Photocatalytic degradation of pharmaceuticals in aqueous solutions and development of new dye sensitised photocatalytic materials
Murphy, Sharon (2012) Photocatalytic degradation of pharmaceuticals in aqueous solutions and development of new dye sensitised photocatalytic materials. PhD thesis, Dublin City University.
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This research examines the photocatalytic degradation of 3 active pharmaceutical ingredients (APIs) Famotidine (FAM), Tamsulosin Hydrochloride (TAM) and Solifenacin Succinate (SOL) using TiO2 and UV light. Photocatalytic degradation studies were monitored with individual HPLC-PDA methods which were developed and validated. The optimum concentration of TiO2 to treat 0.32 L of 0.083 mM drug solution was found to be 0.1 g or 0.2 g, and all three pharmaceuticals could be appreciably degraded within the 3 hour irradiation period (100% FAM and TAM, and ~80 % of SOL). Various parameters were also investigated on the process such as pH, oxidant addition etc. pH was not found to significantly affect the photocatalytic degradation process, although there was notably more adsorption of the drug to the TiO2 surface at more alkaline pHs. This increase in adsorption of the drug at alkaline pH, in the case of SOL, did not lead to an enhanced photocatalytic degradation as is often reported by other authors.
Various intermediates (or degradation products) were observed in LC traces of photocatalytic studies and the addition of hydrogen peroxide to the TiO2/UV process was found to hinder the formation of these intermediates and eliminate them completely within the allotted irradiation time. Concentration studies examined the robustness of the proposed TiO2/UV process to varying concentrations of drug solution. These indicated that the process can completely eliminate concentrations up to 0.2 mM of FAM and TAM, and determined that in the case of SOL only low concentrations of drug can be completely eliminated (0.02 mM).
The intermediates of the optimised photocatalytic degradation experiments were analysed initially by DI-MS, allowing some initial intermediates being proposed. This data was later corroborated with LC-MS/MS data from photodegradation experiments and various intermediates were further proposed based on the masses and fragments obtained from this analysis. Including isomers, over 80 intermediates were discovered between the 3 APIs. Routes of degradation have also been postulated based on the intermediates proposed.
Further to this, composite materials based on TiO2 and dye molecules have been developed in the hope of developing solar/visible light activated photocatalysts. A visible light sensitised photocatalyst based on the porphyrin tetra-(4-carboxyphenyl)porphyrin was successfully developed. This composite was prepared by a simple adsorption method of preparation and was compared in activity to a harsher literature reflux method. Other composites based on novel metallated and metal free phthalocyanine dyes (MPc and Pc) were also prepared and characterised by UV spectroscopy (solution and solid-state) and FT-IR spectroscopy. The photoactivity of these composites was examined with our pharmaceutical targets. These are the first composite materials, to our knowledge, to be tested with actual pharmaceuticals. The porphyrin/TiO2 composite showed good photoactivity in the degradation of FAM, although LC traces indicated that this degradation could merely be a selective conversion to an oxidised/reduced form of FAM. Tests with the other pharmaceuticals further indicated this selectivity toward FAM, as no degradation occurred with either TAM or SOL. MPc/TiO2 and Pc/TiO2 composites were also tested with Famotidine, however these exhibited a poor/negligible degradation compared to the porphyrin/TiO2 composite.
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