Enhancement of hydrolysis through the formation of mixed hetero-metal species: dioxouranium(VI)-cadmium(II) mixtures.

In order to continue the investigation on the formation of hetero-metal polynuclear hydrolytic species, in this paper we report some results (at I = 0.16 mol L(-1) in NaNO3, at t = 25 degrees C by potentiometry, ISE-H+, glass electrode) on the hydrolysis of several mixtures (in different ratios) of the dioxouranium(VI) and cadmium(II) cations. The same experimental and calculation procedure of previously investigated systems was followed, and all measurements were performed by two different operators, using completely independent instruments and reagents. Many different speciation models were considered in the calculations, and a simple statistical analysis of obtained results was proposed too. UO2(2+) and Cd2+ form two hetero-metal polynuclear hydrolytic species, namely UO2Cd(OH)3+ and (UO2)2Cd(OH)4(2+), with logbeta(pqr) = -3.25 +/- 0.25 and -13.75 +/- 0.10, respectively. The formation of hetero-metal hydrolytic species is thermodynamically favored with respect to the homo-metal ones, and causes an enhancement of the percentage of hydrolyzed metal cations; comparisons with previously studied systems reveal that the hydrolytic behavior of UO2(2+)/Cd2+ mixtures is more similar to that observed for UO2(2+)/Cu2+ than for UO2(2+)/(C2H5)2Sn2+, and the tendency to form hetero polynuclear hydrolytic species with dioxouranium(VI) by other cations follows the trend (C2H5)2Sn2+ > Cu2+ > or = Cd2+.