Speciation and partitioning: a survey study of rare earth elements (REEs) in coal fly ash

Rare earth elements (REEs) are crucial for technological advancement in semiconductors, artificial intelligence development, and defense technologies. With the growing demand for and heavy import reliance of the U.S. on REEs, identifying and developing sustainable domestic REE feedstocks such as coal fly ashes (CFAs) is critical. In this study, the chemical speciation of yttrium (Y) as an REE probe was characterized at the bulk- and micro-scales in CFAs using synchrotron X-ray absorption spectroscopy (XAS). The results from bulk X-ray absorption near edge structure (XANES) showed bulk speciation primarily as a Y oxide, followed by Y incorporation into phosphate, iron oxide, and/or glass phases. Micro-X-ray fluorescence microscopy (µ-XRF) and µ-XANES showed varied Y speciation and co-localization with other REEs within the same particle, consistent with the heterogeneous nature of CFA particles. Sequential chemical extraction indicated that REEs were mainly present in the residual fraction typically associated with glass, aluminosilicate, or iron oxide phases. The second highest fraction of REEs was found in the reducible fraction associated with carbonates and partially dissolved iron oxide phases. However, the fractions of certain REE species may have been over- or underestimated due to the intrinsic limitations of XAS analyses. Therefore, a complementary suite of analytical or characterization techniques is needed to fully understand the REE speciation in complex matrices such as CFA and evaluate the related extractability for resource extraction and recovery purposes.