Rare earth element (REE) speciation in municipal solid waste incineration ash

Abstract

A robust and sustainable supply of rare earth elements (REE) is critically needed for clean-energy technologies, which has stimulated substantial interests in REE recovery from waste streams. Municipal solid waste incineration ash (MSWIA) was recently recognized as a potentially important REE resource, yet REE speciation in MSWIA remains poorly understood. This study employed synchrotron X-ray spectroscopy and microscopy techniques to elucidate the speciation of representative REE (Y, Ce, and Nd) in different MSWIA samples. Linear combination fitting of bulk X-ray absorption near edge structure (XANES) data indicated that Y-bearing Al/Fe oxides and phosphates are the primary Y-hosting phases. Micro-XANES of individual Y-containing particles identified by micro X-ray fluorescence (μXRF) mapping revealed notably different Y speciation at micro-scale from the bulk, consistent with the highly heterogeneous nature of MSWIA samples. The main REE-bearing phases in different size-fractionated MSWIA are similar: Y and Nd as oxides and xenotime/monazite, and Ce as apatite and monazite. Our results provide important insights for designing pre-screening processes (e.g., density separation) and optimizing extraction methods (e.g., pH, use of ligands) for cost-effective REE recovery from MSWIA.