Advanced and green ozonation process for removal of clofibric acid in water system: Preparation and mechanism analysis of efficient copper-substituted MCM-48

https://doi.org/10.1016/j.seppur.2018.10.031Get rights and content

Highlights

  • Cu-MCM-48 was prepared by a hydrothermal method.

  • Cu60-MCM-48 exhibited high catalytic activity for ozonation of clofibric acid.

  • Cu60-MCM-48 acted via the hydroxyl radical and superoxide radical mechanism.

  • The HOMO-LUMO energy gap got smaller with the doping of Cu.

  • Main intermediates were identified and the reaction pathway was proposed.

Abstract

This paper described the use of modified mesoporous molecular sieves to remove clofibric acid through catalytic ozonation. The mesoporous silicate MCM-48 and Cu-substituted MCM-48 (Cu-MCM-48) with different Cu contents were prepared by hydrothermal method in surfactant solutions. The catalyst was characterized by XRD, UV–vis, and XPS, indicating that copper was successfully doped in the framework of three-dimensional MCM-48 and Cu-O-Si linkage was formed in Cu-MCM-48 samples. The SEM and TEM graphs exhibited that the cubic arrays were highly homogeneous and well-ordered. However, high Cu contents destroyed the basic structure of pure MCM-48 and metallic nanometric clusters accumulated on the surface of samples. The degradation and mineralization of clofibric acid (CA) were investigated with the presence of Cu modified MCM-48 (Cu-MCM-48) during ozonation. When the molar ratio of Si/Cu was 60, TOC removal reached 41.1% at 60 min oxidation time. The catalytic mechanism of Cu60-MCM-48 was also investigated by using density functional theory (DFT). The radical quenching experiments revealed that hydroxyl radical (radical dotOH) and superoxide radical (radical dotO2) were the reactive oxygen species (ROS) for CA degradation. Cu60-MCM-48 showed favorable removal efficiency in the mild acid condition and remained high Lewis acid amount after reactions according to the results of Fourier transformer infrared (FTIR) of adsorbed pyridine. Furthermore, the possible reaction routes including C1single bondO and C4single bondCl bonds destroying and aromatic ring cleavage were proposed by HPLC-MS. 2-hydroxyisobutiric acid was the main intermediate in ozonation process with the presence of Cu60-MCM-48.

Introduction

In the last few decades, pharmaceuticals and personal care products (PPCPs) continued to release into the aquatic environment due to growing demand, especially in the excessive medical prevention and treatment of some diseases [1]. In China, 70 pharmaceuticals were detected in the five major river watersheds from 2012 to 2013 [2]. These active pharmaceutical ingredients were usually highly persistent and negative impacts on aquatic microorganisms and human health. As the primary metabolite of clofibrate, clofibric acid (CA) of various ng/L in drinking water treatment plant was reported for a long time due to its polar character [3]. A series of reports exhibited that clofibric acid had endocrine disrupting effects in the process of cholesterol synthesis [4]. Scientists took advantage of advanced oxidation processes (AOPs) by generating highly reactive oxidizing radicals with excellent performance to remove and even mineralize these PPCPs even if they were difficult to be degraded in low concentrations [5].

Ozonation was a powerful method to remove clofibric acid and decrease the toxicity, but it had weak mineralization efficiency due to the formation of many refractory intermediates which was difficult to react with ozone. However, this shortage could be overcome by the introduction of efficient catalysts to start advanced oxidation processes (AOP). In the heterogeneous catalytic ozonation system, mineralization and toxic degradation of CA would be improved a lot because of the generation of non-selected reactive oxygen species (ROS) including hydroxyl radicals, superoxide radicals and so on [6], [7]. These highly reactive oxidizing agents were able to react instantaneously and unselectively with various organic compounds [8]. On the other hand, solid catalysts in liquid phase after reaction were cycled and reused, which prevented causing secondary pollution. Therefore, it was expected to design an optional catalyst to achieve quick degradation and mineralization of CA.

