Environmental Pollution 247 (2019) 907e916

Contents lists available at ScienceDirect

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

Co-transport of multi-walled carbon nanotubes and sodium
dodecylbenzenesulfonate in chemically heterogeneous porous
media* 
Miaoyue Zhang a, b, *, Scott A. Bradford c, Jirka Sim
unek d, Harry Vereecken b,b
Erwin KlumppaSchool of Environmental Science and Engineering, Sun Yat-sen University, 510006, Guangzhou, PR China
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, USAd
Department of Environmental Sciences, University of California Riverside, Riverside, CA, 92521, USAbca r t i c l e i n f o

a b s t r a c t

Article history:
Received 4 September 2018
Received in revised form
20 January 2019
Accepted 26 January 2019
Available online 1 February 2019

Multi-walled carbon nanotubes (MWCNTs) are increasing used in commercial applications and may bereleased into the environment with anionic surfactants, such as sodium dodecylbenzenesulfonate(SDBS), in sewer discharge. Little research has examined the transport, retention, and remobilization of
MWCNTs in the presence or absence of SDBS in porous media with controlled chemical heterogeneity,and batch and column scale studies were therefore undertaken to address this gap in knowledge. The
adsorption isotherms of SDBS on quartz sand (QS), goethite coated quartz sand (GQS), and MWCNTs were
determined. Adsorption of SDBS (MWCNTs » GQS > QS) decreased zeta potentials for these materials, andproduced a charge reversal for goethite. Transport of MWCNTs (5 mg L1) dramatically decreased with anincrease in the fraction of GQS from 0 to 0.1 in the absence of SDBS. Conversely, co-injection of SDBS (10and 50 mg L1) and MWCNTs radically increased the transport of MWCNTs when the GQS fraction was 0,0.1, and 0.3, especially at a higher SDBS concentration, and altered the shape of retention proﬁle.
Mathematical modeling revealed that competitive blocking was not the dominant mechanism for the
SDBS enhancement of MWCNT transport. Rather, SDBS sorption increased MWCNT transport byincreasing electrostatic and/or steric interactions, or creating reversible interactions on rough surfaces.
Sequential injection of pulses of MWCNTs and SDBS in sand (0.1 GQS fraction) indicated that SDBS could
mobilize some of retained MWCNTs from the top to deeper sand layers, but only a small amount of
released MWCNTs were recovered in the efﬂuent. SDBS therefore had a much smaller inﬂuence on
MWCNT transport in sequential injection than in co-injection, presumably because of a greater energy
barrier to MWCNT release than retention. This research sheds novel insight on the roles of competitive
blocking, chemical heterogeneity and nanoscale roughness, and injection sequence on MWCNT retention
and release.
© 2019 Elsevier Ltd. All rights reserved.

Keywords:
Multi-walled carbon nanotubes
Sodium dodecylbenzenesulfonate
Competitive blocking
Breakthrough curves
Retention proﬁlesModeling

1. Introduction
Carbon nanotubes (CNTs) consist of rolled-up graphene sheets(Iijima, 1991) that have been used in many commercial applications
such as electrical cables and wires (Janas et al., 2014), hydrogen*

This paper has been recommended for acceptance by Baoshan Xing.
* Corresponding author.School of Environmental Science and Engineering, Sun
Yat-sen University, 510006, Guangzhou, PR China.
E-mail address: zhangmy53@mail.sysu.edu.cn (M. Zhang).
https://doi.org/10.1016/j.envpol.2019.01.106
0269-7491/© 2019 Elsevier Ltd. All rights reserved.

storage (Dillon et al., 1997), solar cells (Guldi et al., 2005), radar
absorption (Lin et al., 2008), and may potentially be employed inenvironmental remediation (Mauter and Elimelech, 2008; Pan andXing, 2012) and water treatment (Camilli et al., 2014; Li et al., 2012;
Zhang et al., 2010). The widespread use of CNTs will undoubtedly
result in their release into the environment (Gottschalk et al.,2009). Published studies have investigated the transport behaviorof CNTs in porous media under various physicochemical conditions,such as solution ionic strength (IS), water content, grain size, input
concentration, dissolved organic matter, and surfactants (Jaisi and908

M. Zhang et al. / Environmental Pollution 247 (2019) 907e916

Elimelech, 2009; Kasel et al., 2013a; Kasel et al., 2013b; Liu et al.,2009; Lu et al., 2013; Lu et al., 2014; Yang et al., 2013; Yuan et al.,2012).
Surfactants are often used to stabilize CNT suspensions (Lu et al.,2013; Lu et al., 2014; Yu et al., 2007), and the mobility of CNTs inporous media was strongly inﬂuenced by the presence of various
stabilizing agents such as sodium dodecylbenzenesulfonate (SDBS),
octyl-phenolethoxylate, and cetylpyridinium chloride. However,these studies were conducted over a limited ranged in CNTs (high)and surfactant (low) input concentrations. The total concentrationof surfactants that discharges in municipal sewer is about
20e70 mg L1 (Matthijs et al., 1999). These surfactant concentrations are much greater than the predicted discharge of CNTs
(Gottschalk et al., 2009), and may be high enough to stabilize CNT
suspensions. However, little research has investigated the inﬂuenceof surfactants on CNT transport when the surfactant concentration
was greater than the CNT concentration. Furthermore, previous
literature considered the simultaneous transport of a mixture ofsurfactants and CNTs, whereas the more environmentally relevant
scenario of sequential release of surfactants and CNTs due to waste
discharge has not yet been investigated.
Previous research with CNTs and surfactants focused on determination of breakthrough curves (BTCs), but did not measure theinﬂuence of surfactants on CNT retention proﬁles (RPs) (Lu et al.,2013; Lu et al., 2014). Liang et al. (2013) demonstrated that thepresence of surfactants had a large inﬂuence on the shape of theRPs for silver nanoparticles in quartz sand, but little inﬂuence on
their BTCs. This was attributed to competitive blocking (e.g., ﬁlling)of silver nanoparticle retention sites by surfactant. Similarly, Becker
et al. (2015) showed that the stabilizing agent can compete for thesame retention sites as quantum dot nanocrystals. Natural porous
media often exhibit surface charge heterogeneity due to Fe and Al
oxyhydroxides with a net positive surface charge (Parks, 1965) andcommon silica minerals with a net negative surface charge(Alvarezsilva et al., 2010) at ambient pH. Nanoparticle attachment
onto positively charged sites can be inhibited by surfactant sorption
which can neutralize or reverse the surface charge (Lin et al., 2012;
Wang et al., 2012c). The inﬂuence of competitive blocking on CNT
BTCs and RPs is therefore expected to be a function of the chemical
heterogeneity of the porous medium. In the absence of surfactant,
Zhang et al. (2016b) demonstrated that increasing the goethitecoated fraction of quartz sand increased the retention of CNTs
due to the combined inﬂuence of surface roughness and positively
charged sites. To the best of our knowledge, no research studies
have examined the inﬂuence of surface roughness and controlled
soil chemical heterogeneities on the transport and fate of functionalized CNTs in the presence of surfactants. Additional researchis needed to assess the potentially signiﬁcant inﬂuence of surfactants on competitive blocking and CNT RPs, especially in porous
media with chemical heterogeneity and roughness.
The objective of this study is to better understand and quantify
the role of anionic surfactant SDBS concentrations (10e50 mg L1)on the transport, retention, and remobilization behavior of functionalized multi-walled carbon nanotubes (MWCNTs, 5 mg L1) inchemically heterogeneous porous media. The sorption afﬁnity for
SDBS to quartz sand (QS), goethite-coated quartz sand (GQS), and
MWCNTs was determined in batch experiments. Column experiments were employed to determine BTCs for both MWCNT and
SDBS, and RPs for MWCNTs in chemically heterogeneous mixtures
of QS and GQS. Kinetic retention, release, and competitive blocking
parameters for MWCNTs and SDBS were determined by inverse
optimization of the collected column data. Furthermore, thisresearch sheds novel insight on the roles of competitive blocking,chemical heterogeneity and nanoscale roughness, and injection
sequence on MWCNT retention and release, and the develop of

