Investigation of the Role of Surface Chemistry and Accessibility of

Nov 21, 2007 - School of Chemical Engineering, Dalian UniVersity of Technology, 158 ... and Fundamental Chemistry Laboratory Center, Dalian UniVersity...
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Langmuir 2008, 24, 11701-11710

11701

Investigation of the Role of Surface Chemistry and Accessibility of Cadmium Adsorption Sites on Open-Surface Carbonaceous Materials Zhanming Gao,†,‡ Teresa J. Bandosz,§ Zongbin Zhao,†,‡ Mei Han,| Changhai Liang,†,‡ and Jieshan Qiu*,†,‡ Carbon Research Laboratory, Center for Nano Materials and Science, State Key Lab of Fine Chemicals, School of Chemical Engineering, Dalian UniVersity of Technology, 158 Zhongshan Road, P.O. Box 49, Dalian 116012, China, Key Laboratory for Micro/Nano Technology and System of Liaoning ProVince, Dalian UniVersity of Technology, Dalian 116023, China, Department of Chemistry, The City College of New York, New York 10031, and The Graduate School of the City UniVersity of New York, New York 10031, and Fundamental Chemistry Laboratory Center, Dalian UniVersity of Technology, Dalian 116024, China ReceiVed NoVember 21, 2007 Carbon nanotubes fabricated by the dc arc discharge method (ADCNTs) and chemical vapor deposition method (CVDCNTs) were oxidized with concentrated HNO3 to modify their surface chemistry. The materials were characterized using SEM, TEM, FTIR, XPS, potentiometric titration, and nitrogen adsorption. The initial and oxidized materials were used as adsorbents of cadmium from aqueous solutions with different pH. Langmuir and Freundlich adsorption models were applied to fit the isotherm data, and both models fit the experimental data very well. The acid oxidation resulted in an increase in the number of oxygen-containing groups without drastic changes in the texture of the adsorbents. Although the small volume of micropores is present, the nanotube structure can be considered as nonporous. The lack of developed microporosity in carbonaceous materials eliminates the inner surface diffusion problems and makes the vast majority of surface groups available for adsorption of cadmium. The availability of these centers depends on the pH of the solution, which controls the protonation level. In spite of the fact that the pH of the solution affects the speciation of cadmium to some degree, the surface chemistry is the predominant force for adsorption at the pH range adopted in the present study, while the texture of materials also affects the nanotube’s cadmium-adsorbing performance.

Introduction 1

Carbon nanotubes (CNTs) have attracted much attention of scientists from various fields of science and technology. This is owing to the combination of such features as their structural perfection, small size, low density, high stiffness, high strength and excellent electronic properties. Examples of CNT applications include material reinforcement, field emission panel display, chemical sensor, drug delivery, and nanoelectronics.2–6 Alhough the internal pores or central cavities of CNTs can be opened by chemical treatment such as oxidation and electrochemical etching,7,8 and the spaces between tubes contribute to porosity, the CNT materials are generally considered nonporous. * Corresponding author. Tel: +86-411-88993970. Fax: +86-41188993991. E-mail: [email protected]. † Carbon Research Laboratory, Center for Nano Materials and Science, State Key Lab of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology. ‡ Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian University of Technology. § The City College of New York. | Fundamental Chemistry Laboratory Center, Dalian University of Technology. (1) Iijima, S. Nature 1991, 354, 56. (2) Ajayan, P. M.; Stephan, O.; Colliex, C.; Trauth, D. Science 1994, 265, 1212. (3) Rinzler, A. G.; Hafner, J. H.; Nikolaev, P.; Nordlander, P.; Colbert, D. T.; Smalley, R. E.; Lou, L.; Kim, S. G.; Tománek, D. Science 1995, 269, 1550. (4) Deheer, W. A.; Châtelain, A.; Ugarte, D. Science 1995, 270, 1179. (5) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K. Science 2000, 287, 622. (6) Ebbesen, T. W.; Lezee, H. J.; Hiura, H.; Bennett, J. W.; Ghaemi, H. F.; Thio, T. Nature 1996, 382, 54. (7) Chen, M.; Yu, H. W.; Chen, J. H.; Koo, H. S. Diamond Relat. Mater. 2007, 16(4–5), 1110. (8) Ito, T.; Sun, L.; Crooks, R. M. Electrochem. Solid-State Lett. 2003, 6(1), C4–C7.

