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An optimized density-based approach for extracting microplastics from soil and sediment samples
Journal Pre-proof An optimized density-based approach for extracting microplastics from soil and sediment samples Xiaoxin Han, Xueqiang Lu, Rolf D. Vogt PII:
S0269-7491(19)32572-2
DOI:
https://doi.org/10.1016/j.envpol.2019.113009
Reference:
ENPO 113009
To appear in:
Environmental Pollution
Received Date: 16 May 2019 Revised Date:
1 August 2019
Accepted Date: 2 August 2019
Please cite this article as: Han, X., Lu, X., Vogt, R.D., An optimized density-based approach for extracting microplastics from soil and sediment samples, Environmental Pollution (2019), doi: https:// doi.org/10.1016/j.envpol.2019.113009. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Graphical abstract:
1
An
Optimized
Density-based
2
Microplastics from Soil and Sediment Samples
3
Xiaoxin Hana,b,c, Xueqiang Lua,b,c*, Rolf D. Vogtc,d
4
a
5
China
6
b
7
Pollution, Tianjin 300350, China
8
c
9
Technology, Tianjin 300350, China.
10
d
for
Extracting
College of Environmental Science and Engineering, Nankai University, Tianjin 300350,
Tianjin Key Laboratory of Environmental Technology for Complex Trans-Media
Tianjin International Joint Research Center for Environmental Biogeochemical
Department of Chemistry, University of Oslo, Oslo, Norway.
11 12
* Corresponding author.
13
E-mail address: [email protected]
14
Approach
15
Abstract
16
Microplastic pollution in the environment has received growing attention worldwide. A
17
major impediment for accurate measurements of microplastics in environmental matrixes is
18
to extract the particles. The most commonly-used method for separation from soil or
19
sediment is flotation in dense liquid based on the relatively low density of plastic particles.
20
This study provides an improved and optimized process for extraction of microplastic
21
particles by modifying the floatation technique and floatation solution. Microplastics in
22
soils and sediments are extracted by adding 200 g dry soil or sediment sample to 1.3 L mix
23
of the saturated NaCl and NaI solutions in a volume ratio of 1:1 and aerating for 40 sec
24
then filtering the supernatant. The accuracy and precision of the new approach is validated
25
by recovery experiments using soil and sediment samples spiked with six common
26
microplastic
27
polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS) and expanded polystyrene
28
(EPS), and comparison with the previous method. The optimized approach is further
29
compared with the previous approach using the real soil and sediment samples.
30
Keywords: microplastics; extracting approach; soil; sediment; air floatation
31
compounds:
polyethylene
(PE),
polyethylene
terephthalate
(PET),
32
33 34
Graphical abstract:
35
1. Introduction
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The term "microplastic" ("MP"), first proposed by Thompson et al. (2004), refers to
37
plastic particle less than 5 mm (Arthur et al., 2009) or 1 mm (Claessens et al., 2011) in
38
length.
39
contaminants (Müller et al., 2018; Xu et al., 2018). Due to its small size, living organisms
40
easily ingest these microplastic particles. Although there is no clear causality that this may
41
increase bioavailability of these compounds, it has raised a public concern. The concern is
42
that the omnipresence of microplastic contamination in the environment (Bergmann et al.,
43
2017; Dekiff et al., 2014; Jayasiri et al., 2013; Tsang et al., 2017; Zhao et al., 2018) might
44
represent an important pathway for pollutant uptake. The contaminated particles might
45
release their harmful absorbed substance when it is passing through the digestion system of
46
the organisms. This could threaten the organisms and even humans through the food chain
47
(Koelmans et al., 2017; Lu et al., 2016). Whether microplastic particles actually represent
48
an emerging contaminant or not needs therefore to be clarified and documented with sound
49
data. The key to investigate and evaluate environmental risk of microplastic contamination
50
is to have a good method for extracting microplastic particles from environmental samples.
Microplastics may accumulate heavy metals (Wang et al., 2017) and organic
51
Soil acts as a preliminary sink of microplastic particles, which may subsequently
52
either be decomposed further to nanoplastic or be remobilized again (Hueffer et al., 2019;
53
Hurley et al., 2018; Liu et al., 2014). Marine sediments are on the other hand important for
54
all contaminants, also for the microplastic compounds. It is estimated that as much as 70%
55
of the marine litter ends up in the seabed (UNEP, 2005). In order to assess the fate of
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microplastics in the environment we need a sound method to accurately extract the
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microplastic particles in soil and sediment samples.
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The most commonly-used approach for extracting microplastic particles from soil and
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sediment samples is based on density separation (Thompson et al., 2004; Browne et al.,
60
2010; Claessens et al., 2011; Hidalgo-Ruz et al., 2012), by means of the density difference
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between microplastics and environmental matrixes. However, the specific operational
62
parameters used for this extraction of microplastic particles differ from study to study and
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are often poorly documented in the literature. The main reason for this is that the method
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has not been optimized and standardized. Data from different studies on concentration of
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microplastic particles in soils and sediments are therefore not possible to compile and
66
compare.
67
The method used for separating the microplastics from soil and sediment samples is
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briefly comprised of three steps: 1) fully mixing the sample with floatation solution; 2)
69
allowing the sample to rest for flotation and settling; and 3) filtration or sieving of the
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supernatant (Hidalgo-Ruz et al., 2012). The extracted amount of microplastics from a given
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soil or sediment sample should thus mainly be influenced by the mass of sample relative to
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the mass of the floatation solution and operational parameters used for the mixing, for
73
example, mass of sample, composition and volume of floatation solution, mixing method,
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mixing time etc. These are therefore the key methodological factors that need to be
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optimized and standardized.
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To decrease the influence of operators, Imhof et al. (2012) and Classens et al. (2013)
77
designed new elutriation apparatuses using a >100 cm pipe with a diameter of >15 cm. As
78
for floatation solution, Imhof et al. (2012) used ZnCl2 solution and Claessens et al. (2013)
79
used tap water and NaI solution for a two-step extraction. Although the new apparatuses
80
could efficiently extract meso-plastic particles and small microplastic particles down to 1
81
µm from sediment samples, large amount of solution is needed due to the large volume of
82
the apparatuses. Nuelle et al. (2014) created a two-step method. Sediment sample need be
83
pre-extracted using the air-induced overflow method based on fluidization in a NaCl
84
solution, and then be extracted using the floatation method in a NaI solution. Although the
85
method can reduce of the usage of NaI, the operation is complicated.
86
In this study, we propose a standard method for extracting microplastic particles from
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soil and sediment samples. Microplastics in soils and sediments are extracted by adding
88
200 g dry soil or sediment sample to 1.3 L mix of the saturated NaCl and NaI solutions in a
89
volume ratio of 1:1. After 40 sec aerating, the supernatant is filtered. The technique is
90
supposed to be optimized in terms of maximizing precision and accuracy, as well as to limit
91
environmental burden of the method. In order to verify the method’s merits, recovery
92
experiments were conducted by spiking soil and sediment samples with six common
93
microplastic materials: polyethylene (PE), polyethylene terephthalate (PET), polypropylene
94
(PP), polyvinyl chloride (PVC), polystyrene (PS) and expanded polystyrene (EPS), with an
95
annual production of 80, 53.3, 52.2, 38.5 and 26.4 million metric tons respectively (ECI,
96
2017; ECI, 2018a,b,c,d).
97 98
2. Material and Experiments
99
2.1. Material and samples
100
Sodium chloride (NaCl, AR, 99.5%), sodium iodine (NaI, 99%) and hydrogen
101
peroxide (H2O2, 35%) were purchased from Aladdin (Shanghai, China), Meryer (Shanghai,
102
China) and Bohua (Tianjin, China), respectively. Solutions of saturated NaCl and NaI were
103
prepared by dissolving an excess of NaCl and NaI pellets in distilled water.
104
For the recovery experiments, plastic particles of <1 mm in size were prepared by
105
shredding and cutting various common plastic products made from PE, PP, PVC, PET, PS
106
and EPS. The plastic particles and their origins are listed in Table 1.
107
Soil samples were randomly collected from the lawn at Jinnan campus of Nankai
108
University, Tianjin, China. Part of these soils was used as a real sample to compare the
109
results of the optimized separation approach in this study to those of the previous approach
110
as described in Thompson et al. (2004). Soils without microplastics were obtained by
111
several elutriations of the soils, which result in the removal of all existing microplastics.
112
Sand was selected as texture for the sediment samples as most sediment samples
113
studied in the literature are from sandy beaches below high tide line (Hidalgo-Ruz et al.,
114
2012). Aquarium sand (grain size 0.5-1.0 mm, Hebei, China) was used to represent
115
sediment sample (Nuelle et al., 2014). The aquarium sand was elutriated several times in
116
order to ensure microplastic free sediments.
