Synthesis of a high-performance Z-scheme 2D/2D WO3@CoFe-LDH nanocomposite for the synchronic degradation of the mixture azo dyes by sonocatalytic ozonation process
Graphical abstract
Synthesis of a high-performance Z-scheme 2D/2D WO3@CoFe-LDH nanocomposite for the synchronic degradation of the mixture azo dyes by sonocatalytic ozonation process
Introduction
Synthetic dye manufacturing has expanded which in turn led to the release of a huge amount of organic azo dyes to the water bodies [1]. Sought to the constant discharge of the effluents and co-exist of diverse kinds of the dye compounds such as Acid blue 9 (AB9) and Acid orange 7 (AO7) in practical wastewater, the concern of the researchers has been increased which arouses from the stability and non-biodegradable property of the azo dyes in water [1], [2]. On the other hand, it is widely known that the water bodies are exposed with a mixture of organic water pollutants; therefore, posing a superior one-step degradation process for the in situ and synchronic degradation of water contaminants has been as the challenging topic for the researchers [3], [4]. Owing to the inadequate capability of the routine treatment technologies, a substantial effort has been put into applying the advanced oxidation processes (AOPs) such as sonocatalytic [5], photocatalytic [6], and catalytic ozonation [7] processes as the more effective degradation processes combined with the novel nanotechnologies. Taking account of the literature, ultrasonic (US) irradiation is one of the main AOPs with an environmentally friendly nature that can produce free active hydroxyl radicals due to the acoustic cavitation phenomena [8]. Accordingly, the ultrasonic waves can go through the water medium and thermolysis of water during cavitation bubbles collapse and produce different kinds of reactive oxygen species (ROS) such as highly active •OH radicals [9]. However, the sole sonolysis demands high energy consumption and it lacks sufficient capability to the thorough degradation of organic compounds [10]. On the other hand, ozonation process has been widely regarded for the degradation of various newly emerged water pollutants due to its capability of producing fewer intermediates, and its robust ability for the complete mineralization of water pollutants by the direct and indirect successive attack of O3 and •OH with strong oxidation potential (2.08 V and 2.8 V, respectively) [11]. Nevertheless, higher treatment costs and lower ozone stability have been documented in the literature which restricted its application for water treatment [9], [11]. Hence, these constraints have been overcome by combining various AOPs which can promote the mineralization performance and the degradation efficiency (DE) of the emerging contaminants in a synergistic manner [12], [13]. It has been proved that in comparison with the sole ozone and sonolysis, combined ozonation-sonolysis processes have demonstrated a desirable DE for the microalga cells. Moreover, scientists have gone further and studied the impact of the nanocatalysts on the US/O3 process [14]. Indeed, the ultrasound irradiation of the semiconductors stimulates the generation of electron-hole pairs in their conduction and valence bands which in turn generates the ROS [9]. Besides, the boosting influence of solid catalysts on the DE of catalytic ozonation processes lies in the higher ozone decomposition due to the presence of more hydroxyl groups on its surface [11]. In this regard, two-dimensional (2D) nanomaterials such as transition metal oxides [15], graphene [16], and layered double hydroxides (LDHs) [17] have recently sparked worldwide attention due to their specific properties such as large surface area, high chemical stability, low cost, and fast charge transfer [18], [19]. LDHs can be expressed as [MII1-xMIIIx(OH)2]x+ [An−]x/m·mH2O, where MII and MIII denote the divalent and trivalent metal cations, respectively, An- implies the anion with n negative charge, and m is the number of intercalated water [20]. However, the mere usage of LDHs as the sonocatalyst lacks the higher performance arising from the fast recombination rate of electron-hole, and low charge carrier mobility [8]. So far, various metal-doped LHDs, as well as their composite with other semiconductors, have been synthesized to restrain the recombination of the photo-generated e- - h+ pairs [17]. Among several types of composite methods, Z-scheme heterojunction plays the principle role in the preparation of effective photocatalyst [12], [13]. The reason is fundamentally related to the recombination of the photo-excited electrons from minor negative conduction band (CB) of semiconductor: (I) and holes in the less positive valence band (VB) of semiconductor (II) which in turn lead to the preservation of the electron and holes with strong redox ability [21]. In this context, many works have been found to use tungsten oxide (WO3) and its composite with other semiconductors as the appropriate catalysts with the medium band gap [22], [23]. As stated in the published articles, WO3 has been acknowledged as the stable n-type semiconductor with the low valence band position which possesses the appropriate privileges suchlike non-toxic, ease for synthesis, and cost-effective [23], [24]. Owing to the aforementioned fantastic characteristics of WO3, it has been ascertained as a potent semiconductor for constructing a high performance Z-scheme photocatalyst [25]. In addition, 2D WO3 nanosheets have specific catalytic activity from their bulk counterparts, wherein, the key factor can be due to the simple transference of generated electron-hole to the surface of the catalyst [26]. Consequently, CoFe-LDH with a high conduction band position was theoretically determined to be the appropriate coupling semiconductor with 2D WO3 to prepare a highly performance Z-scheme photocatalyst [27], [28].
