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PAHs in polystyrene food contact materials: An unintended consequence
Science of the Total Environment 609 (2017) 1126–1131
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
PAHs in polystyrene food contact materials: An unintended consequence Si-Qi Li, Hong-Gang Ni ⁎, Hui Zeng Shenzhen Key Laboratory of Circular Economy, Shenzhen Graduate School, Peking University, Shenzhen 518055, China
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Low-ring PAHs were detected in polystyrene food contact materials (PS FCMs). • PS FCM as a source and sink for PAHs in the ordinary environment were confirmed. • Using of foaming agent results in higher PAHs levels in expand PS than in extrude PS. • Auxochromes and chromophores are propitious to form PAHs in colored PS FCMs. • Higher migration of PAHs from PS FCMs to foods merits special attention.
a r t i c l e
i n f o
Article history: Received 21 June 2017 Received in revised form 29 July 2017 Accepted 29 July 2017 Available online xxxx Editor: Jay Gan Keywords: Polystyrene Food contact material PAH Migration Potential health risk
a b s t r a c t Eight low-ring PAHs were detected in 21 polystyrene (PS) food contact materials (FCMs) samples while high-ring PAHs (N4 rings) were not found. This is because the reaction pathway for formation of high-ring PAHs consists of more steps than it does for low-high PAHs. The concentrations of Σ8PAH were from 18.9±5.16 ng/g for product colorless fruit fork to 476±52.0 ng/g for foam instant noodle container. These data were far beyond levels of PAHs in other plastics. Of the eight PAHs detected, Phe had the highest average concentration, followed by Nap. These two PAHs collectively accounted for over 80% of the Σ8PAH concentrations in all PS FCMs. Levels of Σ8PAH in expanded PS FCMs were higher than those in extruded ones due to utilization of foaming agent. The concentrations of Σ8PAH were lower in colorless PS FCMs than in colored ones. Auxochromes and chromophores contributed to the change of short-chain hydrocarbons to aromatic hydrocarbon. Simulated migration values of PAHs from PS FCMs to food varied widely. The migration value of Σ8PAH with maximum probability was below 10 ng/g, which the maximum tolerated migration level for substance according to the European Union standards. However, higher migration values were possible and the potential health risk should still be concerned because the simulated migration displayed a log-normal distribution. Furthermore, water was used as food simulant would always lead to an underestimate of PAHs migration to real daily food, and then lead to an underestimate of risk. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Because of the advantages of safe, lightweight, strong, easily processed, and stored, plastics are widely used as packaging materials for foods, pharmaceuticals, detergents, and so on (Paraskevopoulou et al., ⁎ Corresponding author. E-mail address: [email protected] (H.-G. Ni).
http://dx.doi.org/10.1016/j.scitotenv.2017.07.262 0048-9697/© 2017 Elsevier B.V. All rights reserved.
2012). However, small molecules, such as residual monomers, plasticizers, and antioxidants, etc., can migrate from package material into the food in contact with them and be potential pollutants (Lin et al., 2017). These low molecular weight impurities may be ingested via food consumption and induce potential human health risk. A large number of studies on migration of monomers and oligomers from plastic food contact materials (FCM) to foods have been published (Helmroth et al., 2002). For example, migration of styrene monomer from polystyrene
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(PS) based food packages to food simulants have been studied and a large number of reports have been published (Genualdi et al., 2014; Lickly et al., 1995; Lin et al., 2017). Also, there were numerous reports about other impurities other than styrene monomer in PS food contact packaging and consumables. Namely, phthalates, alkylphenols, bisphenol A, and di(2-ethylhexyl)adipate tend to migrate from food packaging into food and create potential human health risk (Fasano et al., 2012; Guart et al., 2011). According to a recent study (Rochman et al., 2013b), polycyclic aromatic hydrocarbons (PAHs) could be formed along with the production and processing of PS products. It is speculated that PAHs should be extremely likely to exist in PS FCMs. However, there is no study on the occurrence of PAHs in PS FCMs, let alone the migration of PAHs from PS FCM to food. Possibly, presence of PAHs in PS may be an unintended consequence and it has not aroused the academic circle's attention yet. Actually, researchers only chanced upon the occurrence of PAHs in PS during their field experiments designed to measure sorption of several organic pollutants to plastic debris in the marine environment (Rochman et al., 2013a; Rochman et al., 2013b). PAHs are ubiquitous and persistent organic pollutants generated during the incomplete combustion of modern biomass and fossil fuels (petroleum and coal) (Lima et al., 2005). Because of their hydrophobic nature, PAHs have a low water solubility and a higher tendency to enrich to fatty foods, e.g., dairy food and fish (Lima et al., 2005). PAHs have been considered a category of important pollutants due to their carcinogenicity, mutagenicity and dioxin-like toxicity (Mumford et al., 1995). Previous studies (Xia et al., 2010; Xia et al., 2013) have revealed that human exposure to PAHs mainly via diet and inhalation. Given that, PAHs migration from PS FCMs to food need to concern. As mentioned above, previous studies reported about various impurities in PS products, but no study on the occurrence of PAHs in PS FCMs. Besides, presence of PAHs in virgin PS pellets (Rochman et al., 2013b) cannot provided convincing information of PAHs in PS FCMs because FCMs are final products and have gone through various manufacture processes other than virgin PS pellets. The aims of the present study were to examine PAHs concentrations in PS FCMs and to assess PAHs migration from PS FCMs to food with a stochastic migration model. Results from these experiments will provide valuable data needed for updating dietary intake values for the impurities other than styrene monomer and plastics additives in PS FCMs.
