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

Photolysis on soil surfaces is an important degradation pathway for many agro chemicals. Although the investigation of photochemical pesticide transformation 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 experiments, carried out on soil layers, the observed overall degradation rate is nor 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 describe a theoretical framework to understand combined effects of these different 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 on kaolinite layers of variable thickness and evaluated the results using a mathematical model. Thus, we were able to determine the actual photolysis rate constants which are independent of layer thickness and transport kinetics. The proposed theoretical and experimental concept contributes to the development of a standardized laboratory method.