Transport and mobilization of multiwall carbon nanotubes in quartz sand under varying saturation

ORIGINAL ARTICLE

Transport and mobilization of multiwall carbon nanotubesin quartz sand under varying saturation

Abenezer Mekonen • Prabhakar Sharma •

Fritjof Fagerlund

Received: 26 March 2013 / Accepted: 27 August 2013 / Published online: 19 September 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract In this study, a series of sand packed columns

were used to investigate the mobility of multiwall carbon

nanotubes (MWCNTs) in unsaturated porous media under

unfavorable conditions for deposition. The flow through

column experiments were designed to assess water content,

flow rate, and grain size effect on the mobility of

MWCNTs. It was found that variation in water content had

no significant effect on retention of MWCNTs until it was

lowered to 16 % effective saturation. Thick water films,

high flow rate, and repulsive forces between MWCNTs and

porous media made MWCNTs highly mobile. Different

porous media grain sizes (D50 = 150–300 lm) were used

in this study. The mobility of MWCNTs slightly decreased

in finer grain sands, which was deemed to be an effect of

increase in surface area and the number of depositional

sites, in combination with low-pore water velocity. How-

ever, physical straining was not observed in selected fine-

grain sands and aspect ratio of MWCNTs had low impact

on mobility. Variations in pore-water velocity were pro-

duced by both changes in water saturation and in flow rate.

At high pore-water velocities, the MWCNTs were gener-

ally mobile. However, for the combination of low-pore

water velocity with either low water saturation or small

grain size, some retention of MWCNTs was observed.

Hence, low velocity in combination with flow through

smaller pores increased MWCNT deposition.

Keywords MWCNTs � Mobility � Unsaturated

porous media � Water content � Flow rate

Introduction

Carbon nanotubes (CNTs) are cylindrical tubes of graphene

sheet made from carbon atoms arranged in hexagonal rings

(Monthioux et al. 2007; Petersen et al. 2011a). Single wall

carbon nanotubes (SWCNTs) and multiwall carbon nano-

tubes (MWCNTs) are the main types of CNTs. They have

structure-dependent chemical, mechanical, electrical,

optical, thermal properties and possess unique size (Mauter

and Elimelech 2008). Due to the unique properties of

CNTs, they are widely used in various fields such as,

energy conversion, energy storage, wastewater treatment,

environmental monitoring, green nano-composite design,

and nano-medicine (Flahaut 2011; Tan et al. 2012). As a

result, the use and production of CNTs are continuously

increasing. This can also increase the entrance of CNTs

into the environment either ‘‘accidentally’’ or ‘‘deliber-

ately’’ due to improper disposal during production, usage,

and transport (Klaine et al. 2008; Auffan et al. 2009; Pet-

ersen et al. 2011a). CNTs in the environment have high

potential to contaminate soil, enter into surface water,

groundwater and food chains of living organisms (Klaine

et al. 2008; Perez et al. 2009). In various fields such as

biomedical, CNT surfaces are modified to increase their

stability in solution (Shen et al. 2009) or to change surface

charges, which can affect their behavior in the environment

(Petersen et al. 2011b). A number of studies have been

conducted to assess the risk of CNTs and other carbon-

based nanomaterials on living organisms (such as E. coli)

and ecosystems (Petersen et al. 2011b; Wang et al. 2012).

Though these studies reveal the toxic effects of CNTs to

Electronic supplementary material The online version of thisarticle (doi:10.1007/s12665-013-2769-1) contains supplementarymaterial, which is available to authorized users.

A. Mekonen � P. Sharma (&) � F. Fagerlund

Department of Earth Sciences, Uppsala University,

Villavagan 16, 75236 Uppsala, Sweden

e-mail: [email protected]

123

Environ Earth Sci (2014) 71:3751–3760

DOI 10.1007/s12665-013-2769-1

human cells (Monteiro-Riviere et al. 2005), aquatic

organisms (Kennedy et al. 2008) and reproduction of

earthworms (Scott-Fordsmand et al. 2008), there is cur-

rently no general agreement about toxicity of CNTs

towards living organisms due to variation of CNTs in

shape, size, and composition (Flahaut 2011). However, the

capacity of CNTs to adsorb divalent metals (Rao et al.

2007) and organic chemicals (Pan and Xing 2008) shows

their potential for environmental applications such as waste

water treatment.

