Energy & Fuels 1997, 11, 331-336
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Adsorption onto Fibrous Activated Carbon: Applications to Water Treatment P. Le Cloirec,*,† C. Brasquet,† and E. Subrenat‡ Ecole des Mines de Nantes, Subatech UMR No. 6457, B.P. 20722, 4 rue Alfred Kastler, 44307 Nantes Cedex 3, France, and Actitex, 16 rue Tre´ zel, 92309 Levallois Cedex, France Received September 5, 1996. Revised Manuscript Received November 25, 1996X
The adsorption of polluted waters is performed by activated carbon fibers (ACF). This new material is characterized by scanning electronic microscopy. BET surface areas and pore volumes are determined. Adsorption of natural organics (humic substances) and micropollutants (aromatic compounds such as benzene and toluene) is carried out in a batch or dynamic reactor. Classical models are applied and kinetic constants calculated. The results show that the performance of ACF is significantly higher than that of granular activated carbon (GAC) in terms of adsorption velocity and selectivity for micropollutants. These higher performances are due to some ACF physical properties, such as their high BET surface area and micropore volume. Moreover, the micropores are directly connected on the external surface area of fibers, which allows smaller mass transfer resistance. In a dynamic reactor, the breakthrough curves obtained with ACF beds are particularly steep, suggesting a smaller mass transfer resistance than that of GAC. The adsorption zone in an ACF bed is about 3.5 mm and is not really dependent on the water flow rate within the studied range.
Introduction The Safe Drinking Water Act Amendments of 1986 have identified granular activated carbon (GAC) as the best available technology for removing synthetic organic contaminants from water supplies. Adsorption on GAC is commonly used for the treatment of water and wastewater contaminated by low concentrations of hazardous compounds, especially those of low molecular weight.1,2 Some GAC are also effective in removing higher molecular weight organics such as humic substances (HS),3 but it seems that this ability is governed, in part, by molecular size distribution in relation to adsorbent pore size.4 Recently, a new formulation of activated carbon has been developed: activated carbon fibers (ACF). ACF have received increasing attention in recent years as an adsorbent for purifying water.5,6 This activated carbon is prepared from natural or synthetic precursors,7-9 the raw material having an impact on surface groups and pore size distribution.9,10 First, the precur†
Ecole des Mines de Nantes. Actitex. Abstract published in Advance ACS Abstracts, January 15, 1997. (1) Cheremisinoff, P. N.; Ellerbush, F. Carbon Adsorption Handbook; Ann Arbor Science Publishers: Ann Arbor, MI, 1978. (2) Schulhof, P. J. Am. Water Works Assoc. 1979, 71, 648-661. (3) Christman, R. F.; Gjessing, E. T. Aquatic and Terrestrial Humic Materials; Ann Arbor Science Publishers: Ann Arbor, MI, 1983; 538 pp. (4) Kilduff, J. E.; Karanfil, T.; Weber, W. J., Jr. Environ. Sci. Technol. 1996, 30 (4), 1344-1351. (5) Economy, J.; Daley, M. A.; Mangun, C. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41 (1), 321-325. (6) Baudu, M.; Le Cloirec, P.; Martin, G. Water Sci. Technol. 1991, 23 (7-9), 1659-1665. (7) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Dekker: New York, 1988; 482 pp. (8) Ryu, S. K. High TemperaturessHigh Pressures 1990, 22, 345354. (9) Suzuki, M. Water Sci. Technol. 1991, 23, 1649-1658. (10) Economy, J.; Lin, R. Y. Appl. Polym. Symp. 1976, 29, 199211. ‡
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sor is carbonized at 800-1000 °C to remove noncarbonaceous components and to develop a limited pore volume.7,8 Then, it is physically activated at 800-1100 °C by CO2 and/or steam to increase pore surface area and volume.7 Specific surface areas of ACF are high,11 between 700 and 2000 m2 g-1, and are dependent on raw material and activation conditions.12 ACF are highly microporous (>90% by pore volume is microporous),10 with the micropores directly on the fibers’ external surface8 having an average diameter from 5 to 21 Å.13 The first studies carried out on this new material showed that the ACF seemed very effective in removing contaminants from liquid or air,10,14,15 with adsorption rates and capacities higher than those of GAC. For example, Baudu et al.6 and Thwaites et al.16 showed that initial adsorption rates are 2.5-10 times larger with fibers. Furthermore, due to its high microporosity, ACF seemed selective for the adsorption of low molecular weight compounds.8,17 The main objective of this paper is to assess performance of the ACF in water treatment. Two different approaches are proposed. A batch reactor allows the study of adsorption of a large set of organics present in water and the determination of selectivity of the ACF for low or high molecular weight compounds. The (11) Daley, M. A.; Mangun, C. L.; Economy, J. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41 (1), 326-330. (12) Lemarchand, D. The`se de Docteur Inge´nieur de l’Universite´ de Rennes I, 1981. (13) Sakoda, A.; Kawazoe, K.; Suzuki, M. Water Res. 1987, 21 (6), 717-722. (14) Petkovska, M.; Mitrovic, M. Chem. Biochem. Eng. Q. 1989, 3 (4), 153-159. (15) Cal, M. P.; Larson, S. M.; Rood, M. J. Environ. Prog. 1994, 13 (1), 26-30. (16) Thwaites, M. W.; Stewart, M. L.; McNeese, B. E.; Sumner, M. B. Fuel Process. Technol. 1993, 34, 137-145. (17) Brasquet, C.; Roussy, J.; Subrenat, E.; Le Cloirec, P. Environ. Technol. 1996, 17, 1245-1252.
