Extraction separation of copper and cobalt dependent on intermolecular interaction between Cyanex302 and Cyphos IL101

Journal Pre-proofs Extraction separation of copper and cobalt dependent on intermolecular interaction between Cyanex302 and Cyphos IL101 Yunran Zhang, Jia Tang, Shijun Liu, Fang Hu, Mei Liu, Wei Jin, Jiugang Hu PII: DOI: Reference:

S1383-5866(19)33514-2 https://doi.org/10.1016/j.seppur.2020.116625 SEPPUR 116625

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

7 August 2019 16 January 2020 25 January 2020

Please cite this article as: Y. Zhang, J. Tang, S. Liu, F. Hu, M. Liu, W. Jin, J. Hu, Extraction separation of copper and cobalt dependent on intermolecular interaction between Cyanex302 and Cyphos IL101, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116625

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Extraction separation of copper and cobalt dependent on intermolecular interaction between Cyanex302 and Cyphos IL101

Yunran Zhanga, Jia Tanga, Shijun Liua,*, Fang Hua,*, Mei Liub, Wei Jinc, Jiugang Hu a,d,*

aCollege

of Chemistry and Chemical Engineering, Central South University, Changsha 410083,

China. bCollege

of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua

University, Fushun 113001, China. cSchool

of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China.

dHunan

Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources,

Central South University, Changsha 410083, China.

Corresponding authors: fax: +86-731-88879616 E-mail address: [email protected] (J. Hu), [email protected] (F. Hu), [email protected] (S. Liu).

1

Abstract: The mixed extractants are promising for the separation of complex multi-metal solutions due to their specific intermolecular interaction. In this work, the mixtures of Cyanex302 (C302) and Cyphos IL101 (IL101) were used to separate Cu(II)/Co(II) by adjusting the mole fraction of IL101 (XIL101) and the separation factor of copper over cobalt can be improved compared with the individual C302. The FT-IR combined with NMR analysis indicated that the dimeric C302 can completely dissociate at XIL101>0.3 and form C302-IL101 adduct due to their molecular interactions. Based on the twodimensional correlation analysis of IR spectra (2D-COS IR), four molecular environments of the P-OH group can be found in the mixed systems with varied XIL101, including C302 monomer, C302 dimer, IL101-C302 adduct, adduct of C302 with IL101 ion clusters. Although these formed adducts present an anti-synergism for the extraction of metal ions, the separation of Cu over Co can be greatly improved because the coextraction of Co is evidently depressed. Small-angle X-ray scattering analysis showed that the aggregates of individual IL101 and C302 systems have a similar ellipsoid morphology with different sizes, whereas the mixtures of IL101 and C302 present a spherical structure. These results will be helpful for the design of efficient extractants for the complex separation systems. Keywords: Mixed extractant, Cu/Co separation, intermolecular interaction, 2D-COS IR, SAXS

2

1. Introduction Solvent extraction is widely used in the separation and recovery of valuable metals from the leachates of primary minerals [1, 2] or secondary resources [3] due to its unique advantages such as its simplicity and high efficiency [4]. For various typical mineral leaching solutions, a variety of commercial extractants are now available, including di(2-ethylhexyl)phosphoric acid (D2EHPA), bis(2,4,4- trimethylpentyl) phosphinic acid (Cyanex272), tributoxyphosphine oxide (TBP), trioctylphosphine oxide (TOPO), quaternary ammonium (Aliquat336), and quaternary phosphorus salts (Cyphos IL101), and so on. When the low-grade and complex ores are exploited, several metal ions with similar properties always coexist in the liquors [5]. The complex separation problem of multi-metal ions makes the existing commercial extractants far from meeting demands. However, it has been well known that the mixing of two or more extractants is a feasible and convenient way to overcome the separation difficulty because the mixed extractants will exhibit different degrees of synergistic or anti-synergy effects through the intermolecular interactions [6, 7]. Moreover, some commercial extractants also have been developed by mixing to meet the specific separation aims, such as LIX984N (LIX860N+LIX84) [8], LIX64N (LIX64N+LIX63) [9], LIX973 (LIX84+LIX860) [10], etc. The synergistic effect of ketoxime and aldoxime in these mixed extractants can benefit for copper extraction from various aqueous media. Hui et al. [11] found that the mixture of neutral Cyanex923 and organophosphorus acid shows a significant synergistic effect on Ce(IV) extraction, while presents an antagonistic effect for Th(IV) 3

