Water Research 109 (2017) 358e366

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Water Research
journal homepage: www.elsevier.com/locate/watres

Roles of cation valance and exchange on the retention andcolloid-facilitated transport of functionalized multi-walled carbon
nanotubes in a natural soil 
Miaoyue Zhang a, d, Scott A. Bradford b, *, Jirka Sim
unek c, Harry Vereecken a,aErwin Klumppa

Agrosphere Institute (IBG-3), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
United States Department of Agriculture, Agricultural Research Service, U. S. Salinity Laboratory, Riverside, CA 92507, USA
Department of Environmental Sciences, University of California Riverside, Riverside, CA 92521, USAdInstitute for Environmental Research (Biology V), RWTH Aachen University, Worringerweg 1, 52074 Aachen, Germanybca r t i c l e i n f o

a b s t r a c t

Article history:
Received 18 August 2016
Received in revised form
28 November 2016
Accepted 28 November 2016
Available online 29 November 2016

Saturated soil column experiments were conducted to investigate the transport, retention, and release
behavior of a low concentration (1 mg L1) of functionalized 14C-labeled multi-walled carbon nanotubes(MWCNTs) in a natural soil under various solution chemistries. Breakthrough curves (BTCs) for MWCNTS
exhibited greater amounts of retardation and retention with increasing solution ionic strength (IS) or in
the presence of Ca2þ in comparison to Kþ, and retention proﬁles (RPs) for MWCNTs were hyperexponential in shape. These BTCs and RPs were well described using the advection-dispersion equation with a term for time- and depth-dependent retention. Fitted values of the retention rate coefﬁcientand the maximum retained concentration of MWCNTs were higher with increasing IS and in the presence of Ca2þ in comparison to Kþ. Signiﬁcant amounts of MWCNT and soil colloid release was observedwith a reduction of IS due to expansion of the electrical double layer, especially following cation exchange (when Kþ displaced Ca2þ) that reduced the zeta potential of MWCNTs and the soil. Analysis of
MWCNT concentrations in different soil size fractions revealed that >23.6% of the retained MWCNT mass
was associated with water-dispersible colloids (WDCs), even though this fraction was only a minor
portion of the total soil mass (2.38%). More MWCNTs were retained on the WDC fraction in the presenceof Ca2þ than Kþ. These ﬁndings indicated that some of the released MWCNTs by IS reduction and cationexchange were associated with the released clay fraction, and suggests the potential for facilitatedtransport of MWCNT by WDCs.
© 2016 Published by Elsevier Ltd.

Keywords:
Multi-walled carbon nanotubes
Soil
Breakthrough curves
Retention proﬁlesCation exchange
Soil fractionation

1. Introduction
Carbon nanotubes (CNTs) are allotropes of carbon with acylindrical-shaped nanostructure (Iijima, 1991). They have beenutilized in numerous commercial applications (Gohardani et al.,2014) such as electrical cables and wires (Janas et al., 2014), solar
cells (Guldi et al., 2005), hydrogen storage (Jones and Bekkedahl,1997), radar absorption (Lin et al., 2008) and considering as absorbents for environmental remediation and water treatment* Corresponding author. United States Department of Agriculture, Agricultural
Research Service, U. S. Salinity Laboratory, 450 W. Big Springs Road, Riverside, CA
92507, USA.
E-mail address: scott.bradford@ars.usda.gov (S.A. Bradford).
http://dx.doi.org/10.1016/j.watres.2016.11.062
0043-1354/© 2016 Published by Elsevier Ltd.

(Camilli et al., 2014; Li et al., 2012; Mauter and Elimelech, 2008; Pan
and Xing, 2012; Zhang et al., 2010) due to their unique electric,chemical, and physical properties. The widespread commercial andpotential environmental applications will undoubtedly result intheir release into the subsurface. Current studies have investigated
the transport behavior of CNTs in different porous media, which isaffected by various physical and chemical conditions including
ionic strength (IS), water content, grain size, input concentrationand dissolved organic matter (Kasel et al., 2013a, 2013b; Tian et al.,2012; Yang et al., 2013). However, most of these studies wereconducted in model soil systems such as rigorously cleaned sand or
glass beads, which do not reﬂect the full complexity and heterogeneity of natural soils (e.g. complex grain size distribution andpore structure, and surface roughness and chemical heterogeneity)