To assess the impact of CNTs on ecosystems and human

health, understanding their toxicity and transport behavior

in soil is crucial (Cullen et al. 2010). Several column

experiments have been conducted to investigate the

mobility of CNTs in saturated porous media. According to

those recent studies, surface properties of CNTs (Wang

et al. 2008; Jaisi et al. 2008; Liu et al. 2009; Tian et al.

2011, Tian et al. 2012b) and porous media (Jaisi and

Elimelech 2009; Mattison et al. 2011; Tian et al. 2012a)

control their mobility and deposition. In addition, physical

straining has also been suggested as a mechanism which

can govern the mobility and deposition of CNTs in satu-

rated porous media (Jaisi et al. 2008; Jaisi and Elimelech

2009). However, physical straining was not reported in

several other studies on CNTs mobility (Liu et al. 2009;

Tian et al. 2011, Tian et al. 2012a; Mattison et al. 2011).

Even though a number of studies have been conducted, the

different factors which govern the transport and fate of

CNTs in saturated porous media are not yet fully under-

stood and require further investigation (Jaisi and Elimelech

2009).

Understanding the mobility and fate of CNTs in unsat-

urated soil is crucial to determine the risk they impose on

ecosystems, and improve their environmental application.

Although there are a number of studies focused on the fate

and transport of relatively spherical particles in the unsat-

urated zone (e.g., Shang et al. 2008; Sharma et al. 2008a;

Torkzaban et al. 2008; Tiraferri et al. 2011), much fewer

studies have been made on the transport of cylindrical

particles (such as CNTs) in unsaturated zone (Tian et al.

2012b). It has been suggested that studies on nanomaterials

can be based on existing knowledge of the behavior and

toxicology of colloids (Klaine et al. 2008). Although col-

loids (i.e., bigger size and spherical shape) differ from

CNTs, knowledge of their transport and fate can be used as

a reference to study fate and mobility of CNTs in unsatu-

rated porous media. Studies on colloid transport in unsat-

urated porous media have found that deposition/attachment

to solid–water interfaces (SWI) (Schijven and Hassani-

zadeh 2000), air–water interfaces (AWI) (Wan and Wilson

1994a; Sharma et al. 2008a; b), air–water–solid interfaces

(AWSI) (Gao et al. 2008), physical straining (Bradford

et al. 2007; Torkzaban et al. 2008) and film straining (Wan

and Tokunaga 1997) are the possible retention mechanisms

for colloids in unsaturated porous media. These studies

have shown that the mobility and fate of colloids in

unsaturated porous media can be complicated by additional

factors and are different from saturated porous media due

to the existence of an air phase in the porous medium.

However, specific studies on MWCNTs in unsaturated

porous media are lacking. Therefore, the aim of this study

was to investigate how different physical and chemical

factors affect the mobility of cylindrical nanoparticles (e.g.,

MWCNTs) in unsaturated porous media. One-dimensional

column experiments were designed to specifically assess

the impact of moisture content variation, porous media

grain size and flow rate on the mobility and retention of

MWCNTs. In addition, aspect ratio and functionalization

effect on the mobility of MWCNTs were investigated.

Experiments were conducted under unfavorable conditions

for deposition of MWCNTs and using clean quartz sand.

Materials and methods

Porous media

Quartz sands used in this study were obtained from Sibelco

Nordic, Baskarp, Sweden and sieved into three size dis-

tributions: fine (125–177 lm), medium (177–250 lm), and

coarse (250–350 lm) to achieve uniform grain size distri-

bution based on their supplied particle size analysis. The

quartz sand was washed sequentially by hydrochloric acid

(0.1 M) and hydrogen peroxide (5 %) to remove any

impurities from the surfaces of the sand (Mattison et al.

2011). The sand was rinsed at least five times with

deionized water to obtain neutral pH and remove any

remaining impurities. Then, the sand was oven dried

(105 �C) for 24 h and stored. The bulk densities of the

sieved sands were in the range of 1.48–1.58 g/cm3, and

porosities were in the range of 0.40–0.44 (see Table 1).

Soil moisture characteristics of packed sands were deter-

mined using the capillary fringe method, and hydraulic

conductivity was determined by the falling head method

(Dane and Hopmans 2002; Sharma et al. 2008c). The zeta

potential of the sand was -53.4 mV at pH 7 (SurPASS

Electrokinetic Analyzer, Anton Paar). The surface area of

the sand was calculated by assuming an average spherical

diameter of packed sand grains in the column as shown in

Table 1.

