kaolinite and 4-nitroanisole

The kinetics of direct photochemical transformations of organic compounds in light absorbing and scattering media has been sparsely investigated. This is mostly due to the experimental difficulties to assess the major parameters: light intensity in porous media, the reaction quantum yield and the molar absorption coefficient of the adsorbed compound, epsilon(i) (lambda). Here, we propose a method for the determination of the molar absorption coefficient of compounds adsorbed to air-dry surfaces using the Kubelka-Munk model for the description of radiative transfer. To illustrate the method, the molar absorption coefficients of three compounds, i.e. 4-nitroanisole (PNA), the herbicide trifluralin and the flame retardant decabromodiphenyl ether (DecaBDE), were determined on air-dry kaolinite. The measured diffuse reflectance spectra were evaluated with the Kubelka-Munk model and with previously determined Kubelka-Munk absorption and scattering coefficients (k and s), for kaolinite. For all compounds the maximum absorption band was found to be red shifted and the corresponding epsilon(i) (lambda) values were significantly greater than those determined in solvents. Together with the absorption and scattering coefficient of the medium, the measured epsilon(i) (lambda) can be used to determine the quantum yield of the photochemical reaction in this medium from experimentally determined reaction kinetics.

Topics: Adsorption; Anisoles; Chemistry, Physical; Halogenated Diphenyl Ethers; Kaolin; Models, Chemical; Organic Chemicals; Phenyl Ethers; Polybrominated Biphenyls; Trifluralin

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.

Topics: Adsorption; Anisoles; Diffusion; Energy Transfer; Kaolin; Kinetics; Minerals; Organic Chemicals; Photolysis; Photons; Porosity; Quantum Theory; Soil; Time Factors; Water