Langmuir 2004, 20, 2233-2242
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Acid/Base-Treated Activated Carbons: Characterization of Functional Groups and Metal Adsorptive Properties J. Paul Chen*,† and Shunnian Wu†,‡ Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, and Institute of High Performance Computing, 1 Science Park Road, #01-01 The Capricorn, Singapore 117528 Received May 16, 2003. In Final Form: December 21, 2003
Surface modification of activated carbons by various physicochemical methods directs an attractive approach for improvement of heavy metal uptake from aqueous solutions. Activated carbons were modified with HCl and HNO3 optionally followed by NaOH. The effects of surface modifications on the properties of the carbons were studied by the specific surface area, carbon pH, and total acidity capacity as well as by scanning electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The modifications bring about substantial variation in the chemical properties whereas the physical properties remain nearly unchanged. NaOH causes an increase in the content of hydroxyl groups, while the HCl treatment results in an increase in the amount of single-bonded oxygen functional groups such as phenols, ethers, and lactones. The HNO3 modification generates a large number of surface functional groups such as carbonyl, carboxyl, and nitrate groups. The HNO3 modification significantly increases the copper adsorption, while the HCl treatment slightly reduces the copper uptake. Most of the copper ions are adsorbed rapidly in the first 2 h; the adsorption equilibrium is established in around 8 h. An intraparticle diffusion model successfully describes the kinetics of copper adsorption onto the carbons.
Introduction Heavy metal adsorption onto activated carbons generally depends on their physical and chemical properties. Physical properties of carbons include their specific surface area, size, and porosity, whereas chemical properties are mainly determined by their surface functional groups, including carboxyls, carboxylic anhydrides, phenols, lactones, lactols, carbonyls, quinones, and quinone-like structures.1 Modification of carbon surfaces by physicochemical methods has been studied so that metal removal can be enhanced to meet increasingly stringent environmental regulations.2 Modification Approaches. Carbon surface can be modified to develop desirable physicochemical properties by adequate choice of activation procedures. It is even possible to prepare carbons with designated proportions of micro-, meso-, and macropores. Chemical Approaches. Carbons can be treated by acids, bases, or oxidizing agents to produce favorable chemical and physical properties for different applications (e.g., separation and catalysis). Gas phase oxidation mainly increases the content of hydroxyl and carbonyl surface groups, while liquid phase oxidation enhances especially that of carboxylic groups.3 Surface properties of activated carbons can be modified by inorganic acids. The modification mainly changes the surface chemistry of the carbons and sometimes alters their specific surface area and porosity. For example, treatment of carbon by phosphoric acid can cause a high * Corresponding author. E-mail:
[email protected]; jchen.
[email protected]. Fax: +1-831-303-8636; +65-6872-5483. Tel.: +65-6874-8092. † National University of Singapore. ‡ Institute of High Performance Computing. (1) Boehm, H. P. Carbon 1994, 32, 759-769. (2) Chen, J. P.; Wu, S. N.; Chong, K. H. Carbon 2003, 41, 1979-1986. (3) Figueiredo, J. L.; Pereira, M. F. R.; Freitas, M. M. A.; Orfao, J. J. M. Carbon 1999, 37, 1379-1389.
surface area and high degree of porosity.4-6 Both strongly and weakly acidic oxygen functional groups can be introduced by acid modification.7 Oxidation of activated carbons by a H2O2 solution increases hydroxide groups in the oxidized products when the solution pH is not controlled. However, when the solution pH is fixed at 2.5 or 11.5, the hydroxide groups decrease while other oxygen groups (e.g., carbonyl and carboxyl) increase.8 HNO3 treatment increases the quantity of acidic surface functional groups.9 Various surface oxygen groups and structures containing N-O bonds (nitro groups and nitrate complexes) are developed.10 The effect of the nitrate acid treatment on the surface area of carbons is not conclusive. Mazet et al. showed a significant increase in the surface area,11 while Gomez-Serrano et al. observed a slight variation.10 Polymers and chelating agents have been selected to modify activated carbons. The density of positive surface charges on the carbons can be increased after they are grafted by cationic polymers.12,13 The quantity of surface functional groups is also changed.14 (4) Toles, C. A.; Rimmer, S.; Hower, J. C. Carbon 1996, 34, 14191426. (5) Toles, C. A.; Marshall, W. E.; Johns, M. M. Carbon 1997, 35, 1407-1414. (6) Girgis, B. S.; Ishak, M. F. Mater. Lett. 1999, 39, 107-114. (7) Dandekar, A.; Baker, R. T. K.; Vannice, M. A. Carbon 1998, 36, 1821-1831. (8) Gomez-Serrano, V.; Acedo-Ramos, M.; Lopez-Peinado, A. J.; Valenzuela-Calahorro, C. Fuel 1994, 73, 387-395. (9) Noh, J. S.; Scharwz, J. A. Carbon 1990, 28, 675-682. (10) Gomez-Serrano, V.; Acedo-Ramos, M.; Jopez-Peinado, A. J.; Valenzuela-Calahorrro, C. Thermochim. Acta 1997, 291, 109-115. (11) Mazet, M.; Farkhani, B.; Bauhu, M. Water Res. 1994, 28, 16091917. (12) Preston, D. R.; Bitton, G.; Farrah, S. R. Appl. Environ. Microb. 1990, 56, 295-297. (13) Gajardo, R.; Diezs, J.; Jofre, J.; Bosch, A. J. Virol. Methods 1991, 31, 345-351. (14) Julien, F.; Baudu, M.; Mazet, M. Water Res. 1998, 32, 34143424.
