The role of photochemical transformations in the aggregation and deposition of carboxylated multiwall carbon nanotubes suspended in water

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The role of photochemical transformations in the aggregation and deposition of carboxylated multiwall carbon nanotubes suspended in water Yu Sik Hwang a, Xiaolei Qu b, Qilin Li a b

b,*

Future Environmental Research Center, Korea Institute of Toxicology, Jinju 660-844, South Korea Department of Civil and Environmental Engineering, Rice University, 6100 Main Street MS-519, Houston, TX 77005, United States

A R T I C L E I N F O

A B S T R A C T

Article history:

The photochemical transformation of carboxylated multiwall carbon nanotubes (COOH–

Received 2 August 2012

MWCNTs) in water and the subsequent impact on their aggregation and deposition behav-

Accepted 7 December 2012

iors were examined. The photochemical transformation of COOH–MWCNTs was investi-

Available online 19 December 2012

gated under UVA (300–400 nm) irradiation, the main component of UV light in solar irradiation. Time-resolved dynamic light scattering measurement and quartz crystal microbalance with dissipation monitoring were used to study the initial aggregation and deposition kinetics. Characterization of the physicochemical properties of the COOH– MWCNTs before and after irradiation revealed a loss in surface oxygen after UV irradiation, most likely a result of decarboxylation of the nanotube surface. These changes in surface chemistry greatly reduced the colloidal stability of COOH–MWCNTs in NaCl solutions. No noticeable changes in particle surface zeta potential and stability were observed in CaCl2 solutions after irradiation. Consistent with the decreased colloidal stability in NaCl solutions, the irradiated COOH–MWCNTs had a notably higher deposition than the initial COOH–MWCNTs in NaCl solutions when aggregation did not occur. Our results suggest that the photochemical transformation plays an important role in the transport of carbon nanotubes in natural aquatic systems.  2012 Elsevier Ltd. All rights reserved.

1.

Introduction

Since Iijima’s landmark paper [1] on carbon nanotubes (CNTs), they have attracted much attention because of their unique electronic, thermal, optical and mechanical properties. The use of CNTs in commercial applications is now wide spread including nano-electric devices, composite materials, and automotive parts and their potential applications in many areas are expected to increase exponentially in the foreseeable future [2]. As use of CNTs become increasingly common, human and the ecosystem can be potentially

exposed to CNTs through various pathways during manufacturing, transport, use, disposal and recycle of CNTs or CNTs containing products [3]. Increasing studies have showed that they are not only toxic to organisms and human being [4,5], but also influence the behavior of contaminants such as heavy metals and nonpolar organic compounds [6,7], which raises concerns over their fate and transport in natural aquatic environment [8]. To better quantify the potential risk to human health and the environment requires understanding processes such as aggregation and deposition involved in the fate and transport

* Corresponding author: Fax: +1 713 348 2026. E-mail address: [email protected] (Q. Li). 0008-6223/$ - see front matter  2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.12.012

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of nanoparticles. In general, it is understood that aggregation and deposition behaviors of colloidal particles depend on particle physicochemical properties (e.g., size, shape, surface charge, and coating), solution chemistry (e.g., pH, ionic strength, electrolyte composition and interacting organic matter), and hydrodynamic conditions (e.g., flow velocity) [9,10]. Several studies have investigated the aggregation and deposition behaviors of CNTs in water. Saleh et al. [11,12] examined the influence of solution chemistry on the aggregation kinetics of single-wall and multiwall CNTs (SWCNTs and MWCNTs, respectively) dispersed by successive sonication. Smith et al. [13,14] systematically studied the effects of surface oxidation on MWCNT colloidal stability using a suite of MWCNTs with different surface densities of oxygen-containing functional groups in monovalent electrolyte (NaCl) solutions. Yi and Chen [15] investigated the influence of surface oxidation on the aggregation and deposition kinetics of MWCNTs in the presence of monovalent and divalent electrolytes. These studies showed that the aggregation and deposition of CNTs were controlled by electric double layer repulsion and van der Waals attraction in a manner consistent with the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory and the degree of surface oxidation was a strong determinant of CNT colloidal stability [11–15]. Also, the dependency of CNT aggregation and deposition on surface oxidation was shown to vary with the electrolyte composition. The surface oxidation of CNTs used in these studies was either a result from sample preparation (e.g., sonication) or intentional surface modification. However, when CNTs enter aquatic environments, additional surface modification and/or chemical transformation may occur due to interactions with various environmental components such as sunlight and dissolved oxygen. Recognizing the impact of such natural transformation on CNT aggregation and deposition is critical to accurately predict their fate and transport in the environment. Several studies have shown evidence of photochemical reactions of carbon nanomaterials in the aqueous phase. C60 fullerene nanoparticles (nC60) were found to undergo photochemical oxidization in the presence of dissolved oxygen under sunlight or UV irradiation [16–18]; the surface oxidation was found to greatly influence nC60 aggregation and deposition in the aqueous phase [19,20]. More recently, Chen and Jafvert [21,22] reported that functionalized SWCNTs produced three reactive oxygen species (ROS) including singlet oxygen upon sunlight and UVA lamp irradiation and suggested that potential photo-transformation of CNTs may occur. The objective of the study reported here is to demonstrate the photochemical transformation of CNTs upon sunlight irradiation and determine its impact on physicochemical properties of CNTs as well as the subsequent influence on CNT aggregation and deposition. The photochemical transformation of carboxylated multiwall carbon nanotubes (COOH– MWCNTs) in water was investigated under UVA (300– 400 nm) irradiation, the main component of UV light in solar irradiation. Time-resolved dynamic light scattering (DLS) measurement and quartz crystal microbalance with dissipation monitoring (QCM-D) were used to characterize the aggregation and deposition kinetics of COOH–MWCNTs before and after irradiation. Our results showed decreases of the oxygen amount on the surface of the COOH–MWCNTs after UV

