9586
Langmuir 2006, 22, 9586-9590
Temperature Dependence of Herbicide Adsorption from Aqueous Solutions on Activated Carbon Fiber and Cloth M. A Ä ngeles Fontecha-Ca´mara,† M. Victoria Lo´pez-Ramo´n,† Miguel A. A Ä lvarez-Merino,† and ,‡ Carlos Moreno-Castilla* Departamento de Quı´mica Inorga´ nica y Orga´ nica, Facultad de Ciencias Experimentales, UniVersidad de Jae´ n, 23071 Jae´ n, Spain, and Departamento de Quı´mica Inorga´ nica, Facultad de Ciencias, UniVersidad de Granada, 18071 Granada, Spain ReceiVed June 10, 2006. In Final Form: August 24, 2006 Diuron and amitrole adsorption from aqueous solution on an activated carbon fiber and an activated carbon cloth were studied as a function of temperature. Diuron adsorption was greater than that of amitrole and increased with rising temperature, whereas amitrole adsorption decreased when the temperature increased. Endothermicity of diuron adsorption was due to an increase in the planarity and diffusion of diuron molecules with higher temperatures. However, the exothermicity found for amitrole was due to the increase in amitrole solubility and in vibrational energy of adsorbed molecules with higher temperature. External mass transfer resistance was also found to play an important role in diuron adsorption on activated carbon cloth.
Introduction Adsorption of organic pollutants from aqueous solutions on carbon materials is temperature dependent. Since adsorption is a spontaneous process, a decrease in uptake is expected when the temperature increases. However, there are numerous examples in the literature (see, i.e., refs 1-11) of the opposite effect, resulting in an apparently endothermic adsorption. Varied explanations of this phenomenon have been proposed. Thus, Corkill et al.1 observed that adsorption of n-alkyl polyoxyethylene glycol monoethers from aqueous solutions on Graphon increased with rising temperature. This is opposite to the effect normally observed for the physical adsorption of a single component (e.g., a gas on a solid). According to the authors, this discrepancy arises because adsorption from a two-component system is influenced not only by conventional adsorbate-adsorbent interactions but also by adsorbate-water interactions, resulting in the adsorption of different hydrated species at different temperatures. Seidel et al.2 found that phenol uptake at 55 °C was greater than at 20 °C, which was attributed to an increase in the package density of phenol molecules in micropores. This idea was previously developed by Chiou et al.,3 who found for 11 different * Corresponding author. Tel: +34-958-243-323. Fax: +34-958-248526. E-mail:
[email protected]. † Universidad de Jae ´ n. ‡ Universidad de Granada. (1) Corkill, J. M.; Goodman, J. F.; Tate, J. R. Trans.Faraday Soc. 1966, 62, 979-986. (2) Seidel, A.; Tzscheutschler, E.; Radeke, K. H.; Gelbin, D. Chem. Eng. Sci. 1985, 40, 215-222. (3) Chiou, C.; Manes, M. J. Phys. Chem. 1974, 78, 622-626. (4) Costa, E.; Calleja, G.; Marijua´n, L. Adsorpt. Sci. Technol. 1988, 5, 213228. (5) Ravi, V. P.; Jasra, R. V.; Bhat, T. S. G. J. Chem. Technol. Biotechnol. 1998, 71, 173-179. (6) Garcı´a-Araya, J. F.; Beltra´n, F. J.; A Ä lvarez, P.; Masa, F. J. Adsorption 2003, 9, 107-115. (7) Srivastava, V. C.; Swamy, M. M.; Mall, I. D.; Prasad, B.; Mishra, I. M. Colloids Surf., A 2006, 272, 89-104. (8) Mishra, S. K.; Kanungo, S. B.; Rajeev. J. Colloid Interface Sci. 2003, 267, 42-48. (9) Terzyk, A. P.; Rychlicki, G. Colloids Surf., A 2000, 163, 135-150. (10) Terzyk, A. P.; Rychlicki, G.; Biniak, S.; Lukaszewicz, J. P. J. Colloid Interface Sci. 2003, 257, 13-30. (11) Terzyk, A. P. J. Colloid Interface Sci. 2004, 275, 9-29.
