Buoyant microplastic settling mediated by clay flocculation

A significant fraction of plastic waste entering the ocean does so through muddy rivers and estuaries which carry suspended clay and silt. Individual particles of clay are micron-sized, and so take hours to settle just a few centimetres. However, when clay suspended in fresh water comes into contact with salt water, the particles flocculate to form effectively larger particles that settle rapidly. Through laboratory experiments, we examine the interaction of clay with buoyant plastic particles in salt water solutions. The results show that, in the presence of surfactants, clay can flocculate onto the plastic particles making them effectively more dense and causing a significant fraction of them to settle. In separate laboratory experiments in which solid particles precipitate out of a solution, we find that the descending particles can either form a settling front or act collectively to induce counter-rotating cells in the ambient fluid. Challenges for numerically modelling these experiments will be discussed.