Langmuir 1999, 15, 587-593
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Experimental Study of Water Adsorption on Activated Carbons Issa I. Salame and Teresa J. Bandosz* Department of Chemistry, The City College of New York and The Graduate School of The City University of New York, New York, New York 10031 Received April 28, 1998. In Final Form: October 26, 1998 Two carbons of different origins (wood and coal) were oxidized with nitric acid. The materials were characterized using sorption of nitrogen, Boehm titration, and potentiometric titration. The water adsorption isotherms were measured at various temperatures close to ambient (relative pressure from 0.001 to 0.3). From these isotherms heats of adsorption were calculated using virial equation. The results showed that the isosteric heats of water adsorption are affected by surface chemical heterogeneity only at low surface coverage. The shapes of heats obtained indicate strong water-water interactions as a result of adsorption on secondary sites and cluster formation. In all cases the limiting heat of adsorption equal to the heat of water condensation (45 kJ/mol) was obtained.
Introduction Activated carbons are sorbents which are widely used in numerous processes of gas and water purification.1 Their applications are governed by such characteristics as surface area, pore size distribution, and surface chemistry. For example, for adsorption of small molecule gases, microporous carbons are used, whereas larger molecules are removed from water using mesoporous materials. Moreover, in some processes, such as removal of H2S, the catalytic influence of the carbon surface is enhanced by its impregnation with various chemicals such as NaOH, KOH, KI, or KMnO4.2 Activated carbons are characterized by a certain degree of surface chemical heterogeneity. It is related to the presence of heteroatoms such as oxygen, nitrogen, hydrogen, and phosphorus. The content of these elements varies depending upon the nature of organic precursor and the method of activation.3 Heteroatoms are fixed in the carbon matrix in the from of groups analogous to organic species such as carboxyls, carbonyl, phenols, ketones, amines, phosphates, etc.4,5 The presence of heteroatoms in various arrangements results in existing apparent acidity/basicity of the activated carbon surface. It follows that molecules which interact with carbon in a specific way will be adsorbed stronger and in greater amounts when chemical groups are present.6 Chemical groups may also have an effect on sorption of nonpolar molecules by creating obstacles for physical adsorption and prevent the molecule from occupying the most energetically favorable position on the surface.7 * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon: Marcel Dekker: New York, 1988. (2) Turk, A.; Sakalis, E.; Rago, O.; Karamitsos, H. Ann. N. Y. Acad. Sci. 1992, 661, 221. (3) Marsh, H., Heintz, E. A., Rodriguez-Reinoso, Eds. Introduction to Carbon Technolgies; University of Alicante: Alicante, Spain, 1997. (4) Boehm, H. P. In Advances in Catalysis; Academic Press: New York, 1966; Vol. 16. Boehm, H. P. Carbon 1994, 32, 759. (5) Puri, B. R. In Chemistry and Physics of Carbon; Walker, P. J., Jr., Ed.; M. Dekker: New York, 1970; Vol. 6. (6) Avgul, N. N.; Kiselev, A. V. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6. (7) Bandosz, T. J.; Jagiełło, J.; Schwarz, J. A. Langmuir 1993, 9, 2528.
Activated carbons are very often used as adsorbents from gaseous mixtures containing water vapors such as air. It is well-known that moisture in air may play a role in a competitive adsorption of a pollutant. The mechanism of water adsorption was proposed by Dubinin and Serpinsky.8 Their interpretation of the shape of experimental isotherms is based on interactions of water molecules with either pure carbon- or oxygen-containing surface species. According to DS theory water molecules are first adsorbed on primary adsorption centers, oxygen groups, and then adsorption on secondary centers, adsorbed water molecules, occurs. Thus an increase in the water vapor pressure leads to the formations of clusters of associated water molecules via hydrogen bonding.9,10 It was proposed that below p/p0 ) 0.3 surface chemistry governs the adsorption process, while at higher relative pressure microporosity becomes an important factor.11 The number of primary adsorption centers can be calculated by fitting the experimental isotherm to the equation formulated by Dubinin and Serpinsky (D-S).8 Many experimental results along with their interpretation have been published based on D-S approach. In some cases, especially for carbons with a large number of chemical groups, the attempts to fit the experimental data to the proposed isotherm failed.12,13 Recently Carrasco-Marin and coworkers have presented an approach linking the enthalpies of immersion to the distributions of oxygen groups.14 The authors also demonstrated that water adsorption isotherm may be described as the sum of DubininAstakhov isotherms of types I and V. A new approach leading to the understanding of the phenomena of water adsorption on activated carbons employs Monte Carlo Grand Canonical computer simulations.15,16 Recent results obtained by Gubbins and coworkers showed a good agreement of molecular simula(8) Dubinin, M. M.; Serpinsky, V. V. Carbon 1981, 19, 402. (9) Foley, N. J.; Thomas, K. M.; Forshaw, P. l.; Stanton, D.; Norman, P. R. Langmuir 1997, 13, 2083. (10) Hanzawa, Y.; Kaneko, K. Langmuir 1997, 13, 5802. Iiyama, T.; Nishikawa, K.; Otowa, T.; Kaneko, K. J. Phys. Chem. 1995, 99, 10075. (11) Rodriguez-Reinoso, F.; Molina-Sabio, M.; Munecas, M. A. J. Phys. Chem. 1992, 96, 2707. (12) Evans, M. J. B. Carbon 1987, 25, 81. (13) Bandosz, T. J.; Jagiełło, J.; Schwarz, J. A., Krzyzanowski, A. Langmuir 1996, 12, 6480. (14) Carrasco-Marin, F.; Mueden, A.; Centeno, T. A.; Stoeckli, F.; Moreno-Castilla, C. J. Chem. Soc., Faraday Trans. 1997, 93, 2211.
