Ion in soils

Both pH and the availability of nutrient ions in soil play important roles in rhizo-sphere dynamics and are often dependent on one another. Nutrient ions move in soil toward plant roots either by mass flow with the soil water or by diffusion. Mass flow is the result of bulk convective movements of the soil solution toward roots, whereas diffusion occurs in response to a concentration gradient for a particular ion, which results from its absorption by the root and depletion from the... 

Most primary and secondary minerals found in soil systems are barely soluble in the soil solution. The amount of mass from the bulk phase to hydrated ions in soil solution is negligible compared to the total mass of the solid phase. In arid and semi-arid soils, concentrations of most trace metals in soil solution may be controlled by their carbonates and to some extent by their hydroxides. Other than carbonates, trace elements in arid and semi-arid soils may also occur as sulfate, phosphate or siliceous compounds, or as a minor component adsorbed on the surface of various solid phase components. The solubility of carbonates, sulfates and other common minerals of trace elements in arid and semi-arid soils will be discussed in Chapter 5. Badawy et al. (2002) reported that in near neutral and alkaline soils representative of alluvial, desertic and calcareous soils of Egypt, the measured Pb2+ activities were undersaturated with regard to the solubility of... 

Mitchell L.G., Grant C.A., Racz G.J. Effect of nitrogen application on concentration of cadmium and nutrient ions in soil solution and in durum wheat. Can J Soil Sci 2000 80 107-115. 

Figure 1.2. Common important inorganic anions and cations ions in soil.

The alkali and alkaline earth metals are examples of relatively simple cations that occur in only one oxidation state and are surrounded by water (see Figure 6.1). The most common of these ions in soil, in order of decreasing abundance, are calcium (Ca2+), magnesium (Mg2+), potassium (K+), and sodium (Na+). Sodium is typically present in very small amounts in high-rainfall areas, whereas it may have a relatively high concentration in low-rainfall areas. 

Figure 6.4. Reactions of aluminum ions in soil solution with the release of protons.

It is possible to extract or remove ionic species, both anions and cations, from soil using ion exchange resins. Both anion and cation exchange resins have been used as well as combinations of the two. Resins can be added to the soil and mixed, or they can be contained in a bag (Procedure 11.11), on a strip, or in capsules buried in soil. Mixing resins with soil allows for more intimate contact with soil and with the soil solution. However, one is faced with separation of the resin from soil at the end of some extraction time. Resins in bags, on strips, or as capsules can easily be removed from soil. However, the resins do not have as intimate contact with soil in this procedure. Good relationships between all these methods and standard extraction methods have been obtained and all approaches have found utility in determining the amounts of various ions in soil. 

It is common to concentrate organic components extracted from soil before analysis is conducted. Concentration of ionic species is not as common. However, the use of ion exchange resins to remove ionic species from soil is a well-established ion removal method. Although this method is not commonly discussed in terms of concentration of ions found in soil, it can lead to increased ion concentration and increased ability of analytical methods to measure trace amounts of ions in soil [26],... 

The modern history of ion exchange began in about 1850 when two English chemists, Thompson(4) and Way(5), studied the exchange between ammonium ions in fertilisers and calcium ions in soil. The materials responsible for the exchange were shown later to be naturally occurring alumino-silicates 6. History records very much earlier observations of the phenomenon and, for example, Aristotle(7), in 330 BC, noted that sea-water loses some of its salt when allowed to percolate through some sands. Those who claim priority for Moses(8) should note however that the process described may have been adsorption ... 

Table 4.2. Results obtained in the determination of four ions in soils by using the sensor of Cardwell et al and various standard methods...

Fluoride is a natural component of most types of soil, in which it is mainly bound in complexes and not readily leached. The major source of free fluoride ion in soil is the weathering and dissolution of fluoride rich rock that depends on the natural solubility of the fluoride compound in question, pH, and the presence of other minerals and compounds and of water. The major parameters that control fluoride fixation in soil through adsorption, anion exchange, precipitation, formation of mixed solids and complexes are aluminium, calcium, iron, pH, organic matter and clay [19,20]. 

Barium can also form salts with acetate, nitrate, chloride, and hydroxide ions in soil. The mobility of barium in soils increases upon formation of these water soluble salts (Bodek et al. 1988). In general, the solubility of barium compounds increases with decreasing pH. 

