Microplastic in soil–current status in Europe with special focus on method tests with Austrian samples

Abstract

Within the last decade the production of plastic steadily increased and so did the amount of plastic waste. When bigger plastic pieces enter the environment, they are fragmented over time due to mechanical and environmental forces. The occurring and the directly released microplastic cause severe problems on soil organisms, due to alteration of physical properties and chemical interactions in the habitat. Main emissions sources of microplastic are different kinds of abrasions (road traffic, packaging, fibers of textiles during washing), waste disposal and drifts. Remains of mulching foils and protection nets spoil agricultural soil as well as the application of compost, sewage sludge and digestate, which may contain microplastic. Once released, microplastic accumulates much stronger in terrestrial than in aquatic systems. Spectroscopic, microscopic and thermo-analytical methods are commonly used to analyze microplastic in soil. The main challenges are to differentiate between soil matrix and plastic particles and to get rid of disturbing organic compounds. Unfortunately, there is no soil without plastic, no environmental blind sample to allow the finding of method limits. Inter-laboratory cooperation and data collection should allow estimation and comparison of emissions not only on European but on global scale. Investigations of Austrian samples provided a first orientation for regulations and measures to avoid further environmental pollution.

Full text

AIMS Environmental Science, 7(2): 174–191
DOI: 10.3934/environsci.2020011
Received: 29 November 2019
Accepted: 08 April 2020
Published: 15 April 2020
http://www.aimspress.com/journal/environmental
Review
Microplastic in soil–current status in Europe with special focus on
method tests with Austrian samples
Katharina Meixner, Mona Kubiczek and Ines Fritz*
Institute of Environmental Biotechnology; Department of Agrobiotechnology, IFA-Tulln; University
of Natural Resources and Life Sciences, Vienna; Konrad Lorenz Straße 20, 3430 Tulln, Austria
* Correspondence: Email: ines.fritz@boku.ac.at; Tel: +4314765497442.
Abstract: Within the last decade the production of plastic steadily increased and so did the amount
of plastic waste. When bigger plastic pieces enter the environment, they are fragmented over time
due to mechanical and environmental forces. The occurring and the directly released microplastic
cause severe problems on soil organisms, due to alteration of physical properties and chemical
interactions in the habitat. Main emissions sources of microplastic are different kinds of abrasions
(road traffic, packaging, fibers of textiles during washing), waste disposal and drifts. Remains of
mulching foils and protection nets spoil agricultural soil as well as the application of compost,
sewage sludge and digestate, which may contain microplastic. Once released, microplastic
accumulates much stronger in terrestrial than in aquatic systems. Spectroscopic, microscopic and
thermo-analytical methods are commonly used to analyze microplastic in soil. The main challenges
are to differentiate between soil matrix and plastic particles and to get rid of disturbing organic
compounds. Unfortunately, there is no soil without plastic, no environmental blind sample to allow
the finding of method limits. Inter-laboratory cooperation and data collection should allow estimation
and comparison of emissions not only on European but on global scale. Investigations of Austrian
samples provided a first orientation for regulations and measures to avoid further environmental
pollution.
Keywords: soil; compost; digestate; microplastic; microplastic identification; microplastic particle
quantification

