Interactions between trace metals and plastic production pellets under estuarine conditions

Pre-production pellets are an abundant form of plastic waste in the marine environment whose principal impacts arise from inadvertent ingestion by various organisms when mistaken for food. Pellets also represent a carrier for both organic and metallic contaminants through their adsorption to the modified plastic surface. In the present study, we examine the adsorption of trace metals (Cd, Co, Cr, Cu, Ni, Pb) to both virgin and beached (aged) pellets under estuarine conditions in order to better understand the role of plastic materials on the transport and behaviour of metals from river to ocean. Metals added to river water and sea water adsorbed to both pellet types with isotherms defined by either the Langmuir or Freundlich model. With increasing pH in river water, adsorption of Cd, Co, Ni and Pb increased, adsorption of Cr decreased and adsorption of Cu was relatively invariant. Along a salinity gradient, created by mixing river and sea waters in different proportions, adsorption of Cd, Co and Ni decreased, adsorption of Cr increased and adsorption of Cu and Pb exhibited a minimum towards the fresh water end-member. In all experiments and for all metals, adsorption was considerably greater to beached pellets than to virgin pellets, presumably because of the weathering of and adsorption and attrition of charged minerals by the former. Speciation considerations suggest that adsorption to the pellet surface largely involves metal cations or oxyanions (e.g. HCrO₄^- and CrO₄^2-), although additional forms of Cu and Pb (e.g. organic complexes) may also be involved. Despite mass-normalised adsorption constants being lower than equivalent values defining the adsorption of metals to sediments, microplastics should be regarded as a component of the suspended load of estuaries whose precise role on contaminant transport requires further study.