rXXXX American Chemical Society A dx.doi.org/10.1021/es103769k | Environ. Sci. Technol. XXXX, XXX, 000–000

ARTICLE

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Solubility and Batch Retention of CeO2 Nanoparticles in SoilsGeert Cornelis,*,† Brooke Ryan,† Mike J. McLaughlin,†,‡ Jason K. Kirby,‡ Douglas Beak,#,‡ andDavid Chittleborough§

†School of Agriculture, Food and Wine, University of Adelaide, SA 5005 Adelaide, Australia‡CSIRO Land and Water, Centre for Environmental Contaminants Research, Advanced Materials Transformational CapabilityPlatform, PMB 2, SA 5064 Glen Osmond, Australia§School of Earth and Environmental Sciences, University of Adelaide, SA 5005 Adelaide, Australia

bS Supporting Information

’ INTRODUCTION

Cerium oxide (CeO2) nanoparticles (CeO2 NPs) are used inmany new applications that take advantage of the high oxygenstorage and the UV absorbing capacity of CeO2 NPs and the lowredox potential of the CeIV/CeIII redox couple.1 CeO2 NPs areused particularly in the automotive industry as catalysts reducingthe particulate outputs of diesel engines,2 but are also found in avariety of consumer, pharmaceutical, and agricultural products.3

The increased production of CeO2 NPs has raised a number ofconcerns because similarly to other manufactured nanoparticlesthere are many uncertainties regarding the exposure, fate, andeffects of CeO2 NPs on organisms in the environment.4 Thenatural environment may be exposed to CeO2 NPs from exhaustcatalysts after deposition on soils, when they are collected withroad runoff,5 or by industrial wastewaters that contain CeO2

NPs.6 Very fine (<1 μm) exhaust particulates cause a very diffusepollution,5 and CeO2 NP contamination does not cause asignificant cerium (Ce) enrichment in natural waters.6 This ispossibly because biosolids accumulate up to 95% of CeO2NPs inwastewater treatment plants2 which curbs CeO2 NP dischargeinto the aquatic environment. However, a much less diffuseCeO2 NP contamination may occur when biosolids from waste-water treatment plants are applied to agricultural land as soilamendments. Application of biosolids to land has been posited to

be themain environmental exposure pathway in the case of manyothermetal based nanoparticles.7Depending on production volumeand physicochemical properties, exposure rates of manufacturednanoparticles to soils have been projected to increase by micro-grams or several nanograms each year.7

Recent studies have shown that CeO2 NPs negatively affectthe growth of plants8 and soil nematodes9 by producing reactiveoxygen species, which necessitates risk assessment of CeO2 NPsin soils, but the bioavailability of soil contaminants may dependon their retention by soil particles. In the case of nanoparticles,retention can be regarded as an ensemble of time-dependentaggregation and deposition reactions that are influenced by acomplex array of soil properties, such as pH, ionic strength (IS),dissolved organic carbon (DOC), and clay content.10�12 Dis-solution may change the risk associated with CeO2 NP exposure,because dissolved Ce is not considered to be highly toxic to soilorganisms and is sometimes used to fertilize soils.13 Availabletransport and bioavailability models of nanoparticles are cur-rently in a state of development, and evaluations of nanoparticle

Received: November 8, 2010Accepted: March 2, 2011Revised: February 25, 2011

ABSTRACT: There is a paucity of information on the environmental fate ofcerium oxide nanoparticles (CeO2 NPs) for terrestrial systems that may beexposed to CeO2 NPs by the application of biosolids derived from waste-water treatment systems. Using ultrafiltration (UF), dissolution, and none-quilibrium retention (Kr) values of citrate-coated (8 nm diameter) CeO2

NPs and partitioning (Kd) values of dissolved CeIII and CeIV were obtained

in suspensions of 16 soils with a diversity of physicochemical properties.Dissolution of CeO2 NPs studied in solutions was only significant at pH 4and was less than 3.1%, whereas no dissolved Ce was detected in soils spikedwith CeO2 NPs. Kr values of CeO2 NP were low (median Kr = 9.6 L kg�1)relative toKd values of dissolved Ce

III and CeIV (median Kd = 3763 and 1808L kg�1, respectively), suggesting low CeO2 NP retention in soils. Surfaceadsorption of phosphate to CeO2 NP caused a negative zeta potential, whichmay explain the negative correlation of log Kr values with dissolved phosphate concentrations and the significant reduction of Kr

values upon addition of phosphate to soils. The positive correlation of Kr values with clay content suggested heterocoagulation ofCeO2 NPs with natural colloids in soils. Co-addition of CeO2 NPs with biosolids, on the other hand, did not affect retention.

