Selective Recovery of Rare Earth Elements from Coal Fly Ash

Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Selective Recovery of Rare Earth Elements from Coal Fly Ash Leachates Using Liquid Membrane Processes Ryan C. Smith,*,† Ross K. Taggart,† James C. Hower,‡ Mark R. Wiesner,† and Heileen Hsu-Kim*,† †

Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina 27708, United States Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40511, United States

‡

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S Supporting Information *

ABSTRACT: Coal combustion residues and other geological waste materials have been proposed as a resource for rare earth elements (REEs, herein defined as the 14 stable lanthanides, yttrium, and scandium). The extraction of REEs from residues often generate acidified leachates that require highly selective separation methods to recover the REEs from other major soluble ions in the leachates. Here, we studied two liquid membrane processes (liquid emulsion membranes, LEM, and supported liquid membranes, SLM) and compared them to standard solvent extraction techniques for selective recovery and concentration of REEs from a leachate of coal fly ash. All separation methods involved an organic solution of di(2-ethylhexyl)phosphoric acid dissolved in kerosene or mineral oil and an acid strippant solution of 5 M nitric acid for the liquid-based separations. The LEM configuration, which separated REEs by immersing an acid-in-oil emulsion in the ash leachate, resulted in similar recovery percentages of individual REEs as the conventional solvent extraction approach. The recovery of REEs in the SLM configuration, which involved the impregnation of the solvent in a hydrophobic membrane, was slower than the LEM process. However, the SLM process was notably more selective for the heavy (and higher value) REEs, while the conventional extraction and LEM processes were more selective for the light REEs. A flux-based model of the extraction processes suggested that recovery rates were limited by REE affinity for the solvent chelator in the SLM, while the rates of REEs separation via LEM were limited by diffusive mass transfer across the liquid membrane. Altogether, these results help to identify specific steps in the recovery process that future work should target in the development of scalable liquid membrane separations for REE recovery.

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INTRODUCTION

and other industrial and construction applications, and the remaining portion being disposed of in ash ponds and landfills.22 Technologies for the extraction and recovery of REEs from coal fly ash are not well developed, and this knowledge gap creates a need to understand the feasibility of this alternative source of REEs. The first step in extraction typically produces a complex acidic leachate containing total REEs at 0.5−5 mg L−1, with individual REE concentrations ranging from 0.001 to 1 mg L−1. The leachate mixture will also comprise other major solutes including ions of sodium, silicon, aluminum, iron, and calcium in the range of 100−1000 mg·L−1 in the acid mixture (e.g., typically diluted HCl or HNO3). Other potential “low grade” feedstocks (e.g., leachates of mine wastes, geological brines) have similar levels of matrix complexity.11−20 As such, the 102- to 105-fold difference in concentrations between individual REE and major

Rare earth elements (REEs) are considered critical materials for a variety of modern technologies including the consumer electronics, automotive, energy, and defense industries.1−7 With the history of an unstable REE global supply market as well as environmental concerns of conventional REE mining practices, there exists a need to develop alternate REE sources and improve upon existing methods of REE production.8−10 Recently, researchers have begun investigating the potential for coal fly ash, coal residuals, geological and marine brines, and other waste residues as alternative resources for REEs.11−20 For example, previous research by our group has shown that the REE content of coal fly ash is typically greater than average crustal earth values; however, the enrichment depends on the type of coal feedstock used to generate the ash. Coal fly ashes from Appalachian Basin coals were typically greater in REE content (average 600 mg of REEs per kg ash) relative to fly ashes of other major coal producing regions in the U.S.21 Within the United States, over 97 Mt of coal combustion products are created each year with greater than half being utilized for concrete, drywall, © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

January 24, 2019 March 22, 2019 March 25, 2019 March 25, 2019 DOI: 10.1021/acs.est.9b00539 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

