Abstract
MIL-53(Fe) is a metal organic framework that has been recently considered a heterogeneous photocatalyst candidate for the degradation of water pollutants under visible or solar radiation, though stability studies are rather scarce in the literature. In this work, MIL-53(Fe) was successfully synthesized by a solvothermal method and fully characterized by X-ray diffraction (XRD), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), N2 adsorption–desorption isotherm, Thermogravimetric analysis coupled with mass spectrometry (TGA-MS), UV-visible diffuse reflectance spectroscopy (DRS), elemental analysis and wavelength dispersive X-ray fluorescence (WDXRF). The effects of pH, temperature, solar radiation and the presence of oxidants (i.e., electron acceptors) such as ozone, persulfate and hydrogen peroxide on the stability of MIL-53(Fe) in water were investigated. The as-synthetized MIL-53(Fe) exhibited relatively good stability in water at pH 4 but suffered fast hydrolysis at alkaline conditions. At pH 4–5, temperature, radiation (solar and visible radiation) and oxidants exerted negative effect on the stability of the metal–organic framework (MOF) in water, resulting in non-negligible amounts of metal (iron) and linker (terephthalic acid, H2BDC) leached out from MIL-53(Fe). The photocatalytic activity of MIL-53(Fe) under simulated solar radiation was studied using phenol and metoprolol as target pollutants. MIL-53(Fe) on its own removed less than 10% of the pollutants after 3 h of irradiation, while in the presence of ozone, persulfate or hydrogen peroxide, complete elimination of pollutants was achieved within 2 h of exposure to radiation. However, the presence of oxidants and the formation of some reaction intermediates (e.g., short-chain carboxylic acids) accelerated MIL-53(Fe) decarboxylation. The findings of this work suggest that MIL-53(Fe) should not be recommended as a heterogeneous photocatalyst for water treatment before carrying out a careful evaluation of its stability under actual reaction conditions.
Highlights
Advanced oxidation processes (AOPs) have been extensively used for the degradation of organic pollutants in water
Efforts have been focused on developing new visible-light-responsive photocatalysts for the removal of contaminants from water with improved properties such as high adsorption capacity and photocatalytic activity under solar radiation or visible light, easy post-recovery and reusability
Because of the HO scavenging effect of tert-butanol a steadily accumulation of H2 BDC and practically no formation of short-chain carboxylic acids (SCCAs) were observed along the experiment. These findings suggest that photogenerated HO might attack iron nodes of MIL-53(Fe) leading to the release of both iron and linker to solution
Summary
Advanced oxidation processes (AOPs) have been extensively used for the degradation of organic pollutants in water In these processes, combinations of oxidants, catalysts and/or radiation of different wavelengths produce powerful secondary oxidizing species (e.g., free radicals such as hydroxyl radical, HO ), which can even mineralize organic contaminants. Efforts have been focused on developing new visible-light-responsive photocatalysts for the removal of contaminants from water with improved properties such as high adsorption capacity and photocatalytic activity under solar radiation or visible light, easy post-recovery and reusability. In this sense, some types of the so-called metal–organic frameworks (MOFs) are arising growing interest as potential heterogeneous photocatalysts [2].
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