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Characterization of the interaction of rare earth elements with P507 in a microfluidic extraction system using spectroscopic analysis
Chemical Engineering Journal 356 (2019) 453–460
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Characterization of the interaction of rare earth elements with P507 in a microfluidic extraction system using spectroscopic analysis
T
⁎
Kaihua Chena,b, Yuan Hea,b, C. Srinivasakannanc, Shiwei Lia,b, Shaohua Yina,b, , Jinhui Penga,b, Shenghui Guoa,b, Libo Zhanga,b a
State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, Yunnan 650093, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan 650093, China c Chemical Engineering Department, The Petroleum Institute, Khalifa University of Science and Technology, Abu Dhabi, P.O. Box 253, United Arab Emirates b
H I GH L IG H T S
role of lactic acid in the extraction process is analyzed. • The characterization of extracted compound is studied by spectroscopic analysis. • The • The structure of extracted complexes is confirmed.
A R T I C LE I N FO
A B S T R A C T
Keywords: Rare earths P507 Spectrum Complex extraction Microreactor
An attempt to characterize the interaction of rare earth elements with P507 in a microfluidic extraction system containing lactic acid as complexing agent is made using spectroscopic (FT-IR, UV–Vis, NMR and MS) analysis. The role of lactic acid in the extraction process is analyzed. The extraction mechanism confirmed that lactic acid does not involve in the extraction reaction. A comparative analysis of the composition of the extracts using FTIR, UV–Vis, NMR and MS methods, indicates that the extracted complexes of rare earth ions located in the center is formed by the cation exchange process between PeOeH and RE3+, and coordination process between P]O and RE3+.
1. Introduction Rare earth elements (REEs) are composed of lanthanide series elements in the periodic table, i.e. from lanthanum (La) to lutetium (Lu), together with yttrium (Y) and scandium (Sc) which possess unique physical and chemical characteristics [1,2]. Owing to REEs unique structures (4f orbitals) and distinctive physical/chemical properties, they find application in high-tech industries, such as superconductor [3,4], photonic device [5], permanent magnet material [6,7], agriculture area [8], etc. Currently, solvent extraction with mixer settler or extraction column is being widely employed as a reliable separation method of rare earth ions due to its simplicity, kinetics, and applicability [9]. The organophosphorous acids, di (2-ethylhexyl) phosphoric acid (D2EHPA, P204) and 2-ethylhexyl phosphonic acid mono-2ethylhexyl (HEHEHP, P507) are being utilized as the popular extractants for separating lanthanide commercially [10]. However, the
conventional acidic extractants always needs to be saponified by NH3·H2O, NaOH, or Ca(OH)2 to facilitate the cation exchange of rare earths. This method results in a large amount of waste water containing NH4+, Ca2+, and Na+, which cause discharge of ammonium nitrogen pollutants having high levels of total dissolved solids (TDS) [11–13]. Additionally the conventional separation unit operations such as mixersettler, packed column, and centrifugal extractor, have many drawbacks, e.g., large equipment size, slow extraction rate, high energy consumption, and large solvent holdup, all leading to high separation cost [14]. In order to overcome the above problems, scientific community has been in search of alternative separation processes so as to improve the economics of commercial manufactures. It has been identified that one of the ways to improve the separation is to modify the aqueous phase by adding complexing agents [15]. In presence of the water-soluble complexing agent, part of the metals in aqueous phase combine with the
⁎ Corresponding author at: State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, Yunnan 650093, China. E-mail address: [email protected] (S. Yin).
https://doi.org/10.1016/j.cej.2018.09.039 Received 20 December 2017; Received in revised form 28 August 2018; Accepted 6 September 2018 Available online 07 September 2018 1385-8947/ © 2018 Elsevier B.V. All rights reserved.
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4.5 × 10−9 m3 s−1). The slug flows are generated at this site which flow within the reaction tube, monitored by a high-speed camera, connected to a computer for data storage. The rare earth ions concentration in the aqueous phase is determined by a Prodigy high dispersion inductively coupled plasma spectrometer (Leeman, America), and that in organic phase is measured by mass balance. Distribution ratio (D) is defined as the ratio of concentrations of metal ion in the organic and aqueous phases. The loaded organic phase containing REEs is used to analyze the molecular structure by spectrums.