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dissolution in the soil environment are lacking. The risk asso-ciated with nanoparticles in soils, in particular CeO2 NPs, thusremains unclear.

The present study evaluated, for the first time, the solubility ofCeO2 NPs, Ce

III and CeIV ions, and bulk CeO2 in conditions re-presentative of soils. The partitioning (Kd) values of ionic Ce

III

and CeIV, and bulk CeO2, and the retention (Kr) values of CeO2

NPs were obtained in 16 Australian soils. Whereas Kd valuesoften assume an equilibrium betweenmetal concentrations in theliquid and solid phases, although it is well recognized that Kd valuesare time-dependent, Kr values express the fraction of nanoparti-cles occurring as single particles or aggregates smaller than 0.45μm after 24 h and thus do not assume equilibrium. Kr values willbe relatively high or low depending on if the physicochemicalconditions prevailing in the soil suspension increase or decreasethe aggregation and/or deposition rate of CeO2 NPs.

14 We usedthis approach to investigate the dominant mechanisms determin-ing CeO2 NP retention in soils by relating Kr values to thephysicochemical properties of 16 different soils. We also inves-tigated the effect of the coaddition of biosolids on CeO2 NPretention, which presents the most realistic exposure scenario ofsoils to CeO2 NP, and the effect of phosphate. Phosphate mayalter the surface properties of CeO2NP

1 or precipitate with Cenþ

as sparingly soluble phosphates15 that could affect CeO2 NP dis-solution in soils, e.g., after a fertilizer application.

’EXPERIMENTAL SECTION

Preparation and Characterization of CeO2 NP Stock Sus-pensions. The primary particle diameter of uncoated CeO2 NPpowder (20 nm, MTI Corporation, Richmond, CA) and CeO2

powder (nominal particle diameter <5 μm, Aldrich, St. Louis,MO) was estimated from crystallite sizes obtained by X-raydiffraction (XRD) using the Sherrer equation16 and from surfacearea determinations obtained by Brunauer�Emett�Teller (BET)-N2 adsorption analysis, in both cases assuming a spherical shape anda density of 7.21 g cm�3 in the case of BET-N2 analysis.CeO2 NPs (71.4 μM) were suspended in deionized (DI)

water or in a 0.5 mM trisodium citrate solution, which was pH-adjusted to 10 using 0.1 M sodium hydroxide (NaOH). Bothsuspensions were sonicated for 3 min at 1800 W L�1. Suspen-sions containing citrate were filtered using 0.20 μm membranefilters (Sartorius) according to Cornelis et al.,14 whereas suspen-sions in DI water were centrifuged at 2800 g for 15min to removeaggregates larger than 0.20 μm, because low Ce concentrations(ca. 0.1 μM) were found if 0.20 μm filtration was used instead ofcentrifugation. Polymeric membranes are negatively charged at

pH > 217 and may thus adsorb CeO2 NPs that are that arepositively charged in unamended DI water (see text).The effect of citrate and phosphate on the z-averaged hydro-

dynamic diameter and zeta potential as a function of pH (2�10)was examined in CeO2 NP suspensions containing only water,0.035mMphosphate, 0.5mMcitrate, or both 0.035mMphosphateand 0.5 mM citrate. The pH was adjusted using dilute NaOH ornitric acid (HNO3) solutions. z-Averaged hydrodynamic dia-meters were estimated from the diffusion in water obtained fromthe autocorrelation of Doppler shifts of scattered light over timeobtained by dynamic laser scattering (DLS, Malvern Zetasizer).Zeta potentials were estimated from electrophoretic mobilitymeasurements obtained by DLS using the Smoluchowski ap-proximation that is valid for nanoparticles.18 CeO2 NP from citratestock suspensions were also visualized using transmission elec-tron microscopy (TEM, Phillips CM200 at 120 keV) equippedwith energy-dispersive X-ray spectroscopy (EDX).Dissolution Experiments. Dissolution experiments were un-

