Catalytic Effects of Functionalized Carbon Nanotubes on

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Catalytic Effects of Functionalized Carbon Nanotubes on Dehydrochlorination of 1,1,2,2-Tetrachloroethane Weifeng Chen,† Dongqiang Zhu,‡ Shourong Zheng,‡ and Wei Chen*,† †

College of Environmental Science and Engineering/Ministry of Education Key Laboratory of Pollution Processes and Environmental Criteria/Tianjin Key Laboratory of Environmental Remediation and Pollution Control, Nankai University, Tianjin 300071, China ‡ State Key Laboratory of Pollution Control and Resource Reuse/School of the Environment, Nanjing University, Nanjing, Jiangsu 210093, China S Supporting Information *

ABSTRACT: The environmental implications of carbon nanomaterials have received much attention. Nonetheless, little is known about how carbon nanomaterials might affect the abiotic transformation of organic contaminants in aquatic environments. In this study, we observed that three functionalized multiwalled carbon nanotubes (MWCNTs)including a hydroxylated MWCNT (OH-MWCNT), a carboxylated MWCNT (COOH-MWCNT), and an aminated MWCNT (NH2-MWCNT)all had strong catalytic effects on the dehydrochlorination of 1,1,2,2-tetrachloroethane (TeCA) at three different pH (7, 8, and 9); notably, the most significant effects (up to 130% increase in reaction rate) were observed at pH 7, at which reaction kinetics was very slow in the absence of MWCNT. The primary mechanism was that the −NH2 group and the deprotonated −COOH and −OH groups serve as bases to catalyze the reaction. Modeling results indicate that at any given pH the transformation kinetic constants of MWCNTadsorbed TeCA were up to 2 orders of magnitude greater than the respective kinetic constant of dissolved TeCA. The overall catalytic effects of the MWCNTs depended both on the basicity of the surface functionalities of MWCNT and on the adsorption affinities of MWCNT for TeCA. Interestingly, Suwannee River humic acidselected as a model dissolved organic matterhad negligible effects on the dehydrochlorination kinetics, even though it is rich in surface O-functionalities. An important environmental implication is that carbon nanotubes released into the environment might significantly affect the fate of chlorinated solvents.

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INTRODUCTION With the rapid increase in the worldwide production and use of carbon nanotubes (CNTs)it was estimated that the annual production of multiwalled carbon nanotubes (MWCNTs) had reached 390 tons in 2008 and would further increase to 3400 tons in 20101the possibilities of their environmental release and the associated implications have received much attention.2−5 A number of studies have shown that CNTs can interact strongly with organic contaminants (e.g., polycyclic aromatic hydrocarbons, chlorinated benzenes, antibiotics, etc.), owing to their large surface areas, strong surface hydrophobicity, and unique graphitic structures and surface functionalities.6−10 While it has been demonstrated that the CNTs−contaminant interactions may significantly affect the transport and bioavailability of environmental contaminants,11−13 little is known about whether such interactions can affect the abiotic transformation of organic contaminants in the environment. Findings from several recent studies indicate that carbon nanomaterials may play important roles in mediating abiotic transformation reactions of environmentally relevant organic contaminants.14,15 For example, it was shown that a trace level © 2014 American Chemical Society

of reduced graphene oxide can significantly facilitate the reduction of nitrobenzene in aqueous solution, and both the graphitic surfaces and the zigzag carbons played important roles.15 CNTs are often functionalized or derivitized to introduce an array of surface functionalities,16 such as hydroxyl, carboxyl, and amino groups. Many of these surface functionalities can exert different degrees of acidity and basicity, depending on the pH. We hypothesize that when released into the environment, such functionalized CNTs might significantly affect the nucleophilic and hydrolytic reactions of organic contaminants, which are often acid-catalyzed and/or basecatalyzed.17 Even though this has not been tested, the hypothesis seems to be reasonable based on the findings of several other studies.14,18,19 For example, in a study aimed to regenerate spent granular activated carbon used to treat 1,1,2,2tetrachloroethane (TeCA)-contaminated groundwater, Mackenzie et al.19 observed that at pH 9.2 the hydrolysis reaction of Received: Revised: Accepted: Published: 3856

