Experimental and mathematical investigation of cotransport of clay and microplastics in saturated porous media.

Microplastics in the subsurface cause groundwater contamination, thereby posing potential risks to human health and the ecosystem. Clay particles are ubiquitous in the subsurface and can interact and alter the transport behavior of microplastics. Hence, it is essential to understand the effect of clays on the transport behavior of microplastics to estimate the groundwater contamination potential. This study investigated the individual transport and cotransport of clay and microplastics under different pore-water velocities and sand types in saturated porous media through column experiments and mathematical modeling. Copresence of suspended microplastics retarded the transport of clay due to the preferential attachment of clay over microplastics on grain surfaces and the formation of clay-microplastic heteroaggregates which have a greater retention in sand than free clay and free microplastics. However, in contrast, cotransport with clay enhanced the transport of microplastics due to the lower affinity of microplastics than clay for deposition on grain surfaces and the lesser mass fraction of microplastics than clay in the heteroaggregates. The cotransport of clay and microplastics was successfully simulated using a two-way coupled model, which accounted for the retention of free clay and free microplastics in the sand, kinetics of clay-microplastics heteroaggregation, and heteroaggregate retention in the sand. The rates of heteroaggregation and heteroaggregate retention in sand decreased with increasing velocity and grain size, resulting in increased transport of clay and microplastics.