Selenate anions

Ridley H, CA Watts, DJ Richardson, CS Butler (2006) Resolution of distinct membrane-bound enzymes from Enterobacter cloacae SKLDla-1 that are responsible for selective reduction of nitrate and selenate anions. Appl Environ Microbiol 12 5173-5180. 

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. 

The effect of variation of solvent and concentration of reactants in the Sjy l reaction between benzene selenate (PHSe ) and aryl halides has been studied. Evidence was found to suggest that the radical anion derived from combination of the benzene selenate anion and aryl radicals (see scheme 4) is formed reversibly and can also dissociate to generate phenyl radical and aryl selenate anion. 

A current area of interest is the use of AB cements as devices for the controlled release of biologically active species (Allen et al, 1984). AB cements can be formulated to be degradable and to release bioactive elements when placed in appropriate environments. These elements can be incorporated into the cement matrix as either the cation or the anion cement former. Special copper/cobalt phosphates/selenates have been prepared which, when placed as boluses in the rumens of cattle and sheep, have the ability to decompose and release the essential trace elements copper, cobalt and selenium in a sustained fashion over many months (Chapter 6). Although practical examples are confined to phosphate cements, others are known which are based on a variety of anions polyacrylate (Chapter 5), oxychlorides and oxysulphates (Chapter 7) and a variety of organic chelating anions (Chapter 9). The number of cements available for this purpose is very great. 

Figure 3 Gradient separation of anions using suppressed conductivity detection. Column 0.4 x 15 cm AS5A, 5 p latex-coated resin (Dionex). Eluent 750 pM NaOH, 0-5 min., then to 85 mM NaOH in 30 min. Flow 1 ml/min. 1 fluoride, 2 a-hydrox-ybutyrate, 3 acetate, 4 glycolate, 5 butyrate, 6 gluconate, 7 a-hydroxyvalerate, 8 formate, 9 valerate, 10 pyruvate, 11 monochloroacetate, 12 bromate, 13 chloride, 14 galacturonate, 15 nitrite, 16 glucuronate, 17 dichloroacetate, 18 trifluoroacetate, 19 phosphite, 20 selenite, 21 bromide, 22 nitrate, 23 sulfate, 24 oxalate, 25 selenate, 26 a-ketoglutarate, 27 fumarate, 28 phthalate, 29 oxalacetate, 30 phosphate, 31 arsenate, 32 chromate, 33 citrate, 34 isocitrate, 35 ds-aconitate, 36 trans-aconitate. (Reproduced with permission of Elsevier Science from Rocklin, R. D., Pohl, C. A., and Schibler, J. A., /. Chromatogr., 411, 107, 1987.)...

Generally, oxo-selenates can be classified according to the oxidation state of the selenium atom as oxu-selenates(IV), and o.Yo-selenates(VI). The same applies for the respective tellurates. Besides this systematically correct naming, chemists usually use the terms selenites and tellurites instead of o. o-selenates(IV) and oxo-tellurates(IV), and selenates and tellurates instead of OAO-selenates(VI) and oxo-tellurates(VI). Therefore, both nomenclatures will be used in parallel throughout this chapter. Compared to the respective sulfur species, the oxo-selenate(IV) and oxo-tellurates(IV) ions are very stable so that numerous compounds with these anions have been prepared. 

Wells, J.M. and D.H.S. Richardson. 1985. Anion accumulation by the moss Hylocomium splendens uptake and competition studies involving arsenate, selenate, selenite, phosphate, sulphate and sulphite. New Phytol. 101 571-583. 

Dr. Clifford The alkali tellurates are, of course, quite different chemically from the selenates and the sulfates, which are definitely monomeric. I think even the alkali tellurates are highly insoluble, for example, which would seem quite unlikely if this were a monomeric anion. On the other hand, if this is polymeric, then the oxygens should be nonequivalent, and depending on how much quadrupole splitting can be expected, one might see some which isn t reported. 

Selenium has a complex chemistry in the environment because of its multiple oxidation states and variable surface adsorption properties. Qualitatively it is analogous to sulfur occurring in the oxidation states +6 (selenate, Se04 ), +4 (selenite, SeOs "), 0 (elemental selenium) and —2 (Se, selenide) The Se anion closely resembles S (radii 0.20 and 0.185nm, respectively) and is often associated with sulfide minerals. Also, like S, Se is subject to volatilization through biological methylation. 

Waychunas, G.A. fuller, C.C. Davis, J.A. Big-ham, J.M. Schwertmann, U. (1995a) EX-AFS and simulation study of selenate and arsenate anion positions in coprecipitated schwertmannites. Abstract Amer. Geophys. Union... 

In addition to the halide systems there are a number of thallium(I) salts of thallium(III) complex anions in which the overall stoichiometry implies, incorrectly, the presence of thallium(II) species. These include the sulfate, selenate,369 acetate370 and oxalate371 derivatives. An X-ray determination has confirmed that Tl(OAc)2 is indeed Tl[Tl(OAc)4], in which the anions are linked through seven-coordinate Tl+, with some evidence for a stereochemically active inert pair.370 In T1[T1(0H)(S04)2], the structure consists of sheets of linked anions with Tl+ ions.372... 

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