Effect of Ozone Treatment on Surface Properties of Activated Carbon H. Valde ´s,* ,† M. Sa ´ nchez-Polo, ‡ J. Rivera-Utrilla, ‡ and C. A. Zaror † Departamento de Ingenierı ´a Quı ´mica (F. Ingenierı ´a), Universidad de Concepcio ´ n, Correo 3, Casilla 160-C, Concepcio ´ n, Chile, and Departamento de Quı ´mica Inorga ´ nica (F. Ciencias), 18071-Universidad de Granada, Spain Received June 18, 2001 The combined use of ozone and activated carbon has recently started to be developed for the treatment of toxic effluents. However, the effect of ozone on the properties of activated carbon is not fully elucidated. A study was undertaken of modifications in the surface properties of a commercial activated carbon produced by its ozonation during different time periods. Surface chemistry of the activated carbon samples was characterized by selective neutralization, temperature-programmed desorption, X-ray photoelectron spectroscopy, and pH of the point of zero charge. Surface area and volume of micropores and mesopores were obtained from nitrogen adsorption isotherms at 77 K. Adsorption properties were determined by methylene blue adsorption index. Results show that the higher the ozone dose, the higher is the oxidation of the carbon and the greater is the number of acid groups present on the carbon surface, especially carboxylic groups, whereas the pH of the point of zero charge decreases. The surface area, micropore volume, and methylene blue adsorption all reduce with higher doses. These results are explained by the ozone attack on the carbon and the fixation of oxygen groups on its surface. 1. Introduction Activated carbon has been widely used as a powerful adsorbent in wastewater and gases treatment. The chemical reactivity of the activated carbon surface has recently been emphasized, because it has been demon- strated to catalyze oxidation, reduction, halogenation, dehydrogenation, polymerization, and nucleophilic reac- tions, among others. 1-5 Activated carbon reactivity is directly related to its chemical properties, and the chemical reactivity with organic substrates can be attributed to the presence of carboxylic groups, hydroquinone free radicals, quinones, metallic ions, and nitrogenous impuri- ties. 1 Recently, the combined use of ozone and activated carbon in single treatment processes has been proposed as an attractive option for the destruction of toxic organic compounds. 4,6,7 The great oxidation power of ozone together with the high adsorption capacity of activated carbon effectively eliminates organic contaminants of high toxicity and low biodegradability 7 and prevents their mutagenic activity. 8 Combined ozone-activated carbon treatment processes have been shown to modify the chemical composition of activated carbon surface groups. 9,10 These reactions may determine the adsorption capacity of the activated carbon, its regeneration efficiency, and the process economic feasibility. Thus, greater knowledge of the surface chem- istry of activated carbon could lead to the development of more specific and effective adsorbents for a given process. The purpose of this study was to investigate modifica- tions in both the surface chemistry and textural charac- teristics of activated carbon caused by the action of ozone. This work is part of a wider ongoing project to investigate the possibilities of the combined use of ozone and activated carbon in the elimination of contaminants from water. 2. Experimental Section 2.1. Carbon Treatment. Filtrasorb 400 (Calgon Carbon Corp.), a commercial activated carbon of bituminous origin, was used, denominated “F”, with particle size of 500-800 μm. The carbon was washed with deionized water, oven-dried at 170 °C for 24 h, and stored in a desiccator until its use. The activated carbon was treated with ozone in a fixed bed reactor loaded with 2 g of carbon under a constant ozone flow of 76 mg of O3/min, operating at 25 °C and 1 atm. Ozone was produced in an Ozocav ozone generator and fed to the reactor for different exposure times (10, 20, 30, 60, and 120 min). Inlet and outlet ozone was monitored by UV spectrophotometry (Spectronic Genesis 5) with a gas flow cell at 253.7 nm. After each treatment time, the carbon was withdrawn from the reactor and oven-dried, following a thermal program that allowed drying for 1 h at 60 °C, 1 h at 100 °C, 1 h at 150 °C, and 24 h at 170 °C. Dried samples were stored in a desiccator until use. Each treated sample was assayed for chemical and physical characterization. The O3-treated carbon samples were denominated “F” followed by the number of minutes of the treatment. 2.2. Chemical Characterization. 2.2.1. Surface Func- tional Group Determination. Selective Neutralization Analy- sis. Acid and basic functional groups of the activated carbon surface were determined by the method proposed by Boehm. 11 Solutions of NaHCO3 (0.02 N), Na2CO3 (0.02 N), NaOH (0.02 and 0.1 N), and HC1 (0.02 N) were prepared using deionized * To whom correspondence should be addressed: Fax: 56-41- 247491. E-mail: hvaldez@diq.udeccl. † Universidad de Concepcio ´n. ‡ Universidad de Granada. (1) Chen, C. C.; Lin, C. E. Anal. Chim. Acta 1996, 321, 215-218. (2) Juntgen, H.; Kuhl, H. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; Vol. 22, p 145. (3) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Marcel Dekker: New York, 1988. (4) Cooney, D. O.; Zhenpeng, X. AIChE J. 1994, 40 (2), 361-364. (5) Marsh, H. Introduction to Carbon Science; Butterworth: London, 1989. (6) McKay, G.; McAleavey, G. Chem. Eng. Res. Des. 1988, 66, 532- 536. (7) Zaror, C. A. J. Chem. Technol. Biotechnol. 1997, 70, 21-28. (8) Mondaca, M. A.; Carrasco, V.; Zaror, C. A. Bull. Environ. Contam. Toxicol. 2000, 64, 59-65. (9) Sutherland, I.; Sheng, E.; Braley, R. H.; Freakley, P. K. J. Mater. Sci. 1996, 31, 5651-5655. (10) Zaror, C. A.; Soto, G.; Valde ´ s, H.; Mansilla, H. Wat. Sci. Technol. 2001, 44, 125-130. (11) Boehm, H. P. Adv. Catal. 1966, 16, 179-274. 2111 Langmuir 2002, 18, 2111-2116 10.1021/la010920a CCC: $22.00 © 2002 American Chemical Society Published on Web 02/21/2002