complexing agent, thus hindering the extraction by masking effect. Within our group, the extraction and separation of rare earths were attempted utilizing complexing agent such as lactic acid or citric acid to avoid saponification of extractants, highlighting improved extraction capacity and separation selectivity [16–18]. Furthermore, the complexing agent is reserved in the raffinate after the extraction process, which could be extracted and recycled in a new extraction process, and this recycling is beneficial to reducing industrial pollution also caters the demand of “green chemistry”. Additionally the advantages of microstructured devices over macro scale reactors, are being highlighted due to high interface to volume ratio and short diffusion path, especially for microfluidic solvent extraction (μSX) of rare earth elements. Nishihama et al. [19] have reported micro solvent extraction system to separate lanthanides in Pr/Nd and Pr/Sm binary solutions, wherein it was stated that both lanthanides are extracted together first, and then the lighter lanthanide extracted in the organic phase alternatively while the heavier one in the aqueous phase attaining extraction equilibrium. Kubota et al. [20] have studied the extraction of rare earths with PC88A on a microreactor, and reported unhindered flow of two phases while keeping an aqueous-organic interface. Leblebici et al. [21] have reported successful recycling of rare earths from lamp phosphors using a milli-channel mixer. Chen et al. [22] have introduced gas into a simple one-step microfluidic device to enhance the extraction and enrichment of REEs with a low concentration (less than 100 ppm) from waste water at a high phase ratio (greater than 50:1). Hou et al. [23–25] have reported high efficiency of extraction of La, Ce and Pr with EHEHPA using membrane dispersion micro-extractor, with short residence time. Kolar et al. [26] have studied the microfluidic solvent extraction of rare earth elements from a mixed oxide concentrate using Cyanex 572 solvent, found good selectivity. Thus, compared with the batch solvent extraction, μSX presents the advantages of precise control of contact time of two phases, leading to high speed and high performance separation without mechanical stirring, mixing or shaking. However, the application of microfluidic reactors to liquid–liquid extraction of rare earths in the presence of complexing agent (lactic acid), as well as extraction mechanism in microchannel has not been reported. In this work, the extraction mechanism of rare earths from chloride solution in the presence of complexing agent lactic acid in microchannel, with P507/sulfonated kerosene has been assessed, and the extracted complex has been characterized using spectroscopic techniques including FT-IR, UV–Vis, NMR and MS spectroscopies, to understand the extraction performance of μSX.
D=
Cin−Cout Cout
(1)
where Cin and Cout are the aqueous phase concentrations at the inlet and outlet, respectively. 2.3. Characterization of extracted complexes FT-IR analysis: Fourier transform infrared (FT-IR) spectrum of the extracted complexes is recorded at room temperature in the range of 4000–400 cm−1 with an infrared spectrometer (Nicolet-740, USA) using KBr pellet technique, is used to understand the molecular structure. UV–Vis analysis: The extracted complexes are placed into quartz cell to measure the absorption peaks with the sulfonated kerosene as the reference solution in a UV–Vis spectrophotometer (Optizen 2120UV, Meacsys. Co., Ltd), and UV–Vis spectra are recorded within the wavelength range from 280 nm to 800 nm with a scan speed of 300 nm/min. NMR analysis: NMR experiments are performed in CDCl3 on a Bruker AV-600 spectrometer equipped with a TXI cryoprobe (Bruker, Fällanden, Switzerland). The chemical shifts of 1H NMR and 31P NMR spectra are referenced to the internal standard 10% tetramethylsilane (TMS) and external standard 85% phosphoric acid. MS analysis: Firstly, 0.5 mL sample is dissolved in acetonitrile, and uniformly mixed by ultrasonic oscillation. Secondly, the ionization mode such as electrospray ionization (ESI) is chosen. MALDI-HRMS are analyzed with an Axima Performance Maldi-TOF mass spectrometer (Shimadzu Biotech, Japan). 3. Results and discussions 3.1. Extraction process
2. Materials and methods
In principle, the ion exchange mechanism is responsible for the extraction of rare earths using P507 (abbreviated as H2A2) at lower acidic range; usually the transfer of a rare earth ion is accompanied by release of three hydrogen ions from the organic phase under these conditions, as shown in Eq. (2).
2.1. Materials The organic phase is prepared by dissolving 2-ethylhexyl phosphonic acid mono-(2-ethylhexyl) ester (HEH/EHP, P507; Luoyang Zhongda Chemical Industry Co., Ltd.) in sulfonated kerosene. The aqueous phase is prepared by dissolving rare earth oxide (99.95%, Guosheng Rare Earth Co., Ltd., Jiangsu) in hydrochloric acid and lactic acid solutions. All chemicals are of reagent grade received from Sinopharm Chemical Reagent Co. Ltd., China.
