Formation of secondary Fe-oxyhydroxide phases during the dissolution of chlorite – effects on uranium sorption

Abstract

The formation of Fe-oxyhydroxide colloids resulting from the simulated weathering of chlorite (CCa-2) was studied in batch experiments. The detected colloids showed a particle diameter of 30–80 nm and were composed of Fe and O. On the basis of their chemistry and their appearance, they were identified as ferrihydrite colloids. Mössbauer spectroscopy measurements were used to quantify the increase in Fe(III) which formed during the batch experiment, i.e. the chlorite platelets in contact with water for 2 months. The detected increase in Fe(III) was attributed to the formation of the newly formed ferrihydrite. Further SEM investigations revealed that the colloids were preferentially attached to the {h k 0} edge surfaces and only to a minor extent to the {0 0 1} basal plane surfaces as isolated Fe-colloids. Additional sorption experiments were conducted to study the effect of newly forming secondary mineral phases on the sorption behavior of U(VI) on chlorite. Two different chlorites with different tendencies for forming secondary phases were studied. The first one was a hydrothermally altered chlorite (Grimsel chlorite) and the second one was an unaltered chlorite (CCa-2 chlorite). The Grimsel chlorite was found to be more resistant to weathering than the CCa-2 chlorite which tends to form a secondary phase as a result of weathering reactions. The U(VI) sorption on the CCa-2 chlorite showed a maximum at pH 6.3–7.5 with up to 87.9 % of the initially added U (1 × 10 −6 M U(VI)) adsorbed. In contrast the Grimsel chlorite showed a sorption maximum at pH 5.8–6.9 with a maximum of 70% U(VI) adsorbed. This greater sorption capacity of the CCa-2 chlorite was attributed to newly forming secondary phases, in particular Fe-oxyhydroxides, emerging during the sorption experiments as a result of weathering reactions. Since adsorbed ferrihydrite particles are well-known environmental sinks for migrating heavy metals, they may enhance the adsorption capacity of the chlorite, and the U transport may thus be significantly retarded. On the other hand, mobile ferrihydrite colloids, which are also a product of chlorite weathering, may enhance U migration.