Adsorption of Copper Acetate onto Pretreated Activated Carbons over


Sep 23, 2009 - Department of Chemical and Materials Engineering, Pakistan Institute of Engineering and Applied Sciences, P.O. Nilore, Islamabad 45650,...
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Ind. Eng. Chem. Res. 2009, 48, 9804–9808

Adsorption of Copper Acetate onto Pretreated Activated Carbons over a Wide Concentration Range Amjad Farooq,*,†,‡ Philippe Westreich,‡ Naseem Irfan,† and Jeff Dahn‡ Department of Chemical and Materials Engineering, Pakistan Institute of Engineering and Applied Sciences, P.O. Nilore, Islamabad 45650, Pakistan, and Department of Physics and Atmospheric Science, Dalhousie UniVersity, Halifax, NoVa Scotia B3H 3J5, Canada

The commercially available microporous activated carbon Kuraray GC was pretreated in two different ways: one was heated to 950 °C in 5% hydrogen and the other boiled in nitric acid for 5 h. BET surface area measurements and Boehm titrations were done to find the surface area and the numbers of surface functional groups for each carbon. A large range of linearly varying concentrations of aqueous copper acetate solutions was made and stirred with each sample of pretreated carbon. Atomic absorption spectroscopy, titrations, and weight measurements were employed to measure the amount of copper acetate actually adsorbed onto the respective carbon samples. The number of acidic functional groups on the carbon surface as determined by Boehm titrations in the case of the hydrogen treated sample and the nitric acid treated sample do not match with the usual Langmuir parameters both at low and high concentrations. Two distinct isotherm fittings have been obtained for both carbons; double Langmuir isotherms for the hydrogen-treated sample, and a low concentration Langmuir isotherm combined with a high concentration Freundlich isotherm for the nitric acid treated sample. The results have also been compared with the results for untreated Kuraray GC. 1. Introduction One of the most important developments in the field of impregnating activated carbon (AC), achieved years ago, was the use of copper as an impregnant to improve the ability of AC to remove toxic gases.1 The soaking method of impregnation involves stirring the AC in an aqueous solution of a metallic salt having a particular concentration. Typically, an impregnant loading of 0.1-20% by weight is required. This type of loading range requires that the adsorption onto the carbon be performed from highly concentrated solutions. One of the main goals of the present work is to understand the adsorption of concentrated solutions of copper acetate on modified activated carbons in order to optimize the impregnation process. While there is much generally available literature regarding the removal of metals using activated carbon from metal solutions of concentrations below 1 mmol/L, there is a lack of scientific literature on adsorption from concentrated solutions. Scores of articles are available which discuss the environmental remediation of aqueous industrial waste streams containing heavy metals.2-7 Almost all of these discuss and calculate Langmuir adsorption isotherm constants which are very different from the respective constants determined for the high concentration adsorption.8,9 Adsorption of high concentrations of copper acetate, which was identified as a preferentially adsorbing salt in previous work,8 has been studied on a hydrogen treated and an acid treated carbon in this study.

as follows. The GC was heated to 950 °C in 5% hydrogen (balance argon) at a rate of 40 °C/min and, after staying at that temperature for 3 h, was left to cool down to room temperature in the same atmosphere. This sample is referred to as H2TGC in this text. Nitric acid treated GC was also used in this study and is referred to as NITGC. The nitric acid treatment9 was done by boiling GC in 5 mol/L nitric acid for 5 h and then rinsing with distilled water until the pH of the wash was greater than 5. The BET surface areas, micropore volumes, and the total pore volumes of the GC, H2TGC and NITGC, determined by using a Micromeritics ASAP 2010 gas adsorption analyzer, are given in Table 1. Boehm titration results are all given in Table 2, including previously reported results for GC and NITGC.9 Boehm titration results for the NITGC tested about a year after the acid treatment were found to be within the error values mentioned by Westreich et al.9 Seventeen solutions of linearly varying concentrations of copper acetate monohydrate (obtained from Aldrich, ACS reagent grade) were prepared. For each concentration, 4.0 g of H2TGC (dried overnight at 120 °C) was added in two vessels, each containing 100 mL of solution. The 34 resulting solutions and carbon were stirred for 1 h. After leaving the vessels undisturbed for 24 h, two aliquots of each solution were drawn Table 1. BET Surface Areas, Micropore Volumes, and Total Pore Volumes of Three AC Samples

