Complexation of Pu(IV) with the natural siderophore desferrioxamine B and the redox properties of Pu(IV)(siderophore) complexes

The bioavailability and mobility of Pu species can be profoundly affected by siderophores and other oxygen-rich organic ligands. Pu(IV)(siderophore) complexes are generally soluble and may constitute with other soluble organo-Pu(IV) complexes the main fraction of soluble Pu(IV) in the environment. In order to understand the impact of siderophores on the behavior of Pu species, it is important to characterize the formation and redox behavior of Pu(siderophore) complexes. In this work, desferrioxamine B (DFO-B) was investigated for its capacity to bind Pu(IV) as a model siderophore and the properties of the complexes formed were characterized by optical spectroscopy measurements. In a 1:1 Pu(IV)/DFO-B ratio, the complexes Pu(IV)(H2DFO-B)(4+), Pu(IV)(H1DFO-B)(3+), Pu(IV)(DFO-B)(2+), and Pu(IV)(DFO-B)(OH)(+) form with corresponding thermodynamic stability constants log beta(1,1,2) = 35.48, log beta(1,1,1) = 34.87, log beta(1,1,0) = 33.98, and log beta(1,1,-1) = 27.33, respectively. In the presence of excess DFO-B, the complex Pu(IV)H-2(DFO-B)(2)(2+) forms with the formation constant log beta(2,1,2) = 62.30. The redox potential of the complex Pu(IV)H-2(DFO-B)(2)(2+) was determined by cyclic voltammetry to be E-1/2 = -0.509 V, and the redox potential of the complex Pu(IV)(DFO-B)(2+) was estimated to be E-1/2 = -0.269 V. The redox properties of Pu(IV)(DFO-B)(2+) complexes indicate that Pu(III)(siderophore) complexes are more than 20 orders of magnitude less stable than their Pu(IV) analogues. This indicates that under reducing conditions, stable Pu(siderophore) complexes are unlikely to persist.