Rate equation for the degradation of nitrobenzene by ‘Fenton-like’ reagent

Abstract

This paper describes the effect of temperature and initial concentration of H₂O₂, Fe(II), PhNO₂ and dissolved oxygen on the degradation rate of PhNO₂ in homogeneous aqueous solution by ‘Fenton-like’ reagent ([H₂O₂]_o≫[Fe(II)]_o). The oxidation products o-, m- and p-nitrophenol were found as intermediates in the ratio 1:1.3–2.8:1.4–2.7 as compared with PhNO₂ when conversion of the latter was less than 25%. This fact suggests that hydroxylation of PhNO₂ was promoted by HO radicals. The reaction was investigated in a completely mixed-batch reactor under a wide range of experimental conditions: pH ∼3.0; 278–318 K; 1.5<[H₂O₂]_o<26.5 mM; 0.04<[Fe(II)]_o<1.1 mM; 0.3<[PhNO₂]_o<2.5 mM; and 0<[O₂]_o<1.4 mM. The activation energy for the degradation of PhNO₂ was determined to be 59.7 kJ mol⁻¹. The degradation rate of PhNO₂ follows pseudo-first-order kinetics. The results of this study demonstrate that the degradation rate of PhNO₂ in the ‘Fenton-like’ system could be predicted with sufficient precision by the equation R_D=1.05×10¹¹exp(−59.7/RT)[H₂O₂]_o^0.68[Fe(II)]_o^1.67[PhNO₂]_o^−0.32. The second-order rate constant for the overall rate of H₂O₂ decomposition by Fe(III) was found to be 0.83 M⁻¹ s⁻¹ at 298 K. The value of the steady-state HO radical concentration in the ‘Fenton-like’ reaction was found to be ∼10⁻¹³ M, as estimated by two independent methods.

Introduction

Nitroaromatic compounds are widely used as raw materials in many industrial processes, such as the preparation of pesticides, explosives, textiles and paper. Consequently, these compounds are found as water pollutants as a result of their release in industrial wastewater (Beltran et al., 1998, Sarasa et al., 1998). Nitroaromatic compounds are not only important as constituents in industrial waste streams, but also as hazardous wastes that have contaminated soils, groundwater, sludges and other hazardous solid wastes regulated under the Resource Conservation and Recovery Act (US EPA, 1980, Sittig, 1985). Remediation of wastewaters containing these pollutants is very difficult, since they are usually resistant to biological degradation (O'Connor and Young, 1989).

Oxidation processes that involve the generation of the highly reactive hydroxyl radical (HO) are of current interest for the destruction of organic pollutants in surface and groundwaters, and industrial wastewaters. Generation of HO radicals by the dark reaction of H₂O₂ with ferrous salt (known as Fenton's reagent) has been the subject of numerous studies during the last decade (Arnold et al., 1995, Pignatello and Day, 1996, Chen and Pignatello, 1997, Hislop and Bolton, 1999). The oxidizing species generated in the Fenton reaction have been discussed by many investigators, but are still controversial (Walling, 1975, Stubbe and Kozarich, 1987, Bossmann et al., 1998, MacFaul et al., 1998, Goldstein and Meyerstein, 1999, Kremer, 1999). The recognition of the HO radical as the active intermediate is not yet universal, and doubts as to its very existence in the system have even been raised (Bossmann et al., 1998, Kremer, 1999). In other studies of Fenton's reagent, it is generally considered that the reaction between H₂O₂ and Fe(II) in acidic aqueous medium (pH ≤3) produces HO radicals [Eq. (1)] and can involve the steps presented below [Eq. (1)–Eq. (6)] (Haber and Weiss, 1934, Barb et al., 1951a, Barb et al., 1951b, Walling, 1975). The rate constants are reported at 298 K in M⁻¹ s⁻¹ for a second-order reaction rate (Lin and Gurol, 1998, De Laat and Gallard, 1999).

