Metal complexes in soil solution

TABLE 11.4. Results of a M1NTEQA2 Calculation of the Speciation of Soluble Heavy Metal Complexes in Soil Solutions of Selected Soils of South Australia as a Percentage of the Total Component... 

Studies involving trace metal speciation in soil solutions require the values of stability or association constants of complexes of the trace metals with a number of inorganic and organic ligands which exist in these environments. Much of these data can be obtained readily from published compilations ( -7 ). However, experimental data on association constants for complexes of the trace metals with ligands such as 063, ... 

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... 

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. 

In addition, dissolved organic carbon (DOC) is also an important soil solution solute affecting speciation and bioavailability of many trace elements in soil solution. Many trace elements and heavy metals complex with dissolved organic carbon. This is especially important in arid and semi-arid environments since high soil pH increases the solubility of organic molecules and accordingly increases concentrations of dissolved organic carbon in soil solution. 

Soil pH is the most important factor controlling solution speciation of trace elements in soil solution. The hydrolysis process of trace elements is an essential reaction in aqueous solution (Table 3.6). As a function of pH, trace metals undergo a series of protonation reactions to form metal hydroxide complexes. For a divalent metal cation, Me(OH)+, Me(OH)2° and Me(OH)3 are the most common species in arid soil solution with high pH. Increasing pH increases the proportion of metal hydroxide ions. Table 3.6 lists the first hydrolysis reaction constant (Kl). Metals with lower pKl may form the metal hydroxide species (Me(OH)+) at lower pH. pK serves as an indicator for examining the tendency to form metal hydroxide ions. 

The principle of hard and soft Lewis acids and bases, proposed by Pearson (1963), is useful to describe these reactions. A Lewis acid is any chemical species that employs an empty electronic orbital available for reaction, while a Lewis base is any chemical species that employs a doubly occupied electronic orbital in a reaction. Lewis acids and bases can be neutral molecules, simple or complex ions, or neutral or charged macromolecules. The proton and all metal cations of interest in subsurface aqueous solutions are Lewis acids. Lewis bases include H, O, oxyanions, and organic N, S, and P electron donors. A list of selected hard and soft Lewis acids and bases found in soil solutions is presented in Table 6.1. 

With respect to trace metal cations, it is common in soil solutions for the total cation anion ratio of the reactants to be very low because complexing anions are... 

In Chapter 1 the broad statement is made that the rates of metal complexation reactions are generally high. A more refined conclusion can be drawn from Table 2.3, which lists the time scales over which a number of complex formation and dissociation reactions occur that are important in soil solutions and other natural waters.7 Perusal of these data makes clear the point that although they are usually very rapid, complexation reactions do span a time scale ranging over at least 10 orders of magnitude. Thus the kinetics of these reactions can be very important to understanding the aqueous speciation of metals and ligands in detail. 

The decomposition of DOM in soil will lead to the re-immobilization of DOM-associated metals. Under aerobic incubation conditions, DOM decomposition could be observed clearly with an increase in incubation time. As a result, the amount of Cu in soil solution decreased drastically, especially for green manure DOM (Figure 10.9). This may be due to the breakdown of the soluble Cu-DOM complex through microbial degradation, leading to the re-adsorption of Cu onto the soil matrix. 

Bacteria and fungi were isolated from soil on the basis of metal tolerance and C source, grouped on the basis of morphological, growth and physiological parameters and examined for ability to alter the solubility and form of metals in exocellular solutions and alter metal solubility in soil. Soluble Ni complexes were subsequently characterized in detail. 

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