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Article (refereed) - postprint
This is the peer reviewed version of the following article:
Horton, Alice A.; Dixon, Simon J. 2018. Microplastics: an introduction to
environmental transport processes. Wiley Interdisciplinary Reviews: Water, 5
(2), e1268, which has been published in final form at
https://doi.org/10.1002/wat2.1268
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Microplastics: an introduction to environmental transport
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processes
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Article type: Primer
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Alice A. Horton*
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alihort@ceh.ac.uk
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Centre for Ecology & Hydrology, Maclean Building, Benson Lane, Crowmarsh Gifford, Wallingford,
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Oxfordshire, OX10 8BB, UK.
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ORCID: 0000-0001-6058-6048
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Conflicts of interest: none
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*Corresponding author
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Simon J. Dixon
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s.j.dixon@bham.ac.uk
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School of Geography, Earth and Environmental Science, University of Birmingham, Edgbaston,
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Birmingham, B15 2TT
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ORCID: 0000-0003-3029-8007
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Conflicts of interest: none
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Abstract
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Microplastic pollution is widespread across the globe, pervading land, water and air. These
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environments are commonly considered independently, however in reality these are closely
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linked. This review gives an overview of the background knowledge surrounding sources, fate
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and transport of microplastics within the environment. We introduce a new ‘Plastic Cycle’
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concept in order to better understand the processes influencing flux and retention of
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microplastics between and across the wide range of environmental matrices. As microplastics
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are a pervasive, persistent and potentially harmful pollutant, an understanding of these
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processes will allow for assessment of exposure to better determine the likely long-term
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ecological and human health implications of microplastic pollution.
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Keywords: plastic pollution, plastic cycle, sediment, soil, freshwater, fate
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1. Introduction
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Plastic has many appealing characteristics to manufacturers and consumers, including
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being versatile, lightweight, durable, cheap and watertight. As a result, production of plastic
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has increased enormously since the introduction of commercially available plastics. In 1950
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an estimated 1.7 Mt were produced,
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with production estimates for the year 2015 ranging
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between 322 Mt and 380 Mt.
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An estimated 8300 million metric tons (Mt) of virgin plastic has
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been manufactured to date.
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Today, around 40% of plastic produced is for packaging, with
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these items generally designed for a single use before disposal.
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Unfortunately, this surge in
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the use of plastic has led to a massive increase in plastic items being released to the
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environment, due to intentional or unintentional losses.
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It is estimated that around 60% of all
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plastics ever made have accumulated in landfill or the natural environment.
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Plastic items are manufactured in all shapes and sizes, with the smallest sizes (< 5mm)
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considered to be ‘microplastics’. Those specifically manufactured to be of this small size are
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called ‘primary microplastics’ and are produced as ‘nurdles’ (small pellets used as a raw
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material to make plastic products, Fig. 1), glitter and microbeads, which are added to
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cosmetics and personal care products. Once in the environment, plastic items can break down
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and therefore even large items may eventually form hundreds if not thousands of ‘secondary
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microplastics’ in the form of fragments, fibres or films (Fig. 1). There are a number of
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mechanisms by which this breakdown can occur, including mechanical degradation such as
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road wear, tyre abrasion, physical weathering of large items and washing of synthetic textiles,
5-
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chemical degradation (e.g. exposure to acids or alkalis) and UV degradation (exposure to
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UV radiation). Biological degradation can also occur in the presence of organisms with the
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capacity to ingest and degrade plastics, for example waxworms,
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mealworms,
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and some
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microbes.
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Additionally, over time the plasticisers added to plastics during manufacture to
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give them their flexible and durable properties leach out, rendering the plastic brittle and more
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susceptible to degradation.
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Fig 1. Images of different types of plastic particles a) pellets/nurdles, b) fibres and c) fragments. Scale
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bars are approximate.
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2. Presence and sources of microplastics within the environment
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There are many ways in which plastics can be released to the environment, either as
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primary microplastics or as larger plastic items (‘macroplastics’) which will break down to form
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secondary microplastics (Fig. 2). Primary microplastics from domestic products, such as
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microbeads, can be present in waste water and subsequently discharged to rivers, while
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nurdles can be lost to freshwaters during production processes. Examples of secondary
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microplastic sources include intentional release (illegal dumping), mismanaged waste (litter)
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or unintentional losses (e.g. fishing gear and loss of shipping cargo),
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with the magnitude of
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different sources and pathways for microplastic release varying between the terrestrial,
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freshwater and marine environments.
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2.1. Microplastics on land
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All plastic is manufactured on land and, other than maritime or fishing uses, it is also
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where the majority of plastic is used in consumer products. The pathways for release of waste
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consumer products to land include direct littering and inefficient waste management e.g. loss
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during the waste disposal chain, industrial spillages, or release from landfill sites (Figure 2a).
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Modern agricultural practises make use of plastic in a variety of ways including as mulches,
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which can degrade in situ, in addition to bale twine and wrapping which can be improperly
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disposed of.
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These items can degrade to form secondary microplastics within the
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environment.
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Microplastics may also be released directly to land along with sewage sludge applied to
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agricultural land as a fertiliser. Wastewater treatments plants are quite effective at removing
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microplastic particles from the wastewater stream, often with ~99% removal,
18-20
and many of
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these particles will settle to the sludge. It is estimated that throughout Europe, between 125-
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850 tons of microplastics per million inhabitants are added annually to agricultural soils as a
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result of sewage sludge application.
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Horton et al.
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calculated that 473,000-910,000 metric
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tonnes of plastic waste is retained within European continental environments (terrestrial and
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freshwater) annually, which includes microplastics derived from sewage sludge, in addition to
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predicted inputs of litter and inadequately managed waste. Where plastics are not transported
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from land to rivers or the sea, this could lead to massive accumulation. However, few studies
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have investigated abundance of microplastics within terrestrial environments, or linked
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abundance to input pathways, therefore it is not currently possible to directly link accumulation
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with specific environmental characteristics or anthropogenic activities.
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2.2. Microplastics in freshwater environments
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Freshwaters represent the most complex system regarding microplastic transport and
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retention, as they receive microplastics from the terrestrial environment, function as conduits
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for microplastics to the marine environment (Figure 2b), act as a means of microplastic
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production through breakup of larger items and act as sinks retaining microplastics in
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sediments. Additionally, ‘freshwater’ represents rivers, streams, ditches, lakes and ponds, all
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with very different characteristics.
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Larger plastic items can enter the freshwater environment through inadequate waste
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disposal, either through littering or loss from landfill and transported from land via wind or
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