US Coal Fly Ash Study Part 1: High Rare Earth Element Levels Could Contribute To USA Feedstock

Trends in the Rare Earth Element Content of U.S.-Based Coal Combustion Fly Ashes

Editor’s Note: Although this study is dated (2016), the concepts, deep research and conclusions are insightful. is working on Part 2 of research for processing REEs from coal ash. 
Department of Civil and Environmental Engineering and §Earth and Ocean Sciences Division, Nicholas School of the Environment, Duke University, Durham, North Carolina 27708, United States
Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40511, United States
Environ. Sci. Technol., 2016, 50 (11), pp 5919–5926
DOI: 10.1021/acs.est.6b00085
Publication Date (Web): May 26, 2016
Copyright © 2016 American Chemical Society
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ACS AuthorChoice – This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.


Abstract Image


Coal combustion residues are a major economic and environmental burden in the United States due to their abundance and potential to leach toxic metals at disposal sites. However, coal ash has also been effectively reused for commercial purposes. A proposed application is to mine coal ash for rare earth elements (REEs), which are critical to the automotive, energy, electronics, and defense industries. REEs consist of the lanthanide series, yttrium, and scandium. (“Total REEs” refer to all 16 elements, herein, unless otherwise noted. Promethium is not included as it does not occur naturally.) REEs are relatively abundant elements with total REE average crustal content between 160 and 205 mg kg–1.(1-4) However, REEs are not found in nature as pure metals and must be isolated from the host minerals, which do not often occur in easily exploitable deposits.(5-7) REEs also do not occur individually as mineral constituents due to their chemical similarity.(8)There are approximately 200 distinct rare earth minerals, but of these only bastnaesite, monazite, xenotime, and ion-adsorbed clays are mined for commercial REE production.(9) Currently, more than 86% of REE production, nearly half of known reserves, and the majority of REE processing and separation occur in China.(10) With this monopoly and recent instabilities in the REE global supply market, the United States government has outlined the need for alternative sources,(11)especially for critical elements such as Nd, Eu, Tb, Dy, Y, and Er.(12)
The U.S. generates more than one-third of its electricity from coal.(13) As a result, approximately 115 million metric tons (t) of coal combustion products (CCPs) are generated annually, and this sum includes 45 million t of fly ash. Only 45% of CCPs are reused for applications such as concrete and other construction materials.(14) The remainder is held in landfills or wet impoundments where the leaching of toxic trace elements (e.g., As, Se, Cr) is a concern for groundwater and surface water quality.(15-17) Ash impoundments are also susceptible to catastrophic failures, resulting in large ash spill events such as the Tennessee Valley Authority Kingston Fossil Plant in 2008 and the Duke Energy Dan River Steam Station in 2014.(16-18) The magnitude of these spills and the heavy economic and environmental costs associated with disposal have renewed efforts to find beneficial reuse opportunities that extend the value chain for coal ash.
The recovery of REEs from fly ash has several notable advantages over traditional REE ores. First, it is a readily available waste product with strong environmental incentives and an established market for beneficial reuse. Second, fly ash does not require extensive excavation, which is capital intensive and can be environmentally destructive. REE mines can generate large volumes of waste rock with high contents of uranium and thorium, a challenge with respect to radioactive wastes.(10, 19, 20) Finally, fly ash is a fine powder, which makes it ideal for chemical processing and eliminates several costly ore processing steps (although it does preclude most physical beneficiation of the ash).(21) There is already precedent for metal recovery from fly ash, including gallium, germanium, and aluminum.(22, 23)
While there are potential advantages, the economic feasibility of REE recovery from coal ash is uncertain. This is due to a lack of information on the REE contents of the different types of coal ash produced across the U.S. and how ash characteristics are related to extractability of the REEs. The REE content of coal ash depends heavily on the geological origin of the feed coal.(24-26) Therefore, prioritization of ash sources for REE extraction requires an understanding of geochemical characteristics that are indicative of high REE content in fly ashes. Previous studies have shown that many coal deposits around the world contained high REE levels, and the respective ashes were also enriched in REEs.(19) For example, Seredin found REE contents of up to 1290 mg kg–1 in the Pavlovka coal deposit (Russian Far East), with up to 1% REE in the resulting ash.(19, 27) Two studies of the Kentucky Fire Clay coal bed revealed REE contents between 1965 and 4198 mg kg–1 (ash basis) in coal samples and 1200 to 1670 mg kg–1 in fly ash from a power plant burning this coal source.(24, 25) The Fire Clay Coal may not be typical, however, as the REE enrichment was largely due to a volcanic ash fall during the formation of the coal seam.(25) Recent surveys published by the U.S. Department of Energy indicated total REE contents (not including Sc) of 41 to 1286 mg kg–1 in U.S. coals.(28) While REEs have been reported in laboratory-produced ashes, the REE contents of fly ashes produced at power plants are not widely documented, particularly for utilities in the U.S.
This research aimed to expand previous findings through the analysis of REEs in fly ashes and other CCPs obtained from a broad survey of U.S. power plants. In addition to the total REE content (as determined by whole rock acid digestion), nitric acid-extractable REE content was measured as a means to gauge the accessibility of REEs in fly ash subjected to industrially-relevant leaching processes. Trends in total and extractable REE content of fly ashes were examined in relation to variations in coal origin, major element content, and collector row at each power plant. These data were used to identify possible strategies to leach REEs from the ash during recovery operations and understand the potential valuation of the fly ash with respect to the REE reserves.

Materials and Methods

Coal Combustion Product Samples

CCP samples were collected in the time frame between 1994 and 2015 from 22 U.S. coal-fired power plants located in Kentucky (13 plants); Missouri (3); Texas (3); and individual plants located in South Carolina, Georgia, and New Mexico (Table S1). Fly ash samples were also obtained in 2015 from two power plants in South Africa. Fly ash samples were taken from multiple electrostatic precipitator (ESP) or baghouse rows for most of the power plants to track the fractionation of REEs in fly ash particles as they passed through the collection system. For many plants, ash samples were collected from multiple operational boilers. In addition, several plants were revisited for sampling over the years. The collection represented over 100 fly ash samples generated from feed coals from the Appalachian Basin (8 power plants), Illinois Basin (6), Powder River Basin coals (7), a 70/30 Illinois-Powder River blend (1), and one sample each from San Juan Basin and Gulf Coast coal sources. More than 20 samples of bottom ash, mechanical fly ash, silo ash, and ash from landfills and disposal ponds were obtained from a selection of the same power plants and were also analyzed for total REE and acid-extractable REE content.

