Occurrence, behavior and effects of nanoparticles in the ... · Review Occurrence, behavior and...

Environmental Pollution 150 (2007) 5e22www.elsevier.com/locate/envpol

Review

Occurrence, behavior and effects of nanoparticles in the environment

Bernd Nowack a,*, Thomas D. Bucheli b

a Technology and Society Laboratory, Empa e Materials Science and Technology, Lerchenfeldstrasse 5, CH-9014 St. Gallen, Switzerlandb Agroscope Reckenholz Tänikon, Research Station ART, Reckenholzstrasse 191, CH-8046 Zurich, Switzerland

Received 1 June 2007; accepted 3 June 2007

The behavior and the effects of natural and engineered nanoparticles in the environment are reviewed.

Abstract

The increasing use of engineered nanoparticles (NP) in industrial and household applications will very likely lead to the release of suchmaterials into the environment. Assessing the risks of these NP in the environment requires an understanding of their mobility, reactivity, eco-toxicity and persistency. This review presents an overview of the classes of NP relevant to the environment and summarizes their formation,emission, occurrence and fate in the environment. The engineered NP are thereby compared to natural products such as soot and organic col-loids. To date only few quantitative analytical techniques for measuring NP in natural systems are available, which results in a serious lack ofinformation about their occurrence in the environment. Results from ecotoxicological studies show that certain NP have effects on organismsunder environmental conditions, though mostly at elevated concentrations. The next step towards an assessment of the risks of NP in the envi-ronment should therefore be to estimate the exposure to the different NP. It is also important to notice that most NP in technical applications arefunctionalized and therefore studies using pristine NP may not be relevant for assessing the behavior of the NP actually used.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Engineered nanoparticles; Colloids; Soot; Analysis; Environmental behavior; Ecotoxicity; Risk assessment

1. Introduction

Particles in the nano-sized range have been present on earthfor millions of years and have been used by mankind for thou-sands of years. Soot for instance, as part of the Black Carboncontinuum, is a product of the incomplete combustion of fossilfuels and vegetation; it has a particle size in the nanometeremicrometer range and therefore falls partially within the‘‘nanoparticle’’ domain. Recently, however, nanoparticles(NP) have attracted a lot of attention because of our increasingability to synthesize and manipulate such materials. Today,

Abbreviations: BC, black carbon; CB, carbon black; CNT, carbon nano-

tubes; CTO, chemo-thermal oxidation; DOC, dissolved organic carbon; FFF,

field-flow fractionation; nC60, nano-C60 clusters; NP, nanoparticles; nZVI,

nano zero-valent iron; PM, particulate matter; SEC, size-exclusion chromatog-

raphy; TOC, total organic carbon.

* Corresponding author. Tel.: þ41 71 274 76 92; fax þ41 71 274 78 62.E-mail address: [email protected] (B. Nowack).

0269-7491/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.envpol.2007.06.006

nanoscale materials find use in a variety of different areassuch as electronic, biomedical, pharmaceutical, cosmetic, en-ergy, environmental, catalytic and material applications. Be-cause of the potential of this technology there has beena worldwide increase in investment in nanotechnology re-search and development (Guzman et al., 2006). Data on thecurrent use and production of NP are sparse and often conflict-ing. One estimate for the production of engineered nanomate-rials was 2000 tons in 2004, expected to increase to 58,000tons in 2011e2020 (Maynard, 2006).

The forecasted huge increase in the manufacture and use ofNP makes it likely that increasing human and environmentalexposure to NP will occur. As a result NP are beginning tocome under scrutiny and the discussion about the potential ad-verse effects of NP has increased steadily in recent years; infact it has become a top priority in governments, the privatesector and the public all over the world (Roco, 2005; Hellandet al., 2006; Siegrist et al., 2007). Several reviews have

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6 B. Nowack, T.D. Bucheli / Environmental Pollution 150 (2007) 5e22

summarized the recent developments in this field. Most atten-tion has thus far been devoted to the toxicology and health im-plications of NP (e.g. Oberdörster et al., 2005; Kreyling et al.,2006; Lam et al., 2006; Nel et al., 2006; Helland et al., inpress), while the behavior of NP in the environment (Biswasand Wu, 2005; Wiesner et al., 2006; Helland et al., in press)and their ecotoxicology (Colvin, 2003; Moore, 2006; Oberdör-ster et al., 2006a) have been less often reviewed. However, nosystematic description of natural and anthropogenic NP andtheir occurrence, fate and effects on the environment is yetavailable. This review therefore gives an overview on the be-havior of NP in the environment, both manufactured and unin-tentionally produced, and also makes comparisons betweenthem and natural NP. The knowledge gained with natural NPwill be of invaluable help for assessing the behavior andeffects of engineered NP in the environment.

In the following, first certain types of NP, their distributionand prevalence in the environment are highlighted. Theanalysis of NP is discussed next, followed by selected specificenvironmental processes in which certain NP play a key role.Finally the effects of NP on organisms and their ecotoxicityare presented.

2. Nanoparticle types

2.1. Classification

Nanotechnology is defined as the understanding and controlof matter at dimensions of roughly 1e100 nm, where uniquephysical properties make novel applications possible (EPA,2007). NP are therefore considered substances that are lessthan 100 nm in size in more than one dimension. They canbe spherical, tubular, or irregularly shaped and can exist infused, aggregated or agglomerated forms. Fig. 1 shows howthe NP fit into other size-dependent categories that havebeen used for many decades. The commonly used definitionof ‘‘dissolved’’ is in most cases operationally defined by allcompounds passing through a filter, in many cases with a cutoffat 0.45 mm. The colloidal fraction is defined as having a sizebetween 1 nm and 1 mm (Buffle, 2006), therefore overlappingwith the NP. Separation between dissolved, colloidal and par-ticulate matter in natural waters is normally given by the avail-ability of analytical methods that can distinguish among these

Fig. 1. Definitions of different size classes relevant for nanoparticles.

fractions without introducing artifacts during the measurementprocess (Lead and Wilkinson, 2006). Alternatively, Gustafssonand Gschwend (1997a) have also suggested a ‘‘chemcentric’’definition of colloids and proposed that a colloid is any con-stituent that provides a molecular milieu into and onto whichchemicals can escape from the aqueous solution and whoseenvironmental fate is predominantly affected by coagulationebreakup mechanisms, as opposed to removal by settling. Par-ticles in the atmosphere have normally been classified accord-ing to their size, e.g. PM10 or PM0.1, the latter correspondingto the definition of NP.

NP can be divided into natural and anthropogenic particles(Table 1). The particles can be further separated based on theirchemical composition into carbon-containing and inorganicNP. The C-containing natural NP are divided into biogenic,geogenic, atmospheric and pyrogenic NP. Examples of naturalNP are fullerenes and CNT of geogenic or pyrogenic origin,biogenic magnetite or atmospheric aerosols (both organicsuch as organic acids and inorganic such as sea salt). Anthro-pogenic NP can be either inadvertently formed as a by-prod-uct, mostly during combustion, or produced intentionallydue to their particular characteristics. In the latter case, theyare often referred to as engineered or manufactured NP. Exam-ples of engineered NP are fullerenes and CNT, both pristineand functionalized and metals and metal oxides such asTiO2 and Ag. Engineered NP are the main focus of the currentresearch on NP in the environment, but some of them occuralso naturally, e.g. as inorganic oxides or fullerenes. In thesections below the different types of natural and engineeredNP are presented.

