Lepidocrocite effect

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. 

Fig. 14.12 The effect of silicate and seeding with goethite on the transformation of lepidocrocite to goethite in M KOH at 80 °C The figures on the curves give the Si concentration in mmol (Schwertmann Taylor, 1972 a with permission).

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... 

Cornell, R.M. Giovanoli, R. (1988 a) Acid dissolution of akaganeite and lepidocrocite the effect on crystal morphology. Clays Clay Min. 36 385-390... 

Processings of iron oxides at room temperature. II. Mechanochemical reaction effects on the structure and surface of pure, synthetic lepidocrocite. Mat. Res. Bull. 17 1017-1023... 

A.I. Bordia, R.K. Korshin, G.V. Christensen T.H. (2000 a) Aging of iron (hydr)oxides by heat treatment and effects on heavy metal binding. Environ. Sci. Techn. 34 3991-4000 Sorensen, S. Thorling, L. (1991) Stimulation by lepidocrocite (y-FeOOH) of Fe(II)-depen-... 

Zhang, X. Zhang, F. Mao, D. (1999) Effect of iron plaque outside roots on nutrient uptake by rice Oryza sativa L.) Phosphate uptake. Plant and Soil 209 187-192 Zhang,Y Charlet, L. Schindler, P.W. (1992) Adsorption of protons, Fe(II) and Al(IIl) on lepidocrocite (y-FeOOH). Colloids Surfaces 63 259-268... 

Mao, H,-K. Bell, P. M. (1974b) Crystal field effects of ferric iron in goethite and lepidocrocite Band assignment and geochemical applications at high pressure. Ann. Rept. Geophys. Lab., Yearb. 73, 502-7. 

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

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

Galvez, N., Barron, V and Torrent, J. (1999) Effect of phosphate on the crystallization of hematite, goethite, and lepidocrocite from ferrihydrite. Clays Clay Min. 47 304-311. 

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... 

Many of these oxoanions can form, depending on concentration and pH, various surface complexes. This ability may explain the different effects observed under different solution conditions. For example, Bondietti et al. (33) found that phosphate at low pH (where mononuclear complexes are probably formed) accelerated EDTA-promoted dissolution of lepidocrocite, whereas at near-neutral pH conditions (where binuclear complexes are presumably formed), phosphate was an efficient inhibitor. Furthermore, because of the several geometries involved, the extent of comer sharing or edge sharing by adsorbed oxoanions may differ with the type of oxide and with allotropic modifications of the same metal oxide. 

Case Examples. The effects of various oxoanions on EDTA-pro-moted dissolution of lepidocrocite (y-FeOOH) have been studied by Bondietti et al. (33). EDTA was chosen as a reference system because it is dissolution-active over a relatively wide pH range. Phosphate, arsenate, and selenite markedly inhibit the dissolution at near-neutral pH values. At pH <5 phosphate, arsenate, and selenite accelerate the dissolution. It is presumed that the bi-nuclear surface complexes formed at near-neutral pH values by these oxoanions (Table II) inhibit the dissolution. Figure 8a displays data on the effect of selenite on EDTA-promoted dissolution, and Figure 8b shows that calculations on surface speciation by Sposito et al. (35) support the preponderance of binuclear selenite surface complexes in the neutral-pH range. Mononuclear surface species prevail at lower pH values. 

Figure 8. The effect of selenite on the EDTA-promoted dissolution of y-FeOOH 0.5 gIL). Part a At low pH the dissolution rate is increased by selenite at pH 7 it is strongly inhibited. Concentration of the ligands is given in inol/L. Part b Surface speciation on lepidocrocite as a function of pH according to Sposito et al. (35). These data suggest that binuclear selenite surface complexes are formed in the neutral pH range (from reference 33).

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. 

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... 

With the experimentally determined values n = 0.27 and m 0.17, j can be estimated j = n/m 1.6. This result indicates that, between pH 3 and 5, catalysis of detachment of the reduced surface iron centers by protons may be the predominant pH effect in this heterogeneous photoredox process, because the estimated j value of 1.6 is not too different from the theoretically expected value of 2 for reduced surface iron centers. However, readsorption of the photochemically formed Fe(II), blocking surface sites for the adsorption of oxalate, also has to be taken into account. Because the extent of adsorption of Fe(II) at the surface of lepidocrocite is expected to increase with increasing pH (11), this effect becomes increasingly important. Thus, at pH 6 a large fraction of the photochemically formed Fe(II) may become readsorbed at the reconstituted lepidocrocite surface before oxalate gets adsorbed. This process may explain the relatively low value of log k0 at pH 6. 

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