Environmental Pollution 180 (2013) 152e158

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Environmental Pollution
journal homepage: www.elsevier.com/locate/envpol

Limited transport of functionalized multi-walled carbon nanotubes
in two natural soils
 Daniela Kasel a, *, Scott A. Bradford b, Jirí Sim
unek c, Thomas Pütz a, Harry Vereecken a,
Erwin Klumpp aabc

Agrosphere Institute (IBG-3), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
US Salinity Laboratory, Agricultural Research Service, United States Department of Agriculture, Riverside, CA 92507, USA
Department of Environmental Sciences, University of California Riverside, Riverside, CA 92521, USAa r t i c l e i n f o

a b s t r a c t

Article history:
Received 8 February 2013
Received in revised form
8 May 2013
Accepted 16 May 2013

Column experiments were conducted in undisturbed and in repacked soil columns at water contents
close to saturation (85e96%) to investigate the transport and retention of functionalized 14C-labeled
multi-walled carbon nanotubes (MWCNT) in two natural soils. Additionally, a ﬁeld lysimeter experiment
was performed to provide long-term information at a larger scale. In all experiments, no breakthrough of
MWCNTs was detectable and more than 85% of the applied radioactivity was recovered in the soil
proﬁles. The retention proﬁles exhibited a hyper-exponential shape with greater retention near thecolumn or lysimeter inlet and were successfully simulated using a numerical model that accounted for
depth-dependent retention. In conclusion, results indicated that the soils acted as a strong sink for
MWCNTs. Little transport of MWCNTs is therefore likely to occur in the vadose zone, and this implies
limited potential for groundwater contamination in the investigated soils.
Ó 2013 Elsevier Ltd. All rights reserved.

Keywords:
Undisturbed soil
Carbon nanotube
Lysimeter
Retention proﬁleTransport modeling

1. Introduction
Carbon nanotubes (CNT) are tubular nanoparticles with nanoscale diameters and micro-scale lengths composed of aligned
benzene rings (Iijima, 1991; Mauter and Elimelech, 2008; Petersen
et al., 2011). Two types of CNT are most commonly distinguishedand produced: single-walled carbon nanotubes (SWCNT) are individual graphene tubes, and multi-walled carbon nanotubes(MWCNT) are tubes within tubes consisting of more than two
carbon walls (Sinnott, 2002). Due to their exceptional electrical,
chemical, and physical properties, CNT are used in numerous applications (Jaisi and Elimelech, 2009; Mattison et al., 2011). Thiswidespread use will result in their release to the environment
through point sources (e.g., production facilities, landﬁlls, or
wastewater treatment plants), nonpoint sources (e.g., abrasion ofmaterials containing CNT), accidental release (e.g., during transport), or intentional release (e.g., for groundwater remediation)(Jaisi and Elimelech, 2009; Köhler et al., 2008; Nowack and Bucheli,
2007; Pan and Xing, 2012).

* Corresponding author.
E-mail addresses: d.kasel@fz-juelich.de (D. Kasel), scott.bradford@ars.usda.gov
 (S.A. Bradford), jiri.simunek@ucr.edu (J. Simunek), t.puetz@fz-juelich.de (T. Pütz),h.vereecken@fz-juelich.de (H. Vereecken), e.klumpp@fz-juelich.de (E. Klumpp).
0269-7491/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.envpol.2013.05.031

To date, there is still a lack of knowledge on the fate and effectsof CNT in the environment (Saleh et al., 2008). Ecotoxicologicalstudies on CNT have revealed potential for bioaccumulation (e.g.,daphnia magna) (Petersen et al., 2009a), health risks to different
organisms (e.g., rainbow trout and rats) (Farré et al., 2009; Handy
et al., 2008; Lam et al., 2006; Ma-Hock et al., 2009), and growth
inhibition of algae due to shading and agglomeration of cells with
CNT (Schwab et al., 2011). Furthermore, CNT act as strong adsorbents for organic pollutants (Chen et al., 2007; Li et al., 2012; Lu
et al., 2005; Peng et al., 2003) and may therefore affect the fate
and mobility of these chemicals in natural environments. A CNT
facilitated transport (co-transport) of organic pollutants could leadto enhanced migration of contaminants (Cheng et al., 2005) but
adsorption of organic pollutants onto CNT may also decreasebioavailability (Petersen et al., 2009b). To assess potential risks tohumans and other organisms, it is important to gain knowledge on
the environmental fate of CNT. Thus, information on CNT transportand deposition in natural soils is needed.
In general, CNT can be released as agglomerates or as individual
particles. Recently, the release of free-standing individual CNTsfrom CNT-embedded nanocomposites was demonstrated(Schlagenhauf et al., 2012). The environmental behavior of nanoparticles is frequently controlled by their colloidal stability. Functionalized CNT are of special interest because the modiﬁcation will

