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Colloids and Surfaces A: Physicochem. Eng. Aspects 401 (2012) 29– 37

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

jo ur nal homep a ge: www.elsev ier .com/ locate /co lsur fa

ransport and deposition of ZnO nanoparticles in saturated porous media

ujia Jianga, Meiping Tonga,∗, Ruiqing Lua, Hyunjung Kimb

The Key Laboratory of Water and Sediment Sciences, Ministry of Education, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, PR ChinaDepartment of Mineral Resources and Energy Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea

r t i c l e i n f o

rticle history:eceived 26 December 2011eceived in revised form 2 March 2012ccepted 3 March 2012vailable online 12 March 2012

eywords:inc oxide nanoparticlesuartz sand

a b s t r a c t

The impact of ionic strength and cation valence on the transport and deposition kinetics of ZnOnanoparticles in saturated porous media was systematically investigated in this research. Packed col-umn experiments were performed over a series of environmentally relevant ionic strength in both NaCl(ranging from 1 to 20 mM) and CaCl2 (ranging from 0.1 to 1 mM) solutions. Solution chemistries (ionicstrength and ion types) greatly affected the transport of ZnO nanoparticles in saturated quartz sand. Flatbreakthrough plateaus were observed at relatively low ionic strength in both NaCl (1 and 5 mM) and CaCl2(0.1–0.5 mM) solutions, whereas, ripening was observed at high ionic strength (10 and 20 mM in NaCl,and 1 mM CaCl2) conditions. Deposition of nanoparticle increased with increasing solution ionic strength

ransport behavioryper-exponential decreaseggregation

in both monovalent and divalent salt solutions. The presence of divalent ions in solutions increasednanoparticle deposition in quartz sand. Under all examined conditions, nanoparticles mainly retained atsegments near the column inlet. The retained ZnO nanoparticle concentrations versus transport distancedecreased faster than the theory prediction of log-linear decrease under all examined conditions. Ourstudy found that concurrent aggregation of ZnO nanoparticles occurred during the transport process,

hype

which contributed to the

. Introduction

Due to their enhanced physico-chemical properties relative toheir bulk counterparts, nanoparticles have drawn great atten-ions in the fields of science and engineering in past several years1]. Massive investment in nanotechnology has led to the rapidmerging of new nanoparticles such as metal oxide nanoparti-les [2]. Although great application promise of nanoparticles haseen demonstrated, understanding the transport and fate of theseanomaterials especially metal oxide nanoparticles in natural envi-onment is still incomplete.

ZnO nanoparticles are one of the most popular metal oxideanoparticles that have been wide applications in various

ndustries, such as electronic engineering [3,4], environmentalngineering [5,6] and cosmetics industry [2]. Recent studies havehowed that ZnO nanoparticles were toxic to plants [7,8], bacteria9,10], rodents [11], as well as human cell lines [12]. The poten-ial risks of ZnO nanoparticles to the environmental and publicealth after released into natural environment have been foundo be greatly correlated to their distributions of concentration and

article sizes in natural environment [13,14]. Understanding theransport and aggregation behaviors of ZnO nanoparticles undernvironmentally relevant conditions is therefore necessary.

∗ Corresponding author. Tel.: +86 10 62756491; fax: +86 10 62756526.E-mail address: tongmeiping@iee.pku.edu.cn (M. Tong).

927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2012.03.004

r-exponential retained profiles.© 2012 Elsevier B.V. All rights reserved.

A few studies have investigated the aggregation behavior ofZnO nanoparticles. Zhang et al. [15] examined the stability of ZnOnanoparticles at pH 8 and found particles were quickly aggregatedafter suspending in tap water. Zhang et al. [16] also showed thatthe addition of a relatively weak electrolyte concentration (0.01 MKCl) could result in the aggregation of ZnO nanoparticles in neutralwater. By comparing the aggregation kinetics of two types of ZnOnanoparticles (spherical and irregular shaped ZnO) under variousconditions, Zhou and Keller [17] found that the aggregation of thenearly spherical ZnO exhibited strong dependence on the solutionionic strength, while the influence of ionic strength on aggrega-tion of the irregularly shaped ZnO was minimal. By dispersing ZnOnanoparticles in samples taken from eight different aqueous mediaassociated with seawater, lagoon, river, and groundwater, Kelleret al. [18] showed that the electrophoretic mobility of nanoparti-cles in a given aqueous media was controlled by ionic strength andthe presence of natural organic matter.

