Formation of iron oxyhydroxide particles during the dissolution of chlorite: Effect on uranium sorption

The abandoned uranium mines in the Western Erzgebirge in Germany are mostly related to the rock phyllite. Currently, many of these mines are being flooded. The flood water, which contains concentration of uranium penetrates through cracks and fissures of the phyllite and leads to a dissolution of several minerals. One of the major components in the phyllite is an iron-rich chlorite [1], which dominates the sorption behaviour for heavy metals.
During the dissolution of the chlorite a reaction causes the loss of octahedral layer cations, primarily Fe. Ferrous iron is released in the aqueous solution, where it is oxidized rapidly to ferric iron. Hydrolysis of ferric iron leads to Fe-oxyhydroxide, which precipitates from the solution as due to the low solubility of iron in the pH region higher 4 [2]. By scanning electron microscopy, these precipitates of Fe-oxyhydroxide are detected as small spherical particles of ferrihydrite that are preferentially situated as immobile coatings on the most reactive edge surfaces of the chlorite crystals. During the dissolution of chlorite a mobile component of ferrihydrite is formed, too. These are colloids, which are found under similar geochemical conditions. The mobile colloids are aqueous species that do not adsorb to rock surfaces. The adsorption of contaminants, for example uranium onto the colloidal particles of ferrihydrite influences and enhances the rate of contaminant transport. In previous investigations the uptake of uranium on ferrihydrite reaches almost 95-100 % in a pH range from 5.0 to 7.5 [1]. Conclusively, the formation of secondarily formed ferrihydrite as mobile colloids and as immobile coatings during the dissolution of chlorite in the rock phyllite is an important process which has to be considered in risk management.
References:
[1] Arnold, T. et al. (1998): Chemical Geology 151,
129-141.
[2] Zänker, H. et al. (2000): Aquatic Geochemistry
(submitted).