MWCNT retention proﬁles. This knowledge can be useful for environmental applications and risk management of MWCNTs in thepresence of surfactant and various amounts of soil chemical
heterogeneity.
2. Materials and methods
2.1. Porous media and MWCNTs
Quartz sand (QS, 240 mm), which was used in experiments(Quarzwerke GmbH, 50226 Frechen, Germany) was puriﬁed
following a protocol in the literature. The preparation of goethitecoated quartz sand (GQS) has been described in a former study(Zhang et al., 2016b). The chemically heterogeneous porous medium was prepared by combining various amounts of QS with aknown mass fraction of GQS (l, the mass ratio of GQS in the mixed
porous medium; i.e., l ¼ 0 and 1 for a porous medium with only QS
and GQS, respectively). The goethite coating was veriﬁed to bestable by measuring negligible amounts of iron (inductively
coupled optical emission spectrometry, Agilent) in the efﬂuent of acolumn packed with chemically heterogeneous sand (l ¼ 0.3) under steady-state ﬂow and solution chemistry (1 mM KCl) conditions. Zhang et al. (2016b) examined the roughness and chemicalcomposition of GQS using a scanning electron microscope (SEM,
Supra50VP, Carl Zeiss NTS GmbH, Germany) that was equippedwith energy dispersive x-ray (EDX, SDD-Detector, Oxford Instruments, UK), and determined speciﬁc surface areas of QS andGQS.
SDBS surfactant was purchased from Sigma-Aldrich ChemieGmbH (Munich, Germany). SDBS stock solutions (100 mg L1) wereprepared by adding 50 mg surfactant into 500 ml Milli-Q water. The
synthesis, functionalization, and characterization of radioactively(14C) labeled and unlabeled MWCNTs (Bayer Technology Services
GmbH, Leverkusen, Germany) were previously described by Kasel
et al. (2013a,b). In brief, the morphological properties of MWCNTs
was determined using a transmission electron microscope andoxygen containing functional groups were determined using X-ray
photoelectron spectrometry. The MWCNTs have a median diameterof 10e15 nm and a median length of 200e1000 nm (Pauluhn,
2010), an average hydrodynamic diameter of approximately
180 nm (1 mg L1, in 1 1 mM KCl), and a speciﬁc density of
1.641 g cm3. The point of zero charge for oxidized MWCNTs has
been reported by Han et al. (2008).
Stock suspension of 14C-labeled MWCNTs (100 mg L1) was
prepared by suspending MWCNTs in Milli-Q water and ultrasonicating for 15 min at 65 Watts using a cup horn sonicator(Branson Soniﬁer ® W-250, Danbury, USA). Then, stock suspensionof 14C-labeled MWCNTs was diluted using different SDBS concentrations (0, 10, and 50 mg L1) at the desired ionic strength (1 mM
KCl). To ensure thorough dispersion, mixtures of MWCNT and SDBS
were ultrasonicated for 15 min at 65 W and then ultrasonicatedagain for 10 min before use in characterization, batch, and column
experiments discussed below.
The hydrodynamic radius does not reﬂect the real geometricparticle diameter for non-spherical particles like MWCNTs€v et al., 2008; Pecora, 2000). Nevertheless, it can be used
(Hassello
for comparison of the stability of MWCNTs suspensions in thepresence of surfactant. The hydrodynamic radius of the MWCNTs
suspensions in 1 mM KCl at different SDBS concentrations (0, 10,
and 50 mg L1) was therefore measured using a Zetasizer Nano
(Malvern Instruments GmbH, 71083 Herrenberg, Germany)
immediately after suspension preparation and at 1 h. The hydrodynamic radius of the MWCNTs suspensions at different SDBS
concentrations (0, 10, and 50 mg L1) was 334.6, 315.1, and 306.3
nm, respectively, and in the same range at 0 and 1 h. The MWCNTs

