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Nanomaterials to microplastics: Swings and roundabouts
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Nanomaterials to microplastics: Swings and roundabouts J.J. Scott-Fordsmand a,∗ , J.M. Navas b , K. Hund-Rinke c , B. Nowack d , M.J.B. Amorim e,∗ a
Department of Bioscience, Aarhus University, Vejlsoevej 25, DK-8600 Silkeborg, Denmark National Institute for Agricultural and Food Research and Technology (INIA), Department of Environment, Ctra. de la Coru˜ na Km 7.5, E-28040 Madrid, Spain c Fraunhofer Institute for Molecular Biology and Applied Ecology IME, Auf dem Aberg 1, 57392 Schmallenberg, Germany d Empa, Swiss Federal Laboratories for Materials Science and Technology, Technology and Society Laboratory, Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland e University of Aveiro, Department of Biology and CESAM, 3810-193 Aveiro, Portugal b
a r t i c l e
i n f o
Article history: Received 25 July 2017 Received in revised form 25 August 2017 Accepted 6 September 2017 Available online xxx Keywords: Nanoparticles Microplastics Risk assessment
a b s t r a c t In recent years, the testing of nanomaterials during their use in products has been increasingly emphasized, as this will provide a more realistic risk assessment (RA) compared with RA based on pristine nanomaterials. We show that (i) using such an approach for a “realistic” RA is increasing the complexity of the RA, (ii) several testing-aspects render this approach more challenging than the conventional methods, (iii) interpretation of the results becomes difficult, and (iv) the resulting RA may need to be evaluated carefully as it yields improved understanding of the short-term fate of the individual product, but long-term consequences may be neglected. © 2017 Elsevier Ltd. All rights reserved.
Sustainable development Sustainable development plays a key role in the field of nanotechnology, where regulators, producers and insurance companies strive to get appropriate methods to assess the potential impact. The “normal” approach of ensuring sustainability involves risk assessments (RAs) based on tests with pristine nanomaterials (NMs) [1,2]. However, in recent years, the importance of testing nanomaterials as they appear during their use in products has been increasingly emphasized [3], i.e. the testing should be performed with the NMs as they are present in their product matrix. To perform such testing, the products are shredded into nano- and micro-fragments mimicking wear and tear. The argument for this life-cycle approach is that each product undergoes a number of product-specific transformations during its life-cycle [3–6], and consideration of these transformations provides an estimate of the actual exposure and hazard. The above arguments seem logical and appear to provide for an improved (i.e. “realistic”) approach. However, the testing of NMs in nano- and micro-fragments is extremely
∗ Corresponding authors. E-mail addresses: [email protected] (J.J. Scott-Fordsmand), [email protected] (M.J.B. Amorim).
difficult, may conceal realistic longer-term worse-case scenarios, and needs new approaches for read-across. Starting as a pristine and free NM, these can be incorporated into a product. The product may subsequently release fragments and free NMs during transport, use, or at the end-of-life stage. If the product matrix is a polymer (e.g. car bumper, plastic bag, or food packaging), the fragments containing the NM can be considered nano- or microplastics, with sizes ranging from ∼0.001 to 0.1 m and 0.1–5,000 m [7], respectively. However, since NMs are embedded in a wide range of materials, various types of fragments are obtained. These may by themselves have harmful effects e.g. if high aspect ratio fragments are released such as nano-enabled fibers [8,9]. A more realistic RA approach compared with conventional methods must therefore include the exposure and hazard testing of fragments. Here we describe some of the difficulties associated with this approach. Reliability and repeatability of the tests is a key regulatory requirement which can be achieved by (among others) using reference materials. Reference-type pristine NMs are relatively easy to obtain, at least for the simple metal/-oxide types e.g. JRC [10] and NIST. In contrast, reference-type nano-embedded and nonnano fragmented materials (e.g. nano- and microplastic) are not yet available [11], but should preferably be available from a reference repository similar to the nano-counterpart. A variety of techniques
