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Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 517References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 517

1. Introduction

Burgeoning human population has put immense pressure onthe consumer products. In order to meet the ever growing needsof the society, nanotechnology has touched many spheres of utilityservices, including consumer products, health care, transportation,energy and agriculture (Taton et al., 2002; Cui et al., 2001). Nano-technologies allow the production and manipulation of minute ob-

jects that measure as little as one billionth of a meter (thenanometer). Nanotechnologies got started in the early 1980s withthe appearance of a new type of microscope (atomic force micro-scope), which allowed not only the observation of units of atomsand molecules, but also their physical manipulation and the rela-tive scale of comparison as presented in Fig. 1. In 1991, the manip-ulation of carbon atoms, arranged in the form of a tube, made itpossible to create the first nanotube six times lighter than steel,but 100 times stronger. The nanomaterials form three types of architecture according to the end use are presented in Table 1.

The increased use of nanomaterials introduces the nanoparti-cles intentionally/unintentionally into the waste streams, viz.wastewater treatment facilities. A list of daily use nanomaterialsand corresponding nanoparticles is presented in Table 2. The im-pact that wastewater treatment has on nanomaterials, or con-versely, the impact that nanomaterials have on wastewatertreatment, is largely unknown. Moreover, questions remain onthe efficient way to remove these nanoparticles from industrialwastewaters and sewage treatment plants. Recent research sug-

gests that some nanoparticles escape from treatment plants and

are discharged into natural water bodies. These nanoparticles canremain in the environment for long periods and can be potentiallytoxic to the aquatic life (Oberdörster, 2004; Velzeboer et al., 2008).Upon release, nanomaterials are likely to interact with aquatic sur-faces and biological species as well as aggregate, depending on theinterplay between electrostatic and van der Waals interactions(Thess et al., 1996; Saleh et al., 2008).

At this juncture, there is an urgent need to analyze the possiblebehavior and fate of nanoparticles in wastewater treatment facili-ties and wastewater sludge. There is an extreme paucity of litera-ture in this field as evident from Fig. 2 which illustrates variouspublications in the field of nanoparticles drawn from different re-search journals. Hence, the review discusses the peer-reviewed lit-erature published so far addressing the issues and raisingquestions on the potential impact of these nanoparticles. Some-times, the statements are based on extrapolation from non-waste-water based processes to wastewater processes, however, soundevidence has been provided in this regard. The review takes intoconsideration only the anthropogenic nanoparticles, also called,‘‘engineered nanoparticles” (as given in Table 1), as natural nanopar-ticles have been already studied and reviewed in details elsewhere.

2. Nanoparticle toxicity

The nanomaterials possess large surface area per unit of volumewhich lends novel electronic properties relative to conventional

Fig. 1. Comparison of nanoparticles with macroscale particles (Modified from: National Nanotech Initiative, US EPA White paper, 2005).

Table 1

Different forms of nanoscale architecture.

3D 2D 1D

Fullerenes Carbon nanotubes andnanofilaments

Nanolaminated or compositionally modulatedmaterials

Colloidal particles Metal and magnetic nanowires Grain boundary filmsNanoporous silicon Oxide and carbide nanorods Clay plateletsActivated carbons Semiconductor quantum wires Semiconductor quantum wells and superlatticesNitride and carbide precipitates in high-strength low-alloy steels Magnetic multilayers and spin valve structuresSemiconductor particles in a glass matrix for non-linear optical

componentsLangmuir–Blodgett films

Semiconductor quantum dots (self-assembled and colloidal) Silicon inversion layers in field effect transistors

Quasi-crystals Paints, rusts in pipes

S.K. Brar et al. / Waste Management 30 (2010) 504–520 505

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chemicals. These special properties that make nanomaterials use-ful may also cause some nanomaterials to pose hazards to humansand the environment, under certain conditions. A number of authors have published literature on characterization, fate, andtoxicological information of nanomaterials and proposed researchstrategies for evaluation of safety of nanomaterials (Dunfordet al., 1997; Morgan, 2005; Holsapple et al., 2005; Thomas andSayre, 2005; Balshaw et al., 2005; Tsuji et al., 2006; Borm et al.,2006; Powers et al., 2006; Franklin et al., 2007; Kennedy et al.,2008) and hence this does not form major part of this review.The major toxicological concern is the fact that some of the manu-

factured nanomaterials are redox active (Colvin, 2003), and othersare transported across cell membranes and especially into mito-

chondria (Foley et al., 2002). Lam et al. (2004) demonstrated thatcarbon nanotube products induced dose-dependent epithelioidgranulomas in mice and, in some cases, interstitial inflammationin the animals of the 7-day post-exposure groups. Oberdörster(2004) indicated that nanomaterials (Fullerenes, C60) induced oxi-dative stress in a fish model. Hence, toxicity of nanoparticles that iscoming to the fore is posing questions on the sustainability and fu-ture of nano-products. An in-depth critical review in this contexthas been recently published by Klaine et al. (2008) discussing thekey aspects of their fate, behavior, disposition, and toxicity, witha particular focus on manufactured nanomaterials. Brunner et al.

(2006) reported nanoparticle-specific cytotoxic mechanism for un-coated iron oxide and partial detoxification or recovery after

0

500

1000

1500

2000

2500

3000

3500

Synthesis Air Wastewater Toxicity Environmental

Remediation

Wastewater

sludge

Biosolids

N o . o f p u b l i c

a t i o n s

Fields of publication

79.3 %

14.8 %

0.6 %

4.9 %0.4 %

Fig. 2. Number of publications in different disciplines of nanoparticles, until December 2008. (Note, the number of publications for the possibility of nanoparticlecontamination in wastewater sludge are really minimal, top includes percent of the publications; it is a collective survey done on science direct website for different journalsand hence the publications thereof; Water Research, Water, Science and Technology, Chemosphere, Biochemical and Biophysical Research Communications, Waste

Management, Air, Water and Soil Pollution, Journal of Environmental Quality, Aquatic Toxicology, Water Environment Research, Journal of Hazardous Materials.)

Table 2

Production of nanoparticles from different sources and respective applications.

Source Type of nanoparticle Quantity used interms of tons

Application/uses

Metals and alkalineearth metals

Ag High Antimicrobials, paints, coatings, medical use, food packagingFe High Water treatmentPt group metals High CatalystsSn Unknown Paints

Al High Metallic coating/platingCu Unknown MicroelectronicsZr HighSe Low Nutraceuticals, health supplementsCa Low Nutraceuticals, health supplementsMg Low Nutraceuticals, health supplements

Metal oxides TiO2 High Cosmetics, paints, coatingsZnO Low Cosmetics, paints, coatingsCeO2 High Fuel catalystSiO2 High Paints, coatingsAl2O3 Low Usually substrate bound, paintings

Carbon materials Carbon black High Substrate bound, but released with tyre wearCarbon nanotubes Medium–High Used in a variety of composite materialsFullerenes (C60-C80) Medium–High Medical and cosmetics use

Miscellaneous Nanoclay High Plastic packagingCeramic High Coatings

Quantum dots Low Different compositionsOrganic nanoparticles Low Vitamins, medicines, carriers for medicines and cosmetics, food

additives and ingredients

Safety of Manufactured Nanomaterials: About,” UOECD Environment Directorate, OECD.org, 18 July 2007 . Small Sizes that Matter: Opportunities and Risks of Nanotechnologies, Joint report of the Allianz Center for Technology and the OECD InternationalFutures Programme, ed. Dr. Christoph Lauterwasser, OECD.org 18 July 2007 < http://www.oecd.org/dataoecd/37/19/37770473.pdf >.

506 S.K. Brar et al. / Waste Management 30 (2010) 504–520

http://www.oecd.org/about/0,3347,en_2649_37015404_1_1_1_1_1,00.htmlhttp://www.oecd.org/about/0,3347,en_2649_37015404_1_1_1_1_1,00.htmlhttp://www.oecd.org/dataoecd/37/19/37770473.pdfhttp://www.oecd.org/dataoecd/37/19/37770473.pdfhttp://www.oecd.org/about/0,3347,en_2649_37015404_1_1_1_1_1,00.htmlhttp://www.oecd.org/about/0,3347,en_2649_37015404_1_1_1_1_1,00.html

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treatment with zirconia, ceria, or titania. Likewise, Rothen-Rutish-auser et al. (2006) combined different microscopic techniques tovisualize fine and nanoparticles in red blood cells: (I) fluorescentparticles by laser scanning microscopy combined with digital im-age restoration, (II) gold particles by conventional transmissionelectron microscopy and energy filtering transmission electronmicroscopy, and (III) titanium dioxide particles by energy filteringtransmission electron microscopy. By using these differing micro-scopic techniques particles60.2lm and nanoparticles in red bloodcells were detected. The authors reported that surface charge andthe material of the particles did not influence their entry. These re-sults suggested that particles may penetrate the red blood cellmembrane by an unknown mechanism different from phagocyto-sis and endocytosis. Toxicity of nanoparticles was manifested byinflammation probably resulting from oxidative stress.

