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Microplastic pollution in table salts from China Dongqi Yang, Huahong Shi, Lan Li, Jiana Li, Khalida Jabeen, and Prabhu Kolandhasamy Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03163 • Publication Date (Web): 20 Oct 2015 Downloaded from http://pubs.acs.org on October 27, 2015
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Microplastic pollution in table salts from China
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Dongqi Yang† , Huahong Shi*†, Lan Li‡, Jiana Li†, Khalida Jabeen†, Prabhu Kolandhasamy†
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State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China
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Research Center for Analysis and Measurement, Donghua University, Shanghai 201620, China
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Running title: Microplastics in Salts
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*Address all correspondence to: State Key Laboratory of Estuarine and Coastal Research,
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East China Normal University, Shanghai 200062, China；
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ABSTRACT: Microplastics have been found in seas all over the world. We hypothesize that
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sea salts might contain microplastics because they are directly supplied by seawater. To test
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our hypothesis, we collected 15 brands of sea salts, lake salts and rock/well salts from
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supermarkets throughout China. The microplastics content was 550-681 particles/kg in sea
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salts, 43-364 particles/kg in lake salts and 7-204 particles/kg in rock/well salts. In sea salts,
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fragments and fibers were the prevalent types of particles compared with pellets and sheets.
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Microplastics measuring less than 200 µm represented the majority of the particles,
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accounting for 55% of the total microplastics, and the most common microplastics were
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polyethylene terephthalate, followed by polyethylene and cellophane in sea salts. The
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abundance of microplastics in sea salts was significantly higher than that in lake salts and
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rock salts. This result indicates that sea products, such as sea salts, are contaminated by
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microplastics. To the best of our knowledge, this is the first report on microplastic pollution in
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abiotic sea products.
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KEYWORDS: marine debris; microplastic; sea salt; seawater; human health
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1. Introduction
Marine debris has become a global concern due to its vast and growing threat to the
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marine and coastal environment.1, 2 The degradation of large, individual plastic items
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ultimately leads to millions of microplastic pieces, which are likely the most numerous plastic
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debris in the ocean today.3 Microplastics are defined as plastic materials or fragments
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measuring less than 5 mm and have been found not only in the marine environments but also
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in rivers, lakes, and even in ice.4-9 Several recent studies showed that the coast of China is a
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hotspot of microplastic pollution 2,10-14. 3
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Sea products are among the main sources of food for human beings. The pollution of
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microplastics in the sea will undoubtedly lead to the pollution of sea products. Microplastics
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can even be transferred from sea products to humans through the food chain, which increases
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the potential health risks to humans. Recently, reviews by Seltenrich15 and Bouwmeester et
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al.16 highlighted the importance of investigating the potential risk of transferring microplastics
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from the food chain to humans. Microplastics have been detected in a large variety of marine
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organisms, such as mussels and fish.7,14 To date, however, there has been no report on
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microplastic pollution in abiotic sea products. Table salts provide essential elements for
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humans. Salt materials primarily come from the sea, saline lakes, saline rocks and saline
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wells. Subsequently, table salts can be classified into sea salts, lake salts, rock salts and well
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salts according to their sources. However, rock and well salts are typically labeled as
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rock/well on the packages of table salts and are regarded as the same type in markets in
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China. Sea salt is typically produced by crystallization due to the combined effects of wind and
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sunlight. Before sea salt crystallizes, seawater circulates along a series of successive ponds
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with increasing levels of salinity due to the continuous evaporation of water. Sea salts may
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contain anthropogenic contaminants present in the seawater if they remain after the
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concentration and crystallization processes.17 Therefore, it is necessary to monitor the
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presence of contaminants in sea salts. Seawater is widely polluted by microplastics; thus, we
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hypothesize that sea salts might contain microplastics.
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To test this hypothesis, we collected different brands of sea salts from random
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supermarkets throughout China. We also collected lake salts and rock/well salts for
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comparisons. The abundance, type and composition of the microplastics were measured and
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analyzed.
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2. Materials and methods
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2.1 Collection of table salts
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Fifteen brands of table salts were collected from supermarkets in China during October
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and November of 2014. These salts represent three main types of table salts according to their
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source in China. An average package with a weight range from 240-500 g was chosen. Each
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type of salt originates from a different location (Supporting Information (SI) Figure S1). One
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blank extraction group without salt was tested simultaneously to correct the potential
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procedural contamination. Three replicate packages were used to compare among different
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brands of the same type of salt. Five replicate brands were used for the comparison among the
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different types.
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2.2. Quality control of the experiments
To avoid contamination, all of the liquid (tap water and hydrogen peroxide) was filtered
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using 1 µm pore size filter paper prior to use. All containers and beakers were rinsed three
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times with filtered water. The samples were immediately covered when they were not in use.
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All the experimental procedures were finished as soon as possible.
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2.3. Hydrogen peroxide treatment
Approximately 240-250 g of table salts from one package of salts was directly placed into
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a 1 L glass bottle at a height of 35 cm. The sample in one bottle was regarded as a replicate,
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and four replicate bottles were prepared for each brand. Approximately 100 mL of 30% H2O2
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was added to each bottle to digest the organic matter. The bottles were covered and placed in
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an oscillation incubator at 65 oC at 80 rpm for 24 h and then at room temperature for 48 h.