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concentrations and temperature; NH2-MWCNT was obtained by first covalently amidating COOH-MWCNT with ammonium hydroxide and then decarboxylating the product at high temperature. G-MWCNT contained more than 99.9% (wt:wt) MWCNT and less than 0.1% impurities (mainly ash); OHMWCNT, COOH-MWCNT, and NH2-MWCNT contained more than 95% MWCNT and less than 5% impurities. The outer diameters of the MWCNTs ranged from 8 to 15 nm, and the average length was approximately 50 μm. The Brunauer−Emmett−Teller (BET) specific surface area of the MWCNTs was determined by multipoint N 2 adsorption−desorption using a Micromeritics ASAP2010 accelerated surface area and porosimetry system (Micromeritics Co., USA). Surface elemental analysis was performed with an X-ray photoelectron spectroscopy (XPS) (PHI-5000 Versa Probe, Japan). Fourier-transform infrared (FTIR) analysis was performed using a Bruker TENSOR 27 apparatus (Bruker Optics Inc., Germany). The Boehm titration method was used to determine the concentrations of the surface acidic and basic functional groups of the MWCNTs,24,25 and the pH drift method was used to determine the point of zero charge (pH pzc ). 26 Selected physicochemical properties of the MWCNTs are summarized in Table 1. TeCA (above 99%) and TCE (above 99%) were purchased from Sigma Aldrich (St. Louis, MO, USA). SRHA was obtained from the International Humic Substances Society and is reported to be composed of 52.63% C (wt), 4.28% H, and 42.04% O. The distribution of functional groups is carboxylic (28%), aromatic (31%), aliphatic (29%), and heteroaliphatic (13%). Adsorption Isotherm and Kinetics Experiments. The adsorption isotherm and kinetics experiments of TeCA and TCE to the MWCNTs were conducted using a batch adsorption approach developed in our previous studies;6,27 the detailed procedures are given in the Supporting Information (SI). Each isotherm contained six individual data points, and each data point was run in triplicate. Sorption affinity of TeCA to SRHA was determined using a solid-phasemicroextration-facilitated approach,28 and the detailed procedures are given in the SI. Reaction of TeCA in Homogeneous Aqueous Solution and in the Presence of MWCNT. The dehydrochlorination reaction experiments carried out are summarized in Table 2. Reaction kinetic experiments were conducted both in homogeneous aqueous solution (in the absence of MWCNT) and in the presence of MWCNT. To initiate a kinetic experiment, approximately 275 mL of an aqueous solution (containing 0.05 mol/L K2HPO4/KH2PO4 as the pH buffer and 0.01 mol/L NaN3 as the bioinhibitor) was added to a 275ml amber glass vial. The pH of the solution was adjusted to 7.0, 8.0, or 9.0 with HNO3 or NaOH. For the experiments involving

TeCA (at an initial concentration of 100 mg/L) was enhanced in the presence of activated carbon (at a loading of 500 mg/L), even though they did not discuss the specific mechanisms controlling the catalytic effects. While CNTs and activated carbons differ markedly in physicochemical properties (e.g., pore structures and distribution of surface functionalities), it is expected that CNTs likely can exhibit greater catalytic effects due to the high surface activities associated with nanomaterials. To test the above-mentioned hypothesis, we evaluated the catalytic effects of several functionalized MWCNTs on the dehydrochlorination reaction of TeCA. TeCA is one of the most commonly used chlorinated solvents and a known human carcinogen.20 The dehydrohalogenation reaction of chlorinated solvents is one of the most environmentally relevant reactions via which natural attenuation of chlorinated solvents occurs.21,22 It is commonly assumed that the dehydrochlorination of TeCA follows a β-elimination (E2) mechanism,19,22 in which OH− attacks the hydrogen atom attached to the βcarbon, resulting in the breaking of a C−Cl bond and the formation of a CC bond, and trichloroethylene (TCE) is the sole transformation product:23

Four commercially available MWCNTs, including a hydroxylated MWCNT (OH-MWCNT), a carboxylated MWCNT (COOH-MWCNT), an aminated MWCNT (NH2MWCNT), and a graphitized MWCNT (G-MWCNT) were tested. The effects of these MWCNTs on the reaction kinetics of TeCA were examined under three different pH conditions (pH 7, 8, and 9). The combined effects of the basicity of the surface functionalities of MWCNT and the adsorption affinity of MWCNT for TeCA on the transformation kinetics of TeCA were analyzed. Additionally, the effects of Suwannee River humic acid (SRHA), selected as a model dissolved organic matter, on the reaction kinetics of TeCA at pH 7 were also tested, to further understand the effects of surface basicity and adsorption affinities on reactivity and to illustrate the environmental implications of this study.

