Research Article Clay and Soil Photolysis of the Pesticides Mesotrione and Metsulfuron Methyl Marie Siampiringue, 1 Pascal Wong Wah Chung, 1,2 Moursalou Koriko, 3 Gado Tchangbedji, 3 and Mohamed Sarakha 1 1 Institut de Chimie de Clermont-Ferrand, Clermont Universit´ e and Universit´ e Blaise Pascal, Equipe Photochimie, BP 80026, 63171 Clermont Ferrand, France 2 Clermont Universit´ e, ENSCCF, BP 10448, 63171 Clermont Ferrand, France 3 Laboratoire GTVD, Facult´ e des Sciences, Universit´ e de Lom´ e, 05 BP 796 , Lom´ e, Togo Correspondence should be addressed to Mohamed Sarakha; mohamed.sarakha@univ-bpclermont.fr Received 31 July 2013; Revised 21 November 2013; Accepted 5 December 2013; Published 29 January 2014 Academic Editor: Teodoro Miano Copyright © 2014 Marie Siampiringue et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Photolysis may represent an important degradation process of pollutants at the surface of soil. In the present work, we report a detailed study on the degradation of two pesticides: mesotrione and metsulfuron methyl using a sunlight simulator. In a frst step, we studied the photochemical behaviour at the surface of clays from the kinetic as well as from the analytical point of view. In both cases, the quantum yields were found to be higher when compared to those obtained in aqueous solutions. Te efect of iron(III), water, and humic substances contents was studied. In the former cases, an increase of the degradation rate was observed while an inhibition was observed with the latter owing to a flter efect phenomenon. In a second step, we studied the photodegradation at the surface of natural soil and identifed the generated byproducts. Tey appear to mainly arise from photohydrolysis process. 1. Introduction Chemicals such as pesticides can be introduced into the envi- ronment as a result of their application for plant protection. Tus, the contamination of groundwater, rivers, soils, and also atmosphere is an inevitable efect of their application. Te negative ecological consequences related to these con- taminants are ofen assigned to their residence time and bioavailability. Owing to these environmental efects, there is an increase of the research activities toward the methods which could help in the study of the fate and also the elimi- nation of such substrates. In recent years, various methods for water or air purifcation as well as soil decontamination have been developed including chemical, electrochemical, or pho- tochemical processes [1–5]. In several cases, sunlight degra- dation may represent one of the main destructive pathways for pesticides afer their application into the environment. Since several kinds of these contaminants present absorption spectra with a nonnegligible overlap with that of solar light, an inexhaustible source, the photochemical process becomes of great interest. Under such conditions they can easily undergo photochemical transformation upon exposure to the solar light by direct absorption, namely, direct process [4, 6, 7]. Tis also leads to the formation of various by-products that can be more harmful than the parent compound. In the case where the contaminants do not absorb solar light, they may still undergo phototransformation through indirect reactions. In this case, other substances, added or naturally present in a specifed medium, play the role of photoinducers or/and photosensitizers [8–12]. Tus, reactive species, such as excited states and reactive oxygen species (ROS), permit the degradation of the target pollutants. Indirect photochemical processes may also occur at the surface of soils owing to the presence of organic matter originating from plant debris in various stages of decay [9, 10]. Such substance may contribute for the degradation of the pesticide through the formation of reactive species, such as Hindawi Publishing Corporation Applied and Environmental Soil Science Volume 2014, Article ID 369037, 8 pages http://dx.doi.org/10.1155/2014/369037