Ecological risks of microplastics are far worse than previously thought

12 January 2023

A research team at Hong Kong Baptist University has revealed the complex and unique role of microplastics in the accumulation and transformation of contaminants in natural environments. This study provides the first evidence into how the co-occurrence of organic chemicals affects their ecotoxicological effects.

Although many metals and organic chemicals have been found in environmental microplastic samples worldwide, there had been no reports which have assessed the oxidation states of microplastic-bound metals with or without coexisting chemicals. Without this information, researchers may underestimate the ecological risks of microplastics as carriers for trace metals and other contaminants in real environments.

In response to this knowledge gap, the research team led by Professor Kelvin Leung examined the roles of organic chemicals in the sorptions and oxidation states of microplastic-bound metals and ultimately their ecotoxicological impacts. Polystyrene, chromium and benzophenone-type UV filters were used as the model microplastics, metal, and organic chemicals, respectively.

Environmental contaminants are not just sticking to the [microplastic] particles, but also interacting with each other, ultimately altering the toxicity. ... Our work could signal the tip of a hidden iceberg.

Professor Kelvin Leung, Department of Chemistry, Hong Kong Baptist University

The results showed that the quantity of chromium sorption by polystyrene microplastics was apparently higher when benzophenone-type UV filters were present. This enhanced affinity was attributed to the formation of chromium-UV filter complexes together with multilayer sorption on the microplastics’ surfaces. Subsequent measurements showed that such interactions can alter the chemical forms of microplastic-bound metals, as a higher abundance of chromium-6 was identified on the particles in the presence of a UV filter. The researchers also found that significant inhibitions of algae growth were observed when these complexes were present in the form of microplastic-bound chemicals. The team inferred that these chromium-organic complexes can be transformed into chromium-6, a more toxic compound, on the polystyrene-microplastic surfaces and thus cause more harm to algae growth.

Microplastics may serve as radical sources to convert trivalent chromium-organic complexes into more toxic chromium-6 species. Credit: Hong Kong Baptist University, Environmental Science & Technology Letters

“Previous research has well-documented that [microplastics] are able to accumulate and concentrate other environmental contaminants on their surfaces, hence posing threats to aquatic life and human health,” Leung said. “Our past and ongoing studies reveal that environmental contaminants are not just sticking to the [microplastic] particles, but also interacting with each other, ultimately altering the toxicity. For instance, the surprising results in this study unveil an overlooked capability of [microplastics] –that is, the ability to transform adhered contaminants into a more harmful form. Our work could signal the tip of a hidden iceberg. More comprehensive assessments should be conducted as microplastics continue to accumulate in our air, water, and food.”

The research results were recently published in Environmental Science & Technology Letters.