The concentrations of phenanthrene or nitrobenzene in supernatant will be determined by a high performance liquid chromatography (HPLC). The difference between the initial (Co) and the final equilibrium (Ce) concentrations of phenanthrene or nitrobenzene in solution will be used to determine the concentrations sorbed by the microplastics (Cm). A distribution coefficient (Kd; L/kg) for each individual data point within the linear range of adsorption isotherms will be calculated as Kd=Cm/Ce, and then averaged. Samples arising from the experiments (acid digests of the pellets after the latter had been removed) will be analysed for 65Cu and 66Zn by collision cell–inductively coupled plasma–mass spectrometry (ICP-MS).