Abstract-The adsorption of a cu 5.4 x lo-' M aqueous solution of methylene blue has been examined on five activated carbon samples. The equilibrium ...
Carbon Vol. 35, No. 3, pp. 365-369, 1997 Copyright Q 1997ElsevierScienceLtd Printed in Great Britain All rights reserved 0008.6223/97$17.00+ 0.00
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ADSORPTION OF METHYLENE BLUE FROM AN AQUEOUS SOLUTION ON TO ACTIVATED CARBONS FROM PALM-TREE COBS J. AvoM,‘,*
J. KBTCHA MBADCAM,~ C. NOUBACTEP~ and
P. GERMAIN~ “Laboratoire de Chimie Physique, Faculte des Sciences, Universite de Yaounde I, Boite Postale 812 Yaounde, Cameroon ‘Laboratoire de Thermochimie Minerale, INSA de Lyon, 20 Avenue Albert Einstein, 69621 Villeurbanne Ctdex, France (Received 26 December 1995; accepted in revised form I October 1996)
Abstract-The adsorption of a cu 5.4 x lo-’ M aqueous solution of methylene blue has been examined on five activated carbon samples. The equilibrium concentrations (C,) were determined by spectrophotometry studies. The analysis of Freundlich adsorption isotherms obtained provides the adsorption capacity of each carbon sample. 0 1997 Elsevier Science Ltd. All rights reserved Key Words-A.
Activated carbons, C. adsorption, spectrophotometry.
1. INTRODUCTION
biomass resources. Conversion of the cobs to a valueadded product such as activated carbon will help to solve part of the problem of waste-water treatment in Cameroon. In the present work, we report the determination of the adsorption capacity for various powdered activated carbons. It involves the adsorption of an aqueous methylene blue solution (ca 5.4 x 10m5 M). Experimental data obtained from these studies are fitted into the Freundlich equation, and these values are correlated with the specific surface area (S) obtained by N,-BET method, and with the iodine number (I).
Activated carbons are still probably the most widely used adsorbents in industry. They are recommended for the general removal of organic molecules from water, where special hydraulic requirements must be considered. Elsewhere, granular activated carbon (GAC) adsorption is a recognized technology in waste-water and drinking-water treatment. Most practical applications of activated carbons require sorbents having a large volume of very fine pores. The presence of micropores substantially influences its sorption properties because the amount adsorbed on the macropore surface is negligible in comparison to that for the micropores [ 11. Therefore, characterization of the activated carbons has become one of the most important problems in adsorption technology. Determinations of pore structure are made by fitting either the Langmuir or BET equation [2] to the isothermal equilibrium data obtained. However, these values are not a true indication of the adsorption capacity of a charcoal during liquid-phase adsorption studies [3]. It is therefore more logical to determine the porous structure by combining both the gas-phase and the liquid-phase adsorption equilibrium data. The literature indicates that the adsorption of phenol [4], methylene blue [5,6], caffeine [7] and iodine [8] from the aqueous phase is a useful tool for product control in the manufacture of activated carbon. Palm-tree cobs are important byproducts of the oil industry in tropical countries such as Cameroon. They have been chosen as raw materials as part of a research program aimed at utilizing Cameroonian
2. EXPERIMENTAL 2.1 Sample preparation Two sets of activated carbons were studied. The first set was made of one charcoal (Cl) supplied by the CNPS Hospital in Yaounde (Cameroon) and used for water treatment. The second set was made up of four charcoals (C2-CS) prepared from palmtree cobs. The cobs were dried for 24 hours in an oven at 110°C. The dried cobs were then treated in a 60% (w/w) zinc chloride aqueous solution for 30 minutes and carbonized in a closed reactor [9]. The temperature was raised from 25 to 600 or 700°C (250°C h-i) and maintained or not maintained at this maximal temperature. Table 1 summarizes the maximum temperature conditions and time held at this temperature for carbon samples C2-C5. All the samples were first ground and sieved to obtain a 63 pm fraction (diameter ~63 pm). The uniform fractions were dried in an oven at 100-l 10°C for 24 hours and then stored in a CaCl, dessicator
*Corresponding author. 365
366
J. AWM rt ul.
Table 1. Maximum
temperature
(T,,,)
and residence time
lt7) of carbon samples C2-C5 Carbonization (T,,,? r,)
Charcoal
6OO’C,0 minutes 700 ‘C, 0 minutes 7OO’:C,2 hours 700 C, 5.5 hours
c2 C3 c4 c5
at room studied.
