Chapter 3.7 The reaction of anions and cations with metal oxides as models for their reaction with soil

Abstract

Publisher Summary One of the reasons why the reactions between ions and metal oxides are studied is because they are thought to be important in controlling the availability of nutrients in soils or in controlling the movement of pollutants into the biosphere. There are two strands to this argument. One is that many oxides are present in soils. Taylor and others list six iron oxides, five aluminium oxides and thirteen manganese oxides. (The term “oxide” is used loosely here to include oxyhydroxides). Of the iron oxides, goethite is most commonly found in soils of many climatic regions. The presence of oxides is apparent simply from observing one off the most obvious properties of soils—their color. Soil color is partly related to the amounts of organic matter in the soils but also to the amount and particle size of the oxides present. Iron oxides are especially important with goethite giving the yellow-browns and haematite the reds. The names, “red earth,” “terra rossa,” “terre rosse,” and “kraznozem,” which have been applied to soils in various languages, all show the effectiveness of redness in catching peoples' attention. The second strand of the argument is that oxides are very good at reacting with a very large range of cations and anions. Dzombak and Morel list over 50 studies of reactions of inorganic cations and anions with hydrous ferric oxides. This list makes it clear that such reactions could be very important in controlling the supply of both essential nutrients and of toxic ions to plants. This chapter brings some of these studies to attention. However, the chapter shows how studies using simple materials such as samples of individual oxides may be used to understand behavior of more complex materials such as soils.