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Fig. 1. Pictures of various polystyrene food contact materials.
Under finally optimized conditions obtained by our preliminary experiment, 2 g of each PS plastic sample was cut into fragments with a stainless scissors and then put into a 30-mL centrifuge glass tube, surrogate standards were added into each sample as well. The samples were treated by ultrasonic wave for 30 min with 10 mL mixture of acetone and dichloromethane (1:1 in volume) as extraction reagent. After 15 mL hexane was added, the mixture was stirred sufficiently in order to precipitate the polymer. Liquid contents of each sample were centrifuged for 20 min at 4000 rpm at 25 °C to remove solid precipitate. The supernate was concentrate to approximately 1 mL. A C18 SPE cartridge (Agilent Technologies, Inc. USA) was preconditioned with 5 mL dichloromethane and then 5 mL hexane. For sample cleanup, the extract was injected into the preconditioned cartridge entirely and eluted with 5 mL of mixture of hexane and dichloromethane (4:1 in volume). The final extract was concentrated by a rotary evaporator to approximately 1 mL. Three replicates of each sample were conducted. Internal standards were added to each extract prior to instrumental analysis. 2.2. Instrumental analysis and quality control Concentrations of PAHs were qualified with a Shimadzu Model 2010 gas chromatograph coupled with a Model QP2010 plus mass
2. Materials and method 2.1. Sample collection and extraction A total of 21 kinds of PS FCM samples (three replicate samples for each one) with assigned sample numbers (SN) of 1–21 were purchased from the local supermarkets of Shenzhen, China, which were recognized by the PS mark imprinted on the products (Table S1 of the Supplementary Data). Each individual product was also photographed with a view to formulate its use (Fig. 1). The chemical constituents of PS were analyzed and identified using Fourier transform infrared (FT-IR) spectroscopy with NIST library (Fig. 2). All samples were covered with aluminum foil to avoid contamination. The samples were stored at −4 °C until further analysis. Internal standards (2-fluorobiphenyl and p-terphenyl-d14) and surrogate standards (naphthalene-d8, acenaphthene-d10, phenanthrened10, chrysene-d12 and perylene-d12) were purchased from Dr. Ehrenstorfer Gmbh (Augsburg, Germany). A standard solution of 16 US Environmental Protection Agency priority PAHs was purchased from Chem Service (West Chester, PA), including naphthalene (Nap), acenaphthylene (Acy), acenaphthene (Ace), fluorene (Fle), phenanthrene (Phe), anthracene (Ant), fluoranthene (Flu), pyrene (Pyr), chrysene (Chr), benzo[a]anthracene (BaA), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), indeno[1,2,3c,d]pyrene (InP), dibenzo[a,h]anthracene (DBA), and benzo[g,h,i] perylene (BghiP).
Fig. 2. Fourier transform infrared spectra of standard polystyrene, expanded polystyrene instant noodle bowl, extrude polystyrene drink cup.
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spectrometer (Shimadzu, Japan). In the selected ion monitoring mode, electron ionization was utilized. A 30 m DB-5MS fused silica capillary column (0.25 mm i.d., 0.25 mm film thickness; J&W Scientific, Folsom, CA) was used to achieve the separation of gas chromatograph. The mass selective detector was operated in the selected ion monitoring mode for quantitation, and selected samples containing high levels of the analytes were analyzed in the full-scan mode for peak confirmation. Column temperature was programmed from 60 °C to 200 °C at a rate of 10 °C/min, followed by a ramp to 214 °C at a rate of 2 °C/min, to 254 °C at a rate of 5 °C/min (held for 2 min), and to 290 °C at a rate of 18 °C/min (held for 17 min). Injector temperature was programmed for 100 °C to 280 °C at a rate 180 °C/min and held for 50 min. Samples were injected using an automatic sampler and the injector was used in a splitless mode and the carrier gas was high purity helium. Laboratory procedural blanks and spiked blanks samples were processed with each batch of eight samples to monitor procedural contamination. The former are the solvent used in sample processing and used to detect any external contamination during the laboratory process. They are processed along with practical sample. The latter are prepared from the solvent used in sample processing, which is spiked with the target-analyte standards at desirable concentration levels. These samples are used to assess the performance of the entire analytical procedure without the influence of matrix interference. For PAHs, recoveries of the surrogate standards (spiked concentration: 100 ng/mL) were 79 ± 21%, 74 ± 18%, 68 ± 12%, 67 ± 8.5%, and 81 ± 5.9% for naphthalened8, acenaphthene-d10, phenanthrene-d10, chrysene-d12, and perylened12, respectively. Concentrations were determined using the internal calibration technique. The lowest concentration level from the calibration curve was defined as the reporting limit. In the present study, reporting limit for each individual PAH was 2.5 ng/g PS. All the reported concentrations were not corrected with the recoveries of the surrogate standards. 3. Results and discussion 3.1. PAHs in PS Table S2 in the Supplementary Data summarized the concentrations of eight PAHs (Nap, Acy, Ace, Fle, Phe, Ant, Flu, and Pyr) detected in the PS FCM samples. These eight PAHs have the lowest molecular weight of the 16 US EPA priority PAHs. Among them, Nap and Phe were completely detected in all of the 21 PS FCMs (detection rate = 100%), Fle was frequently detected (detection rate N75%), Flu and Pyr were usually detected (detection rate N 35%), and the detection rate of Acy, Ace and Ant were between 10% to 20%. Overall, high-ring PAHs (N4 rings) were not detected in PS FCMs. A possible explanation is that the reaction pathway for formation high-ring PAHs consists of more chemical steps than it does for low-high PAHs. Although short chain hydrocarbon and free radicals can generate PAHs under certain conditions (Wang and Frenklach, 1997), a series of complex reactions reduce chemical yield sharply (Dias, 2013). The sum of the 8 PAH compounds was defined as Σ8PAH. Concentrations of Σ8PAH were18.9 ± 5.16 ng/g for product colorless fruit fork (SN 15) and 476 ± 52.0 ng/g for foam instant noodle container (SN 1) (Fig. 3). These data were far beyond concentrations of PAHs detected in other plastics, such as polyethylene terephthalate (not detect to 1 ng/g), polyethylene (not detect to 13 ng/g), polyvinyl chloride (not detect to 2 ng/g), polypropylene (2 to 6 ng/g) (Van et al., 2012). When PAHs in PS FCM qualified here were compared to those measured in virgin PS pellets by Rochman et al. (2013b), we found that their data (79 to 97 ng/g) fall within the range of our measured Σ8PAH levels. A large range of PAHs levels in PS FCMs may imply the various manufacturing process. Of the eight PAHs detected, Phe was detected with the highest average concentration which reached up to 166 ng/g with a range from 11.5 ng/g to 503 ng/g (Fig. 4 and Table S2), followed by Nap with the average concentration 45.6 ng/g (range: 2.53–109 ng/g). These two PAHs collectively accounted for 88% (range: 59%–99%) of the Σ8PAH
Fig. 3. Concentration of Σ8PAH in polystyrene food contact materials.
