Article
pubs.acs.org/est

Retention and Remobilization of Stabilized Silver Nanoparticles in an
Undisturbed Loamy Sand Soil
Yan Liang,† Scott A. Bradford,*,‡ Jiri Simunek,§ Marc Heggen,∥,⊥ Harry Vereecken,† and Erwin Klumpp††

Agrosphere Institute IBG-3, Forschungszentrum Jülich GmbH, Jülich 52425, Germany
United States Department of Agriculture, Agricultural Research Service, U. S. Salinity Laboratory, Riverside, California United States§
Department of Environmental Sciences, University of California, Riverside, California United States∥Peter Grünberg Institute PGI-5, Jülich 52425, Germany⊥
Ernst Ruska-Centrum ER-C, Forschungszentrum Jülich GmbH, Jülich 52425, Germany‡

S Supporting Information*

ABSTRACT: Column experiments were conducted with
undisturbed loamy sand soil under unsaturated conditions(around 90% saturation degree) to investigate the retention of
surfactant stabilized silver nanoparticles (AgNPs) with various
input concentration (Co), ﬂow velocity, and ionic strength(IS), and the remobilization of AgNPs by changing the cation
type and IS. The mobility of AgNPs in soil was enhanced withdecreasing solution IS, increasing ﬂow rate and input
concentration. Signiﬁcant retardation of AgNP breakthroughand hyperexponential retention proﬁles (RPs) were observed in almost all the transport experiments. The retention of AgNPs
was successfully analyzed using a numerical model that accounted for time- and depth-dependent retention. The simulated
retention rate coeﬃcient (k1) and maximum retained concentration on the solid phase (Smax) increased with increasing IS anddecreasing Co. The high k1 resulted in retarded breakthrough curves (BTCs) until Smax was ﬁlled and then high eﬄuent
concentrations were obtained. Hyperexponential RPs were likely caused by the hydrodynamics at the column inlet which
produced a concentrated AgNP ﬂux to the solid surface. Higher IS and lower Co produced more hyperexponential RPs becauseof larger values of Smax. Retention of AgNPs was much more pronounced in the presence of Ca2+ than K+ at the same IS, and theamount of AgNP released with a reduction in IS was larger for K+ than Ca2+ systems. These stronger AgNP interactions in thepresence of Ca2+ were attributed to cation bridging. Further release of AgNPs and clay from the soil was induced by cationexchange (K+ for Ca2+) that reduced the bridging interaction and IS reduction that expanded the electrical double layer.
Transmission electron microscopy, energy-dispersive X-ray spectroscopy, and correlations between released soil colloids and
AgNPs indicated that some of the released AgNPs were associated with the released clay fraction.

■

INTRODUCTION
The transport and fate of nanoparticles (NPs) in the subsurface
environment has been a topic of signiﬁcant scientiﬁc interest1
because soils and aquifers act as the primary ﬁlter systems toprotect water resources. NPs have sometimes been found to behighly mobile under environmentally relevant conditions2−4
and may potentially contaminate groundwater. Colloid and NP
mobility in the subsurface environment are generally
considered to be determined by physicochemical interactions
and size-related processes.5−7 Retention of colloids/NPs in
porous media is controlled by the coupling of physicochemical
properties of the particle and the collector surface, solution
chemistry, and hydrodynamics of the system.8,9 Filtration
theory10 has commonly been employed to predict thedeposition of NPs on collector surfaces. However, deviations
from ﬁltration theory predictions have frequently been
demonstrated, e.g., the production of asymmetric or retarded
breakthrough curves (BTCs) and uniform, nonmonotonic, andhyperexponential retention proﬁles (RPs).11,12 Remobilization© 2013 American Chemical Societyof deposited colloids/NPs from solid to aqueous phases hasreceived only limited attention. Existing data indicate that
release may occur with an increase in the hydrodynamic force13
or a reduction in the adhesive force.14 In particular, decreasing
the solution ionic strength (IS)5,15 and/or replacement ofdivalent cations by monovalent cations16 can induce colloid/
NP release by reducing the depth of the secondary energy
minimum and cation bridging, respectively.
Most research studies have investigated NP/colloid transport
in highly idealized systems consisting of repacked, homogeneous, coarse textured porous media under water-saturated3,5,17or unsaturated condition.18−20 This previous research hasprovided fundamental knowledge and understanding of theinﬂuence of many physicochemical factors on NP transport andReceived: May 7, 2013
Revised: September 23, 2013
Accepted: October 9, 2013
Published: October 9, 2013
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ArticleTable 1. Experimental Parameters and the Mass Recovery for Column Experimentsa
recovery, %Figure 1

