© 2009 by Taylor & Francis Group, LLC
155
7
Treatment of
Nanoparticles in
Wastewater
Kim M. Henry
AMEC Earth & Environmental
Kathleen Sellers
ARCADIS U.S., Inc.
Commercial products incorporating nanomaterials eventually reach the end of their
usable life. Sunbathers w
ash sunscreen containing titanium dioxide (TiO
2
) nanopar-
ticles from their skin; antimicrobial silver particles drain from washing machines in
therinsecycle;paintsandcoatingsake;ormaterialsarelandlled.Whathappens
tothosenanoparticlesattheendofproductlife?Inshort,nooneknows.Initialatten-
tionhasfocusedonthefateofnanoparticlesinwastewatertreatment.Nanoparticles
canenteramunicipalwastewatertreatmentplantasaresultofcommercialuseand
discharge. Wastewater discharges from manufacturing processes also can contain
nanoparticles.Asillustratedbyexamplesinthischapter,however,thedischargeand
fateofnanomaterialsisdifculttoquantify.
CONTENTS
7.1 Mass Balance Considerations 156
7.1.1 CaseStudy:SilverCare™ Washing Machine 157
7.1.2 Case St udy: Socks with Na no Silver 159
7.2 Treatment P roc esses 160
7.2.1 Sedimentation 160
7.2.2 Coagulation and Flocculation 161
7.2.3 Activated Sludge 162

7.2.4 Sand Filters 164
7.2.5 Membrane Separation 165
7.2 .6 Disin fection 165
7.3 Summary 165
References 166
© 2009 by Taylor & Francis Group, LLC
156 Nanotechnology and the Environment
Thesameuniquepropertiesthatmakenanomaterialssopromisinginawide
varietyofindustrial,medical,andscienticapplicationsmayposechallengeswith
respect to wastewater treatment.
In 2
004, because the toxicity of nanomaterials and
theirfateandtransportintheenvironmentwerenotwellunderstoodatthetime,
theBritishRoyalSocietyandtheRoyalAcademyofEngineeringrecommended
that “factories and research laboratories treat manufactured nanoparticles and nano
-
tu
besasiftheywerehazardous,andseektoreduceorremovethemfromwaste
streams” [1].
Although t
he body of research regarding the toxicity, fate, and trans-
portofnanoparticleshasgrown[2],literaturesurveysin2006and2007indicatethat
thebehaviorofnanomaterialsduringwastewatertreatmenthasnotbeenwellstudied
[3, 4].
Anabstractforaresearchprojecttoevaluatetheremovalofvarioustypesof
nanoparticles during wastewater treatment, which was funded by the U.S. EPA’s
National Center for Environmental Research (NCER) for the period from 2007 to
2010, states: “Today, almost no information is available on the fate of manufactured
nanoparticles during biological wastewater treatment” [5].
This chapter discusses the potential for various treatment processes to remove

nanoparticles from waste streams.
Ag
eneral description of each process is provided,
as well as an evaluation of how particular properties of nanomaterials can reduce
or enhance the effectiveness of the process.
Research 
ndingsareprovidedwhere
available, or an indication is given as to whether research is ongoing at the time
of writing this book.
While t
he primary focus is treatment processes in a typical
municipal wastewater treatment plant, many of these processes are used in industrial
wastewater treatment.
Certain p
rocesses also may apply to drinking water treatment
and,whererelevant,thendingsfromwatertreatmentresearcharealsodiscussed.
7.1 MASS BALANCE CONSIDERATIONS
Concernsoverthepresenceofnanoparticlesinwastewaterstreams,whichcould
eventually accumulate in sewage sludge or discharge to the environment in treated
wastewater, must be put into context.
The c
oncentration of a nanomaterial in waste-
water depends primarily on:
The amount of local production or use of commercial products containing
nanomaterials
Whe
therthenanomaterialsarexedinamatrix(suchasthecarbonnano-
tu
besinatennisracket)orfree(suchasTiO
2

