Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 10/8/2007
Publication Date: 3/7/2008
Citation: Laird, D.A., Koskinen, W.C. 2008. Triazine Soil Interactions. In: LeBaron, H., editor. The Triazine Herbicides. St. Louis, MO: Elsevier Science. p. 275-299. Interpretive Summary: The triazines are an economically important class of herbicides that have been and continue to be widely used in agriculture. The trazines are also commonly found as an environmental contaminant in surface and ground water. Over the years there has been an extensive amount of research on interactions between triazines and soils. This book chapter summarizes what is known about mechanisms of interaction between triazines and soils. Triazines are adsorbed on surfaces of both soil organic matter and soil clay minerals. The extent of adsorption of triazines by soils can range from all to nothing, but adsorption is strongest when the surface properties of the clay or organic matter match the chemical properties of the specific triazine molecule. Adsorption on surfaces of soil particles helps break down triazine molecules through a process known as hydrolysis. This review will help scientists to better understand the fate of triazines in the environment and help policy makers and regulatory agencies to develop better rules for application and use of triazine herbicides.
Technical Abstract: The fate of triazine herbicides in soils is controlled by three basic processes: transformation, retention, and transport. Sorption of triazines on surfaces of soil particles is the primary means by which triazines are retained in soils. Soils are very complex mixtures of living organisms, various types of organic matter, and mineral particles. These soil constituents have many different types of surfaces. On a very general level, the various surface sites in soils may be classified as ionic, polar, and nonpolar. The ionic and polar sites interact with polar functional groups on triazine molecules. However, these sites also have a high affinity for water, and triazines must compete with water for these sites. Water is very competitive, and generally outcompetes the chlorotriazines for ionic and polar soil surface sites. The methoxy-, methylthio- and hydroxytriazines are somewhat more competitive against water for the ionic and polar soil surface sites than the chlorotriazines. The nonpolar sites on soil surfaces have a low affinity for water and therefore readily interact with nonpolar portions of triazine molecules (the alkyl side chains). Triazines interact with soil most strongly when the different functional groups on the triazine molecules are closely matched with active sites on the soil surface. Transformation of triazines is primarily the result of degradation caused by microorganisms. However, triazines are also subject to a slow chemical degradation process known as hydrolysis. Chemical hydrolysis is relatively fast in acidic and alkaline soils, but it is relatively slow in neutral soils. In neutral soils, the rate of chemical hydrolysis of triazines increases when the triazine is adsorbed on the surfaces of soil particles.