypothesize that Cr uptake and its oxidation state when bound to MP particles are significantly affected by coexisting UV-filtering molecules. In addition, we investigated the effect of weathering on the interaction of MP with Cr and UV filter molecules for a more realistic simulation. As mentioned earlier, the toxicity of a given metal is highly dependent on its oxidation state, which is further influenced by the formation of metal-organic complexes. In view of this, the toxicity of various combinations of MP, Cr, and UV filter molecules was evaluated using the microalgae, Chlorella vulgaris, as a model species.

The oxidation state of the MP-bound metals changed with higher Cr(VI) abundances identified on PSMPs co-existing with UV-filtering molecules. More importantly, our toxicity assessment found that PSMP-bound Cr with a higher oxidation state showed more severe inhibition of microalgal growth. Notably, the toxicological effects of Cr-UV filter complexes were only observed in PSMPs, thereby reaffirming the unique role of MPs in accumulating and transforming pollutants in natural environments. Overall, this study provides the first evidence for how the coexistence of organic chemicals affects the adsorption behavior and oxidation state of MP-bound metals, and ultimately their ecotoxicological effects.

We found that BP-type UV filters can coordinate with Cr(III) to form metal-organic complexes, and these Cr-UV filter complexes can be converted into more toxic Cr(VI) species on the surface of PSMP particles, thereby protecting microbes. Algae growth is more harmful. Our results reveal the complex role of MPs in real chemical mixtures - not only accumulating but also transforming pollutants.

Given the abundance of metals and organic chemicals found on the surface of environmental MP samples, including Cr and organic UV filters, the potential toxicity of the newly identified MPs in this study should be carefully considered in future MP-metal interaction studies Activity. Second, qualitative (i.e., chemical form of the metal) and quantitative (i.e., total metal concentration) analysis of adherent components is essential for accurate assessment of ecological risks associated with metal and organic pollutants under real environmental conditions. In the end, our work may herald the beginning of a larger problem. A more comprehensive assessment of how MP-bound metal complexes could affect human health should be done, especially as microplastics continue to accumulate in our drinking water sources and tap water.

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