Exposure of microplastics to organic matter in waters enhances microplastic encapsulation into calcium carbonate

Plastic pollution in water ecosystems is threatening the survival of wildlife. In particular, microplastics may be encapsulated into calcium carbonate, a crucial building block of hard tissue in many species such as molluscs, corals, phytoplankton, sponges, echinoderms, and crustaceans. Actually little is known on the effect of humic acids, a common component of dissolved organic matter, on the encapsulation of microplastic into calcium carbonate. Here, we precipitated calcium carbonate with humic acids and polystyrene microspheres. The precipitation process was followed by measuring pH during the reaction. Composition, structure, morphology, surface properties and microspheres encapsulation extent were analysed by infrared spectroscopy, X-ray powder diffraction, atomic force microscopy, scanning electron microscopy, total organic carbon analysis, thermogravimetric analysis, nuclear magnetic resonance spectroscopy, electrophoretic and dynamic light scattering. Results show, for the first time, that encapsulation of polystyrene microspheres into calcite crystals occurs only after the treatment of the microspheres with humic acids, leading to encapsulation of about 5% of the initial microspheres mass. On the contrary, untreated microspheres did not encapsulate in calcium carbonate. Our findings imply that exposure of microplastics to dissolved organic matter in water ecosystems could result in enhanced encapsulation into the exoskeleton and endoskeleton of aquatic organisms.