Unlike the studies regarding the aggregation kinetics of ZnOnanoparticles, to date, very few investigations have directedtoward understanding the deposition and transport behavior ofZnO nanoparticles under environmentally relevant conditions. Byemploying a quartz crystal microbalance with dissipation (QCM-D),our recent study [19] investigated the influence of ionic strength

and salt composition on the deposition of ZnO nanoparticles onflat silica surface. This study showed that increasing solution ionicstrength increased ZnO nanoparticle deposition and the deposi-tion was more enhanced in the presence of divalent ions. Very

3 Physic

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ecently, Ben-Moshe et al. [20] examined the transport behavior ofnO nanoparticles in packed spherical porous media (glass beads).y comparing the breakthrough curves obtained under different

onic strength in NaCl solutions, these authors found increasingonic strength enhanced the deposition of nanoparticles. However,etained profiles (the retained nanoparticle concentrations versusransport distance), a more important ‘indicator’ of nanoparticleransport behavior from which the mechanism(s) controlling thenO transport can be derived, have not been investigated in thesetudies. Moreover, the transport of ZnO nanoparticles in irregularorous media (quartz sand) that are more common in natural envi-onment relative to spherical porous media (glass bead) has nevereen examined.

Hence, this study was performed to fully understand the mech-nisms controlling the deposition and transport behaviors of ZnOanoparticles in packed irregular porous media by monitoring bothhe breakthrough curves and the retained profiles. The transportxperiments were performed under series of environmentally rel-vant ionic strengths in both monovalent and divalent solutions.ur study showed that solution chemistries had great influencen the transport behavior of ZnO nanoparticles. In all examinedonditions both in monovalent and in divalent salt solutions,he retained nanoparticle profiles displayed hyper-exponentialecreases with transport distance. Simultaneous aggregation ofanoparticles occurred in the packed porous media was found torive the observed hyper-exponential retained profiles.

. Materials and methods

.1. Nanoparticle suspension preparation

ZnO nanoparticle stock suspension (100 mg L−1) was preparedy suspending nanopowders (20 ± 5 nm in diameter, purity greaterhan 99.5%, Zhejiang Hongsheng Material Technology Co., China)n Milli-Q water (Q-Gard1, Millipore Inc., MA) and sonicated for0 min with a sonicating probe (Ningboxinzhi Biotechnology LTD.,hina) [19]. Nanoparticle suspensions (5 mg L−1) were made byiluting stock suspension into salt solutions (NaCl or CaCl2) at pH.0 ± 0.1 (adjusted with 0.1 M NaOH). After preparation, nanoparti-le suspensions were stirred at 200 rpm for 3 h and then vigorouslyhaken using a vortex mixer at high speed for 15 min prior to eachransport experiment. It should be noted that to ensure the pHf nanoparticle suspension stabilized at 8.0 during the transportxperiment, nanoparticle suspension required continuously slightdjustment with 0.1 M NaOH within 2 h after preparation until thenal pH of 8.0 was obtained and not changed afterwards. The stockuspension was freshly prepared each time before column experi-ent.The detailed characterization of ZnO nanoparticles has been

escribed in our latest published study [19] and also provided inhe Supplementary Information. The corresponding zeta poten-ials and particle sizes of ZnO nanoparticle suspension (5 mg L−1)ere measured in both NaCl and CaCl2 solutions with Zetasizerano ZS90 (Malvern Instruments, UK). Measurements were per-

ormed right before and after the column experiments and repeated0–40 times at room temperature (25 ◦C). The results for zetaotentials and particle sizes of ZnO nanoparticles at different ionictrength conditions in both NaCl and CaCl2 solution were depictedn Figs. S1 and S2, respectively.

.2. Porous media

The porous media used for ZnO nanoparticle transport exper-ments was irregular quartz sand (ultrapure with 99.80% SiO2)Hebeizhensheng Mining LTD., China) with sizes ranging from 417

ochem. Eng. Aspects 401 (2012) 29– 37

to 600 �m (the median diameter of 510 �m). The procedure usedfor cleaning the quartz sand was provided in the previous publi-cation [21], as well as in the Supplementary Information. The zetapotentials of the crushed quartz sand were also measured in bothNaCl and CaCl2 solutions under the experimental conditions andrepeated 9–12 times.

2.3. Porous media experiments

ZnO nanoparticle transport experiments in porous media wereperformed at ionic strength ranged from 1 to 20 mM and from 0.1 to1 mM in NaCl and CaCl2 solutions, respectively. These series of solu-tion chemistries examined in this study can be commonly foundin groundwater [22]. The cylindrical Plexiglass columns (20 cm inlength and 4.0 cm in inner diameter) were wet-packed with cleanedquartz sand, and detailed information for this packing procedurehas been described in Tong et al. [21], which was also providedin the Supplementary Information. The porosity of packed columnwas approximately 0.42.

