Surface Bond Structure of Uranium Adsorbed onto Mine Water Colloids

The sorption complexes of uranium to colloidal ferrihydrite (Fh) were investigated by EXAFS and ATR FT-IR spectroscopy in mildly acidic solution (pH 5.5) in presence and in absence of atmospheric CO2. EXAFS has shown that sulfate, silicate, and carbonate are not involved as ligands bidentately bound onto the adsorbed U. Hence a new surface bond structure of U on hydrous ferric oxides is proposed which tightly fits the experimental EXAFS data. However, ATR FT-IR spectra suggest ternary uranyl carbonato surface complexes under these conditions open to the atmosphere. The conflicting spectroscopic results can be harmonized by proposing a monodentate linkage of the carbonate ligands.