Introduction

Rare earth elements (REEs) consist of scandium, yttrium, and 15 lanthanide elements1. These elements are critical to a wide range of modern technologies, especially green energy applications2,3,4,5. For example, neodymium, praseodymium, and dysprosium have unique magnetic properties and are essential to produce high-performance permanent magnets used in wind turbines, electric vehicles, and hard disk drives2,6,7. Lanthanum and cerium are important for catalysts in petroleum refining and automotive catalytic converters8,9,10. Yttrium, europium, and terbium, with unique luminescent properties, are key components in high-quality phosphor production11,12,13. As of 2022, the global REE reserves were estimated at 130 million metric tons, and annual REE production reached approximately 300,000 tons, with China contributing nearly 70% of the global supply14. However, geopolitical tensions and growing efforts to reduce dependence on China have introduced volatility into the REE market14,15,16. As the global demand for REEs is rising steadily, it has become increasingly important to develop sustainable strategies to recycle REEs from waste materials. One promising source is coal fly ash (CFA), a byproduct of coal combustion, which is abundant and often enriched with REEs17,18,19,20,21. Approximately 1.143 billion tons of CFA is generated worldwide each year, with an average utilization rate of 60%21. When combined with the substantial quantities in storage, CFA is a considerable resource for potential REE recovery.

Over the past decades, ionic liquids (IL) have emerged as alternatives for conventional organic solvents in extraction processes22,23,24,25. ILs are molten salts that exist in a liquid state at room temperature or with mild heating (below 100 °C)26,27,28,29,30. They are typically non-flammable, thermally stable, recyclable, and have negligible vapor pressure22,28,29,30. Certain ILs have been successfully implemented in pilot- and industrial-scale processes such as BASF’s BASIL (biphasic acid-scavenging utilizing ionic liquids) process, Eastman’s IL-catalyzed isomerization, and the IL-based cooling and compression technologies by ILTEC and Linde31. In the context of REE recovery, numerous ILs have excellent laboratory extraction performance with synthetic REE solutions, while a few have been successfully applied to real feedstocks32,33,34,35,36. For example, Yang et al.37 used the non-functional IL [C4mim][Tf2N] as a diluent for the extractant, N, N-dioctyldiglycol amic acid (DODGAA), to extract REEs from phosphor-powder leachates of spent fluorescent lamps. Emam and El-Hefny38 synthesized a bifunctional IL, [AL336][Cy572], by combining Aliquat 336 and Cyanex 572, and applied it for Nd extraction from the raffinate of permanent-magnet scrap. In our previous work, we have demonstrated that REEs can be selectively extracted from CFA solids using the IL betainium bis(trifluoromethylsulfonyl)imide, [Hbet][Tf2N] (Fig. 1)39,40,41,42. Compared to the IL systems that required an organic diluent or an additional extractant, water-saturated [Hbet][Tf2N] can be directly used for REE extraction from both aqueous and solid feedstocks, such as CFA, bauxite residue, fluorescent-lamp phosphors, and permanent magnets43,44,45.

Fig. 1: Scheme of [Hbet][Tf2N]-based extraction systems.
figure 1

a Extraction from Coal fly ash (CFA) solids. b Extraction from CFA leachates.

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The carboxyl-functionalized [Hbet][Tf2N] exhibits particularly high solubility for REE oxides46,47,48. In water-saturated [Hbet][Tf2N], water markedly promotes the dissolution of REE oxides, facilitating the coordination between trivalent REE ions and the oxygen atoms of the betaine groups (as shown in Eq. 1)45,49,50,51,52,53.

$${{{\rm{REE}}}^{3+}}_{\left({AQ}\right)}\,+{3\left[{\rm{Hbet}}\right]\left[{{\rm{Tf}}}_{2}{\rm{N}}\right]}_{\left({IL}\right)}\rightleftarrows {\left[{{\rm{REE}}\left({\rm{bet}}\right)}_{3}\right]\left[{\left({{\rm{Tf}}}_{2}{\rm{N}}\right)}_{3}\right]}_{\left({IL}\right)}+{3{{\rm{H}}}^{+}}_{\left({AQ}\right)}$$
(1)

Fan et al.54 attributed this selective dissolution to lattice disruption and variations in lattice energy among metal oxides. They introduced a U/x value, where U is the lattice energy and x is the number of metal atoms in MxOy, to describe the oxide solubility in water-saturated [Hbet][Tf2N]. As a result, lanthanide sesquioxides with U/x values below 7000 kJ/mol were found to dissolve completely, whereas MO2 oxides with U/x values greater than 10,000 kJ/mol remained essentially insoluble54.