After packing, the columns were pre-equilibrated with at least10 pore volumes of nanoparticle-free salt solutions at desired ionicstrength and pH. Following pre-equilibration, 3 pore volumes ofZnO nanoparticle suspension were injected into the column, fol-lowed by elution with 5 pore volumes of salt solution at the sameionic strength and pH. The suspensions and solutions were injectedinto the columns in up-flow mode using a syringe pump (HarvardPHD 2000, Harvard Apparatus Inc., Holliston, MA). The influentconcentration of ZnO nanoparticle suspension was 5 mg L−1, whichwas determined by measuring the zinc concentration via coupledplasma mass spectrometry (ICP-MS) (XSeries II, Thermo Scientific)after acid digestion by dissolving in 5% HNO3 and diluting by a factorof 10. At this pH (pH < 1), the release of Zn2+ from ZnO nanoparticleswas above 92% [23,24]. The calibration curve of ZnO nanoparti-cle suspensions was provided in the Supplementary Information(Fig. S3). The pore water velocity of all experiments was set to be8 m day−1 (2.93 mL min−1) to represent fluid velocities in coarseaquifer sediments, forced-gradient conditions, or engineered fil-tration systems.

Samples from the column effluent were collected continuously(∼5 mL) in 10 mL sterile centrifuge tubes. Following the transportexperiment, the sand was excluded from column under gravity anddissected into 10 segments (each 2 cm long). Specific volumes ofMilli-Q water were added into each segment. The effluent samplesand supernatant samples from recovery of retained nanoparticleswere analyzed by measuring the zinc concentration via ICP-MSafter dissolving in 5% HNO3 and diluting by a factor of 10. Thearea under the breakthrough-elution curve was integrated to yieldthe percentage of ZnO nanoparticles that exited the column. Thepercentage of ZnO nanoparticles recovered from the sediment wasdetermined by summing the ZnO nanoparticles recovered from allsegments of the sediment and dividing by total number (concen-tration) injected. Summing the percentages of both retained andexited nanoparticles resulted in the overall recovery (mass balance)of the ZnO nanoparticles (Table 1).

2.4. Particle tracking model

The transport and retention of nanoparticles was modeled usingan advection–dispersion equation that includes removal from andre-entrainment to the aqueous phase

∂C = −v∂C + D

∂2C − kf C + �b krSr (1)

∂t ∂t ∂x2 �

where C is the concentration of nanoparticles in the aqueous phase(nanoparticles per unit volume of fluid), t is the travel time, x is thetravel distance, v is the flow velocity, D is the dispersion coefficient

X. Jiang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 401 (2012) 29– 37 31

Table 1Column experiments conditions, mass balances, and model parameters for simulations using the particle-tracking model.

Salt composition Ionic strength (mM) Mass recovery (%) Retained (%) kf (h−1) kr (h−1) fr

NaCl 1 92.7 27.3 0.90 0.40 0.505 91.4 50.3 1.70 0.40 0.70

10 105.6 75.5 2.30 0.30 0.3020 98.4 85.4 3.80 0.10 0.30

CaCl2 0.1 103.7 42.6 1.10 0.50 0.80

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0.3 94.3

0.5 86.7

1 91.0

f the colloid particles, � is the porosity, �b is the bulk density ofediment, and kf and kr are rate coefficients for nanoparticles depo-ition to and re-entrainment from the solid phase, respectively.r is the reversibly retained nanoparticles concentration on solidhase (nanoparticles per unit mass of sediment) and can be furtherxpressed as

r = Sfr (2)

here S is the total deposited nanoparticles concentration, and frs the fraction of reversibly retained nanoparticles.

A one dimensional discrete random-walk particle-trackingodel (developed by Dr. Timothy Scheibe at PNNL) was used to

olve Eq. (1) under the condition of the column experiments, andetails of implementation of the governing equation were given

n Zhang et al. [25]. Best fits to both the breakthrough-elutionurves and the profiles of retained nanoparticles were applied toield the parameters values in the particle tracking simulationTable 1). It should be noted that mass recoveries of nanoparti-les from transport experiments were between 86.7% and 105.6%,ith the majority between 91.4% and 98.4% (Table 1). The goodass balance of the transport experiments indicated that all the

anoparticles recovered from effluent and retained on the sandere changed into Zn2+ ion completely.