irradiation. Furthermore, such photochemical transformation was found to have a significant effect on the aggregation and deposition behaviors of the COOH–MWCNTs.

2.

Experimental section

2.1.

Materials

COOH–MWCNTs (purity greater than 95%) were purchased from NanoLab, Inc. (Newton, MA) and used without further purification. COOH–MWCNTs were chosen because they easily disperse in water and are used in a number of commercial applications [14]. Based on the information provided by the vender, the COOH–MWCNTs were prepared by refluxing pristine MWCNTs synthesized by a conventional chemical vapor deposition (CVD) method in a concentrated HNO3/H2SO4 mixture. The vender-reported outer diameter and length of the nanotubes were 15 ± 5 nm and 1–5 lm, respectively. Reagent-grade NaCl and CaCl2 were obtained from Aldrich Chemicals (Milwaukee, Wisconsin). All other chemicals were reagent grade with purity greater than 99%. All aqueous samples were prepared using double deionized water (18.2 MO cm resistivity at 25 C) produced by a Barnstead Epure water purification system (Dubuque, IA).

2.2.

Aqueous COOH-MWCNT suspension preparation

The aqueous COOH–MWCNT stock suspension was prepared by direct sonication. An aliquot of 5 mg COOH–MWCNT was added to 100 mL ultrapure water in a 200 mL glass beaker. The mixture was put in an ice bath and sonicated with a sonicating probe (Vibra-Cell VCX 500, Sonics & Material, Newtown, CT) for 30 min (power input of 50 W for 10 min, 75 W for 10 min, and 100 W for 10 min). A stable dispersion was obtained after sonication and was stored in dark at 4 C until use. All experiments were performed using the same stock suspension within 3 months after preparation of the stock. Dynamic light scattering measurements showed no change in hydrodynamic diameter of the stock suspension within this period.

2.3.

Irradiation experiments

Irradiation experiments were carried out in a Luzchem LZC4V Photoreactor (Luzchem, Ottawa, Ontario, Canada). Four 8 W black lamps (Hitachi FL8BL-B) with an emitting spectrum in the UVA range (350 ± 50 nm) was employed to mimic the UVA fraction of the solar spectrum. The light intensity at the center of the reactor where the sample suspension was located was measured at approximately 2 mW/cm2 using a Radiometer (Control Company, Friendswood, TX). This is comparable to the UVA intensity of sunlight measured at the ground level on a sunny summer day in Houston, Texas. In each experiment, two Erlenmeyer flasks each containing 50 mL of sample were placed on the merry-go-round at the center of the photoreactor and rotated to ensure uniform exposure. All suspensions contained 4 mg/L COOH–MWCNT with unadjusted pH at 6.0 ± 0.2. No detectable changes in pH were observed during the irradiation experiments. A ventilation system circulated ambient air through the reactor

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to maintain air-equilibrated aerobic condition at 25 ± 2 C. As the photochemical reaction proceeded, samples were withdrawn at predetermined times and stored in dark at 4 C for further analyses and aggregation and deposition kinetic experiments. Singlet oxygen (1O2) production was measured to verify that COOH–MWCNTs produce reactive oxygen species (ROS) upon irradiation. For 1O2 production measurement, the suspensions containing 4 mg/L COOH–MWCNT and 0.2 mM furfuryl alcohol (FFA) as a scavenger for 1O2 [23] were buffered at pH 5, 6.9, and 8.5 by adding phosphate buffers (adjusted by changing the relative molar ratios of NaH2PO4 and Na2HPO4) to a total phosphate concentration of 10 mM. As the reaction proceeded, sample aliquots of 1 mL were withdrawn at predetermined times using a syringe, filtered through 0.22 lm pore size PTFE filters (Millipore, Billerica, MA). The remaining FFA was measured at 230 nm using a high-performance liquid chromatography (Waters 2695, Milford, MA) equipped with a C-18 column (Nova-Pak C18) and a photodiode-array detector (Waters 996, Milford, MA). The HPLC was operated using a mobile phase consisting 40% methanol and 60% water at a flow rate of 0.5 mL/min. The temperature was 25 C and sample injection volume was 30 lL.