In addition, [Hbet][Tf2N] has an upper critical solution temperature of 55 °C, above which it becomes fully miscible with water. An elevated temperature promotes the formation of a homogenous phase and reduces IL viscosity, thereby accelerating the extraction kinetics through improved ion mobility and enhanced mass transfer49,50,55. Upon cooling to room temperature, the mixture reverts to two distinct phases. This thermomorphic behavior enables the combination of REE extraction and separation into a single step, with most leached REEs partitioning into the IL phase, and non-REEs remaining in the aqueous phase. A subsequent stripping of the REE-loaded IL phase regenerates [Hbet][Tf2N] for repeated use39,49,50. The CFA solid-IL extraction method has been applied to ten different CFA samples, and the results confirmed that the process can maintain a high REE recovery efficiency with strong selectivity over bulk and trace elements across diverse CFA types39,42.

This work extends the application of [Hbet][Tf2N] to CFA leachates (Fig. 1). Acid leaching is an early step widely used in extracting REEs from coal combustion ashes56,57,58. Numerous studies have achieved high REE recovery using mineral acids, particularly hydrochloric acid and sulfuric acid18,35,58,59,60,61. Cao et al. reported that HCl leaching of REEs from CFA followed a chemically controlled mechanism, in which H+ ions diffused through the surface layer and reacted with the REE-bearing aluminosilicate matrices56. However, such strong acids have poor selectivity, resulting in high concentrations of bulk elements in the leachates57,62. Meanwhile, organic acids such as citric, lactic, tartaric, and methanesulfonic acids have been explored to selectively dissolve REEs via coordination with carboxylate ligands35,63. In contrast to the leaching mechanism of inorganic acids, Rezaei et al. found that citric acid leaching followed an interfacial transfer and diffusion across the product layer mechanism, in which metal release was controlled by ion diffusion through the reacted ash layer63.

In this context, the [Hbet][Tf2N] system shows potential for downstream REE separation due to its preferential affinity for REEs over other elements52. Also, the IL anion [Tf2N]- is not readily displaced by common aqueous anions such as SO42-, Cl-, and NO3 -64, so it will remain in the IL phase when exposed to highly acidic leachates. In this work, we investigated four types of leachates generated from a Class C CFA using HCl, HNO3, H2SO4, and citrate solutions18,59,60,65. Citrate is a biodegradable and environmentally friendly complexing ligand and was included to explore the potential of [Hbet][Tf₂N] in separating REEs from an organic-rich matrix63. Since the type of leaching reagents was previously identified as the most significant parameter in CFA leaching63, we first evaluated the leaching efficiency of each reagent followed by examining the performance of each leachate-IL system and the effect of different extraction parameters.

Results

Acid leaching

The SEM images in Figure S1 show the morphology of CFA before and after different leaching methods. The as-received CFA particles showed a spherical morphology with a median diameter D50 of 12.3 µm. After HCl leaching, agglomeration of fractured particles was observed together with some spherical particles showing rougher surfaces, resulting in an increased D50 of 35.3 µm. Similar features were observed after HNO3 leaching, showing a combination of agglomerated and rough spherical particles, with a comparable D50 of 36.4 µm. The H2SO4-leached CFA displayed a markedly altered morphology characterized by a combination of elongated, spike-shaped structures and finer particles, resulting in a significantly larger D50 of 77.6 µm. Citrate leaching mostly preserved the spherical structures with slight surface roughness, and the D50 increased to 29.1 µm.

