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Focus topics on microplastics in soil: Analytical methods, occurrence, transport, and
ecological risks
Jia Li, Yang Song, Yongbing Cai
PII:
S0269-7491(19)33588-2
DOI:
https://doi.org/10.1016/j.envpol.2019.113570
Reference:
ENPO 113570
To appear in:
Environmental Pollution
Received Date: 4 July 2019
Revised Date:
1 November 2019
Accepted Date: 3 November 2019
Please cite this article as: Li, J., Song, Y., Cai, Y., Focus topics on microplastics in soil: Analytical
methods, occurrence, transport, and ecological risks, Environmental Pollution (2019), doi: https://
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Retention mechanisms
Potential sources, transport pathways, and ecological risks
1
Focus topics on microplastics in soil: Analytical methods, occurrence,
2
transport, and ecological risks
3
Jia Lia,*, Yang Songb, Yongbing Caic
4
a
5
China
6
b
7
Chinese Academy of Sciences, 71 East Beijing Road, Nanjing 210008, PR China
8
c
9
China
10
School of Environmental Science and Engineering, Yangzhou University, Yangzhou 225127,
Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science,
College of Resource and Environment, Anhui Science and Technology University, Anhui 233100,
Abstract
11
Microplastics with extremely high abundances are universally detected in marine and
12
terrestrial systems. Microplastic pollution in the aquatic environment, especially in ocean, has
13
become a hot topic and raised global attention. However, microplastics in soils has been largely
14
overlooked. In this paper, the analytical methods, occurrence, transport, and potential ecological
15
risks of microplastics in soil environments have been reviewed. Although several analytical
16
methods have been established, a universal, efficient, faster, and low-cost analytical method is still
17
not available. The absence of a suitable analytical method is one of the biggest obstacles to study
18
microplastics in soils. Current data on abundance and distribution of microplastics in soils are still
19
limited, and results obtained from different studies differ significantly. Once entering into surface
20
soil, microplastics can migrate to deep soil through different processes, e.g. leaching, bioturbation,
21
and farming activities. Presence of microplastics with high abundance in soils can alter
22
fundamental properties of soils. But current conclusions on microplastics on soil organisms are
23
still conflicting. Overall, research on microplastics pollution in soils is still in its infancy and there
24
are gaps in the knowledge of microplastics pollution in soil environments. Many questions such as
25
pollution level, ecological risks, transport behaviors and the control mechanisms are still unclear,
26
which needs further systematical study.
27
Keywords: Microplastics; Soil pollution; Analytical method; Transport; Ecological risks
28
Introduction
29
Plastics, as one kind of synthetic polymer materials with high chemical stability and strong
1
30
plasticity, are widely used in packaging, construction, textile, pharmaceutical, agricultural
31
production, and electronics manufacturing industries (Thompson et al., 2009; Andrady, 2011). The
32
global plastic production reached to 348 million tons in 2017 (Plastics, 2018). The high
33
consumption of plastics is accompanied by large amounts of plastic wastes. However, only a small
34
fraction (6–26%) of plastic wastes is recycled (Alimi et al., 2018). Depending on the particle size,
35
plastic wastes in environments can be divided into large plastic (>5 mm), microplastic (0.1 µm–5
36
mm), and nanoplastic (<0.1 µm) (Barnes et al., 2009; Anderson et al., 2016; Alimi et al., 2018).
37
Compared with large debris, microplastics may be more harmful due to their high abundances,
38
smaller particle size, and long rang transport (Andrady, 2011; Law and Thompson, 2014).
39
Microplastics are universally detected in marine and terrestrial systems in recent decades
40
(Thompson et al., 2004; Barnes et al., 2009; Cole et al., 2011; Lee et al., 2013; Cozar et al., 2014;
41
Auta et al., 2017; Zhang and Liu, 2018). The published studies showed that microplastics could
42
pose threats to the whole ecosystem (Andrady, 2011; Peng et al., 2017; de Souza Machado et al.,
43
2018b). For instance, microplastics are considered as vectors for various toxins such as heavy
44
metals, hydrophobic organic pollutants, and pharmaceutical and personal care products (Guo et al.,
45
2012; Turner and Holmes, 2015; Wu et al., 2016; Li et al., 2018). Due to their sizes similar to
46
algae or mineral grain, microplastics can be easily ingested by organisms with different trophic
47
level, and accumulate along the food webs (Lee et al., 2013; Wright et al., 2013; Huerta Lwanga et
48
al., 2016, 2017). After ingestion, the adsorbed pollutants and/or the toxic additives (e.g.
49
plasticizers, organotin compounds, alkylphenols, nonylphenol, bisphenol A) contained in the
50
polymer can be transfered to organisms (Teuten et al., 2009, Bakir et al., 2014; Koelmans et al.,
51
2014), and then causing negative effects to organisms (Lusher et al., 2017; Lo and Chan, 2018).
52
Microplastic pollution has received increasing attention and become a hotspot in the field of
53
ecological and environmental science research.
54
Currently, research on microplastic pollution is overwhelmingly focused on the marine
55
system (Rillig, 2012). As early as 1974, a study has reported the presence of microplastic particles
56
(0.2–3.4 mm) in the surface waters of the Atlantic Ocean (Colton et al., 1974). In 2004, Thompson
57
et al. (2004) called for attention to marine microplastics contamination again. Subsequently, more
58
and more literatures about the source, analytical method, abundance, spatial and temporal
2
59
distribution, transport behavior and ecological effects of microplastics in the marine environment
60
were published (Barnes and Milner, 2005; Bhattacharya et al., 2010; Moret-Ferguson et al., 2010;
61
Browne et al., 2011; Wang et al., 2016; Zhang, 2017). Compared with ocean, terrestrial
62
environment is a more significant “sink” for microplastics. It is estimated that annual plastics
63
released to land were 4–23 times higher than that released to oceans (Horton et al., 2017).
64
However, microplastic pollution in soils has been largely overlooked. One key reason is believed
65
that a suitable analytical method for microplastics in soils is still unavailable (Rillig, 2012;
66
Scheurer and Bigalke, 2018).
67
Actually, soil pollution caused by the large plastic debris is nothing new. “White pollution” in
68
soils caused by plastic bag or film mulch is well known (Liu et al., 2014). Many studies have
69
proved the limited degradation of plastics in soils (Albertsson, 1980; Arkatkar et al., 2009). But
70
these large plastics persisting for decades in soils can break into smaller plastic residues (Krueger
71
et al., 2015; Briassoulis et al., 2015). Nowadays, only few studies have reported the occurrence of
72
microplastics in soil environments (David et al., 2018; Liu et al., 2018; Scheurer and Bigalke,
73
2018; Zhang and Liu, 2018; Zhou et al., 2018; Lv et al., 2019). Nizzetto et al. (2016b) estimated
74
that more than 700,000 tons of microplastics entered into soil annually in Europe and North
75
America, which was more than the global burden of microplastics in oceanic surface water
76
(93,000–236,000 tons). Once entering into soils, a complex and heterogeneous system,
77
microplastics may undergo different environmental processes and cause various ecological risks
78
(de Souza Machado et al., 2018a; He et al., 2018; Hurley and Nizzetto, 2018).