10.1021/la0348463 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/13/2004
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Chen and Wu Table 1. List of Carbons in the Studya
carbon type
brief description of modification
DI-AC HA37-AC NA10-AC NA30-AC NA50-AC NA69-AC SH-AC HA37-SH-AC NA69-SH-AC
Filtrasorb 200 washed by DI water DI-AC treated by concentrated HCl and washed by DI water DI-AC treated by 10% HNO3 at a ratio of 1 g/10 mL and washed by DI water DI-AC treated by 30% HNO3 at a ratio of 1 g/10 mL and washed by DI water DI-AC treated by 50% HNO3 at a ratio of 1 g/10 mL and washed by DI water DI-AC treated by concentrated HNO3 at a ratio of 1 g/10 mL and washed by DI water DI-AC treated by 1 M NaOH and washed by DI water HA37-AC treated by 1 M NaOH and washed by DI water NA69-AC treated by 1 M NaOH and washed by DI water
a AC, activated carbon; DI, deionized; HA, hydrochloric acid; NA, nitric acid; and SH, sodium hydroxide. The numerical value in the abbreviation indicates the volume percentage.
Physical Approaches. Carbons can be treated physically for improvement of their properties for different applications. It is reported that heat treatment in an inert atmosphere can selectively remove some of these functional groups. Surface carboxylic acid groups of carbons disappear after treatment in H2 at 723 K.7 De la Puente et al. observed the removal of carboxylic acid groups at the temperature of 400-623 K.15 The elimination of stable ether groups was found at temperatures above 823 K. At higher temperatures, the concentrations of carboxylic acid groups decrease and, subsequently, ketone and quinone groups disappear.16 With a further increase in the temperature, elimination of both the ether groups and the aliphatic structures occurs.17 Menendez et al. modified an activated carbon by a microwave device in a nitrogen flow.18 Most acidic oxygen groups are removed from the carbon surface, resulting in a significant increase in the carbon pH. Metal Adsorption onto Modified Activated Carbon. Heavy metal adsorption onto activated carbons is due to a series of adsorption reactions between the metal ions and the organic functional groups in the carbons. The adsorptive behaviors may be significantly altered after the surfaces of carbons are modified.19-22 Heat-treated activated carbons can significantly improve the adsorption capacity.10 Anodic oxidation of carbon results in enhancement of both uptake capacity and kinetics for Cr(VI).19 Air oxidation can cause an improvement in copper adsorption.20 In the copper adsorption onto a sulfuric acid-modified carbon, it was observed that the concentration of acidic surface oxides on the carbon surfaces increased and cation exchange reactions occurred.21 On the other hand, the copper adsorption onto a carbon treated in an ammonia atmosphere is due to the formation of surface complexes with the nitrogen- and oxygen-containing functional groups.22 Motivation and Scope of This Study. Very few experimental studies were reported in the literature on the metal adsorption onto activated carbons modified with combined acid and base. Interactions among metal ions and functional groups in activated carbons have not been well-understood. Hence, it is worthwhile to conduct a series (15) De la Puente, G.; Pis, J. J.; Menendez, J. A.; Grange, P. J. Anal. Appl. Pyrolysis 1997, 43, 125-138. (16) Shin, S.; Jang, J.; Yoon, S. H.; Mochida, I. Carbon 1997, 35, 1739-1743. (17) Pastor-Villegas, J.; Gomez-Serrano, V.; Duran-Valle, C. J.; HigesRolando, F. J. J. Anal. Appl. Pyrolysis 1999, 50, 1-16. (18) Menendez, J. A.; Menendez, E. M.; Iglesias, M. J.; Garcia, A.; Pis, J. J. Carbon 1999, 37, 1115-1121. (19) Park, S. J.; Park, B. J.; Ryu, S. K. Carbon 1999, 37, 12231226. (20) Toles, C. A.; Marshall, W. E.; Johns, M. M. Carbon 1999, 37, 1207-1214. (21) Mostafa, M. R. Adsorpt. Sci. Technol. 1997, 15, 551-557. (22) Biniak, S.; Pakula, M.; Szymanski, G. S.; Swiatkowski, A. Langmuir 1999, 15, 6117-6122.
of experimental studies to understand the adsorption mechanisms. Filtrasorb 200 from Calgon Corp. is one of the commonly used granular activated carbons. It was selected in this study to provide a general procedure for modification of activated carbons (H-typed carbons) for industrial applications. The carbon was first treated with both nonoxidative and oxidative acids and, subsequently, modified with base. Copper was chosen as a model metal because it is a typical toxic metal. Because adsorption of other metal ions such as lead and nickel is very similar to that of copper,23 the results from this study can be used for the treatment of these metal ions. The alterations in the physicochemical properties of activated carbons with different surface-treatment methods were examined; the copper adsorption equilibrium and kinetics were studied. It is expected that other activated carbons modified by the approaches described in this paper would exhibit adsorption behavior similar to that of the modified Filtrasorb 200. Surface-modified carbons would have better adsorptive behavior than untreated carbons for metal adsorption. Experimental Methods and Materials Materials. Filtrasorb 200 carbon from Calgon Corp. (Pittsburgh, PA) was ground into particles with 20-32-mesh size. Copper chloride dihydrate from J. T. Baker (Phillipsburg, U.S.A.) and sodium nitrate, sodium hydroxide, sodium acetate, acetic acid, hydrochloric acid, nitric acid, brucine dihydrate, and silver nitrate from Merck (Germany) were used. All chemicals are of reagent grade. Sodium hydroxide and hydrochloric acid of 0.1 M were used for the pH adjustments. Modification Approaches of Activated Carbons. The carbons were treated by the various approaches described in the following and subsequently dried in a Memmert oven at 110 °C for 2 h. The carbons then were stored in a sealed container at room temperature. The activated carbon was thoroughly washed with deionized (DI) water until constant pH of the washed liquid was achieved. The washing removed the fine carbon particles. The carbon was denoted by DI-AC (Table 1). In the HCl modification, the DI-AC carbons were treated by a concentrated HCl solution (37% HCl) at a ratio of 1 g/10 mL. The HCl-carbon suspensions were shaken at 60 °C for 6 h. The carbons were then washed thoroughly with the DI water until the halide in the washing water was not detected by the AgNO3 titration. The carbons were denoted by HA37-AC (Table 1). In the HNO3 modification, the DI-AC carbons were respectively treated by 10, 30, 50%, and concentrated HNO3 solutions (69% HNO3) at a ratio of 1 g/10 mL. The HNO3-carbon suspensions were shaken at 60 °C for 6 h. The carbons were washed thoroughly with the DI water until the nitrate in the washing water was not detected by brucine titration. The carbons were denoted by NA10AC, NA30-AC, NA50-AC, and NA69-AC. (23) Chen, J. P.; Lin, M. S. Water Res. 2001, 35, 2385-2394.
