Rare Earth Elements Recovery and Waste Management of Municipal Solid Waste Incineration Ash

The advancements in high-tech products and pursuit of renewable energy demand a massive and continuously growing supply of rare earth elements (REE). However, REE production from mining is heavily restricted by technoeconomic limitations and global geopolitical tensions. Municipal solid waste incineration ash (MSWIA) has been recently recognized as a potential alternative for REE recovery. This study applies and optimizes a green modular treatment system using organic ligands for effective REE recovery and concentration from MSWIA with minimal generation of secondary wastes. Citrate extracted >80% of total REE at pH 2.0 and ∼60% at pH 4.0. A subsequent oxalate precipitation step selectively concentrated >98% of extracted REE by ∼7–12 times compared to raw MSWIA. Waste byproducts were upcycled to synthesize zeolites, resulting in an overall solid waste volume reduction of ∼80% and heavy metal immobilization efficiency of ∼75% with negligible leaching, bringing the dual benefits of REE recovery and waste management. This work serves as a pioneer study in REE recovery from an emerging source and provides system level insights on the practicality of a simple three-step treatment system. Compared to existing literature, this system features a low chemical/energy input and a light environmental footprint.


Text S2. Municipal solid waste incineration ash (MSWIA) samples
Municipal solid waste incineration bottom ash (MSWIA) samples were obtained from a Waste-to-Energy facility located in Northwest USA and size fractionated using standard testing sieves with size openings of 2 mm, 600 µm, and 106 µm.Prior to characterizations, MSWIA sample was grinded to fine powders using a pestle and mortar.The morphology of sizefractionated ash sample was examined using scanning electron microscopy (SEM).The mineral composition of the ash sample was examined using X-ray diffraction (XRD).The concentrations of trace metals in the ash samples were measured using X-ray fluorescence (XRF).A portion of the MSWIA sample was also digested using a Milestone Ultrawave Single Reaction Chamber microwave digestion system, and the digestate was analyzed for REE concentration using inductively coupled plasma mass spectrometry (ICP-MS).

Text S3. X-ray fluorescence spectrometry (XRF)
The elemental composition of major elements in the raw MSWIA sample were determined using a Rigaku Primus IVi wavelength-dispersive sequential X-ray fluorescence spectrometer (Tokyo, Japan).Dried MSWIA sample was fused as lithium borate glass discs.Lithium borate fusion was prepared with a dilution of 1:20 (sample:flux) using flux type GF-65-5I (65:35 blend of lithium tetraborate to lithium metaborate, 0.5 wt.% lithium iodide non-wetting agent purchased from Premier Lab Supply; Port St. Lucie, USA).Fusion was performed in Pt-Au crucibles using a two-position XRFuse2 fusion oven (Premier Lab Supply).The fused sample was cooled to glass discs in Pt-molds for XRF analysis.The operation voltage and current for XRF were 50 kV and 50 mA, respectively.Calibrations were monitored by drift corrections, standards check, and measurement of certified reference materials.Results were normalized to loss of ignition (LOI) that was measured by thermogravimetric analysis (TGA).

Text S5. Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX)
The morphology of raw MSWIA sample, oxalate precipitates, and zeolite products was examined using a Hitachi SU8320 SEM (Hitachi, Japan) coupled with Oxford X-Max N EDX (Oxford Instruments, U.K.).Samples were mounted on a carbon tape.One layer of carbon coating (34 nm thickness) was applied to all samples using a Quorum Q150V Plus coater (Quorum Technologies, U.K.).SEM images were taken at 20 kV, 10 µA, and a working distance of 8.0 mm.Elemental maps and point spectra were taken at 20 kV, 10 µA, and a dwell time of 200 ms.

Text S6. X-ray diffraction (XRD)
The mineralogy and phase identification of the raw ash sample, oxalate precipitates, and zeolite products were analyzed by XRD using a Panalytical Empyrean multipurpose diffractometer equipped with a PIXcel 3D-Medipi detector (Malvern, UK).Data were collected from 10 to 80 ˚2θ with a step size of 0.03 ˚2θ.Cu Kα (1.5406Å) was used as the radiation source.The operation voltage and current were 45 kV and 40 mA, respectively.The collected XRD patterns were processed using HighScore and refined using Rietveld methods with phase ID references patterns sourced from the International Centre for Diffraction Data (ICDD) database.

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Table S7.The concentrations of trace metals in the TCLP leachates (in comparison to EPA drinking water standards) and wastewaters of this threestep system with and without waste upcycling (unit: ppb).

Figure S6 .
Figure S6.Additional SEM images of (a-c) zeolite-A and (d-f) zeolite-B showing different degrees of crystallization and particle sizes.

Figure S8 .
Figure S8.Elemental maps and point spectrum of the zeolite-B particle in Figure S5d.

Table S1 .
Compositions of non-REE elements in raw MSWIA sample measured by XRF.

Table S2 .
Leaching efficiencies of non-REE and REE without citrate at pH 4 within 24 h.
Figure S5 a-b.SEM images of oxalate product

Table S4 .
1onic radii of non-REE elements and REE in their most common oxidation states and coordination numbers (retrieved fromShannon 1976).1

Table S5 .
Concentrations of REE in raw MSWIA and oxalate product (10 mM oxalate) and corresponding enrichment factors.

Table S6 .
Mineralogical characteristics of synthesized zeolites determined by XRD.