Triflimide Effect in Solvent Extraction of Rare-Earth Elements from Nitric Acid Solutions by TODGA

ABSTRACT Outer-sphere interactions play an important role in the liquid-liquid extraction and separation of metal ions and they could be used to tune the efficiency and selectivity of recovery. Here, we report the extraction behaviour and separation of trivalent rare-earth elements from aqueous nitric acid solutions with N,N,N’,N’-tetra-n-octyl diglycolamide (TODGA), which was preliminarily contacted with an aqueous solution of N-H acid bis[(trifluoromethyl)sulfonyl]imide (triflimide, HTf2N; Tf = CF3SO2) resulting in an adduct TODGA⋅HTf2N. We examined in detail the extraction system composed of TODGA acidified with HTf2N and 1,2-dichloroethane as diluent. The effect of various experimental conditions such as the nature of diluent, aqueous nitric acid concentration, and TODGA⋅HTf2N concentration in the organic phase was studied. It has been found that the distribution coefficient values for metal ions are much greater, especially at low aqueous acidity in the extraction systems with the acidified organic solvent phase. Furthermore, the heavier lanthanides were effectively extracted and the intergroup lanthanide selectivity was greatly improved (separation factors for the Lu/La pair increased by 40 and 2 times for the two systems under study, respectively).


Introduction
Rare-earth elements (REE) have diverse uses related to their remarkable properties. [1]Applications include key sectors such as aerospace, telecom and electronics. [2]REE have also become essential elements for low-carbon technology. [3]Based on the economic importance and supply risk the EU Commission includes light REE (elements with atomic numbers 57 through 64) and heavy REE [4] (atomic numbers 65 through 71) in the list of critical raw materials. [5]In fact, the supply of REE is highly concentrated in China, which provides 98% of the EU's supply of REE.The growing demand for REE in many high-tech and renewable energy sectors has led to an increase in global mining production. [6]owever, the extraction and separation of REE from primary sources require large quantities of water and produce a large volume of waste, causing environmental damage and raising questions about the sustainability of the energy transition concerning the resources. [7,8]Besides, REE are not recycled in large quantities but this could be if recycling became mandated or very high prices of REE made recycling feasible. [9]Therefore, the development of more efficient, cost-effective and environmentally friendly processes for the separation and recovery of REE from primary and secondary sources [10] is of key importance.13] Solvent extraction is a well-known technique which usually used to separate individual REE in an aqueous solution or produce mixed REE solutions or compounds. [14]17] TODGA is solvating extractant, which binds more strongly to the smaller heavy lanthanide ions compared to the larger light ones, which could be used for their separation.[20][21][22][23][24][25][26][27][28] However, it is important to develop a comprehensive understanding of the speciation of the organic phase with TODGA as well as the associated chemical extraction mechanisms to allow further process optimization in terms of efficiency and selectivity.
In the literature, there is an investigation on the role of hydrophobic anions namely tetrakis [3,5-bis(trifluoromethyl)phenyl] borate anions (TFPB − ) introduced in the solvent phase as H + TFPB − in the extraction process of lanthanides with TODGA.The author reported that these weakly coordinated bonding anions enhance metal ion efficiency and selectivity. [29]There are also several studies on the introduction of a small quantity of hydrophobic ionic liquids with Tf 2 N − anionic part in the conventional extraction systems for lanthanide ions.In these studies, the synergistic effect between neutral organic ligands and the ionic liquid was reported. [21,30,31]However, the partial aqueous solubility of ionic liquid is an important drawback of this kind of extraction system.
In this work, we present the results of a study of the extraction behaviour of yttrium(III) and 14 lanthanide(III) ions (Ln(III)) from aqueous acidic solutions with extraction solvent containing TODGA, which was preliminarily acidified by the contact with the aqueous solution of N-H acid bis[(trifluoromethyl)sulfonyl]imide (HTf 2 N).We hypothesized that the introduction of this acid with bulky and hydrophobic anions, Tf 2 N − would yield an extraction system with improved efficiency and selectivity.We examined the general extraction behaviour of metal ions and the lanthanide selectivity.The underlying extraction mechanism in the extraction system under study is discussed.