dertaken on CeO2 NPs from citrate stock suspensions, ionic CeIII

from CeIII nitrate (CeNO3 3 6H2O, Aldrich, St. Louis, MO) dis-solved in DI water, CeIV from ammonium�cerium(IV) nitrate[(NH4)2Ce(NO3)6, Fluka, St. Louis, MO] dissolved in DI water,and bulk CeO2 at concentrations of 0.93 μM, 0.46 μM, 0.46 μM,and 7.1 mM, respectively. The dissolution was examined as afunction of time in 0.1 mM sodium chloride (NaCl) or artificialsoil solution (AS) [Supporting Information (SI) Table S1]. Thesample pHwas buffered at 4, 7, or 9 using 0.1 mM sodium acetate(NaCH3COO), 0.1 mM 2-(N-morpholino) ethanesulfonic acidsodium salt (MES), or 0.1 mM sodium borate (Na2B4O7 3 10H2O)buffers, respectively. After equilibrating for 0, 1, 14, and 28 dayson an end-overend shaker, 2 mL samples were transferred to1 kDa centrifugal ultrafiltration (UF) devices (Pall-Gellman), andafter centrifugation at 4000 g for 15 min the cerium concentra-tion in filtrates was measured using inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7500ce). All UF deviceswere preconditioned with 0.1 M copper nitrate [Cu(NO3)2] tominimize ionic Cenþ adsorption by membrane surfaces.14 Thesolution pHwas checked at each sampling time and adjusted to 4,7, or 9 using dilute hydrochloric acid (HCl) or NaOH solutionswhich brought the average IS of NaCl and AS solutions to 0.2 mMand 34.4 mM, respectively (calculated using PHREEQC).19

Soil Sampling and Characterization. Sixteen soils with vary-ing physical and chemical characteristics were selected fromlocations in Australia (Table 1). All soils were pooled samplesfrom the top 15 cm of the soil profile and were air-dried at 25 �Cand passed through a 2 mm sieve prior to analyses. Soil pH andDOC were measured in 2 mM KNO3 (1:10 m/v soil:solutionratio) following end-overendmixing for 24 h. Soluble/exchangeable

Table 1. Number of Soils Tested (n) and Spiking Rate Used in Partitioning Experiments and the Calculated Median, Minimum,and Maximum Kd and Kr Values for Different Forms of Cerium

Ce addition n

spiking conc

(μmol kg�1)

median Kd

or Kr (L kg�1)

Q25aKd

or Kr (L kg�1)

Q75bKd

or Kr (L kg�1)

geogenic Ce 16 10 498 6796 21 956

CeO2 NPs, low rate 16 9.29 9.61 4.05 102

ionic CeIII 16 9.14 3763 890 6075

ionic CeIV 16 9.14 1808 341 4242

bulk CeO2 16 11 700 136 355 47 350 871 697

CeO2 NPs, high rate 5 12 600 780 759 888a First quartile. bThird quartile.

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major cation (e.g., calcium, magnesium, potassium, and sodium)and anion (e.g., sulfur and phosphorus) concentrations were deter-mined in 0.45 μm filtrates using inductively coupled plasma-opticalemission spectroscopy (ICP-OES, Spectro) and were used toestimate ionic strength (IS) in the 2 mMKNO3 extracts. Particlesize, total carbon, total chloride, cation exchange capacity (CEC),and oxalate-extractable Al and Fe were determined according tostandard methods.20,21 The field moisture capacity of soils wasdetermined using a tension plate procedure (100 cmwater suction).Total Ce concentrations in soils were determined after strongacid extraction in aqua regia (US-EPA 3051A22) and measuredby ICP-MS. The determined Ce concentration of a certifiedreference standard (0.349 mmol kg�1) (ERM-CC690), used asquality control, was in close agreement with the certified value of0.348 mmol kg�1.Kr and Kd Values. The retention of CeO2 NPs was expressed

