ABSTRACT

Development of new technology for water purification deserves increasing attention as a consequence of the problems derived from water scarcity. Among these technologies, photocatalysis is specially interesting, as it can employ solar radiation as a source of energy.

Two organic sensitisers, 2,4,6-triphenylpyrylium cation and acridine yellow G have been tested as photocatalysts in order to improve the performance of solar photocatalysis, as these compounds are able to absorb efficiently in the visible range of the spectrum.

Two phenolic compounds, ferulic acid and caffeic acid have been used as model pollutants. The percentages of degradation were in the range 60-80 % using both photocatalysts, improving the performance of titanium dioxide. These reactions have scaled up at pilot plant showing the applicability of these processes even at pre-industrial scale. Furthermore, toxicity and biodegradability studies demonstrate that total detoxification of the effluent and important enhancement on the biodegradability can be obtained.

The involvement in the photo-oxidative process of transitory species such as hydroxyl radical or singlet oxygen has been ruled. Photophysical measurements have demonstrated the deactivation of the singlet and triplet excited states of the studied photocatalysts and the rate constants have been calculated; in all cases this values are limited by the diffusion of the species in the solvent. On the other hand the electron transfer process from the excited states has been demonstrated to be thermodinamically favourable. All these calculations indicate that triplet excited states seem to be the key species in the photocatalytic process.

Finally, triphenylthiapyrylium and acridine yellow G have been used to remove persistent toxic compounds, namely the commercial pesticides Sevnol® (Carbaryl)  and Ultracid® (Methidathion). In this case triphenylthiapyrylium proved to be more efficient and important removal of the pesticide produced a significant improvement of the biocompatibility of the effluent.