Figure 2
Figure 3

Figure 4
Figure 5a

Co (mg L−1)

q (cm min−1)

IS (mM)

φ

θw

λ

10
10
10
10
1
10
10

0.006
0.006
0.006
0.006
0.006
0.02
0.006

1
5
10
1
1
1
1

0.407
0.429
0.407
0.407
0.438
0.397
0.407
ISC (mM)

0.944
0.947
0.900
0.944
0.949
0.851
0.944
MC

III, 10
IV, 10I, 10
II, 10
III, 10

0.02
0.02
0.02
0.02
0.02

1, K+
5, K+
10, K+

<1%
<1%
<1%

ISA (mM)

MA

0.431
0.453
0.452
0.431
0.462
0.467
0.431
MB H2O

10, Ca2+
10, K+
1, Ca2+
5, Ca2+
10, Ca2+

<1%
9.1
3.9
<1%
<1%

16.0
31.7
3.8
4.6
16.0

Msand

Meff

37.9
70.5
13.9
78.7
3.4
88.6
37.9
70.5
14.4
96.2
64.3
40.0
37.9
70.5
MD H2OISE (mM)
ME

1.5
10.8
16.7

100, K+
100, K+
100, K+

5.0
<1%
<1%

Mtotal
108.4
92.6
92.0
108.4
110.6
104.3
108.4
MF H2O

9.4
7.7
4.2

Figure 1 ionic strength eﬀect; Figure 2: concentration eﬀect; Figure 3: ﬂow rate eﬀect; Figure 4: release of AgNPs by ionic strength reduction;
Figure 5a: release of AgNPs by cation exchange and ionic strength reduction; Co, AgNP input concentration; q, Darcy velocity; IS, ionic strength; φ,porosity; θw: water content; λ, water saturation; Meff, Msand, and Mtotal are mass percentages recovered from eﬄuent, sand, and total, respectively; MA− MF are the mass percentages recovered from Steps A−F in release experiments.
aimportant processes that aﬀected contaminant migration.21,34
To date, no studies have reported on the transport, retention
and remobilization of AgNPs in undisturbed and/or unsaturated soil systems. In particular, the potential of AgNPs
cotransported by natural soil colloids has not yet been studied.
The objective of this study is to better understand theretention and remobilization behavior of stabilized AgNPs in anatural soil, near water saturation (90%) conditions torepresent the maximum transport potential in the vadosezone. Undisturbed soil column experiments were conducted for
diﬀerent solution chemistries, input AgNP concentrations, andﬂow velocities, and a mathematical model was employed toquantitatively assess the eﬀects of these physicochemical factors
on retention. Following AgNP transport and retention, release
experiments were performed by systematically changing the IS
and cation type of the eluting solution. Results provide valuable
insight on the transport, retention, and release behavior of
AgNPs in a natural soil over a range of environmentally relevant
conditions. This information is needed to better assess the riskof AgNP exposure to ecosystems and to develop strategies for
waste management and remediation.

deposition. However, simpliﬁed porous media such as glass
beads and clean sands are not able to account for the fullcomplexity and heterogeneity of natural soils (e.g., pronounced
surface roughness, highly nonuniformed soil particle size
distributions, the complex pore structure of soil, and surface
chemical heterogeneity due to a wide variety of inorganic andorganic compounds). Previous studies concerning the mobilityof colloids/NPs in soil6,7,21 have indicated that the pore
structure and soil properties strongly aﬀect particle retention in
the subsurface environment. In addition, the presence of
diﬀerent electrolyte types, which generally occurs in soils, hasbeen proven to play an important role in NP stability andmobility in porous media.7,22,23 In particular, divalent cations
can enhance aggregation and deposition in comparison tomonovalent ions.6,24
The environmental fate of silver nanoparticles (AgNPs) is of
special concern because of their strong antimicrobial activity,and considerable amounts of research have therefore addressed
this issue.2,25−31 Previous studies have indicated that theinteraction of AgNPs and sand is controlled by physicochemical
conditions of the system.2,25,26 The surface modiﬁcation of
AgNPs and the presence of stabilizers signiﬁcantly inﬂuenced
their transport behavior under environmental conditions andpotentially extended their toxicity in ecosystems.26,31−33
Heterogeneity on the collector surface strongly inﬂuenced
AgNP transport.25,33 In particular, we recently demonstrated incolumn studies that some of the AgNPs interacted with quartz
sand in a weak secondary minimum, while other AgNPs
strongly interacted in a primary minimum due to microscopic
heterogeneity. The breakthrough curves exhibited blocking
behavior (a decreasing retention rate with time) as the
maximum retained concentration on the solid phase (Smax)
ﬁlled.33 Furthermore, RP shapes transitioned from hyperexponential, to nonmonotonic, and to uniform during blocking
as Smax was ﬁlled by retained AgNPs and free surfactant
molecules.33 Mechanical straining and chemical interactions
between AgNPs and the soil surface have also been suggested
to play important roles in AgNP retention in a disturbed sandy
soil.7 Studies on the mobility of micrometer-sized colloids
pointed out that the pore structure of undisturbed soil andleaching of naturally occurring particles during inﬁltration were■

