Hydrous species reaction

Another geochemical reaction that has been investigated extensively as a geospeedometer by high-temperature geochemists is the hydrous species reaction in rhyolitic melt. As the H2O component dissolves in silicate melt, it partially reacts with oxygen in the melt to form OH groups (Reaction 1-10) ... 

S3.1.3 Ceospeedometry based on the hydrous species reaction in rhyoiitic meit... 

Only two high-temperature homogeneous reactions have been investigated in detail for their kinetics by geochemists. One is the Fe-Mg order-disorder reaction in orthopyroxene, and the other is the hydrous species interconversion reaction in rhyolitic melt. The two reactions have been applied as geospeedometers in various geochemical and meteoritic problems. Because they are often encountered in geochemical kinetics literature, the two reactions are discussed in depth below. 

Hence, there are at least two hydrous species in silicate melt, H2O molecules (referred to as H20m in this section) and OH groups (referred to as OH). Total H2O will be referred to as H20t. The equilibrium constant for the interconvert reaction is... 

As can be seen from the interconversion reaction between two hydrous species H20m and OH (Reaction 3-79), the coefficient for OH is 2. This seemingly trivial point turns out to be the key in understanding H2O diffusion. Because of this factor of 2, the equilibrium constant K involves the square of OH concentration but only the first power of H20 i concentration (Equation 3-80b). That is, the species concentrations of OH and H20 i are not proportional to each other, and neither concentration is proportional to total H2O concentration (H20t) in the entire concentration range. Solving Equations 3-80a,b for the two unknowns of H20m and OH yields... 

The geospeedometer based on the kinetics of interconversion of hydrous species in rhyolitic melt is also well developed although the reaction mechanism and rate law are not known. The empirical calibration covers a cooling rate range of 50 Klyx to 100 K/s, about eight orders of magnitude. Theoretically, this... 

As is well documented, formation of chemisorbed oxygen species on a Pt surface at V > 0.75 V occurs in an inert atmosphere on Pt in contact with an aqueous, or hydrous polymer electrolyte, by anodic discharge of water molecules to form OHads on metal sites, according to the Reaction (1.3). It is this chemisorbed oxygen species, derived from water discharge, that will be considered in the following discussion. Significantly, the Reaction (1.3) is associated with a redox potential K(H20)/Pt-OHads which is quite different from the redox potential for the faradaic ORR process,... 

Employing potential-pH and cyclic voltammetric characterization techniques, Burke and coworkers [48, 73] have advanced the notion that incipient surface hydrous oxide species mediate the electrooxidation reactions of reducing agents of interest to electroless deposition. Thus, in the case of electroless Ni-B, an interfacial, basic, cationic Ni(I) hydrous oxide species would mediate the oxidation of DMAB, as shown in part here ... 

When water is added to a metal oxide, it may react to produce the hydroxide, but the reaction may not be complete. Thus, if the metal has a +3 charge, the product may consist of a mixture of M203, M(OH)3, M(OII)/, and M203 -xH20. The first of these is an oxide, the second and third are hydroxides, and the last is a hydrated oxide (also known as a hydrous oxide). In many cases there is a complex equilibrium involving all of these species, so the exact nature of the products when a metal oxide reacts with water may be variable in composition. 

Hydrolysis and Adsorption. Some years ago, a theory was advanced, that hydrolyzed metal species, rather than free metal ions, are adsorbed to hydrous oxides. The pH-dependence of adsorption (the pH edge for adsorption is often close to the pH for hydrolysis) was involved to account for this hypothesis. As Figs. 2.7b and c illustrate, there is a correlation between adsorption and hydrolysis but this correlation is caused by the tendency of metal ions to interact chemically with the oxygen donor atoms with OH, and with S-OH. The kinetic work of Hachiya et al. (1984) and spectroscopic information are in accord with the reaction of (free) metal ions with the surface. 

Barrer and Mainwaring (20) report the use of metakaolin as the aluminosilicate raw material for reaction with the hydroxides of K and Ba as well as the binary base systems Ba-K and Ba-TMA to form zeolites. Zeolite phases previously synthesized in the analogous hydrous aluminosilicate gel systems were crystallized with KOH, including phillipsite-, chabazite-, K-F-, and L-type structures. The barium system yielded two unidentified zeolite phases (Ba-T and Ba-N) and a species Ba-G,L with a structural resemblance to Linde zeolite L. Ba-G,L was reported previously by Barrer and Marshall (21) as Ba-G. Similar phases were formed in the Ba-K system and in the TMA-Ba system where, in addition, erionite-type phases were formed. The L-type structures are said to represent aluminous analogs of the zeolite L previously reported (22). 

It is well known that hydrolyzed polyvalent metal ions are more efficient than unhydrolyzed ions in the destabilization of colloidal dispersions. Monomeric hydrolysis species undergo condensation reactions under certain conditions, which lead to the formation of multi- or polynuclear hydroxo complexes. These reactions take place especially in solutions that are oversaturated with respect to the solubility limit of the metal hydroxide. The observed multimeric hydroxo complexes or isopolycations are assumed to be soluble kinetic intermediates in the transition that oversaturated solutions undergo in the course of precipitation of hydrous metal oxides. Previous work by Matijevic, Janauer, and Kerker (7) Fuerstenau, Somasundaran, and Fuerstenau (I) and O Melia and Stumm (12) has shown that isopolycations adsorb at interfaces. Furthermore, it has been observed that species, adsorbed at the surface, destabilize colloidal suspensions at much lower concentrations than ions that are not specifically adsorbed. Ottewill and Watanabe (13) and Somasundaran, Healy, and Fuerstenau (16) have shown that the theory of the diffuse double layer explains the destabilization of dispersions by small concentrations of surfactant ions that have a charge opposite to... 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

In many instances, the products obtained from stoichiometrically similar reactions differ significantly in composition and structure depending on reaction conditions. For example, the correct formulation of the nominal species Ba(thd)2, over which a great deal of controversy has occurred, is dependent on whether its preparation is conducted by a hydrous or anhydrous method. The product from the reaction of the metal hydroxide... 

Delocalized H+ counterions are denoted with a subscript f, while H+ species which transfer between tbe film and bulk solution during the redox reaction are identified by the subscripts s. Thus, for each electron injected into the film there is a simultaneous transfer of one proton, i.e. Hs +, from the solution bulk into the hydrous oxide material, while at the same time there is a transfer locally of 1.5 protons into the ligand sphere of the central metal ion for each electron added to the latter. Proton transport is likely to occur via a Grotthus-type mechanism in these films and is much more likely than OH movement as suggested by other authors [144]. 

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