Gatti is the authority on nanopathology who discovered the ef-fect of nanoparticles on human health for the first time. The authorhenceforth reported several effects of the engineered nanoparticleson human health, including effects on colon, blood, endothelial cellfunction, muscles, skin, sarcoma development (Gatti and Rivasi,2002; Gatti, 2004, 2005; Gatti et al., 2004, 2008; Sabbioni et al.,2004; Peters et al., 2004; Hansen et al., 2006; Cross et al., 2007).Antimony trioxide nanoparticles have been found to affect the hu-man hematopoietic progenitor cells at 5 lg/ml concentration (Bre-goli et al., 2009. A study examined the relationship between agroup of workers’ experiencing vague symptoms in relation tothe nanoparticle exposure (Song et al., 2009; Gilbert, 2009). Polyac-rylate, comprising nanoparticles, was confirmed in the workplace.Pathological examinations of patients’ (in this case, seven youngfemales, 18–47 years) lung tissue displayed nonspecific pulmonaryinflammation, pulmonary fibrosis and foreign-body granulomas of pleura, thus confirming chronic toxicity of the nanoparticles onlungs. However, there are some unanswered questions in thisstudy especially pertaining to the type of nanoparticle, their con-centration and as to if they were the true initiators and also the sta-tistical count is not significant enough to make a real-time

conclusion.Nanosceptics even find certain similarities between nanoparti-

cles and carbon nanotubes, one of the most popular nanoparticles,and asbestos fibers. In view of a rapid development of nanotech-nologies, it is essential to establish adequate criteria for risk assess-ment that could protect against potential harmful effects arisingfrom specific properties of substances occurring in the form of nanoparticles. The ecotoxicological data of various products of dai-ly use, considered to be potential source of nanoparticles, suggestvaried toxicity of different nanoparticles as enumerated in Table 3.The toxicity of certain well-known metals and metal oxide nano-particles is known and understood. However, there is still scarcityof literature on many of the aquatic and more particularly terres-trial organisms which may bring in new information on the poten-

tial interaction of these nanoparticles. More efforts in this directionwould bridge the knowledge gap as there is a transition from ter-restrial to aquatic environment. Such information would also provehandy for regulatory agencies to determine the threshold limits of these nanoparticles.

3. Application of nanomaterials in decontamination of organic

compounds – potential source in wastewater and sludge

Nanomaterials have been extensively used for rapid or cost-effective cleanup of wastes when compared to current conven-tional approaches (Shan et al., 2009). The application benefits arederived from the nanoparticle characteristics: enhanced reactivity,

surface area, sub-surface transport, and/or sequestration charac-teristics of nanomaterials.

Research has shown that nanoscale iron particles are very effec-tive for the transformation and detoxification of a wide variety of common environmental contaminants, such as chlorinated organicsolvents, organochlorine pesticides, and polychloro biphenyls (Elli-ott and Zhang, 2001; Zhang, 2003; Glazier et al., 2003; Ivanov et al.,2004; Quinn et al., 2005; Mauter and Elimelech, 2008). Modifiediron nanoparticles, such as catalyzed and supported nanoparticleshave been synthesized to further enhance the rate and efficiency of remediation with recent developments in both laboratory and pilotstudies, including: (1) synthesis of nanoscale iron particles (10–100 nm, >99.5% Fe) from common precursors such as Fe(II) andFe(III); (2) reactivity of the nanoparticles towards contaminantsin soil and water over extended periods of time (e.g., weeks); (3)field tests validating the injection of nanoparticles into aquifer,and (4) in situ reactions of the nanoparticles in the sub-surface.

In addition to the zero-valent iron, other nanosized materials,such as metallo-porphyrinogens have been tested for degradationof tetrachlorethylene, trichloroethylene, and carbon tetrachlorideunder anaerobic conditions (Dror et al., 2005). Titanium oxidebased nanomaterials have also been developed for potential usein the photocatalytic degradation of various chlorinated com-pounds (Chen et al., 2005). Enhanced retention or solubilizationof a contaminant may be helpful in a remediation setting. Nanom-aterials may be useful in decreasing sequestration of hydrophobiccontaminants, such as polycyclic aromatic hydrocarbons (PAHs),bound to soils and sediments. The release of these contaminantsfrom sediments and soils could make them more accessible toin situ biodegradation; however, the fate of nanoparticle bound or-ganic compounds is still a puzzle. Nanomaterials made frompoly(ethylene) glycol modified urethane acrylate have been usedto enhance the bioavailability of phenanthrene (Tungittiplakornet al., 2005). In fact, various magnetic nanoparticles have been pre-pared by chemical co-precipitation method and used for the re-moval of Cr(VI) from synthetic electroplating wastewaterencompassing excellent adsorbent capacities (Hu et al., 2007).

Literature is abundant with studies on utilization of nanoparti-

cles for adsorption of various pollutants, mostly metals and dyes(Dai et al., 1999; Hu et al., 2005; Chin et al., 2006; Messina et al.,2006; Huang et al., 2007; Li et al., 2007a; Wang, 2007; Blaneyet al., 2007; Xu et al., 2007a,b). Nanoparticles, such as poly(amido-amine) dendrimers can serve as chelating agents, and can furtherenhance ultrafiltration of a variety of metal ions (Cu(II), Ag(I),Fe(III), and others) by attaching to functional groups, such as pri-mary amines, carboxylates, and hydroxymates (Diallo et al.,2005). Arsenite and arsenate have been precipitated in the sub-surface using zero-valent iron, making arsenic less available (Kanelet al., 2005). At this crux, the point to be investigated is the role of nanoparticles on adsorption to the pollutants in secondary pollu-tion, which is going to be another reality in the future (discussedlater).

Currently, about 150% of nitrogen required for plant uptake isapplied as fertilizer (Frink et al., 1999). Fertilizers and pesticidesthat incorporate nanotechnology may result in less agriculturaland lawn/garden runoff of nitrogen, phosphorous, and toxic sub-stances, which is potentially an important emerging applicationfor nanotechnology. These potential applications are still in theearly research stage (World Resources Institute, 2000; OECD,2001; USDA, 2003). Applications involving dispersive uses of nanomaterials in wastewater might have far reaching implicationson aquatic life and humans (Rao et al., 2004; Brar et al., 2009).

Nanotechnology does possess the potential to improve the envi-ronment, both through direct applications of nanomaterials to de-tect, prevent, and remove pollutants, as well as indirectly by usingnanotechnology to design cleaner industrial processes and create

environmentally responsible nano-products. However, in this pur-suit of efficient clean-up of the environmental compartments,


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some of these nanoparticles are being deliberately loaded into theenvironment also comprising wastewater and down the line, willend up in soils, sediments, water finally posing their way intothe flora and fauna via food chain. To begin with, it is imperativeto understand the physical regime of nanoparticles and inter-par-ticle interactions which govern their overall behavior in the waste-water treatment plants.

3.1. Physical regime of nanoparticles

The principal parameters of nanoparticles are their shape(including aspect ratios where appropriate), size, and the morpho-logical sub-structure. Nanoparticles are present as an aerosol(mostly, solid or liquid phase in air), a suspension (mostly, solidin liquids) or an emulsion (two liquid phases) (Anon, 2003). Inthe presence of chemical agents (surfactants), the surface andinterfacial properties can be modified. Such agents can indirectlystabilize against coagulation or aggregation by conserving particlecharge and by modifying the outmost layer of the particle (canhappen during various steps of wastewater treatment plants asthere is frequent use of coagulants and polyelectrolytes for solidsremoval). Depending on the growth history and the lifetime of ananoparticle, very complex compositions, possibly with complexmixtures of adsorbates (for instance, in this case, it can be waste-water sludge), are expected. Research suggests that carbon-basednanoparticles adsorb to materials commonly used in wastewatersystems. Duncan et al. (2008) reported that fullerenes can adsorbto natural organic matter. The natural organic matter caused disag-gregation of nC60 crystals and aggregates under typical solutionconditions of natural water, leading to significant changes in parti-cle size and morphology (Steven et al., 2001; Xie et al., 2008). Sucheffects magnified with increasing natural organic matter concen-tration and it may act as a carrier in the transport and toxicity of C60 in waste streams. Organic matter is commonly found indomestic wastewater in the form of suspended and dissolved sol-ids (humic and fulvic acids). Consumer products, and debris and

could also be expected to sorb on these nanoparticles. Another as-pect of nanoparticle fate in wastewater is that fullerenes have thepotential to sorb to organic contaminants, such as naphthalene(Cheng and Tomson, 2005). While naphthalene is commonlyknown as a gasoline contaminant and is unlikely to surface inwastewater, the finding nonetheless demonstrates that carbon-based nanoparticles could sorb to other contaminants which, if not removed during wastewater treatment, could be dischargedwith the effluent into the environment. Although no direct studieshave been reported on sorption of nanoparticles onto biofilms lin-ing pipe walls as well as basins within the treatment plant, stillprevious studies on particle entrapment in biofilms are a soundproof for the same. No data was also reported regarding the sorp-tion of other types of nanoparticles to infrastructure materials.