nanoparticlesinsunscreen)
Theamountofthefreenanomaterialintheproduct
The fraction that is washed down the drain
The degree of agglomeration or adsorption occurring in aqueous solution
thatchangestheformofthenanoparticleorremovesitfromsolution
Theextentofdilution
Nostudieshavebeenpublishedofwhichtheauthorsareawarethatattemptto
quantify the discharge of nanomaterials into wastewater treatment plants.
Given t
he
recentgrowthoftheindustry,thewidevarietyofmaterialsenteringthemarket,and
thecondentialityoftheirformulation,thiscomesasnosurprise.
Two ca
se studies
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© 2009 by Taylor & Francis Group, LLC
TreatmentofNanoparticlesinWastewater 157
illustrate both the potential for nanomaterials to enter wastewater streams and the
difcultyinmakingsuchanestimatewhenthedetailsofproductmanufactureare
proprietary. Coincidentally, b
oth examples concern the discharge of silver when
washing clothes.
7.1.1 CASE STUDY: SILVERCARE™ WASHING MACHINE
Samsung’s SilverCare™ option on several models of washing machine uses silver
ions to sanitize laundry. Samsung r

eportedly spent $10M to develop this technology
[6]. The d
etails of the technology are, understandably, proprietary. Company litera-
ture describes the technology in several ways. According t
ooneaccount[6],thesys-
temelectrolyzespuresilverintonano-sizedsilverions“approximately75,000times
smallerthanahumanhair”;assumingthatahumanhairisapproximately60to120
micrometers (μm) wide [4], then the silver nanoparticles would be on the order of 1
nm in diameter.
Elsewhere[7],Samsungdescribedtheirsystemasfollows:
“[A] grapefruit-sized device alongside the [washer] tub uses electrical currents to nano-
shavetwosilverplatesthesizeoflargechewinggumsticks.Ther
esultingpositivelycharged
silver atoms — silver ions (Ag
+
)—areinjectedintothetubduringthewashcycle.”
Thesetwodescriptionsdifferenoughtomakeitunclearwhetherthesilverisreleased
asatruenanoparticle(ca.1nmdiameter)orasionicsilver.(Silverhasanatomic
diameterof0.288nmandanionicradiusof0.126nm[8],andthussilverionsare
smaller than the nanoparticle size range of 1 to 100 nm.) Based o
n the electrolysis
process, both may be present. Key a
ndMaas[9]indicatethatelectrolysisofasilver
electrodeindeionizedwaterproducescolloidalsilvercontainingbothmetallicsilver
particles(1to25wt%)andsilverions(75to99wt%).Thes
ilver particles observed in
colloidalsilvergenerallyrangeinsizefrom5to200nm;aparticle1nmindiameter
would consist of 31 silver atoms. This i
nformation suggests — but certainly does not
conclusively prove — that the SilverCare™ washing m

achine discharges a mixture
ofsilverionsandsilvernanoparticles.Silverions,ratherthannanoparticles,may
comprise most of the mass.
Samsung has offered several indications of the amount of silver released when
washing a load of clothing.
T
heir p
roduct literature notes that electrolysis of silver
generatesupto400billionsilverionsduringeachwashcycle[6,10].Thet
wo chew-
ing-gum sized plates of silver reportedly last for 3000 wash cycles [10]. Finally,
S
amsung reportedly has indicated that using a SilverCare™ washing machine for a
yearwouldrelease0.05gsilver[11].
Withrespecttothesanitizingfunctionthatthisreleaseofsilverprovides,Sam-
su
ng has indicated that the silver ions “eradicate bacteria and mold from inside the
washer” and “stick to the fabric” of clothes being washed to provide antibacterial
functionforupto30days[10].
A
S
amsung representative stated that “silver nano
ions can easily penetrate ‘non-membrane cell’ [sic] of bacteria or viruses and sup-
presstheirrespirationwhichinturninhibit[sic]cellgrowth.Ont
he other hand,
Silver Nano is absolutely harmless to the human body” [6].
WhileSamsunghasmarketedthisantibacterialactionasabenettocustomers,
someconsumershavebecomeconcernedaboutthepotentialconsequencesofusing
© 2009 by Taylor & Francis Group, LLC
158 Nanotechnology and the Environment

