On mechanical fragmentation of single-use plastics in the sea swash zone with different types of bottom sediments: Insights from laboratory experiments.

Mechanical fragmentation of four commonly used plastics, from 2-cm squares or cubes to microplastics (MPs, <5 mm), is experimentally investigated using a rotating laboratory mixer mimicking the sea swash zone with natural beach sediments (large and small pebbles, granules, sand). Macro-samples were prepared from brittle not-buoyant PS (disposable plates), flexible thin film of LDPE (garbage bags), highly buoyant foamed PS (building insulation sheets), and hard buoyant PP (single-use beverage cups). With a great variety of behaviors of plastics while mixing, coarser sediments (pebbles) have higher fragmentation efficiency than sands (measured as the mass of generated MPs), disregarding sinking/floating or mechanical properties of the samples. It is confirmed that, under swash-like mixing with coarse sediments, the MPs tend to burry below the sediment surface. The obtained relationship between the mass of MPs and the number of items is similar to that for MPs floating at the ocean surface.