Elucidating the role of microplastics as reservoirs of brominated flame retardants in river sediment

Microplastics (MPs) are emerging contaminants in aquatic ecosystems and have been suggested to act as vectors of organic pollutants such as brominated flame retardants (BFRs). In this study, river sediments were collected upstream and downstream of a wastewater treatment plant (WWTP) over a 12-month period. Concentrations of eight polybrominated diphenyl ethers (PBDEs) and eight novel brominated flame retardants (NBFRs) were determined. In addition to bulk sediment analysis, BFRs were - for the first time - measured in the MPs isolated from each sediment sample, and the residual sediment remaining after MP removal. The proportion of BFRs associated with MPs was very low, typically < 1 % of total sediment loading. Consistent with this, MP mass and BFR concentrations were not significantly correlated, except for BDE-183 (r = 0.405, p = 0.049). Partition coefficients (K_d) reveale_d significant differences among BFRs, with BDE-209 and DBDPE differing from several lower-brominated congeners, consistent with logKow. Downstream of the WWTP, higher MP and BFR concentrations were observed, with significantly elevated Σ₈PBDE and BDE-209 levels (p = 0.034). Month-to-month fluctuations were evident, with spikes in November 2022, but no clear seasonal trend. Overall, MPs likely play a limited role as reservoirs of BFRs in this urban UK river system.