The leaching efficiency of each reagent is shown in Fig. 2 and compared with the total amounts leached from the CFA to the aqueous and IL phases in a previous ash-IL extraction study42. The total REE leaching efficiencies, calculated as the sum of REEs detected in the leachate divided by the total REE concentrations in CFA, were 83% ± 4%, 90% ± 7%, 67% ± 7%, and 67% ± 3% when using HCl, HNO3, H2SO4, and citrate, respectively (Fig. 2a). These results are consistent with the values reported in earlier studies18,59,60,65. Class C fly ashes typically show high REE extraction in acid leaching due to their high calcium oxide contents that can be readily dissolved by acids18,59. In comparison, the ash-IL extraction yielded a lower total REE leaching efficiency of 60% ± 2%, largely due to reduced individual REE leaching. REEs such as Pr, Tb, and Ho, which were not previously leached by the ash-IL extraction, were detected in the acid leachates. The higher REE leaching efficiency is attributed to the high acidity of the acid-based methods56,66,67. The pH of the leaching reagents was negative for the three mineral acids and approximately 2 for the citrate solution. Under such highly acidic conditions, abundant hydrogen ions promote the dissolution of the glassy aluminosilicate matrix in the CFA particles, enhancing the release of REEs distributed throughout the matrix as surface area increases during dissolution59,66,67. In citrate leaching, the combination of acidic condition and the strong chelating ability of citrate supports a high REE leaching65,68.

Fig. 2: Comparison of leaching efficiency for different leaching reagents and the ash-IL extraction system.
figure 2

a Leaching efficiency, L%, of REEs. b Leaching efficiency, L%, of selected bulk and trace elements.

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However, the acid leaching process is non-selective. Along with REEs, substantial amounts of non-REEs were also released into the leachates for subsequent leachate-IL extraction (Fig. 2b). Specifically, during ash-IL extraction, elements such as Si, Ti, Cu, and Cd either remained entrapped within the CFA matrix or exhibited low partitioning into the IL or aqueous phases (L < 10%). In contrast, these elements were present in significantly higher concentrations in the acid leachates. Therefore, it is important to investigate the capability of [Hbet][Tf2N] to selectively extract REEs from the complex aqueous matrix of leachate-IL systems.

Effect of leachate pH

The general extraction mechanism of [Hbet][Tf2N] for REEs was previously shown in Eq. 1. When substantial amounts of H+ are present in the aqueous phase, the formation of the REE-IL complexes is not favored. Given that the [Hbet]+ cation has a pKa of 1.83, aqueous solutions with an initial pH below 1.5 tend to reach an equilibrium pH with [Hbet][Tf2N] at a similarly low value51,69, which is not desirable for efficient extraction. All leachates used in this study were acidic or highly acidic, so their pH were adjusted to higher levels (pH around 3, 7, and 11, respectively) to examine the effect of leachate pH on extraction performance. [Hbet][Tf2N] can buffer aqueous solutions to an equilibrium pH of ~1.3 when the initial pH ranges from 1.5 to 1051. More basic conditions can lead to deprotonation of [Hbet][Tf2N], which breaks the cation-anion hydrogen bond and forms a homogenous AQ-IL phase46,51.

Interestingly, in our leachate-IL systems, the binary phases were consistently observed even at pH as high as ~11. One of the main reasons for such behavior is the presence of salting-out ions that significantly reduce the solubility of [Hbet][Tf2N] in the water phase. For cations and anions commonly found in leachates, the salting-out effect increases with ion hydration strength and follows the order: Mg2+ > Ca2+ > Li+ > Na+ > K+, and SO42− > Cl− > NO3−, respectively45,64. Consequently, in leachates with high concentrations of dissolved ions, less [Hbet][Tf2N] is solubilized into the aqueous phase, maintaining phase separation of the leachate-IL system even under high-pH conditions.

As shown in Fig. 3, increasing the leachate pH from acidic (~3) to neutral (~7) and further to alkaline conditions (~11) enhanced the REE distribution in the IL phase for extraction from the HCl, citrate, and HNO3 leachates. In HCl leachates (Fig. 3a), REEs, except Sc, exhibited similar behavior. At acidic pH, most REEs were poorly extracted by [Hbet][Tf2N] with a distribution below 1. At neutral pH, they were approximately evenly distributed between the leachate and IL phases (D ≈ 1). At basic pH, they were predominantly extracted by [Hbet][Tf2N] with distribution ranging from 1 to 10. Sc in HCl leachates consistently showed a strong affinity for the IL phase across all pH conditions, with its distribution increasing from 28 ± 1 at acidic pH to 167 ± 4 under alkaline conditions.