79
This paper provided a review of the existing literatures reporting microplastic pollution in
80
soils, focusing on analytical methods, occurrence, transport, and ecological risks. We discussed the
81
advantages and constraints of available analytical methods for the extraction-identification of
82
microplastics in soils. Then, we reviewed current reports on the occurrence, distribution, transport
83
process, and ecological risks of microplastics in soils. Lastly, we discussed current gaps in
84
knowledge regarding understanding of microplastic pollution in soils and proposed several
85
perspectives for future studies.
86
2 Analytical methods of microplastics in soils
87
An accurately analytical method is the foundation of research on microplastics. Generally,
3
88
analytical methods of microplastics in soils contain four steps, i.e. extraction, clean-up,
89
identification, and quantification. Recently, several new methods without extraction and clean-up
90
can directly detect microplastics in soils. The available analytical methods were summarized in
91
Table 1. The advantages and limitations of each analytical method were evaluated.
92
2.1 Extraction
93
The density values of frequently detected microplastics ranging from 0.8 to 1.4 g cm−3
94
(Hidalgo-Ruz et al. 2012), which are smaller than soils (2.6–2.7 g cm−3) (Suthar and Aggarwal,
95
2016). Therefore, density fractionation methods were widely used to extract microplastics from
96
complex soil matrix. However, microplastics can be strongly absorbed or embedded by soil
97
aggregates (Zhang and Liu, 2018), thus decreasing the extraction efficiencies of microplastics. To
98
overcome this problem, several procedures including ultrasonic treatment, stirring, aeration, and
99
continuous flow were conducted to destroy those attachments during extraction (Table 1).
100
Currently, different density solutions have been used including water, NaCl, CaCl2, ZnCl2, and
101
NaI (Table 1). Among them, water is harmless and easily available, however it could be just used
102
for separating microplastics with density < 1.0 g cm−3. NaCl is also easily available, and Na
103
benefits the dispersion of particles, but the maximum solution density of NaCl is still low (1.2 g
104
cm−3). The concentrated ZnCl2 has a density of 1.55 g cm−3, however this solution is corrosive and
105
toxic. The solution density of NaI is high enough (1.8 g cm−3), but NaI is expensive. It seems that
106
CaCl2 solution is relatively suitable to separate microplastics from soils. But the divalent Ca ions
107
would have bridged the negative charges of the organic molecules which may promote the
108
extraction of soil organic material (Scheurer and Bigalke, 2018). That is, all the commonly used
109
density solutions have their limitations. As shown in Table 1, the recovery rates of various
110
microplastics by using density separation method were higher than 90%, indicating that this
111
method was efficient. Density separation method was simple and widely used, however it may be
112
not suitable to separate those more smaller plastic particles (< 10 µm) (Claessens et al., 2013).
113
Recently, Fuller and Gautam (2016) developed a method based on pressurized fluid
114
extraction (PFE), which can extract microplastics from solid matrix (e.g. municipal waste and soil).
115
The PFE based extraction method has several benefits including fully automation, low cost, and
116
high efficiency. In addition, this method can efficiently extract plastic particles less than 30 µm.
4
117
The limitation of this method is sensitivity. It is a challenge for quantifying microplastic samples
118
accurately due to small extracted sample amounts.
5
Table 1. Available analytical methods of microplastics in soils.
Extracting solution
NaCl (1.19 g cm-3)
Separation method
Stir for 30 min and
treat by ultrasonic for
2 min, then settling
for 24 h.
Repeat
Three
times
Extraction
Density
fractionation
Clean-up
H2O2 (30%)
Identification method
Visual identification
using microscopy,
µ-FT-IR spectroscopy
Quantification
Counting
NaCl (1.19 g cm-3)
Stir for 30 min, then
settling for 24 h.
Three
times
Density
fractionation
H2O2 (30%)
Visual identification
using microscopy,
µ-FT-IR spectroscopy
NaI (1.8 g cm-3)
Treat by ultrasonic for
20
min,
then
centrifuging for 10
min at 2300 rpm.
Use a continuous flow
and
floating
separation apparatus
At least
two times
Density
fractionation
H2O2 (35%),
NaOH
(0.5M)
Two
times
Density
fractionation
—
Distilled water
NaCl (1.20 g cm-3)
ZnCl2 (1.55 g cm-3)
Stir and centrifuge
Three
times
Density
fractionation
Distilled water
Shake and settling
Two
times
Density
fractionation
Distilled water
Stir and treat by
ultrasonic for 2 h,
then settling for a
night.
At least
four
times
Density
fractionation
NaCl (1.2 g cm-3)
NaI (1.6 g cm-3)
Reference
Liu et al.,
2018
Counting
Method validation
50 g clean soils were spiked with 20
items of 9 different types of
microplastic particles (1-5 mm).
Except for PET and PVC, mean
recoveries of other polymers
were >90%.
NM
Visual identification
using microscopy
Counting
NM
Zhang and
Liu, 2018
Visual identification
using microscopy,
FT-IR spectroscopy
Counting
Zhou et
al., 2018
Visual identification
using a stereo
microscope
Counting
—
Visual identification
using polarized light
microscopy
Counting
Commercial
polypropylene
or
polyethylene particles (0.2-5 mm)
were mixed with field-cleaned
sands. Recoveries were 97%.
Microplastics (0.5-4.1 mm) were
added to 10 different soil samples.
Recovery of acrylic fibers was 49%,
other polymers (Polyester, Nylon,
polyethylene,
and
polyvinyl
chloride) were >77%.
NM
—
Heating and visual
identification using
microscopy
Counting,
Weighing
6
Polypropylene (<400 µm) and
polyethylene (<150 µm) particles
were added to three different soil
samples at five concentration
gradients (0.05%, 0.1%, 0.2%,
Lv et al.,
2019
Corradini
et
al.,
2019b
Zubris and
Richards,
2005
Zhang et
al., 2018
0.5% and 1.0%, w/w). The mean
recoveries were 86%.
NaCl (1.2 g cm-3)
CaCl2 (1.5 g cm-3)
a)
sedimentation
cylinder
method;
b)
use
a
self-constructed MP
separator;
c) Stir for 10 min,
then centrifuging for
30 min at 3450 G;
—
Three or
four
times
Density
fractionation
KClO (13%),
NaOH
(50%),
H2SO4
(96%),
HNO3
(65%), H2O2
(30%)
Raman spectroscopy,
FT-IR spectroscopy
Weighing
10 Polypropylene particles (0.5-1
mm) were added to 50 g of sand.
Recoveries ranged from 93% to
98%.
Scheurer
and
Bigalke,
2018
One
Pressurized
fluid extraction
—
FT-IR spectroscopy
Weighing
Fuller and
Gautam,
2016
—
—
—
—
—
—
TGA−MS
10-50 mg selected microplastics
particles (1 mm) were added to
municipal waste material. Average
recoveries ranged from 845% to
94%.
NM
—
—
—
—
—
—
vis-NIR
Spectroscopy
Methanol, Hexane,
Dichloromethane
David et
al., 2018
Corradini
et
al.,
2019a
Note: “—” mean “Not Conducted. “NM” mean “Not Mentioned”. µ-FT-IR: Micro-Transformed Infrared Spectroscope; FT-IR: Transformed Infrared Spectroscope; TGA-MS:
Thermogravimetry-Mass Spectrometry; NIR: near-infrared.
