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Journal ArticleDOI

Microplastics as vectors for environmental contaminants: Exploring sorption, desorption, and transfer to biota.

TL;DR: HOC sorption to and desorption from MPs and the underlying principles for their interactions are explored and intrinsic and extrinsic parameters influencing these processes are discussed and focus on the importance of the exposure route for diffusive mass transfer.
Abstract: The occurrence and effects of microplastics (MPs) in the aquatic environment are receiving increasing attention. In addition to their possible direct adverse effects on biota, the potential role of MPs as vectors for hydrophobic organic chemicals (HOCs), compared to natural pathways, is a topic of much debate. It is evident, however, that temporal and spatial variations of MP occurrence do (and will) occur. To further improve the estimations of the role of MPs as vectors for HOC transfer into biota under varying MP concentrations and environmental conditions, it is important to identify and understand the governing processes. Here, we explore HOC sorption to and desorption from MPs and the underlying principles for their interactions. We discuss intrinsic and extrinsic parameters influencing these processes and focus on the importance of the exposure route for diffusive mass transfer. Also, we outline research needed to fill knowledge gaps and improve model-based calculations of MP-facilitated HOC transfer in the environment. Integr Environ Assess Manag 2017;13:488–493. © 2017 SETAC

Summary (2 min read)

Microplastics as Vectors for Environmental Contaminants : Exploring Sorption, Desorption, and Transfer to Biota

  • Microplastics as Vectors for Environmental Contaminants : Exploring Sorption, Desorption, and Transfer to Biota.
  • Page 1 of 17 Microplastics as vectors for environmental contaminants: Exploring sorption, desorption, and transfer to biota.

Keywords

  • Microplastics, contaminants, hydrophobic organic chemicals (HOCs), exposure, transfer, ecotoxicity Page 3 of 17 I. INTRODUCTION Interactions between microplastics (MPs) and organic contaminants take place before, during, and after their release to natural environments.
  • The role of MPs as contaminant vectors has been the topic of experimental studies and review papers, both supporting and challenging this hypothesis.
  • Some argue that plastic debris and HOCs form complex cocktails that increase the overall bioavailability of HOCs to aquatic organisms and ultimately to humans (Vethaak and Leslie 2016).
  • The authors discuss the impact of different uptake routes on MP-facilitated HOC transfer into biota, including direct MP contact exposure (internal and external).
  • Finally, the authors point to knowledge gaps and research required for a more comprehensive understanding and modelling of these processes.

EXTRINSIC PROPERTIES

  • Due to their hydrophobicity and lipophilicity, HOCs sorb to non-polar phases in natural aquatic environments, including sediment particles, suspended organic matter, and MPs.
  • Recent tests with seven different HOCs and four polymers showed that the sorption process depends strongly on the polymer type (Hüffer and Hofmann 2016).
  • Polymers often contain both crystalline and amorphous (i.e., non-crystalline) regions.
  • Page 8 of 17 Desorption of adsorbed and absorbed molecules will depend on many factors and will generally decrease with increasing partition ratios and increasing binding strength.
  • Even if seemingly counter-intuitive, these mechanisms suggest that desorption and leaching of HOCs from MPs often will occur to a greater extent and faster when they are absorbed rather than adsorbed.

III. TRANSFER OF HYDROPHOBIC ORGANIC CHEMICALS BETWEEN

  • A conceptual framework for mechanisms involved in the role of MPs as vectors for HOCs and their transfer into aquatic organisms has been proposed (Koelmans et al. 2016).
  • What is not implicitly included in these model frameworks, however, is direct contact exposure.
  • HOCs sorbed to MPs are thus expected to be Page 10 of 17 transferred more rapidly to biota through digestive fluid than through water, and direct contact of MPs with exterior or interior parts of the organism might be an important and so far overlooked uptake route.
  • In oceans, the mass of water is estimated to be a factor of 1013 higher than the mass of plastic, and the volume of organic carbon in marine coastal environments was modelled to be more than 107 times greater (Koelmans et al. 2016; Gouin et al. 2011).

IV. LINKING LABORATORY AND ENVIRONMENTAL CONDITIONS

  • Environmental sampling experiments provide a relatively unbiased view of the sorption of HOCs to MPs under given environmental conditions.
  • Directly linking cause and effects, for both chemical processes and biological effects, is challenged by the vast number of parameters that will have varied at a given location over time.
  • Laboratory experiments, on the contrary, apply Page 12 of 17 conditions that may mimic, but not replicate, environmental conditions.
  • Field sampling and laboratory testing are thus complementary.