M. Zhang et al. / Environmental Pollution 247 (2019) 907e916909

solution. Efﬂuent solutions were collected using a fraction collector(FoxyJr.®, Teledyne Isco Inc., Lincoln, USA) every 30 s. The efﬂuent
concentrations of bromide were measured by using a highperformance liquid chromatograph (STH 585, Dionex, Sunnyvale,
CA, USA) equipped with a UV detector (UV2075, Jasco, Essex, UK).
Following completion of the tracer experiment, the packed
columns were employed to study the transport of a 2.6 PV pulse of
MWCNT suspension (5 mg L1, 1 mM KCl) in the presence of various
SDBS concentrations (0, 10, and 50 mg L1) through a selectedchemically heterogeneous porous medium (l ¼ 0, 0.1, and 0.3). The
same protocol was employed for tracer and MWCNT transport experiments. The efﬂuent concentrations of MWCNTs and SDBS were
determined using approaches outlined in section 2.2. After recovery of the MWCNT breakthrough curve (BTC), the MWCNT
retention proﬁle (RP) was determined by excavating sand in
approximately 0.5e1 cm thick increments, drying, homogenizing
using a mill, combusting using a biological oxidizer, and thenmeasuring the MWCNT concentration with the LSC. A summary of
the experimental conditions and mass balance information isprovided in Table 1. All column experiments were replicated andexhibited good reproducibility.

suspensions were therefore considered to be less aggregated as
SDBS concentrations increased and stable during this time interval.
The zeta potentials of MWCNTs, crushed QS, and goethite in
1 mM KCl and at different surfactant concentrations was alsomeasured using a Zetasizer Nano. Fig. S1 of the supporting information (SI) provides results. The pH values of MWCNTs suspensions
at different SDBS concentrations (0, 10, and 50 mg L1, 1 mM KCl)
were measured using a pH meter (Mettler Toledo MP230 pH
meter).
2.2. Determination of concentrations of MWCNTs and SDBS
Concentrations of 14C-labeled MWCNT in various SDBS solutions(0e50 mg L1) were determined using a liquid scintillation counter(LSC) (PerkinElmer, Rodgau, Germany). Efﬂuent concentrations of
SDBS in batch and column experiments discussed below were
measured using a UVeVis spectrometer (Beckman DU-640, U.S.A)
at a wavelength of 230 nm. However, both SDBS and MWCNTs haveabsorbance at a wavelength of 230 nm. Calibrations curves ofabsorbance and concentration of MWCNTs, SDBS, and SDBSMWCNTs (MWCNTs modiﬁed by SDBS) were therefore established at a wavelength of 230 nm (Fig. S2). The additivity of individual absorbance of MWCNTs, SDBS, and SDBS-MWCNTs can beapplied to quantitatively determine the concentrations of SDBS in
the efﬂuent. Details pertaining to these calculations are given in thesection S1 of the SI.

2.5. Numerical modeling 
The HYDRUS-1D computer code (Simunek et al., 2008) was used
to simulate the transport and retention of MWCNTs and SDBS in thecolumn experiments. The aqueous and solid phase mass balance
equations for MWCNTs are given in this model as:

2.3. Batch experiments
Adsorption isotherms of SDBS on QS (240 mm), GQS (240 mm,
l ¼ 0.1 and 0.3), and MWCNTs were determined in batch trials
following a published protocol (OECD, 2006). Details pertaining tothese calculations are given in the section S2 of the SI.
2.4. Column transport experiments



vC
v
vC
vðvCÞ
¼
D

 jk1 C þ rb kd S
vt
vz
vz
vz

[1]

vðrb SÞ
¼ qw jk1 C  rb kd S
vt

[2]

where qw [e] is the volumetric water content, C [N L3, N and L
denote the number of MWCNTs and units of length, respectively] is
the aqueous phase MWCNT concentration, t is time [T, T denotes
time units], z [L] is the distance from the column inlet, D [L2T1] isthe hydrodynamic dispersion coefﬁcient, v [L T1] is the pore-watervelocity, j [-] is a dimensionless function to account for time- anddepth-dependent blocking, k1 [T1] is the ﬁrst-order retention coefﬁcient, kd [T1] is the ﬁrst-order detachment coefﬁcient, rb [ML3, M denote units of mass] is the soil bulk density, and S [N M1] isthe solid phase MWCNT concentration. The ﬁrst and second termson the right hand side of Eq. [1] account for dispersive andStainless steel columns with a 3 cm inner diameter and 12 cm
length were uniformly wet packed with a selected mixture of QS
and GQS (l ¼ 0.0, 0.1, and 0.3). The packed columns were equilibrated by ﬂushing with approximately 30 pore volumes (PVs) of
background electrolyte (1 mM KCl) solution before initiatingtransport experiments. In order to characterize the column's hydraulic conditions, a non-reactive tracer (1 mM KBr in 1 mM KCl)was injected into the column at a steady-state Darcy velocity of
0.71e0.73 cm min1 for approximately 2.6 PVs (90 mL), followed bycontinued elution at the same velocity with tracer-free 1 mM KCl

Table 1
Experimental conditions and mass recoveries from efﬂuent for all column experiments. The ionic strength was 1 mM KCl, d50 ¼ 240 mm, and the input concentration of
MWCNTs was 5 mg L1.
Fig.No.

1,2
1,3,4
1,2
1,3
2,3
2
3
3
4

l

0
0.1
0
0.1
0
0
0.1
0.3
0.1

Disp.

Co (MWCNTs)

Co (SDBS)q[cm2min1][mg L1][mg L1][cm min1]

0.28750.07080.01850.01950.28750.01080.07080.03990.0418

5
5
0
0
5
5
5
5
5

0
0
10
10
10
50
10
10
10

0.71
0.73
0.72
0.72
0.72
0.72
0.73
0.73
0.72

Porosity

0.43
0.44
0.43
0.43
0.42
0.45
0.41
0.42
0.45

Msurf

Meff

MsolidMtotal

[%]

[%]

[%]

[%]

NA
NA
95.50
92.77
91.55
94.97
86.37
84.39
80.01

17.51
2.41
NA
NA
87.18
90.49
64.22
0.32
2.47

87.08
98.39
NA
NA
10.20
9.88
34.90
97.17
97.22

104.60100.79NANA
97.38
100.38
99.12
97.49
99.70

Fig. No. is the number of Fig. NA denotes not applicable. l, is the mass ratio of goethite coated quartz sand (GQS) in the porous medium; when l ¼ 0, the porous medium isquartz sand. Disp.is the estimated longitudinal dispersivity. Co (MWCNTs) and Co (SDBS) are the input concentrations of MWCNTs and SDBS, respectively. q is the Darcy velocity.
Msurf is the efﬂuent percentage of SDBS recovered from the column experiment. Meff, Msolid, and Mtotal are the efﬂuent percentage, the retained percentage, and the total
percentage of MWCNTs recovered from the column experiment, respectively.

