Elsevier

Chemosphere

Volume 227, July 2019, Pages 401-408
Chemosphere

Wet scrubber coupled with heterogeneous UV/Fenton for enhanced VOCs oxidation over Fe/ZSM-5 catalyst

https://doi.org/10.1016/j.chemosphere.2019.03.160Get rights and content

Highlights

  • Fe/ZSM-5 exhibits high toluene adsorption and efficient catalytic activation of H2O2.

  • Gaseous toluene can be efficiently removed and mineralized in this coupled process.

  • Many intermediates were produced in the water while no intermediates were discharged in the outlet gas.

  • The toluene degradation possibly includes physical and chemical process.

Abstract

The traditional treatment processes for volatile organic compounds (VOCs) removal generally suffered several disadvantages, such as secondary air-pollutants. To overcome these issues, wet scrubber coupled with heterogeneous UV/Fenton was developed for gaseous VOCs (i.e. toluene) removal. ZSM-5 supported iron oxide (Fe/ZSM-5) was prepared as a multifunctional catalyst for activation of H2O2 and enhancement of gas-liquid mass transfer. Toluene was removed efficiently by this coupled process with the removal efficiency of 85% during 120 min. Many intermediates were detected in the solution by GC-MS while no intermediates were observed in the outlet gas, suggesting that wet scrubber coupled with heterogeneous UV/Fenton could significantly reduce secondary air pollutants. The possible mechanism of toluene oxidation was proposed including the physical adsorption by Fe/ZSM-5 and radical dotOH oxidation. This study provides an environmentally benign and highly efficient chemical scrubbing process for gaseous VOCs removal.

Introduction

In recent years, volatile organic compounds (VOCs) have been heavily involved in the severe haze pollution in quickly developing countries such as China and India (Dai et al., 2017; Zhang et al., 2018). It also presents great threat that VOCs are the vital precursors of ozone and chemical smog, which influence the atmospheric oxidative capacity (Lelieveld et al., 2008). Furthermore, the potential toxicities of VOCs have considerable effects on human health such as asthmatic symptoms and cancers (Kamal et al., 2016). Therefore, it is extremely urgent to control VOCs. Traditional technologies, such as catalytic combustion, photocatalytic oxidation (PCO) and ozone-assisted catalytic oxidation (OZCO), are regarded as efficient VOCs treatment processes in gas phase (Pengyi et al., 2003;Huang et al., 2017;Ji et al., 2017; Fang et al., 2018). However, these destruction technologies strongly depend on the surface properties of catalysts and the humidity of the reaction system (Huang et al., 2017). Moreover, the accumulation of intermediates on the solid catalysts could occupy the active sites and result in catalytic deactivation (Einaga et al., 2002).

For contrast, wet scrubber coupled with advanced oxidation processes (AOPs) allow a better prospect for gaseous VOCs elimination with the specific requirements (Kavitha and Palanivelu, 2004; Liu et al., 2017a, 2017b; Mirzaei et al., 2017), mainly involving physical and chemical processes. Wet scrubber is the physical system that absorbs gaseous VOCs into solution. AOPs are the chemical processes that generates high active species (e.g., radical dotOH, SO4•- and radical dotCl) in solution (Guo et al., 2017; Liu et al., 2017a, 2017b). Therefore, this coupled process is divided into three steps as followed: gaseous VOCs are firstly transferred into solution via absorption process (Xie et al., 2018), and then, the dissolved VOCs are oxidized by the active species generated from Fenton, UV/PMS or UV/chlorine (Zepp et al., 1992; Poza−Nogueiras et al., 2018). Finally, the purified stream is separated from aqueous phase to gaseous phase (Handa et al., 2013). For example, UV/Fenton is regarded as one of the efficient AOPs, which could produce radical dotOH from H2O2 triggered by UV irradiation and/or Fe2+ (eqs. (1), (2)).H2O2 + hv → 2 radical dotOHFe2+ + H2O2 → Fe3+ + radical dotOH +radical dotOH-

The UV/Fenton process showed high performances for gaseous toluene and dichloromethane degradation with removal efficiencies of more than 90% and 65%, respectively (Feitz et al., 2002; Tokumura et al., 2012). However, it remains a big challenge for hydrophobic VOCs, which are hardly transferred into liquid phase due to their poor solubility in water. In that case, VOCs mass-transfer enhancement should be respected. Generally, the addition of adsorbent micro-particles, such as activate carbon or molecular sieves, could reduce liquid film resistance and thus enhance gas-liquid mass transfer of VOCs (Chauveau et al., 2013;Zhang et al., 2015; Lee et al., 2016). Among these, ZSM-5, one of the molecular sieves with well-developed pores and large surface area, is generally used as an efficient adsorbent for VOCs treatment or good support for many catalysts.

