OXIDATION STATE AND ENVIRONMENTAL CONTROLS OF U INCORPORATED IN FE (OXYHYDR)OXIDES

Growing experimental evidence shows incomensurate metals such as U(VI, V) and Tc(IV) can be incorporated into iron (oxyhydr)oxides either during co-precipitation or redox sensitive transformation of ferrihydrite to stable forms such as goethite. Redox-induced incorporation occurs even at room temp with the oxidation state of incorporated U sensitive to solution Eh.

Reaction relations between incorporated metals and solution species depend on oxidation state of the metal and local charge compensation schemes (CCS). For example, incorporated U(VI) substituting for Fe(III) may be associated with deprotonation of hydroxyl groups and/or Fe(III) vacancy formation. More reduced U(V) or U(IV) may also require local excess electron density. Determination of the local bonding environment of such metals is primarily EXAFS with XANES and XPS constraining oxidation states. XPS proves the most useful for detailed determination of oxidation states of U because satellite structures yield robust signatures for U(IV), U(V) and U(VI) whereas the more commonly used U L_III edge position is not sufficient to resolve U(V). The L_IIIwhite line does contain valence information, but care must be taken to determine the bonding environment which can have a strong effect (e.g., uranyl and uranate display very different white line shapes). While EXAFS encodes information on oxidation states and CCS via local coordination environment, lack of standards for trace U in oxides leads to underconstrained fits and loss of information.

Ab initio MD simulations constrain interpretation of U-L_III EXAFS of U incorporated in goethite. Best fits using dynamical stucture models indicate predominance of U(V) in uranate octahedral coordination with deprotonation and excess electrons providing local CCS. A variety of structures are accessible at RT and must all be considered.