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Comprehensive understandings of rare earth element (REE) speciation in coal fly ashes and implication for REE extractability Pan Liu, Rixiang Huang, and Yuanzhi Tang Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019
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Environmental Science & Technology
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Comprehensive understandings of rare earth element (REE) speciation in coal fly ashes
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and implication for REE extractability
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Pan Liu, Rixiang Huang, Yuanzhi Tang*
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School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Dr,
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Atlanta, GA 30332-0340, USA
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* Corresponding author.
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Email:
[email protected]; Phone: 404-894-3814
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Submit to
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Environmental Science & Technology
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Abstract
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In recent years, recovery of rare earth elements (REE) from coal fly ashes (CFAs) has
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been considered as a promising resource recovery option. Yet, quantitative information on REE
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speciation in CFAs and its correlation with REE extractability are not well established. This
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study systematically investigated REE speciation-extractability relationship in four
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representative CFA samples by employing multiple analytical and spectroscopic techniques
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across micro to bulk scale and in combination with thermodynamic calculation. A range of
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REE-bearing phases are identified, including REE oxides, REE phosphates, apatite, zircon, and
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REE-bearing glass phase. REE can occur as discrete particles, as particles encapsulated in glass
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phase, or distribute throughout the glass phase. Although certain discrepancies exist on the
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REE speciation quantified by X-ray adsorption spectroscopy and acid leaching due to intrinsic
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limitations of each method, both approaches show significant fractions of REE oxides, REE
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phosphates, apatite, and REE-bearing Fe oxides. This study contributes to an in-depth
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understanding of REE speciation-distribution-extractability relationship in CFAs and can help
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identify uncertainties associated with the quantification of REE speciation. It also provides a
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general methodology for future studies on REE speciation in complex environmental samples
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and a knowledge basis for the development of effective REE recovery techniques.
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1. Introduction
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Rare earth elements (including lanthanides and yttrium, as REE hereafter) are critical
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for a wide range of high tech applications due to their unique physicochemical properties.1 The
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growing importance of REE to technologies and economies and the potential risks of supply
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disruptions have led both the United States (US) and the European Union to label REE as
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“critical materials”.2,
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importing REE in 2017.4 Over the recent years, there are great interests in developing
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economically feasible and environmentally friendly approaches for domestic REE recovery.5-7
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With this regard, coal combustion products such as coal fly ashes (CFAs) have recently
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emerged as a promising REE source.8-11 In the US, around 45 Mt CFAs are generated
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annually,10 with only 39% beneficially used and the rest discarded.12 On the other hand, REE
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are enriched in the CFAs after coal burning.8, 13 With an average total concentration of REE in
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CFAs at ~500 ppm, the annual value of REE derived from CFAs is estimated to be $4.3 billion
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in the US.10 Thus, recovery of REE from CFAs is a promising waste recycling option that might
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bring about significant environmental, economic, and societal benefits.
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The US is currently a net importer of REE and spent $150 million
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In order to develop cost effective and environmental friendly REE recovery techniques,
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many critical issues need to be addressed,11 including the following two fundamental questions:
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(1) the speciation of REE in CFAs, such as the chemical species and physical distribution
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and/or association with other phases, and (2) the controlling factors on REE extractability.
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Several previous studies14-16 have suggested that REE are dispersed throughout the amorphous
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aluminosilicate glass phase, thus the glass fraction should be targeted for REE recovery from
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CFAs. Other studies suggested the (predominant) presence of REE in phosphate (e.g.,
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monazite)17, carbonate18, and/or silicate minerals (e.g., zircon)15. Such inconsistency might
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result from the intrinsic heterogeneity of CFAs as well as the limitations of individual analytical
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method. 3 ACS Paragon Plus Environment
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Both micro- and bulk-scale analytical methods have been utilized to study the
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correlation between REE speciation (e.g. chemical species and physical distribution) and
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extractability in environmental samples, including CFAs. The main techniques used to study
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REE speciation include scanning/transmission electron microscopy coupled with energy
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dispersive X-ray spectroscopy15, 17-19, electron probe microanalysis19, laser ablation inductively
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coupled plasma mass spectrometry15, 20, and synchrotron X-ray microscopy and spectroscopy21,
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22.