$$\text{Fe}\text{II}\text{+}\text{H}{}_2\text{O}{}_2\text{→}\text{Fe}\text{III}\text{+}\text{HO}{}^{\mathrm{-}}\text{+}\text{HO}{}^{\mathrm{}}\text{k}{}_1\text{=63}$$

$$\text{Fe}\text{III}\text{+}\text{H}{}_2\text{O}{}_2\text{→}\text{Fe}\text{II}\text{+}\text{H}{}^{\mathrm{+}}\text{+}\text{HOO}{}^{\mathrm{}}\text{k}{}_2\text{=0.002−0.01}$$

$$\text{Fe}\text{II}\text{+}\text{HO}{}^{\mathrm{}}\text{→}\text{Fe}\text{III}\text{+}\text{HO}{}^{\mathrm{-}}\text{k}{}_3\text{=3×10}{}^8$$

$$\text{HO}{}^{\mathrm{}}\text{+}\text{H}{}_2\text{O}{}_2\text{→}\text{HOO}{}^{\mathrm{}}\text{+}\text{H}{}_2\text{O}\text{k}{}_4\text{=2.7×10}{}^7$$

$$\text{Fe}\text{II}\text{+}\text{HOO}{}^{\mathrm{}}\text{→}\text{Fe}\text{III}\text{+}\text{HOO}{}^{\mathrm{-}}\text{k}{}_5\text{=1.2×10}{}^6$$

$$\text{Fe}\text{III}\text{+}\text{HOO}{}^{\mathrm{}}\text{→}\text{Fe}\text{II}\text{+}\text{H}{}^{\mathrm{+}}\text{+}\text{O}{}_2\text{k}{}_6\text{<2×10}{}^3$$

The hypothesis of Haber and Weiss (1934) that the Fenton reaction involves the formation of HO radicals as the actual oxidants has been proved by many techniques, including EPR spectroscopy. Although a considerable number of investigators, using the electron paramagnetic resonance (EPR) spin-trapping technique, have found evidence for the formation of HO radicals from Fenton's reagent (Dixon and Norman, 1964, Buettner, 1987, Rosen et al., 2000), it has also been reported by others (Rush and Koppenol, 1987, Rahhal and Richter, 1988) that this species is not the only oxidizing intermediate, but that some type of high-valent iron-oxo intermediates also exist (Groves and Watanabe, 1986, Kean et al., 1987, Sychev and Isak, 1995, Bossmann et al., 1998, Kremer, 1999). Using EPR spin-trapping, three types of oxidizing species (free HO, bound HO and high-valence iron species, which is probably a ferryl ion, Fe^IVO) were detected by Yamazaki and Piette (1991). In the present work, the principal oxidant is assumed to be HO radical, but others, such as iron-oxo species, cannot be ruled out.

Nitrobenzene is one of the most representative nitroaromatic compounds present in several wastewaters and it is considerably soluble at room temperature. A number of studies on the degradation of PhNO₂ in aerated aqueous solutions by Fenton's reagent have been reported (Lipczynska-Kochany, 1991, Lipczynska-Kochany, 1992), although they are actually ‘Fenton-like’ processes because [H₂O₂]_o≫[Fe(II)]_o. However, in a system containing PhNO₂, H₂O₂ and Fe(II), the effects of temperature and initial concentrations of these components and dissolved oxygen on the degradation rate of PhNO₂ have not been investigated in detail. Nitrobenzene half-lives of 360 min at [PhNO₂]_o=0.1 mM, [H₂O₂]_o=3.9 mM, [FeCI₂]_o=0.035 mM (Lipczynska-Kochany, 1991) and of 250 min at [PhNO₂]_o=0.1 mM, [H₂O₂]_o=8.0 mM, [FeCI₂]_o=0.035 mM (Lipczynska-Kochany, 1992) have been determined, but these were studied only at room temperature and were reported without any experimental details. In spite of the numerous studies of Fenton's reagent, the chemistry and kinetics of PhNO₂ oxidation has not been well elucidated. Due to its importance as an environmental pollutant, this compound was selected for a study on its chemical degradation by ‘Fenton-like’ reagent. The study was conducted with the aim of determining some intermediate products and kinetic parameters for PhNO₂ removal in pure water. Knowledge of the kinetic information is necessary for better the planning of better methods of degradation of the pollutant, as well as for mechanistic studies.