Optima-grade acids and quartz-distilled water were used for HF acid digestion. Screw-cap Teflon vials (Savillex) and zirconium crucibles were used for two different types of total element digestions while 50 mL polypropylene digestion tubes (Environmental Express) were used for HNO3 extractions. After the acid extraction procedures, all samples were stored in 50 mL polypropylene tubes and diluted using Milli-Q water (>18 M?-cm, EMD Millipore) and trace-grade hydrochloric acid (HCl) prior to analysis.
Chemical Analysis Methods

For total REE quantification, CCP samples (34 ± 1 mg) were digested overnight at 90–100 °C in a 1:1 mixture of concentrated HF and concentrated HNO3 (2 mL each). The acidified samples were then dried down completely and redigested overnight at 90–100 °C in a mixture of 15 M HNO3 (1 mL), H2O2 (1 mL; Optima-grade, 30–32%), and quartz-distilled water (5 mL). Following redigestion, the samples were diluted to 50 mL, and metal concentrations were determined via inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700x). Accuracy of the HF-HNO3 extraction method was assessed using the National Institute of Standards and Technology (NIST) standard reference material (SRM) for fly ash, SRM 1633c.
Sodium peroxide (Na2O2) alkaline sintering (as described by the U.S. Geological Survey(29-31)) was employed as a second method for total REE quantification in CCP samples. Ash samples (0.1 g) were placed into zirconium crucibles, mixed with Na2O2 (0.6 g), and heated to 450 °C for 30 min. After cooling, the crucibles and their contents were submerged overnight in 20 mL of Milli-Q water. The next day, the crucibles were removed using forceps and carefully rinsed into the solution using 20 mL of 25% v/v HNO3. Additional Milli-Q water was added to dilute the samples to 50 mL for storage and ICP-MS analysis.
Acid-extractable REE content was estimated using heated HNO3 digestion. Ash samples (0.1–0.5 g) were digested in 10 mL of concentrated HNO3 (?15 M) for 4 h at 85–90 °C. After digestion, samples were diluted for storage and ICP-MS analysis. Major element compositions in each sample (reported as percent oxides: SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, K2O, P2O5, TiO2, and SO3) were determined by X-ray fluorescence (XRF).(24, 32)
During ICP-MS analyses, the primary REE interferences were oxides of lighter elements (including other REE). The formation of these oxides in the plasma was monitored using the 156/140 mass ratio (CeO/Ce) as a proxy and kept below 2% for all ICP-MS runs. Common polyatomic interferences of other elements (e.g., As, Fe) were minimized by introducing helium to the collision cell. Doubly charged interferences (for Se) were counteracted using hydrogen as a reaction gas in the collision cell. Reportable lower limits of quantification for REE were in the single-digit parts-per-trillion range (we defined this as 10 times the instrument detection limit). For all analyses, measured REE values for the samples were generally an order of magnitude or more higher than the limit of quantification.
Statistical Analyses

Data analyses were performed using the statistical software R.(33) Nonparametric statistical tests were used because the data (as a whole and for each basin) were not normally distributed as determined by the Shapiro-Wilk test in R. An initial Kruskal–Wallis test was used to determine if grouping the samples by coal basin yields significant differences between groups (defined as p < 0.05) with respect to total REE content, the percentage as the critical REEs (Nd, Eu, Tb, Dy, Y, and Er), and the HNO3-extractable REE (content and % of total) . If significant differences were found, Dunn’s test was used for post hoc multiple comparisons to determine which basins differed and how. The Wilcoxon rank-sum test (Mann–Whitney U test) was used to test for significant differences between individual pairs of coal basins (defined as p < 0.05).


In the HF:HNO3 digestion of the fly ash SRM, the recovery (n = 9 digestions) of individual REEs was between 89.2% and 102.4% (94.9% on average) for those REEs with reference and information mass fraction values (Sc, La, Ce, Nd, Sm, Eu, Tb, Dy, Yb, and Lu) (Figure S1). The alkaline sintering method had similarly high recoveries for REEs in the fly ash SRM (average recovery between 85.8% and 101.3% of SRM values for individual REE, 126.2% for Sc). However, for a subset of fly ash samples tested for this study, the sodium peroxide sinter method recovered only a fraction of the REE value determined by the HF digestion (Table S2). Therefore, all total REE values reported herein refer to the HF:HNO3 digestions.
Total REE contents for all of the CCP samples in this study ranged from 200 to 1220 mg kg–1 with a median value of 481 mg kg–1 (Figure 1). Just over 40% of the samples exceeded 500 mg kg–1. For most power plants in which multiple fly ash samples were taken, little difference was observed in the total REE content of fly ash collected on the same date from different ESP or baghouse rows of the same boiler unit (Figures S2 and S3). This relatively constant REE content with respect to ESP/baghouse row is consistent with findings from a previous study.(24) In contrast, the contents of other trace elements such as arsenic, selenium, lead, and gallium (which are known to be more volatile than REEs in coal combustion flue gas(24)) increased with ESP collector row (Figure S4).

Figure 1. Percentage of the total REEs (including the lanthanides, yttrium, and scandium) in coal ash samples as (A) critical REEs (Nd, Eu, Tb, Dy, Y, and Er) and (B) the nitric acid-extractable REEs. Samples have been labeled according to the origin of the feed coal: Appalachian (App), Illinois Basin (IL), Power River Basin (PRB), Gulf Coast (GC), San Juan (SJ), and the Highveld/Witbank coalfields of South Africa (RSA). Closed symbols represent fly ash; open symbols are other types of CCPs (stoker ash, economizer ash, mechanical fly ash, bottom ash, silo ash, pond ash, landfill ash, and FGD waste). Error bars are the mean ± standard deviation (n = 2–12) of samples collected from multiple ESP and baghouse rows of the same plant, boiler, and sampling date.

