ARTICLE pubs.acs.org/IECR
Anionic Dye Adsorption on Chemically Modified Ordered Mesoporous Carbons Chun He and Xijun Hu* Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ABSTRACT: The surface chemistry of ordered mesoporous carbon CMK-3 was successfully modified by heat treatment in ammonia gas at a temperature of 1173 K, without destroying its hexagonally ordered mesoporous structure significantly. The adsorption kinetics and equilibrium of three commercial anionic dyes, i.e., orange II, reactive red 2 (RR2), and acid black 1(AB1), on the novel functionalized carbon were investigated. It was found that the ammonia-tailored CMK-3 showed a much higher uptake rate than that of a commercial activated carbon, due to its desirable ordered mesoporous structure and the electron-donating effect of the incorporated nitrogen-containing functional groups, particularly for the adsorption of the large molecule dye with a long chain, AB1. The dye initial concentration and adsorption temperature had little influence on the adsorption rate, while the size and spatial structure of dye molecules and the shaking speed played an important role during the adsorption process. The adsorption of AB1 which has a long chain required the longest time to reach equilibrium. Film mass transfer (the mass transport between the carbon surface and the flowing dye fluid) might be the rate-controlling step. This novel functionalized adsorbent could enhance the adsorption capacity of the three anionic dyes by 90200% and 4060% compared to the commercial activated carbon and unmodified CMK-3, respectively. This significant improvement was attributed to the enhanced dispersive forces between the carbon surface and the dye molecules induced by the nitrogen-containing functional groups. The Freundlich isotherm showed better correlation with the experimental adsorption data in all cases than the Langmuir isotherm as a result of the functional groups induced energetic heterogeneous surface. Slow desorption rate and low desorption efficiency of RR2 indicated that the adsorption of RR2 on the modified CMK-3 was extremely favorable, tending to be weakly reversible.
1. INTRODUCTION Synthetic organic dyes have been extensively used in many industries, such as textile, paper, printing, food, and cosmetics. They are a large and important group of water pollutants in an aquatic ecosystem with over 7 105 tons produced annually.13 The discharge of dye effluents into the environment is currently one of the world’s major environmental problems for both toxicological and esthetic reasons. The highly colored effluents may significantly affect photosynthetic activity in aquatic life by cutting off sunlight penetration, and most of the dyes as well as their metabolites have been identified as either toxic or mutagenic and even carcinogenic.47 However, the nondegradable nature of dyes and their stability toward light and oxidizing agents make them very difficult to treat using conventional wastewater treatment methods.1,8 Therefore, efficient color removal from effluents is significant and has prompted growing research and technological interest over the past few years. Various techniques have been employed for the removal of dyes from wastewater, including physical, chemical, and biological methods.914 Liquid phase adsorption is one of the most convenient and effective treatment methods for removing a wide variety of pollutants present in wastewater.1,7 Although commercial activated carbon is the most widely used adsorbent, the requirement of adequate adsorptive capacity for molecules with larger diameters such as bulky molecules or macromolecules restricts the choice of activated carbon as adsorbent in practical separation processes due to its microporous nature. Large molecules or macromolecules cannot easily penetrate the micropores r 2011 American Chemical Society
(pore size 1, linear when n = 1, and unfavorable when n < 1. Figure 16 presents the adsorption equilibrium isotherms of the three anionic dyes at 298 K on the three carbon adsorbents under unadjusted pH conditions as well as the correlation results with 14077
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Figure 12. Normalized concentration curves in the liquid phase (a, c) and solid phase (b, d) versus contact time for the adsorption of RR2 at 298 K on modified CMK-3 with different dye initial concentrations.
the Langmuir and Freundlich equations. The optimum adsorption parameters of the isotherm equations were found by minimizing the difference between the experimental and theoretical data of the equilibrium solid-phase concentration, qe, using the sum of the squares of the error (SSE) function and summarized in Tables 5 and 6, respectively. SSE ¼
p
ðqe, model qe, exp Þi 2 ∑ i¼1
ð6Þ
The fitting results showed that the Freundlich model provided a better correlation than the Langmuir model, particularly for the ammonia-tailored CMK-3, suggesting that the surfaces of the three carbon samples were heterogeneous and nitrogen-containing functional groups created on the carbon surface increased the surface heterogeneity of CMK-3. This novel functionalized adsorbent could enhance the adsorption capacity of three anionic dyes, namely, orange II, RR2, and AB1, by 90200% and 4060% compared to the commercial activated carbon and unmodified CMK-3, respectively. This significant improvement was mainly attributed to the enhanced dispersive forces between the carbon surface and the dye molecules induced by the basic nitrogen-containing functional groups.
