Adsorption of trace metals by microplastic pellets in fresh water

Environmental context Although microplastics are known to adsorb and transport contaminants in the aquatic environment, there is no information on the significance of the effect in fresh waters. We studied the adsorption of metals to new and aged plastic pellets in river water, and show interactions that are dependent on time, pH and metal concentration. Because metals have a higher affinity for aged pellets than for new pellets, weathered microplastics could represent a significant vehicle for the transport of metals in aquatic environments. Abstract Adsorption of trace metals (Ag, Cd, Co, Cr, Cu, Hg, Ni, Pb, Zn) to new (virgin) and aged (beached) plastic production pellets suspended in river water (pH ~ 6.5) has been studied under laboratory conditions. Over a 7-day period, reaction kinetics largely conformed to a pseudo-first-order reversible mechanism with forward rate constants that were typically at least an order of magnitude greater for beached pellets than virgin pellets. Adsorption isotherms were defined by a linear model in many cases, with maximum distribution coefficients of ~6 mL g–1 for virgin pellets (Hg and Pb) and of the order of 102 mL g–1 for beached pellets (Ag and Pb). However, more significant fits to the data were obtained using Freundlich or Langmuir models with adsorption constants that were orders of magnitude greater for beached pellets than virgin pellets and, regarding the former, that were greatest for Ag, Hg and Pb. Increasing pH of river water resulted in an increase in adsorption of Ag, Cd, Co, Ni, Pb and Zn, a reduction in adsorption of Cr and no clear trend for Cu or Hg, and adsorption was always greater to beached pellets than virgin pellets. The ability of pellets to adsorb metals was attributed to the modification of the surface through the attachment of organic matter during the experiments and, with regard to beached pellets, their long-term pre-modification through photooxidation and attrition of charged material. Interactions at the pellet surface likely involve metal cations, oxyanions (HCrO4–/CrO42–) and organic complexes.