Ecotoxicity of particulate plastics in terrestrial and aquatic ecosystems

Full text - Introduction - Particulate plastics that are less than 5 mm (i.e., microplastics or microbeads; 0.05 – 5 mm) and less than 5 um (i.e., nanoplastics or nanobeads; 0.05 – 5 um) in size pose a potential threat to both terrestrial and aquatic ecosystems (Wijesekara et al., 2018). Particulate plastics may enter the environment through a variety of routes; however, this research focuses on two key pathways: wastewater treatment plants and direct routes. Renewed interest surrounding the application of biosolids for the improvement of soil health has inadvertently increased the risk of particulate plastics entering terrestrial environments. Once in the ecosystem, particulate plastics may have the potential to cause toxicity. Materials and methods - This research firstly quantified the amount of microbeads in biosolids samples. The study also monitored the changes in particulate plastics (i.e., microbeads) surface characteristics when exposed to different media (biosolids, sediment, and soil) and chemicals (Polycyclic aromatic hydrocarbons (PAH) and copper). Biosolids, soil and sediment samples were spiked with 'Pristine' (Polyethylene) microbeads. Spiked microbeads were extracted after two weeks of incubation. It was hypothesised that the microbeads' surface would change depending on their exposure to media. This hypothesis was tested through visual observation, elemental analysis, FTIR and SEM-EDX analysis. The ecotoxicity of microbeads in terrestrial and aquatic ecosystems was examined using soil respiration tests, earthworm toxicity test and Daphnia motility test. It was hypothesised that microbeads would have an adverse effect on these organisms through a decrease in microbial respiration (microorganisms), weight (earthworms), and motility (daphnids). Results and discussion - The results indicated that a number of elements including copper (Cu), titanium (Ti), manganese (Mn), iron (Fe), aluminium (Al), and silicon (Si) were retained by microbeads when they were exposed to biosolids, sand and sediment samples. The elemental data suggest that surface characteristics can change depending on their pathways, and microbeads may therefore act as chemical vectors. Microbeads in soil resulted in indirect toxicity as measured by soil microbial respiration, especially when microbeads were exposed to Cu solution. Earthworms showed no observable effect when exposed to microbeads. Microbeads caused immobilisation of Daphnids, caused by both indirect toxicity when exposed to Cu solution and direct toxicity when exposed to pristine, soil and sediment conditions. Conclusions - Overall, these results suggest that microbeads act as chemical vectors and may cause direct and indirect toxicity, but only within specific organisms, highlighting the need for further research. (author)