Nanofiller Presence Enhances Polycyclic Aromatic Hydrocarbon (PAH

Apr 11, 2017 - This research was supported by the National Science Foundation (NSF grant nos. 1436450 and 1350789). The mention of trade names, produc...
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Nanofiller Presence Enhances Polycyclic Aromatic Hydrocarbon (PAH) Profile on Nanoparticles Released during Thermal Decomposition of Nano-enabled Thermoplastics: Potential Environmental Health Implications Dilpreet Singh,† Laura Arabella Schifman,‡,§ Christa Watson-Wright,† Georgios A. Sotiriou,†,∥ Vinka Oyanedel-Craver,‡ Wendel Wohlleben,⊥ and Philip Demokritou*,† †

Center for Nanotechnology and Nanotoxicology, T. H. Chan School of Public Health, Harvard University, 665 Huntington Avenue, Boston, Massachusetts 02115, United States ‡ Department of Civil and Environmental Engineering, University of Rhode Island, 1 Lippitt Road, Kingston, Rhode Island 02881, United States § National Risk Management Research Laboratory, Office of Research and Development, United States Environmental Protection Agency, Cincinnati, Ohio 45268, United States ∥ Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, Stockholm 17177, Sweden ⊥ BASF SE, Material Physics, Ludwigshafen 67056, Germany S Supporting Information *

ABSTRACT: Nano-enabled products are ultimately destined to reach end-of-life with an important fraction undergoing thermal degradation through waste incineration or accidental fires. Although previous studies have investigated the physicochemical properties of released lifecycle particulate matter (called LCPM) from thermal decomposition of nanoenabled thermoplastics, critical questions about the effect of nanofiller on the chemical composition of LCPM still persist. Here, we investigate the potential nanofiller effects on the profiles of 16 Environmental Protection Agency (EPA)priority polycyclic aromatic hydrocarbons (PAHs) adsorbed on LCPM from thermal decomposition of nano-enabled thermoplastics. We found that nanofiller presence in thermoplastics significantly enhances not only the total PAH concentration in LCPM but most importantly also the high molecular weight (HMW, 4−6 ring) PAHs that are considerably more toxic than the low molecular weight (LMW, 2−3 ring) PAHs. This nano-specific effect was also confirmed during in vitro cellular toxicological evaluation of LCPM for the case of polyurethane thermoplastic enabled with carbon nanotubes (PU-CNT). LCPM from PU-CNT shows significantly higher cytotoxicity compared to PU which could be attributed to its higher HMW PAH concentration. These findings are crucial and make the case that nanofiller presence in thermoplastics can significantly affect the physicochemical and toxicological properties of LCPM released during thermal decomposition.

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INTRODUCTION

NEPs incorporate engineered nanomaterials (ENMs, less than 100 nm in at least one dimension)21 for superior performance over their micrometer-sized counterparts. ENMs, because of their small size and therefore high surface area-tovolume ratio,21 have been implicated in various adverse biological6,7,21−36 and environmental effects.37−44 In addition, the high specific surface area of ENMs enables them to act as catalysts in various chemical and biomolecular reactions.45 As a

Advances in nanotechnology have led to an increased proliferation of nano-enabled products (NEPs) in the consumer market sectors such as cosmetics,1 printer toners,2−7 building and construction materials,8 nanopaints and coatings,9,10 car tires,11 biomedical12−15 and electronic devices,16 nanonutraceuticals,17 and agriculture,18,19 to name a few. Estimates have projected nanotechnology market value to reach US $4.4 trillion by 2018.20 Although nanotechnology clearly has a significant economic impact, the environmental health and safety implications of nanotechnology are not clearly understood yet. © 2017 American Chemical Society

Received: Revised: Accepted: Published: 5222

December 20, 2016 March 10, 2017 April 11, 2017 April 11, 2017 DOI: 10.1021/acs.est.6b06448 Environ. Sci. Technol. 2017, 51, 5222−5232

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Environmental Science & Technology