concentrations in 21 PS FCMs (Fig. 5 and Table S2). And Acy had the lowest average concentration among the eight PAHs which was 1.68 ng/g with a range from not detected to 11.3 ng/g (Fig. 4). Actually, the residues of PAHs must not be found in the FCMs according to the level set by European Union (Simoneau et al., 2016). If this is the standard, PS FCMs of interest in the present study did fail to reach the standard. Apparently, level of Σ8PAH in expanded PS FCMs (SN 1 and 12) was higher than that in extruded PS FCMs (other samples except for SN 1 and 12). Specifically, the average concentrations of Σ8PAH were 223 ng/g (range: 15.3–656 ng/g) for extruded PS FCMs and 425 ng/g (range: 311–534 ng/g) for expanded PS FCMs (Fig. 6), respectively. This difference may come from the differences of assistant materials and manufacturing process. Expanded and extruded PS is insoluble and non-hygroscopic. The use of them helps to prevent food wastage. Expanded PS is the polystyrene as main raw materials, made by the foam blowing agent interior has countless blockade of the porous materials. The blowing agents usually contain short-chain aliphatic hydrocarbons (Yang et al., 1996) and can generate PAHs with free radicals under certain conditions (Wang and Frenklach, 1997). Interestingly, the concentrations of Σ8PAH were lower in colorless and transparent PS FCMs (SN 7, 15, and 16) than those in colored PS FCMs (other samples except for SN 7, 15, and 16). For instance, the average concentration of Σ8PAH in dyed PS FCMs (264 ng/g with a range of 24.8–656 ng/g) was nearly three times that of colorless PS FCMs (97.4 ng/g with a range of 15.3–166 ng/g) (Fig. 6). Usually, a colored molecule has chromophore and auxochrome and the two put together if they are in a conjugated system will create color (Rachuta et al., 2017; Sromovsky et al., 2017). Chromophore is usually electron-withdrawing, and auxochromes are normally electron-donating. Unsaturated bonds and chromophores contribute to the growth of short-chain hydrocarbons to aromatic hydrocarbon (Wang and Frenklach, 1997).
Fig. 4. Level of eight detected PAH congeners in polystyrene food contact materials.
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according to corresponding molar mass of the migrants, AP, and T (Brandsch et al., 2002); next, calculate the migration using Eq. (2) (Chung et al., 2002); after that, repeat step 2 a large number of times (10,000 times in the present study); finally, plot a histogram of the obtained migration values. 10454 DP ¼ 104 exp AP −0:1351Mr 2=3 þ 0:003M r − T
ð1Þ
where DP is diffusion coefficient of migrant within FCMs. The parameter AP has the role of a “conductance” of the polymer matrix towards the diffusion of the migrant (Brandsch et al., 2002). Mr is molar mass of the migrant (Table S3). T is temperature (298 K). M F;t 2 Dt 0:5 Dt ¼ − MP;0 LP π αLP 2
Fig. 5. The concentration profiles of PAH congeners in polystyrene food contact materials.
The detection of PAHs in PS FCMs reconfirmed that PS is either source or sink for PAHs (Rochman et al., 2013a). On one hand, plastics are most commonly derived from petrochemicals and petroleum materials usually contains some PAH congeners, especially, lighter PAHs (Soclo et al., 2000). On the other hand, the precursors of PAHs (e.g., styrene and benzene) generated during the course of PS production are propitious to form PAHs (Kwon and Castald, 2008; Lithner et al., 2011). Actually, heating again to process and shape end products from PS pre-production pellets also results in more PAHs. Moreover, PS also can be a sink for PAHs via sorption in the marine environment (Van et al., 2012). It is therefore speculated that the presence of PAHs in PS FCMs should be attributed to two causes: formation during production and sorption during use. This may pose a potential risk of dietary exposure to PAHs migrated from PS FCMs. 3.2. Migration simulation Detection of PAHs in PS FCMs triggered concerns that the food contacting with PS is on the edge of the likelihood of contamination. A large number previous works have confirmed that monomers and additives in FCMs can migrate to foods (Brandsch et al., 2002; Paraskevopoulou et al., 2012; Verzera et al., 2009; Zabaniotou and Kassidi, 2003). It is therefore speculated that PAHs migration from PS FCMs to food should be no doubt that it will happen (Genualdi et al., 2014; Paraskevopoulou et al., 2012). To obtained migration of probability distributions, A Monte Carlo procedure was conducted with the following steps: first, calculate the diffusion coefficients of eight detected PAHs in PS FCMs using Eq. (1)
Fig. 6. Comparison of PAHs in different polystyrene food contact materials (expanded polystyrene versus extrude polystyrene; colorless versus colored polystyrene).