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have been described to produce fragments that mimic aged and released materials e.g. sanding, cryo-milling, and other abrasion techniques [3,5,12,13]. The resulting fragment size depends on the method, the conditions and the material fragmented, e.g. sanding provides finer fragments than cryo-milling [3], materials with higher stiffness generally deform less than softer materials [13], and nano-enabled and non-nano materials may produce a different size-distribution under realistic conditions [12]. After a standard size-distribution has been obtained, the shape and surface-area conformity of the materials must also be verified [14], which may prove to be extremely difficult as heterogeneous primary fragments are typically obtained, thereby hampering the reference control of the testing. To mimic realistic wear and tear these primary fragments can be subjected to an artificial weathering and ageing process, thereby yielding secondary fragments e.g. secondary nano-enabled microplastics [15,16]. However, diverse primary fragments subjected to subsequent weathering or ageing steps will also give rise to non-standard secondary fragments and the process may even increase the heterogeneity [16]. The primary or secondary fragments may decompose more or less completely depending on the stability of the matrix material. This will allow the embedded NMs to be released over time (Fig. 1). The embedded NMs are very similar to pristine material, however as the fragments release the NMs may also be transformed by environmental conditions [16], yielding an NM exposure that is different to that of the pristine NMs combined with an exposure to the remaining nano- or micro-fragments. Actual hazard and exposure testing begins after a suitable material including reference, weathered and aged material are obtained. During testing, it is required that NMs are homogeneously distributed in the test systems, which can be challenging in many cases [17,18]. Large fragments with differing densities will result in a high level of heterogeneity. For example, dispersion of fragments is often impossible in liquid media because their density differs from that of the test media; this is especially true for polymer based fragments e.g. microplastics [19]. Hence, the fragments will either float or sink, resulting in little or no actual exposure of the test organism. This could either be interpreted as a challenge of testing the materials, but can also mean that no actual exposure of the material to the organism is possible in the test system. Thus from the view-point of an exposure-driven RA, there is no risk to be expected due to missing organism exposure. However, owing to currents, turbidity, and the occurrence of other constituents in “natural” waters, the fragments (especially when weathered) may remain mixed within the water column in the real world [19], resulting in exposure whereas the test system does not show exposure. Fragments mixed into a solid environmental medium may yield changes in the characteristics/properties of the medium, e.g. porosity. This is especially important for fragments with only small amounts (e.g. 0.1–2% w/w) of NMs. For example, in regulatory testing, 100 mg/L or 1000 mg/Kg of the active chemical is recommended for the initial testing, the limit-test [20,21]. A fragment containing 0.1% NM (i.e. 1/1000) would therefore require an addition of 1000 g fragment/Kg medium, which would have a negative physical/diluting effect on the test system. It is acknowledged that in the regulatory system, limit testing may be tempered for poorly dissolvable substances. The aforementioned heterogeneity usually leads to lack of exposure or high standard deviations, which in turn decreases the statistical power required for linking the exposure and the associated effects within the test. An acceptable homogeneous distribution of fragments in the test media can be verified by quantitative measurements of the NMs in the exposure media. However, few methods exist that can quantitatively characterize NMs in solid matrices, and the existing techniques for a few metal-based NMs are available in only a few