Multiwall carbon nanotubes

The MWCNTs, used in this study, were obtained from

Cheap Tubes Inc. The diameter and length of MWCNT

3752 Environ Earth Sci (2014) 71:3751–3760

123

used in most of these experiments were 20–30 nm and

0.5–2 lm, respectively. MWCNTs of diameter 30–50 nm

and length 10–20 lm were also used for comparison. To

increase the hydrophilicity and stability of MWCNTs in

aqueous solution, MWCNTs were functionalized using a

3:1 (v/v) ratio of sulfuric acids (97 % purity) and nitric

acids (70 % purity) (Mattison et al. 2011). The MWCNTs

were then filtered and washed from the acid mixture.

The MWCNT solution was prepared by adding 10 mg

of MWCNTs in 1 liter of 0.1-mM background solution.

The background solution was prepared with pH 7 (adjusted

using 0.1-M HCl and 0.1-M NaOH solution) and ionic

strength of 0.1 mM (using 0.1 mM NaCl) to produce

unfavorable conditions for the deposition of MWCNTs. A

mono-dispersed and stable MWCNT solution was prepared

for the column experiments using ultra-probe sonication

(ultrasonic homogenizer, Biologics Inc. Model 3000) for

30 min of sonication (at 40 % of 300 W power), which

provided stable solution for more than double the time

period of our experiments. The zeta potential of the func-

tionalized and less functionalized MWCNTs was -45.42

and -47.36 mV, respectively, at the chosen solution

chemistry (Zeta-sizer, Malvern Instruments). Tracer

experiments were conducted using a conservative tracer

(Brilliant blue FCF, Dr. Ehrenstorfer GmbH).

Experimental setup

Quartz sands were packed in a custom made plexiglass

column of 5-cm diameter and 10-cm length. A sprinkler

made of 12 needles (22 gauges, 0.16 mm inner diameter)

and a water pump (IPC8, Ismatic, Peristaltic pump) were

used to supply inflow solutions from the top of the unsat-

urated column (similar to Sharma et al. 2008b). At the

outlet of the column, a 25-lm membrane filter of 55-cm

bubbling pressure (Nylon net filter, Merck KGaA,

Darmstadt, Germany) was used to support the sand and

create an appropriate suction (Fig. 1).

Before the MWCNTs experiments, tracer experiments

were conducted to determine the hydraulic conditions in

the column at different water contents, flow rates, and sand

sizes. The effluent samples from the column were collected

by a fraction collector (CF-2, Spectrum Labs), and the

absorbance of the samples was measured by a

spectrophotometer (DR 5000, Hach Lange AB) at a

wavelength of 400 nm for MWCNTs and 300 nm for the

tracer.

A series of column experiments were conducted under

unfavorable condition (pH = 7 and 0.1-mM ionic strength)

for deposition. Prior to packing the sand, suction was

created at chosen height to achieve the desired water sat-

uration in the column. Following this, the packed sand was

flushed with 150 ml (3–5 pore volume) of background

solution before MWCNT solution was injected into the

column. Then the sands were flushed with 150 ml of

MWCNT solution (phase I), background solution (phase

II), and deionized water (phase III), respectively, at a

chosen flow rate.

All the different experiments were repeated two times.

Eight column experiments (Experiments 1–4) were con-

ducted at four different saturation conditions (16–100 %)

using coarse sand (D50 = 300 lm) and at the same flow

rate (2.5 ml min-1) to assess the effect of porous media

water content on mobility of MWCNTs (Table 2). In

addition, flow rate effects were investigated by testing three

different flow rates (Experiments 3, 5, and 6) at 25-cm

suction (31 ± 5 % effective saturation) and using coarse

sand (D50 = 300 lm). To compare the mobility of

Fig. 1 Experimental setup of column studies

Table 1 Hydraulic conductivity, porosity, and surface area of sands used in the column experiments

Sand average diameter (lm) Porosity (-) Bulk density (g/cm3) Hydraulic conductivity (cm/sec) Total surface area (cm2)

300 (Coarse) 0.4 1.58 0.012 23,560

211 (Medium) 0.44 1.48 0.0077 31,265

150 (Fine) 0.44 1.48 0.0056 43,980

Surface area of sand calculated by assuming all sands are the same in size and spherical in shape

Environ Earth Sci (2014) 71:3751–3760 3753

123

MWCNTs in different sand sizes, six column experiments

using three different grain size distributions (Experiments

2, 7, and 8) were conducted at constant water content

(70 ± 2 %) and using the same flow rate (2.5 ml min-1).

