Microplastic Pollution in Table Salts from China

Abstract

Microplastics have been found in seas all over the world. We hypothesize that sea salts might contain microplastics because they are directly supplied by seawater. To test our hypothesis, we collected 15 brands of sea salts, lake salts and rock/well salts from supermarkets throughout China. The microplastics content was 550-681 particles/kg in sea salts, 43-364 particles/kg in lake salts and 7-204 particles/kg in rock/well salts. In sea salts, fragments and fibers were the prevalent types of particles compared with pellets and sheets. Microplastics measuring less than 200 μm represented the majority of the particles, accounting for 55% of the total microplastics, and the most common microplastics were polyethylene terephthalate, followed by polyethylene and cellophane in sea salts. The abundance of microplastics in sea salts was significantly higher than that in lake salts and rock salts. This result indicates that sea products, such as sea salts, are contaminated by microplastics. To the best of our knowledge, this is the first report on microplastic pollution in abiotic sea products.

Full text

Microplastic Pollution in Table Salts from China
Dongqi Yang,
†
Huahong Shi,*
,†
Lan Li,
‡
Jiana Li,
†
Khalida Jabeen,
†
and Prabhu Kolandhasamy
†
†
State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China
‡
Research Center for Analysis and Measurement, Donghua University, Shanghai 201620, China
*
SSupporting Information
ABSTRACT: Microplastics have been found in seas all over the
world. We hypothesize that sea salts might contain microplastics,
because they are directly supplied by seawater. To test our hypothesis,
we collected 15 brands of sea salts, lake salts, and rock/well salts from
supermarkets throughout China. The microplastics content was 550−
681 particles/kg in sea salts, 43−364 particles/kg in lake salts, and 7−
204 particles/kg in rock/well salts. In sea salts, fragments and fibers
were the prevalent types of particles compared with pellets and
sheets. Microplastics measuring less than 200 μm represented the
majority of the particles, accounting for 55% of the total
microplastics, and the most common microplastics were polyethylene
terephthalate, followed by polyethylene and cellophane in sea salts.
The abundance of microplastics in sea salts was significantly higher
than that in lake salts and rock/well salts. This result indicates that sea products, such as sea salts, are contaminated by
microplastics. To the best of our knowledge, this is the first report on microplastic pollution in abiotic sea products.
1. INTRODUCTION
Marine debris has become a global concern due to its vast and
growing threat to the marine and coastal environment.
1,2
The
degradation of large, individual plastic items ultimately leads to
millions of microplastic pieces, which are likely the most
numerous plastic debris in the ocean today.
3
Microplastics are
defined as plastic materials or fragments measuring less than 5
mm and have been found not only in the marine environments
but also in rivers, lakes, and even in ice.
4−9
Several recent
studies showed that the coast of China is a hotspot of
microplastic pollution.
2,10−14
Sea products are among the main sources of food for human
beings. The pollution of microplastics in the sea will
undoubtedly lead to the pollution of sea products. Microplastics
can even be transferred from sea products to humans through
the food chain, which increases the potential health risks to
humans. Recently, reviews by Seltenrich
15
and Bouwmeester et
al.
16
highlighted the importance of investigating the potential
risk of transferring microplastics from the food chain to
humans. Microplastics have been detected in a large variety of
marine organisms, such as mussels and fish.
7,14
To date,
however, there has been no report on microplastic pollution in
abiotic sea products. Table salts provide essential elements for
humans. Salt materials primarily come from the sea, saline lakes,
saline rocks, and saline wells. Subsequently, table salts can be
classified into sea salts, lake salts, rock salts, and well salts
according to their sources. However, rock and well salts are
typically labeled as rock/well on the packages of table salts and
are regarded as the same type in markets in China.
Sea salt is typically produced by crystallization due to the
combined effects of wind and sunlight. Before sea salt
crystallizes, seawater circulates along a series of successive
ponds with increasing levels of salinity due to the continuous
evaporation of water. Sea salts may contain anthropogenic
contaminants present in the seawater if they remain after the
concentration and crystallization processes.