SilverCare™ products.Initialeffortstomarketthewashingmachinemetwithresistance
inGermanyandthewashingmachinewastakenoffthemarketinSwedenforabrief
timeduetoconcernsoverthepotentialtoxiceffectsofdischargingsilvernanoparticles
from the use of these machines to wastewater treatment plants [11, 12]. Chapter 4 dis-
cusses regulatory actions in the United States regarding such washing machines.
Attemptstoquantifythedischargeofsilverfromusingthewashingmachine
— and thus illuminate the potential effects on a municipal wastewater treatment
plant—providearangeofanswersbasedontheavailabledata.Inadditiontothe
information provided above regarding the mass and potential form of silver released,
the following assumptions about wastewater generation were used to complete a con-
servative mass balance:
Eachwashcycleuses12.68gallonsofwater[13].
The typical residence generates approximately 70 gallons of wastewater per
person per day [14].
Afour-personhouseholddoestwoloadsoflaundryperdayonaverage.
Allthesilvergeneratedinthewashingmachineentersthesewage.
Further,theauthorsmeasuredthesizeofastickofgumatapproximately0.2by1.8
by7.2cm,assumedthatthedensityofasilverbarwas10.4g/cm
3
[8], and conser-
vativelyassumedthattheentiremassofsilverinthetwoplateswouldbeentirely
consumed within the 3000-cycle lifetime.
Asarstapproximation,theamountofnanosilverparticlesthatcouldentera
wastewater treatment plant from the use of SilverCare™ in w
ashing clothes could
range from 0.001 micrograms per liter (μg/L) to an extreme upper bound concentra-
tionof9μg/L.Thelowestestimateisbasedonthereportedreleaseof0.05gsilver
peryearandtheassumptionthatonly25%ofthemasswouldcomprisenanoparticles
(rather than ions) of silver. The highest estimate is based on complete consumption
ofthetwosilverplatesduringtheunitlifetimeandtheassumptionthat75%ofthe

silverwasinnanoparticulateform.Theactualconcentrationofnanoparticleswould
be lower than either of these estimates due to adsorption and agglomeration. Labora-
to
ryexperimentswithsolutionsof25-nmand130-nmsilverparticlesshowedthat
uponvortexmixing,thesilveragglomeratedintoparticlesrangingupto16μmin
diameter, well outside the nanoparticle range [15]. Further, the mass balance calcula-
tionsdonotaccountfordilutionbysourcesofwastewaterotherthandomesticsew-
agefromhomesusingSilverCare™ washing machines. Dilution from other sources
wouldalsodecreasetheconcentrationofsilvernanoparticles.Thus,theupperbound
estimateof9μg/Lshouldberegardedasanextremeupperbound.
Whateffectcouldthisdischargeofsilverhaveonthemicroorganismsina
wastewater treatment plant? As d
escribed previously, silver has antimicrobial prop-
erties. At the time this book was written, the authors could not identify published
benchmarksthatenabledthemtodirectlycomparetheestimateddischargeofsilver
nanoparticlestolevelsthatareeither“safe”or“toxic”tomicroorganismsatasew-
agetreatmentplant.Theacuteambientwaterqualitycriterionforsilver,whichwas
not derived specically for nanoparticles, is 3.2 μg/L [16]. This concentration is
comparable to the upper bound estimate of the discharge of silver nanoparticles into
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© 2009 by Taylor & Francis Group, LLC
TreatmentofNanoparticlesinWastewater 159
wastewater from using the SilverCare™ system; however, as noted above, that upper
bound estimate was quite conservative. As d
escribed below, research on the toxicity
of silver nanoparticles provides further relevant information.
Rojoetal.[17]assayedthetoxicityofcolloidalsilvernanoparticlesinthe5-to

20-nmsizerangetozebrashembryos.
T
hey t
ested solutions containing between
1and5000μg/Lsilvernanoparticles.Theirinitialtestsshowednoeffectondevel-
opment or survival of the embryos in the rst 2 weeks. Subsequent e
xperiments
monitored effects on eight selected genes. At t
he highest nanosilver concentrations
tested,theresearchers“foundacleareffectongeneexpressioninmostcases.”Those
c
oncentrations were, however, orders of magnitude higher than the estimated levels
of silver nanoparticles in wastewater described above.
Otherresearchershaveworkedwithmammaliancelllinestotestthetoxicityof
silver nanoparticles.
Hussain e
t al. [18] tested the effect of solutions containing 10 to
50 μg/L silver nanoparticles (15 nm) on PC-12 cells. This n
euroendocrine cell line
originated from Rattus norvegicus (N
orwegian rat). The research team observed
decreasedmitochondrialfunctioninthePC-12cellsuponexposuretothesilver
nanoparticles. Skebo e
tal.[15]showedthatratlivercellscouldinternalizesilver
nanoparticles (25, 80, 130 nm) but that agglomeration of nanoparticles can limit
cell penetration. Finally, B
raydich-Stolleetal.[19]testedtheeffectsof15-nmsilver
nanoparticlesonacelllineestablishedfromspermatogoniaisolatedfrommice.The
n
anoparticles reduced mitochondrial function and cell viability at a concentration

