Elucidating temporal trends in environmental contamination with microplastics and organophosphate flame retardants

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 implications for the bioavailability of such chemical additives, this is a significant research gap that should be addressed. Specifically, the impact on additive bioavailability of several factors including the type of plastic polymer and the concentration as well as the physicochemical properties of chemical additives. To further understand the interaction between microplastics and chemical plastic additives in freshwater sediments, the possibility of correlation between microplastic concentrations and the concentration of OPEs in the same sediment samples was investigated. Freshwater sediment samples were collecetd from a range of locations across the world and from 3 rivers and an urban canal in the West Midlands area of the UK on a monthly basis over a 12-month period. UK riverine samples were collected both upstream and downstream of WWTP discharge points.
\nFollowing extraction and quantification of microplastics using density separation and the Nile Red method, the mean number of MPs/kg (dw) for all four UK study locations ranged from 67-267 (Worcester and Birmingham canal), 133-283 (River Tame), 66.67-317 (River Severn), and 67-233 (River Sowe) per month. The highest mean number of MPs/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. The most dominant morphological group in all the isolated MPs across all four study locations over the 12 months period were fragments (71%) and the least abundant fibres (9%). All target OPEs were detected at all four study locations following instrumental analysis on an Agilent 5975C GC coupled to an Agilent 5975C MSD fitted with a 30 m DB-5 MS column with detection frequencies of: (tris (chloroethyl) phosphate (TCEP), tris (2-chloroisopropyl) phosphate (TClPP), tris (1,3-dichloro-2 propyl) phosphate (TDClPP), tri-n-butyl phosphate (TNBP), tris (2-butoxyethyl) phosphate (TBOEP), 2-ethylhexyl diphenyl phosphate (EHDPP), tris (phenyl) phosphate (TPHP), and tri-m-tolyl phosphate (TmTP)) ranging from 58% to 100%. The concentration of Σ\\(_8\\)OPEs ranged from 107 ng/g (dw) (in the Worcester-Birmingham canal) to 52 ng/g (dw) in both Rivers Severn and Sowe). The highest ∑\\(_8\\)OPE concentrations for all study locations were recorded during autumn months (September, October, and November) and concentrations of ∑\\(_8\\)OPEs in sediment samples from the River Severn, River Tame, and River Sowe all varied inversely with the river level and flow rate. The potential risk posed by target OPEs in all four study locations was found to be low, except for a moderate risk identified for EHDPP in the Worcester-Birmingham canal.
\nFor sediment samples from Nepal, Greece, Poland, Swaziland, Spain, Mozambique, Switzerland, Canada, Malaysia, Republic of Korea, Chile, Nigeria, and India, ∑\\(_8\\)OPE concentrations ranged from 13 ng/g in the Republic of Korea to 95 ng/g (dw) in Nepal (with TDCIPP (62 ng/g (dw)) contributing substantially to the high concentration of ∑\\(_8\\)OPEs in Nepal). The detection frequency of all eight target OPEs in 11 out of the 14 countries was 100% while for the other three (Swaziland, Mozambique, and Republic of Korea), the detection frequencies were 80%.
\nThe mean number of MPs/kg (dw) at the various countries from which samples were obtained ranged from 1217 MPs/kg (dw) in Spain (Francoli River and Cànoves stream) to 0 MPs/Kg in some locations (Thames Ontario (Kintore Creek West), Canoves in Spain and Kali Gandaki River in Nepal). The percentage of fragments per kg of sediment (dw) ranged from 100% in Nepal to 40% in the Republic of Korea. The highest percentage of fibres was recorded in South Korea (60%) and the lowest recorded in India (2%). Spheres were only identified in four countries (Greece, United Kingdom, Nigeria, and India) with percentage abundances of 10%, 3%, 20% and 3% respectively. The average length of MPs in all the countries where sediment samples were collected was < 1500 µm. In fact, for 6 out of the 14 countries studied, ave-age MP length was < 300 µm and was less than 1000 µm for all but one country.

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