Solvent extraction procedure
In this study, all solvent extraction batch experiments were carried out at room temperature (20-25°C).A stock REE(III) solution was prepared by dissolving metal nitrate salt.The initial aqueous concentration of the metal ions was 2⋅10 −6 M. The organic phase was prepared by dissolving measured quantities of TODGA⋅HTf 2 N compound and/or TBP in the molecular diluent.TODGA⋅HTf 2 N was prepared by contacting several times TODGA diluted in DCE with a solution of LiTf 2 N in HCl.Then, the molecular solvent was evaporated and the obtained compound was analyzed for the sulfur content using inductively coupled plasma atomic emission spectrometry (ICP-AES) on an ICAP-61 spectrometer (Thermo Jarrel Ash).All extraction samples were prepared using precisely weighed aliquots of each phase in centrifuge tubes equipped with sealing plugs.Generally, a 1:1 volume ratio of organic and aqueous phases was employed.The biphasic mixtures obtained by combining the aqueous and organic phases were shaken vigorously using a rotor mixer at a rate of 60 rpm for 1 h.It was checked in the preliminary trials if this time of phase contact was sufficient to achieve the equilibrium.To promote phase separation, the extraction tubes were centrifuged, and the aliquots of each phase were sampled for further analysis.The values of the metal (M) distribution ratio D M were calculated as the concentration of the metal ion in the organic phase to that in the aqueous phase, [M] org /[M] ([M] indicates the concentration of metal).The subscript "org" denotes the metal species in the organic phase and its omission represents the species in the aqueous phase.Duplicate experiments showed that the reproducibility of the D M measurements was within 10%.The concentration of the metal ions in the aqueous phase was determined using inductively coupled plasma mass spectrometry (ICP-MS) using an X-7 mass spectrometer (Thermo Electron, USA) according to the previously described procedure. [33]

Extraction of HTf 2 N acid by TODGA molecules into various organic diluents
N-H acid bis[(trifluoromethyl)sulfonyl]imide (triflimide, (CF 3 SO 2 ) 2 NH, also abbreviated as HTFSI, HFTSA, or HTf 2 N) is a monoprotic acid, which is widely used in organic synthesis as a catalyst, precatalyst, promoter, etc.. [34] HTf 2 N is soluble in water and most organic solvents.In aqueous solutions, it is partially protonated and the pK a value to describe the equilibrium acidity of HTf 2 N in water was determined to be 1.7 [35] and 0.16: [36] TODGA ligand is known to interact with molecular acid as shown in Equation ( 2): where L is the TODGA ligand, A − is an anionic part of acid, m and n are the numbers of TODGA and extracted acid molecules, respectively, involved in the complex formation with TODGA, and "org" represents the organic phase.In the previous studies, it was reported that the nitric acid dissolves in the organic phase in the form of TODGA⋅HNO 3 and TODGA⋅(HNO 3 ) 2 species. [37,38]TODGA molecules extract similar HTf 2 N acid molecules from an aqueous solution.Dissolved in DCE diluent, TODGA is reported to form the 1:1 complexes with triflimide. [39]irst, we studied the extraction of HTf 2 N using TODGA from the aqueous acidic solution of LiTf 2 N.In particular, the distribution ratio values for Tf 2 N-containing species (D Tf 2 N ) as a function of the total TODGA concentration (0.0-0.01 M) diluted in various molecular diluents were measured at constant initial HCl concentration (0.1 M) in the aqueous phase.It should be noted that logD values for HTf 2 N extraction by the pristine organic diluents (i.e. in the absence of TODGA) were found to be equal to 0.667 and 0.03, for octanol and nitrobenzene, respectively, and negligible for others.Similarly, pure octanol is reported to extract significant amounts of nitric acid as HNO 3 •Octanol and HNO 3 •(Octanol) 2 adducts. [28,40,41]It can be seen from the data in Table 1 that the nature of the organic diluent strongly affects the extraction efficiency of HTf 2 N with TODGA.For aprotic diluents, the distribution ratio for HTf 2 N increases in the order: nonane < toluene < DCE < nitrobenzene.This difference in the extraction behaviour for HTf 2 N using TODGA correlates with the characteristics of organic diluents, namely with the dielectric constant as an indicator of their polarity.It is apparent that the increase in the dielectric constant of the organic diluent used results in the increase of the HTf 2 N extractability with TODGA.A similar trend was observed for the extraction of nitric acid with TODGA in various diluents, [19] and more generally for mineral acids extraction by neutral extractants such as TBP.
The extraction of HTf 2 N acid using TODGA into the organic phase is increased by an increase in the TODGA concentration indicating the complex formation between TODGA and HTf 2 N molecules.The effect of the initial concentration of TODGA in the organic phase on the logD Tf 2 N values for TODGA diluted in DCE, toluene, and nonane diluents is shown in Figure 2. The conditional constant for HTf 2 N extraction with TODGA into the organic phase (Equation 2, where A − is Tf 2 N − ) can be written as: Here, we assumed that the initial extractant concentration in the organic phase is The dielectric constants of organic diluents and distribution ratio values for Tf 2 N species (D Tf 2 N ) for 0.002 M TODGA in various organic diluents.Aqueous phase: 0.00152 M LiTf 2 N dissolved in 0.1 M HCl solution.V A /V O = 1/1.