between5and10μg/mL(or5000and10,000μg/L).Ther
esearchers estimated the
EC50, or the concentration that would provoke a response half-way between the
baseline and maximum response, at 8750 μg/L. This l
evel is orders of magnitude
higher than the rst approximation estimates of silver nanoparticles in wastewater
from using the SilverCare™ s
ystem.
7.1.2 CASE STUDY: SOCKS WITH NANO SILVER
Several manufacturers market socks impregnated with nanosilver particles as an
antibacterial agent. Westerhoff’s [
20] team at Arizona State University measured the
amountofsilverthatvedifferentbrandsofsockscouldreleasewhenwashed.They
simulatedwashingbyplacingthesocksindeionizedwaterfor24hours(hr)onan
orbital mixer, removing, drying, and then rewashing the socks three times (for a total
of four wash cycles).
F
our o
f the test socks initially contained silver at 2.0 to 1360
μg/g sock. The 
fthsockcontainednomeasurablesilver.Theamountofsilverthat
leachedoutofthesilver-bearingsocksafterfoursimulatedwashcyclesrangedfrom
0to100%.Thec
oncentrationofsilverinthewashwaterrangedfromlessthan1to
600μgin500mLwashwater,orupto300μg/L.Ther
esearch team noted that it
was difcult to distinguish between silver ions, silver nanoparticles, and aggregated
silver nanoparticles in the wash water.
These initial laboratory results are difcult to extrapolate to the concentration
ofsilverthatmightresultinsewagefromwashingsockscontainingsilvernanopar

-
ticles. As n
oted above, the typical wash cycle uses more than 12 gallons of water
(ratherthan500mL)andrunsformuchlessthan24hr,suggestingthatdilutionand
© 2009 by Taylor & Francis Group, LLC
160 Nanotechnology and the Environment
ashorterleachingtimemightresultinlowerconcentrationsthanweremeasuredin
theexperiment.Thedifferenceinthevolumeofwashwateralonemightaccountfor
dilutionbyafactorof25;additionaldilutionbyothersourcesofwastewaterwould
reducetheconcentrationstillfurther.Themostdifcultvariabletoquantifywould
bethenumberofsockswashedperloadoflaundry(althoughasanyparentwould
attest,thatvariablecouldincreasetheestimateddischargeofsilverbyatleastan
orderofmagnitudeovertheestimatefromwashingasinglesock).
As these examples show, estimating the discharge of nanomaterials from the
use of commercial products is no simple matter. The m
assorconcentrationreleased
totheenvironmentdependsontheamountandavailabilityofthematerial,among
otherfactors,andsuchproprietaryinformationcanbedifculttoobtain.Thep
os-
sibleeffectsofexposurecanonlybeinferredfromthedevelopingtoxicologicaldata-
base. Some r
esearchisbeginningtoproduceinformationonthepossiblefateof
nanomaterials once released; the next section of this chapter describes the fate of
nanomaterialsinamunicipalwastewatertreatmentplant.
7.2 TREATMENT PROCESSES
Municipal wastewater treatment plants are designed to accelerate the natural pro-
cesses that remove conventional pollutants, such as solids and biodegradable organic
material, from sanitary waste. Treatment processes include:
Physical treatment, to screen out or grind up large-scale debris, to remove
suspendedsolidsbysettlingorsedimentation, and to skim off oating

greases
Bi
ologicaltreatment,topromotedegradationorconsumptionofdissolved
organicmatterbymicroorganismscultivatedinactivated sludge or trick-
ling lters
Chemical treatment, to remove other constituents by chemical addition, or
to destroy pathogenic organisms by disinfection
Ad
vanced treatment, to remove specic constituents of concern by such
processes as activated carbon, membrane separation, or ion exchange
Similarprocessesareusedindrinkingwatertreatment.Coagulation,bythe
addition of alum and other chemicals, removes suspended solids that cause turbidity
and objectionable taste and odors. The oc formed during coagulation is removed
by sedimentation. Sand lters or
other porous media such as charcoal subsequently
remove smaller particles that remain in suspension. (While more commonly used in
water treatment than wastewater treatment, some wastewater treatment systems do
incorporatesandltration.)Disinfection removesbacteriaormicroorganisms[21].
Processes indicated in italic font above are discussed with regard to their poten-
ti
al to remove nanoparticles from waste streams.
7.2.1 SEDIMENTATION
Sedimentationorsettlingisintendedtoremovesuspendedinorganicparticlesthatare
1μminsizeorgreater.Becauseoftheirsize,freenon-agglomeratednanoparticles
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© 2009 by Taylor & Francis Group, LLC
TreatmentofNanoparticlesinWastewater 161

