Suspended clay and surfactants enhance buoyant microplastic settling

Most of the plastic waste that enters rivers and the oceans is unaccounted for. Approximately half of the world's produced plastics are buoyant in water, meaning that processes must take place that effectively increase their density, causing them to settle out of solution. One such mechanism is biofouling, in which organic matter grows on the surface of plastics, making them denser. Here we present a new mechanism supported by laboratory experiments for buoyant plastic settling in which particles of clay adhere to the surface of the plastic, mediated by the presence of surfactants. Although the plastic particles in our experiments were a hundred times larger than the micrometer-sized clay particles, we show that clay can adhere to the plastic with sufficient mass to cause the plastic to sink. This occurs even though the plastic is electrically neutral. It is hypothesized that the hydrophobic tails of the surfactant molecules are attracted to the plastic while the hydrophilic heads attract the clay. A greater fraction of plastic sinks if the surfactant concentration is larger. Our findings suggest that microplastic settling is enhanced in muddy rivers due to interactions with naturally occurring or discharged surfactants, even in the absence of biofouling. Surfactants cause clay particles to adhere to plastic surfaces and may facilitate enhanced settling of microplastics in the environment, suggest insights from laboratory-based experiments.