extraction compared with the individual extractant. So the mixed extractants not only enhance the extraction efficiency of Ce(IV) but also improve the selectivity of Ce(IV)/Th(IV). Similarly, Quinn et al. [12] investigated the extraction and separation of U(VI) and Fe(III) from mixed sulfate/chloride media. By using the mixed extractants of Alamine336 and Cyanex272, an antagonistic effect for Fe(III) extraction and a synergism effect for U(VI) extraction were observed, thereby effectively improving the separation of U(VI)/Fe(III). On the other hand, some commercial bi-functional extractants are also developed by combining acidic and basic extractants [13], such as Cyphos IL104 and Cyphos IL202, which are potentially capable of extracting both cations and anions [14]. Although various binary extractants have been used to achieve the separation of metal ions, the insights into the synergistic or anti-synergy effects of mixed extractants should be deeply disclosed to develop the efficient extractants for the complex separation needs. It is well known that amphiphilic molecules are prone to selfassembling in organic solvents to form reverse micelles or microemulsions [15, 16]. For the mixture of acidic and basic ligands, previous studies showed that various species such as ion-pairs, H-bonding complexes, and solvation adducts can be formed [17]. Liu et al. [18] studied the interactions between organophosphorus acid (D2EHPA, PC88A, and Cyanex272) and tertiary amine (Alamine336) by measuring viscosities and dielectric constants. The results indicated that there is strong component interaction between D2EHPA and Alamine or PC88A and Alamine 336, while comparatively weak interaction between Cyanex272 and Alamine336. Tkac et al. [19] found that the mixed 4

extractant of D2EHPA and CMPO has an obvious intermolecular interaction during the extraction of trivalent f-block ions. The addition of neutral CMPO destroys the acidic HDEHP dimers and new HDEHP-CMPO adduct forms by hydrogen bond interaction between the POH of HDEHP and phosphoryl groups of CMPO, which could reduce the concentration of free CMPO available for complexation with metal ions and induce a lower distribution ratio. Torkaman et al. [20] also found that the improvement of extraction and stripping efficiency of Sm(III) depends on the formation of new dimmers between D2EHPA and Cyanex301. In this study, the mixtures of Cyanex302 (C302) and CyphosIL101 (IL101) were used to achieve efficient separation of Cu(II) over Co(II) in ammoniacal solution by adjusting mole fraction of two extractants. The intermolecular interaction of the mixed extractants was studied with FT-IR and NMR spectroscopies. The fingerprint features overlooked in one-dimensional IR spectra were further disclosed with two-dimensional correlation spectroscopy (2D-COS) analysis. In addition, the morphology change of the mixed extractants before and after extraction was revealed based on the SAXS data. Therefore, the relationship between metal separation behaviors and intermolecular interaction for the mixed systems was disclosed.

2. Experimental 2.1 Reagents and samples The commercial extractant of bis(2,4,4-trimethylpentyl) monothiophosphinic acid (Cyanex302, denoted as C302) was kindly offered by Cognis Corporation. The ionic 5

liquid extractant of trihexyl(tetradecyl) phosphonium chloride (Cyphos IL101, denoted as IL101) is supplied by the J&K(China). The molecular structures of Cyanex302 and Cyphos IL101 were shown in Fig. 1. Toluene is used as a diluent. The extractants and diluents were used without further purification. Aqueous feed solutions containing 0.02 mol/L CuSO4, 0.02 mol/L CoSO4, and 2.0 mol/L (NH4)2SO4 were prepared and the desired pH was adjusted with H2SO4 or NaOH solutions. Cl-

O P

P+

SH

Cyanex302

Cyphos IL101

Fig. 1 Molecular structures of the used extractants. 2.2 ATR-IR measurement and 2D correlation analysis For the measurements of ATR-IR spectra, a series of mixed extractants were prepared with C302 and IL101, where the total extractant concentration was kept at 0.1 mol/L and the mole fraction of IL101 component (XIL101) increased from 0 to 1 with an interval of 0.1. The ATR-IR spectra were recorded over a range of 4000-650 cm -1 by 64 scans at ca. 25 °C using a Nicolet iS50 Fourier transform infrared spectrometer. Each sample was measured three times to check the spectral repeatability, and the average spectra were used for further analysis. Because of the overlapping structure information that is not easily observed in the one-dimensional infrared spectrum, a two-dimensional (2D) correlation technique is used to analyze the final IR spectra [21]. The spectral data of mixed extractants after 6

baseline-correcting were converted using the Hilbert transform matrix method on Matlab7.0 software to construct a 2D-dependent IR spectrum [22]. 2.3 NMR Measurements The 1H and

31P

NMR spectra of the mixed extractants were recorded on a 400

MHz NMR spectrometer (Bruker AscendTM). Toluene-d8 was used as the lock solvent for 1H NMR. Phosphoric acid (P(H3PO4) ≥85%) was used as an external standard for 31P

NMR.

2.4 SAXS Measurements Small-angle X-ray scattering (SAXS) data were collected on the BL16B1 beamline station at Shanghai Synchrotron Radiation Facility (SSRF), China. The distance between the sample and the detector was 1770 mm, and the energy of the light source was 10 keV. The sample was tested in a liquid cell with 2 mm thick. The 2D patterns were recorded at ca. 25 °C with a test time of 120s per sample. The original 2D SAXS data were converted into one-dimensional data using the Fit2d software package [23], I(q) versus q, where q (cm−1) = (4πsinθ/λ), 2θ is the scattering angle, and λ is the wavelength of the X-rays [24]. The one-dimensional data were then analyzed and converted into p(r)-r using ATAS software. The pure solvent was measured in the same condition to deduct the background of the solvent and container. 2.5 Extraction Experiments The feed solution of pH 7.5 was used as an aqueous phase. Before the extraction, the fresh organic solutions with varied mole fractions of IL101 (XIL101) were prepared, where the total extractant concentration was kept at 0.1 mol/L. Equal volumes (5 mL) 7

