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Comparative analysis of the adsorption and fractionation of rare earth elements on kaolinite, palygorskite, montmorillonite, and synthetic clay minerals, A
Rose, Natalie Catherine
Rose, Natalie Catherine
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2023
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Abstract
Knowing the geochemical behavior of rare earth elements (REE) in clay mineral-dominated systems is
essential to understanding observed REE fractionation in clay-hosted REE deposits. Previous work has
focused on the behavior and distribution coefficients (log Kd) of single REE on individual clay minerals
and do not have the ability to show fractionation of REE. Testing if fractionation is driven by processes
involving clay minerals requires systematic study of adsorption behaviors for REE as a
function of varying clay mineral properties.
In this study, four Clay Mineral Society source clays, kaolinite (KGa-1), palygorskite (PFl-1),
Na-montmorillonite (SWy-2), and synthetic mica-montmorillonite (Ba-SYn-1), selected on the basis of
their surface areas and cation exchange capacities (CEC), were reacted with REE-enriched fluids to
determine the log Kd of each REE-clay mineral combination. The REE used in the study, Nd, Gd, and Yb,
represent light, middle, and heavy portions of the lanthanide series. Each clay mineral was also
equilibrated with a REE mix to evaluate REE fractionation. Adsorption isotherms were determined using
purified and size-fractionated clay minerals at 1-atm pressure, constant T = 25°C and pH that was buffered
between 6.4-7.3. All adsorption isotherms followed the Freundlich equation.
Measured log Kd values, defined as the concentration ratio clayREE/aqueous phaseREE, are the same for
individual REE and for KGa-1, PFl-1, and SWy-2, ranging from 1.42 to 2.75. Log Kd values for Ba-SYn-1
are higher, at 2.73, but do not vary between the REE. When REE are in competition, KGa-1 shows a
slight preference for the heavier REE: log Kd for Nd is 2.01-2.57; log Kd values for Gd and Yb are
1.74-2.54. Other clay minerals show no fractionation of the REE mix.
The results of this study represent an internally consistent set of REE distribution coefficients obtained
under identical conditions as well as data regarding the fractionation of REE on different clays. With the
possible exception of Nd on kaolinite, the results of this study show that the observed fractionation of REE
in clay mineral dominated environments is not attributable to clay mineral type, surface area, or CEC, but
rather some other property of a weathering profile.
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