Fig. 3: Effect of leachate pH on REE distribution.
figure 3

a–d Distribution, D, of REEs during extraction from HCl, citrate, HNO3, and H2SO4 leachates at different pH levels.

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A similar trend was observed for most REEs in citrate leachates (Fig. 3b). While the general REE extraction increased with pH, Sc showed a lower affinity for the IL phase in this system (D < 12 across all pH conditions). Additionally, the improvement in REE distribution from neutral to basic pH was less significant compared to the HCl system.

In HNO3 leachates (Fig. 3c), Sc extraction also improved with increasing pH, as the distribution rose from 15 ± 0.4 to 55 ± 4. However, other REEs were largely not extracted by [Hbet][Tf2N] regardless of pH adjustment (mostly 0.1 < D < 1). A possible explanation is that the 15 M nitric acid used in leaching was the most concentrated among all reagents, resulting in a high concentration of nitrate ions in the leachate. These anionic nitrate ions may form stable ion pairs with cationic REE3+ ions via electrostatic interactions, reducing the availability of free REE3+ ions for transfer into the IL phase. Similar trends were observed for extracting trivalent transition metals (Rh3+ and Ru3+) from [Hbet][Tf2N]-HNO3 systems70, where increasing the HNO3 concentration from 0.3 M to 3 M resulted in a decreased metal distribution into the IL phase (log D decreased by ~0.7 for Rh3+ and ~0.4 for Ru3+). An earlier study71 comparing nitrate and chloride media for metal extraction using different carboxylic acids demonstrated that at high pH, the strong coordinating Cl− formed stable chloro-complexes. These complexes helped prevent hydrolysis and precipitation of metal ions, thereby altering extraction behavior.

For H2SO4 leachates (Fig. 3d), none of the REEs, including Sc, were effectively extracted into the IL phase under the tested pH conditions (most D < 0.2). This poor extraction is likely due to the strong binding of sulfate anions to REE3+ cations72,73. According to Wood72, the recommended stability constants (logβ) for REE-sulfate complexes at 25 °C are ~3.6 for LnSO4+ and ~5 for Ln(SO4)2-, while the values for LnCl2+ complexes range from 0.23 to 0.48. The REE-sulfate complexes are highly stable across a wide temperature and pH range73, making the REE complexation with [Hbet][Tf2N] more difficult.

The effect of leachate pH on the extraction of non-REEs is shown in Figure S2. While higher pH levels slightly increased the distribution of non-REEs in the IL phase across all four leachate systems, the majority of them remained predominantly in the leachates (D ≈ 0.1) under the tested conditions, confirming the strong selectivity of [Hbet][Tf2N] for REEs in a highly complex mixture.

Effect of adding ascorbic acid to leachate-IL extraction

In our previous study on ash-IL extraction, adding ascorbic acid (AA) to the aqueous phase after CFA removal significantly limited the co-extraction of iron into the IL phase40. AA can reduce Fe(III) to Fe(II), which poorly forms complexes with [Hbet][Tf2N] compared to Fe(III). When 25 mM AA was added to the aqueous phase, the distribution of iron in the IL phase decreased dramatically from ~75 to ~0.1640, and this suppressed distribution was consistently observed across six additional CFA samples42. Based on these findings, 25 mM AA was added to each leachate, and the results for HCl and citrate leachates are shown in Fig. 4. Results for HNO3 and H2SO4 leachates are not shown as the overall extraction by [Hbet][Tf2N] was not effective in those systems. Initially, iron preferred to partition into the IL phase for both HCl (D = 12 ± 2) and citrate leachates (D = 1.5 ± 0.02, Fig. 4a). The notably higher distribution observed in HCl leachates can be explained by the much lower stability of Fe(III)-chloride complexes compared to Fe(III)-citrate complexes65. Upon adding 25 mM AA, the iron distribution decreased significantly to ~0.2 in both leachates, corresponding to ~13% co-extraction shown in Fig. 4b. Meanwhile, REE recovery remained largely unaffected by the presence of AA (~50%, Fig. 4b), with a minor drop of less than 2% in both leachates.