2.4.

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Particle characterization

The initial and irradiated COOH–MWCNTs were thoroughly characterized for particle size, morphology, electrophoretic mobility, UV/Vis absorbance, Raman spectrum and surface chemistry. The hydrodynamic diameters of the initial and irradiated COOH-MWCNT were measured at 23 C by dynamic light scattering (DLS) using a Zen 3600 Zetasizer Nano (Malvern, Worcestershire, UK) equipped with a monochromatic coherent He–Ne laser with a fixed wavelength of 633 nm. The intensity of scattered light was measured at 173. Each sample was measured 6 times consecutively for the hydrodynamic diameter. The electrophoretic mobility of nanotubes was measured in folded capillary cells (Malvern, Worcestershire, UK) by phase analysis light scattering (PALS) at 23 C using the same instrument. Ten measurements were carried out for each sample. The morphology and size of the nanotubes were characterized using a JEOL-2100 field emission gun transmission electron microscope (TEM) (JEOL Inc., Peabody, MA). Samples for TEM analysis were prepared by depositing 3 lL sample suspension on a 400-mesh carbon support film on copper grid (Ted Pella, Redding, CA) and drying the sample at room temperature overnight. UV/Vis absorbance spectra of the nanotube suspensions were obtained using a dual beam, high resolution UV/Vis spectrophotometer (UV-2550, Shimadzu Scientific Instruments, Columbia, MD). The Raman spectra were acquired using a RENISHAW in Via Raman microscope (RENISHAW, Hoffman Estates, IL) with a 514 nm laser focused through a ·50 objective lens. Each sample was scanned 10 times at 50% energy output. The surface chemistry of the nanotubes before and after irradiation was characterized using X-ray photoelectron spectroscopy (XPS). The XPS analyses were carried out using a PHI Quanteria SXM scanning Xray microprobe ULVAC-PHI with an Almono, 24.8 W X-ray source and a 200.0 lm X-ray spot size at 45.0 (26.00 eV).

2.5.

Aggregation kinetics

Aggregation kinetics of the initial and irradiated COOH– MWCNT suspensions in various solution conditions were characterized by monitoring the intensity weighted average particle hydrodynamic diameter (Dh) as a function of time via time-resolved DLS measurement. For all experiments, 0.25 mL of the COOH–MWCNT suspension was transferred to a disposable polystyrene cuvette (Sarstedt, Germany) and the appropriate amount of stock NaCl or CaCl2 solution and DI water were added to achieve the desired electrolyte concentration. The addition of NaCl and CaCl2 had negligible impact on the pH of the suspensions. The total volume of the final suspension was 1 mL and the concentration of COOH– MWCNT suspensions was 1.0 mg/L. The solution was briefly shaken and quickly inserted into the vat of the light scattering unit. The DLS measurement was initiated immediately. Autocorrelation function was accumulated for every 15 s. The initial aggregation rate constant (k) of the COOH– MWCNT suspensions was obtained by fitting the initial linear increase in Dh [24,25] using Eq. (1):   dDh ðtÞ / kN0 ð1Þ dt t!0 where which N0 is the initial number concentration. In our experiments, all samples have the same particle concentration. For most cases, linear least square regression was applied to the Dh(t) data between the initial value Dh(0) to approximately 1.5 Dh(0). However, at low electrolyte concentrations, the aggregation happened very slowly; the regression analysis was conducted to less than 1.5 Dh(0) but over more than 20 min. To quantify the aggregation kinetics, the attachment efficiency (a, the inverse of the stability ratio W) was calculated by normalizing the initial aggregation rate in Dh at the electrolyte concentration of interest by the initial rate of increase in Dh under diffusion-limited conditions, where the rate is independent of electrolyte concentration [2]:   dDh ðtÞ dt 1 k  t!0 ¼ a¼ ¼ ð2Þ dDh ðtÞ W kfast dt

t!0:fast

Although the DLS size measurement and the above equations assume spherical particles, the results should be valid for comparative studies of the same type of particles. Several studies have shown that this approach could be applied to characterize aggregation behavior of CNTs [11–15].

2.6.