V. KNOWLEDGE GAPS AND FUTURE RESEARCH DIRECTIONS

  • The mechanisms and kinetics of degradation of MPs, and the consequences for MPs as HOC vectors, are not well understood (Eerkes-Medrano et al. 2015; Rocha-Santos and Duarte 2015) and should thus be further investigated.
  • The processes of adsorption versus absorption of HOCs to MPs need to be better understood, especially with regards to how sorption and desorption kinetics may differ between MPs and other naturally occurring particulate matter, and how this may influence transfer into biota.
  • For micro- and especially nano-sized particles, whose sizes are minute compared to the matrix they are in, this poses a range of challenges inherent to the related scientific field of nanotoxicology.
  • To more precisely assess the relative importance of MPs as vectors for HOCs, compared to natural pathways, there is a clear need for more appropriate sampling methods to avoid underestimations of actual environmental MP concentrations.

CONCLUSION

  • While the relative role of MPs as vectors for HOCs to organisms is generally considered minor in comparison to that of natural exposure pathways (such as water, food, and natural particulate matter) under present conditions, it is important to emphasize that: MP concentrations and environmental conditions change over time; and, spatiotemporal MP hotspots do (and will) occur.
  • To better evaluate the role of MPs as pathways for HOC transfer into biota under such temporally and spatially varying conditions, an improved understanding of the governing processes is needed.
  • Sorption/desorption processes differ between different polymers as well as between MPs and various natural particulate matter.
  • Additionally, weathering will change HOC sorption and desorption.
  • How these factors influence the role of MPs as HOC vectors is a topic of further research.

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Microplastics as Vectors for Environmental Contaminants : Exploring Sorption,
Desorption, and Transfer to Biota
Hartmann, Nanna B.; Rist, Sinja; Bodin, Julia; Jensen, Louise Helene Søgaard; Nørgaard Schmidt, Stine;
Mayer, Philipp; Meibom, Anders; Baun, Anders
Published in:
Integrated Environmental Assessment and Management
Link to article, DOI:
10.1002/ieam.1904
Publication date:
2017
Document Version
Peer reviewed version
Link back to DTU Orbit
Citation (APA):
Hartmann, N. B., Rist, S., Bodin, J., Jensen, L. H. S., Nørgaard Schmidt, S., Mayer, P., Meibom, A., & Baun, A.
(2017). Microplastics as Vectors for Environmental Contaminants : Exploring Sorption, Desorption, and Transfer
to Biota. Integrated Environmental Assessment and Management, 13(3), 488-493.
https://doi.org/10.1002/ieam.1904

Microplastics as contaminant vectors – exploring the processes – Postprint version
Page 1 of 17
Microplastics as vectors for environmental contaminants: Exploring
sorption, desorption, and transfer to biota
Accepted for publication in: Integrated Environmental Assessment and Management
DOI:10.1002/ieam.1904
Nanna Bloch Hartmann
, *
, Sinja Rist
, Julia Bodin
, Louise Helene Søgaard Jensen
, Stine
Nørgaard Schmidt
, Philipp Mayer
, Anders Meibom
, Anders Baun
Technical University of Denmark, Department of Environmental Engineering, Bygningstorvet
B115, DK-2800 Kgs Lyngby, Denmark
Laboratory for Biological Geochemistry, School of Architecture, Civil and Environmental
Engineering at Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne,
Switzerland.
* corresponding author , nibh@env.dtu.dk

Microplastics as contaminant vectors – exploring the processes – Postprint version
Page 2 of 17
ABSTRACT
The occurrence and effects of microplastics (MPs) in the aquatic environment are receiving
increasing attention. In addition to possible direct adverse effects on biota, their potential role as
vectors for hydrophobic organic chemicals (HOCs), compared to natural pathways, is a topic of
much debate. It is evident, however, that temporal and spatial variations of MP occurrence do (and
will) occur. To further improve the estimations of the role of MPs as vectors for HOC transfer into
biota under varying MP concentrations and environmental conditions, it is important to identify and
understand the governing processes. Here, we explore HOC sorption to and desorption from MPs
and the underlying principles for their interactions. We discuss intrinsic and extrinsic parameters
influencing these processes, and focus on the importance of the exposure route for diffusive mass
transfer. Also, we outline research needed to fill knowledge gaps and improve model-based
calculations of MP-facilitated HOC transfer in the environment.
Keywords
Microplastics, contaminants, hydrophobic organic chemicals (HOCs), exposure, transfer,
ecotoxicity