At the nanoparticle–liquid interface, polyelectrolytes have beenutilized to modify surface properties and the interactions betweenparticles and their environment. They have been used in a widerange of technologies, including adhesion, lubrication, stabiliza-tion, and controlled flocculation of colloidal dispersions (Liufuet al., 2004). Polyelectrolytes have been used for decades in waste-water treatment plants (Verma et al., 2006) which eventually endup in wastewater sludge and might suffer similar fate. Hence,nanoparticles in wastewater might encounter change in chargedispersion due to interaction with polyelectrolytes and thus con-tribute to a completely changed behavior.

At some point between the Angstrom level and the micrometrescale, the simple picture of a nanoparticle as a ball or dropletchanges. Both physical and chemical properties are derived from

atomic and molecular origin in a complex way. For example, theelectronic and optical properties and the chemical reactivity of

small clusters are completely different from the better knownproperty of each component in the bulk or at extended surfaces.This situation would also arise in wastewater treatment plantswhere the nanoparticles can be present in different forms. More-over, complex quantum mechanical models are required to predictthe evolution of such properties with particle size, and typicallyvery well defined conditions are needed to compare experimentaland theoretical predictions.

3.2. Do nanoparticle–nanoparticle interactions produce free

nanoparticles?

At the nanoscale, particle–particle interactions are either dom-inated by weak Van der Waals forces, stronger polar and electro-static interactions or covalent interactions. Particle aggregation isdetermined by the inter-particle interaction, depending on the vis-cosity and polarizability of the fluid. For nanoparticles in liquids,particle charge can be stabilized by electrochemical processes atsurfaces (Huber et al., 1996; Selvan et al., 1999). The nanoparti-cle–nanoparticle interaction forces and nanoparticle–fluid interac-tions play a key role in describing physical and chemical processes,

and the temporal evolution of free nanoparticles (will be the casein nanoparticle–wastewater interactions). However, they are diffi-cult to characterize due to the small amount of molecules actuallypresent on the surface active layer. Surface energy, charge and sol-vation are additional relevant parameters to be considered. Due tothe critical role of the nanoparticle–nanoparticle interaction andthe nanoparticle–fluid interaction, the term free nanoparticle canbe easily misunderstood. The interaction forces, either attractiveor repulsive, crucially determine the fate of individual and collec-tive nanoparticles (Gomoll et al., 2000; McManus et al., 2000). Thisinteraction between nanoparticles may form aggregates and/oragglomerates that influence the real behavior (Akane et al.,1990). In gas suspensions, aggregation is crucially determined bythe size and diffusion, and coagulation typically occurs faster than

in the liquid phase as the sticking coefficient is closer to unity thanin liquids. The former can occur in the case of digestion processeswhich are used for processing wastewater sludge so that the biogasthat is formed might contain the nanoparticle in the coagulatedform resulting in secondary pollution of air besides presence inwastewater.

3.3. Are particles desired in wastewater treatment plants?

Particles represent undesired pollutants in most of the processwaters. Apart from the mass of suspended matter which is oftenused as bulk parameter, many other quality indicators are stronglyassociated with particles, such as hygienic contaminants and ad-sorbed chemicals. On one hand, particles may negatively interfere

in various treatment processes and supply systems, while particu-late matter in the form of biomass is a necessary prerequisite in theunit operations involving secondary treatment steps (Wang et al.,2005). Nevertheless, the removal of particulate matter will beone of the most crucial steps in water and wastewater treatment.In order to understand the behavior of particles in water andwastewater and to develop and design efficient treatment facilities,the characteristics of particles have to be known on the basis of individual solids and whole particle populations. In wastewatertreatment, particles are of extremely heterogeneous nature withrespect to size, density, shape, chemical composition, shearstrength, surface charge, etc. which represents information thatis in most cases not precisely available. Fig. 3 represents the sche-matic of a typical wastewater treatment plant tracing the fate of

nanoparticles all the way from the source through the unit opera-tions and finally to the sink, aquatic streams.


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To begin with, how do these nanoparticles find their way intowastewater treatment plants? Source of nanomaterials principallyoriginate from the collection systems in municipalities where hugeamounts of nanoparticles are released daily as seen in Table 4. ThisTable represents per capita usage in European scenario, but this isalso a worldwide phenomenon. The presented data clearly men-tions the gravity of nanopollution that is being caused by the dailyactivities. The scenario will be more severe if industrial use data isappended to it. Engineered nanoparticles have been on the marketfor some time and are commonly used in cosmetics, sportinggoods, tires, stain resistant clothing, sunscreens, toothpaste, foodadditives, and others (Colvin, 2003; Product inventory, 2007).These nanomaterials, and new more deliberately fabricated nano-particles, such as carbon nanotubes, constitute a small minorityof environmental nanomaterials. In fact, the quantity of man-made

nanoparticles ranges from well-established multiton per year pro-duction of carbon black (for car tires) to microgram quantities of fluorescent quantum dots (markers in biological imaging) whichcan eventually end up in wastewater being fed to the wastewatertreatment plants.

4. Pertinence of nanoparticles in wastewater and wastewater sludge

Water desalination, wastewater reuse and water disinfectionhave been the principal segments of the markets with high growthand profit potentials. Nanotechnology will bring about a completetransformation in most of the technologies in the wastewater mar-kets in the future (Environment Industry Survey, 2006).

Fig. 3. Typical wastewater treatment plant and possible fate (removal processes) of nanoparticles in each of the unit operations: (1) Collection system: use of products suchas, cosmetics, fragrances, pharmaceuticals, electronic systems and other products which comprise nanoparticles; (2) and (3) bar screen and grit removal: some nanoparticlesmay be removed by the mechanism of adsorption onto large debris; (4) primary sedimentation system: via sedimentation and settling; (5) secondary treatment systems(fixed film or suspended growth system): via settling, interaction with organic matter, microbial interactions; (6) secondary sedimentation: via sedimentation andaggregation; (7) sludge thickener: concentration of nanoparticles; digester: via microbial interactions; organic matter interactions; sludge dewatering: via adsorption andaggregation; landfills: via adsorption, leaching leading to groundwater and sub-surface water contamination; (8) disinfection process: via oxidant interactions; (9) releaseinto the receiving waters; and (10) advanced tertiary treatment: via oxidation and adsorption.

Table 4

Usage scenarios for emission of different products comprising nanomaterials into wastewater treatment plants.

Product type Emission (g/pc/d) Comments References

Antiperspirant 0.35 Based on 50% population use of the product at dose = 0 .7 g/d Loretz et al., 2005, 2006Body lotion 1.2 20% population use of the product at dose = 6.0 g/d Loretz et al., 2005, 2006Body wash 0.32 Home chemical usage survey in Denmark Eriksson et al., 2003Cleaners 0.3 Home chemical usage survey in Denmark Eriksson et al., 2003Deodorants 0.08 Home chemical usage survey in Denmark Eriksson et al., 2003Face cream 1.64 20% population use of the product at dose = 8.2 g/d Loretz et al., 2005, 2006Hair conditioner 0.47 Home chemical usage survey in Denmark Eriksson et al., 2003Hair styling products 0.10 Home chemical usage survey in Denmark Eriksson et al., 2003Lime deposit removers 0.11 Home chemical usage survey in Denmark Eriksson et al., 2003Paint 0.09–0.36 ml/pc/yr Data on usage in the UK and release of 1–4% during cleaning European Commission, 2004Laundry detergents 10.1–20.5 Data for USA, Sweden, Denmark, Finland and Norway Eriksson et al., 2002Oral hygiene products 0.7 Survey of home chemical usage in Denmark Eriksson et al., 2003Perfume 0.05 Dose = 0.5 g/d and 10% of population use product Loretz et al., 2005, 2006Shampoo 1.83–6.30 Data for Sweden and Denmark Eriksson et al., 2002, 2003Shaving foam 0.07 Survey of home chemical usage in Denmark Eriksson et al., 2003Soap 2.5 Data for Sweden Eriksson et al., 2002Skin care products 1.3 Survey of home chemical usage in Denmark Eriksson et al., 2003Softeners 16.4 Date based on use in Europe Eriksson et al., 2002Sunscreen 3.0 Dose = 30 g/d and 10% of population use product Loretz et al., 2005, 2006Window cleaners 0.03 Survey of home chemical usage in Denmark Eriksson et al., 2003

pc refers to ‘‘per capita”.