There are a number of works that indicate that the presence of Fe ions in soils enhance the transport and fate of NAC explosives. When sterilized and unsterilized soils were used [188], transformations were observed under both conditions biologically and abiotically, suggesting the presence of a purely abiotic transformation component. TNT has been demonstrated to disappear in... 

Thus, under equilibrium conditions, the emf of the double electrode-pair system is determined solely by electric potential differences developed at the two liquid junctions that involve KC1 salt bridges. The two Ej may differ because of the effect of soil colloids. Thus the fact that this emf can develop is known as the suspension effect.40 Only ionic transport processes across the liquid junctions need be taken into account in order to evaluate E. Ionic transport processes across the semipermeable membrane between the suspension and the solution are not germane. Moreover, since neither Ej nor Ej can be calculated by strictly thermodynamic methods, the interpretation of E must be made in terms of specific models of ionic transport across salt bridges contacting suspensions and solutions. Thus the relation between E and the behavior of ions in soil suspensions is not direct. 

In general, the nature of ions in soil-water systems depends on type and oxidation state(s) of the elements involved (Table 5.1). The oxidation state of elements in nature can be estimated following the three rules stated below (Stumm and Morgan, 1981) ... 

Picric acid, one of the breakdown products of tetryl, is soluble in water and is expected to leach through soil to groundwater in substantial amounts (Army 1987d). It is expected to dissociate in water, especially when present in low concentrations. Picric acid may also form complexes with metal ions in soil, causing some of the chemical to remain bound (Army 1987d). Picric acid that is bound to soil may- be transformed via photolysis if present at the soil surface (Army 1987d). 

What is the concentration of AP ions in soil solution at the pH values calculated in Worked example 5.4 for soil solution at (a) pC02... 

Na > Li. Ionic concentration must also be taken into account, as these preferences can be overcome by high concentrations of an ion in soil solution. For example, sodium ions would dominate the exchange sites in soil flooded by seawater. 

The exchange process can be written as a chemical reaction, so, for example, for ions in soil solution exchanging with Ca on the surface (X)... 

The concentration of some ions in soil solution is controlled mainly by the presence of poorly soluble solid phase components. Good examples of this are iron and aluminium, both of which are highly insoluble at the pH of most soils, and which are controlled by the solubility of the... 

Oxalic acid will readily degrade in aquatic ecosystems and is expected to also degrade in soil. Under typical environmental conditions (pH 5-9), oxalic acid will exist as the oxalate ion in soil and water (pfCai 3nd pfCa2 values are 1.25 and 4.28, respectively). 

Abundant evidence exists for the downward translocation of metal ions in soil as soluble complexes with organic matter (Simonson, 1959). The process has been termed cheluviation by Swindale and Jackson (1956). Complex-ation results in differential movement of metal ions according to their ability to form coordination complexes with organic ligands iron, aluminum, and other strongly bound elements being eluted to a greater extent than weakly bound ones. 

Biomolecules are present in soils, especially in the rhizosphere, because of root exudates and microbial metabolites (Lynch, 1990a,b Marschner, 1998). Furthermore, in view of the stability constants of these biomolecular ligands with metals (NIST, 2004), a substantial fraction of the soluble metal ions in soil solution may be complexed with a series of biomolecules commonly present in microbial metabolites and root exudates. The concentrations in the rhizosphere of such metals as Cu, Mn, Zn, and Co vary with the seasons and are related to the presence of complexing biomolecules (Nielson, 1976 Linehan et al., 1989). The pH of the rhizosphere soil is generally lower than that of the corresponding... 

With respect to ion-exchange kinetics, the great complexity of soils has resulted in a penchant for the easy approximations of homogeneous models, even where the premise of a quasi-continuum of liquid and solid is hard to accept. The theory of heterogeneous systems offers the Nernst-Planck equations, but these also can provide no more than an approximation for migration of ions in soil constituents. 

Toxic heavy metal ions and radionuclides can be deposited on the soil surface where they are sorbed to a considerable extent also by soil organic matter. To improve predictions of the behavior of these ions in soils we investigated in stirred batch... 

In most soil solutions, the ionic strength I is low (<0.01), so that the extended Debye-Huckel equation is applicable for the correction of ionic concentrations to the more thermodynamically meaningful activities. Typically, conductivity measurements are used to estimate the ionic strength, a much less laborious procedure than measurement of each cation and anion present in solution. More problematic, however, is the detection and measurement of complexing anions and molecules (ligands). As will be shown later in this chapter, their presence can result in activities of metal ions in soil solution being much lower than measured concentrations would suggest. 

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