Lepidocrocite oxidation rate

Mn(II) oxidation is enhanced in the presence of lepidocrocite (y-FeOOH). The oxidation of Mn(II) on y-FeOOH can be understood in terms of the coupling of surface coordination processes and redox reactions on the surface. Ca2+, Mg2+, Cl, S042-, phosphate, silicate, salicylate, and phthalate affect Mn(II) oxidation in the presence of y-FeOOH. These effects can be explained in terms of the influence these ions have on the binding of Mn(II) species to the surface. Extrapolation of the laboratory results to the conditions prevailing in natural waters predicts that the factors which most influence Mn(II) oxidation rates are pH, temperature, the amount of surface, ionic strength, and Mg2+ and Cl" concentrations. 

Under otherwise similar conditions, low oxidation rates appear to promote magnetite and goethite, whereas high rates favor lepidocrocite. Magnetite formation probably requires slow oxidation because complete dehydroxylation of the precursor (green rust) prior to complete oxidation is only possible if sufficient time is available if, on the other hand, complete oxidation is fast and precedes dehydroxylation, lepidocrocite forms in preference to magnetite (Schwertmann Taylor, 1977). Dehydroxylation and oxidation appear to be competing reaction steps. 

Carlson, L. Schwertmann, U. (1981) Natural ferrihydrites in surface deposits from Finland and their association with silica. Geochim. Cosmochim. Acta 45 421-429 Carlson, L. Schwertmann, U. (1987) Iron and manganese oxides in Finnish ground water treatment plants. Wat. Res. 21 165-170 Carlson, L. Schwertmann, U. (1990) The effect of CO2 and oxidation rate on the formation of goethite versus lepidocrocite from an Fe(II) system at pH 6 and 7. Clay Min. 25 65-71... 

Figure 1. The oxidation rate of H2S by lepidocrocite is pseudo-first-order with respect to H2S. The experimental pseudo-first-order rate constant k<,/ is plotted as a function of the surface area concentration of y-FeOOH. The reaction rate depends on the surface area (A).

In summary, the reaction of H2S with y-FeOOH is a fast surface-controlled process. Equations 8 and 9 can be used to estimate an upper limit of sulfide oxidation rates in sediments with reactive iron (assuming reactive iron to be represented by lepidocrocite). The surface-area concentration A of reactive iron can be calculated according to... 

Very rapid oxidation is essential for formation of feroxyhyte. As the oxidation rate is lowered, lepidocrocite and/or magnetite may form. 

The surface adsorption model described in Chapter 3 (Section 3.4.4) has been used to describe observed rates of surface catalysis of the oxidation of Mn + by silica and two iron oxide solids (Davies and Morgan, 1989). The observed oxidation rate constants are compared with that for the maximum suggested rate of the homogeneous reaction in Table 9.7. Iron oxides were the most effective of the catal5dic surfaces at a concentration of 20 [im, lepidocrocite enhanced the reaction rate by nearly a factor of 10. The observed rate law for the surface reaction was... 

Oxide composition and lattice structure influences the coordin-ative environment of surface sites, and should have an impact on rates of ligand substitution. Hematite (Fe203), goethite (a-FeOOH), and lepidocrocite (y-FeOOH), for example, are all Fe(III) oxide/ hydroxides, but may exhibit different rates of surface chemical... 

This paper discusses the oxidation of Mn(II) in the presence of lepidocrocite, y-FeOOH. This solid was chosen because earlier work (18, 26) had shown that it significantly enhanced the rate of Mn(II) oxidation. The influence of Ca2+, Mg2+, Cl", SO,2-, phosphate, silicate, salicylate, and phthalate on the kinetics of this reaction is also considered. These ions are either important constituents in natural waters or simple models for naturally occurring organics. To try to identify the factors that influence the rate of Mn(II) oxidation in natural waters the surface equilibrium and kinetic models developed using the laboratory results have been used to predict the... 

