Direct spectroscopic speciation of the complexation of U(VI) in acetate solution

Abstract: As a result of systematic UV-Vis absorption spectroscopy studies in the U(VI) acetate system, the single component spectrum of [UO₂CH₃COO]⁺ with characteristic parameters was evaluated and applied in quantitative deconvolution of multicomponent spectra. Free acetate concentrations were obtained by the use of geochemical and probabilistic modelling codes. A total of 51 UV-Vis spectra were collected in a wide range of experimental conditions where coordination of U(VI) by acetate ion was indicated by characteristic variations in the spectra structure as compared to UO₂²⁺. Using chemometric data analysis, the resulting factor structure was evaluated to obtain a subset of 14 spectra holding only one coordinated species next to UO₂²⁺_(aq). The molar absorption coefficient for the U(VI) monoaceto species was estimated as ε₄₁₈ = 17.8 ± 1 dm³ mol^-1 cm^-1. Spectral deconvolution was used to obtain an estimate of the species concentrations which allowed to calculate for each sample the free acetate concentration, the total U(VI) amount and, eventually, to estimate the formation quotient lg β₁₁ = 2.8 ± 0.3 of UO₂(CH₃COO)⁺.