organic compounds a greater uptake at temperatures above their melting points. Costa et al.4 also reported that phenol adsorption increased with higher temperatures. They attributed this phenomenon to the large percentage of micropores in the carbons used since the higher temperature would favor pore diffusion, thereby increasing adsorption into them. Ravi et al.5 found that adsorption of phenol, cresols, and benzylic alcohol on activated carbons was endothermic, which they attributed to the partially chemical nature of the adsorption and the polymerization of adsorbed molecules. Garcı´a-Araya et al.6 also observed an increase in the adsorption of gallic, p-hydroxybenzoic, and syringic acids with rising temperature, which they ascribed to activated diffusion into the micropores and chemisorption. However, Srivastava et al.7 proposed that the endothermicity of phenol adsorption on bagasse fly ash was due to phenol chemisorption and not to micropore diffusion since the latter can be ruled out after a long contact time between adsorbent and solution. Mishra et al.8 described an increase in sodium dodecylbenzene sulfonate uptake on an original and demineralized subbituminous coal with a rise in temperature from 30 to 70 °C but did not explain this phenomenon. Terzyk et al.9-11 recently studied the effect of temperature on the adsorption of different adsorbates (e.g., paracetamol, phenol, acetanilide, aniline, and the anilinium cation on original activated carbons and on activated carbons that were chemically treated to increase their surface oxygen complexes). They reported a higher adsorption with temperature between 27 and 47 °C. They concluded that the dispersion interactions are temperature independent, whereas the interactions between adsorbate dipoles and surface oxygen groups decrease with a rise in temperature. For instance, an increase in temperature weakens the hydrogen bonds.12 Therefore, the increase in uptake with temperature was explained to be due to a decrease in interactions between polar molecules of either the solute or the solvent and surface groups located at the entrance of micropores where so-called micropore filling takes place. Therefore, the rise in temperature increases accessibility to the micropores and adsorption by a dispersion mechanism. (12) Vinogradov, S. N.; Linnell, R. H. Hydrogen Bonding; Van NostrandReinhold: New York, 1971.
10.1021/la061666v CCC: $33.50 © 2006 American Chemical Society Published on Web 10/06/2006
Temperature Dependence of Herbicide Adsorption
Langmuir, Vol. 22, No. 23, 2006 9587
Table 1. Characteristics of the Carbon Adsorbents W0 L0 surface surface SBET Vmesopore (DFT) (DFT) acidity basicity carbon (m2/g) (cm3/g) (cm3/g) (nm) (meq/g) (meq/g) pHPZC ACF ACC
1709 2128
0.017 0.028
0.733 0.865
1.41 1.41
0.44 0.16
0.40 0.50
7.0 8.0
In the present study, the effect of temperature on the adsorption of two herbicides on advanced carbon materials (i.e., activated carbon fiber and activated carbon cloth) was investigated. Contamination of surface and groundwater with herbicides may become a major concern due to the intensive and widespread use of these chemicals in agriculture. Therefore, the treatment of water contaminated with herbicides is an important environmental protection task. The two herbicides studied were diuron and amitrole, which are widely used in different cultures. From a practical point of view, the effect of temperature on adsorption is of great interest since the temperature of water in nature ranges from 30 °C in tropics to almost 0 °C in cold regions, and the temperature range of industrial wastewaters is even wider. Experimental Procedures Two commercial activated carbons were used in this work: an activated carbon fiber (ACF) and an activated carbon cloth (ACC), both from Kynol Europe. Surface area and porosity of samples were obtained from N2 adsorption isotherms at -196 °C, which were measured in a Quantachrome Autosorb system. Samples were outgassed at 110 °C overnight under a dynamic vacuum of 10-6 Torr. BET surface area, SBET, and mesopore volume were obtained from the N2 adsorption isotherms. Determination of the micropore size distribution, mean micropore width, L0, and micropore volume, W0, were obtained from the DFT method applied to the N2 adsorption isotherms. Amounts of surface acid and basic groups were obtained by titration with NaOH and HCl, respectively. Surface charge determination and pH at the point of zero charge, pHPZC, were determined by potentiometric titrations. All these characteristics were described elsewhere.13 Herbicides diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea) and amitrole (3-amino-1,2,4-triazole), supplied by Sigma Aldrich with a purity of 99%, were used in this study. These herbicides were characterized by potentiometric titrations to determine their speciation diagrams as a function of pH. The herbicide sizes were determined from the CIF file deposited in the Cambridge Structural Database. Their molecular area and dipolar moment were determined by using the SCF AM1 semiempirical method of the Gaussian03 program. Adsorption isotherms of both herbicides on the carbons used were obtained with 0.05-0.1 g of carbon and 200 mL of herbicide solutions at different concentrations. Adsorption was carried out at temperatures between 15 and 45 °C at an electrolyte concentration (KCl) of 0.01 M and at pH 7 using a buffer composed of potassium phosphate monobasic and sodium phosphate dibasic. Suspensions were mechanically shaken at the selected temperature. Equilibrium concentrations were determined by ultraviolet spectrophotometry at a wavelength of 248 nm for diuron and 201 nm for amitrole.
Results and Discussion Characteristics of the Adsorbents. The surface area, porosity, and surface chemical properties of the carbon materials used are compiled in Table 1, and the micropore size distribution is depicted in Figure 1. Surface area and micropore volume of ACC are larger than those of ACF, although both carbons have a similar mean micropore width, L0. Surface acidity and basicity are similar in ACF, producing a pHPZC of 7. However, surface basicity is higher than surface acidity in ACC, which has a pHPZC of 8. (13) Moreno-Castilla, C.; A Ä lvarez-Merino, M. A.; Lo´pez-Ramo´n, M. V.; RiveraUtrilla, J. Langmuir 2004, 20, 8142-8148.
Figure 1. Pore size distribution. ACF (4) and ACC (O). Table 2. Characteristics of Diuron and Amitrole property
diuron
amitrole
water solubility at 25 °C (g/L) log Kow dipolar moment (D) molecular area (nm2/molecule)
0.042 2.85 7.55 0.75
280 -0.97 1.24 0.39
Characteristics of the Herbicides. Some characteristics of the adsorbates are compiled in Table 2. Diuron is much more insoluble in water and more hydrophobic than amitrole. The log Kow value gives the hydrophobicity of the adsorbates, where Kow is their partition coefficient in octanol-water. The dipolar moment of diuron is also larger than that of amitrole. Therefore, the adsorption of diuron on a hydrophobic carbon surface is expected to be greater than that of amitrole because of greater hydrophobic and van der Waals interactions with the former herbicide. Molecular dimensions of the two adsorbates differ. Dos Santos et al.14 used high level ab initio calculations to demonstrate that diuron can exist with four different molecular conformations in the gas phase. Conformations are classified as trans or cis according to the relative position of the N-H and CdO groups in the H-N-CdO peptide group, with the two trans conformations being planar. The most stable structure is one of the trans conformations, which is depicted in Figure 2 with its dimensions. This conformation has been taken into account in the calculation of the diuron molecular area given in Table 2. Dos Santos et al.14 also studied the effect of water on molecular conformation shape and equilibrium caused by the formation of hydrogen bonds between water and diuron. Conclusions reached were that the formation of hydrogen bonds affects the geometry of the most stable conformation. Thus, the phenyl ring and urea group were twisted by 31°, resulting in a loss of planarity and an increase in height, as shown in Figure 2. In addition, the dipolar moment of diuron decreased. Conversely, amitrole does not have different molecular conformations and has only a planar structure; the shape and dimensions of this molecule are also depicted in Figure 2. Although this molecule would be hydrated in aqueous solutions, its molecular dimensions would not change as drastically as those of diuron. This molecule is smaller than that of diuron, as can be seen from its dimensions and molecular area. Therefore, the microporosity of both carbon materials will be more accessible to the amitrole than to the diuron molecules. However, other factors besides the molecular size of the adsorbate control the adsorption process in aqueous solutions.15 Speciation diagrams are important to determine the presence of different species as a function of solution pH: protonated, neutral, or deprotonated. Thus, diuron is neutral above pH 6, (14) Dos Santos, H. F.; O’Malley, P. J.; De Almeida, W. B. Theor. Chem. Acc. 1998, 99, 301-311. (15) Moreno-Castilla, C. Carbon 2004, 42, 83-94.