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2.4. Floatation and filtration
Approximately 800 mL of filtered water was added to each bottle. A glass rod was used to
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stir the salts in the bottle until they were completely dissolved. The salt solution in one bottle
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of the four replicates was immediately transferred onto a piece of 5 µm pore size, 47 mm
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cellulose nitrate filter paper using a vacuum system. The filter paper was then placed into a
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clean petri dish with a cover and was dried at room temperature to observe the total number of
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particles. The three other replicate bottles containing the salt solution were covered with glass
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lids and held overnight. The supernatants of the salt solutions were transferred onto 5 µm pore
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size, 47 mm cellulose nitrate filter papers. The filter papers were placed in clean petri dishes
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with covers and were dried at room temperature for further microplastic analysis. The
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material deposited at the bottom of the bottles was also transferred into other petri dishes for
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microscopic observation.
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2.5. Visual observation of microplastics under a microscope
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The filters were observed under a Carl Zeiss Discovery V8 Stereomicroscope
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(MicroImaging GmbH, Göttingen, Germany), and images were obtained with an
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AxioCam digital camera. A visual assessment was performed to identify the types and colors
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of microplastics according to the physical characteristics of the particles. Some particles were
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randomly selected for verification using micro-Fourier Transform Infrared Spectroscopy
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(µ-FT-IR). The abundance of microplastics was calculated based on the microscopic
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observation and was confirmed with µ-FT-IR.
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2.6. Identification of microplastics with µ-FT-IR
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The plastic-like particles on the filter paper were randomly selected for µ-FT-IR analysis
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(Thermo Nicolet iN10 MX) in transmittance mode. The spectrum range was set to 4000-675
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cm-1 with a collection time of 3 s and with 16 co-scans for each measurement. The spectral
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resolution was 8 cm-1 for all samples, and the aperture size ranged from 50 x 50 µm to 150 x
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150 µm, depending on the size of the particles. The 15 table salt packages were also identified
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using a µ-FT-IR microscope in attenuated total reflection mode. All spectra were collected at
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a resolution of 8 cm-1 using a diamond MicroTip accessory from 4000-675 cm-1, with a
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collection time of 3 s and with 16 co-scans.
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All the spectra were then compared with the library (Hummel Polymer and Additives,
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Polymer Laminate Films) to verify the polymer type. The spectrum analysis followed the
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method of Woodall et al.13 Briefly, matches with a quality index ≥ 0.7 were accepted.
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Matches with a quality index < 0.7, but ≥ 0.6 were individually inspected and interpreted 7
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based on the proximity of their absorption frequencies to those of chemical bonds in the
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known polymers. Matches with a quality index < 0.6 were rejected.12
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2.7 Data analysis
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The mean differences of the abundance of microplastics among groups were determined by
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one-way analysis of variance (ANOVA) followed by Tukey’s HSD test (homogeneous
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variances) or the Tamhane-Dunnett test (heterogeneous variances), along with multiple
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comparisons. A 0.05 significance level was chosen.
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3. Results
3.1 Particles in table salts
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In the process of handling the samples, contamination with airborne microplastics was
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prevented. The procedural blanks only contained 4.4 ± 2.1 particles/filter of microplastics,
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which is equal to 18 particles/kg when the average weight of salt (240-250 g) in each bottle is
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considered in the control group. When the entire salt solution was filtered, the color of the
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filter paper for sea salts was much darker than the filter papers for lake salts and rock/well
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salts (Figure 1A-C). When the supernatants of the salt solutions were filtered, the particle
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density on the filter papers for sea salts was higher than the particle density on the filter
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papers for lake salts and rock/well salts (Figure 1D-F). Multiple types of particles, including
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fibers, fragments and pellets, occurred in table salts (Figure 1D-F). The most diverse colors
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were observed in the fibers followed by the fragments. The most common colors were black,
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red, blue and white. Some sand particles were also found at the bottom of the bottles (Figure
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1G-I).
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Figure 1. Photographs of the total particles isolated from table salts. A-C, the particles in
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the salt solution without separation; D-F, the particles in the supernatant of the salt
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solutions. More particles were observed in sea salts (D) than lake salts (E) and rock/well
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salts (F); G-I, the particles at the bottom of the bottle after removal of the supernatant.
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Scale bar=10 mm (A-C) or 0.2 mm (D-I).
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3.2 Abundance, type and size of microplastics in table salts
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The number of microplastics was 550-681 particles/kg in sea salts, 43-364 particles/kg in
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lake salts and 7-204 particles/kg in rock/well salts (Figure 2A-C). Different brands showed no
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significant differences in the abundance of microplastics in sea salts (p>0.05) (Figure 2A).
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Microplastic content was less in L3 than in L2 and L4 in lake salts (Figure 2B), and it was
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less in RW/2 and R/W4 than in R/W5 in rock/well salts group (p
