Uranium redox processes and uptake by plant cells

Recently we have shown that uranium can be taken up by plant cells. Fractionation studies showed that the uranium was present in nearly all cell compartments. Nevertheless, luminescence measurements showed that the speciation of the uranium in the several cell compartments differs from each other.
One of the major remaining questions concerns to the ways of uranium uptake. Recently published work /1,2/ proposed that the uranium uptake is influenced by the iron uptake. As it is known that the iron uptake occurs via reduction of the iron(III) into iron(II), we conclude that uranium uptake should also by accompanied by a redox process. First measurements by laser-induced photo-acoustic spectroscopy gave evidence for the presence of uranium(IV) inside the cells.
The formation of uranium(IV) from uranium(VI) is a more complicated redox process, as the oxo-cation uranium(VI) has to be transformed into an oxo-hydrate form. Electrochemically this process is irreversibly. In systems existing at nearly neutral pH additionally hydrolysis or complex formation of the uranium ions occur.
On the other hand the formed uranium(IV) can also be formed by a disproportionation step from uranium(V).

2UO2+ + 4H2O  UO2(OH)2 + U(OH)4 + 2H+

From electrochemical point of view the formation of uranium(V) is a reversible process and the redox potential uranium(VI)/uranium(V) is of the same order as the redox potential iron(III)/iron(II) (values for acidic solution).

UO22+ + e-  UO2+ E0 = - 0.16 V
Fe3+ + e-  Fe2+ E0 = - 0.77 V

However, these values are strongly influenced by pH and complex formation. Uranium(VI) as well as uranium(IV) was detected in cells of Brassica napus.
Therefore we tried to prove the oxidation state uranium(V) by electrochemical and spectroscopic methods.