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Abbreviations: ABS: acrylonitrile butadiene styrene; AUT: Austria; DM: dry matter; DDT:
dichlorodiphenyltrichloroethane; EU: European Union; FTIR: Fourier-transformation infrared
spectroscopy; GC-MS: gas chromatography coupled with mass spectrometry; GER: Germany; HCH:
hexachlorocyclohexane; HDPE: high-density polyethylene; LDPE: low-density polyethylene; MEPs:
meso plastic; MP: micro plastic; NIR: near infrared; PA: polyamide; PAH: polycyclic aromatic
hydrocarbons; PAN: polyacrylonitrile; PC: polycarbonate; PCB: polychlorinated biphenyl; PE:
polyethylene; PES: polysulfone; PET: polyethylene terephthalate; PMMA: poly(methyl
methacrylate); PP: polypropylene; PS: polystyrene; PS: polystyrene; PU: polyurethane; PVA:
Poly(vinyl alcohol); PVC: polyvinyl chloride; SBR: styrene-butadiene rubber; TDS: thermal
desorption; TED: thermal extraction desorption; TEM: Transmission Electronic Microscopy; TGA:
thermogravimetric analysis; THF: tetrahydrofuran; UV: ultraviolet
1. Introduction
Within a time span of 13 years (2004 to 2018) the production of plastics steadily increased–
globally from 224 to 360 million tones and in Europe from 60 to 64 million tones [1,2]. This rise in
plastic production is accompanied by a similarly increased amount of plastic waste. About 26 million
tones plastic waste are produced each year in Europe and only 6% are collected for recycling, 31%
are landfilled and 39% are incinerated. It has to be assumed that the residual 24% end up in the
environment, where it accumulates in nearly all habitats [5] and causes severe damages [6]. If these
current trends continue, a total of 12,000 megatons of plastic waste will have ended up on landfills
and in the environment by the year 2050 [4,7].
1.1. Definition of microplastic
The term “microplastic” was used the first time in the year 2008. Until now it is just a rough
orientation because it is based on physical properties, formal or pragmatic considerations, including
e.g., shape, size, material, delimitation to nanoparticles and measuring techniques. [8].
Depending on the reference the definition of microplastic varies–which indicates the
classification as an artificial system. In the past, the definition included any solid, water insoluble
plastic particle in a size between 0.001 mm and 1 mm and is deriving from consumer products, but
did not cover rubber and fibers [9].
A more recent definition includes rubber and synthetic fibers, since the term plastic (defined as
synthesized, not metallic polymer with a high molecular weight, being assembled of recurring
monomers) already covers rubber, elastomers as well as textile and technical fibers [10]. A further
definition says that plastics are a subgroup of polymers, which only include thermoplastics (e.g.,
polypropylene (PP), polyvinylchloride (PVC); can be softened and remolded) and thermosets (e.g.,
polyurethane (PU), epoxy and phenol resins, polystyrene (PS); sometimes also called duroplastics,
cannot be softened by heat or pressure once they are formed). Nevertheless, recent research covers
elastomers (e.g., rubber) and products based on synthetic polymers as well as modified natural
polymers in the definition [11,12].
Other classifications are based on the particle sizes: usual ranges are 5–1 mm (large
microplastic), 1–0.5 mm, 0.5–0.1 mm, 0.1–0.05 mm, 0.05–0.01 mm, 0.01–0.005 mm and 0.005–

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0.001 mm, which are linked to the numbers of particles that make up a total weight of 14.13 mg [11].
Frequently, microplastic particles are defined to be smaller than 5 mm or between 5 mm and 0.001
mm. Furthermore, the subdivision into big (5–1 mm) and small microplastic (1–0.001 mm) is often
used. The term nanoplastic is used for particles smaller than 0.1 µm [13], being below the range of
small microplastic. [10].
The definition of microplastic is not focused on environmental aspects, neither is it limited to
substance groups with human-and ecotoxicological relevance, nor upper and lower limits exist [8].
1.2. Microplastic emission sources
Microplastic can be categorized with regard to its origin into type A, type B and secondary
microplastic. While type A is part of a product i.e., friction bodies in cosmetics or powder for laser
sintering; type B arises during the utilization of products i.e., abrasion from tires or synthetic fibers
released during washing of textiles. Secondary microplastic occurs through fragmentation of
macroplastic, whose origins are, among others, mainly landfills, waste treatment, agricultural
applications and littering. In the environment, plastic is fragmented into smaller pieces due to
temperature changes and freezing, UV radiation, microorganisms, oxidation or mechanical stress [4].
The two ways, how plastic gets into the environment, can be summarized, from a personal point of
view, as intended and unintended (Figure 1). Agricultural applications and additives in industrial
products are examples for intended release [14], while unintended release includes littering, losses,
dumping and tire abrasion, respectively. Often overlooked, packaged food waste which ends up in
the organic waste bin contributes to the unintentional release of microplastic in soil [15].
Figure 1. Overview of intended (e.g., mulching) and unintended (e.g., littering,
landfilling, abrasions, organic fertilizer) ways of how plastic is entering the environment
(© BOKU, IFA-Tulln).

