Lepidocrocite surface areas

Fig. 14.7 Fractional conversion of lepidocrocite (surface area 122 g ) to maghemite and hematite by dry grinding (Gomez-...

Surface areas of synthetic lepidocrocite range from 15 to 260 m g depending on the conditions of synthesis (Schwertmarm, 1973). Low area samples are obtained by... 

A well crystallized lepidocrocite with smooth edges (surface area 32.5 m g ) was nonporous and displayed a reversible type II N2 adsorption isotherm (Gomez-Villa-cieros et ah, 1984). Poorly crystallized, high surface area material also has a type II adsorption isotherm, but with B type hysteresis this material contained mesopores 2-20 nm across (Crosby et al., 1983 Madrid and De Arambarri, 1985). Lepidocrocite crystals with highly serrated terminals had a surface area of 67 m g of which 13% could be attributed to micropores ca. 1.5 nm across (Weidler, 1995). 

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. 14.4 Isothermal decomposition of lepidocrocite (specific surface area 14 m g ) in vacuum. (Giovanoli. Brutsch, 1975 with permission)...

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).

Figure 2. Experimental pseudo-first-order rate constant k0z,v (normalized to the surface area concentration A) for the reaction of H2S with lepidocrocite plotted as a function of pH. Straight lines a and b correspond to eqs 8 and 9, respectively. kw and kj, are the empirical rate constants.

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... 

Goethite, hematite, and lepidocrocite Batch adsorption of As(V), As(lll), MMAA and DMAA on natural minerals (coarse-grained and very low He-Ar surface area). As adsorption generally goethite > lepidocrocite >> hematite (pH 2-12, maximum often pH 5-8). At pH 7 on goethite, As(lll) > MMAA > DMAA > As(V) ( ). FA (up to 50 mg L 1) tended to reduce As adsorption Bowel I (1994)... 

Figure 4. Effect of duration of heating on specific surface area of products prepared from lepidocrocite at various temperatures...

The surface areas of the various specimens estimated by applying the Brunauer, Emmett, and Teller equation (4) to the initial adsorption isotherms and using the value of 18 sq. A. (14) are compared with the surface areas calculated from the adsorption of cyclohexane in Table I. The surface areas calculated from the total adsorption of methanol on lepidocrocite as well as its products formed by... 

Figure 5. Effect of temperature of preparation on limiting surface areas of products prepared from lepidocrocite (I) and goethite (II)...

How precisely the above methods will separate ferrihydrite from better crystalline oxides depends on the form and crystallinity of the latter. A positive relationship was found between Fco/Fct on the one hand and the surface area and XRD line width on the other for 14 synthetic goethites (range of FCo/Fct = 0.003-0.05) and 15 synthetic lepidocrocites (range of Foo/Fet = 0.06-077) (Schwertmaim, 1973). This shows that whereas goethites (and hematites) are essentially insoluble in oxalate irrespective of their crystal size, the method can only be used for well crystalline lepidocrocites. The same applies to the use of dilute strong acids. 

Properties 60% lepidocrocite, 40% ferrihydrite, XRD results available, BET specific surface area 222 m /g [1611]. 

Ferrihydrite, [5Fe203 9H2O], once called "amorphous ferric hydroxide", has been reported in bog iron and drainage ditches and in environments where other compounds prevent the formation of goethite and lepidocrocite (Schwertmann, 1988). It occurs as bulky precipitates containing water, adsorbed ions, and organic material. It appears as small (5.0-10.0 nm), spherical particles with a high surface area (200-350 m /g Schwertmann and Taylor, 1977). Ferrihydrite is probably the form of iron present in the B horizon of true Podzols. 

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