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MATERIALS AND METHODS Materials. Functionalized and graphitized MWCNTs were purchased from Organic Chemicals Co. (Chengdu, Sichuan Province, China). Based on the information provided by the manufacturer, G-MWCNT was obtained by treating high-purity MWCNT in inert gas at 2800 °C for 20 h; OH-MWCNT and COOH-MWCNT were obtained by oxidizing high-purity MWCNT with KMnO4 in H2SO4 solutions, at different acid

Table 1. Selected Physicochemical Properties of Multiwalled Carbon Nanotubes (MWCNT) MWCNT

SABETa (m2/g)

Cb (wt %)

G-MWCNT OH-MWCNT COOH-MWCNT NH2-MWCNT

146 160 185 155

98.2 94.1 97.0 97.7

Ob Nb (wt %) (wt %) pHpzcc 1.8 5.9 3.0 1.8

0.5

5.2 3.3 3.5 9.8

lactone (μmol/m2)

carboxyl (μmol/m2)

phenolic hydroxyl (μmol/m2)

total acidic groups (μmol/m2)

total basic groups (μmol/m2)

0.19 0.20 0.15 -

1.00 0.56 -

0.064 0.54 0.021 -

0.26 1.7 0.73 -

0.42 0.27 0.22 0.88

a

SABET = surface area measured using the Brunauer−Emmett−Teller (BET) method. bAnalyzed with X-ray photoelectron spectroscopy. cpHpzc = point of zero charge measured with the pH drift method. 3857

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Table 2. Protocols of Reaction Experiments in Homogeneous Aqueous Solution and in the Presence of Multiwalled Carbon Nanotubes (MWCNT)

exp. no. 1a 1b 1c 1d 1e 2a 2b 2c 2d 2e 3a 3b 3c 3d 3e 4a 4b

type of MWCNT a

G-MWCNT OH-MWCNT COOH-MWCNT NH2-MWCNT -a G-MWCNT OH-MWCNT COOH-MWCNT NH2-MWCNT -a G-MWCNT OH-MWCNT COOH-MWCNT NH2-MWCNT SRHAb COOH-MWCNT+SRHAb

pH 7.0 7.0 7.0 7.0 7.0 8.0 8.0 8.0 8.0 8.0 9.0 9.0 9.0 9.0 9.0 7.0 7.0

first-order kinetic model

two-phase kinetic model based on Freundlich isotherm

two-phase kinetic model based on Langmuir isotherm

kobsc,d (h−1)

R2

ksc (h−1)

R2

ks/ka

ksc (h−1)

R2

ks/ka

0.9811 0.9867 0.9744 0.9829 0.9599 0.9814 0.9911 0.9926 0.9874 0.9931 0.9977 0.9994 0.9992 0.9993 0.9994 0.9833 0.9866

0.021 0.030 0.028 0.041 0.31 0.25 0.13 0.42 2.4 1.2 0.29 0.041

0.9723 0.9826 0.9569 0.9908 0.9916 0.9837 0.9921 0.9991 0.9988 0.9916 0.9981 0.9884

57.3 80.1 75.6 9.90 73.6 62.5 30.9 12.7 72.1 37.4 9.42 114

0.038 0.048 0.077 0.059 0.39 0.37 0.36 0.51 3.3 2.4 1.0 0.062

0.9731 0.9824 0.9694 0.9927 0.9956 0.9892 0.9939 0.9989 0.9990 0.9821 0.9980 0.9897

106 128 212 14.0 92.1 89.0 85.9 14.5 94.6 67.8 29.9 172

3.6 3.6 5.1 6.5 8.3 4.2 4.3 5.2 6.2 5.9 3.5 4.4 4.9 7.0 4.4 3.5 7.5

× × × × × × × × × × × × × × × × ×

−4

10 10−4 10−4 10−4 10−4 10−3 10−3 10−3 10−3 10−3 10−2 10−2 10−2 10−2 10−2 10−4 10−4

a

No MWCNT added (i.e., experiments conducted in homogeneous aqueous solution in the absence of MWCNT). bSRHA = Suwannee River humic acid. cValues obtained by fitting reaction kinetic data using eqs 1, 5, or 6. dFor the experiments conducted in homogeneous aqueous solution (i.e., in the absence of MWCNT), kobs = ka.