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Compost contains plastic due to incorrect disposal of plastic into the organic waste bin and since
the removal through manual sorting and sieving before and after composting is insufficient.
Digestate arises from anaerobic treatment of energy crops and agricultural residues but also of food
waste, expired food, organic waste and residual or municipal waste. The packages are removed
before the organic content enters the biogas plant, but plastic residues still remain and are then
present in the digestate. A study focusing on organic fertilizers in Germany showed that 35 billion to
2.2 trillion microplastic particles larger than 1 mm are released into environment per year by organic
waste treatment plants (composting plants, biogas plants). Digestate deriving from the treatment of
municipal organic waste contains the highest number of particles (895 particles kg−1 DM), while
high-quality compost shows the lowest numbers (25 particles kg−1 DM). The enrichment of non-
digestible material and therewith of microplastic is much higher in anaerobic than in aerobic
treatment. Reasons are the higher mass reduction in anaerobic systems [16] and maybe the fact that
digestate is partly recirculated in the biogas plant. Additionally, accumulation and size distribution of
microplastic may be affected by the way digestate is treated [17].
In wastewater treatment plants 90% of the microplastic–fibers of textiles, particles of cosmetic
and hygiene products [13,18]–are transferred into sewage sludge [19] and remains there independent
from the treatment procedures [18]. It mostly consists of nylon, polyester and acrylic fibers. Other
sources in sewage sludge are water soluble, persistent polymers, which are for example used as
transparent foils for dishwasher tabs [15]. In Europe, one third of the sewage sludge is used as
fertilizer on agricultural land and 40% are landfilled [10]. While the first is a direct source of
contamination, the second may leach microplastic in the environment in a slow and non-predictable
way.
Especially in those locations where not enough water is available, treated sewage water is often
used for irrigating field crops. Fibers from textiles and hygiene products are in such a case directly
applied to the soil. On the other side, fields may be flooded by rivers, which transport and deposit
plastic debris of all sizes on the soil [10]. The release of significant amounts of microplastic directly
on the soil and the mechanical filtering effect are the main reasons for the 4-to 23-fold higher
accumulation of microplastic in terrestrial compared to aquatic systems [8,13]. Besides floods, runoff
and deposition from the atmosphere are important transport mechanism which may repeatedly lead to
irregular pollution hotspots [13,19].
In Austria and in Germany, as in the whole EU mainly type B microplastic (89%) is emitted
(Table 1), which primarily includes emissions from traffic, infrastructure and buildings (62%)
followed by private consume and by commercial end-users (24%) and production industry (14%).
Littering in Austria is estimated to account for just 0.5% of the total plastic waste, due to an efficient
waste management system [8,10].

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Table 1. Top emission sources of microplastic and their emitted masses per capita and year for the EU, Austria [20] and Germany [8]. The
quantification mainly followed a top-down approach based on production and consumption data. Emissions in t a−1 were calculated based on
the number of inhabitants (state 1 January 2019) Austria: 8.9 million, Germany: 83 million, EU: 513.5 million [21]. *For microplastic type
specification see chapter 1.2
No.
Source
Type*
Emissions
(g cap−1 a−1)
(t a−1)
AUT
GER
EU
AUT
GER
EU
1
Tire wear
B
1223.1
1228.5
1784.8
10835.0
101989.1
916461.0
2
Release at waste disposal
(compost, rubble and metal crushing, plastic recycling,
landfills)
Secondary 301.5 302.8 439.9 2671.0 25138.2 225889.0
3
Abrasion of bitumen in asphalt
B
227.0
228.0
331.2
2011.0
18928.4
170088.0
4
Losses of plastic pellets
A
181.2
182.0
264.4
1605.0
15109.5
135772.0
5
Drifts of sport fields
(artificial turf for football and hockey, riding arenas,
coopetition lanes, playgrounds)
B 131.2 131.8 191.5 1162.0 10941.9 98323.0
6
Release at construction sites
(abrasion during demolition work, processing of plastics,
abrasion/cutting losses of insulation)
B 116.6 117.1 191.5 1033.0 9721.5 98323.0
7
Abrasion of shoe soles
B
108.5
109.0
158.4
961.0
9049.1
81314.0
8
Fragmentation of plastic packaging
B
98.7
99.1
144.0
874.0
8227.2
73929.0
9
Abrasion of road marking
B
90.6
91.0
132.2
803.0
7554.7
67886.0
10
Fiber abrasion during washing of textiles
(household linen, launderette, commercial laundries)
B 76.4 76.8 111.6 677.0 6375.9 57293.0
11
Release as a consequence of intended plastic use
(mulching foil, others not researched) [22]
Secondary 5.6 6.5 n.f. 50 543 n.f.