RE3(a)+ + 3H2 A2(o) ↔ REA3·3HA (o) + 3H+(a)
(2)
The hydrogen ions affect negatively the rare earth ions extraction due to the increase of acidity in aqueous phase. So it is important to control the pH to keep the equilibrium, and two different methods are commonly used for this propose. As for the first method, the pH in the aqueous phase is adjusted (e.g. by buffering or continuously adding NaOH or NH3·H2O) which proceeds based on the Eq. (2). In the second method, using saponification procedure by neutralizing acidic extractant with aqueous ammonia, the acidic extractant can be converted to ammonium salt (Eq. 3), accordingly, the hydrogen bonds in dimers are broken. Although saponification can enhance the extraction capacities of acidic extractants, the release of ammonium ion to aqueous phase causes serious pollution (Eq. 4).
2.2. Microfluidic setup and extraction The schematic of microfluidic extraction of rare earth ions is shown in Fig. 1. The microchannel has an extraction length (L) of 12.5 cm, a width (W) of 300 μm and a depth (D) of 100 μm with the rectangular section (made of polydimethylsiloxane (PDMS) by pouring and composed of three stages: inlet, channel and outlet plates). The aqueous and organic phases are introduced using two syringes connected to the microfluidic device with PEEK capillary tubing on a double syringe pump (LSP02-1B, Baoding Longer Precision Pump Co., Ltd.), fed into a Y-junction microreactor at equal flow rate (5.6 × 10−10 to 454
NH3·H2 O(a) + H2 A2(o) ↔ NH 4 ·HA2(o) + H2 O(a)
(3)
RE3(a)+
(4)
+ 3NH 4 ·HA2(o) ↔ RE(HA2)3(o) +
3NH+4(a)
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Fig. 1. Schematic diagram of the experimental setup.
In order to overcome the environmental pollution, the present work proposes a complex extraction method, that is, lactic acid (HLac) and hydrochloric acid to dissolve rare earth oxides (Eq. 5). During extraction, rare earth complexes dissociate at the interface due to the larger stability constant of the complex between RE3+ and extractant compared with RE3+ and ligand; at the same time, the released hydrogen ions combine with Lac− to form HLac which buffers the effect of higher acidy change on the extraction [27,28].
D=
[RE3 +](o) [RELac n H(m+ n− 3) (HA2)m ](o) = 3 [RE3 +]t [RE3 +] + ∑i [RE(Lac)3i − i ]
(11)
Taking Eq. (7) into Eq. (11), then
D=
K [H2 A2(o) ]m [Lac (a) ]n Y ·[H+]3 − n
(12)
Taking logarithms of Eq. (12),
log D + log Y = log K + m log[H2 A2](o) + n log[Lac−]−(3−n) log[H+]
3RE2 O3 + 12H+ + 6HLac = 2RE3(a)+ + 2RE(Lac)+2 + 2RE(Lac)2 + + 9H2 O
(13)
(5)
3+
The stoichiometry of complex formation reaction between RE and extractant H2A2 is assessed by studying the dependence of logD + logY with respect to pHeq, log [H2A2] and log [Lac] at constant extraction concentration (Fig. 2). As shown in Fig. 2a, the straight lines with slope of about 3.0 for rare earths, suggests that rare earths extraction releases three hydrogen ions. Fig. 2b shows a linear relationship with a slope around 3.0, suggests that one rare earth ion is extracted with three extractant molecules. At the same time, it is confirmed that lactic acid does not take part in the extraction reaction (Fig. 2c) as the slope less than 0.5, also indicates that lactic acid is not extracted into the organic phase. Based on the slope analysis, rare earth ions extraction using complexing method with P507 can also be expressed as Eq. (2).
