Goethite dissolution

The rate of disproportionation increases with decreasing pH and rising temperature and these conditions also favour oxide reduction. To achieve a reasonable rate of dissolution, one has to compromise on the pH. A pH of 3 is used in kaolin bleaching (Jepson, 1988), whereas in soil analysis the system is usually buffered with citrate and bicarbonate at ca. pH 7 (Mehra Jackson, 1960). Citrate also complexes the dissolved Fe " and prevents its precipitation as Fe" sulphide. For the dithionite/EDTA system, Rueda et al. (1992) found a maximum efficiency at pH 5-6 and an activation energy for goethite dissolution of 70 kj mol". They stressed the importance of adsorption of S2O4 on the surface to ensure reduction of Fe ". 

Feroxyhyte Goethite Dissolution/reprecipitation Alkaline solution... 

Cocozza C., Tsao C. C. G., Cheah S.-F., Kraemer S. M., Raymond K. N., Miano T. M., and Sposito G. (2002) Temperature dependence of goethite dissolution promoted by trihydroxamate siderophores. Geochim. Cosmochim. Acta 66, 431-438. 

Reichard, P. U., Kraemer, S. M., Frazier, S. W., and Kretzschmar, R. (2005). Goethite dissolution in the presence of phytosiderophores rates, mechanisms, and the synergistic effect of oxalate. Plant Soil 276, 115-132. 

Concentrated sulfuric acid is added directly to limonite ore slurry. This substantially dissolves the nickel containing goethite matrix causing over 90% of the nickel and cobalt to leach along with substantial amounts of iron. Leach temperatures close to the boiling point of the liquor are attained, due to the heat of dilution of the acid. The goethite dissolution reaction is represented by equation 1. 

Table 8.3. Acid-consuming reactions in solution, at soil-water interface, and used by microorganisms carbonate buffering (Eqs. 7-9), ion exchange (Eq. 10), silicate weathering (Eqs. 11-12), gibbsite dissolution (Eq. 13), goethite dissolution (Eq. 14), denitrification (Eq. 15) and desulfurication (Eq. 16 Stumm and Morgan 1981, MattheB 1990)...

As in dissolution, a chemical and structural change can occur from hydrolysis as the ions replaced by or OH may be of a different size so that the crystal structure is stressed and weakened. An example of this is the weathering of feldspar or goethite by H ... 

The evolved hardpans (Fig. 1b) are composed by rhythmic alternation of submillimetric goethite-rich (ochreous) and hematite-rich (red) layers. This layering is the result of a complex evolution of the pristine authigenic Fe-oxides and -oxyhydroxides during which the mineral phases are cyclically involved in transformation processes including recrystallization, dissolution and reprecipitation (Carbone etal. 2005). 

Dissolution of goethite by oxalate in the presence of different concentrations of ferrous iron. The reaction mechanism proposed is that of Fig. 9.3d. The change in the concentration of Fe(III) is given (preconditioning of the surface introduces some incipient Fe(III)). pH = 3.0, goethite 0.46 gIt, oxalate 0.001 M. 

Cornell, R. M., and P.W. Schindler (1987), "Photochemical Dissolution of Goethite in Acid/Oxalate Solution", Clays and Clay Minerals 35, 347-352. 

Larson O, Postma D (2001) Kinetics of reductive bulk dissolution of lepidocrocite, ferrihydrite, and goethite. 

The three most abundant minerals forms are Fe203 (hematite), Fe(OH)j (hydrous ferric oxide or ferrihydrite), and FeO(OH) (goethite). The chemical reactions describing their dissolution and K in slightly acidic water at 25°C, 1 atm, and the ionic strength of... 

Fig. 3.2 Fraction of various metals released versus Fe released during acid dissolution of synthetic metal-substituted magnetites (upper six plots Sidhu et al., 1978, with permission), goethites and hematites (lower plots Lim-Nunez dikes, 1987 with permission).

Pores may be present as structural features (e. g. between domains) or as a result of aggregation of particles. They may also be the result of partial dehydroxylation (oxide hydroxides) or dissolution. Although the shapes of pores can be quite variable, there are some definite, basic forms. The commonest of these are 1) slit shaped, the walls of which may or may not be parallel 2) ink bottle which are closed upon all sides but one from which a narrow neck opens and 3) cylindrical. Upon partial dissolution, pores bounded by well-defined crystal planes (e. g. 102 in goethite) develop (Chap. 12). 

XANES spectra provide information about contaminants present at too low a level to produce EXAFS spectra. It has been used to investigate the oxidation of As on Mn-goethite (Sun et al. 1999). This technique has also provided information about the valence of Fe in Fe oxide films during cathodic reduction in a borate buffer (Schmicki et al. 1996), about the dissolution of Fe oxide films in acidic solutions (Vir-tanen et al. 1997) and about the orientation of styrene molecules adsorbed on FeO (111) and Fe304 (111) (Wuehn et al. 2000). 

The extent to which a sparingly soluble solid dissolves is expressed by the solubility product. This describes the equilibrium established between the solid and the concentration of its ions in a saturated solution. Consider, for example, the dissolution of goethite in water ... 

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