Biocalcification Induces the Preferential Settling of Small Buoyant Microplastics in Freshwater

The vertical transport of microplastics (MPs) in freshwater is poorly understood. In this study, the effects of biocalcification on the settling behavior of buoyant MPs in the presence of Microcystis aeruginosa and Ca²⁺ were investigated via incubation experiments. Here, we show that the formation of biogenic calcite on the surface of MPs resulted in an increase in density and, subsequently, the settling of MPs, with its effect varying significantly with the size and aging of MPs. Specifically, biogenic calcite preferentially facilitated the sinking of smaller MPs compared to larger ones, mainly due to their different adsorption affinities for extracellular polymeric substances (EPS), which provide nucleation sites for calcite. Notably, aging further enhanced this size-dependent settling of MPs, which was primarily attributable to differences in the selective binding of macromolecules in EPS. Compared to the pristine MPs, the aged ones tend to acquire more polar macromolecules from EPS, which resulted in greater complexation interactions between the MPs and Ca²⁺ and consequently increased biogenic calcite precipitation and enhanced settling of MPs. The findings of this study highlight the importance of biogenic calcite-mediated settling of MPs in freshwater environments.