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The DI-AC, HA37-AC, and NA69-AC carbons were further treated by a 1 M NaOH solution for at least 2 h. These carbons were then washed with the DI water to remove the excess sodium hydroxide until no variation in the pH of the washed liquid was detected. They were referred to as SH-AC, HA37-SH-AC, and NA69-SH-AC. Specific Surface Area. A NOVA 300 BET Analyzer (QuantaChrome, U.S.A.) was used to determine the specific surface area of the activated carbons. Before the adsorption isotherms were obtained, the activated carbon samples were purged with pure nitrogen gas overnight at a temperature of 150 °C to remove any contaminant and moisture that may have been present in the carbons. Carbon pH. A NaNO3 solution with a concentration of 0.1 M was first prepared; its blank pH was found to be 6.2. A total of 0.5 g of carbon was then added into 25 mL of the prepared NaNO3 solution. An Accumet Basic pH meter (Fisher Scientific, Singapore) was used to measure the carbon pH after the suspension was shaken for 48 h. Total Acidity Capacity (TAC). The carbons with a weight w (g) were titrated with sodium hydroxide solutions to determine their TACs (mmol/g). The samples were shaken for 48 h in a sodium hydroxide solution that had a concentration (cNaOH) of 0.1 M and a volume (vNaOH) of 50 mL in sealed polyethylene flasks. The solutions were then left for 6 h for settling of the carbon particulates. The supernatant was filtered by a 0.45-µm Teflon membrane filter. A filtrate with a volume (vS) of 10 mL was added to a standardized HCl solution that had a concentration (cHCl) of 0.1 M and a volume (vHCl) of 15 mL; excess HCl was determined by titration with standardized 0.1 M NaOH. The volume of NaOH consumed was vt. In the test, the sodium hydroxide solutions without carbons were referred to as blanks. The TAC can be calculated according to the following equation:
vNaOHcNaOH - (TAC × w) vs + cNaOHvt ) cHClvHCl vNaOH
(1)
Scanning Electron Microscopy. The surface morphology of the carbons was visualized by a scanning electronic microscope (SEM; JEOL, JSM-5600V, Japan). The SEM enables the direct observation of the changes in the surface microstructures of the carbons due to the modifications. Fourier Transform Infrared (FT-IR) Spectroscopy. FTIR spectroscopy was used to determine the vibration frequency changes in the functional groups in the carbons. The spectra of carbons were collected by an FTS-135 spectrometer (Bio-Rad, U.S.A.) within the range of 400-4000 cm-1. Specimens of various activated carbons were first mixed with KBr and then ground in an agate mortar (Merck, for spectroscopy) at an approximate ratio of 1/100 for the preparation of pellets (weight of 100 mg). The resulting mixture was pressed at 10 tons for 5 min. Sixteen scans and 8-cm-1 resolutions were applied in recording the spectra. The background obtained from the scan of pure KBr was automatically subtracted from the sample spectra. All spectra were plotted using the same scale on the absorbance axis. X-ray Photoelectron Spectroscopy (XPS). XPS (Kratos Axis Hsi, Perkin-Elmer Corp., U.S.A.) was applied to determine the surface complexes on activated carbons. The XPS spectra were obtained by applying the energy source of monochromatic Mg KR radiation (1253.6 eV), which was operated at 15 kv and 10 mA. The residual pressure in the analysis chamber was 5 × 10-8 Pa. The wide scans were conducted from 0 to 1200 eV with a pass energy of 80 eV. High-resolution scans of activated carbons were performed over the 282-294-eV range for C(1s) with the pass energy of 40 eV. The spectra were decoded by the curvefitting program with the subtraction of Shirley background and the assumption of a ratio of Gaussian (100%)-Lorentzian (0%). For calibration purpose, the C(1s) electron bond energy corresponding to graphitic carbon was referenced at 284.5 eV. When compared with the testing results in the reference, chemicals were identified as the same if the difference of the peak position was within 0.5 eV.
Table 2. Surface Properties of Various Activated Carbons carbon type
specific surface area (m2/g)
carbon pH
TAC (mmol/g)
DI-AC SH-AC HA37-AC HA37-SH-AC NA69-AC NA69-SH-AC
648 664 659 662 636 647
6.88 10.71 5.71 8.49 3.74 7.49
0.15 0.16 0.19 0.19 0.51 0.37
Adsorption Equilibrium. A series of experiments for the determination of adsorption isotherms and pH effects was carried out. In the isotherm experiments, a 100 mL of buffered 1 × 10-3 M CuCl2 solution with different amounts of activated carbons, ranging from 0.01 to 2.5 g, was added into flasks. The buffered solution with a constant pH of 4.9 was comprised of 0.07 M sodium acetate (NaAc) and 0.03 M acetic acid (HAc). Sodium chloride of 0.01 M was added; with consideration of the presence of NaAc/ HAc, the ionic strength of the solution was 0.11 M. The solution was shaken with the temperature controlled at 25 °C for 48 h to obtain equilibrium. Concentrations of metal ions were measured by an inductively coupled plasma emission spectrometer (ICP-ES; Perkin-Elmer Optima 3000, U.S.A.). The samples were acidified with concentrated nitric acid and filtered with 0.45-µm Whatman Autovial filters (U.S.A.) before analysis for the concentrations. The following Langmuir equation was used to determine the copper isothermal adsorption capacity:
qe )
qmaxbCe 1 + bCe
(2)
where qe is the amount of copper adsorbed at equilibrium (mg/g), Ce is the equilibrium concentration of copper in solution (mg/L), qmax is the maximum adsorption capacity, and b is the Langmuir constant. The model parameters were determined by a nonlinear regression approach. In the pH-effect experiment, 100 mL of 1 × 10-4 M CuCl2 solutions were prepared and added into different flasks. The solution pH was adjusted by adding hydrochloric acid or sodium hydroxide instead of using the buffered solution (sodium acetate and acetic acid). The activated carbon with the mass concentration (m) of 1.0 g/L was then added to each flask. Other procedures were the same as those in the isotherm experiments. Adsorption Kinetics. A total of 10 g of activated carbon was added to 1000 mL of buffered 1 × 10-4 M CuCl2 solution. The buffered solution and the ionic strength were the same as those in the adsorption equilibrium study. The solutions were then stirred at a constant rate. Samples were taken at various time intervals and analyzed by the ICP-ES.
