Integrated Environmental Assessment and Management — Volume 13, Number 3—pp.

488–493
488 Received: 6 January 2017 | Returned for Revision: 20 January 2017 | Accepted: 7 February 2017

Invited Commentary

Microplastics as Vectors for Environmental Contaminants:

Exploring Sorption, Desorption, and Transfer to Biota
Nanna B Hartmann,*y Sinja Rist,y Julia Bodin,z Louise HS Jensen,z Stine N Schmidt,y
Philipp Mayer,y Anders Meibom,z and Anders Bauny
yTechnical University of Denmark, Department of Environmental Engineering, Kgs Lyngby, Denmark
zLaboratory for Biological Geochemistry, School of Architecture, Civil and Environmental Engineering at Ecole Polytechnique

de
Fe
rale de Lausanne (EPFL), Lausanne, Switzerland

EDITOR’S NOTE:
This is 1 of 15 invited commentaries in the series “Current Understanding of Risks Posed by Microplastics in the
Environment.” Each peer-reviewed commentary reflects the views and knowledge of international experts in this field and,
collectively, inform our current understanding of microplastics fate and effects in the aquatic environment.

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.  C 2017 SETAC

Keywords: Plastic debris Hydrophobic organic chemicals (HOCs) Exposure Transfer Ecotoxicity

INTRODUCTION hydrophobic organic chemicals (HOCs) form complex cocktails

Interactions between microplastics (MPs) and organic that increase the overall bioavailability of HOCs to aquatic
contaminants take place before, during, and after their organisms and ultimately to humans (Vethaak and Leslie 2016).
release to natural environments. These interactions occur as a This hypothesis has been challenged with the argument that
result of product formulation (e.g., by addition of plasticizers plastics play a minor role as contaminant vectors compared to
and preservatives) as well as unintentionally in, for example, natural particles, such as suspended organic particulates and
wastewater, urban runoff, and landfill leachate containing natural prey, because of their relatively low abundance in the
complex mixtures of other environmental contaminants. environment (Koelmans et al. 2016). On the basis of data from
Recent studies have demonstrated the ability of MPs to published studies, calculations have been made to assess the
carry environmental contaminants (e.g., Rochman et al. 2013; overall relative importance of MPs as contaminant vectors
Velzeboer et al. 2014). This ability has led to the hypothesis compared to other naturally occurring sorbents (Bakir et al.
that, in addition to direct effects of interactions with biota, 2016; Koelmans et al. 2016). Models of increasing complexity
MPs may also play a role in aquatic ecotoxicology as vectors have been developed and applied, taking into account the
for toxic substances. partition ratios between solid phases (including plastic) and
The role of MPs as contaminant vectors has been the topic of water, MP age distribution, the relative abundance, and
experimental studies and review papers, both supporting and ingested amounts of MPs (Koelmans et al. 2016) as well as
challenging this hypothesis. Some argue that plastic debris and 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
* Address correspondence to nibh@env.dtu.dk
inevitably increase in the future (temporal increase) and also vary
Published 25 April 2017 on wileyonlinelibrary.com/journal/ieam.
depending on specific local-scale conditions (spatial variations).

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Microplastics as Contaminant Vectors: Exploring the Processes—Integr Environ Assess Manag 13, 2017 489

Figure 1. Overview of the topics covered in the present paper: HOC sorption to and desorption from MPs (I); the influence of intrinsic and extrinsic properties on
HOC distribution to MPs (II); transfer of HOCs between MPs and biota, including direct contact transfer (III); the role of in situ and laboratory studies when
exploring these processes (IV); and outlook on knowledge gaps and future research directions (V). HOC ¼ hydrophobic organic chemical; KOW ¼ octanol-water
partition ratio; KPL ¼ plastic-water partition ratio; MW ¼ molecular weight; NOM ¼ natural organic matter.

