sources, distribution, environmental fate, and ecological ...1chemical safety division, department...

Critical Reviews in Environmental Science and Technology, 45:277–318, 2015Copyright © Taylor & Francis Group, LLCISSN: 1064-3389 print / 1547-6537 onlineDOI: 10.1080/10643389.2013.852407

Sources, Distribution, Environmental Fate,and Ecological Effects of Nanomaterials

in Wastewater Streams

ANITHA KUNHIKRISHNAN,1 HO KYONG SHON,2,3

NANTHI S. BOLAN,3,4 IBRAHIM EL SALIBY,2

and SARAVANAMUTHU VIGNESWARAN2

1Chemical Safety Division, Department of Agro-Food Safety, National Academy ofAgricultural Science, Wanju-gun, Jeollabuk-do, Republic of Korea

2School of Civil and Environmental Engineering, University of Technology, Sydney, Australia3Cooperative Research Centre for Contamination Assessment and Remediation of the

Environment, Adelaide, Australia4Centre for Environmental Risk Assessment and Remediation, University of South Australia,

Mawson Lakes, Australia

Engineered nanomaterials (ENM) are manufactured, as opposed tobeing an incidental by-product of combustion or a natural process,and they often have unique or novel properties that emerge fromtheir small size. These materials are being used in an expanding ar-ray of consumer products and, like all technological developments,have both benefits and risks. As the use of ENM in consumer prod-ucts becomes more common, the amount of these nanomaterialsentering wastewater stream increases. Estimates of nanomaterialsproduction are in the range of 500 and 50,000 tons per year forsilver and titanium dioxide (TiO2) alone, respectively. Nanoma-terials enter the wastewater stream during the production, usage,and disposal of nanomaterial-containing products. The predictedvalues of nanomaterials range from 0.003 (fullerenes) to 21 ngL−1 (nano-TiO2) for surface waters, and from 4 ng L−1 (fullerenes)to 4 µg L−1 (nano-TiO2) for sewage treatment effluents. Therefore,investigating the fate of nanomaterials in wastewater streams iscritical for risk assessment and pollution control. The authors aim

Address correspondence to Nanthi S. Bolan, Cooperative Research Centre for Contam-ination Assessment and Remediation of the Environment, Adelaide, Australia 5095. E-mail:[email protected]

Color versions of one or more of the figures in the article can be found online atwww.tandfonline.com/best.

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278 A. Kunhikrishnan et al.

first to identify the sources of nanomaterials reaching wastewaterstreams, then determine their occurrence and distribution, and fi-nally discuss their fate in relation to human and ecological health,and environmental impact.

KEY WORDS: nanomaterials, wastewater, ecotoxicity, environ-mental fate, aquatic system, management

1. INTRODUCTION

Nanomaterials are becoming more common in our daily used products. Anextensive number of consumer end-products are being manufactured usingengineered nanomaterials (ENM). According to the Global Industry Analysts’recent report (GIA, 2010), the global market for nanomaterials will reachUS$6.2 billion by 2015. The United States, Western Europe and Japan willremain the largest markets, while demand in China continues to grow expo-nentially (Table 1). It is forecasted that the global nanomaterials demand willrise by 21% annually through 2013 (World Nanomaterials, 2010). Cosmetics,medicine, food and food packaging, paints, and coatings are some examplesof the widespread use of ENM.

As the use of ENM in consumer products becomes more common, theamount of these nanomaterials entering wastewater stream increases. Forexample, titanium dioxide (TiO2) and silver (Ag) nanoparticles are increas-ingly used in commercial products and have a high likelihood of enteringmunicipal wastewater treatment plants (WWTPs; Shon et al., 2007; Kim et al.,2011). Effluents from these treatment plants flow into rivers and lakes wherenanoparticles may pose an ecological risk. Therefore investigating the fateof nanomaterials in wastewater streams is critical for risk assessment andpollution control.

Figure 1 shows the flowchart of nanomaterials research in wastewater.The scientific approach to investigate the nanomaterials behavior cannot beachieved before carrying out an extensive review of the scientific literaturewith particular attention given to the following issues: (a) production andsynthesis of engineered nanoparticles; (b) transport, detection, and fate inwater and wastewater; and (c) impact on ecosystems, biotoxicity, and humantoxicity.

In addition to the discharge of nanomaterials to wastewater streams,increasingly nanomaterials are used for wastewater treatment that may alsoreach the environment (Shon et al., 2006; Tiwari et al., 2008; Wani et al.,2011). The concentration of a nanomaterial in wastewater depends primarilyon (a) the amount produced or used locally, (b) the concentrations of fixedand free nanomaterial in the commercial product, (c) the fraction that reachesthe wastewater stream and the extent of dilution, and (d) the degree of

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Point sources ofengineered

nanoparticles

Productsconsumption and

use

Industrial productionand discharge

Domestic use

Transport, detectionand fate

Water systems,wastewater

Analyticaltechniques

Monitoring andevaluation

Toxicity

Biotoxicity EcotoxicityEffect on human

health

FIGURE 1. Flowchart of the scientific research of nanomaterials in wastewater.

agglomeration or adsorption occurring in wastewater streams that changesthe form of the nanoparticle or removes it from solution. There have beenincreasing concerns about uncertainties regarding these discharges and therisks that nanomaterials may pose to human health and the environment.

Due to their unique properties, nanomaterials can be harmful to biologi-cal systems because these systems are usually adapted to deal and selectivelyfilter macroparticles but not nanoparticles. The fate of ENM is not well iden-tified in wastewater and water systems before reaching to ecosystems. Aslong as health risks associated with nanomaterials consumption through wa-ter use remains controversial, eliminating the possible exposure of human towater containing ENM is advisable. On the contrary, the development andadoption of water treatment techniques for removal of nanomaterials arethought to be a promising solution.

The applications of ENM in water industry are shown in Table 2.Nanoparticles are very effective for the transformation and detoxificationof a wide variety of environmental contaminants, such as chlorinated or-ganic solvents, organochlorine pesticides, and polychloro biphenyls. Variousmagnetic nanoparticles have been used for the removal of metals, for ex-ample, hexavalent chromium from synthetic electroplating wastewater (Huet al., 2007). Arsenite and arsenate are precipitated in the subsurface using

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Fate and Ecological Effects of Nanomaterials in Wastewater 281

TABLE 2. Applications of engineered nanomaterials in water industry

Nanomaterials Application Target pollutant References

Nano-Fe Transformation anddetoxification ofpollutants

Chlorinated organicsolvents,organochlorinepesticides,polychlorophenyl

Elliott and Zhang(2001); Zhang(2003); Glazier et al.(2003); Ivanov et al.(2004); Quinn et al.(2005); Mauter andElimelech (2008)

Metallo-porphyrinogens

Degradation ofpollutants

Tetrachlorethylene,trichloroethylene,carbon tetrachloride

Dror et al. (2005)

TiO2 Adsorption andPhotocatalyticdegradation

Methylene blue El Saliby et al. (2011)

Magnetic nanoparticles Chemicalcoprecipitation ofheavy metals

Cr(VI) Hu et al. (2007)

Gold/iron oxideaerogels

Adsorption andPhotocatalyticdegradation

Azo-dyes Wang et al. (2007)

Bi12TiO20 supportedon nickel ferrite

Photocatalyticdegradation

Methyl orange Xu et al. (2007)

Poly-dendrimers Metal chelation Cu(II), Ag(I) andFe(III)

Diallo et al. (2005)

zero-valent iron (Fe), making arsenic less available (Kanel et al., 2005). TheClean Water Act (CWA, 1997) governs discharges of pollutants into watersof the United States. In its Nanotechnology White Paper (2007), the U.S.Environmental Protection Agency states that depending on the toxicity ofnanomaterials to aquatic life, aquatic dependent wildlife, and human health,as well as the potential for exposure, nanomaterials may be regulated un-der the CWA. Wastewater containing nanomaterials is subject to effluentlimitations, whether technology-based or water quality based, set forth inan National Pollutant Discharge Elimination System (NPDES) permit estab-lished under CWA Section 402. This review aims to (a) identify the sourcesof nanomaterials reaching wastewater streams, (b) quantify their occurrenceand distribution, and (c) discuss their fate in relation to human and ecologicalhealth, and environmental impact.

