Soil solutions metal speciation

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. 

Trace element speciation in soil solution is affected by total metal concentrations in soils. Free Cu2+ activity increases with total Cu content in soils from Quebec and New York (Sauve et al., 1997). Total free Cu activity in soils could be predicted from total Cu content and soil pH ... 

The environmental standards based on total heavy metal concentration in the soil solution seem the most important criterion for the exposition of further compartments of the environment. The additional effects connected with metal speciation and complexations were not considered in the study. 

Koopmans GF, Groenenberg JE. Effects of soil oven-drying on concentrations and speciation of trace metals and dissolved organic matter in soil solution extracts of sandy soils. Geoderma 2011 161 147-158. 

The mobility of metals in soil solutions is controlled by several processes (1) desorption or dissolution (rate depends on the solubility of metal-mineral form) (2) diffusion (depends on speciation of metal, soil oxidation/reduction potential, and pH) (3) sorption or precipitation (depends on soil solution concentration and rhi-zosphere effects) and (4) translocation in the plants (depends on plant species, soil solution concentration, and competing ions) (McBride... 

In operationally defined speciation the physical or chemical fractionation procedure applied to the sample defines the fraction isolated for measurement. For example, selective sequential extraction procedures are used to isolate metals associated with the water/acid soluble , exchangeable , reducible , oxidisable and residual fractions in a sediment. The reducible, oxidisable and residual fractions, for example, are often equated with the metals associated, bound or adsorbed in the iron/manganese oxyhydroxide, organic matter/sulfide and silicate phases, respectively. While this is often a convenient concept it must be emphasised that these associations are nominal and can be misleading. It is, therefore, sounder to regard the isolated fractions as defined by the operational procedure. Physical procedures such as the division of a solid sample into particle-size fractions or the isolation of a soil solution by filtration, centrifugation or dialysis are also examples of operational speciation. Indeed even the distinction between soluble and insoluble species in aquatic systems can be considered as operational speciation as it is based on the somewhat arbitrary definition of soluble as the ability to pass a 0.45/Am filter. 

Ionic strength, pH and electron activity (pE) are the three major characteristics of the soil solution commonly recognised as affecting metal speciation. However, reaction kinetics and the relative concentrations and complexing affinities of cations and anions may be equally important (but are sometimes overlooked). 

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. 

An example of a speciation calculation involving calcite formation is shown in Table 3.1 for a soil solution containing the same metals and ligands as in the example in Table 2.6, but at pH 7.9 instead of 5.6. The nominal total concentration of Ca (5.25 mol m 3) is predicted to be partitioned as 56% calcite and 44% aqueous species at equilibrium. Thus the solubility of Ca is predicted to be 2.3 mol m 3 (-- 0.44 x Ca,) under the conditions of the calculation. Note that about 12% of this solubility is contributed by metal-ligand complexes. As an additional hit of analysis, the IAP for calcite, (Ca2 )(C()2 ), in the soil solution can be calculated, given the values of the concentrations of Ca-, ... 

Recently, attempts have been made to develop biomimetic methods, simulating plant uptake of metals. An example of such a method is DGT (diffusive gradients in thin films), developed by Zhang et al. (2001), for measuring metal availability to plants. In this case, metal accumulation in a chelex layer is measured. By taking into account thickness of the diffusive layer covering the chelex layer and contact time with the soil sample, it is possible to estimate the available metal concentration in the soil solution. The DGT method may also be used to estimate metal speciation in surface water (Zhang 2004). 

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, ... 

Mattigod, S. V. and Sposito, G. Chemical modeling of trace metal equilibria in contaminated soil solutions using the computer program GEOCHEM, j[n Jenne, E.A., ed., "Chemical Modeling in Aqueous Systems. Speciation, Sorption, Solubiliiy, and Kinetics." Amer. Chem. Soc., 1978 (This volume). 

Speciation of Trace Metals in the Soil Solution and Natural Waters... 

---