Kinetics reductive dissolution

Reductive dissolution kinetics of Mn02 (MnlV) and MnOOH (Mnlll) are presented below for demonstration purposes. The chemistry of manganese in nature is rather complex because three oxidation states are involved [Mn(II), Mn(III), and Mn(FV)] and form a large number of oxides and oxyhydroxides with various degrees of chemical stability (Bricker, 1965 Parc etal., 1989 Potter and Rossman, 1979a,b). One... 

Reduction-dissolution kinetics of manganese-oxides in excess H202 plus H2S04 (assuming complete Mn-oxide surface coverage by the reductant, irreversible electron transfer, and instantaneous product release) can be expressed by... 

The data in Figure 7.13 show reductive-dissolution kinetics of various Mn-oxide minerals as discussed above. These data obey pseudo first-order reaction kinetics and the various manganese-oxides exhibit different stability. Mechanistic interpretation of the pseudo first-order plots is difficult because reductive dissolution is a complex process. It involves many elementary reactions, including formation of a Mn-oxide-H202 complex, a surface electron-transfer process, and a dissolution process. Therefore, the fact that such reactions appear to obey pseudo first-order reaction kinetics reveals little about the mechanisms of the process. In nature, reductive dissolution of manganese is most likely catalyzed by microbes and may need a few minutes to hours to reach completion. The abiotic reductive-dissolution data presented in Figure 7.13 may have relative meaning with respect to nature, but this would need experimental verification. 

The efficiency of detachment is a key parameter in the overall photochemical reductive dissolution kinetics, because in oxic environments detachment of reduced surface metal centers may be in competition with their oxidation (9, 10). 

Metal/environment interface—V ne cs of metal oxidation and dissolution, kinetics of reduction of species in solution nature and location of corrosion products him growth and him dissolution, etc. 

In general there does not appear to be any direct correlation between the rate of the chemical dissolution of oxides and the rate of scale removal, although most work on oxide dissolution has concentrated on magnetite. For example, Gorichev and co-workers have studied the kinetics and mechanisms of dissolution of magnetite in acids and found that it is faster in phosphoric acid than in hydrochloric, whereas scale removal is slower. Also, ferrous ions accelerate the dissolution of magnetite in sulphuric, phosphoric and hydrochloric acid , whereas the scale removal rate is reduced by the addition of ferrous ions. These observations appear to emphasise the importance of reductive dissolution and undermining in scale removal, as opposed to direct chemical dissolution. 

The kinetic behavior of the reductive dissolution mechanisms given in Figure 2 can be found by applying the Principle of Mass Action to the elementary reaction steps. The rate expression for precursor complex formation via an inner-sphere mechanism is given by ... 

Segal, M.G. Sellers, R.M. (1982) Kinetics of metal oxide dissolution. Reductive dissolution of nickel ferrite by tris (picolinato) vana-dium(ll). J. Chem. Soc. Faraday Trans. 1. 78 1149-1164... 

Grygar T (1995) Kinetics of electrochemical reductive dissolution of iron(III) hydroxy-oxides. Collect Czech Chem Commun 60 1261-1273. 

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). 

Perhaps one of the most unknown areas in kinetics of soil chemical processes is redox dynamics (Chapter 8). Some work on reductive dissolution of manganese oxides [Mn(III/IV)] and oxidation of As(V), Cr(III), and Pu(III/IV) by oxides has appeared. However, a comprehensive understanding of redox kinetics in heterogeneous systems is lacking. 

In this chapter, reductive dissolution rates and mechanisms of oxides will be discussed. Additionally, oxidation kinetics of As(III), Cr(III), and Pu(III) by Mn(III/IV) oxides will covered. 

The kinetics of the reductive dissolution mechanisms shown in Fig. 8.1 can be derived using the principle of mass action. The kinetic expression for precursor complex formation by way of an inner-sphere mechanism (Stone, 1986) is... 

The kinetics of reductive dissolution reactions is made complicated (relative to the conceptual picture developed in Section 3.1) by electron transfer processes, similar to the way in which these processes bring complexity to... 

The dissolution reaction in Eq. 3.59b can be regarded as an example of a ligand-promoted process, in that adsorbed bicarbonate species are likely to play a role as intermediates in the kinetic analysis of the reaction.5 Ligand-promoted dissolution reactions are a principal basis for the reductive dissolution processes described in Section 3.4 (see Eq. 3.46). The sequence of steps is analogous to that in proton-promoted dissolution ... 

Metallic corrosion occurs because of the coupling of two different electrochemical reactions on the material surface. If, as assumed in the discussion of iron dissolution kinetics above, only iron oxidation and reduction were possible, the conservation of charge would require that in the absence of external polarization, the iron be in thermodynamic equilibrium. Under those conditions, no net dissolution would occur. In real systems, that assumption is invalid, and metallic dissolution occurs with regularity, keeping corrosionists employed and off the street. 

In this expression, i is current density, p is density, n is the number of electron equivalents per mole of dissolved metal, M is the atomic weight of the metal, F is Faraday s constant, r is pit radius, and t is time. The advantage of this technique is that a direct determination of the dissolution kinetics is obtained. A direct determination of this type is not possible by electrochemical methods, in which the current recorded is a net current representing the difference between the anodic and the cathodic reaction rates. In fact, a comparison of this nonelectrochemical growth rate determination with a comparable electrochemical growth rate determination shows that the partial cathodic current due to proton reduction in a growing pit in A1 is about 15% of the total anodic current (26). 

