Impacts of microplastics in freshwater systems

Plastics, in particular microplastics (particles with dimensions < 5 mm), are a widespread and persistent pollutant constituting an emerging scientific and societal issue. Its characteristics allied to an inadequate management contributes to their accumulation in aquatic systems, reaching high densities. Moreover, they can also interact with environment affecting economy, human health and aesthetics. However, most of scientific studies have been focused in marine environment while scarce knowledge exists regarding freshwater systems, including in Portugal. Hence, this study aimed to contribute to fill this gap of information both in uniformization of methodologies of isolation of microplastics (MPs) in water samples as well as on the MPs’ characterization in a Portuguese freshwater system. Thus, the first part of this study aimed to assess the effectiveness of distinct separation methods including density separation methods (sucrose, olive oil and zinc chloride) as well as organic matter degradation methods (hydrogen peroxide and multienzymatic detergent). For that, artificial samples containing the eleven most common types of plastics were prepared, subjected to the different methods and then polymers were detected, quantified and identified using a stereoscope microscope and Fourier transform infrared spectroscopy (FTIR). Among the several tested methods, the most cost-effective was the method of wet peroxide oxidation with addition of zinc chloride. Hence, this study highlights the importance of the use of zinc chloride both in the processing of sediment and water samples. In a following step, the abundance and distribution of MPs in the water and sediment of Antuã river were determined by applying the separation method identified as the most effective previously. The abundance of MPs in water varied from 5 – 8.3 mg m-3 or 58 – 193 items m-3 in March and from 5.8 – 51.7 mg m-3 or 71 – 1265 items m-3 in October. In sediments, the abundance of MPs varied from 13.5 – 52.7 mg kg-1 or 100 – 629 items kg-1 in March and from 2.6 – 71.4 mg kg-1 or 18 – 514 items kg-1 in October. It shows that this river is severely impacted by MPs, in orders similar to that found in marine/coastal environments. A spatial and temporal variation was observed dependent on seasonal conditions, flow velocity and anthropogenic pressure. Thus, this study emphasizes the importance of rivers as carriage systems of MPs, and highlight the potential impacts of MPs as emerging contaminants on freshwater systems.