Leland and Bard (1987) found that the different iron oxides induced photooxidation of oxalate and sulphite at rates that varied by up to two orders of magnitude. For oxalate, the rate was greater for maghemite than for hematite, but this order was reversed for sulphite. Lepidocrocite (layer structure) induced faster oxidation of both compounds that did the other polymorphs of FeOOH (tunnel structures) the authors considered that the rate differences were probably associated with structural differences between the adsorbents. 

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). 

Fig. 13.3 X -ray diffractograms of Fe oxides produced at RT by hydrolysing a 0.1 M Fe(N03)s solution at a different rates (left) and by oxidizing a 0.1 M FeCl2 solution at pH 7 in the presence of various Si concentrations (right) Fh ferrihydrite Gt goethite Lp lepidocrocite. (Schwertmann et al.l 999 with permission Schwertmann Cornell 2000).

In this study we performed initial rate experiments, reacting H2S with lepidocrocite (23). The consumption of H2S was measured continuously by using a pH2S electrode cell (25). To avoid interferences of pH buffer solutions with the iron oxide surface, the pH was stabilized by using a pH-stat that added appropriate amounts of HC1 to the solution. The added volume, which was also continuously monitored, provided information about the amount of protons consumed during the reaction. Dissolved iron was measured only in some runs. 

The extent to which H2S contributes to the release of ferrous iron into pore-water solution through dissolution of reactive ferric oxides such as lepidocrocite or amorphous ferrihydrite remains unclear. According to Can-field (19), liberation of ferrous iron in sediments stems mainly from microbial dissolution of ferric oxides. The release rates of Fe2+ measured in his study range between 3 X 10"6 and 4 X 10 5 M per day, at the lower limit of the theoretical interval. 

The reaction can be carried out over the pH range 6-14. Between pH 6-7 goethite and lepidocrocite result a pure product of either ean be obtained by adjusting the rate of oxidation and the concentration of carbonate in the system (Sehwertmann, 1959 b Carlson and Sehwertmann, 1990). At pH >8 magnetite is obtained and at pH 14, pure goethite is produced. With very rapid oxidation (e.g. by H2O2) feroxyhyte is obtained. 

Synthesis from Fe systems involves oxidative hydrolysis of Fe" solutions. The initial precipitate may be a so-called green rust (Bernal et al., 1959 Taylor, 1980). As various Fe oxides (lepidocrocite, goethite, fer-oxyhyte and magnetite) may be produced by this method, careful control of factors such as the rate of oxidation, pH and the nature of the anion present is necessary to ensure formation of pure goethite. 

A mixed 0.064 M FeCl3-FeCl2 solution with a Fe /Fe of 9 is oxidized in a closed vessel at 20 °C and at a constant pH of 7 using an air flow rate of 10 mFmin (Taylor and Schwertmann, 1974) (For equipment see lepidocrocite synthesis). 

However, some caution is required when transferring rate constants for surface catalysis to the field situation. Most kinetic experiments (16, 61) on the surface-catalyzed oxidation of Mn(II) have been performed with well-crystallized minerals such as goethite or lepidocrocite (a- or y-FeOOH). It has been shown (62) that the catalytic effect of amorphous hydrous ferric oxide on the oxidation of Fe(II) by 02 is much larger than the promotion by a- or y-FeOOH. The estimated abiotic half-lives in Table V should therefore be regarded as upper boundaries. 

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). 

Under our experimental conditions, the overall rate constant of the photochemical reductive dissolution of lepidocrocite in the presence of oxalate is pH-dependent. Thus, the pH dependence of the rate reflects more than the pH dependence of oxalate adsorption at the lepidocrocite surface. Various pH effects may account for this observed pH dependence of ka. One possibility is catalysis of detachment of the reduced surface iron centers by protonation of their neighboring hydroxo and oxo groups. The following question then arises How does the observed rate constant, ka, depend on surface protonation The general rate expression of the proton-catalyzed dissolution of oxide... 

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