Degradation of nitrobenzene by high-gravity intensified heterogeneous catalytic ozonation with Mn-Fe/ZSM-5 catalysts

https://doi.org/10.1016/j.cep.2021.108642Get rights and content

Highlights

  • Mn-Fe/ZSM-5 as the packing of rotating packed bed (RPB) was proposed.

  • Nitrobenzene wastewater degradation by O3 with Mn-Fe/ZSM-5 catalyst in the RPB was studied.

  • Compared with the bubbling reactor, the TOC removal rate increased by 35.5% in the RPB.

  • •OH is the dominant position in the degradation of NB under this system.

Abstract

In this study, high-gravity intensified heterogeneous catalytic ozonation is proposed for degradation of nitrobenzene (NB) in wastewater using Mn-Fe/ZSM-5 as the packing of the high-gravity rotating packed bed (RPB). The effects of inlet O3 concentration, liquid flow rate, initial pH, and high-gravity factor on the removal rate of total organic carbon (TOC) at room temperature are investigated and compared with that in a bubbling reactor (BR), and the degradation mechanism is also discussed. The results show that under the optimal conditions of the high-gravity factor β = 20, initial O3 concentration CO3 = 25 mg/L, liquid flow rate QL = 60 L/h, and initial pH = 6, the removal rate of TOC reaches 72% after 40 min. The removal rate of TOC in RPB is 33.5% higher than that in BR. The radical quenching experiment shows that •OH plays a dominant role in high-gravity intensified heterogeneous catalytic ozonation with Mn-Fe/ZSM-5. It is concluded that high-gravity intensified heterogeneous catalytic ozonation with Mn-Fe/ZSM-5 can effectively improve the degradation efficiency of organic wastewater.

Introduction

Nitrobenzene (NB) is an important industrial chemical used in the manufacturing of petrochemical products, drugs, dyes and explosives [1,2]. As it is highly carcinogenic, teratogenic and mutagenic in nature [3] and exposure to NB irritates the skin, eyes and respiratory tract and even damages the liver, kidneys and central nervous system [4], it is listed as one of priority pollutants in many countries [5]. Concerns have also been expressed about the possible migration of NB from the soil and surface water to groundwater. Nevertheless, the high electron-withdrawing capacity of the nitro group makes degradation of NB a challenging task [6]. In recent year, heterogeneous catalytic ozonation has emerged as an effective advanced oxidation process for degradation of refractory organic pollutants, as the hydroxyl radicals generated from the catalytic decomposition of O3 are strong oxidizing agents and can react non-selectively with organic pollutants [7].

Non-noble metal oxides such as Mn and Fe oxides have been widely used as catalysts in heterogeneous catalytic ozonation for their high catalytic activity, low cost and easy availability. Mn and Fe oxides have different oxidation states, and mobile electrons are generated in the oxidation/reduction cycle of different valence states of Mn and Fe, which can catalyze the decomposition of O3 to produce more reactive oxygen species [8]. Sun et al. [9] prepared Fe-Mn@BT catalyst using bentonite as the substrate and found that at a Fe-Mn@BT dosage of 15 g/L and an O3 output of 2.0 g/h, the removal rate of phenol and total organic carbon (TOC) was 58.7% and 53.5%, respectively. ZSM-5 has high catalytic activity due to the presence of Lewis acid centers on the surface [10], and thus ZSM-5-supported transition metal oxides have found many applications in gas purification for their excellent catalytic performance [11], [12], [13]. The hydrophobic character of the molecular sieve also makes it possible to adsorb NB in water [14]. However, it should be noted that the powder catalyst has some shortcomings that need to be addressed such as catalyst loss and difficulty in recovery. The active components can be supported on a support that is easily separated and recycled in order to reduce catalyst loss and wastewater treatment cost. As the mass transfer of O3 from the gas to the liquid phase is the control step of the catalytic ozonation of NB [15], the low solubility of O3 in water and insufficient gas-liquid contact can reduce the utilization efficiency of O3 and the degradation of organic pollutants. Thus, there is a need to develop new methods to improve the degradation of organic pollutants by increasing the mass transfer of O3.