2. SOURCES AND RELEASE OF NANOMATERIALSIN WASTEWATER STREAMS

Nanomaterials reach wastewater streams through (a) direct discharge frommanufacturing processes involving nanomaterials, (b) disposal of consumerproducts containing nanomaterial components, (c) direct application of

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nanomaterials in WWTPs, and (d) indirect release from decomposing con-sumer products discarded in landfills and also from natural bodies (Kanget al., 2009).

Engineered nanomaterials are being produced by a variety of processesfor industrial purposes. In general, manufactured nanomaterials can be clas-sified according to their chemical composition and properties. They aresynthesized by different procedures that can be grouped into top-downand bottom-up approaches. The top-down strategy utilizes large particleswith the production of smaller particles mainly by physical methods such asmilling, repeated quenching, and photolithography (Gao, 2004). On the con-trary, bottom-up strategies use molecular materials to manufacture complexclusters through chemical reactions, nucleation and growth processes.

Engineered nanomaterials can be classified according to their core ma-terials into organic and inorganic. Fullerenes (C60, C70 and derivatives) andcarbon nanotubes/nanowires (multiwall [MW] and single-wall carbon nan-otubes [SWCNTs]) are considered as organic nanomaterials. They are com-posed of carbon atoms bound together in different shapes and crystallineforms. Fullerene and CNTs production involve the use of arc discharge tech-nique with different types of electrodes (Ju-Nam and Lead, 2008). But, in-organic nanoparticles can be divided into metal oxides (e.g., Fe, zinc [Zn],Ti, cerium [Ce]), metals (mainly Ag, gold [Au], and Fe), and quantum dots(e.g., cadmium selenide [CdSe]). In general, metal oxide nanoparticles areused in food, material (cosmetics and sunscreens), chemical (catalysis), andbiological (fillers in dental fillings) sciences.

Titanium dioxide, silicon dioxide (SiO2) and aluminum (Al) and Fe ox-ides have been massively produced for many years and many nanoparticu-late versions have been developed for industrial applications such as paintand remediation industries. Metal nanoparticles are also produced in largequantities and are used for sensing, catalysis, transport, water treatment, en-vironmental remediation, and other applications in medical and biologicalsciences (Ju-Nam and Lead, 2008; Ostiguy et al., 2010; Bhatt and Tripathi,2011). They are synthesized through chemical (metal salts as starting mate-rial) and physical (bulk metal as starting material) routes followed by thestabilization of the particles to avoid coalescence. Quantum dots are formedby the combination of Group II and IV elements or Group III and V elementsof the periodic table. They are semiconductors, insulators, metals and mag-netic materials or metallic oxides. They display unique optical and electronicproperties and are modified to be used as drug vectors, diagnostic tools andcan be also combined to antibodies, proteins, and oligonucleotides (Ostiguyet al., 2010).

The benefits of using ENM in medical applications are promising due totheir unique properties such as small size, high surface area to mass ratio andhigh reactivity (Kunzmann et al., 2011; Radad et al., 2012). They are beingsuccessfully used in drug delivery, cancer therapy, neuroprotection, tissue

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TABLE 3. Major engineered nanomaterials used in various industries (Brar et al., 2010; Bhattand Tripathi, 2011)

SourceType of

nanoparticlesRate of use in

industryConsumer products and

applications

Metals and alkalineearth metals

Ag High Textiles, antibacterial socks

Fe High Coagulant in water treatmentPt group metals High CatalystsAu Medium Tumour therapyAl High Metal platingZr High

Metal oxides TiO2 High Cosmetics, skin care, sunscreensZnO Low Bottle coatings, skin care productsCeO2 High Gas sensors, solar cells,

glass/ceramic applicationsCarbon materials Carbon black High Substrate bound

Carbon nanotubes Medium-High Sorption of metalsFullerenes (C60-C80) Medium-High Sorption of organic matters

Miscellaneous Nanoclay High Plastic packagingCeramic High CoatingsQuantum dots Low Medicine, solar cells, photonics,

telecommunicationsOrganic

nanoparticlesLow Food additives

engineering and tissue imaging. For instance, some specific applicationscomprised the delivery of therapeutic compounds and genes to targetedcancer cells, the nanoparticle-based oxidation therapy and the autophagy oftumor cells. This indicates that the use of ENM in medicine will certainlylead to their release to wastewater stream at different stages particularly atdrug manufacturing (pharmaceutical industry wastewater), drug applicationand disposal (hospitals’ wastewater) and discharge from the treated subject(domestic wastewater).

The determination of ENM in textiles and textile wastewaters were dis-cussed by Rezic (2011). The antimicrobial/antibacterial properties of Agnanoparticles encourage their use on textiles to eliminate odors in manyproducts such as underwear, socks, jogging outfits, athletes’ clothes, andmedical and military textiles. The most important sources of ENM releasedto the environment from textiles are textile-industry wastewater and wastew-aters from businesses using ENM-coated textiles such as hospitals and hotels.For example, Ag nanoparticles can enter WWTPs through daily washing fromAg-containing plastics and textiles (Blaser et al., 2008; Geranio et al., 2009).

Table 3 shows a comprehensive list of the most commonly producednanomaterials and their rate of use in modern industry. Inorganic nanopar-ticles are commonly used as a coagulant in water treatment, in textile indus-tries, and tumor therapy, whereas metal oxides are used in cosmetics, gas

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TABLE 4. Origin and distribution of engineered nanomaterials in various wastewater streams

ENM Origin/distribution Use References

Nano-Ag Textiles/industrial anddomestic wastewater

Antimicrobial andantibacterial agent

Gottschalk et al.(2011)

Nano-zero-valentFe

Water treatment/groundwater and rivers

Groundwater remediation Valli et al. (2010)

Synthetic TiO2 Exterior facades/runoff and domesticwastewater

Paint Kaegi et al. (2008)

ZnO Industrial manufacturingand end-userproducts/industrial anddomestic wastewater

Skin care products,sunscreens

Christian et al.(2008)

Quantum Dots Industrial manufacturing,hospitals/industrial anddomestic wastewater

Medical imaging andtargeted therapeutics,security inks

Bhatt and Tripathi(2011)

Organicnanoparticles

Pharmaceutical industryand end-userproducts/domestic andindustrial wastewater

Vitamins and medicinesuse and manufacturing

Brar et al. (2010)

sensors, and solar cells. In general, the most commonly produced nanomate-rial is Ag, being present in almost 259 commercial products. It is followed byfullerenes (82 products), Zn (including ZnO; 30), Si (35), Ti including (TiO2;50), Au (27), and zerovalent Fe (Weinberg et al., 2011). They can be releasedto the environment by either intentional (e.g., putting ENM-containing sun-screen onto the skin) or unintentional routes (e.g., abrasion of nano-textiles;Bhatt and Tripathi, 2011). In both cases, ENM will be transported by waterto end up in wastewater streams, WWTPs, and ultimately in the natural en-vironment. The level and pattern of release from a product depend mainlyon how the ENM are embedded in a product. The ENM in fluids are quicklyand in most cases completely released during the use phase, whereas ENMembedded in solid matrices are gradually and only partially released over aproduct’s lifetime (Kohler et al., 2008). Table 4 shows the origin and distribu-tion of ENM in various wastewater streams. The ENM generally originates inthe WWTP, facades, and industrial products and gets distributed in ground-water, rivers, domestic, and industrial wastewaters.

3. DISTRIBUTION AND DETECTION OF NANOMATERIALSIN WASTEWATER STREAMS

The concentration of detectable ENM in wastewater streams depends pri-marily on (a) the type of wastewater (industrial or domestic), (b) the amountof ENM produced or used locally, (c) the concentrations of fixed and freenanomaterial in the consumed commercial products, (d) the fraction that

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FIGURE 2. Schematic diagram illustrating the origin, environmental fate and ecological effectsof nanomaterials in various wastewater streams.

reaches the wastewater stream and the extent of dilution, and (e) the degreeof agglomeration or adsorption occurring in wastewater streams (Brar et al.,2010; Som et al., 2010). Therefore, the distribution of ENM in various com-ponents of wastewater streams such as effluents and biosolids is primarilydependent on the previously mentioned factors and can also vary with theirsize and functionalization (Guo et al., 2006; Brar et al., 2010). High concen-trations of ENM are found in industrial effluents where the manufacturingprocesses involve the production of nanomaterials with a significant amountalso found in effluent from processes requiring the use of nanomaterials. Ascheme representing the distribution of nanomaterials in wastewater streamsfrom the source to the natural environment is shown in Figure 2.