Reductive Dissolution. Many substances in nature contain the same metal or metalloid, but under different oxidation states. For example, the metalloid arsenic may exist as arsenite (AsIII, As03) or arsenate (AsIV, As04) in the forms of ferrous-arsenite or ferric-arsenate, respectively. Ferrous-arsenite is more soluble than ferric-arsenate for this reason, one may be interested in studying the kinetics of arsenate reduction to arsenite. Similar chemistry applies to all elements present in soil-water systems with more than one oxidation state (e.g., iron, manganese, selenium, and chromium). 

Activation control of an overall dissolution rate can, of course, reside in the reduction process, in the oxidation process, in a mixture of both, or in a mixture including some transport control. The reduction process is usually more influential in determining the overall rate. Thus, in the absence of transport control, the kinetics of the electrode process for reduction of hydrated protons, or water molecules, or dissolved molecular oxygen plays the major role in metal dissolution kinetics. Indeed the literature confirms the conclusion that many of the systems seen in experiment or in practice are diffusion controlled that most of the rest are under mixed diffusion and activation control and that those with some activation control... 

The electroless deposition of metals on a silicon surface in solutions is a corrosion process with a simultaneous metal deposition and oxidation/dissolution of silicon. The rate of deposition is determined by the reduction kinetics of the metals and by the anodic dissolution kinetics of silicon. The deposition process is complicated not only by the coupled anodic and cathodic reactions but also by the fact that as deposition proceeds, the effective surface areas for the anodic and cathodic reactions change. This is due to the gradual coverage of the metal deposits on the surface and may also be due to the formation of a silicon oxide film which passivates the surface. In addition, the metal deposits can act as either a catalyst or an inhibitor for hydrogen evolution. Furthermore, the dissolution of silicon may significantly change the surface morphology. 

Where might this be important As discussed above, biological activity can result in the simultaneous precipitation of mixtures of nanoscale sulfide minerals under certain conditions. Each mineral will exhibit a particular particle size distribution, dependent on the solution composition, bacterial activity, rate of crystal growth, and the nature of electrochemical interactions between the particles. These electrochemical reactions could lead to oxidation of one type of nanophase sulfide mineral of a certain size, and reduction of another type of nanophase sulfide particle or other species in the solution. In this way, a tremendous number of mineral-solution-mineral galvanic cells could develop, with potentially significant impact on dissolution kinetics, growth kinetics, and the mixture of phases observed. In addition to environmental relevance, these processes may shape the mineralogy of low-temperature ore deposits. 

Stone, A. T., and Ulrich, H., 1989, Kinetics and reaction stoichiometry in the reductive dissolution of manganese(lV) dioxide and Co(lll) oxide by hydroquinone Journal of Colloid Interface Science, v. 102, p. 509-522. 

The kinetics of the photochemical reductive dissolution of lepidocrocite (y-FeOOH) with oxalate as the reductant depends strongly on pH both the rate and the overall rate constant, k> decrease with increasing pH. This behavior means that the pH dependence of the rate does not simply reflect the pH dependence of oxalate adsorption at the lepidocrocite surface. Between pH 3 and 5, the log k() values can be fitted with a straight line. The dependence of k on the concentration of surface protons, >FeOH2+, can be estimated from the slope of this line and from the protonation curve of lepidocrocite k0 >FeOHf I6. The value of 1.6, which can be considered only a rough estimate, is not too different from the theoretically expected value of 2 for the proton-catalyzed detachment of reduced surface iron centers (i.e., of surface metal centers with the formal oxidation state of II). 

To evaluate the predominant pH effect on the kinetics of reductive dissolution of oxide minerals, it is indispensable to examine the pH dependence of both the rate, R, and the overall rate constant, ka, of reductive dissolution. The rate constant differs from the rate by the surface concentration of the reductant, R = ka RedadJ. If the rate constant ka is independent of pH, then the pH dependence of the rate is solely due to the pH dependence of adsorption of the reductant at the (hydr)oxide surface. If, on the contrary, the rate constant is dependent on pH, then other pH effects have to be considered. 

The evaluation of the effect of pH on the light-induced reductive dissolution of iron(III) (hydr)oxides is a key to the identification of the important parameters that control the kinetics of dissolved iron(II) formation in atmos-... 

Rate Expression of the Photochemical Reductive Dissolution of y-FeOOH by Oxalate. The kinetics of the light-induced reductive dissolution of oxide minerals obey the general rate expression of surface-controlled reactions. The rate is proportional to the concentration of the adsorbed reductant, as in the case of adsorbed oxalate ... 

Obviously, the kinetics of the overall process will be influenced by both the partial reductive dissolution and the kinetics of the nucleation/growth of the deposit of Co metal. This process may involve intermediate cobalt species in solution or proceed via solid-to-solid conversion to a metal phase, as described by Hasse and Scholz (2001) for the reduction of lead oxide. The properties of the metallic deposit are remarkably different for each of the studied materials, as denoted by the observed differences in their anodic behavior. The oxidation signal at -e0.40 V can be described in terms of total or partial oxidative dissolution processes ... 

Liu, C. et al.. Kinetics of reductive dissolution of hematite by bioreduced anthraqui-none-2,6-disulfonate. Environ. Sci. Technol.. 41, 7730, 2007. 

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