laboratories worldwide. Generally acceptable methods for quantifying the release of NMs from the matrix [7,11,15] are lacking. Similarly, widely accepted methods for the quantitative characterization of fragments (e.g. a standardized reference method for characterising microplastics in complex environments) remain elusive [11]. As discussed above the control of the exposure regime is extremely challenging, and testing becomes no less challenging when dealing with biological species. As seen from Fig. 2, the primary fragments may be the same size as or even larger than many important test organisms. For many organisms, measuring the toxicity and uptake of fragments would be equivalent to measuring the toxicity and uptake of rocks from mountains on cows (Fig. 2) [22]. Although, we are concerned with falling rocks, we are also especially concerned with the elements (e.g. arsenic or lead) that may be released from these eroded rocks. Hence, we should be concerned with the materials released from these fragments over time and the effect of the fragment itself. This means that where nanomaterials may be released there is a need for better test methods that reflect long-term exposure. Thus, the evaluation of nano- and micro-sized fragment toxicities is a combination of long-term physical and chemical effects. The realisation of this concern has just started, especially in the terrestrial system, which requires special attention in risk assessment and communication to stakeholders [23]. After completed testing, a RA is based on the acquired data. The same NMs can be embedded in multiple products and, hence, multiple NM sources may occur in the environment. Linking the hazard and exposure measures to specific fragments (from one product) will lead to discrepancies between Predicted Environmental Concentration (PEC) obtained from material flow or fate modelling studies and No Observed Effect Concentration (NOEC/EC10). The models and thus the PEC values currently do not consider the large fragments and can therefore not be used to determine any risk from matrix-embedded NMs. This indicates that the short- or even longterm NOECs from laboratory studies are non-predictive. Given this, novel probabilistic models that can account for both the release of NMs over time from various types of fragmented materials and for the direct toxicity of such fragments, should be developed. Hence, despite the call for an improved and realistic approach for the exposure and hazard testing of NMs, several drawbacks make this approach more complex than other methods. Care must be taken as a product-based focus may easily yield results that are of little value for RA.
Conclusion and future directions The environmental fate of an individual product can be better understood by testing the product. However, valid hazard testing of fragments is very difficult, and controlled exposure to many organisms is even more challenging than for NMs. Correlating the observed effects with specific NM-descriptors (e.g. surface area, surface charge) and read-across between various NMs becomes difficult, thus there is a need for new approaches based on product use, descriptors of the matrix and of the released NMs. If we understand directly the long-term NM-related exposure and hazard [24–26], this can be supported by product fate-studies based on matrix stabilities. Novel hazard paradigms should include long-term test methods capable of including fragments (e.g. microplastics) which also focus on NMs as released. For this to happen, it would be highly important to have test materials as released and weathered, rather than only intermediate micronized materials. The evaluation of nano- and micro-sized nano-enabled fragment toxicities has just started, especially in terrestrial systems, and the physical effects and chemical combinations are of concern.
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Fig. 1. Illustration of the weathering of fragments (e.g. microplastics) with release of NMs over time. The red to yellow colour represents the change of the NMs from pristine to transformed.
Fig. 2. Illustration of the fragment size compared with that of an environmental organism. Left: actual picture of a soil test using the worm species (Enchytraeus crypticus, light brown in picture) exposed to real Carbon NanoTubes enabled product fragments that have been cryo-milled. Right: an edited picture showing the equivalent for the testing of large organisms, where the fragment would be equivalent to large rocks.
Conflict of interest The funding sources had no part in writing the paper. The authors alone are responsible for the content of the article.