MATERIALS AND METHODS
Below we summarize experimental procedures and protocols.
Additional details are provided in the Supporting Information,SI.
AgNPs and Solution Chemistry. The preparation andcharacterization of AgNP suspensions have been described in
our previous study.33 Brieﬂy, surface stabilized AgNPs(AgPURE) were produced using a mixture of two nonionicsurfactants, polyoxyethylene glycerol trioleate and polyoxyethylene (20) sorbitan mono-laurate (Tween 20) at 4% w/w
for each. Analyses of transmission electron microscopy images
indicated that the AgNPs were spherical in shape and around
99% of the AgNPs were in the size range of 15−20 nm.
Electrolyte solutions were made using Milli-Q water and KNO3
or Ca(NO3)2, and the pH ranged from 6 to 7 during the courseof the transport experiments. The surface charge characteristics
and size of AgNPs in a selected electrolyte solution were
determined using a Nano-Zetasizer apparatus (Malvern
ZetaSizer 4).
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ArticleSoil. Polyvinyl chloride columns (8 cm in inner diameterand 10 cm in length) were ﬁlled with undisturbed soil from theupper 30 cm of an agricultural ﬁeld (Kaldenkirchen, Northrhine-Westphalia, Germany). Soil samples were analyzed forspeciﬁc properties. The soil was classiﬁed as a loamy sand, with
4.9% clay (<0.002 mm), 26.7% silt (0.002−0.064 mm), and68.5% sand (0.064−2.000 mm).35 This soil had a total organic
matter content of 1.1%,35 a cationic exchange capacity of 7.8
cmolc kg−1,36 and a speciﬁc surface area of 1.7 m2 g−1. The clay
fraction contains the clay minerals illite, montmorillonite, andkaolinite. The surface charge characteristics of the soil in a
selected electrolyte solution were determined using a NanoZetasizer apparatus (Malvern ZetaSizer 4).
Transport Experiments. AgNP transport experiments
were conducted at diﬀerent IS (1, 5, and 10 mM), input
concentration values (Co, 1 and 10 mg/L), and Darcy watervelocities (q, 0.02 and 0.006 cm/min) by injecting a pulse(around 2 pore volumes) of AgNP suspension into the column.
A 90% water-saturation was employed in these experiments toreﬂect the inﬂuence of entrapped air on water ﬂow in thevadose zone. A summary of the experimental conditions isprovided in Table 1. After recovery of the AgNP breakthroughcurve (the C/Co value less than 0.5%), the soil from the column
was carefully excavated in approximately 1 cm increments, andthen freeze-dried. BTCs and RPs for AgNPs were obtained
from concentrations of Ag in the eﬄuent and soil samples,
respectively, which were digested in HNO3 and thendetermined by an inductively coupled plasma mass spectrometer (ICP-MS, Agilent 7500ce). Each sample was measured
three times, and the average value was taken for analysis. The
dissolution of AgNPs was determined to be negligible in thisstudy. Consequently, Ag concentrations were directly related to
the number of AgNPs in the sample.
AgNP Release Experiments . A series of experiments was
performed to deduce the eﬀects of cation type andconcentration, IS reduction, and cation exchange on the releaseof AgNPs. The input concentration of AgNPs was 10 mg/L andthe Darcy velocity was 0.02 cm/min for all the release
experiments. Four diﬀerent solution chemistry conditions wereconsidered for the initial AgNP deposition and elution with
AgNP free solution (step A), namely: 0.333 mM Ca(NO3)2(experiment I); 1.65 mM Ca(NO3)2 (experiment II); 3.33 mM
Ca(NO3)2 (experiment III); and 10 mM KNO3 (experimentIV). Similar to the AgNP transport experiments described in
the preceding section, the IS was 1, 5, 10, and 10 mM forexperiments I, II, III, and IV, respectively. The release
experiments I−III were subsequently conducted using thefollowing elution solution chemistry sequence: Milli-Q water(step B); KNO3 at the same IS as in step A (step C); Milli-Q
water (step D); 100 mM KNO3 (step E); and Milli-Q water(step F). Experiment IV consisted of only steps A and B.
Concentrations of Ag, K, Ca, Fe, and Al in select eﬄuent
samples were measured using inductively coupled plasma mass
spectrometry/optical emission spectrometry (ICP-MS/OES).
Concentrations of K and Ca demonstrate the process of cationexchange. Concentrations of Fe and Al reﬂect the amount of
released soil colloids in the eﬄuent. Additional analysis of
selected eﬄuent samples was conducted, including: zeta
potential, transmission electron microscopy (TEM) (Philips
CM 20 FEG), and energy-dispersive X-ray spectroscopy (EDX)(EDAX, Genesis).
Mathematical Model. A one-dimensional form of theconvection-dispersion equation was used to simulate the AgNP