Analysis of Reaction Kinetics Data. The first-order kinetic model was used when comparing the reaction kinetics in different experiments

MWCNT, approximately 10 mg of MWCNT was added to the vial (to give an MWCNT concentration of approximately 36 mg/L) before adding the aqueous solution, and after adjusting pH the vial was placed on an orbital shaker (7 rpm) for 24 h to prewet the MWCNT. Then, 50 μL of a TeCA stock solution (in methanol) was injected into the solution with a microsyringe, to give an initial concentration of 1.0 mg/L. Afterward, the vial was filled with the buffer solution, sealed immediately, and left on an orbital shaker (7 rpm) operated at a constant temperature of 25 °C. At designated time intervals, 1.0 mL of the solution was withdrawn from the vial, transferred to a clean glass vial containing a pH 3 buffer solution (0.05 M KH2PO4; pH adjusted with HNO3) to terminate the reaction, and then extracted with hexane (3:1, v:v). The pH of the solution was also monitored during the sampling. For the experiments involving MWCNT the solution was first filtrated through a 0.22-μm membrane filter (Anpel Scientific Instrument, Shanghai, China) before being extracted. The hexane extract was analyzed to determine the mass of TeCA and TCE. Each kinetic experiment was run in triplicate. In all the kinetic experiments pH changed little (less than 0.03) during the entire duration of the experiment (see SI Figure S1). To examine the effects of SRHA on the dehydrochlorination kinetics of TeCA (Exp. 4a) or the effects of SRHA on MWCNT-mediated reaction of TeCA (Exp. 4b), a stock solution of SRHA was added to the pH 7 buffer solution to give a final SRHA concentration of 36 mg/L SRHA. All other experimental procedures are identical to the above-mentioned procedures for the reaction of TeCA. This set of experiments was also run in triplicate. TeCA and TCE were analyzed with an Agilent 6890N gas chromatography with electron capture detector (GC-ECD) (Agilent Technologies, Santa Clara, CA, USA) equipped with an HP-5 capillary column (30 m × 0.32 mm × 0.25 μm).

C /C0 = exp( −kobs·t )

(1)

where C0 (mg/L) is the initial TeCA concentration; C (mg/L) is the TeCA concentration at a given time t (h); and kobs (h−1) is the apparent first-order kinetic constant for the dehydrochlorination reaction of TeCA. A two-phase kinetic model was developed to compare the reaction kinetics of TeCA adsorbed to MWCNT and the reaction kinetics of dissolved TeCA. For a reaction system containing MWCNT, the change of TeCA concentration in the system can be expressed as dC = − ka·Cw − ks·q·CCNT dt

(2)

where Cw (mg/L) is the concentration of dissolved TeCA; q (mg/g) is the concentration of TeCA adsorbed to MWCNT; ka (h−1) and ks (h−1) are first-order kinetic constants for the transformation of dissolved and adsorbed TeCA, respectively; and CCNT (g/L) is the concentration of MWCNT in the reaction system. Because adsorption of TeCA to all the MWCNTs is kinetically very fast (apparent adsorption equilibrium can be reached within 2−4 h; see SI Figure S2), adsorption to MWCNT is not the rate-limiting step for the MWCNT-mediated reactions. Thus, the correlation between q and Cw can be sufficiently represented with an adsorption isotherm, for example, the Freundlich or Langmuir isotherm q = KF·Cw n

(3)

q = qmax ·Cw /(A + Cw )

(4)

1‑n n

where KF (mg L /g) is the Freundlich affinity coefficient; n (unitless) is the Freundlich linearity index; qmax (mg/g) is the maximum adsorption capacity; and A (mg/L) is the Langmuir 3858

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adsorption constant. Note that under the experimental conditions of this study Cw and C are essentially equal (for example, the mass of TeCA in the adsorbed phase was typically less than 1 to 2% of the total mass of TeCA in the system). Assuming Cw = C, and assuming the q−Cw correlation follows the Freundlich isotherm (eq 3), eq 2 can be solved to obtain a Freundlich-isotherm-based two-phase kinetic model (see detailed derivation in the SI): 1/(1 − n) ⎧⎛ ks·KF·CCNT ⎞ [ka·(n − 1)·t ] ks·KF·CCNT ⎫ 1−n ⎨ ⎬ + − C = ⎜C0 ⎟·e ka ka ⎠ ⎩⎝ ⎭ (5) ⎪

⎪

⎪

⎪

Figure 1. Adsorption isotherms of 1,1,2,2-tetrachloroethane to different multiwalled carbon nanotubes (MWCNT). Error bars, representing standard deviations of triplicates, are not shown for reasons of clarity; error bars are in most cases smaller than the symbols.