following major metal element composition 54.1% as SiO2, 28.4% Al2O3, 10.9% Fe2O3, and 1.28% Ca (sample ID no. for this ash is 93938 App-FA1 in previous studies describing REE chemical speciation and leaching methods).21,32,33 Leachates containing the REEs were produced from this ash by alkaline roasting followed by acid leaching, a method that is similar to the previous report.21 In brief, 4 g of fly ash and 4 g of powdered sodium hydroxide were mixed in a zirconium crucible and heated at 450 °C for 30 min in a muffle furnace. The roasted ash was cooled to room temperature and mixed with 1 L of 0.11 M HNO3 for 12 h on a stir plate. After mixing, solids were allowed to settle for 1 h and the supernatant was decanted from the remaining solids. For all separation experiments, the decanted leachate was used within 8 h of preparation. In addition to the coal ash leachates, the LEM and SLM separations were tested on a synthetic leachate solution that was formulated based on the average composition of leachate solutions generated from four different Appalachian coal fly ashes (Table S1 in the Supporting Information). The use of a synthetic solution enabled consistent testing of the methods, as the alkaline roasting and acid leaching process resulted in some variation of leachate composition. The synthetic leachate was prepared by mixing the hydrated nitrate salts of major metals (Na, Mg, Ca, Al, and Fe ranging from 10 to 6000 mg L−1) and sodium metasilicate nonahydrate in a solution of HNO3 (5% v/ v). Seven REEs (Sc, Y, Nd, Eu, Tb, Dy, and Er) were added from their respective standards (Inorganic Ventures) to a final concentration of 150 μg L−1 each.12 This concentration value is approximately 1−100 times greater than concentrations in fly ash leachates. These REEs were selected for initial testing based on their criticality34 and also to span a spectrum of light, medium, and heavy REEs. A comparison of individual major metal and REE concentrations in the real and synthetic leachates is provided Table S1. Unless otherwise noted, all separation experiments were performed with real coal ash leachates, as described above. Liquid Emulsion Membrane. The LEM process was based on methods by others23,25,35,36 and entailed first a preparation of a water-in-oil emulsion. The oil-phase comprised of 10% (v/v) DEHPA and 1% or 3% (v/v) Span 80 in either kerosene or mineral oil with a total final oil-phase volume of 50 mL. This oil phase was combined with 50 mL of 5 M HNO3 stripping solution, added dropwise under high speed mixing at 7000 rpm (T18 Ultra-Turrax disperser, IKA Works, Inc.) for a total volume of 100 mL for the prepared emulsion. Immediately after preparation, the emulsion was mixed with 500 mL of coal ash leachate and stirred with a magnetic stir bar at 200 rpm for 15 min to 1 h extraction time. After extraction, the leachate/ emulsion mixture was poured into a 2 L glass separatory funnel, held static for 2 h, and decanted into separate aqueous and emulsion phases. After emulsion separation into oil and acid phases, the acid was analyzed for REE and major ion concentrations by inductively coupled plasma mass spectrometry (ICP-MS), as described below. Initial work was first performed using the synthetic leachate to determine the optimum amount of Span 80 for the kerosene and mineral oil emulsions. After the surfactant amount was determined, three separate experiments were performed using a real ash leachate and mineral oil as the diluent for DEHPA and Span 80 (2%). Due to the difference in volume between acid strippant and leachate, the total mass of recovered REEs was compared to the total mass of initial REEs in the leachate.

metal ions poses a considerable challenge for economical recovery of REEs. Here, we present work on two different separation processes that use liquid membranes to recover REEs from ash leachate: liquid emulsion membranes (LEMs) and supported liquid membranes (SLMs; Supporting Information Figure S1). Liquid membrane processes use nonaqueous solvents with dissolved chelating agents to physically separate the aqueous leachate and acidic strippant phases while simultaneously recovering REEs. The aqueous strippant may be stabilized within an emulsion (LEM process), or the nonaqueous solvent may be impregnated in a physical membrane (SLM process).23−26 These processes differ from standard solvent extraction as both REE extraction and recovery occur in the same step. Moreover, the advantage of liquid membrane process configurations is they maximize interfacial area of REE mass transfer while also reducing the required volumes of nonaqueous solvent and chelating agent. Previous work by others has demonstrated recovery of neodymium dissolved from rare earth magnets using supported liquid membranes.27 However, to our best knowledge no studies have applied this separation approach for coal ash leachates or similar low-grade feedstocks of geological origin. For liquid membrane processes, kerosene or other organic liquids are normally used for the nonaqueous phase.28,29 However, working with kerosene presents a hazard and handling issue due to its flammability such that alternative solvents may be attractive. The efficacy of less hazardous solvents and cost trade-offs must be evaluated. The goal of the research was to investigate the application of liquid membrane processes for recovering REEs from leachates of coal fly ash as a representative nontraditional REE resource. The research examined recovery potential as well as selectivity for individual REEs. Recoveries of individual REEs were also compared as a function of LEM and SLM process variables such as separation time or the type of solvent (kerosene or mineral oil). Finally, a mathematical model of mass transfer potential in these systems was developed to better understand rate-limiting process for REE recovery selectivity.

