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000202213 1001_ $$0P:(DE-Juel1)144040$$aBrandt, F.$$b0$$eCorresponding Author$$ufzj
000202213 245__ $$aReplacement of barite by a (Ba,Ra)SO4 solid solution at close-to-equilibrium conditions: A combined experimental and theoretical study
000202213 260__ $$aNew York, NY [u.a.]$$bElsevier$$c2015
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000202213 520__ $$aSolid 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.
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000202213 536__ $$0G:(EU-Grant)269688$$aSKIN - Slow processes in close-to-equilibrium conditions for radionuclides in water/solid systems of relevance to nuclear waste management (269688)$$c269688$$fFP7-Fission-2010$$x1
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000202213 7001_ $$0P:(DE-Juel1)130364$$aKlinkenberg, M.$$b2$$ufzj
000202213 7001_ $$0P:(DE-Juel1)144076$$aRozov, K.$$b3$$ufzj
000202213 7001_ $$0P:(DE-Juel1)130324$$aBosbach, D.$$b4$$ufzj
000202213 773__ $$0PERI:(DE-600)1483679-8$$a10.1016/j.gca.2015.01.016$$gVol. 155, p. 1 - 15$$p1 - 15$$tGeochimica et cosmochimica acta$$v155$$x0016-7037$$y2015
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