Microplastics in soils: Analytical methods, pollution characteristics and ecological risks

Elsevier, TrAC - Trends in Analytical Chemistry, Volume 109, December 2018
He D., Luo Y., Lu S., Liu M., Song Y., Lei L.

Microplastic Pollution in Soil: Analytical Methods, Pollution Characteristics, and Ecological Risks

In the quest for sustainable development, an issue that is increasingly attracting attention is microplastic pollution in soil. Microplastics, tiny fragments of plastic smaller than 5mm, pose a largely unquantified but potentially significant threat to the environment and human health.

The Sustainable Development Goals (SDGs), set by the United Nations, encapsulate the global aspiration to protect our environment. One of these goals, SDG 15, highlights the importance of protecting, restoring, and promoting sustainable use of terrestrial ecosystems. However, microplastic pollution in soil can significantly undermine this goal. This article aims to provide a comprehensive overview of this emerging issue, shedding light on the analytical methods used to detect microplastics, their pollution characteristics, and the ecological risks they pose.

What Are Microplastics?

Microplastics are minute particles of plastic that originate from various sources. They are predominantly created when larger plastic items degrade and fragment into smaller pieces, but they can also be purposefully manufactured, as in the case of microbeads found in certain cosmetic products.

Sources and Pathways of Microplastics to Soils

Microplastics can reach soils through a variety of pathways. Wastewater treatment plants, for instance, are significant sources of microplastics that end up in our soils. When wastewater is treated, microplastics are often not fully removed and can end up in the residual sludge that is frequently applied to agricultural lands as fertiliser.

Other sources of microplastics in soils include the direct littering of plastic waste, atmospheric deposition, and the breakdown of larger plastic items in the environment. The use of plastic mulch in agriculture also contributes to the microplastic load in soils.

Detecting Microplastics in Soil

Detecting and quantifying microplastics in soils is a challenging task due to their small size and the complex composition of soils. However, innovative methods are being developed to overcome these challenges.

One popular method involves density separation, where soil samples are mixed with a high-density solution that causes the microplastics to float, allowing them to be separated and analysed. Other methods include the use of micro-FTIR and Raman spectroscopy, which can identify the specific type of plastic in the microplastics.

These detection methods are continually being refined and improved, but they are not without their limitations. They can be time-consuming, require specialised equipment and expertise, and may struggle to detect the smallest microplastics.

Microplastic Pollution Characteristics in Soil

The characteristics of microplastic pollution in soils can vary greatly depending on the source of the microplastics and the specific conditions of the soil.

In terms of size, microplastics found in soils typically range from a few micrometres to a few millimetres. The size of the microplastics can influence how they interact with the soil and organisms within it.

The type of plastic can also vary, with polyethylene, polypropylene, and polystyrene being the most commonly found. These different types of plastics have different properties and potential impacts on the soil environment.

Ecological Risks of Microplastics in Soils

Microplastics in soils can pose a variety of ecological risks. They can physically alter the soil structure, affecting water movement and root growth. They can also be ingested by soil organisms, potentially causing physical harm or behavioural changes.

Microplastics can also act as vectors for pollutants, as they can adsorb and transport other contaminants, such as heavy metals and organic pollutants, into soils. Furthermore, the degradation of microplastics can lead to the release of harmful substances, such as additives and monomers, into the soil environment.

The ecological risks of microplastics in soils are not just confined to the soil ecosystem itself. There is a risk that microplastics could enter the food chain, with potential implications for human health. For instance, earthworms and other soil invertebrates can ingest microplastics, which could then be passed up the food chain to birds and mammals.

The Global Scale of Microplastic Pollution in Soils

The exact scale of microplastic pollution in soils globally is still largely unknown. This is due in part to the challenges of detecting and quantifying microplastics in soils, as well as the lack of comprehensive global monitoring programmes. However, several studies have demonstrated that microplastic pollution in soils is a widespread problem, with microplastics being found in soils across all continents.

In agricultural soils, the use of plastic mulch and the application of sewage sludge are significant contributors to the microplastic load. In urban environments, atmospheric deposition and the breakdown of plastic waste can lead to the accumulation of microplastics in soils.

Addressing Microplastic Pollution in Soils

Addressing microplastic pollution in soils is a complex task that requires a multi-faceted approach. Preventing the input of microplastics into soils in the first place is a crucial step. This could involve improving wastewater treatment processes to better remove microplastics, promoting the use of alternatives to plastic mulch in agriculture, and reducing overall plastic consumption and waste.

Cleaning up existing microplastic pollution in soils is another challenge. Current methods for removing microplastics from soils are limited and can have unintended negative impacts on the soil ecosystem. Therefore, further research is needed to develop effective and environmentally friendly methods for removing microplastics from soils.

Monitoring and research are also crucial for understanding and addressing microplastic pollution in soils. This includes improving methods for detecting and quantifying microplastics in soils, researching the ecological impacts of microplastics in soils, and developing global monitoring programmes.

Microplastic pollution in soil is an emerging issue that poses significant ecological risks. It undermines our ability to achieve Sustainable Development Goal 15 and threatens the health of our terrestrial ecosystems. Addressing this challenge requires a concerted effort from scientists, policy-makers, industry, and society at large. By improving our understanding of this issue and developing effective strategies to tackle it, we can help protect our soil ecosystems for future generations.