Diluent
The dielectric constant, ε [42,43] logD  5) to be 4.1 ± 0.1 and 3.8 ± 0.2 for DCE and toluene, respectively.For DCE, the obtained logK HA is in good agreement with that previously reported in the literature (logK HA = 4.15 ± 0.3. [39]However, interactions between TODGA, HTf 2 N and diluent in the solvent remain unclear and need further, in particular spectroscopic, investigation.For all further solvent extraction experiments in the present work, we used the organic phase prepared by the dissolution of a weighted amount of TODGA⋅HTf 2 N complex in an organic diluent.This reagent was formed by the repeated contact of the solution TODGA in DCE with an aqueous HCl solution of LiTf 2 N salt.Then, it was isolated and characterized to confirm the 1:1 complex stoichiometry.The reaction of triflimide with several neutral organophosphorus compounds such as triphenylphosphine, triphenylphosphine oxide, etc. gives rise to phosphonium salts of triflimide, which was confirmed by 1 H, 13 С, 19 F, 31 P NMR and IR. [44]In reviewing the literature, no data was found on the reaction of HTf 2 N with diglycolamide ligands.It is possible to hypothesise that two kinds of species could be formed, namely [TODGAH] + [Tf 2 N] − salt and H-complex, TODGA⋅HTf 2 N, as in the case of nitric acid. [38]Further study of the structure of adducts formed between TODGA and HTf 2 N is needed.However, we could draw some analogies between prepared complex compound, TODGA⋅HTf 2 N and so-called hydronium solvate ionic liquids, which have been recently reported in the literature. [45,46]This new kind of ionic liquid consists of a hydronium ion, polyether ligands, and bis[(trifluoromethyl)sulfonyl]amide anion, for example, [H 3 O + ⋅18C6][Tf 2 N] (18C6 = 18crown-6).The hydronium solvate ionic liquids intersect the solvate and protic types of ionic liquids and show strong acidity.In the case of TODGA⋅HTf 2 N, we could consider this reagent as a solvate ionic liquid [LH + ][Tf 2 N − ] because it seems to satisfy the criteria for solvate ionic liquids. [47]We hypothesize that proton hydrates H 3 O + interact with C=O groups of the TODGA molecules via hydrogen bonding resulting in the formation of a complex cation.
A similar approach to solvent extraction was used by Kulyako et al. [48] who demonstrated that diphenyl[dibutylcarbamoylmethyl]phosphine oxide (DPDBCMPO) ligand, taken initially as a solid powder, forms on contact with 4 M HNO 3 an oily liquid complex DPDBCMPO⋅HNO 3 ⋅nH 2 O.It has been shown that the distribution ratio values for REE extracted with this compound are higher than those measured in conventional solvent extraction.As was mentioned earlier, Suzuki et al. [29] studied the effect of H + TFPB − introduced in the solvent phase containing TODGA on the extraction of lanthanides.Another example of the study on the acidified solvent is the work of Bromley et al. [49] in which the authors investigated the cerium extraction kinetics by TODGA pre-contacted with HNO 3 .