ð2Þ
where MF,t is migrant amount in food at time t, MP,0 represents initial migrant amount in PS FCMs. α refers to mass ratio of migrant in food to that in FCM at equilibrium, where α = KFP VF/VP. KFP refers to partition coefficient of migrant between food and FCMs (CF, ∞/CP, ∞). VF/VP values were measured in the present study (ranging from 8.333 to 3333) and the data distribution was examined with a software Crystal Ball version 11.0. The partition coefficient (KFP = 0.001) was defined as the ratio of concentration in food simulant to PS FCMs, is 1 for migrants with high solubility in the food simulant and 0.001 for migrants with low solubility, according to previous study (Baner et al., 1996). Lp thickness of FCMs (ranging from 0.001 to 0.1 cm with a normal distribution). t is migration time. It should be specially explained that to simulate the practical situation, T was selected at 298 K and t was chosen 10 days. More details of the derivation procedures of migration model and the required parameters (Table S3) for simulation were provided in the Supplementary Data. Fig. 7 shown the results of the Monte Carlo procedure for a water food simulant. Apparently, the migration values varied widely, which indicated that both low and high migration values were possible. This is due to the large variation in MP,0 and Lp. The probability distribution of the simulated migration for PAH congeners from PS FCMs to the water food simulant were present Fig. 7. The median values of the migration were estimated to be 0.28, 0.01, 0.01, 0.35, 0, 1.02, 0.02, and 0 for Nap, Acy, Ace, Fle, Ant, Phe, Flu, Pyr, respectively. The cumulative of the frequencies displayed in Fig. S1 shown the probability of observation a migration value less than a given limit, for example, 0.1 ng/g PS. (Fig. 7). Unfortunately, there is no migration limit for PAH so far, to our knowledge. Assumed that the specific migration limit of individual PAH congener is 0.1 ng/g PS, in our example, the probabilities that the migration are less than or equal to this value are 0.04, 0.99, 0.99, 0.79, 0.88, 0.04, 0.97, and 0.89 for Nap, Acy, Ace, Fle, Ant, Phe, Flu, Pyr, respectively. These data corresponds with a chance of 0.96, 0.01, 0.01, 0.21, 0.12, 0.96, 0.03, and 0.11 of exceeding to this limit, for Nap, Acy, Ace, Fle, Ant, Phe, Flu, Pyr, respectively. These results suggested that migrations of Nap and Phe from PS FCMs to food need more concerns. The sum of the median values for those eight PAH congeners (1.70 ng/g) was still below 10 ng/g, which the maximum tolerated migration level for substance according to Regulation (EC) No 450/2009 (European Commission, 2011). However, the simulated migration for all eight PAH congeners displayed the log-normal distribution (Fig. S1). This suggested that higher migration values were possible and the potential health risks merit special attention. Besides, water was used as food simulant in the present study, it is therefore the simulated results cannot reflect the reality of PAHs migration to real daily food. PAHs belong hydrophobic organic pollutants (Ni et al., 2011), they tend to have higher concentrations in fatty food (such as dairy products and fish) than in water. Obviously, initial levels of migrants in PS FCMs, thickness of FCMs, migration time and temperature, all these factors affect migration based on the migration model. Apparently, the migration decreases with the increase of LP, and the degree of influence on the migration decreases
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Fig. 7. Probability distribution of the migration of PAHs from polystyrene food contact material to a water food simulant at 298 K during 10 days of incubation.
with the increasing thickness. Except for Lp, all the other factors have substantial positive impacts to migration of PAHs from PS FCMs to food simulant. For example, migration is favored by increased temperature and migration time. 4. Conclusions Low-ring PAHs were detected in PS FCMs because of formation during production process and sorption during PS FCMs use. Utilization of foaming agent results in higher PAHs levels in expand PS than in extrude PS. Auxochromes and chromophores contribute to the higher concentrations of PAHs in colored PS FCMs compare to colorless PS FCMs. The sum of the median values for migration of the eight PAH was below the maximum tolerated migration level for substance according to the European Union standards. Obviously, using water as food simulant may underestimated the migration values for lots of foods carry essential oils. According to the migration model, initial levels of migrants, thickness of FCMs, migration time and temperature, all these factors affect migration. A certain storage conditions can decrease the potential migration of PAHs from PS FCMs to foods. For instance, short time and low temperature for storage could reduce the migration value. Measurable byproducts and processing aids used in the plastic synthesis processes can remain in the final product. Knowledge of these materials is indispensable for safety assessment of the plastics for use in FCMs. The migration rate of low molecular weight impurities from FCMs into foods depends on the molecular structure and the aggregate nature of the plastic material. To estimate migration of impurities from plastics to food, a basic knowledge of the structure of the plastic and food components and their possible influences on migration is necessary. Acknowledgment This study was financially supported by the Shenzhen Science and Technology Research and Development Fund (JCYJ20150828092938205) and the National Natural Science Foundation of China (No. 41573085). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.scitotenv.2017.07.262. References Baner, A., Brandsch, J., Franz, R., Piringer, O., 1996. The application of a predictive migration model for evaluating the compliance of plastic materials with European food regulations. Food Addit. Contam. 13, 587–601.