17
Therefore, it is
necessary to monitor the presence of contaminants in sea salts.
Seawater is widely polluted by microplastics; thus, we
hypothesize that sea salts might contain microplastics.
To test this hypothesis, we collected different brands of sea
salts from random supermarkets throughout China. We also
collected lake salts and rock/well salts for comparisons. The
abundance, type, and composition of the microplastics were
measured and analyzed.
2. MATERIALS AND METHODS
2.1. Collection of Table Salts. Fifteen brands of table salts
were collected from supermarkets in China during October and
November of 2014. These salts represent three main types of
table salts according to their source in China. An average
package with a weight range from 240 to 500 g was chosen.
Each type of salt originates from a different location
(Supporting Information (SI) Figure S1). One blank extraction
group without salt was tested simultaneously to correct the
potential procedural contamination. Three replicate packages
were used to compare among different brands of the same type
Received: June 30, 2015
Revised: October 11, 2015
Accepted: October 20, 2015
Published: October 20, 2015
Article
pubs.acs.org/est
© 2015 American Chemical Society 13622 DOI: 10.1021/acs.est.5b03163
Environ. Sci. Technol. 2015, 49, 13622−13627

of salt. Five replicate brands were used for the comparison
among the different types.
2.2. Quality Control of the Experiments. To avoid
contamination, all of the liquid (tap water and hydrogen
peroxide) was filtered using 1 μm pore size filter paper prior to
use. All containers and beakers were rinsed three times with
filtered water. The samples were immediately covered when
they were not in use. All the experimental procedures were
finished as soon as possible.
2.3. Hydrogen Peroxide Treatment. Approximately
240−250 g of table salts from one package of salts was directly
placed into a 1 L glass bottle at a height of 35 cm. The sample
in one bottle was regarded as a replicate, and four replicate
bottles were prepared for each brand. Approximately 100 mL of
30% H2O2was added to each bottle to digest the organic
matter. The bottles were covered and placed in an oscillation
incubator at 65 °C at 80 rpm for 24 h and then at room
temperature for 48 h.
2.4. Floatation and Filtration. Approximately 800 mL of
filtered water was added to each bottle. A glass rod was used to
stir the salts in the bottle until they were completely dissolved.
The salt solution in one bottle of the four replicates was
immediately transferred onto a piece of 5 μm pore size, 47 mm
cellulose nitrate filter paper using a vacuum system. The filter
paper was then placed into a clean Petri dish with a cover and
was dried at room temperature to observe the total number of
particles. The three other replicate bottles containing the salt
solution were covered with glass lids and held overnight. The
supernatants of the salt solutions were transferred onto 5 μm
pore size, 47 mm cellulose nitrate filter papers. The filter papers
were placed in clean Petri dishes with covers and were dried at
room temperature for further microplastic analysis. The
material deposited at the bottom of the bottles was also
transferred into other Petri dishes for microscopic observation.
2.5. Visual Observation of Microplastics under a
Microscope. The filters were observed under a Carl Zeiss
Discovery V8 Stereomicroscope (MicroImaging GmbH,
Göttingen, Germany), and images were obtained with an
AxioCam digital camera. A visual assessment was performed to
identify the types and colors of microplastics according to the
physical characteristics of the particles. Some particles were
randomly selected for verification using micro-Fourier Trans-
form Infrared Spectroscopy (μ-FT-IR). The abundance of
microplastics was calculated based on the microscopic
observation and was confirmed with μ-FT-IR.
2.6. Identification of Microplastics with μ-FT-IR. The
plastic-like particles on the filter paper were randomly selected
for μ-FT-IR analysis (Thermo Nicolet iN10 MX) in trans-
mittance mode. The spectrum range was set to 4000−675 cm−1
with a collection time of 3 s and with 16 coscans for each
measurement. The spectral resolution was 8 cm−1for all
samples, and the aperture size ranged from 50 ×50 μm to 150
×150 μm, depending on the size of the particles. The 15 table
salt packages were also identified using a μ-FT-IR microscope
in attenuated total reflection mode. All spectra were collected at
a resolution of 8 cm−1using a diamond MicroTip accessory
from 4000 to 675 cm−1, with a collection time of 3 s and with
16 coscans.