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It is a well-known fact that the particles in wastewater play animportant role in all kinds of water quality and treatment issues. Inaddition to their smaller particle size, another factor that couldcomplicate the fate of nanoparticles in wastewater reclamation isthe functionalization of these particles for several applications (Sa-vage and Diallo, 2005). Functionalization is the process by whichorganic/surfactant molecules are attached to the nanoparticles tokeep them in dispersed state (Guo et al., 2006). As a result, nano-particles may behave very differently than the conventional, dis-solved contaminants as well as micron sized suspended particlesduring wastewater treatment.

Meanwhile, whatever ends in wastewater is bound to finallyreach the wastewater sludge, more so through the agglomerationand/or aggregation and settling mechanisms (Fig. 3) as discussedlater. Besides, wastewater sludge contamination by nanoparticlesis a completely neglected domain. Finally, the wastewater sludgeis spread on agricultural fields as biosolids raising issues on the po-tential leachability of nanoparticles into groundwater and sub-sur-face waters. In actuality, the amount of nanoparticles reaching soiland natural waters depends on the fraction of wastewater that iseffectively treated. In the subsequent sections, we track the path-way of the nanoparticles during various unit operations of waste-water treatment plants. The mechanisms operating in a typicalwastewater treatment plant unit operation are illustrated in Fig. 4.

4.1. Preliminary treatment – bar screens and grit removal

The predominant processes that will dictate the fate of nano-particles will be sorption to the debris and other large particlesleading to their removal. However, it is possible that the majorityof nanoparticles may find their way through wastewater to the pri-mary treatment process.

4.1.1. Primary treatment

Primary settling process is a solid–liquid separation process in-tended for removal of suspended, micron sized or larger inorganic

particles. Under laminar flow conditions, particle settling in sedi-mentation tank can be expressed by the Stoke’s Equation:

V s ¼ g =18l ðqs qÞ d2

ð1Þ

where V s = settling velocity; g = acceleration due to gravity;qs = mass density of the particle; q = mass density of water;

d = diameter of the particle (Crank, 1975). The gravitational forceon a particle is a function of mass and hence, particle size and den-sity. Likewise, buoyancy is a function of particle volume (size) andthe density of the liquid media (Bird et al., 1960). Drag is a functionof particle size, fluid viscosity, and particle velocity.

As indicated in the Stoke’s Equation, settling velocity of parti-cles is an exponential function of particle size. Accordingly, settlingvelocity of nanoparticles will be several orders of magnitudeslower and hence, settling time will be of the orders of magnitudehigher than micron/submicron sized particles of the same material.Hence, without the addition of coagulants and flocculants to en-hance the mean particle size, or without the adsorption of nano-particles to large inorganic particles, nanoparticles are unlikely tobe removed in the primary sedimentation tanks.

However, most of the primary treatment processes compriseextensive use of coagulants (AWWA, 2000) which may lead tothe adsorption of these nanoparticles and their further settling inwastewater sludge. The nanoparticles may be further subjectedto numerous possible mechanisms in the wastewater sludge asillustrated in Fig. 5.

In contrast to the soluble chemicals, particles can settle, diffuse,and aggregate differentially according to their size, density, and

surface physical chemistry. The rate of gravitational settling is sig-nificantly different across particle sizes and densities, although thesettling velocities of 100-nm particles and smaller particles can bequite low. In the case of nanoparticles, the gravitational effect isnegligible and might it become significant only when they are at-tached to other colloidal particles present in wastewater. Bindingto colloidal particles and/other microparticles can also alter thehydrodynamic diameter of nanoparticles, with correspondingchanges in particle density/buoyancy (Tirado-Miranda et al.,2003) and consequently settling rates. In fact, shape influencesgravitational settling through effects on drag and buoyancy.Though nanoparticles have many shapes, fractal agglomerates,cubes, and rods, for instance, most of them can be adequately rep-resented as spheres for the purpose of calculating gravitational set-

tling rates. As a very general rule, particles with aspect ratiosgreater than 2, carbon nanotubes, for example, cannot be repre-sented as spheres for settling calculations (Swaminatan et al.,2006), and alternative strategies should be used (Herzhaft andGuazzelli, 1999). Nevertheless, for convenience, nanoparticles canadsorb and agglomerate/conjugate with other particles present inwastewater leading to gravitational settling as discussed earlier.Limbach et al. (2008) performed a study on a model nanoparticle,cerium oxide compound in a model wastewater treatment plantand found that majority of the nanoparticles could be capturedthrough adhesion to clearing sludge, a significant fraction of theengineered nanoparticles escaped the wastewater plant’s clearingsystem, and up to 6 wt% of cerium oxide was found in the effluentstream. A detailed investigation on the agglomeration of oxide

Fig. 4. Hypothetical representation of mechanisms operating in a typical waste-

water treatment plant process step (different mechanisms show further concen-tration into wastewater sludge).

Fig. 5. Probable mechanisms of nanoparticle accumulation/degradation in waste-water sludge.


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nanoparticles in wastewater streams revealed a high stabilizationof the particles against clearance (adsorption on the bacteria fromthe sludge). Hence, there are strong chances of their accumulationinto sludge and easy passage into the downstream processes.

Agglomeration involves the adherence of single or cluster of particles into larger masses due to attractive forces or chemicalor mechanical binding (Maeakin, 1988). Irreversible agglomeratesof primary particles are called hard aggregates. Examples of thistype of agglomeration can be seen in electron micrographs of anumber of metal oxides (Limbach et al., 2005), C60 fullerenes(Fortner et al., 2005), and carbon nanotubes (Lisunova et al.,2006). Some nanoparticles have been manufactured as single par-ticles; example, amorphous silica and polystyrene beads. Eventu-ally, these particles can interact with each other to form soft(reversible) agglomerates if there is a net attractive pair potential.Many nanoparticles are in some degree of agglomeration in bothdry or solution forms (in this case, in the solution with differenttype of ions in wastewater). Efficient aggregation and proper set-tling of flocs is important for the generation of good-quality efflu-ent in the activated sludge process (Malik et al., 2003). The samemechanism is going to hold good for the nanoparticles which canform aggregates in the wastewater sludge. This fact was supportedby physical agglomeration of nanoparticles (on sludge bacteria),adhesion of sludge to gas bubbles, and occasional transfer of sludgeinto the aeration unit as reported in a recent model wastewatertreatment facility study (Limbach et al., 2008).

Agglomeration shifts the size class distribution of particles fromits initial state to one with a larger mean and in some cases, greaterdispersion of particle size distribution. Agglomerates have a highermass and volume than the individual particles they are composedof and have correspondingly higher gravitational and buoyantforces acting on them. Drag is also increased due to the higher vol-ume and non-spherical shape. Agglomerates are not solid particlesdue to spaces between individual packed particles (Sterling et al.,2005) and therefore have a lower density and surface area:mass ra-tio than the primary particles ( Johnson et al., 1996). The net effect

is that the settling rates for agglomerates are generally higher thanthe smaller primary particles (primary reason for settling in pri-mary sludge), but may be higher or lower than a comparably sizedsingle particle depending on the agglomerate shape and packingdensity.