Recently, there are reports in the literature addressing applications of CNTs in environmental protection as an efficient adsorbent for removal of dioxin, fluoride, nickel, zinc, lead, and cadmium9–15 that have shed some light on the mechanism of adsorption in complex systems. Although various modified CNTs were used to remove metal ions from aqueous solutions, the state of the metal ion adsorbed on the surface of adsorbents has not been addressed in detail, because of the adsorption complexity in aqueous solutions. Even though the adsorption on these materials does not look promising from the viewpoint of the real-world applications, this kind of study is still of interesting because it could shed new light on the mechanism of adsorption in complex systems. The objective of the present work is to analyze the adsorption mechanism of cadmium on carbonaceous nonporous materials, i.e. nanotubes fabricated by the dc arc discharge method (ADCNTs) and chemical vapor deposition method (CVDCNTs). The results are discussed on the basis of the detailed study of the surface chemistry and texture of the concerned CNTs. The texture of the CNT is not affected to a great extent after mild oxidation in acid and can be considered to remain unchanged. The advantage of making use of this kind of materials for (9) Long, R. Q.; Yang, R. T. J. Am. Chem. Soc. 2001, 123, 2058. (10) Li, Y. H.; Wang, S. G.; Cao, A. Y.; Zhao, D.; Zhang, X. F.; Xu, C. L.; Luan, Z. K.; Ruan, D. B.; Liang, J.; Wu, D. H.; Wei, B. Q. Chem. Phys. Lett. 2001, 350, 412. (11) Li, Y. H.; Wang, S. G.; Wei, J. Q.; Zhang, X. F.; Xu, C. L.; Luan, Z. K.; Wu, D. H.; Wei, B. Q. Chem. Phys. Lett. 2002, 3057, 263. (12) Li, Y. H.; Wang, S. G.; Luan, Z. K.; Ding, J.; Xu, C. L.; Wu, D. H. Carbon 2003, 41, 1057. (13) Lu, C.; Liu, C. J. Chem. Technol. Biotechnol. 2006, 81, 1932. (14) Lu, C.; Chiu, H. Chem. Eng. Sci. 2006, 61, 1138. (15) Lu, C.; Chiu, H.; Liu, C. Ind. Eng. Chem. Res. 2006, 45, 2850.

10.1021/la703638h CCC: $40.75  2008 American Chemical Society Published on Web 09/26/2008

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fundamental studies of heavy metal adsorption on carbon adsorbents is the lack of significant microporosity, which makes it possible to eliminate inner surface diffusion limitations affecting the performance of microporous adsorbents. A valuable tool applied to evaluate the cadmium removal mechanism is X-ray photoelectron spectroscopy (XPS), which could yield information of interactions/reactive adsorption between an adsorbate and the active groups on the adsorbent surface.16–19 Formation of a new bond is expected to change the distribution of electrons around the corresponding atoms (Cd); as such, the binding energy of the electrons is shifted to a lower or higher level.

Experimental Section Materials. CNTs used were made by different techniques. The ADCNTs were fabricated by the traditional dc arc discharge method with spectral grade graphite as a raw material.20 CVDCNTs produced by the CVD method were purchased from Shenzhen NANO Tech. Port. Co. Ltd. Both CNTs were refluxed in concentrated HNO3 at 120° for 4 h, and then, the suspensions were washed with distilled water to constant pH. The oxidized CNTs are referred to as ADCNTsox and CVDCNTs-ox. Fourier Transform Infrared Spectroscopy (FTIR). The functional groups of the adsorbents were examined using an AVATAR 360 ESP FT-IR spectrometer (Nicolet Instrument Corp.) equipped with a DTGS KBr detector and controlled by EZ OMNIC 5.0 software. All the data have been measured in the range from 4000 to 400 cm-1 with 64 scans at a resolution of 4 cm-1. Samples were homogeneously crushed with anhydrous potassium bromide in a proportion of 1/50. The powder was pressed at 10 t/cm2 to make a translucent tablet for recording FTIR spectra. X-ray Photoelectron Spectroscopy (XPS). XPS analysis of the CNTs before and after adsorption of Cd (II) ions were conducted on VG ESCALAB MK2 X-ray photoelectron spectrometer (The Amicus, Kratos Analytical Inc.), with an aluminum anode (Al KR ) 1486.6 eV) to determine the elements such as C, O, and Cd atoms present in the samples. The X-ray source was run at a reduced power of 250 W, and the pressure in the analysis chamber was maintained from 6 × 10-7 to 2 × 10-6 Pa during the measurement. The highresolution scans were performed over the 275–300, 400–425, 522–547 eV ranges (C 1s, Cd 3d, and O 1s spectra, respectively). All binding energies were referenced to the neutral C 1s peak at 284.3 eV. Potentiometric Titration. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm Herisau Co.). The instrument was set at the mode when the equilibrium pH was collected. For each run, about 0.10 g of the sample in 50 mL of 0.01 M NaNO3 was placed in a container thermostated at 298 K and equilibrated overnight with the electrolyte solution. To eliminate the influence of atmospheric CO2, the suspension was continuously saturated with N2. The carbon suspension was stirred throughout the measurements. The volumetric standard NaOH (0.1 M) was used as the titrant. The experiments were done in a pH range of 3–10. Each sample was titrated with base after acidifying the sample suspension. The surface properties were examined first using the potentiometric titration method.21–23 Here, it is assumed that the population of sites can be described by a continuous pKa distribution, f(pKa). The experimental data can be transformed into a proton binding isotherm, Q, representing the total amount of protonated sites, which is related to the pKa distribution by the following integral, eq 1 (16) Li, J.; Bai, R. B. Langmuir 2002, 18, 9765. (17) Zhang, X.; Bai, R. B. Langmuir 2002, 18, 3459. (18) Bai, R. B.; Zhang, X. J. Colloid Interface Sci. 2001, 243, 52. (19) Zhang, X.; Bai, R. B. J. Mater. Chem. 2002, 12, 2733. (20) Saito, Y. New Diamond Front. Carbon Technol. 1999, 9(1), 1. (21) Jagiello, J.; Bandosz, T. J.; Schwarz, J. A. Carbon 1994, 32, 1026. (22) Salame, I. I.; Bandosz, T. J. Ind. Eng. Chem. Res. 2000, 39, 301. (23) Jagiello, J. Langmuir 1994, 10, 2778.

