The effect of chlorite dissolution on the sorption behavior of U(VI)

In the Western Erzgebirge in Germany the abandoned uranium mines are mostly related to the rock phyllite. Currently, many of these mines are being flooded. The flood water is contaminated with uranium and penetrates the phyllite through cracks and fissures. The contact of the flood water with the rock leads to dissolution processes. One of the major components in this phyllite is an iron-rich chlorite. Chlorites are 2:1 sheet silicates, with an octahedral sheet sandwiched between two tetrahedral sheets. In contact with water chlorites tend to dissolve leading to a loss of octahedral layer cations, i.e. Al3+ with Mg2+, Fe2+ and Fe3+ substitutions. During the dissolution the chlorites are progressively degenerating to vermiculite and secondary iron minerals. In previous studies these secondary iron minerals have been determined as a poorly-crystalline iron oxide hydroxide, like ferrihydrite, which has a significant influence on the uranium migration behavior.
In flow-through experiments the kinetics of the U(VI) uptake on chlorite were studied to show the sorption behavior of U(VI) during the dissolution of chlorite within the phyllite of the abandoned uranium mines. These flow-through experiments were carried out under oxic conditions and at room temperature, using a NaClO4-solution with an ionic strength of 0.1 M and a pH of 7. An UO2(ClO4)2 solution was added to reach an initial U(VI) concentration of 1*10-6 M. The dissolution rates calculated for Al, Mg and Si are decreasing during the first hours of the experiment. Due to the precipitation of iron oxide hydroxides in a pH-range > 5 the released Fe cations have not been determined in the reacted solution. The dissolution rate of Fe is for this reason remaining low during the experiment. The formed secondary iron minerals are responsible for the remarkable high uptake of U(VI) on chlorite. In the first 24 h period of the experiment 72 % of the totally uranium was sorbed. In the following hours the relative amount of the sorbed uranium decreases, since most of available sorption sites of the chlorite and of the secondarily formed iron oxide hydroxides were already occupied. Additional sorption of U(VI) out of solution on the solid chlorite during the flow-through experiment is therefore decreasing. Conclusively, the chlorite within the phyllite is retarding under oxic conditions most of the U(VI) during the flooding, due to the dissolution of chlorite and the subsequent precipitation of iron oxide hydroxides.