Photodegradation of organic compounds adsorbed in porous mineral layers: determination of quantum yields.

Photodegradation is a key process in governing the residence time and fate of many agrochemicals in top soils. However, the basic knowledge of the photolytic transformation reactions of organic chemicals on soil surfaces is still very poor, particularly regarding the quantum yield. In this work we developed a relatively simple model for the quantification of direct photodegradation processes on porous media on the basis of the Kubelka-Munk model for radiative transfer. With the help of this model, the quantum yield was determined using two different approaches: (i) the evaluation of the disappearance rate of the compound in the whole layer and (ii) the evaluation of the reflectance change of the doped porous medium during irradiation. The first approach proved to be simplest when applied to opticallythin layers where the interference of diffusion kinetics from the nonirradiated part of the layerto the surface is minimal. Here, we report experimental results on the photodegradation of 4-nitroanisole and trifluralin on kaolinite and the first results on goethite. The quantum yield for 4-nitroanisole on kaolinite was found to be on the same order of magnitude as in water, whereas for trifluralin the quantum yield was 10 times smaller than in water. Recommendations for a revision of the presently used OECD/EPA test system are proposed.