All the spectra were then compared with the library
(Hummel Polymer and Additives, Polymer Laminate Films)
to verify the polymer type. The spectrum analysis followed the
method of Woodall et al.
18
Briefly, matches with a quality index
≥0.7 were accepted. Matches with a quality index <0.7 but ≥0.6
were individually inspected and interpreted based on the
proximity of their absorption frequencies to those of chemical
bonds in the known polymers. Matches with a quality index
<0.6 were rejected.
18
2.7. Data Analysis. The mean differences of the abundance
of microplastics among groups were determined by one-way
analysis of variance (ANOVA) followed by Tukey’s HSD test
(homogeneous variances) or the Tamhane-Dunnett test
(heterogeneous variances), along with multiple comparisons.
A 0.05 significance level was chosen.
3. RESULTS
3.1. Particles in Table Salts. In the process of handling the
samples, contamination with airborne microplastics was
prevented. The procedural blanks only contained 4.4 ±2.1
particles/filter of microplastics, which is equal to 18 particles/
kg when the average weight of salt (240−250 g) in each bottle
is considered in the control group. When the entire salt
solution was filtered, the color of the filter paper for sea salts
was much darker than the filter papers for lake salts and rock/
well salts (Figure 1A−C). When the supernatants of the salt
solutions were filtered, the particle density on the filter papers
for sea salts was higher than the particle density on the filter
papers for lake salts and rock/well salts (Figure 1D−F).
Multiple types of particles, including fibers, fragments, and
pellets, occurred in table salts (Figure 1D−F). The most
diverse colors were observed in the fibers followed by the
fragments. The most common colors were black, red, blue, and
white. Some sand particles were also found at the bottom of the
bottles (Figure 1G−I).
3.2. Abundance, Type, and Size of Microplastics in
Table Salts. The number of microplastics was 550−681
particles/kg in sea salts, 43−364 particles/kg in lake salts, and
7−204 particles/kg in rock/well salts (Figure 2A−C). Different
brands showed no significant differences in the abundance of
microplastics in sea salts (p> 0.05) (Figure 2A). Microplastic
Figure 1. Photographs of the total particles isolated from table salts.
A−C, the particles in the salt solution without separation; D−F, the
particles in the supernatant of the salt solutions. More particles were
observed in sea salts (D) than lake salts (E) and rock/well salts (F);
G−I, the particles at the bottom of the bottle after removal of the
supernatant. Scale bar = 10 mm (A−C) or 0.2 mm (D−I).
Environmental Science & Technology Article
DOI: 10.1021/acs.est.5b03163
Environ. Sci. Technol. 2015, 49, 13622−13627
13623

content was less in L3 than in L2 and L4 in lake salts (Figure
2B), and it was less in R/W2 and R/W4 than in R/W5 in rock/
well salts group (p< 0.05) (Figure 2C). The average
concentration of microplastics in sea salts was approximately
3 times higher than that in lake salts and 7 times higher than
that in rock/well salts (Figure 2D) (p< 0.05). No significant
differences for the abundance were observed among the
control, lake salt, and rock/well salt groups (p> 0.05).
The composition of microplastic types varied among
different brands for the same type of table salts. However,
fragments and fibers were the more prevalent types of
microplastic particles in sea salts (p< 0.05) (Figure 2E).
Pellets and sheets accounted for less than 6% of the total
number of microplastics in each of the three types of salts
(Figure 2E−H). The sizes of the microplastic particles ranged
from 45 μm to 4.3 mm in all of the table salts. The
microplastics measuring less than 200 μm represented the
majority, accounting for 55% of the total number of
microplastics in sea salts (p< 0.05) (Figure 2I). The proportion
of microplastics >1 mm in size reached 28.6% in R/W2 and
20.8% in L1 (Figure 2).