Deposition kinetics

Deposition kinetics of the COOH–MWCNTs onto silica surface were studied using QCM-D (Q-Sense E4, Va¨stra Fro¨lunda, Sweden). The QCM-D unit is equipped with four measurement chambers, each holds a 5 MHz AT-cut silica coated quartz crystal sensor. Before use, the silica crystals were soaked in 2% SDS cleaning solution for 12 h and sonicated in a sonication bath for 15 min. They were then rinsed with deionized water, dried with ultra high-purity N2 and cleaned in an ozone/UV chamber (Bioforce Nanoscience, Inc., Ames, IA) for 30 min. Clean crystals were mounted in the measurement chambers with a constant temperature of 20 C. The flow rate

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was set at 0.1 mL/min which results in a laminar flow in the measurement chambers. Before each measurement, the background solution was flowed across the crystal surface until the frequency signal drifts less than 1.8 Hz/h. After a stable baseline was achieved, predetermined amount of electrolyte solution and COOH–MWCNTs suspension were mixed to form samples with 2 mg/L COOH–MWCNT in the electrolyte solution of interest. The resulting suspension was immediately pumped through the measurement chambers, allowing COOH–MWCNT deposition onto the silica crystal surface to be monitored. As the deposition took place, the increasing mass of COOH–MWCNTs on the silica crystal surface hindered the vibration of the crystal and subsequently reduced its frequency. The mass of a thin and rigid deposited layer on the silica crystal surface can be related to the frequency shift as described by the Sauerbrey equation [26]: Dm ¼ 

CDfn n

ð3Þ

where Dm is the deposited mass, Dfn is the frequency shift at overtone n, and C is the crystal constant (17.7 ng/(Hz cm2)). Although the Squerbrey equation does not strictly apply for MWCNT deposition, the frequency shift was found to be proportional to the mass of MWCNTs deposited on the crystal [15]. Thus the deposition rate was quantified using the initial slope (200 s) of the frequency shift at the third overtone, which is the most stable. All deposition experiments were performed in duplicate.

3.

Result and discussion

3.1.

Photochemical production of 1O2

Although COOH-SWCNTs have been shown to produce ROS under UVA irradiation [21], there has been no previous report on the capability of MWCNTs to photochemically generate ROS in the aqueous phase. Fig. 1 shows the 1O2 production in a 4 mg/L COOH–MWCNT suspension under UVA

Remaining FFA concentration (C/C0)

1.2 1.0 0.8 0.6 0.4

pH 5.05 pH 6.90 pH 8.50 Dark control

0.2 0.0 0

20

40

60

80

100

120

Irradiation time (hrs) Fig. 1 – FFA (furfuryl alcohol) degradation indicating 1O2 production by COOH–MWCNTs during irradiation.

irradiation, as reflected by the degradation of FFA. The degradation of FFA was found to follow pseudo-first-order kinetics. Negligible FFA degradation was observed in the control experiments performed in dark or with UVA irradiation under anoxic conditions (data not shown), suggesting the indispensible role of UVA light and dissolved oxygen in the generation of 1O2. The COOH–MWCNTs generated 1O2 at all pHs tested in this study (5.1, 6.9 and 8.5). The 1O2 production rate increased with increasing pH, consistent with the result of a study using COOH-SWCNTs [21], although the increase was smaller at higher pH.

3.2.

Characterization of COOH-MWCNT particles

Fig. S1 illustrates the UV/Vis absorbance spectrum of the COOH-MWCNT suspensions as a function of concentration. The absorbance spectra showed a maximum at approximately 260 nm, which is attributed to the p–p* transition characteristics of MWCNTs [27,28], with a broad band between 200 and 300 nm and gradually decreased from UV to near IR. The spectra were similar to those reported earlier [29]. No change in the UV/Vis spectrum was observed during the longest UVA irradiation time period studied, i.e., 7 days. TEM analyses of the initial COOH–MWCNTs (Fig. 2a and b) showed that these nanotubes have an outer diameter of 10– 20 nm, in reasonable agreement with the vender’s specifications. The hydrodynamic diameter of the COOH–MWCNTs dispersed in deionized water (pH 6 ± 0.2) was 220 ± 10 nm, and remained unchanged during the 7 day UVA irradiation. TEM analyses of the irradiated samples (Fig. 2c and d) also did not show noticeable changes in the physical properties (morphology and size) of the nanotubes, suggesting no physical damage of the nanotubes by the irradiation process. Raman spectra of the initial COOH–MWCNTs and the 7-day irradiated COOH–MWCNTs (Fig. S2) showed a similar ID/IG intensity ratio (0.915 and 0.906, respectively), which indicate the number of surface defects did not change significantly after 7 days of irradiation. XPS analyses, however, revealed notable changes in surface chemistry of the COOH–MWCNTs after 7 days of UVA irradiation (Fig. S3). The total atomic concentration of oxygen decreased from 14.4 ± 0.7% in the initial sample to 11.3 ± 0.3% in the irradiated COOH–MWCNTs, suggesting the loss of oxygen functional groups (e.g., –COOH and –OH) from the surface of the nanotubes likely due to the attack by ROS generated under UVA irradiation. The C(1s) spectra of the initial and 7day irradiated COOH-MWCNT samples were further deconvoluted into three peaks, representing underivatized, monooxygenated carbon, and di-oxygenated carbon (Table 1). The percentage of di-oxygenated carbon (mostly C@O and O–C– O) was significantly reduced by UVA irradiation. Because the initial COOH–MWCNTs are highly carboxylated, most of the lost di-oxygenated carbon is expected to be carboxyl functional groups.

