Influence of Polyethylene and Polystyrene Microplastics on Nitrogen Transformation in Floating Treatment Wetlands

Floating treatment wetlands are an emerging remediation technique for nutrient pollutants such as nitrate in waterbodies receiving runoff from urban and agricultural sources or discharge from wastewater treatment plants. Also prevalently found in these sources, microplastics, plastic particles smaller than 5 mm, are a contaminant of emerging concern due to their persistence in the environment and potential for detrimental ecotoxicological and human health effects. As microplastics have been shown to impact nitrogen transformation processes in wastewater treatment plants, they may impact similar processes in environmental remediation efforts such as floating treatment wetlands. To evaluate the potential impact microplastics may have on nitrogen transformation in these environments, root samples from an established floating treatment wetland were collected and used to inoculate microcosms to study nitrification and denitrification processes. Microplastic treatments included polyethylene (PE) and polystyrene (PS) microspheres at two diameters (30 μm and 200 μm) and three concentrations (10 mg/L, 100 mg/L, and 1000 mg/L). Nitrification microcosms were observed in aerobic conditions over a 24-hour period and denitrification microcosms flushed with nitrogen gas to encourage anoxic conditions were monitored over a week. Nitrite production was analyzed using a microplate method to calculate potential nitrification and denitrification rates. For nitrification experiments, only PE 30 μm was found to have a significantly lower normalized potential nitrification rate than root controls. For denitrification experiments, PE 200 μm and PS 30 μm showed a normalized potential denitrification rate significantly higher than the root control and PS 200 μm showed a rate significantly lower. For both nitrification and denitrification experiments there was a significant difference in the normalized rates between the diameters for both plastic types. Additionally, there was not a significant difference found between the three concentrations (10 mg/L, 100 mg/L, and 1000 mg/L) tested. Further research to observe the impacts on weathering and chemical additives would give a better understanding on how microplastics may interact with microorganisms and influence nitrogen movement in wetlands. Advisor: Shannon Bartelt-Hun