Other
Scientific paper
Oct 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011gecoa..75.5389r&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 75, Issue 19, p. 5389-5402.
Other
1
Scientific paper
The fate of divalent radium (Ra 2+ ) in high ionic strength systems is an important issue with relevant aspects to environmental concerns. As a consequence, the description of the thermodynamic properties of Ra 2+ in such systems is a critical matter which was not addressed yet. This can be done using the Pitzer formalism, which is regarded as the most accurate approach to describe activity coefficients of aquatic species in high ionic strength solutions. Isopiestic measurements, measurements of the electromotive force or the solubility of RaSO 4,S in NaCl solutions, which are essential for the evaluation of the Pitzer ion interaction parameters, were not experimentally tested, due to the high radioactivity concentration which involves such experiments. Therefore, the present study examined the possibility to extrapolate the RaCl 2 cation-anion interaction parameters ( β (0) , β (1) and C φ ) by linear regression of other MCl 2 cation-anion interaction parameters with their respective hydrated ionic radii (M 2+ = Mg 2+ , Ca 2+ , Sr 2+ and Ba 2+ ). γ Ra 2 + calculated with the estimated RaCl 2 parameters revealed that the ratios of activity coefficients γ Mg 2 + / γ > γ / γ > γ / γ > 1 > γ / γ follow the opposite order of the ratios of hydrated ionic radii R Mg 2 + / R < R / R < R / R < 1 < R / R . The activity coefficient of Ra 2+ , γ Ra 2 + , calculated with these estimated values is used to test the limitation of the common assumption that the activity coefficients ratio, γ Ra 2 + / γ , is close to unity. It is found that the common assumption of γ Ra 2 + / γ = 1 may be considered reasonable up to high NaCl concentration. The largest deviation in the γ Ra 2 + / γ ratio from this assumption was of the order of ˜25% in the range of halite solubility (6 mol kg -1 of NaCl). However, experimental results of co-precipitation of Ra 2+ in barite are described with more accuracy when the estimated RaCl 2 Pitzer parameters are used. These results are described in a consecutive paper of this journal.
Ganor Jiwchar
Metz Volker
Rosenberg Yoav O.
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