Gast ::
| Hauptseite > Publikationsdatenbank > Replacement of barite by a (Ba,Ra)SO4 solid solution at close-to-equilibrium conditions: A combined experimental and theoretical study |
| Hauptseite > Publikationsdatenbank > Replacement of barite by a (Ba,Ra)SO4 solid solution at close-to-equilibrium conditions: A combined experimental and theoretical study |
| Journal Article | FZJ-2015-04502 |
; ; ; ;
2015
Elsevier
New York, NY [u.a.]
This record in other databases: 
Please use a persistent id in citations: doi:10.1016/j.gca.2015.01.016
Abstract: Solid solution formation between RaSO4 and BaSO4 has long been recognized as a process which potentially controls the Raconcentration in the environment. Here, we have systematically studied the replacement of pure barite by a (Ba,Ra)SO4 solidsolution in 0.1 M NaCl through batch experiments extending up to 883 days at close-to-equilibrium (CTE) conditions, whichare relevant to disposal of nuclear waste in a deep geological repository. Kinetic and thermodynamic models were applied tosupport the interpretation of the experiments, which were carried out at room temperature and at two distinct solid/liquid ratio(0.5 or 5 g/L). Different stages of recrystallization were observed, based on the rate of removal of Ra from aqueous solution.After a first slow kinetic step, a change in the slope of the aqueous Ra concentration vs. time is observed, suggesting nucleationof a new (Ba,Ra)SO4 phase from supersaturation. If this stage was considered to reflect equilibrium between aqueous and solidsolution, one would infer ideality or even negative interaction parameters (a0 6 0). After this fast nucleation step, in the 0.5 g/Lexperiments the Ra concentrations in the aqueous solution slowly increase, approaching a concentration close to that requiredfor equilibrium with a regular (Ba,Ra)SO4 solid solution with an interaction parameter a0 = 1.0. Therefore, these data suggest anon-equilibrium Ra entrapment during the nucleation phase of the replacement, followed by slow recrystallization toward truethermodynamic solid solution equilibrium. Moreover, an interaction parameter value of a0 = 1.0 was inferred from our experiments,which is in good agreement with theoretical predictions from atomistic simulations.A key result from this study is that aqueous solution and binary (Ba,Ra)SO4 approach full thermodynamic equilibriumwithin laboratory time scales (2.5 years). This justifies assuming complete thermodynamic equilibrium for this system in geochemicalcalculations of processes occurring on geological time scales. This finding is of direct relevance for the safety assessmentof radioactive waste disposal, since it may constrain the solubility and thus the mobility of Ra in such environments.
; Current Contents - Physical, Chemical and Earth Sciences ; IF < 5 ; JCR ; NCBI Molecular Biology Database ; SCOPUS ; Science Citation Index ; Science Citation Index Expanded ; Thomson Reuters Master Journal List ; Web of Science Core Collection
|
The record appears in these collections: |
PDF
PDF (PDFA)