117
Five real sediment samples were collected with a shovel from tidal flats along the
118
western coast of Bohai Bay in September, 2018, which are Dashentang beach (S1), the
119
Yongdingxin River estuary (S2), the Haihe River estuary (S3), the Dagupaiwu River
120
estuary (S4) and the Duliujian River estuary (S5) (Fig. 1). For each site, five sub-samples
121
of about 200 mL were randomly collected and loaded into a ziplock bag. All samples were
122
dried at 60 oC and sieved through a 20 mesh sieve (0.9 mm).
123
The clean soil and sediment samples were used for spiking experiments and the real
124
soil and sediment samples were used for method comparison experiments.
125
2.2. Spiking experiments
126
Recovery is an descriptive factor that may be used to validate the accuracy and
127
precision of an analytical method (Claessens et al., 2011). Recovery of known amounts of
128
microplastic particles added to clean soil and sediment matrixes (i.e. spiking experiments)
129
were therefore conducted to document and verify the merits and improvements by the
130
optimized method.
131
Prior to spiking, the non-presence of microplastic particles in the blank soil and sand
132
matrix samples were verified by extracting microplastics using the herewith prescribed
133
extraction device and process. The 200 g clean soil and sand samples were spiked with ten
134
pieces of the prepared microplastic particles (PP, PET, PE, PVC, PS and EPS). Five
135
duplicate microplastics-spiked samples were used in the recovery experiments, allowing
136
the determination of standard deviation based on the amount of microplastics found in the
137
five sample replicates.
138
2.3. Method comparison experiments
139
The common approach for extracting microplastic particles from soil and sediment
140
samples is the same as the process described in Thompson et al. (2004). Briefly, the
141
extraction process includes three steps: 1), add 250ml sample to concentrated saline
142
solution (1.2kg NaCl L-1), 2), stir for 30 seconds, and 3), filter the supernatant after 2
143
minutes. The optimization of extraction process in this study is described in section 3. To
144
test the extraction efficiency, the optimized approach in this study was compared with the
145
previous approach described in Thompson et al. (2004), using the real soil and sediment
146
samples, i.e. one soil sample from Jinnan campus, Nankai University and five sediment
147
samples from the Bohai Bay coast. The same sample was parallelly extracted by the
148
optimized and previous approaches separately, and the extraction results were compared in
149
Table 3 and Fig. 6.
150 151
2.4. Visual sorting and spectral analysis of microplastics
152
The microplastic particles in the extracted sample were visually identified and separated
153
using tweezers, according to the criteria proposed by Moore et al. (2009). The visually
154
recognized microplastic particles were further identified by attenuated total reflection Fourier
155
transformed infrared spectroscopy (ATR-FTIR, Bruker Tensor II, Germany). The ATR-FTIR
156
spectrum is compared to a reference database to determine the types of microplastics. The
157
ATR-FTIR is a single beam, percent transmission technique that runs 40 scans per sample at a
158
resolution of 0.4 cm−1 and wavelength range from 4000 to 350 cm−1.
159 160
3. Optimization of the separation method
161
3.1 Sample mass
162
The important factors governing the extraction amount of microplastics from a sample
163
matrix are the sample mass and the ratio of sample mass and volume of floatation solution
164
applied for extraction. As listed in Table 2, the reported masses of extracted sample and
165
extraction volumes differ between 30 g to 1 kg, and between 25 mL to 500 mL,
166
respectively. These large differences in the amounts of sample extracted, and the relative
167
volumes of floatation solutions applied, decrease the intra study precision of microplastics.
168
A higher sample mass generally ensures a better precision of detection due to
169
relatively low concentrations and high heterogeneity of microplastics in soils and sediments.
170
However, more floatation solution is required when using more mass of samples. Moreover,
171
representability of natural samples is typically more dependent on sampling strategy rather
172
than sample volume (Hidalgo-Ruz et al., 2012). A single grab sample provides a poor
173
representation of the concentration of microplastics in samples since the spatial distribution
174
of microplastics in soils and sediments appears to be rather heterogeneous. Thus, a
175
composite sample of several discrete samples is more representative than one single grab
176
sample. It is therefore better to use a relatively small amount of a composite sample, rather
177
than a large amount of one single grab sample. Based on this, we chose to use a moderate
178
mass of 200 g dry sample in this study, and proposed a sampling strategy using mixture of
179
5 subsamples within 1 m2.
180
3.2 Floatation solution
181
Simply tap water (Nuelle et al., 2014) has been used for the flotation solution, but
182
most commonly the density of the water is increased by dissolving salts to highly saline
183
solutions. Sodium chloride (NaCl) (Thompson et al., 2004; Claessens et al., 2011; Eriksen
184
et al., 2013; Wang et al., 2017), sodium iodide (NaI) (Claessens et al., 2013; Dekiff et al.,
185
2014; Kapp and Yeatman, 2018), potassium formate (HCOOK) (Zhang et al., 2017),
186
sodium polytungstate (Na6[H2W12O40]) (Martin et al., 2017) or zinc chloride (ZnCl2)
187
(Bergmann et al., 2017; Imhof et al., 2012) are commonly used to make the dense flotation
188
solution. Both ZnCl2 and NaI should be avoided or reduced as they are considered
189
hazardous (WGK 3), according to the German Water Hazard Classification, and HCOOK
190
(WGK1) and Na6[H2W12O40] (no WGK data) are rather expensive. Tap water and a
191
saturated NaCl (WGK1) solution is not dense enough to extract high-density microplastics.
192
The flotation solution needs to have a density greater than 1.50 g cm-3 since the densities of
193
the common microplastics are in the range of 0.015-1.50 g cm-3 (Table 1). An exception is
194
for polytetrafluoroethylene (PTFE) (2.2 g cm-3), commonly known as teflon. Considering
195
both extraction efficiency and economic cost, the optimum flotation solution is a mix of the
196
saturated NaCl and the NaI solutions in a volume ratio of 1:1. This provides a flotation
197
solution with a density of ~1.50 g cm-3. The NaCl-NaI based flotation solution may be
198
filtered and reused at least 5 times.
199
The accuracy and precision of the separation method using the proposed NaCl-NaI
200
flotation solution was tested by recovery experiments of sediment samples spiked with
201
microplastic, using both the saturated NaCl-NaI mix and the commonly used NaCl brine as
202
flotation solution. On average, 90% or more of the spiked microplastic particles were
203
recovered when the NaCl-NaI brine was employed as flotation solution. The average
204
recovery rates of PP, PE, PET, PVC, PS and EPS were 92±11.7%, 78±16%, 90±11%,
205
100±0%, 98±4% and 96±4.9%, respectively (Fig. 2). The exception is for PE, which
206
also gave a relatively poor precision. The low recovery of PE may be attributed to the same
207
white color of PE particles as the small quarts or feldspar pieces in the extracted sample.
208
On the other hand, using the commonly applied saturated NaCl solution resulted in
209
practically no recovery of PVC and PET. This is also found by Hidalgo-Ruz et al. (2012).
210
The poor recovery of these materials is due to that the density of saturated NaCl solution is
211
only 1.2 g cm-3, which is less than the density of PVC and PET (Table 1). Compared to the
212
NaCl solution, the NaCl-NaI solution increased the density of the solution (~1.5 g cm-3)
213
sufficiently in order for that all the tested microplastic particles could be extracted.
214
3.3 Design of extraction setup
215
Either manual stirring or shaking tables are used to mix the sediments and the flotation
216
solution. The reported mixing time varies from 30 s (Thompson et al., 2004) to 2 h (Reddy
217
et al., 2006). These different mixing protocols are likely to affect the degree of mixing, and
218
thus extraction efficiency.
219
To avoid the influence from different mixing methods, air mixing and floatation,
220
which can be controlled by air flow meter, was used instead of manual stirring or shaking.
221
Therefore, as schematically illustrated in Fig. 3, we propose standardized extraction setup,
222
based on air mixing and flotation, to optimize the separation and standardize the procedure.
223
The setup is composed of a flotation solution storage unit (A), air floatation unit (B), and a
224
vacuum filtration unit (C). The solution storage unit consists of a 2 L beaker (A1) and a
225
peristaltic pump (A2). The air flotation unit (B) includes an air pump (B1), an air flow
226
meter (B2), an aeration head (B3), and a plexiglass cylinder with an overflow structure
227
(B4). The cylinder has an internal diameter of 10 cm and a height of 18 cm, and an
228
overflow structure with serrated edge was designed to avoid interference of resuspension of
229
bottom sample. The vacuum filtration unit (C) consists of a filter (0.45 µm, Mili, China)
230
(C1) and a water-circulation vacuum pump (SHZ-DIII, Yuhua, China) (C2).
