Electrochemical and spectroelectrochemical studies on uranyl carbonate and aqua complexes

The electrochemical reduction of uranyl ion has been studied by the cyclic voltammetry (CV) and the spectroelectrochemical method using optically transparent thin-layer electrode (OTTE) in basic carbonate and acidic aqueous solutions. The CV measurements showed that U(V) was formed quasireversibly by the reduction of uranyl ion in both basic carbonate and acidic perchlorate solutions. Uranium(V) was found to be much more stable in the carbonate media and its absorption spectrum was measured by the OTTE cell under the applied potentials varying stepwise from -0.7 to -0.9 V vs. Ag/AgCl (3 M NaCl). On the other hand, in the acidic solution the absorption spectra recorded at 20 s interval by the OTTE cell under the fixed applied potential at -0.4 V vs. Ag/AgCl (3 M NaCl) exhibited only the spectral change from uranyl ion to the final reduction product U(IV). It has been suggested based on the present electrochemical study that uranyl is stable for the redox reaction in the deep groundwater, which is in reductive environment of -0.3 V (vs. NHE) and contains carbonate ion, but is reduced to U(IV) in the acidic groundwater in the same reductive environment. (author)