.5. Derjaguin–Landau–Verwey–Overbeek (DLVO) interactionorce

DLVO theory was used to calculate the total interaction force as function of separation distance. Although other forces may influ-nce the net interaction forces (e.g. hydration) [26], we maintainimplicity in this investigation by focusing on van der Waals andlectric double layer forces. The detailed equations utilized to calcu-ate the retarded van der Waals and electrostatic double layer forcesor both sphere–sphere and sphere–plate configurations were pro-ided in the Supplementary Information. The detailed individualalues employed to derive the DLVO interaction forces could beound in Table S1.

. Results and discussion

.1. Characterization of ZnO nanoparticles

Transmission Electron Microscope (TEM) (JEM-200CX, JEOL)mage showed the morphology of ZnO nanoparticles after dispersedn Milli-Q water (Fig. S4). The shape of nanoparticle was almostpherical as illustrated by TEM. Although the effective diameter ofnO nanoparticles was about 20 nm, particles in suspension existeds large agglomerates (∼300 nm) due to nanoparticle aggregation.he size and zeta potential of ZnO nanoparticles were examined

ver range of solution chemistries and the results were provided inigs. S1 and S2. Besides, the degree of dissolution of ZnO nanopar-icles to zinc irons in influent suspension was measured by ICP-MS.t was found that only ∼10 �g L−1 of Zn2+ was detected in ZnO

48.3 1.20 0.30 0.5063.2 2.30 0.25 0.5575.5 3.40 0.15 0.60

nanoparticles suspension (5 mg L−1). Previous studies conductedby Dange et al. and Bian et al. found that the dissolubility of ZnOhighly depended on nanoparticle size and pH [23,24], and that thelowest dissolution of ZnO nanoparticles was observed at pH 8 (∼1%)[24].

3.2. Breakthrough curves of ZnO nanoparticle

The transport behavior of ZnO nanoparticle in packed quartzsand was examined under series of environmentally relevant ionicstrength in both NaCl (from 1 to 20 mM) and CaCl2 (from 0.1to 1 mM) solutions at pH 8.0. Breakthrough plateaus were flatat relatively low ionic strength in both NaCl (1 and 5 mM) andCaCl2 (0.1–0.5 mM) solutions (Fig. 1), indicating temporal con-stancy in the deposition rate coefficient during the course of theseexperiments. Whereas, breakthrough plateaus decreased with theincrease of experiment duration at relatively high ionic strengthin both NaCl (10 and 20 mM) and CaCl2 (1 mM) solutions (Fig. 1),indicating the temporal increase in the deposition rate coefficient(ripening) during the course of experiments. In both NaCl andCaCl2 solutions, the increase in ionic strength resulted in lowerbreakthrough plateaus. Specifically, the breakthrough plateausdecreased from 0.7 to ∼0.1 in response to the ionic strengthincreased from 1 to 20 mM in NaCl solutions, and decreased from0.6 to ∼0.15 with increasing ionic strength from 0.1 to 1 mM inCaCl2 solutions. The increase of solution ionic strength led to thecompression of electrostatic double layer between nanoparticleand quartz sand, and therefore more deposition of nanoparti-cles occurred and lower breakthrough plateaus were obtained athigh ionic strength. The increased deposition with increasing ionicstrength acquired for ZnO nanoparticle has also been observed forother nanoparticles, i.e. C60, TiO2, and Fe0 [20,27–29].

Comparison of breakthrough curves of ZnO nanoparticlesobtained in CaCl2 solutions with those in NaCl solutions yieldedthat, at the same ionic strengths (1 mM), breakthrough plateau inCaCl2 solutions (Fig. 1B) was lower than that in NaCl solutions(Fig. 1A), indicating that the presence of divalent Ca2+ in solu-tions decreased the transport (or increased the deposition) of ZnOnanoparticles in quartz sand. The observed lower breakthroughplateau in CaCl2 solutions relative to that in NaCl solutions was con-sistent with less negative zeta potentials of nanoparticles in CaCl2solutions (Fig. S1 and Table S1). The results demonstrated that sim-ilar to the observation on flat silica surfaces obtained from QCM-D[19], DLVO-type interactions also played an important role in ZnOnanoparticle transport in quartz sand porous media.

3.3. Retained profiles of ZnO nanoparticle

The retained profiles of ZnO nanoparticles obtained in both NaCl

(from 1 to 20 mM) and CaCl2 (from 0.1 to 1 mM) solutions at pH8.0 were presented in Fig. 2. The magnitude of the retained pro-files of ZnO nanoparticles varied oppositely to the breakthroughplateaus, as expected from mass balance consideration (Table 1).