D. Kasel et al. / Environmental Pollution 180 (2013) 152e158

not only increase their stability in aqueous suspensions but also
increase their mobility in the environment (Mattison et al., 2011).
Several studies have reported on the transport of CNT in watersaturated sand columns (Jaisi et al., 2008; Liu et al., 2009; Mattison
et al., 2011; Tian et al., 2010, 2012). Most of these studies have been
conducted in repacked, homogeneous, coarse textured porous
media in order to understand mechanisms and factors inﬂuencing
CNT retention. Results indicate that CNT transport is sensitive to adiversity of experimental conditions including ionic strength (IS),pore water velocity, and collector grain size. However, mechanisms
inﬂuencing the transport and deposition of MWCNT are still notcompletely understood even in these highly idealized systems(Mattison et al., 2011). Information on MWCNT retention proﬁles isstill very scarce (Kasel et al., 2013; Wang et al., 2012). Determination of retention proﬁles provides mass balance information andgives useful insight on mechanisms controlling retention (Bradford
and Bettahar, 2005).
The transport of CNT in ﬁeld soils has received very limited
attention (Jaisi and Elimelech, 2009). In general, soil is expected to
be a more important sink for nanoparticles than sand because its
chemical composition and pore size distribution are more heterogeneous (Pan and Xing, 2012). However, preferential ﬂow paths insoils (e.g., root or earthworm channels) may enhance transport
through the vadoze zone (Camobreco et al., 1996). Undisturbed soilcores and lysimeters are much closer to environmental conditions
than packed sand columns because they allow consideration ofpreferential ﬂow paths. To our knowledge, there is no study available on the transport and retention of MWCNT in undisturbed soilcores or lysimeters. Furthermore, there are only a few studies on
CNT transport under unsaturated ﬂow conditions (Tian et al., 2011).
Unsaturated and/or variably saturated transport experiments arecloser to natural conditions in the vadose zone. Transport processes
are much more complicated under unsaturated than saturated
conditions because of the presence of the air phase, although
greater retention is expected under unsaturated conditions(Gargiulo et al., 2007b; Tian et al., 2011).
The objective of this research was to investigate transport andretention of MWCNT in natural, undisturbed soils under environmentally relevant conditions like low MWCNT concentrations andlow ﬂow rates. Two soil types (a silty loam and a loamy sand) were
investigated in laboratory column experiments at water contents
close to saturation. Additionally, the nanoparticles were applied toa ﬁeld lysimeter ﬁlled with the loamy sand soil. For all experiments,
MWCNT concentrations in the efﬂuent and in the soils were
determined. Finally, the observed data was numerically modeled.
To our knowledge, this is the ﬁrst study providing information on
MWCNT fate in undisturbed soil and in a lysimeter.
2. Materials and methods

