Goethite stoichiometric

Adsorption of anions on oxides is usually accompanied by the uptake of protons (or the release of hydroxyl ions). The ratio between the number of protons that are coadsorbed and the level of anion adsorption is not usually stoichiometric. Studies of adsorption of oxyanions on goethite as a function of pH appear to indicate that, provided only one adsorbed species is present, the proton anion ratio is related to the mode of adsorption (Rietra et al., 1999). It follows that oxyanions which adsorb with a similar proton/anion stoichiometry, have a similar adsorption mechanism. 

Fe Mossbauer spectra of the mature teeth (Fig. 18) showed two components with different magnetic hyperfine field splitting values. Whereas the major component gave six-line magnetically split spectra characteristic of stoichiometric goethite (a-FeOOH) at all tempera-... 

Iron oxides are most conveniently stored as dry powders. However, after prolonged storage in an air-dry state some metastable forms may transform into more stable ones. For example, ferrihydrite will gradually turn into hematite and goethite when kept in contact with the atmosphere, presumably owing to the presence of adsorbed non-stoichiometric water Fig. 2-1 shows an X-... 

We have seen above (Section 3.2) that precipitation of ferric ions in solution forms an oxyhydroxide that turns into goethite (a-FeOOH) or hematite (a-Fc203) through very different reaction mechanisms. In the presence of ferrous ions, ferric ions form a spinel oxide over a wide range of compositions. Alkalinization at pH > 9 of a Fe(ll)/Fe(lll) = 0.5 mixture forms stoichiometric magnetite Fe304 [102-104]. Small quantities of ferrous ions [Fe(lI)/Fe(III) >0.1] lead to a similar structure but containing vacancies [105,106]. 

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