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A Procedure for Measuring Microplastics using Pressurized Fluid Extraction Stephen George Fuller, and Anil Gautam Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00816 • Publication Date (Web): 12 May 2016 Downloaded from http://pubs.acs.org on May 13, 2016
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Environmental Science & Technology
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A Procedure for Measuring Microplastics
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using Pressurized Fluid Extraction
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Stephen Fuller* and Anil Gautam
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Environmental Forensics, Office of Environment and Heritage, Lidcombe, NSW
5
2141, Sydney, Australia
6
KEYWORDS: Microplastics, pressurized fluid extraction, Fourier transform infrared
7
spectroscopy (FTIR), municipal waste,
8
ABSTRACT: A method based on Pressurized Fluid Extraction (PFE) was developed
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for measuring microplastics in environmental samples. This method can address some
10
limitations of the current microplastic methods and provide laboratories with a simple
11
analytical method for quantifying common microplastics in a range of environmental
12
samples. The method was initially developed by recovering 101% to 111% of spiked
13
plastics on glass beads and was then applied to a composted municipal waste sample
14
with spike recoveries ranging from 85% to 94%. The results from municipal waste
15
samples and soil samples collected from an industrial area demonstrated that the
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method is a promising alternative for determining the concentration and identity of
17
microplastics in environmental samples.
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1. INTRODUCTION
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The proliferation of plastics material is an issue of increasing global concern, as there
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is mounting evidence of the risks posed by microplastics in the environment1-5.
22
Microplastics have been reported in marine waters9-12, freshwaters13-16, marine
23
species17-20, and some terrestrial environments21,
24
knowledge on the distribution of microplastics in these environments, particularly
25
terrestrial landmasses.6-8. In these studies, a wide variety of methods have been used
26
to identify and quantify microplastics which can make comparisons of large-scale
27
results difficult23. Experts have identified the need for an established, uniform method
28
for measuring and reporting microplastics in environmental materials23-25.
29
Whilst plastic fragments larger than 1 mm can be recovered and identified in a
30
straightforward manner, with current methods the assay of plastic particles smaller
31
than 1 mm is more problematic. Existing methods for the assay of microplastic
32
fragments typically involve either a floatation or density separation step using dense
33
salt solutions17, 26. Whilst these methods have gained common usage, the procedures
34
can suffer from a limited scope of plastics types and poor collection efficiencies.
35
Alternative procedures involving specialized equipment and denser salts (NaI and
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ZnCl2)17, 27 have been used for sediments with some success but the denser salts are
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expensive and samples with high organic content, such as wastewater, have been
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reported to cause problems25. Enzymatic digestion, peroxide digestion and acid
39
digestion have been reported with varying degrees of success for high organic content
40
materials such as biota and wastewater25, 28. Many of these methods report numbers of
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fragments of plastic which is important information from a risk assessment
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perspective but can be difficult to relate to a concentration23, 29. The size of plastic
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particles assayed in sediments seem to have a practical limit of approximately 20-40
22
. Despite this there is a lack of
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microns17, 30. Microplastic particles smaller than 20 microns are of concern as they
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can be more readily incorporated in the biosphere through ingestion and other
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mechanisms30. There is also evidence that microplastics particles can absorb chemical
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pollutants, complicating an assessment of their environmental impacts31.
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The application of current assays for the plastic content in complex matrices such as
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industrial and domestic wastes are likely to be problematic. Fine plastic particles can
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be unintentionally generated and sequestered into materials through grinding and
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homogenizing, particularly in large scale composting processes. The fine plastics are
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incorporated into the material making it difficult to quantify and characterize. There is
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a need from environmental regulators for a robust analytical method with a uniform
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reporting unit that can quantitatively determine microplastics in environmental
55
materials. Such a method would facilitate the monitoring of marine and terrestrial8
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microplastics and identify priority areas for investigations.
