Microplastic Exposure Assessment in Aquatic Environments: Learning

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Microplastic exposure assessment in aquatic environments: learning from similarities and differences to engineered nanoparticles Thorsten Hüffer, Antonia Praetorius, Stephan Wagner, Frank Von Der Kammer, and Thilo Hofmann Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04054 • Publication Date (Web): 26 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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Environmental Science & Technology

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Microplastic exposure assessment in aquatic environments: learning from

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similarities and differences to engineered nanoparticles

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Thorsten Hüffer1,§, Antonia Praetorius1,2,§, Stephan Wagner3,§, Frank von der Kammer*1, Thilo

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Hofmann* 1,2

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Research Network, Althanstrasse 14, 1090 Vienna, Austria

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University of Vienna, Department of Environmental Geosciences and Environmental Science

University of Vienna, Research Platform Nano-Norms-Nature, Althanstrasse 14, 1090 Vienna,

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Austria

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3

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Permoserstrasse 15, 04318 Leipzig, Germany

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§

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manuscript.

Department of Analytical Chemistry, Helmholtz Centre for Environmental Research – UFZ,

Thorsten Hüffer, Antonia Praetorius and Stephan Wagner contributed equally to this

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*Corresponding authors:

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Thilo Hofmann: E-Mail: [email protected], Phone: 0043 1 4277 53320 and Frank von der Kammer: E-Mail: [email protected], Phone: 0043 1 4277 53380

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Total word count: 1200 word equivalents + 3766 text = 4966 words

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1 ACS Paragon Plus Environment


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TOC art

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Synopsis/Abstract

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Microplastics (MPs) have been identified as contaminants of emerging concern in aquatic environments

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and research into their behavior and fate has been sharply increasing in recent years. Nevertheless,

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significant gaps remain in our understanding of several crucial aspects of MP exposure and risk

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assessment, including the quantification of emissions, dominant fate processes, types of analytical tools

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required for characterization and monitoring, and adequate laboratory protocols for analysis and hazard

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testing. This Feature aims at identifying transferrable knowledge and experience from engineered

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nanoparticle (ENP) exposure assessment. This is achieved by comparing ENP and MPs based on their

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similarities as particulate contaminants, while critically discussing specific differences. We also highlight

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the most pressing research priorities to support an efficient development of tools and methods for MPs

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environmental risk assessment.

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Introduction

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The field of environmental exposure and risk assessment of emerging contaminants is often dominated

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by certain “hot topics”,1 for example, pharmaceuticals, nanomaterials, or (most recently) microplastics.

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The increased interest in a new, “hot” contaminant class is often based on, or justified by, a significant

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lack of knowledge concerning its behavior in natural environments and/or the toxic potential of the

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emerging contaminant, together with a need to assess the suitability of existing regulations or develop

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new ones. Since an emerging contaminant class is rarely entirely novel and unique, but often shares

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properties with “established” contaminants, a careful assessment of existing knowledge on related

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substances help us to direct our research efforts and make more efficient use of limited research

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resources.

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Plastic in the aquatic environment is an issue of global concern that is particularly evident from the

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growing amount of plastic litter found in the world's oceans.2,

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increase in both public and political attention. As a result, the "Leaders' Declaration" from the 2015 G7

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Summit in Germany acknowledged the global risks posed by marine litter, particularly plastics, to marine

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and coastal life, to ecosystems, and also potentially to human health.4 Marine littering has been

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controlled since the 1970s,5 whereas the debate on the occurrence and the consequences of plastic

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particles with sizes between 1 µm and 5 mm, so-called microplastics (MPs), in the environment has

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received increased attention only in the recent decade.6-8 MPs are not only a concern for marine

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environments, but also for freshwater systems as indicated by preliminary reports on a quantitatively

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similar degree of contamination in both.7, 9, 10

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Despite the rapidly growing body of published research into the impacts of MPs on various

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ecosystems,11 our mechanistic understanding of the behavior of MPs in the environment remains

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limited. To improve our understanding of the occurrence, behavior and transport of MPs in aquatic

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This concern has led to a marked



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systems and to ultimately improve our ability to assess the risks they may present a number of open

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research questions and challenges remain,5, 7, 12, 13 falling predominantly into the domains of:

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(1) Elucidating formation, sources and emission pathways,

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(2) Understanding relevant environmental transformation and transport processes,

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(3) Developing analytical methods for characterization and monitoring,

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(4) Designing representative laboratory experiments.