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Independent from the way we classify microplastic, it would not accumulate in the environment
if it would not had been released. While the unintended release is almost impossible to avoid as it
requires drastic measures, the intentional release can be reduced more easily: Germany, for example,
has introduced incentives to increase the collection of both, single-use and reusable PET bottles.
Many other European countries have already released regulations to end production and distribution
of single-use plastic products, following the EU single-use plastic directive (2018/852/EC) [4,8,15].
However, there is generally no refund system established for other types of plastic packaging and
therefore motivation to reduce littering and illegal waste deposition remains is low for those.
1.3. Legislation regarding plastic release
The circular economy strategy of the EU aims on closing nutrient cycles and minimizing
wastage of valuable resources. For this purpose, organic waste is used directly or preferably after
aerobic or anaerobic degradation to compost or secondary fertilizer (digestate). [23]
In the EU [24] as well as in the US [25] the application of sewage sludge is regulated, depending
on their content of pathogens, heavy metals and harmful substances. There are thresholds and limits
for plastics among the other unwanted substances [19].
Current national legislation in Austria and Germany covers nutrient composition (e.g., nitrogen,
phosphorus, potassium) and the related application limits for organic fertilizers in the same way as
for mineral fertilizers. Upper limits of impurities, such as glass, metal and plastic apply as well. The
limits concerning plastic particles aim towards those larger 1 mm, 2 mm and 20 mm, this means
small microplastic and nanoplastic are not covered. The upper limits depend on the field of
application as well as on the according regulation. In Austria the upper limits of plastic particles
larger 2 mm and 20 mm for agriculture are 0.1–0.2% DM and 0.02% DM, respectively. The lower
value (0.1%) is allocated to the fertilizer directive [26], while the larger values (0.2%) originates
from the compost directive [27]. For landscaping, conservation and re-cultivation layers on landfills
the upper limits of plastic particles larger 2 mm and 20 mm are 0.4% DM and 0.04% DM,
respectively [27]. Currently only the province Vorarlberg sets upper soil limits and precautionary
values by a regional regulation which is in force since 1 January 2019 [28]. The upper soil limits for
plastic particles larger 1 mm are 200 mg kg−1 DM and 10 cm2 m−2 and include plastic as well as
rubber and composite materials. The soil precautionary values for this particle size are 100 mg kg−1
DM and 5 cm2 m−2. Exceeding the soil precautionary values leads to limited application. More
specifically, compost which is applied to soil must not contain plastic particles larger 25 mm,
particles larger 2 mm are limited to 0.1% DM and 15 cm2 L−1. Reasons to issue this regulation were
plastic particles in various sizes on fields after applying farm manure.
Plastic residues are considered to be unwanted compounds in solid and liquid waste such as
digestate, compost and wastewater. Limits for the tolerable amounts exist for both, although
identification and characterization is not mandatory. The category “filterable substances” does
include plastic particles without explicit upper limits but counts towards the total amount of solids in
industrial wastewaters. When industry wastewater is discharged into the sewer, no upper limits apply
as long as the sewer function is not negatively affected [10,29].

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In Germany the situation is similar: the upper limits for impurities, including plastic, are set at
0.4% DM for particles larger 1 mm for waste paper, cardboard, glass, metals and ductile non-
deformable plastics and 0.1% DM for other not degradable materials. These limits are applied for
fertilizer and manure, soil additives, growing media and plant growth stimulants [30]. For particles
larger 2 mm the upper limit of 0.5% DM must not be exceeded, which includes glass, plastic and
metal [31]. All these mentioned directives and regulations aim towards visual appearance by
counting particle of a minimum size but do not cover microplastic nor do they consider possible
effects on soil [15].
It is known for a long time that single use products make up the major share of plastic waste and
litter [7]. For example, already in 1994 the European packaging regulation (94/62/EC) [32] clearly
mentions measures to avoid waste by a priority list, which consist of prevention, re-use, recycling
and recovery in descending order.
The most recent of the legal initiatives of the EU is the directive 2019/904/EC [33]. It prohibits
placing single use plastic products and oxo-degradable plastics on the market. This directive includes
the so called extended producer responsibility, which encourages member states to ensure that
producers of single use plastic products cover the costs for cleaning up, transporting and treating
litter resulting from those products. In addition to this directive the Austrian beverage industry has
voluntarily undertaken several steps to reduce plastic waste. These steps include the wish to increase
the market share of reusable packaging and campaigns to raise public awareness against littering [34].
2. Microplastic in soil
2.1. Resilience of released plastic
Synthetic polymers were invented and designed to best possible withstand a re-integration into
the biological cycle. The commonly communicated retention time of plastic (250–500 years) is just
valid at laboratory conditions, where no energy and carbon source other than plastic is available for
the degrading microorganisms. But in soil, in sediment and in sewage sludge many other organic
substances are present that can be utilized more easily than the plastic polymers, making them the
least attractive substrates in the habitat. Microorganisms always prefer to consume the most easily
within all available nutrient sources (known as the principle of “diauxie” [35]) and step by step,
substance by substance, they pass over to decompose the more hardly degradable substances. This
means, persistent polymers will only then be degraded when no more organic substance like plant
residues or humus is available [15].
2.2. Environmental impact of microplastic
All of the currently published impacts of plastic residues on physical, chemical and biological
properties of soil are assumptions [15]. However, there is no evidence about a positive impact of
plastic on soil: the polymers have no water holding capacity, no ion-exchange capacity, no soil
particle binding, cannot act as carbon source to support microbial growth and do not contain mineral
nutrients. Among the rare findings, polyacrylic fibers negatively impact water stable aggregates and
contribute to the formation of soil clumps; finally leading to erosion [13,36].