2

2. Materials and Methods Commercial grade Kuraray GC (referred to as GC in this text), an acid-washed coconut shell-based granular activated carbon (0.4% w/w ash), with particle mesh size of 12 × 35 (0.5-1.7 mm), was used. Hydrogen treatment was performed * To whom correspondence should be addressed. E-mail address: [email protected] † Pakistan Institute of Engineering and Applied Sciences. ‡ Dalhousie University.

BET surface area (m /g) micropore volume (cm3/g) total pore volume (cm3/g)

GC

H2TGC

NITGC

1560 ( 30 0.53 ( 0.01 0.55 ( 0.01

1630 ( 30 0.55 ( 0.01 0.56 ( 0.01

1330 ( 30 0.46 ( 0.01 0.54 ( 0.01

Table 2. Boehm Titration Results of Three AC Samples (mmol/g AC)

GC H2TGC NITGC

HCI (basic)

KHCO3 (carboxyl)

K2CO3 (lactone)

KOH (phenol)

0.74 ( 0.01 0.61 ( 0.01 0.02 ( 0.01

0.00 ( 0.01 0.00 ( 0.01 1.3 ( 0.1

0.07 ( 0.01 0.00 ( 0.01 2.2 ( 0.1

0.09 ( 0.01 0.03 ( 0.01 3.3 ( 0.1

10.1021/ie900492f CCC: $40.75  2009 American Chemical Society Published on Web 09/23/2009

Ind. Eng. Chem. Res., Vol. 48, No. 22, 2009

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to be analyzed for copper concentration by atomic absorption spectroscopy (AAS) at the Minerals Engineering Center of Dalhousie University. The solutions were also titrated twice with HCl using a Mettler-Toledo DL-21 autotitrator to find the concentration of acetate. In order to determine the impregnant amounts in respective carbons by weight, the remaining solutions were drained and the carefully collected carbon was dried in an oven at 120 °C for 2 h prior to weighing the dried carbon. The adsorption experiments were done without adjusting the pH, and the results were within the error limits mentioned for each. The adsorption values were also within error limits when tested for 24 and 48 h in comparison to the 1 h shaking time results. The same procedure was repeated for NITGC. 3. Results and Discussion Table 1 shows that the surface area of GC has increased upon hydrogen treatment and decreased after the nitric acid treatment. This is because heating in hydrogen to high temperatures removes almost all the acidic functional groups,10 thus increasing the surface area, whereas the acid treatment attaches a large number of acidic functional groups on the surface of AC which can block some of the micropores,9,11,12 thus decreasing the surface area to some extent. The same can be clearly seen from the micro and total pore volumes of the three carbon samples mentioned in Table 1. The reduction in micropore volume of NITGC is likely because of blocking of some of the micropores due to attachment of a large number of acidic surface functional groups after the acid treatment. It is also interesting to note that the total pore volume in the case of NITGC in nearly the same as for GC and the substantial reduction takes place only in the micropore volume which in turn reduces the BET surface area also. Both the micro- and total pore volumes show small increases on hydrogen heat treatment and thus the increase in BET surface area too. It is clear from the Boehm titration results shown in Table 2 that the hydrogen heat treatment leaves the surface of GC with very few phenolic groups and almost free from carboxylic and lactone functional groups.10,13,14 Table 2 also shows that nitric acid treatment has imparted a very large number of functional groups to the surface of GC. The hydrogen heat treatment is only able to reduce the basic groups to some extent in the case of H2TGC whereas the basic groups are almost gone in NITGC after the acid treatment. The adsorption data has been fitted to the Langmuir and the Freundlich isotherms. The linear and Langmuir terms fitted to the preferential adsorption curve are combined in an equation explained by Westreich et al.8 but written here again for clarity: n ) V′C +

N′(C/C0) exp[(µa0

- µs0)/(kBT)] + (C/C0)

