The effect of secondary iron mineral and colloid formation on uranium sorption during the dissolution of chlorite

Chlorite is a common accessory mineral, commonly found in sedimentary and low grade metamorphic rocks like the phyllite in the former uranium mining area of Saxony (Germany). The cur-rent flooding of the mines leads to an interaction be-tween the uranium containing flooding water and the surfaces of the wall rock. During the weathering of phyllite a secondary mineral is formed, which in further investigations [1] was described as ferrihydrite. The source of this secondary mineral is the iron-rich chlo-rite of the phyllite.
Chlorite is a phyllosilicate with a trioctahedral structure, consisting of negatively charged 2:1 layers that alternate regularly with positively charged interlayer sheets [2]. During the weathering of chlorite the main constituents of the octahedral sheets, Mg2+, Fe2+, Fe3+ and Al3+, are released into the aqueous solu-tions. In laboratory experiments the dissolution of chlo-rite was studied with a continuous flow-through reactor and in batch experiments. In this study we will lay em-phasis on the released iron because of its ability to form secondary minerals and colloids.
Experiments: Batch experiments were carried out on an iron-rich chlorite from Flagstaff Hill, California, with a grain size of 63-200 µm. Half a gram of the samples were added to 40 ml deionized water, which was previously adjusted to an ionic strength of 0.1 M with NaClO4. The suspension was aged during 2 months without any pH adjustment. A pH value of 7 was found at the end of the experiment. The freshly formed iron hydroxide particles were separated from the large excess of the micron-size chlorite powder particles by well-defined centrifugations (2500 x g, 1h and 3500 x g, 1h). A defined volume of the centrifugate was filtered through a 50 nm and 15 nm Nuclepore filter in order to fix the ultrafine iron particles on a substrate. The filter membranes were prepared for scanning electron microscopy (SEM).
Results: In SEM investigations spherical particles are detected on the Nuclepore filter. The size of these colloids ranges from 30 to 80 nm. The colloids are mainly arranged as agglomerates, while individual colloids are rarely observed. On several large agglom-erates energy-dispersive X-ray analysis (EDS) were taken in order to get information about the chemistry of these colloids. With this method we were able to detect directly iron colloids of a chlorite suspension at pH 7. The process leading to the formation of colloids can be explained as a consequence of the iron release during the dissolution of chlorite. The dissolved iron oxidizes to ferric iron which subsequently hydrolyses, precipi-tates and forms secondary minerals and iron colloids. The detection of iron colloids was previously described in a suspension of ground phyllite [3], where a com-bined technique of centrifugation at varying centrifugal speed and ICP-MS and AAS of the centrifugates was used. The formation of iron colloids was prevented in the acid region of pH = 4 due to the higher solubility of Fe in this pH region. With decreasing solubility in the neutral and alkaline medium, the released Fe forms iron colloids or precipitates as secondary iron mineral coatings. The formation of iron colloids during the dissolu-tion of chlorite must be taken into consideration in the assessment of contaminant behavior in the nature. Though contributing only little to the mass balance of a water body, iron colloids can show significant adsorp-tion of trace elements because of their high specific surface area and their affinity to heavy metals.
The formation of secondary iron mineral coatings is also of importance for the behavior of contaminants in the nature since coatings significantly change the sur-face properties of the weathering chlorite. The most common secondary iron mineral is ferrihydrite, a poorly crystallized iron mineral with a chemical for-mula of 5Fe2O3 . 9H2O [4] and a large specific surface area. Sorption experiments which were carried out with ur...