The process intensification in ultrasonic, microwave, electric or high-gravity field [16], [17], [18] provides an effective means of improving the heterogeneous catalytic ozonation, and it can act synergistically with the catalyst to improve the degradation of refractory organic pollutants. The high-gravity technique is a promising process intensification technique for absorption, rectification, preparation of nano-materials and wastewater treatment [19], in which a high-gravity field is created by the high-speed rotation of the packing of the rotating packed bed (RPB). Wei et al. [20] have demonstrated that the removal rate of total organic carbon (TOC) by catalytic ozonation with Fe-Mn-Cu/γ-Al2O3 catalysts in RPB was 21% higher than that in a bubbling reactor (BR). This is because in RPB, the liquid can be sheared into thin films, filaments, or small droplets by the high-speed rotating packing in order to reduce film thickness and increase the turbulence, and as a result the gas-liquid mass transfer efficiency can be increased by 1–3 orders of magnitude [21].

In this study, high-gravity intensified heterogeneous catalytic ozonation is used for degradation of NB in wastewater using Mn-Fe/ZSM-5 as the packing of RPB. The effects of inlet O3 concentration, liquid flow rate, initial pH, and high-gravity factor on the degradation of NB are investigated, and then tertiary butanol (TBA) is added to scavenger free radicals in order to elucidate the possible degradation mechanism.

Section snippets

Experimental materials

The Mn-Fe/ZSM-5 catalyst was prepared by impregnation method [14], which was spherical with a diameter of about 3 mm. NB (analytical grade) and TBA (alcohol pure) were provided by Tianjin Beichen Fangzheng Chemical Reagent Factory (China); O3 was produced by using an O3 generator; and other chemicals were of analytical grade. Deionized water was used throughout the experiments.

Analysis method

All experiments were carried out under normal temperature and pressure. NB was dissolved in water to simulate the real

Characterization of Mn-Fe/ZSM-5

As shown in Fig. 2(a), the characteristic diffraction peaks of ZSM-5 are clearly observed at 8.074°, 14.886°, 23.184°, 23.425° and 45.668° with high relative crystallinity in the XRD spectrum of Mn-Fe/ZSM-5 [22]; while the characteristic peaks of Mn and Fe are not detected, implying that their crystallinity is low and they are likely to be amorphous or crystals with diameters less than 4 nanometers [23]. Mn and Fe oxides are mostly present in amorphous or poorly crystalline forms, and they are

Conclusions

In this study, high-gravity intensified heterogeneous catalytic ozonation is proposed for the first time for degradation of NB in wastewater using Mn-Fe/ZSM-5 as the packing of the RPB. For wastewater with a NB concentration of 100 mg/L, the optimal operating conditions are as follows: CO3 = 25 mg/L, QL = 60 L/h, initial pH = 6, and high-gravity factor β = 20. After 40 min, the removal rate of NB and TOC reaches 99.99% and 72%, respectively. The removal rate of TOC in RPB is 33.5% higher than

CRediT authorship contribution statement

Zhixing Li: Investigation, Methodology, Formal analysis, Writing – original draft. Jiaxin Jing: Investigation, Software. Kechang Gao: Data curation, Software. Gaomiao Ren: Investigation, Formal analysis. Jingwen Zhang: Investigation, Data curation. Weizhou Jiao: Project administration, Supervision, Writing – review & editing. Youzhi Liu: Conceptualization, Supervision, Resources.

Declaration of Competing Interest

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.

Acknowledgments

This work was supported by the Fund for Shanxi "1331 Project" (nuc2021–006), Key Research & Development Plan of Shanxi Province (201903D321059), Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province (20200004), Transformation and Cultivation Projects of Scientific and Technological Achievements in Universities of Shanxi Province Institutions (2020CG040).

References (38)

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Intended for: Chemical Engineering and Processing: Process Intensification

Type of contribution: Research paper

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