Effect of diluent nature on the extraction of REE with acidified organic phase containing TODGA
The extraction behaviour of trivalent yttrium and lanthanide ions from aqueous 3 M HNO 3 solutions with TODGA or TODGA•HTf 2 N ligand in six various molecular diluents, namely nitrobenzene (NB), toluene (TL), 1,2-dichloroethane (DCE), octanol (OC), nonane and 30% TBP in nonane was examined.Figure 3 displays the extraction behaviour for Tb(III) as a typical example.It can be observed that the distribution ratio values for Tb(III) strongly depend on the diluent nature, as in the case of HTf 2 N extraction (see Section 3.1).As can be also seen in Figure 3, the use of TODGA⋅HTf 2 N extractant results in greatly higher extraction as compared with extraction using TODGA for the extraction systems with nonane, toluene or DCE as diluent (D Tb(TODGA⋅HTf2N) /D Tb(TODGA) is 1288, 347 or 14, respectively).The extraction efficiency for Tb(III) with TODGA⋅HTf 2 N is higher when aprotic nonpolar diluents such as toluene or nonane were used.The increase in the dielectric constant of the aprotic solvent used results in the decline of the metal extractability that follows nonane > TL > 30% TBP in nonane > DCE > NB diluent trend (Table 1, ε for 30% TBP in dodecane is 3.136. [50]The trend in D M values for each solvent acidified with HTf 2 N is contrary to that reported for the actinide and lanthanide extraction with diglycolamide ligands from HNO 3 solution [24,51,52] and Sr(II) extraction from LiTf 2 N solution with TODGA in organic diluents. [39]However, in these studies, the extraction solvent was generally pre-equilibrated with an aqueous HNO 3 solution.
The lowest D Tb with TODGA⋅HTf 2 N was observed in octanol, a protic non-polar solvent.This could be caused by the strong interactions between HTf 2 N, octanol, and TODGA molecules via hydrogen bonding that reduces organic ligand participation in the complex formation with metal ions.The initial concentration of extractant in the organic phase is 0.005 M for the systems with octanol, nitrobenzene, DCE, 30% TBP in nonane, and toluene or 0.001 M for the system with nonane as diluent.

