Current limitations and challenges in nanowaste detection, characterisation and monitoring

Waste Management 43 (2015) 407–420

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Current limitations and challenges in nanowaste detection, characterisation and monitoring Florian Part a, Gudrun Zecha a, Tim Causon b, Eva-Kathrin Sinner c, Marion Huber-Humer a,⇑ a

Department of Water-Atmosphere-Environment, University of Natural Resources and Life Sciences, Institute of Waste Management, Muthgasse 107, 1190 Vienna, Austria Department of Chemistry, Division of Analytical Chemistry, University of Natural Resources and Life Sciences, Muthgasse 18, 1190 Vienna, Austria c Department of Nanobiotechnology, Institute for Synthetic Bioarchitectures, University of Natural Resources and Life Sciences, Muthgasse 11/II, 1190 Vienna, Austria b

a r t i c l e

i n f o

Article history: Received 13 March 2015 Revised 22 May 2015 Accepted 25 May 2015 Available online 24 June 2015 Keywords: Nanomaterials Nanowaste Nano-enabled products Detection Characterisation Waste treatment

a b s t r a c t Engineered nanomaterials (ENMs) are already extensively used in diverse consumer products. Along the life cycle of a nano-enabled product, ENMs can be released and subsequently accumulate in the environment. Material flow models also indicate that a variety of ENMs may accumulate in waste streams. Therefore, a new type of waste, so-called nanowaste, is generated when end-of-life ENMs and nano-enabled products are disposed of. In terms of the precautionary principle, environmental monitoring of end-of-life ENMs is crucial to allow assessment of the potential impact of nanowaste on our ecosystem. Trace analysis and quantification of nanoparticulate species is very challenging because of the variety of ENM types that are used in products and low concentrations of nanowaste expected in complex environmental media. In the framework of this paper, challenges in nanowaste characterisation and appropriate analytical techniques which can be applied to nanowaste analysis are summarised. Recent case studies focussing on the characterisation of ENMs in waste streams are discussed. Most studies aim to investigate the fate of nanowaste during incineration, particularly considering aerosol measurements; whereas, detailed studies focusing on the potential release of nanowaste during waste recycling processes are currently not available. In terms of suitable analytical methods, separation techniques coupled to spectrometry-based methods are promising tools to detect nanowaste and determine particle size distribution in liquid waste samples. Standardised leaching protocols can be applied to generate soluble fractions stemming from solid wastes, while micro- and ultrafiltration can be used to enrich nanoparticulate species. Imaging techniques combined with X-ray-based methods are powerful tools for determining particle size, morphology and screening elemental composition. However, quantification of nanowaste is currently hampered due to the problem to differentiate engineered from naturally-occurring nanoparticles. A promising approach to face these challenges in nanowaste characterisation might be the application of nanotracers with unique optical properties, elemental or isotopic fingerprints. At present, there is also a need to develop and standardise analytical protocols regarding nanowaste sampling, separation and quantification. In general, more experimental studies are needed to examine the fate and transport of ENMs in waste streams and to deduce transfer coefficients, respectively to develop reliable material flow models. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Nanoscale particles and colloids have always been part of our environment. Such particles are in the size range of 1–100 nm and can originate from natural sources (e.g. as aerosols caused by forest fires or volcanic activities), but manufactured nanoscale particles now also have increasing significance in the environment.

⇑ Corresponding author. Tel.: +43 1 318 99 00; fax: +43 1 318 99 00 350. E-mail address: [email protected] (M. Huber-Humer). http://dx.doi.org/10.1016/j.wasman.2015.05.035 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.

This stems from their obvious commercial interest for a wide range of applications primarily due to their nano-specific and tuneable physicochemical properties. As a result, there are both naturally-occurring and engineered nanomaterials (‘‘NNMs’’ and ‘‘ENMs’’) present in our environment and in waste streams. The recommendation of the European Commission (EC, 2011) addresses also both types: ‘‘a nanomaterial means a natural, incidental or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions are in the size range 1 nm – 100 nm.’’

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ENMs have become a significant topic of research due to their potential toxicity and risks that may arise during their manufacturing, application, use and disposal (EC, 2012; Savolainen et al., 2014). Commonly used ENMs such as nano-TiO2 and -SiO2 are already produced in large volumes; approximately 3000 tons of nano-TiO2 and 5500 tons of nano-SiO2 are produced worldwide per year (Piccinno et al., 2012). In the framework of the Project on Emerging Nanotechnologies (last updated in 2013) more than 1600 consumer products were listed in a database to be containing ENMs (PEN, 2014), while The Nanodatabase (DTU Environment, 2015) currently lists more than 1400 products. The database of NANOWERK (2015) catalogues more than 2600 different types of commercial ENMs indicating that there is already a wide range of uses of ENMs in the processing industry (e.g. as additives or catalysts) and in consumer products – various applications are summarised in detail in recent publications (e.g., EC, 2012; Marcoux et al., 2013; Yang and Westerhoff, 2014). Ultimately, products containing such nanomaterials reach their end-of-life and the majority of ENMs will therefore end up and may subsequently accumulate in waste streams (Caballero-Guzman et al., 2015; Gottschalk et al., 2013; Keller and Lazareva, 2013; Mueller and Nowack, 2008; Sun et al., 2014). Therefore, implementation of suitable analytical methods is crucial to assess potential release pathways of ENMs, both during waste treatment processes and from ‘‘final sinks’’ (e.g. landfills), and to create and validate model parameters such as transfer coefficients that are needed for modelling the fate of ENMs along a product’s life cycle. In order to monitor and assess the fate and behaviour of ENMs in the environment, currently available analytical techniques have to be adapted with respect to the nanoparticle type of interest (e.g., in-/organic, un-/coated, hydrophobic/-philic, colloidally un-/stable?), detection and quantification at low concentrations and differentiation from complex background (Hassellov et al., 2008; Howard, 2010; Kammer et al., 2012; Schaumann et al., 2014; Tiede et al., 2008). The present contribution aims to provide a broad overview on available analytical techniques that are currently applicable for detection and characterisation of ENMs, and the reader will be guided to other selected papers focusing on specific analytical methods. Some analytical techniques are discussed regarding their applicability for the challenging analysis of complex and heterogeneous waste matrices, and case studies that provide examples of current approaches for characterisation and detection of nanowaste are reviewed. In terms of environmental protection as well as occupational health and safety aspects, all dispersion media are considered because end-of-life ENMs are likely to occur in diverse waste streams (Gottschalk et al., 2013; Keller and Lazareva, 2014; Sun et al., 2014), and may be present in the solid, liquid and also gaseous phase.

2. Current challenges in nanowaste management In general, ENMs can be released at any point in the life cycle of a product: during production, use phase, or at the end-of-life stage of products via mechanical, thermal and chemical processes such as abrasion, combustion, corrosion and leaching (Gottschalk et al., 2009; Mueller and Nowack, 2008; Nowack et al., 2013; Petersen et al., 2011). Studies on the use of nano-Ag, -SiO2 or contained in paints as additives indicate that very small proportions of ENMs are released during the usage phase and, subsequently, ENMs remain largely in the product’s matrix (Al-Kattan et al., 2014, 2013; Kaegi et al., 2010, 2008). Currently, very little is known on the potential release of ENMs during the end-of-life phase. Realistic mechanisms for ENM release along the product’s life cycle, such as machining, washing, weathering, incineration, and