of organic and aqueous phases were mechanically shaken for 20 minutes at 25 °C for extraction equilibrium. After settling and phase separation, the loaded organic phases were taken for structural characterization. The metal concentration in the aqueous phases was determined with the inductively coupled plasma-atomic emission spectrometry (ICP-AES, Perkin Elmer 5300DV, America) before and after extraction. The metal concentration in the loaded organic phase was calculated by mass balance. The distribution coefficient (D) was calculated as the ratio of metal concentration in the organic phase to that in the aqueous phase at equilibrium. The separation factor of Cu(II)/Co(II) (βCu/Co) was calculated from the ratio of distribution coefficient between copper and cobalt.

3. Results and discussion 3.1 Extraction and separation of metal ions There is considerable interest in the extraction separation of copper, nickel, and cobalt in the ammoniacal liquors [25, 26]. Although Cyanex302 presents a good extraction potential due to its strong coordination ability with metal ions, the separation of Cu(II) and Co(II) in the ammoniacal media is very difficult. In order to clearly present the regulation role of Cyphos IL101 on the extractability of Cyanex302, individual and mixed systems of C302 and IL101 were evaluated for the extraction of copper and cobalt at pH 7.5. From Fig. 2a, it can be found that the individual IL101 has no extractability for both Cu(II) and Co(II), but both of which can be extracted with individual C302. When the concentration of individual C302 is 0.1 mol/L, the 8

extraction percentage of Cu(II) and Co(II) can reach 99.9% and 90.7%, respectively. Although the appropriate concentration of C302 can enlarge the difference in the extraction efficiency of Cu(II) and Co(II), the evident co-extraction phenomenon indicates that the copper and cobalt cannot be effectively separated with individual C302. The maximum separation factor of Cu(II)/Co(II) is about 109.7 for 0.07 mol/L individual C302 and about 61.4% cobalt is co-extracted with copper. However, when mixing C302 with IL101, a strong anti-synergy effect can be observed for both copper and cobalt. As the mole fraction of IL101 in the mixtures increases (Fig. 2a), the extraction efficiency of both Cu(II) and Co(II) decreases. For instance, when mixing with the equal mole of C302 and IL101, the extraction efficiency decreases from 98.2% to 54.2% for Cu(II), and it decreases from 31.8% to 4.3% for cobalt. Interestingly, at each component, the decreasing extent of extraction efficiency of cobalt is evidently larger than that of copper, indicating that the separation of Cu(II)/Co(II) could be achieved by mixing IL101 with C302. The addition of IL101 has almost no inhibition on copper extraction when the concentration of C302 in the mixtures is larger than 0.07 mol/L. From Fig. 1b, it can be observed that Cu(II)/Co(II) separation factor for the mixed system of 0.07 mol/L C302 and 0.03 mol/L IL101 can be greatly improved from 109.7 to 916.2, where the extraction percentage of Cu(II) is still kept at nearly 100% and the extraction percentage of Co(II) is lower than 10%. Therefore, the antagonistic effect of IL101 in the mixed extractants can achieve the efficient separation of copper over cobalt compared with the individual C302.

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Fig. 2 Extraction (a) and separation (b) behaviors of Cu(II) and Co(II) with C302 in the absence and presence of IL101. Organic phase: the total concentration of the mixed extractant is 0.1 mol/L. Aqueous phase: 0.02 mol/L Cu2+, 0.02 mol/L Co2+, 2.0 mol/L (NH4)2SO4, pH=7.5. Three mineral acids including H2SO4, HCl, and HNO3 were further evaluated for copper stripping from the loaded organic phases containing individual C302 or the mixture of C302 and IL01 (mole fraction=7:3). As shown in Fig. 3, the H2SO4 solution almost cannot strip Cu(II) in both C302 and the mixed C302+IL101 systems, even at 8 mol/L H2SO4. Moreover, only 10.8% Cu(II) can be stripped from the individual C302 system when HCl concentration is 6 mol/L, indicating the regeneration of Cu(II)-loaded C302 organic phase is very difficult [27]. The results show that the complete stripping of Cu(II) from the C302 system needs about 6 mol/L HNO3. Although the stripping of Cu(II) from the mixed system of C302 and IL101 is still difficult when using H2SO4 as the stripping agent, the stripping efficiency of Cu(II) can be well enhanced when HCl and HNO3 are used. About 50% Cu(II) can be stripped by 2 mol/L HCl and almost 4 mol/L HNO3 can effectively regenerate the organic phase containing C302 and IL101. 10

Combined with the results in Fig. 2, the mixing of IL101 and C302 not only improves the separation of Cu(II)/Co(II) but also benefits for the regeneration of the loaded organic phases. These phenomena should mainly attribute to the intermolecular interaction between IL101 with C302.