Extraction behaviour of REE with TODGA⋅HTf 2 N diluted in DCE
Next, we studied in detail the solvent extraction of yttrium(III) and fourteen trivalent lanthanides from aqueous nitric acid solutions with 0.01 M TODGA⋅HTf 2 N diluted in 1,2-dichloroethane (DCE) or nonane.Figure 4 shows the extraction curves for Eu(III) ions with acidified extractant diluted in DCE or nonane as a function of initial HNO 3 concentration in the aqueous phase.This data is compared with distribution ratio values for Eu(III) with TODGA in DCE (no pre-contacting with aqueous acidic phase) obtained in the previous study [21] or in nonane (this work).It is apparent from the data that metal extraction depends strongly on the aqueous acid concentration for all solvent extraction systems under study.However, the extraction trends are different, in particular for the [HNO 3 ] < 4 M, which indicates the different underlying extraction mechanisms.In TODGA/DCE system, the nitric acid dependency is bell-shaped, i.e. it exhibits a maximum, and that is often reported for the extraction of neutral metal-ligand species into the organic phase.Turanov et al. [21] in the previous study on the extraction of Ln(III) ions with TODGA diluted in DCE proposed the following solvating extraction mechanism:   [21] or with the 0.01 M TODGA⋅HTf 2 N diluted in DCE, and 0.001 M TODGA⋅HTf 2 N or 0.001 M TODGA in nonane.The lines are only for an eye guide.
In the case of Eu(III) extraction, the authors hypothesized that n < s in Equation ( 7), i.e. the TODGA species in DCE are probably dominant neutral unbounded TODGA and its adduct with HNO 3 , namely TODGA•HNO 3 .Thus, at low acidity increasing the concentration of NO 3 − ions encourages the extraction of Eu(III) nitrate salts with TODGA as Eu•L s (NO 3 ) 3 into the organic phase.Under highly acidic conditions, there is a competition between H + and Eu(III) for TODGA molecules that leads to decreasing of D Eu values with a further increase of aqueous acidity.In nonane diluent, we suggest that the HNO 3 •L adduct outcompetes TODGA in the complex formation with Eu(III) ions.Thus, the extraction of Eu(HNO 3 •L) n (NO 3 ) 3org species into the organic phase could explain the increase of metal extraction efficiency in the whole investigated range of HNO 3 concentration.
In the case of TODGA⋅HTf 2 N diluted in DCE or nonane system, the extraction curves are drop-down, i.e. the increase in [HNO 3 ] results in the decline of the extraction efficiency for Eu(III).So, at 1 M HNO 3 in the initial aqueous phase, D Eu is close to 437, and it reaches 1.2 at 7 M HNO 3 in TODGA⋅HTf 2 N/DCE.It is interesting to note that a similar decreasing acid dependency, which is unusual for the solvent extraction with solvating extractants, was observed for the extraction of actinide and lanthanide ions with TODGA and hydrophobic ionic liquids used as diluent [23] or adjuvant. [21]ext, we studied the extraction of Ln(III) ions with TODGA•HTf 2 N in DCE from a mixture of HNO 3 and NaNO 3 while varying the concentration of HNO 3 and keeping the concentration of nitrate ions at 5 M. The plots of logD Eu vs. log[H + ] for different Ln(III) yields straight lines with a slope close to −3.05 (Figure 5) indicating the participation of three protons in the extraction, probably via the cation exchange mechanism.
Ligand stoichiometry of the Ln(III) extracted species was determined by slope analysis method. [53]Figure 6 illustrates the relationships between logD Ln and log of acidified extractant concentration, log[TODGA⋅HTf 2 N] in DCE.These are linear dependences with slopes near 3 indicating the formation of 1:3 metal: TODGA species in the solvent phase for all studied lanthanides (Table 2).A similar observation was reported by Shimojo et al., [22] wherein the authors studied the extraction of lanthanide ions with TODGA into hydrophobic [C 2 mim][Tf 2 N] ionic liquid.From the comparison with the results of the prior report on the extraction of trivalent lanthanides from 3 M HNO 3 with TODGA diluted in DCE, [21] in which slope values close to 2 and 3 for La-Nd and Tb-Lu, respectively, were reported, it is apparent that the presence of hydrophobic Tf 2 N moieties in the organic phase leads to the higher stoichiometry of extracted complexes for the light lanthanides.This may be attributed to a more lipophilic environment and the weak coordination ability of Tf 2 N −  -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8 anions that probably allows TODGA to saturate the coordination sphere of metal cations.
Based on the experimental results in this work, a possible explanation for the enhancement of lanthanide ions extraction with acidified solvent phase consisting of TODGA⋅HTf 2 N and DCE or nonane diluent could be the participation of Tf 2 N − anions, more hydrophobic than nitrate ions, in the formation of metal-ligand complexes: In this equilibrium, a trivalent metal cation forms the complex with three molecules of TODGA (Table 2) and this complex cation is extracted together with three Tf 2 N − anions as counterions with the expulsion of three protons from the organic phase to the aqueous phase to keep the charge balance.
