Abstract
Background: V2O5 is significantly soluble in water so the use of this oxide or mixed oxides systems based on V2O5, in photocatalytic processes in water, may release vanadium species in the aqueous medium, generating a certain amount of additional contamination. The present report is devoted to study the physical-chemical properties of V2O5-TiO2 composition and its use in photocatalytic processes in aqueous media. The focus is on the possibility of V2O5 dissolution and influence of this phenomenon on the photocatalysis. The photocatalytic activities of these mixed pigments have been investigated on the discoloration of a dye (Methyl Orange) in aqueous medium under two selected chemical (pH) and photochemical illumination (UV and UV-vis) conditions.
Materials and Methods: Vanadium pentaoxide, V2O5, was prepared by a conventional nonhydrothermal process, and mixed oxides TiO2(P25)/V2O5 were also prepared by using parental oxides of commercial TiO2(P25) and the synthesised V2O5. The thermal stability of the prepared mixed system has been studied by XRD, using an acquisition of X-ray diffractograms obtained during the process of continuous heating from room temperature to 800°C and subsequent cooling at room temperature. Portions of the mixed system TiO2(P25)/V2O5, were calcined, in static conditions, at three different temperatures, 300°C, 500°C and 700°C for 2h, studying the viability to release vanadium, from the use of these systems in aqueous suspensions. The catalysts were well characterized using various techniques such as, X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), Field-Emission -SEM, Transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS). Results: Experimental results indicated that the photo-discoloration of Methyl Orange on TiO2(P25), depends on the pH and the illumination conditions (UV or UV-vis). In addition, a part of the V2O5 component from the mixed oxides TiO2(P25)/V2O5 was totally dissolved having thus two toxic species in the medium, the dye and the released vanadium species, which are progressively removed by the photocatalytic action of the TiO2(P25) component. Conclusion: The fact that V2O5 based materials, from which V2O5 (depending of the method of preparation and of its thermal stability) may be released and dissolved, wholly or partly, in the aqueous reaction medium systems, generating species of vanadium which are toxic, can be directed to its recovery by controlled photocatalytic processes, as described in this paper by using TiO2. Titanium dioxide, TiO2(P25), can at pH=2.0, not only act as a good photocatalytic oxidant in the UV, but under the same conditions vanadium (V) species present in the aqueous medium can be recovered. The synergistic effect of other organic pollutants, such as dyes or isopropanol, acting as sacrificial agents, resulted to be in favor in speeding up the photocatalytic reduction of vanadium (V).Keywords: TiO2/V2O5, photocatalysis, vanadium oxide, titanium oxide, dyes, vanadium (V) recovery.
Graphical Abstract
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