The physicochemical properties of ENM are the main criteria for theircharacterization and detection. The preparation of nanoparticles for partic-ular applications has resulted in nanomaterials of diverse physicochemicalcharacteristics. However, several properties such as chemical composition,mass, particle number and concentration, surface site concentration, size dis-tribution, specific surface area, surface charge/zeta potential, stability, andsolubility are universally accepted for assessing ENM (Klaine et al., 2008;Bhatt and Tripathi, 2011; Weinberg et al., 2011).

The detection of ENM is directed through the selection of a propertest and concentrations, selection of test conditions, introduction of ENM to

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test system and monitoring test conditions. Most common analytical tech-niques for the detection and analysis of ENM include (Tiede et al., 2009a;Tiede et al., 2009b; Jimenez et al., 2011; Weinberg et al., 2011): (a) atomicforce microscopy and electron microscopy for morphology detection; (b)centrifugation, dynamic light scattering, field flow fractionation, filtration,hydrodynamic chromatography, and size exclusion chromatography for thedetermination of size distribution; (c) N2 adsorption (BET) for specific surfacearea and porosity measurement; and (d) electrophoretic mobility, voltamme-try, and X-ray spectroscopy for analyzing the zeta potential, metal speciation,and surface chemical and structure analysis, respectively.

There is no single universal instrument that can track the release, con-centration and transformation of ENM in air, water, and soil. In situ detectionand analysis are the most informative type of analysis however the choiceof specific techniques is crucial to producing meaningful data from the an-alytical techniques described above (Simonet and Valcarcel, 2009). Ex situdetermination of nanoparticles is still the most widely practiced techniquebecause most of the instruments are non-portable. Therefore, samples shouldbe collected, conserved and then tested. The amount of variation can be sig-nificant in terms of aggregation and dispersion as the physicochemical prop-erties of nanoparticles are modified considerably. Nevertheless, techniquesavailable can be useful for detecting engineered nanoparticles in water andprovide reliable database for monitoring and tracking of fate and transport.

The level of current uncertainties over the effects of ENM on humanhealth and the environment necessitates a comprehensive risk assessment,taking into account all the potential exposure situations to ENM that mightarise throughout the life cycle of an ENM or an ENM-containing product(Ostertag and Husing, 2008). Life cycle assessment (LCA) is essentially acomprehensive tool for environmental sustainability assessment. In theory, ittakes into account all inputs (e.g., materials, energy, chemicals, land use) andall outputs (e.g., emissions, solid waste, products) throughout the life cycleof a product—from the extraction of the resources to the final disposal ofthe product (LCA, 2007; Grubb and Bakshi 2011). Life cycle assessment canbe used for comparing a product that includes ENM with similar productswithout ENM, and thus to assess the relative environmental performance ofnanoproducts in comparison with their conventional equivalents.

The product life cycle determines in what product life cycle phases(production, transport/storage, use, and disposal/recycling) and in what en-vironmental compartments or in what technological facilities ENM are re-leased (Davis, 2007; Kohler et al., 2008; Som et al., 2010). Depending on theproduct life cycle, humans or other organisms may also be directly exposedto ENM. For example, ENM unintentionally released from geotextiles willprobably end up in soils and might affect terrestrial organisms, whereas ENMunintentionally released from T-shirts might affect human health (by skin,gastrointestinal tract, or lung uptake) or may end up in wastewater treatment

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facilities from where they may be transferred to soils by sludge/wastewaterapplication. The methodology of life cycle analyses and exposure scenariosshould be examined with regard to the special demands of nanomaterialsand, if necessary, adapted. For exposure assessment, the physicochemicalproperties of released nanomaterials (e.g., chemical composition, particlesize and distribution, solubility, state of agglomeration, form, surface, surfacecharge, hydrophilicity/lipophilicity) should be estimated first. Depending onthese properties, exposure routes with the relevant releases to the variousmedia are estimated.

Gottschalk et al. (2009) calculated the environmental concentrations ofnanomaterials based on a probabilistic material flow analysis from a life-cycleperspective of ENM containing products. They found the simulated modesrange (most frequent values) from 0.003 (fullerenes) to 21 ng L−1 (nano-TiO2)for surface waters, and from 4 ng L−1 (fullerenes) to 4 µg L−1 (nano-TiO2)for sewage treatment effluents. In addition, Mueller and Nowack (2008) useda life-cycle perspective to model the quantities of nano-Ag, nano-TiO2, andCNT in the environment. They used variables such as estimated worldwideproduction volume, location, particle release, and flow coefficient. Theyfound that only in the case of TiO2 the expected concentration in water(0.7–16 µg L−1) was close to or higher than the predicted no effect concen-tration (<1 µg L−1). This suggests that more detailed studies are required topredict the effects of TiO2 in the environment.

Khanna et al. (2008) compared the energy consumption and environ-mental impact of carbon nanofibers with traditional materials, aluminumand steel. They found that, on a per mass basis, carbon nanofibers are muchmore energy intensive, in some cases as much as 300 times more than tra-ditional materials. Healy et al. (2008) presented a traditional LCA of threeprocesses, arc ablation, chemical vapor deposition, and a high-pressure car-bon monoxide process, for manufacturing SWCNTs. One of the problemsthey encountered, as in most other life cycle studies of nanomaterials, isa lack of toxicity information for the nanotubes; therefore, their analysis islimited to providing a base case for the manufacturing processes and theirimpacts in three categories: airborne inorganics, climate change, and acidifi-cation potential.

4. ENVIRONMENTAL FATE OF NANOMATERIALSIN WASTEWATER STREAMS

The industrial utilization of nanomaterials to produce domestic goods hasincreased the potential for their release into wastewater streams. The fateof nanomaterials is determined by several factors that highly influence theirtransport in water and wastewater streams. Dissolution, deposition, sedi-mentation, agglomeration, coating, association, reaction, and decomposition

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are the main physicochemical influences that should be considered whenstudying the fate of ENM (Weinberg et al., 2011). For instance, the transportof metal oxide nanoparticles in water depends on their surface properties,sizes, and interaction with other substances in water (Zhang et al., 2008).This is becoming significant, uncontrollable and difficult to be detected inwastewater streams. The aggregation and interaction of nanoparticles amongthemselves and with other compounds can facilitate the detection mecha-nisms. Brant et al. (2007) suggested that fullerol cluster formation in aqueoussolutions is likely to reach a size of 100 nm or more in some cases. Thesize evolution is dynamic and is affected by pH and temperature. Similarly,the aggregation of metal oxide nanoparticles in tap water (presence of elec-trolytes) results in the formation of macroparticles of size larger than 100 nm.The aggregation is disadvantaged in nanopure water where the dispersionof nanoparticles is dominant (Zhang et al., 2008). Van Hoecke et al. (2011)studied the aggregation of CeO2 nanoparticles in synthetic and natural wa-ters and found that increasing the pH and the ionic strength increased theaggregation while increasing the natural organic matter (NOM) content de-creased it. The physicochemical behavior of TiO2 nanoparticles in aquaticenvironment has been recently assessed using a multidimensional parametertesting (Von der Kammer et al., 2010). Results suggested that the behaviorof nanoparticles was not related to their surface area but to water pH andcomposition.