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For that reason, microplastic is not beneficial for soil and is disparate to the benefits of organic
carbon applied with compost [37].
With the fragmentation of (formerly) bigger pieces into microplastic, the probability to enter
deeper soil layers increases. Further transport by wind and water may be reduced while biological
transport (in the digestion tract of soil organisms) becomes an important factor. Longer dwell times
of the microplastic particles result in longer exposure times for the various affected soil
organisms [4,13,18,19].
The above mentioned changes in physical soil properties do indirectly affect all soil organisms.
For example, a reduced water holding capacity increases water evaporation and reduces the available
amount of water for plant growth. Reduced plant growth results in reduced amounts of root exudates
and in reduced microorganism growth which results in reduced feed for grazing organisms. A small
change of one soil characteristic may have big impact on the whole food web and may even become
economically remarkable via reduced soil fertility [37,38].
Other effects are directly caused by chemicals, which are leaching from the plastic matrix or
were adsorbed at their hydrophobic surfaces. Among the better known compounds are e.g.,
plasticizers like bisphenol A and phthalates, which are just loosely incorporated in the polymer
structure and will leach out over time. Heavy metals and polychlorinated biphenyls (PCBs),
polycyclic aromatic hydrocarbons (PAHs), organochlorine pesticides (e.g., DDT, HCH) and non-
volatile organochlorines from PVC and other chlorinated substances can adsorb at the particles
surface. [13,39]
Preferably animals and to a lesser extent also plants are affected by these compounds.
Earthworms, nematodes, collembolan, isopods and mites directly ingest small microplastic
particles [18] while plants can take up dissolved components, such as phthalates [19]. Hardly any
information is available about the direct uptake of microplastic by plants. However, it is assumed that
nanoplastics can pass the cell membrane (independent from plant or animal) due to their size,
structure and properties and can be transported by the organisms flow systems (root, xylem, blood
circulation) and can even reach and influence sub-cellular structures, such as transpiration, water and
lipid fractions, plasma membrane potential, tonoplast potential, cytoplasm and vacuoles. [18].
Finally, nanoplastic, plastic additives and adsorbed substances not only influence the viability of
microorganisms, animals and plants but also enter on this way the food chain and affect human
health [13,18,19].
3. Quantitative measurement of microplastic
Until now, no standardized methods exist to detect, identify and quantify microplastic in soil.
However, the need for comparable data, gained by standardized sampling, sample extraction and
quantitative measurements is mandatory for any legislative work. Current methods are mainly
categorized into thermo-analytical and spectroscopic measurements. Thermo-analytical methods
include pyrolysis or thermal extraction desorption, which are combined with gaschromatography and
mass spectrometry (Pyr-GC-MS, TED-GC-MS). Thermal extraction desorption (TED) is a
combination of thermogravimetric analysis (TGA) and thermal desorption (TDS). Spectroscopic
measurements include analysis via Raman or Fourier-transformation infrared spectroscopy (FT-IR),

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with different ranges of wavenumbers. The results of all the above methods are given in weight
microplastic per sample weight (mg kg−1). Additionally, light microscopy methods are used, which
result in particle numbers (per sample weight or per volume). Figure 2 gives an overview of
analyzing principles and necessary sample preparation steps, which are currently used to identify
microplastic in soil, compost, sewage sludge, digestate and other organic wastes [15]–a
comprehensive overview with further details is provided in the supplementary materials.
Figure 2. Overview of analysis methods, their achievable results and required pre-
treatment
3.1. General requirements for microplastic analysis
When analyzing microplastic, “plastic-free” conditions are mandatory during all steps (sampling,
sample preparation, detection, etc.) to avoid contamination [11]. This is especially important for
those sample types which contain only small numbers of plastic particles, such as river-or ground-
water. Examining microplastic in solid matrices is challenging, due to their inhomogeneous nature
(mixture of organic and inorganic materials of widely spread particle sizes, and layered
structure) [40]. This makes the necessary separation of plastic from matrix particles (in case of
spectroscopic measurements) difficult. Some materials have typical colors and shapes, while others
look and behave similar. A visual differentiation between organic matrix and plastic components or
degradation products therefore requires long training. Figure 3A shows how plastic and matrix
particles are merged in a soil sample and gives an idea of the challenging task to visually distinguish
between these particle types.