3.3.

Aggregation kinetics

Both the initial and 7-day irradiated COOH–MWCNTs showed aggregation behaviors generally consistent with the DLVO theory, in agreement with findings from previous studies

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Fig. 2 – HR-TEM imaging of initial COOH-MWCNT nanopaticles at (a) at low resolution and (b) at high resolution and UV irradiated COOH-MWCNT nanoparticles at (c) at low resolution and (d) at high resolution.

[2,10,30]. Figs. 3a and 4a present the attachment efficiency (a), i.e., the inverse of the stability ratio, of the initial and 7-day irradiated COOH–MWCNTs in NaCl and CaCl2 solutions, respectively, as a function of the electrolyte concentration. All aggregation profiles are characterized by two distinct aggregation regimes: a reaction-limited aggregation regime (a < 1) at low electrolyte concentration in which electrostatic repulsion hinders aggregation; and a diffusion-limited aggregation regime (a = 1) at elevated electrolyte concentrations where the energy barrier from electrostatic repulsion is completely eliminated. This suggested that electrostatic repulsion between COOH–MWCNT particles is the dominant factor controlling particle aggregation. The critical coagulation concentration (CCC) which represents the electrolyte concentration

at the boundary between the two regimes is a useful metric for evaluating colloidal stability. It can be obtained by extrapolating lines through the two regimes, as shown in Figs. 3 and 4. The initial COOH–MWCNTs (14.4% surface oxygen content) were stable at low NaCl concentrations, with a CCC of 226 mM (Fig. 3a). The CCC value was comparable to previously reported values for highly oxidized MWCNTs with oxygen contents above 10% [14,15]. After 7 days of UVA irradiation, however, the stability of the nanoparticles greatly decreased, with a CCC of 52 mM NaCl. During the 7-day period, the initial particle aggregation rate gradually increased with increasing UVA irradiation time (see Fig. S4). Because the colloidal stability of MWCNTs oxidized by acid wash has been shown to

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Table 1 – XPS C(1s) peak deconvolution summary of initial and 7-days irradiated COOH-MWCNT samples. Binding energy (eV)

Carbon

% C (1s) initial

% C (1s) irradiated

284.5 285.4 288.8

Underivatized C Mono-oxygenated C Di-oxygenated C

50.13 22.02 27.85

53.41 23.12 23.48

10

Attachment Efficiency, α

Initial COOH-MWCNT UV irradiated COOH-MWCNT

10 Initial COOH-MWCNT UV irradiated COOH-MWCNT Attachment Efficiency, α

strongly depend on the degree of surface oxidation [13,14,31], the decreased stability is attributed to the loss of oxygen-containing functional groups on the surface of the irradiated COOH–MWCNTs as suggested by the XPS analyses, particularly carboxyl functional groups. Oxygen-containing functional groups stabilize MWCNTs through electrostatic repulsions and reduced hydrophobic forces in water [19,30]. Comparison of the electrophoretic mobility (EPM) of the initial and 7-day irradiated COOH–MWCNTs provides further evidence for photochemical deoxygenation of the COOHMWCNT surface (Fig. 3b). Both the initial and 7-day irradiated

1

0.1

0.01 0.1

(a)

(a)

1

10

100

CaCl2 Concentration (mM) 0.0

1

(b)

EPM (10-8m2/Vs)

-0.5

0.1

0.01 10

100 NaCl Concentration (mM)

0 Initial COOH-MWCNT UV irradiated COOH-MWCNT

1000

(b)

EPM (10-8 m2 /Vs)

-1

-1.0 -1.5 -2.0 Initial COOH-MWCNT UV irradiated COOH-MWCNT

-2.5 -3.0 0.1

1

10

100

CaCl2 Concentration (mM) Fig. 4 – Aggregation (a) and electrophoretic mobility (b) of the initial COOH–MWCNTs and UV irradiated COOH–MWCNTs in CaCl2.