Finally, low moisture content (17 ± 1 %), high flow rate

(2.5 ml min-1), and coarse sand (D50 = 300 lm) were

chosen to compare MWCNTs of different aspect ratios

(Experiments 4 and 9) and functionalization impact on

mobility of MWCNTs (Experiments 4 and 10) (see Table 2

for a summary of experimental conditions).

Water flow modeling

HYDRUS-1D was used to simulate one-dimensional ver-

tical water flow through unsaturated porous media (Simu-

nek et al. 2008; Elmi et al. 2012). A constant head upper

boundary condition and a suction-controlled lower

boundary condition were applied for modeling water flow

across the column. The simulations were done for all flow

rates, sand sizes, and suction heads used in the column

experiments. Water flow parameters used for simulations

were determined using the RETC-code (Van Genuchten

et al. 1991) and the simulation ran until steady-state flows

were reached.

Both experimental and simulation results showed that

the water content of the porous media decreased as the

applied suction increased (Fig. 2a). For all experimental

conditions, the top of the column was more saturated than

the bottom except for the lowest flow rate at the top with

10-cm suction at the bottom of the column (Pfletschinger

et al. 2012). The average water content determined by

HYDRUS-1D was different from measured values except

for experiments conducted at no suction (Table 2). Water

flow simulations in finer sands at a given suction and flow

rate showed that the moisture distribution in both sands

was approximately similar. At the top of the column, the

finer sand (D50 = 150 lm) was more saturated, whereas at

the bottom it was less saturated than the medium sand

(D50 = 211 lm) (as shown in Fig. 2b).

DLVO energy calculation

Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory

was used to calculate interaction energy between

MWCNTs and porous media. Originally, DLVO theory

was established to calculate the interaction energies of

spherical particles. Since the majority of nanoparticles

(NPs) are non-spherical (e.g., MWCNTs are cylindrical),

change in shape can affect the total interaction force

between nanoparticles and porous media (Phenrat et al.

2010). To calculate the interaction energy between

MWCNTs and porous media, either length or diameter has

been used as an effective size in the recent studies (Liu

et al. 2009; Tian et al. 2011, 2012a, b). In this study, both

length and diameter of MWCNTs were used as effective

sizes to calculate a range for the interaction energy. In this

calculation, the zeta potential of sand was -53.4 mV, and

the Hamaker constant between sand and MWCNT was

9.80 9 10-21 and the zeta potential of MWCNTs were

listed in Table 2.

The total interaction energy calculation showed that the

repulsive interaction between MWCNTs and porous media

exists when either diameter or length of MWCNTs was

used, but the primary energy barriers were higher when

length was used in the calculation and relatively low when

diameter was used (Fig. 3). This indicates that the experi-

mental conditions were unfavorable for deposition and that

deposition in primary energy minima on the sand surfaces

was not significant due to a very high primary energy

barrier. A secondary energy minimum was not observed

between the MWCNTs and porous media for both effective

sizes (length and diameter) within 60-nm separation

Table 2 Experimental conditions selected in this study

Exp Average sand

size (lm)

Suction

(-cm of H2O)

Flow rate

(mL/min)

Pore-water

velocity

(cm/min)

Water content (-) Saturation

(%)

MWCNT

n-potential

(mV)

Water-film

thicknessa

(lm)Measured Modeled

1 300 0 2.5 0.32 0.42 0.45 100 -45.42 –

2 300 10 2.5 0.45 0.284 0.43 70.06 -45.42 20.35

3 300 25 2.5 0.88 0.145 0.283 35.91 -45.42 11.217

4 300 40 2.5 1.7 0.075 0.26 18.59 -45.42 5.209

5 300 25 1.5 0.65 0.117 0.256 28.98 -45.42 8.17

6 300 25 0.5 0.23 0.109 0.21 27.72 -45.42 7.964

7 211 29 2.5 0.43 0.295 0.21 66.91 -45.42 20.34

8 150 39 2.5 0.42 0.287 0.21 68.82 -45.42 20.34

9 300 40 2.5 2.1 0.061 0.26 15.12 -47.36 5.209

10 300 40 2.5 1.88 0.069 0.26 17.01 -45.42 5.209

a Calculated by assuming sand is spherical in shape and covered uniformly to their surface area

3754 Environ Earth Sci (2014) 71:3751–3760

123

distance, indicating that deposition at secondary minima

was also not feasible. (See supporting material for DLVO

theory and interaction energy calculation).