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Q(pH) )

∫-∞∞ q(pH, pKa)

f(pKa) dpKa

(1)

The solution of this equation is obtained using a numerical procedure24 in which the regularization is combined with nonnegativity constraints. On the basis of the spectrum of acidity constants and the history of the samples, the detailed surface chemistry was evaluated. Adsorption Isotherms of Nitrogen. The sorption isotherms of nitrogen at 77 K were determined using an ASAP 2010 (Micromeritics). Before the experiments, the samples were outgassed at 120 °C to a constant vacuum (10-4 Torr), while the exhausted samples were outgassed at 100 °C to minimize vaporization of elemental sulfur and weakly bonded sulfuric acid. From the isotherms, the surface area (BET method), SBET; total pore volume, Vt (from the last point of isotherm at relative pressure equal to 0.99); volume of micropores, Vmic (DR25); mesopore volume, Vmes, and pore size distribution (PSD) were calculated (DFT26,27). Scanning Electron Microscopy (SEM). The SEM examination was carried out using a JEOL JSM-5600LV microscope (JEOL Ltd., Tokyo, Japan). The samples were directly mounted on to the sample holder with a piece of electrically conductive glue. Transmission Electron Microscopy (TEM). The CNTs were examined using TEM (Philips Technai-G2 20, Philips, The Netherlands). The CNTs were dispersed in ethanol under ultrasonic conditions, and then, one drop of the diluted dispersion was placed on a 200-mesh carbon-coated copper grid. Atomic Absorption Spectrometry (AAS). The concentration of cadmium was measured using a UNICAM 969 spectrometer (Unicom Analytical Systems, Cambridge, UK) at a wavelength of 228.8 nm and at 4-s intervals. A lamp with an air/C2H2 flame was set to 50% current. The deuterium lamp was used for background. Working standard solutions (1, 2, 3 mg L-1) were prepared using singleelement stock standard solutions (1 g L-1) (Beijing NCS Analytical Instruments Co., Ltd.) with double-distilled water. The Solaar 32 software was used to control the experiments. The linear coefficient for calibration was over 0.995, the minimum detection limit was 0.0145 mg L-1, and the relative standard deviations and the average values of relative errors were 0.2% and 3.8%, respectively. Adsorption of Cadmium from Solution. Cadmium stock solution (1000 mg L-1) was made by dissolving analytical grade cadmium chloride in double-distilled water. The solution was further diluted to the required concentrations before use. Adsorption experiments were carried out in Erlenmeyer flasks at 298 K. Adsorption isotherms were obtained by adding 0.2 g of the adsorbent into 100 mL of cadmium chloride solutions with the initial concentrations of cadmium ions of 1–5 mg L-1 with an interval of about 1 mg L-1. Values of solution pH of 3.50, 5.50, and 6.50 were chosen in order to avoid undesired hydrolysis and precipitation of Cd2+ and to change the level of deprotonation of strongly acidic groups. The pH values were adjusted by adding a controlled amount of 0.1 M nitric acid or sodium hydroxide solutions. To study the influence of the pH value on adsorption, 0.2 g of the adsorbent was introduced into 100 mL of cadmium chloride solutions containing about 3 mg L-1 of cadmium component. The pH values of the solution were adjusted from 2.00 to 12.00 by adding 0.1 M nitric acid or sodium hydroxide solution. The flasks with the adsorbent and the solution were mounted on a shaker (HZQ-C) that was shaken continuously for 6 h at 298 K. The time required to achieve equilibrium was obtained by measuring the adsorbate concentration versus the adsorption time. Then the suspensions were filtered using 0.45 µm membrane filters. The concentration of ions in the filtrates was immediately analyzed by AAS. The amount of cadmium on the adsorbent was determined by the difference between the initial concentration and the equilibrium (24) Dubinin, M. M. In Chemistry and Physics of Carbon; Walker, P. L., Ed.; Dekker: New York 1966. (25) Lastoskie, C.; Gubbins, K. E.; Quirke, N. J. Phys. Chem. 1993, 97, 4786. (26) Olivier, J. P. J. Pourous Mater. 1995, 2, 9. (27) Rao, M. M.; Ramesh, A.; Rao, G. P. C.; Seshaiah, K. J. Hazard. Mater. 2006, 129, 123.