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
PAHs in polystyrene food contact materials: An unintended consequence Si-Qi Li, Hong-Gang Ni ⁎, Hui Zeng Shenzhen Key Laboratory of Circular Economy, Shenzhen Graduate School, Peking University, Shenzhen 518055, China
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Low-ring PAHs were detected in polystyrene food contact materials (PS FCMs). • PS FCM as a source and sink for PAHs in the ordinary environment were confirmed. • Using of foaming agent results in higher PAHs levels in expand PS than in extrude PS. • Auxochromes and chromophores are propitious to form PAHs in colored PS FCMs. • Higher migration of PAHs from PS FCMs to foods merits special attention.
a r t i c l e
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Article history: Received 21 June 2017 Received in revised form 29 July 2017 Accepted 29 July 2017 Available online xxxx Editor: Jay Gan Keywords: Polystyrene Food contact material PAH Migration Potential health risk
a b s t r a c t Eight low-ring PAHs were detected in 21 polystyrene (PS) food contact materials (FCMs) samples while high-ring PAHs (N4 rings) were not found. This is because the reaction pathway for formation of high-ring PAHs consists of more steps than it does for low-high PAHs. The concentrations of Σ8PAH were from 18.9±5.16 ng/g for product colorless fruit fork to 476±52.0 ng/g for foam instant noodle container. These data were far beyond levels of PAHs in other plastics. Of the eight PAHs detected, Phe had the highest average concentration, followed by Nap. These two PAHs collectively accounted for over 80% of the Σ8PAH concentrations in all PS FCMs. Levels of Σ8PAH in expanded PS FCMs were higher than those in extruded ones due to utilization of foaming agent. The concentrations of Σ8PAH were lower in colorless PS FCMs than in colored ones. Auxochromes and chromophores contributed to the change of short-chain hydrocarbons to aromatic hydrocarbon. Simulated migration values of PAHs from PS FCMs to food varied widely. The migration value of Σ8PAH with maximum probability was below 10 ng/g, which the maximum tolerated migration level for substance according to the European Union standards. However, higher migration values were possible and the potential health risk should still be concerned because the simulated migration displayed a log-normal distribution. Furthermore, water was used as food simulant would always lead to an underestimate of PAHs migration to real daily food, and then lead to an underestimate of risk. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Because of the advantages of safe, lightweight, strong, easily processed, and stored, plastics are widely used as packaging materials for foods, pharmaceuticals, detergents, and so on (Paraskevopoulou et al., ⁎ Corresponding author. E-mail address: [email protected] (H.-G. Ni).
http://dx.doi.org/10.1016/j.scitotenv.2017.07.262 0048-9697/© 2017 Elsevier B.V. All rights reserved.
2012). However, small molecules, such as residual monomers, plasticizers, and antioxidants, etc., can migrate from package material into the food in contact with them and be potential pollutants (Lin et al., 2017). These low molecular weight impurities may be ingested via food consumption and induce potential human health risk. A large number of studies on migration of monomers and oligomers from plastic food contact materials (FCM) to foods have been published (Helmroth et al., 2002). For example, migration of styrene monomer from polystyrene
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(PS) based food packages to food simulants have been studied and a large number of reports have been published (Genualdi et al., 2014; Lickly et al., 1995; Lin et al., 2017). Also, there were numerous reports about other impurities other than styrene monomer in PS food contact packaging and consumables. Namely, phthalates, alkylphenols, bisphenol A, and di(2-ethylhexyl)adipate tend to migrate from food packaging into food and create potential human health risk (Fasano et al., 2012; Guart et al., 2011). According to a recent study (Rochman et al., 2013b), polycyclic aromatic hydrocarbons (PAHs) could be formed along with the production and processing of PS products. It is speculated that PAHs should be extremely likely to exist in PS FCMs. However, there is no study on the occurrence of PAHs in PS FCMs, let alone the migration of PAHs from PS FCM to food. Possibly, presence of PAHs in PS may be an unintended consequence and it has not aroused the academic circle's attention yet. Actually, researchers only chanced upon the occurrence of PAHs in PS during their field experiments designed to measure sorption of several organic pollutants to plastic debris in the marine environment (Rochman et al., 2013a; Rochman et al., 2013b). PAHs are ubiquitous and persistent organic pollutants generated during the incomplete combustion of modern biomass and fossil fuels (petroleum and coal) (Lima et al., 2005). Because of their hydrophobic nature, PAHs have a low water solubility and a higher tendency to enrich to fatty foods, e.g., dairy food and fish (Lima et al., 2005). PAHs have been considered a category of important pollutants due to their carcinogenicity, mutagenicity and dioxin-like toxicity (Mumford et al., 1995). Previous studies (Xia et al., 2010; Xia et al., 2013) have revealed that human exposure to PAHs mainly via diet and inhalation. Given that, PAHs migration from PS FCMs to food need to concern. As mentioned above, previous studies reported about various impurities in PS products, but no study on the occurrence of PAHs in PS FCMs. Besides, presence of PAHs in virgin PS pellets (Rochman et al., 2013b) cannot provided convincing information of PAHs in PS FCMs because FCMs are final products and have gone through various manufacture processes other than virgin PS pellets. The aims of the present study were to examine PAHs concentrations in PS FCMs and to assess PAHs migration from PS FCMs to food with a stochastic migration model. Results from these experiments will provide valuable data needed for updating dietary intake values for the impurities other than styrene monomer and plastics additives in PS FCMs.
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Fig. 1. Pictures of various polystyrene food contact materials.
