Volume 16 [2014]

The photocatalytic degradation of caffeine has been investigated in aqueous suspensions of titanium dioxide (TiO₂) by monitoring process efficiency at varying TiO₂ (25 - 175 mg l^-1) and initial drug concentrations (0.5 - 9.5 mg l^-1). The use of the response surface methodology (RSM) allowed to fit the optimal regions of the parameters leading to the degradation of the pollutant. Also, a single polynomial expression modelling the reaction was obtained. In parallel a set of bioassays (Daphnia magna, Pseudokirchneriella subcapitata and Lactuca sativa) was performed to evaluate the process in terms of detoxification of caffeine oxidation by-products under chronic and acute tests. Results clearly demonstrate that caffeine is quickly degraded, but not mineralized as quickly, and that persistent toxic organic intermediates resist further oxidation.

Tubular halloysite clay mineral and nanocrystalline TiO₂ were incorporated in the preparation of nanocomposite films on glass substrates via sol-gel method at 450ºC. The synthesis involves a simple chemical method employing nonionic surfactant molecule as pore directing agent along with the acetic acid-based sol-gel route without addition of water molecules. Drying and thermal treatment of composite films ensure elimination of organic material and lead to the formation of TiO₂ nanoparticles homogeneously distributed on the surface of the halloysite. Nanocomposite films without cracks of active anatase crystal phase and small crystallite size on halloysite nanotubes are characterized by microscopy techniques and porosimetry methods in order to examine their structural properties. The composite halloysite-TiO₂ films with variable quantities of halloysite were examined as photocatalysts to the discoloration of Basic Blue 41 azo dye in water. These nanocomposite films proved to be very promising photocatalysts and highly effective to dye’s discoloration in spite of small amount of halloysite/TiO₂ catalyst immobilized onto glass substrates. It also has been shown that the efficiency of the halloysite/TiO₂ films could be further improved when silver particles were deposited on their surface after successful adsorption from an aqueous solution of a silver salt and UV reduction of the adsorbed ions.

Recently, considerable efforts have been devoted to overcome a major limitation in semiconductor photocatalysis, the recombination of photogenerated e^--h⁺ pairs which leads to reduced quantum yields. Considering the impacts of various parameters on the photocatalytic degradation efficiency, our attention has been mainly focused on the improvement of the TiO₂ mediated photocatalysis for the degradation of a representative emerging micro-pollutant, DEET (N,N-dimethyl-m-toluamide), a widespread insect repellent. The efficiency of TiO₂ photocatalysis to degrade DEET was investigated in the presence of hydrogen peroxide and persulfate ions as oxidants-sacrificial electron acceptors under simulated solar irradiation (SSL). The degradation rates were found to be strongly influenced by the addition of oxidants. Higher degradation rates were observed in the presence of oxidants with the following order: S₂O₈^2- > H₂O_2. Τhe effect of solution pH in the range of 3–10 was investigated and the photodegradation rate was found to increase along with decreasing pH. Scavenging experiments indicated that that sulfate radicals were predominant species at acidic pH while ^•OH radicals were principally responsible for DEET degradation in alkaline media using SSL/TiO₂/S₂O₈^2-, the most efficient process.

Among eleven commercial organic polymers with different additives and processing techniques, only polypropylene, polystyrene, sheet moulding processed poly (methyl methacrylate) and rigid polyvinyl chloride presented transmittances higher than 80% at 360 nm , which allows irradiation there through. The selected polymers were coated without a calcination step with anatase-TiO₂ obtained by sol-gel. Their weathering was studied during 150 days of exposure to solar radiation, with and without the TiO₂ layer. It was observed that only poly (methyl methacrylate) retains the titania and the good optical and mechanical properties after the natural weathering, and thus is the best candidate for immobilization of TiO₂ for air photocatalytic treatment applications.