7
143
144
2.2 Clean-up
Soil is a complex and heterogeneous system. Some components (e.g. SOM and organic fibers)
145
in soils and microplastics have similar densities (Brady and Weil, 2000; Zhang et al., 2018). These
146
components also can be extracted by density solution, so there is difficulties in separating
147
microplastics from soil matrix (Hidalgo-Ruz et al., 2012; Scheurer and Bigalke, 2018). In addition,
148
microplastics in soils can be surrounded by an ecocorona (Galloway et al., 2017), consisting of
149
microbes and various organic deposits. These attachments could substantially influence the
150
characterization of microplastics (e.g. shape, density, and size) (Chubarenko et al., 2016).
151
Therefore, a clean-up procedure to remove SOM and/or other organic attachments is frequently
152
used. Currently, peroxide digestion (H2O2), alkaline digestion (NaOH), and acid digestion (HNO3,
153
H2SO4) are the dominant clean-up procedures (Table 1). The removal rates of SOM by diverse
154
digestion methods are different. Scheurer and Bigalke (2018) tested different chemicals (HNO3,
155
H2SO4, H2O2, NaOH, KClO) for removing SOM and found that most organic matter were
156
removed in a short time by HNO3 than the other reagents. However, HNO3 treatment caused
157
several plastic materials (e.g. acrylonitrile butadiene styrene, polyamide (PA), and polyethylene
158
terephthalate (PET)) to decompose or disintegrate into smaller debris. For those easily degradable
159
plastic materials, a 1:1 mixture of KOH and NaClO was recommended (Enders et al., 2017). Thus,
160
it is advisable to choose an appropriate digestion method for the targeted microplastics.
161
2.3 Identification
162
Identification of microplastics is usually based on the physical and chemical characterizations
163
of isolated particles in mixtures after the extraction and clean-up steps. Therefore, the commonly
164
used identification methods consist of physical identification (i.e. visual sorting) and chemical
165
identification (e.g. spectral analysis and mass spectrometry) (Table 1).
166
Based on the specific properties (e.g. color, shape or surface texture), microplastics can be
167
identified by naked eyes (Nor and Obbard, 2014; Peng et al., 2017). The commonly used criterias
168
to sorting microplastics were 1) particles that cannot be torn apart; 2) particles that have
169
distinguishable colors; and 3) no visible cellular or organic structures (Nor and Obbard, 2014).
170
Visual sorting of relatively larger microplastics (1–5 mm) offers a simple and fast method for both
171
experts and the non-professional volunteers (Shim et al., 2017). For the identification of smaller
8
172
microplastics (i.e. <1 mm) in soils, stereoscopic or dissecting microscopy with professional image
173
software were widely used (Liu et al., 2018; Zhang and Liu, 2018). However, some smaller
174
particles (<100 µm) with no color or typical shape were difficult to be characterized with
175
confidence as plastics by visual or microscopy identification (Song et al., 2015). According to the
176
changes of physical properties (e.g. shape, transparency) of plastics before and after heating,
177
Zhang et al. (2018) recently established a simple and cost-saving method which could identify
178
polyethylene (PE) and polypropylene (PP) microplastics from soils. This heating method was not
179
affected by the presence of SOM. With the help of the microscope and image software, particles
180
size, shape, and number of microplastics could be determined visually. More importantly, heating
181
method could be used to identify smaller particles (<100 µm). Currently, heating method is only
182
suitable for PE and PP, and its applicability for other plastics still needs confirming. In addition,
183
Zubris and Richards (2005) used polarized light microscopy to identify synthetic fibers in soils.
184
Indeed, this is also a visual identity method based on the different physical characterizations of
185
synthetic and natural fibers under polarized light.
186
Visual sorting was considered to be questionable because it exhibited error rates of 20–70%
187
(Eriksen et al., 2013). In most cases, suspected microplastics were usually picked out for further
188
confirmation by chemical characterization analysis (Liu et al., 2018). Fourier Transform Infrared
189
(FTIR) Spectroscopy is a reliable identification method because it can record the specific chemical
190
bonds of chemicals. Through comparing the obtained spectrums of the targeted polymers with the
191
standard database provided by spectrum library, it enables not only confirmation of plastics, but
192
also identification of plastic types. FTIR spectroscopy and its optimized technology (i.e.
193
micro-FTIR) have been applied to microplastics identification in soils (Fuller and Gautam, 2016;
194
Liu et al., 2018). However, it remains a challenge to apply FTIR in analyzing ultra-fine plastic
195
particles (<1 µm). More importantly, success rate of this method once applied to soil still depend
196
on the effectiveness of removing interfering SOM. Corradini et al. (2019a) explored the
197
possibilities of using the vis-NIR spectra to rapidly evaluate microplastics concentrations in the
198
soil without extraction. Their results showed that vis-NIR technique was suitable to quantify PET,
199
low-density polyethylene (LDPE), and polyvinyl chloride (PVC) in soils, with a 10 g kg−1
200
accuracy and a detection limit ≈ 15 g kg−1. Although the vis-NIR technique is faster and simpler, it
9
201
seems to be useful only for pollution hotspots due to its low accuracy. Furthermore, the same
202
authors ignored the impacts of adsorption and biofouling. Because they mixed the tested
203
microplastics with dry soil samples and recorded the spectra immediately. David et al. (2018)
204
applied Thermogravimetry-Mass Spectrometry (TGA-MS) to develop a more simple and accurate
205
method for the direct quantitative analysis of PET in soils without further sample pretreatment.
206
This method is not affected by SOM, but it cannot provide characterizations (e.g. shape, size, and
207
color) of microplastics besides concentration because microplastics are pyrolyzed. Furthermore,
208
this method is just used for analyzing one type of microplastics. That is, TGA-MS cannot
209
simultaneously analyze various kinds of microplastics in soils.
210
2.4 Quantification
211
According to published papers, quantification of microplastics in soils include counting,
212
weighing, mathematical calculation, and instrumental analysis (Table 1). Among them, counting is
213
the most commonly used quantitative method, and the corresponding unit is N kg-1 or N m-2.
214
Counting is a huge workload, but application of professional image software significantly
215
improves the working efficiency (Li et al., 2018). Compared with counting, weighing seems to be
216
simpler and its corresponding unit is mg kg-1. Nevertheless, weighing is more suitable for soil
217
samples contain high microplastics concentrations. Zhang et al. (2018) found a good linear
218
relationship (R2=0.99, p < 0.001) between microplastics weight and particle volume after heating.
219
They created a mathematical model to roughly calculate the mass of microplastics in the field.
220
Furthermore, several studies directly measured microplastic concentration in soils using
221
instrument (e.g. TGA-MS, vis-NIR) (David et al., 2018; Corradini et al., 2019a). Direct
222
quantification means no extraction procedure is required, but it cannot provide data of physical
223
properties (e.g. shape, size, and color) of microplastics.
