Cations and Oxyanions

There has been considerable interest in the remediation of sites contaminated with U(V1) and Tc(VII) by conversion of these soluble forms into less soluble reduced states that may be precipitated. In assessing results involving the reduction of U(VI) and Tc(VII), it is important to take into consideration (a) intermediate levels of reduction that may be critical and (b) the formation of complexes, for example, with carbonate that may determine the final products and their association with cells or particles. The discussion is assembled by organism, since some of these are able to reduce several oxidants. 

Under anaerobic conditions with H2 as electron donor—when growth did not take place—S. putre-faciens rednced Tc(Vll) to Tc(IV) that was associated with the cell or with Tc(lV) complexes in the presence of carbonate (Wildnng et al. 2000). 

It has been shown that in the presence of nitrate and an electron donor such as acetate, both U(VI) and Tc(VII) can be reduced (Istok et al. 2004). Important conclusions were that (a) whereas reduction of Tc(VII) occurred concurrently with nitrate redaction, reduction of U(VI) took place only after Fe(II) was detected and (b) reoxidation of U(IV) took place in the presence of high—though not with low—concentrations of nitrate. 

Cell suspensions of Geobacter sulfurreducens can conple the oxidation of hydrogen to the reduction of Tc(VII) to insolnble Tc(IV). An indirect mechanism involving Fe(II) was also observed, and was snbstantially increased in the presence of the redox mediator AQDS (Lloyd et al. 2000). 

Geobacter metallireducens is also able to grow by vanadate respiration supported by acetate, and it has been suggested that this could provide a new strategy for removing vanadate from groundwater (Ortiz-Bernad et al. 2004b). 

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Although some metal cations and oxyanions can serve as electron acceptors for growth under anaerobic conditions, reduction may also take place gratuitously, and these reductases have been characterized in a number of organisms. Reduction has been implicated in resistance to metal cations and metalloid oxyanions, which are discussed in Part 4 of this chapter. 

Heavy-metal cations and oxyanions are generally toxic to bacteria although resistance may be induced by various mechanisms after exposure. Attention is drawn to an unusual example in which AF+ may be significant, since the catechol 1,2-dioxygenase and 3,4-dihydroxybenzoate (protocat-echuate) 3,4-dioxygenase that are involved in the metabolism of benzoate by strains of Rhizobium trifolii are highly sensitive to inhibition by AP (Chen et al. 1985). 

EXAFS data showed that cations and oxyanions (e.g. selenite and arsenite) can form two kinds of bidentate, inner sphere complexes on iron oxides depending upon the surface site at which the adsorbate adsorbs (Manceau, 1995 Randall et al.. 

From the above description, it becomes apparent that with such disorder, the sorption of both heavy metal cations and oxyanionic species is favoured. Owing to the lack of knowledge of the structure of the C-S-H gel itself, however, it is unlikely that an exact binding mechanism could be discerned at present... 

Figure 10.17 Adsorption of various metal cations and oxyanions, each at a trace concentration of 5 x 10 M), by ferrihydrite ( Fe(lII) = 10 M) as a function of pH at an ionic strength of 0.1 mol/kg. There are 2 x 10 M of reactive sites on the oxyhydroxide. The dashed curves have been calculated. The plots are after W. Stumm, Chemistry of the solid-water interface, Copy--right 1992 by John Wiley Sons, Inc. Reprinted by permission of John Wiley Sons, Inc.

White, C., and Gadd, G. M. (1998b). Reduction of metal cations and oxyanions by anaerobic and metal-resistant organisms chemistry, physiology and potential for the control and bioremediation of toxic metal pollution. In Extremophiles Physiology and Biotechnology, ed. Grant, W. D., and Horikoshi, T., Wiley, New York, 233-254. 

Alcohols react with boric acid with elimination of water to form borate esters, B(OR)3. A wide variety of borate salts and complexes have been prepared by the reaction of boric acid and inorganic bases, amines, and heavy-metal cations or oxyanions (44,45). Fusion with metal oxides yields... 

Microbial cells may be subjected to stress from a number of sources in their environment. These include antibiotics, metal cations and metalloid oxyanions, aromatic hydrocarbons, chlorophenols,... 

In the range of pH exhibited by most natural water and in the concentration range greater than millimolar few metal ions exist as simple hydrated cations, and relatively few oxyanions exist as simple monomeric species. The hydrated cations are good buffers toward bases, the metallate ions toward acids. As pH is raised in solutions of many hydrated cations, isopolycations are produced, and ultimately, hydrous metal oxides precipitate. As pH is lowered in solutions of many metallate ions, isopolyanions are produced, and ultimately, hydrous oxides precipitate. Salts of the intermediate isopolyions precipitate in some cases. Where the results are unambiguous, the nature of the intermediate species can be described. Kinetics are thought to have been neglected in studies of such solutions up to the present time. 

Anions, including arsenic oxyanions and OH-, migrate toward the anode. The negatively charged cathode attracts metal cations and creates reducing conditions and an alkaline (perhaps pH > 12) front in the surrounding waters (Acar and Alshawabkeh, 1993, 2638) ... 

Calculations of the structure of the mesoionic thiopyrylium-3-olate 74 suggest that the C-S bond lengths are similar to those in the pyrylium cation at ca. 168 pm, perhaps supporting the fully charge-separated betaine structure. However, the charge at oxygen is closer to 0.5 than the 1.0 expected for such a structure. Furthermore, the nucleus-independent chemical shift value is appreciably lower than that for the thiopyrylium cation. These data point toward an ylidic structure with an acceptor moiety rather than an aromatic cation and an exocyclic oxyanion <2002IJQ(90)1055>. 

Many rDA reactions are carried out at temperatures of 150 C or more in solution phase and often at temperatures of 400-600 C using the flash vapor pyrolysis (FVP) method individual conditions are referenced throughout the text. However, an accelerating effect by anionic, cationic and radical substimtion on either the dienophile or at the termini of the diene fragments has been predicted by Carpenter.Experimentally, this prediction has been substantiated only for anionic substitution. In 1967, Hart reported what is likely the first example of an oxyanion-accelerated rDA reaction. Both oxyanionic " and car-banionic substituents accelerate the cycloreversion reaction such that they proceed rapidly at room temperature (for example, equation 3). In addition, acid-catalyzed rDA reactions have been reported in which protonation effectively makes the dienophile fragment of the adduct more electron deficient. Grieco has utilized a room temperature retro aza DA reactitm useful for the N-methylation of dipeptides and amino acid derivatives (equatitm 4). ... 

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