Fig. 3 Stripping behaviors of Cu(II) from the loaded organic phases of (a) C302 and (b) C302-IL101 with HNO3, HCl, and H2SO4. 3.2 ATR-IR analysis of mixed extractants The ATR-IR spectra of individual C302 and mixed C302-IL101 systems at different mole fractions of IL101 (XIL101) were collected to reveal the anti-synergistic role of IL101 on the separation of Cu(II)/Co(II) with C302. It is well known that organic phosphonic acid extractants are usually dimeric in the nonpolar solvent [20, 28]. In Fig. 4a, the IR vibration peak at 2200 cm-1 and 3050 cm-1 belongs to the P-OH bond of C302 dimers. Moreover, when decreasing the C302 concentration, the peak intensity of the P-OH bond is only slightly reduced. Therefore, even at the low concentration of C302, the dimeric structure of C302 also does not dissociate. However, after mixing with IL101, the peaks of C302 at both 2200 cm-1 and 3050 cm-1 relatively decrease and even 11

disappear at XIL101=0.4, indicating the addition of IL101 induces a complete dissociation of dimeric C302 at XIL101>0.3.

Fig. 4 (a) ATR-IR spectra of C302 at varied concentrations and (b) C302-IL101 mixtures at various mole fractions of IL101. The vibration peak at 917 cm-1 belongs to the P-OH bond and the peak shifts at different mole fractions of IL101 are presented in Fig. 5. When a small amount of IL101 is added (XIL101=0.1), the peak position of the P-OH bond is almost unchanged. With continuingly increasing the mole fraction of IL101, the P-OH stretching vibration peak is slowly blue-shifted at XIL101<0.3 but significantly blue-shifted at 0.30.6, the excess IL101 molecules tend to form ion clusters due to its strong electrostatic 12

interaction, thereby the peak of P-OH groups no longer changes. Due to the stronger interaction of C302+IL101 adducts, the coordination between C302 molecules and metal ions can be depressed by the competition of IL101. Moreover, copper ions could have a stronger coordination ability with C302 molecules than cobalt ions due to its Jahn-Teller effect, thus the formation of C302-IL101 adduct is more detrimental for Co extraction but advantageous for Cu/Co separation since fewer cobalt ions are extracted.

Fig. 5 ATR-IR spectra of (a) C302-IL101 mixtures at various mole fractions of IL101 and (b) the corresponding peak shifts for P-OH group of C302 at various XIL101. 3.3 2D-IR analysis of mixed extractants Two-dimensional correlation spectroscopy (2D-COS) analysis has been widely used to improve spectral resolution and distinguish the fingerprint features that may be overlooked in the one-dimensional spectra [29]. Because P-OH of C302 is an important functional group in the extraction process, the 2D-COS IR analysis will focus on the spectral range of 860-960 cm-1. As shown in Fig. 6a, only one strong positive auto-peak can be observed from the synchronous spectrum, indicating the mixing of IL101 and C302 has a strong intermolecular interaction [30]. From the asynchronous spectrum in 13

Fig. 6b, the asymmetric cross-peaks and their intensity difference indicate that the peak of P-OH not only has a strong spectral shift but also has an intensity change under different mole fractions of IL101 [31]. In addition, two positive cross-peaks at [913, 885 cm-1(+)], [927, 910 cm-1(+)] and one negative cross-peak at [(945, 927 cm-1(-)] can be observed. According to Noda’s fundamental rule [29], if the spectral intensity change at v1 is prior to that at v2, the cross-peak with v1 and v2 coordinates is positive, and it is negative if the changing trend of spectral intensity is opposite. So the change order of the cross-peak signals is as follows: 885>913>927 cm-1 and 945>927 cm-1. Therefore, it can be inferred that the peak of the P-OH group at 885 cm-1 is attributed to the C302 dimer state and the peak at 945 cm-1 results from the free C302 molecules [32]. Meanwhile, the peak at 913 cm-1 is attributed to the adduct of C302 and IL101, while the peak at 927 cm-1 should be assigned to the interaction of C302 molecules with ion clusters of IL101. Therefore, there are four different chemical environments of the P-OH group in the range of 860-960 cm-1. From the appearance order of various states of the P-OH group, it can be proved that the dissociation of C302 dimers should result from the electrostatic interaction between C302 and IL101 molecules. Moreover, when IL101 molecules are the predominant species, the C302 molecules can further interact with ion clusters of IL101.