3.3. Types of Microplastics in Table Salts Identified
with μ-FT-IR. One hundred and fifty-two plastic-like particles
were selected and identified using μ-FT-IR. Various types of
microplastics were identified, including polyethylene tereph-
thalate (PET), polyester (PES), polyethylene (PE), poly(1-
butene) (PB), polypropylene (PP), and cellophane (CP)
(Figure 3A−F). Some nonplastic particles, such as cyclohexane
derivatives (CHD) and bentonite (BT), were also identified
(Figure 3G−H). The spectrum matches were at least 90% for
most of the identified particles.
For the 152 selected particles, 84.9% were identified as
microplastics, 6.6% as nonmicroplastics, and 8.6% as
unidentified particles. In sea salts, the most common micro-
Figure 2. Comparison of the abundance (A−D), type (E−H), and size (I−L) of microplastics in table salts (n= 3 for A−C, E−G, and I−J; n= 5 for
D, H, and L). Each value represents the mean ±standard deviation. The letters above the bars indicate significant differences (p< 0.05). If two
arbitrary groups have the same letter, then they are not significantly different. The significant differences are not marked in E−L due to limited space.
The abbreviations on the X-axis are as follows: Con, control group; L, lake salt; R/W, rock/well salt; S, sea salt.
Figure 3. Analysis of microplastics with micro-FT-IR. Abbreviations:
BT, bentonite; CP, cellophane; CHD, cyclohexane derivatives; PB,
poly(1-butene); PE, polyethylene; PP, polypropylene; PES, polyester;
PET, polyethylene terephthalate. The value in the brackets indicates
the matches of the spectra with the standards. The black arrows in the
photographs indicate the particles that were identified. Scale bar = 0.5
mm (A) or 0.25 mm (B−H).
Environmental Science & Technology Article
DOI: 10.1021/acs.est.5b03163
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plastic was PET, followed by PE and cellophane (Table 1). In
lake salts, however, cellophane was the most common
microplastic, accounting for 43.2% of all identified micro-
plastics. In rock/well salts, the most common microplastic was
also cellophane. Eight types of nonplastics were identified (SI
Table S1). The plastic packages for all salts were determined as
PE, except for that of L1 (PET) in this study (SI Figure S2).
4. DISCUSSION
4.1. The Identification of Microplastics. In the present
study, we measured the microplastic pollution in 15 brands of
three types of table salts. In the control groups, we found very
low concentrations of microplastics. The identification of
microplastics was based on a microscopic method and was
partially confirmed with μ-FT-IR,
9
which is one of the most
popular methods used to confirm the composition of
microplastics. The spectrum matches reached 98% in a study
by Frias et al.
19
The plastic packages containing the salts were
analyzed using μ-FT-IR in this study, and the spectrum matches
were also higher than 98% for 14 of the 15 samples. Due to the
complexity of environmental samples, however, the matches are
occasionally not high and even show great variations for the
same or a similar sample. The factors affecting the matches
include environmental degradation and weathering of the
microplastic surfaces, inefficient particle recovery, and mis-
identification of particles.
20
Woodall et al.
18
established a match
of 70% as an acceptable value. In the present study, we
combined microscopic and spectroscopic methods, which was
an effective technique that provided relatively reliable results.
21
Although there is no uniform definition of microplastics, it
has been accepted that some nonplastic synthetic organic
particles are also classified as microplastics.
4,20,22
In the present
study, we followed this accepted classification for microplastics.
For example, cellophane is an organic cellulose-based polymer
used in food packaging and cigarette wrappers as well as as a
release agent in the manufacture of fiberglass and rubber
products. Nowadays, cellophane is often used as coatings
combined with synthetic polymers. The shape of fiber
cellophane found in this study might originate from the
fiberglass products of cellophane or the weathering of broken
cellphone films in the environments. In a previous study,
cellophane and cellulose were regarded as microplastics.
20
In
combination with the morphological features, the correspond-
ing particles matching cellophane and cellulose were classified
as microplastics. Although some particles were identified as
nonplastics, they were closely related to the compositions or
additives of plastics. For example, cyclohexane derivatives are
one of the characteristic compounds added to polymers such as
PP.