3.2. Stoichiometry of the complex formation reaction It is assumed that Lac− is involved in the extraction process, and the overall extraction reaction is expressed by Eq. (6). K
RE3(a+) + mH2 A 2(o) + nLac−(a)⟷RELacn H (m + n − 3) (HA2)m (o) + (3−n)H+(a) (6) The equilibrium constant (K) for the above reaction is given by
[RELacn H(m + n − 3) (HA2)m (o) ][H+]3 − n
K=
[RE3(a+) ][H2 A 2(o) ]m [Lac−(a)]n
Lac
RE3 +
−
+
3+
can complex with RE
iLac−
β
→
RE(Lac)i3 − i
(7)
in the aqueous phase, namely:
0⩽i⩽3
3.3. UV–Vis analysis (8)
A comprehensive UV–Vis spectral analysis was performed to confirm coordination between RE3+ and extractant after extraction. The organic phase and extracted complex UV spectra are shown in Fig. 3 at the absorbance scale of 200–800 nm. As can be seen, the peaks are similar, indicating that the energy of the extracted complex is mainly from the ligands and the ligands structure have not changed when rare earth ions are extracted by P507. In addition, one characteristic absorption band for the extractant P507 is visible in the UV–Vis spectrum at 308 nm (corresponds to the transfer of π-π* electrons in the P]O). After extraction, the position of the band shifted to high wavelength namely “red shift”, meaning that extractant P507 has stronger coordination ability to rare earth ions and more stable extraction complex are formed. Especially, after coordination between P]O and rare earth ions, the electron cloud is shifted to the empty orbital of the rare earth
where β is the cumulative stability constant. Then the total concentration of RE3+ in the aqueous solution is given by 3
[RE3 +]t = [RE3 +] +
∑ [RE(Lac)i3−i] i
(9)
Introducing the definition of complexing degree (Y) and distribution ratio (D), listed as follows:
Y=
[RE3 +]t [RE3 +]
=
[RE3 +] + ∑3i [RE[Lac]3i − i ] [RE3 +] 3
Y = 1 + ∑i βi [Lac−]i
(10) 455
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Fig. 3. UV–Vis spectra of the extractant P507 and RE-P507 extracted compounds. Extraction conditions: [RECl3] = 0.1 mol/L, [P507] = 1.0 mol/L, [HLac] = 0.6 mol/L, pH = 2.5.
3.4. FT-IR spectra analysis In order to further confirm the composition of extracted complexes, infrared spectrum measurement was conducted based on the unsaturated extraction (Fig. 4). By comparing the IR spectra of the rare earth complex with the parent compound P507, some obvious differences are seen: (1) the broad absorption bands (2301–2350 cm−1, and 1666–1697 cm−1) corresponding to the P(O)OH vibrations characteristic in the parent P507 still existed in the complex after extraction [29]; (2) the bands at 1164 cm−1 and 985 cm−1 are assigned to the P]O and PeO vibrations of P507, after extraction, the peak of P]O disappeared whilst the peak of PeO shifted to low wave number, with the formation of P-O-RE groups; (3) in the complex (Fig. 4b), the asymmetric absorption (1192 cm−1) and the symmetric absorption (1160–1174 cm−1) of
occur [30]. The above analysis indicates
that the ligand and rare earth ions are bonded through P]O and PeO− to form complex. 3.5. 1H NMR and
31
P NMR analysis
In order to confirm the conclusion from the FT-IR analysis, 1H NMR and 31P NMR were conducted. Firstly, 1H NMR was used to identify types of hydrogen atoms present in a molecule [31]. The fact of analysis lies in the integrated area under a peak in a particular range of chemical shift (δ) is directly proportional to the relative abundance of a particular type of H attached to a functional group [32]. Areas under the peaks are fitted with the deconvolution software DMfit providing a relative contribution (%) of each functional group designated by the chemical shift regions. Fig. 5 shows the 1H NMR spectra of the extractant P507 and P507-Nd extracted compounds. Spectra of the extractant P507 are divided into 6 regions: 0.713–0.851 (m, 12H, CH3), 1.202–1.392 (m, 16H, CH2), 1.536–1.549 (m, 2H, CH), 1.677–1.739 (m, 2H, CH2P), 3.876–3.920 (m, 2H, CH2O), 11.428 (s, 1H, PeOeH), where the signal of 7.28 ppm is considered to be the signal pointing to solvent CDCl3. In the Nd-P507 extracted compound, the signal at 11.428 ppm disappears, suggesting that the hydrogen in PeOeH is replaced by rare earth ion. 31 P NMR measurements were obtained on the same NMR spectrometer used for the chemical shift determinations. All the 31P chemical shifts reported are relative to 85% phosphoric acid (δ = 0 ppm). Fig. 6 shows the 31P NMR spectra of P507 before and after Nd extraction. As shown in Fig. 6a, the 31P NMR spectra are characterized by a strong,
Fig. 2. Plots of logD + logY versus pHeq (a), log [H2A2] (b), and log [Lac−] (c). Extraction conditions: [RECl3] = 0.003 mol/L.
ions, making the conjugated system improved. Thus, π electron cloud of the centered ion is enhanced, and the degree of delocalization of conjugated π electron of the ligand is also improved, resulting in a decrease in the energy level difference of π-π* electrons, and the “red shift” occurs. Based on the analysis of UV–Vis spectra, one can state that the extractant P507 can coordinate with rare earth ions after extraction.