3.4. Adsorption Mechanisms. It was expected that many factors could play roles in the dye adsorption process, such as the dye structure, textural properties, and surface chemistry of adsorbents and the specific interactions between the adsorbent surface and adsorbate.18,29,31,32 As observed in Figure 16, in all cases the amount adsorbed sharply increased at lower equilibrium solution concentration and then reached a plateau at higher equilibrium solution concentration where the saturated adsorption was reached, indicating a high affinity between the dye molecule and the carbonaceous adsorbent.29 Two mechanisms,18,33 i.e., electrostatic forces and dispersive interactions, were involved in the adsorption of organic dye molecules on carbons. The latter arises from the interactions between the delocalized π electrons of the Lewis basic sites in the basal planes of the carbon and the free electrons of the dye molecules present in the aromatic rings and multiple bonds. For microporous activated carbon, the molecular size and spatial effect of the dye molecules were the crucial factors that accounted for its low adsorption capacity. As dye molecules with larger size could not penetrate the micropores easily, pore blockage might occur due to the aggregation of large molecules in the orifice of small pores,17 inhibiting further mass transport and resulting in lower adsorption amounts. This hindrance for 14078
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Figure 13. Normalized concentration curves in the liquid phase (a, c) and solid phase (b, d) versus contact time for the adsorption of RR2 on modified CMK-3 at different temperatures (C0 = 1200 mg/L).
adsorption of larger molecules on the activated carbon was more distinct with AB1 with a long chain. In addition, the electrostatic repulsive forces between the carbon surface and dye molecules might also contribute to resisting the adsorption of orange II and AB1. It was found that orange II (IEP = 5.29, solution pH 6) and AB1 (IEP = 4.52, solution pH 8) molecules were both negatively charged when they were dissolved in water and the surface of the activated carbon was also negatively charged at the two solution pH values as evidenced by ζ-potential analysis in Figure 8. For ordered mesoporous carbons, the surface in the pores was better utilized in the adsorption process due to less resistance for mass transfer and diffusion of the bulky dye molecules. Consequently, a significant increase in the adsorption capacity was observed as shown in Figure 16, particularly for the larger dye molecule of AB1 (improvement of 100% by CMK-3 and 200% by ammonia-tailored CMK-3 compared to the activated carbon). It was worth noting that the modified CMK-3 showed much higher adsorption capacity of the three anionic dyes compared with the unmodified one, implying that the surface chemistry of the carbons played an important role in the adsorption process and the nitrogen-containing functional groups incorporated by ammonia heat treatment were responsible for the remarkable improvement.
As confirmed by elemental analysis (Table 3), there was a large amount of oxygen-containing functional groups present on the surface of CMK-3. These electron-withdrawing groups could reduce the electron density of the carbon surface and thus decrease the adsorption potential for the dye molecules.19 Furthermore, the oxygen-containing functional groups are usually negatively charged in solution, which could result in an increase in the electrostatic repulsive interactions between the anions of the dyes and the carbon surface. After the CMK-3 sample was treated with ammonia gas at the high temperature of 1173 K, a great amount of oxygen-containing functional groups was removed from the surface and nitrogen-containing functional groups were created on the carbon surface. These nitrogencontaining functional groups could increase the reactivity of the aromatic rings and hence strengthen the dispersive interactions between the dyes and the carbon surface due to their electron-donating effect, and in the meantime, the ionized positively charged functional groups could promote the interactions between the anions of the dyes and the carbon surface, which accounted for the considerable improvement of anionic dye adsorption on the ammoniatailored CMK-3. It could also be deduced that the dispersive forces might predominate in the anionic dye adsorption as a remarkable 14079
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Figure 14. Normalized concentration curves in the liquid phase (a, c) and solid phase (b, d) versus contact time for the adsorption of RR2 at 298 K on modified CMK-3 with different shaking speeds (C0 = 1200 mg/L).
Table 4. Removal Efficiency of Selected Dyes on AC, CMK-3, and Modified CMK-3 adsorbent activated carbon
CMK-3
ammonia-tailored CMK-3
dye
C0 (mg/L)
temperature (K)
shaking speed
removal efficiency (%)
orange II
900
298
full
33
RR2 AB1
600 600
298 298
full full
29 25
orange II
900
298
full
48
RR2
600
298
full
58
AB1
600
298
full
55
orange II
900
298
full
65
RR2
600
298
full
82
RR2
1200
298
full
47
RR2 RR2
1200 1200
298 313
80% full
47 48
RR2
1200
333
full
52
RR2
1800
298
full
33
AB1
600
298
full
82
increase in the adsorption of RR2 and AB1 on the modified CMK-3 was still achieved although the electrostatic repulsive interactions between the anions of the dyes and the carbon surface hindered the adsorption of RR2 and AB1. The dye
molecule and the surface of the ammonia-tailored CMK-3 (pHIEP 6.6) carried the same kind of charge in the RR2 (IEP = 4.41, solution pH 3) and AB1 (IEP = 4.52, solution pH 8) solutions. 14080
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Figure 15. Normalized concentration curves in the liquid phase (a) and solid phase (b) versus contact time for the desorption of RR2 at 298 K from modified CMK-3 with different dye loadings.