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Over the past decade, research on potential environmental implications of engineered nanoparticles

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(ENPs), generally defined as particles with at least one dimension < 100 nm,14 has been addressing very

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similar topics and facing related challenges. Given the particulate nature of both ENPs and MPs, this

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comes as no surprise. Exposure assessment strategies developed for “conventional” contaminants,

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present as individual substances in the form of dissolved molecules or ions, are often not adequate for

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particles. Particulate contaminants are present as more or less stable dispersions (multi-phase systems).

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As a result, their behavior cannot be described by thermodynamics; i.e. the use of equilibrium partition

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coefficients, such as for example the octanol-water partition coefficient Kow, which play an important

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role in exposure predictions of “conventional” organic contaminants are not valid for ENPs and MPs.15

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Their fate needs to be described by kinetic principles of aggregation and transport and can be largely

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based on theories developed in colloid/particle science. This is equally valid for ENPs and for MPs.

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Furthermore, their properties cannot be described by their chemical composition alone, but are also

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dependent on particle size and shape. Additionally, heterogeneities in particle populations, both in

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terms of the chemical composition of the individual particles and the polydispersity in their size

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distributions, often increase the level of difficulty in understanding and predicting fate. Particles on the

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smaller end of the size spectrum are particularly prone to exhibiting specific effects due to their high


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surface to volume ratios. Even bulk materials consisting mainly of >> 100 nm sized particles, in terms of

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mass, often contain a significant nano-fraction, in terms of particle number, at the lower end of their

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size distribution. This unintentional nano-fraction has been discussed several times for ENPs16-18 and the

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relevance of nano-plastics (i.e. plastic particles < 100 nm) in environmental exposure assessment of MPs

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has received increasing attention as well, but so far lacking field data.19-22

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It quickly becomes apparent that parallels can be drawn between these two groups of emerging

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contaminants. In this Feature we relate current challenges and open questions regarding the fate and

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exposure assessment of MPs to lessons learned over a decade of studying ENPs in the environment.23-25

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First, the similarities between ENPs and MPs are used to identify the transferrable knowledge from ENP

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exposure assessment (Table 1). This is then followed by a critical discussion of differences requiring

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specific adjustments for MPs. We aim to support a more rapid development of the tools and methods

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required to advance MP exposure and ultimately risk assessment so that prompt regulatory action can

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be taken, where necessary.

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Table 1: Overview of ENPs and MP characteristics and identification of transferrable knowledge from ENPs to MP fate and exposure assessment.

ENP characteristics

MP characteristics

Transferrable knowledge from ENPs

Sources and emission pathways • intentional release: agriculture, remediation • unintentional release: surface run-off, WWTPs, direct release (e.g. sunscreen from bathing)

• unintentional release: dominated by secondary MPs (via fragmentation of macroplastic), primary MPs via WWTPs

• particle size < 100 nm 3 • density often > 1 g/cm • fate processes: dissolution, interaction with biofilm/NOM, heteroaggregation, surface transformations

• MPs: particle size 1 µm - 5 mm; nano-plastics: < 100 nm 3 • density typically 0.9-1.1 g/cm • fate processes: fragmentation, interaction with biofilm/NOM, heteroaggregation, surface transformation, additives leaching

• mass flow models for ENPs transferrable for primary MPs (e.g. microbeads from cosmetics). Fragmentation data needed to assess secondary MP emissions

Environmental transformation and transport • kinetic fate descriptors essential • particle characteristics & properties of the surrounding medium affect fate • adaptation of ENP models with MP-specific properties • particle ageing has to be accounted for

Analytical methods for characterization & monitoring strategies • size distribution • particle number concentration • surface area & chemistry (e.g. type of coating)

• size distribution • particle number/mass concentration • types and concentrations of additives

• importance of particle size and number concentration • sample preparation essential for particle analysis & characterization methods

Representative laboratory experiments • particulate nature requires special considerations • stable dispersion due to electrostatic or steric stabilization

• particulate nature requires special considerations • usually low density combined with large particle size