[8]
[9]
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[17] J.J. Scott-Fordsmand, P.H. Krogh, J.R. Lead, Nanomaterials in ecotoxicology, Integr. Env. Assess Manag. 4 (2008) 126–128. [18] W. Peijnenburg, A. Praetorius, J. Scott-Fordsmand, G. Cornelis, Fate assessment of ENPs in solid media −current insights and the way forward, Environ. Pollut. (218) (2016) 1365–1369. [19] A. Cózar, F. Echevarría, J.I. González-gordillo, X. Irigoien, B. Úbeda, Plastic debris in the open ocean, 17 19 (2014), http://dx.doi.org/10.1073/pnas. 1314705111. [20] OECD Guidance on Sample Preparation and Dosimetry for The Safety Testing of Manufactured Nanomaterials. Series on The Safety of Manufactured Nanomaterials No. 36, OECD (Organisation for Economic Cooperation and Development), 2012, pp. 1–16, ENV/JM/MONO(2007)10. [21] OECD Series on Testing and Assessment Number 23: Guidance Document on Aquatic Toxicity Testing of Difficult Substances and Mixtures, OECD (Organisation for Economic Cooperation and Development), Paris, 2002. [22] S.D. Veresoglou, J.M. Halley, M.C. Rillig, Extinction risk of soil biota, Nat. Commun. 6 (2015) 8862, http://dx.doi.org/10.1038/ncomms9862. [23] F. Murphy, M. Mullins, K. Hester, A. Gelwick, J.J. Scott-Fordsmand, T. Maynard, Insuring nanotech requires effective risk communication, Nat. Nanotechnol. 12 (2017) 717–719, http://dx.doi.org/10.1038/nnano.2017.162. [24] M.F.M. Gonc¸alves, S.I.L. Gomes, J.J. Scott-Fordsmand, M.J.B. Amorim, Shorter lifetime of a soil invertebrate species when exposed to copper oxide nanoparticles in a full lifespan exposure test, Sci. Rep. 7 (1355) (2017) 1–8, http://dx.doi.org/10.1038/s41598-017-01507-8. [25] R.C. Bicho, T. Ribeiro, N.P. Rodrigues, J.J. Scott-Fordsmand, M.J.B. Amorim, Effects of Ag nanomaterials (NM300 K) and Ag salt (AgNO3) can be discriminated in a full life cycle long term test with Enchytraeus crypticus, J. Hazard. Mater. 318 (2016) 608–614, http://dx.doi.org/10.1016/j.jhazmat. 2016.07.040. [26] M.J.B. Amorim, C.P. Roca, J.J. Scott-Fordsmand, Effect assessment of engineered nanoparticles in solid media - current insight and the way forward, Environ. Pollut. 218 (2016) 1370–1375, http://dx.doi.org/10.1016/j. envpol.2015.08.048. Senior scientist Ph.D., Janeck J. Scott-Fordsmand, Department of Bioscience, Aarhus University, Denmark has 20 years of experience in research on environmental risk assessment. He is continuously advising various EPAs, Nordic Council, EU and OECD on risk related issues, e.g. development of test guidelines, guidance documents, intelligent testing strategies, and actual risk assessments. He has long experience as leader of projects and work packages in European nano-and risk related projects. He chairs the Communities of Research activities on nano-risks. He has published more than 100 papers, governmental and advisory reports. He continuously supervise or co-supervise Post Docs, PhD and Master students. Dr. Navas is a research scientist at the Spanish National Institute for Agricultural and Food Research and Technology (INIA) in Madrid. He obtained his PhD in Biology after defending a thesis on reproductive physiology of fish in 1997 (Spanish Superior Council for Scientific Research, CSIC). As postdoc (Marie Curie contract at the Helmholtz Centre for Environmental Research, UFZ, Leipzig Germany; Marie Curie Return Contract at the CSIC in Castellón, Spain; other contracts in Spain) he worked on the antiestrogenic effects of chemicals in fish. Since 2005 he has a permanent position at INIA and he has concentrated on the study of the mechanisms underlying the action xenobiotics, including nanomaterials, on fish using in vitro approaches. The knowledge obtained serves to the diagnosis of environmental pollution under realistic situations in field studies. He acts regularly as Spanish representative at some OECD and EC bodies related with the regulation of nanomaterials (WPMN).