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and acid leaching26. Although these techniques can provide valuable information (e.g., REE
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species, distribution, and extractability) at different spatial/temporal scale, they also possess
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intrinsic limitations. For example, direct information on REE extractability can be provided by
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sequential chemical extraction, but interpretation of the related mineral phases might be
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problematic, because this empirical method might cause incomplete dissolution of desired
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phases, concomitant dissolution of different phases, or alteration of the original phases. To
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obtain a more complete picture and quantitative information of REE speciation in complex
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matrices such as CFAs, comparison and cross-validation of information obtained by different
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techniques are highly desired.
Extractability evaluations were mainly conducted using sequential chemical extraction23-25
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The goals of this study are to comprehensively characterize and quantify REE species
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and distribution in CFAs using complementary techniques across different spatial scales, and
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to correlate the speciation information with REE extractability. Four representative CFA
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samples were selected by taking into consideration of both CFA chemical composition and
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main coal basins in the US. A systematic method was developed by employing techniques
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across bulk to molecular scales, including chemical (acid leaching and sequential extraction),
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mineralogical (X-ray diffraction), microscopic (scanning electron microscopy coupled with
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energy dispersive X-ray spectroscopy), and spectroscopic methods (synchrotron X-ray
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adsorption spectroscopy). Based on the results obtained, the advantages and disadvantages of 4 ACS Paragon Plus Environment
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each technique and controlling factors on REE extractability were evaluated. The results
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contribute to an improved understanding of REE occurrence in CFAs and their extractability,
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evaluate a general methodology for characterizing REE speciation in complex environmental
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matrix, and provide a knowledge basis for the development of cost effective and
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environmentally friendly REE recovery methods.
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2. Materials and methods
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2.1 Materials
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The American Society for Testing and Materials (ASTM) C 618 defines two classes of
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fly ashes based on chemical characterization: Class F CFAs (with SiO2 + Al2O3 + Fe2O3 ≥ 70
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wt%) and Class C CFAs (with 50 wt% ≤ SiO2 + Al2O3 + Fe2O3 ≤ 70 wt%).27 Based on this
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classification and the major coal basins in the US, four representative CFA samples were
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selected for this study (Table 1), including two CFA samples (F-1 and C-1) from a local coal
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fired power plant located in the southeastern US28 and two CFA samples from the National
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Institute of Standards and Technology (NIST) (SRM-2690 as F-2 and SRM-2691 as C-2
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hereinafter). Samples F-1 and F-2 are Class F CFAs and their feed coals are bituminous coal
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from the Illinois Basin and subbituminous coal from a surface mine in Craig, CO, respectively.
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Samples C-1 and C-2 are Class C CFAs and their feed coals are subbituminous coals from the
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Powder River Basin (details in the Supporting Information SI, Table S1). A suite of REE
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reference compounds (REE-organic complexes, REE oxides, REE carbonates, REE phosphates,
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and natural REE-bearing minerals, etc) were obtained or synthesized using chemicals of ACS
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grade or higher (details in Table S2). The synthesized and natural REE reference compounds
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were confirmed by X-ray diffraction to be pure phases.
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2.2. Composition analysis and chemical extraction of REE in CFAs 5 ACS Paragon Plus Environment
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REE contents in CFAs were determined from total digestion29. Two types of chemical
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extraction methods were used to assess the extractability of REE, including sequential
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extraction and acid leaching. Sequential extraction was conducted following a previous
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procedure.30 Briefly, four REE fractions are defined: (1) acid-soluble (Step-1, e.g., carbonates),
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(2) reducible (Step-2, e.g., Fe-Mn oxides), (3) oxidizable (Step-3, e.g., organic matter and
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sulfide), and (4) residue (e.g., silicates). For acid leaching, REE extractability was investigated
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as a function of pH adjusted using HNO3 and NaOH. At desired pH, 0.2 g CFAs per 100-mL
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H2O were reacted for three days under agitations at room temperature. At the end of each
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chemical extraction step or acid leaching, REE concentration was analyzed using inductively
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coupled plasma mass spectrometry (ICP-MS). Details on total digestion, sequential extraction,
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and ICP-MS measurement are in SI Text S1.
