Phase transformation selenates

Biological activity can be used in two ways for the bioremediation of metal-contaminated soils to immobilize the contaminants in situ or to remove them permanently from the soil matrix, depending on the properties of the reduced elements. Chromium and uranium are typical candidates for in situ immobilization processes. The bioreduction of Cr(VI) and Ur(VI) transforms highly soluble ions such as CrO and UO + to insoluble solid compounds, such as Cr(OH)3 and U02. The selenate anions SeO are also reduced to insoluble elemental selenium Se°. Bioprecipitation of heavy metals, such as Pb, Cd, and Zn, in the form of sulfides, is another in situ immobilization option that exploits the metabolic activity of sulfate-reducing bacteria without altering the valence state of metals. The removal of contaminants from the soil matrix is the most appropriate remediation strategy when bioreduction results in species that are more soluble compared to the initial oxidized element. This is the case for As(V) and Pu(IV), which are transformed to the more soluble As(III) and Pu(III) forms. This treatment option presupposes an installation for the efficient recovery and treatment of the aqueous phase containing the solubilized contaminants. 

This Appendix contains the solubility in water of metal selenates and alkali metal selenites. The solubility of most metal selenites in water is a less well-defined quantity due to the protolysis of the selenite ion and as a consequence metal-hydroxide ion complexation and, in cases, transformation of the normal solid phase to a basic selenite phase occur. Most solubilities of metal selenites have therefore been determined in solutions with an adjusted pH value, mainly in the range pH 2 to 4. Such data have been used to calculate the solubility product of the usually slightly or moderately soluble metal selenites as described in Chapter V and Appendix A. 

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