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Langmuir, Vol. 24, No. 20, 2008 11703 Table 1. Structural Parameters Calculated from the Nitrogen Adsorption sample ADCNT ADCNT-ox CVDCNT CVDCNT-ox

Figure 1. SEM images of (A) ADCNTs, (B) ADCNTs-ox, (C) CVDCNTs, and (D) CVDCNTs-ox.

Vt Vmic Vmes SBET (m2 g-1) (cm3 g-1) (cm3 g-1) (cm3 g-1) Vmic/Vt(%) 12 19 66 89

0.064 0.068 0.196 0.243

0.006 0.007 0.025 0.035

0.058 0.061 0.171 0.208

9 10 13 14

The CNT materials were further examined using TEM, of which the typical images are shown in Figure 2. It can be seen that the ADCNTs material mainly consists of straight multiwalled CNTs that have an average inner diameter of 10 nm, an outer diameter of 30 nm, and a length ranging from hundreds of nanometers to micrometers. In the case of ADCNTs, some carbon particles with graphite layer structure are present. After the oxidation in acids, the morphology of the ADCNTs remains unchanged, implying that the CNT structure is not destroyed by concentrated nitric acid. In the case of CVDCNTs, the curved shapes or tube bending are not uncommon due to two kinds of structural defects. One is related to an insertion of pentagonal and heptagonal rings in the otherwise perfect hexagonal network. Another type of structural defect is due to sp3-hybridization of carbon atoms that originates from the formation of C-H bonds.28 The structure defects and hybridization are expected to provide active sites for adsorption.29,30 The pore structure of the CNT materials was analyzed using N2 adsorption at 77 K. From the N2 isotherms, the structural parameters are calculated and shown in Table 1. The results clearly show that the CNT materials can be regarded as nonporous with a small surface area ranging from 10 to 90 m2 g-1. The maximum ratio of microposity (Vmic/Vt) reached 14% for the CVDCNTs-ox. The acid oxidation brought about some changes

Figure 3. Pore size distributions for the initial and oxidized CNT samples.

Figure 2. TEM images of (A) ADCNTs, (B) ADCNTs-ox, (C) CVDCNTs, and (D) CVDCNTs-ox.

concentration. The filtrates were further analyzed using XPS. Doubledistilled water was used throughout the present work.

Results and Discussion Figure 1 presents the typical SEM images of materials used in the present work, showing that ADCNTs and ADCNTs-ox are rather long and straight whereas CVDCNTs and CVDCNTs-ox are curved. These CNT materials are different in terms of the spatial arrangements, indicating the existence of the differences at least in the volume of large pores. In the case of ADCNTs, the oxidation treatment apparently removes impurities, resulting in “tighter” texture, likely as a result of increased dispersive interactions between the tubes.

Figure 4. FTIR spectra for the CNT materials studied.

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Figure 5. Results of XPS analysis. C1s spectra along with their deconvolution: (A) ADCNTs, (B) ADCNTs-ox, (C) CVDCNTs, and (D) CVDCNTsox.

