Potential Scenarios for Nanomaterial Release and Subsequent alteration in the Environment

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Potential Scenarios for Nanomaterial Release and Subsequent alteration in the Environment
  Critical Review POTENTIAL SCENARIOS FOR NANOMATERIAL RELEASE AND SUBSEQUENTALTERATION IN THE ENVIRONMENT B ERND  N OWACK  ,* y  J AMES  F. R ANVILLE , z  S TEPHEN  D IAMOND ,§ J ULIAN  A. G ALLEGO -U RREA , k  C HRIS  M ETCALFE ,#J EROME  R OSE , yy  N INA  H ORNE , zz  A LBERT  A. K  OELMANS ,§§ kk  and S TEPHEN  J. K  LAINE ## y Swiss Federal Laboratories for Materials Science and Technology, St. Gallen, Switzerland z Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado, USA§Mid-Continent Ecology Division, U.S. Environmental Protection Agency, Duluth, Minnesota k Department of Chemistry, University of Gothenburg, Gothenburg, Sweden#Environmental and Resource Studies and Institute for Freshwater Science, Trent University, Peterborough, Ontario, Canada yy CNRS Europole Me´diterrane´en de l’Arbois, Aix-Marseille University, Aix en Provence, France zz Center for Integrated Nanoscale Materials, University of California, Berkeley, California, USA§§Aquatic Ecology and Water Quality Management Group, Wageningen University, Wageningen, The Netherlands kk IMARES, IJmuiden, The Netherlands##Institute of Environmental Toxicology, Clemson University, Pendleton, South Carolina, USAReceived 2 September 2011; Revised 13 October 2011; Accepted 21 October 2011) Abstract  —  The risks associated with exposure to engineered nanomaterials (ENM) will be determined in part by the processes thatcontrol their environmental fate and transformation. These processes act not only on ENM that might be released directly into theenvironment, but more importantly also on ENM in consumer products and those that have been released from the product. Theenvironmental fate and transformation are likely to differ significantly for each of these cases. The ENM released from actual direct useor from nanomaterial-containing products are much more relevant for ecotoxicological studies and risk assessment than pristine ENM.Released ENM may have a greater or lesser environmental impact than the starting materials, depending on the transformation reactionsandthe material. Almost nothingis known about theenvironmental behavior and theeffects ofreleased andtransformed ENM, althoughthese are the materials that are actually present in the environment. Further research is needed to determine whether the release andtransformation processes result in a similar or more diverse set of ENM and ultimately how this affects environmental behavior. Thisarticle addresses these questions, using four hypothetical case studies that cover a wide range of ENM, their direct use or productapplications, and their likely fate in the environment. Furthermore, a more definitive classification scheme for ENM should be adoptedthat reflects their surface condition, which is a result of both industrial and environmental processes acting on the ENM. The authorsconclude that it is not possible to assess the risks associated with the use of ENM by investigating only the pristine form of the ENM,without considering alterations and transformation processes. Environ. Toxicol. Chem. 2012;31:50–59. # 2011 SETAC Keywords —Nanomaterials-containing products Nanoparticles Environmental transformation Fate Transport Risk assessment INTRODUCTION The risks associated with exposure to engineered nanoma-terials (ENM) will be determined in part by the environmentalprocesses that control fate, transport, and transformation. Theseprocesses will determine exposure levels and toxicity of ENM[1–4]. The specific processes that must be considered are not unique to nanomaterials, but the responses of ENM to theseprocesses are likely to differ considerably from those of chem-ical contaminants historically considered in risk assessments. Inmost cases, ENM will enter the environment contained inproducts and will be released from these products during theirlife cycle through product use, disposal, or weathering [5]. Fateand transport processes will act on the product matrix as well ason the ENM contained within it, both prior to and after releasefrom the matrix. Furthermore, the ENM will subsequentlychange in form and chemistry. It is less likely that ENM willoccur in the environment in their as-manufactured form. Thisfact raises