231
Different aeration (mixing) times almost had no significant effect on the recovery rates
232
of the six spiked microplastic particles (Fig. 4). The aeration time after input of sample was
233
thus set to be 40 s.
234
3.4. Standardization of extraction process
235
The extracting process consists of the following steps:
236
i)
flotation solution (1.3 L) is pumped from the storage unit (A) into the cylinder of the floatation unit (B);
237 238
ii)
air is pumped (B1) through the cylinder with a flow rate of 2 L min-1;
239
iii)
dry sample (200 g) is added to the top of the cylinder;
240
iv)
the cylinder with sample is purged with air for 40 s;
241
v)
purging is stopped and the sample mix is allowed to rest for 5 min to allow the heavy particle fractions to settle down;
242 243
vi)
is drain down into the vacuum filtration unit (C);
244 245 246
the supernatant of the floatation solution (250 mL) with microplastic particles
vii)
the filter membrane is collected to identify the microplastic particles.
3.5. Influence of organic matter
247
All soil and sediment samples contain natural organic matter of which some of it will
248
dissolve into the suspension solution. As there was little organic matter in the sandy
249
sediment sample, only the soil sample was used to test the influence of organic matter on
250
the recovery rates of spiked microplastics. The organic matter in the filtrate was removed
251
by storing the filter membrane with floating particles in 30 mL of a 35% H2O2 solution at
252
room temperature for 7 d (Nuelle et al., 2014).
253
Recovery rates of six types of microplastic particles with and without removing
254
organic matter in soil samples are shown in Fig. 5. Whether treating the samples with H2O2
255
or not, all the average recovery rates for the six microplastics are higher than 90%. For PS,
256
the average recovery rates with or without removal of organic matters is completely the
257
same, and both are 100%. For the others, no significant difference in the average recovery
258
rates between the two series of experiments (p> 0.05, independent two-sample t-test) were
259
observed, indicating that the organic matter in the soil has no significant effect on the
260
extraction efficiency using the approach.
261 262
4. Method comparison
263
As shown in Fig. 6, a real soil sample and five sediment samples with unknown plastic
264
particles were used to compare the optimized extraction approach to the previous approach
265
described by Thompson et al. (2004). The morphology, colors, sizes and types of
266
microplastic particles extracted from different samples using the two approaches were
267
listed in Table 3.
268
Generally, the properties of the microplastic particles extracted from all the samples
269
using the two approaches were similar in morphology, color, size and type. However, the
270
optimized approach generally extracted more microplastic items than the previous
271
approach, except the same item for one sample (S2). Indeed, the amount of microplastics in
272
sample S2 was too low to prove if the new approach increases efficiency or not. If
273
considering the overall performance, the optimized approach shows more accurate and
274
efficient than before. This experimentally validated approach can therefore contribute to
275
obtain a more correct knowledge of the amount of microplastic particles present in soils
276
and sediments.
277 278
5. Concluding remarks
279
The density-based extraction approach for microplastic particles in soil and sediment
280
samples was optimized through improvement and standardization of the extraction process.
281
It is suggested that 200 g sample used for the extraction. A NaCl-NaI mix is proposed to
282
replace the commonly used NaCl as floatation solution, in order to achieve a density
283
greater than the most common plastic materials. The methods of stirring or shaking of the
284
sample with floatation solution is changed to air floatation in the developed design. The
285
optimization and standardization, which enhances the comparability of the data, are
286
validated using recovery experiments on microplastics spiked soil and sediment samples.
287 288
Acknowledgements
289
This work was supported by a grant from Tianjin Science and Technology Program
290
(18PTZWHZ00110).
291 292
References
293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323
Arthur, C., Baker, J., Bamford, H., n.d. September 9-11, 2008. In: Proceedings of the International Research Workshop on the Occurrence, Effects, and Fate of Microplastic Marine Debris. NOAA Technical Memorandum, 49. Bergmann, M., Wirzberger, V., Krumpen, T., Lorenz, C., Primpke, S., Tekman, M.B., Gerdts, G., 2017. High quantities of microplastic in Arctic deep-sea sediments from the HAUSGARTEN observatory. Environ. Sci. Technol. 51. Browne, M.A., Galloway, T.S., Thompson, R.C., 2010. Spatial Patterns of Plastic Debris along Estuarine Shorelines. Environ. Sci. Technol. 44, 3404–3409. Claessens, M., De, M.S., Van, L.L., De, C.K., Janssen, C.R., 2011. Occurrence and distribution of microplastics in marine sediments along the Belgian coast. Mar. Pollut. Bull. 62, 2199. Claessens, M., Van, C.L., Vandegehuchte, M.B., Janssen, C.R., 2013. New techniques for the detection of microplastics in sediments and field collected organisms. Mar. Pollut. Bull. 70, 227–233. Corcoran, P.L., Biesinger, M.C., Meriem, G., 2009. Plastics and beaches: a degrading relationship. Mar. Pollut. Bull. 58, 80–84. Dekiff, J.H., Remy, D., Klasmeier, J., Fries, E., 2014. Occurrence and spatial distribution of microplastics in sediments from Norderney. Environ. Pollut. 186, 248–256. ECI (The Essential Chemical Industry), 2017. Poly(ethene) (Polyethylene) [WWW Document]. URL http://www.essentialchemicalindustry.org/polymers/polyethene.html (accessed 9.6.18). ECI (The Essential Chemical Industry), 2018a. Poly(chloroethene) (Polyvinyl chloride) [WWW Document], URL http://www.essentialchemicalindustry.org/polymers/polychloroethene.html (accessed 9.6.18). ECI (The Essential Chemical Industry), 2018b. Polyesters [WWW Document]. URL http://www.essentialchemicalindustry.org/polymers/polyesters.html (accessed 9.6.18). ECI (The Essential Chemical Industry), 2018c. Poly(phenylethene) (Polystyrene) [WWW Document]. URL http://www.essentialchemicalindustry.org/polymers/polyphenylethene.html (accessed 9.6.18). ECI (The Essential Chemical Industry), 2018d. Poly(propene) (Polypropylene) [WWW Document]. URL http://www.essentialchemicalindustry.org/polymers/polypropene.html (accessed 9.6.18). Eriksen, M., Mason, S., Wilson, S., Box, C., Zellers, A., Edwards, W., Farley, H., Amato, S., 2013. Microplastic pollution in the surface waters of the Laurentian Great Lakes. Mar. Pollut. Bull. 77, 177–182.