release through contact with humans (e.g. sweating), are summarised by Froggett et al. (2014). The authors reviewed some experiments showing that also nanoscale debris can be released from composite materials, regardless of whether ENMs are originally present or absent in a sample. This example highlights one of the challenges of generating realistic models as distinguishing between this nanoscale debris from ENM release stemming from nano-enabled products can be extremely difficult from an analytical perspective. In terms of waste management, Boldrin et al. (2014) as well as Marcoux et al. (2013) state that nanowaste can only be generated in the presence of ENMs, when unused ENMs or contaminated items are directly disposed of during production processes or when ENM containing products reach their end-of-life. Currently, very little is known about the transfer of ENMs from solid waste matrices to the liquid phase like in landfill leachates (Reinhart et al., 2010), during incineration processes (Bouillard et al., 2013; Price et al., 2014), or during recycling processes where ENMs can become airborne. Therefore, approaches and analytical methods are needed to detect ENMs in solid, liquid and gaseous waste samples in order to address these knowledge gaps. Of paramount importance in nanowaste detection and tracking ENMs in complex waste matrices is a sound knowledge of their chemical composition. 61 different chemical elements are known to be used in commercially available ENMs (see Fig. 1). These elements were identified based on market research and on the working paper of the European Commission (EC, 2012). In order to examine the fate of ENMs in waste streams, obtaining detailed chemical information at the point-of-manufacturing of ENMs (i.e., element profiles or fingerprints) is a particularly valuable approach to facilitate later comparison from samples in waste streams. Such matrices are, by their nature, very complex and therefore methodological aspects such as sampling and detection must be adapted to cope with this. Thus, Walser and Gottschalk (2014) suggest that the use of ENM fingerprints based on distinctive abundances of rare earth metals in source material or isotope labelling can be used in order to be able to differentiate ENMs from nanoparticulate emissions or natural background sources of the constitutional elements. For example, Kammer et al. (2012) proposed the use of Ce:Nd ratios to discriminate synthetic CeO2 from their natural counterparts, while Gondikas et al. (2014) determined elemental ratios of Ti with many other rare-earth elements to distinguish nano-TiO2 released from sunscreens from naturally-occurring TiO2 nanoparticles. Grass et al. (2014) even used DNA barcodes to label silica particles in order to trace them in waste water treatment plants. Experiments from Schierz et al. (2012) or Petersen et al. (2008) show the traceability of radiolabeled single wall carbon nanotubes (14C-SWNT). Part et al. (2015) made use of the distinctive fluorescent properties of semiconducting quantum dots in order to trace these surface modified CdTe nanoparticles in landfill leachates. Wagner et al. (2014) pointed out that a major difference of ENMs to NNMs is that ENMs are often surface modified (e.g., by organic coatings or artificial surfactants) in order to increase their colloidal stability aiming to increase the processability in nano-enabled products. In general, surface properties, mobility and dissolution behaviour predominantly determine the fate and transport of ENMs in the environment (Wagner et al., 2014). Currently, very little is known about potential alteration or degradation of organic ENM surface coatings via chemical, physical or biological processes during waste treatment processes. It is also noted that possible transformation and alteration processes can also affect bare (uncoated) ENMs. Such processes may occur along the entire life cycle of nano-enabled products and are summarised by Mitrano et al. (2015) and Ramakrishnan et al. (2015). In terms of risk assessment and nanowaste management, alteration and

F. Part et al. / Waste Management 43 (2015) 407–420

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Fig. 1. Identified elements in ENMs based on EC (2012) and market research. ENMs considered are available commercially in dispersions or in solid form as nanopowders, -beads, -wires and -tubes.

transformation processes during the diverse waste treatment processes must be additionally considered, which makes nanowaste characterisation under realistic conditions even more challenging. In this context, Lowry et al. (2012) and Peijnenburg et al. (2015) discuss sorption, aggregation, dissolution, sulfidation, photooxidation/-reduction and biodegradation as the key processes affecting ENM fate and behaviour in the environment, and ultimately also in waste streams. Mobilisation and transport of ENMs that may occur in solid waste streams are emerging issues in waste management (Reinhart et al., 2010). Particle or contaminant transport is generally influenced by chemical and hydrodynamic processes or parameters such as diffusion coefficients of the particles, inonic strength, pH, valence of ion and redox potential of the matrix (McCarthy and Zachara, 1989; Wagner et al., 2014). Thus the following major issues arising for nanowaste analysis can be summarised: (1) Is there any detailed information on the target analyte or ENM available at the point-of-manufacturing (fingerprints)? (2) Is there any information available, on how ENMs will alter or transform during waste treatment processes? Are ENMs colloidally stable or not, if their environment will be changed (e.g. regarding pH, ionic strength and content of natural organic matter)? If not, are stabilising agents or surfactants additionally needed during nanowaste analysis? (3) How can ENMs ultimately be distinguished or fractionated from NNMs? Which sample preparation and analytical techniques are applicable, reproducible and reliable? All of these issues need to be taken into consideration when choosing appropriate methods for sampling and characterisation of nanomaterials in complex waste matrices. 3. Key material parameters and available analytical methods Currently, the fate of ENMs in the environment and potential hazardous properties of nanowaste are not fully understood. Tiede et al. (2008), Hassellöv and Kaegi (2009), and Ulrich et al. (2012) identified key material properties that influence toxicity, transport and behaviour of nanomaterials in the environment. These parameters are summarised in Table 1 in context with

corresponding analytical techniques for nanoparticle characterisation (see also Table 4: List of abbreviations). A brief description of the target parameters in the broad context of environmental monitoring can be found below. In regard to nanowaste management, current research questions mostly focus on the fate of ENMs in various waste streams, and in some cases on perturbing effects of ENMs on waste treatment processes (see also Section 4). Hereby, concentration, elemental composition, particle size, size distribution, shape and morphology are often determined, before more distinct, but intricate material properties, such as particle crystallinity or surface properties are investigated. In general, but with a great importance for nanowaste management in a broader context, the following material parameters are of relevance:  Particle number concentration and mass concentration are important regarding toxicity and for differentiating between concentrations of species stemming from dissolved ions, nanoparticles or colloids (e.g., metal cations, metallic nanoparticles or larger metal-containing aggregates, with apparent sizes of <1 nm, 1–100 nm, and >100 nm, respectively).  The elemental composition of nanomaterials leads to different toxicological effects. The fate and behaviour of ENMs also depend on the chemical elements that are used for particle’s core, shell or coating (e.g. coated nanocomposites).  Particle size and particle size distribution are critical parameters for characterisation of ENMs as they influence uptake and toxicity mechanisms. For example, some ENMs have the potential to cross biological barriers, such as the blood–brain barrier (Wohlfart et al., 2012).  The shape of nanomaterials can also influence toxicity, transport and behaviour of nanomaterials in the environment, which are also influenced by the aggregation state. Similarly, ENM structure and crystallinity can influence the stability and toxicity of nanomaterials (e.g. rutile or anatase TiO2-ENMs).  Surface properties such as surface area, charge, functionality and speciation influence bioavailability, toxicity and aggregation kinetics of nanomaterials (e.g. citrate-functionalised Ag or CdTe/ZnS core–shell nanoparticles). These key material properties can also be considered as the target parameters for the assessment of the fate and behaviour of

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Table 1 Key parameters for characterisation of nanomaterials in the environment (Hassellöv and Kaegi, 2009; Tiede et al., 2008; Ulrich et al., 2012) and proposed analytical methods (adapted from Bandyopadhyay et al., 2013; Salomon, 2011; Sherwood, 2009; Stephan and Hineman, 2014).

*

ENM characterising parameter

Analytical techniques

Concentration related to particle number or mass

FS, GC–MS, MALS, ICP-MS, SPICP-MS, UV/vis

Elemental composition

EDX, GC–MS, ICP-MS, SP-ICP-MS, XPS

Particle size

AFM, APS*, CE, CLSM, CPC*, DMA*, DLS, ELPI*, FFF, FMPS*, FS, HDC, NTA, MALS, SAXS, SEC, SEM, SMPS*, SP-ICP-MS, TEM, XRD

Particle size distribution

AFM, APS*, CE, CLSM, CPC*, DMA*, DLS, ELPI*, FFF, FMPS*, FS, HDC, NTA, MALS, SAXS, SEC, SEM, SMPS*, SP-ICP-MS, TEM, XRD

Agglomeration/ aggregation state

AFM, DLS, CLSM, FFF, FS, NTA, SEM, SP-ICP-MS, TEM

Shape

AFM, CLSM, FFF, SEM, TEM

Structure/ crystallinity

HR-TEM, SAED, SAXS, XRD

Surface area

AFM, BET, SEM, TEM, XPS

Surface charge

AFM, BET, zeta potential by DLS, XPS

Surface functionality

EELS, FTIR, Raman, XPS

Surface speciation

AFM, Fluorescence labelling, SIMS, STM, XPS

Only for aerosol measurements.

nanomaterials in complex matrices encountered in the environment or in waste matrices. Once ENMs enter the environment or waste streams, their characterisation and detection can principally be divided into three steps (Bandyopadhyay et al., 2013; Singh et al., 2014):  1st step: sampling; including sample preparation and preservation.  2nd step: sample pre-fractionation and/or digestion, followed by separation.

