Reduction of aqueous metal species

The Reduction of Aqueous Metal Species on the Surfaces of Fe(II)-Containing Oxides The Role of Surface Passivation... 

Reduction of Fe(III) occurs on the surfaces of basalt and Fe(II) containing silicates (2, 5). Reduction of Cr(VI) by biotite has been experimentally investigated (4, 5) and the oxidation of structural Fe(II) by interlayer Cu(II) has been observed in both natural (6) and experimentally reacted biotites (7, S). The adsorption of Cr(VI) and partial reduction to Cr(III) also occurs on a-FeOOH that contains small amounts of Fe(II) (P). Recently the reduction of aqueous metal species including Fe(III), Cr... 

In the presence of reducible aqueous metal species, additional reactions resulting in the oxidation of Fe(II) oxide surfaces can occur. Anodic half cell reactions describing the oxidation of Fe(II) on the surfaces of magnetite and ilmenite can be decoupled from the corresponding reductive half cell reactions (equations 6 and 8) and linked to the reduction of aqueous metal species at the mineral surfaces (10). 

Polymer-protected noble metallic colloids can be prepared by the one-step radiation-induced reduction of aqueous metallic ions with 2-propanol using y-irradiator or e-beam irradiator (see. Figures 19.1 and 19.2). In an aqueous solution, water molecules absorb the irradiation energy and generate many reactive species, such as solvated electrons (e q"), H, and OH" Equation 19.1... 

Electrorefining has been used for the purification of many common as well as reactive metals. It has been seen that the emf or the potential required for such a process is usually small because the energy needed for the reduction of the ionic species at the cathode is almost equal to that released by the oxidation of the crude metal at the anode. Some metals, such as copper, nickel, lead, silver, gold, etc., are refined by using aqueous electrolytes whereas molten salt electrolytes are necessary for the refining of reactive metals such as aluminum,... 

White AF, Peterson ML. Reduction of aqueous transition metal species on the surfaces of Fe(II)-containing oxides. Geochim Cosmochim Acta 1996 60 3799-3814. 

Lopez-Gonzalez et al. [218] also failed to cite the study by Humenick and Schnoor [216], but they analyzed in some detail the effect of carbon-oxygen and carbon-sulfur surface complexes on the uptake of mercuric chloride, which is very weakly ionized in aqueous solution. (The effectiveness of sulphurized carbons in removing mercury from air or water streams had been demonstrated earlier by Sinha and Walker [219], Humenick and Schnoor [216], and more recently by G6mez-Serrano and coworkers [208].) Their key results are summarized in Fig. 9. There was a noticeable uptake decrease when the activated carbons were oxidized with H Oi (AO). This decrease was not due to a reduction in the surface area and is contrary to the behavior of cationic metallic species, whose uptake is typically enhanced as a consequence of a lower pHp/x of the oxidized carbon. Upon subsequent heat treatment in helium at 873 K (A-873), the adsorption ca-... 

The species H, ejq, and OH" are very reactive. Both H and e-, are strong reducing agents with redox potentials of (H+/H )=-2.3 Vnhe and E° (H20/e-,)=-2.87 V he. respectively. Therefore, both can reduce metal ions present in the solution to a state of zero valence. This process takes place through the direct reaction of the metal ion with either H" or e-, in the case of monovalent ions. In contrast, the reduction of multivalent metal ions in aqueous solutions is a multistep process where atoms in unusual valence states are initially formed. This initial reduction is followed by further reduction and agglomeration until a stable nanoparticle is obtained (Belloni et al. 1998). 

The second step in the procedure requires the working electrode to be anodicaUy polarized, yielding one of the dashed Hnes shown in Fig. 3.6. The electrode is then cathodicaUy polarized, and the other dashed Hne from Fig. 3.6 is obtained. The anodic polarization usuaUy results in the oxidation of the metal species, whUe the reaction resulting from cathodic polarization depends on the medium. In an aerated solution, the oxygen reduction reaction may be the prime cathodic reaction, while, in the case of deaerated aqueous solutions, hydrogen reduction could be the dominant reaction. In Fig. 3.6, the redox reaction is represented by a general reaction... 

