The original examples of lanthanide tetrad effect in solvent extraction: A new interpretation compatible with recent progress in REE geochemistry.

Abstract

Geochmeical studies reporting lanthanide tetrad effects in natural samples often cite the original examples of tetrad effect depicted by log Kd(org/aq) for Ln(III) in the two solvent-extraction systems (Peppard et al., 1969) and its theoretical interpretation (Jorgensen, 1970; Nugent, 1970). An improved equation for the refined spin-pairing energy theory (RSPET), which has been developed in pursuing tetrad effects in geochemical studies, makes it possible to examine more thoroughly the original examples of tetrad effect by using the digitized log Kd(org/aq) data. The log Kd(org/aq) values are found to be successfully described by the RSPET equation. Our analyses show that the ratios of relative differences in Racah parameters for Ln3+ ions in aqueous and organic phases, ΔE3/ΔE1, are 0.29 ± 0.07 and 0.25 ± 0.06 for the two extraction systems. They are comparable with the spectroscopic observations of nephelauxetic effect in Nd(III) compounds (ΔE3/ΔE1 = 0.23 ± 0.02). The tetrad effects in log Kd(org/aq) are experimental evidence for the thermochemical consequence of the nephelauxetic effect in Ln(III) complexes. Although Ln3+ ions in condensed phases are atomic-like, it is rather surprising that log Kd(org/aq) can easily be reproduced by the simple RSPET equation derived from the Slater-Condon-Racah theory for free Ln3+ ions in vacuum. The effect of coordination change of Ln3+ ion across the Ln3+(aq) series is not obvious in the series variations of log Kd(org/aq), suggesting that the coordination changes in the two Ln(III) complex series in the extraction systems are significantly canceled out in ΔGr°. The two sets of log Kd(org/aq) are analogous to the Masuda-Coryell plot in REE geochemistry with respect to their coordinates. This gives an insight into the fact that the Masuda-Coryell plot also depicts tetrad effects in the REE patterns for geochemical samples.