324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365
Hidalgo-Ruz, V., Gutow, L., Thompson, R.C., Thiel, M., 2012. Microplastics in the Marine Environment: A Review of the Methods Used for Identification and Quantification. Environ. Sci. Technol. 46, 3060–3075. Hu, L., Chernick, M., Hinton, D.E., Shi, H., 2018. Microplastics in Small Waterbodies and Tadpoles from Yangtze River Delta, China. Environ. Sci. Technol. Hueffer, T., Metzelder, F., Sigmund, G., Slawek, S., Schmidt, T.C., Hofmann, T., 2019. Polyethylene microplastics influence the transport of organic contaminants in soil. Sci. Total Environ. 657. Hurley, R.R., Lusher, A.L., Olsen, M., Nizzetto, L., 2018. Validation of a Method for Extracting Microplastics from Complex, Organic-Rich, Environmental Matrices. Environ. Sci. Technol. 52, 7409−7417. Imhof, H.K., Schmid, J., Niessner, R., Ivleva, N.P., Laforsch, C., 2012. A novel, highly efficient method for the separation and quantification of plastic particles in sediments of aquatic environments. Limnol. Oceanogr. Methods 10, 524–537. Jayasiri, H.B., Purushothaman, C.S., Vennila, A., 2013. Plastic litter accumulation on high-water strandline of urban beaches in Mumbai, India. Environ. Monit. Assess. 185, 7709–7719. Kapp, K.J., Yeatman, E., 2018. Microplastic hotspots in the Snake and Lower Columbia rivers: A journey from the Greater Yellowstone Ecosystem to the Pacific Ocean. Environ. Pollut. 241, 1082–1090. Koelmans, A.A., Besseling, E., Foekema, E., Kooi, M., Mintenig, S., Ossendorp, B.C., Redondohasselerharm, P.E., Verschoor, A., Wezel, A.P.V., Scheffer, M., 2017. Risks of Plastic Debris: Unravelling Fact, Opinion, Perception, and Belief. Environ. Sci. Technol. 51, 11513– 11519. Liebezeit, G., Dubaish, F., 2012. Microplastics in beaches of the East Frisian islands Spiekeroog and Kachelotplate. Bull. Environ. Contam. Toxicol. 89, 213–217. Liu, E.K., He, W.Q., Yan, C.R., 2014. ‘White revolution’ to ‘white pollution’—agricultural plastic film mulch in China. Environ. Res. Lett. 9, 091001. Lu, Y., Zhang, Y., Deng, Y., Jiang, W., Zhao, Y., Geng, J., Ding, L., Ren, H.Q., 2016. Uptake and accumulation of polystyrene microplastics in zebrafish (Danio rerio) and toxic effects in liver. Environ. Sci. Technol. 50, 4054. Martin, J., Lusher, A., Thompson, R.C., Morley, A., 2017. The Deposition and Accumulation of Microplastics in Marine Sediments and Bottom Water from the Irish Continental Shelf. Sci. Rep. 7. Mohamed Nor, N.H., Obbard, J.P., 2014. Microplastics in Singapore’s coastal mangrove ecosystems. Mar. Pollut. Bull. 79, 278–283. Moore, E., Lyday, S., Roletto, J., Litle, K., Parrish, J.K., Nevins, H., Harvey, J., Mortenson, J., Greig, D., Piazza, M., 2009. Entanglements of marine mammals and seabirds in central California and the north-west coast of the United States 2001-2005. Mar. Pollut. Bull. 58, 1045–1051. Müller, A., Becker, R., Dorgerloh, U., Simon, F.G., Braun, U., 2018. The effect of polymer aging on the uptake of fuel aromatics and ethers by microplastics. Environ. Pollut. 240, 639–646. Nuelle, M.T., Dekiff, J.H., Remy, D., Fries, E., 2014. A new analytical approach for monitoring microplastics in marine sediments. Environ. Pollut. 184, 161–169. Reddy, M.S., Basha, S., Adimurthy, S., Ramachandraiah, G., 2006. Description of the small plastics
366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389
fragments in marine sediments along the Alang-Sosiya ship-breaking yard, India. Estuar. Coast. Shelf Sci. 68, 656–660. Thompson, R.C., Olsen, Y., Mitchell, R.P., Davis, A., Rowland, S.J., Mcgonigle, D., Russell, A.E., 2004. Lost at Sea: Where Is All the Plastic? Science 304, 838–838. Tsang, Y.Y., Mak, C.W., Liebich, C., Lam, S.W., Sze, E.T., Chan, K.M., 2017. Microplastic pollution in the marine waters and sediments of Hong Kong. Mar. Pollut. Bull. 115, 20–28. UNEP, 2005. UNEP Regional Seas Programme, Marine Litter And Abandoned Fishing Gear. Report to the Division of Ocean Affairs and the Law of the Sea, Office of Legal Affairs, UNHQ. Van, C.L., Vanreusel, A., Mees, J., Janssen, C.R., 2013. Microplastic pollution in deep-sea sediments. Environ. Pollut. 182, 495–499. Wang, J., Peng, J., Tan, Z., Gao, Y., Zhan, Z., Chen, Q., Cai, L., 2017. Microplastics in the surface sediments from the Beijiang River littoral zone: Composition, abundance, surface textures and interaction with heavy metals. Chemosphere 171, 248–258. Xu, B., Liu, F., Brookes, P.C., Xu, J., 2018. Microplastics play a minor role in tetracycline sorption in the presence of dissolved organic matter . Environ. Pollut. 240, 87–94. Zhang, K., Xiong, X., Hu, H., Wu, C., Bi, Y., Wu, Y., Zhou, B., Lam, P.K., Liu, J., 2017. Occurrence and Characteristics of Microplastic Pollution in Xiangxi Bay of Three Gorges Reservoir, China. Environ. Sci. Technol. 51, 3794–3801. Zhao, J., Ran, W., Teng, J., Liu, Y., Liu, H., Yin, X., Cao, R., Wang, Q., 2018. Microplastic pollution in sediments from the Bohai Sea and the Yellow Sea, China. Sci. Total Environ. s 640–641, 637– 645.
390
Figure caption
391
Fig. 1. Sampling locations along the Bohai Bay coast (S1: Dashentang beach, S2:
392
Yongdingxin River estuary, S3: Haihe River estuary, S4: Dagupaiwu River estuary, S5:
393
Duliujian River estuary).
394
Fig. 2.
395
different floatation solutions.
396
Fig. 3. Schematic diagram of the optimized extraction setup. A: solution storage unit (A1:
397
beaker, A2: peristaltic pump), B: air flotation unit (B1: air pump, B2: air flow meter, B3:
398
overflow structure, B4: aeration head), C: vacuum filtration unit (C1: filter, C2:
399
water-circulation vacuum pump).
400
Fig. 4. Recovery of the spiked microplastic particles in sediment samples using different
401
aeration times.
402
Fig. 5. Recovery of microplastic particles, with and without removal of natural organic
403
matter, in soil samples spiked with microplastic particles.
404
Fig. 6. The total numbers of extracted microplastic particles using the optimized approach
405
and the previous approach from different samples.
406
Recovery of six spiked microplastic particles in sediment samples using
407 408
Fig. 1
409 NaCl
NaCl-NaI
Recoveries %
100 80 60 40 20 0
410 411
PP
PE
PET
Fig. 2
412 413 414
415 416
Fig. 3
PVC
PS
EPS
10s
40s
70s
100s
Recoveries %
100 80 60 40 20 0 PP
417
PE
PET
PVC
PS
EPS
Fig. 4
418 419 420 421 422
notnot treated with H2O2 treated with HO 2
treated HO treated withwith H2O2 2
2
2
Recoveries %
100
80
60
40
20
0 PP
PE
PET
423 424 425
Fig. 5
PVC
PS
EPS
426
Soil
Sediment
427 428 429
Fig. 6
430
Table list
431
Table 1. Polymer types, densities, colors, original products and sources of plastic particles
432
for recovery experiments.
433
Table 2. Literature values on mass or volume of samples used for extracting microplastics.
434
Table 3. The sample type, location, morphology, colors, types, sizes and numbers of
435
microplastic particles extracted from different samples using the previous and the
436
optimized approach.
437
438
Table 1 Density Polymer type
(g/cm3)
Color
Original product
Source
PP
0.89-0.91
Dark blue
GC vial cap
Lab
PET
1.29-1.40
Light blue
Water bottle
Local market
PE
0.94-0.97
White
Yoghurt bottle
Local market
PVC
1.3-1.50
Red
Pipe
Local
hardware
store PS
1.04-1.08
Black
Spoon
Local market
EPS
0.015-0.03
White
Styrofoam packaging
Lab
439 440
441
Table 2 Mass or volume of samples
References
30 g
Wang et al., 2017
50 g
Zhao et al., 2018
150-190 g
Corcoran et al., 2009
1 kg
Claessens et al., 2011; Nuelle et al., 2014; Reddy et al., 2006
25 mL
Van et al., 2013
50 mL
Browne et al., 2010
68 mL
Imhof et al., 2012
500 mL
Claessens et al., 2013; Liebezeit and Dubaish, 2012
442
Table 3 Previous approach Sample
Type
Location
Optimized approach
Number Morphology
Color
Type
(items/k
Number Size (mm)
Morphology
Color
Type
g d.w.) Soil
S1
S2
S3
Soil
Sediment
Sediment
Sediment
Jinnan campus, Nankai University
Dashentang beach
Yongdingxin River estuary
Haihe River estuary
fragment
fragment
S4
S5
443
Sediment
Sediment
estuary
Duliujian River estuary
green white, green
PP
75
PP
30
Size (mm)
d.w.) (2.6-0.1)× (0.4-0.1) (6.1-2)× (1.0-0.5)
fragment
fragment
white, green white, green
PP
95
PP
55
(3.2-0.5)× (0.5-0.1) (9.0-0.7)× (0.5-0.1)
fibre
white
PS
10
0.5-1.0
fibre
white
PS
125
0.9-10.0
fibre
white
PS
5
2.0
fragment
white
PP
5
0.5×0.1
PE, PP
195
white,
PE,
green
PP
PS
240
white
PS
295
PP
90
PP
125
fragment fibre
Dagupaiwu River
white,
(items/kg
fragment
white, green white white, green
(4.0-0.2)× (1.0-0.1) 0.3-15.0 (6.0-1.0)× (1.0-0.1)
fragment fibre fragment
white, green
225
fibre
white
PS
100
0.9-8.0
fibre
white
PS
75
fragment
white
PP
5
1.0×0.2
fragment
white
PP
10
fibre
white
PS
5
3.0
fibre
white
PS
5
(6.0-0.8)× (1.8-0.1) 1.1-12.0 (9.0-0.7)× (1.0-0.1) 1.1-12.0 (2.0-0.5)× (0.8-0.2) 3.1
Highlights:
1. An optimized approach for extracting microplastics was proposed. 2. The mixture of NaCl and NaI was used as floatation solution. 3. Aeration was used instead of stirring by hand. 4.The optimization was validated by spiking and comparison experiments.