M. Zhang et al. / Water Research 109 (2017) 358e366(Cornelis et al., 2014). Kasel et al. (2013b) found limited transport offunctionalized multi-walled carbon nanotubes (MWCNTs) in an
undisturbed, natural soil. However, information on CNTs interactions with soil colloids and the potential for colloid-facilitatedtransport of CNT is still limited. Furthermore, the detachment or
release of retained engineered nanoparticles (ENPs) like CNTs fromthe solid phase is important for predicting the ultimate fate andtransport of ENPs in the subsurface, but has received little researchattention.
Previous studies have demonstrated that pore-water chemistry
plays an important role in controlling the retention of ENPs in soils(Badawy et al., 2010; Gao et al., 2006; Tian et al., 2012; Wang et al.
2014, 2015). Retention of ENPs is enhanced at higher ionic strength(IS) because the adhesive force increases with compression of theelectric double layer (EDL) and a decrease in the magnitude of thesurface potential (Elimelech et al., 2013; Israelachvili, 2011; Khilar
and Fogler, 1998). Retention of ENPs is also higher in the presenceof similar concentrations of divalent than monovalent cations(Torkzaban et al., 2012; Yang et al., 2013) because the double layer
thickness and magnitude of the surface potential decreases and the
IS increases to a greater extent (Elimelech et al., 2013; Israelachvili,
2011; Khilar and Fogler, 1998). In addition, adsorbed divalent cations, such as Ca2þ, can enhance retention of ENPs by creating
nanoscale chemical heterogeneity on the solid surface that canneutralize or reverse the surface charge at speciﬁc locations(Grosberg et al., 2002) and/or produce a cation bridge between
negatively charged sites on the surface of clays and ENPs
(Torkzaban et al., 2012). The relative importance of solution IS andcation type on retention of ENPs is therefore expected to depend on
the presence of soil colloids, but these complexities have not yet
been fully resolved for natural soils.
A reduction in solution IS and cation exchange (monovalentdisplacing divalent cations) decreases the adhesive force andthereby induces release of clay particles and ENPs (Bradford andKim, 2010; Grolimund and Borkovec, 2006; Liang et al., 2013;
Torkzaban et al., 2013). The migration of soil colloids can facilitate
the transport of pollutants and/or nanoparticles (Bradford and
Torkzaban, 2008; Grolimund and Borkovec, 2005; Liang et al.,2013; Yan et al., 2016; Zhu et al., 2014). For example, Liang et al.
(2013) provided experimental evidence that release of soil colloids with IS reduction and cation exchange can facilitate thetransport of silver nanoparticles. Water-dispersible colloids (WDCs)
are indicators for mobile soil colloids (de Jonge et al., 2004); e.g.,
WDCs are particles from the soil clay fraction that are less than
2 mm in size that are easily dispersible from soil with aqueous solution (Jiang et al., 2012, 2014). Knowledge of the interaction andassociation between ENPs and WDCs is therefore important for
better understanding the fate of ENPs in soil. The effects of perturbations in solution chemistry on the release and colloidfacilitated transport of MWCNTs in soil have not yet been studied.
The objective of this study is to better understand roles of soil
colloids, solution IS, and cation type on the transport, retention andrelease behavior of functionalized MWCNTs (1 mg L1) in a naturalsoil. Breakthrough curves and retention proﬁles for MWCNTs were
determined in column experiments, and a numerical model was
employed to simulate their fate. The retained concentration of
MWCNTs was subsequently determined for the different soil size
fractions (e.g., sand, silt, and WDCs). Other experiments wereconducted to study the release of soil colloids and MWCNTs with
perturbations in solution chemistry (e.g., IS reductions and cationexchange). Results provide valuable insight on the roles of cation
type, chemical perturbations, and soil colloids on the retention,
release and colloid-facilitated transport of MWCNTs. This knowledge can be useful for environmental applications and risk management of MWCNTs.

359

2. Materials and methods
2.1. Soil and MWCNT
Soil samples were collected from the upper 30 cm of an agricultural ﬁeld site in Germany (Kaldenkirchen-Hülst, Germany),sieved to a fraction < 2 mm, and air dried. The soil was classiﬁed asa loamy sand with 4.9% clay (<2 mm), 26.7% silt (2e63 mm), and68.5% sand (63e2000 mm). It had a median grain size (d50) of
120 mm, a total organic carbon content of 1.1%, a cation exchange
capacity of 7.8 cmolc kg1, a pH value of 5.9 (0.01 M CaCl2), a speciﬁc
surface area of 1.7 m2 g1, and 0.8% iron. The clay faction wascomposed of illite, montmorillonite, and kaolinite minerals (Kasel
et al., 2013b; Liang et al., 2013).
Radioactively (14C) labeled and unlabeled functionalized
MWCNTs (Bayer Technology Services GmbH, Leverkusen, Germany)
were boiled with 70% nitric acid (Sigma-Aldrich Chemie GmbH,
Steinheim, Germany) for 4 h to create additional oxygen-containing
functional groups (e.g., carboxylic groups) on their surfaces (Kasel
et al., 2013a). The functionalization and characterization of
MWCNTs were previously described by Kasel et al. (2013a, 2013b).
In brief, a transmission electron microscope was used to characterize morphological properties of MWCNTs, X-ray photoelectron
spectroscopy was used to identify oxygen containing functionalgroups on MWCNTs, and inductively coupled plasma-mass spectrometry was used to determine the amount of metal catalysts
before and after acid treatment of MWCNTs. The MWCNTs have amedian diameter of 10e15 nm and a median length of
200e1000 nm (Pauluhn, 2010).
Suspensions of MWCNT at selected IS were prepared in deionized water with KCl and CaCl2. These suspensions were dispersed
by ultrasonicating the stock suspension for 15 min at 65 W, andthen repeating this process 10 min before injection into soil columns. This low energy of sonication does not damage the MWCNTs(Li et al., 2012).
The hydrodynamic radius measured by dynamic light scattering(DLS) does not reﬂect the real geometric particle diameter for non€v et al., 2008; Pecora,
spherical particles like MWCNTs (Hassello2000). Nevertheless, it can be used to study the aggregation
behavior of functionalized MWCNTs suspensions. The hydrodynamic radius of unlabeled functionalized MWCNTs suspended in
KCl and CaCl2 at IS ¼ 10 mM were measured 0, 1, and 4 h aftersuspension preparation by DLS using a Zetasizer Nano (Malvern
ZetaSizer 4). The surface charge characteristics and electrophoretic
mobility values of unlabeled functionalized MWCNT (pH z 5.4)
and crushed soil (pH z 6) in selected electrolyte solutions werealso determined using the Zetasizer Nano apparatus.
2.2. Transport and retention experiments
Saturated MWCNT transport experiments were conducted at
different IS (1, 4, and 10 mM KCl) and different cation type at thesame IS (1 mM KCl and CaCl2, molar concentration 0.33 mM L1
CaCl2) by using stainless steel columns (3 cm inner diameter and12 cm length) that were wet-packed with soil. Each side of thecolumn was ﬁtted with a stainless steel plate (1 mm opening) anddouble PTFE mesh (100 mm openings) to support the soil and touniformly distribute the ﬂow. The columns were wet packed byincrementally ﬁlling the column with soil and Milli-Q water, andthen gently tapping the columns with a rubber mallet to ensurecomplete water saturation. Steady-state ﬂow was achieved using aperistaltic pump, with the ﬂow direction from the column bottom
to the top. The saturated hydraulic conductivity and bulk density ofthe packed soil column was approximately 0.73 cm min1 and1.5 g cm3, respectively.