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9588 Langmuir, Vol. 22, No. 23, 2006
Figure 2. Molecular dimensions of (a) diuron, (b) diuron---(H2O)2, and (c) amitrole. Dark gray spheres: Cl; light-gray spheres: H; magenta spheres: O; blue spheres: N; and black spheres: C.
whereas amitrole is neutral in the pH range between 6 and 8.16 Therefore, adsorbent-adsorbate interactions are nonelectrostatic under the experimental conditions used to determine the adsorption isotherms (pH 7).15 This is because the adsorbates are neutral, the pHPZC of ACF is 7, and although the pHPZC of ACC is 8, its surface charge at pH 7 is very slightly positive.13 Adsorption Isotherms of the Herbicides. Adsorption isotherms of diuron at different temperatures on ACF are shown in Figure 3. The Langmuir equation was applied to these isotherms to obtain the adsorption capacity, Xm, and BXm, where B is the Langmuir constant. This parameter is related to adsorbate-adsorbent interactions. Values obtained are given in Table 3 together with the correlation coefficient, R2, of the Langmuir plots. The high R2 values obtained indicate that the Langmuir equation fits very well with the adsorption isotherms. Xm values increase with temperature between 15 and 35 °C. However, Xm values at 35 and 45 °C are similar. The same trend is observed for the variation in BXm. The surface area of ACF occupied by diuron molecules at 35 °C can be calculated using the molecular area of diuron given (16) Fontecha-Ca´mara, M. A. Ph.D. Thesis. Universidad de Jae´n, Jae´n, Spain, 2006.
Figure 3. Adsorption isotherms of diuron on ACF at 15 °C (]), 25 °C (4), 35 °C (0), and 45 °C (O). Table 3. Results of Langmuir Equation Applied to Adsorption Isotherms of Diuron on ACF at Different Temperatures T (°C)
Xm (mg/g)
BXm (L/g)
R2
15 25 35 45
345 ( 4 625 ( 17 909 ( 27 917 ( 15
476 ( 5 2000 ( 54 3336 ( 100 3450 ( 55
0.999 0.993 0.996 0.996
in Table 2. This surface area is 1760 m2/g, which is similar to the SBET value of ACF. This result indicates that diuron is adsorbed on the entire ACF surface in a planar form at 35 °C. This explains
Temperature Dependence of Herbicide Adsorption
Langmuir, Vol. 22, No. 23, 2006 9589
Figure 4. Adsorption isotherms of diuron on ACC at 15 °C (]), 25 °C (4), and 45 °C (O).
Figure 5. Adsorption isotherms of diuron on unwoven-ACC at 25 °C (4) and 45 °C (O).
