Photolytic transformation of organic pollutants on soil surfaces - An experimental approach

Photolysis on soil surfaces is an important degradation pathway for many ag ro chemicals. Although the investigation of photochemical pesticide transfo rmation on soil surfaces is required by registration authorities, knowledge of the relevant processes is limited. The quantification of photolysis on soil surfaces is of higher complexity than it is in solutions. In experimen ts, carried out on soil layers, the observed overall degradation rate is no r only determined by photolysis itself but is also a function of the layer thickness and in many cases of transport processes. In this article we desc ribe a theoretical framework to understand combined effects of these differ ent processes, and we present an experimental setup that allows a separate quantification of actual photolysis and diffusive transport processes. For the two compounds p-nitroanisole and trifluralin we performed experiments o n kaolinite layers of variable thickness and evaluated the results using a mathematical model. Thus, we were able to determine the actual photolysis r ate constants which are independent of layer thickness and transport kineti cs. The proposed theoretical and experimental concept contributes to the de velopment of a standardized laboratory method.