as Kr values (see SI for calculation) and were determined (n = 5)in 16 different soils that were spiked with low spike concentra-tions (9.29 μmol kg�1, spiked from 0.5 mM citrate containingCeO2 NP stock suspensions brought to pH 10) and high spikeconcentrations (12.6 mmol kg�1, spiked by weighing CeO2 NPsinto the soils) after shaking end-overend for 24 h (1:10 m/v soil-solution ratio). Total dissolved Ce concentrations in UF filtrates,and in 0.45 μm filtrates following digestion with HNO3, weredetermined by ICP-MS.The Kd values (n = 3) for ionic CeIII and CeIV at spiking rates

of 9.14 μmol CeIII kg�1 [from Ce(NO3)3] and 9.14 μmol CeIV

kg�1 [from (NH4)2Ce(NO3)6], respectively, were determinedafter shaking end-overend for 24 h with 2 mM KNO3 (1:10 m/vsoil-solution ratio). Bulk CeO2 Kd values (n = 4) were deter-mined at a spiking rate of 11.7 mmol Ce kg�1 in 50 g of soil thatwas subsequently shaken end-overend with 500 mL of 2 mMKNO3 for 24 h. The partitioning of geogenic Ce (n = 3) wasdetermined by shaking soils (1:10 m/v soil:solution ratio) with2 mM KNO3 end-overend for 24 h. Total dissolved Ce con-centrations in 0.45 μm filtrates were determined by ICP-MS.Particles in 0.45 μm filtrates from Kr experiments with the highCeO2 NP spike concentration were visualized and analyzed byTEM-EDX.Kr andKdValues inSoilswithBiosolidAddition.The influence

of CeO2 NP addition with biosolid material on Kr values wasexamined in five soils with varying physical and chemical proper-ties (Emerald Black, Mt. Compass, Tepko, Ingham, and Tullysoils). The biosolid material was collected from an urban waste-water treatment plant at Bolivar, South Australia, dried at 60 �Cto ca. 50%moisture content, and sieved to <2mm (Table 1). Thebiosolid material was spiked with CeO2 NPs at rate of 91 mg Cekg�1 and aged in a temperature-controlled room at 25 �C for oneweek. The CeO2 NP spiked biosolid was then added to soils (n =5) at a 1:100 m/m ratio (on a dry weight basis) that correspondsto 20 t biosolids ha�1 (a typical agronomic rate23) (final CeO2

NP spike rate = 6.50 μmol Ce kg�1). TheKr values for CeO2NPs(also spiked at 6.50 μmol Ce kg�1) and for CeO2 NPs addedwith biosolids (n = 5) were determined after shaking end-overend for 24 h with 2 mM KNO3 (1:10 m/v soil-solutionratio). The total Ce concentrations in UF filtrates and in 0.45 μmfiltrates following HNO3 digestion were determined by ICP-MS.Effect of Phosphate and Citrate. To examine the effect of

phosphate and citrate on CeO2 NP retention, Kr determinations(n = 5) were performed using a method similar to that of theprevious experiment. A 2 mMKNO3 background electrolyte wasadded to the same 5 soils that were then spiked with CeO2 NP

suspensions containing either water, 0.5 mM citrate, 3.5 μMpho-sphate fromKH2PO4, or both 0.5mMcitrate and3.5μMphosphate.The CeO2 NP concentration was higher in all treatments (35.7μmol Ce kg�1) to simultaneously examine the effect of a higherspiking rate on CeO2 NP retention.Statistical Analysis. Multiple linear regression analyses with

soil properties as variables were performed using the GENSTATsoftware package. The data were logarithmically transformed tosatisfy the condition of normality when this was required, asindicated by Shapiro�Wilkes tests. Numerous linear regressionmodels were calculated with onlymodels that had significant (P <0.05) parameter relationships retained. The linear regression modelwith the lowest Akaike information criterion was selected.24

’RESULTS AND DISCUSSION

CeO2 NP and Bulk Ce Characterization. XRD analysis sug-gested a primary particle diameter of 8 nm for CeO2NP, whereas9 nm was estimated on the basis of BET-N2 adsorption. Figure 1shows the pH-dependent average hydrodynamic diameter (dh)and zeta potential of CeO2 NP suspended in water, citrate,phosphate, and citrate/phosphate solutions that were measuredwithin 2 h after suspension preparation. The unamended pH inwater of CeO2 NP suspensions was 4.5 where dh of CeO2 NPaggregates was ca. 130 nm, slightly higher than the dh of 100 nmof CeO2 NP aggregates in citrate stock suspensions that werebrought to pH 10 (Figure 1). A similar aggregate diameter wasfound in citrate stock CeO2 NP suspensions using TEM (SIFigure S1). The 8 nm sized CeO2 NPs in aqueous suspensions