© 1997 American Chemical Society
332 Energy & Fuels, Vol. 11, No. 2, 1997
Le Cloirec et al.
Table 1. Main Characteristics of Activated Carbon Materials sample identifier company presentation precursor BET surface area (m2 g-1) micropore surface areaa (%) micropore volumea (cm3 g-1) micropore volume (%) median pore diameter (Å)
CS 1501 Actitex cloth viscose 1689 87.5 0.665 96.3 6.9
RS 1301 Actitex cloth viscose 1460 77.2 0.506 68.1 7.3
NC 60 Pica granules coconut 963 76.0 0.327 94.4 7.3
a Mesopore surface area and micropore volume were estimated by the t-plot method, the thickness of the film being calculated by the Harkins and Jura equation. Micropore surface area was then estimated by difference between the BET surface area and the mesopore surface area.
possibilities of ACF utilization are studied in a dynamic system. The adsorption capacities are calculated. Experimental Section Activated Carbons. The activated carbons used in this study are commercial products produced by the Pica Co. and the Actitex Co. (France). The main characteristics of the materials used in this study are presented in Table 1. Specific surface areas were determined according to the Brunauer Emmet and Teller (BET) method7 by helium adsorption with a Coulter SA 3100 apparatus. The pore size distributions of the three materials were determined (Figure 1). The activated carbon cloths were characterized with a scanning electronic microscope (SEM) JEOL JSM-6400F. Pictures are presented in Figures 2-4. Figure 2 shows that each fiber is ribbed, with a diameter around 10 µm. From the data presented in Table 1, it seems that CS 1501 is the activated carbon with the highest BET surface area and microporosity. Comparison of Figures 3 and 4 confirms that RS 1301 has more mesoporosity than CS 1501. The mesopore diameters range from 2 to 100 nm in width, with elliptical shapes for the sample RS 1301. However, for the sample CS 1501, even at a 150000× magnification, the fiber surface is smooth and formed of micropores (98%). The main characteristics of the adsorbates are given in Table 2. Humic substances were commercially available, extracted from peat. The concentration measurements were carried out using a UV-vis spectrophotometer from Shimadzu, using the UV region (200-300 nm). Adsorption in Batch Reactors. 1. Kinetic Studies. 1.1. Organics. About 0.5 g of activated carbon in the form of cloths or granules was continuously stirred with 1 L of aqueous solution at 20 ( 1 °C initially containing 100 or 150 mg L-1 of micropollutants. Samples of 5 mL were withdrawn at regular times until a steady state was obtained, and concentration in solution as a function of time was plotted. 1.2. Humic Substances. For the humic substances, the initial concentration was 50 mg L-1, the solution volume was 0.250 L, and the activated carbon weight was 0.2 g. The procedure was the same as with the organics. 2. Isotherms. 2.1. Equilibrium Studies. Activated carbon in the form of GAC or ACF (25-500 mg) was continuously stirred with 250 mL of an aqueous solution with an organic micropollutant concentration of about 100 mg L-1 at 20 ( 1 °C. Equilibrium was reached after a contact time of 48 h for the grains and 12 h for the fibers.6 With HS, the initial concentration was 50 mg L-1. 2.2. Selectivity Studies. To characterize the selectivity of the activated carbon cloth CS 1501, adsorption in batch
Figure 1. Pore size distribution of (a, top) granular activated carbon NC 60, (b, middle) fibrous activated carbon CS 1501, and (c, bottom) fibrous activated carbon RS 1301. reactors was performed with two solutions of 0.5 L: one solution contained only phenol with an initial concentration of about 100 mg L-1, the other was a mixture of phenol and HS, both pollutants with a concentration of 100 mg L-1. After 3 h of stirring, adsorption capacities of phenol were compared. The influence of water matrix on selectivity was studied with deionized and surface water from the Gardon River (Ale`s, France). Table 3 gives some characteristics of these different kinds of water, including total organic carbon (TOC). Adsorption in Continuous Flow Reactor. The adsorption in continuous reactor was performed only with sample CS 1501. A laboratory pilot unit was set up for the continuous flow study (Figure 5). The carbon bed diameter was 2.5 cm, the bed length being 4-16 mm. The dead volume between ACF beds was negligible.
Adsorption onto Fibrous Activated Carbon
Energy & Fuels, Vol. 11, No. 2, 1997 333 Table 3. Main Characteristics of the Different Types of Water water
TOCa (mg L-1)
SSb (mg L-1)
pH
conductivity (µS cm-1)
deionized river
