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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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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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TA
BLE
1.
Glo
bal
pro
duct
ion
volu
mes
and
sourc
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ajor
engi
nee
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nan
om
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Engi
nee
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MPro
duct
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(EN
M)
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ide
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ates
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yA
ust
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,000
70>
500
>15
>10
0048
0—
CeO
20.
55–2
800
35–7
00>
55—
——
——
—Si
O2
55–5
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0—
55–5
5,00
075
——
—13
,500
—Fu
llere
nes
0.15
–80
2–80
0.6–
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——
——
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101
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550
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Quan
tum
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0.6–
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6–5.
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ano
Ag
>55
02.
8–20
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1>
8—
>20
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∼390
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ater
ials
.Ref
eren
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om
ater
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.(2
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280 A. Kunhikrishnan et al.
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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282 A. Kunhikrishnan et al.
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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Fate and Ecological Effects of Nanomaterials in Wastewater 283
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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Fate and Ecological Effects of Nanomaterials in Wastewater 285
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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Fate and Ecological Effects of Nanomaterials in Wastewater 287
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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288 A. Kunhikrishnan et al.
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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Fate and Ecological Effects of Nanomaterials in Wastewater 289
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 sludge: 0.053–0.147 mgkg−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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290 A. Kunhikrishnan et al.
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 sludge: 0.0093–0.068 mgkg−1
U.S. STP effluent: 0.0045–0.033 (µgL−1)
Farre et al. (2010)
Gottschalk et al.(2009)
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)
Gottschalk et al.(2009)
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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Fate and Ecological Effects of Nanomaterials in Wastewater 291
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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292 A. Kunhikrishnan et al.
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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Fate and Ecological Effects of Nanomaterials in Wastewater 293
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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294 A. Kunhikrishnan et al.
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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Fate and Ecological Effects of Nanomaterials in Wastewater 295
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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296 A. Kunhikrishnan et al.
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
rence
son
the
ecoto
xici
tyofnan
om
ater
ials
inva
rious
was
tew
ater
stre
ams
Was
tew
ater
/slu
dge
Typ
eofnan
om
ater
ial
Obse
rvat
ion
Ref
eren
ces
Riv
erw
ater
and
was
tew
ater
effluen
tSW
CN
Ts,
MW
CN
Ts,
aqueo
us
phas
eC60
,an
dco
lloid
algr
aphite
sin
gram
neg
ativ
ean
dgr
am-p
osi
tive
bac
teria.
SWCN
Ts
inac
tivat
edth
ehig
hes
tper
centa
geofce
llsin
monocu
lture
sof
Esc
her
ich
iaco
li,P
seu
dom
ona
sa
eru
gin
osa,
Ba
cillu
ssu
btil
is,an
dSt
aph
yloc
occu
sep
ider
mis
,an
din
div
erse
mic
robia
lco
mm
uniti
es.Ele
vate
ddis
solv
edorg
anic
mat
ter
conce
ntrat
ions
reduce
dth
ebac
teria
atta
chm
enton
SWCN
Tag
greg
ates
by
50%
,butdid
notm
itiga
teto
xici
ty.
Kan
get
al.(2
009)
Was
tew
ater
ZnO
-NM
Inhib
ition
ofnitr
oge
nan
dphosp
horu
sre
mova
lin
duce
dby
hig
her
conce
ntrat
ions
ofZnO
-NM
was
due
toth
ere
leas
eof
Zn
ions
from
ZnO
-NM
dis
solu
tion
and
incr
ease
ofre
activ
eoxy
gen
spec
ies
pro
duct
ion,w
hic
hca
use
din
hib
itory
effe
cton
poly
phosp
hat
e-ac
cum
ula
ting
org
anis
ms
and
dec
reas
ednitr
ate
reduct
ase,
exopoly
phosp
hat
ase,
and
poly
phosp
hat
eki
nas
eac
tiviti
es.