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Fig. 6 Contour maps of (a) synchronous and (b) asynchronous of 2D correlation IR spectra for C302-IL101 mixtures. 3.4 NMR analysis of mixed extractants The local environments of the mixed extractants during the component change can be further distinguished by 1H and 31P NMR spectra. The 1H NMR signals and chemical shift (δ) for the individual and the mixed systems are presented in Fig. 7. The chemical shift at 2.83 ppm belongs to the ethyl group of IL101 molecules (P-CH2). As the mole fraction of IL101 decreases, the 1H chemical shift gradually decreases from 2.83 ppm to 2.28 ppm, as shown in Fig. 7b. It is known that the chemical shift can be caused by several factors such as inductive effect and solvent effect. As disclosed by IR analysis, the C302+IL101 adducts are formed through the ionic bond between P-O- of C302 with P+ of IL101. In this case, the electronic cloud density around P+ (IL101) will gradually increase after mixing with C302 because of its inductive effect, thus inducing the chemical shift of P-CH2 moving to the high field direction. In Fig. 7c, the chemical shifts of two peaks at 1.56 ppm and 1.66 ppm are assigned to two methylene groups attached to P-CH2- for IL101. For the C302+IL101 mixed system, when IL101 is the predominant component in the mixtures (0.6< XIL101<1), only a small number of free 15

C302 can bond with IL101 ion clusters. The intramolecular spin-dipole interaction will split the peak of P-CH2-CH2-CH2 groups in IL101 molecules, so two split peaks can be observed in Fig. 7a at 0.6< XIL101<1. As the mole fraction of IL101 gradually decreases, the 1H chemical shift of P-CH2-CH2-CH2 groups gradually shifts to the high field and the split peaks merge into a single peak while 0.2
Fig. 7 (a) 1H NMR spectra and (b, c) 1H chemical shifts of IL101 in the mixtures at different mole fractions. Because the characteristic functional groups of both C302 and IL101 molecules have P atoms, the intermolecular interaction of C302-IL101 systems can be further justified by 31P NMR analysis. As shown in Fig. 8, the 31P chemical shift for individual C302 and IL101 can be found at δ=94.45 ppm and δ=33.275 ppm, respectively. Obviously, the 31P chemical shift exhibits two different directions for C302 and IL101. As increasing the mole fraction of IL101, the 31P chemical shift of IL101 moves to the low field direction for XIL101<0.6, whereas it moves to the high field direction for XIL101>0.6. Therefore, there are two kinds of main intermolecular interaction in the 16

mixed systems as the varying mole fraction of IL101. At XIL101<0.6, the IL101 can interact with C302 via the intermolecular weak dipole interaction, thereby decreasing the electron cloud density around P atoms on IL101. When XIL101 is larger than 0.6, the slow upfield shift indicates that the interaction between C302 and IL101 is weakened. Because IL101 ion clusters are the dominant species in the mixed system at XIL101>0.6, the aggregation of IL101 molecules will result in the increase of electron cloud density around P atoms, thus the chemical shift moves to the upfield. The 31P chemical shift of C302 only moves toward the high field when increasing the mole fraction of IL101 because the formation of C302+IL101 adducts increases the electron cloud density of P atoms on C302. Therefore, the molecular state and interaction between C302 and IL101 can be effectively adjusted by changing the mole fraction of two extractants. The schematic illustration of separation mechanism of Cu(II)/Co(II) with the mixed C302 and IL101 is shown in Scheme 1. Depended on the adjustable intermolecular interaction, the extraction difference for metal ions will be enlarged. Combined with the extraction data in Fig. 2, it can be found that the intermolecular dipole interaction between C302 and IL101 at XIL101<0.6 obviously depresses the extraction of both Cu(II) and Co(II). When XIL101 is larger than 0.6, the extraction of cobalt ions is more inhibited by the intramolecular spin-dipole interaction between C302 and IL101 clusters, thus the efficient separation of Cu(II)/Co(II) can be achieved compared with individual C302.

17

Fig. 8 (a) 1P NMR spectra and (b) 1P chemical shifts of IL101 and C302 in the mixtures at different mole fractions.

Scheme 1. Separation mechanism of Cu(II)/Co(II) with the mixed C302 and IL101.

3.5 SAXS analysis of mixed extractants SAXS is a very powerful method to disclose the size and morphology of molecular aggregates in solution [33, 34]. The scattering functions (I(q) vs q) of mixed extractants 18

with different molar fractions are shown in Fig. 9a. It can be found that both individual C302 and IL101 have a relatively higher I(q) intensity in the low-q region compared with the mixed C302+IL101 system. These features of the scattering curves reflect that the mixing of C302 and IL101 can decrease the micellar size of individual extractant. In the low-to-high q regions, the scattering intensity of individual C302 has a significant increase, indicating that the C302 aggregates could assemble into a higher-ordered structure [35]. After mixing C302 and IL101, the coincident I(q) in the high-q region suggests the mixed systems have a similar cross-section. Because random interactions between aggregation units can influence the scattering intensity and shape in the lowto-medium q region [36], to understand the structure of the higher-ordered architectures, the scattering contribution of the aggregates must be separated from the scattering envelope. The Guinier plots as shown in Fig. 9b are used to reveal the detail information in the low q2 region (0.031-0.039 Å-2). The value of the gyration radius (Rg) can be determined by Guinier’s law [37]:

(

𝐼(𝑞) = I(0)exp ―

𝑞2𝑅𝑔2 3

)

(1)

𝑞2𝑅𝑔2 𝐿𝑛 𝐼(𝑞) = ― + 𝐿𝑛 I(0) 3

(2)

The calculated Rg values of individual extractants and their mixtures are shown in Table 1. Similar to surfactants, the aggregation of extractant molecules can form microemulsions with an ordered structure to minimize the contact between the polar groups and solvent. The size and structure of microemulsion strongly depend on the chemical structure and interfacial activity of extractant molecules. From Table 1, it can 19

be seen that both individual C302 and IL101 have a larger size than the mixed extractants, and C302 has the biggest size due to the strong intermolecular hydrogen bond in phosphoric acid extractant. Mixing of IL101 and C302 can greatly reduce the size of the C302 aggregates, but the size of microemulsions has no evident change for the mixed systems at each XIL.

