Dissolution of elements and

Another class of complex reactions that has been widely studied is the oxidative dissolution of elemental and compound... 

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). 

Dissolution of Major and Trace Elements by NaOAc-HOAc at Various pHs... 

The contents of trace elements extracted by the buffer solutions depend upon the solution s acid capacity in dissolving carbonate from soils. Trace elements dissolved by the buffer solution increased with decreasing pH of the buffer solution (Table 4.3). Release of trace elements by the buffer solutions at pH 6.0 was much smaller from calcareous soils with more than 30% of CaCC>3. The dissolution of trace elements by the buffers paralleled with the dissolution of Ca and Mg. The correlation coefficients between Ca and trace elements were as follows Cd (0.92), Pb (0.87), Zn (0.90), Ni (0.90), Cr (0.91), V (0.54) and Co (0.70) and between Mg and trace elements were Cd (0.88), Pb (0.80), Zn (0.79), Ni (0.87), Cr (0.58), V (0.69) and Co (0.80), (all with n = 32). 

Several reductions have been achieved by dissolving a nickel-aluminum alloy containing usually 50% of aluminum in 20-50% aqueous sodium hydroxide in the presence of a reducible compound. Since under such conditions elemental hydrogen is generated by dissolution of aluminum, and since Raney nickel is formed in this process, such reductions have to be considered catalytic hydrogenations rather than dissolving metal reductions. Their outcome certainly points to the former type. 

The effect of fluoride ions on the electrochemical behaviour of a metal zirconium electrode was studied by Pihlar and Cencic in order to develop a sensor for the determination of zirconium ion. Because elemental zirconium is always covered by an oxide layer, the anodic characteristics of a Zr/Zr02 electrode are closely related to the composition of the electrolyte in contact with it. These authors found the fluoride concentration and anodic current density to be proportional in hydrochloric and perchloric acid solutions only. In other electrolytes, the fluoride ion-induced dissolution of elemental zirconium led to an increase in the ZrOj film thickness and hindered mass transport of fluoride through the oxide layer as a result. The... 

Consider the dissolution of elemental titaninm, Ti, in molten alnminnm, Al, at various temperatures that lead to the formation of intermetaUic TiAls layers. Fignre 3.2 shows the experimental data for the decrease in thickness of the titaninm as a fnnction of time, and Figure 3.3 shows the increase in thickness of the titaninm alnminide layer with time. From these plots, the rate of dissolntion and rate of formation can be determined, respectively, for each temperatnre. These values are presented in Cooperative Learning Exercise 3.1. Application of the Arrhenius equation (3.12) in both instances... 

Iron frequently has been postulated to be an important electron acceptor for oxidation of sulfide (58, 84,119, 142, 152). Experimental and theoretical studies have demonstrated that Fe(III) will oxidize pyrite (153-157). Reductive dissolution of iron oxides by sulfide also is well documented. Progressive depletion of iron oxides often is coincident with increases in iron sulfides in marine sediments (94, 158, 159). Low concentrations of sulfide even in zones of rapid sulfide formation were attributed to reactions with iron oxides (94). Pyzik and Sommer (160) and Rickard (161) studied the kinetics of goethite reduction by sulfide thiosulfate and elemental S were the oxidized S species identified. Recent investigations of reductive dissolution of hematite and lepidocrocite found polysulfides, thiosulfate, sulfite, and sulfate as end products (162, 163). 

Species distribution studies have shown that trace element (e.g. metals) concentrations in soils and sediments vary with physical location (e.g. depth below bed surface) and with particle size. In these speciation studies the total element content of each fraction was determined using a suitable trace element procedure, for example, solid sample analysis by X-ray emission spectroscopy or neutron activation analysis, or alternatively by dissolution of sample and analysis by ICPOES, AAS or ASV. The type of sample fraction analysed can vary, and a few... 

Complex compounds obtained by oxidative dissolution of elemental metals in the systems M°-ligand-solvent-HX (or halogen-substituted hydrocarbon), are represented by all types of complexes [509]. However, their ratio is not equal they are mainly molecular complexes and metal chelates the % -complexes and di(poly)-nuclear coordination compounds are not numerous. A large number of molecular (neutral and ionic) complexes, chelates, and their adducts, mostly with N-bases, are tabulated in monographs [201,202]. 

The most common method for the production of carbon nanotubes is hydrocarbon-based chemical vapor deposition (CVD) [97] and adaptations of the CVD process [98, 99], where the nanotubes are formed by the dissolution of elemental carbon into metal nanoclusters followed by precipitation into nanotubes [100]. The CVD method is used to produce multiwalled carbon nanotubes (MWCNTs) [101] and double-walled carbon nanotubes (DWCNTs) [102] as well as SWCNTs [103], The biomedical applications of CNTs have been made possible through surface functionalization of CNTs, which has led to drug and vaccine delivery applications [104,105],... 

Upon dissolution of oxides and (hydr)oxides of manganese and iron under reducing conditions at depth, divalent cations of these elements can diffuse upwards through the pore waters to be oxidized and precipitated in near-surface oxic layers, leading to the characteristic near-surface enrichment of manganese and iron in the sediments of many well oxygenated lakes (see Section 3.2.4.3). Phosphorus and... 

The effect of oxygen pressure on the solubihties of siderophile elements in silicate melt has been understood for many years due to metallurgical interest, and has been known to the geological community since early work of Newsom and Drake (1982, 1983) on tungsten and phosphorus. Although there was a brief exploration of the possibility of zero valence dissolution of nickel and possibly cobalt at low /o (Colson, 1992 ... 

Vertical profiles of silicic acid (A) and calcium (B). These elements are released to the dissolved phase of seawater upon the dissolution of opal and carbonate tests. (The silicic acid profile was plotted by using Ocean Data View from WOCE data Ca data are from de Villiers (1994).)... 

Little dissolution of quartz and a fortiori of the siliceous constituents (micas, phytolites, etc.) is possible, in the top layer of the soil. On the other hand, kaolinite which is the exclusive clay mineral in all the soils of the bioclimatic sequence, must be stable, since at pH 6—7 and for dissolved silica near 10 " mol/1, the solutions are saturated with respect to this mineral, when dissolved Al exceeds 10 mol/1, i.e. 10 mg/1 (Gardner, 1970). The addition of Al in the chemical charge of drainage waters might result from the colloidal mobility of this element. 

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