willnotberemovedduringsettling,unlessbytheactionofcoagulantsorocculants
or by the adsorption of the nanoparticles onto large particles [3]. For f
urther discus-
sion of the forces affecting the settling of nanoparticles, see Chapter 6.
7.2.2 COAGULATION AND FLOCCULATION
Coagulation and occulation are typically used to remove solids in water treatment;
certain wastewater treatment applications can include these processes. Coagulation
ca
nfacilitatetheremovalofnanomaterialspriortosedimentationormembrane
separation [3].
Coagulationreferstothenetreductioninelectricalrepulsiveforcesatparticle
surfaces to allow them to agglomerate.
In a
treatment plant, operators rapidly mix
acoagulant(suchasaluminumorironsalts,orlong-chainpolyelectrolytes)intothe
water to destabilize colloids.
Flocculation i
s the process of aggregating those par-
ticles by chemical bridging between particles. After t
he coagulation step, water is
slowlymixedtoallowparticlestocollideandoctoform.Sedimentationr
emoves
theoc,ormembraneseparationcanbeusedtopolishthewater.
Huangetal.[22]performedjarteststoevaluatetheoptimaldosageofthe
coagulant poly-aluminum chlorate (PACl) and the optimum pH required to remove
nanoscale silica from chemical mechanical polishing wastewater generated from
semiconductor manufacturing.
Prior t
o use, the silica present in the polishing slurry
hasauniformparticlesizeof100nm.Aftert

he polishing process, the colloidal sil-
ica particles present in the wastewater range in size from 78 to 205 nm and, without
pretreatment,canpenetrateandclogthemicroltrationmembrane.
The r
esearchers
foundthatsupernatantfromthejartestshadthelowestturbiditywhenthepHwas
around6andtheconcentrationofPAClwasgreaterthan10mg/L.
At p
H6,the
PAClactstoneutralizethenegativelychargedsilicaandtodestabilizethecolloidal
particles. Supernatant r
epresentative of the range of optimal conditions identied in
thesettleabilitytestswasthensubjectedtolterabilitytestingbymeasuringthetime
to pass 50 mL of the supernatant through the microltration membrane.
This t
esting
conrmedthatapHof6andaPAClconcentrationof30mg/Lproducedtheshortest
ltration time. The coagulation enlarged the particle size such that nearly all the par
-
ti
cles were greater than 4000 nm in diameter. Although s
ubsequent microltration
througha500-nmmembraneremovedapproximately95%ofthesilica,silicastill
remainedinthetreatedwastewaterataconcentrationof44mg/L[22].
Kvinnesland and Odegaard [23] studied the effect of different polymers on the
coagulation and occulation of humic substances present in water primarily as
nanoparticles less than 100 nm in size.
For t
hepurposesoftheirstudy,theydened
coagulation as the process by which the nanoparticles formed aggregates that could

beremovedbya100-nmlter,andocculationastheprocessbywhichtheparticles
furtheragglomeratedforremovalbyan11,000-nmlter.
The researchers found that
thevedifferentpolymersachievedthesamemaximumremovalofnanoparticles
via coagulation (approximately 95% removal).
The c
oagulation was achieved by the
additionofcationicchargeregardlessofthetypeofpolymerapplied.Removalo
f
thehumicsubstancesbyocculationvariedaccordingtothechargedensityofthe
different polymers [23].
© 2009 by Taylor & Francis Group, LLC
162 Nanotechnology and the Environment
InaprojectfundedbyNCERfortheperiodfrom2004to2007,Westerhoffet
al.[24,25]areresearchingthefate,transformation,andtoxicityofmanufactured
nanomaterials in drinking water.
As p
artoftheirresearch,theyhaveconducted
jar tests of coagulation, occulation, sedimentation, and ltration to evaluate the
removalofmetaloxidenanoparticlesduringtypicaldrinkingwatertreatmentpro
-
cesses. The metal oxide nanoparticles are present in solution as stable aggregates
that range in size from 500 to 10,000 nm [24]. Metal c
oagulants(alum)andsalt
(magnesium chloride) were added to solutions of commercial metal oxide nanopar-
ti
cles, lab-synthesized hematite nanoparticles, and cadmium quantum dots. Accord-
in
gtoapaperpresentedattheNSTI-Nanotech2007Conference[25],“removalof
nanomaterials by coagulation, occulation and sedimentation processes was rela

