From uranyl minerals to nanoscale cage clusters in aqueous solution - Peter C. BUrns - 2 mars 2015

Peter C. Burns - University of Notre Dame (USA)

Lundi 2 mars 2015, 10 h 30

IMPMC, Université P. et M. Curie, 4, Place Jussieu, 75005 Paris

Salle de conférence, 4^e étage, Tour 22-23, Salle 1

Abstract

From uranyl minerals to nanoscale cage clusters in aqueous solution The alpha radiolysis of water creates peroxide that is incorporated into the two known peroxide minerals,
studtite and metastudtite. Natural radioactivity is sufficient for formation of these minerals as components of complex uranyl mineral assemblages. They are also known to form where anthropogenic nuclear materials, such as the Chernobyl “lava” in Ukraine, and spent fuel rods at the Hanford site, contact water. Under laboratory conditions, they have formed on spent nuclear fuel placed in deionized water, on UO2  when irradiated from an external source, and on UO2  containing short-lived alpha emitters.

We discovered in 2005 that uranyl peroxide polyhedra spontaneously self-assemble into complex nanoscale cage clusters in aqueous systems. The primary difference, relative to systems that produce only studtite, is the presence of alkali or alkaline counterions that template cage formation and balance cage charge in solution. To date we have isolated more than 100 types of uranyl peroxide cage clusters containing as many as 124 uranyl ions, in which uranyl ions are bridged through peroxide, hydroxyl, phosphate, pyrophosphate, nitrate, oxalate, phosphite, transition metal polyhedra, etc. Many of these clusters are remarkably stable in
aqueous solution, where they persist for many months, even under elevated temperature and pressure. Uranyl cage clusters exhibit remarkably high solubility in aqueous solutions. Our solubility measurements have indicated that as much as 175,000 ppm uranium can be present in solution in contact with clusterbearing crystals, which is several orders of magnitude higher than expected for aqueous-uranyl mineral systems. Simple geometrical arguments indicate that the solubility of such clusters is limited only by spatial considerations, as their concentration can be estimated from their diameter and allowance for a electrical
double layer of about 1 nm thickness.

We have studied the reaction of several uranyl minerals with aqueous solutions over a range of pH, both under conditions were clusters could form and conditions where they are not expected to form. Where clusters form in solution (as demonstrated by mass spectrometry), uranyl mineral dissolution rates are dramatically higher and the resulting solutions contain several thousand ppm uranyl.