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Pressurized Fluid Extraction (PFE) is a technique that uses solvents at sub-critical
58
temperature and pressure conditions, principally for the recovery of semi-volatile
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organics from solid materials. The technique has achieved acceptance as a standard
60
extraction technique (US EPA Method 3545A)32 and is commonly utilized in
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environmental laboratories for the extraction of organic pollutants from soils,
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sediments and wastes. The authors considered that by optimizing PFE conditions, it
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may be possible to quantitatively extract and analyze plastics from environmental
64
samples. A technique based on the extraction of plastics through either partially
65
emulsifying or solubilizing plastics would have a significant advantage over existing
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physical separation procedures as the process would be unlikely to be affected by
67
particle sizes. Smaller particles would be expected to be more readily extracted with
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such a process, theoretically enabling the assay of even sub-micron particles. This
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work presents method performance characteristics and two case studies using the
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developed method for the measurement and identification of microplastics.
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2. EXPERIMENTAL
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The plastic materials used were high density polyethylene (HDPE) (Aldrich # 434272,
73
USA),
74
(Aldrich 200557, USA) and poly (vinyl chloride) (PVC) (Aldrich #18621-25G, USA).
75
The materials were powders typically 50 microns in diameter with the exception of
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PS which had a mean size of approximately 1 mm. Other plastics used were
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polyethylene terephthalate (PET), and Polypropylene (PP) which were obtained from
78
various plastic packaging materials and containers. The materials were cut and filed
79
or ground, and sieved through a 1 mm sieve. The identity of the materials was
80
confirmed by a Nicolet 6700 FTIR spectrophotometer (Thermo, USA) prior to use.
81
These five plastics were selected as test materials as they account for approximately
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80% of consumed plastics and over 90% of non-recycled plastics, based on Australian
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usage data33.
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A municipal solid sample collected from a waste facility in Sydney was also used for
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method validation. The facility collects and processes household waste and typically
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uses separation, grinding, pasteurization and stabilization. A number of soil samples
87
collected near an industrial site were also used for a case study.
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A pressurized fluid extractor (Dionex ASE-350, USA) with 34mL stainless steel cells
89
was used. High temperature viton o-rings (Dionex # 056325, USA) were used in the
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end caps. Glass fiber filters (Dionex # 056781, USA) were used in the cell base. High
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purity solvents such as methanol, hexane and dichloromethane (Suprasolv, Merck,
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Germany) were evaluated. Except where it is mentioned, samples were extracted
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using solvent, temperature pressure and other parameters as outlined in Table 1. The
polystyrene-block-poly
(ethylene-ran-butylene)-block-polystyrene
(PS)
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solvents were removed from the extract residues by evaporation under a stream of
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nitrogen (Reacti-Therm, Thermo, US). Dried residues were examined by a Nicolet
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6700 FTIR bench equipped with a Smart iTR (Thermo, USA) for identification of
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plastic type. The single bounce ATR consisted of a diamond ATR crystal with ZnSe
98
lens (active sample area of 1.5 mm2, penetration depth of 2.03 microns at 1000 cm-1).
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Spectral information was collected over a wavenumber range from 4000 cm-1 to 600
100
cm-1 (32 scans at 4 cm-1 resolution). The identity of plastic type was accomplished by
101
comparing the sample spectrum against the library spectra using a correlation
102
algorithm in OMNIC 8.2 software (Thermo, USA). The software provides match
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values ranging from 0 (no match) to 100 (match).
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A combustion method based on Australian Standard 1038.8.1 was used to determine
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total chlorine in the soils, semi-volatile organochlorine compounds were determined
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using GC-MS based on USEPA Methods 8270, and soluble chloride was determined
107
using ion chromatography based on APHA Method 4110.
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Standard laboratory quality control procedures were followed for the case studies,
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including spikes, replicates and method blanks.
