Rate, dissolution reductive

The rate of reduction of a vat dye depends partly on the intrinsic chemical properties of the dye and partly on the size and physical form of the dispersed particles undergoing this reaction. The physical factors are much less important than the chemical aspects [26]. The vatting process entails conversion of the insoluble keto form into the soluble sodium enolate (section 1.6.1). The reaction takes place in two stages at ambient temperature. Extremely rapid reduction to the hydroquinone is followed by slower dissolution in the alkaline solution. At higher temperatures, however, the dissolution rate approximates more closely to the rate of reduction. Temperature and dithionite concentration are the important variables and the rate of reduction is much less dependent on dye or alkali concentration. 

The Rate of reductive Dissolution of Hematite by H2S as observed between pH 4 and 7 is given in Fig. 9.6 (dos Santos Afonso and Stumm, in preparation). The HS" is oxidized to SO. The experiments were carried out at different pH values (pH-stat) and using constant PH2s- 1.8 - 2.0 H+ ions are consumed per Fe(II) released into solution, as long as the solubility product of FeS is not exceeded, the product of the reaction is Fe2+. The reaction proceeds through the formation of inner-sphere =Fe-S. The dissolution rate, R, is given by... 

Rates of reductive dissolution of amorphous manganese (111,1V) oxide particles decrease as the electrode half-wave potentials of the substituted phenols (as reported by Suatoni et al., 1961) increase (4.8 x 10 5 M total manganese, pH 4.4). 

Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. 

Based upon thermodynamic data given in Table I, oxidant strength decreases in the order NijO > Mn02 > MnOOH > CoOOH > FeOOH. Rates of reductive dissolution in natural waters and sediments appear to follow a similar trend. When the reductant flux is increased and conditions turn anoxic, manganese oxides are reduced and dissolved earlier and more quickly than iron oxides (12, 13). No comparable information is available on release of dissolved cobalt and nickel. 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

Few studies have systematically examined how chemical characteristics of organic reductants influence rates of reductive dissolution. Oxidation of aliphatic alcohols and amines by iron, cobalt, and nickel oxide-coated electrodes was examined by Fleischman et al. (38). Experiments revealed that reductant molecules adsorb to the oxide surface, and that electron transfer within the surface complex is the rate-limiting step. It was also found that (i) amines are oxidized more quickly than corresponding alcohols, (ii) primary alcohols and amines are oxidized more quickly than secondary and tertiary analogs, and (iii) increased chain length and branching inhibit the reaction (38). The three different transition metal oxide surfaces exhibited different behavior as well. Rates of amine oxidation by the oxides considered decreased in the order Ni > Co >... 

Oxide mineralogy may influence rates of reductive dissolution in several ways. Hematite (ct-Fe203) and maghemite (y-Fe203), for example, have the same stoichiometry but contain Fe(III) in quite different coordinative environments. Fe(III) in hematite occupies trigonally-distorted octahedral sites, while Fe(III) in maghemite is found in both octahedral and tetrahedral sites (42). Differences... 

Al substitution (0.09-0.16 mol mol ) had no definite effect on the photochemical dissolution of substituted goethite in oxalate at pH 2.6 (Cornell Schindler, 1987). On the other hand, Al substitution depressed the initial (linear) stage of dissolution of synthetic goethites and hematites in mixed dithionite/citrate/bicarbonate solutions (Fig. 12.22) (Torrent et al., 1987). As the variation in initial surface area has already been accounted for, the scatter of data in this figure is presumably due to variations in other crystal properties such as disorder and micropores. Norrish and Taylor (1961) noted that as Al substitution in soil goethites increased, the rate of reductive dissolution dropped (see also Jeanroy et al., 1991). 

Dos Santos Alfonso and Stumm (1992) suggested that the rate of reductive dissolution by H2S of the common oxides is a function of the formation rate of the two surface complexes =FeS and =FeSH. The rate (10 mol m min ) followed the order lepidocrocite (20) > magnetite (14) > goethite (5.2) > hematite (1.1), and except for magnetite, it was linearly related to free energy, AG, of the reduction reactions of these oxides (see eq. 9.24). A factor of 75 was found for the reductive dissolution by H2S and Fe sulphide formation between ferrihydrite and goethite which could only be explained to a small extent by the difference in specific surface area (Pyzik Sommer, 1981). 