Similarly, assuming the q−Cw correlation follows the Langmuir isotherm (eq 4), a Langmuir-isotherm-based two-phase kinetic model can be obtained (see the detailed derivation in the SI): ⎡ ⎢ C ln⎢( 0 )A /(ka·A + ks·qmax ·CCNT) C ⎣

COOH-MWCNT > OH-MWCNT. The relative adsorption affinities among the MWCNTs correlate well with the surface hydrophobicity of the MWCNTs − the low pHpzc values of OH-MWCNT and COOH-MWCNT indicate that the surfaces of the two MWCNTs were more negatively charged compared with those of G-MWCNT and NH2-MWCNT; between OHMWCNT and COOH-MWCNT the latter contained a smaller amount of surface O-functionalities and was less hydrophilic. From the adsorption isotherms it can be inferred that in the MWCNT-mediated reaction experiments TeCA was predominately in the aqueous solution and only a very small amount of the TeCA molecules was adsorbed to MWCNT (typically less than 1 to 2% of the total mass of TeCA in the system). Very similar adsorption patterns and adsorption affinities were observed for TCE, the transformation product of TeCA (see SI Figures S7 and S8 and Tables S1 and S2). MWCNT-Catalyzed Dehydrochlorination of TeCA. The effects of MWCNTs on the reaction kinetics of TeCA are compared in Figure 2 (the TCE formation kinetics and data of mass balance are shown in SI Figures S9−S13). At all three test pH values (7, 8, and 9), reaction kinetics of TeCA was significantly enhanced in the presence of a functionalized MWCNT (i.e., OH-MWCNT, COOH-MWCNT, or NH2MWCNT), regardless of the type of MWCNT used. For example, at pH 7 approximately 11% of TeCA was transformed after 336 h in the absence of MWCNT (i.e., in the reaction kinetics experiment conducted in homogeneous aqueous solution − exp. 1a, Table 2). In the presence of OHMWCNT or COOH-MWCNT, transformation of TeCA was 17% and 18%, respectively, whereas in the presence of NH2MWCNT, transformation reached 24%. Strong catalytic effects of the functionalized MWCNTs were also observed at pH 8 and 9. Contrasting to the strong catalytic effects of the three functionalized MWCNTs, G-MWCNT only exhibited a small catalytic effect on the transformation of TeCA at pH 9 but essentially no effects at pH 7 and 8. An interesting observation was that the relative catalytic effects among the functionalized MWCNTsas indicated by the relative positions of the reaction kinetics curves (Figure 2) and by the fitted kobs values (Table 2)varied at different pH values. At pH 7 the relative catalytic effects followed the order of NH2-MWCNT > COOH-MWCNT > OH-MWCNT; at pH 8 the order changed to COOH-MWCNT > NH2-MWCNT > OH-MWCNT (but the difference between COOH-MWCNT and NH2-MWCNT was very small); and at pH 9 the order changed to COOH-MWCNT > OH-MWCNT > NH2-

⎛ ka·C + ka·A + ks·q ·CCNT ⎞ A /(k a·A + ks·qmax ·CCNT) − 1/ ka ⎤ ⎥ max ⎟⎟ × ⎜⎜ ⎥ ⎝ ka·C0 + ka·A + ks·qmax ·CCNT ⎠ ⎦

■

=t

(6)