the question of whether the study of unaltered ENM,the form generally used in environmental, health, and safety(EHS) investigations, can provide an adequate evaluation of therisk of ENM use.Fate and transport processes that can act on nanomaterials inproducts, and after their release, include photochemical trans-formation, oxidation and reduction, dissolution, precipitation,adsorption, desorption, combustion, biotransformation, andabrasion, among other biogeochemically driven processes[3,6,7]. In addition, ENM are also affected by agglomeration or aggregation and settling [8]. The nature of the ENM surfacewill control aggregation, because all forms of ENM (pristine andaltered) are subject to these processes. We will not, however,consider them in this discussion. Hypothetical case studies willbe used to illustrate how fate and transport processes are likelyto act on ENM as they exist in currently available products. Theexamples include titanium dioxide in sunscreen and paint,nanoscale silver in textiles, composite structures containingcarbon nanotubes, and cerium oxide in the fuel combustedin diesel engines. These products and materials were selectedfor hypothetical case studies because they differ in fundamentalproperties or behaviors critical for assessing the risks of expo-sure to nanomaterials. These include their solubility (insoluble Environmental Toxicology and Chemistry, Vol. 31, No. 1, pp. 50–59, 2012 # 2011 SETACPrinted in the USADOI: 10.1002/etc.726 This paper evolved from discussions held at a SETAC-endorsed Tech-nical Workshopheld at Clemson University in August, 2010. The workshopwas sponsored by the Environmental Protection Agency, Arcadis-US, andthe Clemson University Institute of Environmental Toxicology.* To whom correspondence may be addressed(nowack@empa.ch).Published online 28 October 2011 in Wiley Online Library(wileyonlinelibrary.com).50  TiO 2  vs soluble nano-Ag), redox activity (unreactive TiO 2 vs reactive cerium oxide), and product use pathways to theenvironment. CATEGORIZING NANOPARTICLES The vast body of experimental work with ENM hasbeen conducted with what we term pristine ENM, the as-manufactured form. However, for the purpose of building arisk assessment methodology for products containing ENM, aframework that includes additional categories of ENM shouldbe implemented to reflect the diversity of potential ENM bulk and surface properties (Fig. 1). In addition to pristine ENM(P-ENM), ENM embedded in products should be classified asproduct-modified ENM (PM-ENM). Engineered nanomaterialsacted upon by environmental processes while still associatedwith the product are considered product-weathered ENM(PW-ENM). Finally, after ENM are released from the productand are acted on by environmental processes, they can beclassified as environmentally transformed ENM (ET-ENM).Future laboratory research should strive to obtain data on allfour forms of ENM.Pristine ENM having a wide range of surface coatings havebeen evaluated for toxicity and stability [9,10]. In most cases, P-ENM are industrially modified to form the PM-ENM that areembedded in final products. The modifications to P-ENM allowthem to be homogeneously dispersed or incorporated into thematrix or at the surface of the final product [11]. In many cases,this involves modifying the hydrophilic–hydrophobic proper-tiesfortheuseofP-ENMinorganicsolvents,modifyingsurfacecharge for use with aqueous solvents, or a variety of alterationsto manipulate surface reactivity. In the case of incorporatingENM into a material, the matrix can be the following: a solid,such as nano-TiO 2  in self-cleaning cement; membranes incor-porating nano-SiO 2  or nano-Ag; carbon nanotube-reinforcedcomposites; nano-Ag in textiles; or a liquid, such as TiO 2  incosmetics and paints or CeOx in fuels. For example, the nano-TiO 2  particles incorporated into sunscreens are generally cov-ered by an AlOOH or an SiO 2  layer to prevent any reactiveoxygen species (ROS) production, and a second hydrophobiclayer can be added to allow homogeneous dispersion in thecream (Fig. 2).Addingastructurednanolayeratthesurfaceofacommercialproduct can require different modifications or processes. Forexample, with self-cleaning glass, the nano-TiO 2  layer isdeposited at a high temperature on the glass surface whilebeing formed [12]. In other cases, the nanoparticles are lessstrongly bound to the material, such as incorporating silver