Fig. 4: Effect of ascorbic acid on iron and REE extraction from HCl and citrate leachates.
figure 4

a Distribution, D, of iron in HCl and citrate leachates with and without the addition of 25 mM ascorbic acid. b Recovery, R%, of iron and total REEs from HCl and citrate leachates with and without the addition of 25 mM ascorbic acid.

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Comparison of leachate-IL and ash-IL extraction: distribution of REEs

The extraction performance of each leachate-IL system was evaluated under the same experimental conditions (pH ~ 11, 25 mM AA, and 10 mg betaine/g leachate) and compared with previously reported results for ash–IL extraction using the same CFA42. As previously noted, REE extraction from the HNO3 and H2SO4 leachates was poor, with a low distribution and total REE recovery rates of 19% ± 0.4% and 4% ± 0.3%, respectively (Fig. 5). In contrast, the total REE recovery from HCl and citrate leachates reached 51% ± 0.3% and 47% ± 0.3%, respectively, which is comparable to the 49% ± 0.3% recovery achieved in the ash-IL system (Fig. 5b).

Fig. 5: Comparison of REE extraction performance between leachate-IL and ash-IL systems.
figure 5

Extraction from leachates was performed under the same conditions (pH ~11, 25 mM ascorbic acid, and 10 mg betaine/g leachate). a Distribution, D, of individual REEs in each system. b Recovery efficiencies, R%, of individual and total REEs in each system.

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To further understand the influence of leachate chemistry on REE extraction, simulated AQ-IL extraction experiments were performed by dissolving Nd2O3 in 1.5 M HCl, 1.5 M HNO3, 1.5 M H2SO4, and 0.1 M citrate solutions to reach an Nd concentration of 20 ppm. Each solution was added with 1 wt% additional betaine, and the pH was increased to ~11 using NaOH beads and 10 M NaOH solution. The solutions were then contacted with water-saturated [Hbet][Tf2N] under the same extraction and stripping conditions described in section 2.3. The results showed that Nd recovery from the simulated HCl and HNO3 solutions was ~10% (Figure S3), while recovery from the H2SO4 solution was below 1%. The citrate solution yielded the highest Nd recovery of 74%. These results were consistent with the extraction trends observed for the leachates, with the exception of the HCl leachates. The deviation in the HCl case could be explained by the differences in solution complexity: in the simulated system, Nd3+ and Na+ were the only metal ions present, allowing chloride to complex predominantly with Nd3+. However, the HCl leachate of CFA contains a variety of metal ions at much higher concentrations than REE ions, some of which form more favorable and stable complexes with chloride. This competition reduces chloride binding to Nd3+, resulting in more Nd3+ (and other REE3+ ions) available for extraction into the IL phase.

In addition, the effect of excess betaine was found to be less significant in the leachate-IL systems. In a previous study40, the addition of 10 mg betaine per gram of aqueous solution during ash-IL extraction improved the REE recovery from 35% to 55%. However, in the current study, the same amount of betaine resulted in only a 2% increase in REE recovery for HCl leachates, and a 1% decrease for citrate leachates (Figure S4). This suggests that higher concentrations of betaine may be necessary to enhance REE distribution toward IL in leachate-IL systems. Vander Hoogerstraete et al.50 reported that efficient extraction requires a large excess of betaine relative to the REE concentration, and they achieved a distribution of ~100 for Nd3+ when more than 20 wt% of betaine was added to the aqueous phase. Therefore, the addition of 10 mg/g leachate (~1 wt% betaine) is insufficient for current leachate systems, and higher concentrations should be investigated if necessary.