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• use of reference materials and elucidation of fate under well-defined & realistic conditions • use of aged particles • importance of particle characterization • strategies for keeping particles in suspension

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Elucidating formation, sources and emission pathways

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ENPs are typically incorporated in consumer products or industrial applications to fulfill a specific

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purpose,26,

27

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nanofertilizers in agriculture,28, 29 and for contaminant remediation purposes.30 In contrast, MPs are only

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rarely deliberately incorporated into a product to serve a specific function, but generally occur as so-

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called secondary MPs,24 resulting from the unintentional release and fragmentation of larger pieces of

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plastics.6 An exception here are primary MPs, which are specifically produced as micro-scale particles

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and used predominantly in personal care products, e.g. in the form of microbeads in cosmetic peelings.

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These primary MPs, which make up only about 0.1-3% of the MPs in the natural environment,31 may

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dominate locally, e.g. in waste water effluents of urban environments,32 but are globally of minor

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importance. The predominantly unintentional generation of secondary MPs makes it harder to quantify

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their sources for mass flow models as total production volumes of plastics may be less relevant in this

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context, compared to break-up/fragmentation/release processes during or after the use stage. For the

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release quantification of secondary MP two steps are required: i) quantification of mismanaged plastic

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waste which is released in the aquatic environment,33 and ii) determination of the rates of

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fragmentation of the released plastic waste in the aquatic environment.

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The ENPs produced in the largest volumes typically consist of metals or metal oxides.34, 35 Several mass-

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flow modeling studies provided emission estimates into different environmental compartments during

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their life-cycle (e.g. air, water, soil) at global and regional scales.34, 36-38 These emission estimates are

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mainly based on data or estimates of production volumes or market penetration of ENP-containing

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products and applications, release rates during use and transfer factors between various life-cycle stages

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(e.g. retention in waste water treatment). In principle, similar mass flow modeling approaches can be

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applied to MPs, however, different strategies may be needed to elucidate sources and production

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volumes.

or are intentionally applied to the environment, for example as nanopesticides or



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Understanding relevant environmental transformation and transport processes

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Understanding the transport and transformation processes that contaminants undergo in natural

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environments is essential to assess their fate pathways and predict environmental concentrations.39 For

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this purpose, laboratory40-43 and mesocosm43, 44 studies as well as environmental fate models45 for ENPs

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have been developed in recent years, with varying levels of detail, realism and spatial resolution. Many

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of these approaches could serve as a starting point to study MPs. A river model for ENPs has, for

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example, already been adapted for modeling the fate of MPs.20 This was possible because the

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particulate nature of both ENPs and MPs means that the transport processes for both can be in principle

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described using established concepts from colloid science.15, 20, 46, 47 More specifically, processes affecting

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the fate of particles in aquatic environments are heteroaggregation with suspended particulate matter

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(SPM), sedimentation and deposition on surfaces, resuspension, bed load transport, and various

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transformation and ageing processes (biotic and abiotic) (Figure 1).48, 49 These processes are influenced

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by the properties of the surrounding aquatic medium (pH, ionic strength, composition, temperature,

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sunlight, SPM, natural organic matter (NOM) content, flow and turbulence) as well as the particle

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characteristics (size, shape, density, surface chemistry). 50, 51 The effect of specific differences in MP and

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ENP properties on their fate need to be accounted for when extrapolating approaches developed for

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ENP to MPs.

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Colloidal aggregation and transport processes are strongly influenced by particle sizes and, for the larger

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colloidal entities, also densities.52 MPs typically fall into size ranges similar to SPM (µm to mm) in

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contrast to the much smaller ENPs. Most MPs have very low densities close to the density of water (e.g.

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polyethylene (PE): 0.91-0.95 g/cm3; polypropylene (PP): 0.91-0.92 g/cm3; and polystyrene (PS): 1.01-

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1.05 g/cm3),5,

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Ag 10.5 g/cm3, CuO 6.31 g/cm3). The lower densities of MPs combined with their larger sizes compared

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to ENPs will likely lead to the following differences in aggregation and transport:

53

while ENPs generally have densities greater than 1 g/cm3 (e.g. TiO2 4.26 g/cm3,


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MP-SPM heteroaggregates will be less stable than ENP-SPM heteroaggregates due to the

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MPs’ larger sizes and heteroaggregates’ porosity.54 Therefore aggregate break-up needs

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to be accounted for.