Conflict of interest The authors declare no conflict of interest.
S0269-7491(19)32572-2
DOI:
https://doi.org/10.1016/j.envpol.2019.113009
Reference:
ENPO 113009
To appear in:
Environmental Pollution
Received Date: 16 May 2019 Revised Date:
1 August 2019
Accepted Date: 2 August 2019
Please cite this article as: Han, X., Lu, X., Vogt, R.D., An optimized density-based approach for extracting microplastics from soil and sediment samples, Environmental Pollution (2019), doi: https:// doi.org/10.1016/j.envpol.2019.113009. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Graphical abstract:
1
An
Optimized
Density-based
2
Microplastics from Soil and Sediment Samples
3
Xiaoxin Hana,b,c, Xueqiang Lua,b,c*, Rolf D. Vogtc,d
4
a
5
China
6
b
7
Pollution, Tianjin 300350, China
8
c
9
Technology, Tianjin 300350, China.
10
d
for
Extracting
College of Environmental Science and Engineering, Nankai University, Tianjin 300350,
Tianjin Key Laboratory of Environmental Technology for Complex Trans-Media
Tianjin International Joint Research Center for Environmental Biogeochemical
Department of Chemistry, University of Oslo, Oslo, Norway.
11 12
* Corresponding author.
13
E-mail address: [email protected]
14
Approach
15
Abstract
16
Microplastic pollution in the environment has received growing attention worldwide. A
17
major impediment for accurate measurements of microplastics in environmental matrixes is
18
to extract the particles. The most commonly-used method for separation from soil or
19
sediment is flotation in dense liquid based on the relatively low density of plastic particles.
20
This study provides an improved and optimized process for extraction of microplastic
21
particles by modifying the floatation technique and floatation solution. Microplastics in
22
soils and sediments are extracted by adding 200 g dry soil or sediment sample to 1.3 L mix
23
of the saturated NaCl and NaI solutions in a volume ratio of 1:1 and aerating for 40 sec
24
then filtering the supernatant. The accuracy and precision of the new approach is validated
25
by recovery experiments using soil and sediment samples spiked with six common
26
microplastic
27
polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS) and expanded polystyrene
28
(EPS), and comparison with the previous method. The optimized approach is further
29
compared with the previous approach using the real soil and sediment samples.
30
Keywords: microplastics; extracting approach; soil; sediment; air floatation
31
compounds:
polyethylene
(PE),
polyethylene
terephthalate
(PET),
32
33 34
Graphical abstract:
35
1. Introduction
36
The term "microplastic" ("MP"), first proposed by Thompson et al. (2004), refers to
37
plastic particle less than 5 mm (Arthur et al., 2009) or 1 mm (Claessens et al., 2011) in
38
length.
39
contaminants (Müller et al., 2018; Xu et al., 2018). Due to its small size, living organisms
40
easily ingest these microplastic particles. Although there is no clear causality that this may
41
increase bioavailability of these compounds, it has raised a public concern. The concern is
42
that the omnipresence of microplastic contamination in the environment (Bergmann et al.,
43
2017; Dekiff et al., 2014; Jayasiri et al., 2013; Tsang et al., 2017; Zhao et al., 2018) might
44
represent an important pathway for pollutant uptake. The contaminated particles might
45
release their harmful absorbed substance when it is passing through the digestion system of
46
the organisms. This could threaten the organisms and even humans through the food chain
47
(Koelmans et al., 2017; Lu et al., 2016). Whether microplastic particles actually represent
48
an emerging contaminant or not needs therefore to be clarified and documented with sound
49
data. The key to investigate and evaluate environmental risk of microplastic contamination
50
is to have a good method for extracting microplastic particles from environmental samples.
Microplastics may accumulate heavy metals (Wang et al., 2017) and organic
51
Soil acts as a preliminary sink of microplastic particles, which may subsequently
52
either be decomposed further to nanoplastic or be remobilized again (Hueffer et al., 2019;
53
Hurley et al., 2018; Liu et al., 2014). Marine sediments are on the other hand important for
54
all contaminants, also for the microplastic compounds. It is estimated that as much as 70%
55
of the marine litter ends up in the seabed (UNEP, 2005). In order to assess the fate of
56
microplastics in the environment we need a sound method to accurately extract the
57
microplastic particles in soil and sediment samples.
58
The most commonly-used approach for extracting microplastic particles from soil and
59
sediment samples is based on density separation (Thompson et al., 2004; Browne et al.,
60
2010; Claessens et al., 2011; Hidalgo-Ruz et al., 2012), by means of the density difference
61
between microplastics and environmental matrixes. However, the specific operational
62
parameters used for this extraction of microplastic particles differ from study to study and
63
are often poorly documented in the literature. The main reason for this is that the method
64
has not been optimized and standardized. Data from different studies on concentration of
65
microplastic particles in soils and sediments are therefore not possible to compile and
66
compare.
67
The method used for separating the microplastics from soil and sediment samples is
68
briefly comprised of three steps: 1) fully mixing the sample with floatation solution; 2)
69
allowing the sample to rest for flotation and settling; and 3) filtration or sieving of the
70
supernatant (Hidalgo-Ruz et al., 2012). The extracted amount of microplastics from a given
71
soil or sediment sample should thus mainly be influenced by the mass of sample relative to
72
the mass of the floatation solution and operational parameters used for the mixing, for
73
example, mass of sample, composition and volume of floatation solution, mixing method,
74
mixing time etc. These are therefore the key methodological factors that need to be
75
optimized and standardized.
76
To decrease the influence of operators, Imhof et al. (2012) and Classens et al. (2013)
77
designed new elutriation apparatuses using a >100 cm pipe with a diameter of >15 cm. As
78
for floatation solution, Imhof et al. (2012) used ZnCl2 solution and Claessens et al. (2013)
79
used tap water and NaI solution for a two-step extraction. Although the new apparatuses
80
could efficiently extract meso-plastic particles and small microplastic particles down to 1
81
µm from sediment samples, large amount of solution is needed due to the large volume of
82
the apparatuses. Nuelle et al. (2014) created a two-step method. Sediment sample need be
83
pre-extracted using the air-induced overflow method based on fluidization in a NaCl
84
solution, and then be extracted using the floatation method in a NaI solution. Although the
85
method can reduce of the usage of NaI, the operation is complicated.
86
In this study, we propose a standard method for extracting microplastic particles from
87
soil and sediment samples. Microplastics in soils and sediments are extracted by adding
88
200 g dry soil or sediment sample to 1.3 L mix of the saturated NaCl and NaI solutions in a
89
volume ratio of 1:1. After 40 sec aerating, the supernatant is filtered. The technique is
90
supposed to be optimized in terms of maximizing precision and accuracy, as well as to limit
91
environmental burden of the method. In order to verify the method’s merits, recovery
92
experiments were conducted by spiking soil and sediment samples with six common
93
microplastic materials: polyethylene (PE), polyethylene terephthalate (PET), polypropylene
94
(PP), polyvinyl chloride (PVC), polystyrene (PS) and expanded polystyrene (EPS), with an
95
annual production of 80, 53.3, 52.2, 38.5 and 26.4 million metric tons respectively (ECI,
96
2017; ECI, 2018a,b,c,d).
97 98
2. Material and Experiments
99
2.1. Material and samples
100
Sodium chloride (NaCl, AR, 99.5%), sodium iodine (NaI, 99%) and hydrogen
101
peroxide (H2O2, 35%) were purchased from Aladdin (Shanghai, China), Meryer (Shanghai,
102
China) and Bohua (Tianjin, China), respectively. Solutions of saturated NaCl and NaI were
103
prepared by dissolving an excess of NaCl and NaI pellets in distilled water.
104
For the recovery experiments, plastic particles of <1 mm in size were prepared by
105
shredding and cutting various common plastic products made from PE, PP, PVC, PET, PS
106
and EPS. The plastic particles and their origins are listed in Table 1.
107
Soil samples were randomly collected from the lawn at Jinnan campus of Nankai
108
University, Tianjin, China. Part of these soils was used as a real sample to compare the
109
results of the optimized separation approach in this study to those of the previous approach
110
as described in Thompson et al. (2004). Soils without microplastics were obtained by
111
several elutriations of the soils, which result in the removal of all existing microplastics.
112
Sand was selected as texture for the sediment samples as most sediment samples
113
studied in the literature are from sandy beaches below high tide line (Hidalgo-Ruz et al.,
114
2012). Aquarium sand (grain size 0.5-1.0 mm, Hebei, China) was used to represent
115
sediment sample (Nuelle et al., 2014). The aquarium sand was elutriated several times in
116
order to ensure microplastic free sediments.