2.2. Organic colloids

The colloidal matter in natural waters includes particles,macromolecules and molecular assemblies and is defined ashaving a size of about 1 nme1 mm (Buffle, 2006), andtherefore falls to some extent into the size range of NP. Thismacromolecular colloidal organic matter is polymeric, poly-functional and polydisperse (Guo and Santschi, 1997). Envi-ronmental colloids include three major types of compounds:inorganic colloids, humic substances and large biopolymerssuch as polysaccharides and peptidoglycans. Colloids mustbe seen as an essential building block of the abiotic mediumsupporting life in general (Buffle, 2006). Although the knowl-edge of the structure and the environmental impact of naturalcolloids have significantly increased in recent years, theirprecise function and composition are still poorly defined.

2.3. Soot

Natural and anthropogenic combustion processes that takeplace both in stationary and mobile sources emit a wide varietyof particles. Of these particles only the so-called ‘‘ultra-fine’’particles correspond to the standard definition of NP. This re-view is limited to the ‘‘soot’’ fraction out of the Black Carbon(BC) combustion continuum, and we use soot as the overallterm for nano-sized BC. Soot as a product of re-condensation

7B. Nowack, T.D. Bucheli / Environmental Pollution 150 (2007) 5e22

Table 1

Classification of nanoparticles

Formation Examples

Natural C-containing Biogenic Organic colloids Humic, fulvic acids

Organisms Viruses

Geogenic Soot Fullerenes

Atmospheric Aerosols Organic acids

Pyrogenic Soot CNT

Fullerenes

Nanoglobules, onion-shaped nanospheres

Inorganic Biogenic Oxides Magnetite

Metals Ag, Au

Geogenic Oxides Fe-oxides

Clays Allophane

Atmospheric Aerosols Sea salt

Anthropogenic (manufactured,

engineered)

C-containing By-product Combustion by-products CNT

Nanoglobules, onion-shaped nanospheres

Engineered Soot Carbon Black

Fullerenes

Functionalized CNT, fullerenes

Polymeric NP Polyethyleneglycol (PEG) NP

Inorganic By-product Combustion by-products Platinum group metals

Engineered Oxides TiO2, SiO2Metals Ag, iron

Salts Metal-phosphates

Aluminosilicates Zeolites, clays, ceramics

processes during incomplete combustion of fossil and renew-able fuels is mainly emitted into the atmosphere (Goldberg,1985), from where it distributes hemisphere-wide and isdeposited onto soils and water bodies.

Carbon Black (CB) is an industrial form of soot used in var-ious applications such as filler in rubber compounds, primarilyin automobile tires. The particle size of CB is partially in thenanometer range with average values between 20 and 300 nmfor different materials (Blackford and Simons, 1987; Sirisinhaand Prayoonchatphan, 2001; Tscharnuter et al., 2001).

2.4. Natural and unintentionally produced fullerenes andCNT

Although fullerenes and CNT are considered as engineeredNP, they are also natural particles (fullerenes) or have closerelatives in the environment (CNT). Whereas some of thesefullerenes are of interstellar origin that have been brought toearth by comets or asteroids (Becker et al., 1996, 2001), themajority is believed to have formed from polycylic aromatichydrocarbons (PAH) derived from algal matter during meta-morphosis at temperatures between 300 and 500 �C and inthe presence of elemental sulfur (Heymann et al., 2003), orduring natural combustion processes.

2.5. Natural and unintentionally produced inorganic NP

Natural inorganic NP can have atmospheric, geogenic orbiogenic origin. Inorganic NP are present everywhere in soilsand geologic systems (Banfield and Zhang, 2001; Waychunaset al., 2005). NP are also ubiquitous aerosols in the atmosphereand they are precursors for the formation of larger particles,

which are known to strongly influence global climate, atmo-spheric chemistry, the visibility and regional and global trans-port of pollutants (Anastasio and Martin, 2001). Primaryatmospheric NP can be, for example, soil dust and sea salt,although the major mass fraction is composed of coarse parti-cles. The average particle size of airborne mineral dust, basedon mass, is between 2 and 5 mm. However, based on the num-ber of particles, the average size is approximately 100 nmwith a considerable number below this value (Hochella andMadden, 2005).

A special class of unintentionally produced NP is com-posed of platinum and rhodium containing particles producedfrom automotive catalytic converters. Although most Pt andRh are attached to coarser particles, about 17% was foundto be associated with the finest aerosol fraction (


functionalizations, a variety of different CNT are obtained thathave very different properties (Dai, 2002; Niyogi et al., 2002).Especially biological and medical applications explore the po-tential of modifying the properties of CNT (Bianco and Prato,2003).

Both fullerenes and CNT therefore have to be considered asa class of compounds where each single fullerene or CNT canhave very different properties depending on the functionaliza-tion or the synthesis and cleaning method. This will have im-portant implications for assessing the environmental behaviorof these compounds.

2.7. Engineered polymeric NP

NP synthesized from organic polymers have gained wide-spread interest in medicine as carriers for drugs. The possibil-ity to control size, surface charge, morphology andcomposition make polymers especially well suited for design-ing NP with tailored properties. These NP are taken up bya wide variety of cells and are studied for their ability to crossthe bloodebrain barrier (Koziara et al., 2003).

Several types of polymeric NP have also been developedand proposed for soil and groundwater remediation. Micelle-like amphiphilic polyurethane particles have a hydrophilicouter side and a hydrophobic inner core and are thereforevery well suited for the removal of hydrophobic pollutants(e.g. phenanthrene) from soils (Kim et al., 2000, 2003a,b,2004a,b; Tungittiplakorn et al., 2004, 2005). The NP areable to extract the PAH in a similar manner to surfactant mi-celles but unlike the micelles they do not sorb to soil particles.Another polymeric nanoscale material is dendrimers that func-tion as water-soluble chelators (Xu and Zhao, 2005, 2006).

2.8. Engineered inorganic NP

Engineered inorganic NP cover a broad range of substancesincluding elemental metals, metal oxides and metal salts. Ele-mental silver is used in many products as bactericide (Moroneset al., 2005), whereas elemental gold is explored for many pos-sible applications and its catalytic activity (Brust and Liely,2002). The use of nanoscale zero-valent iron (nZVI) forgroundwater remediation ranks as the most widely investi-gated environmental nanotechnological application (Nowack,in press). Metallic iron is very effective in degrading a widevariety of common contaminants such as chlorinated meth-anes, brominated methanes, trihalomethanes, chlorinated eth-enes, chlorinates benzenes, other polychlorinatedhydrocarbons, pesticides and dyes (Zhang, 2003). Successfulresults from field demonstrations using nZVI have been pub-lished, with injection of 1.7e400 kg of NP into the groundwa-ter (Elliott and Zhang, 2001; Quinn et al., 2005). To dateapproximately 30 projects are under way where nZVI isused for site remediation (Li et al., 2006a).