Figure 1. Average hydrodynamic diameters (top) and zeta potential(bottom) as a function of pH of CeO2 NP suspensions in water, citrate,phosphate solution or a citrate�phosphate solution. DLS cannotaccurately measure average hydrodynamic diameters higher than 1 μmso these values are not shown.

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thus always formed aggregates at the given sonication and centri-fugation settings.Figure 1 shows that CeO2 NP formed large aggregates within

2 h at pH values near the isoelectric point (IEP, estimated by zetapotential = 0). The IEP of CeO2 NP was shifted from pH 6 inwater only to pH < 2 when citrate was added, possibly becauseorganic molecules bind strongly to CeO2 NP surfaces,25 and theionization of carboxyl groups of adsorbed citrate (pKa = 3.13, 4.76,9.2) resulted in a net negative surface potential for nanoparticles.26

Phosphate also binds strongly to CeO2 NP and affects their surfaceproperties,1 but the lower concentration added (0.053 mM phos-phate relative to 0.5 mM citrate) and the weaker ionization (pKa =2.12, 7.21, 12.37) resulted in less negative potentials compared tocitrate at the same pH. Addition of citrate together with phosphatemade the surface potential only slightly more negative, probablybecause metal oxides preferentially adsorbed phosphate relativeto citrate.27 The solution composition and pH surface potentialappeared to significantly affect the stability of CeO2 NPs.In the case of bulk CeO2, XRD analysis suggested a 140 nm

primary particle diameter, whereas BET-N2 adsorption analysissuggested 166 nm.Dissolution.Dissolution of CeO2NPs was lower than or close

to detection limits (0.1 nM) at pH 7 and 9, but 3.1% (i.e., 29 μM)CeO2 NPs dissolved at pH 4 in AS after 28 days of equilibration(SI Figure S2). Geochemical modeling in PHREEQC (with anupdated thermodynamic database for Ce28,29) showed that thisCe concentration was much higher than equilibrium with CeO2

and Ce(OH)4, the most likely CeIV precipitates.29 Part of the Ce

dissolved from CeO2 NP therefore possibly occurs as CeIII,which is much more soluble than CeIV (SI Figure S2). Moreover,a fraction of the total Ce in CeO2 NPs in fact occurs as CeIII.30

However, the reduction rate of CeIV to CeIII is slow,29 and CeIII

can also be oxidized to CeIV, which may explain the slowdissolution rate of CeO2 NP reported previously1 and observedhere as well. CeO2 NPs appear to be more soluble in AS relativeto 0.1 mMNaCl, possibly because both ionic CeIII and CeIV formion pairs with nitrate, chloride, and sulfate,31�33 which enhancestheir solubility.Solubility and Partitioning of Ionic CeIII, Ionic CeIV, and Bulk

CeO2 in Soils. The basic statistics for the Kd values of geogenicCe, ionic CeIII, ionic CeIV, and bulk CeO2 in 16 Australian soils(SI Table S3) can be found in Table 1. Geogenic Ce appears to besparingly soluble in soils, probably because it is precipitated inphosphate minerals.34 Bulk CeO2 was found to be sparinglysoluble in soils (<0.1% of total Ce added), which resulted in highKd values in soils (>10 000 L kg

