Enhanced Mineralization of Phenol and other Hydroxylated Compounds in a Photocatalytic Process assisted with Ferric Ions
Heterogeneous photocatalysis represents a promising low-cost solution to the extensively spread problem of contaminated wastestreams. Photocatalytic reactions take place at a slow rate nonetheless. Efforts have focused on the improvement of photocatalytic reactors design and on the enhancement of the photocatalyst activity through various doping techniques. There have been some important contributions on the latter approach. The doping procedures often involve lengthy procedures that call for expensive dopants and high calcination temperatures, and thus, hamper their use for large-scale application. Inexpensive techniques are needed to keep heterogeneous photocatalysis as a cost-effective alternative. In this dissertation it is demonstrated that the addition of iron (Fe), an environmentally friendly and highly abundant metal, can enhance the rates of photocatalytic mineralization of phenol and similar hydroxylated aromatics. Phenol and its aromatic intermediates are first considered in this research in the absence of Fe cations (unpromoted PC reaction) to elucidate the overall phenol kinetic reaction network. Based on experimental observations, the unpromoted PC reaction of phenol is represented with a series-parallel reaction network incorporating all intermediate species and their interactions. Intermediate compounds include aromatics (ortho-dihydroxybenzene, para-dihydroxybenzene and 1,4-benzoquinone) and carboxylic acids (fumaric acid, maleic acid, oxalic acid and formic acid). iii 7 Then, the extent of influence of Fe cations on the oxidation and mineralization rates of phenol and other hydroxylated aromatics is evaluated. It is shown that at reaction conditions 5 ppm of Fe cations is the optimum iron concentration to maximize the oxidation and mineralization rates of phenol and its aromatic intermediates. A mechanism for the improvement based on the adsorbed Fe cations onto TiO2 surface is proposed. Furthermore, it is demonstrated that an overall series-parallel kinetic reaction network also describes the photocatalytic oxidation in the presence of 5 ppm of Fe cations (Fe-assisted PC reaction). The proposed reaction network incorporates the same intermediate species as the unpromoted PC system with the model pollutant and intermediates concentration profiles varying significantly. Additionally, two kinetic models (KM#1 and KM#2) that can be applied to both unpromoted PC and Fe-assisted PC systems are developed. Both models are validated and the parameters are estimated using the experimental data. The models fit the experimental data very well. The improvement on the mineralization rate using Fe cations is demonstrated with overall quantum yield calculations. Finally, in this dissertation it is shown that a wide variety of organic molecules can be mineralized more rapidly using Fe cations as reaction enhancers.
Photocatalysis, Kinetic modeling, Phenol photodegradation, Ferric ions, Hydroxylated aromatics, Overall quantum efficiencies.
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