With these observations, the total REE contents for samples from multiple rows were averaged to produce a single value to represent the fly ash generated by that boiler unit at the time of sampling. The data shown in Figure 1 correspond to these average values with the respective standard deviations (Note: several units were revisited over multiple periods.). These averaged content values were herein used for the statistical evaluations.
The fraction of critical REEs (Nd, Eu, Tb, Dy, Y, and Er) in U.S. CCPs ranged from 28.5% to 41.0% with a median of 36.5% (Figure 1A). In the three fly ash samples from South Africa, 30.4% of the REE total was comprised of the critical elements. These percentages are consistent with previously reported values in fly ashes from Poland (35.6–36.1%) and the U.K. (33.4–35.8%).(34)The critical REE content of Chinese coal ashes was generally lower (16.8–41.2%), while the critical REE content of Russian (31.5–63.2%) and Mongolian (48.6%) coal ashes greatly exceeded the samples in this study due to their very high yttrium content (up to 3540 mg kg–1 in Russian coal ashes).(19)
The heated nitric acid extraction efficiency relative to total REE content was highly variable, ranging from 1.6% to 93.2% with a median value of 29.6% (Figure 1B). In absolute terms, the HNO3-extractable REE mass content ranged from 7.4 mg kg–1 to 372 mg kg–1 with a median of 126.9 mg kg–1.
As shown in Figure 1, the total REE content, critical REE (%), and extractable REE (%) for all of the CCP samples appeared to cluster according to the origin of the feed coal used to produce the ash samples. Fly ash samples represented the largest proportion of the CCP samples. Therefore, we examined the fly ash data further by feed coal origin (Figure 2, Table S3).

Figure 2. Fly ash REE parameters differed by the origin of the feed coal: Appalachian Basin (App), Illinois Basin (IL), and Powder River Basin (PRB). (A) Total REEs (lanthanides, Y, and Sc); (B) critical REEs (Nd, Eu, Tb, Dy, Y, and Er); (C) and (D) nitric acid-extractable REE content. Each data point represents the average of samples from multiple ash collector rows of the same power plant, boiler, and sampling date. Letters (e.g., “a” and “b”) denote statistically significant differences between groups (see Table S4 for summary statistics).

The total REE content of fly ash samples differed significantly by coal basin (Tables S4 and S5). Fly ashes from Appalachian basin coals had significantly higher total REE content (591 ± 98 mg kg–1) than those of Illinois Basin (403 ± 119 mg kg–1) and Powder River coals (337 ± 69 mg kg–1). The median difference (Hodges-Lehmann estimator) in total REE between Appalachian and Powder River ashes was 228.7 mg kg–1; the median difference was 197.3 for Appalachian and Illinois Basin ashes. Total REE contents of Illinois Basin and Powder River coal ashes were not significantly different from each other (p = 0.37, Table S4). Differences between each coal basin group for the nonparametric analysis are listed in Table S4.
The critical REE content (as a percentage of total REE) differed significantly between coal basins (Kruskal–Wallis test, ?2 = 15.8, p = 3.8 × 10–4). Powder River basin ashes had significantly lower critical REE percentages than ashes from Appalachian coals (median difference of 3.82%) or Illinois Basin coals (median difference of 3.39%). Appalachian and Illinois Basin ashes did not differ significantly in critical REE content (median difference of 0.11%, p = 0.78). The nitric acid-extractable REE percentage also differed significantly by coal basin (Kruskal–Wallis test, ?2 = 18, p = 1.2 × 10–4), with the extractable REE fraction in Powder River basin coal fly ashes significantly greater than those of Illinois Basin and Appalachian coals (Figure 2C, Table S4). The median differences in extractable REE percent between Powder River Basin ashes and ashes from Appalachian and Illinois Basin sources were 36.3% and 42.1%, respectively. The median difference between Appalachian and Illinois ash percent extractability was only 7%.
As can be seen in Figure 3, the major elements comprising the fly ash samples differed significantly by coal basins (Table S5). Appalachian coal fly ashes had significantly higher SiO2and Al2O3 content relative to both Illinois Basin and Powder River coal ashes (Table S4). Illinois Basin coal ashes had significantly higher Fe2O3 content than Appalachian or Powder River ashes (median differences of 10.9% and 13.6%, respectively). Powder River ashes were characterized by relatively high CaO content (21% and 20% median difference with Appalachian and Illinois Basin ashes, respectively). These major element signatures of the Appalachian coal fly ash (high Si and Al, low Ca), Illinois Basin coal fly ash (high Fe, low Ca), and Powder River coal fly ash (low Si and Al, high Ca) are consistent with expectations for these coal sources.(35)

Figure 3. Major element composition (reported as % oxides) of fly ash samples grouped according to the origin of the feed coal: Appalachian (App), Illinois Basin (IL), and Powder River (PRB). Each data point represents the average of samples from multiple ash collector rows of the same power plant, boiler, and sampling date. Letters (e.g., “a” and “b”) denote statistically significant differences between groups (Table S4).


Total REE Content and Trends in Fly Ash

Our examination of the total REE contents in the samples of this study suggested trends according to origins of the feed coal. The Appalachian and Powder River ashes formed two distinct clusters (Figure 1), while Illinois Basin ashes straddled the two groups. Approximately 70–80% of the U.S. coal production occurs in mines located in these three basins.(36) The markedly higher REE content of fly ashes derived from Appalachian coals suggests that ashes from Appalachian sources should be prioritized in any REE recovery operation. Importantly, the elevated REE contents in Appalachian coal ashes were present across a variety of coals burned by six different utility power plants between 1994 and 2014. This implies that generic coals burned in the U.S. (not just specific coal seams such as the Fire Clay Coal) can produce fly ashes with greater than 500 mg kg–1 total REE (or approximately >600 mg kg–1 as total rare earth oxides). While these values do not meet the cutoff grade of >1000 mg kg–1 (expressed as rare earth oxide) suggested by Seredin and Dai (2012), they still represent significant added value to other metal recovery operations (e.g., aluminum recovery with simultaneous REE extraction).(19)
Total REE content also correlated strongly with the fraction of aluminum oxides comprising the fly ash (r2 = 0.71) (Figure 4A). This trend suggests that the same geological factors responsible for determining aluminum oxide content in coal ashes may have an influence on REE enrichment. However, it is not clear whether these processes are related or simply the result of the source coal geology.

Figure 4. Total and extractable REE content of coal fly ashes depended on the fractions of major elements comprising the ash. (A) Higher Al2O3 content (as observed in Appalachian basin fly ashes) correlated with higher total REE levels. (B) High CaO content indicated greater extractability via HNO3. The coal sources were Appalachian (App), Illinois Basin (IL), Powder River Basin (PRB), a blend, San Juan basin, Gulf Coast, and the Highveld/Witbank coal fields in South Africa (RSA).