Generally, the solubility, dispersability, biological and abiotic processes,interactions between nanoparticles and natural, and anthropogenic chemi-cals in water control the fate of discharged ENM. Some nanoparticles willaggregate and settle and others are biodegradable while the rest can form sta-ble colloids and be transported to relatively far distances from their originalsource (Brar et al., 2010). Engineered nanomaterials are driven in wastewaterstreams by water flow from the source to the treatment plant (Table 5). Asmentioned previously, the physical dimensions and the chemical propertiesof ENM are controlled by several factors. Therefore, the prediction of ENMbehavior is challenging in complex and heterogeneous environments. Never-theless, a significant amount of ENM in wastewater will reach the wastewatersludge through the aggregation and settling mechanisms. Westerhoff et al.(2008a) reported the aggregation of TiO2 and CdSe quantum dots by coagu-lation using alum. Subsequently, 90% of aggregates were then filtered using0.45 µm membrane. However, coagulation is not always an effective toolwhere 20–80% removal of oxides of Ti, Fe, Zn, Ni, and Si can be achieved(Zhang et al., 2008). It has often been noticed that the majority of ENM reach-ing the wastewater streams tend to accumulate in the solid sludge duringwastewater treatment as shown in Figure 3 (Kiser et al., 2009). For example,Shafer et al. (1998) noticed that the total Ag concentration in wastewater in-fluent ranged from 1.78 to 105 µg L−1, while the wastewater effluent containsonly between 0.028 and 5.5 µg L−1 (Shafer et al., 1998). This indicates that

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TABLE 5. Selected reference on the distribution of engineered nanomaterials in various com-ponents of wastewater treatment system

ENM Origin Distribution References

TiO2 Cosmetics,sunscreen,industrialproducts,industrial waste,modern paints

Headwork: 141–615 (µg L−1)Treated effluent: 2–18 (µg L−1)Headwork: 843 (µg L−1)Primary effluent: 99 (µg L−1)Secondary effluent: 35 (µg L−1)Tertiary effluent: 36 (µg L−1)Primary solids: 803 (µg L−1)Secondary solids: 8464 (µg L−1)Untreated waste water: 10.2 (µg

L−1)Treated waste water: 10.1

(µg L−1)Biosolids: 180.7 (µg L−1)

Westerhoff et al.(2011)

Westerhoff et al.(2009); Kiser et al.(2009)

Khosravi et al.(2012)

Headwork: 185 (µg L−1)Treated water: 0.7–16 (µg L−1)Europe Sewage Treatment Plant

(STP) Sludge: 100–433 mg kg−1

Europe STP effluent: 2.5–10.8 (µgL−1)

U.S. STP sludge: 107–523 mg kg−1

U.S. STP effluent: 1.37–6.70 (µgL−1)

Garcıa et al. (2012)Mueller and

Nowack (2008)Nowack et al. (2009)

Ag Clothing, textiles,industrial waste

Headwork: 2–18 (µg L−1)Wastewater influent: 1.78–105 (µg

L−1)Wastewater effluent: 0.028–5.5 (µg

L−1)Wastewater sludge: 7–39 mg kg−1

Treated water: 0.02–0.1 (µg L−1)Europe STP sludge: 1.68–4.44 mg

kg−1

Europe STP effluent: 0.042–0.111(µg L−1)

U.S. STP sludge: 1.55–5.86 mgkg−1

U.S. STP effluent: 0.0164–0.0747(µg L−1)

Garcıa et al. (2012)

Yang et al. (2012)

Mueller andNowack (2008)

Nowack et al. (2009)

CNT Commercialproducts,industrial waste

Treated water: 0.0005–0.0008 (µgL−1)

Europe STP sludge:0.047–0.129 mg kg−1

Europe STP effluent:0.0114–0.0315 (µg L−1)


U.S. STP effluent: 0.0066–0.0184(µg L−1)

Mueller andNowack (2008)

Nowack et al. (2009)

(Continued on next page)

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TABLE 5. Selected reference on the distribution of engineered nanomaterials in various com-ponents of wastewater treatment system (Continued)

ENM Origin Distribution References

Fullerenes(C60,C70)

Effluent C60: 0.0005 −0.019 (µg L−1)Effluent C70: 0.0017 – 0.181 (µg L−1)Europe STP sludge: 0.0088–0.055 mg

kg −1

Europe STP effluent: 0.0042–0.026(µg L−1)


U.S. STP effluent: 0.0045–0.033 (µgL−1)

Farre et al. (2010)


ZnO Europe STP sludge: 13.6–57.0 mgkg−1

Europe STP effluent: 0.340–1.42 (µgL−1)

U.S. STP sludge: 17.4–57.7 mg kg−1

U.S. STP effluent: 0.22–0.74 (µg L−1)


the majority of Ag in wastewater is accumulated in sludge. Similarly, Wanget al. (2012) noticed that concentrations of Ag and Ti were much higher inbiosolids than the settled effluent (Figure 4). Nanosilver can be adsorbed tosludge and embedded in sludge to form new products such as Ag2S (Kimet al., 2010; Kaegi et al., 2011).

Engineered nanomaterials in wastewater streams reach natural ecosys-tems by sludge disposal in terrestrial system and effluent discharge endingin aquifers and rivers. The first and second scenarios are shown in Figure 5

Effluents

Tit

aniu

m c

on

cen

trat

ion

(g

L-1

)

0

1000

2000

8000

10000

Solids1 2 3 4 5 6 7

µ

FIGURE 3. Distribution of titanium in effluents and solids during wastewater treatment (1– headworks; 2 – primary effluent; 3 – secondary effluent; 4 – tertiary effluent; 5 – primarysolids; 6 – aeration basin; 7 – secondary solids (Kiser et al., 2009).

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0 5 10 15 20 25 30Days of Operation

0

1

2

3

4

Co

nce

ntr

atio

n (

mg

L-1

)C

on

cen

trat

ion

(m

g L

-1)

InfluentSettled effluentBiosolids

(a)

0 5 10 15 20 25 30Days of Operation

0

2

4

6

8

10InfluentSettled effluentBiosolids

(b)

FIGURE 4. Distribution of (a) silver and (b) titanium nanomaterials in wastewater influent(headworks), settled effluent, and biosolid (Wang et al., 2012).

and can be predicted through wastewater sludge leachate (biosolids forsoil amendment) and effluent discharge volume. Recently, Gottschalk et al.(2011) studied different ENM transport scenarios in rivers using nano-ZnO,nano-Ag, and nano-TiO2. The transport models were highly governed by thegeographical distribution of the ENM release points and the spatially vari-able dilution due to rivers’ flow. There are increasing evidences that the fateof ENM will have significant impact on the biotic and abiotic conditions ofmicro- and macroenvironments. The effect of ENM release on the growth,

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FIGURE 5. Two scenarios for the discharge of nanomaterials into the environment afterwastewater treatment.

health, and reproduction of plants and animals will certainly have severeimplications on the life of affected human population.

5. ECOLOGICAL EFFECTS AND HUMAN RISKSOF NANOMATERIALS IN WASTEWATER STREAMS

Engineered nanomaterials represent a real challenge to environmental toxi-cologists and pathologists. During evolution, living organisms have not en-countered such nanomaterials and do not have well adapted defensive mech-anisms to deal with their adverse properties and toxicity (Chae et al. 2009;Bhatt and Tripathi, 2011). The main causes of nanoparticles’ toxicity are dueto (a) chemical toxicity of materials, (b) high reactivity and small size, and(c) shape (Bystrzejewska-Piotrowska et al., 2009). Additionally, the ease oftransport of ENM in ecosystems (dissolved or aggregated) enables them tointerfere in the life cycle of living organisms (Figure 6). The abundance ofENM in wastewater discharge facilitates the contact with aquatic animals andmicroorganisms.

In Europe, the current approach for measuring the environmental riskassociated with nanoparticles is based on the quotient of a predicted envi-ronmental concentration and a predicted non-effect concentration. However,this quotient has many limitations and sometimes does not reflect the realbehavior of nanoparticles for many reasons listed by Quik et al. (2011). Quiket al. (2011) studied the methods that have been used to assess the exposure

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FIGURE 6. Effect of agglomeration of nanomaterials on their transport.

of aquatic organisms to manufactured nanoparticles and concluded that cur-rent exposure models need modification by accounting for dissolution andsedimentation in the assessment methods of colloidal nanoparticles behavior.This is evident as the uptake and accumulation of nanoparticles in organismsdepend on the availability and mobility of nanoparticles in solution. The LCAof ENM is another tool to determine their impact on the environment, whichwas discussed by Hischier and Walser (2012). It can be analyzed using dif-ferent models and approaches such as the USEtox (Rosenbaum et al., 2008),TRACI (Bare et al., 2003; Singh et al., 2008), and Eco-indicator 99 (Goedkoopand Spriensma, 2000). The last two models rely on the ecotoxicity potentialor factor of a unit quantity of chemical released into the environment toassess the LCA of ENM.