RESULTS AND DISCUSSION Surface Properties of MWCNTs. The X-ray photoelectron spectroscopy results (SI Figure S3) show that G-MWCNT contained only 1.8% of surface oxygen, whereas OH-MWCNT and COOH-MWCNT contained a greater amount of surface oxygen − 5.9% and 3.0%, respectively; the surface nitrogen content of NH2-MWCNT was 0.5%. The Fourier-transform infrared spectra (SI Figure S4) confirm the existence of CO in carbonyl or carboxyl and −OH in phenol on G-MWCNT, OH-MWCNT, and COOH-MWCNT, as well as the existence of −NH2 group and −OH in phenol on NH2-MWCNT. The characteristics of the surface functionalities of the MWCNTs are better shown with the Boehm titration results (Table 1). GMWCNT, OH-MWCNT, and COOH-MWCNT differed both in the type and in the abundance of surface O-functionalities. G-MWCNT contained only small amounts of surface Ofunctionalities, primarily in the form of lactone. The surface Ofunctionalities of COOH-MWCNT are dominated by −COOH and lactone (0.56 and 0.15 μmol/m2, respectively), and the amount of −OH was negligible. OH-MWCNT contained much greater amounts of surface O-functionalities than COOH-MWCNT − not only a high concentration of −OH (0.54 μmol/m2) but also a significant amount of −COOH (1.0 μmol/m2). The Boehm titration results indicate that the amounts of surface acidic groups of NH2-MWCNT were negligible. The four MWCNTs differ significantly in pHpzc (SI Figure S5 and Table 1), ranging from as low as 3.3 for OHMWCNT to as high as 9.8 for NH2-MWCNT. The differences were apparently linked to the different types and abundance of surface functionalities of the MWCNTs. Adsorption Affinities of Different MWCNTs. The adsorption isotherms of TeCA to the different MWCNTs are shown in Figure 1. The adsorption data were fitted with the Freundlich and Langmuir isotherms (SI Figure S6), and the fitted isotherm parameters are summarized in SI Tables S1 and S2. The four MWCNTs exhibited different adsorption affinities for TeCA, with the order of G-MWCNT/NH2-MWCNT > 3859

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Figure 2. Dehydrochorination kinetics of 1,1,2,2-tetrachloroethane in homogeneous aqueous solution and in the presence of different multiwalled carbon nanotubes (MWCNT). The lines were plotted by fitting the data using the first-order kinetic model (eq 1) or two-phase kinetic model (eq 5). Curve fitting was carried out with SigmaPlot 10.0 using the least-squares-root approach. Error bars indicate standard deviations of triplicates.

adjacent to each other −OH can increase the acidity of −COOH,32 the deprotonated −COOH groups on OHMWCNT are weaker bases than the deprotonated −COOH groups on COOH-MWCNT. The weak catalytic effects of GMWCNT observed at pH 9 were likely because under relatively strong basic conditions the lactone groups can undergo hydrolysis reaction to form deprotonated carboxyl and hydroxyl groups.33 Comparison of Reaction Kinetics between Adsorbed TeCA and Dissolved TeCA. To further understand the catalytic effects of the different MWCNTs and to compare their relative effectiveness at different pH, the apparent reaction kinetic constants of adsorbed TeCA, ks, associated with different MWCNTs were obtained by fitting the kinetic data in Figure 2 with eqs 5 and 6, with ks as the only fitting parameter. The values of the isotherm parameters (SI Tables S1 and S2) in the equations were obtained from the adsorption data, and the ka values (Table 2) were obtained by fitting the kinetic data of TeCA in homogeneous aqueous solution. The fitted ks values are summarized in Table 2. In general, the ks values obtained with the Freundlich-isotherm-based kinetic model (eq 5) and the Langmuir-isotherm-based kinetic model (eq 6) are comparable. Because the Freundlich isotherm provided better fits for the adsorption data within the concentration ranges covered in the reaction kinetics experiments (Figure S6), the ks values obtained using eq 5 are used in the discussion below. A striking observation was that at any given pH the ks values associated with the functionalized MWCNTs are significantly greater than the respective ka value, by nearly 2 orders of magnitude (see the ks/ka values in Table 2). This indicates that transformation of TeCA on the surface of MWCNTs is kinetically much faster than transformation of freely dissolved TeCA. Thus, the large differences between the ks and ka values at a given pH can well explain the strong catalytic effects of the functionalized MWCNTs observed in this study. Notably,

MWCNT (the difference between OH-MWCNT and NH2MWCNT was small). Thus, it appears that at relatively low pH the amino group is a stronger catalytic surface functionality than the carboxyl and phenolic groups, but with the increase of pH it gradually becomes less effective in catalyzing the transformation of TeCA than the other two types of surface functionalities. Intriguingly, even though OH-MWCNT contained a greater amount of surface carboxyl groups than did COOH-MWCNT (Table 1), it was always less effective in catalyzing the transformation of TeCA than was COOHMWCNT. The dehydrochlorination reaction of TeCA is a basepromoted reaction.22 Thus, one way to understand the relative catalytic effects of different functionalized MWCNTs is to compare the basicity of the different surface functionalities under different solution pH. Within the test pH range the amino groups on the surface of NH2-MWCNT should always be in its neutral form and function as a base,29 which likely can catalyze the transformation of TeCA. The likely pKa range of the carboxyl groups on the surfaces of carbon is 3−6;30 however, oxidized carbons may contain a complex mixture of acidic functionalities and the pKa of carboxyl groups can be up to 8.31 Accordingly, at pH 7 a considerable fraction of the surface carboxyl groups on COOH-MWCNT and OHMWCNT should be deprotonated. These deprotonated carboxyl groups become relatively strong conjugate bases and likely can catalyze the reaction of TeCA. Note that with the increase of pH the deprotonation of the carboxyl groups becomes more profound, and thus the catalytic effects of the deprotonated carboxyl groups increase with pH. This is consistent with the fact that COOH-MWCNT and OHMWCNT had smaller catalytic effects than NH2-MWCNT at pH 7 but stronger effects at pH 9. The pKa range of the phenolic groups on the surfaces of CNTs is likely around 9− 10.31 Thus, phenolic groups can only deprotonate and catalyze the reaction at relatively high pH. Additionally, because when 3860