Fig. 9 (a) SAXS data and (b) Guinier plots for the SAXS profiles obtained for individual and C302+IL101 mixtures. Table 1 Gyration radius of the mixed extractants at different molar fractions. C302

XIL101=0.3

XIL101=0.5

XIL101=0.7

IL101

Rg(Å)

5.61

1.87

3.40

3.72

4.17

Variance

±0.009

±0.079

±0.063

±0.084

±0.014

The p(r) function reflects the distribution of the distances between the components, where r is the distance and p(r) is the relative amount of the specific distance that occurs [35]. As shown in Fig. 10a, the curves of individual C302 and IL101 present an asymmetrical bell-shaped peak, and the mixed systems present symmetrical arch20

shaped peaks. These phenomena suggest that individual C302 or IL101 has an ellipsoid structure and the mixed systems have a spherical structure [36, 38]. Moreover, the maximum linear extent (MLE) of the aggregates was evaluated to reflect the maximum length of aggregates [39]. As shown in Fig. 10b, the individual C302 has the MLE value of about 2.91 Å, whereas it is about 1.82 Å for individual IL101. After mixing IL101 with C302, the MLE for the mixed systems has an evident decrease and has a similar value of about 1.4 Å. Therefore, when mixing IL101 with C302, the structure of microemulsions changes from ellipsoid to spherical with the reduction of size, indicating that the addition of IL101 can destroy the ordered structure of C302.

Fig. 10 (a) P(r) functions and (b) Maximum linear extent (MLE) for different molar ratio mixtures. The maximum linear extent (MLE) of the tails corresponds to rmax.

3.6 The structure analysis of the loaded organic phase In order to clarify the structural changes of the mixed systems during the extraction process, the ATR-IR spectra of C302+IL101 (XIL=0.3) before and after loaded Cu(II) 21

are shown in Fig. 11a. Once coordinating with Cu(II), the absorption peak at 917 cm-1 (P-OH) of both C302 and C302+IL101 mixture obviously disappears and two shoulder peaks at 1043 cm-1 and 1080 cm-1 (P=O) appear at the same time. Therefore, the loading of Cu(II) can induce a tautomerism from thiono (-P(=S)OH) to thiol (-P(=O)SH) of C302 molecules for both individual and mixed systems [40], which will benefit for Cu(II) extraction because sulfur atom is a stronger electron donor than oxygen. The SAXS scattering data and p(r) functions of C302+IL101 (XIL101=0.3) before and after Cu(II) extraction were further analyzed (dry organic phase and Cu(II)-loaded organic phase). For comparison, the SAXS scattering data of dry organic phase after contacting with the aqueous phase without copper ions were also collected (wet organic phase). From Fig. 11b, the SAXS curves of Cu(II)-loaded organic phase and wet organic phase are almost coincident from low-q to high-q region. These scattering features indicate that the mixed C302+IL101 systems have a similar size after contacting with aqueous phase with and without Cu(II). From the p(r) function in Fig. 11c, the MLE of both wet organic phase and Cu(II)-loaded organic phase are almost double than that of the dry organic phase. The symmetrical change of arch-shaped peaks indicates the spherical structure of the dry organic phase becomes larger after contacting with the aqueous phase, which means that the mixed C302+IL101 system can coordinate with water, and the water molecules can be exchanged by Cu(II) during extraction.

22

Fig. 11 ATR-IR and SAXS data of C302-IL101 mixture (XIL101=0.3) before and after Cu(II) extraction. (a) ATR-IR spectra, (b) SAXS curves, and (c) p(r) functions.

4. Conclusions In this work, the separation of copper over cobalt can be effectively improved by the mixed system of C302 and IL101 compared with the individual C302. Moreover, the relationship between metal separation behaviors and intermolecular interaction was disclosed. The FT-IR combined with NMR analysis indicated that interactions between IL101 and Cyanex302 compete with dimeric C302, and the dimeric C302 molecules completely dissociate at XIL101>0.3. Based on the two-dimensional correlation analysis of IR spectra, four molecular environments of P-OH group are disclosed in the mixed systems at varied XIL101, including C302 monomer, C302 dimer, IL101-C302 adduct, adduct of C302 with IL101 ion clusters. Although these formed adducts present an antisynergism on the metal extraction, the separation of Cu over Co can be greatly improved because the co-extraction of Co is evidently depressed. SAXS analysis 23

showed that individual IL101 and C302 aggregates have a similar ellipsoid morphology with a different size, whereas the mixtures of IL101 and C302 have a similar spherical structure at varied XIL101. Moreover, the mixed C302+IL101 systems have a similar size and spherical morphology after contacting with aqueous phase with and without Cu(II).

