Solar Photocatalytic Treatment of Oil Sands Process Water by Bismuth Tungstate Based Semiconductor Photocatalysts
Creators: Meng,Lingjun
Publisher: name: University of Alberta Library
Year: 2022

<p>Bitumen extraction in Alberta produces large quantities of oil sands process water (OSPW), which contains highly recalcitrant organics such as naphthenic acids (NAs). NAs are known to be the major contributors to the OSPW toxicity that need effective treatment before being released into the environment. Photocatalysis is a minimally invasive, sustainable, and economical approach to effectively treat wastewater by using renewable solar energy in a semi-passive treatment strategy. However, photocatalysts are limited by their bandgap and/or capability to separate the photoinduced charge carriers. Theoretically, photocatalytic activities are improved by the use of semiconductors with narrow bandgap and Z-scheme heterojunction systems. In this thesis, novel visible-light-driven bismuth tungstate (Bi2WO6) based semiconductor photocatalysts were prepared, and the application of the catalysts for the treatment of OSPW and the fundamentals in the treatment process were investigated. <br>
       Firstly, three different morphologies of Bi2WO6 photocatalysts were prepared by the hydrothermal method. The prepared catalysts were characterized to obtain their structural, textural, and chemical properties and tested for the degradation of model NAs and real OSPW under simulated solar irradiation. The flower-like structure exhibited the highest specific surface area and total pore volume. Accordingly, flower-like Bi2WO6 displayed the highest photocatalytic activity for the degradation of NAs, by achieving complete degradation of cyclohexanoic acid (CHA) at a fluence-based rate constant of 0.0929 cm2/J. The effect of metallic ions on the degradation rates of S-containing and N-containing NAs varied, whereas heteroatom appeared as a main reactive site. The by-products of heteroatomic NAs were identified and the degradation pathways were reported for the first time. The concentration changes of each byproduct were further estimated by mass balance. <br>
        Then, a novel photocatalyst Ag/NiO/Bi2WO6 with hierarchical flower-like Z-scheme heterojunction was synthesized. The photocatalyst exhibited excellent stability and activity over a wide light spectrum. The as-prepared composites were used in the remediation of OSPW, and a complete removal of aromatics, classical NAs, and heteroatomic NAs was observed after 6 h of the photocatalytic treatment. The acute toxicity of OSPW was completely eliminated after only 2 hours of treatment. In the photocatalytic system, h+, O2•−, and hydroxyl radical (•OH) were found to be the major oxidative species. The enhanced photocatalytic efficiency appeared to be the result of unique Z-scheme electron transfer among electron mediator Ag, NiO, and Bi2WO6 and the surface plasmon resonance effect near Ag, which was further supported by the Density Function Theory (DFT) calculations of the electronic properties of Ag/NiO/Bi2WO6 heterostructure. <br>
       OSPW inorganic fraction (IF) is a complex saline solution comprising inorganic ions and trace metals. These ions and metals are known to influence the remediation efficiency of NAs in OSPW. In this research, different photocatalytic performances were tested for six model NA compounds mixtures in buffer and OSPW-IF using Bi2WO6 and Bi2WO6/NiO/Ag. The distinct water matrices significantly affected the removal of model NA compounds. Further experimental analysis suggested that chloride and bicarbonate could commonly produce the inhibited effects for photocatalytic pollutant elimination. The addition of catalysts only accelerated the degradation rate of 1-adamantanecarboxylic acid (ACA) but did not change its degradation pathway. However, it reduced the chronic toxicity by generating lesser toxic byproducts as observed via ecological structure activity relationships. Additionally, the transformation products of 4,5-dihydronaphtho[1,2-b]thiophene-2-carboxylic acid (DTCA) were proposed for the photocatalytic system. <br>
       In recent years, oxidant-assisted photocatalysis has attracted extensive attention as it can overcome the shortcomings of traditional advanced oxidation and photocatalysis for the decontamination of organic pollutants. In this thesis, photocatalytic performances for hydrogen peroxide (H2O2), peroxymonosulfate (PMS), and potassium permanganate (KMnO4) were compared for the treatment of OSPW using z-scheme photocatalyst Bi2WO6/NiO/Ag. Briefly, H2O2 and PMS assisted photocatalysis significantly promoted the degradation of aromatics in OSPW as compared to the control, while KMnO4 assisted photocatalysis inhibited the degradation. Accordingly, the detoxification of OSPW was only observed for H2O2 and PMS assisted photocatalysis. The main reactive species were h+, O2•−, •OH and electron (e-) in H2O2 assisted photocatalysis; and sulphate radical (SO4•−), h+, O2•−, •OH in PMS assisted photocatalysis. Therefore, H2O2 and PMS displayed a dual role as an electron acceptor and radical precursor in the oxidant-assisted photocatalysis.</p>