Several factors influence the rate and extent of agglomeration:(a) particle concentration affects the rate and degree of agglomer-ation via particle-to-particle interactions; (b) zeta potential; (c)shape, and (d) hydrophobicity/hydrophilicity can also impactagglomeration rates by influencing repulsive or attractive (adhe-sive) properties. Fluid characteristics (van Oss et al., 1978) andthe extent and method of mixing also affect agglomeration (Lim-bach et al., 2005). The presence of polyelectrolytes on the particles(which is the case in primary treatment process due to addition of

coagulants) can create a steric repulsive force and reduce the netattractive interactions between particles, altering the agglomera-tion state. Microflocculation (perikinetic flocculation) will domi-nate in governing the aggregation of nanoparticles brought aboutby the random thermal motion of fluid molecules (Brownian mo-tion). Microflocculation has significant influence on transport of particles that are in the size range from 0.001 to 1 lm. Moreover,a suspension is a dispersion of solid particles in a liquid and nano-particles will form a colloidal suspension. In the suspension of large particles, for example 10 lm or larger, only hydrodynamicinteractions dominate the suspension flow properties and particlepacking behavior. However, in colloidal suspensions comprisingnanoparticles, inter-particle as well as hydrodynamic interactionsplay a key role in determining the flow and particle packing prop-

erties. Aggregation phenomenon is also affected by the presence of different ions in the solution (wastewaters have plenty of them, in

primary treatment, further aided by the commonly used coagu-lants, such as alum Fe3O4 that are added). Nanoparticles are likelyto undergo fast diffusive aggregation at higher ionic concentra-tions. In fact, fullerene nanoparticles have been reported to formbigger aggregate structures under high salt conditions, eventuallysettling out from the bulk phase of aquatic systems (Chen andElimelech, 2006).

The ionic strength of environmental systems may vary consid-erably, and this will also affect particle aggregation and thus trans-port. While aggregation of nanoparticles occurs over a wide rangeof conditions, the aggregates are mobile (Park et al., 2004). Thismust be taken into consideration for designing nanoparticle-basedenvironmental remediation systems, analyzing life cycles of nano-particles used in commercial products, and determining potentialexposure to nanoparticles for health and impact studies fromwastewater and wastewater sludge. Transport distances may belarge enough for particles to cross redox zones, to move to regionsof different solution chemistry or surface charge, and to cross tra-ditional barriers designed for contaminant removal. This scenariomay be prevalent in wastewater treatment plants while traversingfrom one unit operation to another.

Hence, the particle size and concentration dependence of agglomeration would alter the relationships between particle size,concentration, and settling for agglomerating nanoparticles. Theaggregation and adsorption state of nanoparticles imposes ques-tions on whether the nanoparticles will be truly mobile in thewastewater environment leading to further contamination of otherenvironmental compartments. Likewise, other important particlecharacteristics, such as zeta potential and mean hydrodynamicdiameter, which can directly or indirectly affect particle settlingshould be a subject of extensive investigation in order to determinethe fate of nanoparticles in wastewater treatment processes. Theanalytical methods have to be developed and strengthened in thisregard. The analysis will become more complicated when specific-ity of the nanoparticle has to be evaluated.

4.2. Secondary treatment process

4.2.1. Role of physical interactions

The secondary treatment processes comprise fixed filmand sus-pended growth systems and corresponding secondary sedimenta-tion systems. Both are aerobic systems involving entrainment of air for microbial degradation of the organic matter. The secondarytreatment process environment is populated by microorganismswhere there is a possibility that the nanoparticles will adhere tomicrobial cell surfaces or microbe-associated extracellular poly-meric substances. Such a film formation can have vital effects onmetabolic activity, for example, communication may be restrictedbetween the cell and its surrounding environment (‘‘quorum sens-ing ”). In fact, when polyelectrolyte-neutral block copolymers of

nanoparticles are mixed in solutions with oppositely charged spe-cies (e.g. surfactant micelles, macromolecules, proteins, etc.), thereis the formationof stable ‘‘supermicellar” aggregates thatarea com-bination of both components ( Jeong et al., 2003; Castelnovo, 2003;Berret et al., 2004; Burgh et al., 2004; Berret and Oberdisse, 2004).

Furthermore, the fixed film systems might be clogged by thepresence of nanoparticles which may occupy active sites in thefixed membrane. It is possible that in clogging conditions whichmay occur due to presence of these nanoparticles during transportin fixed film systems, nanoparticle aggregates may act differentlythan the solid particles. As the pores become clogged, the shearforces due to fluid flow would increase. The solid particles willnot be subject to breakage due to the shear forces, nanoparticleaggregates may break apart, and re-entrainment may be greater

than for solid particles as was observed in literature in the case of clay nano-composites (Oh et al., 2006). Furthermore, nanoparticle


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aggregates may be subject to shear forces that may break up aggre-gates and reform them with other nanoparticles or nanoparticleaggregates, an aspect that is not accounted for in current literature.In addition, the shear forces required for nanoparticle disaggrega-tion are generally not known and which will also influence theirtransport during secondary treatment.

4.2.2. Interaction with microorganismsAnother major interaction that the nanoparticles will experi-

ence during secondary treatment process is the interaction withthe microorganisms present. The processes, media properties,and characteristics of particles that propel transport of nanoparti-cles to microorganisms comprise: diffusion, gravitational settling,and agglomeration. Agglomeration and gravitational settling havebeen already discussed in greater details. Hence, diffusion will beexplored in further details. Diffusion is the spontaneous, passivemovement of particles from high chemical potential to low chem-ical potential. There is no net transport by diffusion in systems atequilibrium (Kikuo et al., 2006). Particles in microbial systemsare not necessarily in equilibrium. Particles adhere to microorgan-isms or are taken up by microorganisms, creating a concentration

gradient in the unstirred layer immediately above the microbialcells. This gradient drives diffusional transport of particles. Ratesof diffusional transport are a function of particle size and the vis-cosity of the media (in this case, wastewater or wastewatersludge). Smaller particles diffuse more rapidly than the large parti-cles which will be the likely fate of nanoparticles. However, nor-mally other media and particle characteristics (for instance,charge and surface chemistry) do not affect diffusional transportto a great extent. Diffusional transport can be estimated from thediffusion coefficient (D, cm2/s) as given in Eq. (2):

D ¼ RT

N 6pld ð2Þ

where R = gas constant (8.314 J/K/mol), T = temperature (K),

N = Avogadro’s number, l = solution viscosity (kg/m/s), and d = par-ticle diameter (m). Additionally, D is an inverse function of particlesize and is not a function of particle density (Einstein, 1905; Bergq-vist et al., 1987). As the diffusion coefficient is inversely related toparticle size, delivery by diffusional transport is less important forlarger particles (above 100 nm); the time for a 1000-nm particleto diffuse 1 cm is 3 years compared with only 1 day for 1-nm par-ticles. Larger particles will be transported principally by gravita-tional settling, thus making diffusion a principal transportphenomenon in secondary treatment.

No systematic studies have been performed to date to evaluate:(i) removal of nanoparticles in secondary treatment processes, andmore so, (ii) impact of nanoparticles to the biomass. Only a few iso-lated studies are found on the subject which lead to some cohesive

conclusions. Zero-valent iron nanoparticles which find wide appli-cation in removal of various pollutants in wastewater treatmentplants was found to have a strong bactericidal effect under de-aer-ated conditions (Lee et al., 2008a). On the contrary, air saturationrequired much higher nano-Fe0 doses due to the corrosion and sur-face oxidation of nano-Fe0 by dissolved oxygen. Hence, these nano-particles will undergo inactivation in the secondary treatmentprocesses and if escaped can cause inhibition downstream duringanaerobic digestion of wastewater sludge. Ivanovet al. (2004)eval-uated retention of nano/micro latex beads (0.1, 0.6 and 4.2lm)and cells of Escherichia coli by microbial self-aggregated granulesin wastewater treatment plants. Microbial granules are sphericalbiofilm structures where microbes are attached to each other andembedded in an extracellular matrix (Ivanov et al., 2004). Approx-

imately 10% of 0.1 and 0.6 lm particles were removed by the bio-film after 60 min of incubation. Furthermore, these particles

penetrated only to the top 250–300 lm from the edge of the gran-ule. Cells of E. coli on the other hand penetrated to approximately800lm of the biofilm. Results from these studies indicated thatsome nanoparticles can be removed by adsorption on activatedsludge. However, more studies are required to evaluate the extentof nanoparticles adsorption to activated sludge, their removal insecondary clarifiers as well as their fate during sludge digestionprocesses. Some inferences in this context canbe drawn from asep-tic culture studies.