Acknowledgements This work was financially supported by the National Key Research and Development Program of China (No. SQ2019YFC190391), the Hunan Provincial Science and Technology Plan Project (Nos. 2016TP1007 and 2019JJ30031) and the Major Scientific Research Projects of China Petrochemical Corporation (119014-2).

References [1] S. Espiari, F. Rashchi, S. Sadrnezhaad, Hydrometallurgical treatment of tailings with high zinc content, Hydrometallurgy, 82 (2006) 54-62. [2] M. Cox, Solvent extraction in hydrometallurgy, in:

Solvent Extraction Principles

and Practice, Revised and Expanded, CRC Press, 2004, pp. 466-515. [3] J. Li, X. Li, Q. Hu, Z. Wang, J. Zheng, L. Wu, L. Zhang, Study of extraction and purification of Ni, Co and Mn from spent battery material, Hydrometallurgy, 99 (2009) 7-12. [4] H. Ota, M. Matsumiya, T. Yamada, T. Fujita, S. Kawakami, Purification of rare earth

bis(trifluoromethyl-sulfonyl)amide

salts

by

hydrometallurgy

and

electrodeposition of neodymium metal using potassium bis(trifluoromethylsulfonyl)amide melts, Sep. Purif. Technol., 170 (2016) 417-426. [5] F. Anjum, M. Shahid, A. Akcil, Biohydrometallurgy techniques of low grade ores: A review on black shale, Hydrometallurgy, 117-118 (2012) 1-12. [6] M. Tian, N. Song, D. Wang, X. Quan, Q. Jia, W. Liao, L. Lin, Applications of the 24

binary mixture of sec-octylphenoxyacetic acid and 8-hydroxyquinoline to the extraction of rare earth elements, Hydrometallurgy, 111-112 (2012) 109-113. [7] A.H. Bond, M.L. Dietz, R. Chiarizia, Incorporating size selectivity into synergistic solvent extraction: a review of crown ether-containing systems, Ind. Eng. Chem. Res., 39 (2000) 3442-3464. [8] V. Sridhar, J. Verma, S.A. Kumar, Selective separation of copper and nickel by solvent extraction using LIX 984N, Hydrometallurgy, 99 (2009) 124-126. [9] B.D. Pandey, V. Kumar, D. Bagchi, D.D. Akerkar, Extraction of nickel and copper from the ammoniacal leach solution of sea nodules by LIX 64N, Ind. Eng. Chem. Res., 28 (1989) 1664-1669. [10] F.J. Alguacil, P. Navarro, Non-dispersive solvent extraction of Cu(II) by LIX 973N from ammoniacal/ammonium carbonate aqueous solutions, Hydrometallurgy, 65 (2002) 77-82. [11] N. Panda, N.B. Devi, S. Mishra, Extraction of neodymium(III) using binary mixture of Cyanex272 and Cyanex921/Cyanex923 in kerosene, J. Radioanal. Nucl. Chem., 296 (2013) 1205-1211. [12] J.E. Quinn, K.H. Soldenhoff, Process for uranium recovery using Cyanex272, Hydrometallurgy, 152 (2015) 7-12. [13] R. Giernoth, Task‐specific ionic liquids, Angew. Chem., 49 (2010) 2834-2839. [14] L. Guo, Y. Liu, C. Zhang, J. Chen, Preparation of PVDF-based polymer inclusion membrane using ionic liquid plasticizer and Cyphos IL 104 carrier for Cr (VI) transport, J. Membrane Sci., 372 (2011) 314-321. [15] H.-F. Eicke, Surfactants in Nonpolar Solvents: Aggregation and Micellization., Top. Curr. Chem., (1980) 85-145. [16] H.F.M. Eicke, Z., Temperature-Dependent Coalescence in Water-Oil Microemulsions and Phase Transitions to Lyotropic Mesophases, J. Colloid Interface Sci., (1981) 151-158. [17] A.M. Eyal, E. Bressler, R. Bloch, B. Hazan, Extraction of metal salts by mixtures of water-immiscible amines and organic acids (acid-base couple extractants). 1. A review of distribution and spectroscopic data and of proposed extraction mechanisms, 25