Expanding our fundamental understanding of the processes molecules dissolved and retained only by relatively weak
involved in MP-facilitated transport of contaminants is therefore van-der-Waals forces. Such partitioning is largely driven by the
necessary, allowing us to develop more accurate models and hydrophobicity of the compound, “pushing” it out of the water
thereby evaluate the role of MPs as contaminant vectors under and into the matrix, leading to partition ratios that, for
varying and case-specific conditions. HOCs and polymers, generally correlate well with their
A conceptual overview of the topics covered in the present octanol-to-water partition ratios (Mayer et al. 2000; O’Connor
paper is illustrated in Figure 1. In the following sections, we et al. 2016). On the contrary, surface adsorption can involve a
describe the processes governing the interactions between wide range of different interaction forces, including van-der-
MPs and HOCs and their subsequent uptake into aquatic Waals, ionic, steric, п-п interactions, and covalent bonds. At
organisms, including HOC sorption to and desorption from low concentrations of the chemical, adsorption generally leads
MPs and parameters affecting HOC distribution. We discuss to much higher partition ratios compared to absorption due to
the impact of different uptake routes on MP-facilitated HOC the stronger interaction forces on the surface. However, at
transfer into biota, including direct MP contact exposure higher concentrations of the chemical, absorption often takes
(internal and external). Finally, we point to knowledge gaps over as the dominant retention process due to the much larger
and research required for a more comprehensive under- volume to accommodate the molecules (Luthy et al. 1997;
standing and modeling of these processes. Cornelissen et al. 2005).
Although this issue was initially a matter of debate within
SORPTION AND DESORPTION: THE INFLUENCE analytical chemistry (Hawthorne et al. 2000; Vaes et al. 2000),
OF INTRINSIC AND EXTRINSIC PROPERTIES experimental studies have now demonstrated that hydro-
Due to their hydrophobicity and lipophilicity, HOCs sorb to phobic organic contaminants (such as polycyclic aromatic
nonpolar phases in natural aquatic environments, including hydrocarbons [PAHs] and polychlorinated biphenyls) can be
sediment particles, suspended organic matter, and MPs. The absorbed into various polymers, including silicone (Mayer
distribution of HOCs between different types of particulate et al. 2000) and low density polyethylene (LDPE) (Lohmann
matter and the aqueous phase is a function of the intrinsic 2012). This process is the mechanistic basis for most passive
properties of the given HOC as well as the particulate matter sampling techniques for HOCs (Booij et al. 2016). Recent
(under given environmental conditions). The distribution is tests with 7 different HOCs and 4 polymers showed that the
governed by equilibrium partitioning and molecular diffusion sorption process depends strongly on the polymer type
and quantified by plastic- and HOC-specific partition ratios (H€
uffer and Hofmann 2016).
(Mayer et al. 2000; O’Connor et al. 2016). Transport of the HOCs within the polymer matrix depends
Overall, the sorption of a compound to a sorbent can take on a number of factors, such as the free volume within the
place by absorption or by adsorption. Absorption involves the polymer and the segmental mobility of the polymer chains
partitioning of molecules into a sorbing matrix, keeping the (Rusina et al. 2010), defined as the ability of the polymer

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490 Integr Environ Assess Manag 13, 2017—NB Hartmann et al.