Zhen
get
al.(2
011)
Seaw
ater
TiO
2U
nder
low
inte
nsi
tyU
V,re
activ
eoxy
gen
spec
ies
inse
awat
erin
crea
sed
with
incr
easi
ng
nan
o-T
iO2
conce
ntrat
ion.This
incr
ease
may
lead
toin
crea
sed
ove
rall
oxi
dat
ive
stre
ssin
TiO
2-c
onta
min
ated
seaw
ater
,an
dca
use
dec
reas
edre
silie
ncy
ofm
arin
eec
osy
stem
s.
Mill
eret
al.(2
012)
Was
tew
ater
trea
tmen
tpla
nt
Ag
Silv
erw
asre
leas
edfrom
com
mer
cial
cloth
ing
(sock
s)in
tow
ater
,an
den
ded
up
inw
aste
wat
ertrea
tmen
tpla
nts
.The
hig
hsi
lver
conce
ntrat
ion
may
limit
the
sludge
dis
posa
las
agricu
ltura
lfe
rtili
zer.
Ben
nan
dW
este
rhoff
(200
8)
Was
tew
ater
trea
tmen
tsy
stem
Ag-
NM
Ag-
NM
,Ag+
ions
(AgN
O3),
and
AgC
lco
lloid
s,al
lat
1m
gL−
1
Ag,
inhib
ited
resp
irat
ion
ofau
totrophic
nitr
ifyi
ng
org
anis
ms
by
86%
,42
%,an
d46
%,re
spec
tivel
y.
Choiet
al.(2
008)
Was
tew
ater
trea
tmen
tsy
stem
Ag-
NM
Origi
nal
was
tew
ater
bio
film
sw
ere
hig
hly
tole
rantto
the
Ag-
NM
trea
tmen
t.Red
uct
ion
ofbio
film
bac
teria
afte
rth
eap
plic
atio
nof20
0m
gA
gL−
1Ag-
NM
was
insi
gnifi
cantaf
ter
24h.Rem
ova
loflo
ose
lybound
extrac
ellu
lar
poly
mer
icsu
bst
ance
sre
duce
dth
evi
abili
tyofw
aste
wat
erbio
film
s.W
hen
trea
ted
aspla
nkt
onic
pure
cultu
re,bio
film
sw
ere
hig
hly
vuln
erab
leto
Ag-
NM
.
Shen
gan
dLi
u(2
011)
(Con
tin
ued
onn
ext
page
)
297
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15
TA
BLE
7.
Sele
cted
refe
rence
son
the
ecoto
xici
tyofnan
om
ater
ials
inva
rious
was
tew
ater
stre
ams
(Con
tin
ued
)
Was
tew
ater
/slu
dge
Typ
eofnan
om
ater
ial
Obse
rvat
ion
Ref
eren
ces
Was
tew
ater
trea
tmen
tsy
stem
CeO
2,TiO
2,Ag,
and
Au
CeO
2-N
Mca
use
dth
egr
eate
stin
hib
ition
for
ord
inar
yhet
erotrophic
org
anis
ms
(OH
O)
and
amm
onia
oxi
diz
ing
bac
teria
(AO
B).
The
Ag-
NM
cause
dan
inte
rmed
iate
and
slig
htin
hib
ition
for
OH
Oan
dAO
B,re
spec
tivel
y,w
her
eas
Au
and
TiO
2-N
Ms
cause
donly
slig
ht
or
no
inhib
ition
for
both
the
bio
mas
ses.
Gar
cıa
etal
.(2
012)
Act
ivat
edsl
udge
Cu
Most
pro
bab
lenum
ber
test
and
resp
irat
ion
dat
ain
dic
ated
that
10Cu
2+m
gL−
1w
ere
toxi
cto
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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Fate and Ecological Effects of Nanomaterials in Wastewater 299
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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300 A. Kunhikrishnan et al.
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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Fate and Ecological Effects of Nanomaterials in Wastewater 301
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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302 A. Kunhikrishnan et al.
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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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
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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imno
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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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Fate and Ecological Effects of Nanomaterials in Wastewater 305
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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306 A. Kunhikrishnan et al.
(>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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308 A. Kunhikrishnan et al.
Cooperative Research Centre for Contamination Assessment and Remediationof the Environment (CRC CARE).
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