Ind. Eng. Chem. Res., 33 (1994) 1067-1075. [18] Y. Liu, M.S. Lee, Determination of viscosity and dielectric constant for studying the interactions in binary mixtures of organophosphorus acid and tertiary amine, J. Mol. Liq., 222 (2016) 233-238. [19] P. Tkac, G.F. Vandegrift, G.J. Lumetta, A.V. Gelis, Study of the interaction between HDEHP and CMPO and its effect on the extraction of selected lanthanides, Ind. Eng. Chem. Res., 51 (2012) 10433-10444. [20] R. Torkaman, M. Moosavian, M. Torab-Mostaedi, J. Safdari, Solvent extraction of samarium from aqueous nitrate solution by Cyanex301 and D2EHPA, Hydrometallurgy, 137 (2013) 101-107. [21] I. Noda, Generalized two-dimensional correlation method applicable to infrared, Raman, and other types of spectroscopy, Appl. Spectrosc., 47 (1993) 1329-1336. [22] S. Šašić, A. Muszynski, Y. Ozaki, New insight into the mathematical background of generalized two-dimensional correlation spectroscopy and the influence of mean normalization pretreatment on two-dimensional correlation spectra, Appl. Spectrosc., 55 (2001) 343-349. [23] G. Yao, T. Duan, M. An, H. Xu, F. Tian, Z. Wang, The influence of epitaxial crystallization on the mechanical properties of a high density polyethylene/reduced graphene oxide nanocomposite injection bar, RSC Adv., 7 (2017) 21918-21925. [24] G. Fournet, A. Guinier, Small angle scattering of X-rays, in, John Wiley & Sons, New York, 1955, pp. 7-78. [25] P.K. Parhi, S. Panigrahi, K. Sarangi, K.C. Nathsarma, Separation of cobalt and nickel from ammoniacal sulphate solution using Cyanex 272, Separation and Purification Technology, 59 (2008) 310-317. [26] K.C. Nathsarma, P.V.R. Bhaskara Sarma, Processing of ammoniacal solutions containing copper, nickel and cobalt for metal separation, Hydrometallurgy, 33 (1993) 197-210. [27] N.E. El ‐ Hefny, J.A. Daoud, Extraction of Copper(II) by CYANEX 302 in Kerosene from Different Aqueous Media, Solvent Extraction and Ion Exchange, 25 (2007) 831-843. 26

[28] R.J. Ellis, T. Demars, G. Liu, J. Niklas, O.G. Poluektoy, I.A. Shkrob, In the Bottlebrush Garden: The Structural Aspects of Coordination Polymer Phases formed in Lanthanide Extraction with Alkyl Phosphoric Acids, J. Phys. Chem. B, 119 (2015) 11910-11927. [29] I. Noda, Advances in Two-Dimensional Correlation Spectroscopy (2DCOS), in: J. Laane (Ed.) Frontiers and Advances in Molecular Spectroscopy, Elsevier, 2018, pp. 4775. [30] S. Morita, H. Shinzawa, I. Noda, Y. Ozaki, Effect of band position shift on movingwindow two-dimensional correlation spectroscopy, J. Mol. Struct., 799 (2006) 16-22. [31] M.A. Czarnecki, Two-dimensional correlation spectroscopy: effect of band position, width, and intensity changes on correlation intensities, Appl. Spectrosc., 54 (2000) 986-993. [32] P. Larkin, Infrared and Raman spectroscopy: principles and spectral interpretation, Elsevier, 2017. [33] E. Roche, M. Pineri, R. Duplessix, A. Levelut, Small‐angle scattering studies of nafion membranes, J. Polym. Sci., 19 (1981) 1-11. [34] D.J. Yarusso, S.L. Cooper, Microstructure of ionomers: interpretation of smallangle x-ray scattering data, Macromolecules, 16 (1983) 1871-1880. [35] R.J. Ellis, M. Audras, M.R. Antonio, Mesoscopic Aspects of Phase Transitions in a Solvent Extraction System, Langmuir, 28 (2012) 15498-15504. [36] O. Glatter, The interpretation of real-space information from small-angle scattering experiments, J. Appl. Crystallogr., 12 (1979) 166-175. [37] G. Porod, O. Glatter, O. Kratky, Small angle X-ray scattering, in: O.G.a.O. Kratky (Ed.), Academic Press, London, 1982, pp. 17. [38] O. Glatter, Evaluation of small-angle scattering data from lamellar and cylindrical particles by the indirect transformation method, J. Appl. Crystallogr., 13 (1980) 577584. [39] O. Glatter, X. Neutrons, rays and Light: Scattering Meth-ods Applied to Soft Condensed Matter, in, Elsevier: Amster-dam, 2002. [40] K.C. Sole, T.L. Ferguson, J.B. Hiskey, Solvent extraction of silver by Cyanex 272, 27

Cyanex 302 and Cyanex 301, Solvent Extr. Ion Exc., 12 (1994) 1033-1050.

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Statement of Author Contributions Yunran Zhang: Data curation, Investigation, Roles/Writing - original draft. Jia Tang: Data curation. Shijun Liu: Supervision, Methodology. Fang Hu: Data curation. Mei Liu: Resources, Investigation. Wei Jin: Investigation, Writing - review & editing. Jiugang Hu: Conceptualization, Investigation, Supervision, Writing - review & editing.

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Conflict of Interest Statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Highlights

Efficient separation of Cu over Co is achieved with the mixture of C302 and IL101. Intermolecular interaction of the mixtures was disclosed by IR, NMR and SAXS. Four molecular environments of P-OH group were distinguished by 2D-COS IR analysis. Individual IL101 and C302 aggregates have a similar ellipsoid morphology. The morphology of IL101 and C302 mixtures is a similar spherical structure.

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