Potential ecological risk assessment of microplastics in coastal sediments: Their metal accumulation and interaction with sedimentary metal concentration.

Metal pollution in sediments has long been recognized, while sediments are also a long-term sink for microplastics (MPs). MPs may also adsorb environmental pollutants, including metals, as well as leaching polymer components and chemicals used during production. A comprehensive survey of 21 locations around Qatari coastline investigated abundance of MPs in high-shore intertidal sediments and concentration of metals both on MPs and sediment particles. MPs abundance ranged from 3 to 156 MPs particles·kg (12-624 MPs particles·m) with polyethylene being the most abundant (27.4 %). MPs showed physical morphologies, with 76 % displayed signs of chemical degradation as confirmed by the carbonyl absorption peak profile, possibly due to exposure to harsh environmental conditions on the Arabian Gulf shores. Most metals analyzed were found at higher concentrations in sediments than the same metals adsorbed to MPs. The average metal concentration ranged from 0.26 (Cd) to 3122.62 μg∙g (Sr) in sediments while 0.22 (Mo) to 30.26 μg∙g (Sr) in MPs. The calculated metal Pollution Load Index (Sed PLI, range 0.57-2.38) for sediments indicates unpolluted to moderately polluted levels, while the Potential Ecological Risk Index (Sed PERI, range 6.9-2220) indicates a relatively considerable ecological risk for metal pollution in sediments in some of the coastal areas surveyed. PLI values calculated for metals associated with MPs (MPs PLI, range 1.1-7.5), suggests relatively moderate pollution, while the PERI for metals in MPs (MPs PERI, range 25.2-1811) has similar ecological risk in terms of metal pollutants in MPs as for sediments. This may be effective in providing relative spatial indices of pollution load and risk for metals associated with MPs, which could potentially inform establishment of an appropriate assessment framework, where MPs are increasingly abundant in coastal sediments. However, this does not account for the relatively lower abundance of MPs compared to sediments.