Drug release profiles from the tablets in various dissolution media are shown in Fig. 2. In all cases the release rates decreased initially from the control (distilled water) as electrolyte concentration increased, until a minimum release rate was obtained. As the electrolyte concentration further increased the release rates similarly increased until a burst release occurred. These initial decreases in release rates were probably coincident with a decrease in polymer solubility, in that as the ionic strength of the dissolution medium is increased the cloud point is lowered towards 37°C. It may be seen from Table 5 that minimum release rates occurred when the cloud point was 37°C. At this point the pore tortuosity within the matrix structure should also be at a maximum. It is unlikely to be an increase in viscosity that retards release rates since Ford et al. [1] showed that viscosity has little effect on release rates. Any reduction in hydration, such as that by increasing the concentration of solute in the dissolution media or increasing the temperature of the dissolution media, will start to prevent gelation and therefore the tablet will cease to act as a sustained release matrix. 

Modifications of the chemical nature of the catalyst under cathodic load are also possible. Sulphides can be reductively dissolved with liberation of H2S [139]. Oxides can be progressively reduced with loss of the specific activity [140]. In the latter case, an additive can be used to diminish the rate of reduction. Intermetallic compounds or alloys may exhibit preferential dissolution of one of the components during cathodic performances in concentrated alkali [141],... 

The potential sources of variability for gastric residence in the fed mode are the size of the pyloric opening versus the tablet size, the rate of reduction in the size of the tablet by dissolution and antral grinding by the stomach, and inter- and intraindividual variations in the duration of the fed mode, particularly as a function of caloric and fat content. The dominant limitation in the use of food for gastric retention is the minimal total fat content. 

The oxidant strength of transition metal oxides usually decreases in the order Ni304 > Mn02 > MnOOH > CoOOH > FeOOH. The rate of reductive dissolution in sediments and natural waters follows a similar order. 

An analogous rate expression can be written for the outer-sphere mechanism. From Hq. (8.2), it can be predicted that high rates of reductive dissolution are enhanced by high rates of precursor complex formation... 

A number of studies have appeared in the literature on reductive dissolution of Mn(III/IV) oxides, particularly by organic reductants. Rates of reductive dissolution by hydroquinone (Stone and Morgan, 1984a), substituted phenols (Stone, 1987b), and other organic reductants (Stone and Morgan, 1984b) have been determined. 

Stone (1987a) studied reductive dissolution of Mn oxides by oxalate and pyruvate an example with 1.00 x 1(T4 M oxalate is shown in Fig. 8.2. The rate of dissolution was directly proportional to organic reductant concentration and increased as pH decreased. For oxalate, the rate of reductive dissolution at pH 5.0 was 27 times faster than at pH 6.0. For pyruvate, a similar change in pH increased the rate only by a factor of 3. 

As can be seen from Fig. 8.2 for oxalate and, although not shown, also for pyruvate, the apparent order with respect to [H+], a, decreased with pH. Rates of reductive dissolution of Mn(III/IV) oxides by phenols were characterized by a values that also decreased with pH and were >1.0 at pH > 6.0 but were 0.5 as pH 4 was approached (Stone, 1987a). 

Recently, workers (2) have been examining the equilibrium and kinetic factors that are important at the oxic-anoxic interface. The kinetic behavior is difficult to characterize completely due to varying rates of oxidation and absomtion above the interface and varying rates of reduction, precipitation and dissolution below the interface (2.51. Bacterial catalysis may also complicate the system (1). Although one can question the importance of abiotic thermodynamic and kinetic processes at this interface, we feel it is useful to use simple inorganic models to approximate the real system. Recently, the thermodynamics and kinetics of the H2S system in natural waters has been reviewed (0. From this review it became apparent that large discrepancies existed in rates of oxidation of H2S and the thermodynamic data was limited to dilute solution. In the last few years we have made a number of thermodynamic (7.81 and kinetic (9 101 measurements on the H2S system in natural waters. In the present paper we will review these recent studies. The results will be summarized by equations valid for most natural waters. 

As is clear from the above discussion, reduction of surface-located Fe(III) (which may or may not lead to oxide dissolution) is associated in most instances with oxidation of the electron donor at the particle surface and many of the same factors that influence the rate of reductive dissolution will also affect the rate of donor oxidation. Leland and Bard [138] found that the rate constants of photooxidation of oxalate and sulfite varied by about two orders of magnitude with different Fe(III) oxides and concluded that this appears to be due to differences in crystal and surface structure rather than to differences in surface area, hydrodynamic diameter or band gap . 

The dissolution is completely inhibited when the rate of reduction by H2 is equal to the rate of dissolution of oxidized UO2. 

THE RATE OF reductive dissolution of oxide minerals such as iron(III) ind manganese(III,IV) (hydr)oxides depends strongly on pH it generally increases with decreasing pH (1-3). Several phenomena may contribute to this pH dependence. 

Last but not least, the pH dependence of the thermodynamic driving force may account for the pH dependence of the observed rate constant. Reductive dissolution of lepidocrocite with oxalate as the reductant is an ex-... 

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