At the individual REE level, Sc, La, and Dy showed extremely high distribution (D > 100, Fig. 5a and Table S1) in the ash-IL system, indicating nearly all the leached elements were extracted by [Hbet][Tf2N], with no detectable amounts remaining in the aqueous phase. In the HCl and citrate systems, these elements continued to preferentially partition into the IL phase, though with a reduced distribution (DSc = 15 ± 0.6 and 11 ± 0.2, DLa = 2.1 ± 0.05 and 1.6 ± 0.08, and DDy = 3.2 ± 0.08 and 1.7 ± 0.06, respectively, Table S1), likely due to the competition from high concentrations of coordinating anions in the leachates that interfered with complexation by [Hbet][Tf2N]. However, since the acid-based methods leached greater overall amounts of REEs from CFA compared to the ash-IL system, the recovery of Sc, La, and Dy was improved: RSc increased by 9% and 20%, RLa increased by 13% and 10%, and RDy increased by 8% and 17% in HCl and citrate systems, respectively. In contrast, some REEs showed modest declines in both distribution and recovery: RY decreased by 10% and 14%, RCe decreased by 4% and 6%, and RNd decreased by 7% and 17% in HCl and citrate systems, respectively. Notably, Pr, Tb, and Ho were not leached from the CFA to the aqueous or IL phase in the ash-IL system, but after being leached by HCl and citrate, they were successfully extracted by [Hbet][Tf2N] (RPr = 28% ± 1% and 35% ± 0.3%, RTb = 62% ± 3% and 2% ± 1%, and RHo = 36% ± 4% and 47% ± 3%, respectively). The enhanced recovery of these elements helped offset the reduced recovery of others, resulting in a comparable overall REE recovery to that of the ash-IL system. These results indicated the continued effectiveness of [Hbet][Tf2N] for REE extraction from HCl and citrate leachates.

Comparison of leachate-IL and ash-IL extraction: distribution of bulk and trace elements

As described in the earlier section, non-REEs primarily remained in the leachates under high-pH conditions that favored REE distribution into the IL. Figure 6 shows the distribution and recovery of selected bulk and trace elements in both leachate-IL and ash-IL systems. During ash-IL extraction, no detectable amount of Si was found in the IL phase, so both the distribution and recovery efficiency were zero. In the leachate-IL systems, only trace amounts of Si were detected in the IL phase, with recovery efficiency below 2% for all leachates. For the rest of the five bulk elements analyzed (Mg, Al, Ca, Ti, and Fe), their distribution was consistently lower in the leachate-IL systems than in the ash-IL system, except for Ca in H2SO4 leachates, which was slightly higher (0.55 ± 0.01 compared to 0.38 ± 0.002, Table S2). In particular, Al and Ti showed relatively high distribution in the IL phase during ash-IL extraction (DAl = 3 ± 0.08 and DTi = 6 ± 0.5, Fig. 6a and Table S2), but their distribution significantly declined in the leachate systems (DAl = 1.5 ± 0.02 and 0.7 ± 0.05; DTi = 1.1 ± 0.007 and 0.02 ± 0.004 in HCl and citrate systems, respectively, Table S2). Ti showed slightly elevated recovery in the HCl and H2SO4 leachates (RTi = 23% ± 0.1% and 13% ± 0.3%, respectively, in Fig. 6b) compared to the ash-IL system (RTi = 8% ± 0.01%), due to its significantly higher concentrations in these leachates. Other than Ti, the recovery of bulk elements generally became lower in leachate-IL systems: RMg decreased by 16% and 15%, RAl decreased by 13% and 19%, RCa decreased by 3% and 5%, and RFe decreased by 11% and 10% in HCl and citrate systems, respectively.

Fig. 6: Comparison of selected bulk and trace elements extraction performance between leachate-IL and ash-IL systems.
figure 6

Extraction from leachates was performed under the same conditions (pH ~11, 25 mM ascorbic acid, and 10 mg betaine/g leachate). a Distribution, D, of selected bulk and trace elements in each system. An * indicates elements with very low concentrations in the IL phase but no detectable amounts remaining in the leachate after extraction. b Recovery efficiencies, R%, of selected bulk and trace elements in each system.

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Among the trace elements, most of them exhibited distribution below 1, indicating they largely remained in the leachates after extraction. The distribution and recovery of V and Mn became comparatively lower, with RV decreasing by 0% and 20%, and RMn decreasing by 29% and 18% in HCl and citrate systems, respectively. Cu, Cd, and Pb were either not extracted (R = 0%) or minimally extracted (R < 2%) by [Hbet][Tf2N] when directly in contact with CFA. However, during the leachate-IL extraction, despite a low distribution, their high concentrations in the leachates resulted in an increased recovery. Overall, the leachate-IL systems showed a reduced extraction of bulk elements but elevated extraction of certain trace elements.