In this companion study, the potential nanofiller effects on the organic chemistry of the released LCPM from thermal decomposition of nano-enabled thermoplastics (Table S1, Supporting Information) and its toxicological profile were assessed. Specifically, we looked at how the presence of nanofiller impacts the distribution of the 16 Environmental Protection Agency (EPA)-priority PAH species66 (Table S2, Supporting Information) in the LCPM that is released during the ramping of decomposition temperature from ambient temperature (25 °C) to 800 °C. PAHs are important byproducts of thermal decomposition of carbonaceous materials such as plastics,67 and some of them are classified as probable or possible human carcinogens by the EPA.68 This is the first study, to the best of our knowledge, showing the effect of nanofillers embedded in industrially relevant NEPs (specifically, nano-enabled thermoplastics) on the PAHs adsorbed onto released LCPM particles from thermal decomposition. Of note, this study does not aim to simulate real-world incineration of nano-enabled products in an industrial waste incinerator; instead, it is a fundamental study on the thermal decomposition behavior of NEPs with a specific focus on understanding the role of nanofiller in PAH formation.

result, there is a growing concern about the fate of ENMs incorporated in NEPs as these products traverse through the manufacture−consumer use−end-of-life continuum of the product lifecycle. These ENMs may be released freely or as part of the product matrix over the lifecycle and may be accompanied by release of certain gaseous emissions (such as semivolatile organic compounds (sVOCs)), depending on the specific lifecycle scenario that the product undergoes, e.g., mechanical stress,46 weathering (ultraviolet light and precipitation).47,48 Published data for mechanical degradation of various nano-enabled thermoplastics make the case that the physicochemical properties of released lifecycle particulate matter (LCPM) are determined primarily from the matrix of the product rather than the nanofiller.46,49,50 The end-of-life of NEPs via incineration/thermal decomposition is an increasingly important scenario because of ever increasing volumes of nanowaste. A recent study estimated that approximately 20 000 t of ENMs are expected to end up in municipal incineration facilities worldwide on an annual basis, especially in developed countries such as North America and Europe.51,52 Therefore, it is important to address the possible environmental health and safety implications of the complex byproducts53 resulting from incineration/thermal decomposition of NEPs. There have been scarce studies investigating the thermal decomposition end-of-life scenario of NEPs. The limited studies done so far investigate the incineration of pure ENMs54,55 or nanomaterial-spiked wastes56,57 and primarily focus on characterizing the size and concentration of released aerosols.58−62 Addressing the lack of standardized methodologies to study thermal decomposition of NEPs, the authors recently developed an integrated exposure generation system (INEXS) (Supporting Information, Figure S1)63 that is a versatile platform to perform thermal decomposition of various NEPs under controlled combustion conditions and investigate the physicochemical, morphological and toxicological properties of the byproducts (i.e., released aerosol or LCPM and residual ash). The platform has been used already to investigate thermal decomposition of various nano-enabled thermoplastics,64,65 employing both carbonaceous (carbon nanotubes (CNTs), carbon black, organic pigment) and inorganic (iron oxide, Fe2O3; titania, TiO2) nanofillers with various mass loadings in the thermoplastic matrices. Sotiriou et al.64 and Singh et al.65 demonstrated that there was no detectable release of carbonaceous nanofillers in the released aerosol during thermal decomposition, but there was nanofiller-related release observed for the metal oxide nanofillers (Fe2O3 and TiO2) that was dependent on the nanofiller loading. The released aerosol concentration and size were affected by both the thermoplastic matrix and the nanofiller loading. The chemical composition of the released LCPM particles, however, was primarily organic (>99 wt %) irrespective of the presence or loading of nanofiller and hence apparently governed by the host polymer matrix. While in terms of the elemental and organic carbon (EC/OC) contents of LCPM our published study65 makes the case that the matrix of thermoplastic and not the nanofiller itself is the determinant, questions remain in terms of the chemical speciation of organic compounds present in LCPM particles, especially when metal and metal oxides are used as nanofillers that can have potential catalytic effects on the formation of organic species such as polycyclic aromatic hydrocarbons (PAHs).