Nitrate ions complex Ln(III) cations in water: The Ln(III) conditional extraction constant for Equation (7) including the equilibria Equation ( 8) can be expressed as: where β 1 and β 2 are the stability constants of the lanthanide(III) nitrate complexes formation.The values of the Ln(III) conditional extraction constant were calculated from Equation (8) using β 1 and β 2 values obtained from the literature. [54]logK ex values for several Ln(III) are presented in Figure 7.These values corroborate well with the results of our previous study on the extraction of Ln(III) ions using TODGA diluted in DCE from the aqueous neutral LiTf 2 N solutions.As can be seen in Figure 7, in this system the Ln(III) conditional extraction constant for the postulated LnL 3 (Tf 2 N) 3 species are higher than logK ex values obtained in this work, while the dependency logK ex = logK 3 (LiTf 2 N) is close to linear, that further support the proposed extraction mechanism described by Equation (7).
Let us now return to the data in Figure 4 on the dependency of D Ln as a function of [HNO 3 ] in the aqueous phase for TODGA•HTf 2 N diluted in DCE or nonane.There are two likely causes for the decreasing acid dependency for lanthanide ions with increasing acidity.The first one is the competition between HTf 2 N and HNO 3 molecules for TODGA molecules with increasing HNO 3 concentration in the aqueous phase.and from a mixture of HNO  [21] or 0.01 M TODGA⋅HTf 2 N solutions in DCE (left) and with 0.001 M TODGA or 0.001 M TODGA⋅HTf 2 N solutions in nonane (right).The lines are only for an eye guide.
extraction of Ln(III) ions with TODGA in DCE [21] or nonane (this work), (TODGA + [C 4 mim][Tf 2 N]) in DCE [22] are shown for the sake of comparison.A local minimum at Gd was detected in each lanthanide pattern.In all presented extraction systems, D Ln values are gradually increased from La to approximately Ho, and then they are similar for Er -Lu.This trend is consistent with the results of previous studies, which reported the preference of TODGA for binding smaller over larger trivalent lanthanide ions. [51]The D values in the TODGA⋅HTf 2 N/DCE system were higher than those in TODGA/DCE for Nd -Lu.It is interesting to compare the extraction data for Ln(III) in TODGA⋅HTf 2 N/DCE and (TODGA + [C 4 mim][Tf 2 N]) in DCE in [21] that shows close distribution ratio values for these two extraction systems with.
The separation factors defined as SF Ln1 / Ln2 = D Ln1 /D Ln2 are summarized in Figure 9 and Table S1-S4.It can be seen that in general, the SF for adjacent pairs of Ln(III) ions are close in all extraction systems.These results are in agreement with previous research, which attributed this issue to the persistence of the homoleptic inner-sphere metal-diglycolamide complex in different chemical environments. [55]However, the trans-lanthanide selectivity was greatly improved in the TODGA⋅HTf 2 N/DCE system.La(III), Sm(III), and Lu(III) were selected as a light, a middle, and a heavy Ln to evaluate the separation efficiency of the extraction systems.For instance, the separation factors for Lu/La and Lu/Sm were near 40 and 2 times higher with TODGA⋅HTf 2 N extractant in the organic phase than with TODGA in DCE (SF Lu/La are 10,965 and 275, SF Lu/Sm 48 and 28, for TODGA⋅HTf 2 N/DCE and TODGA/DCE system, respectively).When nonane was used as a diluent, this effect is less pronounced for Lu/La separation (SF Lu/La are 4677 and 1096 for TODGA⋅HTf 2 N/nonane and TODGA/nonane, respectively), whereas TODGA⋅HTf 2 N/nonane is less selective than TODGA/nonane (SF Lu/Sm are 8 and 28 for TODGA⋅HTf 2 N/nonane and TODGA/nonane.We suggest that this  D Ln values are decreased for heavy lanthanides, namely for Ho-Lu.The value of D Ln increased in the order: H 2 SO 4 < HNO 3 < HCl < H 3 PO 4 for light lanthanides, and H 2 SO 4 < HCl < HNO 3 <H 3 PO 4 for heavy lanthanides.These trends are probably the result of the TODGA aggregation contribution in the different acidic conditions, [24] as well as different hydration energies of the relevant anions.In a recent study on the lanthanide chloride and nitrate clusters in hydrocarbon solutions formed during liquid-liquid extraction with diglycolamide ligands, Brigham et al. reported that the extraction efficiency was impacted by anion type and solvent character, which was attributed to outersphere effects in stabilizing the ion cluster. [55]s can be seen in Tables S2, S4-S7 the most performant TODGA⋅HTf 2 N/DCE system in terms of the selectivity for HREE over light and middle elements is that in contact with the HNO 3 acidic media (for instance, SF Lu/La is near 10,965 for HNO 3 , and 159, 118 and 105 for HCl, H 2 SO 4 and H 3 PO 4 , respectively).

