Complex formation in the ternary U(VI)-F-L system (L = carbonate, oxalate and picolinate)

The formation of ternary complexes and their equilibrium constants, in the system U(VI)-F--L, where L is one of the Ligands picolinate, oxalate or car bonate have been investigated by potentiometric titrations using both F- an d H+ selective electrodes. The experimental results indicated that very sta ble ternary complexes are formed with the composition UO2LFq, q = 1 to 3, a nd UO2L2F for L = picolinate and oxalate. These complexes have a pentagonal bipyramid coordination geometry, with the five exchangeable donor atoms in a plane perpendicular to the linear UO2-group. The equilibrium constants h ave been determined at 25 degrees C in a 1.00 M NaClO4 ionic medium. The eq uilibrium constants for the stepwise addition of F- to UO2L to form UO2LFq, q = 1 to 3, and to UO2L2 to form UO2L2F indicates that the prior coordinat ion of L to U(VI) has a fairly small effect on the subsequent bonding of fl uoride, except for a statistical effect determined by the number of availab le coordination sites. These results indicates that ternary complexes might be important for the speciation and transport of hexavalent actinides in g round and surface water systems.