Lyon et al. (2005) and Fortner et al. (2005)evaluated the toxicityof nanoscale carbon fullerene (nano-C60) to two facultative nitratereducing soil bacteria, E. coli (gram negative) and Bacillus subtilis(gram positive). Nano-C60 is unique type of carbon nanoparticlesthatare candidates fora variety of applications includingdrug deliv-ery and energy conversion (Tufenkji and Elimelech, 2004). Nano-C60, at 0.4 mg/l concentration, inhibited the growth of both themicroorganisms when grown in a basic media. Furthermore, when4 mg/l of nano-C60 were added to these bacterial cells during theirlog growth phase, their respiration rate significantly slowed down.Although these studies were performed using soil microorganisms,results suggested that nanoparticles have the potential to inhibitactivatedsludgeprocess duringwater reclamation. Likewise, a studycarried out by An et al. (2007) demonstrated that the growth of microorganisms was significantly hindered by the silver nanoparti-clecoating.The potential fornanosilver to adversely affect beneficialbacteria in the environment, especially in soil andwater is of partic-ularconcern. In fact,nanosilverfinds useinthelining of refrigeratorsand washing machines which can provide an active source for con-tamination. There is also a risk that the use of silver nanoparticles(‘‘nanosilver”) will lead to the development of antibiotic resistanceamong harmful bacteria (Silver, 2003; Press release, 2006; SamsungSilver Nano Health System, 2007). This would then require the sep-arationofnanosilverfromeffluentduringthesewagetreatmentpro-cess because of the danger that nanosilver would adversely affectbeneficial bacteriain generalandsoil bacteriain particular (digestedsludge is marketed as an agricultural fertilizer) (Oberdörster et al.,

2005). Additionally, unextracted nanosilver could pollute the sea,riversand lakes,poisoning a variety of waterorganisms. This in turnwill greatly increase silver concentrations in treatment-plant dis-charges, leading to adverse effects, such as bioaccumulation in fishand killing of aquatic life (Tang et al., 2004; Lubick, 2008). Further-more, thereis a possibility that nanoparticles and persistent organicpollutants and other hazardous metals may form associations andspread together, thereby amplifying their toxicity. Fullerene whenphotosensitizedin thedisinfectionprocessesof a typicalwastewatertreatment plant has also been found to enhance viral inactivationrates through the generation of superoxide and singlet oxygen(Badireddy et al., 2007). A study in which the researchers dosedthe high concentration of 50,000 mg of C60 to 1 kg of sludge didnot affect themicrobeschosenin thethree domainsof a genetic tree-

bacteria, archaea and eukaryotes (Nyberg et al., 2008). Activity wasassessed by monitoring production of CO2 and CH4. Findings sug-gested that C60fullerenes hadno significant effecton the anaerobiccommunity over an exposure period of a fewmonths. However, theslow growing nitrifying bacteria community is greatly affected bynanoparticles. Size-dependent inhibition by Ag nanoparticles andevaluation of the relationship between the inhibition and reactiveoxygen species demonstrated toxicity (Choi and Hu, 2008), thusquestioningtheir releaseinto thewastewaterstreams.Furthermore,electron micrographs of wastewater microbial community illus-trated attachment of Ag nanoparticles to the microbial cells, proba-blycausingcell wall pitting (Choi et al., 2008). The results suggestedthat nitrifying bacteria were especially susceptible to inhibition byAg nanoparticles, and the accumulation of Ag nanoparticles could

have detrimental effects on the microorganisms in wastewatertreatment.


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However, research is incomplete on the effects of nanoparticleson other phyla such as, invertebrates and vertebrates from terres-trial and aquatic habitats. Since nanoparticles from both domesticand industrial products will be released into the environment, e.g.wastewater, it is essential to investigate the impact on such speciesand further downstream on the ecosystem.

In fact, microorganisms and nanoparticles can develop symbi-otic or similar synergistic relationships under specific environmen-tal conditions. Microorganisms also have the potential to beinfluential catalytic agents. They can alter the oxidation state of numerous elements (NSF, 2004). They can promote the removalof specific molecules containing nanoparticles. Microorganismscan also catalyze reactions that can lead to nanoparticle aggrega-tion or molecule conjugation aiding in the removal. Thus, on onehand, the nanoparticles have an adverse impact on the microbialaction and on the other hand, they synergize with microorganismsto assist in auto-aggregation and finally removal. Similar types of mechanisms are also possible in wastewater sludge which is awarehouse of diverse microbial systems and may result in numer-ous mechanisms – complexation, aggregation, among others(Fig. 5).

In a typical wastewater treatment process, the nanoparticlescan adsorb to other nanoparticles and or macro and micropollu-tants leading to agglomerates which can form complexes (stableor unstable) with extracellular polymeric substances resulting inincreased or decreased reactivity as demonstrated in Fig. 4. Biolog-ical uptake is a potential possibility for the microbial biomass pres-ent in the secondary treatment process, resulting in eitherinhibition of the process or enhanced sorption/transformation of the nanoparticles. The nanoparticles form aggregates which willeither deposit in the wastewater sludge and depending on the pro-cess conditions can undergo desorption and or complexation/ioni-zation. This is a grave concern to any wastewater treatment facilityas biological treatment is the backbone of the facility, and withoutit significant reduction in contaminant loads would not be achiev-able. Presence of microorganisms in secondary treatment pro-

cesses and eventual release of microbial exudates, such aspolysaccharides, among others may fuel the possibility of biodeg-radation of these nanoparticles.

4.2.3. Biodegradation processes

Biodegradation of nanoparticles may result in their breakdownas typically seen in biodegradation of organic molecules, or may re-sult in changes in the physical structure or surface characteristicsof the material. The potential and possible mechanisms of biodeg-radation of nanosized particles have just begun to be investigated.As is the case for other fate processes, potential for biodegradationwill strongly depend on the physical and chemical nature of theparticle. Nanomaterials are composed of inherently non-biode-gradable inorganic chemicals, such as ceramics, metals and metal

oxides, and are not expected to biodegrade. However, a recent pre-liminary study found that C60 and C70 fullerenes were taken up bywood decay fungi after 12 weeks, suggesting that fullerene carbonhad been metabolized (Filley et al., 2005). For other nanomaterials,biodegradability may be integral to the material’s design and func-tion. This is the case for some biodegradable polymers being inves-tigated for use in drug transport (Madan et al., 1997; Brzoska et al.,2004), for which biodegradability is mostly a function of chemicalstructure and not particle size.

Biodegradability in waste treatment and the environment maybe influenced by a variety of factors. Recent laboratory studies onC60 fullerenes have indicated the development of stable colloidstructures in water that demonstrate toxicity to bacteria underaerobic and anaerobic conditions (CBEN, 2005; Fortner et al.,

2005). Further studies are needed to determine whether fullerenesmay be toxic to microorganisms under environmental conditions.

One must also consider the potential of photoreactions and otherabiotic processes to alter the bioavailability and thus biodegrada-tion rates of nanoparticles. In summary, not enough is known toenable precise predictions on the biodegradation of nanomaterialsin the wastewater environment and further testing and researchare needed. A clear focus and comprehensive studies in the samecontext might throw light on some of the unanswered issues.

4.2.4. Interaction with other pollutants

Nanoparticles such as, C60 have unique properties compared tolarger particles, and so they may have a very different effect on thetoxicity and availability of pollutants. Researchers from the Techni-cal University of Denmark and the University of Copenhagen, Den-mark tested the effect of four common pollutant chemicals:atrazine, methyl parathion, pentachlorophenol and phenanthreneon green algae and freshwater crustaceans (Baun et al., 2007). Theyfound that the presence of C60 nanoparticles affected the availabil-ity of the toxic chemicals to the organisms. C60 increased the tox-icity of phenanthrene to algae at lower concentrations, forinstance, but decreased the toxicity against crustaceans. Likewise,C60 made pentachlorophenol less toxic to both algae and crusta-ceans. C60 had little effect on the toxicity of the other two pollu-tants tested. Nanoparticles also affected the rate and quantity of the pollutant taken in by the organisms. Clumps of C60 itself alsogot stuck to the crustacean bodies and inside their digestive sys-tems. Nevertheless, microorganisms and the extracellular materialassociated with them are thought to play key roles in determiningthe nanoparticle chemical speciation and its mobility in the envi-ronment (Kemner et al., 2004). This study is the first of its kindto demonstrate the influence of C60-aggregates on aquatic toxicityand bioaccumulation of other environmentally relevantcontaminants.

Hence, nanopollution in wastewater treatment plants must takeinto account not just the toxicity of the particles themselves, butalso the possible interaction with other environmental contami-nants. Moreover, further research into the effect of nanoparticles’

different phases (in particular their behavior in wastewater as theyform suspensions or clumps of molecules known as aggregates) isalso relevant for their potential toxicity in the wastewater treat-ment plants and further the aquatic environment.

Colloids are known to control to a large extent, the transport of pollutants in various environmental compartments. For example,the colloidal pumping model (Honeyman and Santschi, 1992) cangive a clear explanation for the fate of pollutants through the envi-ronment: pollutants are hypothesized to bind preferentially tosmall colloids which eventually aggregate into particles that arelarge enough to settle. In groundwater and other porous media,colloid-facilitated transport may be important in increasing dis-tances travelled by pollutants and pathogens with respect to thosepredicted for non-colloid bound components. The same hypothesis

can be extended to wastewater treatment plants when they passfrom one unit operation to other experiencing different hydrody-namic regimes as they form colloidal suspensions.