The transition metals can be reduced in basic aqueous solutions via other mechanism. For example, the metal carbonyls could be attacked by the hydroxide ions and the metal reduced to metal hydride species by the elimination of carbon dioxide to yield a hydride, which could then be deprotonated with the excess of hydroxide ions (Figure 17). The reduction of the metal complexes by CO in aqueous phase is indeed a very important step in the Reppe-type catalysis and water-gas shift reactions. 

The reduction on aqueous transition metal species at the surfaces of Fe(II)-containing oxides is defined in terms of heterogeneous redox reactions that occur concurrently with oxidation and weathering of the mineral surfaces. Electrochemical measurements made on mineral electrodes document that such reactions are coupled half cells in which reductive dissolution can be decoupled and substituted for reduction of aqueous species. This is confirmed experimentally in aqueous/mineral suspensions in which Cr(VI), V(V) Fe(III) and Cu(II) are reduced to lower valance states. 

Oxo-bridged metal complexes can be considered an extension of metal-aquo/hydroxo/oxo systems, and in fact these species often interconvert in the redox reactions of aqueous metal ions however, the pH-dependent reduction potentials of oxo-bridged complexes have enjoyed a special distinction because of their relevance to the proposed mechanisms for the oxidation of water by a tetramanganese oxo cluster in photosystem II.Thorp and co-workers characterized the pH-dependent redox mechanisms of mixed-valence manganese oxo-bridged complexes such as [(NN)2Mn (0)2Mn (NN)2], where NN represents bpy or phen. This family of complexes... 

Rate-determining steps leading to I(IV) and I(III) are postulated, with subsequent rapid reduction of intermediate iodine species. An induction period followed by oxidation of the [Fe(phen)3] complex is a feature of the reaction of that complex with bromate ions. The rates of the corresponding reactions with CI2 and affected by Cl", Br", or hydrogen ion. The oxidations occur via the one-electron transfer steps and an analysis of the data and those for other metal ion reductants has been made using a Marcus theory approach. The kinetics of the peroxodisulfate oxidation of two [Fe(II)(a-diimine)3] complexes have been investigated in binary aqueous-solvent mixtures.The rate data have been dissected into initial and transition state energies. Comparisons between the relative contributions to these parameters for redox and substitution reactions remain a topic of interest. 

Tellurium and cadmium Electrodeposition of Te has been reported [33] in basic chloroaluminates the element is formed from the [TeCl ] complex in one four-electron reduction step, furthermore, metallic Te can be reduced to Te species. Electrodeposition of the element on glassy carbon involves three-dimensional nucleation. A systematic study of the electrodeposition in different ionic liquids would be of interest because - as with InSb - a defined codeposition with cadmium could produce the direct semiconductor CdTe. Although this semiconductor can be deposited from aqueous solutions in a layer-by-layer process [34], variation of the temperature over a wide range would be interesting since the grain sizes and the kinetics of the reaction would be influenced. 

The standard electrode potentials , or the standard chemical potentials /X , may be used to calculate the free energy decrease —AG and the equilibrium constant /T of a corrosion reaction (see Appendix 20.2). Any corrosion reaction in aqueous solution must involve oxidation of the metal and reduction of a species in solution (an electron acceptor) with consequent electron transfer between the two reactants. Thus the corrosion of zinc ( In +zzn = —0-76 V) in a reducing acid of pH = 4 (a = 10 ) may be represented by the reaction ... 

Consider some vanadium ions in aqueous solution. Pale violet solutions of vanadium(ii) salts contain the [V(H20)6] ion. The vanadium(ii) center is only weakly polarizing, and the hexaaqua ion is the dominant solution species. Aqueous vanadium(ii) solutions are observed to be unstable with respect to reduction of water by the metal center. In contrast, vanadium(ni) is more highly polarizing and an equilibrium between the hexaaqua and pentaaquahydroxy ion is set up. The of 2.9 means that the [V(OH2)6] ion (Eq. 9.17) only exists in strongly acidic solution or in stabilizing crystal lattices. 

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