117
Five real sediment samples were collected with a shovel from tidal flats along the
118
western coast of Bohai Bay in September, 2018, which are Dashentang beach (S1), the
119
Yongdingxin River estuary (S2), the Haihe River estuary (S3), the Dagupaiwu River
120
estuary (S4) and the Duliujian River estuary (S5) (Fig. 1). For each site, five sub-samples
121
of about 200 mL were randomly collected and loaded into a ziplock bag. All samples were
122
dried at 60 oC and sieved through a 20 mesh sieve (0.9 mm).
123
The clean soil and sediment samples were used for spiking experiments and the real
124
soil and sediment samples were used for method comparison experiments.
125
2.2. Spiking experiments
126
Recovery is an descriptive factor that may be used to validate the accuracy and
127
precision of an analytical method (Claessens et al., 2011). Recovery of known amounts of
128
microplastic particles added to clean soil and sediment matrixes (i.e. spiking experiments)
129
were therefore conducted to document and verify the merits and improvements by the
130
optimized method.
131
Prior to spiking, the non-presence of microplastic particles in the blank soil and sand
132
matrix samples were verified by extracting microplastics using the herewith prescribed
133
extraction device and process. The 200 g clean soil and sand samples were spiked with ten
134
pieces of the prepared microplastic particles (PP, PET, PE, PVC, PS and EPS). Five
135
duplicate microplastics-spiked samples were used in the recovery experiments, allowing
136
the determination of standard deviation based on the amount of microplastics found in the
137
five sample replicates.
138
2.3. Method comparison experiments
139
The common approach for extracting microplastic particles from soil and sediment
140
samples is the same as the process described in Thompson et al. (2004). Briefly, the
141
extraction process includes three steps: 1), add 250ml sample to concentrated saline
142
solution (1.2kg NaCl L-1), 2), stir for 30 seconds, and 3), filter the supernatant after 2
143
minutes. The optimization of extraction process in this study is described in section 3. To
144
test the extraction efficiency, the optimized approach in this study was compared with the
145
previous approach described in Thompson et al. (2004), using the real soil and sediment
146
samples, i.e. one soil sample from Jinnan campus, Nankai University and five sediment
147
samples from the Bohai Bay coast. The same sample was parallelly extracted by the
148
optimized and previous approaches separately, and the extraction results were compared in
149
Table 3 and Fig. 6.
150 151
2.4. Visual sorting and spectral analysis of microplastics
152
The microplastic particles in the extracted sample were visually identified and separated
153
using tweezers, according to the criteria proposed by Moore et al. (2009). The visually
154
recognized microplastic particles were further identified by attenuated total reflection Fourier
155
transformed infrared spectroscopy (ATR-FTIR, Bruker Tensor II, Germany). The ATR-FTIR
156
spectrum is compared to a reference database to determine the types of microplastics. The
157
ATR-FTIR is a single beam, percent transmission technique that runs 40 scans per sample at a
158
resolution of 0.4 cm−1 and wavelength range from 4000 to 350 cm−1.
159 160
3. Optimization of the separation method
161
3.1 Sample mass
162
The important factors governing the extraction amount of microplastics from a sample
163
matrix are the sample mass and the ratio of sample mass and volume of floatation solution
164
applied for extraction. As listed in Table 2, the reported masses of extracted sample and
165
extraction volumes differ between 30 g to 1 kg, and between 25 mL to 500 mL,
166
respectively. These large differences in the amounts of sample extracted, and the relative
167
volumes of floatation solutions applied, decrease the intra study precision of microplastics.
168
A higher sample mass generally ensures a better precision of detection due to
169
relatively low concentrations and high heterogeneity of microplastics in soils and sediments.
170
However, more floatation solution is required when using more mass of samples. Moreover,
171
representability of natural samples is typically more dependent on sampling strategy rather
172
than sample volume (Hidalgo-Ruz et al., 2012). A single grab sample provides a poor
173
representation of the concentration of microplastics in samples since the spatial distribution
174
of microplastics in soils and sediments appears to be rather heterogeneous. Thus, a
175
composite sample of several discrete samples is more representative than one single grab
176
sample. It is therefore better to use a relatively small amount of a composite sample, rather
177
than a large amount of one single grab sample. Based on this, we chose to use a moderate
178
mass of 200 g dry sample in this study, and proposed a sampling strategy using mixture of
179
5 subsamples within 1 m2.
180
3.2 Floatation solution
181
Simply tap water (Nuelle et al., 2014) has been used for the flotation solution, but
182
most commonly the density of the water is increased by dissolving salts to highly saline
183
solutions. Sodium chloride (NaCl) (Thompson et al., 2004; Claessens et al., 2011; Eriksen
184
et al., 2013; Wang et al., 2017), sodium iodide (NaI) (Claessens et al., 2013; Dekiff et al.,
185
2014; Kapp and Yeatman, 2018), potassium formate (HCOOK) (Zhang et al., 2017),
186
sodium polytungstate (Na6[H2W12O40]) (Martin et al., 2017) or zinc chloride (ZnCl2)
187
(Bergmann et al., 2017; Imhof et al., 2012) are commonly used to make the dense flotation
188
solution. Both ZnCl2 and NaI should be avoided or reduced as they are considered
189
hazardous (WGK 3), according to the German Water Hazard Classification, and HCOOK
190
(WGK1) and Na6[H2W12O40] (no WGK data) are rather expensive. Tap water and a
191
saturated NaCl (WGK1) solution is not dense enough to extract high-density microplastics.
192
The flotation solution needs to have a density greater than 1.50 g cm-3 since the densities of
193
the common microplastics are in the range of 0.015-1.50 g cm-3 (Table 1). An exception is
194
for polytetrafluoroethylene (PTFE) (2.2 g cm-3), commonly known as teflon. Considering
195
both extraction efficiency and economic cost, the optimum flotation solution is a mix of the
196
saturated NaCl and the NaI solutions in a volume ratio of 1:1. This provides a flotation
197
solution with a density of ~1.50 g cm-3. The NaCl-NaI based flotation solution may be
198
filtered and reused at least 5 times.
199
The accuracy and precision of the separation method using the proposed NaCl-NaI
200
flotation solution was tested by recovery experiments of sediment samples spiked with
201
microplastic, using both the saturated NaCl-NaI mix and the commonly used NaCl brine as
202
flotation solution. On average, 90% or more of the spiked microplastic particles were
203
recovered when the NaCl-NaI brine was employed as flotation solution. The average
204
recovery rates of PP, PE, PET, PVC, PS and EPS were 92±11.7%, 78±16%, 90±11%,
205
100±0%, 98±4% and 96±4.9%, respectively (Fig. 2). The exception is for PE, which
206
also gave a relatively poor precision. The low recovery of PE may be attributed to the same
207
white color of PE particles as the small quarts or feldspar pieces in the extracted sample.
208
On the other hand, using the commonly applied saturated NaCl solution resulted in
209
practically no recovery of PVC and PET. This is also found by Hidalgo-Ruz et al. (2012).
210
The poor recovery of these materials is due to that the density of saturated NaCl solution is
211
only 1.2 g cm-3, which is less than the density of PVC and PET (Table 1). Compared to the
212
NaCl solution, the NaCl-NaI solution increased the density of the solution (~1.5 g cm-3)
213
sufficiently in order for that all the tested microplastic particles could be extracted.
214
3.3 Design of extraction setup
215
Either manual stirring or shaking tables are used to mix the sediments and the flotation
216
solution. The reported mixing time varies from 30 s (Thompson et al., 2004) to 2 h (Reddy
217
et al., 2006). These different mixing protocols are likely to affect the degree of mixing, and
218
thus extraction efficiency.
219
To avoid the influence from different mixing methods, air mixing and floatation,
220
which can be controlled by air flow meter, was used instead of manual stirring or shaking.
221
Therefore, as schematically illustrated in Fig. 3, we propose standardized extraction setup,
222
based on air mixing and flotation, to optimize the separation and standardize the procedure.
223
The setup is composed of a flotation solution storage unit (A), air floatation unit (B), and a
224
vacuum filtration unit (C). The solution storage unit consists of a 2 L beaker (A1) and a
225
peristaltic pump (A2). The air flotation unit (B) includes an air pump (B1), an air flow
226
meter (B2), an aeration head (B3), and a plexiglass cylinder with an overflow structure
227
(B4). The cylinder has an internal diameter of 10 cm and a height of 18 cm, and an
228
overflow structure with serrated edge was designed to avoid interference of resuspension of
229
bottom sample. The vacuum filtration unit (C) consists of a filter (0.45 µm, Mili, China)
230
(C1) and a water-circulation vacuum pump (SHZ-DIII, Yuhua, China) (C2).