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Stronger effect of MP properties on MP-SPM heteroaggregates. Slower sedimentation of

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MP-SPM heteroaggregates compared to ENP-SPM heteroaggregates or pure SPM, due to

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lower MP density and higher MP volume fraction in heteroaggregate. Higher potential for

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MPs to be transported far from their emission source.

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•

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SPM acts as a collector for ENPs, whereas MPs could act as collectors of SPM or as substrates for biofilm growth, a scenario that is most unlikely for ENPs.

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•

ENP movement is dominated by Brownian motion/diffusion due to their small sizes,37

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whereas MPs experience stronger shear forces during laminar and turbulent flow making

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them much more susceptible to resuspension from sediments than ENPs.55

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The heteroaggregation attachment efficiencies (i.e. the probability of heteroaggregate formation upon

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collision with SPM) of ENPs and MPs will vary according to their different surface characteristics but are

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expected to follow similar trends, depending on the conditions in the surrounding aquatic medium (e.g.

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increased heteroaggregation with increasing ionic strength and stabilization in the presence of NOM).20

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56 57

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dependent on their composition. Some ENPs undergo fast surface transformation and/or dissolution

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reactions (e.g. dissolution/sulphidation of silver and copper oxide NPs), 58,59.60,61,62 whereas other remain

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largely inert (e.g. titanium dioxide and cerium oxide NPs).63 In contrast, MPs made of polymeric material

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(e.g. PE, PP or PS) are not expected to dissolve in the environment. They can, however, be affected by

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aging processes, for example when exposed to sunlight. Aging of ENPs results mainly in surface

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transformations (e.g. oxidation64 or loss of particle coating), whereas aging of MPs typically leads to

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more brittle particles that could break up into smaller, possibly nanoscale particles (nano-plastics),65

Just like the attachment efficiencies, the transformation behavior of both MPs and ENPs are largely



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together with the leaching of various additives incorporated during production of the original plastic

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materials.66

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Homoaggregation of both ENPs and MPs in aquatic environments is likely to be negligible due to the low

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particle concentrations expected in surface waters compared to the concentrations of naturally

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occurring SPM. This can be illustrated by comparing the mean inter-particle distance between MPs and

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SPM in a typical freshwater body. Assuming a particle number concentration of 316 particles/1000 m³

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for MP particles with a size of 1 mm (as found in the Danube River by Lechner et al.67), the average

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distance between MP particles would be 1.8 m. In contrast, the mean distance to SPM, present with a

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typical mass concentration of 30 mg/L (corresponding to 2.6 × 10particles/1000 m³, assuming a

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density of 2.0 g/cm³ and a particle size of 0.1 mm) is equal to 2.0 × 10 m, approximately four orders

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of magnitude less than the mean distance between MP particles.


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Figure 1: Overview of the most relevant transformation and transport processes affecting ENPs (black circles) and MPs (black

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Developing analytical methods for characterization and monitoring

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In case of particulate contaminants such as MPs and ENPs the characterization and quantification in

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laboratory settings, technical products and natural environments has to go beyond total mass

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quantification towards a more detailed analysis to reveal information on particle size (distribution),

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possible coatings, shape and aggregation state.68,

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improvements in terms of sensitivity and selectivity of microscopic, chromatographic and spectroscopic

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instruments have led to the identification of promising approaches, which however often require a

pentagons) in aquatic systems. NOM: natural organic matter. The green clouds represent naturally-occurring suspended particulate matter (SPM).

69

For ENPs continuous developments and



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combination of complementary analytical techniques to elucidate the ENP characteristics (Figure 2).70

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For example, particle size and particle composition of inorganic ENPs can be determined with a

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hyphenation of field flow fractionation and element selective detectors such as inductively coupled

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mass spectrometry (ICP-MS). Further prominent methods to determine ENP size (distribution) include

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electron microscopy (EM), light scattering and, most recently, single-particle inductively coupled mass

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spectrometry (sp-ICP-MS) (Figure 2). A particular challenge is associated to the fact that the ENPs of

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primary concern are those smaller than 10 nm, which are also the most difficult to analyze since most of

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the analytical methods used are based on the detection of mass.71 The mass scales with the third order

