New insights in the formation processes of Pu(IV) colloids

Abstract

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.

Gas-Phase Coordination Complexes of UVIO22+, NpVIO22+, and PuVIO22+ with Dimethylformamide

“…The first report appeared in 2003, where ESI/MS was used to identify the molecular structure of radiolytic products of irradiated TRPOkerosene systems [16]. More recently, Walther et al employed ESI/MS to characterize tetravalent plutonium polymer growth [18].…”

The redox potential of Pu containing acidic solutions and the fate of "Pu(IV)-colloids": Direct measurement versus optical absorption spectroscopy

“…The reaction Pu(IV) coll ⇔ Pu(IV) aq is directly plausible. The equilibrium with Pu(V) may be understood through the existence of hyperstoichiometric PuO 2+x reported in [16] and further supported by the observation of small mixed valence Pu(IV)/Pu(V) polymers described in [13] (for species > 5nm the term colloid is used rathetr than polymer). Whether Pu(III) or Pu(VI) is formed at the expense of the colloidal fraction is determined by the redox potential in the solution.…”

“…3) show that Pu(V) contributions are negligible in all samples. At time zero all samples contain, presumably amorphous [13], Pu IV -oxo-hydroxide colloids ("colloids", magenta) at a concentration level of some [Pu coll ] = 4×10 -5 M. Over time the largest fraction of these colloids dissolve.…”

“…[Pu(IV) coll ] was obtained from the optical absorption at 630nm. The analytical errors for concentration determination are about 3% for Pu(III), Pu(IV) and Pu(VI), and considerably larger (some 10%) for the Pu(IV) coll fraction, since the absorption cross section depends on age and composition of the colloids [13].…”

Particulate Phases Possibly Conveyed from Nuclear Waste Repositories by Groundwater

“…There has been however noticed that small polymers such as dimers, trimers and tetramers include mixed oxidation states of Pu (Walther et al, 2009). In natural waters with pH ranging between 6 and 8, Pu(IV) prevalently occurs under the form of true colloids.…”