This paper proceeded with two main purposes as follows. Firstly, a novel Z-scheme 2D/2D WO3@CoFe-LDH nanocomposite was synthesized and the as-obtained materials were characterized by applying diverse analytical techniques. To the best of our knowledge, there is no report for the synchronic degradation of AB9 and AO7 through a sonocatalytic ozonation process. Therefore, the second aim of the current article was to study the synergistic effect of the degradation process for two organic dyes. Furthermore, some feasible experiments were fulfilled for determining the optimum values for the operational parameters such as sonocatalyst dosage, ozone gas inlet flow rate, the concentration of mixed two module pollutants, and the initial pH. Moreover, for precise investigation, the predominant ROS involved in the degradation process was studied by using various radical scavengers, and consequently, a proposed mechanism was reported. Finally, for a more thoroughgoing assessment of the process efficiency, the produced intermediates during the sonocatalytic ozonation process, and the toxicity of untreated and treated dye solution was evaluated under gained optimal conditions.
Section snippets
Chemicals
Acid Blue 9 and Acid Orange 7 were purchase from Azar Rang (Tabriz, Iran) and used without the need for purification. Table 1 demonstrates their characterizations. Ethanol (96%) was prepared from Jahan Alcohol Teb (Arak, Iran). Na2WO4. 2H2O, Co(NO3)2. 6 H2O, Fe(NO3)3. 9 H2O, HCl, HNO3, bovine serum albumin BSA, Na2CO3, NaOH, 1,4 benzoquinone, isopropanol, chromium trioxide(CrO3), H2SO4, NaCl, Na2SO4, and H2O2 were obtained from Merck (Germany). Moreover, distilled water and deionized water was
Characterization
Fig. 2a, illustrates the obtained XRD pattern for the as-prepared bare WO3, CoFe-LDH, and WO3@CoFe-LDH composite. All the sharp peaks shown in the XRD patterns of the prepared samples are completely matched to the diffraction peaks of the standard samples which are presented in the literature. Owing to the obtained results mere WO3 displayed the diffraction peaks at 23.2º, 24.1º, 24.8 º, 26.8 º, 29.1 º, 33.9 º, 42.8 º, 47.5 º, 49.5 º, 50.3 º and 55.9 º which are assigned to the (002), (020),
Conclusion
In the present study, a hydrothermal method was utilized to synthesize a novel z-scheme 2D/2D WO3@CoFe-LDH sonocatalyst for the simultaneous degradation of AB9 and AO7 through a sonocatalytic ozonation process. The accomplished XRD, FTIR, SEM, TEM, EDX, X-ray dot-mapping, PL, DRS, XPS, and BET analysis verified the successful fabrication of nanocomposite with the WO3 nanosheets along with the lamellar structure of CoFe-LDH. Afterward, a conspicuously better sonocatalytic ozonation performance
Conflict of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work is funded by the grant NRF-2019R1A5A8080290 of the National Research Foundation of Korea.
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