32 X. Jiang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 401 (2012) 29– 37

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ig. 1. Breakthrough curves for ZnO nanoparticles in both NaCl (A) (ranging from 1o 20 mM) and CaCl2 solutions (B) (ranging from 0.1 to 1 mM) at pH 8.0. Error barsepresent standard deviations from replicate experiments (n = 2).

he increase of solution ionic strength caused greater depositionf nanoparticles in quartz sand in both NaCl (Fig. 2A) and CaCl2Fig. 2B) solutions. Close analysis of Fig. 2 showed that the increasedeposition of nanoparticles due to the increasing ionic strengthainly occurred at segments near the column inlet, which induced

elatively steeper retained profiles of ZnO nanoparticles at highonic strength relative to that obtained at low ionic strength in bothaCl and CaCl2 solutions. At the same ionic strength (1 mM), the

etained nanoparticle concentrations were greater in CaCl2 solu-ions (Fig. 2B) relative to those in NaCl solutions (Fig. 2A). Moreover,he observed excess retention in CaCl2 solutions relative to that inaCl solutions also mainly located near the column inlet, as a result,

elatively steeper slope of retained profiles were observed in CaCl2olutions.

.4. Comparison to simulation

To better understand the transport and retention of ZnOanoparticles in packed quartz sand under all examined ionictrength conditions in both NaCl (from 1 to 20 mM) and CaCl2from 0.1 to 1 mM) solutions, a particle-tracking model was

mployed to fit all experimental results and the correspondingesults in NaCl and CaCl2 solutions from simulation were pro-ided in Figs. S5 and S6, respectively. The breakthrough curvesbtained in both NaCl and CaCl2 solutions for the most part could

20 mM) and CaCl2 solutions (B) (ranging from 0.1 to 1 mM) at pH 8.0. Conc Zn repre-sents the measured zinc ion concentration. Error bars represent standard deviationsfrom replicate experiments (n = 2).

be fitted by the particle transport model using a constant ratecoefficient of deposition (Figs. S5 and S6, left). However, underall examined ionic strength conditions in both NaCl and CaCl2solutions, the retained profiles could not be simulated by the par-ticle transport model using a constant deposition rate coefficient(Figs. S6 and S7, right). Concentrations of retained ZnO nanoparti-cles decreased more rapidly than the log-linear retained profileswhich obtained using a single deposition rate coefficient, sug-gesting that hyper-exponential decreased retained profiles wereobserved under all examined conditions. Moreover, the deviationsfrom log-linearity increased with increasing ionic strength in bothNaCl and CaCl2 solutions (Figs. S3 and S4), which was also demon-strated by the increased steepness of the retained profiles at highionic strength (Fig. 2). The hyper-exponential profiles observed forZnO nanoparticles have also been previously reported for othercolloids, i.e. microsphere, nano-TiO2, and bacteria [30–32].

3.5. Mechanism(s) contributing to hyper-exponential decrease

Several mechanisms such as collector heterogeneity [33–35],distributions in surface properties among the colloid popula-tion [30], deposition in secondary energy minimum [33,36], and

straining [31] have been proposed to cause the observed hyper-exponential decreases in retained profiles. Possible mechanism(s),contributing to the observed hyper-exponential decreases in ZnOnanoparticle retained profiles, would be discussed below.

Physicochem. Eng. Aspects 401 (2012) 29– 37 33

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X. Jiang et al. / Colloids and Surfaces A:

.5.1. Collector heterogeneitiesSome previous studies attributed hyper-exponential deviation

rom classical filtration theory to heterogeneity in collector sur-aces (e.g. [33,35]). In their studies, various metal oxides such asl2O3, Fe2O3, and Na2O were contained in quartz sand as impuri-

ies. These metal oxides that were usually near-neutrally chargedr slightly positive at pH 6–8, would favor the interaction withegative-charged colloids as compared to metal oxide-removedilica surfaces, resulting in much higher initial deposition ratesat those favorite sites) than predicted rates based on the averageollector surface potential. The quartz sands used in our experi-ent, however, were thoroughly cleaned following the procedure

rovided in the Supplementary Information. Therefore, collectoreterogeneity unlikely contributed to hyper-exponential decreasef retention profiles observed in this study.