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to particle size, therefore a 50% reduction in particle size (for spheres) reduces the mass (and hence the

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signal detected) by 87.5%. The broad size range of MPs ranging over three orders of magnitude or more

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requires the use of a variety of techniques in order to cover the entire range of sizes (Figure 2). For MPs

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the development of analytical strategies has so far been focusing on larger size particles (micrometer

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range and above), where techniques such as sieving, optical microcopy and laser obscuration in

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combination with Raman or Fourier transformed infrared spectroscopy or thermoanalytical techniques

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to determine particle composition (polymer type) were applied. Besides particle size and composition,

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additives in the polymer might be of interest in the discussion of MP analysis. However, analytical

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methods to analyze polymer additives are not considered in this Feature.

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The listed techniques which are known from material science are considered as most promising to meet

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the challenges (particle size and composition i.e. polymer type) for analysis of MPs in the size range >

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50 µm in aquatic environmental samples.72 For MP size fractions < 50 µm similar challenges (selectivity

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and sensitivity) as for ENPs arise for the development of analytical methods, where knowledge from

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ENPs could be integrated (Table 1). The possible existence of nano-plastic particles may lead to a further

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increase in similarities as encountered for the detection of ENPs above (Figure 2). For particle size

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determination, field flow fractionation (FFF)73 or liquid chromatography74 techniques might be adapted,


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but will be challenged by particles > 1 µm. Detection techniques for analysis of organic ENPs

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(determination of carbon-based ENPs, i.e. C60 fullerenes in environmental samples by atmospheric

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photoionization with an Orbitrap high resolution MS) are not directly transferable to nano-plastic

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analysis because of insufficient ionization of the polymer matrix. There is clearly a need to develop and

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adapt polymer selective detectors (e.g. Raman spectroscopy hyphenated to particle size separation

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techniques) in order to reveal the existence and fate of nano-sized MPs in environmental matrices.

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Similar to ENPs, MP characterization in the presence of a particulate matrix (as often encountered in

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natural systems and biota) requires sample preparation and/or highly specific and sensitive particle

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analysis. For ENPs determination particle-by-particle analysis using high-resolution and element-specific

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methods such as electron-microscopy or newly developed spectrometric techniques are under

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discussion.75 Furthermore, suitable sample preparation strategies are critical to preserve ENP or MP

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properties, and need to be developed on a case-by-case basis.70 For example, to extract particles from a

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product or environmental matrix, harsh sample treatment strategies may result in a physical or chemical

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alteration of particle properties or induce particle aggregation resulting in misleading size

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measurements.76, 77

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Although a huge effort is currently being directed to developing appropriate analytical protocols for

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determining and characterizing levels of MP contamination in aquatic environments, there remain

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significant discrepancies between different sets of published data due to a lack of standardization (of

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analytical methods and definitions).78,

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concentration units in which the occurrence of MPs in aqueous systems is reported. For marine surface

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samples, these range from mass-based “grams per m3” to number-based “particles per m3” as well as to

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surface area normalized concentrations “grams per m2” and “particles per m2”.80 The debate on

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reporting particle number versus mass-based concentrations among the MP research community is

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reminiscent of similar discussions with respect to ENPs.81 For ENPs, no final agreement on concentration

79

This is particularly evident from the broad variety of



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metrics has been found, but the discussion has resulted in an increased awareness of the challenges

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associated with reporting properties of particulate contaminants.

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244 245 246 247 248

Figure 2: Overview of relevant size ranges for ENPs, natural colloids, MPs and nano-plastics and available analytical techniques to characterize particles by size and/or composition. * only in combination with an energy dispersive x-ray detector (EDX) ** no direct measurement, calculated from mass measurements based on information on particle shape stoichiometry *** only in combination with element specific detection e.g. with ICP-MS

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Designing representative laboratory experiments

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Particulate contaminants require special considerations when it comes to designing adequate laboratory

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experiments to investigate, for example, their behavior or toxicity. Since particles do not form solutions 14 ACS Paragon Plus Environment

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but more or less stable dispersions, their dispersion stability needs to be controlled in the experimental

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set-ups. The dispersion stability will depend on both the properties of the particles and of the medium40

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as well as the particle concentration. At high ionic strength for example, ENPs are prone to aggregation

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since the dissolved ions in solution effectively screen the repulsive charges on the ENP surfaces and

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promote aggregation. In systems where their surface charge is important enough to provide

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electrostatic stabilization against aggregation, the ENPs form stable dispersions, whose movement is

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dominated by Brownian motion and sedimentation is prevented, even in non-stirred systems (Figure

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3a). MPs on the other hand, which are often characterized by a lower density and larger sizes, may float

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in the aquatic media, possibly leading to inhomogeneities in non-stirred systems (Figure 3b). Even in

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cases where MPs have a density larger than water, stirring would be required to avoid sedimentation

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due to increased gravitational settling of large and dense particles (Figure 3c).