(1)

where n is the number of moles of impregnant per gram of carbon, V′ is the volume of solution per gram of carbon that stays on the carbon after the draining step, C is the concentration of the solution used, C0 is 1 M, N′ is the number of moles of adsorption sites per gram of carbon, µ0a - µ0s is the difference between the chemical potentials of adsorbed and dissolved salts in the standard state, kB is Boltzmann’s constant, and T is the temperature. By subtracting the linear V′C term and rearranging eq 1, one can recover a more familiar form of the Langmuir equation:15 n bC )θ) N′ 1 + bC

(2)

Figure 1. Adsorption isotherm for copper acetate on H2TGC, fitted to a sum of two Langmuir isotherms.

where θ is the fractional coverage of impregnant and b ) exp[-(µ0a - µ0s)/(kBT)]. Curve fitting to the data enables the determination of the parameters N′ and µ0a - µ0s. On the basis of the least-squares curve fitting, the data has been divided into two parts, the low concentration and the high concentration data and hence the determination of separate N′ and µ0a - µ0s for both the isotherms has been made possible for H2TGC. Values of V′ were determined through blank experiments with water impregnation only for the hydrogen treated and the acid treated carbons and were found to be 0.84 and 0.94 mL/g, respectively. These values do not compare very well to the total pore volumes given in Table 1 for the H2TGC and the NITGC (0.56 and 0.54 mL/g, respectively). The higher value determined by water impregnation is likely due to excess water on the surface of the activated carbon particles; this value is more appropriate for use with the Langmuir equation, since the carbon is saturated with water when being drained. Some of the adsorption data were fitted to the following linearized form of the Freundlich adsorption isotherm:16 log Cads ) log Cm + (1/n)log Ce

(3)

where Cads is the adsorbed concentration of adsorbate onto adsorbent in moles per gram and Ce is the equilibrium concentration of Cu(II) ions in solution in moles per liter. The value of the constant Cm gives a measure of adsorption capacity. The Freundlich adsorption isotherm takes the heterogeneity of the surface into account as well as an exponential distribution of sites and their energies. It assumes that the heat of adsorption declines logarithmically with coverage. If this model is obeyed, a plot of log Cads vs log Ce should yield a straight line whose slope is equal to 1/n and intercept to log Cm. The value of the Freundlich sorption parameter, 1/n, is always less than unity and depicts adsorption intensity. A steep slope close to unity indicates high adsorptive capacity at higher equilibrium concentrations.17 Figure 1 displays the amount of adsorbed copper acetate onto H2TGC against the equilibrium solution concentration on a log-log scale fitted to a sum of two Langmuir isotherms. The low concentration and the high concentration isotherms are called Langmuir no. 1 and Langmuir no. 2, respectively. The titration and weight data are only shown at high concentrations as it was not possible to use these measurement techniques at low concentrations. A plateau can very clearly be seen in the fitted curve pointing toward the presence of two different mechanisms of adsorption at the low and high concentrations.

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Ind. Eng. Chem. Res., Vol. 48, No. 22, 2009 Table 3. Adsorption Parameters from Langmuir Fits for GC and H2TGC Langmuir no. 1 carbon

N′ (mmol/g AC)

GC H2TGC

0.08 ( 0.01 0.10 ( 0.01

Langmuir no. 2

(kBT)

N′ (mmol/g AC)

(µa0 - µs0)/ (kBT)

-13.5 ( 0.2 -13.3 ( 0.2

2.1 ( 0.2 2.8 ( 0.2

-2.9 ( 0.2 -2.5 ( 0.2

(µa0

µs0)/

Table 4. Adsorption Parameters from Combined Langmuir and Freundlich Fits for NITGC Langmuir

Freundlich

carbon N′ (mmol/g AC) (µa0 - µs0)/(kBT) Cm (mmol/g AC) NITGC

0.15 ( 0.01

-11.5 ( 0.2

5.68 ( 0.05

0.49 ( 0.02 8

Figure 2. Adsorption isotherm for copper acetate on NITGC, fitted to a combination of Langmuir and Freundlich isotherms (AAS data only).