Results and discussion

Effect of porous media water content on mobility

of MWCNTs

To assess the impact of water content on mobility of

MWCNTs, four column experiments were conducted

(Experiments 1–4). For all the different water contents,

MWCNTs were detected in the effluent after less than one

pore volume of flow, and increased quickly to make a

plateau (phase I, in Fig. 4). For all moisture conditions, the

shape of the breakthrough curves for MWCNTs was sim-

ilar, and their maximum relative concentrations (C/C0)

were close to 0.94 ± 0.02. After steady-state concentration

had been reached, the influent was switched to background

solution, as a result, C/C0 decreased to a minimum value

for all moisture conditions (phase II). Finally, to flush out

the retained (5–8 %) MWCNTs, the influent was switched

to deionized water (phase III). The fact that no significant

changes were observed in the effluent concentrations dur-

ing this phase indicates that a decrease in ionic strength had

no significant effect on retained MWCNTs in the column

(Fig. 4).

These findings indicate that MWCNTs were highly

mobile ([92 %) in unsaturated porous media, and the

presence of an air phase in porous media had minimal

impact on their transport even at the lowest water satura-

tion (16 % of saturation) tested in this study. The high

mobility of MWCNTs in unsaturated porous media can be

explained by repulsive forces between MWCNTs and

porous media and is supported by DLVO calculations

showing that surface deposition of MWCNTs was unlikely

at secondary energy minima.

However, water-film straining of MWCNTs in unsatu-

rated porous media may cause retention of MWCNTs.

According to film straining theory of colloids (Wan and

Tokunaga 1997), straining of colloids in unsaturated por-

ous media occurs only if the moisture content is lower than

a ‘‘critical saturation’’ at which the linkage between pen-

dular rings is broken, or when particle size is bigger than

the water-film thickness. In this study, all column experi-

ments were conducted in a water saturation range between

16 and 100 % (see Table 2). Although the water content

Fig. 2 HYDRUS-1D

simulation of water content

distribution in column depth for

different suctions and flow rates

in a coarse sand (300 lm) at

different suction and flow rates

b course sand at 10-cm suction,

medium sand at 29-cm suction,

and fine sand at 39-cm suction

with flow rate of 2.5 ml min-1

and 70 % average water content

Fig. 3 MWCNTs and porous

media interaction energy as a

function of separation distance

at the solution chemistry used in

this experiment: a fully

functionalized and b less

functionalized

Environ Earth Sci (2014) 71:3751–3760 3755

123

varies with depth over the column (Fig. 2), we used aver-

age water content for comparison. For all experimental

conditions, the calculated average water-film thickness

around a single sand grain was thicker than both diameter

and length of the MWCNTs except Experiment 9 (see

Table 2), as discussed in ‘‘Effect of MWCNTs size of their

mobility’’ section. The water-film thickness over the sand

surface was calculated by assuming spherical sand grains

uniformly covered with pore water. With this water-film

thickness, effects of film straining on the mobility of

MWCNTs were unlikely to be observed. The results of this

study indicate that variation of the water saturation in the

range of 16–100 % had negligible effect on retention of

MWCNTs. This is in accordance with preliminary findings

by Tian et al. (2011), indicating that retention of SWCNTs

may occur only at very low (\10 %) water saturation due

to water-film straining.

Corapcioglu and Choi (1996) reported high affinity of

hydrophilic particles toward AWIs, but Tian et al. (2011),

on the other hand, observed repulsive forces between

anionic surfactant-coated SWCNTs and AWI. In this study,

deposition of MWCNTs on AWI was not significant,

because observed retention of functionalized MWCNTs

was very low. Particle deposition/removal by moving

liquid–gas interfaces are more prominent in drying/wetting

events specially when the interface can move over initially

dried surfaces of deposited particles (Sharma et al. 2008b;

Aramrak et al. 2013), but MWCNT-contained suspensions

were replaced by particle-free solution without further

changing the total wet surface area in this study. To con-

firm this finding for MWCNTs, additional studies at much

lower moisture content (\16 %) are needed to further

investigate the effect of pore straining (Jaisi and Elimelech,

2009; Jaisi et al. 2008), AWSI attachment (Crist et al.

2005; Gao et al. 2008), and air–water interface attachment

(Chen et al. 2008).