Conclusions
In the present study, REE ions were extracted with TODGA ligand which was preliminarily acidified by contact with the aqueous solution of N-H acid bis [(trifluoromethyl)sulfonyl]imide.Nitric acid and TODGA⋅HTf 2 N dependencies on distribution ratio values with DCE or nonane diluents were examined.The extraction behaviour of metal ions in the extraction systems with TODGA⋅HTf 2 N differed from that with the nonacidified solvent phase.This is attributed to a difference in the underlying extraction mechanism.It was revealed that REE are extracted via a cation exchange mechanism with TODGA⋅HTf 2 N/DCE solvent.It has been found that the distribution coefficient values for metal ions are much greater, especially at low aqueous acidity in the extraction systems with the acidified organic solvent phase.The heavier lanthanides were effectively extracted and the intergroup lanthanide selectivity was greatly improved in the extraction systems under study.One of the more interesting findings to emerge from this study is that in contrast to TODGA, TODGA⋅ HTf 2 N effectively extracts Ln(III) ions from aqueous HCl, H 2 SO 4 or H 3 PO 4 solutions.

Figure 1 .
Figure 1.The chemical structure for the TODGA extractant and Tf 2 N − anion used in this work.

Figure 2 .
Figure 2. The effect of TODGA concentration in various organic diluents on the extraction of HTf 2 N. [TODGA] org = 0.002-0.01M. Aqueous phase: 0.00152 M LiTf 2 N in 0.1 M HCl solution.V A /V O = 1/1.The solid lines are fits based on Equation (5) (see text for more details).

Figure 3 .
Figure3.Extraction of Tb(III) ions from aqueous 3 M HNO 3 solution with TODGA⋅HTf 2 N or TODGA diluted in various molecular diluents.The initial concentration of extractant in the organic phase is 0.005 M for the systems with octanol, nitrobenzene, DCE, 30% TBP in nonane, and toluene or 0.001 M for the system with nonane as diluent.

Figure 4 .
Figure 4.The effect of the initial HNO 3 concentration in the aqueous phase on the Eu(III) extraction (logD Eu ) with 0.01 M TODGA in DCE[21] or with the 0.01 M TODGA⋅HTf 2 N diluted in DCE, and 0.001 M TODGA⋅HTf 2 N or 0.001 M TODGA in nonane.The lines are only for an eye guide.

Figure 6 .
Figure 6.Extraction of Ln(III) ions (logD Ln ) from aqueous 3 M HNO 3 solution with TODGA⋅HTf 2 N diluted in DCE.The solid lines represent the results of linear fits.

Figure 5 .
Figure 5. logD Eu vs. log[H + ] for the Ln(III) extraction with 0.01 M TODGA⋅HTf 2 N diluted in DCE and in [NO 3 − ] = 5 M in the aqueous phase.

Figure 10 .
Figure 10.Effect of 3.0 M mineral acid (HNO 3 , HCl, H 2 SO 4 or H 3 PO 4 ) in the aqueous phase on the extraction efficiency of lanthanide metal ions with 0.01 M TODGA⋅HTf 2 N solutions in DCE.The lines have no physical meaning and are only guides for the eye.
These findings imply that mono-solvate species L⋅HTf 2 N are extracted with TODGA diluted in DCE and toluene, whereas mono-and disolvate species, L⋅HTf 2 N and L 2 ⋅HTf 2 N are formed in nonane.The values of logK HA were calculated by Equation ( or We performed the nonlinear fitting of the experimental data in Figure2using Equation (5) in Origin 9.1.software.The values of m (m is an average number of TODGA in the extracted species) are 1.06 ± 0.07, 0.93 ± 0.02 and 1.79 ± 0.13 for the extraction system with toluene, DCE and nonane as diluent, respectively.

Table 2 .
The results of slope analysis for Figure6.