.5.2. Colloid population heterogeneitiesTo examine whether the hyper-exponential retained profiles

ould be caused by the heterogeneity in surface property amongolloid population, the zeta potentials of ZnO nanoparticles bothn the influent and in the effluent solutions were determined. The

easurements were conducted for experiments performed in bothow and high ionic strengths (1 and 10 mM NaCl). The average zetaotentials based on 15 measurements for the influent and efflu-nt solutions at ionic strength of 1 mM NaCl were −35.2 ± 1.2 mVnd −34.8 ± 1.3 mV, respectively, whereas, at 10 mM NaCl, the zetaotentials of influent and effluent solutions were −16.2 ± 1.5 mVnd −15.9 ± 1.7 mV, respectively. The insignificant difference ineasured zeta potentials between the influent and effluent implied

hat surface chemical heterogeneity among colloid populationight not contribute to the hyper-exponential retained pro-

les observed at both low and high ionic strengths. To furtheremonstrate that the colloid surface chemical properties did nothange along the flow path, zeta potentials of nanoparticles des-rbed from each segment of the column under representativeolution conditions (1 and 10 mM NaCl) were determined. Equiv-lent zeta potentials were observed for nanoparticles retainedn each segment (data not shown), indicating that nanoparti-les retained along the flow path had similar surface chemicalroperties. The results showed that hyper-exponential retainedrofiles of ZnO nanoparticles were not driven by the chemicaleterogeneity in surface properties among the colloid popula-ion.

Particle size distribution among the colloid population wouldossibly produce the hyper-exponential as well, especially whenanoparticle was large enough to induce the straining. Previoustudies reported that most of the commercial oxide nanoparti-les were bimodal size distribution due to the highly aggregationtate of nanoparticle suspension [15,37]. These aggregates, typi-ally 3000–5000 nm, cannot be separated even by sonication [37].lose analysis of size distribution of ZnO nanoparticles after beingispersed in water yielded that, although the average particle sizeas ∼300 nm, bimodal size distribution (with first peak of ∼200 nm

nd the second peak of ∼5000 nm) was also observed for ZnOanoparticles (Fig. S7). Since the lowest threshold value of ratiof particle to collector for straining was reported to be 0.002 [38],hese large aggregates (dp:dc = 0.0098) might induce straining andhus have contribution to the observed hyper-exponential decreasen retained profiles. However, it should be noted that althougharge aggregates were present in nanoparticle suspensions, theercentage of these large aggregates was very small since these

articles usually cannot be detected during the size measurement.herefore, it is reasonable to deduce that another more impor-ant mechanism dominated the transport and deposition of ZnOanoparticles in packed quartz sand.

Fig. 3. Breakthrough curves (A) and retained profiles (B) for selected elution experi-ment. Conc Zn represents the measured zinc ion concentration. Error bars representstandard deviations from replicate experiments (n = 2).

3.5.3. Secondary energy minimum depositionTo investigate whether the hyper-exponential retained profile

was caused by deposition in secondary energy minima, selectedexperiments (conducted in 5 mM NaCl solution) were preformedwith elution by 3 pore volumes of Milli-Q water, with pH adjustedto 8.0 (following 2 pore volumes of salt solution having the sameionic strength as the nanoparticle solution) and the results werepresented in Fig. 3. Only a very small fraction (∼7%) of previouslyretained nanoparticles was released upon introduction of Milli-Qwater (applied to remove secondary energy minima), suggest-ing that only small portion of deposited particles was associatedwith quartz sand surfaces via secondary energy minima. Further-more, comparison of retained profile eluted with salt solutionversus that eluted with Milli-Q water yielded that the release ofsmall portion of nanoparticles previously deposited in secondaryenergy minima did not induce the obvious change of the shapeof retained profiles (Fig. 3B). The results demonstrated that theobserved hyper-exponential retained profiles were not producedby deposition in secondary energy minima.

3.5.4. Concurrent nanoparticle–nanoparticle aggregation andstraining

Although ripening was only observed at relatively high ionic

strengths in both NaCl and CaCl2 solutions, we speculated that con-current nanoparticle–nanoparticle aggregation might occur in theprocess of transport under all examined ionic strength conditions,contributing to the hyper-exponential retained profiles observed

34 X. Jiang et al. / Colloids and Surfaces A: Physic

Fig. 4. Measured particle size of ZnO nanoparticles desorbed from solid phase as afunction of transport distance in 1 mM (A) and 10 mM NaCl solutions (B). Error barsrw

ftsraiabsf1soMi4eisa∼(stctc

6–250 times higher in flow velocity relative to our study. Moreover,

epresent standard deviations from replicate experiments (n = 2), and each sampleas measured for 10 times.

or all experiments. To verify this speculation, the sizes of nanopar-icles retained in each segment of column at representative ionictrength conditions (1 and 10 mM NaCl) were determined and theesults were presented in Fig. 4. Desorption was performed bydding each segment of sand into salt solutions (with the sameonic strength as nanoparticle solution injected into the column)nd then mildly shaken for a few seconds. To prevent the possi-le nanoparticle aggregation occurring after desorbed into the saltolution, particle size measurements were performed immediatelyollowing the dissection of sediment. Fig. 4 clearly showed that at

mM NaCl, the particle sizes of ZnO nanoparticles retained in allegments were larger than 600 nm, which was greater than 387 nmbserved in the initial influent solutions injected into the column.oreover, among all the examined segments, the nanoparticle size

n the segment near the column inlet was largest, which was around000 nm at 1 mM NaCl solutions. The greater nanoparticle size inach desorbed segment (>900 nm) relative to that in the initialnfluent solutions (∼580 nm) was also held true for 10 mM NaClolutions. The largest particle size for 10 mM NaCl experimentslso occurred in the segment near the column inlet, which was8000 nm. The greater sizes of nanoparticles observed in columns