transport and retention in the undisturbed soil columns asfollows:∂(qC)∂(θwC)∂ ⎛⎜∂C ⎞=θwD ⎟ −− θwψk1C
∂t
∂z ⎝
∂z ⎠
∂z

∂(ρb S)∂t= θwψk1C(1)(2)

where θw [−] is the volumetric water content, C [M L−3, M andL denote units of mass and length, respectively] is the aqueous
phase AgNP concentration, t is time [T, T denotes time units],z [L] is the distance from the column inlet, D [L2T−1] is thehydrodynamic dispersion coeﬃcient, q [L T−1] is the Darcy
water ﬂux, ψ [−] is a dimensionless function to account for
time- and depth-dependent blocking, k1 [T−1] is the ﬁrst-orderretention coeﬃcient of AgNPs from the aqueous to the solid
phase, ρb [M L−3] is the soil bulk density, and S [M M−1] is the
AgNP concentration on the solid phase. The ψ term is given as
follows: 37−β⎛S ⎞⎛ d50 + z ⎞ψ = ⎜1 −⎟⎟⎜Smax ⎠⎝ d50 ⎠⎝(3)where d50 [L] is the median grain size of the porous medium, β[−] is an empirical parameter controlling the shape of thespatial distribution of retained NPs, and Smax [M M−1] is the
maximum solid phase concentration of deposited AgNPs. In
this study, β = 1.532 was chosen for simulation instead of 0.432
that was applied in AgNP transport in sand.33 The ﬁrst term on
the right side of eq 3 accounts for time dependent blocking/ﬁlling of retention sites using a Langmuirian approach,38 while
the second term on the right side describes depth dependent
retention (e.g., a decreasing retention rate with depth).
The pore water velocity and dispersivity in the AgNP
transport simulations were determined from the tracer BTCs
by ﬁtting to the solution of the advection dispersion equation.
The parameters k1 and Smax were obtained from the observed
AgNP transport and retention using the inverse ﬁtting
algorithm39 in HYDRUS-1D. 40 A third type boundary
condition and zero dispersive ﬂux were set for the column
inlet and outlet, respectively, for all simulations. The release
experiments were not simulated because of the confounding
inﬂuence of clay release.

■

RESULTS AND DISCUSSIONAgNP and Soil Properties. The size of AgNPs in IS of 1 to
100 mM KNO3 and Ca(NO3)2 were in the range of 45 to 78
nm without an obvious trend. The zeta potentials of the AgNPs
and soil in the same IS range are presented in SI Figure S1. The
zeta potentials of AgNPs were negative and did not vary muchover the considered conditions. The zeta potentials of the soil
exhibited much more negative charges than the AgNPs.
AgNP Transport and Retention in Undisturbed Soil.
Figures 1−3 show the observed and simulated BTCs and RPs
for AgNPs in the undisturbed soil for various IS (Figure 1),
input concentrations (Figure 2), and ﬂow velocities (Figure 3).
Here BTCs are plotted as normalized eﬄuent concentrations(C/Co) versus eluted pore volumes, and RPs are plotted as
normalized solid phase concentrations (S/Co) with distancefrom the column inlet. Mass balance information for the BTCs,
RPs, and total columns are provided in Table 1. The total
column mass balance (92−111%) was very good. SI Table S1
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ArticleFigure 2. Eﬀect of AgNP concentration on the transport and retentionof AgNPs in an undisturbed loamy sand soil: observed and ﬁtted
breakthrough curves (a) and retention proﬁles (b) of AgNPs under
AgNP input concentrations of 1 and 10 mg/L, respectively. Other
experimental conditions were the same: electrolyte, 1 mM KNO3;
Darcy velocity, 0.006 cm/min.