23
Some particles, such as 1-bromooctadecane, might
originate from laboratory organic synthesis. However, other
particles, such as bentoite and calcium carbonate, are mostly
naturally occurring inorganic compounds. Therefore, it is very
important to investigate the composition of particles in the
identification of microplastics.
4.2. Microplastic Pollution in Sea Salts. The much
higher microplastic contents in sea salts strongly suggest that
they have been contaminated by microplastics. In contrast, lake
salts and rock/well salts were less contaminated by micro-
plastics. This difference might be due to the different pollution
levels of microplastics in salts from different sources. Currently,
many studies have found microplastics in seawater and in
lakes.
9,24
In China, the sources of sea salts are from the coastal
waters in locations where the population density is very high
(SI Figure S1). In recent studies, microplastics have been found
in water and in sediments of the coastal and estuarine
environments, as well as in commercial bivalves from local
markets in China.
10−14
The density of microplastics reached
4137.3 particles/m3in the Yangtze Estuary.
10
The average
abundance of microplastics reached 5595 particles/m2on the
beaches of Hong Kong, which was higher than international
averages.
12
A high content of microplastics (up to 8714
particles/kg) was also found in the coastal sediments from
Hainan Island in China.
13
These results suggest that micro-
plastics are widespread in the coastal and estuarine environ-
Table 1. Types of Microplastics Identified with Micro-FT-IR for the Particles Randomly Selected from Table Salts
sea salt lake salt rock/well salt total
composition of particles no.
a
% no. % no. % no. %
particles measured 52 100 50 100 50 100 152 100
plastic particles 44 84.6
b
44 88 41 82 129 84.9
polyethylene terephthalate (PET) 12 27.3
c
5 11.4 4 9.8 21 16.3
polyethylene (PE) 9 20.5 0 0 2 4.9 11 8.5
polyester (PES) 6 13.6 2 4.5 1 2.4 9 7
poly(1-butene) (PB) 4 9.1 5 11.4 2 4.9 11 8.5
polypropylene (PP) 1 2.3 3 6.8 2 4.9 6 4.7
PE and PP copolymer (PE−PP) 0 0 2 4.5 1 2.4 3 2.3
polymerized, oxidized material(POM) 1 2.3 4 9.1 1 2.4 6 4.7
polyalkene (PAK) 00 49.100 43.1
polyacrylonitrile (PAN) 1 2.3 0 0 1 2.4 2 1.6
poly methyl acrylate (PMA) 12.300 00 10.8
poly(vinyl chloride) (PVC) 00 00 12.410.8
ethylene vinyl acetate (EVA) 00 00 12.410.8
poly(vinyl acetate:ethylene) 3:1 12.300 00 10.8
cellophane (CP) 8 18.2 19 43.2 24 58.5 51 39.5
cellulose (CL) 00 00 12.410.8
nonplastic particles 6 11.5 2 4 2 4 10 6.6
unidentified particles 2 3.8 4 8 7 14 13 8.6
a
Number of particles.
b
The percentage of plastic particles in all the measured particles from each type of salt.
c
The percentage of each type in all the
plastic particles from each type of salt.
Environmental Science & Technology Article
DOI: 10.1021/acs.est.5b03163
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ments of China, which might serve as the primary sources of
sea salts.
The sources of lake salts are remote mountain lakes located
in northwestern China (SI Figure S1). The population density
on the east coast of China, where the sea salts in this study
originate, was 559 people/km2in 2013.
25
However, the
poluation density in the northwest, where the lake salts in
this study originate, was only 12 people/km2. Undoubtedly, the
population density and economic structures will greatly affect
the pollution levels of microplastics in the local environ-
ments.
2,11
Although there are no available data of microplastics
in salt lakes worldwide, including those in China, it is expected
that the microplastic pollution level should be much lower in
salt lakes in the northwest than in the coastal waters along the
east coast of China due to the great differences in the
population density and the developing levels of economy.
