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Dissolution of A2Ti2O7 (A = Y3+, Gd3+, or Lu3+) Pyrochlore by Experiment at pH = 2, T = 90°C: Evidence for Solubility Control Using a Linear Free Energy Model

Published online by Cambridge University Press:  21 March 2011

J. P. Icenhower ,
B. P. McGrail ,
W. J. Weber ,
B. D. Begg ,
N. J. Hess ,
E. A. Rodriguez ,
J. L. Steele ,
C. F. Brown  and
and M. J. O'Hara
J. P. Icenhower
Affiliation:
Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A.
B. P. McGrail
Affiliation:
Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A.
W. J. Weber
Affiliation:
Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A.
B. D. Begg
Affiliation:
ANSTO, PMB1, Menai, New South Wales 2234, Australia.
N. J. Hess
Affiliation:
Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A.
E. A. Rodriguez
Affiliation:
Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A.
J. L. Steele
Affiliation:
Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A.
C. F. Brown
Affiliation:
Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A.
and M. J. O'Hara
Affiliation:
Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A.
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Abstract

We performed a series of dissolution experiments with well-characterized pyrochlore ceramics with the formula A2Ti2O7, where A = Y3+, Gd3+, or Lu3+ in H2O- and D2Obased solutions [pH(D) = 2] at 90°C. Normalized log10 dissolution rates (g·m−2·d−1) in H2O-based solutions increase from Lu2Ti2O7 (−3.2 to –3.3) to Gd2Ti2O7 (−2.6 to –2.9), to Y2Ti2O7 (−1.9 to –2.0). Rates in D20-based solutions are indistinguishable from rates in H2O, indicating that release of elements is probably not diffusion controlled. A recent dissolution model, based on ligand-exchange reactions, suggests that the rate of reaction should increase in inverse order of the cation field strength: Lu < Y < Gd (where the cation denotes the appropriate pyrochlore composition), which is not observed. Evaluation of the thermodynamic stability of the three solids was performed using a linear free-energy model and reported free energies of formation. The calculations indicate that reactivity should follow in the progression Lu < Gd < Y, as observed in the dissolution experiments. Our data indicates, therefore, that dissolution models based on ligand-exchange reactions may not be strictly applicable to simple pyrochlore minerals.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 713: Symposium JJ – Scientific Basis for Nuclear Waste Management XXV , 2002 , JJ6.2
Copyright
Copyright © Materials Research Society 2002

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References

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