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Mesoporous photocatalysts for the degradation of persistent organic pollutants

Final Activity Report Summary - MESOCAT (Mesoporous Photocatalysts for the Degradation of Persistent Organic Pollutants)

Semiconductor particles have been found to act as photocatalysts in a number of environmentally important reactions. Materials such as titanium dioxide have been found to be efficient in pollution abatement systems, reducing both organic (e.g. halogenocarbons, benzene derivatives, detergents, PCBs, etc.) and inorganic (e.g. nitrates, sulphates, cyanates, bromates etc.) pollutants / impurities to harmless species.

The removal of persistent organic pollutants, such as pesticides, solvents, detergents etc, from water is a pressing ecological problem. The control of one class of such contaminants, the pesticides and herbicides, has been the subject of substantial legislation by, inter alia, the European Union. Consequently, the remediation of ground water supplies from such contaminants is receiving a great deal of interest around the world. Thus, the technological focus of this project was to study the application of semiconductor photocatalysis as means to degrade / remove wastewater pollutants such as pesticides and herbicides. In particular, given the involvement of India as a project partner, and the fact that India produces 90 000 metric tonnes of pesticide materials per year, we were interested in the degradation by photocatalysis of the hitherto unstudied pesticides and herbicides: Dinoseb, Dinoterb, Fenamiphos and Glyphosate. To these ends, the scientific objectives of this project are twofold:
(i) to elucidate the degradation route of those pesticides; and
(ii) to understand factors contributing to photocatalyst efficiency.

In pursuit of these objectives, we have found that photocatalysis can be used to efficiently photodegrade dinoseb, dinoterb and glyphosate. In the case of the latter, we have fully elucidated the mechanism of this photocatalytic degradation under a range of solution conditions. Fenaminophos was found to be efficiently degraded to carbon dixide and water by conventional photochemical means and so use of photocatalysis for the destruction of this pesticide was not deemed necessary.

One of the key factors determining degradation route and process efficiency is whether the pollutant / pesticide to be remediated has to adsorb onto the catalyst surface for substrate destruction to occur. Further, if adsorption does occur then, upon illumination and immediately prior to the onset of pollutant destruction, does the extent of that adsorption change from that which occurs in the dark? Such a photoinduced change in extent of adsorption could impact on or enhance the rate of pollutant destruction. Whether a photoinduced adsorption change occurs upon catalyst illumination is one of the great unresolved questions of photocatalysis.

For the first time, we have used the quartz crystal microbalance to measure substrate adsorption at the catalyst surface in real time and most especially during a photodegradation experiment. Importantly, we have found that, in the photodegradation of the pesticide glyphosate, illumination of the photocatalyst allows for greater total adsorption of pollutant to occur at the catalyst surface than at the same surface in the dark, so enhancing the destructive effect of the catalyst. Our studies also show that upon illumination and again in comparison to the dark, higher concentrations of pollutant are needed in the reaction solution under study in order for any noticeable adsorption to take place. However, this may be a consequence of pollutant being photodegraded very shortly after adsorption at the surface, so giving the impression of a lower level of adsorption overall.