Adsorption and Photochemical Behaviour of the Herbicide Monuron on Clay Surfaces

Abstract

The intensive use of pesticides on large areas of agricultural soil has given rise to concern about their fate in the environment. For many years, their presence as well as their environmental impact has attracted much attention due to the wide use of such compounds in agriculture and household applications (Beck et al., 1993; Serrano et al., 2005; Mansour, 1993). Less than 1% of total applied pesticides reach the target pests, the vast majority being dispersed in the different environmental compartments (water, air and soil) but mainly in aquatic environment via agricultural runoff or leaching (Pimentel, 1995). The application of herbicides on agricultural soils is a well-established and effective practice to control weed growth. They represent about 50% of the demand for agricultural chemicals; their prolonged use involves the risk of their retention in crops and soils. Soil is an ultimate reservoir for these products, whether applied directly or received indirectly from spray drift and residues of treated sites (Barbash & Resek, 1996). These products can be the subject of various transformations and they can react with different fractions of soil minerals or organic. Mineral soil colloids play an important role in adsorption of polar organic compounds, mainly due to the high surface areas associated to their small particle size and, in the case of Smectites, because they have internal (expandable) surfaces accessible to water and polar organic molecules such as pesticides (Lagaly, 1994; Cox et al., 1995; Lee & Kim, 2002). These clay minerals possess layered structures and the presence of interlayer space thus gives a sterically constrained reaction environment for pesticide molecules when intercalated (Caine et al., 1999). Iron is one of the most abundant transition metals in soil and is considered to play a large role in photoinduced redox reactions generating active oxygen species such as .OH (Sherman, 1989). The clay surface is also covered with humic substances that represent 60%−70% of soil organic matter, probably via formation of clay–metal– organic complexes. These complexes are characterized by the presence of stable radical species detected by ESR (Senesi & Schnitzer, 1977). Regarding the presence of these substances in soil, they may either contribute to the degradation of the pesticide through the formation of reactive species (Albanis et al., 2002) or be involved as inhibitors by a screen effect phenomenon (Hebert & Miller, 1990).