Nanoparticulate metal oxides are among the most used NP(Aitken et al., 2006). Bulk materials of TiO2, SiO2 and alumi-num and iron oxides have been produced for many years.However, recently they have also been manufactured in

nano-sized form and have already entered the consumer mar-ket, e.g. ZnO in sunscreens (Rittner, 2002). TiO2 NP arewidely used for applications such as photocatalysis, pigmentsand cosmetic additives (Aitken et al., 2006).

A wide variety of other nanomaterials is under vigorous in-vestigation by materials scientists. Nano-sized zeolites (Larluset al., 2006), clays (Yaron-Marcovich et al., 2005) and ce-ramics (Cain and Morrell, 2001) are other NP that havebeen proposed for various applications. Several non-carbonnanotubes have also been synthesized (Pokropivnyi, 2001),for example, TiO2 (Zhang et al., 2006). Quantum dots madefrom semiconductor materials such as CdSe, CdTe or ZnS haveattracted wide interest in areas such as information technology,molecular biology and medicine (Gao et al., 2004).

3. Analysis of NP


Today the development of analytical methods for engi-neered NP is still in its infancy. More experience is availableon aquatic colloids (Lead and Wilkinson, 2006) and it isexpected that many of the techniques applied there will alsobe useful for the determination of engineered NP. Severalapproaches have been used for aquatic colloids (Lead andWilkinson, 2006): microscopic methods (e.g. electron micros-copy, atomic force microscopy), size fractionation (e.g. ultra-filtration, field-flow fractionation (FFF) or centrifugation) andchromatography (e.g. size-exclusion or gel permeation chro-matography (SEC and GPC)). The standard methods forseparating colloids and particles are currently cross flow filtra-tion, but FFF has gained increasing importance for the separa-tion and characterization of aquatic colloids due to the ease ofcoupling the FFF to sensitive detection techniques such asICP-MS (Lyven et al., 2003; Stolpe et al., 2005). Also laser-induced breakdown detection (LIBD) has been used to quan-tify natural aquatic organic NP in lake and drinking watersamples (Wagner et al., 2003, 2004).

3.2. Black Carbon/soot

Extensive evaluations of methods for BC quantification insoils and sediments are available (summarized in (Cornelissenet al., 2005). BC quantification methods in soils and sedimentsroughly fall into three categories: optical, thermal, and chem-ical. The first ones mainly resort to organic-petrographic orclassical light microscopes and will not be discussed here fur-ther, as they are mainly restricted to BC particles >5e10 mm.In most methods, non-BC organic matter is first removed byselective oxidation (thermal or chemical). The remaining car-bon residue is defined as BC and is quantified by various de-tection methods such as microscopic particle counting,titration, coulometry, 13C NMR, molecular marker analysis,or elemental analysis. The most important method artifactsare organic matter charring (overestimation of BC contents),and losses of soot BC during solution handling and ‘‘overoxi-dation’’ (underestimation).


Currently, the chemo-thermal oxidation at 375 �C (CTO;e.g., Gustafsson et al., 1997, 2001a; Elmquist et al., 2004) isa frequently used method and selective for the soot fractionof BC. Despite its possible limitations, this method has pro-vided BC results for sediments that are consistent with geo-chemical characteristics. For instance, the spatial distributionof pyrogenic compounds such as PAH and polychlorinated di-benzodioxins and furans (PCDD/Fs) shows stronger correla-tions with BC than with bulk TOC contents (e.g., Gustafssonand Gschwend, 1997b; Gustafsson et al., 1997; Perssonet al., 2002), and the 14C isotopic composition of CTO-isolatedBC was identical to that of PAH but quite distinct from that ofTOC in both sediments and aerosols (Gustafsson et al., 2001a;Reddy et al., 2002). Further, certain molecular combustionmarkers correlated significantly with CTO-isolated BC con-tents, but not with TOC in a set of Swiss soils (Bucheliet al., 2004).

3.3. Engineered carbon-containing NP

Quantification of C60 is usually performed by UVevismeasurements for high concentrations. Absorption bands ofC60 are located at 336, 407, 540, and 595 nm, but this methodis only applicable to defined laboratory systems. HPLC is usedfor the detection of low concentrations of C60 (Fortner et al.,2005) but also a method for the direct analysis by electrospraytime-of-flight mass spectrometry has been developed (Kozlov-ski et al., 2004). The HPLC method has been used to detectfullerenes in natural samples, e.g. rocks and minerals (Beckeret al., 1994; Heymann et al., 1995; Chijiwa et al., 1999).

No method for the quantification of CNT in natural mediaexists so far. Electron microscopy has been used to identifyCNT in ice cores, but no quantification was done (Esquiveland Murr, 2004; Murr et al., 2004b,c). Several methods existto analyze CNT under well-defined laboratory settings. Thequantification is normally performed by UVevis spectrometryat high concentrations (Sano et al., 2001; Jiang et al., 2003).CNT show strong absorption in the UV and visible regionwith peaks at 253 nm (Jiang et al., 2003), 266 nm(Liu et al., 2006a), 350 nm (Sano et al., 2001) and strongabsorption extending up to 1200 nm (Karajanagi et al., 2006;Liu et al., 2006a). Wrapping of CNT with conventional fluoro-phores (Prakash et al., 2003; Kim et al., 2006), fluorescentpolymers (Didenko et al., 2005) or with DNA-oligonucleotides(Heller et al., 2005) allowed their detection in biomedical ap-plications. SEC with UV-detection was used for the separationof CNT from impurities and amorphous carbon (Duesberget al., 1998a, 1999; Niyogi et al., 2001; Zhao et al., 2001).As mobile phase both organic solvents and aqueous solutionswere applied.

3.4. Natural and engineered inorganic NP

Many techniques have been used to identify and character-ize natural inorganic NP (Burleson et al., 2004). The onesmost often used are transmission electron microscopy andscanning probe microscopy; images of individual NP can be

obtained with both techniques. However, only a minute frac-tion of material is characterized, which means that it can beextremely difficult to ensure that a representative sample is ex-amined. Also the sample preparation and the measurementconditions can often change the structure of the NP (Burlesonet al., 2004). These methods have been used to detect NP pro-duced by bacteria isolated from soils, sediments and waters.

Almost no reports about the analysis of inorganic engi-neered NP have been published so far. One study presentsthe characterization of CdSe quantum dots by SEC (Kruegeret al., 2005). FFF was used to monitor the stability of nano-ZnO in soil suspension (Gimbert et al., 2007).

4. Formation, emission, occurrence and fate of NP in theenvironment

4.1. Overview

Assessing the risks imposed by the use of nanomaterials incommercial products and environmental applications requiresa better understanding of their mobility, bioavailability, andtoxicity. For nanomaterials to comprise a risk, there must beboth a potential for exposure and a hazard that results after ex-posure. The important processes and pathways of NP in theenvironment are depicted in Fig. 2. Release of NP maycome from point sources such as production facilities, landfillsor wastewater treatment plants or from nonpoint sources suchas wear from materials containing NP. Accidental release dur-ing production or transport is also possible. In addition to theunintentional release there are also NP released intentionallyinto the environment. nZVI, for example, is directly injectedinto groundwater polluted with chlorinated solvents. Whetherthe particles are released directly into water/soil or the atmo-sphere, they all end up in soil or water, either directly orindirectly for instance, via sewage treatment plants, waste

Fig. 2. Nanoparticle pathways from the anthroposphere into the environment,

reactions in the environment and exposure of humans.


handling or aerial deposition. In the environment the forma-tion of aggregates and therefore of larger particles that aretrapped or eliminated through sedimentation affects the con-centrations of free NP (Fig. 3). Humans can be either directlyinfluenced by NP through exposure to air, soil or water or in-directly by consuming plants or animals which have accumu-lated NP. Aggregated or adsorbed NP will be less mobile, butuptake by sediment-dwelling animals or filter feeders is stillpossible. Whereas the possibility of biomagnification of NPin the food-chain has been mentioned (EPA, 2007), no dataare currently available.