-
ti
vely difcult.” More t
han 20% of the commercial metal oxide and the laboratory-
synthesized hematite nanoparticles remained in the water following these processes.
Fo
r all the nanoparticles tested, microltration through a 0.45-μm lter following
sedimentation removed additional nanoparticles.
However, 5
to 10% of the initial
concentrationofparticlesremainedaftercompletionofthesimulateddrinkingwater
treatment process [25].
Thepresenceofotherconstituentsinthewatercanaffectthecoagulationand
occulation of nanoparticles.
In a
presentationtotheNationalInstituteofEnviron-
mental Health Sciences, Westerhoff suggests that dissolved organic matter (DOM)
present in water may stabilize nanoparticles by inhibiting the formation of aggre
-
gates.TheDOMthusaffectstheremovalofnanoparticlesduringsedimentation
and ltration [20]. For e
xample,Fortneretal.[26]haveconductedresearchonthe
factors that affect the formation of nano-C60, the water-stable aggregate that forms
when fullerenes (C60) come in contact with water.
Their r
esearch shows that the pH
ofthewateraffectstheparticlesizeofthenano-C60,andtheionicstrengthaffects
thestabilityofthenano-C60insolution[26].
Similarly, multi-walled carbon nanotubes are hydrophobic and would be
expected to aggregate and settle out in water.

However, r
esearchers at the Georgia
InstituteofTechnologyhaveobservedthatmulti-wallednanotubesadsorbtoorganic
material that occurs naturally in river water, forming a suspension that persisted for
the month-long period of observation.
Thenaturalorganicmatterappearedtobea
betterstabilizingagentthansodiumdodecylsulfate,asurfactantoftenappliedin
industrial processes to stabilize carbon nanotubes [27].
This t
ype of interaction of
nanoparticleswithconstituentsinnaturalwaterswouldlikelyaffecttheirremoval.
7.2.3 ACTIVATED SLUDGE
Somenanoparticlescanberemovedbyadsorptiontoactivatedsludge[3].Aresearch
projectfundedbyNCERfortheperiodfrom2007to2010willaddressthefateof
manufactured nanoparticles during biological wastewater treatment.
The i
nvestiga-
tors (Westerhoff, Alford, and Rittman of Arizona State University) indicate that the
objectiveoftheirresearchistoquantifytheremovaloffourtypesofnanoparticles
(metal-oxide, quantum dots, C60 fullerenes, and carbon nanotubes) during wastewa
-
ter treatment. Batch a
dsorption experiments will be performed using whole biosol-
ids, cellular biomass only, and extracellular polymeric substances from biological
© 2009 by Taylor & Francis Group, LLC
TreatmentofNanoparticlesinWastewater 163
reactors and full-scale wastewater treatment reactors. Nanoparticles also will be
added to laboratory-scale bioreactors to quantify biotransformations to the nanopar-
t
i

cles and toxicity to the microorganisms. E
lectron m
icroscopy imaging will be used
toevaluatetheinteractionsbetweenthenanoparticlesandthebiosolids[5].
No NCER progress reports were available for the research of Westerhoff, Alford,
andRittmanatthetimeofwritingthisbook.
H
owever, t
he investigators hypothesize
in their research abstract that “dense bacterial populations at wastewater treatment
plants should effectively remove nanoparticles from sewage, concentrate nanopar
-
ticles in biosolids, and/or possibly biotransform nanoparticles. T
he r
elatively low
nanoparticle concentrations in sewage should have negligible impact on the waste-
w
at
er treatment plant’s biological activity or performance” [5]. P
reliminary r
esults
[20]hintatthepossiblebehaviorofC60fullerenesinsewagetreatment.I
ni
nitial
tests, the research team mixed a solution of C60 aggregates and biomass in water,
then ltered the solids and measured C60 levels to determine the amount sorbed to
biosolids. T
hese r
esults were incorporated into mass balance modeling that simulated
the operation of a wastewater treatment plant at steady state. T

