Microplastics in surficial sediments from some UK rivers and canals: seasonal and spatial variation and relationship with concentrations of organophosphate esters†

The relationship between microplastics (MPs) and organic pollutants such as organophosphate esters (OPEs), which are widely used as plastic additives is poorly understood. Given the potential toxic implications for such chemical additives, this is a substantial research gap. To address this, we collected 12 sets of freshwater sediment samples from 3 rivers and an urban canal in the West Midlands of the UK between November 2019 and April 2021. Riverine sediment samples were collected both upstream and downstream of waste water treatment plants (WWTPs) discharge points. The mean number of MPs per kg (dw) for all four UK study locations ranged from 67–267 (Birmingham and Worcester Canal), 133–283 (River Tame), 66.67–317 (River Severn), and 67–233 (River Sowe) per month. The highest mean number of MPs per kg was recorded in the River Tame. Overall, 57% of the mean number of MPs identified across all three locations over the 12 months period were detected downstream of the WWTPs while 43% were upstream; however this difference was not significant. Using previously reported concentrations of OPEs in the same samples by our research group, we observed a significant negative correlation (p < 0.05) between mean MPs number and OPE concentrations in sediment at the River Tame over a 12 months sampling period. Additionally, a significant negative correlation was observed between concentrations of Tris(2-butoxyethyl) phosphate (TBOEP)-the most abundant OPE and mean particle number (r = −0.309, p = 0.004). A similar negative correlation was observed between Tri-n-butyl phosphate (TnBP) concentrations and mean MPs per kg at the River Severn. These results suggest that the sources of MPs and OPEs in these waterways may differ and imply that MPs could potentially adsorb OPEs from the sediment in the samples studied.