When normalized to the REE content of the upper continental crust (UCC),(37, 38) the individual REEs showed differing enrichment trends between coal basins (Figure S5). The enrichments of Appalachian basin coal ashes tended to be greater in medium and heavy REE, especially Gd, relative to the light REEs. Illinois Basin and South African ashes were also enriched in Gd, but light, medium, and heavy REEs otherwise had similar enrichment factors. Powder River Basin ashes had relatively even enrichment across REEs relative to the UCC. A notable exception is an observed enrichment of Eu; however, we cannot rule out the possibility that this data reflects 137Ba16O interference during ICP-MS analyses.
Fly ash represents a complex mixture of the unburned coal material along with glassy aluminosilicate particles that condensed in and were captured from the flue gas stream. Little is known regarding which components of the fly ash matrix may be enriched with REEs, if at all. The lack of difference in REE contents as a function of ESP/baghouse row implies that the REEs were not partitioning onto the fly ash particles from the gas phase. This process is known to occur with volatile elements such as arsenic, selenium, lead, and gallium, which tend to adsorb to particle surfaces at cooler temperatures, resulting in higher contents in fly ash collected at later rows (shown in Figure S4).(24) Instead the REEs are likely distributed throughout the glassy particles, as demonstrated in a previous study that examined the distribution of cerium within cross sections of two fly ash samples.(39) Future investigations into the mineralogy and speciation of REEs in fly ash could provide insight into the correlations between REE and other elemental contents as well as methods for extraction.
Critical Rare Earth Fraction

The 16 REEs are typically grouped together because of their colocation in geological media. However, the abundances of the individual REEs relative to each other do not coincide with their utility. The supply challenges are primarily for the critical REEs, whereas excessive REEs (e.g., Ce, Ho, Tm, Yb, and Lu) are produced in surplus.(19)
The data in this study demonstrated that the critical REEs were greatest for the Appalachian and Illinois basin samples relative to the Powder River Basin samples. Regardless of the differences in critical REE contents of the various coal basins, the balance of critical and excessive REE in the coal ashes studied was superior to those of bastnaesite ores currently being mined, including the world’s largest REE deposit at the Bayan Obo formation in China, which has less than 10% critical REE.(19) Although the absolute REE contents in fly ash were approximately two orders-of-magnitude less than those of conventional ore, the higher fraction of critical REE in fly ash may represent a major advantage. Continued REE production from the same deposits may not alleviate the shortage of critical REEs and could result in the overproduction of excessive REEs (particularly cerium).(9) Our findings show that the coal fly ashes currently being produced in the U.S. would provide more than three times the critical REE mass per kg of total REE extracted than conventional ores. This makes fly ash an attractive source for REEs if the cost per kg of REE extracted can match that of REE mining (the cost of which will increase as REE mineral deposits are exhausted).
REE Reserves and Valuation of Fly Ash

The total REE “reserve” available in the fly ash depends not only on the content of REEs in the ash but also on the quantity of available fly ash. In 2014, 46% of fly ash produced in the U.S. was reused for beneficial purposes such as construction and road materials, while the remainder was generally discarded (Table S6).(40) Assuming that fly ash coal sources mirror the relative coal production by coal basin (22.3% in Northern/Central Appalachia, 11.7% Illinois, and 36.1% Powder River for 2013),(41) then the amount of unused fly ash produced annually is approximately 5.5, 2.9, and 8.9 million t for the Appalachian, Illinois, and Powder River Basin coals, respectively (Table S6). After converting the measured contents of each REE to rare earth oxide (REO) units (Table S7), we estimate that the annual total tonnage of rare earths (reported as REO) in unused fly ash is 4000 t for the Northern and Central Appalachian Basin, 1280 t for the Illinois Basin, and 3630 t for the Powder River Basin, for a total of approximately 8910 t as REO (Table S8). This total fly ash “reserve” of REOs produced annually at U.S. coal-fired power plants is the same order of magnitude for annual REO production at the Mountain Pass Mine (3473 t REO in 2013; 4769 t in 2014).(42) (Note that this mine ceased operations in 2015 due to bankruptcy filings by the parent corporation).
The total annual value of the rare earths derived from the fly ash reserve was estimated at $4.3 billion, based on 2013 prices for select REOs (Table S8). The valuation would be smaller if only a portion of the unused fly ash was usable for REE recovery (a more likely scenario). However, legacy ash in ash ponds and other impoundments could be a resource that was not part of this valuation.
The REO value per metric ton of ash was $329 for Appalachian-derived ashes, $238 for Illinois Basin ashes, and $203 for Powder River ashes (Table S9). These per-ton REO values provide a basis for evaluating the use of fly ash as an alternative REE resource. The costs of recovery, which would include transportation as well as separation technologies, should be scaled in relation to this valuation. Moreover, only a subset of fly ashes would be feedstock candidates, as shown by the variations of REE contents and extractability in this study. Therefore, these valuations will require adjustments according to the recovery technologies that are currently under development. Other opportunities for added value, such as the recovery of other metals and reuse opportunities for wastes after extraction, will likely need to be incorporated in the REE recovery process.
This analysis also shows that the potential value of the REEs in fly ash is driven by specific elements. The significant value of scandium ($3700–5000 per kg in 2011–2013), as well as high prices of critical REEs such as neodymium, dysprosium, europium, and terbium, contributes to more than 80% of the overall REE value in fly ash (Tables S8 and S9). This underscores the importance of the higher critical REE fraction in fly ash, which is one notable advantage over conventional ores. Efforts to optimize and commercialize REE recovery processes may benefit by focusing on these elements.
The other major contributor to the total value of REE in fly ash is the sheer volume of the fly ash waste stream. While the REO value per metric ton of fly ash is only a few hundred dollars, the amount of fly ash generated and not reused is substantial: approximately 27 million t per year. Moreover, in the U.S. coal ash is disposed of in approximately 1000 landfills and surface impoundments, which have been accumulating coal combustion residuals for years and sometimes for decades.(43) Waste management rules recently issued by federal and local authorities could provide additional incentives for REE recovery, particularly for ash impoundments that need to be excavated.
Nitric Acid Extractable Rare Earth Content

