Processes at interfaces: the effect of carbonate on the dissolution of iron and uranium oxides

Full text of publication follows: Iron oxides and hydroxides can strongly affect the mobility of radionuclides and, in particular of uranium in oxic groundwaters. The iron cycle is one of the most dynamic systems in the geosphere. Precipitation and dissolution of iron(III) oxy-hydroxides is normally a fast process, and transformations of the ferric phases can lead to immobilisation-mobilization of the trace components associated to these solid phases. Therefore, the understanding of the effect of the different parameters affecting the dynamics of the iron(III) system is paramount to address the migration of the trace components affected by this system. One of the parameters that importantly affects the behaviour of iron(III) solids is carbonate. This ligand, besides forming strong complexes with some radionuclides such as uranium, also forms stable aqueous species with Fe(III). Although the uranium-carbonate complexation has been extensively studied, and the stability of the different species formed is well established, the speciation of Fe(III) in the presence of carbonate is not sufficiently well understood and, therefore, its effect on the mobilization of associated radionuclides is neither sufficiently well addressed. The work presented is focused on the understanding of the effect of carbonate in the solubility and kinetics of dissolution of iron(III) oxides. The results of laboratory experiments on the effect of carbonate on the solubility and kinetics of dissolution of ferri-hydrite and hematite are presented. From the interpretation of the experimental data, the following conclusions have been drawn: 1.- The solubility of ferri-hydrite and hematite is enhanced in the presence of CO2 mainly due to the formation of two different aqueous complexes, FeOHCO3 (aq) and Fe(CO3)33- with the following stability constants: Fe3+ + yH2O + zCO32- ↔ Fe(OH)y(CO3)z3-y-2z + yH+; logβ0 FeOHCO3 = 10.76 ± 0.38; logβ0 Fe(CO3)33- = 24.24 ± 0.42. 2.- The rate of dissolution of ferri-hydrite, a U(VI)-bearing ferri-hydrite and of schoepite (U(VI) oxide) is enhanced under the presence of CO2. Fractional orders on the hydrogeno-carbonate concentration have been determined for all the studied solids, what is likely an indication of a surface-mediated and hydrogeno-carbonate-promoted dissolution mechanism. The empiric rate laws determined from the experiments are: Ferri-hydrite: rFe =6.45.10-10. [HCO3-]0.26 [mol.m-2.h-1]; Co-precipitate: rU =2.09.10-8. [HCO3-]0.74 [mol.m-2.h-1]; Schoepite: rU =5.13.10-5. [HCO3-]0.41 [mol.m-2.h-1]. (authors)