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COOH-MWCNT than on OH-MWCNT (by a factor of approximately 2). Thus, even though under relatively basic conditions (i.e., pH 8 and 9) TeCA adsorbed to OH-MWCNT can react more rapidly than TeCA adsorbed to COOHMWCNT (as indicated by the greater ks values associated with OH-MWCNT), the overall catalytic effects of OH-MWCNT are counterbalanced by its weaker adsorption affinities for TeCA. Effects of Dissolved Organic Matter on Reaction Kinetics of TeCA. It is necessary to note that aquatic environments are rich in dissolved organic matter, which contains a variety of surface O-functionalities; thus, it seems to be reasonable to anticipate that dissolved organic matter can catalyze the dehydrochlorination of TeCA in a similar manner as functionalized MWCNTs. To test the hypothesis, the effects of SRHAa model dissolved organic matteron the transformation of TeCA were evaluated at pH 7 (at which the most profound catalytic effects of the MWCNTs were observed), and the results are shown in Figure 5 and Table 2 (the TCE formation kinetics and data of mass balance are shown in SI Figure S14). Interestingly, SRHA (at 36 mg/L) exhibited no catalytic effects. The results are consistent with the findings of Georgi et al.,34 in which a humic acid obtained from Carl Roth GmbH (Germany) (at 2 g/L) had no effects on the dehydrochlorination reactivity of TeCA. As discussed above, the catalytic effects of the functionalized MWCNTs depend not only on the surface functionalities that serve as bases but also on the strong adsorption affinities for TeCA. Thus, the negligible effects of SRHA observed in this study were likely attributable to the much weaker sorption of TeCA to SRHA than to the MWCNTs (the Kd value of TeCA to SRHA was only ∼26 L/kg, approximately 1 order of magnitude smaller than the respective Kd values to the MWCNTs). Furthermore, the strong catalytic effects of the functionalized MWCNTs appeared to be not affected by the presence of SRHA, as tested using COOH-MWCNT at pH 7 (Figure 5). Dissolved organic matter may influence the catalytic effects of MWCNTs in two different ways: it can enhance the dispersion of MWCNTs (thus inhibiting the bundling of MWCNTs),35,36 causing more surface functional groups to be available as active catalytic sites; it can also adsorb onto MWCNTs,36,37 which may cover a fraction of adsorption/catalytic sites of TeCA. The overall effects of SRHA observed in this study were likely from the simultaneous contributions from the two factors. While more research is needed to fully understand the roles of dissolved organic matter, the fact that strong catalytic effects of functionalized MWCNTs were unaffected by the presence of SRHA underscores the environmental relevance of such carbon nanomaterial-catalyzed reactions. Environmental Implications. The findings of this study underline the possibility that carbon nanomaterials, when released into the environment, may significantly affect the hydrolytic reactions of chlorinated solvents. For reactions involving nucleophiles, the types and abundance of the functional groups on carbon nanomaterials play pivotal roles in mediating the reactivity. It is noteworthy that the three functionalized MWCNTs used in this study all contained very low concentrations of surface functionalities (especially NH2MWCNT). It is likely that carbon nanomaterials containing greater amounts of surface functionalities will exhibit even greater catalytic effects than what were observed in this study. Likewise, the extent of catalytic effects will likely be greater for contaminants that can adsorb more strongly to carbon

