Synthesis of photocatalysts/heterojunctions for abatement of a specific pollutant in aqueous media

Ashiegbu, Darlington Chidera
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The current organic pollutant abatement practices are mostly unsustainable, inefficient, ineffective and in some instances inadequate. In some cases, these applications have been known to convert contaminants from one phase to another, causing extra costs in order to eliminate secondary pollutants. Photocatalysis is capable of in-situ production of OH, and O2- in addition to the production of electrons and holes for the remediation of organic pollutants into less harmful products. Metal oxide semiconductor photocatalysts like ZnO and WO3 are of importance in wastewater remediation under solar irradiation (UV/visible) due to their unique features including non-toxicity, easy synthesis, suitable band edge positions and photogeneration of active oxygen species in aqueous media. BiOI, which is a ternary metal oxide semiconductor, has also received excellent reviews in this regard, owing to its visible light absorption property and stability. Unfortunately, these photocatalysts are plagued by the same problem of quick charge carrier recombination. This work reports the semiconductor assisted photodegradation of 2-chlorobiphenyl which is a chemically and biologically stablepollutant in addition to its persistence in aqueous media. Previous studies of 2CBP only utilized TiO2 for the degradation of 2CBP in very low concentration. This thesis consists of five main parts. The first part is concerned with the synthesis of ZnO, WO3, BiOI and heterostructures of ZnO-WO3 and ZnO-BiOI with different weight loadings of WO3 and BiOI respectively. Direct, facile and fast synthesis techniques were used in the synthesis of all the semiconductor photocatalysts in this work. The second part deals with the characterization of all the as synthesized photocatalysts. Various characterization techniques such as surface morphology (SEM), elemental analysis (EDS), specific surface area measurements (BET), crystallinity (XRD) and optical studies (UV-vis spectrophotometer) were employed. The mean crystallite sizes were established from the diffraction peaks using the Debye-Scherrer relation, while the optical BG were estimated using Tauc plots from the UV-vis absorption spectra. Observations from the SEM images showed diverse morphologies for all the as prepared photocatalysts. EDS results revealed only the presence of the reference elements with good stoichiometric values, thus depicting purity of the samples and excellent synthesis. The XRD patterns showed crystalline patterns with no impurity peaks. The XRD data also showed that the introduction of WO3 and BiOI into the ZnO lattice were successful. Slight red shifts in the absorbance of ZnO as WO3 and BiOI were incorporated into ZnO were observed. The absorption tails of the heterostructures were well inside the visible light region. All the calculated band gaps decreased/narrowed with increased content of WO3 and BiOI into ZnO (5, 10 20 %), with the exception of ZnO-WO3 10 %. The calculated band gaps were 3.24, 2.0, 1.23, 3.08, 3.0, 2.20, 2.56, 3.16, 2.17 eV representing the band gap values for ZnO, WO3, BiOI, ZnO-BiOI 5 %, ZnO-BiOI 10 %, ZnO-BiOI 20 %, ZnO-WO3 5 %, ZnO-WO3 10 % and ZnO-WO3 20 % respectively. Nitrogen adsorption-desorption isotherms indicated that nearly all the composites were of the Type IV isotherm. Increments in the surface area measurements were observed in the heterostructures when compared with the bare and undoped composites. The third part of this thesis focused on the application of the as synthesized photocatalysts in the photocatalytic degradation of 2-chlorobiphenyl (2CBP). BiOI (49 %) was observed to exhibit the best photocatalytic activity when compared with ZnO and WO3 for the bare and undoped catalysts, while ZnO-BiOI 10 % (57 %) showed the best photo-activity for all the catalysts in the degradation of 2CBP. ZnO-WO3 10 % (53 %) also showed significant activity in this regard. The fourth part of the thesis deals with reaction kinetics of the pollutant destruction by the various photocatalysts The kinetics of the photocatalytic reactions were fitted to a pseudo-first order kinetics model. The ZnO-BiOI 10 % composite exhibited the highest rate constant of 0.0054 min-1 . Observations from the kinetic modelling also show that ZnO-WO3 10 % exhibited significant photocatalytic activity when compared to ZnO-WO3 20 % and ZnO-WO3 5 % (0.0049, 0.0016, 0.0019 min-1 , respectively). BiOI showed the highest photocatalytic performance among the undoped photocatalysts, with a calculated rate constant of 0.0029 min1 , which was over 2 fold higher than its bare counterparts (ZnO – 0.0012; WO3-0.0010 min-1). The R-square values obtained affirm that the photocatalytic degradation experiments all follow a pseudo-first order kinetic model. The fifth part of this study focused on the influence of different process parameters and conditions on the photodegradation of 2CBP, when using the most efficient catalyst (ZnO-BiOI 10 %.) Photodegradation of 2CBP decreased under very acidic conditions (pH 2: 20 %) and decreased more significantly in alkaline media (pH 11.3: 5 %), The most efficient activity was observed at the natural pH of the 2CBP solution (pH 7.2: 57 %). When the concentration of 2CBP was varied at 20, 35 and 50 ppm, photodegradation efficiency reduced. At lower concentrations of 5 and 10 ppm, there was an observed improvement in the photocatalytic activity. When the initial catalyst dose was doubled to 70 mg, the photocatalytic efficiency improved to 71 %. At much higher catalyst doses of 85 (68 %), 100 (30 %) and 150 (4 %) mg, the photocatalytic activity reduced. Thus, the optimum efficiency in this work was observed at 35 mg catalyst weight, 10 ppm pollutant concentration at a pH of 7.2. A proposed photodegradation mechanism of ZnO-BiOI 10 % was put forward for the degradation of 2CBP.
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Science in Engineering to the Faculty of Engineering and the Built Environment, School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, 2022