910

M. Zhang et al. / Environmental Pollution 247 (2019) 907e916

advective transport of MWCNTs, respectively. Third and forth terms
in Eq. [1] are used to describe retention and release to/from thesolid phase, respectively. The value of j is given in this work as:



j¼ 1

S þ G SsSmaxd50 þ zd50

3. Results and discussion

b
[3]

where Ss [Ns M1; Ns denotes the number of surfactant] is the solid
phase concentration of the surfactant SDBS, Smax [N M1] is the
maximum solid phase concentrations of MWCNTs, G [-] is the area
conversion factor between SDBS and MWCNTs, d50 [L] is the mediansand grain diameter, and b [-] is an empirical parameter that controls that depth-dependency of the retention rate coefﬁcient. The
value of b was set to 0.765 based on our previous study with
MWCNTs in chemically heterogeneous sand (Zhang et al., 2016a;
Zhang et al. 2016b; Zhang et al. 2017).
Similar to the MWCNT, the aqueous and solid phase mass balance equations for the surfactant SDBS are given in this model as:



vCs
v
vCs
vðvCs ÞD
 k1s js Cs þ rb kds Ss
¼

vz
vz
vt
vz

[4]

vðrb Ss Þ¼ qw k1s js Cs  rb kds Ss
vt[5]

where the superscript s on parameters indicates that they areassociated with the surfactant SDBS. In the absence of experimental
information about the depth dependency of SDBS retention, thevalue of js is given as:S þG Sjs ¼ 1  s maxs
Ss

absence of the competitive species, and then optimizing values of G
and Gs to both MWCNT and SDBS data sets.

[6]

Eq. [3] and [6] allow for the possibility of single species andcompetitive Langmuirian blocking. Langmurian blocking (Adamczyk
et al., 1994) is modelled using the ﬁrst term on the right hand side ofthese equations when G and Gs are set to zero, whereas competitive
blocking (Becker et al., 2015) is considered when G and Gs are greaterthan zero. The parameter G accounts for differences in the crosssectional areas of the particles and the porous medium surface
that contributes to retention of MWCNTs and SDBS. When Smax andSmax
encompass the same area on the porous media surface, thesvalue of G ¼ 1/Gs ¼ As/A (Becker et al., 2015), where As [L2] and A [L2]are the cross-sectional areas of SDBS and MWCNT, respectively.
HYDRUS-1D includes provisions for inverse parameter estimation using a nonlinear least squares optimization routine. The
bromide tracer data was simulated using the solution of theadvection dispersion equation (Eq. [1] with k1 ¼ 0), and values of v
and D were determined by inverse optimization. Values of k1, kd,
and Smax were determined by inverse optimization to the BTC and
RP for MWCNTs. Values of k1s, kds, and Smax
were determined bysinverse optimization to the BTC for SDBS. The single-species
transport (SST) of MWCNTs and SDBS were described with Eqs.
[1e3] and [4e6], respectively, by setting G and Gs to zero. Threemodeling approaches (denoted as M1, M2, and M3) were considered when both MWCNTs and SDBS were simultaneously present.
The M1 approach neglects competitive blocking by setting G and Gs
to zero, and then optimizing transport and retention parameters to
MWCNT and SDBS data sets as if they were independently obtained.
The M2 approach allows for a small amount of competitive blocking
by ﬁxing the retention and release parameters to those determinedfrom M1, and then optimizing values of G and Gs to both MWCNTand SDBS data sets. The M3 approach allows for the maximum
amount for competitive blocking by ﬁxing retention and releaseparameters for MWCNTs and SDBS that were determined in the

3.1. Adsorption of SDBS on QS, GQS and MWCNTs
The functionalization of MWCNTs produced an increased
amount of groups containing oxygen (e.g. carboxylic groups) incomparison to the pristine MWCNTs (Kasel et al., 2013a; Wei et al.,2007), resulting in its negative zeta potential in 1 mM KCl solution(Fig. S1). Zeta potentials of MWCNTs, QS, and goethite decreased
with increasing SDBS concentration, suggesting that the increased
electrostatic repulsion from adsorbed SDBS can enhance the stability of functionalized MWCNTs. The pH of the MWCNTs suspensions slightly increased from 5.39, 5.81, and 6.42 with an increase in
the SDBS concentrations of 0, 10, and 50 mg L1, respectively, due tothe generation of more hydroxyls radicals (Duan et al., 2018). Thisvariation in pH is expected to inﬂuence the interaction between
MWCNTs and QS/GQS by decreasing the zeta potential of thesesurfaces and creating a greater energy barrier to interaction in a
primary minimum (Wang et al., 2012a). Zhang et al. (2016a,b)
calculated interaction energies for MWCNTs on QS and GQS surfaces with different surface roughness properties in 1 mM KCl bytreated them as equivalent solid spherical particles. Small fractionsof nanoscale roughness were demonstrated to play a dominant roleon MWCNT interaction energies on both QS and GQS; e.g., reducing
the energy barrier height and the depths of primary and secondary
minima.
Adsorption isotherms of SDBS on QS, GQS, and MWCNTs areshown in Fig. S3. Adsorption of SDBS followed an order:
MWCNTs » GQS > QS. It is worth noting that the adsorption afﬁnityof SDBS on GQS increased as l increased. That can be explained by
the enhanced surface area and positive charge of goethite (e.g.,
Fig. S1). The adsorption isotherm initially increased sharply, thenreached a plateau at an equilibrium surfactant concentration of
about 5 mg L1. This behavior is consistent with observations reported by Han et al. (2008). Adsorption of SDBS on MWCNTs can be
explained by hydrophobic interactions between the hydrophobic
chains on the surfactant and hydrophobic sites on the MWCNTs(Matarredona et al., 2003), and the benzene ring of SDBS induces
Pi-Pi interactions with MWCNTs (Tan et al., 2008). The SDBS isotherms on QS, GQS, and MWCNTs were described using Langmuir
and Freundlich models (Fig. S3) with R2 > 0.87. The adsorption
isotherms on MWCNTs were close to High-afﬁnity type (Fig. S3).
3.2. Single-species transport of MWCNTs and SDBS
Column experiments were performed to understand thesingle-species transport behavior of MWCNTs and SDBS in chemically heterogeneous porous media (l ¼ 0 and 0.1). The experimental
conditions and mass balance information are presented in Table 1.
The parameters d50 ¼ 240 mm, IS ¼ 1 mM KCl, and q ¼
0.71e0.73 cm min1 were the same for all experiments. The input
concentration (Co) was 5 mg L1 for MWCNTs and 10 mg L1 for SDBS.
The total recovered mass from the BTCs and RPs for MWCNTs
was very good (>97%). The MWCNT efﬂuent mass balance (Meff)
strongly decreased from 17.5% to 2.4% (Table 1) as l increased from0 to 0.1. Similarly, the MWCNT retained mass balance (Msolid)
increased from 87.1% to 98.4% as l increased from 0 to 0.1. Observed
and simulated BTCs for MWCNTs are plotted as normalized efﬂuent
concentrations (C/Co) versus pore volumes in Fig. 1a. The BTCs werevery-well described by the model (R2 > 0.97), and parameters k1
and Smax/Co both increased with l (Table 2). Blocking was clearly
evident in the BTCs, whereas release was very minor (kd ¼ 0.0001

