The build-up of microplastics in the ecosystem, as well as within our bodies, is an increasingly concerning matter. However, forecasting where these omnipresent particles will gather, and thus where cleanup efforts should be targeted, has proven challenging due to the numerous elements that influence their distribution and settling.
Recent findings from MIT indicate that a significant factor in locating where microparticles are likely to concentrate relates to the presence of biofilms. These thin, adhesive biopolymer layers are released by microorganisms and can gather on surfaces, such as sandbanks or shorelines. The research revealed that, under identical conditions, microparticles are less inclined to settle in sediment enriched with biofilms, since if they land there, they are more likely to be resuspended by moving water and swept away.
The accessible research results are published in the journal Geophysical Research Letters, in a paper authored by MIT postdoctoral researcher Hyoungchul Park and civil and environmental engineering professor Heidi Nepf. “Microplastics have certainly been in the news frequently,” Nepf remarks, “and we do not completely grasp where the accumulation hotspots are likely to occur. This study provides some insights” into the various factors that may lead these particles, along with smaller particles in general, to cluster in specific areas.
Most investigations examining the transportation and depositing of microparticles have been carried out over unstressed sand, says Park. “However, in reality, there exists a multitude of microorganisms, such as bacteria, fungi, and algae, and when they adhere to the streambed they produce certain sticky substances.” These materials are called extracellular polymeric substances, or EPS, and they “can considerably influence the characteristics of the channel bed,” he states. The new research aimed to specify the impact of these substances on the transport of microparticles, including microplastics.
The study utilized a flow tank with a base lined with fine sand, and occasionally with vertical plastic tubes mimicking the presence of mangrove roots. In some trials, the base was purely sand, while in others it was combined with biological material to replicate the natural biofilms prevalent in various riverbed and seashore habitats.
Water mixed with minuscule plastic particles flowed through the tank for three hours, after which the bed surface was photographed under ultraviolet light that caused the plastic particles to fluoresce, allowing for a quantitative assessment of their concentration.
The findings unveiled two distinct phenomena that influenced the extent of plastic accumulation on the various surfaces. The area surrounding the rods simulating above-ground roots created turbulence that hindered particle settlement. Furthermore, as the volume of simulated biofilms in the sediment bed increased, the accumulation of particles diminished.
Nepf and Park deduced that the biofilms occupied the gaps between the sand grains, creating less space for the microparticles to settle. The particles were more exposed as they penetrated less deeply amongst the sand grains, making them much easier to be resuspended and transported by flowing water.
Nepf adds: “The biofilm obstructs the plastics from settling in the bed as they cannot penetrate deeply. They merely remain on the surface, where they can be lifted and relocated. Therefore, if I dumped a large quantity of microplastics in two rivers, one with a sandy or gravelly bottom, and one muddier with more biofilm, I would anticipate that more of the microplastics would be retained in the sandy or gravelly river.”
All of this is complicated by additional factors, such as water turbulence or the texture of the bottom surface, she mentions. Yet it offers a “valuable perspective” for researchers studying the effects of microplastics in natural environments. “They are attempting to identify what types of habitats these plastics inhabit, and this provides a structure for categorizing those habitats,” she states. “It offers direction on where to seek more plastics versus fewer.”
As an example, Park suggests that within mangrove ecosystems, microplastics may preferentially gather in the sandy outer edges while the inner zones feature sediment with more biofilm. Consequently, this research implies that “the sandy outer regions may serve as potential hotspots for microplastic accumulation,” he asserts, making this a priority area for monitoring and conservation.
The study received support from Shell International Exploration and Production through the MIT Energy Initiative.