224
2.5 Method validation
225
To test the reliability of the analytical methods, researchers usually carried out recovery
226
experiments (Table 1). That is, microplastics with known amount or weight were added to clean
227
soil or sand samples. These samples were treated using corresponding extraction methods. Then,
228
the recovery rates of microplastics could be calculated based on the initial amount and the final
229
extraction amount. As shown in Table 1, the reported recoveries of current methods were relatively
10
230
high, and all the researchers supposed that their analytical methods were perfect enough. However,
231
it is not hard to find that tested microplastics used in current recovery experiments are easy to be
232
extracted and identified. Because most of these tested microplastics are unaged plastics (Liu et al.,
233
2018; Zhang et al., 2018; Zhou et al., 2018), and/or relatively large particles in millimeter range
234
(Fuller and Gautam, 2016; Liu et al., 2018; Scheurer and Bigalke, 2018; Zhou et al., 2018).
235
Meanwhile, mediums used in current recovery experiments are sand (Scheurer and Bigalke, 2018;
236
Zhou et al., 2018) or only one kind of soil (Liu et al., 2018). These mediums, in a way, were not
237
representative. Recently, Corradini et al. (2019b) reported that predicated recovery rates decreased
238
with increasing of soil organic matter. Therefore, although the recoveries of microplastics based
239
on current analytical methods were high enough, the recovery experiments may be questionable
240
and the tested microplastics could not replace environmental microplastics. If possible, some kinds
241
of standard surrogates should be developed in future studies and adding them to each soil samples
242
to be analyzed.
243
3 Sources and concentrations of microplastics in soils
244
The possible sources of plastics in soils were recently reviewed by several studies (Nizzetto
245
et al., 2016c; Blasing and Amelung, 2018; Chae and An, 2018; Hurley, et al., 2018; Rochman et al.,
246
2019). Plastic film mulching, sewage sludge landfill, application of compost, irrigation and
247
flooding of waste waters, car tires debris, and atmospheric deposition were considered as major
248
contributors of microplastics in soil environments (Fig. 1).
atmospheric
deposition
UV
irrigation
runoff
fragmentation
ingestion
adhere
root
decompose
soil cracks
leaching
249
mulching
sewage sludge compost
landfill
egestion
groundwater
11
plowing
bioturbation
by plant root
250
Fig. 1. Sources and transport of microplastics in soils.
251
Currently, only few studies reported the occurrence and abundance of microplastics in soil
252
environments (Table 2). In industrial soils from Sydney, Australia, concentrations of microplastics
253
ranged from 300 to 67,500 mg kg−1 (Fuller and Gautam, 2016). Scheurer and Bigalke (2018)
254
found microplastics at concentrations of up to 55.5 mg kg−1 (593 N kg−1) in soil samples from 26
255
floodplain sites in Switzerland. In Chile, microplastics concentrations in agricultural field applied
256
with sludge ranged from 0.57-12.9 mg kg−1 (Corradini et al., 2019b). Zhang and Liu (2018)
257
investigated the concentration of plastics in four croplands and one riparian forest buffer zone in
258
Yunnan Province, China. They found that the concentration of plastic particles (0.05–10 mm)
259
ranged from 7100 to 42,960 N kg−1 (mean value was 18,760 N kg−1). Among them, 95% of these
260
sampled plastics were in the microplastics size range (0.05–1 mm). Liu et al. (2018) studied
261
microplastics in farmland soils from twenty vegetable fields around the suburbs of Shanghai,
262
China. They reported that the abundance of microplastics was 78.00 ± 12.91 and 62.50 ± 12.97 N
263
kg−1 in shallow (0–3 cm) and deep soils (3–6 cm), respectively. Another investigation conducted in
264
Shanghai showed the same order of magnitude for microplastics in rice soils (16.1 ± 3.5 N kg−1),
265
but a low concentration in aquaculture soils (4.5 ±1.2 N kg−1) (Lv et al., 2019). Zhang et al. (2018)
266
reported that mean concentrations of microplastics in agricultural field, fruit field, green house
267
field were 140, 440, and 180 N kg−1 respectively. In the study of Zhou et al. (2018), concentrations
268
ranging from 1.3 to 14,712.5 N kg−1 (dry weight) of microplastics were found in 120 soil samples
269
collected from coastline in Shandong province, China. Comparing the studies using the same units
270
of measurement, abundances of microplastics in soils differed significantly (Table 2). Actually,
271
those areas with high microplastics concentrations usually have typical pollution sources. For
272
instance, the highest concentrations in Yunnan Province is related to application of more sewage
273
sludge and irrigation with wastewater (Zhang and Liu, 2018). Another possible reason is related to
274
the different analytical methods. Although recovery experiments were performed in each study to
275
verify the extraction procedure (Table 1), the recovery experiments were questionable due to using
276
unaged and relatively larger plastic particles. Thus, differences caused by various analytical
277
methods should not be ignored.
278
Table 2. Investigation on microplastics abundances in soils.
12
Location
Sydney,
Soil type
Industrial soil
Size
< 1 mm
Abundance (depth)
Ref.
−1
300–67500 mg kg
Fuller and
Australia
Switzerland
Gautam, 2016
Floodplain soil
< 2 mm
−1
Scheurer and
55.5 mg kg (0–5 cm)
−1
593 N kg (0–5 cm)
Mellipilla, Chile
Agricultural field
< 1 mm
Bigalke, 2018
−1
0.57-12.9 mg kg (0-25 cm)
Corradini et al.,
2019b
Shanghai, China
Aquaculture soils
20 µm–5
−1
4.5 ±1.2 N kg
Lv et al., 2019
40±126–320±329 N kg−1 (0–10 cm)
Zhang et al.,
8±25–540±603 mg kg−1 (0–10 cm)
2018
mm
Loess plateau,
Agricultural field
China
Fruit field
< 5 mm
−1
Green house field
80±193–120±169 N kg (10–30 cm)
24±51–460±735 mg kg−1 (10–30 cm)
Shandong
Coastal soil
< 5 mm
1.3–14,712.5 N kg−1 (0–2 cm)
Province, China
279
Zhou et al.,
2018
Yunnan
Farmland,
0.05–10
Province, China
Forest buffer zone
mm
−1
7100–42960 N kg (0–10 cm)
Zhang and Liu,
2018
4 Transport of microplastics in soils
280
As shown in Fig. 1, transport behaviors of microplastics in soils are complex. It has been
281
supposed that microplastics on the surface soils may be lost by surface runoff or wind (Nizzetto et
282
al., 2016a). Nevertheless, Zubris and Richards (2005) found evidence for downward translocation
283
of fibers by unknown mechanisms. Recent researchers have detected microplastics in both topsoil
284
and deep soil (Table 2). These results indicate that microplastics could move vertically in soils.
285
4.1 Transport pathways of microplastics in soils
286
Soil is a porous media with macro-pores and meso-pores in the µm range (Blasing and
287
Amelung, 2018), which makes the migration of dissolved chemicals or small particles in soils
288
possible. Several studies have demonstrated that the small particles can transport along soil pores
289
through leaching. For example, Grayling et al. (2018) reported that particles with a size range of
290
0.1–6.0 µm in diameter can move vertically in soil column. For those relatively larger microplastic
291
particles, soils will presumably retain them and act as a sink. However, the presence of external
292
forces (e.g. bioturbation and farming activity) may contribute to larger microplastic particles
293
movement in soils. Recent research has reported that microplastic particles can be moved and
294
distributed by two collembola species (i.e. Folsomia candida and Proisotoma minuta) in a
295
laboratory arena (Maaß et al., 2017). Zhu et al. (2018a) showed that mite (i.e. Hypoaspis
296
aculeifermoved) can also move and disperse the commercial PVC particles (80–250 µm) in the
13
297
plates. Rillig et al. (2017b) observed microplastics could stick to the earthworms. So, they
298
supposed that attachment to the outside of the earthworm was a possible transport mechanism.