10.1021/la980492h CCC: $18.00 © 1999 American Chemical Society Published on Web 12/18/1998
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tions with the experimental results.16 This allows one to observe the process of cluster formation and its dependence upon the density and strength of oxygen surface groups. Moreover, the effectiveness of sorption of other gases such as methane in the presence of water can be also investigated.15 The objective of this paper is to describe the process of water adsorption on activated carbons and to discuss the effects of surface chemistry and pore structure on water uptake as well as on energetics of the process given by isosteric heats of adsorption. Carefully measured isotherms at temperatures close to ambient allowed us to evaluate the energetics of water-carbon, water-functional groups and water-water interactions. Understanding these phenomena is very important to control the processes when changes in the moisture content of air may affect the effectiveness and feasibility of the application of activated carbons as sorbents and separation media. Experimental Section Materials. Two activated carbons of different origins were chosen for this study: WVA 1100 (Westvaco, wood based, H3PO4 activation) and Xtrusorb (Calgon, coal based, steam activation). They are referred to as W and C, respectively. The carbons were oxidized with nitric acid.17 Briefly 10 g of carbon was stirred with 100 mL of 15 N (73%) HNO3 for 24 h at room temperature. Then samples were washed in a Soxhlet apparatus to remove excess of oxidizing agent and other watersoluble species. After oxidation, samples are referred to as W-O and C-O. Methods. Boehm Titration. The oxygenated surface groups were determined according to the method of Boehm.4 One gram of carbon sample was placed in 25 mL of the following 0.05 N solutions: sodium hydroxide, sodium carbonate, sodium bicarbonate, and hydrochloric acid. The vials were sealed and shaken for 24 h, and then 5 mL of the each filtrate was pipetted and the excess of base and acid was titrated with HCl and NaOH, respectively. The numbers of acidic sites of various types were calculated under the assumption that NaOH neutralizes carboxyl, phenolic, and lactonic groups; Na2CO3 neutralizes carboxyl and lactonic groups; and NaHCO3 neutralizes only carboxyl groups. The number of surface basic sites was calculated from the amount of hydrochloric acid that reacted with the carbon. Potentiometric Titration. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm). The instrument was set at the mode when the equilibrium pH was collected. Subsamples of the carbons of about 0.100 g in 50 mL of 0.01 M NaNO3 were placed in a container thermostated at 298 K and equilibrated overnight with the electrolyte solution. To eliminate the influence of atmospheric CO2, the suspension was continuously saturated with N2. The carbon suspension was stirred throughout the measurements. Volumetric standards of NaOH and HCl (0.1 M) were used as titrants. The experiments were done in the pH range of 3-10.13,18-20 Each sample was titrated by both acid and base, starting from the initial pH of the suspension. Sorption of Nitrogen. Nitrogen isotherms were measured at 77 K using a ASAP 2010 (Micromeritics). Before the experiment (15) Maddox, M.; Ulberg, D.; Gubbins, K. E. Fluid Phase Equlib. 1995, 104, 145. Ulberg, D. E.; Gubbins, K. E. Mol. Phys. 1995, 84, 1139. Muller, E. A.; Rull, L. F.; Vega, L. F.; Gubbins, K. E. J. Phys. Chem. 1996, 100, 1189. (16) Bandosz, T. J.; Blas, F. J.; Gubbins, K. E.; McCalumm, C. L.; McGrother, S. C.; Sowers, S. L.; Vega, L. F. In Recent Advances in Catalytic Materials; MRI Symposium Proceedings; Rodriguez, N. M., Soled, S. L., Hrbek, J., Eds.; MRS: Warendale, PA, 1998; Volume 497, pp 231-242. (17) Bandosz, T. J.; Jagiełło, J.; Schwarz, J. A. Anal. Chem. 1992, 64, 891. (18) Bandosz, T. J.; Jagiełło, J.; Contescu, C.; Schwarz, J. A. Carbon 1993, 31, 1193. (19) Jagiełło, J.; Bandosz, T. J.; Schwarz, J. A. Carbon 1994, 32, 1026. (20) Bandosz, T. J.; Buczek, B.; Grzybek, T.; Jagiełło, J. Fuel 1997, 76, 1409.
Figure 1. Nitrogen adsorption isotherms. Table 1. Structural Parameters Calculated from Sorption of Nitrogen sample
SBET (m2/g)
Vmic (cm3/g)
V
