Chemical absorption process for degradation of VOC gas using heterogeneous gas–liquid photocatalytic oxidation: Toluene degradation by photo-Fenton reaction Masahiro Tokumura, Rina Nakajima, Hussein Tawfeek Znad, Yoshinori Kawase * Research Center for Biochemical and Environmental Engineering, Department of Applied Chemistry, Toyo University, Kawagoe, Saitama 350-8585, Japan article info Article history: Received 28 March 2008 Received in revised form 5 June 2008 Accepted 5 June 2008 Available online 23 July 2008 Keywords: Photocatalytic degradation Chemical absorption Photo-Fenton reaction Toluene Effluent gas abstract A novel process for degradation of toluene in the gas-phase using heterogeneous gas–liquid photocata- lytic oxidation has been developed. The degradation of toluene gas by photo-Fenton reaction in the liquid-phase has experimentally examined. The photo-Fenton reaction in the liquid-phase could improve the overall toluene absorption rate by increasing the driving force for mass transfer and as a result enhance the removal of toluene in the exhaust gas. The toluene concentrations in the inlet gas were var- ied in the range from 0.0968 to 8.69 g m 3 with initial hydrogen peroxide concentration of 400 mg l 1 and Fe dose of 5.0 mg l 1 . It was found that toluene in the inlet gas was almost completely dissolved into water and degraded in the liquid-phase for the inlet toluene gas concentration of less than 0.42 g m 3 . The dynamic process of toluene gas degradation by the photo-Fenton reaction providing information for reaction kinetics and mass transfer rate was examined. Toluene removal kinetic analysis indicated that photo-Fenton degradation was significantly affected by H 2 O 2 concentration. The experimental results were satisfactorily described by the predictions simulated using the simplified tanks-in-series model combined with toluene removal kinetic analysis. The present results showed that the proposed chemical absorption process using the photo-Fenton heterogeneous gas–liquid photocatalytic oxidation is very effective for degradation of volatile organic gases. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Volatile organic compounds (VOCs) in effluent gas from indus- tries have been known as the most hazardous materials which af- fect public health and welfare owing to their toxicity potential, carcinogenicity and stability (Li et al., 2005; Bouzaza et al., 2006; Zuo et al., 2006). Recently emission control of VOCs has become a major concern in air pollution prevention. Treating effluent gas including VOCs to harmless level is an arduous process. The photo- catalytic degradation of VOCs presents a promising solution of this environmental problem and receives great interest in pollution control. Most studies have been performed with TiO 2 powder pho- tocatalyst in combination with UV light (Li et al., 2005; Zuo et al., 2006; Menesi et al., 2008). The photo-Fenton process, which is one of the advanced oxidation processes, is recognized as a power- ful photocatalytic degradation technology for VOCs in wastewater and has been extensively studied (e.g., Lee and Yoon, 2004; Chen et al., 2005; Hrvoje et al., 2006; Ipek et al., 2006; Tokumura et al., 2006). The photo-Fenton reaction may offer a promising technology even in the effluent gas treatment for removal of VOCs owing to its greater efficiency in degradation. The degradation of VOCs in effluent gas by the photo-Fenton reaction in the liquid-phase or heterogeneous gas–liquid photocat- alytic oxidation of VOCs involves mass transfer with simultaneous chemical reaction (Fig. 1). The first step accounts for the mass transfer of VOCs from the gas-phase to the liquid-phase. This is fol- lowed by the oxidative reaction due to highly reactive hydroxyl radicals generated in the liquid-phase by the Fenton reagents and UV light. The photo-Fenton process which consists of a combi- nation of the Fenton reagents (Fe 2+ and H 2 O 2 ) and light energy can be described by the following two reactions (Kavitha and Palaniv- elu, 2004; Nogueira et al., 2005). Fe 2þ þ H 2 O 2 ! Fe 3þ þ Å OH þ OH  ðFenton reactionÞ ð1Þ Fe 3þ þ H 2 O þ hm ! Fe 2þ þ Å OH þ H þ ð2Þ The first reaction (Reaction (1)) is a reaction of Fe 2+ with H 2 O 2 , which generates powerful reactive species OH radicals and oxidize Fe 2+ to Fe 3+ . In other words, the hydroxyl radical generation in the Fenton process is due to iron-catalyzed decomposition of H 2 O 2 . In addition to the Fenton reaction the formation of hydroxyl radicals also occurs by the photo-Fenton reaction. The second reaction of photo-Fenton process (Reaction (2)) is a reaction of Fe 3+ with water which occurs when the light of wavelength from 300 to 650 nm is irradiated (Chacon et al., 2006). In this reaction,OH radicals are generated and Fe 3+ is reduced to Fe 2+ . These two oxidation– 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.06.021 * Corresponding author. Tel.: +81 492391377; fax: +81 492131031. E-mail address: ykawase@toyonet.toyo.ac.jp (Y. Kawase). Chemosphere 73 (2008) 768–775 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere