Graduation Year

2014

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Chemical Engineering

Major Professor

John N. Kuhn

Keywords

aquaculture, geosmin, MIB, photocatalysis, titania

Abstract

Water is a vital natural resource. To develop more sustainable water systems, we must focus efforts on the removal of persistent contaminants. Aqueous organic contaminants include azo dyes, halogenated organics (e.g. pesticides), and algal and bacterial metabolites. The latter are common to surface waters and freshwater aquaculture systems and can cause taste and odor problems. Two of the principal organoleptic compounds are geosmin and 2-methylisoborneol (MIB). Traditional oxidation treatment methods, utilizing chlorine, hydrogen peroxide, and potassium permanganate, have been employed with varying levels of efficacy for removal of these and other organic contaminants. Advanced Oxidation Processes (AOPs) have greater potential for the removal of persistent contaminants than traditional methods due to their higher pollutant removal rates, their ability to degrade a variety of organic material, and their ability to completely mineralize compounds [1].

An emerging AOP technology is the use of titania based photocatalysts for water treatment. Titanium dioxide (TiO2) is an effective, inexpensive, and stable photocatalyst used for the decomposition of aqueous organics. Titania is primarily activated by the ultraviolet portion of the spectrum due to its energy band gap of 3.0-3.2 eV (depending upon crystalline structure). Photocatalytic efficiency can be enhanced or tuned through the use of semiconductor dopants and the variance of titania crystal structure (i.e. anatase to rutile ratios). Metal oxides, like indium vanadate (InVO4), may enhance reaction rates through new interfacial reaction sites and electron scavenging, transport, and storage. InVO4 has been shown to have four sub-bandgap transitions, of which three are in the visible range [2]. In this work, the synthesis of InVO4-TiO2 composite semiconductors is examined to shift photo-initiation into the visible portion of the spectrum. Parametric studies of the visible spectrum photodegradation of methyl orange, an azo dye, and 2-chlorophenol provide a basis for analysis. Methyl orange was utilized to ascertain the effect of pure and mixed phase titania in the semiconductor composites.

The TiO2 photodegradation of geosmin and MIB has been previously demonstrated in small-scale batch slurry reactions. Slurry systems require the downstream separation of catalyst from the liquid. Laboratory trials use centrifugation or micro-filtration. Alternatively, immobilization of the photocatalyst could allow scale-up of the process. Here, titania was immobilized on glass plate substrates using an ethanol spray technique.

Finally, naturally tainted waters may contain a number of constituents in addition to the target compounds. In recirculating aquaculture systems, the water contains natural organic matter (NOM), ammonia, nitrite/ nitrate, and carbonate species. These constituents may block light penetration, block reaction sites, scavenge hydroxyl radicals, or affect the surface chemistry of the catalyst. Further, geosmin and MIB concentrations are extremely low, in the ppt range. Naturally tainted waters from MOTE Marine Laboratory Aquaculture Research Park are treated in the laboratory and in situ to demonstrate TiO2 degradation efficiency for trace concentration geosmin and MIB degradation in a complex water matrix.

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