Metals speciation in soil solution

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, ... 

Sposito, G., and Bingham, F. T. (1981). Computer modeling of trace metal speciation in soil solutions correlation, with trace metal uptake by higher plants. J. Plant Nutr. 3, 35-49. 

The technique has recently found application in studies of metal speciation in soil solutions and pore waters. A Teflon or Kel-F cell is employed with a 30-50 ml donor solution separated from a 200 pi acceptor solution by a strong acid cation-exchange membrane (e.g., Nafion-117). Unless metal concentrations are high, the necessary detection limits may be difficult to achieve. 

Groenenherg, J. E., G. F. Koopmans, and R. N. J. Comans. 2010. Uncertainty analysis of the nonideal competitive adsorption—Donnan model Effects of dissolved organic matter variahihty on predicted metal speciation in soil solution. Environmental Science Technology 44, no. 4 1340-1346. 

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 ... 

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... 

Hirsch, D., and Banin, A. J. (1990). Cadmium speciation in soil solutions the residual effect of sewage sludge on heavy metal content of tobacco and peanut. J. Environ. Qual. 19, 366-372. 

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. 

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 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. 

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). 

Knight, B., Zhao, F. J., McGrath, S. P., and Shen, Z. G. (1997). Zinc and cadmium uptake by the hyperaccumulator Thlaspi caerulescens in contaminated soils and its effects on the concentrations and chemical speciation of metals in soil solution. Plant Soil 197, 71-78. 

The hioavailability of metal to plants and soil organisms is mainly through the solution phase. However, soil solutions are in intimate contact with the soil solid phase and are therefore influenced by mineral equilibria and exchange and sorption processes that involve soil organic matter and mineral phases, as well as complexation by organic matter in both the solid phase and that dissolved in the soil solution. Le Chatelier s principle of chemical equilibrium would suggest that metal hioavailability in soils is thus controlled by both solution speciation and the metal associated with the solid phase. 

Numerous processes take place in soil solution, including plant uptake, ion complexation, adsorption and desorption, and precipitation and dissolution (Figure 2.1). As shown in Figure 2.1, Mo solid phases dissolve upon contact with water and provide dissolved Mo in soil solution. The free molybdate ion (Mo04 ) reacts with metals to form complexes and ion pairs in soil solution. Plants absorb dissolved Mo, mainly as Mo04 from soil solution. Removal of Mo04 by plants disrupts the electroneutrality of a soil solution. This causes desorption and adsorption of Mo by oxides, as well as dissolution and precipitation of Mo solid phases in soil solution, until charge balance is reached. The speciation of dissolved Mo in soil solutions must be understood in order to quantitatively describe the availability, toxicity, adsorption, and pre-... 

The study of metal speciation in the soil solution has been encouraged by the free metal hypothesis in environmental toxicology (Lund, 1990). This hypothesis states that the toxicity and hioavailability of a metal is related to the activity of the free aqua ion. Although this hypothesis is gaining popularity in studies of soil-plant relations (Parker et al, 1995), some evidence is now emerging that the free metal ion hypothesis may not be valid in all situations (Tessier and Turner, 1995). Therefore, the role of metal-organic and metal-inorganic complexes in metal uptake merits attention. 

The need to assess the speciation of trace metals and to estimate their bioavailability in the plant root environment is crucial in order to establish the extent of trace metal contamination in soils and to address their potential adverse effects. Although the bioavailability of metals is best predicted by their liquid phase speciation, the assessment of the trace metal fractionation in the solid phase is still essential because the replenishment rate of metals in the soil solution is controlled by solid phase metals (Minnich et al., 1987 Zhang et al., 2001 Krishnamurti et al, 2002). For instance, Zhang et al (2001) showed that Cu concentrations in plant parts correlated best with the effective Cu concentration, which was defined as the Cu concentration in the soil solution combined with a... 

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