Lepidocrocite and akaganeite

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). 

Although 2-line ferrihydrite has been used for dissolution studies, 6-line ferrihydrite has, to date, not been investigated. Fischer (1976) compared the dissolution behaviour of three 2-line ferrihydrites in 0.2 M oxalate and found the slowest dissolution rate for a slowly precipitated sample and faster dissolution for rapidly precipitated samples (hydrolysed by fast addition of NH3 or by bacterial oxidation of Fe citrate). Adsorbed silicate reduced the dissolution rate in oxalate probably by blocking surface Fe sites (Schwertmann Thalmann, 1976). 

The rate of reductive dissolution of monodispersed hematite by ascorbic acid (up to 0.5 10 M, 25 °C) increased with increasing coverage by adsorbed ascorbate from 2.4 min at pH 4 to 6.6 min at pH 3 (Suter et al., 1991). Al substitution depressed the rate of reductive dissolution of hematite (Fig. 12.22) (Torrent et al., 1987). The scatter of the data was large and could not be accounted for by variations in other properties of the samples. Cu substitution (0.09 mol mol ) did not influ- 

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