Goethite nucleation

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. 

The foreign species act in solution and usually retard nucleation or growth of goethite by competing with soluble Fe " species for sites on the subcritical nucleus or on the growing crystal. This mechanism is independent of the presence of ferrihydrite. 

Gunneriusson, L. (1994) Composition and stability of Cd(II) chloro and Cd(II) hydroxo complexes at the goethite (a-FeOOH)/water interface. J. Colloid Interface Sd. 163 484-492 Gunten, U. von Schneider, W. (1991) Primary produds of oxygenation of iron(II) at an oxic/ anoxic boundary nucleation, agglomeration and ageing. J. Colloid Interface Sd. 145 127-139... 

Fe and Fe solutions should be used immediately after preparation. If Fe solutions are allowed to stand, some nucleation of goethite can take place, even at room temperature. If Al-substituted goethite is being prepared, for example, the presence of such seed crystals of goethite in an aged Fe solution will prevent a imiformly substituted product from being obtained. Aged Fe° solutions will, of course, be partly oxidized. 

Examples of both homogeneous and heterogeneous nucleation exist in the iron oxide system. Goethite precipitates directly from soluble ferric species in solution. Its formation can, however, be assisted by addition of seed crystals of goethite to the system. As the interplanar spacings in the... 

Bruemmer et al. (55) studied Ni, Zn, and Cd sorption on goethite, a porous iron oxide known to have defects within the structure in which metals can be incorporated to satisfy charge imbalances. At pH 6, as reaction time increased from 2 hours to 42 days (at 293K), sorbed Ni increased from 12 to 70% of Ni removed from solution, and total increases in Zn and Cd sorption over this period increased 33 and 21%, respectively. The kinetics of Cd, Zn, and Ni were described well with a solution to Pick s second law (a linear relation with the square root of time). Bruemmer et al. (55) proposed that the uptake of the metal follows three-steps (i) adsorption of metals on external surfaces (ii) solid-state diffusion of metals from external to internal sites and (iii) metal binding and fixation at positions inside the goethite particle. They suggest that the second step is the rate-limiting step. However, they did not conduct microscopic level experiments to confirm the proposed mechanism. In view of more recent studies, it is likely that the formation of metal-nucleation products could have caused the slow metal sorption reactions observed by Bruemmer et al. (55). 

Here P and G are the minerals pyrite and goethite while X, F and T represent the mobile oxygen, Fe and thiosulfate species. Initially in the aquifer G = 0 and upon mobilization of F, a supersaturation with respect to goethite may occur so that G is nucleated. The interesting aspect of this situation in the present context is that the G content behind the advancing pyrite redox front can be oscillatory. 

The quantity g allows for nucleation of G. If F.X > Q, Qn being the critical value of the goethite equilibrium constant Q for nucleation, then g is a small number go whereas g vanishes other-... 

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