Soil solution competing anion

The effects of these factors can be illustrated by considering the formation of a 1 1 complex in a hypothetical soil solution at constant ionic strength in the presence of equimolar concentrations of the reactants, in the absence of competing cations and anions, and at pH = pKl = 4 for the dissociation of the monoprotic acid from which the complexing anion is derived (Fig. 9.1). Equilibrium modelling (using TITRATOR Cabaniss, 1987) indicates that >50% of the total cation concentration will not be complexed with the anion unless the pK for the formation of the species is approximately >7.7. The pK value is more than halved,... 

Arsenic sorption is a function of several chemical factors, including solution pH, competing anion concentration, and Ee oxide concentration in the soil. Impact of soil oxalate extractable Ee contents on As sorption in two Piracicaba soils was determined previously (Table 2). The solubility of amorphous forms is greater than it is for highly crystalline forms. The amount of Ee in the soil solution was determined by the colorimetric method using o-phenanthroline [26]. 

It is well documented that, selenate is taken up by plant roots from soil solution by a process of active transport (Brown and Shrift 1982). It competes with sulfur for uptake, both anions using a sulfate transporter in the root plasma membrane (Arvy 1993). Organic forms of Se, such as selenomethionine, are also taken up actively by plant roots. In contrast, transport of selenite does not appear to require the use of a sulfur transporter (Abrams et al. 1990). Subsequent translocation of Se within the plant is related to the form in which the element is supplied to the root. Se04 is more easily transported from the roots and much more is accumulated in the leaves than either SeOs" or organic selenium. Much of the SeOs is retained in the roots where it is rapidly converted into organic forms, particularly selenomethionine (Zayed et al. 1998). Distribution of Se in various tissues differs between accumulator and nonaccumulator plants. In the former, the Se is accumulated especially in young leaves, but later appears at higher levels in seeds than in other tissues, while, in nonaccumulators, such as cereals, levels in seeds and roots are usually the same as reviewed by Reilly (2(X)6). 

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