Microplastics: Understanding the pollution we can’t always see

Visible plastic pollution in our oceans has garnered media attention and public worry. But it is the microplastics now abundant in our ecosystem which may pose direct threats to human health and ecosystem functioning. Experts at the University of Birmingham are investigating the scale of microplastic pollution and potential impacts they may cause.

Macroplastics Are Only Part of the Story

Concerns about plastic pollution have sparked high-profile campaigns worldwide, and emotive images of marine animals ensnared in plastic waste fill the media. The vast majority of this waste reaches the oceans via rivers and waterways, but these ‘macroplastics’ are only one part of the story.

Microplastics — particles less than 5mm in diameter — are increasingly abundant in the ecosystem too. Some, known as primary microplastics, have been manufactured to be small particles, such as cosmetic 'beads'. Secondary microplastics result from the breakdown of larger fragments of plastics, through the mechanical abrasive effects of being washed down a river, battered by ocean waves or put through washing machines in the case of synthetic fleece clothing.

Sources of microplastics include waste-water inputs, run-off from roads and sealed surfaces or inappropriate waste management. Waste-water treatment can reduce the amount of plastics, but a large number of particles still end up in rivers and residual biosolids may be used as fertilizers, meaning that microplastics find their way into soils and groundwater via pathways not yet well understood.

Microplastics may pose greater risks to human and environmental health than the more visible macroplastics, because they can be ingested, inhaled or absorbed by organisms and move through the food chain; potential health consequences are poorly understood. Some plastic additives such as bisphenol-A (BPA) are recognized as potentially harmful to the endocrine system and largely phased out of food packaging.

There are two important implications for research. First, we need to better understand the mechanisms by which plastics are transported through our rivers to the oceans; how they change along these pathways and accumulate in rivers, sediments, soils and organisms. Second, research is needed into the effects of such plastics on ecosystems, including organisms at all levels of the food chain. Academics at the University of Birmingham are addressing these concerns in a series of inter-disciplinary research projects.

Studying Rivers as Transporters and Transformers of Plastics

How plastics are transported, accumulated and break down in freshwaters is a focus for Professor Stefan Krause, a hydrologist interested in biogeochemical cycling, contaminant transport and ecohydrological feedbacks in complex landscapes.

Krause’s work on river flow dynamics and particularly exchange fluxes between rivers and their streambed sediments shows how plastics behave and ‘age’ as they are transported.

“Rivers are not just transporters of plastic but are transformers of plastic,” he explains. “Low density plastics such as polystyrene or polyethylene floating at the surface of the river are exposed to sunlight, causing chemical and physical changes that bypass higher density plastics suspended in the water or sediment on the river bed.

“Sediment-bound plastics may take years to make their way into the oceans, by which time their interactions with biological materials will have changed their properties and behavior; contaminants may have adsorbed on their surfaces and they may have caused long-term exposures to vulnerable organisms.”

Understanding how plastics are transported and transformed in water is a first step for researchers who want to quantify their potential impacts and risks, providing the basis for determining how plastics affect human health.

Professor Dr. Stefan Krause, Professor of Ecohydrology and Biogeochemistry

Nanoplastics Combine to Create New Substances

Professor Iseult Lynch is an environmental nanoscientist focusing on behaviour of nanoplastics, the very smallest of particles, which range from 1 - 100 nanometers (nm) and microplastics. As particles reduce in size, the proportion of surface area to volume increases. The chemical behavior of particles this small is then driven more by the properties of their surface area than by their constituent materials.

Microplastics have already found their way into zooplankton, fish, invertebrates and mammalian digestive systems. There are concerns they may accumulate in organisms via a ‘Trojan Horse’ effect in which they bind to other entities rather than enter through simple inhalation or ingestion; Lynch’s work is identifying ways in which nanoplastics can bind to organic materials to create new substances. Contaminants adhere to the surface of microplastic particles and biofilms that may have formed around them, and which are a food source for some organisms. These particles are then consumed by organisms that would not normally ingest plastics.

Lynch’s research examines the particles’ impacts on daphnia, or water fleas. These are important sentinel organisms for the impact of water pollution, as they are filter-feeders. Micro- and nanoplastics in the water can enter their gut and cause stiffening or constipation. Lynch's research demonstrates that daphnia release proteins that attach to the particles in a process known as adsorption.

“It is thought that this adsorption fundamentally changes the behaviour of these nanoplastics,” explains Lynch. “These coated nanoparticles appear to have certain toxic effects on their host organisms, probably related to enhanced retention in the gut. This is unlikely to manifest as acute toxicity, being more likely to lead to subtle, longer-term effects such as slow starvation or impaired development.”

The contaminants of concern are not only the plastics themselves, but the kinds of compounds that might stick to their surfaces, like heavy metals and pharmaceutical products. Toxicity effects are not well understood and, thus, an important avenue for research. Academics must also focus on improving technologies for waste water treatment works, which continue to release large proportions of microplastics into rivers and, via biosolid fertilizers and irrigation, potentially into agriculture.

Professor Iseult Lynch, Professor of Environmental Nanosciences

Possible Uses of Micro and Nanoplastics

Despite concerns that micro and nanoplastics are deleterious for human health, experts are also interested whether nanoscience could have human and environmental benefits. Nanoplastics’ affinity to bind with toxic compounds could help remove these substances from water, although the challenge will be containing the nanoplastics once they have bound to contaminants.

Both Krause and Lynch see a hopeful future as researchers across disciplinary boundaries work together. Hydrologists, ecologists, sedimentologists, fluvial geomorphologists, economists, human geographers and clinical scientists working with chemists, environmental scientists and ecotoxicologists will help us to better understand the impact of microplastics.