A Procedure for Measuring Microplastics using Pressurized Fluid

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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

2

using Pressurized Fluid Extraction

3

Stephen Fuller* and Anil Gautam

4

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

9

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

16

method is a promising alternative for determining the concentration and identity of

17

microplastics in environmental samples.

18

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1. INTRODUCTION

20

The proliferation of plastics material is an issue of increasing global concern, as there

21

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

36

ZnCl2)17, 27 have been used for sediments with some success but the denser salts are

37

expensive and samples with high organic content, such as wastewater, have been

38

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

41

fragments of plastic which is important information from a risk assessment

42

perspective but can be difficult to relate to a concentration23, 29. The size of plastic

43

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

45

can be more readily incorporated in the biosphere through ingestion and other

46

mechanisms30. There is also evidence that microplastics particles can absorb chemical

47

pollutants, complicating an assessment of their environmental impacts31.

48

The application of current assays for the plastic content in complex matrices such as

49

industrial and domestic wastes are likely to be problematic. Fine plastic particles can

50

be unintentionally generated and sequestered into materials through grinding and

51

homogenizing, particularly in large scale composting processes. The fine plastics are

52

incorporated into the material making it difficult to quantify and characterize. There is

53

a need from environmental regulators for a robust analytical method with a uniform

54

reporting unit that can quantitatively determine microplastics in environmental

55

materials. Such a method would facilitate the monitoring of marine and terrestrial8

56

microplastics and identify priority areas for investigations.

57

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

59

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

61

environmental laboratories for the extraction of organic pollutants from soils,

62

sediments and wastes. The authors considered that by optimizing PFE conditions, it

63

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

66

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

68

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

70

developed method for the measurement and identification of microplastics.

71

2. EXPERIMENTAL

72

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

76

PS which had a mean size of approximately 1 mm. Other plastics used were

77

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

82

80% of consumed plastics and over 90% of non-recycled plastics, based on Australian

83

usage data33.

84

A municipal solid sample collected from a waste facility in Sydney was also used for

85

method validation. The facility collects and processes household waste and typically

86

uses separation, grinding, pasteurization and stabilization. A number of soil samples

87

collected near an industrial site were also used for a case study.

88

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

91

purity solvents such as methanol, hexane and dichloromethane (Suprasolv, Merck,

92

Germany) were evaluated. Except where it is mentioned, samples were extracted

93

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

95

nitrogen (Reacti-Therm, Thermo, US). Dried residues were examined by a Nicolet

96

6700 FTIR bench equipped with a Smart iTR (Thermo, USA) for identification of

97

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).

99

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

103

values ranging from 0 (no match) to 100 (match).

104

A combustion method based on Australian Standard 1038.8.1 was used to determine

105

total chlorine in the soils, semi-volatile organochlorine compounds were determined

106

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,

109

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,

115

acetone, hexane, DCM) and temperatures in the range of 60 oC - 200 oC, a two-step

116

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

120

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

122

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

124

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

128

o

129

The optimized extraction conditions are presented in Table 1. Slight variations in

130

instrument parameters did not have a significant effect on the performance of the

131

extraction.

132

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

133

The optimized extraction conditions were used to recover the spiked plastics from the

134

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

136

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

138

(SD = 0.17, n=3). It is possible, the plastics may have encapsulated some of the

139

solvent and caused greater than 100% recovery but the recoveries were within typical

140

environmental laboratory acceptance criteria of 80%-120% for organic analytes and

141

therefore considered satisfactory.

142

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

143

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

162

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

177

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

186

material, which contained a wide range of chemical and physical contaminants

187

including fats and oils, organic carbon, heavy metals, cations, anions, plastics and

188

glass.

189


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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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266

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]

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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


A Procedure for Measuring Microplastics using Pressurized Fluid

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