M. Zhang et al. / Environmental Pollution 247 (2019) 907e916

911

Fig. 1. Observed and ﬁtted breakthrough curves (BTCs, a and c) and retention proﬁles (RPs, b) of MWCNTs and SDBS in porous media at different l (l, the mass ratio of goethitecoated quartz sand in the porous medium). (a) BTCs of MWCNTs at l ¼ 0 and 0.1; (b) RPs of MWCNTs at l ¼ 0 and 0.1; (c) BTCs of SDBS at l ¼ 0 and 0.1. The input concentrations of
MWCNTs and SDBS were 5 and 10 mg L1, respectively. The ionic strength was 1 mM KCl. The Darcy velocity is 0.71e0.73 cm min1.

Table 2Fitted model parameters.
Fig. No.

Model

AIC

G

Smax/Co
3 1

[cm g

l ¼ 0, Co (MWCNTs) ¼ 5 mg L1, Co (SDBS) ¼ 0 mg L1
1,2
SST
l ¼ 0.1, Co (MWCNTs) ¼ 5 mg L1, Co (SDBS) ¼ 0 mg L1
1,3,4
SST
l ¼ 0, Co (MWCNTs) ¼ 0 mg L1, Co (SDBS) ¼ 10 mg L1
1,2
SST
l ¼ 0.1, Co (MWCNTs) ¼ 0 mg L1, Co (SDBS) ¼ 10 mg L1
1,3
SST
l ¼ 0, Co (MWCNTs) ¼ 5 mg L1, Co (SDBS) ¼ 10 mg L1
2,3
M1
S4
M2
S4
M3
l ¼ 0, Co (MWCNTs) ¼ 5 mg L1, Co (SDBS) ¼ 50 mg L1
2
M1
S4
M2
S4
M3
l ¼ 0.1, Co (MWCNTs) ¼ 5 mg L1, Co (SDBS) ¼ 10 mg L1
3
M1
S5
M2
S5
M3
l ¼ 0.3, Co (MWCNTs) ¼ 5 mg L1, Co (SDBS) ¼ 10 mg L1
3
M1
S5
M2

]

Gs

Smax
/Co
s

k1
1

[-]

[min

]

3 1

[cm g

]

k1s

kds
1

[-]

[min

]

[min

R2 BTC

R2RP

R2s BTC

1]

101.901.351

NA13.000

NA

NA

NA

NA

0.970

0.876

NA

52.9527.450NA

19.910

NA

NA

NA

NA

0.991

0.647

NA

2.06

NA

NA

NA

0.263

NA

0.258

0.200

NA

NA

0.986

119.90

NA

NA

NA

0.384

NA

0.362

0.200

NA

NA

0.986

308.00
306.10
304.80

0.063
0.063
1.351

0.000
0.002
2.169

1.426
1.426
13.000

0.110
0.110
0.263

0.000
0.031
43.880

0.064
0.064
0.258

0.126
0.126
0.200

0.988
0.988
0.989

0.964
0.964
0.869

0.970
0.970
0.961

308.40
306.70
167.20

0.061
0.061
1.351

0.000
0.034
47.0401.377
1.377
13.000

0.090
0.090
0.263

0.000
0.166
151.200

0.061
0.061
0.258

0.163
0.163
0.200

0.987
0.987
0.983

0.944
0.945
0.985

0.977
0.977
0.985

231.00231.10
211.70

0.211
0.211
27.4500.000
4.16E-04
0.625

9.277
9.277
19.910

0.134
0.134
0.384

0.000
0.014
296.700

0.067
0.067
0.362

0.009
0.009
0.200

0.943
0.944
0.944

0.689
0.689
0.971

0.968
0.968
0.703

132.20
1.00

1.800
1.800

0.000
0.091

51.43051.430

0.131
0.131

0.000
0.040

0.070
0.070

0.008
0.008

0.963
0.963

0.139
0.139

0.962
0.964

Fig. No. is the number of Fig.; SST is the simulated model based on single-species transport of MWCNTs or SDBS; l, the mass ratio of goethite coated quartz sand (GQS) in the
porous medium; when l ¼ 0, the porous medium is quartz sand. AIC is Akaike information criterion; k1, the ﬁrst-order retention rate coefﬁcient of MWCNTs; kd ¼ 1.0E-04
min1, the ﬁrst-order release rate coefﬁcient of MWCNTs; Smax/Co, the normalized maximum solid phase concentration of deposited MWCNTs; k1s, the ﬁrst-order retention
rate coefﬁcient of SDBS; kds, the ﬁrst-order release rate coefﬁcient of SDBS; Smax
/Co, the normalized maximum solid phase concentration of deposited SDBS; G, the areas
conversion factor between MWCNTs and SDBS; Gs, the area conversion factor between SDBS and MWCNTs; R2 BTC, R2RP, and R2s BTC reﬂect the correlation of observed and ﬁtted
data for BTC of MWCNTs, RP of MWCNTs, and BTC of SDBS; NA - denotes not applicable.