231
Different aeration (mixing) times almost had no significant effect on the recovery rates
232
of the six spiked microplastic particles (Fig. 4). The aeration time after input of sample was
233
thus set to be 40 s.
234
3.4. Standardization of extraction process
235
The extracting process consists of the following steps:
236
i)
flotation solution (1.3 L) is pumped from the storage unit (A) into the cylinder of the floatation unit (B);
237 238
ii)
air is pumped (B1) through the cylinder with a flow rate of 2 L min-1;
239
iii)
dry sample (200 g) is added to the top of the cylinder;
240
iv)
the cylinder with sample is purged with air for 40 s;
241
v)
purging is stopped and the sample mix is allowed to rest for 5 min to allow the heavy particle fractions to settle down;
242 243
vi)
is drain down into the vacuum filtration unit (C);
244 245 246
the supernatant of the floatation solution (250 mL) with microplastic particles
vii)
the filter membrane is collected to identify the microplastic particles.
3.5. Influence of organic matter
247
All soil and sediment samples contain natural organic matter of which some of it will
248
dissolve into the suspension solution. As there was little organic matter in the sandy
249
sediment sample, only the soil sample was used to test the influence of organic matter on
250
the recovery rates of spiked microplastics. The organic matter in the filtrate was removed
251
by storing the filter membrane with floating particles in 30 mL of a 35% H2O2 solution at
252
room temperature for 7 d (Nuelle et al., 2014).
253
Recovery rates of six types of microplastic particles with and without removing
254
organic matter in soil samples are shown in Fig. 5. Whether treating the samples with H2O2
255
or not, all the average recovery rates for the six microplastics are higher than 90%. For PS,
256
the average recovery rates with or without removal of organic matters is completely the
257
same, and both are 100%. For the others, no significant difference in the average recovery
258
rates between the two series of experiments (p> 0.05, independent two-sample t-test) were
259
observed, indicating that the organic matter in the soil has no significant effect on the
260
extraction efficiency using the approach.
261 262
4. Method comparison
263
As shown in Fig. 6, a real soil sample and five sediment samples with unknown plastic
264
particles were used to compare the optimized extraction approach to the previous approach
265
described by Thompson et al. (2004). The morphology, colors, sizes and types of
266
microplastic particles extracted from different samples using the two approaches were
267
listed in Table 3.
268
Generally, the properties of the microplastic particles extracted from all the samples
269
using the two approaches were similar in morphology, color, size and type. However, the
270
optimized approach generally extracted more microplastic items than the previous
271
approach, except the same item for one sample (S2). Indeed, the amount of microplastics in
272
sample S2 was too low to prove if the new approach increases efficiency or not. If
273
considering the overall performance, the optimized approach shows more accurate and
274
efficient than before. This experimentally validated approach can therefore contribute to
275
obtain a more correct knowledge of the amount of microplastic particles present in soils
276
and sediments.
277 278
5. Concluding remarks
279
The density-based extraction approach for microplastic particles in soil and sediment
280
samples was optimized through improvement and standardization of the extraction process.
281
It is suggested that 200 g sample used for the extraction. A NaCl-NaI mix is proposed to
282
replace the commonly used NaCl as floatation solution, in order to achieve a density
283
greater than the most common plastic materials. The methods of stirring or shaking of the
284
sample with floatation solution is changed to air floatation in the developed design. The
285
optimization and standardization, which enhances the comparability of the data, are
286
validated using recovery experiments on microplastics spiked soil and sediment samples.
287 288
Acknowledgements
289
This work was supported by a grant from Tianjin Science and Technology Program
290
(18PTZWHZ00110).
291 292
References
293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323
Arthur, C., Baker, J., Bamford, H., n.d. September 9-11, 2008. In: Proceedings of the International Research Workshop on the Occurrence, Effects, and Fate of Microplastic Marine Debris. NOAA Technical Memorandum, 49. Bergmann, M., Wirzberger, V., Krumpen, T., Lorenz, C., Primpke, S., Tekman, M.B., Gerdts, G., 2017. High quantities of microplastic in Arctic deep-sea sediments from the HAUSGARTEN observatory. Environ. Sci. Technol. 51. Browne, M.A., Galloway, T.S., Thompson, R.C., 2010. Spatial Patterns of Plastic Debris along Estuarine Shorelines. Environ. Sci. Technol. 44, 3404–3409. Claessens, M., De, M.S., Van, L.L., De, C.K., Janssen, C.R., 2011. Occurrence and distribution of microplastics in marine sediments along the Belgian coast. Mar. Pollut. Bull. 62, 2199. Claessens, M., Van, C.L., Vandegehuchte, M.B., Janssen, C.R., 2013. New techniques for the detection of microplastics in sediments and field collected organisms. Mar. Pollut. Bull. 70, 227–233. Corcoran, P.L., Biesinger, M.C., Meriem, G., 2009. Plastics and beaches: a degrading relationship. Mar. Pollut. Bull. 58, 80–84. Dekiff, J.H., Remy, D., Klasmeier, J., Fries, E., 2014. Occurrence and spatial distribution of microplastics in sediments from Norderney. Environ. Pollut. 186, 248–256. ECI (The Essential Chemical Industry), 2017. Poly(ethene) (Polyethylene) [WWW Document]. URL http://www.essentialchemicalindustry.org/polymers/polyethene.html (accessed 9.6.18). ECI (The Essential Chemical Industry), 2018a. Poly(chloroethene) (Polyvinyl chloride) [WWW Document], URL http://www.essentialchemicalindustry.org/polymers/polychloroethene.html (accessed 9.6.18). ECI (The Essential Chemical Industry), 2018b. Polyesters [WWW Document]. URL http://www.essentialchemicalindustry.org/polymers/polyesters.html (accessed 9.6.18). ECI (The Essential Chemical Industry), 2018c. Poly(phenylethene) (Polystyrene) [WWW Document]. URL http://www.essentialchemicalindustry.org/polymers/polyphenylethene.html (accessed 9.6.18). ECI (The Essential Chemical Industry), 2018d. Poly(propene) (Polypropylene) [WWW Document]. URL http://www.essentialchemicalindustry.org/polymers/polypropene.html (accessed 9.6.18). Eriksen, M., Mason, S., Wilson, S., Box, C., Zellers, A., Edwards, W., Farley, H., Amato, S., 2013. Microplastic pollution in the surface waters of the Laurentian Great Lakes. Mar. Pollut. Bull. 77, 177–182.