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3. RESULTS
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3.1. Method development using glass beads and municipal waste material
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Instrument conditions were optimized using plastic spiked onto glass beads (4 mm,
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Crown Scientific Australia). The typical cell loading was 10-20 mg of plastic material
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and approximately 40 g of glass beads. After evaluating various solvents (water,
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acetone, hexane, DCM) and temperatures in the range of 60 oC - 200 oC, a two-step
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extraction procedure was developed. An initial extraction step was devised using
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methanol at a temperature of 100 oC to remove all the extractable semi-volatile 5 ACS Paragon Plus Environment
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organic compounds such as fats and oils. Extracts recovered from the methanol
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extraction were found to contain none of the spiked plastics. The second extraction
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used dichloromethane to recover the microplastic fraction. Pressurized fluid extraction
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using dichloromethane at a cell temperature of 100 ºC and a pressure of 1500 PSI as
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specified in USEPA method 3545A32 was found to quantitatively recover PET and PS
123
from glass beads, however PP and PE yielded poor recoveries. Optimum extraction
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temperatures for these plastics was determined to be 180 to 190 ºC. An extraction
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temperature as low as 160 oC was sufficient to extract PE, however long term usage
126
under these conditions with a high sample loading of PE can cause blocking of a 100
127
micron frit in the static valve. The use of higher extraction temperatures of 180-190
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o
129
The optimized extraction conditions are presented in Table 1. Slight variations in
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instrument parameters did not have a significant effect on the performance of the
131
extraction.
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Table 1: Optimized extraction conditions for PFE
C and reduced loading of plastic material prevented this problem occurring.
Parameter
Pre-extraction
Micro-plastics extraction
Extraction solvent (ºC)
Methanol
Dichloromethane
Extraction temperature (ºC)
100
180
Heating time (min)
5
9
Static time (s)
5
5
Cycles
3
2
Rinse volume (%)
45
80
Purge time (s)
75
75
System rinse volume (mL)
15
15
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The optimized extraction conditions were used to recover the spiked plastics from the
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glass beads. The collected DCM extracts were evaporated to dryness and were 6 ACS Paragon Plus Environment
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measured gravimetrically. Recovery results, as shown in Table 2, shows greater than
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100% recovery for many of the samples indicating positive interference, however
137
results for laboratory control blanks showed average microplastic content of 0.09 mg
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(SD = 0.17, n=3). It is possible, the plastics may have encapsulated some of the
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solvent and caused greater than 100% recovery but the recoveries were within typical
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environmental laboratory acceptance criteria of 80%-120% for organic analytes and
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therefore considered satisfactory.
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Table 2: Recovery data using spiked glass beads Plastic type
HDPE
PP
PVC
PS
PET
Spike amount (mg)
Average Recovery
Recovered amount (mg)
(%)
11.8
12.4
49.9
48.3
12.9
13.0
49.4
45.1
37.4
44.0
7.7
12.7
36.3
42.6
8.1
13.2
19.5
18.5
47.1
50.3
25.6
19.8
49.0
51.3
17.9
19.2
10.30
10.81
20.8
20.4
11.48
11.63
20.8
20.9
42.2
54.8
24.3
22.2
46.5
61.9
102
101
111
107
111
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The recovered plastic residues were analyzed by FTIR to confirm their identity. The
144
residues were different in appearance to the spiking material indicating changes
145
during hot solvent dissolution/emulsification and reprecipitation process34,
146
FTIR spectra of the residues were found to be similar to the original spiking material
147
and could readily be identified using a standard FTIR database, indicating that no
148
significant chemical changes has occurred during the extraction processes. Evidence
35
. The
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original spike material is presented in Figure 1. 0.2 PE Standard 0.2 PE Extracted at 195 °C 0.2 PET Standard PET Extracted at 195 °C 0.1 -0.0 0.2 PP Standard 0.05 PP Extracted at 195 °C 0.00
Abs Abs
0.1
Abs
PVC Standard
0.1
0.2 PS standard
Abs
Abs
Abs
Abs
Abs
150
Abs
for the spectral similarity between the extracted plastic residue at 195 oC and the
Abs
149
0.2
PVC Extracted at 195 °C
PS Extracted at 195 °C 3500
3000
153
2000
1500
1000
Wavenumbers (cm-1)
151 152
2500
Figure 1: Comparison of FTIR spectra of extracted plastic with the spike material. 3.2.Method validation using a municipal waste sample
154
The optimized extraction procedure was validated on a sample collected from a
155
municipal waste processing facility. Some characteristics of the waste material are
156
presented in Table 3.