299
Huerta Lwanga et al. (2017) also proved earthworm can contribute to microplastics movement in
300
soils, but they attributed this mechanism to the ingestion/excretion by earthworms. Bioturbation
301
by plant roots (e.g. root movement, root expansion, water extraction by roots) has a significant
302
impact on soil particle transport (Gabet et al., 2003). Similarly, the transport of microplastics could
303
also be influenced by plant roots. Furthermore, when the root decomposes, it leaves macropores
304
approximately the size of the root, which will facilitate the transport of microplastics in soils.
305
However, this is just an inference, and future studies should be conducted to reveal the effects of
306
plant root on microplastics transport. Farming activity such as plowing will bring about an
307
inversion of the surface soil and subsurface soil (Rillig et al., 2017a). Accordingly, microplastics
308
in surface soil will easily be brought to deep soil. In addition, harvesting of rhizome (e.g. potatoes,
309
carrots) may also facilitate the downward movement of microplastics. Lastly, it is known that dry
310
climate will lead to the appearance of soil cracks, which could open entryways for microplastics to
311
reaching deep soils. A recent study proved that wet-dry circles could accelerate microplastics
312
downward movement (O'Connor et al., 2019). Undoubtedly, above-mentioned external forces can
313
also promote transport of small microplastic particles. However, these external forces usually have
314
limited auxiliary effects on vertical transport of microplastics. For instance, conventional tillage
315
practices affect only the topsoil (20–30 cm) (Rillig et al., 2017a). By contrast, leaching, which is
316
defined as infiltration of water contained suspended or dissolved topsoil materials into the deepsoil,
317
has more significant facilitation for microplastics transport vertically in soils. As reported by Cey
318
et al. (2009), microplastics with average diameter of 3.7 µm could move downward to over 70 cm
319
deepsoil through leaching. It has even been predicted that microplastics may end up in shallow
320
groundwater with the help of leaching (Blasing and Amelung, 2018).
321
4.2 Influencing factors and retention mechanisms
322
An essential requirement for downward leaching of microplastics is that their sizes are
323
smaller than the diameter of soil pores, otherwise microplastics will be captured by soil. Therefore,
324
leaching of microplastics in soils with higher porosity, especially more macropores, is more likely
325
to happen. Soil texture has been experimentally shown to directly affect transport of microplastics
14
326
(Bradford et al., 2002; Cey et al., 2009; Rahmatpour et al., 2018). Because soil texture (grain size)
327
could influence its pore size. Studies showed that increasing ionic strength could significantly
328
promote the retention of microplastics in quartz sand media (Pelley and Tufenkji, 2008; Treumann
329
et al., 2014). This can be attributed to compression of the double layer thickness under high ionic
330
strength condition which produces a lower energy barrier and greater depths in the primary and
331
secondary minima (Bradford and Torkzaban, 2012). Similar impacts can also be expected for ionic
332
strength of soil pore water, even though there is no experimental evidence yet. Further, many
333
studies have shown that surface roughness of medium, biofouling, organic matter, saturation, and
334
hydrodynamic condition can affect transport and retention of microplastics in quartz sand (Pelley
335
and Tufenkji, 2008; Majumdar et al., 2014; Treumann et al., 2014; Mitzel et al., 2016). Compared
336
with homogeneous quartz sand, soil is a complex and heterogeneous medium. How does
337
physicochemical properties of soils and leaching condition affect the transport and retention of
338
microplastics? These questions have not been well solved, which creates obstacles for fully
339
understanding the transport of microplastics in soils. Future studies should be conducted from
340
simple medium to complex medium, that is, from pure quartz sand to sea sand, to sandy soil, then
341
to clay.
342
The movement of microplastics in soils is largely dependent on their properties (e.g. size,
343
shape, density). Previous studies have shown that the size and hydrophobicity of microplastics can
344
affect their transport in soil (Pelley and Tufenkji, 2008; O'Connor et al., 2019). Rillig et al. (2017b)
345
stated that the smallest plastic particles moved downward the most since that the small particles
346
could pass through soil pores and eventually reach deep soil. At present, the shape of most
347
commonly used microplastics in relevant experiments were sphere and particle (Zhuang et al.,
348
2005; Treumann et al., 2014; Huerta Lwanga et al., 2017; Rillig et al., 2017b). These studies
349
showed that microplastics with the two shapes could easily move to deep soils. Rillig et al. (2017a)
350
supposed that other shapes (e.g. fiber and film) would behave differently from microsphere. A
351
recent study indicated that microfibers could help them to entangle soil particles more efficiently
352
to form clods (Zhang et al., 2019). In addition, O'Connor et al. (2019) indicated that microplastics
353
with low density were difficult to leach downward. Currently, no studies have been able to explain
354
the effect of shape on microplastics migration in soils. The effects of the type and surface structure
15
355
of microplastics on their migration and retention in soil require further investigation.
356
After entering into soils, microplastics may undergo many processes such as attachment,
357
detachment, sedimentation, or incorporation into soil aggregates (Treumann et al., 2014; Rillig et
358
al., 2017a; Zhang and Liu, 2018), which can restrict the movement of microplastics. Previous
359
studies on colloid transport in quartz sand and glass bead revealed that straining and
360
physicochemical deposition (including collision and attachment) were the key processes
361
controlling the transport and retention of microplastics (Wan and Tokunaga, 1997; Gamerdinger
362
and Kaplan, 2001; Bradford et al., 2002; Zhuang et al., 2005; Bradford and Torkzaban, 2012).
363
These studies supposed that attachment at solid-liquid interface, film straining, air-water
364
interfacial capture, and pore exclusion were the dominant mechanisms (Fig. 2). Contribution of
365
these mechanisms depend on microplastics properties (e.g. particle size and surface structure) and
366
environmental factors (e.g. pore size, ionic strength, and saturation). For instance, straining is
367
more important for relatively larger microplastics, while attachment is more significant for smaller
368
microplastics (Bradford et al., 2002). Above-mentioned studies could provide implications for
369
exploring the transport mechanism of microplastics in soils. However, the physicochemical
370
properties of quartz sand were quite different from soils, such as particle size distribution, surface
371
charge, surface roughness, mineral composition and pore size. The controlling mechanisms of the
372
migration and retention of microplastics in heterogeneous soils may be more complex. It is
373
necessary to combine column experiments with numerical simulation as well as microscopic
374
imaging technology in future studies.
Pore exclusion
Straining
Physicochemical deposition
Film straining
375
376
377
Fig. 2. Retention mechanisms of microplastics in sand.