In addition, the transport of colloids in unsaturated

(Sharma et al. 2008a; Wan and Tokunaga 1997) and

MWCNTs in saturated porous media (Liu et al. 2009) can

depend on water velocity. Since Experiments 1–4 were

conducted at the same flow rate (2.5 mL/min) but at dif-

ferent water saturations, the pore-water velocity increased

with decreasing saturation, as shown in Table 2. Higher

pore-water velocity should support the mobility of

MWCNTs. However, at the same time, lower water satu-

ration means that the water flow takes place through a

subset of smaller sized pores, since water resides in the

smallest pores in a partially saturated medium. In smaller

pores, the particle mobility may be reduced and hence, this

is a competing effect to the increased velocity at lower

water saturation. The results from Experiments 1–4 show

that the velocities are large enough to keep MWCNTs

mobile. Similarly, Liu et al. (2009) found that the mobility

of MWCNTs in saturated porous media was affected by the

pore-water velocity, and that MWCNTs were highly mobile

at pore-water velocities [4 m/day. This is consistent with

our experimental observations and also in agreement with

other colloid literature. The following experiments further

investigate the effect of velocity and pore size by varying,

first, the flow rate and then the grain size.

Effect of flow rate on mobility of MWCNTs

Three column experiments (Experiments 3, 5, and 6) were

conducted to investigate the effect of flow rate on mobility

and retention of MWCNTs in unsaturated porous media

(Fig. 5). After MWCNTs were injected into the column,

they were detected in the effluent after less than one pore

volume, which suggests that some dispersion occurred

during their movement through the column. For all flow

rates, the relative concentration in the effluent increased

quickly and reached a steady-state/maximum value (phase

I), then after the influent switched to background solution,

they decreased to a minimum value (phase II). Deionized

water was used to remobilize retained MWCNTs, but no

changes were observed in the effluent (phase III). At high

flow rate (2.5 mL min-1), the mobility of MWCNTs was

high, and the maximum/steady-state relative concentration

in the effluent was close to 0.95. For low flow rates, 1.5 and

0.5 mL min-1, retention of MWCNTs increased as flow

rate decreased, and the effluent maximum relative con-

centration decreased to 0.88 and 0.75, respectively (Fig. 5).

This shows that the mobility of MWCNTs was a function

of flow rate.

Fig. 4 MWCNTs breakthrough curves in coarse sand (300 lm) at pH

7 and ionic strength 0.1 mM for effective saturation of a 16 %,

b 36 %, c 70 %, d 100 %

3756 Environ Earth Sci (2014) 71:3751–3760

123

In this study, the variations in flow rate also resulted in

moderate changes in water content in the column, from

0.145 at 2.5 mL/min to 0.109 at 0.5 mL/min. However, the

pore-water velocity still deceased with decreasing flow rate

(as shown in Table 2). A possible explanation to the higher

retention observed for lower flow rates (e.g., Experiment 6)

in Fig. 5 is a possible combination of a low-pore water

velocity (0.23 cm/min) with a low water saturation, where

the water flow is restricted to a subset of smaller pores. It

can be noted in Experiment 1 with low-pore water velocity

(0.32 cm/min) that the medium was fully saturated and the

water flow had full access of large pores as well.

Another possible explanation to the retention and low

mobility of MWCNTs can be slow exchange between

immobile and mobile zones of the pore water occurring at

the low flow rates. Findings of Gao et al. (2006) suggest the

exchange between the zone of immobile and mobile water

under steady water flow is a mechanism that governs

mobility of colloids at low flow rates. In this study, high

retention of MWCNTs was observed at low flow rate. At

high flow rates, higher mobility of MWCNTs would be

favored by higher hydrodynamic forces increasing the drag

force on MWCNTs.

Grain size effect on mobility of MWCNTs

Three column experiments were conducted (Experiments

2, 7, and 8) to investigate the impact of sand grain size on

mobility and retention of MWCNTs in unsaturated porous

media (Fig. 6). Breakthrough curves of MWCNTs with

three types of porous media follow a similar trend on both

sides of the limb. For all three cases (D50 = 300, 211,

150 lm), MWCNTs were detected in the effluent after less

than one pore volume. In the coarse sand (50 = 300 lm),

MWCNTs were highly mobile and the maximum relative

concentration was close to 1 (*0.95), whereas for the

medium (D50 = 210 lm) and fine (D50 = 150 lm) sands,

the maximum relative concentration of MWCNTs

decreased to 0.90 and 0.86, respectively (Fig. 6).

