Components, rocks, soil surfaces

In this chapter, the relationship of geological origins and interfacial properties of bentonite clay will be reviewed first. Then we will discuss the migration of water-soluble substances in rocks and soil, and the effect of sorption on the migration. A linear model will be derived by which the quantity of ion sorbed on rocks can be estimated when the mineral composition and sorption parameters of the mineral components are known. Surface acid-base properties of soils will be discussed, and the sorption of an anion (cyanide ion) will be shown on different soils and sediments. 

The reservoir representing the land (2) is defined as the amount of P contained in the upper 60 cm of the soil. This rather narrow definition of the land reservoir is made because it is through the upper portions of the soil system that the major interactions with the other P reservoirs occur. Specifically, most plants receive their nutritive P needs from the upper soil horizons and the return of P to the soil system by the decomposition of plant matter is also concentrated in this upper soil zone. Similarly, the major interactions with the atmosphere, ground-waters, and rivers occur near the soil surface. And, finally, phosphate in the form of fertilizer is applied directly to the soil surface. Thus, in attempting to represent the land and its interaction with other reservoirs, the surface soil horizon most directly interacts with all components and best represents the d)mamical nature of this reservoir. Phosphorus in soils deeper than 60 cm and in crusted rocks is included in the sediment reservoir (1). This reservoir accounts for all of the particulate P that exchanges with the other reservoirs only on very long time-scales. 

The component of the Earth s surface comprising the rock, soil, and sediments. It is a relatively passive component of the climate system, and its physical character- istics are treated as fixed elements in the determination of climate. 

Because of the mainly detrital character of soil micas, all mica species known as rock components may occur in soils. Quantitatively, however, the proportion of different mica species within particular soils varies under the influence of environmental conditions. On a large scale, this is shown by the fact that although in the earth s crust trioctahedral micas are more abundant than dioctahedral micas, the reverse is true in soils. This is usually ascribed to the greater susceptibility of trioctahedral micas to weathering. Trends of weathering in soil profiles show the change in predominance from trioctahedral to dioctahedral layer silicates as the soil surface is approached. 

The nuclei of iron are especially stable, giving it a comparatively high cosmic abundance (Chap. 1, p. 11), and it is thought to be the main constituent of the earth s core (which has a radius of approximately 3500 km, i.e. 2150 miles) as well as being the major component of siderite meteorites. About 0.5% of the lunar soil is now known to be metallic iron and, since on average this soil is 10 m deep, there must be 10 tonnes of iron on the moon s surface. In the earth s crustal rocks (6.2%, i.e. 62000ppm) it is the fourth most abundant element (after oxygen, silicon and aluminium) and the second most abundant metal. It is also widely distributed. 

In addition to straightforward precipitation reactions, components may dissolve and react with components already present, including atoms on colloidal surfaces. For example, phosphate may dissolve from phosphate rock and react with iron present in the soil solution or on particle surfaces to form an iron phosphate that is insoluble. 

If sediment was collected from a particular waterway, the distribution of the element of interest between different components of the sediment was found to vary with the degree of exposure to air and the temperature of any drying stages (Rapin et al., 1986 Kersten and Foerstner, 1986). The minor elements present in sediments (and soils) are not uniformly distributed. Part can be present as mineral fragments derived from the original parent rock, while other parts can be associated with distinct component phases such as carbonate compounds, hydrous oxides of Fe, Al, Mn and organic matter. Some fractions are loosely sorbed on particle surfaces or are held on ion exchange sites. 

These programs are able to model the geological systems soil/rock-aqueous solution systems that is the concentration and distribution of the thermodynamically stable species can be determined based on the total concentrations of the components and the parameters just mentioned. In addition, the programs can also be used to estimate thermodynamic equilibrium constants and/or surface parameters from the concentrations of the species determined through experiments. Thermodynamic equilibrium constants can be found in tables (Pourbaix 1966) or databases (e.g., Common Thermodynamic Database Project, CHESS, MINTEQ, Visual MINTEQ, NEA Thermodynamical Data Base Project (TDB), JESS, Thermo-Calc Databases). Some programs (e.g., NETPATH, PHREEQC) also consider the flowing parameters. 

On the surfaces of geological formations, different precipitation processes can be observed. The first one occurs when the concentration of some components reaches the value of the solubility product, the solution becomes oversaturated, and a new solid phase is precipitated (Section 1.2.3). The quantity of the precipitate depends only on the concentration of the solution. The precipitation takes place in a solution without the necessary presence of a solid surface. When, however, a solid phase, rocks, or soil is originally present, the precipitate is formed on it, and thus the total composition of the solid phase changes. When the precipitation forms colloidal particles, especially in diluted solutions, they can be adsorbed on the solid, if it is present. This process is governed by the so-called theory of colloid adsorption (Derjaguin and Landau 1941 Verwey and G. Overbeek 1948). 

Human exposure to any particular chemical through drinking-water requires a source of that chemical and a pathway from the source of contamination to the consumer. Pathways for transport of chemicals may be through natural features such as aquifers, surface water bodies, soils and rock, or overland flow, or through human-made components of water supply systems,... 

Specific adsorption on well defined materials has been the subject of many reviews [8-13]. Specific adsorption plays a key role in transport of nutrients and contaminants in the natural environment, and many studies with natural, complex, and ill defined materials have been carried out. Specific adsorption of ions by soils and other materials was reviewed by Barrow [14,15]. The components of complex mineral assemblies can differ in specific surface area and in affinity to certain solutes by many orders of magnitude. For example, in soils and rocks, (hydr)oxides of Fe(IH) and Mn(IV) are the main scavengers of metal cations and certain anions, even when their concentration expressed as mass fraction is very low. Traces of Ti02 present as impurities are responsible for the enhanced uptake of U by some natural kaolinites. In general, complex materials whose chemical composition seems very similar can substantially differ in their sorption properties due to different nature and concentration of impurities , which are dispersed in a relatively inert matrix, and which play a crucial role in the sorption process. In this respect the significance of parameters characterizing overall sorption properties of complex materials is limited. On the other hand the assessment of the contributions of particular components of a complex material to the overall sorption properties would be very tedious. 

Petroleum is a normal component of sedimentary rocks of marine origin and every sedimentary rock, no matter where it occurs, contains some deposits. Petroleum takes advantage of any permeable rock pointing toward the surface to rise as high as can. Hydrocarbon seepages, asphalts found on the surface of the soil, bituminous shales, bituminous limestones, and fossil paraffins are all evidence of this. They may indicate underground deposits of petroleum but often they are proof of evaporation and natural exhaustion over time. 

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