Aluminum from nuclear fuel cladding plays a commanding role in high level radioactive waste chemistry. Solid phases are composed primarily of the Al (oxy)hydroxide minerals boehmite [Al(OOH)] and gibbsite [Al(OH)3], along with poorly defined solid solutions of phases involving Fe and Cr. For solutions in contact with these solids, unexpected speciation, redox chemistry, and interfacial reactivity have been documented. The coordination geometry of Al(III) in gibbsite and boehmite is predominantly octahedral (Oh), and similar geometries are expected for Fe(III) and Cr(III). However, when these solids dissolve into solution under highly alkaline conditions, Al(III) and possibly Fe(III) and Cr(III), form symmetrical tetrahedral hydroxyanions M(OH)4-. Changes in metal ion coordination from Oh in the solid to Td in solution appear to limit the rates of dissolution and precipitation, resulting in conditions far from equilibrium. In situ NMR (17Oand 27Al) studies of the structural changes of Al(III) species in concentrated NaOH solutions will be presented, along with the impact of other counterions (Li+ and K+) to determine the effects of ion size and charge on the distortion of Al(OH)4-. Ex situ ultra-high field 27Al NMR demonstrates that boehmite has only octahedral Al sites on the surface but both penta- and tetra-Al sites are present on the surface of gibbsite, and their relative amount increase as particle size decreases; this may have implications for the recalcitrant dissolution of boehmite in radioactive tank waste processing. In addition, the use of Al, O and Na K-edge, along with Fe and Cr L-edge x-ray absorption spectroscopy to study the coordination numbers, bond distances, type of neighbors and local disorder for Al-phases at the interface with solution will be discussed.

Behavior of nanoscale Al (oxy)hydroxide minerals under highly alkaline conditions as a function of metal substitution, surface coordination and solution chemistry

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.