The REE resource potential for fly ash strongly hinges on the ease of extraction from the ash. The HNO3-extractable REE content in the coal ashes varied considerably in our sample set and was not necessarily related to the total REE content. For example, the sample with the highest total REE content (1220 mg kg–1) was a stoker ash from a Kentucky power plant burning an Appalachian Basin coal (Figure 1). However, only 3% of the total REEs in this sample were extracted via heated nitric acid.
In contrast, the fly ash samples derived from Powder River basin coals were among some of the lowest in total REEs for all of the fly ash samples (Figure 2A). However, these samples were notable for their superior recovery by nitric acid extraction (Figure 2C). These results coincided with their significantly higher calcium content (Figure 4B). This trend could be due to the major element composition of Power River Basin-based coals and their respective ashes, which are generally high in Ca content. Nevertheless, for other fly ashes such as those burned from Illinois Basin coals, the acid-extractable REE fraction appeared to correlate with the %CaO value; this relationship was not apparent for Appalachian fly ashes. Calcium-bearing particles (such as calcium oxides) were likely to be more soluble in nitric acid than the other major oxides comprising fly ash, such as silicon and aluminum oxides. It is not clear, however, if the REEs were present in the Ca-bearing phases of the fly ash particles. Previous research has indicated that REE are dispersed throughout the glassy matrix of the fly ash particles.(39) Therefore, high calcium content likely caused a greater fraction of each ash particle to dissolve during the nitric acid extraction process, perhaps exposing more surface area that could release REEs from the fly ash matrix. If the REEs were diffusely distributed within coal ash particles, one could reasonably expect an increase in specific surface area for the particles to lead to gains in REE recovery. Historically, one of the primary difficulties in extracting metals from fly ash has been incomplete dissolution. New REE recovery technologies will need to address this challenge, perhaps by focusing on methods to decompose the overall fly ash matrix rather than dissolve the individual REE-bearing minerals entrained within the aluminosilicate glasses.
The concentration of REEs in the HNO3 leachate before dilution was typically between 3 and 7 mg L–1 (approximately 10 mg L–1 for Powder River ashes). It should be noted that the ratio of acid (10 mL) to sample mass (?0.1 g) was chosen to optimize analysis, not extraction efficiency. Future development of leaching methods will need to minimize costs of reagents for the extraction processes. The fly ashes with high Ca contents might have an advantage in this respect in that sufficiently high extraction efficiencies could be achieved with smaller volumes of acid.
Overall, the results showed that ashes with high calcium content (e.g., class C fly ashes) such as those derived from Powder River Basin coals may be promising for REE extraction despite their lower overall REE levels. Notably, the extractable mass concentrations of REE for Powder River fly ashes were not significantly different from the extractable REE mass for Appalachian ashes (Figure 2D, Table S4). This suggests that higher extraction efficiency may be able to compensate for lower REE content in some ashes. Illinois Basin ashes had significantly less extractable REE mass than either Appalachian or Powder River Basin ashes (Table S4), indicating that ashes from this coal source may be the least promising of the three major basins.
Due to the nonspecific nature of the nitric acid digestion, many elements were leached from the fly ash in addition to the REEs and often in higher concentrations. Major impurities in the leachate include those elements comprising the fly ash particles themselves: Na, K, Al, K, Ca, and Fe (Si was not measured but is also expected in high concentrations). Trace elements of interest present in concentrations comparable to or exceeding the total REE included Sr, Ba, V, Cr, Mn, and As. Co, Ni, Cu, Ga, and Mo concentrations did not exceed total REE but were similar in concentration to Ce. Some of these impurities, such as Ga, are valuable in their own right. Nitric acid digestion was highly effective at mobilizing As and Se from the fly ash, recovering more than 80% of the total values in most samples. These preliminary results suggest that recovery of REE from coal fly ash would reduce the toxic metal content of the solid wastes, perhaps lowering the risks of solids disposal or reuse. However, these metals in leachates as well as large acid volumes are significant challenges that need to be addressed in the development of recovery technologies.
Implications for Fly Ash as an Alternative REE Resource

This research represents one of the first extended studies reporting REE contents and their extractable fraction for a broad variety of coal ashes generated at power plants across the United States. These results demonstrated widely variable total REE contents that appeared to depend on the origin of the feedstock coal which produced the ash. Power plants do not always burn coal from one source and can change suppliers over time or burn mixtures. Nevertheless, characteristics of the fly ash product (e.g., major element content) are often quantified by ash producers and recyclers as a way to determine suitability for the resale market. In this case, the percent major oxides reported by XRF data could provide indications of REE content, extractability, and other insights for power plants burning multiple coal sources.
The results of this study also highlight the importance of extraction efficiency in REE recovery in addition to the total REE content. To take advantage of the higher REE content identified in Appalachian Basin ashes, it will be necessary to develop scalable, high-efficiency recovery processes. These processes might need to be tailored specifically to the type of fly ash. Outside of the Powder River samples, nitric acid digestion was not an efficient means of leaching REE from fly ash. The maximum percent recovery by nitric acid for all of the ashes except for the Power River Basin ashes was only 44.1%, and many recoveries were significantly lower, even though heat (85–90 °C) and concentrated (?15 M) HNO3 were used. In contrast, the Na2O2 sinter pretreatment and leach with 25% v/v (?4 M) HNO3 yielded much greater leaching efficiency (generally >80% of HF values) than HNO3 alone (Table S5). The dramatic increase in REE extractability from Na2O2 sintering (as compared to HNO3 digestion alone) suggests that pretreatment with an alkaline agent (such as lime roasting, a common metallurgical process) would be an effective way to liberate REEs without resorting to HF digestion, which may be too hazardous for large-scale industrial use.
In summary, the ease of extractability may matter more than the total REE content, which has been the major focus thus far in the research literature. More attention is needed for improved methods of extraction and recovery. In this respect, the economic feasibility and environmental sustainability of coal fly ash as an alternative REE resource will hinge upon future developments in scalable recovery technologies.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b00085.