157
Table 3: Composted waste material characteristics (typical range) Moisture content (% w/w)
22-30
Ash content (% w/w, dry weight)
30-45
Oil and grease (g/kg, dry weight)
28-70
158
Prior to the assay for microplastics the material was dried, sieved to remove particles
159
greater than 1 mm, then ground with a mortar and pestle to promote homogeneity
160
prior to subsampling. The sample size extracted was 2 g. The remaining space in the 8 ACS Paragon Plus Environment
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cell was filled with glass beads. The collected dichloromethane extracts were
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evaporated to dryness, rinsed with methanol (washing discarded), weighed and
163
analyzed by FTIR for plastic identification. Replicate analysis of the dried residue
164
found an average microplastics concentration of 23000 mg/kg (SD =2000, n=7). A
165
typical FTIR spectrum of the residue from a waste processing facility is shown in
166
Figure 2. The residue was typically found to be an amorphous material that can break
167
up into flakes. FTIR traces from different spots on the material indicated homogeneity
168
of the extracted plastic. Each FTIR trace is collected from an area of 1.5 mm2, defined
169
by the area of the diamond crystal. Additional tests to confirm the residue as plastic
170
were loss on ignition and optical microscopy. These results and micrographs of a
171
typical plastic residue are included in the supporting information document. Municipal Waste replicate 1
Abs
0.3
0.2
0.1
0.20 Municipal Waste replicate 2
Abs
0.15
0.10
0.05
0.5 Municipal Waste replicate 3
Abs
0.4 0.3 0.2 0.1 3500
172
3000
2500
2000
1500
1000
Wavenumbers (cm-1)
173
Figure 2: FTIR spectra of various areas of the extract from a waste sample
174
A more detailed analysis of the FTIR spectra (Figure 3) indicated the presence of
175
polystyrene and polycarbonate in the samples. The detection of polycarbonate
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indicated that the method may be applicable to a wider range of plastic materials than
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initially validated. Microplastic in municipal waste material Abs
0.4
0.2
Abs
0.3 PS standard 0.2 0.1
PS substracted from sample Abs
0.2
0.1
0.0
Abs
0.4 Polycarbonate standard
0.2
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1)
178 179 180
Figure 3: FTIR spectra of the extracted plastic from a composted waste collected from a waste processing facility
181
The precision and accuracy of the analytical method was verified by spiking the
182
municipal waste material with known amounts of selected plastics (10-50 mg).
183
Recoveries were estimated by subtracting the previously calculated total microplastics
184
content of the material from the spiked amount. The recovery results are shown in
185
Table 4. Average recoveries were found to be greater than 80% in municipal waste
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material, which contained a wide range of chemical and physical contaminants
187
including fats and oils, organic carbon, heavy metals, cations, anions, plastics and
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glass.
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Table 4: Spike results from a municipal waste Plastic type
HDPE
PP
PVC
PS
PET
191 192 193
Spike amount (mg)
Average Recovery
Recovered amount (mg)
(%)
9.44
10.29
30.21
25.75
9.48
9.8
23.87
18.80
13.15
12.50
8.55*
8.07
12.81
9.01
4.43*
6.73
37.44
34.63
30.45
36.24
36.48
35.40
27.16
31.05
47.95
45.86
26.87
43.64
48.54
45.85
20.31
37.07
35.91
42.29
28.10
34.04
35.99
44.26
21.08
25.77
87
84
94
90
89
*, low recovery in this sample was considered a random error and not used for average recovery
3.3.Case study –microplastic in soils collected from an industrial area
194
The validated method was used in a case study with the purpose of identifying the
195
source of very high chlorine levels in soil samples collected from around a Sydney
196
industrial area. A field fluorescence assay indicated high levels of chlorine, however
197
semi-volatile organic testing found no organochlorine compounds in the samples.
