Adsorption of uranyl ions and microscale distribution on Fe-bearing mica

The interaction of uranyl ions (UO22+) with Fe-bearing mica is an important determinant of the mobility of U species in granitic rocks or their weathered terrains, especially with regard to radionuclide migration. To understand their interaction, U sorption experiments were conducted on both fresh and oxidized biotites, especially examining an effect of oxidized mineral surfaces which was treated by H2O2 for 3 weeks. The U sorption onto the biotite reached a maximum at around pH 7.0, and the amount adsorbed by the oxidized biotite was much larger than that by the fresh one. The difference of the adsorption capacity between the two biotites may be attributed to an increase in the specific surface area by oxidization, which accompanies some slightly peeled off and reactive surfaces with amorphous precipitates. During the U adsorption reaction, there was a continuous depletion of K+ ions from the interlayer space. At the same time structural Fe was released and oxidized near the edges, forming non-detectable very small goethite particles. Such an incipient feeble crystalline Fe (hydr)oxide phase was only detected by scanning electron microscopy (SEM), but its location interestingly corresponded to the U adsorbed position on the biotite surface, implying its strong adsorption of U. Besides, the crystallized Fe (hydr)oxide seems to be relatively more effective in the adsorption of U as compared with the amorphous one on the H2O2 treated biotite.