  • Additional information on the sources of CCP samples for this study, results of statistical analyses of the data, data for the reserve and valuation estimates, and additional plots of the REE data (PDF)

  • Spreadsheets of all total and individual REE data for each sample in this study (XLSX)

Trends in the Rare Earth Element Content of U.S.-Based Coal Combustion Fly Ashes

Showing 1/2: es6b00085_si_002.xlsx
1 Sc La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu Total REE Critical Uncritical Excessive PctCrit
2 Sample Plant Unit Type Row Basin mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 %
3 SRM 1633c 37.6 87 180 87 19 4.67 3.12 18.7 7.7 1.32
4 HF SRM 1 38.10361272136116 78.85830929326318 177.3054722773942 20.490174361384405 86.05936888547302 19.3279010145631 4.214072989212377 20.296920589418114 2.9693565730773908 16.579095264049705 98.61445659994318 3.375408685501941 9.265195970585824 1.2755645827116637 7.8216633753497105 1.1686919577693187 585.7252651410582 217.70154628234152 138.97330525862878 190.94680087872683 39.75400631447186
5 HF SRM 2 37.72515166808801 84.35388359621388 188.05646420856357 21.911561459343716 86.99254291029335 19.386102665576953 4.513252527270607 20.52064076123497 3.1154794948904625 17.947742313957008 103.02937262576012 3.561167684769791 9.86807121637695 1.365262840505195 8.293221799520232 1.2333016024103252 611.8732193747752 225.4664610885485 146.1721884823695 202.50941813576912 39.269741338523794
6 HF SRM 3 37.81951254451667 82.11905394215724 179.8268688014239 21.113899073270794 83.63231266121448 18.80797906228333 4.4366705878767245 19.915955745047683 3.033817735614898 17.232146048951375 100.85495758899275 3.4086969835109415 9.532494097155869 1.3108668438972682 7.996562562052318 1.1867184975865723 592.2285127755528 218.7223987198061 141.95688782275903 193.729713688471 39.45145165909266
7 HF SRM 4 35.40006083138632 84.96341988550589 191.04312654804482 22.1339835905136 87.4141201947147 20.04116816882559 4.540581872711691 20.84142380341338 3.175386109095213 17.946127770858528 104.62342316524777 3.549023867589485 9.903203370812426 1.3539447677578014 8.388361111878234 1.2107760285153453 616.5281310868709 227.60284248344033 147.97999544825845 205.54523232378568 39.16569412717889
8 HF SRM 5 34.829126268217195 78.51179981466748 179.9205091792818 21.212038626284826 84.29038490100189 18.87629554829682 4.343753126798686 20.042525101635835 3.0633810298372977 17.344238045627456 94.61348503507125 3.4426582623987425 9.411073581342066 1.2856954750808887 7.954302155080087 1.1931026012955348 580.3343687519176 213.06631571967864 138.64265909088496 193.79626767313704 39.058527604534426
9 HF SRM 6.1 34.003408384525294 73.46218885439458 170.97218606957017 19.71208194499269 79.31288599866096 17.996924105821197 4.190679587257135 18.658852397309502 2.8711039929437425 16.25972864220117 89.53886711664649 3.249579399118538 8.97219142678459 1.227416948778725 7.6522349254226025 1.1456936927557968 549.226023487183 201.1454567644941 129.83004730251798 184.24711103564584 39.040494510206216
10 HF SRM 6.2 35.12722506752463 74.05910034625343 172.492680971797 20.004879582422088 78.89967130757627 17.822605351266716 4.114785408004269 18.557030026536566 2.8565260682143876 16.293422956233133 93.74158020841074 3.2853673883026566 8.948171492596238 1.2223193495606015 7.600314750485507 1.1167783178736104 556.1424585930578 204.85415744103503 130.44361530647882 185.71746077801936 39.318266388269784
11 HF SRM 7 36.596433283268084 76.17121558368412 171.09894476278086 20.636426709767942 85.90094873423513 18.71034710234338 4.379784853225956 21.75930843681382 3.0468580204624116 16.44729297338088 92.24665262489735 3.3131880366513826 8.925608488392292 1.2842604054003306 7.602028153274369 1.1585205488597425 569.277818717438 210.94714569459404 137.27729783260926 184.45694190696668 39.600998169413856
12 HF SRM 8 35.07263353618407 74.11229946561164 167.21167450326553 20.30699590548009 84.46194467849395 18.556268637432005 4.362587311852729 21.721045423595722 2.9205171858932895 16.287296022336132 90.67650101032241 3.230129870664322 8.841399141444219 1.2451892763553865 7.559212815451622 1.1665318789224248 557.7322266633056 207.55024535034275 134.69660943211946 180.41273834465926 39.71040579367349
13 HF SRM 9 36.396961168985655 76.79636950711702 179.7037614498876 20.704157127969232 81.96107352046008 18.50528528786864 4.394462898969616 20.035160999766866 3.0564915439612284 16.819025980379735 94.1304447375574 3.3267927623327522 9.47646471373438 1.2841675571517812 7.983771089653654 1.193305843378802 575.7676961891742 209.83796339506242 136.04097292272178 193.4917987024046 38.90421740942399
15 92277 W FA App 42.93483117927798 117.85163681913156 266.24533913886904 28.92037157248438 113.21970736251755 23.44382564416786 4.187984051505075 27.054368606731845 3.8988493272627833 22.35842367464851 123.9108248347335 4.630569344738988 12.935090735495985 1.841965585762292 11.418987613511458 1.698100705092158 806.5508761959311 280.5108799861634 197.27020264251567 285.8349623879739 36.73454503958805
16 92278 W BA App 38.33990043444066 109.27559655396173 246.72676853853412 26.76797657310668 104.52282674050355 21.764773880986052 3.964224819679822 24.07946475257923 3.4566047279814396 19.385463101420033 106.47723672696956 3.9340135549773736 10.933140900401586 1.5957357032373887 9.730546480383397 1.4990121070763947 732.4532855962389 248.739497016956 181.8878117606337 263.4860763842087 35.835571296319934
18 93171-1 H 3 ESP 1 App 28.267732866010732 79.8423330885503 163.21242505621598 18.607625950097113 69.52838003591657 14.704287947564055 2.833990849886561 14.349567362967381 2.212484944191561 12.913854408207914 80.51782985080253 2.593831245217197 7.311964924308166 1.0361315195205179 6.427928234937819 0.9553563930795561 505.3157246774739 175.31850501331328 127.50381434917885 174.22567244897107 36.75070601336939