Results and Discussion Physicochemical Properties. Table 2 gives the specific surface area, the carbon pH, and the TAC of the activated carbons. The variations among the specific surface areas of various activated carbons are within 2.5%, which indicates that the acid- and base-treatment processes do not change the surface area of the carbons. Wang and Lu also reported that the surface area of the carbons did not change after the HCl, HNO3, and HF modification.24 However, our finding here is quite different from that of the activated carbon modification by citric acid (HOOC-CH2-COH(COOH)-CH2-COOH).2 The citric acid treatment reduces the surface area of activated carbon by 34%. Because simple inorganic acids such as HCl and HNO3 have small molecular weights and cannot be adsorbed by organic functional groups in carbons, they do not significantly change the specific surface area of carbons. On the other hand, organic acids such as citric (24) Wang, S.; Lu, G. Q. Carbon 1998, 36, 283-292.
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acid can be adsorbed by activated carbons during the surface modification. As a result of their bigger molecular sizes, these compounds may cause pore blockage in carbons. Hence, the specific surface area of modified carbons significantly decreases. Acid-base titration of activated carbons is normally used for the characterization of surface functional groups.23 It is assumed the surface functional groups in the carbons function as a generalized amphoteric group (SOH) (sometimes referred to as a weak acidic group). The two-pK triple layer surface complex formation model has been successfully used in the modeling of metal adsorption onto activated carbons. Through the titration, the parameters of the surface reactions can be determined. Carbon pH and TAC can be used as a more straightforward approach to compare the chemical properties of carbons. Carbon pH can be treated as an approximate measure of the pH at the point of zero charge (pHPZC). According to a study by Wang and Lu,24 the difference between the carbon pH and the pHPZC is less than 2% for most of the carbons that are treated by acids. It is shown in Table 2 that the pH values for the carbons are different, ranging from 3.74 for NA69-AC to 10.71 of SH-AC. DI-AC had a pH value of 6.88, which is close to the determined value of 7.07 for the same activated carbon by a potentiometric titration method.2 The carbon pH indicates that the surfaces of the DI-AC are positively charged when the solution pH is below 6.88. The treatment of DI-AC by both HNO3 (NA69-AC) and HCl (HA37-AC) greatly reduces the carbon pH values (Table 2). It is known that stronger acidic functional groups are deprotonated at a lower pH while weaker acidic functional groups are deprotonated at a higher pH. This implies that the HNO3- and HCl-treatment processes increase the number of strong and weak acidic functional groups, respectively. Hence, NA69-AC and HA37-AC are negatively charged when the solution pH values are above 3.74 and 5.78, respectively. The NaOH treatment of the carbon (SH-AC) distinctly increases the carbon pH values; this can result in positive charges on the surfaces of the carbons at rather higher solution pH values. The affinities of different functional groups with heavy metal ions are different. Solution chemistry plays an important role in metal speciation.25 Therefore, it is expected that acid or base treatment of carbons would significantly change their heavy metal adsorption behaviors. Sodium hydroxide can be used to target various oxygen functional groups including carboxylic acid, lactone, and phenolic groups; therefore, the TAC determined by sodium hydroxide titration can quantify total oxygen functional groups that can effectively form complexes with metal ions. As shown in Table 2, NA69-AC has a TAC of 0.51 mmol/ g, 240% more than that of DI-AC. This indicates that the HNO3 treatment introduces a large number of oxygencontaining functional groups. As a result, a higher amount of sodium hydroxide is required to neutralize the functional groups in the activated carbon.9 The TAC of HA37-AC is slightly higher than that of DI-AC and much lower than that of NA69-AC. The HCl treatment does not significantly oxidize the functional groups in the DI-AC carbon. This observation is consistent with that in the carbon pH measurement. The NaOH treatment does not alter the TAC values of activated carbons (i.e., SH-AC versus DI-AC and HA37(25) Chen, J. P.; Hong, L.; Wu, S. N.; Wang, L. Langmuir 2002, 18, 9413-9421.