temperature
2.2 Experimental
until
they
were ready
to be
studies
To evaluate the adsorption equilibrium data. experiments were carried out as follows. A series of 200 ml dry volumetric flasks were prepared, each containing a known mass of powdered activated carbon. To each flask was added 25 ml of c~ 4 x 10e4 M methylene blue aqueous solution and distilled water. The total volume of the solution was 200 ml. The flasks were then stoppered and shaken at the same stirring rate for 4 hours. The temperature of the system was kept constant at room temperature. When the equilibrium was reached (a preliminary kinetic study has shown that satisfactory equilibrium conditions were attained after 2 hours), the solutions were separated from the adsorbents by decantation after 4 hours at room temperature, and 5 ml of the supernatant solution was titrated. The adsorbate concentration at equilibrium (Ck) was determined by UV absorption at 625 nm using a P.M.ZK spectrophotometer. Regression analysis of the experimental data of optical density (d) versus concentrations (C) for the standard Beers-law plot gave the molar absorption coefficient (E) of methylene blue as 3.5 x 1Oh 1 mall’ and C=2.90 x 10m5tl with a regression coefficient of r = 0.99.
2.3 Adsorption
isotherms
The equilibrium adsorption data for all five charcoal-methylene blue systems are summarized in Table 2, where A4 is the mass of the carbon sample, C, is the adsorbate concentration at equilibrium, Q. is the amount of methylene blue adsorbed by 1 g of carbon at equilibrium, and P is the discoloration amount expressed as a percentage (P= ((C,- C,),! C,} x loo), C, being the initial concentration of the aqueous methylene blue solution. In Fig. 1 we have plotted the adsorption isotherms Q,=f‘(C,). The discoloration amount P vs the mass of charcoal (M) plots are given in Fig. 2. The empirical Freundlich equation is often used as means of data description. The relation Qe=k(C,)” is merely indicative for the adsorption capacity and intensity. It can easily be verified using the relation in the form In Qe= In k +n In c,, whereby a plot of In Qe vs In C, should be linear. bringing out the validity of the equation. Such curves are plotted in Fig. 3. Table 3 gives the values of k and n obtained
Adsorption of methylene blue on to activated carbons
361
Qe x 105 Cmol/g) loo 1
60
0 0
1
2
3
4
5
Ce x 105 (mol/l) Fig. 1. Adsorption isotherms.
by the method of linear regression of the data, the regression coefficient Y, and the C, range over which a given system adheres to the linearity of the relation. The values of r are all between 0.94 and 0.99, indicating a good mathematical fit.
3. RESULTS AND DISCUSSION The adsorption isotherms (Fig. 1) indicate fairly typical type-l “H” or “high affinity” Giles’ [ 111 classification isotherms as generally refered to in the literature [lo], or type-2 class L (Langmuir type) according to the classification of isotherms for adsorption from solution. It can be assumed that practically all the isotherms obtained fall into this category, implying strong preferential adsorption of the solute, and that these charcoals are microporous. At high concentrations, desorption was noted instead of the expected adsorption for Cl and C5. The interpretation is not clear. Cousins et al. [12] noted a similar effect in the determination of solution adsorption isotherms for toluene solutions of triethylenediamine, quinuclidine and triethylamine at 298.15 K with a commercial activated microporous carbon. Close inspection of Fig. 2 reveals that discoloration occurs in two steps. In the first step (0< P85%), the slope tends to fall and levels off at P= 100%. This would be the maximum mass of activated carbon necessary to decolorize the methylene blue aqueous solution. The Cl, C3 and CS systems would need less activated carbon to decolorize a methylene blue aqueous solution (ca 5.4 x lo-’ M), whereas the C2 and C4 systems would need twice as much activated carbon to decolorize the same solution. We can conclude, therefore, that the adsorption capacities of the samples increase with the slope of the first step in the order Cl > C3 > C5 > C2 > C4. However, when the slopes are close, the curves can intersect (C3 and C5) and the former classification depends on the mass of charcoal. For example, for M= 12 mg, Cl >C3> C5 >C2 >C4 but for M=28 mg, C3 interchanges its position with C5. The curves plotted in Fig. 3 for values of P > 50% obey linearity fairly well. This permits us to deduce Ml, the theoretical mass of charcoal necessary for the discoloration of 1 1 of the initial concentration of aqueous solution of methylene blue. The values obtained are given in Table4. Taking into account the values of the specific surface areas (S) found by the N,-BET measurements at 77 K and of the iodine number (I) determined elsewhere [13] (Table4), it