The dispersion and ecotoxicity of ENM in wastewater will be influencedby the nature and physicochemical properties of ENM, and wastewater so-lution chemistry parameters, including pH, ionic strength, and dissolved or-ganic carbon (DOC) concentration (Bolan et al., 2011; Kunhikrishnan et al.,2012). For example, Kang et al. (2009) have shown that higher conductivityand divalent cation concentrations (e.g., Mg and Ca) in wastewater effluentsmake it likely that the ENM are more aggregated than in river water, therebyinfluencing their ecotoxicity. Similarly, elevated concentrations of dissolvedand suspended organic matter in most wastewater samples have a stronginfluence of the ecotoxicity of ENM. Adsorption of DOC by ENM is likely toresult in a protective coating thereby reducing the deposition and attachment

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SR

Mo

rtal

ity

(%)

0

20

40

60

80

100

Ef YR DLControl

Sp

ecif

ic U

V a

bso

rban

ce

FIGURE 7. Effect of dissolved organic carbon coating of quantum dot nanomaterials onthe mortality of Daphnia; the specific UV absorbance for the coated materials is given. Thecorresponding sources of dissolved organic carbon samples are SR: Suwannee River; Ef:Damyang wastewater treatment plant; YR: Yeongsan River; DL: Dongbuk Lake (Lee et al.,2011).

of bacteria on the surface of ENM. Choi et al. (2009) noticed that ecotoxic-ity of ENM decreased with increasing hydrophobicity which they attributedto enhanced adsorption of DOC. Similarly, Lee et al. (2011) observed thatmortality of Daphnia resulting from nanomaterials decreased with increasingconcentration of DOC (Figure 7). Yang et al. (2012) have shown that Ag-NM at moderate concentrations (e.g., <40 mg L−1) have negligible impacton anaerobic digestion and methanogenic assemblages during wastewatertreatment. Thus the inhibitory effect of ENM on wastewater treatment pro-cess and the toxicity of ENM in aquatic environments are partially mitigatedby NOM coatings on ENM.

In aquatic animals, the route of entry of nanoparticles can be by directpassage across gills and other external surface epithelia. Table 6 summarizesthe uptake of nanomaterials by biota. At the cellular level, endocytosis isthe main passage to intracellular medium. In plants, nanoparticles mainlyadhere to the root surface. Meyer et al. (2010) used traditional and novelanalytical methods to study the intracellular uptake and associated toxic-ity of Ag nanoparticles by Caenorhabditis elegans. Significant aggregation,extra-organismal dissolution of Ag nanoparticles, organismal uptake, andtransgenerational transfer were observed. Some studies have shown the roleof Ag+ in prevention of DNA replication and permeability of cell membrane(Feng et al., 2000). The chemical stability of nanoparticles is another indicatorfor their cytotoxicity and genotoxicity. Chemically stable metallic nanopar-ticles showed no cytotoxicity while metallic nanoparticles with strong oxi-dizing or reducing power can be cytotoxic as revealed by in vitro analysis(Auffan et al., 2009). However, fullerenes were reported to be cytotoxic to

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TABLE 6. Selected references on the uptake of nanomaterials by biota

Type ofnanomaterials

Uptake organs/organisms Observations References

Fe3O4 Roots/pumpkin(Cucurbita maxima)

45% of ENM were stored in rootsand 0.6% in leaves.

Zhu et al. (2008)

Fullerene C70 Roots and leaves/rice(Oryza sativa)

Fullerenes are mobile in thevascular system

Lin et al. (2009)

ZnO Roots/ryegrass No upward translocation fromroots to shoots. Nanoparticlesadhere to root surface.

Lin and Xing(2008)

Cu Roots/bean and wheatseedlings

Linear relationships was recordedbetween the concentration ofnanoparticles in the growthmedia and the accumulation inplant tissues

Lee et al. (2008)

Polystyrenenanospheres,CdSe/ZnSquantum dots

Protoplasts fromsycamore culturedcells

40 nm polystyrene nanospheresand 20 nm were easily absorbedby the cultured cells

Etxeberria et al.(2006)

ENM Gut/animals ENM can enter gut cells by:diffusion through cellmembranes, throughendocytosis, adhesion

Geiser et al. (2005)Kim et al.(2006); Lin et al.(2006); Baunet al. (2008)

mammalian cells and this was attributed to their lipophilicity. Modifying thesurface of fullerenes can render them less toxic by reducing their lipophilicity(Sayes et al., 2004).

The use of nanomaterials in different applications will increase the riskto human exposure (Hoyt and Mason, 2008). There is still little informationavailable on the effect of nanomaterials/nanotechnology on human health.Generally, nanoparticles can enter the human body through various routesincluding skin absorption, ingestion and inhalation (Umwelt Bundes Amt,2006). Skin adsorption occurs after using products such as sunscreens andcosmetics that contain compounds like TiO2 (U.S. Environmental ProtectionAgency, 2009). Titanium dioxide enters the skin through the epidermal layerand it is believed that sunlight exposure might cause deep absorption ofnanoparticles (Smijs and Pavel, 2011). Ingestion of nanoparticles is not acommon route but can happen after a hand-to-mouth contamination or as aconsequence of inhalation. Ingested nanoparticles traverse the digestive sys-tem and might end up in the blood stream, which will carry them to otherorgans such as kidneys, liver, or even the brain. The adverse effect of nano-materials on the metabolism of organs is not well understood but their de-position is believed to cause malfunction due to the particular characteristicsof nanomaterials. Recently, Savolainen et al. (2010) reported that the healthconcerns of ENM on individuals are due to their effects on lungs, circulation,brains and also their genotoxic and possible carcinogenic effects. The mostcommon route of human exposure is inhalation as it can readily occur when

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nanoparticulate dust is present in the workplace (Lee et al., 2011). Nanoparti-cles enter lungs through the respiratory tract and are deposited in the alveoli.This occurs because the small size of nanoparticles prevents their detectionby macrophages that usually envelop toxic compounds and remove themfrom the lungs (Hoyt and Mason, 2008). The toxicity of nanoparticles is muchmore pronounced because they have higher surface area and can react moreeasily than microparticles. Some studies have reported that CNTs can dam-age the lungs if inhaled, while the damage caused to the liver and brain byfullerenes has been examined in other studies (Shelley, 2005). Animal cellsare normally not equipped to resist contaminants at the nanoscale thereforetheir impact is believed to be high as cells will be fully exposed to suchnanomaterials.

Table 7 highlights the ecotoxicity of ENM in different wastewaterstreams. Many studies demonstrated the toxic effects of ENM to microor-ganisms and marine organisms. The antibacterial effects of ENM dependon physicochemical properties and environmental factors, which vary de-pending on the type of ENM, exposure conditions, and type of bacteria.There are various modes of antibacterial toxicity, including attacks on thecell wall, cytoplasmic membrane, protein synthesis, and nucleic acid syn-thesis. The microbial toxicity of ENM, for instance, Ag-NM is dependent onphysicochemical properties such as size and shape. Smaller sized particles(≤10 nm) were highly toxic (Rai et al., 2009; Sotiriou and Pratsinis, 2010)because the small size increases the generation of Ag+ (Sotiriou and Pratsi-nis, 2010). Triangular-shaped nanoparticles were more toxic than sphericaland rod-shaped nanoparticles because they had a higher density of atomsper unit area on the edges. Shrivastava et al. (2007) postulated that the ma-jor mechanism through which Ag-NM manifest antibacterial properties wasby anchoring to and penetrating the bacterial cell membrane. Musee et al.(2011) argues that the antibacterial activity of ENM in WWTPs means thatsome chemicals that are bacterially decomposed can escape at increasedconcentrations into the receiving environment. In addition, ENM may po-tentially be discharged from the WWTPs and pose a risk to the integrity ofreceiving environments due to their antibacterial activity.

Zheng et al. (2011) conducted short-term exposure experiments to de-termine whether ZnO-NM caused adverse impacts on biological nitrogen(N) and phosphorus (P) removal in unacclimated anaerobic-low dissolvedoxygen sequencing batch reactor. Figure 8 highlights the effect of ZnO-NMon the concentrations of ammonium, nitrate and soluble orthophosphateduring the treatment at 180 min. Compared with the absence of ZnO-NM,the presence of 10 and 50 mg L−1 of ZnO-NM decreased total N removalefficiencies from 81.5% to 75.6% and 70.8%, respectively. The effluent Pconcentrations increased from nondetectable to 10.3 and 16.5 mg L−1, re-spectively, which were higher than the influent P (9.8 mg L−1), suggestingthat higher concentration of ZnO-NM induced the loss of normal P removal.