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2.3 Mineralogical, microscopic, and spectroscopic analysis of CFAs
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X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray
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spectroscopy (SEM-EDX), micro X-ray fluorescence (μXRF) imaging and micro X-ray
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absorption near edge structure spectroscopy (μXANES), and bulk XANES analyses were
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conducted on the CFA samples. For XANES analyses, Nd and Y were chosen as representative
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light REE (LREE, from La to Sm) and heavy REE (HREE, from Eu to Lu plus Y), respectively.
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μXRF imaging and μXANES spectra of Y K-edge, Nd LIII-edge, and Nd LII-edge were
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collected at Beamline 2-3 of the Stanford Synchrotron Radiation Lightsource (SSRL, Menlo
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Park, CA). Bulk XANES spectra were collected at Beamlines 5-BM-D at the Advanced Photon
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Source (APS, Lemont, IL). Spectra of reference compounds were also collected for liner
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combination fitting (LCF) analysis. Details on XRD, SEM-EDX, μXRF, and bulk/micro
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XANES data collection and LCF analysis are in SI Text S2.
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3. Results and discussion
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3.1 Chemical and mineralogical characteristics of CFAs
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The element contents of CFA samples are listed in Tables 1 and S1 and compared to
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common CFAs from the major coal basins in the US. Samples F-1 and F-2 show chemical
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composition typical of Class F CFAs that are enriched in SiO2 (55%), Al2O3 (25%), and Fe2O3
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(5–12%), while samples C-1 and C-2 as Class C CFAs are relatively depleted in the above
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elements but abundant in alkaline oxides (26–28% CaO and 1–5% MgO). Total REE content
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in Class F CFAs is similar to that of Class C CFAs (250–320 ppm), and comparable to common
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CFAs in the US (Table S1). REE patterns of the four CFA samples normalized (with subscript
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N) by the Upper Continental Crust (UCC, representing the natural REE background)31 are
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shown in Fig. S1, which are in line with the general REE patterns of CFAs. REE are M-type
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enriched (LaN/SmN < 1 and GdN/LuN > 1)8 in both Class F and C CFAs without obvious Ce
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anomaly (defined as 𝐶𝑒 ∗ = 2𝐶𝑒𝑁 (𝐿𝑎𝑁 + 𝑃𝑟𝑁)). Samples C-1, C-2, and F-2 are characterized
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with positive Eu anomaly (𝐸𝑢 ∗ = 2𝐸𝑢𝑁 (𝑆𝑚𝑁 + 𝐺𝑑𝑁)), while sample F-1 does not show
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evident anomaly.
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The mineralogical composition of CFAs was investigated by XRD (Fig. S2).
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Crystalline phases identified in Class F CFAs include quartz (SiO2), mullite (Al6Si2O13),
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hematite (Fe2O3), and magnetite (Fe3O4). Class C CFAs have a more complex mineralogy, with
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quartz, anhydrite (CaSO4), tricalcium aluminate (Ca3Al2O6), lime (CaO), and periclase (MgO)
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being identified. A broad hump at around 20–30 2 suggests the presence of amorphous
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aluminosilicate glass, a common major component (50–80 wt%) in CFAs.32 SEM analysis
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observed different morphologies of CFA particles, with the predominance presence of
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spherical and cenospherical particles ranging at 1–100 μm (Fig. S3). Such morphological
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features are typical for CFAs due to particle melting and decomposition during coal burning
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process.33 In addition, considerable amounts of fine particles (e.g., Fe oxides) were found to be 7 ACS Paragon Plus Environment
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embedded within glass phase in both Class F and C CFAs (Figs. S3 and S4).
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3.2 REE speciation by SEM-EDX
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SEM-EDX observed two types of REE occurrence in CFAs, including pure REE
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phosphates and minor REE in lime and zircon. Fig. 1a shows a REE phosphate particle (~30-
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μm size) that is enriched in LREE, while the one in Fig. 1b (~10-μm size) is enriched in HREE
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with close association with aluminosilicate glass. These two REE phosphate particles might be
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monazite/rhabdophane (LREE-enriched phosphate) and xenotime/churchite (HREE-enriched
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phosphate), respectively. Monazite and xenotime are anhydrous phosphate minerals, while
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rhabdophane and churchite are hydrated phosphate minerals. It is difficult to differentiate them
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by SEM-EDX. REE phosphates have also been identified previously using transmission
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electron microscopy17, 34 and electron probe microanalysis19. A lime (CaO) particle containing
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minor REE (REE oxide content < 0.5 wt%) is shown in Fig. 1c. This phase contains minor
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HREE and its spherical shape suggests that it might be relics of REE-bearing CaCO3 after
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burning. Both REE-bearing CaCO3 and CaO have been previously identified.18, 19 A zircon
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(ZrSiO4) particle (REE oxide content < 0.3 wt%) is shown in Fig. 1d, which is encapsulated in
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glass phase. Although zircon is relatively abundant in the CFAs, only a portion of observed
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zircon particles (4 out of 16) contains EDX-detectable REE. Other potential REE-bearing
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phases, such as apatite19 and glass phase14, were observed but with no REE detected by EDX.