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M. Zhang et al. / Water Research 109 (2017) 358e366

overnight digesting, diluting, and using inductively coupled opticalemission spectrometry (ICP- OES). Because radioactive samples(14C) are not allowed for ICP-OES measurement, the release experiments were repeated using unlabeled functionalized MWCNTs
for this analysis. The efﬂuent from the release experiment was also
analyzed by DLS for soil colloids. A conservative tracer (KBr)
experiment was repeated (Section 2.2) after a release experiment toassess potential changes in the hydrodynamic properties of thecolumn.

The soil in the column was equilibrated before initiating thetransport experiment by injecting approximately 30 pore volumes(PVs) of a selected background electrolyte solution at a slow, constant Darcy velocity of 0.18 cm min1. Conservative tracer (KBr) and
MWCNT transport experiments were then conducted sequentiallyat the same IS and cation type as the equilibrium phase, but at ahigher, constant Darcy velocity of 0.71e0.73 cm min1. The conservative tracer experiment consisted of injecting a 2.1 PVs pulse(90 mL) of KBr solution, followed by continued eluting with thesame bromide-free electrolyte solution for another 5.5 PVs. The
efﬂuent concentrations of bromide were determined using a highperformance liquid chromatograph (STH 585, Dionex, Sunnyvale,
CA, USA) equipped with a UV detector (UV2075, Jasco, Essex, UK).
Transport experiments for MWCNTs (input concentration,
1 mg L1) were conducted in a similar manner as the conservative
tracer; e.g., injection of a 2.1 PVs pulse of MWCNT suspension,
followed by elution with the same particle-free electrolyte solution
for another 5.5 PVs. The 14C-labeled efﬂuent concentrations of
MWCNT were determined using a liquid scintillation counter (LSC)(PerkinElmer, Rodgau, USA). After recovery of the breakthroughcurve (BTC), the MWCNT retention proﬁle (RP) was determined.
Soil samples were excavated from a column in 0.5e1 cm increments, dried, crushed, combusted using a biological oxidizer at
900  C (OX 500, R.J. Harvey Instrumentation Corporation, Tappan,
NY, USA), and retained concentrations of MWCNTs in the soil were
determined by LSC measurement. A summary of the experimental
conditions is provided in Table 1.

2.4. Soil fractionation
Soil fractionation experiments were conducted to further
elucidate the interactions between 14C-labeled functionalized
MWCNTs and the soil. The initial deposition phase was conductedsimilar to the MWCNT transport experiments discussed above(Section 2.2) at the same IS (1 mM KCl or CaCl2). After recovery of
the MWCNT breakthrough curve, all the soil was excavated fromthe column, dried, and then the soil particle-size was fractionatedquaris and Lewandowski (2003). In
following the method of Se
brief, 100 g of dried soil was added to a 1 L Duran bottle (Schott,Mainz, Germany) containing 200 mL of the same electrolyte solution as the transport experiments (1 mM KCl or CaCl2) and horizontally shaken at 150 rpm for 6 h. An additional 600 mL of thesame electrolyte solution (1 mM KCl or CaCl2) solution was thenadded to this bottle and mixed before sedimentation. The pipettemethod was then used to separate the soil into three size classes
based on their sedimentation time according to Stoke's law (aparticle density of 2.65 g cm3 was assumed): (i) >20 mm (sandsize) after 6 min; (ii) 2e20 mm (silt size) after 12 h; and (iii) <2 mm(water-dispersible colloids, WDCs) after 12 h. Triplicate 50 mL
samples of the WDC suspension were further fractionated bycentrifuging at 2525  g for 4 min. Stoke's Law calculations indicated that the resulting supernatant consisted of electrolyte solution and the WDC fraction that was <0.45 mm, whereas the pellet
was the WDC fraction that was 0.45e2 mm. Concentrations of
MWCNTs that were associated with the sand, silt, 0.45e2 mm, and<0.45 mm fractions were determined by LSC in a similar manner tothe transport experiments. Concentrations of MWCNTs in the column efﬂuent and residual soil samples from columns that were notused for soil fractionation were measured to ensure mass balance.
All the experiments were replicated and exhibited similar results.