Kerstin Hund-Rinke graduated on the effect of pesticides on soil microflora at the Technical University in Munich, Germany. Since 1988 she has been working in the Ecotoxicology department at the Fraunhofer Institute for Molecular Biology and Applied Ecology in Schmallenberg where she heads the research group Soil protection; nanomaterials focusing on the ecotoxicological/ecological evaluation of soil quality, microbial biodiversity and the bioavailability of pollutants. She is involved in environmental risk assessments for chemicals, pesticides, biocides, products, nanomaterials, and wastes. She is a member of several scientific committees such as the German delegation in the OECD WPMN (Working Party of Manufactured Nanomaterials) and the Scientific Advisory Board on Fertilizer Issues initiated by the German Federal Ministry of Food and Agriculture. Prof. Dr. Bernd Nowack holds a MSc. (1992) and a PhD (1995) in environmental sciences from ETH Zürich. He is leading the Environmental Risk Assessment and Management group at Empa, the Swiss Federal Laboratories for Materials Science and Technology, and is adjunct professor at ETH Zurich. His current research deals with the chances and risks of engineered nanomaterials, nanobiomaterials and microplastics, using different approaches: development and application of methods for material flow modelling, environmental risk assessment and life cycle assessment; experimental studies about release of materials from products and investigations about their behavior and effects in the environment. Bernd Nowack has published more than 150 peer-reviewed publications and is listed in The World’s most influential scientific minds 2015 from Thomson Reuters in the category Environmental Sciences/Ecology. He acted as co-advisor of 15 PhD projects, is founding co-Editor-in-Chief of the journal NanoImpact and is Associate Editor of the journal Environmental Pollution. Dr. Mónica Amorim, PhD in Biology (2004) at the University of Aveiro, Portugal, where she is a researcher since 2006. She is the head of the Ecotoxicogenomics laboratory of CESAM (Centre for Environmental and Marine Studies). She was president of SETAC Europe in 2015. SETAC, the Society of Environmental Toxicology and Chemistry, probably the largest world ecotoxicology society (∼6000 members). Her research deals with the environmental hazards of nanomaterials, using a systems toxicology approach, investigating the mechanisms and integrating effects of different levels of biological organization, using high-throughput −omics tools. Mónica Amorim has published 90 papers in international peer reviewed journals, has more than 100 platforms and posters at international conferences; has supervised 10 PhD students and 6 Post-Docs. Coordinated the Post-Graduation Practical approach to ecotoxicogenomics since 2007.
Please cite this article in press as: J.J. Scott-Fordsmand, et al., Nanomaterials to microplastics: Swings and roundabouts, Nano Today (2017), http://dx.doi.org/10.1016/j.nantod.2017.09.002
ARTICLE IN PRESS
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Nano Today xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Nano Today journal homepage: www.elsevier.com/locate/nanotoday
News and opinions
Nanomaterials to microplastics: Swings and roundabouts J.J. Scott-Fordsmand a,∗ , J.M. Navas b , K. Hund-Rinke c , B. Nowack d , M.J.B. Amorim e,∗ a
Department of Bioscience, Aarhus University, Vejlsoevej 25, DK-8600 Silkeborg, Denmark National Institute for Agricultural and Food Research and Technology (INIA), Department of Environment, Ctra. de la Coru˜ na Km 7.5, E-28040 Madrid, Spain c Fraunhofer Institute for Molecular Biology and Applied Ecology IME, Auf dem Aberg 1, 57392 Schmallenberg, Germany d Empa, Swiss Federal Laboratories for Materials Science and Technology, Technology and Society Laboratory, Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland e University of Aveiro, Department of Biology and CESAM, 3810-193 Aveiro, Portugal b
a r t i c l e
i n f o
Article history: Received 25 July 2017 Received in revised form 25 August 2017 Accepted 6 September 2017 Available online xxx Keywords: Nanoparticles Microplastics Risk assessment
a b s t r a c t In recent years, the testing of nanomaterials during their use in products has been increasingly emphasized, as this will provide a more realistic risk assessment (RA) compared with RA based on pristine nanomaterials. We show that (i) using such an approach for a “realistic” RA is increasing the complexity of the RA, (ii) several testing-aspects render this approach more challenging than the conventional methods, (iii) interpretation of the results becomes difficult, and (iv) the resulting RA may need to be evaluated carefully as it yields improved understanding of the short-term fate of the individual product, but long-term consequences may be neglected. © 2017 Elsevier Ltd. All rights reserved.