According to Mattison et al. (2011) for water saturated

conditions, an increase in surface area and number of

depositional sites increase the retardation of MWCNTs in

fine sands. Also in the unsaturated experiments conducted

here, the surface area increased as the sand sizes were

decreased (see Table 1), which in turn increased the

number of depositional sites and adsorption capacity of the

media (Canales et al. 2013). In addition, even though all

column experiments were conducted at the same flow rate

(2.5 mL/min), pore-water velocities in the medium and fine

sands (0.42 cm/min) were slightly lower than that in coarse

sand (0.44 cm/min) because of a slight difference in

porosity. The experimental results indicated that MWCNTs

were mobile in both sand types. The difference between the

breakthrough curves indicating somewhat more deposition

of MWCNTs in the finer sands (Fig. 6) was deemed to

occur mainly due to differences in sand surface area and

number of depositional sites although a slight effect of the

lower pore-water velocity cannot be excluded.

In previous studies, physical straining (grain-to-grain

straining) has been suggested as a factor that affects

deposition and transport of CNTs (Jaisi and Elimelech

2009; Jaisi et al. 2008). Findings by Bradford et al. (2007)

indicate that colloid physical trapping increases with

increasing colloid size to grain size (dp/dg) ratio and sug-

gest that straining could occur at even low dp/dg ratio

(0.003). In another study using SWCNTs, adp/dg ratio of

0.0008 was reported as the effective diameter ratio for

straining (Jaisi et al. 2008). In this study, the calculated

MWCNT length to sand-grain-size ratio was much greater

than the critical limit, but MWCNT diameter to sand-grain-

size ratio was less than this critical limit (see Table 3).

Fig. 5 MWCNTs breakthrough curves in coarse sand (300 lm) at pH

7 and ionic strength 0.1 mM for different flow rate. Data point shows

the average value from replicate experiments

Fig. 6 Breakthrough curves comparison of MWCNTs (diame-

ter = 20–30 nm and length = 0.5–2 lm) in coarse (D50 =

300 lm), medium (D50 = 211 lm) and fine (D50 = 150 lm) sand

at pH 7, ionic strength 0.1 mM, effective saturation 70 %, and flow

rate 2.5 ml min-1. Data point shows the average value from replicate

experiments

Environ Earth Sci (2014) 71:3751–3760 3757

123

According to Auset and Keller (2006), straining effects

could be reduced if cylindrical nanoparticles can be ori-

ented parallel to the stream lines in the porous media. The

results of this study may not indicate straining because

retention of MWCNTs was only observed to a relatively

small degree with decreasing sand size. For the case of this

study, MWCNTs may be retained in both sand types if they

were oriented perpendicular to stream channel. However,

the high flow rate and also unfavorable conditions for

deposition have likely reduced the straining effect. The

results in this study suggest that a combination of low-pore

water velocity and small pore sizes available for flow

(Experiments 6 and 8) increases retention of MWCNTs.

This is in agreement with previous studies suggesting that a

combined effect of water content, DLVO, and hydrody-

namic forces influences the straining of colloids (Torkz-

aban et al. 2008), which should be further investigated for

CNTs. However, we performed two separate experiments

with different set of MWCNTs size to confirm the straining

effect of retention and two types of MWCNTs for the effect

of their interaction energies (DLVO energy) in the fol-

lowing sections.

Effect of MWCNTs size on their mobility

To further investigate the possible effects of straining on

retention, we also investigated the effect of MWCNT

aspect ratio on mobility; two different types of MWCNTs

were used in this case, and two column experiments were

conducted all together (Experiments 4 and 9). The calcu-

lated water-film thickness was approximately 5.2 lm in the

selected experiments (Table 2). Although the diameter of

MWCNTs was much smaller for both cases, but the aver-

age length of MWCNTs in Experiment 9 was three times

higher than the water-film thickness. Breakthrough curves

of both MWCNTs are similar in shape, and the maximum

relative concentration (C/C0) was varied only by 4 % from

each other (Fig. 7). The results showed that the shorter

MWCNTs were more mobile in porous media under the

experimental conditions of this study, as possible cause of

physical straining. The two types of MWCNTs only differ

by their dp/dg ratios (MWCNTs length/diameter to sand

size ratio) (Table 3). According to previous studies, aspect

ratio of CNTs and collector properties can cause physical

straining (Jaisi et al. 2008; Jaisi and Elimelech 2009).

However, the observed result shows minimal sign of

physical straining. It is anticipated that the difference in the

mobility of MWCNTs due to aspect ratio occurs if an

experiment can be conducted at very low moisture condi-

tions and/or in very fine porous media.