Figure 6. Results of XPS analysis. O 1s spectra along with their deconvolution: (A) ADCNTs, (B) ADCNTs-ox, (C) CVDCNTs, and (D) CVDCNTsox.

in the porosity. The surface and the volume of pores have increased by 35% because of the removal of impurities and the formation of some defects on the tube walls. Nevertheless, the majority of the surface is still in large pores. The CVDCNTs have higher

surface area than ADCNTs, which is in agreement with the SEM and TEM observation discussed above. The PSDs calculated using DFT are shown in Figure 3, revealing that the majority of pores in all CNT samples have

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Langmuir, Vol. 24, No. 20, 2008 11705

Figure 7. (A) Proton uptake curves for the initial and oxidized samples and (B) distributions of acidity constants. Table 2. pKa of the Species Present on the Surface and the Numbers of Groups pKa (numbers of groups, mmol g-1) sample

pH

ADCNTs ADCNTs-ox CVDCNTs CVDCNTs-ox

8.03 4.67 6.70 3.82

4–5

5–6 5.56 (0.005)

5.00 (0.013) 4.23 (0.002) 4.50 (0.059)

5.67 (0.054)

6–8 7.39 6.48 6.22 6.75

sizes bigger than 30 Å, and the acid oxidation causes only small changes mainly in the volume of pores bigger than 300 Å. In the case of CVDCNT samples, a small volume of pores with sizes of about 10 Å is present, which increases slightly after the oxidation treatment. These pores originated from the spatial arrangement of curved nanotubes and the defects in the tube walls. The spatial arrangement is related to the shape of the tubes, which leads to a more heterogeneous texture of CVDCNTs in comparison to ADCNTs. The PSDs of the carbon materials were obtained by adopting the slit shape model of pores, which clearly reveals the variation trends in porosity. FITR has been widely used as a rapid yet reliable method to qualitatively determine the surface chemistry of carbonaceous materials. The acid oxidation, even though it does not result in visible changes in the porosity, is expected to change the surface chemistry of the CNT materials. Figure 4 is the FTIR spectra for the CNT adsorbents studied. In the case of the ADCNTs, the bands at 1080 cm-1 and 1730 cm-1 and the broad band of over 3000 cm-1 are due to the stretching vibrations of C-O, the stretching vibrations of -CO- in saturation ester, and hydroxyl groups,31 respectively. This indicates that even in ADCNTs oxygen-containing functional groups such as hydroxyl, carboxyl, and carbonyl groups are present. These functional groups may have their origin in the weak ambient air oxidation of the ADCNTs surface. For the ADCNTs-ox sample, all peaks are well-defined. The variation in the peak intensity implies that the quantities of hydroxyl, carboxyl and carbonyl on the surface of ADCNTs-ox increase. In the case of CVDCNTs, the 1180 cm-1 band represents the stretching vibration of C-O groups, and the bands at 1730 cm-1 and over 3000 cm-1 can be ascribed to the stretching vibrations of -CO- and hydroxyl groups. The intensities of all bands increase after the oxidation treatment as a result of the generation of more functional groups. It is expected that these groups will function as the adsorption centers for cadmium via a cation exchange process.32,33 (28) Bulusheva, L. G.; Okotrub, A. V.; Asanov, I. P.; Fonseca, A.; Nagy, J. B. J. Phys. Chem. B 2001, 105, 4853. (29) Sa´nchez-Polo, M.; Rivera-Utrilla, J. EnViron. Sci. Technol. 2002, 36, 3850. (30) Rivera-Utrilla, J.; Sa´nchez-Polo, M. Water Res. 2003, 37, 3335. (31) El-Hendawy, A. A. Carbon 2003, 41(4), 713.

8–9

(0.010) (0.036) (0.053) (0.071)

8.05 8.93 8.14 8.17

(0.010) (0.017) (0.025) (0.021)

9–10

9–11

9.64 (0.025) 10.12 (0.058) 9.73 (0.108) 9.27 (0.028)

9.75 (0.031)

all 0.050 0.124 0.188 0.264

XPS is another powerful tool to determine the surface chemistry of materials. Figures 5 and 6 present the C 1s and O 1s XPS spectra of the CNT materials. An asymmetric tailing that indicates the contribution of oxygen-containing functional groups can be clearly seen for the initial and oxidized CNT samples,34–36 even though for the oxidized CNTs this feature seems to be much more pronounced. The C 1s peak is deconvoluted into four symmetric peaks of the Gaussian type, as shown in Figure 5. For the CNTs studied, except for a main peak at 284.3 eV due to the graphitic carbon, the three other peaks associated with C-C are assigned to C-O bonds of functional groups such as hydroxyl and/or ethers (at 286.1 eV), CdO as in the carbonyl group (at 287.6 eV), and -O-CdO characteristic of carboxylic and/or ester groups (at about -289.1 eV), respectively.37,38 The O 1s XPS spectrum for both kinds of nanotubes (Figure 6) was deconvoluted into four peaks with binding energies of 531.1, 532.3, 533.3, and 534.2 eV that are assigned to the OdC (at 531.1 eV); carbonyl oxygen atoms in esters, anhydrides, hydroxyls or esthers (at 532.3 eV); ether oxygen in esters and anhydrides (at 533.3 eV); and the oxygen atom in carboxylic groups (at 534.2 eV).37,39 As revealed by the peak areas, the carboxylic acids seem to be the predominant surface-oxygen-containing species, which is further supported by the FTIR result and the low pH of the oxidized nanotubes. For the initial CNT samples

Figure 8. The variation of absorption rate of Cd2+ with adsorption time (pH 6.50, at 298 K).

