A Thermodynamic Approach for Assessing the Environmental

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A Thermodynamic Approach for Assessing the Environmental Exposure of Chemicals Absorbed to Microplastic Todd Gouin,*,† Nicola Roche,† Rainer Lohmann,‡ and Geoff Hodges† † ‡

Safety and Environmental Assurance Centre, Unilever, Colworth Science Park, Sharnbrook, Bedfordshire, U.K., MK44 1LQ Graduate School of Oceanography, University of Rhode Island, South Ferry Road, Narragansett, Rhode Island, 02882 United States

bS Supporting Information ABSTRACT: The environmental distribution and fate of microplastic in the marine environment represents a potential cause of concern. One aspect is the influence that microplastic may have on enhancing the transport and bioavailability of persistent, bioaccumulative, and toxic substances (PBT). In this study we assess these potential risks using a thermodynamic approach, aiming to prioritize the physicochemical properties of chemicals that are most likely absorbed by microplastic and therefore ingested by biota. Using a multimedia modeling approach, we define a chemical space aimed at improving our understanding of how chemicals partition in the marine environment with varying volume ratios of air/water/organic carbon/polyethylene, where polyethylene represents a main group of microplastic. Results suggest that chemicals with log KOW > 5 have the potential to partition >1% to polyethylene. Food-web model results suggest that reductions in body burden concentrations for nonpolar organic chemicals are likely to occur for chemicals with log KOW between 5.5 and 6.5. Thus the relative importance of microplastic as a vector of PBT substances to biological organisms is likely of limited importance, relative to other exposure pathways. Nevertheless, a number of data-gaps are identified, largely associated with improving our understanding of the physical fate of microplastic in the environment.

’ INTRODUCTION Societal demand for plastic material continues to increase, with global plastic manufacture and consumption estimated to be 308 million tonnes annually.1 It is notable that approximately 50% of all plastic used is for single-use disposable applications.2 In Europe, a combination of plastic recovery programs, including mechanical recycling, feedstock recycling, and energy recovery (i. e., combustion of waste for energy generation) have resulted in a recovery rate of only 39% of plastic waste generated in 2003.2 Relatively poor plastic recovery rates, combined with inappropriate disposal of plastic products, is thus a significant factor influencing the potential for plastic debris to enter the environment.3,4 Unfortunately, there is a paucity of data quantitatively assessing the release rates of plastic to the terrestrial and aquatic environments. Nevertheless, based on a number of semiquantitative and/or qualitative assessments, the observation of plastic debris and its temporal and spatial accumulation, particularly in the marine environment, imply cause for concern.3-11 Whereas a number of studies report the effects of macroplastic (>5 mm) on the marine environment (see for instance refs 3 and 12), concerns regarding the potential effects of microplastic (99%. The lines represent 33% partitioning to air (blue), water (green), and organic carbon (brown). The dimensions of the model environment used in calculating the distributions are representative of the coastal environment, which includes 55 m3 of polyethylene and 6.91  108 m3 of organic carbon (Supporting Information).

the results of the food-web model describe the uptake of chemical across the GI tract as a steady-state process, which therefore does not consider the kinetics of desorption of chemicals from the plastic. Kinetic factors that likely influence this process will be related to the gut retention time of the organism and the extraction efficiency of the gastrointestinal fluids for polyethylene plastic. A more sophisticated dynamic model would thus be required to directly assess desorption of chemicals from the plastic and uptake across the GI tract, with respect to time. Consequently, the model used in this study is limited to assessing the relative importance of exposure to chemicals from the natural diet versus digestion of plastic debris, and to identify potential data-gaps in our understanding using a steady-state food-web model. To directly assess the influence of adding microplastic to the Arnot and Gobas32 bioaccumulation food-web model, we have modeled a suite of hypothetical organic chemicals with varying partition coefficients of one log unit of between 1 and 8 for log KOW, and -4 and 2 for log KAW. All chemicals are assumed to be perfectly persistent. The microplastic is assumed to be buoyant, and therefore resides in the water column, where it absorbs chemicals, based on their log KOW, and is then consumed by organisms at a rate of 10% of an organisms’ diet. Differences in the chemical concentration for each of the organisms in the foodweb with and without plastic are then derived as an estimate of the relative influence that inclusion of microplastic in an organisms’ diet may have on the bioaccumulation potential of an HOC. 1468

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Figure 2. Plot of the mass fraction versus log KAW and log KPE-W, and illustrating the distribution of 32 chemicals for which measured log KPE-W are available (squares). Also illustrated are regions representing FA (blue), FW (green), and FPE-W (brown), where chemicals will partition >99%. The lines represent 33% partitioning to air (blue), water (green), and polyethylene (brown). Part (a) represents an open ocean environment with 1200 m3 of polyethylene and (b) represents an open ocean environment with 106 m3 of polyethylene.