912

M. Zhang et al. / Environmental Pollution 247 (2019) 907e916

min1). Similar MWCNTs transport behavior and trends in model
parameters have been reported by Zhang et al. (2016b). These authors discussed mechanisms of MWCNT retention in QS and GQS in
the absence of surfactant. In brief, their results indicated thatnanoscale roughness played an important role in contributing to
MWCNT retention in QS by reducing or eliminating the energy
barrier, whereas microscopic roughness altered the lever arms forapplied hydrodynamic (decreases) and resisting adhesive (increases) torques. These same factors also contributed to MWCNT
retention in GQS, but additional retention occurred with increasing
l due to more electrostatically favorable sites.
Observed and simulated RPs for MWCNTs are given as
normalized solid-phase concentrations (S/Co) versus column depth
in Fig. 1b. Most previous studies have reported hyper-exponential
shaped RPs for MWCNTs (Kasel et al., 2013a; Kasel et al., 2013b;
Wang et al., 2012b; Zhang et al., 2016a; Zhang et al., 2017), and ourmodeling approach which considered depth-dependent retention
provided a better description of the RPs than the uniform modelwith b ¼ 0 (data not shown). However, Fig. 1b shows nonmonotonic
RPs for the MWCNTs. Nonmonotonic RPs have previously been
observed for other colloids and attributed to a number of potential
factors such as variations in the pore-scale velocity distribution(Bradford et al., 2011), solid phase colloid migration (Yuan andShapiro, 2011), colloid distribution properties (Bradford et al.,2006; Tong et al., 2008), competitive blocking (Becker et al.,2015), and reversible blocking (Leij et al., 2016). Competitive
blocking does not occur in the single-species transport experiments, solid phase migration is expected to be more difﬁcult for rod
shaped MWCNTS than spherical colloids, and the MWCNT
detachment rate was determined to be very small. Consequently,
the most likely of these explanations are common variations in the
pore-scale velocity or MWCNT distribution properties.
Arrival of the SDBS BTCs in Fig. 1c was slightly delayed incomparison to the conservative tracer, approached a constant C/Co
value of 1, and then exhibited considerable amounts of low concentration tailing. The SDBS BTC for l ¼ 0.1 had a greater delay andconcentration tailing than that for the l ¼ 0 condition. The SDBS
efﬂuent mass balance (Msurf) slightly decreased from 95.5% to 92.8%(Table 1) as l increased from 0 to 0.1, which was expected frombatch experiments (Fig. S3). This observation suggests that some of
the SDBS was irreversibly sorbed onto the sand surfaces, and thisamount increased with l. The SDBS BTCs were well described using
the model that considered retention, release, and blocking(R2 > 0.986). Both k1s and Smax
/Co increased with l. Similar tos
MWCNTs, blocking played an important role in SDBS transport. In
contrast to MWCNTs, SDBS had much higher release rates (kds ¼ 0.2
min1), which indicates signiﬁcant amounts of reversible retention.
3.3. Simultaneous transport of MWCNTs and SDBS in QS
Fig. 2 presents observed and simulated (M1 model) BTCs
(Fig. 2a) and RPs (Fig. 2b) for MWCNTs in QS when the SDBS concentration equaled 0, 10, and 50 mg L1. The experimental information and mass recoveries from the efﬂuent and solid phase arepresented in Table 1. The value of Meff strongly increased from 17.5%
to 90.5% (Table 1) as the input concentration of SDBS increased
from 0 mg L 1 to 50 mg L1. Similar to results of Tian et al. (2011)and Lu et al. (2013), the MWCNTs were highly mobile in the presence of 10 or 50 mg L1 SDBS with values of Meff equal to 87.2% and90.5%, respectively. In addition, the RPs of MWCNTs became hyperexponential in the presence of SDBS, whereas they were nonmonotonic in the absence of SDBS. Consequently, both BTCs andRPs of MWCNTs demonstrated that SDBS can enhance MWCNTstransport in QS. Potential explanations will be discussed below.
Observed and simulated (M1) BTCs for 10 and 50 mg L1 SDBS in

the QS are given in Fig. 2c. Similar to Fig. 1c in the absence of
MWCNTs, SDBS was highly mobile (Msurf>91.6%) in the QS that
contained retained MWCNTs. Values of Msurf were comparable tothose in the absence of MWCNTs (95.5%). Consequently, only asmall fraction of SDBS was adsorbed onto the MWCNT surfaces(<3.9%) and to the QS (<4.5%), although much higher adsorption
per unit mass occurs for MWCNTs than QS (Fig. S3). Similar to
Fig. 1c, the BTCs were slightly delayed in comparison to the conservative tracer, approached a constant C/Co value of 1, and thenexhibited considerable amounts of low concentration tailing.
However, the delay in breakthrough decreased with increasing
SDBS concentration, presumably due to a more rapid ﬁlling ofavailable retention sites. The values of Msurf increased from 91.6% to
95% as the input concentration of SDBS increased from 10 to
50 mg L1 for a similar reason.
Several reasons can explain the enhancement of MWCNT
transport in the presence of SDBS. Sorption of SDBS onto the surfaceof the QS and the MWCNTs (Fig. S3) produces a more negative
surface charge (Fig. S1), which would increase the electrostatic
repulsion between MWCNTs in suspension and with the QS. In
addition, sorption of organics onto surfaces has been reported toenhance suspension stability and diminish retention as a result ofsteric repulsion (And and Sticher, 1997; Flynn et al., 2012; Yanget al., 2014), alteration of nanoscale roughness properties(Bradford et al., 2017), and/or competitive blocking (Becker et al.,2015; Lin et al., 2012; Wang et al., 2012c). Note that the M1
model provided a reasonable description of the transport andretention behavior of MWCNTs and SDBS in Fig. 2. Table 2 summarizes ﬁtted and statistical parameters for the M1, M2, and M3
models (simulation results are shown in Fig. S4) that allowed for no,
limited, and maximum competitive blocking, respectively. Valuesof R2 and the Akaike information criterion (AIC) (Akaike, 1974)
indicate that the M1 model was preferred. Consequently, competitive blocking is not expected to play the dominant role inenhancing MWCNT transport in the presence of SDBS. A viable
alternative explanation is due to alternation of interaction energies
between MWCNTs and QS by SDBS sorption. In particular, SDBS
sorption can increase the energy barrier to MWCNTs interactions
by increasing electrostatic and/or steric interactions, or by eliminating the energy barrier and creating a shallow (reversible) primary minimum through modiﬁcation of the surface roughness
properties. In support of this hypothesis the values of k1 and Smax/Co
for the M1 model were greatly reduced in the presence than theabsence of SDBS (Table 2).
3.4. Simultaneous transport of MWCNTs and SDBS in GQS
Signiﬁcant mobility of MWCNTs was observed in the QS in thepresence of SDBS (Fig. 2a and b). Additional experiments wereconducted to better understand the effect of SDBS on MWCNT
transport in the presence of goethite coatings. Fig. 3 presents
observed and simulated (M1 model) BTCs (Fig. 3a) and RPs (Fig. 3b)
for MWCNTs in sand with l ¼ 0, 0.1, and 0.3 when the SDBS concentration equaled 10 mg L1. The experimental conditions andmass recoveries are presented in Table 1, and the M1 model andstatistical parameters are given in Table 2.
The value of Meff strongly decreased from 87.2% to 0.3% as lincreased from 0 to 0.3 when the SDBS concentration was
10 mg L1 (Table 1). This observation indicates that geothite coatings had a large inﬂuence on MWCNT transport even in the presence of SDBS. Table 2 indicates that the M1 model provided a good
description of the BTCs for MWCNTs in sand with different values ofl and SDBS. Values of k1 and Smax/Co increased with l (Table 2),
indicating that GQS was a preferential retention site even in thepresence of SDBS. Blocking was observed in the MWCNTs BTCs