 3rd step: characterisation and/or quantification of the analytes of interest. During method development and adaption, all steps must be considered in detail because each stage of the characterisation process can lead to measurement artefacts, loss of material or alteration of the analytes. The sampling of representative waste samples, which are very heterogeneous with respect to element composition, size fractions etc., is particularly challenging in this regard and usually the most error-prone step in waste analysis (Ferrari et al., 2006). Additionally, it is extremely difficult to differentiate ENMs that are expected in low concentrations from NNMs and the natural background during analysis (Duester et al., 2014; Hassellov et al., 2008; Howard, 2010; Kammer et al., 2012; Tiede et al., 2008; Walser and Gottschalk, 2014).

3.1. Sample preparation While many suitable methods for nanoparticle separation and detection are available for the analytical characterisation of pristine nanomaterials, environmentally-sourced samples typically require clean-up or enrichment of nanoparticles from their matrices due to elevated concentrations of natural colloids and particulate organic matter that can hamper analytical protocols. Moreover, it is often of great interest to preserve the aggregation state and coatings of nanomaterials during this process as this is required to yield reliable information regarding the stability and potential toxicity and environmental behaviour of the specific nanomaterial. For these reasons, variants of filtration and centrifugation approaches are frequently employed prior to subsequent analysis steps. The central problems associated with simple cut-off filtration for aqueous samples are described by Howard (2010). Particles (which may also be non-spherical in shape) in the nanometer-sized range require significant centrifugation force to be isolated from dissolved matter (e.g. humic substances) in the aqueous matrix. Ultrafiltration, ultracentrifugation and density gradient centrifugation have thus emerged as successfully applied and practicable techniques for preparation of aqueous samples (Bolyard et al., 2013; Hennebert et al., 2013; Kaegi et al., 2008; Kammer et al., 2012; Plathe et al., 2010). For example, Bolea et al. (2010) and Hennebert et al. (2013) applied gravitational settling to remove microscale particles (>5 lm) contained in compost eluates and landfill leachates. After this, the liquid waste samples were centrifuged for 1 min (removal of particles >6 lm or >4 lm, based on the density of SiO2 and TiO2 particles, respectively) or for 30 min to remove particles >6 lm. According to Leenheer (1981), the centrifugation time can be calculated based on the angular velocity of the centrifuge, the particle diameter, the density differences between the particles and the medium and its viscosity. Kaegi et al. (2008) also used ultracentrifugation in order to remove TiO2 particles >300 nm in urban run-offs. Bolyard et al. (2013) and Hennebert et al. (2013) applied ultrafiltration to remove nanoparticles >3 nm while this ultrafiltrate was used to determine proportions of dissolved metal species. Other notable approaches for preparation of aqueous samples include dialysis (to remove small molecules or salts) and split-flow thin-cell fractionation (SPLITT) for removal of particles >1 lm (Howard, 2010). However, no clear protocols for separating ENMs from natural organic matter currently exist, and Howard (2010) emphasised that more work has to be done to examine nanoparticle recovery and particle enrichment methods with respect to quantitative aspects. Similarly, Kammer et al. (2012) stated that examples of full quantitative analysis of nanoparticulate fractions and thorough method evaluation are lacking.

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3.2. Separation techniques The following section briefly summarises the principles of relevant separation approaches suitable for analysis of aqueous environmental samples in order to obtain time-resolved fractograms and particle size distribution. For a more detailed overview of variants and applications in environmental analysis, the reader is directed to the reviews by Fedotov et al. (2011) and Howard (2010). The techniques described here are considered to be of particular relevance for nanowaste management as they have some potential to separate ENMs from matrix components present in the natural background. Size exclusion chromatography (SEC) utilises the physical accessibility of pore space in specially designed chromatographic packing materials to fractionate differently sized entities (both dissolved and undissolved) according to their hydrodynamic radius in solution. SEC is amenable for metal species, such as CdS, Au, Fe2O3 or SiO2 nanoparticles, less than 100 nm in diameter (Fedotov et al., 2011; Howard, 2010; Laborda et al., 2009), but can be hindered by unwanted interaction between the chromatographic particles toward solution components found in environmental matrices. Persson et al. (2006) showed the applicability of SEC for the analysis of landfill leachates in order to characterise dissolved organic matter (<0.45 lm), such as proteins, humic or fulvic acids that may also interact with nanowaste. Capillary zone electrophoresis (CZE or CE) is a powerful analytical technique facilitating chemical separation based on size-to-charge ratio of nanoscopic entities and molecules present in solution. Employment of a suitable electrolyte-containing liquid medium allows bulk liquid flow to be generated by application of an electric potential across the length of an open-tubular capillary (electroosmotic flow). CE has proven to be an excellent approach for the separation of a range of ENMs as long as they are in a charged state in solution (e.g. Liu et al., 2005; Schnabel et al., 1997) and is a promising technique for environmental studies (Celiz et al., 2011; Zanker and Schierz, 2012). Despite the limited development of CE for the analysis of ENMs in nanowaste, the use of this technique for routine analysis of waste samples has been demonstrated. For example, the U.S. EPA (2007) established a method to separate dissolved inorganic anions in landfill leachates from larger particulate fractions. Techniques falling into the category of field flow fractionation (FFF) rely upon the application of a force field (typically gravimetric or centrifugal) perpendicular to the direction of flow, yielding separation of particles present in the fluid according to their mobilities under the force exerted by the particular field. The most versatile variants of FFF are flow-FFF approaches (e.g. asymmetric flow-FFF, or AF4) which is a technique for the analysis of nanomaterials within environmental samples (Fedotov et al., 2011; Kammer et al., 2011; Loeschner et al., 2013), but must be tailored to the size range of the particles of interest. Furthermore, fouling of membranes can be a limitation for reproducible analysis of environmental samples (Bendixen et al., 2014; Bolea et al., 2010). Bolea et al. (2010) demonstrated the successful application of AF4 on eluates generated from compost samples. A complementary approach for sized-based characterisation is so-called hydrodynamic chromatography (HDC), which can be performed in open-tubes or in columns packed with porous or non-porous beads (Striegel, 2012). In this technique, particles are differentially separated in the flowing stream and larger (heavier) particles remain near the centre of the flow streams and thus reach the end of the column before smaller particles. This approach is highly suitable for characterisation of larger particles (>50 nm) or agglomerates of particles and can be coupled with elemental mass spectrometry for the analysis of environmental samples (Lewis, 2015; Tiede et al., 2009a, 2010).

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CE, FFF and HDC are particularly attractive for analysis of ENMs in the environment due to the lack of destructive forces upon particles during separation. This is important to properly assess the aggregation states and stability of ENMs present in the sample. Currently, no publication could be found, applying CE, SEC, AF4 or HDC on complex waste matrices with focus on the separation of nanowaste from natural organic matter. Nevertheless, some studies have already shown the potential of these techniques to characterise natural organic matter in liquid waste samples (Bolea et al., 2010; EPA, 2007; Persson et al., 2006; Tiede et al., 2010), and could therefore also be developed further for nanowaste characterisation. 3.3. Aerosol measurement techniques With respect to health and safety in waste treatment plants a relevant issue will become techniques for measuring ultrafine particles or nanowaste in the gaseous phase. Target parameters including particle size, size distribution and number concentration in aerosols can be determined using techniques such as aerodynamic particle sizer (APS), electrical low pressure impactor (ELPI), fast mobility particle sizer (FMPS) or scanning mobility particle sizer (SMPS). Their applicability regarding operating size ranges (from 5 nm to 10 lm) was demonstrated by Price et al. (2014). The authors stated that particle morphology plays an important role when choosing the appropriate instrument as for example fluffy or lose agglomerates might disintegrate due to acceleration in APS (Price et al., 2014). Buonanno and Morawska (2015) have reviewed studies on ultrafine particle emissions from waste incineration processes without special focus on ENMs. Irrespective of whether ENMs are present in combusted waste matrices or not, APS, ELPI, APS, FMPS and SMPS have been the instruments of choice (Buonanno et al., 2009; Cernuschi et al., 2012; Derrough et al., 2013; Maguhn et al., 2003; Ozgen et al., 2012; Ragazzi and Rada, 2012; Vejerano et al., 2014; Walser et al., 2012; Zeuthen et al., 2007). In the context of nanowaste characterisation, the applicability of these techniques in combination with sampling methods is described in more detail in Section 4.3 (e.g., Vejerano et al., 2014 or Walser et al., 2012). APS, ELPI, APS, FMPS and SMPS are often performed in combination with methods for elemental analysis (e.g. ICP-MS or ICP-OES) or imaging techniques. In the framework of future studies and in regard to occupational health and safety aspects, such combination of techniques could be used to monitor, for example, ultrafine particles and airborne nanowaste that may are generated during waste recycling. 3.4. Imaging techniques Particle shape, size, structure and aggregation state of ENMs can be probed via microscopy-related techniques, whereby resolution power depends on the diffraction (Abbe) limit. Due to the wavelength of visible light, optical microscopy is not applicable for ENM characterisation (Tiede et al., 2009b). Therefore, electron sources are used for high resolution microscopy such as transmission electron microscopy (TEM) or scanning electron microscopy (SEM), whereby resolution down to 0.1 nm can be achieved. SEM and TEM are performed under vacuum conditions and can suffer from a range of imaging artefacts. For sample preparation, nanowaste can be placed on a carbon layer for heat dissipation and conductivity that is the top layer of the TEM grid or SEM stub. Other sample preparation steps, such as coating, drying, staining, freezing or embedding, can also be required which often leads to perturbed samples (Tiede et al., 2009b). WETSEM™ (QuantomiX) or environmental SEM (ESEM or SEM in low vacuum mode) can be used for hydrated samples and aim to reduce many of the potential measurement artefacts (Manero et al., 2003; Muscariello et al.,

