Nitrobenzene degradation in aqueous solution using ozone/cobalt supported activated carbon coupling process: A kinetic approach
Graphical abstract
Introduction
Recently, with the increasing variety of manufactured products, the water pollution has become a global issue of concerns [1], [2]. The use of biological process for water treatment involves several problems [3]. Furthermore, the presence of toxic refractory molecules with low biodegradability characters prevents the use of such treatments [3], [4]. It is, therefore, urgent to achieve new effective treatment processes to resolve these problems. Advances in wastewater treatment have led to the development of new methods known by advanced oxidation processes (AOPs) [5]. AOPs can be generally defined as aqueous phase oxidation methods based on the generation of highly reactive species such as hydroxyl radicals (OH°) [6]. These radicals react rapidly with molecules present in water in an unselective way [7]. The application of AOPs such as ultrasonic, photo-oxidation, photocatalytic oxidation plasma, Fenton, photo-Fenton, wet oxidation, and ozone/ultraviolet (UV) has concerned many investigations for the degradation and mineralization of a wide range of organic compounds [8]. Due to their complexity and high costs, these processes are rarely used as a possible solution for water treatment [8]. Ozonation process appears as an appropriate AOP for water treatment [9]. Ozone is well-known as a powerful oxidant; however it reacts slowly with some organic compounds such as inactivated aromatic compounds [10] and rarely leads to total mineralization [1]. Many researches are focused on developing catalysts in order to provide fast degradation and high mineralization level by ozone decomposition on high active species [11]. The combination of ozonation process with other agents such as UV, H2O2, or homogeneous catalyst (Mn2+, Fe3+, Fe2+, Ag+, Zn2+, and Co2+) can lead to an important degradation efficiency [11], [12], [13]. However, in this case, when dissolved metal salts in aqueous solutions or metal oxides were used as catalysts a secondary polluting problem can be exist [10]. Since, these catalysts have to be removed after the oxidation reaction of the organic compounds [10]. Therefore, these technologies are rarely selected as promising methods [8].
Actually, to increase the efficiency of simple ozonation process, heterogeneous catalytic ozonation processes, as a promising AOP, have been investigated [14]. Among frequently effective heterogeneous catalysts used in heterogeneous ozonation process mainly include: Zeolithe [15] Al2O3 [16], TiO2 [17] and AC [18]. The coupling of ozone and activated carbon was proven to be an effective method to degrade organic contaminants [19], [20]. The use of activated carbon can promote the decomposition of dissolved ozone into free hydroxyl radicals (OH) [21], [22], [23]. Thus, in the presence of activated carbon, ozonation process can lead to the oxidation of micropollutants either by through molecular ozone reaction or with generated OH [17]. The basic functional groups (e.g., chromene, pyrone and pyrrole) on the surface of the AC, metals in the structure and/or on its surface, or the electrons in the graphene planes were suggested as the possible active sites during ozonation [24], [25]. In this process, carbon can act as an adsorbent, a reactive support, and free-radical initiator [20]. Nevertheless, to furthermore improve the AC catalytic activity and stability as catalyst to be used on catalytic ozonation, AC was used as a catalyst support for metal as well as for their oxides [10]. AC exhibits excellent physical and chemical characteristics for supported catalyst preparation used for the oxidation of organic compounds [6], [10]. Activated carbon is stable under both acidic and alkaline environments and is a relatively cheap material [26]. It can be synthesized from low cost materials (agricultural industrial wastes) [26]. Transition metals catalysts such as Fe, Mn, Ru, Ce, Co, Ni are gaining prominence, nowadays, to be supported on AC [14]. Synthesized catalyst has been applied to increase the ozone decomposition and generated highly reactive free radicals [8]. Both the catalyst and the support play important roles in heterogeneous catalysis [27]. Supported catalyst is common used in the oxidation of a great number of organic compounds [1], [28] such as nitrobenzene (NB) [10], [26], [29]. This molecule is considered to be highly toxic; unfortunately, it is resistant to oxidation by biological treatment processes due to its carcinogenesis and mutagenesis nature [29]. It is also considered as a hard biodegradable compound and inhibitor for activated sludge [30]. This important class of industrial chemicals is widely used in the synthesis of many products, including dyes, polymers, pesticides, and explosives [31]. The release of these compounds into the environment is the result of their widespread application and improper disposal [10]. The oxidation of nitrobenzene was achieved using various supported catalyst [26].
In a previous study, O3/Cu/AC coupling was proven to be efficient methods to remove NB [10]. These preliminary experimental results proved that copper supported catalyst was very effective for enhancing the degradation of refractory organic pollutants. 82% removal of TOC was achieved in aqueous solution.
This study is focused on the treatment of bio-recalcitrant organic compound by coupling processes involving new supported cobalt catalyst. Nitrobenzene was selected as the target organic component to be treated by the coupled process. It is resistant to be treated with ozone molecule with low reaction rate constant and highly reactive with generated hydroxyl radicals [32]. Cobalt supported catalyst was prepared using wetness impregnation method over olive stones activated carbon (OSAC) prepared by physical process. The effect of cobalt catalyst on the coupling O3/AC process performance was investigated. The presence of radical scavengers on the process rate was also assessed to highlight the mechanism of ozone decomposition during catalytic ozonation reaction of NB.
Section snippets
Olive stones activated carbon preparation
In this study, activated carbon (OSAC) was prepared from crushed olive stones in the size range of 1.25–3.00 mm by physical processes according to the method developed by Souad Najjar et al. [33]. Initially, raw material was abundantly washed with hot distilled water to remove any impurities then dried at ambient temperature. Thereafter, olive stones were thermally carbonized under a continuous nitrogen flow. The carbonization temperature was fixed at 600 °C for two hours. The char obtained from
Experimental setup
Nitrobenzene degradation was performed in a 1-L gas–liquid stirred reactor of controlled temperature equipped with a mechanically stirrer to favor mass transfer between the three phases and to limit AC attrition. In each experiment, the reactor was filled with 1 L of a nitrobenzene aqueous solution (10 mg/L), and 1 g of OSAC or Co/OSAC was then added. The reactor was continuously fed with an ozonated oxygen stream. The temperature in the reactor was controlled and kept constant at 25 °C. pH was
Specific surface area and textural properties
Adsorption-desorption isotherms of nitrogen at 77 K on OSAC and Co/OSAC supported catalyst are shown in Fig. 2. As it can be observed from Fig. 2, the raw OSAC fits the typical type I/IV isotherm according to the IUPAC classification, which is characteristic of microporous materials with a contribution of mesoporosity [37]. Moreover, the presence of a broad knee at the relative pressure of P/P0 = 0.4, suggests the presence of micropores [6]. The isotherm indicates the presence of a type-H4
Conclusions
The feasibility of using supported catalyst on olive stones activated carbon in ozonation process was studied in this paper, in order to develop a new hybrid process for the treatment of refractory molecules by the combination ozone/supported catalyst process. The results showed that Co/OSAC catalyst prepared by impregnation method was more efficient for catalytic ozonation activity than OSAC alone in the degradation of NB. Co/OSAC catalyst could also enhance the TOC removal efficiency. Hence,
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