chains to move and attain different physical conformations. MPs. The lower the hydrophobicity and MW of a compound,
These properties, in turn, are related to the glass transition the faster the diffusive mass transfer toward steady state or
temperature, degree of cross-linking, and crystallinity of the equilibrium (M€ uller et al. 2001; Pascall et al. 2005; Rochman
polymer. Crystallinity is a descriptor for the polymer et al. 2013). This implies that the diffusive mass transfer of, for
structure; the more ordered and fixed, the higher the example, naphthalene (logKow ¼ 3.3, MW ¼ 128 g/mol) will
crystallinity. Increasing crystallinity results in decreasing be faster than that of, for example, benzo[a]pyrene (logKow
capacity and rate of absorption of a contaminant into the ¼ 6.13, MW ¼ 252 g/mol). Furthermore, the planarity of the
polymer matrix (Mato et al. 2001; Karapanagioti and Klontza molecule influences how close it can move to the particle
2008). Polymers often contain both crystalline and amor- surface, thus facilitating adsorption (Velzeboer et al. 2014).
phous (i.e., noncrystalline) regions. The amorphous regions, Desorption of adsorbed and absorbed molecules will
where sorption of HOCs generally occurs (Teuten et al. 2009), depend on many factors and will generally decrease with
can be either “glassy” or “rubbery,” depending on the glass increasing partition ratios and increasing binding strength.
transition temperature of the polymer. Glassy amorphous This behavior is consistent with the general perception that
polymers (e.g., polyvinyl chloride [PVC] and polystyrene [PS]) absorbed molecules in amorphous organic matter desorb
are more condensed and cross-linked than rubbery amor- readily, whereas strongly adsorbed molecules are character-
phous polymers (e.g., polyethylene [PE] and polypropylene ized as slowly desorbing or even desorption resistant
[PP]); the former therefore have lower diffusivity than the (Cornelissen et al. 2005; Mayer et al. 2011). It is also
latter. Furthermore, glassy polymers have internal pores consistent with the wealth of studies showing that plastic
(“nanovoids”), creating strong adsorption sites and contrib- additives, dissolved within the plastic matrix, can relatively
uting to slow HOC release rates (Teuten et al. 2009). H€ uffer easily leach out and into other media with sufficient
and Hoffman recently reported that for PE, a rubbery solubilizing capacity. Even if seemingly counterintuitive,
polymer, absorption dominated, whereas for the glassy these mechanisms suggest that desorption and leaching of
polymers, PS, PVC, and polyamide, adsorption was found to HOCs from MPs often will occur to a greater extent and faster
be dominant (H€ uffer and Hofmann 2016). These findings when they are absorbed rather than adsorbed.
emphasize that sorption processes are closely linked to the The conditions of the surrounding environment,
polymer structure. especially pH, temperature, and ionic strength, can modulate
Another important factor that determines HOC sorption plastic–contaminant interactions. Lower pH and higher
to polymers is the distance between the polymer chains. temperature increase desorption of HOCs from MPs (Bakir
The greater the distance, the easier it is for chemicals to et al. 2014). On the contrary, higher salinity increases the
diffuse into or through the matrix (Pascall et al. 2005) and partitioning to the plastic polymer (Karapanagioti and
the higher the sorptive capacity (Rochman et al. 2013). For Klontza 2008; Velzeboer et al. 2014). Furthermore, salinity
this reason, for example, PE is expected to have a greater influences the agglomeration or aggregation state of MPs
sorptive capacity in comparison to PP (Rochman et al. which, in turn, can change properties such as total size and
2013). Similarly, additives can change the structure of the surface area (Velzeboer et al. 2014). It may be that these
polymer and thus its sorption capacity (Endo et al. 2005). factors can result in differences in the sorption of HOCs to
Surface polarity plays a role as HOCs interact more strongly MPs between freshwater and marine environments and also
with nonpolar surfaces (Mato et al. 2001). Microplastic cause differences in intraorganismal sorption and desorption
shape and size define the surface-to-volume ratio and behavior, compared to that occurring in the surrounding
diffusional length scales, which in turn determine the time water.
to reach equilibrium and the rate of absorption and
desorption (Teuten et al. 2009). TRANSFER OF HYDROPHOBIC ORGANIC
Weathering of MPs, which is the integrated result of CHEMICALS BETWEEN MICROPLASTICS
environmental conditions and exposure time, can modify AND BIOTA
their properties. Photo-weathering, which causes bond A conceptual framework for mechanisms involved in the
breakages in the polymer matrix and subsequent formation role of MPs as vectors for HOCs and their transfer into aquatic
of cracks, increases the surface area and pore size, resulting in organisms has been proposed (Koelmans et al. 2016). This
increased diffusivity and sorption of HOCs. In contrast, framework describes different scenarios whereby HOCs are
reactions with O2 can increase the surface polarity, which released from MPs internally (after uptake) or externally (in the
decreases the affinity for HOCs (Endo et al. 2005; Teuten water phase or into natural food or prey), followed by uptake
et al. 2009). Weathering can also lead to an increase in into the organism. An alternative model has been proposed
crystallinity of the polymer (Mato et al. 2001; Karapanagioti by Bakir et al. (2016), including also considerations of
and Klontza 2008), thereby reducing sorption of HOCs. differences in pH, temperature, and the role of gut
Microplastics in the aquatic environment are furthermore surfactants. What is not implicitly included in these model
prone to fouling, whereby biomaterials can serve as frameworks, however, is direct contact exposure. Here, we
additional sorbents (Endo et al. 2005). argue that these processes should not be overlooked in the
The hydrophobicity, molecular weight (MW), and molar evaluation of MPs as HOC vectors. In the organism’s external
volume of HOCs are crucial properties for their sorption to environment, HOCs can be transferred from MPs to biota