Figure 1. Eﬀect of ionic strength on the transport and retention of
AgNPs in an undisturbed loamy sand soil: observed and ﬁtted
breakthrough curves (a) and retention proﬁles (b) of AgNPs under 1,
5, and 10 mM KNO3, respectively. Other experimental conditions
were the same: input concentration, 10 mg/L AgNPs; Darcy velocity,
0.006 cm/min.

summarizes the ﬁtted model parameters, the 95% conﬁdence
intervals for the ﬁtted parameters, and the Pearson’s correlationcoeﬃcient (R2) between observed and simulated data. Overall,
the simulated curves were able to capture the main features of
AgNP transport and retention (R2 > 0.959).
The BTCs for AgNPs were retarded in comparison with thetracer (data not shown). This retardation is also evident in
Figures 1−3. In particular, the center of mass of the AgNP
breakthrough front frequently occurred after 1 pore volume.
These retarded breakthrough curves can be quantiﬁed with the
ﬁtted parameters k1 and Smax/Co in eqs 1−3 which are related
to the retention rate and capacity of AgNPs on the porous
media, respectively. A high value of k1 produces complete
retention until Smax/Co is ﬁlled. Consequently, more retarded
BTCs are expected for higher k1 and larger Smax/Co. However, if
k1 is too low, then breakthrough will occur (no retardation) andthe BTC will slowly increase with time as the favorable
retention locations are gradually ﬁlled.
More retarded BTCs, lower eﬄuent concentrations, andgreater retention occurs at higher IS in Figure 1. This can be
explained by an increase in k1 and Smax/Co with IS (SI Table
S1) that occurs due to compression of the electrical double
layer thickness that produces a shallow secondary minimum
and enhances the eﬀects of nanoscale heterogeneities.41 Higher
values of Co (input concentration) produced less retardation,higher eﬄuent concentrations, and less retention in Figure 2.
One potential explanation is due to blocking which increases
the rate of ﬁlling of Smax with a larger Co.42 In addition, theretained AgNPs could hinder further retention due to therepulsive AgNP−AgNP interactions especially when moreparticles were retained at higher Co.33 An observed decreasein k1 and Smax/Co with larger Co supports this hypothesis (SIFigure 3. Eﬀect of ﬂow rate on the transport and retention of AgNPs
in an undisturbed loamy sand soil: observed and ﬁtted breakthroughcurves (a) and retention proﬁles (b) of AgNPs under Darcy velocity of
0.006 and 0.02 cm/min, respectively. Other experimental conditions
were the same: electrolyte, 1 mM KNO3; input concentration, 10 mg/
L AgNPs.

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Articlesigniﬁcant for nanoparticles as a result of nanoscale chemicaland physical heterogeneity on the soil surface.41 Nanoscale
heterogeneity is well-known to be signiﬁcant for soils due toroughness and adsorbed clays, natural organic matter (NOM),
metal oxides, ions, and variations in mineralogy.48 The
nanoscale−scale heterogeneity on the soil surface may produce
primary minimum interactions for AgNPs depending of thecross-sectional area of the heterogeneity and the solution IS.41
However, charge heterogeneity on the soil surface will tend to
be masked by NOM49−51 present in the loamy sand (1.1%)35
or free surfactant in the AgNP suspension.33 Furthermore, theeﬀect of nanoscale heterogeneity on AgNP retention is
expected to be diminished under low IS conditions.41 Both ofthese factors can help to explain the high mobility of AgNPs in
the loamy sand when the IS was 1 mM (Figure 1). Collectively,
these results suggest that AgNP transport in the loamy sand soil
was mainly controlled by primary minimum interactions fromsoil heterogeneities that were diminished by NOM, especially
under low IS conditions.
AgNP Remobilization. Experiments I−IV were conducted
to better understand the eﬀect of cation type, IS reduction, andcation exchange on the release of AgNPs in soil. Figure 4