-2

-3

-4 10

100 NaCl Concentration (mM)

Fig. 3 – Aggregation (a) and electrophoretic mobility (b) of the initial COOH–MWCNTs and UV irradiated COOH–MWCNTs in NaCl.

COOH–MWCNTs were negatively charged over the range of NaCl concentration studied (10–200 mM), and the negative EPM decreased with increasing NaCl concentration due to charge screening as is commonly observed with charged colloidal particles in aqueous solutions. Over the entire range of NaCl, the irradiated COOH–MWCNTs were notably less negatively charged than the initial COOH–MWCNTs, which was consistent with the decreased stability of COOH–MWCNTs after UVA irradiation. Considering that oxygen-containing functional groups other than carboxyl groups are unlikely to dissociate at pH 6, the dissociated carboxyl groups are

expected to be the main contributor to the negative surface charge of both the initial and the UV irradiated MWCNTs [13]. Therefore, the decrease in negative surface charge suggests loss of carboxyl groups during UVA irradiation. Interestingly, in CaCl2 solutions, the CCC values of CaCl2 for the initial and irradiated COOH–MWCNTs were similar, 0.86 and 0.83 mM, respectively (Fig. 4a). These values were also much smaller than those of NaCl due to charge neutralization by Ca2+. The stability profiles of the COOH–MWCNTs before and after UV irradiation were very similar. The EPM measurement (Fig. 4b) also revealed that the initial and UV irradiated COOH–MWCNTs had similar negative surface charge over the entire range of CaCl2 concentrations tested (0.3–10 mM) despite the difference in surface oxygen content. Because the initial COOH–MWCNTs have more carboxyl groups, these results suggest that Ca2+ was more effective in neutralizing negative charges on the initial COOH–MWCNT surface than on the 7-day irradiated COOH–MWCNTs. Similar results were reported in the work of Yi and Chen [15] where the aggregation kinetics of two MWCNTs with different degrees of surface oxidation was similar in CaCl2. They suggested that Ca2+ has a higher tendency to form complexes with adjacent carboxyl groups on highly oxidized MWCNTs than with isolated carboxyl groups on less oxidized MWCNTs. This offers a plausible explanation to the similar stability of the initial and irradiated COOH–MWCNTs observed in our study. Additional work is necessary to test this hypothesis. Furthermore, the analysis of CCCCaCl2/CCCNaCl ratio suggests that the aggregation of the 7-day irradiated COOH–MWCNTs (CCCCaCl2/CCCNaCl ratio = 25.97) follow the Schulze–Hardy Rule, which states that the CCC of highly charged particles is proportional to Z6 (Z is the valence of the counter-ion) [30]. In contrast, the behavior of the initial COOH–MWCNTs deviated from the Schulze–Hardy Rule, with a CCCCaCl2/ CCCNaCl ratio of 28.03. Because the Schulze–Hardy Rule is only valid for inert electrolytes, i.e., electrolytes that do not specifically interact with the particle surface, this difference indicates the loss of surface carboxyl groups during UV irradiation that can specifically complex with Ca2+. Similar values of the CCCCaCl2/CCCNaCl ratio were reported by Yi and Chen for surface oxidized MWCNTs: CCCCaCl2/CCCNaCl was 27.9 for highly oxidized MWCNTs and 25.7 for the less oxidized MWCNTs [15].

3.4.

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Deposition kinetics

Deposition rates of COOH–MWCNTs depend on both the mass transport of COOH–MWCNTs to the surface and the deposition attachment efficiency (i.e., the probability of a successful attachment resulting from a collision between COOH– MWCNTs and the silica surface). Fig. 5 shows the deposition rate of the initial and irradiated COOH–MWCNTs as a function of NaCl concentration. Bell shaped relationship was found between the deposition rate and the ionic strength for both COOH–MWCNTs due to the concurrent aggregation at high ionic strength [2,15]. The mass transfer rate of both COOH–MWCNTs remained constant at low ionic strength (<20 mM) where the COOH–MWCNTs were stable against aggregation as suggested by Fig. 3a. The increasing ionic strength reduced the energy barrier between COOH–MWCNTs

0.5

0.4

Deposition Rate (Hz/min)

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Initial COOH-MWCNT UV irradiated COOH-MWCNT

0.3

0.2

0.1

0.0 10

100

NaCl Concentration (mM) Fig. 5 – Deposition of the initial COOH–MWCNTs and UV irradiated COOH–MWCNTs in NaCl.