324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365
Hidalgo-Ruz, V., Gutow, L., Thompson, R.C., Thiel, M., 2012. Microplastics in the Marine Environment: A Review of the Methods Used for Identification and Quantification. Environ. Sci. Technol. 46, 3060–3075. Hu, L., Chernick, M., Hinton, D.E., Shi, H., 2018. Microplastics in Small Waterbodies and Tadpoles from Yangtze River Delta, China. Environ. Sci. Technol. Hueffer, T., Metzelder, F., Sigmund, G., Slawek, S., Schmidt, T.C., Hofmann, T., 2019. Polyethylene microplastics influence the transport of organic contaminants in soil. Sci. Total Environ. 657. Hurley, R.R., Lusher, A.L., Olsen, M., Nizzetto, L., 2018. Validation of a Method for Extracting Microplastics from Complex, Organic-Rich, Environmental Matrices. Environ. Sci. Technol. 52, 7409−7417. Imhof, H.K., Schmid, J., Niessner, R., Ivleva, N.P., Laforsch, C., 2012. A novel, highly efficient method for the separation and quantification of plastic particles in sediments of aquatic environments. Limnol. Oceanogr. Methods 10, 524–537. Jayasiri, H.B., Purushothaman, C.S., Vennila, A., 2013. Plastic litter accumulation on high-water strandline of urban beaches in Mumbai, India. Environ. Monit. Assess. 185, 7709–7719. Kapp, K.J., Yeatman, E., 2018. Microplastic hotspots in the Snake and Lower Columbia rivers: A journey from the Greater Yellowstone Ecosystem to the Pacific Ocean. Environ. Pollut. 241, 1082–1090. Koelmans, A.A., Besseling, E., Foekema, E., Kooi, M., Mintenig, S., Ossendorp, B.C., Redondohasselerharm, P.E., Verschoor, A., Wezel, A.P.V., Scheffer, M., 2017. Risks of Plastic Debris: Unravelling Fact, Opinion, Perception, and Belief. Environ. Sci. Technol. 51, 11513– 11519. Liebezeit, G., Dubaish, F., 2012. Microplastics in beaches of the East Frisian islands Spiekeroog and Kachelotplate. Bull. Environ. Contam. Toxicol. 89, 213–217. Liu, E.K., He, W.Q., Yan, C.R., 2014. ‘White revolution’ to ‘white pollution’—agricultural plastic film mulch in China. Environ. Res. Lett. 9, 091001. Lu, Y., Zhang, Y., Deng, Y., Jiang, W., Zhao, Y., Geng, J., Ding, L., Ren, H.Q., 2016. Uptake and accumulation of polystyrene microplastics in zebrafish (Danio rerio) and toxic effects in liver. Environ. Sci. Technol. 50, 4054. Martin, J., Lusher, A., Thompson, R.C., Morley, A., 2017. The Deposition and Accumulation of Microplastics in Marine Sediments and Bottom Water from the Irish Continental Shelf. Sci. Rep. 7. Mohamed Nor, N.H., Obbard, J.P., 2014. Microplastics in Singapore’s coastal mangrove ecosystems. Mar. Pollut. Bull. 79, 278–283. Moore, E., Lyday, S., Roletto, J., Litle, K., Parrish, J.K., Nevins, H., Harvey, J., Mortenson, J., Greig, D., Piazza, M., 2009. Entanglements of marine mammals and seabirds in central California and the north-west coast of the United States 2001-2005. Mar. Pollut. Bull. 58, 1045–1051. Müller, A., Becker, R., Dorgerloh, U., Simon, F.G., Braun, U., 2018. The effect of polymer aging on the uptake of fuel aromatics and ethers by microplastics. Environ. Pollut. 240, 639–646. Nuelle, M.T., Dekiff, J.H., Remy, D., Fries, E., 2014. A new analytical approach for monitoring microplastics in marine sediments. Environ. Pollut. 184, 161–169. Reddy, M.S., Basha, S., Adimurthy, S., Ramachandraiah, G., 2006. Description of the small plastics
366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389
fragments in marine sediments along the Alang-Sosiya ship-breaking yard, India. Estuar. Coast. Shelf Sci. 68, 656–660. Thompson, R.C., Olsen, Y., Mitchell, R.P., Davis, A., Rowland, S.J., Mcgonigle, D., Russell, A.E., 2004. Lost at Sea: Where Is All the Plastic? Science 304, 838–838. Tsang, Y.Y., Mak, C.W., Liebich, C., Lam, S.W., Sze, E.T., Chan, K.M., 2017. Microplastic pollution in the marine waters and sediments of Hong Kong. Mar. Pollut. Bull. 115, 20–28. UNEP, 2005. UNEP Regional Seas Programme, Marine Litter And Abandoned Fishing Gear. Report to the Division of Ocean Affairs and the Law of the Sea, Office of Legal Affairs, UNHQ. Van, C.L., Vanreusel, A., Mees, J., Janssen, C.R., 2013. Microplastic pollution in deep-sea sediments. Environ. Pollut. 182, 495–499. Wang, J., Peng, J., Tan, Z., Gao, Y., Zhan, Z., Chen, Q., Cai, L., 2017. Microplastics in the surface sediments from the Beijiang River littoral zone: Composition, abundance, surface textures and interaction with heavy metals. Chemosphere 171, 248–258. Xu, B., Liu, F., Brookes, P.C., Xu, J., 2018. Microplastics play a minor role in tetracycline sorption in the presence of dissolved organic matter . Environ. Pollut. 240, 87–94. Zhang, K., Xiong, X., Hu, H., Wu, C., Bi, Y., Wu, Y., Zhou, B., Lam, P.K., Liu, J., 2017. Occurrence and Characteristics of Microplastic Pollution in Xiangxi Bay of Three Gorges Reservoir, China. Environ. Sci. Technol. 51, 3794–3801. Zhao, J., Ran, W., Teng, J., Liu, Y., Liu, H., Yin, X., Cao, R., Wang, Q., 2018. Microplastic pollution in sediments from the Bohai Sea and the Yellow Sea, China. Sci. Total Environ. s 640–641, 637– 645.
390
Figure caption
391
Fig. 1. Sampling locations along the Bohai Bay coast (S1: Dashentang beach, S2:
392
Yongdingxin River estuary, S3: Haihe River estuary, S4: Dagupaiwu River estuary, S5:
393
Duliujian River estuary).
394
Fig. 2.
395
different floatation solutions.
396
Fig. 3. Schematic diagram of the optimized extraction setup. A: solution storage unit (A1:
397
beaker, A2: peristaltic pump), B: air flotation unit (B1: air pump, B2: air flow meter, B3:
398
overflow structure, B4: aeration head), C: vacuum filtration unit (C1: filter, C2:
399
water-circulation vacuum pump).
400
Fig. 4. Recovery of the spiked microplastic particles in sediment samples using different
401
aeration times.
402
Fig. 5. Recovery of microplastic particles, with and without removal of natural organic
403
matter, in soil samples spiked with microplastic particles.
404
Fig. 6. The total numbers of extracted microplastic particles using the optimized approach
405
and the previous approach from different samples.
406
Recovery of six spiked microplastic particles in sediment samples using
407 408
Fig. 1
409 NaCl
NaCl-NaI
Recoveries %
100 80 60 40 20 0
410 411
PP
PE
PET
Fig. 2
412 413 414
415 416
Fig. 3
PVC
PS
EPS
10s
40s
70s
100s
Recoveries %
100 80 60 40 20 0 PP
417
PE
PET
PVC
PS
EPS
Fig. 4
418 419 420 421 422
notnot treated with H2O2 treated with HO 2
treated HO treated withwith H2O2 2
2
2
Recoveries %
100
80
60
40
20
0 PP
PE
PET
423 424 425
Fig. 5
PVC
PS
EPS
426
Soil
Sediment
427 428 429
Fig. 6
430
Table list
431
Table 1. Polymer types, densities, colors, original products and sources of plastic particles
432
for recovery experiments.
433
Table 2. Literature values on mass or volume of samples used for extracting microplastics.
434
Table 3. The sample type, location, morphology, colors, types, sizes and numbers of
435
microplastic particles extracted from different samples using the previous and the
436
optimized approach.
437
438
Table 1 Density Polymer type
(g/cm3)
Color
Original product
Source
PP
0.89-0.91
Dark blue
GC vial cap
Lab
PET
1.29-1.40
Light blue
Water bottle
Local market
PE
0.94-0.97
White
Yoghurt bottle
Local market
PVC
1.3-1.50
Red
Pipe
Local
hardware
store PS
1.04-1.08
Black
Spoon
Local market
EPS
0.015-0.03
White
Styrofoam packaging
Lab
439 440
441
Table 2 Mass or volume of samples
References
30 g
Wang et al., 2017
50 g
Zhao et al., 2018
150-190 g
Corcoran et al., 2009
1 kg
Claessens et al., 2011; Nuelle et al., 2014; Reddy et al., 2006
25 mL
Van et al., 2013
50 mL
Browne et al., 2010
68 mL
Imhof et al., 2012
500 mL
Claessens et al., 2013; Liebezeit and Dubaish, 2012
442
Table 3 Previous approach Sample
Type
Location
Optimized approach
Number Morphology
Color
Type
(items/k
Number Size (mm)
Morphology
Color
Type
g d.w.) Soil
S1
S2
S3
Soil
Sediment
Sediment
Sediment
Jinnan campus, Nankai University
Dashentang beach
Yongdingxin River estuary
Haihe River estuary
fragment
fragment
S4
S5
443
Sediment
Sediment
estuary
Duliujian River estuary
green white, green
PP
75
PP
30
Size (mm)
d.w.) (2.6-0.1)× (0.4-0.1) (6.1-2)× (1.0-0.5)
fragment
fragment
white, green white, green
PP
95
PP
55
(3.2-0.5)× (0.5-0.1) (9.0-0.7)× (0.5-0.1)
fibre
white
PS
10
0.5-1.0
fibre
white
PS
125
0.9-10.0
fibre
white
PS
5
2.0
fragment
white
PP
5
0.5×0.1
PE, PP
195
white,
PE,
green
PP
PS
240
white
PS
295
PP
90
PP
125
fragment fibre
Dagupaiwu River
white,
(items/kg
fragment
white, green white white, green
(4.0-0.2)× (1.0-0.1) 0.3-15.0 (6.0-1.0)× (1.0-0.1)
fragment fibre fragment
white, green
225
fibre
white
PS
100
0.9-8.0
fibre
white
PS
75
fragment
white
PP
5
1.0×0.2
fragment
white
PP
10
fibre
white
PS
5
3.0
fibre
white
PS
5
(6.0-0.8)× (1.8-0.1) 1.1-12.0 (9.0-0.7)× (1.0-0.1) 1.1-12.0 (2.0-0.5)× (0.8-0.2) 3.1
Highlights:
1. An optimized approach for extracting microplastics was proposed. 2. The mixture of NaCl and NaI was used as floatation solution. 3. Aeration was used instead of stirring by hand. 4.The optimization was validated by spiking and comparison experiments.
Conflict of interest The authors declare no conflict of interest.