Colloidal organic matter is regularly around 10 times moreabundant than larger particulate organic matter (filter defined>0.7 mm) in surface seawater (Sharp, 1973; Benner et al.,1992), and makes up about 10e40% of the filter passingDOC (Gustafsson et al., 2001b). Aggregation of colloids inwater bodies and settling eliminates compounds from the watercolumn and in soils the aggregation leads to porous microstruc-tures that are essential for the development of microorganismsand for nutrient bioavailability. Colloids have also significanteffects on pollutant, nutrient and pathogen chemistry, transportand bioavailability and may themselves be bioavailable (Leadand Wilkinson, 2006). They are internally and externallyporous and charged. The overall charge is a function of pH andionic strength and the surface is also hydrated to a variabledegree, with hydrophilic and hydrophobic regions.

4.3. Soot

Incomplete combustion of fossil fuels leads to re-condensa-tion reactions in the gas phase that produce highly aromaticglobular structures in grape-like aggregates (soot) (Goldberg,1985). Besides soot, incomplete combustion of biomass alsoyields less structured, carbonized residues from parental mate-rials (chars, charcoal). The characteristics of BC vary widely

Fig. 3. Release of NP from products and (intended or unintended) applications:

(a) release of free NP, (b) release of aggregates of NP, (c) release of NP em-

bedded in a matrix and (d) release of functionalized NP. Environmental factors

(e.g. light, microorganisms) result in formation of free NP that can undergo

aggregation reactions. Moreover, surface modifications (e.g. coating with nat-

ural compounds) can affect the aggregation behavior of the NP.

(Goldberg, 1985; Middelburg et al., 1999; Schmidt and Noack,2000; Morawska and Zhang, 2002), but generally, BC particleshave a three-dimensional structure composed of stacked aro-matic sheets, particle sizes in the nanometeremicrometerrange, high but variable specific surface areas ranging from2 to 776 m2/g, and sometimes plant residues (Cornelissenet al., 2005). Estimates of BC production from biomass burn-ing range from 0.05 to 0.27 Gt/year (Kuhlbusch and Crutzen,1995) and from fossil fuel combustion from 0.012 to 0.024 Gt/year (Penner et al., 1993). Reliable data on inventories of sootas a fraction of BC in final recipient media such as sedimentsand soils are largely missing or highly uncertain.

4.4. Fullerenes and CNT

Fullerenes have been detected in geological materials dat-ing back 1.85 billion years (Becker et al., 1994). Most notablyfullerenes are present in clays from the Cretaceous/Tertiaryand the Permo-Triassic boundary, where they are believed tohave formed in the extensive wildfires that resulted from themeteorite impact (Heymann et al., 1994; Chijiwa et al.,1999). Fullerenes have also been found in fulgurites, a glassyrock that forms when lightning strikes the ground (Heymannet al., 2003). CNT and fullerenes also occur in 10,000-year-old ice cores from Greenland (Esquivel and Murr, 2004;Murr et al., 2004b,c), indicating that even before the wide-spread use of fires by humans these compounds were presentin ambient air. Natural CNT have been found in a coalepetro-leum mix extracted from an oil well (Velasco-Santos et al.,2003). This finding shows that high temperature is not a prereq-uisite for synthesis of CNT, but that they can be produced un-der natural and albeit geologic conditions of elevated pressureand temperature. CNT have also been sampled from fuel-gascombustion streams (Bang et al., 2004; Murr et al., 2004a)and can be formed in alloys and steels during productionand processing (Chernozatonskii et al., 1997; Reibold et al.,2006).

Fullerenes also occur in emissions from coal-burning powerplants (Utsunomiya et al., 2002), in petrol, diesel and fuel-gassoot (Ishiguro et al., 1997; Lee et al., 2002; Murr and Soto,2005). They have also been found in quite high concentrations(3 mg/kg to 1 g/kg) in Chinese ink sticks made from sootobtained by the slow burning of oils (Heymann et al., 2003).

The stability of the fullerenes under geologic conditions forhundreds of millions of years shows that they are truly recal-citrant in the environment. However, this also implies that lifehas been exposed to low concentrations of these compoundsfor hundreds of millions of years.

Pristine fullerenes are virtually insoluble in pure water(Heymann, 1996; Beck and Mandi, 1997), but fullerenes de-rivatized with ionizable or hydrophilic groups exhibita much greater water solubility (Wudl, 2002). Not derivatizedfullerenes can be solubilized in water by forming clusters ofC60 with a size of about 25e500 nm (nC60; nano-C60). Thecolor, hydrophobicity, and reactivity of individual C60 are sub-stantially altered in this aggregate form. These aggregates arecrystalline in order and remain as underivatized C60


throughout the formation/stabilization process that can later bechemically reversed (Fortner et al., 2005). The standardmethod of producing nC60 clusters uses C60 dissolved in or-ganic solvents (Scrivens et al., 1994; Deguchi et al., 2001;Fortner et al., 2005), but it is also possible to produce nC60by extended vigorous mixing (several weeks to months) ofC60 in water (Brant et al., 2005; Labille et al., 2006). The par-ticle size can be affected by formation parameters such as thepH of the water. Once formed, nC60 remains stable in solutionat or below ionic strengths of 0.05 I for months. nC60 carrya strong negative charge (zeta potential of about �50 mV)and they remain negatively charged in the presence of mono-valent and also divalent cations such as Ca and Mg in the con-centration range between 0.01 and 10 mM (Brant et al., 2005).The zeta potential increased to zero at about 100 mM ionicstrength (Brant et al., 2005), resulting in aggregation intoeven larger clusters, whereas at lower concentrations the sizeof nC60 remained constant (Fortner et al., 2005). The negativecharge of nC60 is likely acquired through charge transfer froman organic solvent or surface hydrolysis reactions involving thehydration shell of the cluster and the strong electron-acceptingproperties of C60 (Brant et al., 2005). Almost all studies of theenvironmental behavior and effects of fullerenes have beencarried out with this form of C60.

Single, dissolved molecules of fullerenes are also formedby interaction with host molecules. Cyclodextrin, for example,can solubilize C60 in a hosteguest interaction (Anderssonet al., 1992; Sundahl et al., 1993). Fullerenes are also easilytaken up by micellar or colloidal solution of tensides (e.g.Triton X-100) or phospholipids (Hungerbuhler et al.,1993; Beeby et al., 1994; Bensasson et al., 1994; Guldi et al.,1994).