Under finally optimized conditions obtained by our preliminary experiment, 2 g of each PS plastic sample was cut into fragments with a stainless scissors and then put into a 30-mL centrifuge glass tube, surrogate standards were added into each sample as well. The samples were treated by ultrasonic wave for 30 min with 10 mL mixture of acetone and dichloromethane (1:1 in volume) as extraction reagent. After 15 mL hexane was added, the mixture was stirred sufficiently in order to precipitate the polymer. Liquid contents of each sample were centrifuged for 20 min at 4000 rpm at 25 °C to remove solid precipitate. The supernate was concentrate to approximately 1 mL. A C18 SPE cartridge (Agilent Technologies, Inc. USA) was preconditioned with 5 mL dichloromethane and then 5 mL hexane. For sample cleanup, the extract was injected into the preconditioned cartridge entirely and eluted with 5 mL of mixture of hexane and dichloromethane (4:1 in volume). The final extract was concentrated by a rotary evaporator to approximately 1 mL. Three replicates of each sample were conducted. Internal standards were added to each extract prior to instrumental analysis. 2.2. Instrumental analysis and quality control Concentrations of PAHs were qualified with a Shimadzu Model 2010 gas chromatograph coupled with a Model QP2010 plus mass
2. Materials and method 2.1. Sample collection and extraction A total of 21 kinds of PS FCM samples (three replicate samples for each one) with assigned sample numbers (SN) of 1–21 were purchased from the local supermarkets of Shenzhen, China, which were recognized by the PS mark imprinted on the products (Table S1 of the Supplementary Data). Each individual product was also photographed with a view to formulate its use (Fig. 1). The chemical constituents of PS were analyzed and identified using Fourier transform infrared (FT-IR) spectroscopy with NIST library (Fig. 2). All samples were covered with aluminum foil to avoid contamination. The samples were stored at −4 °C until further analysis. Internal standards (2-fluorobiphenyl and p-terphenyl-d14) and surrogate standards (naphthalene-d8, acenaphthene-d10, phenanthrened10, chrysene-d12 and perylene-d12) were purchased from Dr. Ehrenstorfer Gmbh (Augsburg, Germany). A standard solution of 16 US Environmental Protection Agency priority PAHs was purchased from Chem Service (West Chester, PA), including naphthalene (Nap), acenaphthylene (Acy), acenaphthene (Ace), fluorene (Fle), phenanthrene (Phe), anthracene (Ant), fluoranthene (Flu), pyrene (Pyr), chrysene (Chr), benzo[a]anthracene (BaA), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), indeno[1,2,3c,d]pyrene (InP), dibenzo[a,h]anthracene (DBA), and benzo[g,h,i] perylene (BghiP).
Fig. 2. Fourier transform infrared spectra of standard polystyrene, expanded polystyrene instant noodle bowl, extrude polystyrene drink cup.
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spectrometer (Shimadzu, Japan). In the selected ion monitoring mode, electron ionization was utilized. A 30 m DB-5MS fused silica capillary column (0.25 mm i.d., 0.25 mm film thickness; J&W Scientific, Folsom, CA) was used to achieve the separation of gas chromatograph. The mass selective detector was operated in the selected ion monitoring mode for quantitation, and selected samples containing high levels of the analytes were analyzed in the full-scan mode for peak confirmation. Column temperature was programmed from 60 °C to 200 °C at a rate of 10 °C/min, followed by a ramp to 214 °C at a rate of 2 °C/min, to 254 °C at a rate of 5 °C/min (held for 2 min), and to 290 °C at a rate of 18 °C/min (held for 17 min). Injector temperature was programmed for 100 °C to 280 °C at a rate 180 °C/min and held for 50 min. Samples were injected using an automatic sampler and the injector was used in a splitless mode and the carrier gas was high purity helium. Laboratory procedural blanks and spiked blanks samples were processed with each batch of eight samples to monitor procedural contamination. The former are the solvent used in sample processing and used to detect any external contamination during the laboratory process. They are processed along with practical sample. The latter are prepared from the solvent used in sample processing, which is spiked with the target-analyte standards at desirable concentration levels. These samples are used to assess the performance of the entire analytical procedure without the influence of matrix interference. For PAHs, recoveries of the surrogate standards (spiked concentration: 100 ng/mL) were 79 ± 21%, 74 ± 18%, 68 ± 12%, 67 ± 8.5%, and 81 ± 5.9% for naphthalened8, acenaphthene-d10, phenanthrene-d10, chrysene-d12, and perylened12, respectively. Concentrations were determined using the internal calibration technique. The lowest concentration level from the calibration curve was defined as the reporting limit. In the present study, reporting limit for each individual PAH was 2.5 ng/g PS. All the reported concentrations were not corrected with the recoveries of the surrogate standards. 3. Results and discussion 3.1. PAHs in PS Table S2 in the Supplementary Data summarized the concentrations of eight PAHs (Nap, Acy, Ace, Fle, Phe, Ant, Flu, and Pyr) detected in the PS FCM samples. These eight PAHs have the lowest molecular weight of the 16 US EPA priority PAHs. Among them, Nap and Phe were completely detected in all of the 21 PS FCMs (detection rate = 100%), Fle was frequently detected (detection rate N75%), Flu and Pyr were usually detected (detection rate N 35%), and the detection rate of Acy, Ace and Ant were between 10% to 20%. Overall, high-ring PAHs (N4 rings) were not detected in PS FCMs. A possible explanation is that the reaction pathway for formation high-ring PAHs consists of more chemical steps than it does for low-high PAHs. Although short chain hydrocarbon and free radicals can generate PAHs under certain conditions (Wang and Frenklach, 1997), a series of complex reactions reduce chemical yield sharply (Dias, 2013). The sum of the 8 PAH compounds was defined as Σ8PAH. Concentrations of Σ8PAH were18.9 ± 5.16 ng/g for product colorless fruit fork (SN 15) and 476 ± 52.0 ng/g for foam instant noodle container (SN 1) (Fig. 3). These data were far beyond concentrations of PAHs detected in other plastics, such as polyethylene terephthalate (not detect to 1 ng/g), polyethylene (not detect to 13 ng/g), polyvinyl chloride (not detect to 2 ng/g), polypropylene (2 to 6 ng/g) (Van et al., 2012). When PAHs in PS FCM qualified here were compared to those measured in virgin PS pellets by Rochman et al. (2013b), we found that their data (79 to 97 ng/g) fall within the range of our measured Σ8PAH levels. A large range of PAHs levels in PS FCMs may imply the various manufacturing process. Of the eight PAHs detected, Phe was detected with the highest average concentration which reached up to 166 ng/g with a range from 11.5 ng/g to 503 ng/g (Fig. 4 and Table S2), followed by Nap with the average concentration 45.6 ng/g (range: 2.53–109 ng/g). These two PAHs collectively accounted for 88% (range: 59%–99%) of the Σ8PAH
Fig. 3. Concentration of Σ8PAH in polystyrene food contact materials.