Figure 3. CEMC SFU Agro food-model results for reductions to body burden mass for a piscivorous fish consuming 10% microplastic. The encircled areas represent the relative positions of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs).

’ RESULTS AND DISCUSSION Equilibrium Partitioning and Chemical Space Diagrams. The volume ratio of air/water/organic carbon/polyethylene for the marine coastal environment is 6.6  1012:6.55  1010:1.26  107:1. Figure 1 is a plot of the mass fraction versus log KAW and log KOC, which graphically illustrates the equilibrium partitioning space of a number of environmentally relevant chemicals for which measured KOC 34 data are available, within the marine coastal environment defined in this study (Supporting Information). Only those substances with log KOC > 5.5 are likely to partition significantly (>99%) to organic carbon. Based on the volume ratios between organic carbon and polyethylene in this system, in which the organic carbon present in the system is 1.26  107 times greater than the polyethylene, it is reasonable to expect that the majority of partitioning in this system will be to the organic carbon fraction. Indeed, the results shown in Figure 1

do not include polyethylene because partitioning to polyethylene in this system is 7. Figure 2a illustrates the equilibrium partitioning of 32 chemicals, for which measured KPE-W values are available,22,33 in an open-ocean environment with an air/water/polyethylene ratio of 3  1011:6.0  109:1 (i.e., volume of polyethylene = 1200 m3). Results shown in Figure 2a suggest that, based on the relatively small volume of polyethylene in the environment, organic chemicals of environmental relevance partition between the air and water, with significant cycling between these two compartments. If we assume, however, that the amount of plastic in this system will continue to accumulate as a result of continuous discharge, then it is possible that the volume of plastic will increase to a level where a significant fraction of chemical may partition into it. Figure 2b illustrates the equilibrium partitioning in an open-ocean environment where the volume of plastic has increased by 3 orders of magnitude (i.e., 106 m3). In this system we begin to observe that substances with log KPE-W > 5 will now have the potential to partition >1% to polyethylene. Thus in a system where there are large quantities of plastic, and limited natural organic matter, significant partitioning of bioaccumulative HOCs into polyethylene plastic will occur. It is notable that this observation assumes that the presence of organic matter in the water, such as phytoplankton, is negligible. Where populations of phytoplankton are elevated, however, increased partitioning to the phytoplankton will likely occur, consistent with observations of Dachs et al.,35 and thus the relative partitioning of HOCs to polyethylene will decrease accordingly. The results in Figures 1 and 2 effectively illustrate the output of a sensitivity analysis, which highlights the relative importance of the volume ratios between air, water, naturally occurring organic carbon, and polyethylene plastic with respect to how the partitioning behavior of chemicals will change with varying volume ratios. The results clearly imply that increasing quantities of plastic will result in competition for the sorption of hydrophobic substances to 1469

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Figure 4. Sensitivity analysis of dietary assimilation efficiency of NLOM and lipid content of forage fish defined in the CEMC SFU Agro food-model, with respect to changes in body burden concentrations of perfectly persistent hypothetical chemicals with varying partitioning properties.

naturally occurring organic carbon. Thus quantifying the amounts of plastic in the environment is needed to improve our understanding of the relative importance of this process. It is notable that since this is an equilibrium partitioning approach, the size of the plastic does not influence the results illustrated. Thus the results relate to both macro- and microplastic material. The time to equilibrium, however, will differ depending on the size of the material, with microplastic reaching equilibrium sooner than macroplastic. It has been suggested that substances absorbed into plastic as a result of their partitioning behavior may be transported in the environment from source regions and may cause a negative impact on remote environments. Zarfl et al.36 have assessed the relative importance of this transport mechanism to the Arctic Ocean. Their results, however, suggest that this transport process, as compared with the transport of pollutants via atmospheric and oceanic currents, is orders of magnitude smaller.36 Thus, the movement of plastic is not likely to be a significant vector for transporting HOCs from source to remote regions. Nevertheless, Zarfl et al.36 imply that it may be possible that highly hydrophobic organic chemicals (for instance log KOW > 6.5), which typically partition strongly to sediment, and therefore have limited transport potential, may have enhanced mobility if absorbed to buoyant microplastic material. Thus, concern regarding the relative importance of microplastic as a vector for PBT substances remains, particularly given the suggestion that an organism that consumes microplastic material, to which HOC have absorbed, can result in enhanced uptake by the organism.16 Results illustrated in Figures 1 and 2 thus suggest that the potential for enhanced biological uptake, as a result of consuming microplastic, will likely be limited to chemicals with log KOC/Log KPE-W > 6.5, since these are the substances that have higher potential for partitioning to polyethylene plastic, and thus will have the greatest potential to cause a detrimental effect. It is notable, however, that gastrointestinal absorption efficiency of HOCs decreases beginning at about log KOW > 5.5.32,37 Thus assessing an organisms’ potential to extract highly HOCs of environmental concern, absorbed to microplastic, will be complicated by a number of competing processes, which are explored in greater detail below. Simulating Biological Uptake and Bioaccumulation. Figure 3 illustrates the chemical space related to reductions in body