153

stability of CNT in the aqueous phase (Smith et al., 2009). Transmission electron
microscope (TEM, Philips CM20 FEG, FEI Company, 5651 GG Eindhoven, theNetherlands) images revealed that the size distribution of MWCNT was very heterogeneous. The hydrodynamic radius of MWCNT dispersed in 1 mM KCl was
determined to range between 170 and 210 nm using dynamic light scattering (DLS,
Zetasizer Nano, Malvern Instruments GmbH, 71083 Herrenberg, Germany). The
electrophoretic mobility for MWCNT in 1 mM KCl solution (pH 8.5) was measured
using the Zetasizer Nano to be 2.85  108 m2 V1 s1.
For transport experiments, nanoparticle suspensions with concentrations of
10 mg L1 were prepared by adding 1 mg of 14C-labeled functionalized MWCNT to
100 mL of 1 mM KCl solution. The suspension was sonicated for approximately
10 min at 65 W using a cup horn ultrasonicator (Branson SoniﬁerÒ W-250, Danbury,
CT 06813-1961, USA) until no aggregates were visible. Afterward, ﬁve 0.5 mL aliquots were each added to 2.5 mL of scintillation cocktail (Insta-Gel Plus, PerkinElmer, 63110 Rodgau, Germany) and 2 mL of water, shaken, and the radioactivity
was measured using a liquid scintillation counter (LSC, PerkinElmer, 63110 Rodgau,
Germany). The low standard deviation of the radioactivity measured in the ﬁvereplicates indicated that the MWCNT suspensions were homogeneous. For eachtransport experiment a fresh suspension was prepared.
2.2. Soil
Soil samples from the upper 30 cm of two well characterized agricultural ﬁeld
sites in Germany were used as natural porous media for transport experiments. In
particular, a loamy sand (Gleyic Cambisol) from the test site in Kaldenkirchen-Hülst(KAL) and a silty loam (Orthic Luvisol) from the test site in Merzenhausen (MRZ)(Unold et al., 2009a) were employed. The electrophoretic mobility of the soils (0.1 wt%) in 1 mM KCl solution (pH 8.5) was measured using the previously described
Zetasizer Nano. The physicochemical characteristics of the two soils are summarizedin Table 1.
Undisturbed soil samples were collected in columns made of polyvinyl chloride(PVC) with a length of 10 cm and an inner diameter of 8 cm using a metal adaptor
with a sharp front. The adaptor was mounted on the bottom of the PVC column andplaced on top of the soil. The column was then pushed stepwise into the plough layerof the soil using a water-level. Between the pushing steps, the surrounding soil was
removed. Finally, the soil at the end of the column was cut using a knife to obtain aneven surface. For repacked soil columns, the soil was ﬁrst sieved to a fraction<2 mm, air dried, and then the soil was wet packed into the column.
2.3. Experimental setup
The soil columns were mounted on acrylic glass plates covered with a highconductivity 20 mm nylon mesh. The air-entry value of the nylon mesh wasapproximately 50 mbar. The undisturbed soil columns were slowly saturated fromthe bottom with 1 mM KCl. The water-saturated columns (repacked and undisturbed) were equipped with two tensiometers (inserted at 2.5 and 7.5 cm below thesoil surface) to monitor the pressure head inside the columns. Software controlled
equipment (Unold et al., 2009a) was used to achieve unit gradient, steady-state ﬂow
conditions in the columns. In brief, the columns were irrigated from the top with
1 mM KCl and suction was applied to the bottom. A unit gradient in the matrix
potential (monitored using the two tensiometers) was achieved by automaticallychanging the irrigation rate and the bottom suction. The irrigation rate was recorded
by weighing the storage bottle of the irrigation solution. The electrical conductivityof the leachate was measured over the duration of the experiment.
The columns were ﬂushed with 1 mM KCl until the conductivity of the leachate
and the matrix potentials (water contents) were constant. Afterward, a 1 h pulse of
10 mM KCl was applied to the columns as conservative tracer and its breakthroughcurve was detected by measuring the conductivity of the leachate. The columns
were ﬂushed again with 1 mM KCl until the conductivity remained constant atbackground levels. Following the tracer experiment, 5 mL of MWCNT suspension(10 mg L1) were pipetted evenly on top of the soil. The columns were ﬂushed again

2.1. Carbon nanotubes
Transport experiments were performed using radioactively (14C) labeled
MWCNT (Bayer Technology Services GmbH, 51368 Leverkusen, Germany) with a
speciﬁc radioactivity of appr. 3.2 MBq mg1. The average outer diameter of the
MWCNT was 10e15 nm and the average length was 200e1000 nm (Kasel et al.,2013). Prior to transport experiments, the MWCNT were functionalized by boilingin 70% nitric acid (SigmaeAldrich Chemie GmbH, 89555 Steinheim, Germany) for 4 h
under reﬂux (Nagasawa et al., 2000). The MWCNT were removed from the acid byﬁltering through a 0.45 mm polytetraﬂuoroethylene (PTFE) membrane and rinsing
with deionized water until a neutral pH-value was achieved in the ﬁltrate (Mattison
et al., 2011). After drying the functionalized MWCNT at 45  C in an oven, the material
was stored at room temperature until needed.
Kasel et al. (2013) characterized many properties of the MWCNT using unlabeled
functionalized material. A brief summary is provided below. The acid treatment wasdemonstrated to remove a signiﬁcant amount of the metal catalysts (using ICP-MS)
and induced oxygen containing functional groups (via XPS-spectra, appr. 6 wt%
oxygen) on the MWCNT surfaces. These functional groups are known to enhance theTable 1
Physical and chemical properties of the soils from two experimental test sites Kaldenkirchen-Hülst (KAL, loamy sand) and Merzenhausen (MRZ, silty loam).