he r
esults indicated
that22%ofC60wouldadsorbtobiosolidsandtheremainderwouldbedischargedin
the efuent.
Westerhoff [20] noted that the model estimates must be validated with
laboratoryandeldmeasurements.
Ivanov et al. [28] conducted research to evaluate whether microbial granules
presentinabiolmcouldremovenano-andmicro-particlesfromwastewaterand
whether calcium enrichment, which is typically applied to wastewater with high
organicloading,couldenhancetheremovalofsmallparticles.
C
alcium i
ons enhance
theformationofmicrobialaggregatesbydecreasingthenegativesurfacechargeof
thecells.Therefore,particleremovalbymicrobialgranuleswasevaluatedfordif
-
fe
rent calcium concentrations. T
wo l
aboratory-scale sequencing batch reactors, one
with no calcium supplement and the other with a calcium concentration of 100 mg/L,
were inoculated with aerobic sludge and operated in parallel.
The inuent consisted
of synthetic wastewater. A
erobic g
ranules from the reactors were incubated with
particlesuspensionsofdifferentsizes:100-nmuorescentmicrospheres,420-nm
uorescent microspheres, and stained cells of
Escherichia coli. Researchers used a
confocal laser scanning microscope, a ow cytometer, and a uorescence spectrom-

e
t
ertomeasuretherateofparticleremovalandtheaccumulationofparticlesinthe
microbial granules.
Theresultsshowedthattheadditionofcalciumdidnotenhance
the removal of microspheres from the wastewater. M
icrospheres w
ere adsorbed to
the surface of the granules but the depth of penetration did not vary with the calcium
concentration, as it did for the
E. coli cells [28]. Ivanov et al. concluded that the
behavior of inorganic nanoparticles in aerobic wastewater treatment is different from
thebehaviorofbiologicalcells.
Researchers have shown that at certain concentrations, some nanoparticles may
betoxictobacteria.
Forexample,Fortneretal.[26]haveshownthatnano-C60inhibits
the growth of bacterial cultures at concentrations of 0.4 mg/L or more and decreases
aerobic respiration rates at 4 mg/L.
Other research supports the antibacterial activity
of nano-C60 water suspensions, indicating that suspensions formed by four different
processes exhibited minimum inhibitory concentrations ranging from 0.1 to 1.0 mg/
L[29].
A
sn
oted previously, silver also can have antimicrobial activity.
© 2009 by Taylor & Francis Group, LLC
164 Nanotechnology and the Environment
7.2.4 SAND FILTERS
Brownian diffusion is the dominant mechanism governing the transport of nanopar-
ticles through the granular lter. As t

heypassthroughthelter,nanoparticlesare
removedfromtheuidstreambyseveralprocesses,including:
1. Brownian diffusion causes the nanoparticles to agglomerate into larger par
-
ti
cles or to agglomerate with the lter grains.
2. Nanoparticles are immobilized by gravitational sedimentation because
theirdensityishigherthanthatoftheltermedium,ortheowvelocityis
reducedwithinthelterbed.
3. Nanoparticles are intercepted by physical contact with the lter medium
[30].Attachmentofparticlestotheltermediumisaffectedbyavarietyof
forces, described by the term “attachment efciency,” as discussed further
below [31].
The attachment efciency (
F)istheratiooftherateofparticledepositiontothe
rate of particle collisions with the lter medium [31]. This parameter is governed
byvariousphenomena,includingvanderWaalsforces,theforcesofsolvency,and
electrostatic repulsive forces (see Chapter 6).
When F is less than unity, conditions
are not conducive to particle attachment. When F
equals unity, no barriers to particle
attachment exist. When F
isgreaterthanunity,particlesmaybeattractedtothesur-
face of the lter medium over small distances. However, f
or very small nanoparticles
lessthan2nminsize,therelativeeffectsoftheforcesgoverningtheparameterF can
b
e unpredictable and different from those of larger particles. If s
maller nanoparticles
aggregate to form colloidal material, as has been observed for C60 fullerenes and