Figure 3. Adsorption isotherm for copper acetate on GC and H2TGC fitted to a sum of two Langmuir isotherms and on NITGC fitted to a combination of Langmuir and Freundlich isotherms.

Figure 2 displays the amount of adsorbed copper acetate onto NITGC against the equilibrium solution concentration (AAS data only) on a log-log scale fitted to a combination of Langmuir and Freundlich isotherms. These data do not fit to the double Langmuir isotherms as in the case of H2TGC owing to the absence of a clear plateau. The H2TGC data also fails to fit the combined Langmuir and Freundlich isotherm. Fitting combined isotherms is sometimes necessary to explain complex data sets.18,19 Figure 3 displays, on a log-log scale, the adsorption data for copper acetate on GC, H2TGC, and NITGC fitted to a sum of two Langmuir isotherms for GC and H2TGC and to a combination of Langmuir and Freundlich isotherms for the NITGC on the same graph. The GC data and fit taken from ref 8 have been plotted for the sake of comparison. For the GC and H2TGC, all the respective AAS, titration, and weight data have been combined for the sake of simplicity under one symbol. Only the AAS data are shown in the case of the NITGC. It can be seen that the NITGC displays very different behavior from the GC and H2TGC. NITGC has almost double the adsorption of GC and H2TGC in the range 10-4-10-2 mol/L. Table 3 displays the values of N′ and (µ0a - µ0s)/(kBT) both for the low concentration Langmuir no. 1 and for the high concentration Langmuir no. 2 determined for GC and H2TGC. It is clear that the values of N′ and (µa0 - µs0)/(kBT) for Langmuir no. 1 are much different from the respective Langmuir no. 2

1/n

values. As already discussed by Westreich et al., these very distinct values indicate that there are two entirely different adsorption mechanisms in action at the low and high concentrations in these AC samples. Also, it can be seen that the values of the respective parameters for both samples are very close to each other. Table 4 shows the values of the respective adsorption parameters for the combined Langmuir and Freundlich fit to the NITGC data. From the Langmuir parameters, it can be said that, at lower concentration, the Langmuir isotherm fits the data very well, implying monolayer adsorption. From the Freundlich parameters, it can be said that owing to the presence of a large number of acidic functional groups on the NITGC surface, and at higher salt concentrations, the Freundlich isotherm explains the adsorption well. It is generally understood that the low concentration adsorption is being caused by the interaction of functional groups present on the surface of the AC. This is proved by the Boehm titration result (0.09 mmol/g) and the N′ value (0.08 mmol/g) for Langmuir no. 1 being very close to each other in the case of GC. But the same respective parameters are much further apart in the case of NITGC (3.3 vs 0.15 mmol/g). One possible explanation is that a greater number of functional groups (as in NITGC) gives more possible sites for Cu2+ adsorption, and as the Cu2+ is adsorbed, H+ is released, causing the surrounding solution pH to decrease. This increase in H+ concentration of the solution results in a shift of the chemical potential, making it more energetically unfavorable to continue to remove H+ from the carbon surface sites. This is why the N′ value in the case of NITGC is much smaller than the Boehm titration result. The solution pH of NITGC after stirring was around 4, which is a sufficiently low value to affect the adsorption process. It has also been quoted that lower metal adsorption occurs at lower pH values.2-7,20,21 This is because of the dependence of the dissociation of surface functional groups on the solution pH and with an increase in pH, more surface functional groups dissociate to provide binding sites for metal ions thus resulting in higher adsorption.22 In addition, the hydrolysis products do not contribute to the adsorption, as a metal ion is hydrolyzed only when its hydroxide has a lower pH than the pH of the experimental condition and it is known that the hydroxide of copper is a strong base.23 The lower adsorption at lower pH is also due to the electrostatic repulsion of positively charged cations (Cu2+) with the carbon surface. Also, it has been found that a different sorption mechanism acts at different pH values for copper.24 In the case of H2TGC however, it can be seen by comparing the Boehm titration result (0.03 mmol/g) and the N′ value (0.10 mmol/g) for Langmuir no. 1, that the functional groups are almost nil due to the heat treatment in hydrogen but still the N′ value (0.10 mmol/g) for this carbon sample is approximately

