Nitroaromatic Hydrocarbon Ozonation in Water. 2. Combined


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Ind. Eng. Chem. Res. 1998, 37, 32-40

Nitroaromatic Hydrocarbon Ozonation in Water. 2. Combined Ozonation with Hydrogen Peroxide or UV Radiation Fernando J. Beltra´ n,*,† Jose´ M. Encinar,† and Miguel A. Alonso‡ Departamento de Ingenierı´a Quı´mica y Energe´ tica, Universidad de Extremadura, 06071 Badajoz, Spain, and Departamento de Ingenierı´a Quı´mica, Universidad de Castilla La Mancha, 13400 Almade´ n, Ciudad Real, Spain

The combined ozonation of nitrobenzene and 2,6-dinitrotoluene with hydrogen peroxide and UV radiation has been studied. Effects of variables such as pH, hydroxyl radical scavengers, ozone feed rate, or hydrogen peroxide to ozone feed molar ratios have been investigated. Results are also compared to those obtained from UV radiation and single ozonation (Beltra´n et al., 1998). Quantum yields of nitrobenzene and 2,6-dinitrotoluene at 254 nm were found to be 7.0 × 10-3 and 2.2 × 10-2 mol einstein-1, respectively. Removal rates of nitrobenzene through single or combined ozonation with hydrogen peroxide at concentration lower than 10-2 M are similar, while under the same conditions combined ozonation of 2,6-dinitrotoluene leads to higher removal rates than single ozonation. Also, the latter compound is removed faster by UV radiation alone and UV/O3 oxidation than by single ozonation. Rate constants of the reaction between the hydroxyl radical and nitrobenzene and 2,6-dinitrotoluene were found to be 2.9 × 109 and 7.5 × 108 M-1 s-1, respectively. Nitrophenols and 2,6-dinitrobenzaldehyde appear to be some of the principal intermediates formed in the treatment of nitrobenzene and 2,6-dinitrotoluene, respectively, regardless of oxidation techniques applied. Apart from economic considerations UV/O3 oxidation is the technique among those studied leading to the highest removal rates. Introduction Following the conclusions reached in the preceding paper (Beltra´n et al., 1998) the removal rate of nitroaromatics from water by ozonation alone (single ozonation) is a free radical process involving reactions between nitroaromatics and hydroxyl radicals generated during ozonation. Thus, due to the importance of these reactions, it seemed advisable to continue this study with other oxidation techniques like the combination of ozone and hydrogen peroxide or UV radiation (combined ozonation) able to generate a higher hydroxyl radical concentration (Glaze et al., 1987). A priori, these techniques should increase the removal rates of nitroaromatic hydrocarbons in water. Particularly, the oxidation with ozone and UV radiation presents the advantage of three different ways of degradation: direct ozonation (though in the case of nitroaromatic hydrocarbon oxidation is negligible), direct photolysis, and free radical oxidation. So far, the combined ozonation with hydrogen peroxide or UV radiation of nitroaromatic compounds in water has been the subject of a few works (Glaze et al., 1992; Guittoneau et al., 1990) where experimental results are interpreted in a qualitative way. The most extensive work on this matter is due to Guittoneau et al. (1990) who reported a comparative study on the oxidation of p-chloronitrobenzene with O3/UV and H2O2/ UV oxidation systems. The main objectives of this paper is to analyze the impact of different variables on the O3/H2O2 or O3/UV * To whom correspondence should be addressed. Telephone: 34-24-289387. Fax: 34-271304. E-mail: [email protected] unex.es. † Universidad de Extremadura. ‡ Universidad de Castilla La Mancha.

oxidation of nitrobenzene (NB) and 2,6-dinitrotoluene (DNT), to identify oxidation byproducts, and to determine quantum yields of nitroaromatics and the rate constants of their reaction with the hydroxyl radical. In a forthcoming paper, a mechanism and a kinetic model of the advanced oxidation will be presented. Experimental Part Most of O3/H2O2 oxidation experiments were carried out in the standard reactor described in the preceding paper (Beltra´n et al., 1998), while experiments involving UV radiation were developed in a 1000-cm3 glass annular photochemical reactor also described in detail in a previous work (Beltra´n et al., 1995). This reactor has a TNN 15/32 Hanau low pressure mercury vapor lamp. The actual intensity of incident radiation was found to be 3.8 × 10-6 einstein L-1 s-1 determined by actinometry with hydrogen peroxide (Nicole et al., 1990) following the procedure indicated in a previous work (Beltra´n et al., 1995). According to this and reactor geometry, the flux of incident radiation was 3.3 × 10-8 einstein cm-2 s-1. Both reactors operated in semibatch mode with a gas mixture of ozone-oxygen being continously fed in the nitroaromatic aqueous solution. More details for oxidation runs, materials used, analytical methods applied for ozone and nitroaromatic hydrocarbon concentration, and identification of oxidation products were as in preceding papers (Beltra´n et al., 1995, 1998). Hydrogen peroxide, on the other hand, was obtained from Merck and its concentration determined iodometrically (when >10-2 M) and with a fluorescence method (if