Formation of neptunium(IV)-silica colloids at near-neutral and slightly alkaline pH.
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.
Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf , Bautzner Landstrasse 400, 01328 Dresden, Germany.
Richard
R
Husar
Stephan
S
Weiss
Christoph
C
Hennig
René
R
Hübner
Atsushi
A
Ikeda-Ohno
Harald
H
Zänker
D000468:Alkalies / Q000737:chemistry*; D003102:Colloids / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008027:Light; D009391:Nephelometry and Turbidimetry; D009405:Neptunium / Q000737:chemistry*; D010316:Particle Size; D017785:Photons; D012542:Scattering, Radiation; D012822:Silicon Dioxide / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D013997:Time Factors; D014461:Ultracentrifugation; D014462:Ultrafiltration; D056928:X-Ray Absorption Spectroscopy
D000468:Alkalies; D003102:Colloids; D009405:Neptunium; D012822:Silicon Dioxide