>600 nm for 1 mM and >900 nm for 10 mM) relative to that in saltolutions (387 nm for 1 mM and 580 nm for 10 mM) indicated thathe increase of nanoparticle sizes occurred during the travel pro-

ess. Here, it is necessary to clarify that we do not want to asserthat this aggregation only took place in the column. Nanoparti-les in the stock influent have slight changes after about 2 hour’s

ochem. Eng. Aspects 401 (2012) 29– 37

injection. Fig. S2 showed the size changes of ZnO nanoparticlesin NaCl solution after injection process (solid triangle). Specifi-cally, nanoparticle size increased from 387.1 to 412.3 nm in 1 mMNaCl solution and from 583.1 to 636.4 nm in 10 mM NaCl solution,respectively. However, the sizes of nanoparticles desorbed from theeach segment of column were still much larger than those in theinfluent.

Comparison of the particle sizes obtained at low and high ionicstrengths showed that the nanoparticle sizes in each segment weregreater at 10 mM relative to those at 1 mM NaCl solutions, indi-cating that the nanoparticle–nanoparticle aggregation was morepronounced at high ionic strength. This observation may explainthe observed ripening at high ionic strengths (10 and 20 mM NaCl)(Fig. 1). For both examined solution conditions, the nanoparticlesizes decreased with increasing transport distance, which followedthe trends of retained nanoparticle concentration versus the trans-port distance. These observations clearly showed that for bothlow and high ionic strength conditions, the sizes of nanoparti-cles significantly increased after injection into the packed column,which strongly indicated that nanoparticle–nanoparticle aggre-gation occurred during the transport process. This concurrentaggregation had two important effects posing on ZnO nanoparticletransport. First, as stated previously, small portion of initial largeaggregates were present in nanoparticle suspension, which couldpossibly induce the straining effect after introduced into the col-umn. Particle–particle aggregation resulted in ZnO nanoparticlesprone to deposition on retained large aggregates, which furthernarrowed the pore throat (grain to grain contacts) in the porousmedia and more nanoparticles would be trapped at this loca-tion [39,40]. As a result, this could be one of the contributionsto large aggregates that detected in the column inlet. The secondeffect of concurrent aggregation was that after initial nanoparti-cle attachment, the following nanoparticle subsequently attachedto previously deposited ZnO nanoparticles, which would also beexpected to plug pore throats and retain vast majority of nanopar-ticles near entry surfaces of porous media [41]. In this case, largeaggregates could be detected as well near the column inlet (Fig. 4).Since the large aggregates in aqueous solution were only in quitesmall fraction after preparation, the later effect should be morepronounced in this study. Saleh et al. tracked the whole pro-cess of large aggregates formation by employment of microfluidicflow cells for zero valent iron nanoparticle transport in porousmedia [39]. Pore plugging induced most influent nanoparticle trap-ping in the column inlet (straining) and accordingly resulted inhyper-exponential retained profiles observed under all examinedconditions.

The concurrent aggregation phenomenon has also beenreported for other metal oxide nanoparticles like TiO2 and Al2O3[28,32,42]. However, in these studies, nonmonotonic retention pro-files were exhibited instead of hyper-exponential decrease [32,42].Both hyper-exponential and nonmonotonic retained profiles havebeen observed as result of straining (physical entrapment) by Brad-ford et al. [31,38,41,43,44]. Nonmonotonic retained profiles likelyoccurred when colloid aggregation took place during transport[44]. However, whether the small aggregates would depart fromthe deposited large aggregates, which was the critical step fornonmonotonic formation, were dependent on colloid concentra-tion, injection time, pore size, and flow velocity. Previous studiesthat observed the nonmonotonic type of nanoparticle retentionprofiles were all conducted at relatively high influent concentra-tion (20–100 mg L−1) and flow velocity (0.36–14 cm min−1) [32,42],which was about 4–20 times greater in influent concentration and

the nanoparticle injection duration in previous studies (6–80 PV)was also much longer than our study [32,42]. All these discrepan-cies resulted in the different expression of concurrent aggregation

X. Jiang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 401 (2012) 29– 37 35

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hyper-exponential decrease) in nanoparticle retention profilesomparison to previous studies (nonmonotonic decrease). Li et al.roposed that the form of deviation (from classic filtration) ofetained profiles was highly sensitive to system conditions, whichould switch from hyper-exponential to nonmonotonic even minorhanges of solution chemistry, surface properties of colloid or col-ector [45].