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MATERIALS AND METHODS

Pure and Nano-enabled Thermoplastics. The industrially relevant nano-enabled thermoplastics used in this investigation were synthesized by our industrial collaborators and also in-house and are summarized in Table S1 (Supporting Information). Polyurethane (PU), polyethylene (PE), polypropylene (PP), polycarbonate (PC), and ethylene-vinyl acetate (EVA)-based thermoplastics were employed. It is worth noting that thermoplastics with no nanofillers were also synthesized and used as control materials. The different nanofillers used for our investigation are currently used in real-world industrial applications such as CNTs in electronics,69 iron oxide (Fe2O3) in biomedical devices,70 titania (TiO2) in food packaging,71 and diketopyrrolopyrrole organic pigments as building blocks for organic semiconducting polymers.72 It is worth noting that certain types of nanofillers, especially the metallic ones (Fe2O3 and TiO2), may display catalytic behavior that may affect released LCPM properties and potential toxicological outcomes. Observation of potential nanofiller effects across these different nanofiller types will help make the case for such effects for other similar categories of nanofillers as well. It is however worth noting that the data presented for these polymers and nanofillers might not be generalizable to other polymer/nanofiller combinations. Briefly, the CNTs used in our study are multiwalled and rope-like agglomerates with an average diameter of ∼10 nm and length from 99 wt %)

nanofiller and increasing nanofiller loading on the equivalent toxicological concentration of a HMW PAH called benzo[a]pyrene (BaP). Thermal decomposition (TD = 800 °C) of PUCNT results in a significantly greater e-BaP PAH concentration than pure PU (PU-CNT,: 69.7 ppm; PU, 43.6 ppm, p < 0.05). Similarly, PC-CNT exhibits a nearly 7 times higher e-BaP PAH concentration than pure PC (PC-CNT, 332.2 ppm; PC, 48.4 ppm, p < 0.01). PC-CNT also shows the highest e-BaP PAH concentration among all thermoplastics investigated. Interestingly, the e-BaP concentration for PP-CNT is significantly lower than that for pure PP (PP-CNT, 21.9 ppm; PP, 58.1 ppm, p < 0.0001), even though PP-CNT shows significantly higher total and HMW PAHs as compared to pure PP (Figures 1 and 3). For the EVA polymer, the e-BaP PAH concentration first decreases as TiO2 loading in EVA polymer increases from 0 to 2 wt % and then increases as the loading is increased from 2 to 15 wt % (EVA, 18.3 ppm; EVA-2%, 6.4 ppm; EVA-5%, 20.4 ppm; EVA-15%, 31.8 ppm, p < 0.0001), indicating a significant nanofiller loading effect.

Figure 5. In vitro toxicological evaluation of the released LCPM (PM0.1) from the thermal decomposition of PU and PU-CNT while ramping from 25 °C to a final temperature of 800 °C using multiple end points, namely, (a) cytotoxicity and (b) metabolic activity at different delivered doses following 24 h exposure in human small airway epithelial cells. 5226