5 Ecological risks of microplastics on soil ecosystem
16
378
With the concern raised by many researchers regarding the risks posed by microplastics in
379
aquatic environments (Bhattacharya et al., 2010; Setala et al., 2014; Bouwmeester et al., 2015;
380
Batel et al., 2016; Green et al., 2016), some studies also focused on ecological risks of
381
microplastics on soil ecosystem. However, recent studies on the toxic effects of microplastics on
382
soil ecosystem were still in the early stage. That is, a scientific conclusion about whether
383
microplastics could contaminant soil ecosystem is still not available.
384
5.1 Effects of microplastics on soil properties
385
As a kind of solid pollutants, microplastics could alter fundamental properties of soils. Liu et
386
al. (2017) studied the response of soil dissolved organic matter to PP microplastic addition in
387
Chinese loess soil. They found that the lower level (7% W/W) of microplastic addition had a
388
negligible effect on the nutrient contents (e.g. DOC, DON, DOP, NH+4 , NO-3 , PO3-4 ) in DOM
389
solution, while the higher level (28% W/W) of microplastic addition significantly increased the
390
nutrient contents. de Souza Machado et al. (2018b) studied the potential of microplastics to disturb
391
soil structure. They exposed a loamy sand soil to environmentally relevant nominal concentrations
392
(up to 2% W/W) of four common microplastic types (polyacrylic fibers, PA beads, polyester fibers,
393
and PE fragments) for 5 weeks. Their results showed that microplastics affect the bulk density,
394
water holding capacity, and water stable aggregates of soils. However, different microplastics
395
showed different impacts on these indicators. For instance, soils contaminated with polyester
396
fibers showed a significant decrease in bulk density and water stable aggregates with increasing
397
polyester concentrations, while none of the other microplastics elicited similar effects. Meanwhile,
398
the same authors also noted that microplastics in soils may pose further effects due to these tested
399
indicators (i.e. bulk density, water holding capacity, and water stable aggregates) correlates with
400
soil physical quality and rootability. However, Zhang et al. (2019) reported that polyester
401
microfibers ( 0.3% W/W) did not alter soil bulk density and saturated hydraulic conductivity. The
402
different results in two studies may be attributed to the different test concentrations of
403
microplastics. Furthermore, Zhang et al. (2019) also found that polyester microfibers reduced the
404
volume of <30 µm pores, while increased the volume of >30 µm pores. Microfibers could enter
405
micropores and then occupied the space of micropores. The linear shape of polyester microfibers
406
can help them to entangle soil particles more efficiently to form clods. Therefore, the increase in
17
407
clods caused by polyester microfibers can also make more soil macropores. Recently, Rillig (2018)
408
argued that microplastics in soils make a hidden contribution to soil carbon storage. Because
409
plastics are mostly carbon (e.g. PS or PE are almost 90% carbon). It should be noted that this
410
fraction of carbon may interact differently with soil microbes because they are likely not
411
functionally similar to natural soil organic matters.
412
5.2 Ecological risks of microplastics on soil microbial community and plants
413
Previous study has shown that microplastics in aquatic environments are a distinct microbial
414
habitat and may be a novel vector for the transport of unique bacterial assemblages (McCormick et
415
al., 2014). Although there is no research on reaction of microplastics and microbes in soils yet, we
416
could suggest that microplastics may change soil microbial community during their transport in
417
soils. Microplastics showed high adsorption capacity for antibiotics, heavy metals, and other toxic
418
pollutants (Turner and Holmes, 2015; Wu et al., 2016; Li et al., 2018). These contaminants will
419
affect microbes adhered to microplastics. Further, Sun et al. (2018) reported that the existence of
420
microplastics
421
bacteria/phage-harbored resistance genes (ARGs). Awet et al. (2018) documented short-term
422
detrimental impacts of PS nano-plastics on soil microbe. de Souza Machado et al. (2018b, 2019)
423
studied the effects of various microplastics (PA, polyester, PE, PP, PS, PET, polyacrylic) on soil
424
microbial activity. They found that soil microbial activity varied among microplastic types. For
425
instance, the general microbial metabolic activity was increased by PA, PE, and polyester while
426
decreased by PS and PET. However, Judy et al. (2019) reported that there was little evidence the
427
microplastics (PE, PVC, and PET) affected soil microbial community diversity. Obviously,
428
researches on effects of microplastics on soil microbial community are still in the early stage.
(polyolefin
film)
inhibited
the
dissipation
of
soil
antibiotics
and
429
Plant performances depend significantly on root colonizing microbes, including N-fixers,
430
pathogens and mycorrhizal fungi (Powell and Rillig, 2018). Thus, microplastics could influence
431
plant growth via affecting soil microbes. Likewise, microplastics may influence plant growth
432
directly or indirectly. As reported by de Souza Machado et al. (2019), microplastics could affect
433
plant (Allium fistulosum) root traits, leaf traits, and total biomass, but the positive and negative
434
effects varied among microplastics types. Liao et al. (2019) demonstrated the toxic impacts of PS
435
(5 µm) on the growth of wheat (Triticum aestivum). Qi et al. (2018) reported that microplastic
18
436
residues affected the wheat (Triticum aestivum) during both vegetative and reproductive growth.
437
They also found the biodegradable plastic mulch had stronger negative effects as compared to PE.
438
Recently, Rillig et al. (2019) proposed several potential mechanistic pathways through which
439
microplastics could affect plant performance. Their paper provided guidelines for future studies on
440
this topic.
441
5.3 Ecological risks of microplastics on soil animals
442
5.3.1 Direct ecological risks of microplastics on soil animals
443
Like to other contaminants, microplastics may have a direct toxic effect on soil animals
444
(Table 3). Huerta Lwanga et al. (2016) studied the survival of the earthworm (Lumbricus terrestris)
445
exposed to LDPE microplastics (<400 µm) in sandy soil at different concentrations (0, 7, 28, 45,
446
60% W/W). They found that small plastic particles (<50 µm) can be easily ingested by
447
earthworms. Mortality was higher at 28, 45, and 60% W/W than at 7% W/W and in the control
448
(0%). Growth rate was significantly reduced at high microplastic concentrations (>28% W/W).
449
They supposed that the ecological effect mechanisms of microplastics on earthworms were
450
dilution of ingested food and changing food quality. It should be noted that microplastics
451
concentration used in their study was very high. Their conclusions may not be suitable for actual
452
environmental concentration. Other studies also demonstrated that microplastics have no
453
significant impacts on mortality and growth of earthworms at relatively lower concentrations
454
(<20%) (Hodson et al., 2017; Rodriguez-Seijo et al., 2017; Wang et al., 2019). In a recent study,
455
Zhu et al. (2018b) reported that PVC microplastics exposure (0.1% W/W) could alter feeding
456
behavior of soil collembolan (Folsomia candida), and then inhibited their growth and reproduction.
457
Lei et al. (2018) showed that PS microplastics (1 mg L-1) could accumulate in the intestine of
458
nematodes, then resulted in decreasing of survival rate, body length, and reproduction and caused
459
intestinal damages and oxidative damages. They also emphasized that there was strong association
460
between microplastic particle size and its toxicity. These are important researches indicating that
461
lower concentration of microplastics exposure will cause adverse effects on growth of soil animals.