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Microplastics as Contaminant Vectors: Exploring the Processes—Integr Environ Assess Manag 13, 2017 491

partitioning being the governing process that not only

determines the distribution of the HOCs between water and
sorbent but also is crucial for the release and bio-uptake of
the chemicals.
The relative role of MPs as a vector for hazardous
contaminants to organisms has been found negligible in
comparison to natural exposure pathways (Gouin et al. 2011;
Bakir et al. 2016; Koelmans et al. 2016; Ziccardi et al. 2016).
This finding has mainly been attributed to the huge differ-
ences in mass between MPs and other sources. 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 C in marine
coastal environments was modeled to be more than 107
times greater (Gouin et al. 2011; Koelmans et al. 2016). As a
consequence, for example, it is predicted that sorption to PE
would account for less than 0.1% of the HOCs in the oceans
Figure 2. Four different diffusive mass transfer processes for HOCs between (Gouin et al. 2011). There may, however, be specific scenarios
microplastics (or naturally occurring sorbents) and organisms via water (I), via in which MPs will play a larger role as a pathway for HOC
intraorganismal fluids (II), and by direct contact exposure, either external (III) or
transfer into biota than identified through such calculations
internal (IV).
(Bakir et al. 2016).
First, it is very likely that the actual occurrence of MPs in the
either via the aqueous phase or via direct contact exposure environment is currently underestimated due to sampling
from MPs adhering to the exterior of the organism (i.e., the method issues (Eerkes-Medrano et al. 2015; Kooi et al. 2016)
skin or exoskeleton) (see Figure 2). If MPs with sorbed HOCs and a lack of consensus regarding sampling methods (Rocha-
are taken up by biota, HOCs can be transferred to the tissue Santos and Duarte 2015). To illustrate the importance of this
either via intraorganismal fluids (e.g., gut fluid) or via direct issue, it was found that by decreasing the net mesh size, a
contact exposure from MPs adhering to the interior of the substantial increase in numbers of collected particles was
organism (e.g., the gut or gill walls). Uptake via (pore) water observed (Set€ al€
a et al. 2016). Quantification of MP occur-
and intraorganismal fluids requires desorption of the HOCs rence is therefore highly influenced by the choice of sampling
from the sorbent before uptake of the freely dissolved method. Second, the use and discard of plastics is increasing
molecules. Desorption of molecules succeeded by uptake as at a high rate, resulting in an increased environmental
freely dissolved molecules as well as external or internal occurrence of MPs (da Costa et al. 2016). In addition, specific
direct contact exposure are (largely) governed by diffusion local and small-scale conditions may apply. Microplastics are
and partitioning. not evenly distributed in the environment and several studies
Diffusive mass transfer of a given HOC can differ greatly have reported local hotspots, in which the fraction of MPs is
between various exposure media. Using a microscale much higher than in the oceans as a whole. Such hotspots
experimental system with a clean and a PAH-loaded silicone include the North Pacific Gyre (Moore et al. 2001), industrial
disk, the diffusive mass transfer of PAHs was found to be areas in the vicinity of plastic production plants (Nor
en 2007),
much higher through the gut fluid of a sediment-dwelling and coastal sediments (Carson et al. 2011; Lee et al. 2015).
worm than through water in 48-h experiments, but the On a local scale, MPs could therefore present a significant
diffusive mass transfer was highest at direct contact between phase for the interaction with HOCs and subsequently for
the disks (Mayer et al. 2007). Hydrophobic organic chemicals their transfer to organisms. Finally, to predict the role of MPs
sorbed to MPs are thus expected to be transferred more as vectors for HOCs it is important not only to understand the
rapidly to biota through digestive fluid than through water, distribution of HOCs between different particulate matter,
and direct contact of MPs with exterior or interior parts of the but also the ability of organisms to distinguish between
organism might be an important and so far overlooked natural food sources and MPs.
uptake route.
In natural environments, there are many phases that LINKING LABORATORY AND ENVIRONMENTAL
dissolve and bind HOCs to varying degrees, including water, CONDITIONS
dissolved organic C, colloid C, black C, natural fibers, The rapidly growing body of literature about the possible
bacteria, detritus biogenic silica (mainly diatoms), and other vector function of MPs can be roughly divided into 2 groups:
plankton (Dean et al. 1993; Ladewig et al. 2015; Koelmans 1) in situ studies reporting findings of MPs with associated
et al. 2016). The described processes of (de)sorption and contaminants in the environment (e.g., Endo et al. 2005;
diffusive mass transfer also hold true for many other small- Llorca et al. 2014), and 2) laboratory experiments attempting
sized sorbents of HOCs in natural environments (Koelmans to create robust experimental designs for studying vector
et al. 2016). It should however be noted that (de)sorption effects and analyzing the mechanisms and dynamics of
processes will differ widely between sorbents and HOCs, with MP–contaminant interactions and to measure biological