someotherparticles,thebehaviorofthematerialwithinagranularlterwilldiffer
fromtheresponsepredictedbasedonthesizeoftheoriginalmanufacturedparticle.
Therefore,researchershaveconcludedthatdirectmeasurementofthemobilityof
nanoparticlesiscurrentlythemostaccuratemeansbywhichtoquantifytheirbehav
-
io
rinporousmedia[32].
Nanoparticlemobilitywithinaporousmediumisafunctionnotonlyofsize,but
also of surface chemistry [32].
Lecoanet, Bottero, and Wiesner [30] conducted labo-
ratoryexperimentstoquantifythemobilityofeightdifferentmanufacturednanoma-
te
rialsinaporousmediumofglassbeads,whichtheresearchersindicatedwouldbe
representativeofawatertreatmentplantlterorasandygroundwateraquifer.
Their
r
esults indicated that different forms of nanoparticles with the same composition have
different mobilities.
For e
xample, of the carbon-based particles tested, single-walled
nanotubes and fullerols (hydroxylated C60) passed through the porous medium more
rapidly than the colloidal aggregate form of C60
knownasnano-C60.Thesolubi-
lizedformsoftheparticlesaremoremobilethanthesuspendedform[33].
Conditions in the waste stream, such as pH and ionic strength, will also affect
the behavior of nanoparticles in water and the attachment efciency of nanoparticles
passing through a lter medium [31].
As n
otedabove,Fortneretal.[26]observed
thatthepHandionicstrengthofwateraffect,respectively,theparticlesizeandsta-

bi
lityofthenano-C60insolution.
© 2009 by Taylor & Francis Group, LLC
TreatmentofNanoparticlesinWastewater 165
Finally, surface coatings applied to manufactured nanoparticles also will affect
their mobility in porous media. Typical s
urface coatings include polymers, poly-
electrolytes,andsurfactants,andareoftenappliedwiththeintentionofimproving
thedeliveryormobilityofthenanoparticles.
Because t
hese coatings can affect the
surface charge of the nanoparticles or stabilize the particles against aggregation,
theymayreducetheabilityoftheltermediumtoremovethenanomaterialsfrom
the waste stream [32].
7.2.5 MEMBRANE SEPARATION
Membrane separation is the general process in which contaminants are removed from
auidasitpassesthroughamicroporousmembrane.Specicm
embrane processes
aredistinguishedbythesizeoftheporesorthesizeoftheparticlesretainedbythe
membrane, as follows [34]:
Microltration (MF): 100 to 10,000 nm
Ultraltration (UF): 1 to 100 nm
Nanoltration(NF):0.1to1nm
Reverseosmosis(RO):lessthan0.1nm
Microltration of individual or agglomerate nanoparticles of 100 nm or more in
sizecanresultinfoulingthemembrane.
Particles l
ess than 100 nm in size can pass
through the membrane.
Thesmallerparticlesmustbepretreatedbycoagulationpriortothemicroltra

-
ti
on(seediscussionabove),ortreatedbyothermeans[3].Figure7.1s
hows the ranges
overwhichthesevariousformsofltrationcangenerallybeeffective.
7.2.6 DISINFECTION
A research project funded by the NCER for the period from 2005 to 2008 focuses
onthefateandtransformationofC60nanoparticlesinwatertreatmentprocesses.
In
the2006progressreport,theinvestigators,KimandHughes,documentedthe
resultsofapplyingdissolvedozone,acommondisinfectantreagent,toasuspension
containing the aggregate nano-C60.
The p
roductsofthistreatmentwerehighlyoxi-
dized, soluble fullerenes [38], suggesting that disinfection has the effect of rendering
a stable aggregate more soluble and thus potentially more mobile.
Future r
esearch
activities will include applying ultraviolet radiation and chlorine to water containing
nano-C60 [38].
7. 3 S U M M A R Y
Atthisearlypointinthenanotechnologyrevolution,weknowlittleaboutthefateof
nanomaterialsattheendofusefulproductlife.Theamountofnanomaterialreleased
totheenvironmentmaybelimitedbytherelativelylowconcentrationsoffreenano
-
mat
erials in many products; the mobility of those nanomaterials, once released,
maybelimitedbyagglomerationandadsorption.However,fewrelevantdatanow
•
•

•
•
© 2009 by Taylor & Francis Group, LLC
166 Nanotechnology and the Environment
exist,andmanufacturers’needtoprotectcondentialbusinessinformationcanlimit
access to relevant data.
Manyinitialconcernsabouttheend-of-lifefateofnanomaterialsfocusonwaste
-
water treatment. Initial research shows some potential for removal in various unit
processes. The extent of that removal, and the potential toxic effects of those nanoma
-
te
rials, vary substantially between materials. Particle size, concentration, and surface
properties, as well as the other characteristics of the wastewater, can affect removal.
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