2.3. Release experiments
Additional experiments were performed to study the release
behavior of MWCNTs with IS reduction and cation exchange at aconstant Darcy velocity of 0.71e0.73 cm min1. The initial deposition phase (step A) was conducted using MWCNTs (1 mg L1) in
10 mM KCl (experiment I), 1 mM CaCl2 (experiment II), and 10 mM
CaCl2 (experiment III) solutions in an analogous fashion to
MWCNTs transport experiments discussed above (Section 2.2).
Release experiments II and III were then initiated by changing theeluting solution chemistry in the following sequence: Milli-Q water(step B); KCl at the same IS as in step A (step C); Milli-Q water (stepD); 100 mM KCl (step E); and Milli-Q water (step F). Each solution
chemistry step was conducted for 7.6 PVs. Experiment I consisted ofonly steps A and B. The 14C-labeled efﬂuent and soil concentrationsof MWCNT were measured in the same manner as in transport
experiments. Cation exchange and soil colloids were quantiﬁed
during the release experiments by measuring the efﬂuent concentrations of K, Ca, Fe, and Al by adding 5 mL of aqua regia for2.5. Numerical modeling
The one-dimensional MWCNT transport was described by the 
HYDRUS-1D code (Simunek et al., 2016) using the advection-

Table 1
Experimental conditions, hydraulic parameters and mass balance information for all column experiments.
IS [mM]
Fig. 2

Fig. 3Fig. 4

þ

1, K
4, Kþ
10, Kþ
1, Kþ
1, Ca2þ

Co [mg L1]q [cm min1]

Disp. [cm]4

Meff [%]

M*eff [%]

Msoil [%]

M*soil [%]Mtotal [%]M*total [%]

1
1
1
1
1

0.72
0.72
0.71
0.72
0.72

0.604
0.364
0.417
0.604
0.711

0.5
0.51
0.5
0.5
0.5

45.8
13.8
4
45.8
4

47.9
11.7
2.9
47.9
3.6

48.8
77.2
90.3
48.8
91.8

44.2
85.9
90.5
44.2
93.2

94.6
91.0
94.3
94.6
95.8

92.1
97.6
93.4
92.1
96.8

ISC [mM]

MC [%]MD H2O [%]ISE [mM]

ME [%]MF H2O [%]Msoil [%]

Mtotal [%]

21.7
11.6

45.1
50.6
64.8

91.9
90.5
88.4

q [cm min1]

4

ISA [mM]

MA [%]

MB H2O [%]I, 0.71
II, 0.72
III, 0.73

0.47
0.49
0.48

10, Kþ
1, Ca2þ
10, Ca2þ

4.6
4.2
0.03

42.2
7.7
0.3

1, Kþ
10, Kþ

0.6
0.02

5.3
11

100, Kþ
100, Kþ

0.4
0.6

Fig. 1: ionic strength effect; Fig. 2: cation type effect; Fig. 3: release of MWCNT by ionic strength reduction and cation exchange; Co, MWCNT input concentration; q, Darcy
velocity; IS, ionic strength; 4,porosity; Disp.is the estimated longitudinal dispersivity; Meff, Msoil, and Mtotal are mass percentages recovered from efﬂuent, soil, and total,
respectively; MA  MF are the mass percentages recovered from efﬂuent (Steps AF) in release experiments.
M*eff, M*soil, M*total are mass percentages of the replicate experiments recovered from efﬂuent, soil, and total, respectively. The data from the replicate experiments were notshown in Figs. 1 and 2.

M. Zhang et al. / Water Research 109 (2017) 358e366

dispersion equation with a kinetic retention site (Bradford et al.,
2003; Kasel et al., 2013a, 2013b):

vC
vS
v2 C
vC
q þ r ¼ qD 2  q
vt
vt
vx
vx

(1)

where q is volumetric water content [L3L3], r is the bulk density ofthe soil [ML3; where L and M denote units of length and mass,
respectively], C is MWCNT concentration in the efﬂuent [NL3;
where N denotes number], t is the time [T; where T denotes units oftime], x is the spatial coordinate [L], D is the hydrodynamic
dispersion coefﬁcient [L2T1], q is the Darcy velocity [LT1], and S[NM1] is the solid phase concentrations of MWCNTs. The solid
phase mass balance is given as:rvS¼ qjksw C  rkrs S
vt(2)

where ksw [T1] is the ﬁrst-order retention rate coefﬁcients, krs[T1] is the ﬁrst-order release rate coefﬁcients, and j [] aredimensionless functions that account for time- and depthdependent retention. The parameter j is equal to:



j¼ 1



d50 þ x b
Smax
d50
S

(3)

where Smax [NM1] is the maximum solid phase concentration, b[] is a ﬁtting parameter which controls the shape of the retention
proﬁle. The value of b ¼ 0.765 was selected based on reported results for MWCNTs (Kasel et al., 2013a, 2013b).
HYDRUS-1D includes a provision to determine model parameters by inverse optimization to experimental data. Values of q andthe dispersivity (lÞ were determined by ﬁtting to the conservative
tracer BTC, whereas retention model parameters (ksw , krs , and Smax )were obtained by optimization to BTCs and RPs for MWCNTs.