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492 Integr Environ Assess Manag 13, 2017—NB Hartmann et al.

effects (e.g., Browne et al. 2013; Lee et al. 2014; H€

uffer and organisms represent a new frontier. To more precisely assess
Hofmann 2016). the relative importance of MPs as vectors for HOCs,
Environmental sampling experiments provide a relatively compared to natural pathways, there is a clear need for
unbiased view of the sorption of HOCs to MPs under given more appropriate sampling methods to avoid underestima-
environmental conditions. However, directly linking cause tions of actual environmental MP concentrations. Finally,
and effects, for both chemical processes and biological selectivity for food-versus-plastic for different aquatic organ-
effects, is challenged by the vast number of parameters that isms should be further investigated, and food webs, which are
will have varied at a given location over time. Laboratory potentially vulnerable to MP facilitated HOC uptake, should
experiments, on the contrary, apply conditions that may be identified.
mimic, but not replicate, environmental conditions. They can
be designed to give a clearer analysis of causality, but the CONCLUSION
duration of a laboratory experiment is often substantially Although the relative role of MPs as vectors for HOCs to
shorter than the time scale during which MPs are present in organisms is generally considered minor in comparison to that
the environment (Cole et al. 2015; Hall et al. 2015). Field of natural exposure pathways (such as water, food, and natural
sampling and laboratory testing are thus complementary. particulate matter) under present conditions, it is important to
When exploring the role of MPs as HOC vectors in controlled emphasize that MP concentrations and environmental con-
laboratory studies, certain aspects of environmental realism ditions change over time, and spatiotemporal MP hotspots do
can be included with regard to parameters that may be (and will) occur. To better evaluate the role of MPs as pathways
crucial to HOC sorption. These aspects include, for example, for HOC transfer into biota under such temporally and spatially
MP weathering prior to the test, test duration, and variations varying conditions, an improved understanding of the
in water chemistry, as well as performing tests in (synthetic) governing processes is needed. For example, sorption and
intraorganismal fluids. desorption processes differ between different polymers as
well as between MPs and various natural particulate matter.
KNOWLEDGE GAPS AND FUTURE RESEARCH Additionally, weathering will change HOC sorption and
DIRECTIONS desorption. How these factors influence the role of MPs as
The mechanisms and kinetics of degradation of MPs, and HOC vectors is a topic of further research. Here, we have
the consequences for MPs as HOC vectors, are not well highlighted direct contact exposure as a route of HOC transfer
understood (Eerkes-Medrano et al. 2015; Rocha-Santos and from MPs into biota, a process that should not be overlooked
Duarte 2015) and should thus be further investigated. when working toward a better understanding of MPs as HOC
Weathering of MPs should be considered in laboratory vectors in the environment.
studies when evaluating HOC sorption and desorption, in Acknowledgment—We would like to thank the Technical
addition to testing of pristine MPs. The processes of University of Denmark for funding through the DTU-EPFL
adsorption versus absorption of HOCs to MPs need to be collaborative PhD grant of S Rist.
better understood, especially with regard to how sorption Disclaimer—The authors declare no conflicts of interest.
and desorption kinetics may differ between MPs and other Data availability—Data available upon request to corre-
naturally occurring particulate matter, and how this may sponding author Nanna Hartmann (nibh@env.dtu.dk).
influence transfer into biota. Studies that directly correlate
HOCs and MP uptake, thus implying a vector effect (Tanaka
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