M. Zhang et al. / Environmental Pollution 247 (2019) 907e916

913

Fig. 2. Observed and ﬁtted BTCs (a and c) and RPs (b) of MWCNTs and SDBS with or without additional SDBS or MWCNTs in quartz sand (l ¼ 0). (a) BTCs of MWCNTs with adding 0,
10, and 50 mg L1 of SDBS; (b) RPs of MWCNTs with adding 0, 10, and 50 mg L1 of SDBS; (c) BTCs of SDBS without MWCNTs, and with MWCNTs at 10 and 50 mg L1 of SDBS. The
input concentration of MWCNTs was 5 mg L1. The ionic strength was 1 mM KCl. The Darcy velocity is 0.71e0.73 cm min1.

(Fig. 3a), and MWCNTs exhibited greater delay in breakthroughwith increasing l because they take longer to ﬁll a larger value of
Smax/Co. The RPs were hyper-exponential in shape and welldescribed using the M1 model when l ¼ 0 and 0.1 (Fig. 3b). In
contrast, the RP for the l ¼ 0.3 condition was non-monotonic inshape and exhibited a peak value at a depth of about 5 cm thatwas not accurately captured by the M1 model. A non-monotonic RP
was also observed for the l ¼ 0 and 0.1 systems in the absence ofSDBS (Fig. 1b), which was attributed to variations in the pore-scale
velocity distribution (Bradford et al., 2011) and/or MWCNT distribution properties (Bradford et al., 2006; Tong et al., 2008). In
addition to these processes, competitive blocking (Becker et al.,2015) and reversible blocking (Leij et al., 2016) are expected tocontribute to the development of non-monotonic RPs in the presence of SDBS and this likely explains the peak MWCNT value at agreater depth.
Comparison of results for l ¼ 0.1 in the presence (Fig. 3a and b)
and absence (Fig. 1a and b) of SDBS reveals that the surfactant
greatly enhanced the transport of MWCNT (e.g., Meff increased from
2.4% to 64.2%). Similar to QS (Fig. 2), this enhanced transport of
MWCNTs is attributed to alteration of the interaction energy by
SDBS sorption (e.g., increasing electrostatic and/or steric interactions, or by creating reversible interactions on rough surfaces).
The value of Msurf for SDBS was equal to 91.6%, 86.4%, and 84.2%
when l ¼ 0, 0.1, and 0.3, respectively (Table 1). This decrease in Msurf
with an increase in l reﬂects the presence of more irreversible
retention sites for SDBS on a geothite surface and/or on retained
MWCNTs (which also increased with an increase in l). The
observed and simulated BTCs for SDBS in sand with l ¼ 0, 0.1, and
0.3 are shown in Fig. 3c. The BTCs for SDBS exhibited similar

behavior to those shown in Figs. 1c and 2c, and were well describedusing the M1 model (Table 2). Analogous to the SDBS in QS (Figs. 1cand 2c), the values of k1s and Smax
/Co for the M1 model weres
reduced in the presence than the absence of MWCNTs (Table 2).
Similar to simultaneous transport of MWCNTs and SDBS in QS, theﬁtted and statistical parameters from M2 and M3 models in GQS
are summarized in Table 2 and shown in Fig. S5.
3.5. Remobilization of retained MWCNTs in GQS by SDBS injection
An additional experiment was conducted to investigate thepotential for injection of 10 mg L1 of SDBS to remobilize MWCNTs
that were initially retained in sand with l ¼ 0.1 in the absence of
surfactant (Fig. 4). The initial retention phase for MWCNTs was
conducted in an analogous fashion to Fig. 1 and this l ¼ 0.1 dataset
was also included in Fig. 4 for comparison purposes. The solution
chemistry sequence for remobilization was meant to simulate theprocess of using SDBS to remediate soil contaminated by MWCNTs.
Even though the mobility of MWCNTs was limited in sand withl ¼ 0.1 (Meff ¼ 2.4%), it still appeared again in the outﬂow at low
levels after SDBS injection (Fig. 4a). In addition, the concentrationsof retained MWCNTs were also obviously decreased in the top
layers and increased in the deeper layers of the column due to the
SDBS elution (Fig. 4b). This shift demonstrated that injection of
SDBS facilitated the remobilization of MWCNTs.
Mass balance information in Table 1 indicates that co-injectionof MWCNTs and SDBS produced a much greater recovery in the
efﬂuent (Meff ¼ 64.2%) in comparison to sequential injection of
MWCNTs and SDBS (Meff ¼ 2.5%). The high recovery of MWCNTs
during co-injection was attributed in sections 3.2 and 3.2 to the914