3. Results and discussion
3.1. MWCNT suspension stability
The hydrodynamic radius of MWCNTs in KCl and CaCl2 solutionsat an IS ¼ 10 mM was measured 0, 1, and 4 h after suspension
preparation by DLS. The hydrodynamic radius was always within
the measurement error, and did not show a systematic trend withthe cation type or time. The MWCNT suspensions were therefore
considered to be stable during the injection phase (approx. 18 min)of all transport and release experiments discussed below.

3.2. Zeta potential
Fig. 1 presents plots of measured zeta potentials (Fig. 1a) andelectrophoretic mobility (Fig. 1b) values for MWCNTs and soil as afunction of IS and cation valence (Kþ and Ca2þ). Both MWCNTs andsoil exhibited a net negative charge for the considered solution
chemistry conditions. This result indicates that net electrostaticinteractions between the MWCNTs and soil were repulsive. The
zeta potential was generally similar in magnitude for MWCNTs andsoil under the same solution chemistry condition. Furthermore,increasing the solution IS and cation valence tended to decrease themagnitude of both zeta potentials. These trends are consistent withpublished literature (Elimelech et al., 2013; Liang et al., 2013;
McNew and LeBoeuf, 2016).

361

3.3. Transport and retention of MWCNTs
Fig. 2 presents observed and simulated BTCs and RPs for functionalized MWCNTs when the IS ¼ 1, 4, and 10 mM KCl. The BTCs
(Fig. 2a) are plotted as the normalized efﬂuent concentration (C/Co;
where Co is the inﬂuent suspension concentration) versus porevolumes, whereas the RPs are plotted as normalized solid phase
concentration (S/Co) as a function of distance from the column inlet.
The experimental conditions, hydraulic parameters (q and l), andmass balance information for these experiments are presented inTable 1. Simulations provided an excellent description of BTCs and
RPs with a Pearson's correlation coefﬁcient (R2) > 0.95. Table 2
provides a summary of ﬁtted retention model parameters.
The total mass balance (Mtotal) of column experiments in Fig. 2
was >91% (Table 1). The mass percentage recovered from theefﬂuent (Meff) strongly decreased from 45.8% to 4.0% as the IS
increased from 1 to 10 mM (KCl), resulting in a corresponding increase in the solid phase mass percentage (Msoil) from 48.8% to90.3%. Fitted values of ksw and Smax =Co also increased with IS,
whereas krs decreased with IS. These trends are attributable to anincrease in the adhesive force for MWCNTs on soil with IS due tocompression of the double layer thickness and a decrease in themagnitude of the zeta potential (Fig. 1), and is consistent with otherstudies (Jaisi et al., 2008; Tian et al., 2012; Yang et al., 2013). It
should also be mentioned that the inﬂuence of nanoscale heterogeneities on colloid retention increases with an increase in IS(Bradford and Torkzaban, 2013).
BTCs for MWCNTs (Fig. 2a) exhibited increasing breakthroughconcentrations over time due to blocking; e.g., ﬁlling of a limited
number of retention sites with continued MWCNT injection. The
breakthrough time for MWCNTs was also delayed in comparison
with the tracer (data not shown). This occurs because ksw was
sufﬁciently high to produce complete retention until available
retention sites ﬁll enough to induce breakthrough (e.g., Leij et al.,2015). This delay in breakthrough increases with IS because alarger value of Smax =Co takes longer to ﬁll. Other studies that haveinvestigated the effect of IS on CNTs transport in clean sands or
glass beads have not observed this result (Jaisi et al., 2008; Tianet al., 2012; Yang et al., 2013), but it has been previous demonstrated for other nanoparticles (e.g.,Sasidharan et al., 2014). Thisapparent discrepancy is due to the strong dependency of blocking
on ksw , Smax , Co, and the input pulse duration (Leij et al., 2015). In
particular, delay in the breakthrough will not occur if ksw ; Co, andthe input pulse duration is too low, or if Smax is too high.
The RPs for MWCNTs (Fig. 2b) exhibited a hyper-exponential
shape that was well described using the model with a depthdependent retention function (Eq. (3)). This indicates that agreater retention rate occurred near the column inlet then theoutlet. Hyper-exponential RPs have previously been observed for
MWCNTs (Kasel et al., 2013a, Kasel et al. 2013b). Liang et al. (2013)
also observed hyper-exponential RPs for spherical silver nanoparticles (diameter of 15 nm) in this same soil. A variety of potential
explanations for hyper-exponential RPs have been identiﬁed in theliterature, including: straining (Bradford et al., 2002, 2003;
Bradford and Bettahar, 2006), colloid aggregation (Chen andElimelech, 2006, 2007), chemical heterogeneity on the soil andcolloid (Tong and Johnson, 2007; Tufenkji and Elimelech, 2005),
and system hydrodynamics (Bradford et al., 2009; Li et al., 2005). It
is difﬁcult to ascertain the potential contribution of each factor tothe observed RP. However, MWCNTs retention still resulted in a
hyper-exponential shape even under low IS conditions (1 mM KCl)
that should minimize the contribution of colloid aggregation andchemical heterogeneity. Results for theoretical calculations thatconsider forces and torques that act on colloids near heterogeneous
surfaces, and comparison of retention in batch and column studies

