Rare Earth Element Adsorption to Clay Minerals: Mechanistic Insights and Implications for Recovery from Secondary Sources

Abstract

The energy transition will have significant mineral demands and there is growing interest in recovering critical metals, including rare earth elements (REE), from secondary sources in aqueous and sedimentary environments. However, the role of clays in REE transport and deposition in these settings remains understudied. This work investigated REE adsorption to the clay minerals illite and kaolinite through pH adsorption experiments and extended X-ray absorption fine structure (EXAFS). Clay type, pH, and ionic strength (IS) affected adsorption, with decreased adsorption under acidic pH and elevated IS. Illite had a higher adsorption capacity than kaolinite; however, >95% adsorption was achieved at pH ∼7.5 regardless of IS or clay. These results were used to develop a surface complexation model with the derived binding constants used to predict REE speciation in the presence of competing sorbents. This demonstrated that clays become increasingly important as pH increases, and EXAFS modeling showed that REE can exist as both inner-and outer-sphere complexes. Together, this indicated that clays can be an important control on the transport and enrichment of REE in sedimentary systems. These findings can be applied to identify settings to target for resource extraction or to predict REE transport and fate as a contaminant.

Rare Earth Element Speciation in Coal and Coal Combustion Byproducts: A XANES and EXAFS Study

“…The first peak at ∼1.7 Å corresponded to the first O shell. EXAFS fitting invoked Y–O bonding over a range of 2.19–2.28 Å for thortveitite and 2.31–2.38 Å for xenotime, both of which were consistent with literature values. , The thortveitite values were also similar to what has been found for inner-sphere Y–O bonding for Y adsorption to illite and kaolinite, which may imply that Y in the coal and ashes are related to clay minerals. Apart from the PR and A3 xenotime models, the Y–O bond distances between the fly and bottom ashes were remarkably alike, and the bond distances in the ashes were generally shorter than those of the respective source coals.…”

“…Synchrotron-based XAS was utilized to investigate the bulk speciation and coordination environment of Y in coal, fly ash, and bottom ash. Data were collected at the Canadian Light Source (CLS) in Saskatoon, Saskatchewan on the Hard X-ray Micro-Analysis (HXMA) beamline following previous methods. , The CLS has a storage ring energy of 2.9 GeV and a beam current of 220 mA. The Y K-edge (17,038 eV) was analyzed as a proxy for REE as it has similar ionic properties to heavy REE (HREE), it is sufficiently abundant in the samples, has a K-edge binding energy that can be probed within the energy range of this beamline, and is not susceptible to interferences, unlike the L-edge of most lanthanides.…”

Separation of Adjacent Light Rare Earth Elements Using Silica Gel Modified with Diglycolamic Acid

“…), and biosorbents (cellulose-based adsorbents, chitosan-based adsorbents, etc.) [ 6 , 7 , 8 , 9 , 10 , 11 ]. Most of these adsorbents are designed to separate only REEs or REEs from other impurities such as Fe or Al.…”