Untreated CNT as received from the producing companiesare not dispersable in water (Chen et al., 2004). A standardprocedure for solubilizing CNT is cutting pristine CNT instrong acid under sonication. The resulting shortened CNTare hydroxylated at the ends and at defects and can reasonablywell be suspended in water, however, they rapidly aggregate inthe presence of cations (Sano et al., 2001). Solubilized CNTcan be obtained in a similar way to the fullerenes by interac-tion with organic molecules, e.g. by wrapping them with linearpolymers such as surfactants (Duesberg et al., 1998b; Jianget al., 2003; Moore et al., 2003). Biopolymers such as alginicacid (Liu et al., 2006b), starch (Star et al., 2002), proteins(Karajanagi et al., 2006) and phospholipids (Wu et al., 2006)have been found to solubilize CNT. A recent study shows thathumic and fulvic acids are able to solubilize single CNT andthat a stable suspension in water containing humic acids canbe obtained (Hyung et al., 2007).

Underivatized CNT were reported to have a point of zerocharge of around pH 7 and a zeta potential of less than30 mV (Hu et al., 2005). Acid-treated and therefore hydroxyl-ated CNT were negatively charged over the whole pH rangewith a zeta potential between �30 and �70 mV (Hu et al.,2005). The low or even absent charge on pristine CNT aroundneutral pH values is the main reason for their inability to forma stable suspension in water.

4.5. Natural inorganic NP

Nucleation is a process that can result in the formation ofsecondary aerosols, which are to a large extent in the nanopar-ticulate size range (Biswas and Wu, 2005). The key species inthe atmosphere are sulfuric acid, nitric acid and organic gases.The growth of the NP is the main process that affects their life-time in the atmosphere, accomplished by the condensation oflow volatile gases, coagulation with other NP and surface re-actions that increase particle mass (Anastasio and Martin,2001). Anthropogenic emissions have more than doubled theflux of fine particles into the atmosphere (Anastasio andMartin, 2001). The dominant anthropogenic source is primaryemissions from combustion sources and secondary aerosol for-mation from the oxidation of species such as gaseous SO2 ororganic compounds (Kittelson, 1998).

Chemical weathering processes of silicates, oxides andother minerals frequently produces NP such as amorphous sil-ica, hydrous aluminosilicates such as allophane, clays such ashalloysite, and oxides such as magnetite and hematite. UO2NP particles are formed by reaction of uranyl with greenrust, a Fe(II)/iron(III) phase (O’Loughlin et al., 2003). Inacid mine drainage, the very low pH solutions derived fromweathering of sulfide-rich rocks forms a variety of differentNP when mixed with natural waters due to changes in temper-ature, pH and higher oxygen concentrations. Ferrihydrite,amorphous iron oxide, and goethite (a-FeOOH) are typical ex-amples of NP formed during this process. Polynuclear com-plexes and nanoclusters of aluminum, e.g. Al13 or Al30(Casey et al., 2001; Furrer et al., 2002), and of sulfides, e.g.Cu4S6 nanoclusters, have been found in natural waters.

Microorganisms can also generate NP through the genera-tion of metabolic energy by pathways involving inorganicions that participate in redox reactions. Oxidation of Fe(II) re-sults in the formation of iron oxide NP. A variety of differentmanganese oxide NP are formed, both inside and outside thecell (Matsunaga and Sakaguchi, 2000). Also sulfides can beproduced by the action of sulfate-reducing bacteria, e.g.ZnS, but also UO2 NP have been observed on the surface ofbacteria (Gilbert and Banfield, 2005).

The physical properties (e.g. melting point, electricalconductivity) and reactivity (e.g. catalytical activity, sorptioncapacity) of such NP vary dramatically as a function of sizeand can be very different from the bulk material (Maddenand Hochella, 2005). The minerals in the size range of approx-imately a few to several tens of nanometers are in the transi-tional range where properties are expected to be variable anddeviate from bulk behavior (Waychunas et al., 2005).

5. Selected specific processes of NP in the environment

5.1. Aggregation of organic colloids and NP in water

In the environment natural colloids or NP interact amongthemselves and with other natural NP or larger particles(Figs. 2 and 3). The formation of aggregates in natural systemscan be understood by considering physical processes, i.e.


Brownian diffusion, fluid motion and gravity. Aggregation isparticle-size dependent and results in efficient removal ofsmall particles in environmental systems (Omelia, 1980). Toquantify the stability of NP in the environment we have to pre-dict the stability of their suspension and their tendency to ag-gregate or interact with other particles (Mackay et al., 2006).Twenty nanometer-sized nZVI particles aggregated, for exam-ple, within 10 min to micrometer-sized clusters (Phenrat et al.,2007). The nature of the NP is modified by adsorption pro-cesses (Fukushi and Sato, 2005) and especially the surfacecharge plays a dominant role (Kallay and Zalac, 2001,2002). Cations, for example, are able to coagulate acid-treatedCNT with critical coagulation concentrations of 37 mM forNa, 0.2 mM for Ca and 0.05 mM for trivalent metals (e.g.La3þ) (Sano et al., 2001). Aggregation of CNT added as sus-pension to filtered pond water has been reported (Zhu et al.,2006c).

NP are not necessarily released as single NP (see Fig. 3). Inmany applications NP are embedded in a matrix and release ofNP will occur through release of matrix-bound NP (Koehleret al., in press). As many NP are functionalized, release offunctionalized NP is also possible. In the environment the re-leased NP are affected by environmental factors such as light,oxidants or microorganisms. This can result in chemical or bi-ological modification or degradation of the surface functional-ization or the embedding matrix and may result in free NP.The surface of pristine NP can also be modified by environ-mental factors (e.g. coating by organic matter) or functional-ized by chemical or biological processes. The effect ofhumic and fulvic acids to inhibit the aggregation of CNThas been recently shown (Hyung et al., 2007) and also nZVIis efficiently coated with humic acids (Giasuddin et al.,2007). Nanoparticulate ZnO which was coated with the surfac-tant sodium dodecyl sulfate was stable in soil suspension for14 days without changes in particle size distribution (Gimbertet al., 2007).

5.2. Behavior of NP in porous media

The transport of colloids in porous media and the colloid-facilitated transport of contaminants have received a lot ofattention in the past (McGechan and Lewis, 2002; Sen andKhilar, 2006). The movement of colloids e and therefore alsoof NP e in porous media is impeded by two processes: strain-ing or physical filtration where the particle is larger than thepore and is trapped and true filtration where the particle isremoved from solution by interception, diffusion and sedimen-tation. However, particles removed from solution by such pro-cesses can readily become resuspended upon changes in thechemical or physical conditions (e.g. changes in pH, ionicstrength, flow rate) (Sen and Khilar, 2006).

Several studies have investigated the transport of a widerange of engineered NP through porous media (Lecoanetet al., 2004; Lecoanet and Wiesner, 2004; Dunphy Guzmanet al., 2006). Particles smaller than 100 nm are predicted tohave very high efficiencies of transport to collector surfacesdue to Brownian diffusion. If all particleecollector contacts

were to result in particle attachment to the collector, thesesmall particles would be retained to a large extent by the po-rous medium. However, nano-sized silica particles were notappreciably removed and also anatase NP were only removedbetween 55 and 70%, depending on the flow velocity (Lecoa-net and Wiesner, 2004). Fullerol (hydroxylated fullerene) andsurfactant-stabilized CNT were almost completely mobile andonly removed to a very low percentage (Lecoanet et al., 2004).The most efficient removal was observed for an iron oxide andfor fullerene clusters (nC60). The deposition efficiency fornC60 increased with time and after 60 pore volumes virtuallyno nC60 were detected in the effluent, an effect ascribed to‘‘filter ripening’’, the increased filter efficiency by depositionof particles (Cheng et al., 2005a). These studies show thatthe collector efficiency for NP can be very different and thatespecially the surface-modified NP displayed high mobilities.Also the environmental conditions are important, though, andefficient removal of titania NP was observed close to the pH atthe point of zero charge (Dunphy Guzman et al., 2006).