Clay (<2 mm)
Silt (2e63 mm)a
Sand (>2000 mm)a
pH (0.01 M CaCl2)aTotal organic mattera
Cation exchange capacitya
Ironb
Electrophoretic mobility
a

a
b

Unit

KAL

MRZ

% mass% mass% mass

4.9
26.7
68.5
5.9
1.1
7.8
0.8
2.7E-8

15.4
78.7
5.9
6.2
1.3
11.4
1.5
3.2E-8% masscmolc kg1%m2 V1 s1Data adapted from Unold et al. (2009a).
Data adapted from Kasteel et al. (2010).

154

D. Kasel et al. / Environmental Pollution 180 (2013) 152e158

with 1 mM KCl and the leachate was collected using a fraction collector. The leachate
was analyzed using a liquid scintillation counter and the experiment was stopped
when the radioactivity in the efﬂuent was below the detection limit.
After termination of the transport portion of the experiment, the soil was
removed in 1 cm thick increments, dried at 45  C in an oven, and then homogenizedusing a mill. Five replicates (500 mg) of each sample were combusted at 900  C using
a biological oxidizer (OX 500, R.J. Harvey Instrumentation Corporation, Tappan, NY
10983, USA). Before measuring the radioactivity using the liquid scintillation
counter, the emerging 14CO2 was dissolved in vials ﬁlled with scintillation cocktail(OxisolvÒ, MERCK KGAA, 64293 Darmstadt, Germany). Table 2 summarizes column
properties determined from the conservative tracer experiments and mass balance
information for MWCNT.
2.4. Lysimeter
A stainless steel lysimeter with a surface area of 0.5 m2 and a depth of 1.15 m wasused for larger scale transport experiments (Burauel and Führ, 2000). The lysimetercontained an undisturbed soil monolith from the test site in Kaldenkirchen-Hülst(KAL). It represents a long-term outdoor experiment on a larger scale. The lysimeter
was equipped with two time-domain reﬂectrometry (TDR) probes at depths of 5 and30 cm to observe the water content during the experiment. The lysimeter wascovered with a stainless steel plate to prevent evaporation, precipitation, and plant
growth. It was artiﬁcially irrigated twice a week with 5 L of tap water to achieve a
precipitation rate of 1200 mm y1. Slight ponding at the lysimeter surface wasobserved immediately after irrigation.
First, a pulse of 5 L of potassium bromide solution (appr. 16.5 g KBr in total) wasapplied as a conservative tracer uniformly onto the soil’s surface. One week later, thesame procedure was repeated with 5 L of 14C-MWCNT suspension (appr. 25 mg
MWCNT in total). The leachate ran into a stainless steel pan at the bottom of thelysimeter and was collected in regular intervals. The bromide concentration in theleachate was determined using a high performance liquid chromatograph (STH 585,
Dionex, Sunnyvale, CA, USA) equipped with a UV-detector (UV2075, Jasco, Essex,
UK). The liquid scintillation counter was again used to determine the radioactivity.
After termination of the experiment (8 month after MWCNT application), soil
samples were taken at different points using a Pürckhauer soil corer (Schierholz
et al., 2000) to obtain information on the MWCNT distribution in the soil proﬁlefollowing a similar approach to the columns.
2.5. Mathematical modeling
The HYDRUS-1D code was used to analyze the transport experiments. It is a
ﬁnite element model which can simulate the one-dimensional movement of water, heat, and multiple solutes in variably saturated media (Simunek et al., 2008). The
inverse ﬁtting of model parameters to experimental breakthrough curves (BTCs) andretention proﬁles (RPs) was performed using a nonlinear least squares optimization
routine in the code. The column’s porosity and dispersivity (Table 2) were obtainedby ﬁtting the advective dispersive equation (ADE) parameters to the conservative
tracer BTC for each column.
The transport of MWCNT was simulated using the ADE with terms for one sitekinetic retention. The total mass balance equation is written as (Gargiulo et al.,2007a):


vqC
vS
v
vC
vqC
qD
þr
¼

vt
vt
vx
vx
vx

(1)