Chen and Wu
SH-AC versus HA37-AC). The TAC of NA69-SH-AC is 147% higher than that of DI-AC but 27% lower than that of NA69-AC. This could be due to the elimination of some acidic functional groups by NaOH. Morphology of Carbons. Figure 1 shows the effects of NaOH, HCl, and HNO3 treatments on the activated carbon surface morphologies. The DI-AC surface is covered with numerous submicrometer particles; there are many distinct cleavages on the surface. The acid and base treatments cause insignificant changes on the surface morphology of DI-AC. Some of the particles on the surfaces of the DI-AC are removed and some cleavages are eroded. The treatments, at the same time, appear to generate additional smaller cavities. FT-IR. FT-IR analysis can be used to determine the presence of the surface functional groups in the activated carbons. The effects of NaOH, HCl, and HNO3 treatments on the surface functional groups of DI-AC were examined. It can be seen from Figure 2 that several absorption peaks appear in the four FT-IR spectra of the carbons with different intensities. In the range of 3200-3600 cm-1, the bands of stretching O-H vibrations are revealed. This may be attributed to surface hydroxyl groups and physically absorbed water.26 The weak splitted peaks in 22502400 cm-1 can be assigned to double-bonded carbonoxygen groups. The strong adsorption bands in 15001600 cm-1 is suggested for the overlapping of aromatic ring stretching vibrations with the bands of carboxylate moieties.26,27 The broad peak in 1000-1200 cm-1 may be attributed to different function groups containing singlebonded oxygen atoms such as phenols, ethers, and lactones. The NaOH treatment (SH-AC) increases the relative concentrations of surface functional groups corresponding to the peaks in the range of 3200-3600 cm-1, which suggests a relative increase of hydroxyl groups. This may be due to the surface reaction occurring to lactone groups shown in eq 3.28
The HCl treatment (HA37-AC) increases the relative concentration of the peak at 1000-1200 cm-1. This implies a relative increase in single-bonded oxygen functional groups such as phenols, ethers, and lactones. A possible reaction, illustrated in eq 4, may occur.29
The two neighboring oxygen atoms are postulated to constitute one basic site, which is of a pyrone-type structure. These two atoms are proposed to be preferably (26) Zawadzki, J. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Dekker: New York, 1989; Vol. 20, pp 147-380. (27) Puziy, A. M.; Poddubnaya, O. I.; Martı´nez-Alonso, A.; Sua´rezGarcı´a, F.; Tasco´n, J. M. D. Carbon 2002, 40, 1493-1505. (28) Pendleton, P.; Wu, S. H.; Badalyan, A. J. Colloid Interface Sci. 2002, 246, 235-240. (29) Leon, C. A. L. Y.; Solar, J. M.; Calemma, V.; Radovic, L. R. Carbon 1992, 30, 797-811.
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Figure 1. SEM photographs of carbons: (a) DI-AC; (b) SH-AC; (c) HA37-AC; and (d) NA69-AC.
The HNO3-treated carbon (NA69-AC) shows a significantly different trend in the FT-IR spectra. Firstly, the appearance of a sharp absorption peak at 1380-1400 cm-1 indicates the abundant introduction of carboxylcarbonate structures26,30 and nitrate groups.31 Secondly, a remarkable increase in the relative intensity of the peak located at 3200-3600 cm-1 indicates a significantly increased content of hydroxyl groups in the carbon, which may be due to the increase of carboxylic groups. These clearly indicate that the oxidation by HNO3 generates a large number of surface functional groups such as carbonyl, carboxyl, and nitrate groups. The aliphatic side chains on the activated carbon surfaces are especially susceptible to oxidation.32 It can be assumed that oxygen and nitrogen functional groups are incorporated in the carbon basal planes with the nitric acid oxidation similar to the oxidation of aromatic hydrocarbons such as 9,10dihydrophenanthrene, diphenylmethane, and benzene. These are illustrated in eqs 5-7. This is in distinct Figure 2. FT-IR spectra of various activated carbons.
located in two different rings of a graphitic layer so as to favor resonance stabilization of the positive charge.
(30) Biniak, S.; Szymanski, G.; Siedlewski, J.; Swiatkowski, A. Carbon 1997, 35, 1799-1810. (31) Zielke, U.; Huttinger, K. J.; Hoffman, W. P. Carbon 1996, 34, 983-998. (32) Vinke, P.; van der Eijk, M.; Verbree, M.; Voskamp, A. F.; van Bekkum, H. Carbon 1994, 32, 675-686.
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Table 3. Surface Elemental Composition (atom %) for Various Carbon Samples carbon type
C
O
DI-AC SH-AC
92.70 92.04
7.30 7.96
a
N
carbon type
C
O
N
a a
HA37-AC NA69-AC
91.95 82.49
8.05 10.84
a 6.67
Not determined.
discrepancy with the activated carbon treatment by HCl or NaOH.
XPS. XPS was employed to study the shifts of the binding energy (BE) of the respective coordination atom [O(1s)] and its neighboring atom [C(1s)] in various carbon samples due to the surface modifications. The surface element compositions of DI-AC as well as NaOH-, HCl-, and HNO3-treated carbons obtained from the XPS analysis are presented in Table 3. It should be noted that these values reflect the sample composition only over a depth of about a few nanometers. The increase in the oxygen content due to the NaOH treatment is found to be rather small (9%). This observation is consistent with the reaction shown in eq 3. The HCl treatment increases the oxygen content by 10%. A possible mechanism for the increase in oxygen content by the HCl treatment is due to the chemisorption of water molecules by delocalized π electrons in the carbon basal plane according to eq 8:
Cπ + H3O+ T Cπ - H3O+
(8)
where Cπ is defined as “a graphitized carbon surface platelet having maximum itinerant π electrons”.29 The decrease in the pH value of HA37-AC (Table 2) also supports this hypothesis. The HNO3 treatment markedly decreases the percentage of surface graphitic and aromatic carbon from 92.70 to 82.49% and increases the oxygen percentage from 7.30 to 10.84%. In addition, 6.67% nitrogen is introduced, which supports the possible reaction demonstrated in eq 7. Because of the enhancement in the oxygen and nitrogen contents, it is expected that the HNO3 treatment can significantly increase the heavy metal adsorption capacity. Typical high-resolution XPS spectra of the C(1s) region are shown in Figure 3. It can be seen that various oxide surface functional groups are present in all the activated carbons. The C(1s) signal shows an asymmetric tailing, partially due to the intrinsic asymmetry of the graphitic peak and to the contribution of oxygen surface complexes.33 (33) Darmstadt, H.; Roy, C.; Kaliaguine, S. Carbon 1994, 32, 13991406.