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Figure 3: Schematic representation of ENPs and MPs in laboratory experiments. The effect of particle size and density is depicted by comparing small particles (nanometer scale) of high density (a), large particles of low density (b) and large particles with high density. Experiments using realistic particle concentrations often fall below analytical detection thresholds (d), while suspensions with concentrations high enough to be detected are prone to fast particle aggregation (e).

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The strong influence that the properties of the surrounding aquatic medium exert on the behavior of

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particulate contaminants needs to be taken into consideration. Important aspects of ENP fate

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assessment are i) the use of well-defined aquatic media that are complex enough to be representative

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of environmental conditions but can be reproduced in standard laboratories,41,

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knowledge of the ENP characteristics. This will be equally important in similar investigations for MPs. An

275

additional challenge, which may be relevant for both ENPs and MPs is the fact that laboratory

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experiments typically require the use of particle concentrations that are orders of magnitude greater

277

than those likely to be found in natural environments,12 because the detection limits of currently

278

available analytical methods make it impossible to work at realistic concentrations (Figure 3d).70

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However, using unrealistically high particle concentrations leads to an exaggerated importance of

280

homoaggregation in laboratory experiments (Figure 3e) relative to realistic scenarios, which needs to be

281

critically accounted for in the experiment interpretation. Techniques used to keep ENPs in a state of

282

dispersion (e.g. sonication, or the addition of NOM or other stabilizers) may be transferrable to MPs.

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This is expected to be especially relevant for the smaller MPs and nano-plastics that are particularly

284

prone to aggregation, whereas larger particles (> 10 µm) are likely to form looser aggregates which are

285

more affected by shear forces than by surface effects.

286

Another important topic is the effect of particle ageing (i.e. a result of one or more physical or chemical

287

transformations during their life cycle) on ENP behavior.83,

288

synthesized for laboratory experiments, and which have been used in most studies to date, may often

289

not be a good representative of the ENP encountered in real environments. Consequently, including

290

aged ENPs in fate and toxicity testing protocols is being suggested increasingly.85,

291

considerations are highly relevant as well, since the well-defined MP particles typically used in

292

laboratory experiments to date are likely not representative of the MPs found in nature and having

293

undergone various fragmentation and ageing steps.12 Fragmentation is a less relevant process for the

84

82

and ii) a detailed

Pristine ENPs, as can be purchased or


86

For MPs these

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fate of individual ENPs. However it may play a significant role for the release of ENPs from nano-enabled

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products and materials. Concepts to quantify release of ENPs due to fragmentation of the product in

296

different life-cycle stages have been addressed in various guidelines.87 These concepts can potentially be

297

adapted for fragmentation of plastic material under environmental conditions.

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The way forward with MPs

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Given their particulate nature, which significantly distinguishes both ENPs and MPs from dissolved

300

contaminants and meanwhile represents their strongest commonality, it is clear that knowledge from

301

ENPs fate research can be transferred to MPs based on their similarities (Table 1). The advances made in

302

ENP exposure assessment, both with respect to the tools and methods developed as well as the

303

experience in handling particulate contaminants, can serve as a strong basis for developing exposure

304

assessment approaches for MPs. With this example, the importance of interdisciplinary research teams

305

becomes apparent. By omitting, or not actively seeking, a greater involvement of researchers from other

306

related disciplines such as polymer/material, colloid/nano sciences and hydrology the MP research

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community would certainly be missing out on a significant contribution that could help to provide

308

improved, faster, and more efficient MP environmental fate, exposure and risk assessment. Most

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recently, there have been first signs of explicit discussions on how knowledge of ENP research could be