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MATERIALS AND METHODS Kerosene (purum) or mineral oil (white, light) were compared as the nonaqueous phases for the separation experiments. Kerosene is often used at the industrial scale for liquid−liquid separations. Mineral oil presents a reduced hazard of flammability due in part to its lower volatility. Di-2-ethylhexylphosphoric acid (DEHPA, 97% purity) was dissolved to a concentration of 10% (v/v) into either kerosene or mineral oil solvent. DEHPA is a strong metal chelator for REEs that is frequently used for industrial metal separations and is considered a less hazardous metal carrier than other industrially relevant organophosphorus compounds such as tributyl phosphate.30,31 The acid strippant solution for all separations comprised of 5 M HNO3 (≥99.999% purity, trace metal basis). For the liquid emulsion membrane, Span 80, a hydrophobic surfactant, was used as a stabilizer for the emulsion. All chemicals were purchased from Sigma-Aldrich and used without further purification. Fly Ash Leachate. Leachates were produced from a fly ash sample collected in 2014 from a storage silo of a coal-fired power plant in Kentucky. This ash sample was produced from a coal feedstock originating from the Central Appalachian Basin region. In a previous study,32 we report that this sample contained total REE content of 703 mg kg−1 and comprised the B

DOI: 10.1021/acs.est.9b00539 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 1. (A) Average percent recovery (n = 3) of major ions and REEs using solvent extraction with 10% DEHPA (v/v) in kerosene after 24 h extraction. Error bars indicate standard deviation. (B) Mass recovery via liquid emulsion membrane (at 15, 30, and 60 min mixing time) utilizing liquid phases of 10% DEHPA and 2% Span 80 in mineral oil, and 5 M HNO3 as the acid strippant. (C) Average percent recovery (n = 3) of major elements and REEs via supported liquid membrane (SLM) using 10% DEHPA in kerosene from a leachate of coal fly ash after 24 h. Error bars indicate standard deviation. Bar colors correspond to major ions (blue), REEs (green), and radionuclides (red).

Supported Liquid Membrane. A 10% (v/v) solution of DEHPA in kerosene or mineral oil was prepared and passed through a 47 mm 0.22 μm PVDF membrane (EMD Millipore) by vacuum filtration until the DEHPA solution had soaked through the entire membrane. PVDF was chosen for the membrane due to its high hydrophobicity and use by other researchers.37 The membrane was removed from the vacuum filtration apparatus and soaked in the remaining 10% DEHPA mixture at least 12 h. Then the membrane was placed in an Hcell reactor system (Item # MFC 250.40.0, Adams & Chittenden Scientific Glass) consisting of two 250 mL flanged glass jars that allowed placement of the prepared 47 mm membrane between the two glass chambers (see Figure S2 for a schematic). Ash leachate and 5 M HNO3 stripping solution (250 mL each) were added to opposite sides of the H-cell reactor and stirred at 700 rpm. Initial testing with synthetic leachate focused on determining the optimal amount of time to perform extraction, and the effect on recovery due to the difference between using kerosene and mineral oil as the support phase for DEHPA. For testing with real leachate, only kerosene was used as the diluent for DEHPA. Triplicate SLM extractions were performed on replicate aliquots of a fly ash leachate over 24 h; initial studies on extraction demonstrated equilibrium was reached after 20 h (Figure S3). The strippant concentration after 24 h was analyzed for major and trace elements via ICP-MS, as described below.

The concentrations of the triplicate separations were averaged for each element. Conventional Solvent Extraction. In addition to the two liquid membrane processes, standard solvent extraction was also performed in triplicate using both kerosene and mineral oil at the same ratios of solvent, acid, and coal ash leachate used for the liquid emulsion experiments. A 100 mL aliquot of real coal ash leachate was separately mixed with 10 mL of 10% DEHPA in mineral oil for 24 h. The organic and aqueous portions were decanted, and 4 mL of mineral oil was mixed with 4 mL of 5 M HNO3 for another 24 h and the acid was analyzed for REE content by ICP-MS. Elemental Analyses. Major and trace element concentrations were analyzed in the leachate feedstocks and separation products by ICP-MS (7900, Agilent Technologies). Elements Ca and Si were quantified under hydrogen reaction gas conditions, while all other elements, including the REEs, were quantified under helium reaction gas mode. Instrument calibrations were performed at the start of a sample batch, and internal standards of Rh and In were spiked in each sample and used to correct for shifts in signal intensity during a batch run. Barium oxides are known to contribute to the mass signal for europium,38,39 and silicon oxides contribute to the mass signal for scandium.40 Due to the high concentrations of both barium and silicon in ash digests, it is necessary to account for these C


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Table 1. Estimated Composition, on a Dry Mass Basis, for Major Elements and Total REEs in Initial Leachate and Final Strippant Products of Liquid Emulsion Membrane (LEM), Supported Liquid Membrane (SLM), and Conventional Solvent Extractiona 15 min LEM

Na Mg Al Si K Ca Fe REE

30 min LEM

60 min LEM

SLM

Kerosene Solvent

initial leachate (mg kg−1)

final strippant (mg kg−1)



initial leachate (mg· g−1)