Table 4. RPA % of C(1s) Peaks carbon type
BE of 284.5
BE of 286.0
BE of 287.2
BE of 288.5
BE of 290.0
BE of 291.5
DI-AC SH-AC HA37-AC NA69-AC
67.13 66.73 66.35 57.54
11.69 15.11 15.62 16.16
6.22 5.98 5.50 7.98
5.38 4.88 5.19 9.99
5.1 4.21 3.66 4.48
4.48 3.09 3.68 3.85
Deconvolution of the C(1s) spectra yields six peaks with different BE values representing carbon in aromatic and aliphatic groups (284.5 eV), in phenolic, alcohol, ether, or C-N groups (286.0 eV), in carbonyl, quinone, or CdN groups (287.2 eV), and in carboxyl or ester groups (288.5 eV). Carbonate or absorbed carbon dioxides and plasmon are responsible for the BEs of 290.0 and 291.5 eV, respectively. The relative peak area percentage (RPA %) of aromatic and graphitic carbon as well as carbons bonded in different oxygen-containing groups was calculated and ascribed to the fraction of surface functional groups (Table 4). It can be seen that the HCl and NaOH treatments (HA37-AC and SH-AC) do not greatly change the RPA % of aromatic and graphitic carbon (284.5 eV); however, one can observe a relatively large increase in the RPA % of the peak at 286.0 eV corresponding to single-bonded oxygen functional groups and a decrease of the peaks corresponding to the double-bonded oxygen functional groups and carboxylic groups in C(1s) of SH-AC (287.2 and 288.5 eV). The C(1s) of HA37-AC shows an even greater increase of the peak at 286.0 eV. These observations indicate that treatment by HCl or NaOH may primarily change carbonyl or carboxyl groups to phenol or lactone groups instead of the generation of additional new functional groups. This provides supportive evidence to eqs 3 and 4. As demonstrated in Table 4, the HNO3 treatment results in a distinct decrease in the RPA % of aromatics and aliphatics and the increase in the RPA % of all single- and double-bonded oxygen-containing groups and carboxylic groups. The RPA % of the carbon in aromatics and aliphatics reduces to 57.54%, lower than 67.13% of the DI-AC. These findings support the proposed reactions in eqs 5-7. Copper Adsorption Equilibrium. The copper solution chemistry was first studied by MINEQL+.34 According to the built-in database, copper precipitates of Tenorite (CuO) and Cu(OH)2 are formed according to the following reactions:
Cu2+ + H2O ) CuO + 2H+
pK ) 7.64
(9a)
Cu2+ + 2OH- ) Cu(OH)2
pK ) 8.67
(9b)
It is noted that Cu(OH)2 is first formed and subsequently converted to Tenorite. Our previous modeling study of copper adsorption onto the Filtrasorb 200 carbon demonstrates that the following adsorption reaction plays an important role:23
SOH + Cu2+ ) SO-Cu+ + H+
pK ) 1.85 (10)
Comparison of eqs 9a and 9b with 10 clearly shows that the latter is the dominant reaction. In the isothermal experiments, the initial copper concentration ranged from 1 × 10-4 to 1 × 10-3 M in pH 4.9 maintained by buffered solutions. In the pH-effect experiments, the initial con(34) Schecher, W. D.; McAvoy, D. C. MINEQL+ Chemical Equilibrium Modeling System, version 4.5 for Windows; Environmental Research Software: Hallowell, ME, 2001.
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Figure 3. XPS spectra of C(1s) of activated carbons: (a) DI-AC; (b) SH-AC; (c) HA37-AC; and (d) NA69-AC.
centration was 1 × 10-4 M at pH 1.6-5.7 (no buffered solutions). Hence, the copper precipitation unlikely occurred in both cases. In the experiments, the copper supernatant was collected, acidified, filtrated, and finally analyzed by the ICP-ES. The measured concentration was virtually the total metal concentration in the solution. If the previous precipitation reactions contribute the metal removal, the measured concentration should be higher at the higher pH. However, the pH-effect experiments did not support this assumption because the lower concentration was observed in a higher pH range (see Figure 5). Another evidence was that no precipitation was observed in the experiments. In the isothermal adsorption experiments, NaAc and HAc were used to maintain a constant pH. Using NaAc/ HAc for pH control in adsorption has been a common practice.5,20 It has to be pointed out that the presence of NaAc/HAc would change the copper solution chemistry. Speciation of copper in their presence was conducted by using MINEQL+.34 Given that the total copper concentration is 1 × 10-4 M, the percentages of Cu2+, CuAc+, CuAc2, and CuAc3- in the solution (before adsorption) are 5.15, 46.6, 40.3, and 7.86%, respectively. Additional adsorption of NaAc and HAc may take place because both are organic compounds. However, the buffered solution can provide a rather simple approach to compare the adsorptive behaviors of different modified carbons given that the copper speciation is the same. It is not a major concern because the copper adsorption behaviors onto different carbons have the same buffered solution background (NaAc/HAc). The copper adsorption is due to the formation of metal surface complexes between the functional groups and the
copper (Cu2+, CuAc+, CuAc2, and CuAc3-) with the possible reactions given as follows:
SOH + Cu2+ ) SO-Cu+ + H+
(10)
SOH + CuAc+ ) SO-CuAc + H+
(11a)
SOH + CuAc2 ) SOH-Cu-Ac2
(11b)
SOH + H+ + CuAc3- ) SOH2-CuAc3
(11c)
As a result of the different surface modifications, the characteristics of functional groups (SOH) are changed. It has to be pointed out that the formation of SOH2+ in eq 11c is reasonable because of the weak acid properties of the carbons (i.e., adsorption of hydrogen ions). Copper adsorption isotherms are illustrated in Figure 4a,b. It can be seen that the Langmuir isotherm well describes the adsorption data. The parameters (qmax and b) for the different activated carbons are listed in Table 5. The maximum copper adsorption capacity (qmax) at constant pH 4.9 follows a descending order of NA69-AC > NA69-SH-AC > NA50-AC > NA30-AC > NA10-AC > SH-AC > DI-AC > HA37-SH-AC > HA37-AC. qmax for the DI-AC is close to that for the Filtrasorb 200 (no buffered solution).35 It is shown that the HNO3 treatment significantly improves the copper adsorption. This is consistent with the remarkable increase in oxygen and nitrogen compositions (Table 3) and the enhancement of the amounts of (35) Chen, J. P.; Wang, L. Chemosphere 2004, 54, 394-404.