Ind. Eng. Chem. Res., Vol. 48, No. 22, 2009 Table 5. pH Values at Different Initial Concentrations As Measured for Copper Acetate Solution before and after Stirring with NITGC initial solution conc. (mol/L)

initial pH (copper acetate solution)

final pH (after stirring with NITGC)

drop in pH

0.3 0.05 0.02

5.17 5.65 5.84

4.66 4.21 3.55

0.51 1.44 2.29

Table 6. pH Values at Different Initial Concentrations As Measured for Copper Acetate Solution before and after Stirring with H2TGC initial solution conc. (mol L-1)

initial pH (copper acetate solution)

final pH (after stirring with H2TGC)

drop in pH

0.3 0.05 0.02

5.17 5.65 5.84

4.5 4.85 5.1

0.67 0.8 0.74

the same as the N′ value for the untreated GC (0.08 mmol/g). This means that the acidic functional groups, found to be very few in this sample, had no significant participation in adsorption, and it is likely that the edges of graphene layers served as the adsorption centers in this case. Others have also reached the same conclusion by considering that their sample had few acidic groups and by monitoring pH as a function of adsorption.2 The graphene layers, Cπ, act as Lewis basic centers, where protonation takes place. In fact, one cannot rule out that this mechanism could account for some of the adsorption on untreated GC as well. There can even be more than one adsorption mechanism in action at the same time.2 Table 3 shows that the values of (µa0 - µs0)/(kBT) for Langmuir no. 1 for GC and H2TGC range from -13.3 to -13.5 whereas the same values for Langmuir no. 2 range from -2.9 to -2.5. Clearly, Langmuir no. 2 values are much smaller in magnitude than the values of Langmuir no. 1. The values of N′ for the two carbons are bigger for Langmuir no. 2 than for Langmuir no. 1 (Table 3). This signifies that, for these two samples, the high concentration adsorption, which is taking place due to a much weaker interaction, cannot be attributed to the presence of functional groups on the surface of the respective carbons. One possible mechanism can be the interaction of organic acetate ions with aromatic centers on the carbon8,25 in the form of pibond stacking or the separation of polar groups from nonpolar groups. Another possible mechanism is the formation of Cu(OH)2 on carbon surface sites.23 For NITGC, the drop in solution pH after stirring was much more for the lower concentrations than for the higher concentrations as can be seen in Table 5. This means that, at lower concentrations, more acidic pH hinders the adsorption process resulting in lower N′ value at lower solution concentration (Table 4). At higher solution concentrations (above 0.1 mmol/L), the higher pH favors higher adsorption thus giving a very high value of the Freundlich parameter Cm (Table 4). The reasons that have been suggested above for the lower adsorption at low concentration for NITGC are all valid for its higher adsorption at high concentration.2-7,20-23 This is the reason why we do not see a clear plateau in the NITGC curve (Figure 3) when compared to GC and H2TGC.

