Akaganeite dissolution

Aicagan ite. can incorporate up to 0.06 mol mol Cu and much smaller amounts of Cr, Mn, Co, Ni and Zn in the structure (Inouye et al., 1974 Holm, 1985 Buch-wald Clarke, 1989). Incorporation of Al, Cr and Ga has also been reported (Lorenz Kempe, 1987). Cornell (1992) produced akaganeite from acid Si-containing Fe " solutions and found by congruent dissolution that up to 0.04 mol mol Si could be incorporated. The Si species were probably located in the tunnels (0.5 nm ) of the akaganeite structure. 

Dissolution curves for akaganeite in 0.5 M HCI at 76 °C were sigmoidal. TEM showed that acid attack was concentrated along the [001] direction initially the tapered ends of the spindle-shaped crystals (see Fig. 4.15 a) became squared and, as dissolution continued, the crystals became shorter (Fig. 12.24a-c). With further dissolution the crystals were gradually hollowed out (Fig. 12.24d) and it is the resulting increase in surface area that is thought to be responsible for the shape of the dissolution curve (Cornell Giovanoli, 1988 a). 

Fig. 12.24 Morphological changes of somatoidal akaganeite C7stals after a) 10 b) 25 c) 50, Cr-shadowed and d) 80% dissolution (Cornell, Ciovanoli, 1988a, with permission).

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

This material contains 2.3 mol% Si. Si is released congruently with Fe upon dissolution and is, therefore, considered to be homogenously distributed within the akaganeite crystals, probably in the tunnels (Cornell, 1992). 

Because of the similar thermodynamic stability of goethite and hematite, formation of hematite competes with nucleation of goethite which proceeds via dissolution of the ferrihydrite precursor. The lower the temperature, the more likely it is that goethite will form. Goethite nucleation can therefore be inhibited by preheating the oven and also all solutions before they are combined. The presence of chloride should be avoided because it promotes the formation of akaganeite (P-FeOOH). However, chloride concentrations below 0.02 M can be tolerated at temperatures of around 100 °C. 

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