Catalytic ozonation for the degradation of nitrobenzene in aqueous solution by ceramic honeycomb-supported manganese
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 (MnOx/GAC) [23], [24], ceramic honeycomb [25], [26] and synthetic goethite [27]. Moreover, except for MnOx/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.
Section snippets
Materials and reagents
Monoliths of ceramic honeycomb (Shanghai Pengyinaihuo Material Factory, China) were used as the catalyst and the framework of catalyst. These blocks have the following characteristics: the shape of a cylinder with a diameter of 50 mm and a length of 50 mm, wall thickness 0.4 mm, the cell density 400 square cells per square inch and the weight of a single block of ceramic honeycomb was 34.6–35.4 g. All the monolithic blocks were cleaned before the catalytic ozonation process and the modified
Degradation efficiency of nitrobenzene in the different processes
The experiments were performed in the different processes to investigate the degradation efficiency of nitrobenzene, including ozonation alone, ozonation/ceramic honeycomb, ozonation/Mn–ceramic honeycomb, adsorption of ceramic honeycomb and adsorption of Mn–ceramic honeycomb, under the condition of the applied specific ozone dose of 20 mg ozone/mg nitrobenzene. The results are represented in Fig. 1.
As shown in Fig. 1, the concentrations of nitrobenzene all decrease with the increasing reaction
Conclusions
It has been demonstrated that the degradation efficiency of nitrobenzene in aqueous solution by ozonation alone is accelerated in the presence of ceramic honeycomb and Mn–ceramic honeycomb as the catalysts, and the more pronounced degradation efficiency is achieved in the process of ozonation/Mn–ceramic honeycomb. The results suggest that the adsorption of the organic model substances on the surface of catalysts is not compulsory for effective catalytic ozonation, in fact it may scarcely be
Acknowledgement
The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (Grant No. 50578051).
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