Table S1). Higher eﬄuent concentrations and less retention
were observed in Figure 3 for higher q (ﬂow velocity). Similar
to experimental observations, ﬁltration theory predicts that k1 is
proportional to q to the 1/3 power.43 However, the overall rateof advection is proportional to q, so a decrease in retention is
expected with an increase in q. Experimental values of Smax/Co
also decrease with higher q similar to predicted trends when
conducting a balance of applied hydrodynamic and resistingadhesive torques to determine the fraction of the solid surface
that contributes to colloid immobilization.41
The above results indicate that AgNP transport and retention
in soil are highly dependent on blocking, which has also been
demonstrated to play an important role in homogeneous quartz
sand.33 Similar to Figures 1−3, BTCs for AgNPs in quartz sandwere observed to decrease with IS, and to increase with Co andq. In contrast to Figures 1−3, retardation was not observed toplay a signiﬁcant role in the quartz sands. Furthermore, the RPs
in the loamy sand (Figures 1−3) were always hyperexponential
in shape and a signiﬁcant fraction of the retained AgNPs(approximately 70%) was in the top 0−3 cm. In contrast,
blocking caused the RP shapes to transition from hyperexponential, to nonmonotonic, and to uniform in the quartz
sand.33 These diﬀerences can partially be ascribed to the higher
values of Smax/Co in the current work (>1.94 cm3 g−1). In
comparison, values of Smax/Co were frequently as low as 0.732
cm3 g−1 in the quartz sand.33 In addition, as conﬁrmed by the
BET surface area analysis, the interacting surface area in the
loamy sand (1.7 m2 g−1) is much greater than in the quartz
sand (less than 0.2 m2 g−1). Both of these factors tend todiminish the relative importance of blocking in loamy sand incomparison to quartz sand; e.g., it takes more AgNPs to ﬁll alarger value of Smax/Co.44,45
Several explanations have been proposed in the literature for
hyperexponential RPs, including: aggregation,37 straining,11chemical heterogeneity on the particle,45 and system hydrodynamics.46 These hypotheses will be brieﬂy examined below.
Our TEM images taken from the column eﬄuent (SI FigureS2) demonstrated that the AgNPs were still well dispersed.
Furthermore, the BTCs did not exhibit ripening behavior(increasing retention rate with time). This information suggeststhat AgNP aggregation was insigniﬁcant during the transport
experiments. Straining was neglected because of the small ratioof the AgNP diameter to the soil grain size.11 Charge
heterogeneity on the AgNP surface was an unlikely explanation
because the zeta potentials for AgNPs in inﬂuent and eﬄuent
solutions were very similar. Alternatively, variations in the porescale velocity can provide a viable explanation for thehyperexponential RPs when the ﬂux adjacent to the solid
surface near the column inlet is the dominant mass transfer
mechanism to the solid-water interface.46 This eﬀect is reported
to be more pronounced in ﬁner textured porous media,46 whichis consistent with the higher value of β in eq 3 for AgNPs inloamy sand (β = 1.532) than quartz sand (β = 0.432).33 A
higher value of β indicates a stronger depth dependence of theretention coeﬃcient (k1).
Liang et al.33 reported that the AgNPs attached to quartz
sand in a weak secondary minimum and a strong primary
minimum. Similar interactions are expected for AgNPs in the
loamy sand. Secondary minimum interactions are very shallow
for AgNPs under low IS conditions.33 Consequently, the kinetic
energy of diﬀusing AgNPs is expected to be suﬃciently large toovercome most of the shallow secondary minimum interactions.47 Conversely, primary minimum interactions may be

Figure 4. Breakthrough and release behavior of AgNPs in an
undisturbed loamy sand soil. Deposition (step A) occurred at an IS= 10 mM using Ca(NO3)2 (experiment III) or KNO3 (experimentIV), whereas release (step B) was initiated by eluting with Milli-Q
water. The Darcy velocity was 0.02 cm/min and the input AgNP
concentration was 10 mg/L.

presents BTCs when AgNPs were deposited and eluted (stepA) in the presence of KNO3 or Ca(NO3)2 at an IS of 10 mM,
and then release of AgNPs was initiated in step B by areduction in IS (Milli-Q water). Retention of AgNPs in step A
was more pronounced in the presence of Ca(NO3)2 than
KNO3, even though the IS was the same. This can be explainedby bridging complexation between soil grains and functionalized nanoparticles in the presence of Ca2+.24 Natural mineralsurfaces such as clays are known to show a stronger aﬃnity fordivalent than monovalent cations.16 Release of AgNPs by IS
reduction (step B) was more pronounced when the AgNPs
were retained (step A) in the presence of monovalent cation.
Mass balance information indicates that the recovery in step B
was 31.7% for KNO3 solution and only 16% for Ca(NO3)2(Table 1). This behavior indicates that some of the AgNPs
were interacting in a reversible minimum that was eliminatedby a reduction in the IS. However, the resistance to AgNP
release with IS reduction was apparently greater in the presenceof Ca(NO3)2 than KNO3, suggesting that cation bridging
created stronger primary minimum interactions.5 Our exper12233