362

M. Zhang et al. / Water Research 109 (2017) 358e366

Fig. 1. Zeta potentials (a) and electrophoretic mobility (b) values of MWCNTs (pH z 5.4) and soil (pH z 6) as a function of the solution (KCl and CaCl2) ionic strength.

Fig. 2. Effect of ionic strength on the transport and retention of MWCNTs in soil: observed and ﬁtted breakthrough curves (a) and retention proﬁles (b) of MWCNTs under 1, 4, and10 mM KCl, respectively.

Table 2Fitted model parameters.

Fig. 2

Fig. 3IS [mM]b

Smax/Co [cm3g1]

S.E. Smax/Co

ksw [min1]

S.E. ksw

krs [min1]

S.E. krs

R2

1, Kþ
4, Kþ
10, Kþ
1, Kþ
1, Ca2þ

0.765
0.765
0.765
0.765
0.765

5.45
5.98
6.10
5.45
9.33

1.51
NF
4.65E-01
1.51
NF

10.30
28.22
49.69
10.30
44.54

5.89E-01
6.88E-01
1.17Eþ00
5.89E-01
1.39Eþ00

6.80E-03
5.70E-03
4.56E-03
6.80E-03
3.31E-03

5.00E-04
4.59E-04
1.30E-03
5.00E-04
1.34E-03

0.975
0.958
0.991
0.975
0.971

Fig. 1 ionic strength effect; Fig. 2: cation type effect; R2 reﬂects the correlation of observed and ﬁtted data; NF - denotes not ﬁtted. ksw, the ﬁrst-order retention rate coefﬁcient;krs, the ﬁrst-order release rate coefﬁcient; Smax/Co, normalized maximum solid phase concentration of deposited MWCNTs; S.E. e standard error.

indicate that retention of MWCNTs is controlled by surface straining near macroscopic roughness locations and grain-grain contacts
under low IS conditions (Bradford and Torkzaban, 2015; Zhang
et al., 2016). Similarly, other studies have concluded that straining
played a dominant role in the retention of MWCNTs (Jaisi et al.,2008; Kasel et al., 2013a, 2013b; Wang et al., 2012).
The effect of cation type on the transport and retention of
MWCNTs in soil is presented in Fig. 3. In this case, MWCNTs were
deposited and eluted at the same IS ¼ 1 mM in the presence ofmonovalent Kþ or divalent Ca2þ cations. Fig. 3 presents observed
and simulated BTCs and RPs. Table 2 summarizes the ﬁtted retention model parameters that provided an excellent description ofthis data (R2 > 0.97). Retention of MWCNTs (Table 1) and ﬁtted
values of ksw and Smax =Co (Table 2) were much higher in thepresence of Ca2þ than Kþ at the same IS. In comparison to Kþ, Ca2þproduces a stronger adhesive force due to the smaller magnitude of
the zeta potential (Fig. 1), localized neutralization and/or reversal of
surface charge (Grosberg et al., 2002), and cation bridging(Torkzaban et al., 2012) that all enhances retention of MWCNTs.
Fitted values of Smax =Co clearly indicate that many more retention
sites were available in the presence of Ca2þ than Kþ (an increase of
171%). This large increase in the value of Smax =Co in the presenceCa2þ also inﬂuenced the blocking behavior in the MWCNT BTC. In
particular, a greater delay in the breakthrough time and a slower
rate of increase in the BTC with time was observed (Fig. 3b). Similar
to RPs in the presence of Kþ (Fig. 2b), the RP in the presence of Ca2þalso exhibited a hyper-exponential shape. This observation suggests that similar retention mechanisms were operative in thepresence of both monovalent and divalent cations.
3.4. Release of MWCNTs
Very little release of MWCNTs was observed under steady-statesolution chemistry conditions (Figs. 2 and 3, and Table 2).
Conversely, signiﬁcant amounts of colloid and nanoparticle release
have been reported when transient solution chemistry reduce theM. Zhang et al. / Water Research 109 (2017) 358e366

363

Fig. 3. Effect of cation type on the transport and retention of MWCNTs in soil: observed and ﬁtted breakthrough curves (a) and retention proﬁles (b) of MWCNTs under 1 mM KCl
and CaCl2, respectively.