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An overview of the REE speciation observed by SEM-EDX reveals three critical
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features regarding REE occurrence in CFAs (Table S3). First, different phases might
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preferentially
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monazite/rhabdophane, and HREE in xenotime/churchite, lime, and zircon (Fig. 1). This
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phenomenon might be due to the gradually decreasing cation radii of REE with increasing
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atomic number (i.e., lanthanum contraction), which changes the compatibility of individual
accommodate
LREE
or
HREE.
LREE
are
more
enriched
in
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REE element for a specific mineral phase. Second, as shown in Figs. 1 and S5, REE-bearing
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phases can occur as discrete particles or be closely associated with glass phase (at the edge or
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totally encapsulated). The close association of REE-bearing phases with glass phase is likely
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due to the capture of REE-bearing phases during glass cooling and agglomeration21, and it has
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also been noticed by several previous studies (e.g., ref.17, 21). Third, REE-bearing phases might
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be subjected to decomposition during combustion. Although REE phosphates and zircon are
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typically thought to be stable even at high temperature (e.g., ref19), Fig. S6 shows
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decomposition characteristics of REE phosphates (particle shattering) and zircon (holes and
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melting features), as compared to Figs. 1a and 1d, respectively.
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3.3 REE speciation by synchrotron microscopy and spectroscopy
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Characteristics of REE reference compounds. Y XANES spectra of reference
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compounds are plotted in Figs. S7. Spectra of Y2O3, Y-churchite (YPO4·H2O), apatite
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(Ca5(PO4)3(OH, F)), REE-bearing calcite (CaCO3), monazite ((Ce, Th, La, Nd)PO4), xenotime
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((Y, Dy)PO4), zircon (ZrSiO4), and REE-bearing glass all show a peak at 17056.5 eV and a
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shoulder at 17066 eV, but the relative intensities are different, and with additional features at
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17080–17140 eV. These different spectral features reflect different structures and local
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coordination environments of the REE reference compounds (such as the first shell
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coordination, summarized in Table S2). On the other hand, Y spectra of Y3+(aq), Y-tengerite
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(Y2(CO3)3·2-3H2O), bastnäsite ((Ce, La, Nd)CO3(OH, F)), REE-bearing hematite (Fe2O3), and
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REE-organic complexes are similar to each other, showing a single peak at around 17056.5 eV,
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although the peak position is slightly right-shifted for Y3+(aq) and left-shifted for bastnäsite.
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Nd spectra were not collected for some reference compounds (e.g., bastnäsite and
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monazite) due to interference of Ce LII-edge signal on Nd LIII-edge and Sm LIII-edge signal on
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Nd LII-edge. For reference compounds with Nd spectra obtained, their LIII- and LII-edge spectra 9 ACS Paragon Plus Environment
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are generally similar in appearance with a characteristic intense main peak (Fig. S8), making it
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difficult to distinguish REE speciation based on Nd XANES. Thus, Y was used as the primary
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“probe” for characterizing REE speciation. Although some Y spectra of REE-reference
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compounds are similar, giving the possibility that some subtle differences might still be helpful
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in differentiating REE speciation, all thirteen Y reference spectra were used for the linear
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combination fitting (LCF).
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μXRF and μXANES analyses. Representative µXRF maps were plotted in Fig. 2 and
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SI Figs. S9 and S10. Most Y hotspots observed have a particle size of 60%)
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at those hotspots by LCF. Five hotspots are identified as REE oxides (Table S4) and they have
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a relatively smaller particle size (generally 3.5 (REE oxides, carbonates)
pH 3.51.5 (apatite)
pH