concentrations in 21 PS FCMs (Fig. 5 and Table S2). And Acy had the lowest average concentration among the eight PAHs which was 1.68 ng/g with a range from not detected to 11.3 ng/g (Fig. 4). Actually, the residues of PAHs must not be found in the FCMs according to the level set by European Union (Simoneau et al., 2016). If this is the standard, PS FCMs of interest in the present study did fail to reach the standard. Apparently, level of Σ8PAH in expanded PS FCMs (SN 1 and 12) was higher than that in extruded PS FCMs (other samples except for SN 1 and 12). Specifically, the average concentrations of Σ8PAH were 223 ng/g (range: 15.3–656 ng/g) for extruded PS FCMs and 425 ng/g (range: 311–534 ng/g) for expanded PS FCMs (Fig. 6), respectively. This difference may come from the differences of assistant materials and manufacturing process. Expanded and extruded PS is insoluble and non-hygroscopic. The use of them helps to prevent food wastage. Expanded PS is the polystyrene as main raw materials, made by the foam blowing agent interior has countless blockade of the porous materials. The blowing agents usually contain short-chain aliphatic hydrocarbons (Yang et al., 1996) and can generate PAHs with free radicals under certain conditions (Wang and Frenklach, 1997). Interestingly, the concentrations of Σ8PAH were lower in colorless and transparent PS FCMs (SN 7, 15, and 16) than those in colored PS FCMs (other samples except for SN 7, 15, and 16). For instance, the average concentration of Σ8PAH in dyed PS FCMs (264 ng/g with a range of 24.8–656 ng/g) was nearly three times that of colorless PS FCMs (97.4 ng/g with a range of 15.3–166 ng/g) (Fig. 6). Usually, a colored molecule has chromophore and auxochrome and the two put together if they are in a conjugated system will create color (Rachuta et al., 2017; Sromovsky et al., 2017). Chromophore is usually electron-withdrawing, and auxochromes are normally electron-donating. Unsaturated bonds and chromophores contribute to the growth of short-chain hydrocarbons to aromatic hydrocarbon (Wang and Frenklach, 1997).
Fig. 4. Level of eight detected PAH congeners in polystyrene food contact materials.
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according to corresponding molar mass of the migrants, AP, and T (Brandsch et al., 2002); next, calculate the migration using Eq. (2) (Chung et al., 2002); after that, repeat step 2 a large number of times (10,000 times in the present study); finally, plot a histogram of the obtained migration values. 10454 DP ¼ 104 exp AP −0:1351Mr 2=3 þ 0:003M r − T
ð1Þ
where DP is diffusion coefficient of migrant within FCMs. The parameter AP has the role of a “conductance” of the polymer matrix towards the diffusion of the migrant (Brandsch et al., 2002). Mr is molar mass of the migrant (Table S3). T is temperature (298 K). M F;t 2 Dt 0:5 Dt ¼ − MP;0 LP π αLP 2
Fig. 5. The concentration profiles of PAH congeners in polystyrene food contact materials.
The detection of PAHs in PS FCMs reconfirmed that PS is either source or sink for PAHs (Rochman et al., 2013a). On one hand, plastics are most commonly derived from petrochemicals and petroleum materials usually contains some PAH congeners, especially, lighter PAHs (Soclo et al., 2000). On the other hand, the precursors of PAHs (e.g., styrene and benzene) generated during the course of PS production are propitious to form PAHs (Kwon and Castald, 2008; Lithner et al., 2011). Actually, heating again to process and shape end products from PS pre-production pellets also results in more PAHs. Moreover, PS also can be a sink for PAHs via sorption in the marine environment (Van et al., 2012). It is therefore speculated that the presence of PAHs in PS FCMs should be attributed to two causes: formation during production and sorption during use. This may pose a potential risk of dietary exposure to PAHs migrated from PS FCMs. 3.2. Migration simulation Detection of PAHs in PS FCMs triggered concerns that the food contacting with PS is on the edge of the likelihood of contamination. A large number previous works have confirmed that monomers and additives in FCMs can migrate to foods (Brandsch et al., 2002; Paraskevopoulou et al., 2012; Verzera et al., 2009; Zabaniotou and Kassidi, 2003). It is therefore speculated that PAHs migration from PS FCMs to food should be no doubt that it will happen (Genualdi et al., 2014; Paraskevopoulou et al., 2012). To obtained migration of probability distributions, A Monte Carlo procedure was conducted with the following steps: first, calculate the diffusion coefficients of eight detected PAHs in PS FCMs using Eq. (1)
Fig. 6. Comparison of PAHs in different polystyrene food contact materials (expanded polystyrene versus extrude polystyrene; colorless versus colored polystyrene).