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TA

BLE

7.

Sele

cted

refe

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son

the

ecoto

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tyofnan

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Was

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onn

ext

page

)

297

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15

TA

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both

colif

orm

and

amm

onia

oxi

diz

ing

bac

teria

inw

aste

wat

eran

da

55%

dec

reas

ein

resp

irat

ion

rate

.N

oin

hib

itory

effe

cts

or

dec

reas

ein

resp

irat

ion

rate

wer

eobse

rved

with

the

additi

on

ofsa

me

amountofCu-N

M.

Gan

esh

etal

.(2

010)

Sludge

from

anae

robic

was

tew

ater

trea

tmen

tsl

udge

Fulle

rene

(C60

)Fu

llere

nes

had

no

sign

ifica

ntef

fect

on

anae

robic

com

munity

ove

ran

exposu

reper

iod

ofa

few

month

s.Abse

nce

ofto

xici

tyw

asin

dic

ated

by

no

chan

gein

met

han

oge

nes

is.D

enat

uring

grad

ient

gelel

ectrophore

sis

resu

ltssh

ow

edno

evid

ence

ofsu

bst

antia

lco

mm

unity

shifts

.

Nyb

erg

etal

.(2

008)

Sludge

from

was

tew

ater

trea

tmen

t

Au

Tro

phic

tran

sfer

toN

icot

ian

ata

bacu

mL.

cvX

an

thian

dM

an

du

case

xta,an

dbio

mag

nifi

catio

nofAu-N

Mw

ere

obse

rved

by

mea

nfa

ctors

of6.

2,11

.6,an

d9.

6fo

rth

e5,

10,an

d15

nm

trea

tmen

ts,

resp

ectiv

ely.

Judy

etal

.(2

011)

Act

ivat

edsl

udge

MW

CN

Ts

Res

pirat

ion

inhib

ition

was

obse

rved

for

both

unsh

eare

dan

dsh

eare

dm

ixed

liquor

when

MW

CN

Ts

wer

epre

sent,

how

ever

,gr

eate

rre

spirat

ion

inhib

ition

was

obse

rved

for

shea

red

mix

edliq

uor.

The

hig

hes

tco

nce

ntrat

ion

ofM

WCN

Ts

exhib

ited

the

hig

hes

tre

spirat

ion

inhib

ition.

Luongo

and

Zhan

g(2

010)

Act

ivat

edsl

udge

Ag-

NM

Cer

tain

mic

robia

lsp

ecie

sin

the

activ

ated

sludge

wer

ehig

hly

sensi

tive

toAg-

NM

(1m

gL−

1 ),al

though

no

reduct

ion

ince

llcu

ltura

bili

tyw

asdet

ecte

dduring

24h

trea

tmen

t.Conve

rsel

y,one

log

unit

reduct

ion

with

no

mic

robia

lco

mm

unity

stru

cture

chan

ges

was

obse

rved

for

unse

ttle

dsl

udge

flocs

afte

r24

h.St

udy

sugg

ests

Ag-

NM

can

impac

tth

em

icro

bia

lco

mm

unity

dep

endin

gon

the

stru

cture

ofth

eflocs

,an

dsp

atia

ldis

trib

utio

nofm

icro

org

anis

ms.

Sun

etal

.,20

13

298

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NH4+ - N

Co

nce

ntr

atio

n (

mg

L-1

)

0

5

10

35

40

Control1 mg L-1 NPs10 mg L-1 NPs50 mg L-1 NPs

SOPNO3- - N

1 2 3 4 1 2 3 4 1 2 3 4

FIGURE 8. Effect of nanomaterials on the concentrations of ammonium, nitrate, and solubleorthophosphate during wastewater treatment (Zheng et al., 2011).

Inhibition of N and P removal induced by higher concentrations of ZnO-NMwas due to the release of Zn ions from ZnO-NM dissolution and increaseof reactive oxygen species production, which caused inhibitory effect onpolyphosphate-accumulating organisms and decreased nitrate reductase, ex-opolyphosphatase, and polyphosphate kinase activities.

Using autotrophic nitrifying organisms, Choi et al. (2008) observed thatAg-NM (average size = 14 ± 6 nm), Ag+ ions (AgNO3), and AgCl (averagesize = 0.25 mm) colloids, all at 1 mg L−1 Ag, inhibited respiration by 86%,42%, and 46%, respectively. Of all the Ag species tested, Ag-NM presentedthe highest inhibition on nitrifying bacterial growth (Figure 9). Their resultssuggest that nitrifying bacteria are susceptible to inhibition by Ag-NM, andthe accumulation of Ag-NM could have detrimental effects on the microor-ganisms in wastewater treatment. Luongo and Zhang (2010) examined thetoxicity of MWCNTs on the microbial communities in activated sludge us-ing respiration inhibition test on both unsheared and sheared mixed liquor.Respiration inhibition was observed for both unsheared and sheared mixedliquor when MWCNTs were present, however, greater respiration inhibitionwas observed for the sheared mixed liquor (Figure 10). The toxicity observedby the respiration inhibition test was determined to be dose-dependent; thehighest concentration of MWCNTs exhibited the highest respiration inhibi-tion.

Garcıa et al. (2012) tested the activity of ordinary heterotrophic organ-isms (OHO) and ammonia oxidizing bacteria (AOB) in the presence andabsence of metal oxide (CeO2 and TiO2) and zero-valent metal (Ag and Au)nanomaterials after different exposure times. The CeO2 NM caused a strong

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0 0.2 0.4 0.6 0.8 1Silver concentration (mg L-1)

0

20

40

60

80

100

Inh

ibit

ion

(%)

Ag NPsAg+

AgCl

FIGURE 9. Inhibition of nitrification as a function of silver concentration in various forms(Choi et al., 2008).

0 1 2 3 4Concentration (g L-1)

0

20

40

60

80

Inh

ibit

ion

(%)

Sheared mixed liquorUnsheared mixed liquor

FIGURE 10. Effect of carbon nanotubes on inhibition of respiration in sheared and unshearedmixed liquor (Luongo and Zhang, 2010).

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Inh

ibit

ion

(%

)

0

20

40

60

80

100

120

Au Ag TiO2 CeO2 Au Ag TiO2 CeO2

OHO AOB

1 hr

4 hr

FIGURE 11. Effect of various nanomaterials on the inhibition of ordinary heterotrophic or-ganisms (OHO) and ammonium oxidising bacteria (AOB) after different exposure time (Garcıaet al., 2012).

inhibition for OHO (nearly 100%, after 4 h) and AOB biomasses. The Ag-NM caused an intermediate inhibition for OHO after 4 h (33%) and a slightinhibition for AOB, whereas Au and TiO2 nanoparticles caused only slightor no inhibition for both the biomasses (Figure 11). In another study, theeffect of Ag ions, nano- and microparticles (with and without stabilizers) ona luminescent biosensor bacterium Pseudomonas putida originally isolatedfrom activated sludge was assessed by Dams et al. (2011). The bacteriumcarrying a stable chromosomal copy of the lux operon (luxCDABE) detectedthe toxicity of ionic and particulate Ag over short-term incubations rangingfrom 30 to 240 min. The results of IC50 values obtained at different time in-tervals showed that highest toxicity (lowest IC50) was obtained after 90 minincubation for all toxicants and it was considered the optimum incubationfor testing. The data showed that ionic Ag was the most toxic followedby nano-Ag particles with micro-Ag particles being least toxic (Figure 12).They reported that the release of nanomaterials is likely to have an effect onthe activated sludge process using a common sludge bacterium involved inbiodegradation of organic wastes.