�1). The Kd values for ionic CeIII

and CeIV were lower, but they were not significantly differentfrom each other on the basis of a paired t-test (p > 0.05) on allsoils. Both sets of Kd values were correlated positively with CEC,whereas the Kd values for ionic CeIII were also found to becorrelated with the soluble (<0.45 μm) phosphate concentrationaccording to the following equations

log Kd, CeIII ¼ 4:126þ 0:726x logðCECÞþ 0:243x log½PO4� ðr2 ¼ 0:64Þ ð1Þ

log Kd, CeIV ¼ 2:486þ 0:805x logðCECÞ ðr2 ¼ 0:55Þ ð2Þwhere log Kd values are in L kg�1 and [PO4] is the measuredsoluble phosphate concentration in <0.45 μm soil extracts in molL�1. These relationships are also shown in SI Figure S3. Thecorrelation of logKd,CeIII with log[PO4] accounted for 16% of the

total sum of squares and is most likely a consequence of the lowsolubility of CeIII-phosphates that control the solubility of CeIII

in soils.34 However, 49% of the total sum of squares of logKd,CeIII

was accounted for by log(CEC), which was also the only soilparameter that correlated significantly with log Kd,CeIV values.The adsorption of bothCeIII andCeIV bymetal oxides and clays34,35

and interchanges between CeIV and CeIII (see SI) explain thesimilarity of their respective Kd values.CeO2 NP Dissolution and Retention in Soils. The Ce con-

centrations found in <1 kDa filtrates ofKr experiments of 16 soilsspiked with 9.29 μmol kg�1 as CeO2 NPs were lower than 0.2μM (or <2% of Ce added). Either the solubility of CeO2 NPs insoils is limited or dissolved Ce is partitioned strongly to the solidphase, considering the strong observed partitioning of both CeIII

and CeIV. Conversely, up to 90% (median 50%) of the addedCeO2NPs (9.29 μmol kg�1) remained suspended (<0.45 μm) asCeO2 NPs or as CeO2 NP aggregates after 24 h. Up to 300 timesmore Ce was found in <0.45 μm filtrates of soil suspensionsspiked with 12.6 mmol kg�1 as CeO2 NPs compared to the samesoils spiked with bulk CeO2 at a concentration of 11.7 mmol kg

�1.A comparably smaller proportion of CeO2 NP will occur as

single particles or aggregates smaller than 0.45 μm after 24 hwhen aggregation or deposition is slow. It can thus be argued thatdifferences in the physicochemical parameters that cause differ-ences in aggregation kinetics are reflected by the variation in theKr values. The multiple linear regression that best explained thevariation in Kr values was

log Kr, CeO2NP ¼ � 3:96þ 0:038xðclay%Þ � 0:4262x log½PO4�þ 0:3xpH ðr2 ¼ 0:72Þ

where log Kr values are in L kg�1. 52% of the total sum of squares

was accounted for by clay%, the clay percentage of the soils, 14%by log [PO4], and 6% by the pH. This relationship is also shownin SI Figure S3. IS is probably less important in this study, becausethe calculated values (SI Table S2) are all much lower than typicalcritical coagulation concentrations of engineered nanoparticles.36�38

However, heterocoagulation, the aggregation of dissimilar parti-cles, has been found to occur at much lower IS than homocoagula-tion, especially when collision between oppositely charged particlesoccurs.39,40 The mineralogy of the clay (<2 μm) fraction in soilsis dominatedby layer lattice clays and amorphous iron and aluminumoxides that are either predominantly positively charged, or havepositive charges at crystal edges, at the pH values measured in thesoil suspensions (SI Table S2).41,42 Aggregation with kaolinite,for instance, influences the transport of negatively charged nano-sized viruses in soils.43 Conversely, CeO2NPs are predominantlynegatively charged in soils, due to adsorption of phosphate and/or organic molecules such as citrate (Figure 1). The above equationthus suggests that CeO2 NPs are electrostatically attracted byclay edges, Al or Fe oxides, thus forming aggregates despite thelow IS. EDX analysis found that CeO2 NP aggregates in 0.45 μmfiltrates of soils spiked with 12.6 mmol kg�1 CeO2 NPs werealways associatedwithAl, silicon, Fe,Ca, andK (SI Figure S4), whichagain suggests a close association between CeO2 NPs and clays.The addition of phosphate has been shown to promote the

transport of Al0 and titania (TiO2) nanoparticles in soils, whichwas explained by the increased surface charge on these nano-particles upon phosphate adsorption.12,44 However, Figure 1shows that the zeta potential of citrate stabilized CeO2 NPs didnot change significantly upon phosphate addition, and homo-coagulation of CeO2 NPs in the present soil suspensions may be