ð2Þ
where MF,t is migrant amount in food at time t, MP,0 represents initial migrant amount in PS FCMs. α refers to mass ratio of migrant in food to that in FCM at equilibrium, where α = KFP VF/VP. KFP refers to partition coefficient of migrant between food and FCMs (CF, ∞/CP, ∞). VF/VP values were measured in the present study (ranging from 8.333 to 3333) and the data distribution was examined with a software Crystal Ball version 11.0. The partition coefficient (KFP = 0.001) was defined as the ratio of concentration in food simulant to PS FCMs, is 1 for migrants with high solubility in the food simulant and 0.001 for migrants with low solubility, according to previous study (Baner et al., 1996). Lp thickness of FCMs (ranging from 0.001 to 0.1 cm with a normal distribution). t is migration time. It should be specially explained that to simulate the practical situation, T was selected at 298 K and t was chosen 10 days. More details of the derivation procedures of migration model and the required parameters (Table S3) for simulation were provided in the Supplementary Data. Fig. 7 shown the results of the Monte Carlo procedure for a water food simulant. Apparently, the migration values varied widely, which indicated that both low and high migration values were possible. This is due to the large variation in MP,0 and Lp. The probability distribution of the simulated migration for PAH congeners from PS FCMs to the water food simulant were present Fig. 7. The median values of the migration were estimated to be 0.28, 0.01, 0.01, 0.35, 0, 1.02, 0.02, and 0 for Nap, Acy, Ace, Fle, Ant, Phe, Flu, Pyr, respectively. The cumulative of the frequencies displayed in Fig. S1 shown the probability of observation a migration value less than a given limit, for example, 0.1 ng/g PS. (Fig. 7). Unfortunately, there is no migration limit for PAH so far, to our knowledge. Assumed that the specific migration limit of individual PAH congener is 0.1 ng/g PS, in our example, the probabilities that the migration are less than or equal to this value are 0.04, 0.99, 0.99, 0.79, 0.88, 0.04, 0.97, and 0.89 for Nap, Acy, Ace, Fle, Ant, Phe, Flu, Pyr, respectively. These data corresponds with a chance of 0.96, 0.01, 0.01, 0.21, 0.12, 0.96, 0.03, and 0.11 of exceeding to this limit, for Nap, Acy, Ace, Fle, Ant, Phe, Flu, Pyr, respectively. These results suggested that migrations of Nap and Phe from PS FCMs to food need more concerns. The sum of the median values for those eight PAH congeners (1.70 ng/g) was still below 10 ng/g, which the maximum tolerated migration level for substance according to Regulation (EC) No 450/2009 (European Commission, 2011). However, the simulated migration for all eight PAH congeners displayed the log-normal distribution (Fig. S1). This suggested that higher migration values were possible and the potential health risks merit special attention. Besides, water was used as food simulant in the present study, it is therefore the simulated results cannot reflect the reality of PAHs migration to real daily food. PAHs belong hydrophobic organic pollutants (Ni et al., 2011), they tend to have higher concentrations in fatty food (such as dairy products and fish) than in water. Obviously, initial levels of migrants in PS FCMs, thickness of FCMs, migration time and temperature, all these factors affect migration based on the migration model. Apparently, the migration decreases with the increase of LP, and the degree of influence on the migration decreases
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Fig. 7. Probability distribution of the migration of PAHs from polystyrene food contact material to a water food simulant at 298 K during 10 days of incubation.
with the increasing thickness. Except for Lp, all the other factors have substantial positive impacts to migration of PAHs from PS FCMs to food simulant. For example, migration is favored by increased temperature and migration time. 4. Conclusions Low-ring PAHs were detected in PS FCMs because of formation during production process and sorption during PS FCMs use. Utilization of foaming agent results in higher PAHs levels in expand PS than in extrude PS. Auxochromes and chromophores contribute to the higher concentrations of PAHs in colored PS FCMs compare to colorless PS FCMs. The sum of the median values for migration of the eight PAH was below the maximum tolerated migration level for substance according to the European Union standards. Obviously, using water as food simulant may underestimated the migration values for lots of foods carry essential oils. According to the migration model, initial levels of migrants, thickness of FCMs, migration time and temperature, all these factors affect migration. A certain storage conditions can decrease the potential migration of PAHs from PS FCMs to foods. For instance, short time and low temperature for storage could reduce the migration value. Measurable byproducts and processing aids used in the plastic synthesis processes can remain in the final product. Knowledge of these materials is indispensable for safety assessment of the plastics for use in FCMs. The migration rate of low molecular weight impurities from FCMs into foods depends on the molecular structure and the aggregate nature of the plastic material. To estimate migration of impurities from plastics to food, a basic knowledge of the structure of the plastic and food components and their possible influences on migration is necessary. Acknowledgment This study was financially supported by the Shenzhen Science and Technology Research and Development Fund (JCYJ20150828092938205) and the National Natural Science Foundation of China (No. 41573085). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.scitotenv.2017.07.262. References Baner, A., Brandsch, J., Franz, R., Piringer, O., 1996. The application of a predictive migration model for evaluating the compliance of plastic materials with European food regulations. Food Addit. Contam. 13, 587–601.
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