19 93171-2 H 3 ESP 1 App 29.383296386896394 81.60817051325807 165.50472724856988 18.717728320347938 70.3396617771444 14.854893773514261 2.861956160070301 14.664213088423601 2.2566898039911836 13.219977235947352 82.6378318523963 2.661598480892286 7.585806923624706 1.051156337406566 6.688284199137794 0.9738150862114411 515.0098071878324 178.90192375317426 129.84500569554388 176.87958135221794 36.839406369745795
20 93171-3 H 3 ESP 1 App 29.426697610841096 83.20262645951294 170.55166150409627 19.315999641851366 72.122476651926 15.266815076632975 2.888844236816861 15.019672370887287 2.321138985834131 13.541563714579814 86.0048282487852 2.7710397054129023 7.949331203722918 1.097230081317974 6.923702173376566 1.0224400702927874 529.426067735887 184.82818304166491 132.80511354888458 182.3660735344965 36.965683175848966
21 93171 Avg. H 3 ESP 1 App 29.025908954582736 81.55104335377378 166.42293793629403 18.880451304098802 70.66350615499566 14.94199893257043 2.8615970822579073 14.67781760742609 2.2634379113389587 13.225131786245027 83.05349665066134 2.6754898105074623 7.615701017218597 1.0615059794150195 6.679971535817393 0.9838705165279281 516.5838665337311 179.6828706027175 130.05131119786913 177.82377577856184 36.85193185298805
22 93171 Std. Dev. H 3 ESP 1 App 0.6569582578229282 1.6808749266369938 3.7547877516261567 0.3811920760889142 1.32702324738389 0.29120383631621477 0.027428456338040832 0.33525958988938875 0.05464044203608872 0.3138863972332208 2.767014776479093 0.08941720399392566 0.31973299197549365 0.031837004895353294 0.24799148127985238 0.03465384100283181 12.131999328919415 4.802697423020428 2.656664218793445 4.151524996234528 0.10803453565773156
23 93172 H 3 ESP 1 App 34.38114377311594 91.20413442595809 188.16427813526198 21.491939615013024 80.63384721819013 17.498339314116173 3.4110169141479116 17.485546778602767 2.7132270540967065 16.139253030648728 102.95364489545584 3.2772985572536832 9.2831596350378 1.3122892583237051 8.136180535411228 1.2317312923594237 599.3170304329931 215.13414874757711 147.67996013369003 202.12177777861004 38.081161743774985
24 93174 H 3 ESP 2 App 31.637713211000868 75.37786919148405 155.27385122270147 17.78831503816652 66.95015563664796 14.538564910998334 2.9875995750113615 14.753909851624417 2.335841112133022 13.832502667320217 88.4880509549529 2.8130701068127606 7.968469863030578 1.1206900565730507 7.1116795298368265 1.0589006692074563 504.0371835975017 182.56261980909602 122.45865899227331 167.37819158513156 38.64581381933591
25 93176 H 3 ESP 3 App 36.24415130372522 86.00027251603098 177.5311139619157 20.229122604519528 76.29000581057348 16.719246013623774 3.32580633991817 16.843609901370606 2.679787583096225 15.76089915861513 102.7596728829438 3.225193086846289 9.177333316264841 1.3043422782770893 8.175165145134729 1.1857042779992506 577.4514261808548 209.99350509141166 139.7922510355449 191.42151875017308 38.80093909289866
27 93259 EK 1,4 ESP 1 App 28.751428719098843 54.910761954004336 117.55705389499856 14.169597482595407 54.922683482534104 12.66194780906062 2.769029617445448 13.741441367629754 2.208258658595174 13.79295913571825 81.95539655008035 2.7011673014207367 7.8078012804986265 1.1090205477320492 6.348738813737746 0.9663928995106533 416.37367951466064 163.45612872487195 95.48374861329012 128.68237345739973 42.16892306605002
28 93260 EK 1,4 ESP 2 App 25.08701423202654 44.17028105922258 94.88181145287771 11.208451035574532 43.705511185063195 9.934445603294385 2.2136301053741056 10.51405059796868 1.6972764229595017 10.101887201705324 64.5105593901242 2.1089767933367307 5.83586862705937 0.7935945185558079 4.878532343044986 0.7297561842937785 332.37164675248147 128.0647329322857 75.82722829606017 103.39267129210901 41.67625692240268
29 93261 EK 1,4 ESP 3 App 38.58579525551114 67.24567751054177 152.7880670774552 17.281151597502028 66.77846239432053 14.770504053779625 3.3151111365995956 15.174316368363728 2.396815409041315 14.191349717985634 87.42640109578122 2.944613703921634 8.163130074879986 1.1107791579913222 6.909603398298107 0.9929954554346158 500.07477340740735 182.27126982860824 114.47164953018715 164.7460587931009 39.49634302395295
31 93283 H 1 ESP 1 App 34.7252685661491 75.15096779956052 163.99497749337684 18.396886988820718 69.80843484542146 14.957932757692701 3.370576567351231 14.986605698608662 2.4125647911046397 14.156753202898466 82.09352899114113 2.8990408650664863 8.176485625599804 1.153478882220243 7.156424158148512 1.0717393806245916 514.511666613785 180.01834402351676 123.4923932446826 176.2756607794367 37.52051845489031
32 93284 H 1 ESP 1 App 35.37201461599457 73.45753336350514 160.1387824607874 17.975626226158656 68.65284290598007 14.793718417979326 3.3412787364137095 14.889292199259533 2.3451675443212907 14.08115243918422 81.61317999122039 2.8839217001136657 8.121438911645528 1.1598993369560866 7.177653461172714 1.090858126165893 507.0943604368582 178.1550605287652 121.11617020690264 172.45111508519577 37.766932626172334
33 93285 H 1 ESP 2 App 35.15503022728491 65.15030804629586 143.15471497423135 16.099424290866718 61.278865934566724 13.60563874611151 2.9931824004292458 13.61613426656987 2.1510608233918047 12.667126878558506 73.35725032072115 2.575745259759971 7.221185470566168 1.0251585216607633 6.412043064706155 0.959880146892728 457.42274937261345 159.66867182823358 108.47150534984397 154.12754196725095 37.81218989493291
34 93286 H 1 ESP 2 App 36.56457717449927 69.79372122238757 153.9442632659883 17.3105687541943 66.09536418658858 14.579080726147058 3.2733708651649853 14.684082351600145 2.333607145571023 13.796542241508682 82.10281479068026 2.8671747554429525 8.080455732920116 1.1417113686414355 7.025569745483319 1.0487850644903902 494.6416893913085 175.68215496243366 116.36745305432906 166.02750420004642 38.35209188082788
35 93289 H 2 ESP 1 App 35.37734864490959 78.90224300425291 171.0055386166003 19.247700909679946 73.35238522573154 16.375681726956685 3.5623973323555385 16.536359180199273 2.6648901362959303 15.698024788888084 89.90674426233271 3.1783134120292877 8.954442876962732 1.274716299096561 7.9627761227487355 1.1619430975364884 545.1615056365763 194.13888462256654 131.06198482108883 184.58328754801136 38.082565328867226