456
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Fig. 5. 1H NMR of the extractant P507 and RE-P507 extracted compounds.
Fig. 4. FT-IR spectra of the extractant P507 and RE-P507 extracted compound: (1) P507-kerosene; (2) La-P507 extracted compound; (3) Ce-P507 extracted compound; (4) Pr-P507 extracted compound; (5) Nd-P507 extracted compound. [P507] = 1.0 mol/L, Extraction conditions: [RECl3] = 0.1 mol/L, [HLac] = 0.6 mol/L, pH = 2.5.
narrow peak at about +34 ppm and −2 ppm. Fig. 6b is an expansion of the spectrum shown in Fig. 7a. The 31P of P507 is located at −1.814 ppm, while it shifts to −2.982 ppm in the Nd-P507 extracted compound, indicating that the coordination between Nd and P507 is through P]O [33]. The 31P peak of P507 after extraction of rare earth ions is shifted to higher magnetic field, most probably due to increased positive shielding from π-electrons of P]O. 3.6. MALDI-MS analysis MALDI-HRMS was used to determine the various compounds in the extracted complex. The positive mass spectra of the P507 and the extracted-Nd compound dissolved in methylene chloride were recorded. In general, the P507 belongs to unstable ester compound, so it is easy to decompose into some smaller molecules on electron impact. MS analysis in the positive ion mode for the P507 extractant is shown in Fig. 7, MS-ESI m/z: calculated for ([P507]+1) m/z = 307.4, is found close to 307.2 corresponding to a monomer P507; calculated for (2[P507]+1) m/z = 613.8, is found close to 613.4 corresponding to a dimer P507; calculated for (3[P507]+1) m/z = 920.2, is found close to 919.7 corresponding to a trimer P507. In addition, some other fragment ions also
Fig. 6.
457
31
P NMR of the extractant P507 and RE-P507 extracted compounds.
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Fig. 7. MS analysis of the extractant P507.
the center of the extracted complex. The possible structure of the extracted complex is shown in Fig. 9.
emerged in this spectrogram. The extracted compound was analyzed by the recorded mass spectra of P507-Nd extracted complex, and shown in Fig. 8. As can be seen, the abundance of the molecular ions is less than that of the fragmented ions, suggesting that the extracted compound is not stable and easy to decompose into fragments. As we know, in the extraction reaction the transfer of a rare earth ion is accompanied by release of three hydrogen ions from the organic phase, at the same time 3 molecules of P507 dimer are involved in the extraction step. Analyses for the P507-Nd complex, calculated for (6[P507] + 144−3 + 1) m/z = 1980.4, is found close to 1980.3 corresponding to a P507–Nd complex. In addition, there are many ion peaks of isotope and many different qualities of fragment ions. Based on the above spectra analysis, the extracted complexes is formed by the cation exchange process between PeOeH and Nd3+, and coordination process between P]O and Nd3+, with Nd3+ located in
4. Conclusions The interaction of rare earth elements with P507 in a microfluidic extraction system has been studied by spectroscopic (FT-IR, UV–Vis, NMR and MS) analysis. The saponification of P507 is avoided by adding complexing agent lactic acid to eliminate environmental pollution, wherein lactic acid plays the role of dissolving rare earth oxides and buffering acidity. The stoichiometry of the complex formation reaction is estimated based on the slope analysis of logD + logY versus pHeq and logD + logY versus log [H2A2], which confirmed that lactic acid does not take part in the extraction reaction. A comprehensive analysis of the molecular interactions between P507 and rare earth ions has been based on the FT-IR, UV–Vis, NMR and MS spectrums, which confirmed
Fig. 8. MS analysis of the P507-Nd extracted compound. 458
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Fig. 9. Structure of extracted complex.
the extracted complexes of rare earth ions located in the center formed by the cation exchange between PeOeH and RE3+, and coordination between P]O and RE3+.