and the silica surface by screening their surface charge, resulting in higher attachment efficiency and consequently higher deposition rates. This is qualitatively consistent with the classic DLVO theory [32,33]. However, as the ionic strength was further increased, the deposition rate drastically decreased due to the fast concurrent aggregation of COOH–MWCNTs during the deposition process (around 4 min), which resulted in decreased particle diffusivity as described by the Stokes–Einstein equation. The reduced mass transfer outweighed the enhanced attachment efficiency, leading to a net decrease in deposition rate. In 5 mM NaCl solution, the deposition rates of both COOH– MWCNTs were under the detection limit of our QCM-D system. While the deposition rate of irradiated COOH–MWCNTs was found to be 3 times higher than initial COOH–MWCNTs in 10 mM NaCl solution. Because the UVA irradiation did not change the initial particle size of COOH–MWCNTs, the mass transfer rates of initial and irradiated COOH–MWCNTs were considered to be the same at low ionic strength (<20 mM) where aggregation did not occur. However, the surface potential of COOH–MWCNTs reduced after irradiation as indicated by their lower electrophoretic mobility (Fig. 3b), resulting in lower energy barrier. Thus the higher deposition rate of irradiated COOH–MWCNTs at 10 mM NaCl can be attributed to their lower surface potential, which leads to higher attachment efficiency. In 40 mM NaCl solutions, aggregation took place, which greatly hindered the mass transport as discussed earlier. The impact of aggregation on deposition rates of initial and irradiated COOH–MWCNTs was different due to their different aggregation rates at a given ionic strength less than their CCCs. Initial COOH–MWCNTs were much more stable than the UVA irradiated sample as shown in Fig. 3a. Thus the mass transfer rate of initial COOH–MWCNTs was higher than the irradiated sample at high ionic strength due to their slower aggregation rate and hence higher diffusion coefficient. As a result, the deposition rate of the initial COOH– MWCNTs gradually became higher than irradiated sample as ionic strength increased.

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Conclusions

The change in surface chemistry and consequently the colloidal stability of COOH–MWCNTs upon UVA irradiation indicates the importance of sunlight on the fate and transport of carbon nanotubes in natural aquatic systems. The photochemical transformation of the carbonaceous nanoparticle surface is not yet well understood. In our previous study, oxygen-containing functional groups were introduced onto underivatized fullerene nanoparticle surface after UVA irradiation [19,20], which contradicts to the findings in this work. We speculate that the photochemical transformation depends on the original oxidation state of the carbon surface. Since the COOH–MWCNTs used in this study are highly oxidized, the ROS generated preferably attack the –COOH groups, leading to oxidative decarboxylation.

Acknowledgements This work was supported by the NSF Center for Biological and Environmental Nanotechnology (Award EEC-0647452), the USEPA STAR program (Grant No. 834093), and Korea Institute of Toxicology (KIT).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2012.12.012.

R E F E R E N C E S

[1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354(6348):56–8. [2] Chen KL, Elimelech M. Aggregation and deposition kinetics of fullerene (C-60) nanoparticles. Langmuir 2006;22(26):10994–1001. [3] Nowack B, Bucheli TD. Occurrence, behavior and effects of nanoparticles in the environment. Environ Pollut 2007;150(1):5–22. [4] Helland A, Wick P, Koehler A, Schmid K, Som C. Reviewing the environmental and human health knowledge base of carbon nanotubes. Environ Health Perspect 2007;115(8):1125–31. [5] Migliore L, Saracino D, Bonelli A, Colognato R, D’Errico MR, Magrini A, et al. Carbon nanotubes induce oxidative DNA damage in RAW 264.7 cells. Environ Mol Mutagen 2010;51(4):294–303. [6] Fan QH, Shao DD, Hu J, Chen CL, Wu WS, Wang XK. Adsorption of humic acid and Eu(III) to multi-walled carbon nanotubes: effect of pH, ionic strength and counterion effect. Radiochim Acta 2009;97(3):141–8. [7] Cho HH, Smith BA, Wnuk JD, Fairbrother DH, Ball WP. Influence of surface oxides on the adsorption of naphthalene onto multiwalled carbon nanotubes. Environ Sci Technol 2008;42(8):2899–905. [8] Klaine SJ, Alvarez PJJ, Batley GE, Fernandes TF, Handy RD, Lyon DY, et al. Nanomaterials in the environment: behavior, fate, bioavailability, and effects. Environ Toxicol Chem 2008;27(9):1825–51.

