An insight into laboratory column experiments for microplastic transport in soil

Abstract Plastic litter, most of which is landfilled or improperly disposed to aquatic and terrestrial environments, is a global environmental concern. Large-sized plastics can fragment into smaller pieces due to abrasion or weathering effects generating microplastics (MPs) which are tiny plastic debris smaller than 5 mm. In the aquatic environment, particularly oceans, MPs have been well documented in terms of their occurrence and toxicity. Soils have been shown to be an important sink of MPs, but the latter's fate and transport in the subsurface are not well understood. Laboratory studies have so far mostly employed rigid-wall columns to investigate MP transport through porous media, with a vast majority focusing on glass beads and predominantly sandy soils. Much less knowledge is available on MP transport in clay soils and minerals where transport mechanisms are much slower and the potential for chemical reactivity higher. There is hence a need for establishing experimental protocols for conducting such experiments. The paper presents preliminary results for an investigation assessing whether flexible wall permeameters (FWP) are suitable for the study of the transport of MPs in clayey soils. Key advantages of FWP are that they are widely available and commonly used and that they allow the control of effective stress in the sample. Two key questions addressed in this paper are whether the instruments themselves act as sink for MPs or as source of material that may interfere with the measurement of MPs in the effluent.