Recycling of [Hbet][Tf2N]

The leachate-IL extraction process was repeated five times using recycled [Hbet][Tf2N]. Between each cycle, the IL was washed with small amounts of cold ultrapure water to remove the residual acid following the stripping step. Figure 7 shows the distribution and recovery of representative REEs with high initial concentrations in the CFA, along with six bulk elements. The distribution of REEs dropped after the first cycle but remained relatively stable over the subsequent four cycles. Y and La showed a distribution slightly below 1 after the first cycle. Similarly, the total REE recovery dropped after the first cycle but remained at a similar level in the following cycles. Among the bulk elements, Si consistently showed a minimal recovery below 1% across all cycles. The distribution and recovery of the remaining bulk elements exhibited minor fluctuations from one cycle to the next.

Fig. 7: Performance of fresh and regenerated [Hbet[Tf2N] during extraction from HCl leachates.
figure 7

Each extraction cycle was performed under the same conditions (pH ~ 11 and 25 mM ascorbic acid). a Distribution, D, of selected REEs and bulk elements over five cycles. b Recovery, R%, of selected REEs and bulk elements over five cycles.

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In addition, an IL loss of approximately 8 wt% was observed after each leachate extraction process, and approximately a total IL loss of 14 wt% was observed after the washing process. These results indicate that although the regenerated [Hbet][Tf2N] can be reused without a substantial decline in REE recovery efficiency, the addition of fresh IL is necessary to compensate for the loss when operating over extended cycles.

Discussion

This study successfully demonstrates the application of [Hbet][Tf2N] for preferential extraction of REEs from Class C CFA leachates. Among the four tested leachates, HCl and citrate offered the most effective REE recovery comparable to the performance of previous ash-IL systems, while maintaining strong selectivity over non-REEs. Process modifications, such as increasing leachate pH to ~11 and adding 25 mM ascorbic acid, substantially enhanced REE extraction and reduced iron co-extraction. The addition of betaine showed a limited impact at the tested low concentrations, but its potential benefit at higher levels needs further investigation. To further enhance the separation of REEs from bulk and trace elements, optimization of the [Hbet][Tf2N]-AQ ratio can be considered44,45,55,74,75. Previous studies demonstrated that in water-unsaturated [Hbet][Tf2N], increasing the water content improved the dissolution kinetics of REE oxides by lowering IL viscosity and facilitating proton transfer; however, the introduction of water compromised selectivity by enabling partial solubilization of other inert oxides such as alumina44,45,74. Mawire and van Dyk also reported that increasing the water-[Hbet][Tf2N] ratio from 2 mL: 1 g to 7 mL: 1 g sped up the extraction of scandium oxide but simultaneously caused undesired dissolution of alumina55. These observations emphasize the importance of balancing system composition to maximize REE recovery while minimizing co-extraction of non-REEs.

The regenerated [Hbet][Tf2N] can be reused over multiple cycles with no significant decrease in REE extraction, but replenishment is necessary to offset the inevitable IL losses. Due to the mutual solubility of water and [Hbet][Tf2N], it is inevitable that a small portion of IL dissolves into the aqueous phase during extraction, which necessitates further study of its potential environmental impacts. The cation betaine is a naturally-occurring zwitterionic derivative of amino acid glycine that is biodegradable and environmentally benign76,77,78. However, the anion [Tf2N]- raises greater concern78,79,80,81. While [Tf2N]- provides thermal stability and hydrophobicity to the IL, which are beneficial for extraction, it is a fluorinated and highly persistent species78,79,81. A recent study on the components of lithium-ion batteries has detected [Tf2N]- in water, soil, and sediment near manufacturing facilities, with concentrations comparable to or exceeding those of legacy per- and polyfluoroalkyl substances82. Laboratory studies showed that the exposure to [Tf2N]- can disrupt cell membranes and metabolism in aquatic species at ppt levels79,80,81,82. Sorbent-based treatments using granular activated carbon and ion exchange resin can effectively remove [Tf2N]-, but it remains resistant to oxidative degradation81,82. From the perspective of green IL design, Hulsbosch et al. highlighted that biobased anions could serve as sustainable alternatives to fluorinated anions while retaining the key functional characteristics of ILs78. Therefore, the exploration of other ILs may provide new insights and opportunities to improve extraction performance from both solid and aqueous matrices.