Sustainable development Sustainable development plays a key role in the field of nanotechnology, where regulators, producers and insurance companies strive to get appropriate methods to assess the potential impact. The “normal” approach of ensuring sustainability involves risk assessments (RAs) based on tests with pristine nanomaterials (NMs) [1,2]. However, in recent years, the importance of testing nanomaterials as they appear during their use in products has been increasingly emphasized [3], i.e. the testing should be performed with the NMs as they are present in their product matrix. To perform such testing, the products are shredded into nano- and micro-fragments mimicking wear and tear. The argument for this life-cycle approach is that each product undergoes a number of product-specific transformations during its life-cycle [3–6], and consideration of these transformations provides an estimate of the actual exposure and hazard. The above arguments seem logical and appear to provide for an improved (i.e. “realistic”) approach. However, the testing of NMs in nano- and micro-fragments is extremely
∗ Corresponding authors. E-mail addresses: [email protected] (J.J. Scott-Fordsmand), [email protected] (M.J.B. Amorim).
difficult, may conceal realistic longer-term worse-case scenarios, and needs new approaches for read-across. Starting as a pristine and free NM, these can be incorporated into a product. The product may subsequently release fragments and free NMs during transport, use, or at the end-of-life stage. If the product matrix is a polymer (e.g. car bumper, plastic bag, or food packaging), the fragments containing the NM can be considered nano- or microplastics, with sizes ranging from ∼0.001 to 0.1 m and 0.1–5,000 m [7], respectively. However, since NMs are embedded in a wide range of materials, various types of fragments are obtained. These may by themselves have harmful effects e.g. if high aspect ratio fragments are released such as nano-enabled fibers [8,9]. A more realistic RA approach compared with conventional methods must therefore include the exposure and hazard testing of fragments. Here we describe some of the difficulties associated with this approach. Reliability and repeatability of the tests is a key regulatory requirement which can be achieved by (among others) using reference materials. Reference-type pristine NMs are relatively easy to obtain, at least for the simple metal/-oxide types e.g. JRC [10] and NIST. In contrast, reference-type nano-embedded and nonnano fragmented materials (e.g. nano- and microplastic) are not yet available [11], but should preferably be available from a reference repository similar to the nano-counterpart. A variety of techniques
http://dx.doi.org/10.1016/j.nantod.2017.09.002 1748-0132/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: J.J. Scott-Fordsmand, et al., Nanomaterials to microplastics: Swings and roundabouts, Nano Today (2017), http://dx.doi.org/10.1016/j.nantod.2017.09.002
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have been described to produce fragments that mimic aged and released materials e.g. sanding, cryo-milling, and other abrasion techniques [3,5,12,13]. The resulting fragment size depends on the method, the conditions and the material fragmented, e.g. sanding provides finer fragments than cryo-milling [3], materials with higher stiffness generally deform less than softer materials [13], and nano-enabled and non-nano materials may produce a different size-distribution under realistic conditions [12]. After a standard size-distribution has been obtained, the shape and surface-area conformity of the materials must also be verified [14], which may prove to be extremely difficult as heterogeneous primary fragments are typically obtained, thereby hampering the reference control of the testing. To mimic realistic wear and tear these primary fragments can be subjected to an artificial weathering and ageing process, thereby yielding secondary fragments e.g. secondary nano-enabled microplastics [15,16]. However, diverse primary fragments subjected to subsequent weathering or ageing steps will also give rise to non-standard secondary fragments and the process may even increase the heterogeneity [16]. The primary or secondary fragments may decompose more or less completely depending on the stability of the matrix material. This will allow the embedded NMs to be released over time (Fig. 1). The embedded NMs are very similar to pristine material, however as the fragments release the NMs may also be transformed by environmental conditions [16], yielding an NM exposure that is different to that of the pristine NMs combined with an exposure to the remaining nano- or micro-fragments. Actual hazard and exposure testing begins after a suitable material including reference, weathered and aged material are obtained. During testing, it is required that NMs are homogeneously distributed