6. MANAGEMENT OF NANOMATERIALS IN WASTEWATERTREATMENT

Engineered nanomaterials pose an ecological risk, especially to aquatic or-ganisms (Morimoto et al., 2010; Scown et al., 2010). Risk assessment and

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IC50

Val

ues

(m

g L

-1)

25

125

225

325

425

525

AgNO3 Ag-NP Ag-NPBSA

Ag-NPCA

Ag-MP Ag-MPBSA

Ag-MPCA

0

FIGURE 12. Effect of stabilizers (BSA: Bovine serum albumin; CA: Citric acid) on the microbialtoxicity of silver nanoparticle (Ag-NP) and microparticle (Ag-MP) (Dams et al., 2011).

management require information on both toxicity and exposure. Many stud-ies exist regarding toxicity of nanomaterials, but only a few exposure as-sessments have been reported. Municipal WWTPs are particularly importantsources of contaminant release into the environment, as they provide po-tential pollutant pathways into surface waters, soils, and air through treatedeffluent, biosolids, and plant-generated aerosols (Muller et al., 2007; Limbachet al., 2008; Mueller and Nowack, 2008; Gottschalk et al., 2010; Kunhikrish-nan et al., 2011, 2012, 2013). There is a vital need to understand the fateof nanoparticles during the wastewater treatment process and to look atinnovative techniques to remove them. Research to determine the potentialremoval mechanisms for ENM during wastewater treatment has only recentlybegun (Nyberg et al., 2008; Yin et al., 2009).

Sorption to activated sludge or biomass is a major removal mechanismfor pollutants, including ENM, in conventional activated sludge wastewa-ter treatment plants. Kiser et al. (2010) conducted batch sorption isothermexperiments with activated sludge as sorbent and a total of eight types ofENM as sorbates. Epifluorescence images clearly showed the biosorption offluorescent silica ENM; ENM biosorption increased with increasing ENM ex-posure to biomass. Furthermore, the extent of biosorption varied with thetype of ENM. For example, upon exposure to 400 mg L−1 total suspendedsolids of wastewater biomass, 97% of Ag-NM were removed, probably in partby aggregation and sedimentation, whereas biosorption was predominantlyresponsible for the removal of 88% of aqueous fullerenes, 39% of function-alized Ag-NM, 23% of nanoscale TiO2, and 13% of fullerol-NM (Table 8).Westerhoff et al. (2008b) determined the activated sludge sorption isothermsfor dispersed C60 and SiO2 nanoparticles and reported Freundlich isothermcoefficients were similar to those measured for TiO2 nanoparticles (Kiser

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TA

BLE

8.

Sele

cted

refe

rence

son

the

rem

ova

lpro

cess

ofnan

om

ater

ials

inva

rious

was

tew

ater

stre

ams

EN

MConce

ntrat

ion

(µg

L−1)

Rem

ova

lpro

cess

Obse

rvat

ion

Ref

eren

ces

TiO

2H

eadw

ork

:18

5Ter

tiary

Effl

uen

t:17

Hea

dw

ork

:A

vera

ge-

377

Med

ian-

321

Tre

ated

Effl

uen

t:Ran

ge-

<2–

20

-Prim

ary

and

seco

ndar

yse

dim

enta

tion

-Aer

atio

n-T

ertia

ryfiltr

atio

n-P

rim

ary

grav

ityse

dim

enta

tion

-Bio

logi

caltrea

tmen

t(A

ctiv

ated

sludge

/trick

ling

filte

r/la

goon)

-Mic

rofiltr

atio

n/r

ever

seosm

osi

s/te

rtia

ryfiltr

atio

n/s

ubm

erge

dm

icro

filtr

atio

nm

embra

ne

91%

rem

ova

lofTifr

om

influen

t.

Hig

hva

riab

ility

insi

zeofpar

ticle

sin

hea

dw

ork

low

inef

fluen

t.A

vera

geof98

.3%

rem

ova

lofTifr

om

wat

er.

Wes

terh

off

etal

.(2

009)

Wes

terh

off

etal

.(2

011)

Unsp

ecifi

edW

aste

wat

erbio

mas

sad

sorp

tion

Rem

ova

lof23

%ofTiO

2.

Kis

eret

al.(2

010)

Ag

Unsp

ecifi

ed-W

aste

wat

erbio

mas

sad

sorp

tion

Rem

ova

lof97

%A

g-N

Man

d37

%of

funct

ional

ized

Ag-

NM

.K

iser

etal

.(2

010)

Max

imum

:50

0-A

erat

ion

and

seco

ndar

ycl

arifi

catio

n>

90%

ofci

trat

e-st

abili

zed

Ag

nan

opar

ticle

sin

the

sew

erre

mai

ned

inth

ew

ater

stre

amaf

ter

prim

ary

clar

ifica

tion.H

ow

ever

,ae

ratio

nan

dse

condar

ycl

arifi

catio

nofth

esi

mula

ted

sequen

cing

bat

chre

acto

r(S

BR)

pro

cess

com

ple

tely

rem

ove

dth

eA

gth

rough

outth

e15

-day

exper

imen

t.

Hou

etal

.(2

012)

Influen

t:1.

78–1

05Effl

uen

t:0.

028–

5.56

-Sorp

tion

and

filtr

atio

nby

was

tew

ater

bio

mas

s>

95%

ofA

gw

asse

ques

tere

din

toth

ew

aste

wat

erbio

mas

sSh

afer

etal

.(1

998)

Cu

Max

imum

:10

000

Bio

mas

sTre

ated

:<

1000

Filtr

ate

Tre

ated

:20

00–2

500

-Act

ivat

edsl

udge

bio

mas

s-A

ctiv

ated

sludge

filtr

ates

Usi

ng

bio

mas

sCu-N

Mw

ere

rem

ove

dat

>90

%ef

fici

ency

butw

ithfiltr

ates

only

75–8

0%Sl

udge

only

adso

rbs

15–3

5%to

talof

Cu

rem

ove

d.

Gan

esh

etal

.(2

010)

Influen

t:22

80Effl

uen

t:27

4-C

loud

poin

tex

trac

tion

Usi

ng

CPE

88%

ofCu

nan

opar

ticle

sw

ere

rem

ove

d.

Liu

etal

.(2

010)

(Con

tin

ued

onn

ext

page

)

303

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0 Ja

nuar

y 20

15

TA

BLE

8.

Sele

cted

refe

rence

son

the

rem

ova

lpro

cess

ofnan

om

ater

ials

inva

rious

was

tew

ater

stre

ams

(Con

tin

ued

)

EN

MConce

ntrat

ion

(µg

L−1)

Rem

ova

lpro

cess

Obse

rvat

ion

Ref

eren

ces

Fulle

renes

Unsp

ecifi

ed-W

aste

wat

erbio

mas

sad

sorp

tion

88%

ofaq

ueo

us

fulle

renes

wer

ere

move

dfr

om

sam

ple

s.K

iser

etal

.(2

010)

Conce

ntrat

ion:

5000

–100

,000

-Alu

mco

agula

tion

-Flo

ccula

tion

-Sed

imen

tatio

nan

dfiltr

atio

n

Rem

ova

lre

ached

itspea

kbet

wee

npH

7–8

when

alum

and

CaC

O3

dosa

ges

wer

e10

0,00

0µ

gL−1

.20

%in

crea

seofC60

rem

ova

lw

asobse

rved

afte

rfiltr

atio

n,w

hen

nC60

rem

ova

lby

coag

ula

tion

was

less

than

20%

.H

ow

ever

,w

hen

nC60

rem

ova

lby

coag

ula

tion

was

>60

%,ad

diti

onal

rem

ova

lby

filtr

atio

nw

asm

inim

al.

Hyu

ng

and

Kim

(200

9)

Conce

ntrat

ion:10

0–30

00-A

ctiv

ated

sludge

bio

mas

sRem

ova

lof94

%ofaq

ueo

us

fulle

renes

.K

iser

etal

.(2

012)

CeO

2Contin

uous:

0.1

-Act

ivat

edsl

udge

bio

mas

sA

larg

eportio

nofCeO

2w

asre

move

dth

rough

adhes

ion

toth

ecl

earing

sludge

.H

ow

ever

,6

wt%

ofth

eCeO

2

esca

ped

from

the

trea

tmen

tsy

stem

.

Lim

bac

het

al.

(200

8)

SiO

2M

ass

conce

ntrat

ion:

2,47

0,00

0-F

locc

ula

tion

and

sedim

enta

tion

86–9

9%ofTw

een-c

oat

edSi

O2

wer

ere

move

daf

ter

1.5

hfirs

tly,by

flocc

ula

tion

and

then

by

sedim

enta

tion.

Jarv

ieet

al.(2

009)

Au

100–

3000

-Act

ivat

edsl

udge

bio

mas

sRem

ova

lof91

%ofA

u.