Table 4. Results from the Langmuir Equation Applied to Adsorption Isotherms of Diuron on ACC and Unwoven-ACC carbon ACC unwoven-ACC
T (°C)
Xm (mg/g)
BXm (L/g)
R2
15 25 45 25 45
161 ( 20 263 ( 32 476 ( 11 714 ( 12 1046 ( 19
15 ( 2 29 ( 3 351 ( 8 294 ( 10 1429 ( 90
0.900 0.910 0.997 0.999 0.998
why the Xm and BXm values obtained at 45 °C were the same as those obtained at 35 °C since the whole carbon surface is already occupied by a diuron monolayer at the latter temperature. Adsorption isotherms of diuron on ACC are depicted in Figure 4, and results of the application of the Langmuir equation to these isotherms are given in Table 4. The trend observed in the variation of Xm and BXm with adsorption temperature is the same as that found with ACF. However, there are three differences between these two adsorbents. The first is that diuron adsorption on ACC is much lower than on ACF. Thus, diuron occupies 43% of the ACC surface area at 45 °C, whereas at the same temperature, it occupies the entire ACF surface area. Likewise, BXm values are much lower in the ACC than in the ACF. The second difference is that when the adsorption temperature increases from 35 to 45 °C, Xm and BXm values do not change in ACF but markedly increase in ACC. The final difference is that the correlation coefficient increases with temperature in the case of ACC but remains practically unchanged in the case of ACF. This means that the adsorption isotherms on ACC are better fitted to the Langmuir equation when the adsorption temperature increases from 15 to 45 °C. Therefore, results of diuron adsorption on ACF and ACC indicate that it is apparently an endothermic process, which can be explained as follows. The rise in temperature diminishes the mass transfer resistance in both the liquid and the solid phase and weakens the hydrogen bonds formed among water molecules and between water molecules and solute or adsorbent.17 Therefore, the increase in temperature favors diffusion and dehydration of the diuron molecules, which makes them more planar and gives them a larger dipolar moment. The increase in planarity gives diuron molecules more access to the microporosity of the adsorbents and greater diffusivity in both liquid and solid phase. The increase in dipolar moment leads to enhanced adsorbentadsorbate interactions. In addition, the adsorption potential within the micropores increases the planarity of diuron molecules. In the case of ACF, the increase in planarity of the diuron molecules with increasing temperature would be more important than the reduction in mass transfer resistance due to the small size of the fibers; therefore at 35 °C, the entire ACF surface is covered by a monolayer of diuron molecules, as described (17) Fontecha-Ca´mara, M. A.; Lo´pez-Ramo´n, M. V.; A Ä lvarez-Merino, M. A.; Moreno-Castilla, C. Carbon 2006, 44, 2335-2338.
Figure 6. Adsorption isotherms of amitrole on ACF at 15 °C (]) and 35 °C (0).
previously. Thus, our group previously showed17 that, unlike diuron adsorption on ACF, the adsorption of 3,4-dichloroaniline decreases with higher temperature since hydration of the 3,4dichloroaniline molecules in solution has a smaller effect on their size and polarity because they lack the urea type chain, which is twisted after hydration. However, in the case of ACC, external mass transfer resistance is of major importance and explains the differences in diuron adsorption found between ACF and ACC. Thus, adsorption kinetic studies16 of diuron on both adsorbents at 25 °C found that the external mass transfer coefficients were 1 cm/min and 0.44 × 10-2 cm/min for ACF and ACC, respectively. The reason for this large difference is that the threads of the cloth are composed by a bundle of activated carbon fibers that are very close together. Therefore, in terms of the adsorption kinetics the relationship between activated carbon fiber and activated carbon cloth is similar to that between powder activated carbon and granular activated carbon. For this reason, the ACC was unwoven to reduce the effect of mass transfer resistance on the diuron adsorption. Adsorption isotherms of diuron on the unwovenACC are depicted in Figure 5, and the results are compiled in Table 4. The Xm and BXm values obtained are much greater than those found with ACC and indicate the importance of external mass transfer coefficient in diuron adsorption on ACC. This coefficient at 25 °C increases from 0.44 × 10-2 cm/min in ACC to 0.22 cm/min in the unwoven-ACC. The diuron uptake in unwoven-ACC also increases with higher temperature but now largely due to an increase in the planarity of diuron molecules. The percentage of unwoven-ACC occupied by diuron at 45 °C is 95%. Adsorption isotherms of amitrole on ACF are shown, as an example, in Figure 6, and results of the Langmuir equation are compiled in Table 5. These isotherms have a slightly sigmoidal shape and show that amitrole uptake decreases when adsorption temperature rises from 15 to 35 °C. The same variation with temperature is observed for the BXm value. Hence, unlike diuron adsorption, amitrole adsorption between 15 and 35 °C is exothermic. Xm and BXm values are much smaller than those found for diuron, especially in the case of ACF, despite the
AÄ ngeles Fontecha-Ca´ mara et al.