where q [-] is the volumetric water content, r is the bulk density of the porous media[M L3, where M and L denote units of mass and length, respectively], t is the time [T;T denotes units of time], x is the spatial coordinate [L], q is the ﬂow rate [L T1], C is
the particle concentration in the aqueous phase [Nc L3, where Nc is the number of
MWCNT], S is the solid phase particle concentration [Nc M1], and D is the dispersion
coefﬁcient [L2 T1]. The corresponding solid phase particle mass balance equation is
given as:

r


 b
vS
dc þ x C  k2 rS
¼ qk1
vt
dc

(2)

where k1 [T1] is the ﬁrst-order retention coefﬁcient [T1], k2 is the ﬁrst-orderdetachment coefﬁcient [T1], dc [L] is the median diameter of the soil grains, andb [-] is an empirical variable that controls the shape of the retention proﬁle.
This model was already utilized by Kasel et al. (2013) to simulate BTCs and RPs of
MWCNT in quartz sand, as well as for bacteria and colloid transport by many others(Bradford and Bettahar, 2006; Bradford et al., 2005; Gargiulo et al., 2008). The model
formulation shown in Eq. (2) can account for retention proﬁles that are exponential
or hyper-exponential with depth. The depth-dependent retention term equals 1
when b ¼ 0 and predicts an exponential distribution of retained MWCNT with depth
similar to conventional ﬁltration theory. Conversely, when b > 0 is employed theretention proﬁle of MWCNT exhibits a hyper-exponential shape (e.g., a higher
deposition rate close to the column inlet). The optimal value for b was estimated tobe 0.765 for MWCNT based on literature information (Kasel et al., 2013; Wang et al.,2012). The estimated model parameters for all experiments are listed in Table 3.

3. Results and discussion
3.1. Comparison of two water-saturation levels
Transport and retention of MWCNT in an undisturbed soil coreof a loamy sand (KAL) were investigated at water contents close tosaturation (appr. 96 and 85%, respectively). The negative charge on
both MWCNT and soil, and the low ionic strength conditions suggest that highly unfavorable attachment conditions existed for
MWCNT during the transport experiments.
There was no detectable breakthrough of MWCNT in the soilcore at a water content of 96%. Quantiﬁcation of the retention
proﬁle (Table 2) revealed that 91% of MWCNT were retained in thesoil. The main fraction of MWCNT was retained close to the column
inlet and the retention proﬁle exhibited a hyper-exponential shape.
The same shape was observed by Kasel et al. (2013) for functionalized MWCNT in saturated sand columns and was attributed to adepth-dependent retention mechanism. The determined retention
proﬁle for the soil column was simulated using the previously
described model and is shown in Fig. 1. The retention coefﬁcientaccounting for depth-dependency was able to describe the shape ofthe experimental retention proﬁle (RP) well.
At lower water saturation (appr. 85%), no breakthrough of
MWCNT was observed and 98% of the applied radioactivity was
detected in the soil proﬁle. The shape of the retention proﬁle wasalso hyper-exponential (Fig. 1) and was ﬁtted well using a model
including a depth-dependent retention term (Eq. (2)).
Though both experiments were performed at different watersaturation levels and thus, different amounts of air in the column,
the retention proﬁles were similar. Complete retention occurredalready at 95% water-saturation but reduction of the water content
to 85% changed the MWCNT distribution in the soil. When
comparing the proﬁles (Fig. 1), the MWCNT concentrations in layers
deeper than 2 cm were higher at the higher water content than forthe lower water content. This implies that MWCNT transport wasslightly enhanced at the higher water content. An increase in

Table 2
Experimental conditions, hydraulic parameters, and mass balance information (as fractions of the total applied mass) for all column experiments. The electrolyte was 1 mM
KCl.
Type of core

Soil type

Water saturation[%]

q [cm min1]

Porosity

Disp.a [cm]CNT effb

CNT soilc

Total massbalance

Undisturbed
Undisturbed
Repacked
Undisturbed
Lysimeter

KAL
KAL
KAL
MRZ
KAL

96
85
96
95
n.d.

0.00640.00840.00670.00630.0003

0.37
0.32
0.38
0.49
0.23

0.60
0.69
0.37
2.41
17.88

<0.01
<0.01
<0.01
<0.01
<0.01

0.91
0.98
0.86
1.01
n.d.