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It also makes fitting the double Langmuir isotherm impossible to these data, thus making a combination Langmuir and Freundlich isotherm necessary. On the other hand, for H2TGC, the drop in solution pH after stirring for the same solutions as shown in Table 5 did not vary significantly from the higher to the lower concentration. The final pH values after stirring for the three solutions shown are also not very far from each other, and as we go from lower to higher concentration, the final pH keeps decreasing. This trend is opposite to what we have seen in the case of NITGC (Table 5). These values can be seen in Table 6, and it is clear that the pH has not played a very significant role in this case as compared to the NITGC sample. It is always possible that different adsorption mechanisms act at the same time2 thus making it more complex to predict the exact nature of the adsorbent and adsorbate interaction. The higher concentration adsorption mechanism is still not understood fully, and further studies are needed to find out the whole story. 4. Summary Kuraray GC was pretreated with hydrogen and with nitric acid to produce two AC samples. The former treatment was performed to obtain an AC with very few acidic surface functional groups thus giving it a so-called basic character. The latter was done to obtain a very large number of acidic surface functional groups. The hydrogen treatment increased the BET surface area and the micro- and total pore volume to some extent, while the nitric acid treatment decreased them. A large range of linearly varying concentrations of aqueous copper acetate solutions was made and stirred with each sample of pretreated AC. Atomic absorption spectroscopy, titrations, and weight measurements were employed to determine the amount of copper acetate actually adsorbed onto the respective carbon samples. The data were fitted to a sum of two distinct Langmuir isotherms and to a combination of Langmuir and Freundlich isotherms. The lower concentration results match with the Boehm titration result values in the case of the untreated sample thus confirming the generally understood view that adsorption occurs at the functional group sites. This mechanism does not seem to work for the hydrogen treated sample where most probably the edges of graphene layers served as the adsorption centers. At high concentrations, it is the interaction of organic acetate ions with aromatic centers on the carbon and the hydrolysis products of copper that increase the adsorption. However, for the nitric acid treated GC, at lower concentration, the surface functional sites do not get fully filled with copper acetate and the number of surface functional groups obtained from Boehm titration results are much larger than the Langmuir isotherm constants. Therefore at low concentrations, other factors such as pH play a role in preventing the copper acetate from filling the available functional group sites on the NITGC. At high concentrations for NITGC, the Freundlich parameter Cm gave a very high value which is even higher than the number of surface functional groups as determined by the Boehm titration. Two distinct isotherm fittings have been obtained for

Table 7. Summary of Suggested Interactions at Low and High Concentrations in the Cases of GC, H2TGC, and NITGC suggested mechanism low concentration

high concentration

GC adsorption on functional groups and maybe on edges of graphene layers interaction of organic acetate ions with aromatic centers on the carbon and the hydrolysis products of copper increase the adsorption H2TGC edges of graphene layers and the functional groups served as the adsorption centers NITGC lower solution pH prevents adsorption higher solution pH favors adsorption on functional groups

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both carbons; double Langmuir isotherms for the hydrogentreated sample and a low concentration Langmuir isotherm combined with a high concentration Freundlich isotherm for the nitric acid treated sample. Table 7 displays a summary of the suggested adsorption mechanisms at low and high concentrations for the GC, H2TGC, and NITGC. The high concentration adsorption is not fully understood yet, and further detailed studies are needed to develop a full picture of the impregnation science at a desired impregnant loading. Acknowledgment The authors thank NSERC and 3M Canada Co. for their financial support. The authors thank Drs. Lisa M. Croll and Simon J. Smith of 3M Canada Co. for useful discussions. The authors also thank the Higher Education Commission of Pakistan for making this research collaboration possible through the International Research Support Initiative Program. Supporting Information Available: More experimental data for GC, H2TGC, and NITGC samples. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Bush, V.; Conant, J. B.; Noyes, W. A. Military Problems with Aerosols and Nonpersistent Gases; OSRD, NDRC: Division 10: Washington, DC, 1946. (2) DeMesquita, J. P.; Martelli, P. B.; Gorgulho, H. D. F. Characterization of Copper Adsorption on Oxidized Activated Carbon. J. Braz. Chem. Soc. 2006, 17, 1133. (3) Chen, J. P.; Yiacoumi, S.; Blaydes, T. G. Equilibrium and Kinetic Studies of Copper Adsorption by Activated Carbon. Sep. Technol. 1996, 6, 133. (4) Chen, J. P.; Lin, M. S. Equilibrium and Kinetics of Metal Ion Adsorption onto a Commercial H-Type Granular Activated Carbon: Experimental and Modeling Studies. Water Res. 2001, 35, 2385. (5) Netzer, A.; Hughes, D. E. Adsorption of Copper, Lead and Cobalt by Activated Carbon. Water Res. 1984, 18, 927. (6) Satapathy, D.; Natarajan, G. S. Surface Modification of Granular Activated Carbon by Nitric Acid for the Enhancement of Copper Adsorption. Indian J. Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal. Chem. 2006, 45, 2011. (7) Kucukgul, E. Y.; Kudu, S. Zinc and Copper Adsorption from an Aqueous Solution onto Activated Carbon. Fresenius EnViron. Bull. 2006, 15, 512. (8) Westreich, P.; Selig, S.; Fortier, H.; Dahn, J. R. Two Distinct Langmuir Isotherms Describe the Adsorption of Certain Salts onto Activated Carbon over a Wide Concentration. Carbon 2006, 44, 3145. (9) Westreich, P.; Fortier, H.; Flynn, S.; Foster, S.; Dahn, J. R. Exclusion of Salt Solutions from Activated Carbon Pores and the Relationship to Contact Angle on Graphite. J. Phys. Chem. C 2007, 111, 3680.

