2:15 PM - 2:30 PM
[HRE13-02] Relationship between the chemical species and extraction rates of rare earth elements (REEs) in ion-adsorption type REE mineralization in Japan
Keywords:Rare earth elements; REEs, Ion-adsorption type deposits; IADs, X-ray absorption fine structure; XAFS, Outer-and inner-sphere complexes, Speciation, Extraction rates
Ion-adsorption type rare earth element deposits (REE-IADs) are secondary deposits formed by weathering of felsic rocks such as granite. Although the ore grade of IADs (ΣREE = ~2000 ppm, average 800 ppm) is lower than any other type of REE deposits such as carbonatite (ΣREE = several to several tens of wt.%), high extraction rates (Rex = 50-100%) of the REEs make IADs useful and effective REE sources (Nagasawa et al., 2020).
Direct speciation of yttrium and neodymium by X-ray absorption fine structure (XAFS) revealed that the high Rex of REEs is attributed to the outer-sphere complexation of REEs on clay minerals such as kaolinite (Yamaguchi et al., 2018; Borst et al., 2020). Here, it is true that kaolinite has permanent charges produced by isomorphous substitution for outer-sphere complexation (Schroth and Sposito, 1997), but another study (Ma and Eggleton, 1999) insists that exchangeable cations occur mostly on variable charges produced by deprotonation of hydroxyls exposed at the basal surface and edges, which suggests the inner-sphere complexation.
In this study, therefore, we aimed to reveal the relationship between the adsorption structures and Rex of REEs by analyzing ion-adsorption type mineralized samples with various speciation methods including XAFS combined with transmission electron microscope (TEM). The samples were collected by hand-auger boring in Tanakami Granite, located at Otsu City, Shiga Prefecture, Japan. Typical depth profiles consisting of (i) REE-depleted surface layers, (ii) REE-enriched subsurface layers, and (iii) deeper layers were obtained.
As a result of XAFS and TEM analyses, it was revealed that the formation of outer- or inner-sphere complexes strongly depends on the depth profile of soil pH controlling the distribution coefficient (Kd). The relationship between the chemical species and Rex of REEs are summarized as follows.
(i) Surface layers: REEs mostly exist in crystal structures of residual minerals such as zircon and monazite, as a result of leaching and migration of REEs caused by the intense weathering in the layers. Thus, Rex is the lowest in the depth profile.
(ii) Subsurface layers: REEs mostly exist as outer-sphere complexes to be accumulated on permanent charges of kaolinite due to the low Kd corresponding to the low soil pH in the relatively acidic environment where hydroxyls of kaolinite basal surfaces and edges are less likely to be deprotonated. Thus, Rex is the highest in the depth profile.
(iii) Deeper layers: REEs mostly exist as inner-sphere complexes to be accumulated on variable charges of kaolinite due to the increase of the soil pH and Kd in the layers where hydroxyls of kaolinite basal surfaces and edges are more likely to be deprotonated. Thus, Rex is lower but higher than those of subsurface and surface layers, respectively.
The practical finding here is that even inner-sphere complexes as well as outer-sphere complexes are extractable by lowering the pH condition, which suggests that the adsorption of inner- and outer-sphere complexes is reversible.
Direct speciation of yttrium and neodymium by X-ray absorption fine structure (XAFS) revealed that the high Rex of REEs is attributed to the outer-sphere complexation of REEs on clay minerals such as kaolinite (Yamaguchi et al., 2018; Borst et al., 2020). Here, it is true that kaolinite has permanent charges produced by isomorphous substitution for outer-sphere complexation (Schroth and Sposito, 1997), but another study (Ma and Eggleton, 1999) insists that exchangeable cations occur mostly on variable charges produced by deprotonation of hydroxyls exposed at the basal surface and edges, which suggests the inner-sphere complexation.
In this study, therefore, we aimed to reveal the relationship between the adsorption structures and Rex of REEs by analyzing ion-adsorption type mineralized samples with various speciation methods including XAFS combined with transmission electron microscope (TEM). The samples were collected by hand-auger boring in Tanakami Granite, located at Otsu City, Shiga Prefecture, Japan. Typical depth profiles consisting of (i) REE-depleted surface layers, (ii) REE-enriched subsurface layers, and (iii) deeper layers were obtained.
As a result of XAFS and TEM analyses, it was revealed that the formation of outer- or inner-sphere complexes strongly depends on the depth profile of soil pH controlling the distribution coefficient (Kd). The relationship between the chemical species and Rex of REEs are summarized as follows.
(i) Surface layers: REEs mostly exist in crystal structures of residual minerals such as zircon and monazite, as a result of leaching and migration of REEs caused by the intense weathering in the layers. Thus, Rex is the lowest in the depth profile.
(ii) Subsurface layers: REEs mostly exist as outer-sphere complexes to be accumulated on permanent charges of kaolinite due to the low Kd corresponding to the low soil pH in the relatively acidic environment where hydroxyls of kaolinite basal surfaces and edges are less likely to be deprotonated. Thus, Rex is the highest in the depth profile.
(iii) Deeper layers: REEs mostly exist as inner-sphere complexes to be accumulated on variable charges of kaolinite due to the increase of the soil pH and Kd in the layers where hydroxyls of kaolinite basal surfaces and edges are more likely to be deprotonated. Thus, Rex is lower but higher than those of subsurface and surface layers, respectively.
The practical finding here is that even inner-sphere complexes as well as outer-sphere complexes are extractable by lowering the pH condition, which suggests that the adsorption of inner- and outer-sphere complexes is reversible.