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A)
B)
Figure 3. A) Microplastic fiber among organic and inorganic particles of a soil sample
(400x magnification, © BOKU, IFA-Tulln); B) Inorganic particle or microplastic? When
using microscopic observations it might be difficult to distinguish between microplastic
and inorganic fragments (400x magnification, © BOKU, IFA-Tulln)
One of the first steps is to select the particle size of interest, independent from the quantification
method. For this purpose samples are sieved, either before or after drying. This mainly depends on
the water content of the sample.
Based on the requirements on the subsequently conducted analysis, it is often necessary to
separate plastic from matrix; preferably based on densities. Subsequently, floating plastic is collected
and washed [41]. Unfortunately, plastics with higher densities (e.g., PET, PVC) are lost. Another
option is the chemical removal of organic matrix components by treatment with HNO3 or H2O2. In
case of HNO3 particles of ABS, PA, and PET are also affected [11,42,43].
Samples need to be homogenized by either cutting or milling for some types of analyzes [40,44].
Especially when mills are used, the different characteristics of matrix and plastic particles need to be
considered, since the later are often flexible and resist the grinding process [45].
Thermo-analytical methods are generally fast and simultaneously detect either all (Pyr-GC-MS)
or selectively those components which can be absorbed and desorbed (TED-GC-MS). The
determination limit is mostly below 1 µg kg−1. The methods require little sample preparation but
have high prerequisites for a dust-free environment (TED-GC-MS). The procedures are destructive
and not able to determine particle sizes and numbers. Furthermore, chemical interferences may occur
and measuring times are sometimes long [11,40,44,46,47].
Spectroscopic measurements are fast, non-destructive and, in case of Raman, also contactless
options. The determination limits for µ-Raman and FTIR are particles with a minimum size of 1 or
10 µm, respectively. The needed sample amount is in the range of µg to mg. These methods require a
dust-free environment, complex sample preparation and are not able to quantify amounts or
concentrations. Furthermore, in case of µ-Raman chemical and spectroscopic interferences may
occur [11,46].

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The advantages of microscopic methods are that they are simple, low-cost and that different
shapes (fibers, fragments, films pellets) can be determined. The main disadvantages are that plastic
contaminations cannot be quantified and their identification is difficult which can partly be overcome
by comparing a sample with reference materials (Figure 3B) [15,41,48].
Method specific drawbacks can be avoided by combining microscopic, spectroscopic and
thermo-analytical methods. For example, the combination of Pyr-GC-MS and TEM (Transmission
Electronic Microscopy) allows to identify and quantify microplastic in soil [47]. Another example is
the examination and sorting of particles under the microscope and identification with Raman
spectrometry, FTIR or thermo-chemical measurements [4], such as the examination of particles with
a stereo microscope and subsequent analyses with µ-FTIR [41].
3.2. Methods used for investigating solid samples from Austria
We adapted sample pretreatment on the consistency (granular solid, pasty solid, liquid) and on
the estimated content of organic and inorganic solids (mainly by color and sample type).
Soil and compost samples (granular, high content of inorganic solids) were sieved (< 1 mm) and
elutriated with deionized water in a weight based ratio of 1:10 (sample: water). Sewage sludge
samples (high in filamentous organics) were diluted with deionized water to obtain a fluid
consistency; a weight based ratio of 1:10 (sample: water) was aimed at. For practical reasons, all
samples were weight into centrifuge tubes, the amount was recorded and tubes were balanced
pairwise (for the later centrifugation) with water. This led to non-integer dilution ratios. None of the
digestate samples required dilution with water or any other pretreatment.
Soil, compost and sewage sludge, all three are containing high amounts of disturbing matrix
substances and had to be pretreated by heating at 95 °C for 4 hours in presence of 10 ml 30% H2O2
solution. Colored organics were oxidized and to filamentous aggregates of sludge samples were
disrupted by the procedure. Evaporated water was replaced on weight basis (after H2O2 addition,
before heating). Compost and sewage sludge samples were ready for microscopy after this step.
Soil samples required a density separation with either saturated CaCl2 solution (1.35 g cm−3) or
with saturated ZnCl2 solution (1.45 g cm−3). The solid salts (CaCl2, ZnCl2) were directly added to the
H2O2 treated, suspended samples based on the calculated amount of water. The salts were fully
dissolved and the tubes were then put into the ultra-sonic bath at 25 °C for 5 minutes to break up
aggregates. Centrifugation at ca. 3000 g for 15 minutes separated the mineral particles from the
floating light fraction, which included all plastic. The upper half of the tube content was transferred
into a new tube (via Pasteur pipette) and filled to the original volume with deionized water–therefore
re-suspending the plastic without changing the dilution rate.
The amount of 15 to 20 µL treated sample suspension was inspected and counted in the light
microscope at 100x and at 400x magnification (to cover big and small particles). Digital images were
taken as backup for later revision–mainly to discuss uncertainties in the manual identification, which
proved to be a valuable learning procedure.
We are aware of the high failure rate and subjective decisions, whether a certain particle is
plastic or not [49]. To reduce the percentage of misjudged particles we started to create a reference
database with microscopic images from natural and synthetic fibers and from mechanically broken