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2005; Tiede et al., 2009b). TEM and SEM have been applied for the analysis of waste samples, while solid fractions were diluted using methanol or ethanol and then air dried before analyses (see Section 4). In the future, ESEM or WETSEM™ studies on hydrated waste matrices could be conducted to investigate the prevalent and real aggregation state of nanoparticles. Compared to electron microscopy, scanning tunnelling microscopy (STM) or atomic force microscopy (AFM) are rarely used for liquid or solid environmental samples. However, these imaging techniques can yield high spatial resolution (below 1 nm). STM and AFM can be used for measurements of ENMs dispersed in the liquid or solid phase to determine size, size distribution, mechanical properties, morphology and roughness (Baer et al., 2010). AFM is primarily useful for the analysis of materials with well-defined compositions and thus the measurement of nanowaste samples is often hampered by the sample heterogeneity, where target end-of-life ENMs may also be present in very low concentrations and unknown aggregation states. A more promising imaging technique for ENMs or nanowaste is fluorescence microscopy, especially confocal laser scanning microscopy (CLSM), where a specimen is excited by a laser source and allows visualization of materials with fluorescent properties. Fluorophores or fluorescent ENMs such as organic dyes or semiconducting quantum dots emit visible light or fluorescence radiation due to excitation by laser wavelengths in the UV/Vis region (Schermelleh et al., 2010). CLSM can be applied to environmental samples to examine, for example, the spatial distribution of fluorescent and semiconducting quantum dots such as CdSe/ZnS, CdS/ZnS, CdTe/ZnS, InGaP/ZnS or InP/ZnS. Such types of quantum dots can also be used as biomarkers or nanotracers in order to examine their behaviour and transport in complex environmental matrices (Deerinck, 2008; Part et al., 2015; Petryayeva et al., 2013). Part et al. (2015) showed that hydrophilic CdTe quantum dots, which were used as nanotracers in landfill leachates, could be detected via CLSM without prior sample preparation and a detection limit of 1 lg/L was achieved.

3.5. Spectroscopy-related techniques Composition and concentration of ENMs can be assessed by relatively non-selective, but concentration-sensitive spectroscopic techniques including ultraviolet/visible wavelength (UV/Vis), fluorescence and Raman spectroscopy (FS or Raman). The utility of these techniques therefore relies heavily on the ENMs of interest having distinctive chemical properties that can be distinguished from background levels. FS, for example, is highly amenable for studying semiconducting quantum dots, which have crystalline structures with distinct optical properties. Using FS, quantum dots can be differentiated from the natural background in complex environmental samples due to their narrow and distinctive emission (Part et al., 2015; Petryayeva et al., 2013). Some metallic nanoparticles can also cause discoloration of soils, which yield characteristic absorption spectra according to the particle size (Amendola and Meneghetti, 2009). In the future, such transformation processes could be studied focussing on metallic nanowaste. Part et al. (2015) showed that surface modified and colloidally stable CdTe quantum dots with unique distinctive fluorescence properties can be used to investigate the interaction of such nanotracers with natural organic matter using FS. Schierz et al. (2012) showed that near-infrared FS can be used for detection and quantification of carbon nanotubes in aqueous environmental samples. FS can also be used as a rapid and facile method to characterise nanoparticulate emissions, such as proteins, humic or fulvic acids contained in landfill leachates (Huo et al., 2008; Xiaoli et al., 2012; Xie and Guan, 2015).

Energy-dispersive X-ray spectroscopy (EDX) can be performed with SEM or TEM instrumentation, whereby the recorded X-ray spectrum results from the element-specific release of energy in the form of X-rays following stimulation by an incident beam. EDX is often coupled to SEM or TEM and is a powerful tool for screening heterogeneous waste samples to identify elemental composition (see also Section 4). Nevertheless, EDX is currently not applicable for quantification of single nanoparticles occurring in complex waste samples due to its lack of sensitivity. Electron energy-loss spectroscopy (EELS) is similar to EDX, and uses analysis of electron energy distribution following interaction with a specimen. EELS, also performed with TEM instrumentation, can be used for elemental analysis and examination of surface properties (Egerton, 2011; Farré et al., 2011). EELS is more chemically sensitive than EDX when analysing elements with low atomic number (e.g. carbon, nitrogen or oxygen), but it is less accurate for quantification. EELS could be used, for example, to examine the surface properties of pristine nanotracers before they are applied to waste matrices. When requiring more detailed information about the chemical composition, various forms of elemental spectroscopy which enable sensitive and selective detection for distinctive chemical characteristics of ENMs and nanowaste can be used. Amongst highly robust variants for elemental analysis including atomic absorption spectroscopy (AAS), and techniques using inductively coupled plasma (ICP) such as those utilising atomic emission (AES) or optical emission spectroscopy (OES), the most valuable tool for elemental analysis of ENMs is inductively coupled plasma coupled with mass spectrometry (ICP-MS) as it is capable of profiling samples for nearly all elements present in the periodic table. The high selectivity together with the ultimate sensitivity of ICP-MS can be used to provide accurate quantitative information on the amount, stoichiometric composition and identity of ENMs and nanowaste as well as associated chemical entities as samples can be filtered, then separately digested in acidic media prior to analysis to characterise different size fractions. A number of examples using such elemental spectroscopy techniques for nanowaste characterisation are discussed in Section 4. Emerging more recently, single-particle mass spectrometry (SP-ICP-MS) can also be used for environmental analysis (Jiménez et al., 2011; Laborda et al., 2013a; Lee et al., 2014), but can be limited by measurement uncertainties when analysing multiple isotopes simultaneously (Montano et al., 2014). SP-ICP-MS can also be used for diverse environmental samples, such as waste water, food and diverse biological media in order to determine particle number concentration (Peters et al., 2015). Further applications and challenges in SP-ICP-MS measurements are discussed in the following studies (Gondikas et al., 2014; Laborda et al., 2013b; Reed et al., 2012; Tuoriniemi et al., 2012). 3.6. Scattering and diffraction techniques Light and X-ray scattering techniques can generally employed for the analysis of environmental samples to determine size, size distribution and structure of ENMs. Light scattering techniques, such as dynamic light scattering (DLS), multi-angle light scattering (MALS) or nanoparticle-tracking analysis (NTA), can be used for characterisation of ENMs and NNMs dispersed in liquid samples (Brar and Verma, 2011; Domingos et al., 2009; Gallego-Urrea et al., 2011; Kammer et al., 2005). The detection limits of size are limited depending on the wavelength, used for illumination. Particle size distribution measurements using DLS, MALS and NTA are limited in waste samples that show high polydispersity and/or contain non-spherical particles. Nanostructures and shapes of solid or liquid samples can be examined using X-ray diffraction (XRD) or small-angle X-ray