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Figure 9. Fits of the data obtained to (A) the pseudo-first-order and (B) pseudo-second-order kinetic models of the adsorption process (9, ADCNTs; b, ADCNTs-ox; 2, CVDCNTs; [, CVDCNTs-ox). Table 3. Kinetic Parameters of Cd2+ Adsorbed on the Adsorbents for Two Models at 298 K and pH 6.50 sample

initial Cd(II) concn (mg L-1)

k1(min-1)

ADCNTs ADCNTs-ox CVDCNTs CVDCNTs-ox

2 2 2 2

0.0263 0.0491 0.0164 0.0688

pseudo-first-order qe(mg g-1) 0.0398 0.7581 0.0959 1.0001

r2

qe(mg g-1)

0.3005 0.3769 0.1452 0.3435

0.0381 0.7580 0.0791 0.9457

pseudo-second-order k2(g mg-1 min-1)

r2

10.2293 2.8735 10.8491 4.8528

0.9958 0.9999 0.9985 1.0000

Table 4. Fitting Parameters of Langmuir and Freundlich Adsorption Isotherm Models for Cd2+ at Various pH Values Langmuir sample ADCNTs ADCNTs-ox CVDCNTs CVDCNTs-ox

pH

qm(mg g-1)

KL(L mg-1)

3.5 5.5 6.5 3.5 5.5 6.5 3.5 5.5 6.5 3.5 5.5 6.5

0.0830 0.3728 1.1861 1.8225 0.0952 2.3180 3.1415 3.6160

2.6638 0.8228 4.6105 4.6322 0.6727 1.0532 7.9779 26.3381

before oxidation, although oxygenated groups exist on the surface, the deconvolution suggests fewer carboxylic groups present, which is consistent with the FTIR results. It is important to mention that the effect of oxidation, however visible, is not so well pronounced as in the case of activated carbons. The number and strength of surface groups can be quantitatively obtained from the potentiometric titration study. The proton uptake curves shown in Figure 7A show that, after oxidation, the CNT

Figure 10. Variation of the Cd removal rate with the pH of the solution: (A) CVDCNTs-ox, (B) ADCNTs-ox, (C) CVDCNTs, and (D) ADCNTs.

Freundlich r2

n

KF

r2

0.9581 0.9902 0.9968 0.9651 0.9386 0.9942 0.9924 0.9946

4.1181 2.3883 4.3983 3.1044 1.9673 1.7322 3.8251 4.5467

0.0529 0.1649 0.8777 1.4249 0.0370 1.0919 2.6588 3.5409

0.8367 0.9650 0.9918 0.9931 0.9319 0.9961 0.9986 0.9953

surface becomes more acidic, as evidenced by an increase in proton release (negative values of Q). As revealed by the FTIR and XPS study, the initial CNT samples also contain a measurable number of surface groups. The effect of oxidation is much more pronounced for CVDCNTs than for ADCNTs, which might be related to more defects present in CVDCNTs and their higher surface area. Deconvolution of the proton uptake curves using the SAIES procedure24 leads to the distributions of acidity constants that are presented in Figure 7B, where areas under the peaks represent the number of groups of the strength determined by the pKa of the specific peak. Although the specific assignment of the peaks to the organic functional groups is impossible in the case of carbonaceous surfaces,21–23 it is accepted that the groups with pKa less than 8 represent carboxylic acid, while those with pKa (32) Yue, Z. R.; Jiang, W.; Wang, L.; Toghiani, H.; Gardner, S. D.; Pittman, C. U., Jr Carbon 1999, 37(10), 1607. (33) Kortum, G.; Vogel, W.; Andrussow, K. Dissociation Constants of Organic Acids in Aqueous Solutions; Burtterworth: London, 1961. (34) Kelemen, S. R.; Freund, H. Energy Fuels 1988, 2, 111. (35) Gardner, S. D.; Singamsetty, C. S. K.; Booth, G. L.; He, G. Carbon 1995, 33, 587. (36) Park, S. H.; McClain, S.; Tian, Z. R.; Suib, S. L.; Karwacki, C. Chem. Mater. 1997, 9, 176. (37) Darmstadt, H.; Roy, C.; Kaliaguine, S. Carbon 1994, 32, 1399. (38) Lee, W. H.; Lee, J. G.; Reucroft, P. J. Appl. Surf. Sci. 2001, 171, 136. (39) Hüttinger, K. J.; Zielke, U.; Hoffman, W. P. Carbon 1996, 34, 983.

