Comparative analysis of actinide(VI) carbonate complexation by Monte Carlo resampling methods

Selected data from solubility studies of UO2CO3(s) and NpO2CO3(s) in 0.1 M perchlorate medium are analyzed by balanced Monte Carlo resampling techniques for evaluation of probability density distributions for the solubility products lg K-sp' of AnO(2)CO(3)(s) (An = U, Np) and the formation constants lg beta(101)', lg beta(102)' and lg beta(103)' of solution species AnO(2)COP(3)(0), AnO(2)(CO3)(2)(2-) and AnO(2)(CO3)(3)(4-), respectively. Comparison is made on the basis of non-parametric statistics by Kolmogorov-Smirnov test, Bayesian statistics and Wilcoxon-Mann-Whitney rank order test, thus avoiding assumption of normal probability densities for the variables. Variables for Np(VI) are generally found lower than the respective variables for U(VI), in agreement with both the smaller effective charge of Np(VI) compared to U(VI) and structural properties of actinyl(VI) carbonate species. The quality of data reported in literature is found quite scattered and insufficient to allow a statistically conclusive assessment of the properties of both systems. Large standard deviations reported for analogous Pu(VI) data prevented further interpretation. Hence, this work calls for further studies of these systems.