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less important than heterocoagulation due to the relatively low ISof the present soil suspensions. Phosphate adsorption to naturalcolloids renders their surface potential less positive (SI FigureS5), which may inhibit heterocoagulation, thus reducing CeO2 NPretention.The weak, but significant, positive correlation of log Kr values

with pH, on the other hand, seemingly contradicts heterocoagu-lation, because the surface potential of natural colloids is lesspositive at more alkaline pH values (SI Figure S5), which reducesheterocoagulation with negatively charged CeO2 NPs. However,CeO2 NPs are more negatively charged as pH increases (Figure 1),which promotes heterocoagulation. The role of pH may there-fore be related to the relative increase in negative surface chargebetween CeO2 NPs and naturally occurring colloids, but hetero-coagulation kinetics of CeO2 NPs with natural colloids should beinvestigated at various pH values and dissolved organic carbonconcentrations to better assess the quantitative importance ofthis mechanism in the fate of CeO2 NPs in terrestrial systems.Effects of Biosolids, Spiking Rate, Citrate, and Phosphate

on Kr Values. The Kr values with added biosolids were higher inmost soils (Figure 2). The relatively large increase in theKr values ofTepko and Mount Compass soils may be explained by the lowclay contents of these soils (SI Table S2) that was increasedmuchmore upon addition of biosolids. The most consistent effect ofbiosolids addition was an increase of the pH in all soils (SI FigureS6), which may also have caused higher Kr values.The Kr values also increased if the CeO2 NP addition rate was

increased from9.29 to 35.7μmol kg�1 (Figure 2) and to 12.6mmolkg�1 (Table 1). The higher CeO2 NP concentration may haveincreased the collision efficiency and thus also the fraction ofaggregates larger than 0.45 μm after 24 h. The effect of citrate onCeO2 NP Kr values was limited, because similar Kr values wereobtained for the five selected soils, regardless of whether they werespiked as CeO2 NP suspensions with or without citrate. However,the addition of phosphate significantly lowered Kr values, except inTepko and Mount Compass soils. These soils have a low naturalcolloid concentration, which suggests that themost important effectof phosphate is by reducing the charge on natural colloids ratherthan by increasing the surface potential of CeO2 NPs.

Whereas previous studies on the fate of nanoparticles in soil orsand columns have focused on the composition of the soilsolutions in terms of pH, IS, and dissolved organic carbon, thepresent study suggests that this set should be extended with theconcentration of natural colloids. If the quantitative importanceof heterocoagulation in the fate of NPs is confirmed in furtherresearch, it would imply that the transport of NP in soils andpossibly also in natural waters may be colloid-mediated, similarlyto many dissolved contaminants, which has important environ-mental implications for the type of models that can be used forrisk assessment of nanoparticles.

’ASSOCIATED CONTENT

bS Supporting Information. Additional details include thecalculation methods for Kr and Kd values, all presented in exten-sive tables. Additional figures are also given. This material is avail-able free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Phone:þ61(0)883036578; fax:þ61(0)883036511; e-mail: geert.cornelis@adelaide.edu.au.

Present Addresses

)

U.S. Environmental Protection Agency, National Risk Manage-ment Research Laboratory, 919 Kerr Research Dr., Ada, OK74820, United States.

’ACKNOWLEDGMENT

The authors would like to thank Maria Manjarrez and MargaretYam for their efforts in experimental work. This work was fundedby the Australian Government Department of Sustainability, En-vironment, Water, Population and Communities and the Aus-tralian Research Council (Discovery Project DP0879165). TheU.S. EPA has not subjected this manuscript to internal policyreview. Therefore, the research results presented herein do notnecessarily reflect Agency policy. Mention of trade names ofcommercial products and companies does not constitute endor-sement or recommendation for use.

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Figure 2. Kr values of five selected soils (n = 5) spiked with 9.3 μmolkg�1 CeO2 NPs suspended in citrate, 9.3 μmol kg�1 CeO2 NPsincubated in biosolids, 36 μmol kg�1 CeO2 NPs suspended in citrate,36 μmol kg�1 CeO2 NPs suspended in water, 36 μmol kg�1 CeO2 NPssuspended in phosphate solution, and 36 μmol kg�1 CeO2 NPssuspended in a phosphate solution containing citrate (see text).

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