Microwave Assisted Photocatalytic Treatment of Naphthenic Acids in Water
Naphthenic acids (NAs) are natural constituents of bitumen and crude oil, and predominantly obtained as the by-product of petroleum refining with variable composition and ingredients. Naphthenic acids are composed of alkyl-substituted cycloaliphatic carboxylic acids, with smaller amounts of acyclic aliphatic acids. Naphthenic acids become a significant part of the tailings pond water (TPW) after separation from oil sands material. NAs are soluble in water and are concentrated in TPW as a result of caustic oil sands extraction processes. Tailings ponds near the Athabasca oil sands region near Fort McMurray, Alberta, Canada are contaminated with a variety of toxic organic compounds released in industrial effluent from the oil extraction processes. NAs are among the major water contaminants in those regions because of their toxicity and environmental recalcitrance. They may enter surface water systems due to erosion of riverbank oil sands deposits and through groundwater mixing. Significant environmental and regulatory attention has been focused on the naphthenic acids fraction of oil sands material to address these challenges and potential hazards. Biological, chemical, and photolytic treatments of water contaminated with NAs have been studied, but are either time consuming or involve significant capital investment. There is a growing need to develop more efficient and cost-effective treatment methods. Based on existing literature, microwave and photocatalysis for degradation of naphthenic acids in water may be one solution. A knowledge gap exists in determining the effect of microwave energy and/or photocatalysis on the rate and extent of NAs degradation in contaminated water. Part of this work included evaluation of the physical and chemical properties of NAs. Dielectric properties, important for designing a microwave system, were investigated. Effects of temperature, concentration, and frequency of microwaves on the dielectric properties of NA-water mixtures were studied and were used in designing the treatment systems for NAs. Three laboratory scale systems, (1) photocatalysis, (2) microwave, and (3) microwave assisted photocatalysis systems were designed and developed. Experiments were conducted to determine the NA degradation efficiency of these systems for both commercially available Fluka NAs and those extracted from oil sand process water (OSPW). Effects of water source (deionised and river water) and use of TiO2 catalyst in the degradation process, were also investigated. Degradation kinetics for total NAs as well as individual z-family were calculated. Results show that the three developed treatment systems were able to degrade NAs at a faster rate than the methods reported to date. The concentration of higher molecular weight NAs (z = -4 to -12) decreased more significantly than the lower molecular weight NAs in all the three treatment systems. Toxicity assessments of the NAs samples before and after treatment indicated that photocatalysis and microwave assisted photocatalysis systems decreased the toxicity of Fluka and OSPW NAs completely (up to 5 min IC50 v/v > 90%). The microwave system reduced the toxicity of water containing Fluka NAs from high (5 min IC50 v/v = 15.85%) to moderate (5 min IC50 v/v = 36.45%) toxicity. However, a slight increase in toxicity was noted post-treatment in OSPW NAs. Microwave-assisted photocatalysis was the most rapid degradation system for OSPW NA extracts in water with a half-life of 0.56 h in the presence of TiO2. The microwave system degraded OSPW NAs in water at a more moderate half-life of 3.32 h. The photocatalysis system was the slowest with a half-life of 3.99 h under similar conditions. High and ultra high resolution analysis of NA sample, estimations of cost and further efficiency related research of the developed systems to treat water with microbial load along with chemical contaminants are recommended for future work to further validate these treatment systems.
Photocatalysis, Naphthenic acids, Microwave-Assisted Photocatalysis, Microwave Treatment
Doctor of Philosophy (Ph.D.)
Agricultural and Bioresource Engineering
Agricultural and Bioresource Engineering