adhesive force (e.g., Liang et al., 2013). Additional experiments were
therefore conducted to better understand the effect of IS reduction
and cation exchange on the release of MWCNTs in soil. Step A of
Fig. 4a presents BTCs when MWCNTs were retained and eluted in
the presence of IS ¼ 10 mM KCl (experiment I), and CaCl2 solution at
IS ¼ 1 mM (experiment III) and IS ¼ 10 mM (experiment III).
Transport of MWCNTs was limited in Step A for all of these conditions. Indeed, efﬂuent mass balance during step A (MA) only
equaled 4.6, 4.2, and 0.03% for experiments I, II, and III, respectively.
Step B in Fig. 4a presents the subsequent release curves whenretained MWCNTS were eluted by Milli-Q water. Release was much
more pronounced when the MWCNTs were retained (step A) in thepresence of monovalent Kþ (42.2%) than divalent Ca2þ (<7.7%)
cation. This result indicates that the adhesive interaction wasstronger and less reversible in the presence of Ca2þ than Kþ.
Furthermore, release of MWCNTs was less pronounced when they
were retained in IS ¼ 10 mM CaCl2 (0.3%) than IS ¼ 1 mM CaCl2(7.7%) solution, respectively. This implies that a stronger adhesive
force continued to act on the MWCNTs when they were initiallyretained in the presence of a higher concentration of Ca2þ. A potential explanation for these observations is due to localized charge
neutralization/reversal or cation bridging by Ca2þ which createsstrong interactions with MWCNTs (Jaisi et al., 2008; Liang et al.,2013).
Experiments II and III were continued to further study therelease of MWCNTs that were retained in the presence of Ca2þusing the following elution sequence: KCl at the same IS as in step A(step C); Milli-Q water (step D); 100 mM KCl (step E); and Milli-Q
water (step F). The efﬂuent concentrations of MWCNTs during
each of these steps are shown in Fig. 4a, with corresponding massbalance information given in Table 1 (denoted as MC-MF). Efﬂuent
concentrations of Kþ and Ca2þ, and Al and Fe in a replicate release
experiment for MWCNT in IS ¼ 1 mM CaCl2 solution during step A
are shown in Fig. 4b and c, respectively. The RPs for MWCNTs
following completion of transient release experiments I, II, and III
are shown in Fig. 4d.
MWCNT release always occurred when Milli-Q water was
injected into the column during steps B, D, and F, but graduallyceased when KCl was injected during steps C and E (Fig. 4a). The
amount of MWCNTs that was released with Milli-Q water strongly
depended on the concentrations of injected Ca2þ during step A andKþ during steps C and E. Release of MWCNTs that occurred with IS
reduction during steps D and F were also inﬂuenced by the amountof cation exchange that occurred during steps C and E. Speciﬁcally,
greater amounts of Ca2þ ions were exchanged back into theaqueous phase when a higher concentration of Kþ was injected(Fig. 4b). This exchange process reduced the strength of theadhesive interaction, such that greater amounts of MWCNTs werereleased with a subsequent reduction in solution IS. Consequently,
MD was less when the IS equaled 1 mM (MD ¼ 5.3%) than 10 mM(MD ¼ 11%) KCl during step C. The value of MF was inﬂuenced by: (i)the initial concentration of Ca2þ during deposition (step A); (ii) theamount of cation exchange (steps C and E); and (iii) the previous
amounts of MWCNT that were released (steps B and D). In this case,the value of MF equaled 21.1 and 11.6% when the IS equaled 1 and10 mM CaCl2, respectively, during step A. A similar trend wasobserved for the total recovered mass of MWCNTs during steps A-F,
and this indicates that increased strength of the adhesive force at ahigher Ca2þ concentration (step A) was the dominantconsideration.
Fig. 4d presents the RPs for MWCNTs following completion of
the release experiments. Similar to Figs. 2 and 3, RPs were still
hyper-exponential in shape. However, close inspection of Figs. 2b,
3b and 4d reveals that retained MWCNTs in release experiments
were shifted from the top into deeper layers. This observation
supports the potential for continued slow remobilization of
MWCNTs in the subsurface due to the effects of IS reduction andcation exchange.
Bradford and Kim (2010) examined the release behavior of insitu kaolinite clay from sand due to cation exchange and IS reduction, and observed similar trends as seen for the MWCNTs in Fig. 4.
This observation suggests the potential for colloid-facilitatedtransport of MWCNTs in this study. Indeed, efﬂuent samples
exhibited differences in sample turbidity due to release of soil
colloids, and DLS measurements indicated the presence of soil
colloids with hydrodynamic diameter ranged from 150 to 308 nm.
To better understand the potential association of released MWCNTs
and soil colloids, Fig. 4c presents plots of efﬂuent concentrations of
Al, Fe, and MWCNTs during the release experiment. Note that
release behavior of Fe, Al, and MWCNTs closely follow each otherwith IS reduction and cation exchange. Liang et al. (2013) also
observed this same phenomenon for silver nanoparticles with Al
and Fe. All of these observations strongly support the potential for
colloid-facilitated transport of MWCNTs during the release experiments. However, it should be mentioned that BTCs for the conservative tracer before and after release experiments were nearly
identical (data not shown). This observation suggests that changes
in the soil hydraulic properties due to soil colloid release wereinconsequential during the release experiment.