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scattering (SAXS). With both techniques particle size, size distribution, structures in atomic scale as well as lattice parameters of unknown crystalline phases can be examined. SAXS (in contrast to XRD) allows examination of non-crystalline or polydisperse materials such as polymers, molecules or ENMs in environmental matrices (Als-Nielsen and McMorrow, 2011; Kameya and Lee, 2013). X-ray photoelectron spectroscopy (XPS) is a surface-based technique that can be used to probe oxidation state of ENMs using element-specific spectra (Sciortino et al., 2011). As it is rather challenging to analyse hydrated forms of ENMs, XPS is not commonly applied for the analysis of complex environmental samples. Selected area electron diffraction (SAED) is similar to X-ray diffraction techniques with the difference that an electron diffraction pattern of a very small (selected) area is obtained. SAED is performed using TEM instrumentation and can be used to examine crystalline properties of ENMs (Farré et al., 2011; Mavrocordatos et al., 2004). The application of X-ray related methods, such as XRD, SAXS, XPS and SAED for waste samples is hampered by the heterogeneity of waste samples, where ENMs are expected to occur in very low concentrations. Nonetheless, these methods can be used for characterisation of pristine ENMs at the point of manufacturing. On the contrary, DLS, MALS and NTA have been used for ENMs contained in complex waste matrices (Hennebert et al., 2013; Lozano and Berge, 2012), but their applicability is also limited due to the aforementioned reasons. 4. Experimental case studies on nanowaste In this section almost all published experimental studies focussing on ENMs in waste matrices, are summarised in regard to nanowaste detection and characterisation. Experimental studies on ENMs in waste water samples, such as urban run-off or effluents from waste water treatments plants (WWTPs), are excluded and are discussed in previous contributions (e.g. Barton et al., 2014; Delay and Frimmel, 2012; Doolette et al., 2013; Gartiser et al., 2014; Kaegi et al., 2013; Kent et al., 2014; Limbach et al., 2008; Wang et al., 2012; Westerhoff et al., 2013). Comprehensive reviews on ENMs in WWTPs have already been published by Brar et al. (2010), Neale et al. (2013) and Musee et al. (2011). Focussing on nanowaste characterisation, recent studies about end-of-life ENMs, present and measured in the solid phase, aim to examine the fate and potential transformation processes of ENMs during thermal waste treatment processes (Vejerano et al., 2013, 2014, 2015; Walser et al., 2012). Currently, only few studies are published that investigated nanowaste in the liquid phase, the transfer of ENMs to liquid phase or their behaviour in landfill leachates (Bolyard et al., 2013; Hennebert et al., 2013; Lozano and Berge, 2012; Walser et al., 2012). Recent studies about nanowaste present in the gaseous phase focus predominantly on the fate and behaviour of ENMs during incineration and aims to close the mass balance in waste-to-energy plants or to examine the influence of ENMs on the emission of particulate matter (Buha et al., 2014; Derrough et al., 2013; Walser et al., 2012). The goal of one study was also to investigate potential workplace exposure within a real waste-to-energy plant (Walser et al., 2012). 4.1. Characterisation of nanowaste in the solid phase Walser et al. (2012) conducted a large-scale experiment in an incineration plant for municipal solid waste (capacity of 220,000 t/a) in order to study the fate of cerium dioxide (nano-CeO2) during combustion processes. CeO2-suspensions were synthesised (5% and 1% w/w of nano-CeO2 in ultrapure water and stabilised with artificial surfactants) and sprayed directly into the furnace and onto the solid waste. Particle size and size distribution

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of nano-CeO2 in suspensions were determined by using HR-TEM and X-ray disk centrifugation (mean diameter of 80 nm). These methods are currently not applicable to determine the particle size distribution of nano-CeO2 after incineration due to the heterogeneity of solid residues, where altered or transformed nano-CeO2 are likely to occur in very low concentrations compared to their natural counterparts. After combustion of waste containing nano-CeO2, samples were collected from the bottom ash and the fly ash (discharge from the boiler and after the electrostatic precipitator). For ICP-MS and HR-TEM/EDX analyses, samples from the flue/clean gas after the electric precipitator, the wet scrubber and stack were also analysed. Hereby, additional silica filters were mounted in the incineration plant and these filters were analysed. ICP-MS in combination with thermodynamic calculations was used to quantify nano-CeO2 and determine material flows within the incineration plant (mass balance). Regarding ICP-MS analysis, the authors assumed that more than 95% of nano-CeO2 could be recovered after sample preparation (Ytterbium and Iridium were used as recovery tracers). In addition, HR-TEM/EDX analysis was applied to examine particle structure, morphology and chemical composition following combustion. Samples were diluted with ethanol and placed on TEM grids, consisting of copper and a carbon top layer. The TEM was also operated in scanning mode while using a high-angle annular dark field detector (HAADF-STEM) to improve image contrast. The untreated silica filters were analysed using SEM. As high vacuum conditions in electron microscopy and air drying lead to sample perturbation, it is almost impossible to investigate the real aggregation state of nano-CeO2 that occurs, for example, in hydrated waste samples after the wet scrubber. In order to provide an accurate mass balance and a stochastic fate model for ENMs in waste incineration plants, Walser and Gottschalk (2014) used the data obtained from this previous measurement campaigns and ICP-MS analyses, respectively. The authors state that temporally-resolved measurements of background concentration significantly improve the accuracy of flow models if relatively low concentrations of ENMs are expected in comparison to that of their natural counterparts. On the laboratory scale, Vejerano et al. (2014) investigated the fate of ENMs during incineration, mimicking the disposal of nanowaste from laboratories and hospitals. Therefore, seven different ENM types (TiO2, NiO, Ag, Ce, Fe2O3, CdSe/ZnS quantum dots and C60 fullerenes) were individually mixed with synthesised medical and laboratory waste. ENMs were spiked to different waste fractions (i.e. paper towels, polyethylene from plastic water bottles and PVC from gloves as well as a combination of the materials in equal amounts) in concentrations of 0.1% w/w, 1% w/w and 10% w/w. These waste samples were then incinerated at 850 °C. The bottom ash and also the airborne particulate matter, collected onto polytetrafluoroethylene filters, were subsequently analysed. Before ICP-MS measurements were performed, all samples were filtered (< 0.2 lm). In order to analyse samples containing fullerenes (C60), materials were extracted with toluene and filtered, prior to determination of C60 concentrations via HPLC. TEM/EDX and SAED were used to examine particle size and morphology. All samples were diluted in methanol and droplets were located onto carbon-coated TEM grids (consisting of gold for particulate matter and of copper for bottom ash). Following this, samples were dried under vacuum conditions (at 120 °C for 3 h) and stored under nitrogen atmosphere before TEM analyses. Vejerano et al. (2014) derived so-called emission factors (i.e. number of particles emitted per mass of waste) from their combustion experiments, but they were not able to close the mass balance because of losses within in the combustion system (thermophoretic loss was estimated to be between 10% and 20%) as well as during sample preparation, transfer and handling. The authors also mention that volatile metal chlorides, which can be formed during incineration, were not determined.

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Both studies show exemplarily the challenges of providing an accurate mass balance to be able to determine the ultimate fate of ENMs during waste incineration. Different spiking methods (e.g., ENM suspensions are sprayed onto waste or ENMs, which are embedded in surrogate wastes, are combusted) play an important role regarding their fate and thermal behaviour during incineration. However, such experimental studies are crucial to assess the ultimate fate of nanowaste aiming to derive transfer coefficients and to develop accurate material flow models. 4.2. Characterisation of nanowaste in the liquid phase In the framework of large-scale experiments in a waste incineration plant, Walser et al. (2012) also analysed the quench water, collected from the wet scrubber, aiming to close the mass balance for nano-CeO2 as described above. Certified fly ash standards and nano-CeO2 suspensions were used to validate the digestion method and to obtain recovery rates for cerium during ICP-MS analysis. Ytterbium and iridium were additionally used as recovery tracers. All liquid samples were homogenised via ultra-sonication before ultrafiltration and acid digestion. However, this method offers no possibility to distinguish between nanoparticulate species and dissolved metal ions. Nonetheless, Walser et al. (2012) were able to calculate recovery rates of between 55% and 68% for the entire nano-CeO2 flows that mostly depend on the spiking method used to introduce ENMs into waste (sprayed into the furnace or onto municipal solid waste). Measurement uncertainties, waste sample heterogeneity and variability of the combustion residues were also considered. However, discussion regarding the transferability of the spiking approach to real-world conditions remains to be addressed as, for example, ENMs may be embedded within a solid matrix of a product, which may mean that the spiking approach does not reflect realistic nanowaste emission processes. The study of Lozano and Berge (2012) focused on the fate and behaviour of single-walled carbon nanotubes (SWNTs) in mature landfill leachate. A solution of approximately 12.5 mg/L SWNTs in synthesised landfill leachate was prepared by sonicating the suspension in an ice bath and DLS measurements were conducted to determine the zeta potentials with respect to the pH, ionic strength and humic acid concentration. The Smoluchowski approximation was used to calculate the zeta potential of SWNTs in landfill leachates with different electrolyte concentrations in order to assess their colloidal stability. Furthermore, leaching experiments by using synthesised municipal solid waste (45.5% paper, 9.6% glass, 16.4% plastic, 17.6% food and 10.9% metal) and SWNT suspensions were performed. The SWNT concentration in the respective eluates was measured by UV/Vis spectroscopy (absorbance at a wavelength of 800 nm). This study showed that SWNTs are colloidally stable and mobile under common landfill leachate conditions. The authors determined a SWNT recovery of 62% w/w, but it was not discussed if interferences between SWNTs and natural organic matter or nanoscale debris limited the precision of the applied method for detection and quantification. Bolyard et al. (2013) studied the behaviour of coated nano-ZnO, -TiO2 and -ZnO in mature landfill leachate and assessed the effects on biological processes by spiking these ENMs. Leachate samples were collected from two municipal solid waste landfills in Florida (USA) directly from the pump station and transmission pipe. For leachate exposure, coated ZnO (with triethoxycaprylylsilane), TiO2 (with hydrated silica, dimethicone/methicone copolymer and aluminium hydroxide) and coated Ag (with polyvinylpyrrolidone) were used. Before spiking these ENMs into landfill leachates, HR-TEM analysis was used to determine particle size. In order to assess the effects on anaerobic and aerobic biochemical/biological processes in leachate, high ENM