intotextiles [13]. It becomes clear that the physical, chemical, andbiological properties of the nanomaterials-containing products(PM-ENM)differfromthoseoftheP-ENMfromwhichtheyaremade.A key issue is whether the PW-ENM released during theweathering of products containing nanomaterials have anincreased or decreased reactivity or toxicity relative to thepristine and modified materials. The research challenge is todetermine whether altering commercial nanomaterial productswill release PW-ENM with high reactivity or whether theweathering of the product will lead to a certain kind of passivation through surface amorphization, adsorption, surfaceredox evolution, or other mechanisms. Below, some caseexamples illustrate the fate processes that occur as a result of releases during product weathering. Of course, when PW-ENMare released from the product, they will undergo further trans-formationsintheenvironment,resultinginET-ENMformation. FATE AND TRANSFORMATION PROCESSES Several important alteration and transformation processescan act on the products and ENM. In many cases, theseprocesses act on all four types of ENM defined previously,although the rates and products of reaction may differ for eachENM type. For this reason, in most of the following discussion,we use the generic ENM designation, unless reference is madeto the specific ENM type for clarity. The alteration and trans-formation processes can be combined into what we term fateprocesses, which can occur during use and after products orENM enter the environment. These fate processes have beenincorporated into the hypothetical case studies presented here,in which each process has been labeled numerically. In thefollowing discussion, these numbers are shown in parentheses.In photochemical transformation (process 1), incident lightmay penetrate the product and reach photoreactive ENM,inducingexcitationENM[14,15]andgenerationoffreeradicals [16] or by direct interaction with other components of theproduct [17]. The extent to which this process will influencethecreationofPW-ENM(orpossiblyET-ENM)isrelatedtotheincident light wavelength, the capacity of the light to penetratethe product and the outer layers of the ENM (for example,aggregated or surface-coated particles may have lower lightpenetration efficiency), and the capacity of the photosensitiveportion of the ENM to be excited or photodegraded. Photo-chemical transformations occur at fast rates after the incidentlight has reached the target, and the rate-determining step is the Fig. 1. Categorization of engineered nanomaterials (ENM) into pristineENM (P-ENM; as produced), product modified ENM (PM-ENM; asincorporated into product), weathering and altered EMN (PW-ENM; inproduct and during release), and environmentally transformed ENM(ET-ENM). [Color figure can be seen in the online version of this article,available at wileyonlinelibrary.com]Nanoparticle release  Environ. Toxicol. Chem.  31, 2012 51  mass transfer from the surface of the material to the externalmedia. Photochemical processes may also alter the interactionof ENM with environmental components (for example, photo-activation of TiO 2  may alter its binding to dissolved organicmatter) [18].Materials in a given oxidation state are susceptible tooxidation (process 2) or reduction (process 3) if the reactionis thermodynamically favorable [19]. In general, redox poten-tials are used to estimate the susceptibility of a compound toundergo these processes [20]. Redox reactions are highlyinfluenced by a variety of environmental conditions, includingthe presence of reducing or oxidizing agents that can acceleratethe rates of reaction, the pH of the media that determines thefavorability of the reaction, the presence of the necessaryreactants that will determine whether the redox reaction cantake place, and the presence of adsorbed substances or stabil-izers on the surface of the ENM that will reduce the rates of transformation [21]. For the present discussion, it was decidedto separate oxidation from reduction, because different compo-nents of a composite nanomaterial, once released into theenvironment, may undergo different redox pathways.Dissolution (process 4a) refers specifically to the release of individual ions or molecules that are soluble in water [4,22–24]. The dissolution process can involve reaction of the surfacemolecules and ultimate release of the ionic form [21] or directdissolution of the constituent