Redox speciation of uranium with phenylphosphonic acid (PPA) in aqueous medium

Studies on complexation of uranium with organophosphorous ligands in aquatic systems are important from point of view of mobility of uranium in environment. In the present paper, we report the results of complexation of U(VI) by a model ligand for organophosphorus functionalities in humic substances (HS), that is, phenylphosphonic acid (PPA), using electro analytical techniques. The UO22+ has been found to form 1: 1 and 1: 2 complexes with mono-protonated PPA (HPhPO3-) and 1: 1 complex with non-protonated PPA, (PhPO3)(2-), with the latter complex (UO2 PhPO3) dominating over the other two species. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to investigate the redox behavior of UO2 PhPO3 species and to explore the kinetics of its reduction by evaluating heterogeneous electron-transfer kinetic (D, k(s) and alpha) parameters. The diffusion coefficient (D) value was found to be 6.76 x 10(-5) cm(2) s(-1) and 5.03 x 10(-5) cm(2) s(-1) at pH 5 and 3, respectively, with rate constant, k(s) = 0.304 x 10(-3) cm/ s. Using the DeFord and Hume formalism the stability constant (log beta) of UO2PhPO3 was calculated to be (6.98 +/- 0.12), which is in agreement with the literature data. Electrospray ionization mass spectrometry (ESI-MS) studies corroborated the existence of UO2PhPO3 complex.