The dilution of aqueous neptunium carbonate complexes induces the intrinsic formation of nanocrystalline neptunium dioxide (NpO2) particles, which are characterised by UV/Vis and X-ray absorption spectroscopies and transmission electron microscopy. This new route of nanocrystalline NpO2 formation could be a potential scenario for the environmental transport of radionuclides from the waste repository (i.e. under near-field alkaline conditions) to the geological environment (i.e. under far-field neutral conditions).

The reducing conditions in a nuclear waste repository render neptunium tetravalent. Thus, Np is often assumed to be immobile in the subsurface. However, tetravalent actinides can also become mobile if they occur as colloids. We show that Np(IV) is able to form silica-rich colloids in solutions containing silicic acid at concentrations of both the regions above and below the "mononuclear wall" of silicic acid at 2 × 10(-3) M (where silicic acid is expected to start polymerization). These Np(IV)-silica colloids have a size of only very few nanometers and can reach significantly higher concentrations than Np(IV) oxyhydroxide colloids. They can be stable in the waterborne form over longer spans of time. In the Np(IV)-silica colloids, the actinide--oxygen--actinide bonds are increasingly replaced by actinide--oxygen--silicon bonds due to structural incorporation of Si. Possible implications of the formation of such colloids for environmental scenarios are discussed.

Glutathione (GSH), a ubiquitous intracellular reducing tripeptide, is able to reduce hexavalent uranium, U(vi), to its tetravalent form, U(iv), in aqueous media in vitro, inducing the formation of nanocrystalline mixed-valence uranium oxide particles. After the initial reduction to U(v) and subsequent dismutation, the yielded U(iv) rapidly hydrolyses under near-neutral conditions forming 2-5 nm sized nanoparticles. The latter further aggregate to 20-40 nm chain-like building blocks that finally arrange as network-like structures.

Separation processes based on room temperature ionic liquids (RTIL) and electrochemical refining are promising strategies for the recoveryo fl anthanides from primary ores and electronic waste. However,t hey require the speciation of dissolved elements to be knownw ith accuracy. In the present study,E uc oordination and Eu III /Eu II electrochemical behavior as af unction of water contenti n1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm] [NTf 2 ]) was investigatedu sing UV-visible spectrophotometry, time-resolvedl aser fluorescence spectroscopy,e lectrochemistry,a nd X-ray absorption spectroscopy. In situ measurements were performed in spectroelectrochemicalc ells. Undera nhydrous conditions, Eu III and Eu II were complexed by NTf 2 ,f orming EuÀOa nd EuÀ(N,O) bonds with the anion sulfoxide functiona nd Na toms, respectively.T his complexation resulted in agreater stabilityofEu II ,and in quasi-reversible oxidation-reduction with an E 0 'p otential of 0.18 V versust he ferrocenium/ferrocene (Fc + /Fc) couple. Upon increasingw ater content,p rogressivei ncorporation of water in the Eu III coordination sphereo ccurred. This led to reversible oxidation-reduction reactions, but also to ad ecreasei n stabilityo ft he + II oxidation state (E 0 ' = À0.45 Vv s. Fc + /Fc in RTIL containing 1300 mm water).