Experimental Determination and Modeling of the Speciation of Uranium(VI) in Phosphoric Acid Medium

Phosphoric acid solutions with 0 < [H3PO4] < 8 mol L-1 and 0 < U(VI) < 2 mol L-1 are studied by Infra Red (IR) spectroscopy and modelized using an equation of state adapted for electrolytes. Modeling of H3PO4 solutions without uranium, based on a data set combining literature data and original IR measurement, shows that the dimer H5P2O8- has to be considered for H3PO4 apparent concentration higher than 1 mol L-1. When uranium(VI) is present and in the considered concentration range, the results of the data treatment are consistent with the existence of three complexes: UO2(H2PO4)(+), UO2(H2PO4)(2), and UO2(H3PO4)(H2PO4)(2), the last two ones being largely predominant.