3.5. Soil size fractionation
Soil fractionation was conducted following completion of
MWCNT transport experiments at IS ¼ 1 mM in KCl and CaCl2

364

M. Zhang et al. / Water Research 109 (2017) 358e366

solutions to further investigate the association between MWCNTs
and soil colloids in the presence of different cation types. Fig. 5ashows a plot of the soil mass percentage of the different size fractions. Results indicate that the soil was composed of 2.38% WDCs
(1.51% in the range of 0.45e2 mm and 0.87% less than 0.45 mm),
11.5% silt (2e20 mm), and 86.2% sand (20e2000 mm). This size
fractionation is comparable to that obtained by (Kasel et al., 2013b),with small variations likely due to the use of different soil fractionation methods.
Fig. 5b presents a plot of the recovered mass percentages of
MWCNTs in the various soil size fractions when the IS ¼ 1 mM KCl
and CaCl2. The total mass recovery was calculated to be more than
85%. Mass percentages of MWCNTs in the sand and silt fractions
were only 9.5 and 17.2%, respectively, even though they accounted
for 97.7% of the soil mass. Mass percentage of MWCNTs in the
0.45e2 mm WDC fraction that accounted for 1.51% of the soil mass
was 23.6%. The mass percentage of MWCNTs in the <0.45 mm WDC
fraction and the so-called electrolyte phase (Jiang et al., 2014) was
49.7%. This fraction included MWCNTs associated with very ﬁne soil
colloids and those released during the fractionation procedure.
Fig. 5c presents the retained concentration (S/Co) of MWCNTs indifferent soil fractions in the presence of KCl or CaCl2 when the
IS ¼ 1 mM. The retained concentration of MWCNTs in each soil
fraction was more pronounced in the presence of Ca2þ than Kþ,
especially in the WDC fraction. This observation can be explainedby charge reversal/neutralization (Grosberg et al., 2002) and/or
bridging complexation between soil grains and functionalized
MWCNTs in the presence of Ca2þ (Torkzaban et al., 2012). Similar toexperimental observations, bridging complexation is expected tobe more pronounced with the WDC fraction (Torkzaban et al.,2012).
The above information indicates that the WDC fractions wereenriched in MWCNTs in comparison with the sand and silt fractions, and that the association between WDCs and MWCNTs was
related to the cation valance. This strong association between
WDCs and MWCNTs provides further evidence for colloidfacilitated transport of MWCNTs during the release experiments.
4. Conclusions
Findings in this study provide important insight on the roles ofsolution IS, cation valance, and soil colloids on the transport,
retention, and remobilization of MWCNTs in soils. Experimental
and modeling results indicate that the mobility of MWCNTs was
highly sensitivity to the IS and cation valence, with greater amountsof retention occurring at high IS and with divalent cations. BTCs for
MWCNTs exhibited a delay in breakthrough that increased withsolution IS and cation valence due to blocking. RPs for MWCNTs
showed a hyper-exponential shape likely due to surface straining
processes. Signiﬁcant amounts of MWCNTs could be released byperturbations in solution chemistry that reduced the adhesive force(e.g., IS reduction and cation exchange). This release behavior wasdemonstrated to depend on the concentration of Ca2þ during
MWCNT deposition and the release of soil colloids with a highFig. 4. (a) Breakthrough and release behavior, and retention proﬁles (d) of MWCNTs insoil. Deposition (step A) occurred at an IS ¼ 10 mM using KCl for experiment I, and an
IS ¼ 1 and 10 mM using CaCl2 for experiment II and III, respectively, whereas release
was initiated by ionic strength reduction (steps B, D, and F, Milli-Q water) and cationexchange (steps C and E) as summarized in Table 1. (b) Efﬂuent concentrations of K and
Ca during steps AF in experiment II. (c) Release of MWCNTs and naturally occurring
minerals due to ionic strength reduction (steps B, D, and F, Milli-Q water) and cationexchange (steps C and E) in soil in experiment II. For experiment III, the range of stepA-F is not shown as the blue dotted line due to the different experimental conditions
compared with experiment II. The injection procedure in experiment II is step A(0e7.61 PV), step B (7.61e15.23 PV), step C (15.23e17.51 PV), step D (17.51e22.85 PV),
step E (22.85e25.13 PV), and step F (25.13e30.47 PV).(For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of thisarticle.)M. Zhang et al. / Water Research 109 (2017) 358e366

365

association for MWCNTs. These results suggest the potential for soil
colloids to facilitated the transport of MWCNTs in the subsurface
environment, and thereby pose a potential risk of groundwaterpollution, especially during rainfall or irrigation events that alter
the solution chemistry.
Acknowledgments
The ﬁrst author thanks the China Scholarship Council (CSC) for
ﬁnancial support. The authors would like to acknowledge Stephan€ppchen for bromide measurement, and Andrea Kubika andKoMartina Krause for TOC measurements. Thanks to Herbert Philipp
and Claudia Walraf for their technical assistance, and Xiaoqian Jiang
and Wulf Amelung for the helpful discussion related to soil
fractionation.
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Fig. 5. (a) Mass percentage of each soil sized fraction: sand, silt, 0.45e2 mm WDCs and<0.45 mm WDCs; (b) Mass percentage of MWCNTs in sand, silt and 0.45e2 mm WDCs ofsoil. The <0.45 mm WDCs also contained the MWCNTs in the so-called electrolyte
phase (1 mM KCl and CaCl2); (c) Retained MWCNTs in sand, silt, 0.45e2 mm WDCs and<0.45 mm WDCs including electrolyte phase under different cation type.

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