[9] Wiesner MR, Bottero J-Y. Environ Nanotechnol. New York: McGraw-Hill; 2007. [10] Petosa AR, Jaisi DP, Quevedo IR, Elimelech M, Tufenkji N. Aggregation and deposition of engineered nanomaterials in aquatic environments: role of physicochemical interactions. Environ Sci Technol 2010;44(17):6532–49. [11] Saleh NB, Pfefferle LD, Elimelech M. Aggregation kinetics of multiwalled carbon nanotubes in aquatic systems: measurements and environmental implications. Environ Sci Technol 2008;42(21):7963–9. [12] Saleh NB, Pfefferle LD, Elimelech M. Influence of biomacromolecules and humic acid on the aggregation kinetics of single-walled carbon nanotubes. Environ Sci Technol 2010;44(7):2412–8. [13] Smith B, Wepasnick K, Schrote KE, Bertele AH, Ball WP, O’Melia C, et al. Colloidal properties of aqueous suspensions of acid-treated, multi-walled carbon nanotubes. Environ Sci Technol 2009;43(3):819–25. [14] Smith B, Wepasnick K, Schrote KE, Cho HH, Ball WP, Fairbrother DH. Influence of surface oxides on the colloidal stability of multi-walled carbon nanotubes: a structure– property relationship. Langmuir 2009;25(17):9767–76. [15] Yi P, Chen KL. Influence of surface oxidation on the aggregation and deposition kinetics of multiwalled carbon nanotubes in monovalent and divalent electrolytes. Langmuir 2011;27(7):3588–99. [16] Hou WC, Jafvert CT. Photochemical transformation of aqueous C60 clusters in sunlight. Environ Sci Technol 2009;43(2):362–7. [17] Hwang YS, Li QL. Characterizing photochemical transformation of aqueous nC(60) under environmentally relevant conditions. Environ Sci Technol 2010;44(8):3008–13. [18] Lee J, Cho M, Fortner JD, Hughes JB, Kim JH. Transformation of aggregate C60 in the aqueous phase by UV irradiation. Environ Sci Technol 2009;43(13):4878–83. [19] Qu XL, Hwang YS, Alvarez PJJ, Bouchard D, Li QL. UV irradiation and humic acid mediate aggregation of aqueous fullerene (nC(60)) nanoparticles. Environ Sci Technol 2010;44(20):7821–6. [20] Qu X, Alvarez PJJ, Li Q. Impact of sunlight and humic acid on the deposition kinetics of aqueous fullerene nanoparticles (nC60). Environ Sci Technol 2012;46(24):13455–62. [21] Chen C-Y, Jafvert CT. Photoreactivity of carboxylated singlewalled carbon nanotubes in sunlight: reactive oxygen species production in water. Environ Sci Technol 2010;44(17):6674–9. [22] Chen C-Y, Jafvert CT. The role of surface functionalization in the solar light-induced production of reactive oxygen species by single-walled carbon nanotubes in water. Carbon 2011;49(15):5099–106. [23] Haag WR, Hoigne J, Gassman E, Braun AM. Singlet oxygen in surface waters. 1. Furfuryl alcohol as a trapping agent. Chemosphere 1984;13(5–6):631–40. [24] Holthoff H, Egelhaaf SU, Borkovec M, Schurtenberger P, Sticher H. Coagulation rate measurements of colloidal particles by simultaneous static and dynamic light scattering. Langmuir 1996;12(23):5541–9. [25] Schudel M, Behrens SH, Holthoff H, Kretzschmar R, Borkovec M. Absolute aggregation rate constants of hematite particles in aqueous suspensions: acomparison of two different surface morphologies. J Colloid Interface Sci 1997;196(2):241–53. [26] Sauerbrey G. Verwendung Von Schwingquarzen Zur Wagung Dunner Schichten Und Zur Mikrowagung. Z Phys 1959;155(2):206–22. [27] Henley SJ, Hatton RA, Chen GY, Gao C, Zeng H, Kroto HW, et al. Enhancement of polymer luminescence by excitationenergy transfer from multi-walled carbon nanotubes. Small 2007;3(11):1927–33.

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[28] Ago H, Kugler T, Cacialli F, Salaneck WR, Shaffer MSP, Windle AH, et al. Work functions and surface functional groups of multiwall carbon nanotubes. J Phys Chem B 1999;103(38):8116–21. [29] Wang Q, Han Y, Wang Y, Qin Y, Guo Z-X. Effect of surfactant structure on the stability of carbon nanotubes in aqueous solution. J Phys Chem B 2008;112(24):7227–33. [30] Elimelech M, Gregory J, Jia X, Williams RA. Particle Deposition and Aggregation: Measurement. Oxford, UK: Modelling and Simulation; 1995.

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[31] Chen KL, Elimelech M. Relating colloidal stability of fullerene (C-60) manoparticles to nanoparticle charge and electrokinetic properties. Environ Sci Technol 2009;43(19):7270–6. [32] Verwey EJW. Theory of the stability of lyophobic colloids. Philips Res Rep 1945;1(1):33–49. [33] Derjaguin B, Landau L. Theory of stability of highly charged liophobic sols and adhesion of highly charged particles in solutions of electrolytes. Zh Eksp Teor Fiz 1945;15(11):663–82.