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Langmuir, Vol. 24, No. 20, 2008 11707

Figure 11. Cadmium adsorption isotherms measured at different pH along with the fits to Langmuir (solid lines) and Freundlich (dotted lines) equations: (A) ADCNTs, (B) ADCNTs-ox, (C) CVDCNTs, and (D) CVDCNTs-ox.

more than 8 represent -OH groups attached to the carbon surface.40 It can also be seen that, for the initial CNTs, CVDCNTs are more acidic than ADCNTs, which is demonstrated by more peaks with much higher intensity. After oxidation the heterogeneity of surface increases, and for both CNT materials, new stronger acids are formed, especially in the range of carboxylic groups. These data are summarized in Table 2. The pH values listed in Table 2, in spite of the fact that they do not represent the pHPZC (pH of the point of zero charge), provide comparable information about the average acidity (number and strength) of the CNT samples. The effect of oxidation is similar for both CNT samples, as evidenced by the decrease of about 3 pH units. Moreover, CVDCNTs-ox is the most acidic and ADCNTs is the least acidic, and this should affect their ability to adsorb cadmium as described below. After extensive surface characterization of the nanotubes, the adsorption of cadmium from solution was carried out. The kinetic study shows that the adsorption of cadmium on CNTs is a fast process without visible diffusion limitations and levels off or reaches a plateau after 60 min, indicating the equilibrium time is about 60 min, as shown in Figure 8. Because of this, it is reasonable to analyze the kinetics using pseudo-first-order and pseudo-second-order sorption equations.27,41 Below are the equations used

dqt ) k1(qe - qt) dt dqt ) k2(qe - qt)2 dt

(2) (3)

where k1 and k2 are the rate constants for the pseudo-first-order and pseudo-second-order adsorption processes, respectively, qt

is the amount adsorbed at time t and qe is the amount adsorbed at equilibrium. From the linear forms of these equations, and assuming that qt ) 0 at t ) 0 and qt ) qt at t ) t, the rate constants and qe can be calculated:

log(qe - qt) ) log qe t 1 1 ) + t qt k q 2 qe

k1 t 2.303

(4) (5)

2 e

The fits presented in Figure 9 and Table 3 indicate that the adsorption follows a pseudo-second-order kinetic adsorption model, which is common for removal of metals on carbonaceous materials.27,41–43 The variation of the removal rate of cadmium with an increasing pH of the solution is shown in Figure 10. Although the adsorption of cadmium on the CNT surfaces is expected to occur mainly via a cation exchange process, the pH would strongly influence the electrostatic forces.44 The surface complex formation theory (SCF)45 explains an increase in the metal removal with an increase in the pH, which is due to a decrease in competition between proton and metals species for the surface active sites. With an increase in the pH, the surface positive charge decreases, resulting in a lower Coulombic repulsion of the adsorbent surface. Moreover, if the surface is heterogeneous and organic functional (40) Kadivelu, K.; Namasivayam, C. AdV. EnViron. Res. 2003, 7, 471. (41) Ho, Y. S.; Mckay, G. Process Saf. EnViron. Prot. 1998, 76B, 183. (42) Li, Y. H.; Di, Z. C.; Ding, J.; Wu, D. H.; Luan, Z. K.; Zhu, Y. Q. Water Res. 2005, 39, 605. (43) Li, Y. H.; Di, Z. C.; Luan, Z. K.; Ding, J.; Zuo, H.; Wu, X. Q.; Xu, C. L.; Wu, D. H. J. EnViron. Sci 2004, 16(2), 208. (44) Seco, A.; Marzal, P.; Gabaldon, C. Sep. Sci. Technol. 1999, 34, 1577.

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Gao et al.

Figure 12. Deconvolutions of Cd 3d region XPS: (A) ADCNTs and ADCNTs-ox and (B) CVDCNTs and CVDCNTs-ox.