concentrations of 100 lg/L, 1.0 mg/L, and 100 mg/L were used. Micro- und ultrafiltration were conducted for size fractionation to generate filtrates (cut-offs) >1500 nm, <1500 nm and <1 nm. All filtrates were digested based on Standard Method 3030F, described in Rice et al. (2012), and filtered (<0.45 lm) prior to ICP-OES measurements. Ti, Zn and Ag concentrations were determined for both nanoparticulate and dissolved metal species. However, this approach did not allow characterisation of nanowaste in the size range of 1–100 nm. Bolyard et al. (2013) also used an equilibrium model (Visual MINTEQ) for chemical speciation of metal ions (Zn2+, Ti4+ and Ag+) under the prevailing and changing environmental conditions in such landfill leachate samples. Since nanospecific dissolution rates are currently missing in such equilibrium models, chemical speciation (in particular for ENMs) is hampered. Hennebert et al. (2013) investigated 25 different waste samples in order to detect and characterise colloids and nanoparticles in leachate and eluates. The authors highlighted that natural colloids and nanoparticulate fractions, stemming from solid wastes and occurring in landfill leachates are rarely investigated. In their study, 3 landfill leachate samples and 22 solid waste samples (from waste incineration, soil excavation, packaging and metallurgy) were collected. All solid samples were leached according to the batch test (CEN EN 12457-2:2002) using deionised water in order to generate eluates. DLS measurements on the eluates and landfill leachate samples were performed to determine the zeta potential and particle size distribution. Hennebert et al. (2013) stated that DLS measurements were not applicable on leachates regarding the limit of quantification of particles <100 nm. However, size fractionation was conducted by using micro- and ultrafiltration obtaining three size fractions (cut-offs for: >450 nm, microfiltrates <450 nm and ultrafiltrates < 3 nm). Elemental analysis was carried out using ICP-MS and the digestion method based on CEN EN 13656:2003-01. In addition, TEM/EDX analysis was applied to obtain information on size, form and elemental composition of the particles/aggregates found in the eluates and landfill leachate. Droplets (100 lL) were deposited on carbon coated Cu TEM grids and were air-dried before imaging. Regarding pre-fractionation by microfiltration (<0.45 lm), Bolea et al. (2010) pointed out that these enrichment methods can lead to sample perturbation, whereby a large proportion of particles <0.45 lm can inadvertently be removed, caused by formation of a filter cake that decreases the effective pore size. However, the authors showed that AF4 as separation technique is applicable for compost eluates. Thus, this method could be adapted to landfill leachates and may be used for nanowaste characterisation (with focus on particles from 1 to 100 nm). Regarding visualisation of nanowaste via TEM analyses, sample perturbation caused by high vacuum conditions or drying effects need to be considered in future studies. Bolyard et al. (2013) and Hennebert et al. (2013) found aggregates consisting of diverse nanoparticles and it was almost impossible to distinguish between ENMs and their natural counterparts. Without knowing the fingerprints of ENMs this task is going to be challenging. Nevertheless, Hennebert et al. (2013) were able to identify ENMs contained in real unknown waste samples using TEM/EDX after microfiltration, whereby spherical organic polymers with a size of 80–100 nm were detected. 4.3. Characterisation of nanowaste in the gas phase Walser et al. (2012) conducted small-scale experiments to examine the fate and behaviour of nano-CeO2 during waste incineration, whereby CeO2-suspensions were sprayed onto sawdust to mimic waste matrices (15–22% w/w). Thermogravimetric analysis (TGA) was performed between 40 and 950 °C and combined with online ICP-OES measurements in order to quantify cerium in the

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exhaust gas. Such small-scale experiments do not allow differentiation of volatile fractions and airborne nano-CeO2. Walser et al. (2012) additionally investigated airborne CeO2 particles, collected by silica filters (<2.7 lm), to investigate potential transformations of nano-CeO2 using SEM/EDX. If SEM would be operated in low vacuum mode (or ESEM), the aggregation state could also be investigated close to atmospheric pressure conditions. In terms of occupational health and safety aspects, Walser et al. (2012) additionally conducted workplace measurements in the control room, waste bunker and waste feeding zones of the real incineration plant. A light-scattering laser photometer, a handheld photometer for aerosol monitoring, an electronic diffusion battery (EDB) and a charge-based personal aerosol sampler were used for workplace exposure measurements. Particle size and size distribution were also determined by SMPS for ultrafine aerosol measurements, which is applicable for a broad size range from 2.5 nm to 1 lm. Those methods are already established for workplace measurements. No significant changes in particle number concentration could be measured during incineration experiments using nano-CeO2. In the framework of an incineration experiment at laboratory scale, Vejerano et al. (2013, 2014) were aiming to examine the

influence of ENMs on the formation of PAH, PCDD/F and airborne particulate matter. Waste samples were spiked with ENMs and incinerated at 850 °C (see also Vejerano et al., 2014 in Section 4.1). During combustion, particle number concentration and size distributions of airborne particles in the exhaust gas were measured by a combination of SMPS and APS (with sizes ranges of 14–720 nm and 720 nm–5 lm). Elemental analyses via ICP-MS were conducted for all collected particulate matter and bottom ash (see also Section 4.1). In addition, a micro-analysis particle sampler, consisting of a three-stage impactor (last filter cut-off diameter of 50 nm), were used for collection of particulate matter which was subsequently analysed by TEM. However, the authors could not close the mass balance for all tested ENMs, hence a quantitative ENM fate analysis with high accuracy was not feasible. Derrough et al. (2013) investigated the behaviour of pristine nano-Sn, -Ni and -Ag at high temperatures (850 °C and 1100 °C) in laboratory scale experiments. 600 mg samples containing pristine ENMs were inserted into a laboratory heater setup from which FMPS (5.6–560 nm) was used to measure the particle size distribution in the exhaust gas. Furthermore, SEM micrographs were taken from nano-Ag residues. The authors did not spike ENMs to waste samples in order to simulate real conditions in a waste-to-energy

Table 2 Summary of methods for sampling and sample fractionation/enrichment before nanowaste characterisation. Target analyte/nanowaste

Goals of the study

Applied methods

Dispersion media: Solid

Sampling and sample fractionation/enrichment CeO2 in fine fraction of fly Fate during incineration ash Inhalable dust fraction that may contain CeO2

Fate during incineration and workplace exposure

CeO2 in slag

Fate during incineration

TiO2, NiO, Ag, Ce, Fe2O3, CdSe/ZnS QDs and C60 in residues after incineration

Fate during incineration and effect on emissions of particulate matter

Coated ZnO, TiO2 and Ag in 5 different mature landfill leachates

Effects of ENMs on aerobic and anaerobic biochemical/-logical processes in landfill leachates

Single-walled carbon nanotubes (SWNTs) in suspensions before column/leaching experiments

Colloidal stability and mobility of SWNTs in synthesised MSW landfill leachate while changing ionic strength, humic acid content and pH

Non-target analysis with focus on the nanoparticulate fractions in liquid waste samples

Characterisation of nanoparticulate emissions, respectively nanoscale debris, stemming from solid wastes, regardless the absence or presence of ENMs in waste streams