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higher total PAH content in LCPM than for PU-CNT (0.1 wt % CNTs). This could be attributed to the higher concentration of catalytic metals such as aluminum. It is reasonable to say that the catalytic activity of the nanofillers in the formation of PAHs is not just due to the presence of metal/metal oxides that are well-known combustion catalysts94,111,112 but also because these metal/metal oxides are in the nanoscale. Nanomaterials are more effective combustion catalysts compared to their bulk forms because of their tremendously high surface area to volume ratio and different physicochemical properties.45,56,113,114 In a previous study,56 it was demonstrated that PAH emissions from combustion of waste spiked with nanomaterials were much higher than those from waste spiked with the bulk counterparts at the same loading. In this study, micrometer-sized fillers were not used as comparative controls; therefore, a nanometer vs micrometer comparison of the observed catalytic effects could not be made unfortunately. Such investigations need to be performed in future studies to address the size-specific nature of the observed catalytic effects. The high surface reactivity and catalytic activity of the nanofillers toward PAH formation is further amplified when nanofiller loading in the thermoplastic matrix increases, because of the higher amount of available nanofiller surface area, thus explaining the nanofiller loading effect on total PAH concentration. In addition, it has been shown recently by the authors that metal oxide nanofillers (such as TiO2 and Fe2O3) can be released into the LCPM (aerosol) during thermal decomposition, depending on their initial nanofiller loading in the thermoplastic.65 For example, when TiO2 loading in EVA was increased from 2 to 5 wt %, the TiO2 nanofiller concentration in the released LCPM also increased. The presence of these metals (or their oxides) on the released LCPM surface and their increase in concentration in the LCPM with higher nanofiller loading can further catalyze the formation of PAHs from gaseous precursors, as in a previous study where metallic iron on the surface of growing soot particles was hypothesized to catalyze the formation of PAHs from acetylene gas.86 In summary, the nanofiller and its loading effect on total PAH concentration can be readily explained by the high catalytic surface area offered by the nanofillers in the thermoplastic matrix as well as their presence (especially the inorganic nanofillers, Fe2O3 and TiO2) in the released LCPM particles which further increases the available catalytic surface for PAH formation. Furthermore, in addition to the increase in total PAH content of LCPM aerosol, it was demonstrated in this study that the HMW PAHs (4−6 ring) are also significantly enhanced in the presence of nanofiller (i.e., in the case of CNTs and TiO2). This implies that the presence of the nanofiller promotes the conversion from low to high molecular weight PAHs. Wey et al.100 reported that the presence of heavy metals such as lead (Pb) and chromium (Cr) in the incinerator fly ash can catalyze the formation of multiple-ring (4−6 ring) PAHs by providing more adsorptive sites and reducing the activation energy for reaction between gaseous precursors and LMW PAHs in the gas phase. Another study by Yan et al.102 found that the addition of copper and cupric oxide additives to coal can promote the formation of multiple ring PAHs. Large PAHs are typically formed by reactions between the smaller PAHs (e.g., naphthalene (Nap)) and various resonancestabilized radicals such as cyclopentadienyl radical (C5H5) and indenyl radical (C9H7).115 Therefore, the presence of