198
Further analysis of the soils for total chlorine using Eschka mixture method yielded
199
concentrations up to 30000 mg/kg whereas water soluble inorganic chloride values
200
were less than 100 mg/kg. This indicated that the high chlorine may be due to the
201
presence of non-volatile organic chlorine compounds. As the industrial site was
202
historically used to produce chlorinated plastics, the collected soil samples were
203
analysed using the developed microplastic method. The soils samples were dried at 40
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°C overnight, sieved through 1 mm sieve and stored at 80% library match) in most of the soil samples. The residue FTIR
211
spectra for two of the soil samples indicated the presence of multiple plastic types (PE
212
and PVC and PS and PVC). FTIR spectral comparison is included in the supporting
213
document. Microplastic in the soil from an industrial area 0.10
Abs
0.08
0.06
0.04
0.02
0.08 PVC Standard 0.07
Abs
0.06 0.05 0.04 0.03 0.02 0.01 3500
214
3000
2500
2000
1500
1000
Wavenumbers (cm-1)
215 216
Figure 4: Typical FTIR spectral comparison between the plastic extracted from the sample and a PVC standard
217
A summary of microplastic concentrations measured in a number of soil samples is
218
presented in Table 5 . The soil samples were found to contain 0.03% to 6.7% of
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microplastic. The microplastic concentrations were related to the vicinity of sample
220
locations to the industrial area.
221
Table 5 also shows non-volatile chlorine results that was calculated using results from
222
total chlorine (combustion method) and water soluble chloride (ion chromatography
223
method). The non-volatile chlorine results were found to correlate with microplastic
224
results as shown in Figure 5. The good correlation between two different methods
225
provides an additional evidence for the validity this method. The slope of the
226
correlation curve is also consistent with an average chlorine content for common
227
PVCs based on an assumption that PVC contains 50-60 % (w/w) chlorine36.
228
Table 5: Microplastic results for soil samples collected from an industrial area
Sample
Microplastics (mg/kg)
Total Chlorine (mg/kg)
Water soluble chloride (mg/kg)
Non-volatile chlorine Calculated5
Soil 1 8001 300 14 4 Soil 2 2300 900 12 888 Soil 3 9003 500 11 489 2 Soil 4 500 200 12 Soil 5 24004 700 15 685 Soil 6 5003 400 28 372 4 Soil 7 9200 4700 17 4683 Soil 8 31004 1200 6 1194 4 Soil 9 13400 9100 12 9088 4 Soil 10 7700 5800 19 5781 Soil 11 44004 1700 34 1666 4 Soil 12 10400 5800 40 5760 4 Soil 13 67500 30000 11 29989 Soil 14 3003 600 24 576 3 Soil 15 700 1100 28 1072 Soil 16 10004 700 11 689 4 Soil 17 6900 4500 15 4485 1: FTIR spectral matching identified the presence of polyethylene in the sample. 2: FTIR spectral matching identified the presence of polystyrene and PVC in the sample. 3: FTIR spectral matching identified trace amount of PVC in these samples 4: FTIR spectral matching identified microplastic as PVC 5: Non-volatile chlorine = total chlorine - volatile and semi-volatile organic chlorinated compounds -water soluble inorganic chloride. Semi-volatile organic chlorinated
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compounds have not been reported however they were found to be insignificantly small (< 2 mg/kg)
229 230
Relationship between microplastic content and non-volatile organic chlorine 35000
Total Chlorine (mg/L)
30000
y = 0.4611x R² = 0.9773
25000 20000 15000 10000 5000 0 0
10000
20000
30000
40000
50000
60000
70000
80000
Microplastic content (mg/L)
231 232
Figure 5: Correlation between the amount of microplastic and non-volatile organic
233
chlorine in the soil samples
234
4. DISCUSSIONS
235
The results presented in this work provide evidence that a PFE based extraction
236
method can be used to quantify and identify microplastics in a variety of
237
environmental materials. The method is likely to be applicable to other plastic types
238
as the preliminary results indicated that polycarbonate and polyurethane foam (PUF)
239
can also be extracted using this method. This method was demonstrated to be
240
applicable to municipal waste and soils, but it is likely to be applicable to a variety of 14 ACS Paragon Plus Environment
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other environmental matrices with minor optimization and verification of
242
performance. The PFE technique may also be applicable to liquid matrices by an
243
initial filtering step through glass fiber filters with subsequent PFE extraction of the
244
filters, however this approach is yet to be verified.