681 000 7500 132 000 138 000 11 700 9210 19 100 302

410 500 18 050 376 700 40 810 8396 130 200 3054 3300

754 000 3840 103 000 108 000 12 300 5780 12 000 146

429 200 8794 442 300 25 700 8056 75 570 2660 1710

796 000 7790 73 700 97 400 13 000 7920 2480 176

402 600 16 600 385 400 32 330 8301 143 000 739 3700

821 000 7450 57 000 88 600 14 200 7410 3050 136

702 200 6297 95 200 124 300 20 870 26 960 16 220 5280

753 000 7850 99 000 103 000 12 900 8870 12 900 213

2079 260 966 800 1574 176 23 270 1209 1820

a

Concentrations were calculated by taking the measured aqueous concentrations and assuming the metals formed nitrate salts upon drying. See the Supporting Information spreadsheet file.

substantial mass recovery of Al and Ca was observed (64% and 17%, respectively). However, radioactive elements such as Th or U were not recovered to appreciable extents. Note that the percentage for each element refers to the final element mass in the acid stipping solution normalized to the element mass in the initial feed solution (e.g., the ash leachate). Concentrations values for major ions and REEs for the feed and the strippant product solutions are provided in the SI .xlsx data file. While the percentage recovery provides information for individual elements, the purity of the mixed REE product is easier to compare to REE extraction technologies by calculating a dry mass-based concentration. To estimate this concentration, we used the measured aqueous concentrations of all measured metal and metalloid elements (REEs and major cations) in the strippant and determined the dry mass assuming all metal cations formed nitrate salts upon drying (e.g., La(NO3)3). Si and U were assumed to dry as H4SiO4 and UO2(NO3)2, respectively. For the solvent extraction data, the mixed REE purity was estimated to be 1820 mg·kg−1 (dry mass basis), an 8-fold increase compared to the initial leachate (Table 1). Liquid Emulsion Membrane Processes. The LEM separation process was first tested on a synthetic leachate to establish surfactant concentrations necessary to form a stable emulsion while also maximize REE separation. We observed a decrease in REE recovery when the surfactant concentration increased from 1% to 3% in mineral oil (Figure S4). An excess of surfactant at the oil−water interface might hinder transport of REEs into the oil phase, as indicated by others.23 We also observed that increasing the concentration of Span 80 improved the stability of the emulsion. LEM separations using less than 2% Span 80 did not yield an emulsion that could be stable over for a 60 min reaction and 2 h settling time. Therefore, 2% Span 80 was selected for all subsequent experiments with real ash leachates as a means to maximize both emulsion stability and REE recovery. The recovery percentages of REE from the coal ash leachates by the LEM process were similar to results of the conventional solvent extraction process (Figure 1B). We note that the LEM separation utilized mineral oil as the organic solvent for the DEHPA (10% v/v) instead of kerosene with DEHPA. Regardless, the general trend of greater extraction of the light REEs over the heavy REEs was observed in the LEM process. The mixing time of the LEM process varied between 15, 30, and 60 min with a constant phase separation time of 2 h. Between mixing times for the LEM process, a 60 min extraction time achieved greater than 70% recovery of most REEs, and as

interferences that affect quantitation of Eu and Sc. A detailed explanation of the method for correction can be found in the Supporting Information, but in brief, the percent oxide formation for Ba and Si was calculated and then used to correct the signal intensity at 153 and 45 m/z (mass/charge) for Eu and Sc, respectively. Separation Factor for Individual Metals. For each process, the relative separation efficiency for each REE was quantified by computing individual distribution coefficients for each element and comparing this distribution to a reference metal. Here, we defined the distribution coefficient of a particular metal, D Me , as the ratio between the final concentration in the strippant, [Me]strippant, over the final concentration remaining in the leachate after extraction, [Me]leachate: DMe =

[Me]strippant [Me]leachate

(1)

Previous work by Kim et. al using the SLM process to recover REEs from scrap magnets using TODGA, showed distribution coefficients for Nd on the order of 103.27 Due to the complex nature of the coal ash leachate, we compared the relative selectivity of a particular metal to a reference metal, sodium (Na), by calculating an apparent separation factor, αMe/Na according to αMe/Na =

[Me]strippant [Na]leachate DMe = [Na]strippant [Me]leachate D Na

(2)

Values for αMe/Na were calculated for each REE as well as for major ions (Ca, Fe, and Al). These calculations utilized the following resulting data for each process: (1) the average of triplicate extractions in the conventional solvent, (2) the 30 min mixing time (n = 1) for the LEM process, and (3) the average (n = 3) at the 24 h time point for the SLM process. Na was used as the reference because this metal was the greatest concentration for all dissolved metals in the ash leachate feedstock.