Because the nZVI particles used for groundwater remedia-tion have the strong tendency to aggregate and adsorb to sur-faces of minerals, a lot of effort has been directed towardsmethods to disperse the particles in water and render them mo-bile. In one approach water-soluble starch was used as stabi-lizer (He and Zhao, 2005); in another hydrophilic carbon orpolyacrylic acid delivery vehicles were used (Schrick et al.,2004). Modified cellulose, sodium carboxymethyl cellulose(CMC) was found to form highly dispersed nZVI (He et al.,2007) and also several polymers have been tested and foundto be very effective (Saleh et al., 2007). These modified NPwere found to be mobile under natural conditions, indicatingthe importance of knowing the exact surface properties ofNP for a prediction of their potential mobility in theenvironment.

5.3. NP as adsorbents

The unique structure and electronic properties of some NPcan make them especially powerful adsorbents. Dissolved or-ganic carbon and organic colloids in the sub-micron size rangehave been recognized as a distinct non-aqueous organic phaseto which organic pollutants are partitioning (Burkhard, 2000),which leads to their reduced bioavailability. Most data on dis-solved organic carbon partition coefficients (KDOC) are avail-able for PAH. In general, KDOC values for any individualcompound vary widely over several orders of magnitude, de-pending on the characteristics of the sorbents such as size,conformation, and chemical composition, and probably alsothe experimental method applied. Moreover, they are signifi-cantly lower than predictions from octanolewater partition co-efficients and partitioning relationships developed primarilyfrom natural organic matter of sediments and soils (Burkhard,2000). Still, by their shear abundance, such sorbents may sig-nificantly attenuate the truly dissolved exposures of organicpollutants. Note also that the partitioning of organic pollutantsinto colloidal organic carbon is not restricted to hydrophobicorganic pollutants such as PAH, but also of relevance for


more polar compounds such as steroid hormones (Zhou et al.,2007a) or modern herbicides (Irace-Guigand and Aaron,2003). The distribution of metal ions between solution andcolloids strongly influences metal speciation and thereforemetal bioavailability (Lead and Wilkinson, 2006). Colloidsact therefore as metal buffer that keep free metal ions,e.g. of Cu2þ, within a range that is beneficial for life.

Also soot is a powerful adsorbent for organic compounds.The nonlinear adsorption of organic compounds onto BCcan completely dominate total sorption at low aqueous con-centrations in soils and sediments (Cornelissen et al., 2005).The extremely efficient sorption to BC pulls highly toxic poly-cyclic aromatic hydrocarbons, polychlorinated biphenyls, di-oxins, polybrominated diphenylethers and pesticides intosediments and soils (Koelmans et al., 2006). The increasedsorption is general but strongest for planar and most toxiccompounds at environmentally relevant, low aqueous concen-trations. The presence of BC can explain that the sorption oforganic compounds into soils and sediments is much higherthan expected based on absorption into organic matter alone(Cornelissen et al., 2005).

CNT have received a lot of attention as very powerful adsor-bents for a wide variety of organic compounds from water. Ex-amples include dioxin (Long and Yang, 2001), PAH (Gotovacet al., 2006; Yang et al., 2006b,c), DDT and its metabolites(Zhou et al., 2006c), PBDEs (Wang et al., 2006), chlorobenzenesand chlorophenols (Peng et al., 2003; Cai et al., 2005), trihalo-methanes (Lu et al., 2005, 2006), bisphenol and nonylphenol(Cai et al., 2003b), phthalate esters (Cai et al., 2003a), dyes(Fugetsu et al., 2004), the pesticides thiamethoxam, imidaclo-prid and acetamiprid (Zhou et al., 2006a) and the herbicides nic-osulfuron, thifensulfuron, metsulfuron, triasulfuron (Zhou et al.,2006b, 2007b), atrazine and simazine (Zhou et al., 2006d), anddicamba (Biesaga and Pyrzynska, 2006). It was found that puri-fication (removal of amorphous carbon) of the CNT improvedthe adsorption (Gotovac et al., 2006). The available adsorptionspace was found to be the cylindrical external surface, neitherthe inner cavity nor the inter-wall space of multi-walled CNTcontributed to adsorption (Yang and Xing, 2007). Different tofullerenes no adsorptionedesorption hysteresis was observed,indicating reversible adsorption (Yang and Xing, 2007).

Oxidized and hydroxylated CNT are also good adsorbersfor metals. This has been found for various metals such asCu (Liang et al., 2005a), Ni (Chen and Wang, 2006; Lu andLiu, 2006), Cd (Li et al., 2003; Liang et al., 2004), Pb(Li et al., 2002, 2006b), Ag (Ding et al., 2006), Am(III) (Wanget al., 2005) and rare earth metals (Liang et al., 2005b). Inmost cases adsorption is highly pH-dependent with increasingsorption with increasing pH as expected for adsorption ofmetals onto hydroxyl groups. Adsorption of organometalliccompounds on pristine multi-walled CNT was found to bestronger than for CB (Munoz et al., 2005). Chemically modi-fied NP have been proposed for environmental cleanup andmay therefore be released into the environment (Obare andMeyer, 2004). TiO2 functionalized with ethylenediamine wastested for the ability to remove anionic metals from ground-water (Mattigod et al., 2005).

Fullerenes have also been tested for adsorption of organiccompounds. Adsorption depends to a great extent on the dis-persion state of the C60 (Cheng et al., 2004). Because C60forms cluster in water there are closed interstitial spaceswithin the aggregates where the compounds can diffuse intowhich leads to significant adsorptionedesorption hysteresis(Cheng et al., 2005b; Yang and Xing, 2007). Fullereneswere found not to be very good sorbents for a wide varietyof organic compounds (e.g. phenols, PAH, amines) whilethey are very efficient for removal of organometallic com-pounds (e.g. organolead) (Ballesteros et al., 2000).

Many materials have properties that are dependent on size(Hochella, 2002). Hematite particles with a diameter of 7 nmfor example adsorbed Cu ions at lower pH values than parti-cles with 25 or 88 nm diameter, indicating the uniqueness ofsurface reactivity for iron oxide particles with decreasingdiameter (Madden et al., 2006). However, an investigation ofPb adsorption onto TiO2 NP showed that the bulk material ex-hibited stronger adsorption and higher adsorption capacity(Giammar et al., 2007).