J. AVOM et ul.
368
P (7%) 100
80
0,Ol
0,oo
0,02
0,03
0,04
0,05
0,06 $1 (g)
Fig. 2. Variation Table 3. Freundlich Charcoal Cl C2 c3 c4 c5
of the discoloration
isotherm
data
Kx 10”
n
r
Cc x IO’ (Molly ‘)
3.2 1.2 1.6 0.22 0.68
0.11 0.12 0.12 0.03 0.05
0.97 0.98 0.98 0.94 0.99
0.06 -1.91 0.12-0.92 0.34 -2.21 0.35-2.05 0.18-2.40
Table 4. Results from methylene blue adsorption Nitrogen adsorption (S) and iodine adsorption Charcoal Cl C2 c3 c4 c5
amounts
M, (mg) 48 147 114 321 131
S(m’g
405 664 161 692
r)
(MI). (I)
[(mgg~‘)
520 620 420 740
appears that if I, #I, and S, #S,, their respective isotherms will never intersect, and the values of M, decreases with increasing I and S. However, it appears from I and S studies that we have the classification C5 > C3 > C2 > C4. This classification can be explained by the fact that, although they are all microporous, some of the pores of C5 which are accessible to iodine and nitrogen (molecules having the same shape) are not accessible to methylene blue, which is a larger molecule: its minimum diameter is
as a function
of the mass of charcoal
about 0.80 nm, and the limiting diameter of pores which can admit this molecule has been estimated to be about 1.3 nm [ 14,151. Taking into account the absolute error admitted in the determination of Z, S and M, we conclude that the adsorption capacities of the samples increase in the order Cl > C3 zC5 >C2 >C4, and that in our conditions of treatment, the carbonization up to 700°C of cobs initialy impregnated with zinc chloride, followed by immediate cooling, leads to the most adsorptive charcoal. It is also of interest to examine the n values, because the closer n is to 1, the more homogenous is the surface. Inspection of the n values obtained shows a variation among the charcoals with respect to adsorption of methylenee blue: they all have different surface chemistry.
4. CONCLUSION
The present investigation on the characterization of activated carbons prepared from cobs of palm trees by pyrolisis after chemical activation with ZnCl, reveals that the adsorption capacity of the carbon adsorbents produced depends on the thermal treatment temperature and duration: high temperature and low residence time enhance microporosity. The values of MI, I and S suggest that these three
Adsorption of methylene blue on to activated carbons
369
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Fig. 3. Freundlich isotherms. parameters are predominantly determined by the size of the pores. Further investigations are needed. A~k~zo,L,ledgenzenfs-The authors are grateful to P. Claudy and J. M. Letoffk for helpful discussions.
6. Barton, S. S., Carbon, 1987, 25, 343. 7. Stoeckli, H. F., Fraguiere, M., Huguenin, S., Depraz, M. and Bellerini. L., C&on, 1988, 26, 915. 8. Fernandez-Colinas, J., Denoyel, R. and Rouquerol. J., Adsorption Sci. Tech., 1989,6, 18. 9. Avom, 3. and Hajal, 1. J., Science et Technologie, 1983, 3, 2.
REFERENCES 1. Dubinin, M. M., Carbon, 1983, 21, 359. 2. Brunaiier, S., Emmet, P. H. and Teller, E., J. Am. C’hem. Sot., 1938, 60, 309. 3. Yenkie, M. K. N. and Natarajan, C. S., S’e~urafiu~? Science and Teckwlugy, 1993, 28, 1177. 4. Puri, B. R., in Activated Cnvbon Adsorpfion of Orgunics porn Aqueous Phuse, Vol. 1 ed. 1. H. Suffet and
M. J. McGuire. Ann Arbor Science, Ann, Arbor, MI 1980. p.353. 5. Linge, H. G., Fuel, 1989, 68, 1I I.
IO. Hiester,
N. K., Vermeulen, Th. and Klein, G., in
Chemicul Engineers Humfbook, 4th edn, Chap. 16, eds
11. 12.
13. 14. 15.
R. H. Perry. C. H. Chilton t S. D. Kirkpatrick. MacGraw-Hill, New York, 1963. Giles, C. H., MacEwan, T. H., Nakwa, S. N. and Smith. D., J. Chem. Sot. 1960, 786, 3973. Cousins, H. J., Gardner, P. J. and Matthews, S. J., Carbon, 1992, 30, 1’7. Avom, J., unpublished results. Graham, D., J. Whys. Chem., 1955,59,896. Kipling, J. J. and Wilson, R. B., J. Appt. Chew., 1960, 10, 109.