19
462
Table 3. Recent studies on ecological risks of microplastics on soil animals
Microplastics
Toxin
chemicals
Polybrominated
diphenyl ether
(PBDE)
Test organisms
Test soil
Exposure
concentration
MPs:
1:2000
(W/W)
PBDEs: 83 mg
kg-1
MPs: 0, 7, 28,
45, 60% (W/W)
Exposure
time
7, 14 and
28 d
Evaluating
indicator
Bioaccumulation
Conclusions
Ref.
Earthworm,
Eisenia fetida
Artificial
soil
PBDEs accumulate in organisms ingesting soils
containing biosolids or waste plastics.
Gaylor et al.,
2013
LDPE (<400
µm) particles
-
Earthworm,
Lumbricus
terrestris
Sandy soil
60 d
Mortality;
growth;
activity;
ingestion
Huerta
Lwanga et al.,
2016
Clay loam
MPs: 1 g kg-1
28 and
56 d
Growth;
reproduction;
isotope
composition;
gut microbiota.
Caenorhabditis
elegans
-
MPs: 1 mg L-1
3d
-
Snails,
Achatina fulica
Cultivation
soils
MFs:
0.014,
0.14 and 0.71 g
kg-1
28 d
Growth;
motor behavior;
Oxidative
damage
Food intake;
excretion;
histopathology;
oxidative stress
PVC particle
arsenic
Earthworm,
Metaphire
californica
Farmland
soil
28 d
Bioaccumulation
gut microbiome
High-density
Zn
Earthworm,
Woodland
MPs: 2000 mg
kg-1
As(V): 40 mg
kg-1
Zn-bearing
28 d
Growth;
PE particles can be ingested by earthworms;
Mortality was higher at 28, 45, and 60% W/W
than at 7% W/W and in the control (0%);
Growth rate was significantly reduced at 28, 45,
and 60% W/W.
Micro-PVC altered gut microbiota and
increased bacterial diversity;
Collembolan growth and reproduction were
inhibited;
Micro-PVC enhanced δ15N and δ13C values of
collembolan tissues.
PS microplastics decreased survival rate, body
length and reproduction of nematodes and
caused intestinal damages and oxidative
damages.
MFs were uptake and depurated by the digestive
tract; MFs inhibited food intake and excretion;
0.71 g kg-1 MFs induced villi damages in walls
of gastrointestinal tract; MFs could affect
oxidative stress.
PVC reduced arsenic accumulation in gut and
body tissues.
PVC alleviated the effect of arsenic on the gut
microbiota.
There was no evidence of Zn accumulation,
PVC (80-250
µm) particles
-
Collembolans,
Folsomia
candida
PS (0.1-500
µm) particles
-
PET
(1257.8µm)
fibers
Polyurethane
foam (PUF, <
75 µm) particles
20
Zhu et
2018b
al.,
Lei et
2018
al.,
Song et
2019
al.;
Wang et al.,
2019
Hodson et al.,
Polyethylene
(HDPE, <400
µm) pieces
PE (250 µm-1
mm)
Lumbricus
terrestris
soil
MPs:
(W/W)
0.35%
Earthworm,
Eisenia andrei
OECD
artificial
soil
MPs: 62.5, 125,
250, 500, 1000
mg kg-1
28 and
56 d
bioaccumulation;
mortality
mortality, or weight change.
2017
Survival,
number
of
juveniles;
weight;
histopathological
analysis;
damages;
immune system
response
Ingestion;
antioxidant
defense system
No effect on survival, number of juveniles and,
in the final weight of adult earthworms, but
damages and immune system responses were
confirmed.
RodriguezSeijo et al.,
2017
MPs could be ingested by earthworms;
Exposure to PE or PS particles (20%)
significantly influenced enzyme activity of E.
fetida, while no discernible effect was detected
at low rates ≤10%.
Microplastic decreased bioaccumulation of
PAHs and PCBs in E. fetida.
Wang et al.,
2019
LDPE
(≤300
µm) particles
PS (≤300 µm)
particles
-
Earthworm,
Eisenia fetida
Sandy loam
MPs: 1, 5, 10,
20% (W/W)
14 d
LDPE
(≤300
µm) particle
(PS (≤300 µm)
particles
phenanthrene,
fluoranthene,
pyrene,
benzo[α]
pyrene, PCB52,
PCB70, and
PCB153
Chlorpyrifos
Earthworm,
Eisenia fetida
Sandy loam
MPs: 1, 5, 10%
(W/W)
28 d
Bioaccumulation
Earthworm,
Eisenia fetida
OECD
artificial
soil
MPs(5mm): 16
N kg-1
MPs (250 µm-1
mm): 360-400
N kg-1
14 d
Ingestion;
neurological;
response
LDPE (5 mm
and 250 µm-1
mm) pellets
21
LDPE (5 mm) cannot be ingested by
earthworms.
Data obtained from this study cannot provide a
precise answer to whether LDPE were carriers
of pesticides to biota.
Wang et al.,
2019
RodriguezSeijo et al.,
2019
463
Although lower concentration of microplastics have no effect on mortality and growth of
464
earthworms, histopathological and immune system responses have already been confirmed. For
465
instance, Rodriguez-Seijo et al. (2017) reported that PE pellets (125-1000 mg kg-1) caused tissue
466
and immune system damages of earthworms (E. andrei). Wang et al. (2019) found that exposure to
467
PE or PS particles (20% W/W) significantly influenced enzyme activity of earthworms (E. fetida),
468
while no discernible effect was detected at low rates ≤10% (W/W). Except for earthworms, recent
469
studies also demonstrated the adverse effects of microplastics on immune system of other soil
470
organisms. Zhu et al. (2018b) indicated that PVC particles (1 g kg-1) enhanced δ15N and δ13C
471
values of collembolan tissues. Song et al. (2019) studied the toxic effects of PET fibers on
472
terrestrial snails (Achatina fulica) after 28 d at concentrations of 0.014-0.71 g kg-1 (dry soil
473
weight). They found that PET fibers were uptake and depurated by the digestive tract, and PET
474
fibers could decrease food intake and excretion, induce villi damages in walls of gastrointestinal
475
tract, and influence oxidative stress. This study implied that 0.14 g kg-1 PET fibers caused adverse
476
effects on snails.
477
5.3.2 Indirect ecological risks of microplastics on soil animals
478
Microplastics can accumulate contaminants from soil environments and then may act as
479
vectors to increase pollutants exposure in animals. Several studies have been conducted about this
480
topic, but different results are got. Gaylor et al. (2013) showed that PBDEs leached from
481
polyurethane foam (<75 µm) could be accumulated by earthworms (Eisenia fetida). They also
482
supposed that such earthworms might transfer PBDEs to predators or translocate them from the
483
application site. Adsorption and desorption of Zn on fragmented HDPE bags (<400 µm) were
484
studied by Hodson et al. (2017). However, they reported that there was no evidence of Zn
485
accumulation in earthworms. Similar result was also reported by Rodriguez-Seijo et al. (2019),
486
they cannot be sure whether LDPE were carriers of pesticides to soil biota. Meanwhile, two recent
487
studies demonstrated that microplastics reduced As(V), PAHs, and PCBs accumulation in gut and
488
body tissues of earthworms (Wang et al., 2019; Wang et al., 2019). This is an important finding
489
because it overturns the traditional view that microplastics can increase toxic pollutants
490
bioavailability. Furthermore, Hüffer et al. (2019) studied the impacts of PE microplastics on the
491
transport of atrazine and 4-(2,4-dichlorophenoxy) butyric acid in soils. Their results implied that
22
492
the presence of microplastics in soils could increase the mobility of organic contaminants by
493
reducing the sorption capacity of natural soils. As a result, these organic contaminants may seep
494
into groundwater or other surrounding water sources, and then pose great threats to humans.