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pieces from the most common plastic types (LDPE, HDPE, PET, PP, PES, PC, PS, PVC, PA), from
different types of hair and from typical silt and clay minerals. Additionally, we have been and we
continue collecting published photos.
3.3. Method variants that did not improve the procedure nor the result quality
Variations of the microscopy technique were tested, such as dark field and phase contrast
illumination, counting in the grid of a Thoma chamber with and without color staining of certain
plastic types (such as polyester or polyethylene, etc).
Dyeing of plastics with commercially available products failed because of the specificity of
those dyes and because of the comparably low color uptake of microplastic particles. The resulting
coloring was pale and therefore not helpful for counting.
Exceptional high numbers of plastic particles were counted in all sample types when CaCl2 was
used for density separation compared to ZnCl2 solution. We measured high plastic particle counts in
pure CaCl2 solutions (without sample), independent from the brand and purity grade of the salt. This
was not the case for ZnCl2. We will not use CaCl2 any longer and all analysis already done have been
repeated with ZnCl2 separation.
Filtering the floating layers after density separation resulted in significant losses due to adhesion
of the particles at the filter housing and due to formation of a stable foam in digestate and compost
samples. For that reason, digestate and compost samples should not be density separated.
Counting attempts via Thoma chamber failed, since some particles or agglomerates were bigger
than the chamber high and made the quantitative counting impossible.
Heating the sample on the glass slide after visual inspection (similar to Kofler melting point
determination [48]) can improve the detection rate and the counting. At least the thermoplasts can
clearly be differentiated from minerals based on their melting behavior. Knowing the melting point
may lead to an identification of the plastic type as well. Although not error-free, this attempt is
simple and cost effective. However, we did not evaluate the heating procedure and are currently not
able to provide a deeper insight about appearance of certain plastic types in certain sample types.
3.4. Results from soil, compost, sludge and digestate samples
We investigated 16 compost, 11 soil, 12 digestate and 6 sewage sludge samples from different
locations in Austria. Method development and treatment variations were tested and optimized with
these samples. Table 2 provides an overview of the results obtained with the optimized treatment and
analysis procedures. Some of the samples were analyzed in three replicates with the optimized
procedure, others were analyzed only once per variation; failed procedures (classified as outliers
according to ANOVA statistics) are not presented here.
The average number of plastic particles of 16 compost samples (a total of 22 analysis) was
15.4 Mil per kg sample, with 5.2 Mil minimum and 42.8 Mil maximum. The average of 11 soil
samples (17 analysis in total) was 12.7 Mil plastic particles per kg sample, with 3.1 Mil minimum
and 25.6 Mil maximum. The average of 12 digestate samples (a total of 18 analysis) was 7.1 Mil
plastic particles per kg sample, with 0.6 Mil minimum and 38.7 Mil maximum. The average number

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of plastic particles found in 6 sewage sludge samples (12 analysis in total) was 3.0 Mil per kg sample,
with 0.4 Mil minimum and 6.2 Mil maximum.
The total numbers of plastic particles found in those solid samples was surprisingly high.
Deviations were much higher than those from other types of chemical measurements but were
acceptable when considering that plastic particles will never be homogeneously distributed in solid
(powdery or paste-like) samples.
Although, the methods cannot be considered to be final and quality parameters, such as
repeatability and precision cannot be provided now, our results are a first data set for a comparably
simple and rapid investigation procedure. The microscopic method does not need costly
instrumentation and results can be obtained within 24 hours.
Table 2. Plastic particle counts found in Austrian soil, compost, digestate and sewage
sludge samples by conducting best possible optimized methods for microscopic analysis.
*mean values of three replicates.
Sample type
No. analysis
sample amount
final dilution
plastic particles (g
−1
)
(g)
(1 : x)
count
min
max
compost 1
1
12.7
23.6
5238
compost 2
1
10.4
28.7
9573
compost 3
1
11.6
28.7
12758
compost 4
1
10.9
28.8
8007
compost 5
1
10.0
29.6
11497
compost 6
1
10.7
27.9
42812
compost 7
1
10.5
27.7
13867
compost 8
1
10.4
34.8
23173
compost 9
1
10.1
30.6
28548
compost 10
1
10.5
28.8
30386
compost 11
1
10.1
31.4
19197
compost 12
1
10.3
28.4
9477
compost 13
1
10.8
27.6
12905
compost 14
3
10.2
29.8*
6459*
2077
11263
compost 15
3
10.1
31.5*
7099*
3885
9475
compost 16
3
10.5
28.4*
6161*
1939
10658
soil 1
1
10.5
331.7
18954
soil 2
1
10.1
306.9
8102
soil 3
1
10.1
377.3
19924
soil 4
1
10.6
366.7
13838
soil 5
1
10.8
300.9
11144
soil 6
1
10.6
282.0
3547
soil 7
1
10.3
374.6
14142