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Figure 4. Effect of surface modifications on copper adsorption: (a) single and combined modification; (b) HNO3 modification. The buffered solution for the maintenance of constant pH 4.9 was comprised of 0.07 M sodium acetate and 0.03 M acetic acid; sodium chloride of 0.01 M was added. The points and lines represent experimental data and Langmuir isotherm fittings. Table 5. List of Constants of the Langmuir Equation for Copper Adsorption onto Activated Carbons carbon type
qmax (mg/g)
b (mg/L)-1
correlation coefficient (r2)
DI-AC SH-AC NA10-AC NA30-AC NA50-AC NA69-AC NA69-SH-AC HA37-AC HA37-SH-AC
6.15 6.75 6.93 9.23 9.91 15.34 14.97 5.08 5.38
0.18 0.27 0.91 0.91 0.74 0.23 0.20 0.20 0.38
0.92 1.0 1.0 1.0 1.0 0.99 0.99 0.99 0.99
hydroxyl and carboxylic groups (Figure 2) after the HNO3 treatment. Comparison of qmax values indicates that the NaOH treatment slightly increases the copper adsorption of DI-AC and the HCl-treated activated carbons (SH-AC versus DI-AC and HA37-SH-AC versus HA37-AC) but reduces the copper adsorption of HNO3-treated carbons (NA69-SH-AC versus NA69-AC). This is consistent with the increase in the amount of hydroxyl groups by the NaOH treatment (Figure 2). The HCl treatment, on the other hand, decreases copper ion adsorption. This is in contrast with their increased TAC values (Table 2). The low copper adsorption may be due to the reactions between the organic functional groups and HCl/Cl2. Note that the concentrated HCl (37%) contains a certain amount of Cl2. The nucleophilic substitution, the Hell-Volhard-Zelinsky reaction, and the Markovnikov additions demonstrated in eqs 12a-d could play important roles in the reduction of metal adsorption capacity.
RsOH + HCl ) RCl + H2O RCH2COOH + Cl2 ) RCHClCOOH + HCl
(12a) (12b)
RCHdCH2 + HCl f RCHClCH3
(12c)
RCHdCH2 + Cl2 f RCHClCH2Cl
(12d)
The HCl treatment may change the surface structure of the carbon, which also decreases the copper uptake capacity. In addition, the results from the pH-effect experiments shown in Figure 5c show that the HA37-AC
does not adsorb hydrogen ions, which causes less copper uptake (discussed later). The concentration of HNO3 solution used in activated carbon modification is an important factor affecting the surface functional groups as shown in Figure 4b and Table 5. The concentration remarkably affects the activated carbon oxidation degree, and, thus, the copper adsorption is significantly changed. The activated carbons treated by the concentrated HNO3 solution have a much higher copper adsorption capacity than those treated by the diluted HNO3 solution. This indicates that the concentrated HNO3 solution introduces more surface functional groups (e.g., carboxylic groups) contributive to the copper adsorption. Dilution of the concentrated HNO3 solution to the concentration of 50% reduces the adsorption capacity by 35% (qmax of NA69-AC versus qmax of NA50-AC). However, in the HNO3 concentration range from 30 to 50%, the effect of the HNO3 concentration in the adsorption capacity is rather insignificant. The difference between the copper adsorption capacities of NA50-AC and NA30AC is only 7%. When the HNO3 concentration is further reduced to 10%, the copper adsorption capacity is substantially reduced. The copper adsorption capacity of the NA10-AC is only 13% higher than that of the DI-AC but 55% lower than that of the NA69-AC. Therefore, the concentrated HNO3 solution is much more efficient in the enhancement of copper adsorption. Solution pH plays an important role on ion adsorption onto carbons from aqueous solutions. Figure 5a shows that the copper removal increases with an increase in the initial pH before all activated carbons achieve the maximum copper ion removal. When the initial pH is around 1.6, the copper uptake by the carbons is relatively small. This can be due to the competitive adsorption of hydrogen ions with copper ions. However, when the initial solution pH increases, both HNO3- and NaOH-treated activated carbons show a sharp increase in copper adsorption. When the initial solution pH reaches 3.0, nearly 100% adsorption onto both HNO3- and NaOHtreated activated carbons is obtained. At the same initial pH of 3.0, however, the adsorption percentages of 50 and 30% are achieved for DI-AC and HA37-AC, respectively. The adsorption onto DI-AC reaches 100% when the initial pH is 4.5. A higher initial solution pH does not help to improve copper adsorption onto HA37-AC. Even when
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Figure 5. Solution pH on copper adsorption onto activated carbons, [Cu]0 ) 1 × 10-4 M, m ) 1.0 g/L, [NaCl] ) 0.01 M: (a) initial pH effect; (b) final pH effect; and (c) final pH versus initial pH.