in the test systems, which can be challenging in many cases [17,18]. Large fragments with differing densities will result in a high level of heterogeneity. For example, dispersion of fragments is often impossible in liquid media because their density differs from that of the test media; this is especially true for polymer based fragments e.g. microplastics [19]. Hence, the fragments will either float or sink, resulting in little or no actual exposure of the test organism. This could either be interpreted as a challenge of testing the materials, but can also mean that no actual exposure of the material to the organism is possible in the test system. Thus from the view-point of an exposure-driven RA, there is no risk to be expected due to missing organism exposure. However, owing to currents, turbidity, and the occurrence of other constituents in “natural” waters, the fragments (especially when weathered) may remain mixed within the water column in the real world [19], resulting in exposure whereas the test system does not show exposure. Fragments mixed into a solid environmental medium may yield changes in the characteristics/properties of the medium, e.g. porosity. This is especially important for fragments with only small amounts (e.g. 0.1–2% w/w) of NMs. For example, in regulatory testing, 100 mg/L or 1000 mg/Kg of the active chemical is recommended for the initial testing, the limit-test [20,21]. A fragment containing 0.1% NM (i.e. 1/1000) would therefore require an addition of 1000 g fragment/Kg medium, which would have a negative physical/diluting effect on the test system. It is acknowledged that in the regulatory system, limit testing may be tempered for poorly dissolvable substances. The aforementioned heterogeneity usually leads to lack of exposure or high standard deviations, which in turn decreases the statistical power required for linking the exposure and the associated effects within the test. An acceptable homogeneous distribution of fragments in the test media can be verified by quantitative measurements of the NMs in the exposure media. However, few methods exist that can quantitatively characterize NMs in solid matrices, and the existing techniques for a few metal-based NMs are available in only a few
laboratories worldwide. Generally acceptable methods for quantifying the release of NMs from the matrix [7,11,15] are lacking. Similarly, widely accepted methods for the quantitative characterization of fragments (e.g. a standardized reference method for characterising microplastics in complex environments) remain elusive [11]. As discussed above the control of the exposure regime is extremely challenging, and testing becomes no less challenging when dealing with biological species. As seen from Fig. 2, the primary fragments may be the same size as or even larger than many important test organisms. For many organisms, measuring the toxicity and uptake of fragments would be equivalent to measuring the toxicity and uptake of rocks from mountains on cows (Fig. 2) [22]. Although, we are concerned with falling rocks, we are also especially concerned with the elements (e.g. arsenic or lead) that may be released from these eroded rocks. Hence, we should be concerned with the materials released from these fragments over time and the effect of the fragment itself. This means that where nanomaterials may be released there is a need for better test methods that reflect long-term exposure. Thus, the evaluation of nano- and micro-sized fragment toxicities is a combination of long-term physical and chemical effects. The realisation of this concern has just started, especially in the terrestrial system, which requires special attention in risk assessment and communication to stakeholders [23]. After completed testing, a RA is based on the acquired data. The same NMs can be embedded in multiple products and, hence, multiple NM sources may occur in the environment. Linking the hazard and exposure measures to specific fragments (from one product) will lead to discrepancies between Predicted Environmental Concentration (PEC) obtained from material flow or fate modelling studies and No Observed Effect Concentration (NOEC/EC10). The models and thus the PEC values currently do not consider the large fragments and can therefore not be used to determine any risk from matrix-embedded NMs. This indicates that the short- or even longterm NOECs from laboratory studies are non-predictive. Given this, novel probabilistic models that can account for both the release of NMs over time from various types of fragmented materials and for the direct toxicity of such fragments, should be developed. Hence, despite the call for an improved and realistic approach for the exposure and hazard testing of NMs, several drawbacks make this approach more complex than other methods. Care must be taken as a product-based focus may easily yield results that are of little value for RA.