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RESULTS AND DISCUSSION Conventional Solvent Extraction. The separation and recovery of REEs from the leachates via conventional solvent extraction (with kerosene and DEHPA) varied with the type of REE. The recovery percentages for the lighter REEs (La, Pr, Nd, Sm, and Eu) were generally greater than heavier REEs (Dy, Ho, Er, Tm, Yb, and Lu; Figure 1A). In addition to the REEs, D


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Environmental Science & Technology the mixing time increased individual recoveries approached the recoveries observed with solvent extraction with kerosene, Figure 1B. For all REEs, there was little difference between 15 and 30 min extraction, but a large difference between 30 and 60 min extraction. This difference is especially true for most heavy elements such as Tb, Dy, and Ho that increased from 60%−66% recovery at 15 min extraction to 77%−83% recovery for 60 min extraction. Interestingly, the recoveries of Ca and Al switched between solvent extraction and LEM with Al being recovered more effectively using kerosene solvent extraction and Ca recoveries relatively greater for the mineral oil LEM. The expected dry mass REE contents of the LEM product were approximately 3300, 1710, and 3700 mg kg−1 for 15, 30, and 60 min mixing times (Table 1). These values represent a 10-, 12-, and 21-fold increase in purity relative to the initial feed REE concentrations. For the 15 and 60 min mixing times, REE purity was almost double the dry mass purity for the kerosene solvent extraction (1820 mg kg−1.). Supported Liquid Membrane Process. Initial studies were performed with the synthetic leachate to understand the time scale of the SLM process. The results (Figure S3) showed that the concentrations of REEs in the acid strippant increased within the first 3 h of separation and, after this period, did not change considerably between the 3 and 20 h. Based on these results, we used the 24 h time point to represent an approximate steady state for subsequent SLM experiments with real coal ash leachate solutions. The application of the SLM system for the coal fly ash leachates resulted in relatively higher percentage recoveries for the heavy REEs compared to the light REEs (Figure 1C). For example, the recoveries of Y, Tb, Dy, Ho, Er, Tm, Yb, and Lu were all greater than 75% after 24 h of extraction, while recovery of La, Ce, Pr, and Nd was less than 50%. This general difference between the light and heavy REEs is opposite to the trend observed for the LEM and conventional solvent extraction separations. For the major metal constituents Na, Mg, Al, Si, K, Ca, and Fe, less than 10% was recovered in the strippant solution. When comparing the expected solid phase product between the all recovery processes studied (Table 1), the SLM process resulted in the highest purity with 5280 mg kg−1 total REEs. The expected solid phase REE content of the original leachate was only 136 mg kg−1 indicating that the SLM process was capable of increasing REE content by approximately 39 times. Between all three studied processes, the SLM process provided the highest purity mixed REE product. However, in addition to the desired REEs, high recovery of uranium was also observed. Uranium is commonly extracted during nuclear fuel reprocessing using tributyl phosphate which is similar to the extractant DEHPA used in this study.31 While the total recovery was 75%, the expected amount of uranium in the final dried product only would be 0.39% (see the Supporting Information). Low Recovery of Scandium. For all three liquid separation processes, Sc was not observed in appreciable concentrations in the final acid strippant solution. For example, we observed that in the SLM process the Sc concentration in the leachate feed decreased from an average 29 to 19 μg/L, indicating that 32% of the original Sc was lost from the leachate but only 1% was recovered into the strippant. The Sc complex with DEHPA has a very high stability constant (relative to the other REEs). For example, the estimated binding constant K for the reaction Me3+ + 3 DEHPA2 + Me(DEHP·DEHPA)3 + 3H+ is 1.1 × 107 for Sc3+ (see the Supporting Information) and 1.8 × 103 for Dy3+.41

Difficulties with recovering Sc after extraction with DEHPA have been known by others for almost 50 years.42 If these processes are to be scaled up, economical solutions must be developed for recovering Sc and other heavy REEs from the extractant in order to prevent eventual poisoning of the DEHPA. Separation Factors for REE and Major Metal Ions. In comparing the three processes, we observed two important distinctions: selectivity trends between individual REEs and selectivity of the REE relative to other competing metals such as Ca, Fe, or Al. The apparent separation factors αMe/Na of each REE in the kerosene solvent extraction were always greater than respective αMe/Na values for either liquid membrane processes due to the low recovery of Na during solvent extraction (Figure 2A). The αMe/Na values for the solvent extraction also tended to

Figure 2. Selectivity values for kerosene solvent extraction, LEM process, and SLM process for (top) all REEs and (bottom) major competing metals and light and heavy REEs.