Collection of fly ash (discharge of electrostatic precipitator and boiler) ? ICP-MS, HAADF-STEM and TEM analyses Collection via silica filters with personal sampler (GSP-10 system) according to Swiss standard procedure ? ICP-MS, HAADF-STEM and TEM analyses Collection of 70–210 kg slag ? dry fine fraction after drying at 105 °C ? milling of sample < 40 lm ? 50 g of slag powder ? ICPMS, HAADF-STEM and TEM analyses Collection of airborne nanowaste in polyethylene chamber while stirring with a fan ? SMPS, APS measurements; collection of airborne nanowaste with PTFE filters ? extraction using CH2Cl2 ? ICP-MS, HPLC and TEM analyses; collection and fractionation by micro-analysis particle sampler (3-stage impactor) ? TEM analysis; collection of bottom ash in sample boat using CH2Cl2 ? ICPMS, HPLC and TEM analyses Collection of landfill leachates in HDPE container (from pump station and transmission pipe) ? spiking of coated ENMs (100 lg/L, 1 and 100 mg/L) to mature landfill leachates ? microand ultrafiltration (cut-offs: >1500 nm, <1500 nm and <1 nm) ? ICP-MS analyses Spiking of SWNTs to synthesised landfill leachates (12.5 SWNTs mg/L) during sonicating for 20 min and cooled down via ice bath (no visible phase separation observed); columns were flushed with NaCl to saturate the ion exchange sites before the SWNTs were used as tracer ? DLS before the leaching experiments and UV/Vis after leaching the columns 22 eluates generated by the batch test (EN 12457-2) at liquid-to-solid ratio of 10:1 and 3 landfill leachate samples ? settling for 15 min ? centrifugation for 1 min at 47 g (sedimentation of particles >4 lm and >6 lm (based on particle density of TiO2 and quartz)) ? micro- and ultrafiltration to obtain the fractions > 450 nm, <450 nm and <3 nm ? ICP-MS and TEM/EDX analyses

Liquid

References

Gas

U

U

Walser et al. (2012) Walser et al. (2012) Walser et al. (2012)

U

U

Vejerano et al. (2014)

U

Bolyard et al. (2013)

U

Lozano and Berge (2012)

U

Hennebert et al. (2013)

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Table 3 Summary of analytical techniques used for nanowaste characterisation. Target analyte/nanowaste

Goals of the study

Applied methods

Dispersion media: Solid

Determination of size distribution related to mass or volume CeO2 in suspensions used for Fate during incineration tracing studies Ultrafine aerosols and inhalable Fate during incineration and workplace exposure dust fraction that may contain CeO2 in control room, waste bunker and feeding zones TiO2, NiO, Ag, Ce, Fe2O3, CdSe/ Influence of ENMs on emission of ZnS QDs and C60 in exhaust particulate matter gas Ag, Sn, Ni in exhaust gas Simulation of behaviour/emission process of pristine ENMs at temperatures of 850 °C and 1100 °C, lab-scale Determination of chemical composition and mass concentration CeO2 in residues after Fate during incineration incineration

TiO2, NiO, Ag, Ce, Fe2O3 and CdSe/ZnS QDs in residues after incineration C60 in residues after incineration CeO2 in quench water and CeO2 in residues after incineration

Fate during incineration and effect on emissions of particulate matter

Coated ZnO, TiO2 and Ag in suspension before and after spiking to 5 different landfill leachates

Effects of ENMs on aerobic and anaerobic biochemical/-logical processes in landfill leachates

Single-walled carbon nanotubes (SWNTs) in eluate obtained from column/leaching experiments Non-target analysis with focus on the nanoparticulate fractions in liquid waste samples

Leachability and mobility of SWNTs through columns, packed with synthesised MSW while changing ionic strength, humic acid content and pH

Fate during incineration and effect on emissions of particulate matter Fate during incineration

Characterisation of nanoparticulate emissions, respectively nanoscale debris, stemming from wastes, regardless the absence or presence of ENMs in waste streams

Determination of size, shape, structure/crystallinity and morphology CeO2 before (in suspension) and Fate during incineration after incineration (in residues) TiO2, NiO, Ag, Ce, Fe2O3, CdSe/ Fate during incineration and effect on emissions of particulate matter ZnS QDs and C60 before (in suspension) and after incineration (in residues) Effects of ENMs on aerobic and anaerobic Coated ZnO, TiO2 and Ag in suspension before spiking to biochemical/-logical processes in landfill leachates landfill leachates Characterisation of nanoparticulate Non-target analysis with focus emissions, respectively nanoscale debris, on the nanoparticulate stemming from wastes, regardless the fractions in liquid waste absence or presence of ENMs in waste samples streams Ag in solid residues after Potential transformation processes of ENMs incineration at temperatures of 850 °C and 1100 °C, labscale Evaluation of aggregation/agglomeration state and colloidal stability Colloidal stability and mobility of SWNTs in Single-walled carbon synthesised MSW landfill leachate while nanotubes (SWNTs) in changing ionic strength, humic acid content eluate obtained from and pH column/leaching experiments

HR-TEM, X-ray disk centrifugation

Liquid

Gas

U

3

References

Walser et al. (2012) Walser et al. (2012)

SMPS (2.5 nm–1 lm; 1–107 p/cm , 16 s), electronic diffusion battery (7–400 nm) and charge based person aerosol sampler according to Swiss standard procedures

U

SMPS (14–720 nm); APS (720 nm–5 lm)

U

Vejerano et al. (2014)

CPC (5 nm–3 lm, 0–107 p/cm3, 1 s); SMPS (5–350 nm, 0–107 p/cm3, 3 min); FMPS (5.6–560 nm, 1 s)

U

Derrough et al. (2013)

ICP-MS (after microwave-assisted digestion using HNO3, HCl and H2O2; recovery rates >95%) in combination with stochastic flow model

U

ICP-MS (after digestion using HNO3:HCl (1:3 v/v), HNO3:H2SO4 (1:1 v/v) or HNO3:HF:H2O2 (1:1:1 v/v)), EDX HPLC (after dilution of particulate matter extract with toluene) EDX (coupled to TEM); ICP-MS (after microwave-assisted digestion using HNO3, HCl and H2O2; recovery rates >95%) in combination with stochastic flow model

U

Walser et al. (2012), Walser and Gottschalk (2014) Vejerano et al. (2014)

U U

U

Vejerano et al. (2014) Walser et al. (2012), Walser and Gottschalk (2014) Bolyard et al. (2013)

EDX (coupled to TEM) of pristine ENMs and ICP-OES (after spiking to leachates and after microwave-assisted digestion using HNO3 and HCl (1:1 v/v) based on Standard Method 3030F, described in Rice et al. (2012); recovery rates: 95% of ZnO, 71% TiO2 and 79% of Ag) UV/vis (absorbance at a wavelength of 800 nm; 62% recovery of SWNTs in effluent)

U

U

Lozano and Berge (2012)

EDX (coupled to TEM; 100 lL droplet were deposited on carbon coated Cu TEM grids and air-dried), ICP-MS (microwave digestion method EN 13,656 using HCl, HNO3 and HF as well as H3BO3 for neutralisation of HF)

U

Hennebert et al. (2013)

HR-TEM, HAADF-STEM (after dilution with ethanol)

U

U

Walser et al. (2012)

TEM (after dilution with methanol)

U

U

Vejerano et al. (2014)

HR-TEM and SAED

U

Bolyard et al. (2013)

TEM (100 lL droplet were deposited on carbon coated Cu TEM grids and air-dried)

U

Hennebert et al. (2013)

SEM (material at crucible before and after heating)

DLS (after dilution of spiked eluates, ranging from 1 to 600; zeta potentials deviated from Smoluchowski approximation)

Derrough et al. (2013)

U

U

Lozano and Berge (2012)

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plant. In this case, SEM/EDX or other X-ray related methods could be used to investigate transformation processes such as the sulfidation of nano-Ag to Ag2S or the oxidation to Ag2O (Impellitteri et al., 2013). In general, it is very challenging to measure particle size distribution directly in the flame if the behaviour and re-formation of particles during combustion processes is of interest. For this purpose, Paur et al. (2005) developed a novel particle mass spectrometer that is applicable for the size range of 0.3– 50 nm which can be used for size distribution measurements within the flame.