M. Zhang et al. / Environmental Pollution 247 (2019) 907e916

Fig. 3. Observed and ﬁtted BTCs (a and c) and RPs (b) of MWCNTs and SDBS in porous media at different l. (a) BTCs of MWCNTs without adding SDBS at l ¼ 0.1, and with adding
SDBS at l ¼ 0, 0.1, and 0.3; (b) RPs of MWCNTs without adding SDBS at l ¼ 0.1, and with adding SDBS at l ¼ 0, 0.1, and 0.3; (c) BTCs of SDBS in the absence of MWCNTs at l ¼ 0.1, andpresence of MWCNTs at l ¼ 0, 0.1, and 0.3. The input concentrations of MWCNTs and SDBS were 5 and 10 mg L1, respectively. The ionic strength was 1 mM KCl. The Darcy velocity is
0.71e0.73 cm min1.

Fig. 4. Observed BTCs (a) and RPs (b) of MWCNTs and SDBS without SDBS and with SDBS elution at l ¼ 0.1. Fig. (4a) has two vertical axes. The left axis shows the relative MWCNTs
concentration, while the right axis shows the relative SDBS concentration. The input concentration of MWCNTs and SDBS was 5 and 10 mg L1, respectively. The ionic strength was
1 mM KCl. The Darcy velocity is 0.71e0.73 cm min1.

M. Zhang et al. / Environmental Pollution 247 (2019) 907e916

inﬂuence of SDBS adsorption on the interaction energy between
MWCNTs and the sand (e.g., increases in electrostatic and stericinteractions, and/or alteration of surface roughness properties). The
results from the sequential injection of MWCNTs and SDBS (Fig. 4)
suggest that it is much more difﬁcult to alter the interaction energy
and promote release once the MWCNTs have been retained. This isdue to the difference in the energy barrier for retention and releaseof MWCNTs. In particular, the initial retention of MWCNTs depends
on the energy barrier to the minimum, which was eliminated onelectrostatically favorable surfaces like goethite. Conversely, therelease of MWCNTs depends on the difference in the magnitude of
the minimum and the energy barrier height. Note that the depth ofthe primary minimum is expected to be large on goethite surfacesthat are smooth. Consequently, the energy barrier to release is
expected to be much larger than for retention on goethite coatedsurfaces. Furthermore, adsorption of SDBS increases the energy
barrier to detachment by increasing electrostatic (Fig. S1) and stericinteractions (And and Sticher, 1997; Flynn et al., 2012; Yang et al.,2014). Conversely, alteration of surface roughness properties by
SDBS adsorption can decrease the depths of both primary andsecondary minimum, and thereby decrease the energy barrier todetachment (Bradford et al., 2017). Diffusive release of MWCNT
from a shallow minimum on rough goethite coated sand may occur
when the energy barrier to detachment is decreased to less than afew dimensionless energy units.
The transport behavior of SDBS following the pulse applicationof MWCNT was generally consistent with previously discussed results for SDBS in sand with l ¼ 0.1 in Fig. 1 (absence of MWCNTs)
and 3 (simultaneous injection of MWCNTs and SDBS). However, thevalue of Msurf for SDBS on sand with l ¼ 0.1 was found to depend on
the presence of MWCNTs and the injection sequence. In particular,
the value of Msurf equaled 92.8% in the absence of MWCNTs (Fig. 1),
86.4% when MWCNTs and SDBS were simultaneously injected(Fig. 3), and 80% when MWCNTs and SDBS were sequentiallyinjected (Fig. 4). This decrease in Msurf corresponds to increasing
amounts of MWCNT retention in these systems (Table 1) with ahigh sorption capacity of MWCNTs for SDBS (Fig. S3).
4. Conclusions
The presence of the anionic surfactant SDBS in sewer orwastewater treatment efﬂuent will greatly inﬂuence theco-transport of MWCNTs in chemically heterogeneous sand.
Modeling results indicated that competitive blocking only played asecondary role in enhancing the transport of MWCNTs by SDBS.
Rather, SDBS adsorption onto the surfaces of quartz and goethiteminerals, and especially MWCNTs decreased their zeta potentials
and/or even reversed the charge of positively charged minerals.
This surface modiﬁcation will enhance the electrostatic and/or
steric repulsion, and alter the nanoscale surface roughness properties that strongly reduce the adhesion of MWCNTs to solid surfaces. Both of these factors will greatly enhance the transport andalter the shape of retention proﬁles for MWCNTs in chemicallyheterogeneous media in the presence of SDBS. It is logical toanticipate that surface modiﬁcation by anionic surfactants may
similarly inﬂuence the fate of other colloids and nanoparticles. This
process may potentially be exploited to deliver selected colloids or
nanoparticles to desired locations in the subsurface for remediation
purposes (e.g., zero valent iron), or inadvertently increase the riskof colloid contamination (e.g., pathogenic microorganisms). In
contrast to co-injection, sequential injection of MWCNTs and SDBS
pulses produced only a limited enhancement in the remobilizationof retained MWCNTs presumably due to a greater energy barrier for
release than retention. This implies that discharge of anionic surfactants will have a greater impact on the transport of colloids in915

the aqueous phase than on the release of retained colloids. Theseﬁndings provide new insights on the effects of anionic surfactantson the transport, retention, and release of colloids and nanoparticles in porous media, and their role in remediation scenariosand contamination risks.
Acknowledgements
Funding was provided by the National Key R&D Program ofChina (Project No. SQ2018YFD080023), the 111 Project (Project No.
B18060), the National Natural Science Foundation of China (ProjectNo. 41701547), and China Postdoctoral Science Foundation (ProjectNo. 2017M612806 and 2018T110909). The authors would like to€ ppchen for bromide measurement.
acknowledge Stephan Ko
Thanks to Herbert Philipp and Claudia Walraf for their technicalassistance.
Appendix A. Supplementary data
Supplementary data to this article can be found online athttps://doi.org/10.1016/j.envpol.2019.01.106.
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