0.91
0.98
0.86
1.01
n.d.

n.d. e no data.
aDisp. is the longitudinal dispersivity estimated on basis of the conservative tracer BTC.
b
CNT eff is the total amount of CNT in the liquid phase.
c
CNT soil is the total amount of CNT in the solid phase.

D. Kasel et al. / Environmental Pollution 180 (2013) 152e158

155

Table 3Fitted model parameters for the retention proﬁles of the different experiments. Correlation of observed and ﬁtted data is reﬂected by R2.
Type of core

Soil type

Water
saturation [%]R2bk1 [min1]Standard error k1

k2 [min1]Standard error k2

Undisturbed
Undisturbed
Repacked
Undisturbed

KAL
KAL
KAL
MRZ

96
85
96
95

0.89
0.96
0.86
0.97

0.765
0.765
0.765
0.765

10.93
2.56
0.42
0.21

16.87
1.55
0.59
0.25

0.62E-02
0.16E-02
NF
NF

0.89E-02
0.13E-02
NF
NF

NF e not ﬁtted.

retention was expected with decreasing water-saturation becauseof a larger airewater interfacial area and a greater amount of ﬂow
through small pore spaces (Gargiulo et al., 2008). In contrast to the
expected trend, the ﬁtted retention coefﬁcient (k1) was higher at
the higher water content (Table 3). One potential explanation is due
to insensitivity in k1 when complete retention occurs. This hypothesis is supported by the high standard error for k1 for theexperiment at 85% water content (Table 3).
Nevertheless, for both experiments, no signiﬁcant transport todistances greater than 10 cm occurred and the highest fraction of
MWCNT was retained in the top layer of the soil. In consistency
with our results, Jaisi and Elimelech (2009) reported limited
transport of CNT in a particular fraction of soil (420e1000 mm) inwater-saturated column experiments. Based on their ﬁndings andour results, we assume that there is no transport of functionalized
MWCNT through the studied loamy sand soil at water contents
lower than 85%.
It is known that CNT suspensions can be stabilized by naturalorganic matter (Hyung et al., 2006) and that humic acid canenhance CNT transport (Wang et al., 2008). Nevertheless we did notﬁnd any breakthrough of MWCNT in the loamy sand soil containing
appr. 1% organic carbon. Similarly, Wang et al. (2010) observed
complete retention of Fullerene (C60) nanoparticles in saturated
columns packed with two different soils containing 0.75 and 3.3%
organic carbon, respectively. One possible explanation for the highretention of MWCNT in soils compared to quartz sand might beadsorption via negatively charged carboxylic groups of the MWCNT
onto positively charged metal oxides. However, the iron content of
the soil from KAL was relatively low (Table 1). Thus, we assume thatphysical factors (e.g., straining) play a dominant role for the
MWCNT retention in soil (Jaisi and Elimelech, 2009; Kasel et al.,2013). Since MWCNT are non-spherical particles, they are easily

trapped in small pore spaces and may create additional retention
locations (Kasel et al., 2013; Pan and Xing, 2012). Another reason forenhanced MWCNT retention may be the low, but environmentally
relevant, ﬂow rate (0.006 and 0.008 cm min1) in our experiments.
It is already known, that the ﬂow rate strongly inﬂuences MWCNT
transport and that retention increases with decreasing ﬂow rate
(Liu et al., 2009). In addition, the chemical heterogeneity of soils
may also enhance MWCNT retention compared to quartz sand (Panand Xing, 2012). Aggregation also inﬂuences CNT mobility inporous media (Pan and Xing, 2012) but could not be measured inthis study.
In summary, results show that even at a water content close tosaturation (96%) no detectable breakthrough of functionalized
MWCNT occurred in undisturbed cores of the loamy sand soil from
KAL. The observed high retention of MWCNT in soil at experimental
conditions might be due to an interplay of low ﬂow rate (Liu et al.,2009), soil composition (Fang et al., 2009), heterogeneous pore size
distribution (Pan and Xing, 2012), and the presence of an air phase(Gargiulo et al., 2008). Based on the results of our environmentally
relevant setup (undisturbed soil core, precipitation from the top,point application of MWCNT, leaching from top to bottom, and low
particle concentrations) we expect no signiﬁcant transport of
MWCNT in the loamy sand soil through the vadose zone.
3.2. Comparison of undisturbed and repacked soil
An additional experiment was performed in a repacked column
with soil from KAL at a water content of appr. 96% and comparedwith the experiment performed in the undisturbed column at thesame conditions (Fig. 2). The dispersivity of the undisturbed soilcolumn was almost twice as high as that for the repacked column(Table 2), indicating more heterogeneity in the undisturbed soil

