Important exchangeable cations

In soils of temperate zones, Ca, Mg, H , Al are the main exchangeable cations, to a lesser extent accompanied by ions of K, Na, Mn, Fe, etc., which occur in trace amounts. 

In countries with an arid climate, sodium also becomes one of the main exchangeable cations and it can even become prevalent. A sodium content exceeding by 5% that of all the other exchangeable cations indicates salination of the soil. 

The exchangeable cations in the soil colloidal complex considerably affect not only chemical processes and biological ratios, but also the physical condition and technological characteristics of the soil. 

Soils containing mainly Ca and Mg ions (calcareous soils) are usually most suitable for the cultivation in the Temperate Zone. They show 

Soils with a prevalence of Na ions (alkaline soils), which are formed in arid regions, have very unsuitable characteristics. Soil colloids of organic as well as inorganic character are easily peptized by action of the Na ion adsorption, and the soils lose their structure. On moistening, they swell and become impermeable. After drying, the colloids reduce their volume and the soils tend to hardening and cracking. These soils are very unsuitable for cultivated plants and can hardly be cultivated or improved [1, 5-7]. 

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The important feature is the formation of a coordinatively unsaturated site (cus), permitting the reaction to occur in the coordinative sphere of the metal cation. The cus is a metal cationic site that is able to present at least three vacancies permitting, in the DeNOx process, to insert ligands such as NO, CO, H20, and any olefin or CxHyOz species that is able to behave like ligands in its coordinative environment. A cus can be located on kinks, ledges or corners of crystals [16] in such a location, they are unsaturated. This situation is quite comparable to an exchanged cation in a zeolite, as studied by Iizuka and Lundsford [17] or to a transition metal complex in solution, as studied by Hendriksen et al. [18] for NO reduction in the presence of CO. 

The CEC of soil is important from two perspectives. First, it retards the leaching loss of important cations from soil. Each year, calcium and magnesium are leached out of the soil, causing soils in humid regions to become acidic. This increased acidity must be ameliorated by the application of limestone (CaC03). Second, exchangeable cations are available to plants, and this characteristic allows soil to serve as a store of important soil nutrients such as potassium, calcium, and magnesium. 

Most solutes in soils are to some extent adsorbed on the soil solid only a small fraction is in the solution in the pores. However some adsorbed solutes, particularly exchangeable cations, can have considerable mobility on soil surfaces (see below), so it is important to consider the solid phase pathway as well as the solution. Because the diffusing solute passes rapidly between the solid and solution, the two pathways partly act in series. In such a heterogeneous medium as soil it is not realistic to account for the mobilities and concentration gradients of solutes in all the constituent parts. But if the soil volumes and reaction times... 

Zeolites have many uses, most importantly as cation exchangers (e.g., in water softening), as desiccants (i.e., drying agents), and as solid acid catalysts. 

Soil cation exchangeable capacity is an index used both to evaluate the nutrient and water retention ability of the soil and as an important basis for the amelioration of soil and to apply, rationally, fertiliser. Exchangeable cations absorbed by soil colloid include K+, Na+, Ca2+, Mg2+, Al3+ and H+. K+, Na+, Ca2+ and Mg2+ are exchangeable bases. Al3+ and H+ are exchangeable acids and the sum of these ions is known as the cation exchangeable capacity. Exchangeable Cu2+, Zn2+ and Mn2+ are present at negligible concentrations. 

Spectroscopy. The important parameters, which determine the characteristic evolution of the fluorescence intensities with loading are (1) the particle size of the clays (hectorite vs. laponite) (2) the exchangeable cation (Na+, Ca2+, K+ vs. Cs+) and (3) for BS the pH. The latter is due to the extensive protonation of proflavine on the surface even in neutral suspension (Figure 3 and reference 5). 

The important role of the exchangeable cation in determining the structure of adsorbed water on smectites was discussed in the review of experimental studies of the structure of water adsorbed on smectites [32], It was concluded that the spatial arrangement of the adsorbed water molecules indeed derives mainly from the solvation of exchangeable cations. Despite the great... 

Molecular simulation of the adsorption of gases by the ALPOs was pioneered by Cracknell and Gubbins (1993), who pointed out that the aluminophosphates should be easier to model than the aluminosilicates. There are two important advantages first, the charge neutrality of the framework means that there are no exchangeable cations to be taken into account (this is, of course, also true for pure Silicalite) and second, the modelling is simpler because the pores are unidirectional with no interconnections. 

In montmorillonite, similar to other minerals, when the size of the exchanged cation is similar to the pore sizes in the crystal lattice, cations can build into the crystal lattice and, consequently, they reduce the negative layer charge (Chapter 1, Section 1.3.3.2). Other neutral molecules or cationic substances (Chapter 1, Sections 1.3.3.1 and 1.3.3.2) can also be sorbed in the interlayer space and on the external surfaces as well. They play an important role in defining the internal and total surface area and catalytic properties, and they may have an effect on the hydrophobicity of the mineral, as well as playing an important role in the production of pillared materials, etc. 

Similarly, the conditions needed for hydrogen-cation exchange can also be predicted (the ratio in Equation 2.54 has to be high). This can prove to be important in the treatment of acidic soils, where the quantity of the exchangeable cations can be increased and so the nutrient cycle can be improved. 

Catalytic oxidation-reduction (redox) reactions in zeolites are generally limited to reactions of molecules for which total oxidation products are desired. One important class of such reactions falls under the category of emission control catalysis. This encompasses a broad range of potential reactions and applications for zeolite catalysts. As potential catalysts one may consider the entire spectrum of zeolitic structural types combined with the broad range of base exchange cations which are known to carry out redox reactions. 

Exchangeable cations play an important role in the behavior of lignite coals in coal conversion processes. It is, therefore, important that a fundamental understanding be attained, first of the possible interaction of exchangeable cations in lignites with each other, and, second, of the possible effects on the subsequent catalytic activity of the cations for lignite char gasification. 

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