Microplastics as contaminant vectors – exploring the processes – Postprint version
Page 3 of 17
I. INTRODUCTION
Interactions between microplastics (MPs) and organic contaminants take place before, during,
and after their release to natural environments. These interactions occur as a result of product
formulation (e.g., by addition of plasticisers and preservatives) as well as unintentionally, in e.g.,
wastewater, urban runoff, and landfill leachate containing complex mixtures of other environmental
contaminants. Recent studies have demonstrated the ability of MPs to carry environmental
contaminants (e.g. Rochman et al. 2013; Velzeboer et al. 2014). This has led to the hypothesis that,
in addition to direct effects of interactions with biota, MPs may also play a role in aquatic
ecotoxicology as vectors for toxic substances.
The role of MPs as contaminant vectors has been the topic of experimental studies and review
papers, both supporting and challenging this hypothesis. Some argue that plastic debris and HOCs
form complex cocktails that increase the overall bioavailability of HOCs to aquatic organisms and
ultimately to humans (Vethaak and Leslie 2016). This hypothesis has been challenged with the
argument that plastics play a minor role as contaminant vectors compared to natural particles, such
as suspended organic particulates and natural prey, because of their relatively low abundance in the
environment (Koelmans et al. 2016). Based on data from published studies, calculations have been
made to assess the overall relative importance of MPs as contaminant vectors compared to other
naturally occurring sorbents (Koelmans et al. 2016; Bakir et al. 2016). Models of increasing
complexity have been developed and applied, taking into account the partition ratios between solid
phases (including plastic) and water, MP age distribution, the relative abundance and ingested
amounts of MPs (Koelmans et al. 2016) as well as the role of gut surfactants, pH, and temperature
on HOC desorption (Bakir et al. 2016). With increasing plastic production and use in society,
environmental occurrence of MPs will inevitable increase in the future (temporal increase) and also
vary depending on specific local-scale conditions (spatial variations). Expanding our fundamental
understanding of the processes involved in MP facilitated transport of contaminants is therefore

Microplastics as contaminant vectors – exploring the processes – Postprint version
Page 4 of 17
necessary, allowing us to develop more accurate models and thereby evaluate the role of this
process under varying and case-specific conditions.
A conceptual overview of the topics covered in this paper is illustrated in Figure 1. In the
following sections, we describe the processes governing the interactions between MPs and HOCs
and their subsequent uptake into aquatic organisms, including HOC sorption to and desorption from
MPs and parameters affecting HOC distribution. We discuss the impact of different uptake routes
on MP-facilitated HOC transfer into biota, including direct MP contact exposure (internal and
external). Finally, we point to knowledge gaps and research required for a more comprehensive
understanding and modelling of these processes.
Figure 1. Overview of the topics covered in this paper: I) HOC sorption to and desorption from MPs; II) The influence
of intrinsic and extrinsic properties on HOC distribution to MPs; III) Transfer of HOCs between MPs and biota,
including direct contact transfer; IV) The role of in situ and laboratory studies when exploring these processes; and, V)
Outlook on knowledge gaps and future research directions

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"Microplastics as vectors for enviro..." refers background in this paper

  • ...Furthermore, glassy polymers have internal pores (“nanovoids”), creating strong adsorption sites and contributing to slow HOC release rates (Teuten et al. 2009)....

    [...]

  • ...The amorphous regions, where sorption of HOCs generally occurs (Teuten et al. 2009), can be either “glassy” or “rubbery,” depending on the glass transition temperature of the polymer....

    [...]

  • ...Microplastic shape and size define the surface-to-volume ratio and diffusional length scales, which in turn determine the time to reach equilibrium and the rate of absorption and desorption (Teuten et al. 2009)....

    [...]

  • ...In contrast, reactions with O2 can increase the surface polarity, which decreases the affinity for HOCs (Endo et al. 2005; Teuten et al. 2009)....

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TL;DR: The issue of microplastics in freshwater systems is reviewed to summarise current understanding, identify knowledge gaps and suggest future research priorities.

1,688 citations


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  • ...First, it is very likely that the actual occurrence of MPs in the environment is currently underestimated due to sampling method issues (Eerkes-Medrano et al. 2015; Kooi et al. 2016) and a lack of consensus regarding samplingmethods (RochaSantos and Duarte 2015)....

    [...]

  • ...The mechanisms and kinetics of degradation of MPs, and the consequences for MPs as HOC vectors, are not well understood (Eerkes-Medrano et al. 2015; Rocha-Santos and Duarte 2015) and should thus be further investigated....

    [...]