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ArticleD. Conversely, less than 20% of the injected K+ (100 mM) instep E was adsorbed to the soil and high release peaks for Ca2+
were observed. This clearly indicates that excess amounts of K+
in step E produced substantial release of Ca2+ as a result of
cation exchange. However, very little AgNP release occurred
during cation exchange (C and E), while rather signiﬁcantrelease of AgNPs occurred during IS reduction (D and F)
immediately following cation exchange. Exchange of K+ for
Ca2+ will decrease the potential for bridging complexation,
whereas IS reduction will expand the double layer thickness andeliminate the secondary minimum. These observations suggestthat both cation exchange and IS reduction were needed torelease the AgNPs that were interacting with the soil grains in
the presence of divalent cation.
The amount of AgNPs that were released in step D followed
a similar trend to that shown in step B (Table 1); e.g.,increasing with IS of KNO3 in step C. This observationsuggests that IS reduction was the dominant cause of release inthis case. Conversely, greater amounts of AgNP releaseoccurred in step F for soils exposed to lower concentrationsof Ca(NO3)2 during step A (Table 1). Consequently, a larger
amount of AgNPs was released when the cation exchange wasmore pronounced followed by IS reduction. Bradford andKim52 observed signiﬁcant amounts of kaolinite clay release
following similar processes of cation exchange (Na+ for Ca2+)and IS reduction. Similarly, the eﬄuent was very turbid during
steps B, D, and F. This phenomenon implied the release ofnatural soil colloids from the loamy sand soil. The
remobilization of soil colloids from an AgNP-free soil column
was conﬁrmed by measuring eﬄuent concentrations of Al andFe shown in SI Figure S3. Notice that the release of Al and Fe
in soil showed a very similar trend as the release of AgNPs(Figure 5a), suggesting the potential for AgNP association withsoil colloids. Additional evidence for the role of soil colloids on
AgNP release is provided below.
To further conﬁrm the potential for soil colloids to facilitate
the release of AgNPs, Figure 6a presents plots of Ag, Al, and Fe
concentration for steps A−F. In this case, AgNP deposition andelution during step A occurred in the presence of Ca(NO3)2 at
an IS of 5 mM when Co was 10 mg/L and q was 0.7 cm/min. In
contrast to other experiments, a repacked soil column was
employed under saturated conditions because all of theundisturbed soil columns had already been used. Similar to
SI Figure S3, the release of AgNPs was highly correlated with
Fe and Al concentrations of soil colloids. The association of
AgNPs with soil colloids was further conﬁrmed by TEM
investigation. Figure 6b,c presents representative TEM imagesof soil colloids in the eﬄuent. The black dots on these colloids
were conﬁrmed by EDX to be AgNPs (Figure 6d,e). A rigorous
determination of the colloid-associated fraction of AgNPs was
not attempted because of TEM limitations in sample
preparation and inspection, and the transient solution
chemistry conditions. Nevertheless, the TEM images (Figure
6b,c) and EDX analysis (Figures 6d,e), correlations between
Ag, Fe, and Al (SI Figures S3 and 6a), and available
literature48,53 do support the interaction between soil colloids
and some fraction of the released AgNPs. Additional research isneeded to fully resolve the role of soil colloids (such as clay
minerals, iron oxide-hydroxide, and their complexes) infacilitating the release and transport of AgNPs during transient
solution chemistry conditions.
Environmental Implications. Experiments were designed
to examine the eﬀects of physicochemical factors (IS, cation

imental results also indicate that these primary minimum
interactions were not diminished by long elution at reduced IS.
Figure 5a shows the AgNP BTCs from experiments I−III in
the presence of Ca(NO3)2 when the IS in step A was 1, 5, andFigure 5. (a) Breakthrough and release behavior of AgNPs in an
undisturbed loamy sand soil. Deposition (step A) occurred at an IS =1, 5, and 10 mM using Ca(NO3)2 for experiment I, II, and III,
respectively, whereas release was initiated by eluting with Milli-Q
water (steps B, D, and F) and cation exchange (steps C and E) assummarized in Table 1. (b): Eﬄuent concentrations of K and Ca
during steps A−F. The Darcy velocity was 0.02 cm/min and the input
AgNP concentration (Co) was 10 mg/L.