0
0

-2

Depth [cm]Depth [cm]

-2

-4

-6

-4

-6

KAL und. 96%

KAL und. 96%-8

KAL dist. 96%-8

KAL und. 85%

-10
-10

0
0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

S/C0 [mL g-1]
Fig. 1. Observed and simulated retention proﬁles for MWCNT in undisturbed (und.)loamy sand soil cores from the test site in Kaldenkirchen-Hülst (KAL) at two watersaturation levels (85 and 96%). For detailed experimental conditions see Table 2. The
dots are the experimental data and the lines represent the model ﬁts.

0.01

0.02

0.03

0.04

S/C0

[mL g-1]

0.05

0.06

0.07

Fig. 2. Observed and simulated retention proﬁles for MWCNT in a disturbed (dist.) soilcolumn from the test site in Kaldenkirchen-Hülst (KAL) compared with the previously
shown RP for the undisturbed soil column from the same test site. The watersaturation was around 96%. For detailed experimental conditions see Table 2. The
dots are the experimental data and the lines represent the model ﬁts.

D. Kasel et al. / Environmental Pollution 180 (2013) 152e158

3.3. Comparison of two soil types
In addition to the previously described experiments with aloamy sand, an experiment with a silty loam (MRZ) at conditions
close to water-saturation (96%) was performed. In general, soil from
MRZ was expected to exhibit stronger sorption than the loamy sandsoil from KAL due to a higher clay content which causes a larger
speciﬁc surface area (Unold et al., 2009b). For CNT, straining is
expected to be the dominant retention mechanism in soil (Jaisi andElimelech, 2009). Soil texture (e.g., clay content) also affects thepore size distribution in the soil and thus, straining. For TiO2
nanoparticles, Fang et al. (2009) described a signiﬁcantly increased
retention with increasing clay contents.
The comparison of KAL and MRZ soils at similar experimental
conditions did not reveal any detectable differences in transport
behavior of MWCNT in both soils. No MWCNT breakthrough was
detected and more than 90% was recovered in the soil proﬁle of
MRZ soil. The retention proﬁles in the two soils (Fig. 3) were very
similar despite signiﬁcant differences in their dispersivity andporosity (Table 2). In fact, the MWCNT concentration in layers
0Depth [cm]

-2

110

a)

100
90

Breakthrough [%](Unold et al., 2009b). Additionally, the conservative tracer BTCs
(data not shown) were described well for both repacked and undisturbed soil columns using the classical ADE model. This ﬁnding
indicates that physical non-equilibrium processes (e.g., preferential
ﬂow) did not play a dominant role in our experiments (Unold et al.,2009b). In general, preferential ﬂow will be more prominent infully water-saturated media because macropores drain fast when
the water level falls below full saturation, unless high rainfall intensities occur (Beven and Germann, 1982).
The transport results were similar for both undisturbed andrepacked column experiments. The only difference was that the
MWCNT concentration in layers deeper than 2 cm was higher in theundisturbed soil compared to the disturbed soil (Fig. 2). Thisobservation suggests a higher transport potential in the undisturbed soil. Nevertheless, no breakthrough was found in both columns. In the repacked column, more than 86% of MWCNT wererecovered in the RP which also exhibited a hyper-exponential
shape (Fig. 2). This result indicates that the structure of the undisturbed loamy sand soil was very homogeneous without fractures or macropores, similar to the structure of the repacked soilcolumn.

80
70
60
50
40
30

Tracer

20

MWCNT

10
0
0

20

40

60

80

100

120

140

160

180

200

220

Time [days]
0

b)

-10
-20
-30

Depth [cm]

156

-40
-50
-60
-70
-80
-90
-100
0

0.02

0.04

0.06

0.08

0.1

S/C0 [mL g-1]
Fig. 4. Cumulative breakthrough of tracer (Br) and radiolabeled MWCNT (a) as well as
an averaged retention proﬁle (b) 220 days after application of MWCNT to a lysimeter.
The lysimeter was ﬁlled with an undisturbed loamy sand soil from the test site in
Kaldenkirchen-Hülst.

deeper than 2 cm was slightly higher for the KAL soil that had lessclay. Thus in the absence of preferential ﬂow, we do not expect any
transport of MWCNT through the vadose zone in the silty loam MRZ
soil because of its higher clay content and lower water contents.
In summary, results indicated that the transport behavior of
MWCNT was not signiﬁcantly different for these two soils. We
hypothesize that physical ﬁltration mechanisms are the dominant
processes controlling MWCNT retention in these natural soils.
These processes are expected to be inﬂuenced by water content(Gargiulo et al., 2007a), ﬂow rate (Liu et al., 2009), and particle
concentration (Kasel et al., 2013).

