The solubility and redox reactions of Pu(IV) hydrous oxide were analyzed by comparing total Pu concentrations, oxidation state distributions and simultaneously measured redox potentials under air and under Ar containing only traces of oxygen. At pH>3 the aqueous Pu concentration is dominated by Pu(V) for both solubility studies under air and argon. Combining all information strongly indicates that PuO_2+x(s,hyd), mixed valent (Pu^V)_2x(Pu^IV)_1-2xO_2+x(s,hyd) or (PuO_2.5)_2x(PuO₂)_1-2x(s,hyd) withx= 0.003 (present study) andx= 0.05–0.06 (literature studies under air), is the solubility controlling solid phase. It can be formed by the oxidation of PuO₂(s,hyd) with the oxygen in the system and by co-precipitation of Pu(V) and Pu(IV). The Pu⁴⁺concentration is given by the known solubility product of Pu(IV) hydrous oxide and the PuO₂⁺concentration is described by the solubility product (K_sp= [PuO₂⁺][OH^-]) for the fraction of Pu(V) in PuO_2+x(s,hyd): logK°_sp= −14.0±0.8 at zero ionic strength. Small Pu(IV) colloids/polymers present in neutral to alkaline solutions at a constant level of log[Pu(IV)]_coll= −8.3±1.0 play an important role for the redox potentials in these systems. Similar Pu(V) concentrations and redox potentials were reached from oversaturation in initially Pu(VI) solutions.

The chemical behavior of ternary Ca–UO₂–CO₃ complexes was investigated by using time-resolved laser fluorescence spectroscopy (TRLFS) in combination with EDTA complexation at pH 7–9. A novel TRLFS revealed two distinct fluorescence lifetimes of 12.7 ± 0.2 ns and 29.2 ± 0.4 ns for uranyl complexes which were formed increasingly dependent upon the calcium ion concentration, even though nearly indistinguishable fluorescence peak shapes and positions were measured for both Ca–UO₂–CO₃ complexes. For identifying the stoichiometric number of complexed calcium ions, slope analysis in terms of relative fluorescence intensity versus calcium concentration was employed in a combination with the complexation reaction of CaEDTA²⁻ by adding EDTA. The formation of CaUO₂(CO₃)₃ ²⁻ and Ca₂UO₂(CO₃)₃(aq) was identified under given conditions and their formation constants were determined at I = 0.1 M Na/HClO₄ medium, and extrapolated to infinitely dilute solution using specific ion interaction theory (SIT). As a result, the formation constants for CaUO₂(CO₃)₃ ²⁻ and Ca₂UO₂(CO₃)₃(aq) were found to be log β⁰ ₁₁₃ = 27.27 ± 0.14 and log β⁰ ₂₁₃ = 29.81 ± 0.19, respectively, providing that the ternary Ca–UO₂–CO₃ complexes were predominant uranium(VI) species at neutral to weakly alkaline pH in the presence of Ca²⁺ and CO₃ ²⁻ ions.

Tetravalent plutonium readily undergoes hydrolysis even in highly acidic aqueous solutions. In the past, many attempts were made to quantify hydrolysis species by means of optical absorption spectroscopy. In the present work solutions ranging from 10⁻⁵M to 10^-2M (total Pu) concentration in 0.5 M HCl/NaCl (0.3 < pH_c< 2.1) are carefully investigated by combining absorption-spectroscopy (UV-Vis, liquid core waveguide capillary) and laser-induced breakdown detection, with special emphasis on the limited solubility of Pu(IV). The results clearly indicate that all changes in the absorption spectra originate from the formation of Pu-polyspecies and colloids. The molar absorptivity of mononuclear Pu(IV) hydroxide complexes does not vary with increasing pH_cand ongoing hydrolysis. The normalized absorption spectra of at least the first and the second hydroxide complex (Pu(OH)_n^4-nn= 1, 2) do not differ from those of the hydrated Pu⁴⁺ion.

The high tendency of tetravalent plutonium to form polymeric complexes and colloids is well known but the exact processes underlying their formation are still controversially discussed. In the present work, the nucleation of small polynuclear hydroxide complexes, i.e. , ionic species containing more than one Pu ion, their aggregation and formation of larger colloids (polymers exceeding some 5 nm in size) and finally ripening processes of freshly formed amorphous Pu(IV) colloids towards more crystalline particles are investigated by use of a combination of various spectroscopic techniques. By electrospray mass-spectrometry small polymers such as dimers, trimers and tetramers containing mixed oxidation states of Pu were observed. These polymers might be responsible for the equilibration between the Pu(III)/Pu(IV) and the plutonyl species Pu(V)/Pu(VI) even in dilute solutions in the absence of colloids or precipitates.