irrespective of the nanofiller presence. However, in the present study, we observe a significant nanofiller effect on the speciation profile of OC because of an increase in total and HMW PAH content of the LCPM in the presence of the nanofiller, which is also linked to increased cytotoxicity. This important finding contradicts other lifecycle perspective studies of NEPs that make the case that the matrix is the primary determinant of LCPM physicochemical and toxicological properties and not the nanofiller.46,82 It is now clear from this study that both the polymer matrix and the nanofiller have important roles in determining the released LCPM chemistries and potential toxicity. In total, the presence of nanofiller and increased nanofiller loading not only results in a total PAH content increase but also results in a significant increase in the concentration of HMW (4−6 ring) PAHs that are considerably more carcinogenic than the LMW (2−3 ring) PAHs.81 PAH Formation Mechanisms. Several studies have reported the catalytic effect of metals and metal oxides on the formation of PAHs during combustion processes, which might help to explain the observed nanofiller effect on PAH concentration in released LCPM.56,57,83−104 Certain metals/metal oxides tend to favor PAH production, while others tend to decrease it, and some may be more catalytic than the others. For example, in an earlier study,56 TiO2 nanomaterial spiked in polyethylene plastic waste significantly increased the emission factor of total PAHs compared to the control waste without TiO2 and the PAH emission factor increased when TiO2 loading was increased from 0.1 to 10 wt %, which is in agreement with our results. On the other hand, in the same study,56 the presence of Fe2O3 nanoparticles in surrogate waste decreased the emission factor of particle-bound PAHs compared to the control waste and emissions further decreased as nanofiller loading increased from 0.1 to 10 wt %, whereas in our study, total PAH emissions increased with increasing Fe2O3 loading, although not significantly. Another study86 also reported that the addition of an iron additive to a carbonaceous fuel more than doubled the total PAH concentration, which agrees with our results since there is an increase (although not significant) in total PAHs in the presence of Fe2O3 nanofiller. The increase in total PAH emissions in the presence of CNT-enabled thermoplastics may also be explained in part due to the presence of certain trace metals in CNTs that might play a catalytic role in PAH production. Among the metals found in these CNTs, aluminum (Al, 1.72 wt %), iron (Fe, 0.65 wt %), and molybdenum (Mo, 0.14 wt %) were the most abundant (see Materials and Methods in the Supporting Information). A previous study described the catalytic role of alumina (Al2O3) in the conversion of acetylene gas to PAHs, which was dependent on the role of Al3+ ions as Lewis acid sites.97 It is therefore possible that under the oxygen-rich conditions in the furnace, aluminum in the CNTs oxidized to form aluminum oxide (or alumina)105 that in turn catalyzed formation of PAHs during the combustion of thermoplastics. Alternatively, emissions of PAHs may also come from the CNTs themselves since it is well known that PAHs can form during the manufacture of CNTs (via catalytic chemical vapor deposition),106 which can then strongly adsorb to the CNT surface107,108 and finally desorb when heated during thermal decomposition.109,110 However, it is not possible to confirm this since the raw CNTs were not analyzed for their PAH content. It is worth noting that CNTs also exhibit a nanofiller loading effect on the total PAH emissions. Higher CNT loading in PP-CNT and PC-CNT (3 wt % CNTs each) results in 5227

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from the nano-enabled thermoplastic vs the pure thermoplastic. The higher toxicity can be attributed to the presence of significantly more HMW PAHs in the LCPM from the nanoenabled thermoplastic compared to that from the pure thermoplastic (with no nanofiller). Although it is possible that the results may not be generalizable to other polymer/ nanofiller combinations, nevertheless, the investigated panel of polymer matrices and nanofillers provides important insight into the potential role of nanofillers and their loading in determining the physicochemical and toxicological properties of released LCPM during thermal decomposition. It is however important to note that our INEXS setup does not mimic an actual industrial waste incineration scenario but provides important insights into the potential role of nanofiller properties in the thermal decomposition behavior of nanoenabled thermoplastics and associated environmental health and safety implications. These insights will be useful for incineration facility managers in designing better exposure control strategies in order to minimize exposure for facility workers. This would especially be important in developing countries where adequate emission control equipment may not be installed and where open waste burning is commonly practiced. Moreover, such findings will help in assessing and managing risk during situations like uncontrolled building fires where there may be no engineered exposure controls for public health protection. Future studies should include investigations of PAHs along with other hazardous air pollutants like dioxins and furans under both complete and incomplete combustion scenarios, where the latter is more likely to be encountered in the case of incidental fires in buildings. Nano-enabled building materials, an increasingly important category of nano-enabled products, should be investigated especially if catalysts are inherently associated with the nanofiller (e.g., aluminum in CNTs) or if the nanofiller itself is known to act as a catalyst (such as Fe2O3) and should then be compared to other metal-containing products in order to differentiate the nano-specific effect (due to unique physicochemical properties of ENMs) from the metal-associated catalytic effect. Changing other thermal decomposition parameters such as temperature, heating rate, and residence time would also be important in assessing the factors that lead to formation of these hazardous combustion byproducts. Finally, in addition to fundamentally understanding the mechanisms involved in the thermal decomposition of nano-enabled products, real-world incineration studies on industrially relevant nano-enabled products are warranted that would address knowledge gaps pertaining to environmental health and safety implications of incinerating waste containing NEPs in an industrial incinerator.