  • ...First, it is very likely that the actual occurrence of microplastics in the environment is currently underestimated due to sampling method issues (Kooi et al. 2016; Eerkes-Medrano et al. 2015) and a lack of consensus regarding sampling methods (Rocha-Santos and Duarte 2015)....

    [...]

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TL;DR: Field adsorption experiments using PP virgin pellets demonstrated significant and steady increase in PCBs and DDE concentrations throughout the six-day experiment, indicating that the source of PCBs, DDE, and nonylphenols is ambient seawater and that adsor adaptation to pellet surfaces is the mechanism of enrichment.
Abstract: Plastic resin pellets (small granules 0.1−0.5 centimeters in diameter) are widely distributed in the ocean all over the world. They are an industrial raw material for the plastic industry and are unintentionally released to the environment both during manufacturing and transport. They are sometimes ingested by seabirds and other marine organisms, and their adverse effects on organisms are a concern. In the present study, PCBs, DDE, and nonylphenols (NP) were detected in polypropylene (PP) resin pellets collected from four Japanese coasts. Concentrations of PCBs (4−117 ng/g), DDE (0.16−3.1 ng/g), and NP (0.13−16 μg/g) varied among the sampling sites. These concentrations were comparable to those for suspended particles and bottom sediments collected from the same area as the pellets. Field adsorption experiments using PP virgin pellets demonstrated significant and steady increase in PCBs and DDE concentrations throughout the six-day experiment, indicating that the source of PCBs and DDE is ambient seawater...

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"Microplastics as vectors for enviro..." refers background in this paper

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  • ...Weathering can also lead to an increase in crystallinity of the polymer (Mato et al. 2001; Karapanagioti and Klontza 2008), thereby reducing sorption of HOCs....

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TL;DR: It is advocated that the use of generic organic carbon-water distribution coefficients in the risk assessment of organic compounds is not warranted and that bioremediation endpoints could be evaluated on the basis of freely dissolved concentrations instead of total concentrations in sediment/soil.
Abstract: Evidence is accumulating that sorption of organic chemicals to soils and sediments can be described by “dual-mode sorption”: absorption in amorphous organic matter (AOM) and adsorption to carbonaceous materials such as black carbon (BC), coal, and kerogen, collectively termed “carbonaceous geosorbents” (CG). Median BC contents as a fraction of total organic carbon are 9% for sediments (number of sediments, n ≈ 300) and 4% for soils (n = 90). Adsorption of organic compounds to CG is nonlinear and generally exceeds absorption in AOM by a factor of 10−100. Sorption to CG is particularly extensive for organic compounds that can attain a more planar molecular configuration. The CG adsorption domain probably consists of surface sites and nanopores. In this review it is shown that nonlinear sorption to CG can completely dominate total sorption at low aqueous concentrations (<10-6 of maximum solid solubility). Therefore, the presence of CG can explain (i) sorption to soils and sediments being up to 2 orders of m...

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TL;DR: The chemical interactions of hydrophobic organic contaminants (HOCs) with soils and sediments (geosorbents) may result in strong binding and slow subsequent release rates that significantly affect remediation rates and endpoints.
Abstract: The chemical interactions of hydrophobic organic contaminants (HOCs) with soils and sediments (geosorbents) may result in strong binding and slow subsequent release rates that significantly affect remediation rates and endpoints The underlying physical and chemical phenomena potentially responsible for this apparent sequestration of HOCs by geosorbents are not well understood This challenges our concepts for assessing exposure and toxicity and for setting environmental quality criteria Currently there are no direct observational data revealing the molecular-scale locations in which nonpolar organic compounds accumulate when associated with natural soils or sediments Hence macroscopic observations are used to make inferences about sorption mechanisms and the chemical factors affecting the sequestration of HOCs by geosorbents Recent observations suggest that HOC interactions with geosorbents comprise different inorganic and organic surfaces and matrices, and distinctions may be drawn along these lines,

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  • ...However, at higher concentrations of the chemical, absorption often takes over as thedominant retention process due to themuch larger volume to accommodate the molecules (Luthy et al. 1997; Cornelissen et al. 2005)....

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Frequently Asked Questions (2)
Q1. What contributions have the authors mentioned in the paper "Microplastics as vectors for environmental contaminants: exploring sorption, desorption, and transfer to biota" ?

In this paper, direct contact exposure is used as a route of HOC transfer from MPs into biota, a process which should not be overlooked when working towards a better understanding of MPs as HOC vectors in the environment. 

How these factors influence the role of MPs as HOC vectors is a topic of further research.