Figure 16. Adsorption equilibrium isotherms of (a) orange II, (b) RR2, and (c) AB1 on activated carbon, CMK-3, and modified CMK-3.
3.5. Desorption Studies. In order to evaluate the irreversibility of the adsorption process, three consecutive runs of desorption were conducted by contacting the dye-loaded carbon with deionized water to see how many dye molecules could be desorbed from the exhausted ammonia-tailored carbon. Figure 17 presents the percentage of dye desorbed by three runs of the desorption process. It was found that the
desorption efficiency was low, as only a small amount of dye (around 20%) could be desorbed from the carbon by running the desorption process three times. It was also noted that about 95% of the weakly adsorbed dye molecules was desorbed in the first desorption cycle. The tiny amount of dye desorbed in the second and third desorption cycles was negligible. It appeared that the adsorption of RR2 on the 14081
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Table 5. Langmuir Isotherm Parameters of Dye Adsorption on AC, CMK-3, and Modified CMK-3 parameters adsorbent activated carbon
CMK-3
ammonia-tailored CMK-3
dye
qs (g/g carbon)
b (L/g)
orange II
0.329
RR2
0.285
96.90 7.24
AB1 orange II
0.171 0.385
59.50 337.00
RR2
0.365
16.80
AB1
0.353
219.00
orange II
0.596
4170.00
RR2
0.549
19.90
AB1
0.509
73.30
Table 6. Freundlich Isotherm Parameters of Dye Adsorption on AC, CMK-3, and Modified CMK-3 parameters adsorbent activated carbon
CMK-3
ammonia-tailored CMK-3
K ((g/g carbon)(L/g)1/n)
n
orange II
0.326
17.40
RR2
0.245
4.36
AB1
0.167
27.10
orange II
0.386
18.90
RR2 AB1
0.342 0.353
6.13 18.50
dye
orange II
0.621
12.10
RR2
0.518
12.60
AB1
0.504
19.30
4. CONCLUSIONS The surface chemistry of ordered mesoporous carbon CMK-3 was successfully modified by ammonia heat treatment at the high temperature of 1173 K, without destroying its hexagonally ordered mesoporous structure significantly. Four types of basic nitrogen functional groups were created on the carbon surface, with pyridine-like nitrogen and aromatic amines being the dominant functional groups. These basic surface functional groups made the carbon “more basic”, shifting the pHIEP of CMK-3 from 5.1 to 6.6. Adsorption kinetic studies revealed that the ammonia-tailored CMK-3 showed a much higher uptake rate than that of microporous activated carbon because of its desirable ordered mesoporous structure and the electron-donating effect of the incorporated nitrogen-containing functional groups, particularly for the adsorption of AB1. The size and spatial structure of dye molecules also played a role in the adsorption process. The adsorption of AB1 with a long chain required the longest time to reach equilibrium. The dye initial concentration and adsorption temperature had little influence on the adsorption rate for RR2 adsorption whereas the shaking speed played an important role during the adsorption process, implying that film mass transfer between the solid surface and the flowing fluid might be the ratecontrolling step. This novel functionalized adsorbent could enhance the adsorption capacity of three anionic dyes, namely, orange II, RR2, and AB1, by 90200% and 4060% compared to the commercial activated carbon and unmodified CMK-3, respectively. This significant improvement was mainly attributed to the enhanced dispersive forces between the carbon surface and the dye molecules induced by the basic nitrogen-containing functional groups. The Freundlich isotherm showed better correlation with the experimental adsorption data for all three dyes studied than the Langmuir isotherm as a result of the functional groups induced energetic heterogeneous surface. Low desorption rate and desorption efficiency indicated that the adsorption of RR2 on the modified CMK-3 was extremely favorable, tending to be weakly reversible. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors acknowledge financial support from the Research Grants Council of Hong Kong (CERG, Project No. 613705). ’ REFERENCES
Figure 17. Desorption of RR2 from ammonia-treated CMK-3 by three desorption cycles.
modified CMK-3 was extremely favorable, tending to be weakly reversible. More acidic and alkaline conditions and other solvent regeneration methods using ethanol34 or surfactant6,35 as solvent will be attempted to see if more dye molecules could be desorbed.
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