Nanoparticles of CeO2 (a strong oxidant), have recently beenshown to both decarboxylate and polymerize some small organicmolecules (Cervini-Silva et al., 2005). The environmental releaseof CeO2 into the wastewater treatment plants may therefore poten-tially impact carbon chemistry and microbial interactions. Like-wise, nanoparticulate oxides, such as TiO2, used to degradepollutants and as a disinfectant (Serpone and Khairutdinov,1997; Kuhn et al., 2003; Rincon and Pulgarin, 2003), may havethe potential to induce other chemical transformations and impactphotochemical reactions in the gaseous phase of wastewater treat-ment plants which needs to be investigated.

The main models for understanding the uptake and bioavail-ability of trace metals by organisms are the equilibrium-based free


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ion activity and biotic ligand models (Campbell, 1995; Paquinet al., 2002; Slaveykova and Wilkinson, 2005). While hydrophobicorganic compounds (Galle et al., 2005) uptake is often reduced inthe presence of colloids, the general mechanisms upon which thisinhibition is based are unclear. A reduction in the diffusive masstransport from the solution to microbial cell surface (the larger col-loids are effectively prevented from having access to the microbialcell because of slow diffusion) anda reduced chemical ability of thecolloid–pollutant complex (as compared to the free metal orinorganic complexes) may influence bio-uptake, if these processesbecome rate-limiting (Wilkinson and Buffle, 2004; Li et al., 2007b).This can also affect the microbial removal of nanoparticles inwastewater treatment processes.

The binding of nanoparticles to organic matter, metals, andother contaminants could also have other undesirable conse-quences. For example, nanoparticle concentration in wastewatersludge is also a possibility. Sludge is the result of accumulationof solids removed during clarification. If nanoparticles were boundwith the solids, they would presumably be settled out and concen-trated in the sludge during clarification.

In the other steps of wastewater treatment plant namely, disin-fection, anaerobic digestion, the same processes as discussed ear-lier will predominate. Thus, a clear understanding of theupstream processes will lend a helping hand to understand therespective downstream processes. Finally, when the digesteddewatered sludge will be sent to landfills and as biosolids for agri-cultural application, leachability of nanoparticles is a possibility.Thus, the mechanisms that govern the transport and fate of nano-particles in water and soil andinteraction with plants will predom-inate as discussed henceforth.

5. Transport during land application and/or landfilling of

wastewater sludge

5.1. Fate in soil

The fate of nanoparticles released to soil is likely to varydepending upon the physical and chemical characteristics of thenanoparticle. Nanoparticles released to soil can be strongly boundrendering them immobile. On the other hand, nanoparticles aresmall enough to fit into smaller spaces between soil particles,and might therefore travel further than larger the particles beforebecoming trapped in the soil matrix. The strength of the sorptionof any intentionally produced nanoparticle to soil will be dependenton its size, chemistry, applied particle surface treatment, and theapplication conditions. Studies have demonstrated the differencesin mobility of a variety of insoluble nanosized materials in a porousmedium (Zhang, 2003; Lecoanet and Wiesner, 2004; Lecoanetet al., 2004). Humic substances, common constituents of natural

particles, are known to photosensitize a variety of organic photore-actions on soil and other natural surfaces that are exposed to sun-light. Some organic compounds tend to sorb to soils mainly viainteractions with soil organic matter.

Many reports have shown a positive correlation between theorganic C content and the sorption potential (Chefetz et al., 2000;Ahmad et al., 2001; Gunasekara and Xing, 2003). Therefore, soil or-ganic matter is considered to significantly affect the fate of organiccompounds in the environment. In addition to the amount of soilorganic matter, the sorption affinity, as well as the desorption po-tential of organic compounds have been reported to be controlledby the nature and chemical properties of soil organic matter andkinetics of entrapment of sorbate, in this case, nanoparticles.Meanwhile, the soil organic matter is very similar to organic mat-

ter of sludge. Suspensions of aggregated C60 fullerenes, when ap-plied to soil at varying concentrations showed that the microbial

biomass and respiration rate (an indication of the activity of soilmicroorganisms) were unaffected by nanoparticle treatments( Johansen et al., 2008). Soil protozoans, such as amoeba, wereslightly sensitive, but fast-growing bacteria decreased up to 4-foldin number. Thus, interactions between microorganisms in the soilecosystem are very complicated and the impact of fullerenes onfast-growing bacteria may affect the balance of these interactionsand in turn the overall health and function of the soil.

Generally, nanoparticles interact with microorganisms presentin soil and groundwater through passive and active mechanismsthat alter the chemical form and hence the groundwater transportand soil retention characteristics of the nanoparticles. This willultimately affect the human exposure route and toxicity. In astudy, gold nanoparticles functionalized with citrate were studiedfor their mechanism of interaction with the soil microorganisms,Pseudomonas fluorescens (aerobic) and Clostridium sp. (anaerobic).Changes in the nanoparticle surface chemical functionality andaggregation behavior in water were studied after exposure togrowing and resting bacterial cells (Fitts et al., 2006). Gold nano-particle surface plasmon resonance indicated modification of thecitrate functionality and aggregation. Soft X-ray scanning trans-mission spectromicroscopy revealed that nanoparticles were at-tracted to the bacterial cell surface; nanoparticle–cellinteractions were also studied by transmission electron micros-copy. These batch studies can also explain interactions with bio-films where nanoparticles can undergo redox reactions due tothe presence of three zones, namely, aerobic, anoxic and anaerobic.Furthermore, this will give a clear understanding of the predomi-nant mechanisms of biotransformation in order to predict fateand transport of nanoparticles in the wastewater treatment plants.

Studies of nanoparticle transformations in field situations arefurther complicated by the presence of naturally occurring nano-particles of similar molecular structures and size ranges. Iron oxi-des are one example (Kamat and Meisel, 2003; Manceau et al.,2008). Such kind of sorption and further mobility is also a possibil-ity with nanoparticles as reported recently for aluminum nanopar-

ticles (Darlington et al., 2009). Many factors influence the transportof nanoparticles in the environment. Size, charge, and the agglom-eration rate of nanoparticles in the transport medium are indica-tors of nanoparticle mobility in soil which is possible on landapplication of wastewater sludge based biosolids.

5.2. Fate in water

Fate of nanomaterials in aqueous environments is controlled bysolubility or dispersability, interactions between the nanomaterialand natural and anthropogenic chemicals in the system, and bio-logical and abiotic processes. Waterborne nanoparticles generallysettle more slowly than the larger particles. However, due to theirhigh surface-area-to-mass ratios, nanosized particles have the po-

tential to sorb to soil and sediment particles (Oberdörster et al.,2005a). The sorbed nanoparticles can be more readily removedfrom the water column. Some nanoparticles will be subject to bio-tic and abiotic degradation resulting in removal from the watercolumn. Complexation by natural organic materials, such as hum-colloids can facilitate reactions that transform metals in anaerobicsediments (Nurmi et al., 2005).

For example, researchers at Rice University have reported thatalthough C60 fullerene is initially insoluble in water, it spontane-ously forms aqueous colloids containing nanocrystalline aggre-gates. Currently, processes that control transport and removal of nanoparticles in water and wastewater are being studied to under-stand nanoparticle fate (Moore, 2006; Wiesner et al., 2006).

Likewise, when the wastewater sludge as ‘‘biosolids”, loaded

with these nanoparticles will be applied to agricultural soils, thereis a likelihood of leaching through soils, further entraining through


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the aquifer and affecting plant growth. There are contradicting andscarce reports on this subject which involve over- or under-esti-mated concentrations and hence, data.

5.3. Interactions with plants

Only a few studies are available on the effects of nanoparticles

on higher plants. The majority of the reported studies point to thepositive impacts of nanoparticles on plant growth with a few iso-lated studies pertaining to negative effect. Numerous studies havedemonstrated that TiO2 nanoparticles promoted photosynthesisand nitrogen metabolism, and thus greatly improved growth of spinach at a concentration as low as 20 mg/L (Hong et al.,2005a,b; Yang et al., 2006; Zheng et al.,2005; Lei et al., 2005). Itwas also pointed out that a mixture of nanoscale SiO2 (nano-SiO2) and TiO2 (nano-TiO2) could increase nitrate reductase insoybean (Glycine max), enhanced its abilities of absorption and uti-lization of water and fertilizer, stimulated its antioxidant system,and apparently hastened its germination and growth (Lu et al.,2002). However, after investigating the phytotoxicity of nanoscalealumina (nano-Al2O3) powders with or without phenanthrene

coating, Yang and Watts (2005) concluded that uncoated aluminaparticles inhibited root elongation of corn, cucumber, soybean,cabbage and carrot. This study triggered the claim that nanoparti-cles can exert a negative effect on plants (Murashov, 2006). But, theauthors did not identify dissolution of nano-Al2O3 in solution, andthus, failed to clarify if the phytotoxicity was from nano-Al2O3 oraluminum ion in the aqueous solution.