-4

3.4. Lysimeter-6

KAL und. 96%
MRZ und. 95%

-8

-10
0

0.01

0.02

0.03

0.04

S/C0

[mL g-1]

0.05

0.06

0.07

Fig. 3. Observed and simulated retention proﬁles for MWCNT in an undisturbed silty
loam soil from the test site in Merzenhausen (MRZ) compared with the previously
shown RP for the undisturbed loamy sand soil from the test site in KaldenkirchenHülst (KAL). The water-saturation was around 96%. For detailed experimental conditions see Table 2. The dots are the experimental data and the lines represent the modelﬁts.

The results of the lysimeter experiment revealed that 100% of
the bromide tracer was recovered in the efﬂuent after 107 days. In
dramatic contrast, no radioactivity was detected in the lysimeter
efﬂuent after 220 days (Fig. 4a) even at a relatively high irrigation
rate (1200 mm y1). Therefore, the lysimeter experiment was
stopped and the MWCNT distribution in the soil proﬁle wasdetermined. The corresponding RP (Fig. 4b) exhibited the same
hyper-exponential shape as repacked and undisturbed columns.
The main fraction of MWCNT was retained in the upper 20 cm of
the soil column. Consistent with the ﬁndings of Jaisi and Elimelech(2009), our column experiments indicate that there was no signiﬁcant CNT transport in that undisturbed loamy sand soil underunsaturated conditions.

D. Kasel et al. / Environmental Pollution 180 (2013) 152e158

The lysimeter experiment proved that complete retention of
MWCNT occurred in undisturbed soil at an environmentally relevant irrigation rate for a time period of 8 month. Nevertheless, theinﬂuence of certain weather conditions and environmental factors(e.g., preferential ﬂow paths, uptake by organisms, oversaturation
due to heavy rainfall, drainage and imbibition) needs to be further
considered when evaluating potential risks of MWCNT to humans
and the environment.
4. Conclusions
This study revealed almost complete retention of functionalized
MWCNT in undisturbed cores of two well characterized soils. Experiments showed no detectable breakthrough of MWCNT. Morethan 86% of MWCNT were recovered in the soil proﬁle at conditions
close to saturation. At lower water-saturation, MWCNT retention in
the upper soil layers was enhanced. A long-term study conﬁrmedthat the MWCNT were retained in the top soil layer. Soils are
therefore expected to act as an effective sink for MWCNT. Reasonsfor the high retention of MWCNT in soil might be physical andchemical heterogeneity, particle aggregation, the heterogeneous
soil pore size distribution, and the presence of an air phase. It is
known that organic matter is able to stabilize CNT and enhancetheir transport. This could not be proven in our experiments,
possibly due to the low organic carbon content (appr. 1%) in thesoils. Collectively, our results suggest little potential for MWCNT
transport through the vadose zone and for subsequent groundwater contamination in the considered soils. Nevertheless, MWCNT
transport to deeper soil layers and groundwater under conditionsof heavy rainfall and/or preferential ﬂow cannot be fully excluded.
In addition, transient drainage and imbibition events mightremobilize retained particles.
Acknowledgments
This research was performed within the framework of the‘NanoFlow’-project supported by the German Federal Ministry of
Education and Research. The authors thank Ansgar Weuthen andStephan Sittig for instructions and technical assistance with theexperimental setup. The support of Markus Duschl and Yan Liangespecially in collecting the soil samples is acknowledged. We thankLutz Weihermüller for his helpful instructions with TDR and fruitful
discussions. The practical assistance of Anne Berns, Diana Hofmann,
Stephan Köppchen, Martina Krause, Ulrike Langen, Bastian Niedrée,
Herbert Philipp, and Herbert Rützel, especially in conducting thelysimeter experiment, is highly appreciated.
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