Enhancement of hydrolysis through the formation of mixed hetero-metal species

In order to analyze the formation of hetero-metal polynuclear hydrolytic species, in this paper, we reported some results of an investigation (at I = 0.16 mol L-1 in NaNO3. at t = 25 degreesC by potentiometry, ISE-H+, glass electrode) on the hydrolysis of several mixtures (in different ratios) of two couples of cations: dioxouranium(VI)/copper(II) and dioxoranium(VI)/diethyltin(IV). The elevated total concentrations of cations 0.005 less than or equal to SigmaC(M) mol L-1 less than or equal to 0.05) adopted in these measurements induced us to study again the hydrolysis of uranyl, for which no suitable literature data are available in these particular experimental conditions. All measurements were performed by two different operators. using completely independent instruments and reagents. Many different speciation models were considered in the calculations, including the simultaneous refinement of homo- and hetero-metal species, and a statistical analysis of obtained results was proposed too. Main results can be summarized as follows: UO22+ and Cu2+ form three hetero-metal polynuclear hydrolytic species [(UO2)Cu(OH)(3+). (UO2)Cu(OH)(2)(2+) and (UO2)(2)Cu(OH)(4)(2-), with log beta(pqr), -2.93 +/- 0.01, -7.34 +/- 0.03 and - 13.78 +/- 0.03, respectively], all those common to their simple speciation without the other cation: UO22+ and (C2H5)(2)Sn2+ form seven mixed hydrolytic species [(UO2)(DET)(OH)(3+), (UO2)(DET)(OH)(2)(2+), (UO2)(2)(DET)(OH)(4)(2-). (UO2)(DET)(2)(OH)(4)(2+). (UO2)(2)(DET)(OH)(5)(+), (UO2)(DET)(2)(OH)(5)(+) and (UO2)(2)(DET)(OH)(7)(-), with log beta(pqr) = -2.5 0.2 -4.74 +/- 0.02. -10.70 +/- 0.06. -10.34 +/- 0.03, -15.70 +/- 0.06, -15.58 +/- 10.06 and -27.9 +/- 0.1, respectively] that are of the same kind of those formed by uranyl: formation of mixed hydrolytic species causes a significant enhancement of the percentage of hydrolyzed metal cations, modifying, the solubility and therefore, the bioavailability of these cations. We also determined, for dioxouranium(VI)/copper(II) system, the corresponding complex formation enthalpies and entropies by direct calorimetric measurements. We obtained DeltaH(112) = 47.9 +/- 0.6 and DeltaH(214) 92.9 +/- 0.5 kJ mol(-1). TDeltaS(112) = 6 +/- 1 and TDeltaS(214) = 14 +/- 1 kJ mol(-1) (+/-S.D.), respectively, for the formation of (UO2)(Cu)(OH)(2)(2+) and (UO2)(2)(Cu)(OH)(4)(2-) species (according to reaction 2). We also calculated the single enthalpic and entropic contributes to the extra-stability that these species show with respect to the corresponding homo polynuclear ones. (C) 2004 Elsevier B.V All rights reserved.