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Evaluating the role of microplastics as a vector in metal cycling within the River Thames
Summary
Researchers characterized how microplastics in River Thames water adsorb toxic heavy metals, comparing adsorption capacity across different plastic types and water chemistry conditions. Microplastics consistently adsorbed metals including lead, cadmium, and copper, providing the first data on metal-binding capacity for Thames microplastics and supporting their role as carriers of inorganic pollutants in urban rivers.
The full extent of the toxicological impacts of microplastics within the natural environment is presently not well understood. This study seeks to assess the potential of microplastics as carriers of toxic heavy metals by characterising the adsorption behaviour of metals to microplastics in River Thames water. This is the first study to our knowledge conducting analysis on the metal adsorption capacity of microplastics within Thames water. In this study we observe the adsorption of three metals – Cd (cadmium), Cr (chromium) and Pb (lead), and their adsorption behaviour on aged microplastic surfaces. Overall, experiments proved that metal-microplastic adsorption occurs, with both Cr and Pb found to adsorb and subsequently desorb at higher concentrations than Cd on microplastic surfaces. Isotherm and kinetic modelling based on experimental data in both adsorption and desorption phases was conducted to reveal possible dominant adsorption mechanisms, indicating the adsorption capacity and behaviour of metals on microplastics, alongside overall adsorption behaviour in Thames water. Adsorption modelling implied complex adsorption occurs within Thames water containing both microplastics and natural adsorbents, as Cd and Cr data followed the Sips isotherm model (indicating mixed adsorption mechanisms), though Pb data followed the Freundlich model (indicating physisorption). Equilibrium for Cd could not be conclusively established within the 14 day incubation period used for isotherm experiments, and therefore interpretations of Cd adsorption behaviour should be considered with caution. The fraction of metals adsorbed to aged microplastic samples within Thames water (containing natural adsorbents) was also assessed, revealing that at lower (0.6 mg/L) and very high (9.9 mg/L) metal concentrations, microplastics were competitive adsorbents for metals. Desorption modelling suggests that Cr and Pb adsorption is best described by the PSO (pseudo-second-order) kinetic model, implying involvement of chemisorption. Desorption of Cd, Cr and Pb in simulated digestive fluid, indicated a completely reversible adsorption mechanism, suggesting metals adsorbed onto microplastics may become bioavailable to aquatic fauna once ingested, highlighting the toxic potential of metal-laden microplastics through co-exposure. Overall, these findings support previous findings that plastics within marine or riverine environments should not be thought of as inert (non-reactive in terms of metal adsorption), but rather as potential carriers of metals.