Effect of MWCNT functionalization on mobility

To investigate the effect of functionalization of MWCNTs

on their mobility, two experiments were conducted

(Experiments 4 and 10) using both fully functionalized and

less functionalized (as received from company) MWCNTs

of 20–30 nm diameter and 0.5–2 lm length. The maximum

relative concentration (C/C0) of fully functionalized

MWCNTs was approximately 0.95, while that of less

functionalized MWCNTs was only 0.65 (Fig. 8). The less

functionalized MWCNTs were detected in the effluent later

than the fully functionalized MWCNTs and their break-

through curve is also distorted. This indicates that the fully

functionalized MWCNTs were highly mobile, whereas the

less functionalized MWCNTs were less mobile and

retained in the column.

Since these studies were conducted under the same

conditions and their DLVO profile did not show significant

difference (Fig. 3), the observed differences were attrib-

uted to the functionalization and particularly the addition of

carboxyl (-COOH) groups to the surfaces of the MWCNTs

Table 3 Ratio of MWCNTs

length and diameter with

different types of porous media

Average diameter and length

used to calculate MWCNTs to

sand size ratio

Sand types MWCNT (D = 25 nm and L = 1.25 lm) MWCNT (D = 40 nm and L = 15 lm)

Diameter/sand size Length/sand size Diameter/sand size Length/sand size

Coarse (300 lm) 8.3 9 10-5 41.6 9 10-4 1.3 9 10-4 0.05

Medium (211 lm) 1.2 9 10-4 59.2 9 10-4 1.9 9 10-4 0.07

Fine (150 lm) 1.6 9 10-4 83.3 9 10-4 2.6 9 10-4 0.1

Fig. 7 Different size of MWCNTs in coarse sand (D50: 300 lm), pH

7, ionic strength 0.1 mM, effective saturation 16 %, and flow rate

2.5 ml min-1. Data point shows the average value from replicate

experiments

3758 Environ Earth Sci (2014) 71:3751–3760

123

increasing their hydrophilicity. It has been found that the

addition of surfactants to NP’s surfaces stabilizes their

suspension and reduces aggregation (Phenrat et al. 2010).

According to other studies, aggregation, due to the absence

of energy barriers between the particles, increases the size

and deposition rate of NP aggregates and contributes to the

occurrence of physical straining (Crist et al. 2005; Jaisi

et al. 2008; Jaisi and Elimelech 2009; Phenrat et al. 2010).

Therefore, aggregation is likely to have increased the

deposition and retention of the less functionalized (more

hydrophobic) MWCNTs in this study. According to the

previous studies, retention of hydrophobic colloids in

porous media is higher than that of hydrophilic colloids,

and solid–water and AWSI are suggested as the main site

of desposition (Corapcioglu and Choi 1996; Crist et al.

2005; Wan and Wilson 1994b). In addition, the retention of

colloids increases with surface hydrophobicity (Wan and

Wilson 1994a) which agrees with our experimental

observations, since less functionalized MWCNTs are more

hydrophobic compared to (fully) functionalized MWCNTs.

Conclusions

This study demonstrates that the functionalized MWCNTs

were highly mobile in unsaturated porous media under the

investigated experimental conditions, and the variation in

water saturation had negligible effect on their mobility and

retention for the range of investigated water saturation

(16–100 %). The high mobility of MWCNTs was favored

by thick water-film coating over the sand grains, electro-

chemically unfavorable conditions for deposition, and high

flow rate under which these experiments were conducted.

The results also suggest that the mobility and retention of

MWCNTs can vary with pore-water velocity. Variations in

pore-water velocity were produced by both variation of the

flow rate and variation of the water saturation at a given

flow rate. At high pore-water velocities, the MWCNTs

were highly mobile, and their retention was very low,

whereas at low velocities in combination with either low

water saturation or small grain size, the retention of

MWCNTs was higher. It is possible that MWCNTs

deposited in immobile pore-water zones, and slow

exchange between mobile and immobile zones makes

MWCNTs less mobile at slow flow rates. This study also

showed that MWCNTs were mobile in both the coarse and

fine sands used in this study. However, an increase in

surface area and the number of depositional sites made

MWCNTs less mobile in finer sands. Effects of physical

straining were not observed in fine sands size used in this

study and MWCNTs were highly mobile in unsaturated

porous media regardless of their aspect ratio. This study is

aimed at improving understanding of the transport mech-

anisms for MWCNTs in the vadose zone after their release

into the environment. The study presents an evidence that,

functionalized MWCNTs can be fully mobile in the

unsaturated zone, and thereby can be transported with

infiltrating water to the saturated zone contaminating soil

and groundwater resources.

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