9590 Langmuir, Vol. 22, No. 23, 2006 Table 5. Results from the Langmuir Equation Applied to Adsorption Isotherms of Amitrole on ACF and ACC carbon
T (°C)
Xm (mg/g)
BXm (L/g)
R2
ACF
15 35 15 35
60 ( 2 44 ( 2 62 ( 7 48 ( 4
1.62 ( 0.06 1.35 ( 0.07 1.49 ( 0.14 1.18 ( 0.11
0.996 0.995 0.950 0.980
ACC
smaller molecular dimensions of amitrole than diuron. This is because hydrophobic and van der Waals interactions are much less with amitrole than with diuron since amitrole is less hydrophobic and much more soluble in water and has a smaller dipolar moment. The percentage of surface area occupied by amitrole at 15 °C, taking into account its molecular area, was 9% for ACF and 8% for ACC. Therefore, both adsorbents can remove the same amount of amitrole per unit surface area, unlike in the case of diuron. This is because the size and planarity of amitrole does not change with temperature and its molecule is smaller than that of diuron. Both characteristics of amitrole also affect its external mass transfer coefficient at 25 °C, which shows a value of 0.77 × 10-2 cm/min for ACF and 0.12 × 10-2 cm/min for ACC, a smaller difference between adsorbents than found with diuron.16 The decrease in amitrole uptake with higher adsorption temperature is mainly due to two effects. One is the increase in solubility with temperature, from 210 g/L at 15 °C to 375 g/L at 35 °C, which reduces hydrophobic interactions. The other is the increase in vibrational energy of the adsorbed amitrole molecules at higher temperatures, so that more adsorbed molecules have sufficient energy to overcome the attractive forces and desorb
back into the solution.18 This effect would be very important in the amitrole-carbon system because the BXm values obtained with both adsorbents are very low.
Conclusion Adsorption of diuron on activated carbon fiber or activated carbon cloth is much higher than that of amitrole because the former herbicide has a much lower solubility, much higher hydrophobicity, and larger dipolar moment than the latter. A further difference is that adsorption of diuron is apparently endothermic and that of amitrole exothermic at the temperature range studied. In the former, the endothermicity can be explained by an increase in the planarity and diffusion of diuron molecules with higher temperature. However, the exothermicity found for amitrole is due to the increase in amitrole solubility and in vibrational energy of adsorbed molecules with higher temperature. Results obtained in this study clearly show the major importance of temperature in herbicide adsorption from aqueous solution. There was a large increase in diuron uptake when the activated carbon cloth was unwoven. Thus, at 45 °C, the diuron molecules covered 95% of the surface area of the unwoven cloth versus only 45% of the surface area of the cloth, largely due to the greater external mass transfer coefficient of the unwoven material. Acknowledgment. The authors are grateful to MEC, FEDER, and Junta de Andalucı´a, Projects CTQ2004-07783-C02-02 and RNM 547, for financial support. LA061666V (18) Cooney, D. O. Adsorption Design for Wastewater Treatment; Lewis Publishers: CRC Press LLC, Boca Raton, FL, 1998.