materials, followed by a diffu-sional transport of the dissolved compounds [25]. Precipitation(process 4b) refers to the formation of a new solid material afterthe dissolution and transport of ionic species and reaction ordeposition of these dissolved species with the available ligandsor suspended material that are present in natural waters [26,27].These two processes are regulated by the solubility product( K  sp ), which determines the equilibrium among the ionic spe-cies in the solution. These constants are dependent on the ionicstrength (activity coefficients corrections), ligand availability(such as for metal complex species formation), pH, and temper-ature of the surrounding media [19]. The presence of adsorbedsubstances may decrease or increase the dissolution rates byprotecting the surface from the media or by the removal of surface atoms in processes of adsorption or desorption, respec-tively [25]. Thermodynamic calculations can be used to predictthe stable phases under certain environmental conditions; how-ever, slow kinetics and diffusion rates may retard the attainmentof the final product [28–30]. Adsorption (process 5a) is the process by which substancesattach to the surface of solids by means of Van der Waalsattractions (physisorption), electrostatic interactions (ionexchange), or chemical bonding (chemisorption), as discussedby Dabrowski [31], Rabe et al. [32], and Pan and Xing [33]. In physisorption, the adsorbate is weakly and nonspecificallybound to the surface of the ENM. For ion exchange andchemisorption,eitherachargedinteractionorchemical bondingto specific available surface sites is involved. Adsorption of substances may have two opposing effects on the stabilizationoftheparticles.Ifsurfacecoverageispartial,thenthedispersionmaybedestabilized,andaggregationoccursbyabridgingeffectbetween the free surface and the nonadsorbed functional groupsof the adsorbate, especially in the case of large molecules suchas polymers or humic substances. If, however, the surface of theparticles is totally covered, the dispersion may be stabilized,which will reduce the aggregation induced by both electro-chemical and steric interactions [34]. Electrostatic modifica-tions of colloidal stability can also occur after chemisorption of small inorganic or organic molecules [35]. Furthermore, theparticle may adsorb contaminants or small biological entitiesand act as a vector for their transport in the environment[36–40]. Sorption processes may be particularly important with respect to changing the surface characteristics of the PM-ENMto that of the PW-ENM and ET-ENM.A large quantity of literature is available on the influence of organic matter coatings on the behavior of ENM in naturalsystems, which directly affects the colloidal stability of ET-ENM in suspension [8]. Because natural organic matter (NOM)is a ubiquitous constituent of natural waters, these changes inthe surface properties of PW-ENM and ET-ENM by NOMshould be significant, for example, because they affect agglom-eration of particles [1,41,42]. Desorption (process 5b) of chemisorbed species will occur if the equilibrium with the media is altered by lowering thechemical potential (for example, concentration) of the adsorbedsubstance in the surroundingmedia. Modifing the aqueous ioniccomposition alone may be sufficient for desorption of non-specific physisorbed species [31]. In certain cases, anothersubstance with greater affinity for the surface sites of theadsorbent may interact with the surface and promote desorptionoftheoriginallyadsorbedsubstances,followedbyadsorptionof the new substance. Moreover, the particle itself may undergodeposition (different from sedimentation) onto a collector andexperience immobilization when attached to the external sur-face [1]. Desorption processes will strongly affect the coatingsof the PM-ENM, especially if only weakly bound to the surface.Combustion (process 6) is the process of reactions with air atan elevated temperature and generally implies oxidation of theelemental components of the ENM or even phase transformation Fig. 2. ( Left ) Transmission electron microscopy image of nano-TiO 2  material used in sunscreen. ( Right ) Schematic view of the nanocomposite formulationconsisting of a TiO 2  core and Al(OOH) and polydimethylsiloxane layers.52  Environ. Toxicol. Chem.  31, 2012 B. Nowack et al.  [43]. If the former occurs, then it is a special case of oxidation(process 2) facilitated by heat. This process will occur mainlywhen products containing PM-ENM are incinerated, an excep-tion being cerium oxide in fuels. The presence of other oxidizedconstituents such as byproducts of the combustion processesmay lead to adsorption of foreign substances to the PW-ENM.Biotransformation or biodegradation (process 7) may inducetransformation of the ENM [44] or their alteration products[45]. Biologically mediated transformation may include all of the previously described processes, with the exception of combustion. The rates and relative importance of each processis a result of conditions in the biological compartments, suchas processes after ingestion by multicellular animals [46] orenzymatic reactions mediated by microorganisms [47]. One of the physical processes that will influence the final destination of the PM-ENM is abrasion (process 8) or mechanical erosion.Abrasion is the process by which physical forces, such asturbulent fluid regimes or strong collision of solid materials,induce the breakdown of the srcinal material and may lead tothe release of PW-ENM- or PW-ENM-containing particles indifferentshapesand sizes[48–52].Shearstressestimationsmay indicate the final shape and size of the PW-ENM after under-going abrasion processes. Processes such as oxidation may befacilitated by the mechanical energy introduced by abrasion. RELEASE AND ALTERATION OF ENM The scientific consensus is that producing, using, and dis-posing of ENMs leads to environmental releases of nanopar-ticles[5,53,54].However,verylittleactualdatafromreal-world conditions are available on the emissions of PW-ENM fromproducts and releases into the environment. This is causedprimarily by a lack of techniques and instrumentation capableof detecting and quantifying both ENM emissions and theresultant environmental concentrations.Initial measurements show evidence for the release of PM-ENM and PW-ENM from consumer products. Kaegi et al. [51]presented direct evidence of the environmental release of nano-TiO 2  by leaching from painted house facades under the influ-ence of sun and rain. The same authors recently showedevidence that Ag is released from nano-Ag-containing paints[50]. Several studies have investigated release from consumerproducts during use. Examples include changes in Ag speci-ationintextiles[55]andreleasefromtextilesintowater[56,57], washing liquid [13], sweat [58], and washing machines [59]. Also, different studies have been published on abrasion of particles from coatings [48,49,60] or textiles [61]. One major conclusion of all these studies is that the vast majority of thereleased particles are large agglomerates containing PM-ENMand PW-ENM, but also that single, dispersed PW-ENM can befound. As shown in Figure 3, with two examples of releasedparticles, it is obvious that for both Ag released from paintsand ZnO released from coatings, the released PW-ENM arestill embedded in the matrix, so the environmental behavior of PW-ENM is still determined to a large extent by the propertiesof the matrix. This is, of course, however, product specific. Inother cases PM-ENM and PW-ENM may not be bound stronglyto the matrix.A handful of modeling studies have tried to quantifyPW-ENM release to the environment. Some studies evaluatedrelease to the environment from a restricted set of PM-ENM-containing products during the consumption or use phase[62–65]. Other studies modeled release throughout the whole life cycle of PM-ENM-containing products, including ENMproduction and manufacturing of products, use, recycling, anddisposal [66,67]. Sewage sludge, wastewater, and waste incin- eration of products containing PM-ENM were shown to be themajor pathways through which PM-ENM enter the environ-ment.The original P-ENM incorporated into the products aremodified during product manufacture and use and are alteredand transformed after release by the environmental factorsdiscussed above. An example of an alteration during useis the phase transformation of Ag nanoparticles caused byexposure to bleach under washing conditions [55]. After thePW-ENM reaches the environment, transformation processescan significantly change their behavior. Auffan et al. [17] andLabille et al. [68] have shown that the hydrophobic coatingof a TiO 2 -Al(OH) 3 -polydimethylsiloxane nanomaterial (Fig. 2)used in sunscreen creams was desorbed and oxidized on contactwith water, resulting in a stable aqueous