4.4. Summary of methods for nanowaste characterisation Depending on the type of ENMs and target analytes, ICP-MS and ICP-OES have been used to determine elemental composition and mass concentration of nanowaste (Bolyard et al., 2013; Hennebert et al., 2013; Vejerano et al., 2014; Walser et al., 2012). HPLC was used to quantify fullerenes (C60) dispersed in solid residues after incineration (Vejerano et al., 2014). Lozano and Berge (2012) used UV/Vis spectroscopy to detect carbon nanotubes dispersed in landfill leachates and DLS to evaluate their colloidal stability. Particle size, shape, structure/crystallinity and morphology were mostly determined using TEM or SEM (Bolyard et al., 2013; Derrough et al., 2013; Hennebert et al., 2013; Vejerano et al., 2014; Walser et al., 2012). EDX was often coupled to these techniques to screen the elemental composition during imaging. SMPS, APS, CPC and FMPS were used to determine size distribution of airborne nanowaste related to their mass or volumes (Derrough et al., 2013; Vejerano et al., 2014; Walser et al., 2012). An EDB and a charge-based person aerosol sampler were additionally used to evaluate workplace exposure and inhalable dust fractions within a real waste-to-energy plant (Walser et al., 2012). Before nanowaste could be characterised, airborne species were collected by filters (silica or PTFE) (Vejerano et al., 2014; Walser et al., 2012) and liquid species were enriched using micro- and ultrafiltration (Bolyard et al., 2013; Hennebert et al., 2013). In cases where ENMs need to be transferred to the liquid phase, solid nanowastes were leached at a liquid-to-solid ratio of 10:1 (Bolea et al., 2010; Hennebert et al., 2013; Lozano and Berge, 2012). In the future, nanoparticle extraction methods, described by Plathe et al. (2010, 2013), could alternatively be adapted to solid wastes. A summary of methods that were used for sampling and sample fractionation/enrichment for nanowaste are found in Table 2, while corresponding analytical techniques are outlined in Table 3.

5. Discussion and conclusions Regarding sustainable nanotechnology application and risk assessment of ENMs, it is crucial to examine the ultimate fate and behaviour of ENMs in the environment and in waste management processes. In this regard, particle size, size distribution and particle shape as well as elemental composition currently seem to be key material parameters. Moreover, it is important to consider potential transformation and alteration processes of nanowaste that may occur during the diverse waste treatment options. Although this arose as a main issue, only scarce information is available in published literature regarding how ENMs will alter or transform during waste treatment processes. In addition, persistent particle coatings and their potential transformation in waste streams are of high relevance, because surface properties of ENMs predominantly influence their fate and behaviour in waste matrices. Up to date, no study could be found focussing on the degradation or persistence of surface coatings of nanowaste. Thus, this issue is a challenging task for future research.

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In terms of nanowaste management, recent studies focus on the behaviour and potential transformation processes during waste incineration. In regard to workplace exposure, nanowaste was also investigated in both the solid and gaseous phase. Methods like APS, ELPI, FMPS and SMPS were previously applied to measure size distribution of airborne ENMs and, in general, good agreement was found between the results obtained by the different methods in the diverse studies. To our knowledge, currently no studies have been published to measure airborne nanowaste in the course of recycling processes, e.g. shredding of WEEE or processing of secondary materials. However, such studies need to be conducted in terms of occupational health and safety aspects and must be a major research focus in future. Regarding characterisation of nanowaste in the solid phase, imaging techniques, such as TEM or SEM combined with EDX or ICP-MS analyses have been shown to be powerful tools to determine particle size, morphology and element composition. However, currently it is almost impossible to quantify ENMs in unknown and complex solid waste matrices. Accurate quantification of nanowaste is mainly hampered by the ability to distinguish ENMs from their natural counterparts that are likely to occur in higher concentrations, e.g. Al2O3, CeO2, CuO, SiO2, TiO2, or ZnO natural nanoparticles. Quantification of ENMs in complex matrices is also hindered by the lack of suitable reference nanomaterials. Nevertheless, ICP analyses are promising tools to characterise nanowaste present in both the solid and liquid phase. In cases, where soluble nanoparticulate fractions stemming from solid waste matrices were needed, standardised leaching protocols (liquid-to-solid ratio usually of 10:1) were applied in the screened studies. In future studies it is of importance to examine the transfer of solid nanowaste to the liquid phase. Regarding the transport of ENMs it has to be considered that ENMs may be colloidally stable and mobile in waste streams under the prevailing conditions as has been shown, for example, on carbon nanotubes in landfill leachates. In regard to characterisation of nanowaste in the liquid phase, SP-ICP-MS, DLS or NTA could be applied but these methods are limited regarding particle size limit of detection and quantification of nanowaste. Furthermore, separation techniques, such as AF4, CE, HDC or SEC, are more sensitive and can be coupled to various detection methods (e.g., UV/Vis, MALS or ICP-MS). In particular, AF4 was proven to be a promising tool to determine particle size distribution of nanoparticulate fractions in aqueous waste samples. However, it should be noted that larger particles could lead to clogging of the membrane and to unwanted removal of particles, which are originally smaller than the pore size. The same issues can happen to enrichment methods such as micro- and ultrafiltration. Therefore, robust standardised protocols for enrichment and particle separation still need to be developed and evaluated in order to obtain quantitative information. In general, there is a need to develop and standardise analytical protocols regarding nanowaste sampling, characterisation and quantification. A most significant current limitation and future challenge in nanowaste characterisation and monitoring is the need to differentiate ENMs from their natural counterparts. In this respect, we are far away from robust, applicable and reproducible methods allowing a non-targeted analysis and reliable quantification of nanowaste in unknown waste samples. In this respect, a very promising approach seems to be making use of nanotracers or ENMs with distinctive physicochemical properties, i.e. with unique element or isotopic ratios as fingerprints. Such experimental approaches are crucial to investigate the ultimate fate of ENMs in complex waste matrices and to develop accurate assessment models and material flow analyses, as well as tools for monitoring nanowaste behaviour in the environment. Since it became obvious, that available analytical techniques are not yet capable of providing sufficient evidence for comprehensive

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nanowaste characterisation, waste management has to rely on additional approaches. In-depth information at the point-ofmanufacturing of ENMs, such as elemental composition, surface properties or particle coatings, are needed and must be demanded, e.g., adapted from the extended producer responsibility, which is already quoted in the EU Waste Framework Directive (2008/98/EC), to facilitate risk assessment of ENMs in products and ultimately in nanowaste. Acknowledgements This study was conducted in the framework of the Austrian NANO Environment Health and Safety program (project 844415) coordinated by the Austrian Research Promotion Agency (FFG) and funded by the Austrian Federal Ministry of Agriculture, Forestry, Environment and Water Management (BMLFUW) and the Austrian Ministry for Transport, Innovation and Technology (BMVIT). Appendix Annex See Table 4.

Table 4 List of abbreviations. AAS AES AFM AF4 APS BET CE CLSM CPC DLS EDB EDX EELS ELPI FFF FMPS FS GC–MS HAADF-STEM HDC HPLC HR-TEM ICP-MS ICP-OES MALS (Nano)DMA NTA Raman SAED SAXS SEC SEM SIMS SMPS SP-ICP-MS STM TEM TD-GC–MS UV/Vis XPS XRD

Atomic absorption spectrometry Atomic emission spectrometry Atomic force microscopy Asymmetric flow field flow fractionation Aerodynamic particle sizer Molecular gas adsorption according to Brunauer–Emmett– Teller Theory Capillary electrophoresis Confocal laser scanning microscopy Condensation particle counter Dynamic light scattering Electronic diffusion battery Energy-dispersive X-ray spectroscopy Electron energy loss spectroscopy Electrical low pressure impactor Field-flow fractionation Fast mobility particle sizer Fluorescence spectroscopy Gas chromatography–mass spectrometry Scanning transmission electron microscopy with a highangle annular dark field detector Hydrodynamic chromatography High-performance liquid chromatography High-resolution transmission electron microscopy Inductively coupled plasma mass spectrometry Inductively coupled plasma atomic emission spectroscopy Multi-angle light scattering (Nano) differential mobility analyzer Nano tracking analysis Raman spectroscopy Selected area electron diffraction Small-angle X-ray scattering Size exclusion chromatography Scanning electron microscopy Secondary ion mass spectrometry Scanning mobility particle sizer Single particle inductively coupled plasma mass spectrometry Scanning tunneling microscope Transmission electron microscopy Thermo-desorption GC–MS Ultraviolet–visible spectrophotometry X-ray photoelectron spectroscopy X-ray diffraction

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