The Solid Surfaces in Soils

In addition to the factors considered for water, we need to consider for soil (a) the far greater importance of interactions with solid surfaces and the buffering of ions in solution by ions adsorbed on the surfaces and (b) the more-strongly reducing conditions that develop in soil because of the greater sink for O2, resulting in transformations of soil surfaces as well as of species in solution. 

Redox processes are discussed in detail in Chapter 4. The rest of this chapter deals with solid-solution interactions, firstly for soils in general and then for submerged soils. Recent reviews of solid-solution interactions in soils include Sposito (1994), Sparks (2003) and the relevant chapters of Sumner (2000). 

The layer silicates comprise tetrahedral sheets of silica and octahedral sheets of aluminium and magnesium hydroxide, with varying amounts of the Si, Al and Mg replaced by cations of lower valence giving the lattice a net negative charge. Two basic combinations occur 1 tetrahedral sheet with 1 octahedral (e.g. kaoUnite, halloysite), and 2 tetrahedral with 1 octahedral (e.g. smectite, vermiculite, illite). 

Interchange of Solutes between Solid, Liquid and Gas Phases 

Group Layer structure Typical formula Structural negative charge (molckg Specific surface area )(m kg- ) 

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Any inference concerning the effects of a possibly altered molecular structure of water near the solid surfaces in soil clays must proceed from an acquaintance with the structure of liquid water in bulk and in aqueous electrolyte solutions. In this section, the current picture of the molecular arrangement in bulk water is reviewed. In Sec. 2.2, the same is done for aqueous solutions of inorganic electrolytes. These summaries are followed by discussions of the structure of water near the surfaces of phyllosilicates and the effect of these surfaces on the solvent properties of the water molecule. 

Sposito, G., The Reactive Solid Surfaces in Soils, The Surface Chemistry of Soils, Oxford University Press, New York, 1984. 

The solid phases that exhibit surface reactivity in soils are to be found primarily in the clay fraction. This well-known fact is a consequence of the geometric relationship between particle volume and surface area in a closely packed mass of solid particles, the total surface area increases as the degree of subdivision of the mass increases. For example, in a cubic meter of medium sand particles, each assumed spherical with a diameter of 500 ftm, the total surface area is about 1.2 x lO m. In the same volume of clay particles, however, each with a diameter of 2 fjun, the total surface area is about 3 x 10 m, or 250 times that of the sand particles. It is evident from this comparison that a study of the solid phases in soils in relation to surface chemistry need focus only on those solids that are common in clay fractions. 

The surface reactivity of the solid phases in soils derives from the chemical behavior of surface functional groups in soil clays. A surface functional group is a chemically reactive molecular unit hound into the structure of a... 

There are two principal reasons for this very important characteristic. First, the properties of the solid surfaces in a soil clay often can be altered during the preparation of the clay for a surface area measurement. For example, it may be necessap thoroughly and maintain it... 

The positive adsorption of metal cations by the solid phases in soil can involve the formation of either inner-sphere or outer-sphere surface complexes, or the simple accumulation of an ion swarm near the solid surface without complex formation. These adsorption mechanisms are implied in the development of the concept of surface charge balance (Eq. 3.3) and were illustrated, for the case of surface complex formation, in Figs. 1.8 and 1.10. The quantitative relationship between these mechanisms and measured surface excesses of metals on soil minerals is taken up in Chap. 5. In the present section, emphasis is placed on the qualitative... 

The mechanisms of surface chemical reactions represent a problem in coordination chemistry, which is the study of complexes, molecular units comprising a central group surrounded by other atoms in close association. This book is principally an introduction to the interpretation of surface phenomena in soils from the point of view of coordination chemistry. Therefore the basic concept to be discussed is the surface functional group, the central moiety in surface complexes, whose formation provides the most important mechanism of adsorption by the solid phases in soils. No detailed consideration of adsorption isotherm equations or the thermodynamic theory of ion exchange is presented, except insofar as their tenuous relation with surface coordination chemistry is to be illustrated. The discussion in this book is intended to be self-contained, but a previous exposure to soil physical chemistry, soil mineralogy, and the fundamentals of inorganic chemistry will prove helpful. 

The bulk properties of water and of solutions of electrolytes in water have been reviewed by VON Erichsen [1955]. Furthermore, the theory of ionic solution was the theme of a discussion sponsored and published by the Faraday Society [1957]. An account of the more recent literature is contained in a review on clay-water relationships by Graham [1964] and in an article by Luck [1964]. Therefore, the treatment in this chapter will be limited to the effects of the solid interface in soils and clays on the properties of the liquid phase in the vicinity of the solid surface. 

The cleaning process proceeds by one of three primary mechanisms solubilization, emulsification, and roll-up [229]. In solubilization the oily phase partitions into surfactant micelles that desorb from the solid surface and diffuse into the bulk. As mentioned above, there is a body of theoretical work on solubilization [146, 147] and numerous experimental studies by a variety of spectroscopic techniques [143-145,230]. Emulsification involves the formation and removal of an emulsion at the oil-water interface the removal step may involve hydrodynamic as well as surface chemical forces. Emulsion formation is covered in Chapter XIV. In roll-up the surfactant reduces the contact angle of the liquid soil or the surface free energy of a solid particle aiding its detachment and subsequent removal by hydrodynamic forces. Adam and Stevenson s beautiful photographs illustrate roll-up of lanoline on wood fibers [231]. In order to achieve roll-up, one requires the surface free energies for soil detachment illustrated in Fig. XIII-14 to obey... 

Ecologically, copper is a trace element essential to many plants and animals. However, high levels of copper in soil can be directly toxic to certain soil microorganisms and can disrupt important microbial processes in soil, such as nitrogen and phosphorus cycling. Copper is typically found in the environment as a solid metal in soils and soil sediment in surface water. There is no evidence that biotransformation processes have a significant bearing on the fate and transport of copper in water. 

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