burden concentrations associated with a piscivorous fish that includes a 10% microplastic component of its diet. The bioaccumulation of chemicals is shown to decline, with chemicals having log KOW of between about 6.5 and 7.5 showing >20% reduction in body burden concentrations as a consequence of including 10% microplastic in the diet. Reductions in body burden concentrations are also observed to increase as the amount of microplastic in the diet increases. The observation of reductions in body burden concentrations is due to the high sorption affinity assumed for polyethylene, which results in a fugacity gradient within the GI gut that does not favor transfer to the organism. For instance, organic matter that is degraded in the GI gut results in contaminant concentrations increase per mass of organic carbon, thus there is an increase in fugacity that favors transfer to the organism, if polyethylene is absent. If polyethylene is present in the GI gut, however, which is assumed to be nondegradable, then the fugacity increase will be less due to the sorption affinity of the plastic. Consequently, a reduced body burden concentration can be realized. This hypothesis is consistent with other studies that have demonstrated limited absorption efficiency related to the consumption of nondigestable material.38 Moser and McLachlan,38 for instance, observed that consumption of nondigestable fats, such as olestra, could be an efficient process for reducing chemical body burdens. Thus, the role of microplastic as a vector for enhanced biological exposure of PBT substances appears to be of limited importance. Nevertheless, we further assess the potential for an increase in body burden concentrations by performing a sensitivity analysis on various input parameters defined in the food-web model. Figure 4 illustrates the results with respect to changing the dietary assimilation efficiency of NLOM and the percent lipid in prey fish consumed by the piscivorous fish. Figure 4 illustrates that decreases in body burden mass are less pronounced in piscivorous fish that consumes microplastic along with prey with relatively low lipid content (1%) and which has an increasing potential to digest NLOM. This observation appears to be consistent with the hypothesis described earlier, in which the fugacity of a chemical within the GI gut of the organism can vary depending on the relative fractions of lipid and NLOM present in the system. Variability in the potential for the organism 1470

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Environmental Science & Technology to digest NLOM relative to lipid will thus further influence the fugacity of the chemical in the GI gut, and consequently the transfer of chemical to the organism. Consequently, the design of an experiment aimed at assessing the role of ingested microplastic as a vector for exposure to PBT substances needs to consider not only the kinetics related to the digestion efficiency of the plastic (such as gut residence time and gut extraction efficiency), but also the influence of the natural diet. It is of interest to note that increasing the dietary assimilation efficiency to 95% results in negligible differences in body burden mass between an organism that consumes plastic and one that does not (Figure 4). This is due to the increased capacity of the organism to not only digest plastic polymeric material, but also the increased capacity to digest NLOM associated with its natural diet. Clearly, caution is warranted with respect to this discussion, since the environmental relevance of an organism that is highly efficient at digesting NLOM and which consumes a diet low in lipid content is likely limited. Addressing Limitations and Identifying Research Needs. The theoretical approaches used in this study largely assume thermodynamic equilibrium. Consequently, there is a need to better understand and assess the influence of kinetics with respect to sorption/desorption processes. For instance, what is the influence of rubbery to glass transition of the plastic on the Fickian diffusion of a chemical through the polymeric material? There are some studies that have demonstrated that chemical diffusion through polyethylene decreases with increased crystallinity,39,40 however, further research regarding the relevance of this process as it pertains to sorption/desorption of environmentally relevant chemicals of concern is needed. Additionally, there is a need for improving our understanding of how microbial biofouling influences sorption/desorption processes. From observations based on the use of PRC in passive samplers, for instance, it has been demonstrated that biofouling can significantly influence uptake rates,41 thus improving our understanding of how this process influences both the fate of the plastics and chemicals sorbed to the plastic and could enhance our ability to better assess the relative risk. Apart from the capacity for plastic debris to sorb chemicals of concern, there is also a need to improve our understanding of the fate of the physical presence of the microplastic material in biological organisms. Specifically, what are the gut retention times and what fraction of microplastic consumed by an organism moves across the epithelial tissue into the organism? What influence does the size and shape of the microplastic have with respect to a toxicological response? Thus, while it appears that the role of microplastic as a vector for PBT substances may be relatively small when compared to other exposure pathways, it does appear that there are a number of data-gaps that need to be addressed, particularly those quantifying the sources and sinks of microplastic and an improved understanding of what the effects of the physical presence of the material are on the environment.

’ ASSOCIATED CONTENT

bS

Supporting Information. Descriptions and calculations of model dimensions and the volume of polyethylene used in personal care products, and tables summarizing input parameters used in the CEMC SFU Agro food-web model component. This information is available free of charge via the Internet at http:// pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ44 1234264770; fax: þ44 1234 264744; e-mail: todd. [email protected].

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