the initial pH is set as high as 5.0, only 40% adsorption can be reached. Figure 5a is particularly helpful when the carbons are applied in the treatment of metal waste streams. As long as the initial pH is controlled above 4.5, all the carbons (except HA37-AC) can be effectively used to remove copper from aqueous solutions. The copper adsorption as a function of the final pH can be further elucidated by Figure 5b. As shown, the copper uptake by all the carbons increases when the final pH is higher. The pH edge ranges from 2 to 6.5 (except for HA37AC), similar to that reported in the literature.23 At a final pH of 3-4, adsorption of copper ions onto the carbons follows a descending order: NA69-AC > NA69-SH-AC > HA37-SH-AC > SH-AC > DI-AC > HA37-AC. This order, except for HA37-SH-AC, is the same as that when the solution pH was maintained by the buffered solutions. In the pH-effect experiments, the copper adsorption is mainly due to the formation of metal surface complex SOCu+ shown in eq 10. Comparison of solution pH during the experiments in Figure 5c shows that the final pH is much higher than the initial pH. This observation gives important supportive evidence of a strong adsorption of hydrogen ions by the carbons (SOH2+). Copper Adsorption Kinetics. The kinetics of copper ion adsorption by the activated carbons is illustrated in Figure 6. It can be seen that adsorption follows a two-step process: a fast adsorption in the first 2 h followed by a gradual process of around 8 h until the equilibrium is established. All activated carbons achieved the final copper
Figure 6. Kinetics of copper adsorption onto activated carbons [Cu]0 ) 1 × 10-4 M, m ) 10 g/L. The buffered solution for the maintenance of constant pH 4.9 was comprised of 0.07 M sodium acetate and 0.03 M acetic acid; sodium chloride of 0.01 M was added. The points and lines represent experimental data and modeling results.
removal of 90-98%. Both the HNO3 and the NaOH treatments facilitate the adsorption kinetics (NA69-AC, NA69-SH-AC, and SH-AC), while the HCl treatment slows down the uptake (HA37-AC and HA37-SH-AC). NA69SH-AC and HA37-AC show the fastest and lowest
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adsorption, respectively, under the same experimental conditions. This might be ascribed to the different distribution of surface functional groups in the carbons as well as their surface properties. An intraparticle diffusion model with an assumption of a “two-step mass transport mechanism” was used to describe the metal uptake kinetics. Metal ions first transfer through the external liquid film from the bulk solutions and, subsequently, diffuse inside carbons before finally being adsorbed by functional groups of carbons. It is assumed that the spherical carbon adsorbents are homogeneous in terms of physical and chemical properties.35,36 The governing equation and its corresponding initial and boundary conditions are shown as follows.
∂c Dp ∂ ∂c [p + Fp f ′(c)] ) 2 r2 ∂t r ∂ ∂r
(13)
q ) f(c)
(14)
( )
where p is the porosity of the particle, Fp is the particle density (kg/m3), c is the concentration in the particle pore (M), t is the time (s), Dp is the pore diffusivity (m2/s), and r is the radial distance from the center of the particle (m). It is assumed that the instant local equilibrium (q) shown in eq 14 is established, which can be described by the Langmuir equation. The initial and boundary conditions are
c)0 ∂c )0 ∂r kf ∂c ) (Cb - c*) ∂r Dp
0 e r e rp, t ) 0 r ) 0, t > 0 r ) r p, t > 0
(15) (16a) (16b)
where rp is the radius of the particle (m), kf is the external mass transfer coefficient (m/s), Cb is the bulk concentration (M), and c* is the concentration at the surface of the particle (M). The kinetic model can be solved by a finite difference method available in the literature.36 By comparing the modeling results with the experimental observations, the kinetic parameters (kf and Dp) can be obtained. Simulation of the adsorption kinetics was performed with the modeling results illustrated in Figure 6 and Table 6. One can see that the model successfully describes the experimental data. The values of the pore diffusivity (Dp) (36) Tien, C. Adsorption calculations and modeling; ButterworthHeinemann: Boston, 1994. (37) Xiu, G. H.; Li, P. Carbon 2000, 38, 975-981.
Table 6. Parameters in the Modeling of Adsorption Kineticsa carbon type
kf (m/s)
Dp (m2/s)
carbon type
kf (m/s)
Dp (m2/s)
DI-AC 4 × 10-6 3 × 10-10 HA37-SH-AC 4 × 10-6 3 × 10-10 SH-AC 5 × 10-6 3 × 10-10 NA69-AC 3 × 10-6 4 × 10-10 HA37-AC 2 × 10-6 2 × 10-10 NA69-SH-AC 8 × 10-6 5 × 10-10 a Physical parameters in the modeling: r ) 3.4 × 10-4 m, ) p p 0.6, and Fp ) 889 kg/m3.
are compatible with those of other carbons.35,37 The external mass transfer coefficients, on the other hand, are slightly lower than those for untreated carbons, which may be due to a certain degree of blockage caused by NaAc and HAc. Table 6 also shows that the NaOH treatment improves the kinetic properties of the carbon (e.g., NA69SH-AC versus DI-AC). However, the acid modification causes a negative effect of the adsorption kinetics (e.g., HA37-AC versus DI-AC). Conclusions The treatments by NaOH, HCl, and HNO3 significantly change the chemical properties of carbon, such as carbon pH and TAC, while the specific surface areas are not changed. The XPS and FT-IR studies indicate that the HNO3 modification generates a significantly large number of surface functional groups such as carbonyl, carboxyl, and nitrate groups. NaOH causes an increase in the content of hydroxyl groups. The HCl treatment results in an increase in the amount of single-bonded oxygen functional groups such as phenols, ethers, and lactones. The maximum copper adsorption capacity (qmax) in a NaAc/HAc buffered solution follows a descending order of NA69-AC > NA69-SH-AC > NA50-AC > NA30-AC > NA10-AC > SH-AC > DI-AC > HA37-SH-AC > HA37AC. A higher initial solution pH can cause higher copper adsorption; at an initial pH > 3.0, both HNO3- and NaOH-treated activated carbons attain nearly the maximum adsorption. Most of copper ions are adsorbed rapidly in the first 2 h; the adsorption equilibrium is established in around 8 h. An intraparticle diffusion model with consideration of external mass transfer and diffusion is successfully used to describe the copper adsorption kinetics. Acknowledgment. The financial support provided by the National University of Singapore (NUS) under research Grants R-279-000-104-112, 279-000-034-112, and R-279-000-062-112 is appreciated. The authors would like to thank Professor L. Hong of the Department of Chemical and Biomolecular Engineering, NUS, for his comments on the instrumental analysis. LA0348463