Another study by Lin and Xing (2007, 2008) investigated phyto-toxicology of nanoparticles (multi-walled carbon nanotube, alumi-num, alumina, zinc, and zinc oxide) on seed germination and rootgrowth of six higher plant species (radish, rape, ryegrass, lettuce,corn, and cucumber). Seed germination was not affected exceptfor the inhibition of nanoscale zinc (nano-Zn) on ryegrass and zincoxide (nano-ZnO) on corn at 2000 mg/L. Inhibition on root growthvaried greatly among nanoparticles and plants. Similar study byCanas et al. (2008) investigated the effects of functionalized andnon-functionalized single-walled carbon nanotubes on root elon-gation of six crop species (cabbage, carrot, cucumber, lettuce,onion, and tomato). Non-functionalized carbon nanotubes inhib-ited root length more than functionalized nanotubes. Microscopyimages showed the presence of nanotube sheets on the root sur-faces, but no visible uptake was observed. In another study, Cunanoparticles were toxic to Phaseolus radiatus (mung bean) andTriticum aestivum (wheat) and also were bioavailable (Lee et al.,2008b). A cupric ion released from Cu nanoparticles had negligibleeffects in the concentration ranges of the present study, and theapparent toxicity clearly resulted from Cu nanoparticles. Similarresults were reported with the application of silica, palladium, goldand copper nanoparticles on a soil microbial community and thegermination of lettuce seeds (Shah and Belozerova, 2009).

At this juncture, two schools of thought on positive and nega-tive impacts of nanoparticles on plants are evident. Thus, no con-crete conclusion can be drawn with respect to the nanoparticlesloaded in biosolids applied potentially to these soils and it repre-sents a big lacuna in this field. Moreover, studies reported so farpertain to virgin nanoparticles whereas biosolids will be a mixtureof unchanged and transformed nanoparticles.

6. Future outlook

The discharge of nanoparticles from industrial waste or disposalof such materials from commercial and/or domestic use will inev-

itably occur with increasing production and enter into wastewatertreatment facilities with unknown consequences. The fate of nano-

sized particles in wastewater treatment plants is not well investi-gated at this stage. However, it can be expected that bar screeningand other mechanical treatment methods will be ineffective atremoving any nanoparticles. Wastewater treatment operationswith the highest potential for removing nanoparticles from waste-water are the primary and secondary sedimentation tanks. Never-theless, nanoparticle removal potential will rely on the specificcharacteristics of the nanoparticles. The nanoparticle removal willbe facilitated through: (1) binding with organic matter which isultimately settled out; (2) natural aggregation with one anotherthus improving settling; (3) binding with organic contaminantsand; (4) adherence to selective surfaces. The ability of either of these processes to immobilize or modify the particles will dependon the chemical and physical nature of the particle and the resi-dence times in relevant compartments of the treatment plant.Sorption, agglomeration and mobility of the nanoparticles will bestrongly dictated by pH. Settling of nanoparticles, however, couldbe enhanced by entrapment in the much larger sludge flocs.

Sludges have the potential to contain high concentrations of nanoparticles due to removal of water in a sludge thickening anddewatering process. In addition, contaminants bound to thesenanoparticles (such as heavy metals) could also be present in driedsludge. No studies were found so far that evaluated the potentialfor nanoparticles in dried sludge to become airborne when thematerial was agitated or transferred for disposal. This would alsobe an occupational hazard issue. During biological treatment, somenanoparticles have been found to inhibit or even prevent biologicalactivity. A reduction of biological activity by toxic nanoparticlescould decrease the contaminant removal effectiveness of the entirefacility failing effluent discharge limits. A total failure of the biolog-ical process could be experienced in the worst case. Furthermore,nanoparticles could bind to piping, equipment, basin surfaces, bedifficult to remove, and require the complete shutdown and sani-tization of the infrastructure affected before treatment could bereinitiated. Unfortunately, limited research has been conductedthat would enable further conclusions on the fate of nanoparticles

on surfaces. Nanoparticle fate in sludge sent to landfills or inciner-ators must be evaluated. Major focus should be to understandwhether or not the nanoparticle is destroyed during sludge stabil-ization, incineration, and if nanoparticles could desorb and enterlandfill leachate.

There is a need to evaluate whether or not nanoparticles exhibittoxicity to wastewater and more specifically wastewater sludge. Itstill needs to be determined whether or not the bacteria in the acti-vated sludge is protected significantly enough by the extracellularpolymeric substances and whether or not there is toxicity in theform of respiration inhibition when exposed to nanoparticles.While the mechanisms of particle transport and the impact of par-ticle size during wastewater treatment are well studied for micron-sized pollutants, very little information is currently available on

the fate of nanoscale materials in treatment processes and relevantstudies are still at natal stage.

In general, particles at nanoscale behave differently than mi-cron-suspended as well as totally dissolved chemicals in wastewa-ter treatment processes. As a result: (i) increased release of nanoparticles may affect the existing treatment processes, and/or(ii) the nanoparticles may not be removed at the same efficiencyas their conventional counterparts in the treatment process. In fu-ture, wastewater utilities as well as industries manufacturingnanomaterials and those incorporating nanomaterials in theirproducts and equipment may have to include new treatment unitsor alter their existing operations to remove nanoparticles from thewastewater streams.

As the characteristic properties of nanomaterials are caused by

their high size surface to mass ratio, the aggregation behavior inthe environment is essential to study in detail. It could be


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estimated that under certain conditions in the wastewater envi-ronment the particles tend to aggregate and therefore transportin the environment is restricted e.g. sedimentation processes(Degushi et al., 2001; Brant et al., 2005; Hyung et al., 2007). Param-eters, such as pH, anions and cations (ion type and concentration)and humic acids may influence the surface properties and hencechemical reaction of engineered nanoparticles. Thus, fate of nano-particles in wastewater treatment plants is still not deciphered andneeds in particular, more detailed efforts for the fate in wastewatersludge as it can have a cascading impact on the ecosystem. The dis-position of nanoparticles following wastewater treatment willdetermine their subsequent fate and transport pathways in twoways via: (1) sewage sludge following land application and incin-eration and; (2) discharge water affecting aquatic organisms andecosystems.

7. Conclusions

The increasing use of nanoparticles in everyday products in-creases the potential for their release in water sources and waste-water streams. The release of nanoparticles into the wastewater

streams may have the following implications: (a) when presentin trace levels (lg/l and/or ng/l), based on toxicity study data, somenanoparticles may potentially be added to the list of componentsto be removed prior to water recharge/reuse applications and;(b) when present in higher concentration (mg/l), the nanoparticlesmay impact the performance of waste treatment processes by var-ious mechanisms, including inhibition of microorganisms in sec-ondary treatment process, increasing the turbidity, fouling of membranes or affecting the efficiency of disinfection processes.No methodical studies have been performed till date to evaluatethe presence and removal of nanoparticles during various waste-water treatment processes and consequent presence in wastewatersludge which is the ultimate sink. All cited studies were conductedwith well characterized, ‘‘virgin” materials. However, bacteria in

biological treatment processes are likely be exposed to weatherednanoparticles that have undergone agglomeration and transforma-tion, including loss or acquisition of coatings and impurities thatchange surface chemistry, reactivity, bioavailability and toxicitywhich needs to be addressed. There is a possibility that these nano-particles agglomerate or even get adsorbed to the extracellularpolymers during primary and secondary treatment eventually end-ing up in wastewater sludge. Unfortunately, there will still bemany unanswered questions regarding nanoparticle fate and im-pact on wastewater facility and treatment operations. Another per-tinent issue that needs to be addressed is whether thenanoparticles in wastewater are a significant environmental riskwhen we are still tackling with other traditional pollutants, suchas heavy metals and polycyclic aromatic hydrocarbons.

Acknowledgements

The authors are sincerely thankful to the Natural Sciences andEngineering Research Council of Canada (Discovery Grants A4984and 355254, STP235071, Canada Research Chair) and INRS-ETEfor financial support. The views or opinions expressed in this arti-cle are those of the authors and should not be construed as opin-ions of the US Environmental Protection Agency. We extend ourspecial thanks to Killam Foundation for granting Killam post-doc-toral fellowship to Dr. Mausam Verma.

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