36 93290 H 2 ESP 1 App 37.700898551095875 77.00791010528657 166.86317946604572 18.677432076322862 71.50969916463289 15.526475675983924 3.4081009290607374 15.996398847236579 2.5602243770321085 15.273142092004719 86.38498652310811 3.043047090006755 8.559299901547433 1.214969811707062 7.682088828548642 1.1206339066444115 532.5284873462646 187.695452987386 127.20821670482994 179.92391910295257 37.93148507430568
37 93291 H 2 ESP 2 App 35.825396554806225 68.6010301975516 150.02788420855796 16.891560211098405 64.84935352749203 14.295004478662333 3.1726132308582464 14.156888441962984 2.2947126953130432 13.365851755878143 79.01624595130738 2.7605427174587827 7.803720462961593 1.1052098144464186 6.906808742902405 1.0236695160656621 482.0964925073232 170.50249762381043 113.94448332927533 161.82411499943123 38.20603645859955
38 93292 H 2 ESP 2 App 36.411791879147216 72.83441612937096 160.3363411942053 17.964962745965103 68.7129347362044 15.12589634556261 3.437927415049824 15.368786923460851 2.491672776382126 14.616198296619151 85.02523593534458 2.9996038057442522 8.4998608474094 1.1861872045664545 7.4246083799770695 1.122132074679047 513.5585566896884 182.78383000700947 121.29406214435951 173.06887265917211 38.30767459559047
40 93342 C 3 ESP 1 App 34.254422618593225 88.9452915563984 186.24001776774637 21.1651325827157 80.02888143851051 16.888160929941066 3.444612198449308 16.753904995867867 2.6335768051277664 15.238776888084004 86.11991690896284 3.0654624442271325 8.554558370675604 1.208777104941193 7.562441550194366 1.130165430355043 573.2340995907905 196.02032260981002 143.75249006492302 199.2068642974641 36.368778079163384
42 93375 I 1 ESP 1 App 45.99533442606515 86.97900569083765 189.10401059548823 21.581563887344412 83.3581980857572 18.60063921092691 4.065226480496764 19.293180947908382 3.055590735465912 18.222272774065736 103.82272018239411 3.6844092240149995 10.510513666420367 1.485288704549794 9.241628998678761 1.3770654354101983 620.3766490458245 223.03452192460009 146.45438973701738 204.892402958142 38.83039302423147
43 93376 I 1 ESP 2 App 46.56376307140309 79.89316726780896 173.2518356522985 20.10880154479588 78.83924062979045 17.94061767959294 3.8566156873852804 18.175923990414116 2.865378261678397 17.122233121929998 100.27641762893441 3.513409222244985 10.067465016016971 1.4074153787938248 8.888486775385248 1.3367788227755573 584.1075497512486 213.0273503457355 136.1185104826119 188.3979258514981 39.62976703005071
44 93377 I 1 ESP 3 App 49.766752389515844 79.2278011257141 172.46054563643025 19.970124772604656 77.51566490004666 16.992963759103052 3.7987700077129505 17.16271952102122 2.7959664007874125 16.48498668011497 95.61566546635086 3.413753086662704 9.59388487218221 1.388144257890295 8.44734010683705 1.260164354993163 575.8952473379675 205.80493832719506 133.353609178443 186.96994744281346 39.11685839166707
45 93382 I 2 ESP 1 App 38.47884989638275 82.21396171971173 174.91055962358837 20.09959606518188 76.97440592363607 16.280116329887246 3.5025518016551294 16.22381624101536 2.4828544006209854 14.53057054870971 82.3078826777129 2.906895110827971 8.252371207539898 1.1657788318829376 7.272077664376667 1.1127521107239362 548.7150401534533 188.0506365598747 134.81749035579622 187.36806334139987 36.855605335468205
46 93383 I 2 ESP 1 App 37.14261994287472 81.9874278769172 176.00236553123267 20.028169030056503 76.6466673619552 16.276600526858672 3.4421029095336997 16.36199856189451 2.545873456479523 14.725173178055348 83.65584069126243 2.97404088235276 8.42872564013646 1.1935926679550968 7.528662578534291 1.1464533769363974 550.0863142130353 189.44438323742264 134.65419599572687 188.84511503701123 36.93278333540538
47 93384 I 2 ESP 2 App 39.10285657879896 77.51814353509351 166.50307595091297 19.2002285403618 74.15314789606416 15.996792590220354 3.46265915519795 15.863046133919761 2.5106489161973897 14.301846591174735 82.80097135920427 2.960798476422021 8.405432730672656 1.2006518984715884 7.577341423783144 1.1078964145056933 532.665538191001 185.63470664851116 128.5782107995954 179.3497641640954 37.61117150148855
48 93385 I 2 ESP 2 App 39.40673696634707 76.14279717207728 164.3372756096745 18.90059084860865 73.06747907387552 16.02905269246701 3.443708274911027 15.867715375363346 2.5145266613082846 14.517642374896157 85.0554040174978 3.003673959797185 8.589647828948447 1.194354165946899 7.616148980701029 1.1379763431055543 530.824730345526 187.18840823143722 126.94015608851629 177.28942905922517 38.09148438872941
49 93386 I 2 ESP 3 App 43.92313818628305 77.23230658264212 168.50380711197766 19.407934142004613 74.92584692217412 16.246274994143604 3.5880398845134676 16.58088174609658 2.6066247671827534 15.280414623464434 90.94901585866339 3.1839203694255658 9.024217418743364 1.2731769390280387 7.868809868020626 1.1927563785045923 551.787165792868 196.37415947474153 129.46739746488691 182.0224706669565 38.66668021363822
50 93387-1 I 2 ESP 3 App 39.3892607226472 83.05623526526666 182.49682315496165 20.878377493653097 80.95330907300738 17.785955642250443 3.8828970884040275 17.865479244313867 2.831096017384366 16.65316116675841 98.69400746928451 3.4133405644898818 9.847712740223644 1.3880704322677022 8.596220375386313 1.2879355080349528 589.019881958334 212.86218355506233 139.5860476454841 197.18239003514051 38.72822498071572


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The authors declare no competing financial interest.


We thank the power utility companies and coal ash suppliers for providing samples for this study. The research was supported by the U.S. National Science Foundation programs in Environmental Engineering (CBET-1510965) and Partnerships in International Research and Education (OISE-12-43433). R.K.T. was supported in part by scholarships from the Environmental Research and Education Foundation and the American Coal Ash Association.


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