Heterogeneity of Polymer-Based Active Carbons in Adsorption of Aqueous Solutions of Phenol and 2,3,4-Trichlorophenol K. La ´ szlo ´,* ,† P. Podkos ´cielny, ‡ and A. Da ¸ browski* ,‡ Department of Physical Chemistry, Budapest University of Technology and Economics, H-1521 Budapest, Hungary, and Faculty of Chemistry, Department of Theoretical Chemistry, Maria Curie-Sklodowska University, pl. M. Curie-Sklodowskiej 3, 20-031 Lublin, Poland Received October 28, 2002. In Final Form: April 23, 2003 Heterogeneity effects that accompany adsorption of phenolic compounds from water by polymer-based activated carbons are investigated at different values of solution pH. Activated carbons prepared from poly(ethylene terephthalate) and polyacrylonitrile (APET and APAN, respectively) were used to adsorb phenol and 2,3,4-trichlorophenol in acidic (pH ) 3), unbuffered, and basic (pH ) 11) aqueous solutions. A Langmuir-Freundlich adsorption-isotherm equation was used to estimate the parameters that characterize adsorption of phenols from dilute solutions on heterogeneous surfaces. Adsorption energy distribution functions were calculated by a regularization method. Analysis of these functions for the APET and APAN carbons provides comparative information about their heterogeneity. Introduction The heterogeneity of active carbon surfaces stems from two sources, geometrical and chemical. Geometrical heterogeneity is the result of differences in size and shape of pores, as well as cracks, pits, and steps. Chemical heterogeneity is associated with different functional groups, mainly oxygen groups that are located most frequently at the edges of the turbostratic crystallites, as well as with various surface impurities. Both chemical and geometrical heterogeneities contribute to the unique sorption properties of active carbons. Functional groups and delocalized electrons of the graphitic structure determine the apparent chemical character of an activated carbon surface. 1 Oxygen, for instance, may be present in various forms, such as carboxyls, carbonyls, phenols, lactones, aldehydes, ke- tones, quinines, hydroquinones, anhydrides, or ether structures. These groups may also interact among them- selves. Some groups, e.g., carbonyl, carboxyl, phenolic, hydroxyl, and lactonic, are acidic, while pyrone, chromene, and quinone are basic. 2 Granular or powdered activated carbons are prepared from a variety of raw materials, among which the most frequently employed are hard coal, lignite, lignocellulosic materials, and certain polymers. 2-12 Polymer precursors are used preferentially if carbon with low inorganic content is needed. In addition to the nature of the starting material, the preparation process, including the method of carbon- ization, activation, and/or further treatment, also have a significant effect on the final surface properties. Since carbon has a high adsorption capacity for organic compounds, it is the most commonly used adsorbent for removing these compounds from aqueous media. 13-14 The presence of water further modifies the chemistry of a surface, as its interaction with the specific groups on the carbon surface may modify their chemical behavior. Owing to the amphoteric character of a carbon surface, i.e., to the acidic and/or basic functional groups, the surface properties may be influenced by the pH value of the coexisting bulk liquid phase. When a dissolved chemical species that is to be removed, e.g., by adsorption, bears an acidic and/or basic character, any acidic or basic sites on the carbon may also participate in the interaction. In this paper, the interaction of activated carbon with weak aromatic acids is addressed, since the question of heterogeneity in the adsorption of aromatic compounds from water has until now attracted little general attention. 15-25 Weak aromatic acids such as phenolic compounds are, however, widespread in many industrial * Authors to whom correspondence may be addressed: K. La ´ szlo ´, tel +36-1-463-1893, fax +36-1-463-3767, e-mail klaszlo@ mail.bme.hu; A. Da ¸ browski, tel +48-81-5375605, fax +48-81- 5375685, e-mail dobrow@hermes.umcs.lublin.pl. † Budapest University of Technology and Economics. ‡ Maria Curie-Sklodowska University. (1) Leo ´n Y Leo ´n, C. A.; Radovic, L. R. In Chemistry and physics of carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1994; Vol. 24, p 214. (2) La ´ szlo ´, K.; Szu ¨ cs, A. Carbon 2001, 39, 1945. (3) Mahajan, O. P.; Moreno-Castilla, C.; Walker, P. L., Jr. Sep. Sci. Technol. 1980, 15, 1733. (4) La ´ szlo ´, K.; Bo ´ta, A.; Nagy, L. G. 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