decrease slightly with increasing hydrated ionic radius for the REE. The LEM αMe/Na values remained fairly constant with increasing hydrated ionic radius apart from Ce, Yb, and Lu while αMe/Na values for the SLM process increased with ionic radius. In comparisons of the REEs to major competing metals, different trends emerged where the LEM and SLM processes always show greater selectivity for REEs when compared with solvent extraction, Figure 2B. All three processes demonstrated greater selectivity for REEs over Ca and Fe, but αMe/Na values for Al during solvent extraction were within the same order of magnitude as the light REE Nd. In total, while the αMe/Na values for REEs during solvent extraction were consistently higher than both the LEM and SLM processes, compared to other major competing metals like Al, the LEM and SLM processes demonstrated higher selectivity for REEs Difference between Kerosene and Mineral Oil. The importance of the carrier organic solvent for the liquid membrane processes was further explored by directly comparing E


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Figure 3. Effect of solvent on recovery of selected elements using synthetic leachate for (left) liquid emulsion membrane process with 30 min mixing time (n = 1) and (right) supported liquid membrane processes after 20 h equilibration time (n = 1). Note that REE separation from real leachate with the mineral oil solvent was not reproducible.

kerosene and mineral oil for separation of REE in synthetic coal ash leachates. In the LEM process, the recovery of the REEs (Y, Nd, Eu, Tb, Dy, and Er) was greater in mineral oil than with kerosene as the organic solvent (Figure 3A). In contrast, the SLM process demonstrated a consistently lower recovery using mineral oil as a solvent (Figure 3B). This difference in recoveries was approximately 18% for each REE in the case with 1% Span80 as the surfactant. For the SLM system, the opposite effect of solvent was observed, with kerosene providing REE recovery percentages that were an average 16% greater than REE recovery percentages in the mineral oil SLM system. Previous work by others43 with an amide chelating agent also has demonstrated the impact of solvent on REE recovery. While mineral oil and kerosene are both hydrocarbon mixtures, they may be sufficiently different in polarity, resulting in observable variation in REE mass transfer of the DEHP−metal complex.43 We also note that we had significant difficulty in reproducing the mineral oil/DEHPA system for the SLM process. Despite using the same experimental conditions, we sometimes observed zero recovery of REEs (data not shown). The SLM separations with kerosene were fairly reproducible (e.g., Figure 1C). While the reason for the poor reproducibility of mineral oil in the SLM system remains to be elucidated, we suspect that the miscibility of the DEHPA in mineral oil might be a factor. Model of REE Flux for Liquid Membrane Processes. The relative recovery of individual REEs differed greatly between the LEM and SLM processes. The lighter REEs tended to be recovered more efficiently in the LEM process, while the heavier REEs were more efficiently extracted during the SLM process even though both processes used similar liquid−liquid reagents (10% DEHPA in organic solvent and 5 M nitric acid as the strippant). The transfer of REE ions from the original ash leachate, into the oil phase and then into the acid stripping phase entailed a multiple-step process (Figure 4A) including: (1) loss of hydration shell surrounding the trivalent rare earth ion, (2) complexation of the rare earth ion with DEHPA, a process that depends on the relative binding constant between the metal ion and the ligand, (3) mass transfer of the metal−DEHPA complex through the oil phase, a process driven by diffusive flux across a concentration gradient in the oil phase, (4) release of rare earth ion at the interface of the acid stripping solution and proton exchange with Me(DEHP·DEHPA)3, and (5) back diffusion of protonated DEHPA to the oil-leachate interface. Any one of these steps might be rate limiting for REE ion recovery in the acid stripping step, depending on system configurations such as interfacial surface areas between the leachate-oil and oil-

Figure 4. (A) Schematic of reactions during the separation and concentration of REEs from coal fly ash leachate in the liquid emulsion membrane and the supported liquid membrane processes. (B) Concentration profile of a single REE (purple line) for a transport model where the steady state flux of the metal across each liquid (JL, JO, and JS) was controlled by diffusive transport across a diffusion film layer of thickness δ at each liquid−liquid interface.

strippant phases, the thickness of the oil layer for mass transfer, and the concentration of available chelating agent. In order to identify the potential influences of the system configuration on overall recovery of different REEs, we calculated the potential flux of each REE under steady state conditions as a means to understand the major rate-limiting step for mass transfer. The model assumed that REE transport occurred through five regimes: (i) bulk leachate, (ii) leachate film, (iii) organic phase, (iv) strippant film, and (v) bulk strippant as detailed in Figure 4B. The calculations were performed with the following assumptions: 1 Each flux (JL, JO, and JS) represents the flux of an individual metal. 2 At steady state, the flux across the boundary layer of each phase is equal (JL = JO = JS) and was equal to the product of a diffusion coefficient and a concentration gradient (Fick’s first law). 3 The concentration of the fully protonated DEHPA in the organic phase, (HR)2, is determined by equilibrium between the leachate and the organic phase. 4 After extraction by DEHPA, all metals are then recovered by the acid strippant with no metals remaining in the organic phase. F