Kis

eret

al.(2

012)

ZnO

Dosi

ng

conce

ntrat

ion

per

cycl

e:50

00-P

rim

ary

clar

ifica

tion

-Aer

atio

n-S

econdar

ycl

arifi

catio

n

70%

ofth

edose

dZnO

wer

ere

move

d.

During

sim

ula

ted

SBR

pro

cess

es,

ZnO

wer

eco

mple

tely

rem

ove

din

each

cycl

eth

rough

outth

e11

-day

dura

tion.

Hou

etal

.(2

013)

304

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et al., 2009), suggesting that dispersed C60 and SiO2 nanoparticle removalwill be similar to that for TiO2. Similar removal efficiencies for CNTs alsotake place when humic acids are readily adsorbed (Hyung et al., 2007) andwhen the humic acids’ Freundlich coefficients are similar to that of TiO2

(Esparza-Soto and Westerhoff, 2003).Removal of the colloid-sized material in treated wastewater is affected by

the design and operational efficiency of each unit process such as sedimen-tation, granular media and/or membrane separation. Mass flow modelingof ENM during wastewater treatment considered fixed removal efficiencies(97%) of particles based solely on their size <100 nm or uniform removal dis-tributions (90.6–99.5%) as determined by bench-scale studies in the presenceof biomass (Mueller and Nowack, 2008; Gottschalk et al., 2009). Benn andWesterhoff (2008) examined adsorption of Ag-NM onto wastewater biomassand concluded that typical activated sludge reactors would remove over 99%of Ag-NM in wastewater influent. Recently, Kaegi et al. (2011) confirmed thatAg-NM was sorbed to wastewater biosolids both in the sludge and in the ef-fluent. Once attached, removal of the ENM is related to the management andremoval of the suspended biomass. The WWTP secondary sedimentationunit processes are very effective at settling biomass and thus would removesorbed ENM. This explains the high removals reported in the batch exper-iments that served as the basis for mass flow modeling of ENM (Limbachet al., 2008; Gottschalk et al., 2010). Westerhoff et al. (2011) observed thatthe raw sewage Ti concentrations in their study ranged from 181 to 1233 µgL−1 (median of 26 samples was 321 µg L−1). The WWTPs removed morethan 96% of the influent Ti, and all WWTPs had effluent Ti concentrations ofless than 25 µg L−1. They attributed that the attached (trickling filters) andsuspended (activated sludge) biological treatment processes played an im-portant role in trapping nanoparticles in biomass, which can then be settledor removed via membrane filtration (Table 8).

Removal of ENM during the wastewater treatment will probably occurby sorption to sludge, along with coagulation and flocculation. This is sup-ported by recent findings where Kiser et al. (2009) reported 70–85% removalefficiency for TiO2-NM at eight U.S. WWTPs with most of the TiO2 massdetected in the settled sludge. Similar removal efficiencies and associationsto sludge were reported for CeO nanoparticles (Limbach et al., 2008). Theturbidity of wastewater containing SiO2 nanoparticles was found to decreaseby 99.7% after addition of polyaluminum chloride coagulant in the pH rangefrom 5 to 7.5 (Lin and Yang, 2004). However, outside of this pH range,there was no observable change in turbidity after coagulant addition, indi-cating very little flocculation. While bare SiO2 nanoparticles were found tobe unsettlable in wastewater over typical primary-treatment residence times,SiO2 nanoparticles coated with a surfactant (i.e., Tween 20) readily agglom-erated and were likely to be removed during primary sedimentation (Jarvieet al., 2009). Hou et al. (2012) demonstrated that a significant proportion

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(>90%) of citrate-stabilized Ag-NM would remain in the water stream afterprimary clarification. However, aeration and secondary clarification of thesimulated sequencing batch reactor process completely removed the Ag-NMthroughout the 15-day experiment.

Filter media coated with biofilm extracellular polymers retained moreENM than uncoated filter media to an extent unaccountable for simply byelectrostatic attraction (Tong et al., 2010). Carboxylated and PEG-coatedquantum dots also accumulate in biofilms (Morrow et al., 2010). In thepresence of high concentrations of ENM, biofilm sloughing due to Ag-NMwas observed, whereas carbon nanotubes affect biofilm attachment and ex-hibit potential ENM–biofilm interactions (Fabrega et al., 2009; Upadhyayulaand Gadhamshetty, 2010). Submerged and pressurized microfiltration mem-brane systems are becoming an increasingly common means of achievingsecondary solids separation. These membranes commonly have 0.1–0.4 µmpore sizes and achieve very high levels of colloid removal (Meng et al., 2009).Although the beneficial use of nanoparticles in conjunction with innovativemembrane treatment systems has been identified, only a few reports specif-ically on removal of engineered nanoparticles from wastewater by mem-branes currently exist (Lipp et al., 2009; Guo et al., 2010; Jassby et al., 2010).In one study (Lipp et al., 2009), ultrafiltration membranes (20 nm) were ob-served to remove polystyrene or magnetic nanoparticles (20–250 nm) betterthan microfiltration membranes (100 nm) did. The removal mechanisms forcolloids and membranes are complex and involve not only size exclusion butalso colloid surface charge interactions with bare membrane or membranefoulants; the mechanisms are also affected by water movement (dead-endvs. cross-flow membrane designs).

Musee (2011) illustrated through modeling how the WWTP removalefficiency of ENM from the influent influences the mode of introducingnanoscale pollutants into the environment. For instance, at higher efficiencyregime, most ENM are removed from the influent but are adsorbed into thebiosolids. Conversely, at low efficiency regime of the WWTP—most ENMpasses through untreated—and introduced into the environment through thetreated effluent. This implies that effective techniques for removing or neu-tralizing the ENM in the biosolids need to be developed to ensure continueduse as fertilizer and/or compost (Westerhoff et al., 2013).

7. CONCLUSIONS AND FUTURE RESEARCH NEEDS

Nanomaterials reach wastewater through commercial discharge from indus-tries producing nanomaterial products, domestic discharge of consumerproducts containing nanomaterials, and application of nanomaterials inwastewater treatment process. Increasingly, nanomaterials will be used intreating both potable and wastewater resources, resulting in the potential

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accumulation of residual nanomaterials in end products including biosolidsand treated effluents. Nanomaterials from wastewater streams reach terres-trial and aquatic ecosystems during the disposal of wastewater sludge to soiland treated effluent to rivers and ocean.

The lack of information concerning the fate of ENM in wastewater sys-tems is flagrant, and the poor understanding of the impact of ENM on humanhealth and the environment is placing the water industry in a defensive posi-tion. Life cycle concepts can play a crucial role in dealing with the uncertain-ties encountered in relation to the effects of ENM. Thus, a combination oflife cycle concepts and the current knowledge on effects of ENM on humanhealth and the environment could provide a basis for an adaptive risk assess-ment and decision making by regulators at an early stage of nanoproductsdevelopment. Given the current limited knowledge of nanomaterial dynam-ics in wastewater streams, we consider the following research areas shouldbe pursued:

• Identification of the sources of ENM in various wastewater streams.• Development of analytical methods to quantify and characterize

wastewater-derived ENM.• Monitoring their transport and detecting their impact on the ecosystem

and human health.• Characterization of the exposure routes of human to wastewater-derived

nanomaterials.• Development of guidelines regarding the allowable concentration of ENM

in discharged water and biosolids.• Long-term persistence and stabilization of ENM in environment.• Development of process-based mechanistic models to predict the fate and

impact of ENM in aquatic and terrestrial environments.• Need for screening of biological organisms, in order to find those capable

of reprocessing nanomaterials, as well as hyperaccumulating toxic speciesreleased into the environment.

• Need to consider the time-dependent storage of ENM in products/re-suspended materials, time-dependent dynamics of ENM productionamounts, and nanoproducts’ consumption volumes.

• Improved and innovative results are necessary to help stakeholders pro-mote science-based regulations for nanotechnology, and for nanotechnol-ogy to be on good terms with society.

FUNDING

The Postdoctoral Fellowship Program (PJ009219) at the National Academy ofAgricultural Science, Rural Development Administration, Republic of Korea,supported Dr. Kunhikrishnan’s contribution. This research was funded by the

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Cooperative Research Centre for Contamination Assessment and Remediationof the Environment (CRC CARE).

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