Adsorption of Acetone, Methyl Ethyl Ketone, 1,1,1-Trichloroethane, and Trichloroethylene in Granular Activated Carbons Joa ˜ o Pires,* Moise ´ s Pinto, Ana Carvalho, and M. Brotas de Carvalho Department of Chemistry and Biochemistry, University of Lisbon, Faculty of Sciences, Campo Grande, 1749-016 Lisbon, Portugal The adsorption isotherms of two oxygenated and two chlorinated volatile organic compounds, namely acetone, methyl ethyl ketone, 1,1,1-trichloroethane, and trichloroethylene, were determined at 298 K, by the gravimetric technique, in four granular activated carbons. The textural properties of the adsorbents were evaluated from low-temperature nitrogen adsorpion. The Dubinin-Astakhov equation was used to fit the data, and a discussion of the results was made on the basis of relevant physical properties of the adorbate molecules. Introduction In recent years a particular class of substances, known as volatile organic compounds (VOCs), has attracted the attention of the authorities, the scientific community, and the industries, due to the recognition that a large propor- tion of these compounds, which are used in a variety of processes, can cause severe health effects. Considering only the present 15 E.C. countries, and admitting that recent regulations will be met, an amount higher than 6.5 × 10 6 tons of volatile organic compounds will be released to the atmosphere by the year 2010. In the U.S. it is esti- mated that about 0.5 × 10 6 tons of VOCs are emitted annually. 1,2 The use of activated carbons in gas separation by ad- sorption is a fact known for a long time and does not need to be justified. 3,4 The potentialities of using such materials in the abatement of VOCs in the vapor phase are, obvi- ously, high, particularly due to their high adsorption capacity and their hydrophobicity. 5,6 The limitations of these solids, in relation to their use as adsorbents of volatile organic compounds, are mainly related with temperature constraints, 7 since these materials are flammable. This is an important fact that has to be taken into account, since the regeneration of adsorbents is frequently made by increasing the temperature. One important group of studies concerning the adsorp- tion of VOC molecules in activated carbons was related with the development of suitable adsorbents for sampling and, therefore, for increasing the accuracy of the analytical methods currently used when monitoring the level of a particular volatile organic compound in the atmosphere. 8,9 The adsorption isotherms can be regarded as the primary source of information on a particular adsorbent/adsorbate system whose data can be used to evaluate the suitability of a particular adsorbent, or as a starting point for modeling the adsorption process, that can include the recollection of VOCs or even their selective separation. 10 A number of studies of adsorption of volatile organic compounds in activated carbons, with different insights, can be found in the literature. 10-25 However, the under- standing of such adsorbent/adsorbate systems at a physi- cal-chemistry level, in the majority of cases, is far from clear. This work aims to contribute to the understanding of the role of the main factors, from the adsorbent and from the adsorbate, which influence the adsorption isotherm, a question that has several practical implications. To this effect, four commercial activated carbons and four VOC molecules were selected. The activated carbons had a granular form, since this work is part of a project related with the development of filters, by supporting the adsor- bent materials in polymeric matrixes, and in this way, granular activated carbons are advantageous over, for instance, powder activated carbons. The selected VOCs are representative examples of oxygenated and halogenated solvents, which have wide use in the industry, namely acetone, methyl ethyl ketone (MEK), 1,1,1-trichloroethane (TCA), and trichloroethylene (TCE). Experimental Section Materials. Four commercial activated carbons, labeled as RB1, RB3, RB4, and CarbTech, were used. The former three are indicated for the removal of contaminants from gas flow, and the latter is used in some incinerators of urban residues. The micropore size distributions of the four samples, evaluated from the Horvath-Kawazoe method, 26 are narrow and present a maximum for a width of 0.65 nm, the differences between the samples being within the precision of the method. The surface chemistry of these samples was characterized by the method of Boehm titra- tion, as described in detail elsewhere. 26 Briefly, this method consists of equilibrating a given amount of solid (1 g during 24 h), with different basic or acid solutions (each solution in a given vessel) of NaOH, NaCO 3 , NaHCO 3 , and HCl. After equilibration the excess of acid or base, which did not react with the activated carbon, was titrated with NaOH or HCl, respectively. The concentration of each aqueous solution was 0.05 N. The results, expressed as type of site, total acidity, and total basicity, 26 are summarized in Table 1. Adsorption Isotherms. To determine the adsorption isotherms of nitrogen at 77 K, the samples (about 50 mg) were outgassed at 573 K, for 2 h after a ramp of 10 K/min, under a dynamic vaccum better than 10 -2 Pa. Nitrogen was 99.995% (Air Liquide, France) pure, and the results were * Corresponding author. E-mail: jpiresil@fc.ul.pt. 416 J. Chem. Eng. Data 2003, 48, 416-420 10.1021/je020180n CCC: $25.00 © 2003 American Chemical Society Published on Web 02/11/2003