Journal of Hazardous Materials 165 (2009) 168–173 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Design equation with mathematical kinetic modeling for photooxidative degradation of C.I. Acid Orange 7 in an annular continuous-flow photoreactor Mohammad A. Behnajady ∗ , Nasser Modirshahla, Mohammad Shokri, Behrouz Vahid Research Laboratory, Department of Applied Chemistry, Islamic Azad University, Tabriz Branch, Tabriz, Iran article info Article history: Received 9 June 2008 Received in revised form 9 September 2008 Accepted 23 September 2008 Available online 1 October 2008 Keywords: Advanced oxidation processes (AOPs) UV/H 2 O 2 Annular continuous-flow photoreactor C.I. Acid orange 7 Electrical energy per order (E EO ) abstract The decolorization of C.I. Acid Orange 7 (AO7), an anionic monoazo dye of acid class was investigated using UV/H 2 O 2 process in an annular continuous-flow photoreactor (ACFP) as a function of oxidant, dye concentrations, reactor length and volumetric flow rate. The removal efficiency of AO7 was a function of operational parameters and increased with increasing initial concentration of H 2 O 2 but it was low at high flow rate and initial concentration of AO7. Results indicated that the decolorization rate was pseudo-first order kinetic with respect to the dye concentration. A rate equation for decolorization of AO7 was obtained by kinetic modeling. Design equation for ACFP reactor was obtained with combination of kinetic model and rearranged tubular reactor design equation. Design equation was used for predicting concentration of AO7 and also electrical energy per order (E EO ) at different conditions. The calculated results obtained from design equation and kinetic model were in good agreement with experimental data. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Approximately 10,000 different dyestuffs are used in textile industry and among them, azo dyes such as C.I. Acid Orange 7 (AO7) is used over 50% of all dyes [1]. Textile industry wastewa- ter is heavily charged with unconsumed dyes because 1–15% of the dyes are lost during dyeing process [2]. Some azo dyes and degrada- tion products such as aromatic amines are highly carcinogenic [3]. If these effluents are release to the environment without treatment, they will threat ecosystem [4]. Conventional biological treatment methods are ineffective for decolorization and degradation because of the presence of large degree of aromatics in these dyes. Physical tech- niques, such as adsorption on activated carbon, ultrafiltration, reverse osmosis, coagulation and electrocoagulation are widely used for removal of these pollutants efficiently but they just transport contaminants from water to sludge and produce secondary wastes which need more treatment [5]. The alter- natives to non-destructive physical water treatment processes are advanced oxidation processes (AOPs), because they miner- alize organic water contaminants [6]. There are different AOPs reacting systems which are almost characterized by the effi- cient production of hydroxyl radicals. These radicals are powerful ∗ Corresponding author. Tel.: +98 411 3396025 fax: +98 411 3333458. E-mail address: behnajady@iaut.ac.ir (M.A. Behnajady). and highly reactive oxidant with short-lived time and non- selectively attack to pollutants. Among the AOPs, UV/H 2 O 2 process has been applied successfully to treat different pollutants in water [7–9]. In this process UV light is absorbed directly by H 2 O 2 , hydroxyl radicals are generated by photolysis of perox- ide bond and the highest hydroxyl radical yields are obtained when short-wave ultraviolet radiations (200–280 nm) are used [10]. Various types of kinetic models have been postulated to describe the kinetics of the UV/H 2 O 2 process (Table 1). The first type is the power law model which was tested for many organic compounds. For example Ku and Ho [11], Sundstrom et al. [12] and Elkanzi and Kheng [13] tested and obtained reaction order for various opera- tional parameters. The second type is kinetic model on the basis of reaction mechanism and known chemical and photochemical reactions. Most of kinetic models of second type utilize the pseudo- steady state approximation for the free radical species in the system [8,14,15]; these kinetic models were developed for constant con- centration of H 2 O 2 in the decolorization course. Most of the past efforts on the treatment of dye wastewater by the UV/H 2 O 2 process was mainly studied in a batch or recirculated photoreactor. There- fore, the study on the development of the kinetic modeling of the dye removal by UV/H 2 O 2 process in a continuous photoreactor is scarce. The objective of the present study is to develop a kinetic model and design equation for the decolorization of AO7 using UV/H 2 O 2 process in an annular continuous-flow photoreactor (ACFP). Design 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.09.095