suspension of TiO 2 -Al(OH) 3  nanoparticles. However, because the Al(OH) 3  coatingwasnotaffected,thematerialwasstillnotphotoreactiveanddidnot produce ROS compared with photocatalytic TiO 2 .Soluble ENM undergo dissolution reactions. For example,both thermodynamic calculations and kinetic measurementsindicate that metallic Ag nanoparticles will not persist inrealistic environmental compartments containing dissolved Fig. 3. Particles released from nano-Ag containing paint by natural weathering ( left , the area labeled 3 contains the Ag nanoparticles, whereas areas 1 and 2 areTiO 2 -containing particles) [50] and from a surface coating containing nano-ZnO by abrasion ( right ) [49]. Images reprinted from Kaegi et al. ([50]; # 2010) andVorbau et al. ([49]; # 2009), respectively, with permission from Elsevier.Nanoparticle release  Environ. Toxicol. Chem.  31, 2012 53  oxygen [21]. However, this oxidation process is slow undermost environmental conditions and can require months to reachcompletion. Nano-Ag was recently shown to transform rapidlyunder anaerobic wastewater treatment conditions into insolublesilver sulfides [69]. Phenrat et al. [70] reported that partial or complete oxidation of nanometer-sized zero-valent iron underenvironmental conditions decreased its redox activity, agglom-eration, sedimentation rate, and toxicity to mammalian cells.Also, biological modification of ENM and microbially medi-ated redox processes can change the fate and toxicity of ENMsuch as quantum dots and carbon nanotubes [25,46,71]. HYPOTHETICAL CASE STUDIES To illustrate the major processes and pathways of ENM, wepresent five case studies involving hypothetical scenarios. Thediscussion is illustrated using figures that show the likely flowandtransformationofPM-ENM,startingwiththenanomaterial-containing product and ending with their distribution in envi-ronmental compartments as ET-ENM. In the first step, weillustrate the important processes and the relative rates thatresult in the release of the PM-ENM from the product andhow they alter the form of the released PW-ENM. The releasedPW-ENM, and possibly the srcinal nanomaterial-containingproduct, can then undergo a treatment process or can passdirectly into an environmental compartment. The three mostimportant treatment processes are waste incineration, waste-water treatment, and landfill disposal. These important chem-ical processes act on all ENM classes and are identified by ournumbering scheme. Case study 1: NanoTiO 2  in sunscreen This case study focuses on TiO 2 , an inorganic, insolubleENM present in a dispersed form in the product [18,29]. In the dispersed form, PM-ENM can be released readily from theproduct through normal use and disposal. Because sunscreensare consumed during use and are lost during swimming orwashed off during bathing, almost all of the srcinal product isreleased either into wastewater or directly into rivers [72]. Thisis different from the other four case studies, in which directrelease of the product does not readily occur; rather, releaseresults from accidental spills or less likely pathways (see thediscussion below for CeO 2 ). Figure 4a illustrates the majorreactions involved in TiO 2  alteration and release and trans-formation. In all figures, the initial set of reactions is acting onboth the PM-ENM and the matrix in which it is dispersed.Subsequentreactions areconsidered to be actingonthe releasedPW-ENM. The processes that alter and transform TiO 2  arephotochemistry and adsorption or desorption of coatings [17].Incineration involving high-temperature combustion of TiO 2 -containing products can result in chemical and physical sinter-ing of the TiO 2 . Within a wastewater treatment plant (WWTP),the important processes that affect TiO 2  are dissolution andprecipitation, adsorption and desorption, and biotransformation Fig. 4. ( a–d ) Material flow diagrams showing the release of engineered nanomaterials (ENM) from different products and the transformation reactions duringtransfer from one environmental compartment to another: TiO 2  release from sunscreen and paint, Ag release from textiles, CeO 2  release from fuels and carbonnanotubesreleasefromcomposites.Thenumbersrefertotheprocessesidentifiedinthetext.[Colorfigurecanbeseenintheonlineversionofthisarticle,availableatwileyonlinelibrary.com]54  Environ. Toxicol. Chem.  31, 2012 B. Nowack et al.
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