Abstract
Abstract : South Africa is rich in naturally occurring resources including water, coal, oil, land. Individuals and industries use these resources as raw material inputs on a daily basis. However, they create significant amounts of pollution such as persistent organic pollutants (POPs). Such pollutants can be classified as inorganic, microbial and organic. Organic pollutants such as organic dyes are found in textile effluents that escape to the environment. They are designed to have a high degree of stability to fading upon sunlight exposures, chemicals and microbial attack leading to the ineffectiveness of current conventional wastewater treatment methods. Conventional wastewater treatment methods have been reported to be ineffective in the degradation of textile dyes because of their chemical stability. These methods have low effectiveness, with limited flexibility, they require specialized equipment and further handling of the generated waste. These methods are reported to have the ability to effectively remove colour, but for but lack the ability to completely degrade the dye molecules. Developed methods such as advanced oxidation processes (AOPs) use of photocatalytic semiconductors. These semiconductors have been researched and reported to have the characteristics to effectively treat wastewater by completely degrading a diversity of organic pollutants. A widely used semiconductor is monoclinic tungsten trioxide (WO3). It is viewed as an ideal candidate for photocatalytic applications. It is a photocatalyst that is responsive in the visible region, it absorbs light in the region up to 480 nm. WO3 has small band-gap energy which has been reported to range from 2.4−2.8 eV and high oxidation power of valence band (VB) holes and thus displays enhanced photoabsorption in visible-light irradiation. This gives WO3 the advantage to be used as an indoor pollutant treatment as well as outdoor applications. Hence, this project aims to utilize lanthanum-doped WO3 for the photodegradation of refractory vi organic dyes. Lanthanides as dopants are reported to improve the photocatalytic activity of the catalyst by increasing the adsorption capacity for pollutants, as well as, reducing the electron-hole recombination rates. In this study, pristine tungsten trioxide (WO3) nanoparticles were synthesized using the impregnation method with tungstic acid (H2WO4) and nitric acid as precursors, Lanthanum nitrate hydrate was used as a source of lanthanum (La) dopant. The as-synthesized nanoparticles were annealed at 450ºC for 3 hrs. The nanoparticles were characterized using X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM), coupled with energy dispersive X-ray (EDX), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, zeta potential, UV-visible spectroscopy (UV-vis), X-Ray photoelectron spectroscopy (XPS) and Ion chromatography (IC). Spectroscopic instruments such as XRD, Raman and FTIR confirmed the nanoparticles were composed of monoclinic polymorphs. The spherical morphology was confirmed by TEM and SEM, with EDX confirming the presence of tungsten, oxygen and lanthanum in the samples. The band gap energy obtained from the DRS measurements were found to be 2.45, 2.42 and 2.57 eV for m-WO3, 1-La-WO3, and 5-La-WO3 nanoparticles respectively. XPS was used to determine the valence band maximum (VBM) which was used to calculate the conduction band and estimate the band edge position. XPS band edge positions were in agreement with the UV-vis band edge positions. Zeta potential confirmed the point of zero charge for the nanoparticles to be at pH 3.8. Ion chromatography confirmed the evolution of the chlorides and sulphate ions from the degradation of Methylene blue and Congo red respectively.
M.Tech. (Chemistry)