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(HR)2, the diffusion boundary layer thickness (δL, δO, and δS), and the diffusion coefficients DL, DO, and DS in the leachate, oil, and strippant phases. A complete summary of the methods used to determine each individual parameter is given in the Supporting Information. Model calculations (Figure 5A) suggest that the flux of each metal varies with the configuration of the system (i.e., LEM or

5 The concentration of acid in the strippant phase remains constant (5 M). Complexation of all metals with DEHPA was assumed to occur according the following equilibrium reaction: Me3 + + 3(HR)2 ↔ Me(R·HR)3 + 3H+

(1′)

where the overbar indicates the organic solvent phase. DEHPA dissolved in nonaqueous solvent exists as a dimer, (HR)2, and the lanthanide−DEHPA complex exists as one lanthanide ion with three DEHPA dimers, [Me(R·HR)3].44,45 For each lanthanide, there is an associated equilibrium constant: [Me(R· HR)3 ][H+]3

Keq =

3

[(HR)2 ] [Me3 +]

(2′)

Equation 2 can be rearranged to create a relationship between the metal concentrations at the interfacial regions of the organic phase and both aqueous phases: kL =

kS =

[Me(R· HR)3 ] [Me3 +]L [Me(R· HR)3 ] [Me3 +]S

3

=

Keq[(HR)2 ] [H+]L

3

(3a) 3

=

Keq[(HR)2 ] [H+]S

3

(3b)

where subscripts L and S indicate the leachate and strippant phases, respectively. The flux of an individual metal ion through each phase is assumed to be driven by gradients in concentration C as described by Fick’s First Law: JL =

DL B (C L − C LF) δL

(4a)

JO =

DO F F (CO,L − CO,S ) δO

(4b)

JS =

DS F (CS − CSB) δS

(4c)

Figure 5. Predicted initial fluxes for individual metal ions (normalized to the initial concentration in the leachate) for (A) both liquid membrane processes for a leachate with pH 4 (note log scale) and (B) the LEM process (same data points as part A) showing small variation in flux due to differences in diffusion coefficients.

SLM), its ionic radius, and its binding affinity with DEHPA (i.e., Keq). The most important difference between the two systems is the value for [(HR)2] within the organic phase. For the LEM system at initial leachate pH 4, the calculated concentration of (HR)2 was 0.141 M, resulting in the following value for kL (according to eq 3a′):

where the subscripts L, O, and S indicate the parameters for the leachate, organic, and strippant phases, respectively, and the superscripts B and F indicate the bulk and film layers, respectively. The diffusion coefficients D were calculated from the hydrated ionic radius according to the Stokes−Einstein relationship (Supporting Information). The diffusion film layer δL and δS in the leachate and strippant phases were calculated from the mixing speed while δO was based on microscopic observations of oil film thickness in the emulsion or reported membrane thickness for the SLM system (Supporting Information). By assuming that JL = JO = JS and using eqs 3a and 3b, an overall expression for the flux of an individual metal ion normalized to its bulk concentration in the leachate can be expressed as J = C BL

kL = =

+

δO DO

+

[Me3 +]L

3

=

Keq[(HR)2 ]

Keq(0.1410.141M)3 (10−4)3

[H+]L

3

= 2.81 × 109Keq

(3a′)

As a result of this (HR)2 concentration, at pH 4, the term kL is approximately 109 or greater, depending on the value of Keq. Therefore, the kLδ L term in the denominator of eq 5 is DL

significantly greater than the

kL kLδ L DL

[Me(R· HR)3 ]

δO DO

and

k SδS terms, DS

resulting in the

following simplification:

k SδS DS

(5) +

for

+

The variables kL and kS depend on [H ]L and [H ]S, respectively, and both depend on the unchelated DEHPA concentration [(HR)2]. Therefore, the normalized flux shown in eq 5 depends on nine variables: the pH of the leachate and strippant phases (i.e., [H+]L and [H+]S), concentration of free chelating agent

δ kδ kLδ L ≫ O and S S , DL DO DS

k D J = k δL = L L L L δL CB DL

(6)

This simplification for the LEM system implies that normalized fluxes for the REEs were limited by diffusion through the leachate film. The lanthanide contraction results in a decrease in G