(10) Menendez, J. A.; Phillips, J.; Xia, B.; Radovic, L. R. On the Modification and Characterization of Chemical Surface Properties of Activated Carbon: In the Search of Carbons with Stable Basic Properties. Langmuir 1996, 18, 4404. (11) Shim, J. W.; Park, S. J.; Ryu, S. K. Effect of Modification with HNO3 and NaOH on Metal Adsorption by Pitch-Based Activated Carbon Fibers. Carbon 2001, 39, 1635. (12) Xiao, B.; Thomas, K. M. Adsorption of Aqueous Metal Ions on Oxygen and Nitrogen Functionalized Nanoporous Activated Carbons. Langmuir 2005, 21, 3892. (13) Boehm, H. P.; Diehl, E.; Heck, W.; Sappok, R. Surface Oxides of Carbon. Angew. Chem., Int. Ed. 1964, 3, 669. (14) Boehm, H. P. Some Aspects of the Surface-Chemistry of CarbonBlacks and Other Carbons. Carbon 1994, 32, 759. (15) Langmuir, I. The Adsorption of Gases on Plain Surfaces of Glass, Mica and Platinum. J. Am. Chem. Soc. 1918, 40, 1361. (16) Freundlich, H. On Adsorption in Solutions. Z. Phys. Chem. (Muenchen, Ger.) 1907, 57, 385. (17) Sips, R. On the Structure of a Catalyst Surface. J. Chem. Phys. 1948, 16, 490. (18) Lodewyckx, P.; Van Rompaey, D.; Verhoeven, L.; Vansant, E. F. Water Isotherms of Activated Carbons with Small Amounts of Surface Oxygen Groups Fitting the Mesopore Region. Carbon 2001, 39, 309. (19) Lodewyckx, P.; Raymundo, P. E.; Wullens, H.; Vaclavikova, M.; Beguin, F. Water Isotherms of Structurally Identical Carbons with Different Amounts of Surface Oxygen Groups. Presented at the International Conference on Carbon 08, Nagano, Japan, July 2008; Paper 226. (20) Chu, K. H.; Hashim, M. A. Adsorption of Copper(II) and EDTAChelated Copper(II) onto Granular Activated Carbons. J. Chem. Technol. Biotechnol. 2000, 75, 1054. (21) Basso, M. C.; Cerrella, E. G.; Cukierman, A. L. Activated Carbons Developed from a Rapidly Renewable Biosource for Removal of Cadmium(II) and Nickel(II) Ions from Dilute Aqueous Solutions. Ind. Eng. Chem. Res. 2002, 41, 180. (22) Chen, J. P.; Wu, S. N.; Chong, K. H. Surface Modification of a Granular Activated Carbon by Citric Acid for Enhancement of Copper Adsorption. Carbon 2003, 41, 1979. (23) Budinova, T. K.; Petrov, N. V.; Minkova, V. N.; Gergova, K. M. Removal of Metal Ions from Aqueous Solution by Activated Carbons Obtained from Different Raw Materials. J. Chem. Technol. Biotechnol. 1994, 60, 177. (24) Pesavento, M.; Profumo, A.; Alberti, G.; Conti, F. Adsorption of Lead(II) and Copper(II) on Activated Carbon by Complexation with Surface Functional Groups. Anal. Chim. Acta 2003, 480, 171. (25) Bong, K. K.; Nyrkova, A. N.; Chobanu, M. M.; Shestakov, G. K.; Temkin, O. N. Adsorption of Zinc Acetate from Aqueous Solutions on Activated Carbons. Russ. J. Appl. Chem. 1997, 70, 1872.

ReceiVed for reView March 30, 2009 ReVised manuscript receiVed August 25, 2009 Accepted September 4, 2009 IE900492F