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AIMS Environmental Science Volume 7, Issue 2, 174–191.
Sample type
No. analysis
sample amount
final dilution
plastic particles (g−1)
(g)
(1 : x)
count
min
max
soil 8
1
10.5
306.1
11659
soil 9
3
10.0
31.1*
24368*
22299
25625
soil 10
3
10.0
31.0*
7222*
2308
9809
soil 11
3
10.3
25.2*
5185*
3394
7880
digestate 1
1
11.0
10.0
38667
digestate 2
1
14.0
10.7
11310
digestate 3
1
10.7
10.3
4569
digestate 4
1
13.6
10.1
1679
digestate 5
1
15.3
9.8
1307
digestate 6
1
14.7
10.0
1667
digestate 7
1
15.7
10.0
7222
digestate 8
1
10.5
10.0
1667
digestate 9
3
15.0
7.3*
13173*
10185
14667
digestate 10
3
11.0
10.1
561
digestate 11
3
13.0
8.5*
783*
470
940
digestate 12
3
11.0
10.7*
3178*
1788
4768
sewage sludge 1
1
14.9
7.4
984
sewage sludge 2
1
11.5
9.6
5101
sewage sludge 3
1
13.0
8.5
6205
sewage sludge 4
3
11.7
9.4*
3830*
3134
5223
sewage sludge 5
3
14.4
7.6*
849*
424
1273
sewage sludge 6
3
12.5
8.8*
1304*
489
1956
3.5. Outlook for upcoming method improvement
Reliable, simple and fast analyzing procedures are required for routine investigations. In contrast
to many other authors, we are convinced that it is more important to quantify the amount of plastic
than to identify the particle material. It is highly improbable to track back the contamination source
from the finding of a certain microplastic particle. We doubt the necessity to know the origin of the
plastic particle to estimate the effect of the contamination on a certain ecosystem.
A reliable routine method has to be applicable to various matrixes. For this purpose we use a
light microscope (Olympus BX43, 100x, 400x magnifications) connected to a camera (Canon 1100D)
to detect microplastic in agricultural soil, in soil from public areas, in solid and liquid digestate, in
sewage sludge and in compost. Our simple method allows the characterizing and counting of
particles down to 5 µm. Possible disadvantages of the procedure are, it requires attention, training
and some experience.
The next step in our approach is to improve the separation of plastic from matrix particles, in
order to simplify their counting and analysis. An obvious option is the destructive removal of the

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organic matter with hot HNO3 and other oxidizing agents [41,43,50,51]. We will evaluate in how far
plastic particles will still be detectable, how they keep their shapes and how the treatment will
influence their micro-spectroscopic properties.
4. Conclusions
We expect the plastic contaminations to have negative, at least no positive effects in soil. In
accordance with the precautionary principle, action is urgently needed to avoid further accumulation
of resilient plastic in the environment. Moreover, the effort to analyze solid environmental matrices
is quite high and control samples or blanks are no longer available, as the contamination is
ubiquitous.
We need to continue with the standardization attempt about definition of microplastic, taking all
applications, plastic types like rubber, water soluble plastics and environmental effects into account.
Standardized detection and quantification methods for microplastic in solid matrixes (soil, compost,
sewage sludge, digestate) need to be developed and improved, at least to provide data for comparison
with reports about plastic in marine samples. This would allow evaluation and comparison of
contaminations and types of emissions on a global scale. Finally, legislation must be provided a
database for upcoming regulations and for decision on measures to avoid further environmental
pollution. It is now time for actions.
Acknowledgements
This work is freely financed by the University of Natural Resources and Life Sciences, Vienna.
We gratefully thank Nina Hasselhahn of Hani Design, for creating the graphical overview. All
authors declare no conflicts of interest in this paper.
Conflict of interest
The authors declare no conflict of interest.
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