310

integrated into MP ecology24, toxicity23 environmental fate modelling20, and food safety.88 Based on our

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analysis above, the most pressing research priorities in MPs exposure assessment to be addressed by

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interdisciplinary approaches can be summarized as:

313

•

Investigation of fragmentation processes of macroplastics and rates of secondary MP

314

formation under various natural conditions are important to account for relevant MP source

315

and emission pathways and asses their environmental fate,



316

•

317 318

Investigation of MP heteroaggregation to obtain MP-specific attachment efficiencies and heteroaggregate break-up rates,

•

319 320

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Evaluation of the leaching of additives, and their effects on particle properties and ecosystems,

•

Definition of particle size distribution range(s) and related biological and chemical relevance

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for the environment would facilitate the development of analytical methods and monitoring

322

strategies,

323

•

Establishment of an analytical framework, which sets analytical techniques for defined

324

particle size fractions and analytical parameters (e.g. the use of FT-infrared microscopy for

325

screening of larger MPs and, if positive, analysis of smaller sizes by Raman microscopy),

326

•

Definition of protocols for stable MP dispersions accounting for the large variety of MP

327

particle properties and sizes is required to design representative and comparable laboratory

328

experiments.

329

MPs and ENPs clearly have strong similarities based on their particulate nature. Many of the strategies

330

developed for ENPs can serve as a solid starting point for addressing research questions related to MP

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occurrence and fate in the environments, for the design of appropriate laboratory and analytical

332

methods and can help direct research efforts for MP exposure assessment in (aquatic) environments. At

333

the same time several details need to be uncovered specifically for MPs and have been highlighted in

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the distinct research priorities listed above.

335


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336

Author Information

337

Corresponding author:

338

*Email: [email protected] (T.H.); [email protected] (F.v.d.K)

339 340

Notes:

341

The authors declare no competing financial interest.

342 343

Acknowledgements

344

We thank Gabriel Sigmund for his helpful comments and discussions.

345



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346

Biography

347

Thorsten Hüffer works as a postdoctoral researcher in the Department of Environmental Geosciences at

348

the University of Vienna, focusing on investigations into the phase transfer processes of organic

349

compounds in natural environments and on analytical preconcentration techniques. He is also head of

350

the expert committee on “Plastics in the aquatic environment” within the German Water Chemistry

351

Society.

352

Antonia Praetorius is a postdoctoral researcher in the Department of Environmental Geosciences and

353

the Nano-Norms-Nature Research Platform at the University of Vienna. Her research focuses on

354

assessing and modeling the fate of engineered nanoparticles in natural environments and on

355

interdisciplinary approaches to a sustainable development of nanotechnology.

356

Stephan Wagner is a researcher at the Department of Analytical Chemistry, Helmholtz-Centre for

357

Environmental Research – UFZ, in Leipzig. His research focuses on the development of analytical

358

methods for nanomaterials in consumer products and various environments within the context of risk

359

assessment and the regulatory framework.

360

Frank von der Kammer is currently Vice-Head of the Department for Environmental Geosciences, Senior

361

Scientist and Research Faculty at the University of Vienna. His research interests include environmental

362

colloids, their dynamic behavior and interaction with trace elements, natural nano-scale processes, and

363

nanoparticle characterization in complex samples.

364

Thilo Hofmann is Professor for Environmental Geosciences (Chair) at the University of Vienna. His main

365

research interests are the behavior of organic contaminants in sediments and soils, interactions with

366

nanomaterials, colloid-bound contaminant transport, and the environmental relevance of engineered

367

nanomaterials.


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Figure 1: Overview of the most relevant transformation and transport processes affecting ENPs (black circles) and MPs (black pentagons) in aquatic systems. NOM: natural organic matter. The green clouds represent naturally-occurring suspended particulate matter (SPM). 157x142mm (300 x 300 DPI)




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Figure 3: Schematic representation of ENPs and MPs in laboratory experiments. The effect of particle size and density is depicted by comparing small particles (nanometer scale) of high density (a), large particles of low density (b) and large particles with high density. Experiments using realistic particle concentrations often fall below analytical detection thresholds (d), while suspensions with concentrations high enough to be detected are prone to fast particle aggregation (e). 80x30mm (300 x 300 DPI)


Microplastic Exposure Assessment in Aquatic Environments: Learning

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