Chlorite dissolution relevant to environmental processes in Uranium tailings: influence on the Uranium(VI) migration

In flow-through reactor experiments the dissolution of a ripidolite chlorite (CCa-2) from Flagstaff Hill (El Dorado County, California, USA) was studied with the intention to approach mineral dissolution conditions occuring in the unsaturated zone of an uranium tailing. The experiments were conducted under atmospheric conditions, a temperature of 25 °C, an ionic strength of 0.1 M, and a pH of 7. In addition the experiments were conducted with and without 1.10-6 M uranium and also with and without a humic acid concentration of 5 mg/L.
During the first eight hours of the experiments high initial dissolution rates calculated from Mg, Si, Al and Fe concentrations were determined. These initial dissolution rates are attributed to reactive surface sites on the freshly crushed chlorite platelets /1/. High initial dissolution rates were obtained when using an aqueous solution with a humic acid or uranium concentration. The initial dissolution rates for Mg and Si are 250 times and 400 times greater than the initial dissolution rates determined in experiments with solutions free of humic acid. In uranium containing solutions an initial dissolution rate for Al was 50 times greater than in solutions free of uranium. It seems that humic acid as well as uranium ions initiate a fast dissolution of the chlorite mineral. A constant dissolution rate was obtained after 8 hours which is almost similar to experiments with solutions free of humic acid and uranium. Average dissolution rates of 3,18.10-12 and 1,86.10-11 mol.m-2.sec-1 were calculated for Mg and Si, respectively. The average dissolution rates for Fe and Al of 7,86.10-13 and 2,87.10-12 mol.m-2.sec-1 respectively seem to be very low. In fact they are primarily higher. But however, they are based on the concentration of Fe and Al in solution and do not consider precipitated Fe and Al phases. Due to the low solubility of Fe and Al in aqueous solutions with a pH > 5 Fe-oxyhydroxides and Al(OH)-phases are precipitating. Fe-oxyhydroxide particles have been detected on the {hk0}-faces of chlorite crystals and as well as immobile colloids in the surrounding solution /2/. Due to their very high specific surface area and their affinity to bind heavy metals the formation of secondary Fe-minerals may significantly influence as Fe-coatings and Fe-colloids the migration of uranium and other toxic heavy metals in uranium tailings. Additional EXAFS investigations of sorbed uranium on chlorite platelets were performed to find out if chlorite or the newly formed Fe-phases are responsible for the immobilization of uranium in the environment of uranium tailings.

/1/ Rochelle, C.A. et al., 1995. Mat. Res. Soc. Symp. Vol. 353, p. 149-156.
/2/ Krawczyk-Bärsch et al.: Formation of secondary iron oxyhydoxide phases during the
dissolution of chlorite - effect on uranium sorption. In: Chemical Geology (submitted).