Figure 13. (A) Dependence of the amount of cadmium adsorbed at the pH 6.5 on the pH of the carbon surface (from Table 2) and (B) dependence of qm and KF (from Table 4) at pH 3.5, 5.5, and 6.5 on the amount of groups being dissociated at those pH values. Table 5. Comparison of the Adsorption Capacity at Specific pH to the Number of Groups Dissociated at the Same pH surface density (group Å-2) sample ADCNTs ADCNTs-ox CVDCNTs CVDCNTs-ox

a

a

solution pH

amount adsorbed (mmol g-1)

no. of groups (mmol g-1)

carboxylic groups

all acidic groups

3.5 5.5 6.5 3.5 5.5 6.5 3.5 5.5 6.5 3.5 5.5 6.5

0.0007 0.0033 0.0106 0.0162 ----0.0008 0.0206 0.0279 0.0322

0 0 0.003 0 0.009 0.035 0 0.008 0.032 0 0.078 0.121

0.002 0.002 0.002 0.015 0.015 0.015 0.005 0.005 0.005 0.012 0.012 0.012

0.020 0.020 0.020 0.042 0.042 0.042 0.017 0.017 0.017 0.018 0.018 0.018

Number of groups dissociated at the pH at which the isotherms were measured.

groups containing oxygen are expected to be the cadmium adsorption sites,45,46 increasing the pH will lead to an increase in the number of dissociated groups, which, in our case, are acids of various strength represented by the pKavalues listed in Table 2. For ADCNTs-ox at pH below 3.0, the removal rate is almost zero. Then it gradually increases and reaches more than 95% at pH over 8. For the initial sample at low pH, the adsorption is negligible, but it rises steeply at pH over 8. For CVDCNTs the trend in the rate looks similar, though for the oxidized sample at low pH the removal rate is higher than for the ADCNTs-ox. This can be caused by the more acidic nature of this CNT sample, as further evidenced by the potentiometric titration results.

Although the changes in the speciation of cadmium are expected to occur at pH >8, at which hydrated cations Cd(OH)+ would be formed,29 the changes in adsorption may be mainly attributed to the electrostatic forces and the dissociation of surface acids, which creates new sites for cadmium adsorption. To assess this influence quantitatively, the cadmium adsorption isotherms were measured at pH 3.5, 5.5, and 6.5, respectively. They are presented in Figure 11 along with the fits to Langmuir and Freundlich models that are expressed as follows (A) Langmuir model

(45) Dzombak, D. A.; Morel, F. M. M. Hydrous Ferric Oxide; Wiley: New York, 1990.

(46) Li, Y. H.; Zhu, Y. Q.; Zhao, Y. M.; Wu, D. H.; Luan, Z. K. Diamond Relat. Mater. 2006, 15, 90.

Cd Adsorption Sites on Carbonaceous Materials

q)

qmKLC 1 + KLC

Langmuir, Vol. 24, No. 20, 2008 11709

(6)

adsorption capacity

where C is the equilibrium concentration of cadmium (mg L-1), q is the amount adsorbed (mg g-1), and qm and KL are Langmuir constants related to adsorption capacity and energy of adsorption, respectively. (B) Freundlich model

q ) KFC1⁄n

Table 6. The BET Surface Area of Various Adsorbents and Their Adsorption Capacity of Cadmium

(7)

where KF and n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. The calculated results are summarized in Table 4, showing that both Langmuir and Freundlich models could fit the experimental data, which suggests that the cadmium adsorption on CNT materials is a complex process. An increase in the n value with an increase in the pH indicates an increase in heterogenity of surface sites, which is consistent with an increase in the degree of dissociation of surface acidic groups. This strongly supports the mechanism that functional groups with different pKa are involved in the adsorption process. In order to figure out the state of cadmium adsorbed on the adsorbents, all of the CNT samples exposed to cadmium were examined by XPS spectroscopy. Before examination, all samples were added into a solution with the initial cadmium concentration of 100 mg L-1 and were kept at pH 6.50. After adsorption, the adsorbents were pressed to form tablets for XPS examination. The XPS spectra of Cd 3d are given in Figure 12. The Cd 3d5/2 spectrum of all the CNTs shows a main band centering at 405.6 eV accompanied by a secondary one at higher binding energy (Cd 3d3/2 at 412.4 eV). The characteristic energy of the cadmium element is irregular: the CdO 3d5/2 energy shows a main band centering at 404.0 eV, whereas the Cd 3d5/2 energy shows a main band centering at 404.8 eV.47 The peak centered at 405.6 eV suggests that the cadmium is combined with not only oxygen but also other elements, and their binding energy is higher than 405.6 eV. Compared to the results for the initial CNTs, the Cd 3d peak for the oxidized CNTs shows an offset of +0.3 eV. This further supports the hypothesis about the complexity of cadmium adsorption on the oxidized CNTs. The adsorption process might involve the interactions of cadmium species with different groups involving OH and other species such as chlorine present in the vicinity of the adsorption centers. Even though the amounts of cadmium adsorbed on CNTs are not high (up to 4 mg g-1), the differences in the performance are well-pronounced. As expected, the oxidized CNT samples have higher capacity than the untreated initial CNTs. Moreover, the trend in the amount adsorbed seems to follow the trend in the acidity (Table 2, Figure 13A). The fact that a part of the exponential curve presented in Figure 13A at pH