In contrast to sea salts or lake salts, rock/well salts are
typically collected from the underground in China, even as
deep as hundreds of meters. It is difficult for synthetic polymers
to penetrate into these deep areas. The lowest concentrations
of microplastics in rock/well salts suggest that the transport to
surface water is the primary route of micoplastics introduced
into the environment. However, microplastic pollution has
other routes, such as atmospheric transport.
16
Microplastic
pollution has also been found in honey.
26
The occurrence of
microplastics in rock/well salts suggests that microplastics
might be introduced into table salts during the production
stages of collecting, wind drying, and packaging.
It is reasonable to deduce that the micoplastics in sea salts
primarily arise from the seawater. However, due to the different
methods of investigation, it is difficult to construct precise
relationships of the microplastic types, size, and chemical
composition between the sea salts and seawater. Plankton nets
are typically used in microplastic investigations in seawater, and
particles less than 333 μm are not available due to the size of
the nets. However, we found that the microplastics measuring
less than 200 μm represented the majority of those in sea salts.
These results were in accordance with those found in
commercial bivalves from markets in China.
14
In nine
investigated bivalve species, the microplastics measuring less
than 250 mm consisted of 33−84% of the total microplastics
calculated by species.
14
In most studies, PE is one of the most
common polymers in microplastics in coastal environ-
ments.
27,28
However, we found that PET was the most
common polymer in sea salts. This might be due to the high
density of PET (1.38 g/cm3), indicating that it is likely to settle
in the raw salts during the specialized processes of sea salt
production, such as crystallization.
It is interesting that cellophane was the dominant micro-
plastic in lake salts and rock/well salts in the present study. As a
type of packaging debris, cellophane wrappers have even been
found in Lake Hovsgol, Mongolia.
4
Cellophane wrappers
account for 52% of the total debris ingested by dolphins living
in a fully marine environment and 58% in an estuarine
environment.
22
Cellophane has also been found as the
predominant microplastic in two sample points of the Solent
estuarine complex in the UK.
20
Further studies should be
conducted to elucidate the reasons leading to the high
proportion of cellophane in table salts.
4.3. The Risk of Microplastics in Sea Salts to Human
Health. Microplastics are a pervasive pollutant present in
marine environments worldwide and tend to increase in
concentration over time due to plastic fragmentation.
2,3
Adults
should consume less than 5 g of salts according to new
guidelines issued by the World Health Organization (WHO).
Therefore, for the maximum consumption of sea salts, adults
will ingest approximately 1000 microplastic particles each year
just from table salts. Humans will also ingest microplastics by
consuming other sea products, such as mussels and fish, as well
as other microplastic-contaminated food and water.
29
Although
the amount of microplastics ingested through salt consumption
is much less than that through mollusk consumption by the top
European consumers (11,000 microplastic particles per year),
30
more individuals will be affected because table salts are required
and consumed in our daily diet.
Microplastics are a particular threat to organisms due to their
small size and their capacity to absorb persistent organic
pollutants.
31
The constituents of plastics, as well as the
chemicals and metals they absorb, may ultimately be ingested
by humans through the consumption of seafood. Due to the
pollution of seawater, many contaminants have been found in
sea salts, including plasticizers, such as di(2-ethylhexyl) adipate
and benzyl butyl phthalate.
15
Plastic might be the direct sources
of these contaminants. However, plastics might absorb
contaminants from the seawater and transfer them to the sea
products. Therefore, the presence of marine microplastics in sea
salts might pose a threat to food safety.
In sum, we measured the microplastic pollution in sea salts,
lake salts, and rock/well salts in China. Our results strongly
suggest that the sea salts were contaminated by microplastics,
which mostly arise from the seawater columns. To the best of
our knowledge, this is the first report on microplastic pollution
in abiotic sea products.
■ASSOCIATED CONTENT
*
SSupporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.est.5b03163.
Figure S1, sources of table salts tested in this study;
Figure S2, spectra of the 15 packages of table salts; Table
S1, eight nonplastic particles identified with micro-FT-IR
(PDF)
■AUTHOR INFORMATION
Corresponding Author
*Phone: 86 21 62455593. E-mail: hhshi@des.ecnu.edu.cn.
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (41571467, 21507031).
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