Soil solution ligand

GEERING H.R. and HODGSON J.F. 1969. Micronutrient cation complexes in soil solution III Characterisation of soil solution ligands and... 

Americium will occur in soil in the trivalent state. The transformations that may occur would involve complexation with inorganic and organic ligands (see Section 6.3.1) and precipitation reactions with anions and other substances present in the soil solution. The 241 Am occurring as an ingrowth progeny of 241Pu and trapped in a plutonium matrix will exhibit solubility and biokinetic characteristics of the plutonium, rather than americium. 

Organic complexed Cd is not important in arid soil solution. Hirsh and Banin (1990) observed 5-10% of Cd bound to organic ligands in Israeli arid soil solution. Emmerich et al. (1982) found that organic-Cd complexes constituted 1-4% of Cd in California arid soil solution. However, Villarroel et al. (1993) reported that in a California sludge-treated soil, Cd was mainly present in both free ion and organic complex forms (each accounted for 32-40% and 30-45% of total Cd in soil solution, respectively), followed by the chloride complexes (8-20%), S04-complex (3-10%), and P04-Cd complex (1.5-7.7%). The nitrate Cd complexes were the lowest. Cadmium activities and speciation is not significantly affected by P and N treatments. 

Soil solution to soil ratios also strongly affect distribution of some trace elements such as Zn speciation in arid and semi-arid soils. Fotovat et al. (1997) reported that the proportion of free hydrated Zn2+ to total Zn ranged from 20-65% at field capacity soil water content and decreased with increases in solution to soil ratios, while the proportion of Zn complexed with organic ligands increased dramatically in soils. However, solution to soil ratios do not strongly affect the distribution of Cu speciation in soil solution since Cu primarily occurs as organic complexes in these soil solutions. 

Kerven GL, Ostatek-Boczynski Z, Edwards DG, Asher CJ, Oweczkin J. Chromatographic techniques for the separation of A1 and associated organic ligands present in soil solution. Plant Soil 1995 171 29-34. 

Table 3.6 shows the major inorganic species expected in a solution with a composition typical of natural fresh water. Some calculations for organic ligands in submerged soil solutions are given in Section 3.7. 

Inskeep and Bloom (1986) measured inhibition of calcite precipitation by organic ligands in simulated soil solutions prepared from CaCl2,KHC03 and seeds of CaCOs, and maintained at constant pH and CO2 pressure. The data fitted the rate equation ... 

This sensitivity of precipitation in soil to organic ligands and other inhibitors explains why the soil solutions of submerged soils may be as much as a hundredfold over-satmated with respect to solid phases in the first few weeks following submergence (Chapter 4). 

In soil solutions, the kinetics of competition between ligands for certain metals (or vice versa) may be very important in controlling the prevalent form in solution and hence their availability or toxicity to plants. The formation of a metal... 

An example of a speciation calculation involving metals and ligands that adsorb to form only inner-sphere surface complexes is shown in Table 9.10 for a soil solution at pH 7.5. The adsorption reactions for these metals and ligands are exemplified by the first and eighth rows in Table 9.7 ... 

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 way in which conditional stability constants are used to calculate the distribution of chemical species can be illustrated by consideration of the forms of dissolved Cu(II) in a dilute, acidic soil solution. Suppose that the pH of a soil solution is 6.0 and that the total concentration of Cu is 0.1 mmol m 3. The concentrations of the complex-forming ligands sulfate and fulvic acid have the values 50 and 10 mmol m 3, respectively. The important complexes between these ligands and Cu are CuS04 and CuL where L refers to fulvic acid ligands (see Section 2.3). These illustrative complexes are not the only ones formed among Cu, S04, or L, nor are the three ligands the only ones that form Cu complexes in soil solution.29 Under the conditions assumed, the equation of mole balance for Cu is (cf. Eqs. 2.11 and 2.30)... 

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