The carrier and active component of catalysts should be considered when they were prepared. This family of materials generally called M41S have gained popular in the field of catalytic ozonation, including two-dimensional hexagonal MCM-41 and three-dimensional cubic MCM-48 [9], [10], [11]. In essence, these mesoporous molecular sieves were applied as the carrier of catalysts because they possessed huge surface area and narrow pore size distribution. Moreover, MCM-48 had a gyroid minimal surface and 3D well-organized networks of channel, which favored mass transport [12]. Besides carriers, the selection of active components was also important. The previous works showed that transition metals could be active sites, like Co, Fe, Mn, and Cu, which gained wide utilization in catalytic ozonation field [13], [14], [15], [16], [17]. It was found the CuO/Cu2O, as p-type oxides, also exhibited catalytic efficiency because the redox couple (Cu2+/Cu+) could prompt the electron transfer in the process of reaction and ozone decomposition [18], [19].

Generally, the approach to modifying mesoporous molecular sieves could be classified into two categories: The first one was that active sites were loaded on the surface of carriers, generating weak chemical bond on the surface of catalyst. The second one corresponded to products with strong bond in the framework due to substitution of silicon by metal or other active components. However, metal-leaching of metal loaded catalysts caused secondary pollution. To overcome this, incorporating metal into the framework of MCM-48 became an attractive technology in the process of synthesizing catalysts in this work. Meanwhile, the doping of cooper increased lattice oxygen and generated oxygen vacancies. In turn, the oxygen vacancies enhanced mobility of lattice oxygen and improve catalytic efficiency. The DFT method was employed to study the structure mechanism of catalysts. The highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy gap was a simple indicator of chemical reactivity and kinetic stability [20]. The energy separation value of orbitals reflected electron transfer and catalytic capacity of catalysts. Then, role of redox couple, reactive oxygen species, and nature of acidic active sites were also investigated to learn detailed reaction mechanism about catalysts. H2O2 detection and ozone utilization efficiency were discussed to explore catalytic mechanism in the gas-solid-liquid system. Moreover, the possible reaction routes of CA were proposed with presence of Cu substituted MCM-48. Besides oxalic acid, acetic acid and humic acid, 2-hydroxyisobutiric acid was found as the main intermediates with the presence of Cu-MCM-48 during ozonation of CA.

Section snippets

Catalyst preparation

Pure MCM-48 material was synthesized via a hydrothermal method according to previous research [11]. Cu-MCM-48 was prepared with addition of 5 mL cupric nitrate solution before crystallization. The calcined powder-form samples were designated as CuX-MCM-48, and the X represents different molar ratio of Si/Cu (X = 160, 100, 60, 20). For detecting the influence of active components, cupric nitrate was also incinerated at 550 °C for 6 h to get the cupric oxide.

Experiments and analytical methods

Experiments were conducted in a 1.3 L

Characterization of different samples

The small and big angle XRD patterns of MCM-48 and Cu-MCM-48 were shown in Fig. 1(a) and (b), respectively. The small angle XRD of pure MCM-48 and Cu-MCM-48 exhibited double peaks at 2θ smaller than 3°, and some weak peaks between 3.5 and 5.5° were typical MCM-48 phase when the content of copper was little. These peaks were indexed as the (2 1 1), (2 2 0), (4 0 0), (4 2 0), (3 3 2), and (4 3 1) reflections in the cubic Ia3d space of three-dimensional MCM-48 [22]. However, with the excess

Conclusions

In this study, metal modified mesoporous molecular sieves Cu-MCM-48 were prepared, characterized, and tested for their catalytic efficiency for degradation of clofibric acid in the ozonation. The optimal Si/Cu ratio was 60 among these catalysts and CA mineralization reached to 41.1% at 60 min, Cu60-MCM-48 kept good stability in the mild acid system. The reactive oxygen species including hydroxyl radicals and superoxide radicals were examined by quenching experiments, which improved the

Acknowledgements

Authors were greatly indebted to the Science & Technology Office of Guangzhou (201607010276), the PhD Start-up Fund of Natural Science Foundation of Guangdong Province (2017A030310420) and Major Science and Technology Program for Water Pollution Control and Treatment in China (No. 2017ZX07202-004)

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