Kinetics of Ozone Decomposition by Granular Activated Carbon Pedro M. AÄ lvarez,* ,² F. Javier Masa, ² Josefa Jaramillo, ² Fernando J. Beltra´ n, ² and Vicente Go´ mez-Serrano ‡ Departamento de Ingenierı ´a Quı ´mica and Departamento de Quı ´mica Orga´ nica e Inorga´ nica, UniVersidad de Extremadura, Badajoz 06071, Spain The kinetics of the removal of ozone from air by granular activated carbon (GAC) has been investigated. Twenty GACs with different textural and chemical surface properties were used in this study. The kinetic experiments were carried out in an expanded bed reactor (EBR) and in a packed bed reactor (PBR). A rate equation that takes into account thermal ozone decomposition and removal of ozone by GAC due to chemisorption and catalytic decomposition has been proposed. Deactivation kinetics has also been considered in the reaction model. Intrinsic rate constants for thermal ozone removal (k T ), chemisorption of ozone (k 1 ), and GAC-catalyzed decomposition of ozone (k 2 ), as well as deactivation rate constants (k d1 and k d2 ) have been evaluated from dynamic modeling. At the conditions used in this work, ozone was thermally stable only below 323 K. Above this temperature, ozone partially decomposed even in the absence of GAC. Temperature favored the rate of ozone removal by GAC following Arrhenius behavior while air humidity had a negative effect, likely attributed to the blockage of GAC active sites by water. A relationship between k 1 and surface area, volume of pores larger than 3.5 nm, concentration of surface oxygen complexes (SOC), and ash content has been found, while k 2 depends mainly on surface area, basic SOC concentration, and metal content. The activity of the all the GAC samples used decreased with the ozone exposure time. The formation of acidic SOC due to chemisorption of ozone onto GAC has been pointed to as a major reason for the process deactivation. Effective GAC regeneration has been achieved by thermal treatment at 1123 K. 1. Introduction The knowledge of the interaction of ozone with granular activated carbon (GAC) is a deal of practical interest from health and environmental points of views. First, ozone is one of the six criteria air pollutants identified by the U.S. EPA (national ambient air quality standard ) 80 μg/m 3 , 8 h average). In order to keep ozone concentration below the threshold limits, ventila- tion systems equipped with ozone purifiers are used in indoors environments where ozone is present (e.g., aircraft cabins, office environments, buildings exposed to high outdoor ozone con- centrations, etc.). GAC filters can be efficient ozone air cleaners as ozone is destroyed by chemical reaction with the carbon structure, deposited metal species, and adsorbed compounds (e.g., VOCs). 1-3 In the environmental research area, the oxidation of GACs with ozone to enhance their adsorption and catalytic behavior in specific pollution control applications has attracted the interest of various research in recent years. Thus, for example, a commercial grade GAC has been treated with ozone to increase its adsorption capacity for Cd(II), Hg(II), and Cr(III) from water. 4,5 Also, enhanced adsorption uptake of an ozonized GAC for ammonia and VOCs as methylethylketone and benzene has been observed. 6,7 The reactivation of spent GAC with ozone is another environmental application for which the knowledge of the ozone-GAC reaction is of great interest. 8,9 In a previous work, we showed that a GAC exhausted with phenol could be effectively regenerated by controlling its exposition to gaseous ozone. 8 However, after long exposure time the phenol uptake capacity of the GAC decreased as a result of the formation of acidic surface oxygen complexes (SOCs) from the interaction between ozone and the GAC itself. More recently, this reactivation method has also been applied to regenerate a GAC loaded with benzothiazole. 9 Most of the published studies about the ozone-activated carbon interaction focus on the changes in texture and chemical surface properties of the activated carbon and the impact of these modifications on the adsorption of selected compounds. 10-13 However, little is known about the kinetics of the ozone- activated carbon reaction. The few works on this matter have been carried out at low ozone concentration (typically below 200 ppm). 1,14-16 According to these reports, the reaction mechanism of ozone removal over activated carbon comprises two distinct routes: first, a rapid ozone transformation into SOC on the carbon surface followed by carbon gasification with release of CO and CO 2 and a slow catalytic transformation of ozone into molecular oxygen. The rate of ozone decomposition over activated carbon has been found to range from 10 -9 to * Corresponding author. Tel.: 0034-924-289300 ext. 9032. Fax: 0034-924-289385. E-mail: pmalvare@unex.es. ² Departamento de Ingenierı ´a Quı ´mica. ‡ Departamento de Quı ´mica Orga´nica e Inorga´nica. Figure 1. Schematic of the experimental setup used in kinetic experi- ments: (1) air cylinder; (2) gas flow controller; (3) ozone generator; (4) water bath; (5) ozone analyzer; (6) reaction system; (7) temperature controller. 2545 Ind. Eng. Chem. Res. 2008, 47, 2545-2553 10.1021/ie071360z CCC: $40.75 © 2008 American Chemical Society Published on Web 03/20/2008