6. Effects on organisms

6.1. Uptake and toxicity

A consistent body of evidence shows that nano-sized parti-cles are taken up by a wide variety of mammalian cell types,are able to cross the cell membrane and become internalized(Lynch et al., 2006; Rothen-Rutishauser et al., 2006; Smartet al., 2006). The uptake on NP is size-dependent (Limbachet al., 2005; Chithrani et al., 2006). Aggregation and size-de-pendent sedimentation onto the cells or diffusion towards thecell were the main parameters determining uptake (Limbachet al., 2005). The uptake occurs via endocytosis or by phago-cytosis in specialized cells. One hypothesis is that the coatingof the NP by protein in the growth medium results in confor-mational changes of the protein structure, which triggers theuptake into the cell by specialized structures, limiting uptaketo NP below about 120 nm (Lynch et al., 2006).

Within the cells NP are stored in certain locations (e.g. in-side vesicles, mitochondria) and are able to exert a toxic re-sponse. The small particle size, a large surface area and theability to generate reactive oxygen species play a major rolein toxicity of NP (Nel et al., 2006). Inflammation and fibrosisare effects observed on an organism level, whereas oxidativestress, antioxidant activity and cytotoxicity are observedeffects on a cellular level (Oberdörster et al., 2005). Severalrespiratory and cardiovascular diseases in humans are causedby BC (Avakian et al., 2002; Morawska and Zhang, 2002;Armstrong et al., 2004). Ultra-fine soot globules migratedeep into the lungs and carry very toxic, often carcinogeniccompounds such as polycyclic aromatic hydrocarbons (PAH)(Avakian et al., 2002; Morawska and Zhang, 2002; Armstronget al., 2004). Air pollution related illnesses causes ‘‘prematuredeath’’ among especially asphalt-, coke plant-, and gas-workers, chimney sweeps, carbon electrode manufacturers,


or victims of smoke related accidents, as well as the generalpublic (Armstrong et al., 2004).

6.2. Uptake under environmental conditions andecotoxicity

Most toxicology studies have been carried out with mam-malian cells, and the NP were exposed to a cell culture me-dium containing a mixture of proteins and other biologicalcompounds. Results from such in vitro studies can thereforenot be directly transferred to environmental conditions whereuptake of NP into the aquatic biota is a major concern. Poten-tial uptake routes include direct ingestion or entry across epi-thelial boundaries such as gills or body wall. At the cellularlevel prokaryotes like bacteria may be largely protectedagainst the uptake of many types of NP since they do nothave mechanisms for transport of colloidal particles acrossthe cell wall (Moore, 2006). However, for eukaryotes, e.g. pro-tists and metazoans, the situation is different since they haveprocesses for the cellular internalization of nanoscale or mi-croscale particles, namely endocytosis and phagocytosis(Moore, 2006). The uptake of different NP has indeed beenobserved. CNT were taken up by a unicellular protozoan andwere localized with the mitochondria of the cells (Zhu et al.,2006c). Latex NP were taken up by eggs of the fish Oryziaslatipes and adult fish accumulated the NP in gills and intestinebut particles were also detected in brain, testis, liver and blood(Kashiwada, 2006). Another study reported that C60 adsorp-tion onto the gram-negative E. coli was 10 times higher thanon gram-positive Bacillus subtilis (Lyon et al., 2005). Also in-organic NP are taken up by cells. Nano-sized ZnO, for exam-ple, was internalized by bacteria (Brayner et al., 2006). Alsonano-sized CeO2 particles were adsorbed onto the cell wallof E. coli, but the microscopic methods were not sensitiveenough to discern whether internalization had taken place(Thill et al., 2006).

Ecotoxicological studies show that NP are also toxic toaquatic organisms, both unicellular (e.g. bacteria or protozoa)and animals (e.g. Daphnia or fish). CNT induced a dose-de-pendent growth inhibition in a protozoan (Zhu et al., 2006c)and were found to be a respiratory toxicant in rainbow trout(Smith et al., 2007). However, also growth stimulation of a uni-cellular protozoan by CNT was observed in a growth mediumcontaining a yeast extract medium. This was explained by up-take of CNT-peptone conjugates where the additional peptonewas responsible for the stimulation (Zhu et al., 2006b). Withcopepods purified CNT did not show any effect, whereas un-purified CNT with all their by-products increased mortality(Templeton et al., 2006). CNT coated with lipids were readilytaken up by Daphnia magna and they modified the solubilityof the CNT through digestion of the lipid coating (Robertset al., 2007). Acute toxicity was only observed at the highestconcentration.

For C60 more information about the interaction with severalaquatic organisms is available, e.g. bacteria (Lyon et al., 2005,2006), Daphnia (Lovern and Klaper, 2006) and fish (Oberdör-ster, 2004; Oberdörster et al., 2006b; Zhu et al., 2006a).

However, in most studies only toxicity and sometimes stressbiomarkers were determined. Fullerenes have only little im-pact on the soil microbial community and function, based onsoil respiration, soil enzyme activity and changes in commu-nity structure (Tong et al., 2007). Bacterial cell walls showa physiological adaptation to the presence of fullerenes andthey show a response in lipid composition and membranephase behavior (Fang et al., 2007).

Quite a lot of information is available for nano-sized silverparticles due to their use as bactericides. The cells of bacteriaare damaged in the presence of nano-Ag, finally resulting indeath of the organisms (Sondi and Salopek-Sondi, 2004).The interaction with the cells is size-dependent (Moroneset al., 2005) and seems to depend also on the shape of the par-ticles (Pal et al., 2007). Nano-Ag appears to be significantlymore toxic than Agþ-ions towards E. coli (Lok et al., 2006).

Inorganic nanoparticular TiO2, SiO2 and ZnO had a toxiceffect on bacteria, and the presence of light was a significantfactor increasing the toxicity (Adams et al., 2006). Whereasbulk TiO2 is considered to have no health effects on aquaticorganisms, this is clearly the case for nano-sized TiO2 (Lovernand Klaper, 2006). NP that damage bacterial cell walls havebeen found to be internalized while those without this activitywere not taken up (Stoimenov et al., 2002).

6.3. Interaction of NP with plants

To date research on interaction of NP with plants is almostnon-existent. One study reports the effect of aluminum oxideNP on root elongation in hydroponic studies (Yang and Watts,2005). A slight reduction in root elongation was found in thepresence of uncoated alumina NP but not with NP coated withphenanthrene. It was proposed that the surface characteristicsof the alumina played an important role in phytotoxicity. How-ever, the study is problematic as pointed out by Murashov(2006) because the author did not take into account that solu-ble Al3þ is a potent root toxicant and known to inhibit rootgrowth. The solubility of aluminum oxide is known to increasewith decreasing particle size and modification of the surfaceby adsorbed compounds is known to affect the dissolution rate.

Several articles by Hong et al. (2005a,b), Zheng et al.(2005), Gao et al. (2006) and Yang et al. (2006a) have shownthat nano-sized TiO2 can have a positive effect on growth ofspinach when administered to the seeds or sprayed onto theleaves. Nano-TiO2 was shown to increase the activity of sev-eral enzymes and to promote the adsorption of nitrate and ac-celerate the transformation of inorganic into organic nitrogen.Normal-sized TiO2 did not have these effects.

One study reported the interaction of NP with green algae,which have a cell wall similar to plants (Hund-Rinke and Si-mon, 2006). Only information on toxicity of nano-sizedTiO2 under illumination is reported in this study, whereas nei-ther uptake of the NP nor the dark reaction was studied.