Conclusion and future directions The environmental fate of an individual product can be better understood by testing the product. However, valid hazard testing of fragments is very difficult, and controlled exposure to many organisms is even more challenging than for NMs. Correlating the observed effects with specific NM-descriptors (e.g. surface area, surface charge) and read-across between various NMs becomes difficult, thus there is a need for new approaches based on product use, descriptors of the matrix and of the released NMs. If we understand directly the long-term NM-related exposure and hazard [24–26], this can be supported by product fate-studies based on matrix stabilities. Novel hazard paradigms should include long-term test methods capable of including fragments (e.g. microplastics) which also focus on NMs as released. For this to happen, it would be highly important to have test materials as released and weathered, rather than only intermediate micronized materials. The evaluation of nano- and micro-sized nano-enabled fragment toxicities has just started, especially in terrestrial systems, and the physical effects and chemical combinations are of concern.
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Fig. 1. Illustration of the weathering of fragments (e.g. microplastics) with release of NMs over time. The red to yellow colour represents the change of the NMs from pristine to transformed.
Fig. 2. Illustration of the fragment size compared with that of an environmental organism. Left: actual picture of a soil test using the worm species (Enchytraeus crypticus, light brown in picture) exposed to real Carbon NanoTubes enabled product fragments that have been cryo-milled. Right: an edited picture showing the equivalent for the testing of large organisms, where the fragment would be equivalent to large rocks.
Conflict of interest The funding sources had no part in writing the paper. The authors alone are responsible for the content of the article.
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Kerstin Hund-Rinke graduated on the effect of pesticides on soil microflora at the Technical University in Munich, Germany. Since 1988 she has been working in the Ecotoxicology department at the Fraunhofer Institute for Molecular Biology and Applied Ecology in Schmallenberg where she heads the research group Soil protection; nanomaterials focusing on the ecotoxicological/ecological evaluation of soil quality, microbial biodiversity and the bioavailability of pollutants. She is involved in environmental risk assessments for chemicals, pesticides, biocides, products, nanomaterials, and wastes. She is a member of several scientific committees such as the German delegation in the OECD WPMN (Working Party of Manufactured Nanomaterials) and the Scientific Advisory Board on Fertilizer Issues initiated by the German Federal Ministry of Food and Agriculture. Prof. Dr. Bernd Nowack holds a MSc. (1992) and a PhD (1995) in environmental sciences from ETH Zürich. He is leading the Environmental Risk Assessment and Management group at Empa, the Swiss Federal Laboratories for Materials Science and Technology, and is adjunct professor at ETH Zurich. His current research deals with the chances and risks of engineered nanomaterials, nanobiomaterials and microplastics, using different approaches: development and application of methods for material flow modelling, environmental risk assessment and life cycle assessment; experimental studies about release of materials from products and investigations about their behavior and effects in the environment. Bernd Nowack has published more than 150 peer-reviewed publications and is listed in The World’s most influential scientific minds 2015 from Thomson Reuters in the category Environmental Sciences/Ecology. He acted as co-advisor of 15 PhD projects, is founding co-Editor-in-Chief of the journal NanoImpact and is Associate Editor of the journal Environmental Pollution. Dr. Mónica Amorim, PhD in Biology (2004) at the University of Aveiro, Portugal, where she is a researcher since 2006. She is the head of the Ecotoxicogenomics laboratory of CESAM (Centre for Environmental and Marine Studies). She was president of SETAC Europe in 2015. SETAC, the Society of Environmental Toxicology and Chemistry, probably the largest world ecotoxicology society (∼6000 members). Her research deals with the environmental hazards of nanomaterials, using a systems toxicology approach, investigating the mechanisms and integrating effects of different levels of biological organization, using high-throughput −omics tools. Mónica Amorim has published 90 papers in international peer reviewed journals, has more than 100 platforms and posters at international conferences; has supervised 10 PhD students and 6 Post-Docs. Coordinated the Post-Graduation Practical approach to ecotoxicogenomics since 2007.
Please cite this article in press as: J.J. Scott-Fordsmand, et al., Nanomaterials to microplastics: Swings and roundabouts, Nano Today (2017), http://dx.doi.org/10.1016/j.nantod.2017.09.002