Abstract
In this study, the aqueous behavior of isolated U60 nanoclusters (K16Li25[UO2(O2)OH]60)-19 was studied under several pH conditions and nanocluster concentrations to determine if the nanoclusters exhibit solid phase buffering behavior or if they exhibit behavior more like aqueous complexes. U60 is a cage cluster consisting of 60 (UO2)(O2)2(OH)2 uranyl polyhedral which share OH and O2 groups with their neighboring uranyl polyhedral, resulting in negatively charged cage clusters whose charge is at least partially offset by K+ and Li+ in the aqueous phase. Batch experiments to monitor nanocluster stability were conducted for 16days at pH 7.5, 8.0 and 8.5 at nanocluster suspension concentrations of 1.4, 2.8 and 6.0g/L. The aqueous concentrations of U, Li, and K, determined after 10kDa molecular weight filtration, achieved steady-state with the nanoclusters within 24h. The steady-state aqueous U, Li, and K concentrations were independent of solution pH, however they increased with increasing nanocluster concentration, indicating that the nanoclusters do not buffer the aqueous activities as a bulk solid phase would, but exhibit behavior that is more characteristic of dissolved aqueous complexes. The ion activity product (I.A.P.) value was calculated using two approaches: (1) treating the nanoclusters as a solid phase with an activity of one, and (2) treating the nanoclusters as aqueous complexes with a non-unit activity equal to their concentration in solution. The I.A.P. values that were calculated with non-unit activity for the nanoclusters exhibited significantly less variation as a function of nanocluster concentration compared to the I.A.P. values calculated with a nanocluster activity of one. The results yield a calculated log dissociation constant for the U60 nanoclusters of 9.2+0.2/-0.3 (1σ). Our findings provide a better understanding of the thermodynamic stability and behavior of U60 nanoclusters in aqueous systems, and can be used to estimate the dissociation behavior of nanoclusters under a range of aqueous conditions.
| Original language | English |
|---|---|
| Pages (from-to) | 94-105 |
| Number of pages | 12 |
| Journal | Geochimica et Cosmochimica Acta |
| Volume | 156 |
| DOIs | |
| State | Published - May 1 2015 |
| Externally published | Yes |
Funding
This material is based upon work supported as part of the Materials Sciences of Actinides Center, an Energy Frontier Research Center funded by the U.S. Department of Energy , Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001089 . ICP-OES analyses were conducted at the Center for Environmental Science and Technology at the University of Notre Dame. Electrospray Mass Spectrometry was performed in the Mass Spectrometry and Proteomics Facility at the University of Notre Dame supported by the National Science Foundation ( CHE-0741793 ). Three journal reviews and suggestions made by editor Marc Norman significantly improved the presentation of the research, and are greatly appreciated.