Herein, ZSM-5 supported iron oxide (Fe/ZSM-5) was developed as a multifunctional catalyst for degradation of gaseous VOCs in a heterogeneous the UV/Fenton process. Fe/ZSM-5 was prepared via an impregnation method and evaluated to degradation of gaseous toluene, which was a typical VOC with high toxicity and photochemical activity, and widely emitted from many processes such as printing and coating industries (Wang et al., 2013). Electron paramagnetic resonance (EPR) spectroscopy was used to monitor the generation and evolution of the reactive radicals. Subsequently, the intermediates produced from the toluene degradation were detected via GC-MS and the possible mechanism of heterogeneous UV/Fenton process was proposed.

Section snippets

Chemicals and materials

Hydrogen peroxide (H2O2, 30%, w/w) was obtained from Yongda Chemical Reagents (Tianjin, China). Ferrous sulfate was provided from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). 5,5-dimethyl-1-pyrroline (DMPO, >99.0%) was purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Toluene (chromatographic grade) was obtained from Tianjin Fuyu Fine Chemical. Sulfuric acid, which was used to adjust the pH value of Fenton reagent, was acquired from Guangzhou Chemical Reagent Factory. All

Characterization of catalysts

  • (1)

    SEM and TEM images

The morphology of the catalysts was observed by the SEM as shown in Fig. 2a and b. Both the ZSM-5 and Fe/ZSM-5 particles were uniformly rod-like with the particle sizes from 400 nm to 500 nm. No remarkable change was observed in the size, shape and surface morphology between ZSM-5 and Fe/ZSM-5. The XRD pattern of Fe/ZSM-5 (Fig. S1) further shows that the major peak position and intensity of Fe/ZSM-5 were not changed, which maintained the crystalline structure of ZSM-5 after Fe

Conclusions

Fe/ZSM-5 was prepared by the conventional impregnation method and tested in a batch heterogeneous UV/Fenton process. It presented multifunctional performances with high adsorption and catalytic ability. The typical VOC (gaseous toluene) was removed efficiently by wet scrubber coupled with heterogeneous UV/Fenton process. Fe/ZSM-5 had a synergistic effect on gaseous toluene removal, in which it significantly enhanced toluene gas-liquid mass-transfer, and simultaneously activated H2O2 to generate

Acknowledgement

This research was supported by Guangzhou Science and Technology Project (No. 201504301654288), National Natural Science Foundation of China (21677179), the Innovation Platform Construction of Guangdong and Hong Kong (No. 2017B050504001), the NSFC and Research Grants Council (RGC) of Hong Kong Joint Research Scheme (No. 51561165015 and No. N_HKU718/15) and the National Key Research and Development Program of China (No. 2016YFC0204800).

References (36)

Cited by (30)

  • Gaseous toluene abatement by the heterogeneous Fenton-like process using iron/carbon-coated monolith as catalyst: Proof of concept

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    A slight improvement in the toluene transfer was observed for the adsorption process with support (CM) or catalyst (CM-Fe), having the liquid phase been saturated in 40 min. The increase in toluene removal observed by adsorption was also reported by Xie et al. (2019) when they evaluated the elimination of the same compound using Fe/ZSM-5 as adsorbent. Both materials used herein (CM and CM-Fe) had similar toluene removal profiles (Cout(t)/Cin versus time), yielding the same amount of toluene transferred (2.7 × 10−3 mol – see Table 2), which was already expected since the CM and CM-Fe monoliths have similar SBET data (see Table 1); this result corresponds to an increased amount of transferred toluene by over one and a half–fold when compared to the absorption process.

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