495
5.4 Potential human health risks caused by microplastics
496
As mentioned in section 4, microplastics can transport vertically in soils. Thus, researchers
497
supposed that microplastics especially those in micron range might reach groundwater (Rillig et
498
al., 2017a), and then increased the possibility of entering the body. Meanwhile, a published study
499
has provided field evidence for transfer of microplastics along a terrestrial food chain
500
(soil-earthworm-chicken) (Huerta Lwanga et al., 2017). The authors supposed that microplastics
501
accumulated in chicken has potentially negative consequences for human health. Besides, a recent
502
study indicated that PS microplastics can be absorbed by the roots of lettuce and then transport to
503
stems and leaves (Li et al., 2019). This will absolutely facilitate microplastics to enter human. To
504
date, we still have no evidence that microplastics are harmful to human. All we know is just that
505
soil microplastics may accumulate in body via drinking groundwater or food chain.
506
6 Future perspectives
507
Microplastic pollution in aquatic environments (especially the oceans) has garnered a global
508
concern, while soil systems have received far less scientific attention. There are still gaps in the
509
knowledge of soil microplastic pollution and many questions still remain unclear. That is, before
510
comprehensively revealing soil microplastic contamination, much more studies should be
511
conducted. Here, we highlighted several key gaps in understanding of microplastic pollution in
512
soils based on the published literatures. And then, we proposed several perspectives for future
513
studies.
514
6.1 Major gaps of current studies
515
The methods used to extract, quantify and characterize microplastics from water or sediment
516
samples were adjusted and then used for soil samples (Zubris and Richards, 2005; Liu et al.,
517
2018; Scheurer and Bigalke, 2018; Zhang and Liu, 2018; Zhou et al., 2018). Soil is a
518
complex and heterogeneous media which makes the identification of microplastics from it
519
extremely challenging. Although several analytical methods have been established and
520
proven have their own advantages, a universal, efficient, faster, and low-cost analytical
23
521
method is still not available. Furthermore, the absence of standardized methods will hinder
522
evaluating soil microplastics contamination due to the errors between different analytical
523
methods.
524
Currently, field data on measured microplastic concentration in soil systems are still not
525
widely available, which will limit our understanding of the current state of microplastic
526
pollution in soils. The physicochemical properties of most abundant microplastics in soils has
527
not been reported. Due to the lack of quantitative data of environmental concentrations, it is
528
difficult to assess the ecological risks posed by microplastics in soil system under realistic
529
exposure conditions. Several studies have summarized the possible sources of microplastics
530
in soils (Horton et al., 2017; Alimi et al., 2018; Chae and An, 2018), but the contribution of
531
each source and the total flux of microplastics released into soils remain unknown.
532
To scientifically evaluate the effects of soil microplastic pollution, understanding the
533
transport processes of microplastics in the soil environment is undoubtedly an important task.
534
However, transport behaviors and dominant mechanisms of various kinds of microplastics in
535
the soil environment remain unclear. Recent studies about transport simulation have focused
536
on polystyrene (PS) spheres but abundant PP and PE with different shapes (e.g. fragment,
537
fiber, and film) were detected in soils (Gamerdinger and Kaplan, 2001; Li et al., 2004; Liu et
538
al., 2018). Due to well-defined size and surface charge of spherical microplastics, using PS
539
spheres in experiments is convenient, but results obtained from these simulation experiments
540
are not applicable to real environment.
541
Although several studies have been conducted to reveal the ecological effects of
542
microplastics on soil ecosystems, the risk evaluation system has not been developed. We are
543
not even sure whether variety microplastics (especially the aged microplastics) at
544
environmental concentration have adverse effects on soil organisms, because current
545
conclusions are conflicting. Recent studies have demonstrated that microplastics can alter soil
546
structure and properties (Liu et al., 2017; de Souza Machado et al., 2018b). So, what is the
547
subsequent effect on soil biota? Microplastics can enter to body through soil food chain,
548
however the amount of entering body need to be estimated. Furthermore, the question of
549
whether microplastics have adverse impacts on human need also be answered.
24
550
6.2 Perspectives for future studies
551
Future studies on soil microplastics research still need to focus on four aspects (Fig. 3).
552
Analytical method is the foundation of soil microplastics research. A standard and accurate
553
method will absolutely facilitate investigation of occurrence, distribution, and transport behaviors
554
of various microplastics in soils. Data of abundance, distribution, and fate of microplastics in soils
555
will provide the basis for their ecological risk assessment. However, recent studies on these topics
556
are still limited. Based on the published researches, future studies should be conducted from the
557
following topics.
Ecological risks
analysis
Abundance
and
distribution
Fate and
transport
Optimization of analytical methods
558
559
560
Fig. 3. Topics for future studies.
Developing and optimizing extraction methods to improve extraction efficiency. If possible,
561
establishing a standard extraction method for soil microplastics. Recovery experiments
562
should be optimized and the standard surrogates should be developed. Meanwhile, a database
563
of microplastics characteristics including morphology, chemical component, thermology,
564
mechanical property, and electromagnetism should be built. Research and development of
565
analytical instruments are also an important topic. The combined use of different analytical
566
instruments
567
Chromatography Mass Spectrum) are recommended.
568
(e.g.
Thermogravimetric
Analysis,
Micro-IR
spectroscopy,
and
Gas
Abundance and distribution of microplastics in global soils should be investigated.
569
Developing mathematical models to evaluate the amount of microplastics released into soils
570
from different sources.
25
571
Performing column (i.e. packed-column and undisturbed-column) experiments as well as
572
field experiments to simulate transport behaviors of various kinds of microplastics (different
573
types, shapes, sizes, and surface morphology) in various soil environments, and to determine
574
the key mechanisms and influencing factors. Future studies should be conducted from simple
575
medium to complex medium, that is, from sea sand to sandy soil, then to clay.
576
Assessing the ecological risks posed by various kinds of microplastics (especially those
577
collected from soil environment) in soil ecosystems under realistic exposure conditions. The
578
test species should include microorganism, edaphon, and plant. Lastly, assessment system
579
should be developed to evaluate soil microplastics for human.
580
581
Acknowledgement
582
This study was financially supported by Shandong Key Laboratory of Coastal Environmental
583
Processes, YICCAS (2019SDHADKFJJ12), Research Startup Project of Yangzhou University
584
(137011522), the National Natural Science Foundation of China (41877032).
585
586
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Advantages and limitations of current analytical methods of soil microplastics were
evaluated.
Microplastics could move vertically to deep soils through multiple processes.
Whether microplastics have adverse effects on soil systems or humans remain unclear.
Several perspectives for future studies on microplastic pollution in soils were proposed.