10 mM, respectively. The subsequent release procedures were
Milli-Q water (step B), KNO3 at the same IS as in step A (stepC), Milli-Q water (step D), 100 mM KNO3 (step E), and MilliQ water (step F). A small amount (3.9%) of the injected
AgNPs was transported through the undisturbed soil column
during step A in experiment I. In contrast, the AgNP
concentrations in the eﬄuent from step A were under thedetection limit in experiments II and III. The recoveries of
AgNPs in step B (Milli-Q water) were 3.8%, 4.6%, and 16.0%
for experiment I, II, and III, respectively. Similar to Figure 1,
these observations suggests that a greater amount of AgNPs
were reversibly retained in a secondary minimum with thehigher IS in step A.
When KNO3 was introduced in step C at the same IS as instep A and at a much larger IS of 100 mM in step E, further
AgNP remobilization occurred by IS reduction in steps D and
F. The continued release of AgNPs as IS reduced during steps
D and F was initiated by the cation exchange during steps C
and E, respectively. Figure 5b shows plots of K+ and Ca2+
concentrations in the column eﬄuent for experiments I−III.
Around 95% of the injected K+ mass in step C was retained in
the column even after ﬂushing with Milli-Q water during step12234

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Environmental Science & Technology

ArticleFigure 6. (a) Release of AgNPs and naturally occurring minerals by cation exchange (steps C and E) and ionic strength reduction (steps B, D, andF) in a saturated, repacked soil column. The Darcy velocity was 0.7 cm/min and the input AgNP concentration (Co) was 10 mg/L. (b)−(e):Transmission electron microscopy (b and c) and energy-dispersive X-ray spectroscopy (EDX) (d and e) measurements of AgNPs in the eﬄuent
sampled from release experiment after cation exchange (AgNPs cotransported through soil column with released naturally occurring colloids/clay).
The marked points in (b) and (c) were selected for EDX analysis as shown in (d) and (e), respectively.

type, Co, and q) on the transport, retention, and release of
surfactant stabilized AgNPs in undisturbed soil, near water
saturation (90%) conditions to represent the maximum
transport potential in the vadose zone. Similar to column
studies conducted with repacked quartz sand,33 the mobility of
the AgNPs was highly dependent on the physicochemical
conditions. In contrast, the BTCs for AgNPs in the undisturbed
loamy sand exhibited signiﬁcant amounts of retardation and theshape of the RPs was hyperexponential. The amount ofretardation was dependent on physicochemical properties thatinﬂuenced the retention rate and/or capacity. Release of AgNPs

and soil colloids was promoted by a reduction in IS,
monovalent ions, and cation exchange. An association between
some fraction of the released AgNPs and soil colloids was also
observed. Collectively, our results demonstrate that AgNP
transport studies in ﬁeld soils need to consider retarded BTCs,
hyperexponential RPs, cation exchange, and colloid associations
which are commonly neglected in repacked sand column
experiments. The release processes can produce signiﬁcantamounts of AgNP migration that pose a potential risk ofgroundwater contamination during intermittent rain and12235

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Environmental Science & Technology

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irrigation events that produce transient changes in solution
chemistry.

■

ASSOCIATED CONTENT

S Supporting Information*

Brief description and discussion of the following: (i) characterization of AgNPs; (ii) procedures for collecting undisturbed soil
samples and for running the column experiments; (iii) ﬁtted
values of k1 and Smax/Co (Table S1); (iv) zeta potentials of thesoil grains as a function of KNO3 and Ca(NO3)2 concentrations(Figure S1); (v) transmission electron micrograph of AgNPs in
eﬄuent (Figure S2); and (vi) release of Fe and Al from a
AgNP-free soil column by cation exchange and IS reduction(Figure S3). This material is available free of charge via theInternet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author*Phone: +1 951 369 4857; fax: +1 951 342 4964; e-mail: scott.
bradford@ars.usda.gov.
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
The ﬁrst author thanks the China Scholarship Council (CSC)
for ﬁnancial support. This research was funded by the
NanoFlow project supported by German Federal Ministry of
Education and Research (BMBF). The authors would like toacknowledge Ansgar Weuthen and Claudia Walraf for thetechnical assistance, and Michael Burkhardt for valuablesuggestions.

■

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