Catalytic ozonation for the degradation of nitrobenzene in aqueous solution by ceramic honeycomb-supported manganese

Abstract

Catalytic ozonation of nitrobenzene in aqueous solution has been carried out in a semi-continuous laboratory reactor where ceramic honeycomb and Mn–ceramic honeycomb have been used as the catalysts. The presences of the two catalysts significantly improve the degradation efficiency of nitrobenzene, the utilization efficiency of ozone and the production of oxidative intermediate species compared to the results from non-catalytic ozonation, and the improvement of them is even more pronounced in the presence of Mn–ceramic honeycomb. Adsorptions of nitrobenzene on the two catalytic surfaces have no remarkable influence on the degradation efficiency. Addition of tert-butanol causes the obvious decrease of degradation efficiency, suggesting that degradation of nitrobenzene follows the mechanism of hydroxyl radical (OH) oxidation. Some of the main operating variables like amount of catalyst and reaction temperature exert a positive influence on the degradation efficiency of nitrobenzene. Initial pH also presents a positive effect in the ozonation alone system while the optimum working initial pH is found to be around 8.83 and 10.67 to the processes of ozonation/ceramic honeycomb and ozonation/Mn–ceramic honeycomb, respectively. The surface characteristics measurement of the two catalysts indicates that the loading of Mn increases the specific surface area, the pH at the point of zero charge (pH_PZC) and the density of surface hydroxyl groups, and results in the appearance of new crystalline phase of MnO₂. The results of mechanism research confirm that the loading of Mn promotes the initiation of OH.

Introduction

Nitrobenzene is a major environmental pollutant due to its carcinogenesis and mutagenesis [1], [2]. The commercial uses of nitrobenzene are reduction to aniline, solvent, synthetic products of benzene [3], metal polishes, shoe-black, perfume, dye intermediates [4], [5], plastics, explosives, pharmaceuticals [6], pesticides [7] and a combustible propellant [1]. The production of nitrobenzene in the USA was close to 0.75 billion kg for the year 1995 [1]. Therefore, the remediation of nitrobenzene in aqueous solution is of environmental concern because of its toxicity and the quantity of its production. However, nitrobenzene resists to oxidation by conventional chemical oxidation due to the strong electron-withdrawing property of the nitro-group. Mineralization of nitrobenzene by microorganisms is prevented owing to the effects of toxic and mutagenic on biological systems, which derive from nitrobenzene and its transformation metabolites, such as nitrosobenzene, hydroxylaminobenzene and aniline [2], [4], [8], [9], [10], [11], [12], [13], [14], [15], [16]. In order to afford the inexpensive and effective processes for the water treatment, various chemical reduction treatment and advanced oxidation processes (AOPs) have been studied for the degradation of nitrobenzene in aqueous solution, such as Fe⁰ reduction [1], [2], photocatalysis [17], [18], photoassisted Fenton oxidation [19], supercritical oxidation [20] and so on.

AOPs are characterized by the generation of hydroxyl radical (OH), species of high oxidizing power, that reacts on the matter present in water in an unselective way [21]. In recent years, heterogeneous catalytic ozonation, as a promising AOP, has received much attention in water treatment due to its high oxidation potential. Several researches have been reported on the degradation of nitrobenzene in aqueous solution by the heterogeneous catalytic ozonation processes. The experimental results indicate that removal efficiency of nitrobenzene is significantly promoted in the presence of heterogeneous catalysts compared with that of ozone alone, including nano-TiO₂ [22], Mn-loaded granular activated carbon (MnO_x/GAC) [23], [24], ceramic honeycomb [25], [26] and synthetic goethite [27]. Moreover, except for MnO_x/GAC catalytic ozonation, it is found that the degradation of nitrobenzene follows the OH oxidation mechanism in the other systems mentioned above.

In order to increase the catalytic activity of ceramic honeycomb [25], [26] and develop the convenient operation of manganese catalyst [23], [28], [29], [30] used in the previous study, ceramic honeycomb was modified by loading Mn in the experiments, and the degradation efficiency of organic micropollutant was investigated by ceramic honeycomb-supported Mn catalytic ozonation. Nitrobenzene slowly reacts with molecular ozone (0.09 ± 0.02 M⁻¹ s⁻¹), reacts quickly with OH (2.2 × 10⁸ M⁻¹ s⁻¹) and it does not adsorb on the ceramic honeycomb catalyst surface. Therefore, nitrobenzene, listed as a priority pollutant, is chosen as a model pollutant due to its toxicity of the central nervous system and its refractory nature to conventional chemical oxidation, and the special indication of OH. Furthermore, the objective of the present investigation was to compare the degradation efficiency of nitrobenzene in the different processes (ozonation alone, catalytic ozonation and adsorption of catalysts), and to elucidate several control options for the degradation efficiency of nitrobenzene (addition of OH scavenger, amount of catalyst, reaction temperature, initial pH) and to confirm preliminarily the reaction mechanism.