245
Some of the benefits of this technique include simplicity, cost, speed, and uniformity
246
in reporting concentration results. The extraction component of the method can be
247
fully automated, minimizing operator skill requirements. The sensitivity of the
248
method is expected to be limited by the precision and accuracy of gravimetric
249
techniques. The authors found that a few milligrams could be accurately measured,
250
therefore by using larger sample amounts through multiple cell extractions or larger
251
volume extraction cells the method should realize part per million sensitivity. The
252
method results are directly presented as a concentration which is a more standardized
253
basis for comparison of data than particle numbers. The method will efficiently
254
extract plastic particles less than 30 microns, which can be difficult to measure by
255
floatation and other physical separation procedures. The recovered residue is
256
amenable to further characterization by a range of techniques such as FTIR, GC
257
pyrolysis, thermogravimetric analysis and differential scanning calorimetry.
258
Some limitations to this method are the inability to measure size fractions of
259
microplastics in samples, however it is possible to separate material into different size
260
fractions using sieves and apply the PFE technique to those size fractions. The
261
extraction procedure obliterates the morphology of microplastic particles, therefore
262
physical characterization tests to assist with source attribution will not be possible on
263
the extract residues. Similarly, samples with multiple types of microplastics will
264
produce residues with complex FTIR spectra that may require skills in spectral
265
deconvolution to identify constituent polymers or required advanced techniques for
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identification. Additional work is necessary to verify the applicability of the method
267
to resins, adhesives and other plastics.
268
In conclusion, a simple pressurized fluid extraction method has been successfully
269
evaluated and applied to the assay of five common thermoplastics in glass beads, soil
270
and municipal waste. The method recovery and precision were deemed satisfactory to
271
realize good quantitative data. The concentration, number, type and morphology of
272
plastics are important information in the assessment of microplastic pollution. This
273
PFE method is envisaged to be complementary to existing methods to deliver these
274
information requirements.
275
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AUTHOR INFORMATION
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Corresponding Author
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*Stephen Fuller
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Building 1, 480 Weeroona Rd, Lidcombe, NSW, Australia
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Phone: 61 2 9995 5071
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Fax: 61 2 99955004
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E-mail:
[email protected] 393
Author Contributions
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The manuscript was written through contributions of all authors. All authors have
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given approval to the final version of the manuscript.
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Funding Sources
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The authors would like to acknowledge New South Wales (NSW) Government,
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Office of Environment and Heritage, Environmental Forensic section for their
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support.
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Notes
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The authors declare no competing financial interest
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ACKNOWLEDGMENT
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The authors would like to acknowledge New South Wales (NSW) Government,
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Office of Environment and Heritage, Environmental Forensic section for their
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support.
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SUPPORTING INFORMATION
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This information is available free of charge via the Internet at http://pubs.acs.org
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ABBREVIATIONS
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SYNOPSIS
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An alternative method based on Pressurized Fluid Extraction (PFE) technique was
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developed for measuring microplastics in environmental samples.
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Table of Contents/Abstract Graphics
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PHOTOS TAKEN BY ANIL GAUTAM
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