Spectroscopic investigations of Np(V/VI) redox speciation in hyperalkaline TMA-(OH, Cl) solutions

The redox chemistry of Np(V/VI) was investigated in similar to 0.6 M tetramethylammonium hydroxide/chloride (TMA-(OH, Cl)) solutions with 9 <= - log[H+] <= 13.5. Redox conditions were defined by the absence or presence of ClO- as oxidizing agent (Na-salt, 5 x 10(-3) M and 5 x 10(-2) M). The high total Np concentration ([Np](tot) similar to 2 x 10(-3) M) led to the precipitation of solid phases in some of the samples. The carbonate concentration (as impurity of TMA-OH) was 2-3 x 10(-3) M. UV-vis/NIR spectra obtained from the supernatant in TMA-(OH, Cl) solutions and absence of ClO- showed clear Np(V) features, identified as NpO2+, NpO2CO3- and (NpO2)(x)(CO3)(y)(OH)(z)(x-2y-z). No NIR features were observed within 800 nm <= lambda <= 1300 nm for samples with ClO-. XANES edge energies and features of these samples confirmed the predominance of Np(V) in the absence of ClO- and Np(VI) in the presence of ClO-, by comparison to XANES reference spectra of Np(III/IV/V/VI) prepared within the present work by in-situ electrolysis. A similar Np redox distribution was observed for the solid phases based on XANES and EXAFS measurements. EXAFS spectra indicative of (NpO2OH)-O-V(s) and (NpO3)-O-VI center dot xH2O(s) were obtained for samples in absence and presence of ClO-, respectively. The formation of a Na-Np(VI) phase in 5 x 10(-2) M ClO- and - log[H+] similar to 12 was also indicated from the EXAFS, chemical analysis and SEM-EDS. These results indicate that Np(VI) aqueous species and solid compounds prevail far below the oxidation border of water in alkaline solutions and also far below the E-H border calculated with the current NEA data selection [1]. These observations are further supported by correlations of literature thermodynamic data for actinides (U, Np, Pu and Am), which predict the formation of NpO2(OH)(3)(-) and NpO2(OH)(4)(2-) aqueous species with stability constants (log* beta(1,3)degrees and log* beta(1,4)degrees) similar to those available for U(VI).