Trace Element Solubility In The Rhizosphere

The key abiotic mechanisms that control solubility will now be discussed and how these processes can be represented analytically will be outlined. For any trace element, only some fraction of the total concentration will be in soil solution, with the remainder bound to the soil matrix. Mass balance of this distribution gives

where

M is the total concentration (mg/kg) 8 is the volumetric water content (m3/m3) C is the trace element concentration in the soil solution (mg/L) p is the bulk density of the soil (t/m3) S is the concentration bound to the soil matrix (mg/kg)

The solubility of a trace element in the rhizosphere is often described in a simple way by a distribution coefficient (Kd) where

In some cases, however, as the total concentration of trace elements in a soil increases, unlike the representation of Equation 6.2, the soil's ability to adsorb these further loadings decreases, due to a saturation of the chemical-binding sites in the soil. The observations of sorption of most trace elements can be described using a Langmuir (Equation 6.3) or a Freundlich (Equation 6.4) isotherm, which accounts for this nonlinearity in sorption.

o KQArC

where

K is the adsorption constant

Q is number of sorption sites (mol/m3)

Ar is the atomic mass of the trace element (g/mol)

n is the Freundlich exponent

If n = 1, then Equation 6.4 collapses to the linear model of Equation 6.1.

In the Langmuir case, when Q is finite, as in the case of soils and sediments, the value of S approaches QAr as the concentration of trace element in soil solution increases. Figure 6.1 shows the adsorption of Cd by a silt loam at a range of Cd concentrations in soil solution. In this case, the isotherm is described by the Langmuir equation (Equation 6.3) with values of Q and K equal to 28.7 and 0.14, respectively.

The solubility of trace elements in the rhizosphere is a function of the soil's chemical and physical properties. Most trace element ions carry a positive charge and can therefore be retained by the negative binding sites of the soil's matrix. The soil's cation exchange capacity (CEC), which indicates the number of negative charges per unit mass, provides an indication as to the soil's potential to retain positively charged ions. The negatively charged binding sites for trace elements occur on organic matter, clays, and the oxides of Fe, Mn, and Al, which make up the soil's matrix.

Trace elements that carry a negative charge, such as F-, Br-, and the oxyanions AsO2- and CrO42-, can bind electrostatically to positively charged sites in the soil matrix, as occurs in variably charged soils. This is measured by the soil's anion exchange capacity (AEC). In many temperate soils, the AEC is so small as to be insignificant. Therefore, many negatively charged ions, such as Br-, can move freely with soil moisture unaffected by exchange. They can be used as chemical "tracers" of water movement through soil.

However, some highly weathered soils, and those that contain significant quantities of such volcanic minerals as allophane and imogolite can have a significant AEC. The AEC depends on pH. Some trace element anions, such as arsenate and selenate, also form specific chemical bonds with soil components. This results in their adsorption exceeding the AEC of the soil. The strength of the

FIGURE 6.1 The effect of Cd2+ concentration on soil adsorption. The soil used was a silt loam pH 5.7, an organic matter content of 6.3%.

FIGURE 6.1 The effect of Cd2+ concentration on soil adsorption. The soil used was a silt loam pH 5.7, an organic matter content of 6.3%.

bond between the binding site of the soil and the trace element is a function of the size and charge on the trace element ion or complex. Smaller ions, with a higher charge, form the strongest bonds.

Although a small percentage of a soil's clay fraction carries a permanent negative charge, the charge carried by organic matter and, as mentioned some variable-charge clay minerals is pH dependent. Therefore, pH also profoundly affects the binding of trace elements in the rhizosphere. For positively charged ions such as Cd2+, soil acidification invariably results in increased trace element solubility due to increased competition from H+ ions at the negatively charged binding sites [1] (Figure 6.2). Conversely, soil adsorption of some trace elements such as Zn2+ can lower soil pH by releasing H+ ions from bound surfaces [2]. Negatively charged trace elements, or trace element complexes, tend to be more soluble at a higher pH. The adsorption of trace elements onto variable charge minerals such as Fe, Al, and Mn oxides is also pH dependent. As these materials assume more negative charge under alkaline conditions, their capacity to absorb trace elements, which is generally positively charged, increases [3].

Trace elements may be displaced from exchange sites by other ions attracted from the soil solution. The extent of this competition for binding sites depends on the type and concentration of the trace element as well as that of the competing ion. As a general rule, trace elements such as Cd2+, which has an atomic radius (r) of 0.97A, can be displaced by other ions of a similar size and charge in soil solution, such as Ca2+ (r = 0.99A). Therefore, soil amendments such as phosphates that are designed to immobilize heavy metals may actually promote the solubility of some co-contaminants such as As.

Trace element adsorption onto charged exchange sites is not the only mechanism governing trace element solubility in the rhizosphere. The extent, soluble-insoluble partitioning, and mobility of soil organic matter play an important role in the solubility and environmental fate of trace elements. Metal complexation by organic matter can promote or reduce metal solubility, depending on the solubility of the organic ligand.

Although the exact composition of dissolved organic matter is variable and complex, a large portion of this mobile material is composed of fulvic and humic acids. Minor components can also include macromolecular hydrophilic acids, carbohydrates, and carboxylic and amino acids [4]. Dissolved organic matter has been demonstrated to promote heavy-metal solubility [5] and mobility,

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