overall the strongest catalytic effects of the functionalized MWCNTs were observed at pH 7 (see the ks/ka values in Table 2), at which the reaction in the homogeneous aqueous solution (in the absence of MWCNTs) was very slow. The relative catalytic strength of different MWCNTs at a given pH is also evident when comparing the fitted ks values. At pH 7 the ks values associated with COOH-MWCNT and NH2MWCNT are similar (0.030 and 0.028 h−1, respectively) and are approximately 33 to 42% greater than the ks value associated with OH-MWCNT (0.021 h−1). This trend is reasonable in that the −NH2 groups and the partially deprotonated −COOH groups serve as bases, whereas the presence of −OH groups may weaken the basicity of the deprotonated −COOH groups of OH-MWCNT. However, with the increase of pH the catalytic strength of OH-MWCNT and COOH-MWCNT becomes increasingly greater than that of NH2-MWCNT, because within the test pH range speciation of carboxyl and phenolic groups (but not the amino group) is significantly affected by the changes of pH. This trend is particularly remarkable at the highest test pH (pH 9), at which the ks values of OH-MWCNT (2.4 h−1) and COOH-MWCNT (1.2 h−1) are 8.3 and 4.1 times greater than the ks value of NH2MWCNT. A closer look at the changes of ka and ks values with pH (Figure 3) indicates that increasing pH by one unit more or

Figure 3. Changes of kinetic constants of dissolved 1,1,2,2tetrachloroethane (ka) and kinetics constants of the multiwalled carbon nanotubes (MWCNT)-adsorbed 1,1,2,2-tetrachloroethane (ks) with pH.

less resulted in the increase of ka value by a factor of 10. However, only the ks values associated with OH-MWCNT showed a similar pH-dependent trend, whereas for COOHMWCNT and NH2-MWCNT the ks values increased less appreciably with pH. The different ks−pH trends among the three functionalized MWCNTs are consistent with their differences in the types and abundance of functional groups, as mentioned above. Note that based on the fitted kobs values in Table 2, it appears that at all three pH COOH-MWCNT had stronger catalytic effects than did OH-MWCNT. This trend seems to be contradictory to the differences in ks values between the two MWCNTs, in that only at pH 7 was a greater ks value observed for COOH-MWCNT, whereas at both pH 8 and 9 the ks value observed for OH-MWCNT was considerably greater than the ks value observed for COOH-MWCNT. The seemingly contradictory observation was likely attributable to the different adsorption properties between the two MWCNTs. In Figure 4 the adsorption affinities of the two MWCNTs over the concentration ranges of TeCA relevant to the transformation kinetics experiments are compared. It can be seen that at any given pH adsorption of TeCA was consistently stronger on 3861

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Article

Figure 4. Comparison of adsorption affinities between hydroxylated multiwalled carbon nanotubes (OH-MWCNT) and carboxylated multiwalled carbon nanotubes (COOH-MWCNT) within the concentration ranges covered in the reaction kinetics experiments. The q values were calculated using the Cw values observed in the reaction experiments and KF and n values obtained in the isotherm experiments (Table S1).

adsorption isotherms of TCE; mass balance data in kinetic experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: 86-22-6622-9516. E-mail; [email protected]. cn. Notes

The authors declare no competing financial interest.

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Figure 5. Effects of Suwannee River humic acid (SRHA) on dehydrochorination kinetics of 1,1,2,2-tetrachloroethane. The lines were plotted by fitting the data using the first-order kinetic model (eq 1) or the two-phase kinetic model (eq 5). Curve fitting was carried out with SigmaPlot 10.0 using the least-squares-root approach. Error bars indicate standard deviations of triplicates.

ACKNOWLEDGMENTS This project was supported by the Ministry of Science and Technology (Grant 2014CB932001) and the National Natural Science Foundation of China (Grants 21237002 and 21177063).

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nanomaterials. Furthermore, the results of this and related studies14,15 indicate that it is possible to take advantage of the strong catalytic effects of carbon nanomaterials through the designing of specific surface functionalities to treat water contaminated with recalcitrant chlorinated compounds. Future studies should be carried out with chlorinated compounds of different adsorption properties and different propensities for hydrolytic reactions, under wider environmental conditions.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Adsorption experiments of TeCA to MWCNTs; determination of sorption affinity of TeCA to SRHA; derivation of two-phase kinetic model; summary of adsorption model coefficients; changes of pH with time in the reaction experiments; adsorption kinetics of TeCA to MWCNTs; XPS spectra, FTIR spectra and pHpzc values of different MWCNTs; fitting of adsorption data with Freundlich and Langmuir isotherms; 3862

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