Reduction potentials arsenic

Fig. 33.2. Factors controlling reaction rate (expressed per kg water, as r/nw) in the simulation of bacterial arsenic reduction, including kinetic factors FD and Fa, thermodynamic potential factor FT, and biomass concentration [X], Biomass concentration determines the rate early in the simulation, but later the thermodynamic drive exerts the dominant control.

Given the following standard reduction potentials at 25°C, (a) balance the equation for the reaction of H2M0O4 with elemental arsenic in acidic solution to give Mo3+ and H3As04, and (b) calculate E° for this reaction. 

Arsenic(III), antimony(III), and tin(II) ions can be oxidized to arsenic(V), antimony(V), and tin(IV) ions respectively. On the other hand, the latter three can be reduced by proper reducing agents. The oxidation-reduction potentials of the arsenic(V)-arsenic(III) and antimony(V)-antimony(III) systems vary with pH, therefore the oxidation or reduction of the relevant ions can be assisted by choosing an appropriate pH for the reaction. 

Figure 9 Yeast5. cerevisiae cell with small daughter cell bud and proteins of arsenate reduction and transport. Acr2p the yeast cytoplasmic arsenate reductase. Acr3p the potential-driven membrane arsenite efflux protein, equivalent to bacterial ArsB. Ycflp the novel As(III)-3 GSH adduct carrier than transports the adduct complex into the cell vacuole compartment, functioning as an ATPase.

Both NT-26 and P. arsenitoxidans are aerobes, and arsenite serves as the source of reducing equivalents to be transferred via an electron transport chain to oxygen to form water. In anaerobic bacteria, arsenate could serve as the terminal electron acceptor in the presence of an electron source of lower reduction potential. This appears to be the case in a strain of anaerobic bacterium, MIT-13, which reduces arsenate when lactate is present as an electron source (22). 

This illustrates that the deposited metal dissolves in the mercury electrode. Other elements, such as selenium, arsenic, mercury, silver, gold, or tellurium, which either do not form amalgam or oxidize at potentials anodic to that of mercury can be measured by ASV using a variety of solid electrodes. The deposition step is usually carried out under conditions of forced convection to facilitate the transport of the metal ions to the electrode surface. The convective transport is achieved mainly by electrode rotation, solution stirring, and flow. The deposition potential (Ed in Figure 1) should be 0.3-0.4V more negative than the reduction potential of the metal ion to ensure an efficient deposition process. The duration of the preconcentration step is selected according to the... 

The standard electrode potential for zinc reduction (—0.763 V) is much more cathodic than the potential for hydrogen evolution, and the two reactions proceed simultaneously, thereby reducing the electrochemical yield of zinc. Current efficiencies slightly above 90% are achieved in modem plants by careful purification of the electrolyte to bring the concentration of the most harmful impurities, eg, germanium, arsenic, and antimony, down to ca 0.01 mg/L. Addition of organic surfactants (qv) like glue, improves the quaUty of the deposit and the current efficiency. 

The amount of HEU that becomes avadable for civdian use through the 1990s and into the twenty-first century depends on the number of warheads removed from nuclear arsenals and the amount of HEU in the weapons complex that is already outside of the warheads, ie, materials stockpdes and spent naval reactor fuels. An illustrative example of the potential amounts of weapons-grade materials released from dismanded nuclear weapons is presented in Table 7 (36). Using the data in Table 7, a reduction in the number of warheads in nuclear arsenals of the United States and Russia to 5000 warheads for each country results in a surplus of 1140 t of HEU. This inventory of HEU is equivalent to 205,200 t of natural uranium metal, or approximately 3.5 times the 1993 annual demand for natural uranium equivalent. 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

The reactions discussed above show that arsenic(fV) is of redox amphoteric character and a stronger reducing agent than arsenic(in), but at the same time it is a stronger oxidant than arsenic(V). Partners of the oxidation-reduction reactions of arsenic(fV) known so far can be seen in Table 13. It follows from the redox amphoteric character that the oxidation potentials of couples involving arsenic species are in the order... 

This order of the potentials indicates that arsenic(IV) is an intermediate in the oxidation of arsenicflll) and reduction of arsenic(V) which is unstable from thermodynamic point of view and disproportionates easily according to... 

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