Article


phase. For the SLM process, total volume requirements for the organic solvent phase was orders of magnitude lower than conventional solvent extraction. Thus, REE recovery rate depended mostly on relative affinity for the solvent carrier (e.g., DEHPA). These findings are informative for future investigations of LEM or SLM processes that could focus on these components of the separation process through experimentation with different chelating agents and the organic diluents. Both LEM and SLM processes demonstrate high separation factors for the REEs in the presence of other competing metal ion species. Future work should focus on further purification and recovery of REEs from the strippant solutions in addition to reducing the total acid requirements. Full scale application of these processes will also require an understanding the effects of pH for both leachate and strippant solution on REE recovery. While this work was focused on leachates of coal fly ash, our findings are applicable to other dilute mixed metal waste streams. Future work could focus on applying these methods to other secondary feedstocks such as acid mine drainage, geothermal brines, medical/laboratory wastewater, or seawater. These brine mixtures all pose similar challenges to coal ash leachates in relation to matrix and difficulty in recovering REEs, and liquid membrane processes could be advantageous due to the relatively high selectivity for REE.

ionic radius with increase in atomic number along the lanthanide series resulting in an increase in hydrated ionic radius and a decrease of the diffusion coefficient. The diffusion coefficients along the lanthanide series were calculated to fall within an order of magnitude of each other and, therefore, yielded relatively small differences between fluxes among REEs (Figure 5B) that was not sufficient to explain the observed results. A likely reason for this inconsistency is the model assumption of equal fluxes across all three phases, an assumption that would not account for observations of certain REE (e.g., Sc or Lu) immobilized on the organic phase and not recovered by the strippant phase as the equilibrium constant for Lu is 5 orders of magnitude larger than La. Thus in the LEM system, diffusion into the organic phase may be the overall rate-limiting step for the recovery of light REEs and dissociation of the Me(R·HR)3 complex may be ratelimiting for the heavy REEs. For the SLM system, the predicted concentration of (HR)2 at pH 4 was 9.9 × 10−6 M resulting in kL = 9.7 × 10−4 Keq. As a consequence, the δO term was the largest term in the DO

denominator of eq 5 and flux simplified to the following: for

kδ δO kδ ≫ L L and S S , DO DL DS KeqDO[(HR)2 ]3 kLDO kL J = = = δO δO δO[H+]3 C BL DO

■

(7)

ASSOCIATED CONTENT

S Supporting Information *

From this simplification, differences in flux between the REEs in the SLM system could be explained by variations in both the metal-DEHPA binding constant, Keq, and the diffusion coefficient DO. The value of DO decreases with increasing ionic radius (i.e., from light to heavy REE); however, the range of values are within 1 order of magnitude. In contrast, the value of Keq increases by 5 orders of magnitude along the lanthanide series. These results indicate the DEHPA concentration [(HR)2] is smaller in the SLM system than the LEM, and that REE competition for (HR)2 is the key factor affecting recovery and flux in the SLM system. The limiting amount of free (HR)2 is a direct result of the small total volume of solvent present in the membrane. As such, affinity toward free (HR)2 will be the deciding factor for overall flux favoring heavier REEs over lighter REEs. While the flux calculations can compare the REEs for their recovery potential, the model inadequately predicts observations of certain metals. For example, the model indicates a high flux of Sc. However, in our LEM and SLM separations, we observed less than 1% of the original Sc in the strippant phase, and a minimum of 40% Sc remaining in the original leachate depending on the method of recovery used. Sc was likely immobilized in the organic phase, probably because of the strong binding affinity with DEHPA.42 Implications and Future Work. The unique chemical properties of REEs make them both critical and irreplaceable in industrial applications. Due to the difficulty and environmental impact of REE mining, developing methods for the recovery and concentration of REEs from alternate sources such as coal ash is crucial. Compared to the conventional kerosene-based solvent extraction, the LEM process was shown to be a potential alternative that could be implemented with mineral oil, a less hazardous solvent than kerosene. Using the developed model, we demonstrated that the recovery rates of REEs appeared to be controlled by diffusional mass transfer of the REE across the oil

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.9b00539. Schematic of the LEM and SLM processes, a table of the four coal ash sinters used as a basis for the synthetic leachate recipe, schematic of the SLM system, kinetics of SLM process, methods for correction of ICP-MS analysis, method estimation of the solid phase composition of the strippant after evaporation, effect of solvent choice on LEM process, and the methods used to estimate the parameters for the flux model (PDF) Spreadsheet of the initial and final concentrations of leachate and strippant for all processes (XLSX)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ryan C. Smith: 0000-0003-2591-416X James C. Hower: 0000-0003-4694-2776 Mark R. Wiesner: 0000-0001-7152-7852 Heileen Hsu-Kim: 0000-0003-0675-4308 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the U.S. National Science Foundation (No. CBET-1510965 and CBET-1510861) and the U.S. Department of Energy (DE-FE0026952).

■

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Selective Recovery of Rare Earth Elements from Coal Fly Ash

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