.6. DLVO interpretation of nanoparticle–nanoparticleggregation

To better understand the mechanisms controlling the observedransport behavior of ZnO nanoparticle in packed quartz sandspecially the observed hyper-exponential retained profilesnder all examined conditions, nanoparticle–nanoparticle andanoparticle–silica (quartz sand) interaction force profiles inoth NaCl and CaCl2 solutions were derived and presented inigs. 5 and S8, respectively. Under all examined conditions,anoparticle–nanoparticle and nanoparticle–silica interactionsere repulsive, indicating unfavorable deposition conditions. The

nergy barrier decreased with increasing ionic strength in bothaCl and CaCl2 solutions. It is clearly shown in Figs. 5 and S8 that

he nanoparticle–nanoparticle interaction (dashed line) was lessepulsive than nanoparticle–silica interaction (solid line) underll examined conditions. DLVO interaction profiles indicated thathe mobile nanoparticles tended to attach onto the previouslyeposited nanoparticles rather than onto quartz sand, thus yieldinganoparticle–nanoparticle aggregation in column by overcominghe relatively smaller energy barrier via Brownian motion and/or

ollision forces [46]. The attachment of mobile nanoparticles to thereviously deposited ones was more prone to occur near the col-mn inlet at which the concentrations of mobile nanoparticles wereelatively high due to the continuous injection of nanoparticle

Seperation Distance (nm)

d nanoparticle–silica (solid line) surfaces in NaCl solutions (1–20 mM).

suspension into the packed column. Therefore, the sizes ofnanoparticle–nanoparticle aggregates near the column inlet werethe largest along the flow path (Fig. 4). Furthermore, close inspec-tion of Figs. 5 and S8 demonstrated that nanoparticle–nanoparticleinteraction was more sensitive to the changes in ionic strengthrelative to that of nanoparticle–silica. This observation indicatedthat increasing ionic strength favored nanoparticle–nanoparticleinteraction relative to nanoparticle–silica interaction, whichtheoretically supported the observation that nanoparticle in eachsand segment were greater at high ionic strength (10 mM NaCl)relative to those at low ionic strength (1 mM NaCl) (Fig. 4) and alsoexplained the phenomena that ripening was occurred at high ionicstrengths (10 and 20 mM NaCl, and 1 mM CaCl2) (Fig. 1).

The above results showed that the aggregation of nanopar-ticles occurred in the packed porous media had great sig-nificance in the transport behavior of ZnO nanoparticles.Nanoparticle–nanoparticle aggregation was more significant athigh ionic strengths and in the presence of Ca2+ in solutions. Aggre-gation of nanoparticles during the transport process contributed tothe hyper-exponential retained profiles observed under all exam-ined conditions in our study, and can also explain the observedstraining for other metal-based nanoparticles [e.g. Refs. 20, 47].

4. Conclusions

This study systemically investigated the transport and depo-sition behaviors of ZnO nanoparticles, one of the most popularmetal oxide nanoparticles, in packed quartz sand over series of

environmentally relevant ionic strength in both monovalent anddivalent salt solutions. The transport kinetics of ZnO nanoparticleswere highly related to ionic strength and ion valence. Specifi-cally, increasing solution ionic strength increased the deposition

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f nanoparticles on quartz sand. The presence of divalent ions inolutions distinctly enhanced nanoparticle deposition. Concurrentggregation played an important role in nanoparticle transportnd distribution in porous media. This simultaneous nanoparticleggregation during the transport process increased the nanoparti-le size, constricted the pore space, and finally plugged the passagen porous media. Pore plugging induced most influent nanoparticlesrapping in the column inlet (straining) and accordingly resulted inyper-exponential retained profiles observed in all examined con-itions. The above observations indicated that classical filtrationheory could not accurately estimate the distribution or transportistance of ZnO nanoparticles after released into the environment.articles would more readily retain in the porous media near tohe injection or discharge sites, with shorter travel distance thanheory predicted.

cknowledgements

This work was supported by the National Natural Science Foun-ation of China under Grant No. 40971181. We acknowledge threenonymous reviewers and for their useful and constructive com-ents. We are also grateful to Professor Alistair Borthwick from theniversity of Oxford for his kind help in English Editing.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.colsurfa.2012.03.004.

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