2 research outputs found
Rare Earth Element Adsorption to Clay Minerals: Mechanistic Insights and Implications for Recovery from Secondary Sources
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
Field- and Lab-Based Potentiometric Titrations of Microbial Mats from the Fairmont Hot Spring, Canada
<p>Potentiometric titrations are an effective tool to constrain the protonation constants and site concentrations for microbial surface ligands. Protonation models developed from these experiments are often coupled with data from metal adsorption experiments to calculate microbial ligand-metal binding constants. Ultimately, the resulting surface complexation models can be used to predict metal immobilization behavior across diverse chemical conditions. However, most protonation and metal-ligand thermodynamic constants have been generated in laboratory experiments that use cultured microbes which may differ in their chemical reactivity from environmental samples. In this study, we investigate the use of <i>in situ</i> field potentiometric titrations of microbial mats at a carbonate hot spring located at Fairmont Hot Springs, British Columbia, with the aim to study microbial reactivities in a natural field system. We found that authigenic carbonate minerals complicated the potentiometric titration process due to a “carbonate spike” introduced by the contribution of inorganic carbonate mineral dissolution and subsequent carbonate speciation changes during the transition from low to high pH. This inhibits the determination of microbial surface ligand variety and concentrations. Our preliminary study also highlights the need for developing novel probes to quantify <i>in situ</i> microbial mat reactivity in future field investigations.</p