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Hydration change reaction of light REE (super 3+) (aq) series; I, An important constraint on REE(III) partitioning and complex formation reactions in low-temperature to hydrothermal condition

Hydration change reaction of light REE (super 3+) (aq) series; I, An important constraint on REE(III) partitioning and complex formation reactions in low-temperature to hydrothermal condition

The Journal of Earth and Planetary Sciences, Nagoya University 53: 33-50

The hydration state of light REE (super 3+) (aq) is changing across the series, and each light REE (super 3+) (aq) is a mixture of two distinguishable species of nonahydrate and octahydrate in equilibrium with respect to the hydration change reaction of light REE (super 3+) (aq). Two sets of thermodynamic data for the hydration change of light REE (super 3+) (aq) at 25 degrees C and 1 atm have been reported: one is the data set for the reaction [REE(H (sub 2) O) (sub 8) ] (super 3+) (aq)+H (sub 2) O(1) = [REE(H (sub 2) O) (sub 9) ] (super 3+) (aq) evaluated by Miyagawa et al. (1988) on the basis of the hydration enthalpies and free energies for the two hydrate series calculated from electrostatic bonding energies and Born's hydration theory. The other is the data set for the conventional reaction that octahydrate REE (super 3+) (aq) = REE (super 3+) (aq), which was evaluated by Kawabe (1999a) from the thermodynamic data for solution of the fully isomorphous series of REE(ES) (sub 3) .9H (sub 2) O (ES = C (sub 2) H (sub 5) SO (super -) (sub 4) ) and the partly isomorphous ones of REECl (sub 3) .nH (sub 2) O (n = 7 for La-Pr and n = 6 for Nd-Lu) and their least-squares fittings to the improved RSPET (refined spin-pairing energy theory) equation. Here is shown that the thermodynamic parameters for the two reactions are readily convertible, and that the standard-state Delta G (super 0) (sub r) values for [REE(H (sub 2) O) (sub 8) ] (super 3+) (aq)+H (sub 2) O(1) = [REE(H (sub 2) O) (sub 9) ] (super 3+) (aq) from Kawabe (1999a) are more acceptable than those calculations by Miyakawa et al. (1988) when referred to the X-ray diffraction studies of hydrated REE (super 3+) in REE(III) chloride solutions. The Delta G (super 0) (sub r) values at elevated temperatures up to 250 degrees C have been calculated using the standard-state Delta H (super 0) (sub r) and Delta S (super 0) (sub r) for the reaction and the equation by Helgeson (1967 and 1969). The theoretical estimates are compatible with our recent experimental results of REE(III) partition coefficients between Fe(III) oxyhydroxide and aqueous solution in hydrothermal conditions. The series variations of logarithmic equilibrium constants for all the geochemical reactions involving REE (super 3+) (aq) are affected by the hydration change of REE (super 3+) (aq), but the hydration change effect can be separated from the over-all series variations using the thermodynamic parameters calculated in this study.

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Accession: 019152981

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