nanofiller (e.g., TiO2) in the released LCPM might act to stabilize these radicals, thereby reducing the high energy barrier required for conversion from small to large PAHs.56 Also, it was reported in a previous study83 that certain metal additives added to a hydrocarbon/air flame decreased the concentration of hydroxyl (OH) radicals that are responsible for PAH destruction via oxidation.86 Hence, it is possible that the metal/ metal oxide-related nanofillers in our study suppressed OH production, thereby increasing the total PAH as well as HMW PAH concentration. Estimated BaP Toxic Potential of PAHs. In the presence of nanofiller, the e-BaP PAH concentrations increase for most nano-enabled thermoplastics compared to the pure ones, except for PP-CNT, which showed a significant decline with respect to PP. These observations can be readily explained since nanofiller presence increases formation of the HMW PAHs, which commonly possess higher toxic equivalency factors75 than the LMW PAHs. For PP-CNT, even though the HMW PAHs increase in comparison to PP, the concentration of the most toxic (highest TEF) PAH, i.e., dibenz[a.h]anthracene (DaA), was nondetectable in the LCPM from PP-CNT, leading to its lower e-BaP concentration than PP. Thus, based on this equivalent measure of toxicity that is entirely chemical based (in this case, a mixture of PAHs), the released LCPM from nano-enabled thermoplastics can be predicted to be more toxic than that from pure thermoplastics because of its higher e-BaP concentration as observed. It may be noted, however, that this prediction of LCPM toxicity is only based upon its PAH chemical content rather than the complete toxicological profile of LCPM nanoparticles. Released LCPM Cellular Toxicity in the Presence of Nanofiller for the Case of PU-CNT. In order to link the observed nanofiller effects on the PAH content and speciation to the released LCPM toxicity, a cellular toxicological assessment was also performed for the case of PU and PUCNT. Investigation of the in vitro toxicity of the released LCPM from PU and PU-CNT against SAEC indeed shows that PU-CNT LCPM is significantly more toxic than that from pure PU as confirmed by cytotoxicity and metabolic activity (Figure 5). This is in agreement with the significantly higher e-BaP PAH concentration estimated for PU-CNT compared to PU (Figure 4). It is thus possible that the increase in total PAH content and especially the HMW and more toxic PAHs in the presence of nanofiller (such as CNT in PU matrix) results in higher toxicity of the released LCPM particles from thermal decomposition of nano-enabled thermoplastics. Although only the PU/PU-CNT thermoplastics were investigated for toxicity, overall results of the study indicate that the presence of nanofiller plays an important role in influencing the physicochemical and hence toxicological properties of released LCPM for the specific lifecycle scenario of thermal decomposition. This study demonstrates that the presence of nanofiller and an increase in nanofiller loading in nano-enabled thermoplastic materials significantly enhance the concentration of total PAHs adsorbed onto released nanoparticulate matter during thermal decomposition. More importantly, the concentration of the more toxic 4−6 ring HMW PAHs also significantly increases along with the total level of PAHs. The catalytic properties of the metal/metal oxides combined with the presence of a high number of active surface sites in these nanofillers could be the likely drivers of higher PAH production. Toxicological evaluation reveals a higher toxicity of the released LCPM

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b06448. Materials and Methods; schematic of INEXS; TEM scans of nano-enabled thermoplastics; summary of the thermoplastics and nanofillers used; summary of the 16 EPApriority PAHs investigated; in vitro dose metrics (administered and delivered doses) for LCPM (PM0.1) particles; colloidal characterization of LCPM (PM0.1) particles in deionized (DI) water and small airway basal media (SABM) (PDF) 5228

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AUTHOR INFORMATION

Corresponding Author

*Tel: +1 617 432 3481, E-mail: [email protected]. ORCID

Dilpreet Singh: 0000-0003-2090-1693 Laura Arabella Schifman: 0000-0003-4700-5530 Georgios A. Sotiriou: 0000-0001-5040-620X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Science Foundation (NSF grant nos. 1436450 and 1350789). The mention of trade names, products, or services does not convey and should not be interpreted as conveying official EPA approval, endorsement, or recommendation.

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