In-situ remediation of groundwater pollution using photocatalysis
|This dissertation addresses the use of photocatalytic technology for the in situ remediation of groundwater. This work is the continuation of a project started in the Geotechnical Research Group at Cambridge University in 2000, focused on developing technology to address the problem of groundwater contamination by methyl tertiary-butyl ether (MTBE). MTBE is a widely used gasoline additive that has leaked into the ground in many locations and is currently causing significant problems with drinking water supplies in the United States. Unlike most gasoline components, MTBE is water soluble, resistant to biodegradation, and has low affinity for both soil and air; as a result it spreads much more rapidly than other pollutants and is harder to clean up. Prior research by May Chan (PhD, Cambridge 2005) demonstrated the feasibility of using a photocatalytic immobilizedcatalyst reactor to mineralize MTBE in water. However, field trials of a prototype reactor did not produce the results expected from laboratory testing.
A photocatalytic reaction occurs when a semiconducting material absorbs a photon, generating excited electrons and holes, which can (respectively) reduce or oxidize reactants on the semiconductor surface. An important application of this technology is the oxidative mineralization of aqueous pollutants. Unlike other methods of water purification (such as chlorination, air stripping, or absorption by activated carbon) photocatalytic mineralization does not require or produce any hazardous chemicals that must in turn be disposed of. Titanium dioxide (TiO2) is the photocatalyst of choice because it is inexpensive, biologically and chemically inert, highly photoactive, and very stable.
Several approaches were taken to understand the context and technical feasibility of in situ photocatalytic remediation of groundwater. The current status of MTBE contamination and the state of the art of photocatalysis were reviewed. Based on these findings, the immobilization of the TiO2 catalyst was identified as an important focus for experimental research. A series of kinetic experiments was performed to compare the performance of TiO2 immobilized on different catalyst substrates under groundwater conditions. From these tests, glass fibre was chosen as a promising substrate and used in an experiment modelling the performance of the first field reactor. Results showed that glass fibre had high initial photocatalytic activity which degraded over time. Methods of improving catalyst durability were also briefly investigated. The final component of the research was a theoretical study on the fundamental behaviour of photogenerated electrons and holes. This component of the research is only in the preliminary stages, but has potential to greatly improve photocatalytic efficiency by physically separating the oxidation reactions through the use of conductive catalyst substrates. The research presented in this dissertation will provide the basis for the design of future photocatalytic reactors for groundwater cleanup.