The few data that are available indicate that at least inor-ganic, oxidic NP are able to interact with plant cells or greenalgae with a similar cell wall structure. To date no informationabout cellular internalization has become available. Plant cell


walls differ from those of mammalian cells, for which inter-nalization of NP is known (see above). Possible interactionsof NP with plant roots are adsorption onto the root surface, in-corporation into the cell wall, and uptake into the cell. The NPcould also diffuse into the intercellular space, the apoplast, andbe adsorbed or incorporated into membranes there. Plant cellscarry a negative surface charge, which allows the transport ofnegatively charged compounds into the apoplast. The Caspar-ian strip poses a barrier to the apoplastic flow and transportand only symplastic transport is possible into the xylem. How-ever, this barrier is not perfect and compounds can enter thexylem through holes or damaged cells without ever crossinga cell membrane and be further transported to the shoots.This process has been found to be the dominant process forthe uptake of metal complexes with chelators such as EDTAand their translocation to the shoots (Nowack et al., 2006;Tandy et al., 2006). Based on this knowledge about root phys-iology we could therefore hypothesize that negatively chargedNP could enter the apoplasm of the root cortex and eventuallyalso the xylem, but are not taken up by the cells.

6.4. Interactions among organisms, NP andcontaminants

The interaction of NP with toxic, organic compounds canboth amplify as well as alleviate the toxicity of the com-pounds. In contrast to harmful effects, NP can therefore alsohave an advantageous role in the environment. The possibleinteractions in systems with organisms, NP and pollutantsare shown schematically in Fig. 4. The influence of pollutantson organisms is well studied (Fig. 4a) and the interaction withNP alone has been described in sections above (Fig. 4b). In theternary system organismepollutanteNP two possible ways ofinteraction are possible: Fig. 4c shows that the NP may adsorbor absorb the pollutant, therefore reducing its free concentra-tion and hence reducing the toxicity of the pollutant. If the

Fig. 4. Scheme of the interactions of pollutants, NP and organisms (algae as

example). (a) Adsorption and uptake of pollutant, (b) adsorption and uptake

of nanoparticle, (c) adsorption (or absorption) of pollutants onto NP and reduc-

tion in pollutant uptake by organisms and (d) adsorption of NP with adsorbed

(or absorbed) pollutant and possible uptake of pollutant-NP.

NP with the adsorbed pollutants are taken up by the cells(Fig. 4d), then a toxic effect could be the consequence, eithercaused by the NP, the pollutant or in a synergistic way by both to-gether. However, it could also be that no effect is observed if thebound pollutant is not bioavailable and the NP itself is not toxic.

The strong sorption of organic compounds to BC wasshown to be a dominant factor for the low and variable biotato sediment accumulation factors and the limited potentialfor microbial degradation (Cornelissen et al., 2005). BC gen-erally comprises about 9% of total organic carbon in sedi-ments and may reduce uptake in organisms by up to twoorders of magnitude (Koelmans et al., 2006). Decreased bio-availability of organic pollutants in sediments and benthic in-vertebrates in presence of BC was reported in a number ofpapers (Maruya et al., 1997; Jonker et al., 2004; Lohmannet al., 2004; Rust et al., 2004; Sundelin et al., 2004; Voparilet al., 2004). Much less has been done with plankton fromthe open water column (Knauer et al., 2007).

Growth stimulation of a unicellular protozoan by CNT wasobserved in a growth medium containing a yeast extract me-dium and was explained by uptake of CNTepeptone conju-gates and the additional peptone was responsible for thestimulation (Zhu et al., 2006b). A slight reduction in root elon-gation was found in the presence of uncoated alumina NP butnot with NP coated with phenanthrene (Yang and Watts,2005). It was proposed that the surface characteristics of thealumina played an important role in phytotoxicity and that ad-sorption of phenanthrene shielded the toxic alumina surfacefrom the plant roots. When exposed to As(V)-contaminatedwater in the presence of nano-sized TiO2, carp accumulatedconsiderably more As than without NP (Sun et al., 2007).

6.5. Environmental risk assessment of NP

Environmental exposure varies on the basis of conditionssuch as the way in which materials are handled in the work-place, how nanomaterials partition to various phases (e.g., wa-ter and air), the mobility of nanomaterials in each of thesephases, their persistence, and the magnitude of the sources.A lot of research is currently devoted to these topics. This ba-sic information about the behavior and toxicity is needed, butis not sufficient to allow for a realistic risk assessment of NP inthe environment. What is also needed is an evaluation of theexpected quantities and concentrations of NP in environmentalsystems. To date nothing is known about this issue, neitherfrom an analytical point of view (e.g. actual measurementsof NP in the environment) nor with respect to theoretical ormodeling studies. Only few products containing NP are actu-ally on the market, but this is expected to change rapidly inthe next years as more and more nano-products are sold. Itis therefore not only necessary to get an overview on currentexposure, but also more important to anticipate future scenar-ios on the use of nano-products and exposure to released NP.As a starting point to risk assessment, exploring the sourcesand environmental pathways helps to identify relevant applica-tions and situations where a subject deserving protection mayface exposure to NP (Reijnders, 2006; Koehler et al., in press).


7. Conclusions

There is currently a lot of attention being paid to the behav-ior and effects of engineered NP, but there is still only limitedsolid information. Nonetheless, there is quite a vast amount ofwork being done with natural NP, e.g. soot and aquatic col-loids, and the knowledge gained there will be of invaluablehelp in assessing the fate of engineered NP. Analyticalmethods developed for natural NP are now beginning to be ap-plied to engineered NP. The discussion of NP in the environ-ment is therefore different from organic micropollutants,where often advances in analytical techniques and detectionof compounds in the environment have been the starting pointfor scientific and public discussions. For NP the public discus-sion predates the possibility of their analysis.

Compared to conventional or other emerging contaminants,NP particles pose some new challenges for scientists. Whereasit is already obvious that particle size plays an important rolewith respect to toxicity, much less is known how size affectsthe behavior and reactivity of NP. It is also important to realizethat many engineered NP are functionalized and this signifi-cantly affects their behavior. Changes in functionalization byenvironmental factors or the coating of the surface by naturalcompounds is clearly an important process in the environmentwhich has, however, been studied only marginally so far.These new issues and concepts need to be incorporated intothe experimental design and the choice of appropriate analyt-ical methods (Holsapple et al., 2006; Powers et al., 2006).When applying established toxicity tests to NP the resultsshould always be interpreted with great care especially toavoid the reporting of false positive results (Wörle-Knirschet al., 2006).

Finally, the use of limited resources such as Pt, Pd, andother scarce metals in nanoform might pose a problem becauseonce applied and emitted, these metals are inadvertently lost tomankind because they are dispersed to yield infinitely lowconcentrations in the environment (Gordon et al., 2006). Recy-cling will in many cases not be possible and future generationswill not have these elements at their disposal anymore. In con-trast to organics, the number of metal elements is finite andsubstitution often not possible.

Acknowledgements

The research for this article was carried out in the frame-work of the project ‘‘NanoRisk’’, funded by the Swiss FederalOffice of Public Health (BAG), the Federal Office for theEnvironment (FOEN), the Federal Office for ProfessionalEducation and Technology Innovation Promotion Agency(OPET-CTI) and EMPA Materials Science & Technology.

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