In addition, while citrate proved to be an effective leaching agent, its relatively high cost compared with inorganic acids presents a limitation for large-scale applications. Future studies should investigate more economical organic acid alternatives or explore bioleaching strategies to reduce reagent costs and enhance process sustainability. Overall, integrating conventional leaching with IL-based extraction provides a promising and sustainable strategy for REE recovery from coal combustion byproducts.

Methods

CFA sample

The Class C CFA sample 93927 was collected from a power plant that burned Powder River Basin coal. The composition was reported by previous studies18,83 and is shown in Table 1.

Table 1 Elemental composition of the CFA sample
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Acid leaching

All the chemicals used in this study were of ACS grade or higher and were used without further purification. Ultrapure water (18.2 MΩ-cm) was produced from a Milli-Q water purification system. The CFA leachates were prepared by adopting methods summarized in Table 2. During the citrate leaching, the pH was maintained at around 2 using HCl and NaOH solutions. After each leaching process, the leachate and residual solids were separated by vacuum filtration. The residual solids were rinsed with ultrapure water and dried overnight in a 70 °C oven. The CFAs as received and after each acid leaching method were characterized using a scanning electron microscope (Hitachi SU8230), and their morphology images are shown in Figure S1. The particle size was determined using a Mastersizer 3000 + . The HCl and HNO3 leachates were diluted with ultrapure water for storage and subsequent extraction. The H2SO4 and citrate leachates were stored and used for extraction without dilution. All the leachate samples were diluted with 5% HNO3 for ICP-OES analysis. The leaching efficiency of each reagent was calculated as

$$L\,\left( \% \right)=\frac{{M}_{{leachate}}}{{M}_{{total}}}$$
(2)

where Mleachate is the concentration (mg/L) of an element in the leachate, and Mtotal is the theoretical concentration (mg/L) assuming complete dissolution of that element in the leachate.

Table 2 Summary of leaching methods
Full size table

Leachate-IL extraction

[Hbet][Tf2N] was synthesized according to a literature method46 by reaction between equimolar amounts of dissolved HbetCl and LiTf2N in water. The leachate-IL extraction process is depicted in Fig. 8. To examine the effects of leachate pH, ascorbic acid (AA) reduction of iron, and additional aqueous betaine, the leachate pH was adjusted using NaOH beads and 10 M NaOH solution. A concentrated AA solution and solid betaine chloride were added to the leachate, resulting in 25 mM AA and 10 mg betaine/g leachate39,40. After that, 4.25 g of leachate was mixed with 5.75 g of water-saturated [Hbet][Tf2N] in a glass vial to obtain a 1:1 IL/water mass ratio. The vial was then heated in an 85 °C oil bath with stirring at 600 rpm for 3 h. A heating temperature of 85 °C was selected to ensure complete miscibility between the IL and water phases and to maintain consistent extraction conditions with the ash–IL extraction42. According to Stoy et al., REE leaching and distribution were enhanced at 75 °C and 85 °C relative to lower temperature ranges41. After cooling to room temperature, the leachate phase was separated and diluted with 5% HNO3 for ICP-OES analysis. The remaining IL phase was then mixed with 1.5 M HCl solution at a 1:1 IL/acid mass ratio in a new glass vial and heated in an 85 °C oil bath with stirring at 600 rpm for 1.5 h. Upon cooling, the HCl phase was removed and diluted with 5% HNO3 for ICP-OES analysis. The IL was reused for more extraction and stripping cycles by washing with small amounts of cold ultrapure water.

Fig. 8
figure 8

IL extraction and stripping scheme for leachate-IL systems.

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The distribution of an element between the IL and leachate phases was calculated as

$$D=\frac{{M}_{{IL}}}{{M}_{{remain}}}$$
(3)

where MIL is the elemental concentration (mg/L) stripped from the IL phase by 1.5 M HCl solution, and Mremain is the concentration (mg/L) remaining in the leachate after extraction.

The recovery efficiency of the leachate-IL extraction method was calculated as

$$R\,\left( \% \right)=\frac{{M}_{{IL}}}{{M}_{{total}}}$$
(4)