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solvent, Inorg. Chim. Acta 130 (1987) 119–123. [31] S. Decesari, M.C. Facchini, S. Fuzzi, E. Tagliavini, Characterization of water-soluble organic compounds in atmospheric aerosol: a new approach, J. Geophys. Res. 105 (2000) 1481–1489. [32] V. Kumar, A. Goel, P. Rajput, Compositional and surface characterization of HULIS by UV-Vis, FTIR, NMR and XPS: wintertime study in Northern India, Atmos. Environ. 164 (2017) 1–8. [33] J.C. Liang, R.C. Tian, C.Y. Rong, Study on the extraction mechanism of extracting MPA from H2SO4 solution with TBP, Chin. J. Rare Met. 2 (1984) 32–35 (in Chin.).
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Characterization of the interaction of rare earth elements with P507 in a microfluidic extraction system using spectroscopic analysis
T
⁎
Kaihua Chena,b, Yuan Hea,b, C. Srinivasakannanc, Shiwei Lia,b, Shaohua Yina,b, , Jinhui Penga,b, Shenghui Guoa,b, Libo Zhanga,b a
State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, Yunnan 650093, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan 650093, China c Chemical Engineering Department, The Petroleum Institute, Khalifa University of Science and Technology, Abu Dhabi, P.O. Box 253, United Arab Emirates b
H I GH L IG H T S
role of lactic acid in the extraction process is analyzed. • The characterization of extracted compound is studied by spectroscopic analysis. • The • The structure of extracted complexes is confirmed.
A R T I C LE I N FO
A B S T R A C T
Keywords: Rare earths P507 Spectrum Complex extraction Microreactor
An attempt to characterize the interaction of rare earth elements with P507 in a microfluidic extraction system containing lactic acid as complexing agent is made using spectroscopic (FT-IR, UV–Vis, NMR and MS) analysis. The role of lactic acid in the extraction process is analyzed. The extraction mechanism confirmed that lactic acid does not involve in the extraction reaction. A comparative analysis of the composition of the extracts using FTIR, UV–Vis, NMR and MS methods, indicates that the extracted complexes of rare earth ions located in the center is formed by the cation exchange process between PeOeH and RE3+, and coordination process between P]O and RE3+.
1. Introduction Rare earth elements (REEs) are composed of lanthanide series elements in the periodic table, i.e. from lanthanum (La) to lutetium (Lu), together with yttrium (Y) and scandium (Sc) which possess unique physical and chemical characteristics [1,2]. Owing to REEs unique structures (4f orbitals) and distinctive physical/chemical properties, they find application in high-tech industries, such as superconductor [3,4], photonic device [5], permanent magnet material [6,7], agriculture area [8], etc. Currently, solvent extraction with mixer settler or extraction column is being widely employed as a reliable separation method of rare earth ions due to its simplicity, kinetics, and applicability [9]. The organophosphorous acids, di (2-ethylhexyl) phosphoric acid (D2EHPA, P204) and 2-ethylhexyl phosphonic acid mono-2ethylhexyl (HEHEHP, P507) are being utilized as the popular extractants for separating lanthanide commercially [10]. However, the
conventional acidic extractants always needs to be saponified by NH3·H2O, NaOH, or Ca(OH)2 to facilitate the cation exchange of rare earths. This method results in a large amount of waste water containing NH4+, Ca2+, and Na+, which cause discharge of ammonium nitrogen pollutants having high levels of total dissolved solids (TDS) [11–13]. Additionally the conventional separation unit operations such as mixersettler, packed column, and centrifugal extractor, have many drawbacks, e.g., large equipment size, slow extraction rate, high energy consumption, and large solvent holdup, all leading to high separation cost [14]. In order to overcome the above problems, scientific community has been in search of alternative separation processes so as to improve the economics of commercial manufactures. It has been identified that one of the ways to improve the separation is to modify the aqueous phase by adding complexing agents [15]. In presence of the water-soluble complexing agent, part of the metals in aqueous phase combine with the
⁎ Corresponding author at: State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, Yunnan 650093, China. E-mail address: [email protected] (S. Yin).
https://doi.org/10.1016/j.cej.2018.09.039 Received 20 December 2017; Received in revised form 28 August 2018; Accepted 6 September 2018 Available online 07 September 2018 1385-8947/ © 2018 Elsevier B.V. All rights reserved.
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4.5 × 10−9 m3 s−1). The slug flows are generated at this site which flow within the reaction tube, monitored by a high-speed camera, connected to a computer for data storage. The rare earth ions concentration in the aqueous phase is determined by a Prodigy high dispersion inductively coupled plasma spectrometer (Leeman, America), and that in organic phase is measured by mass balance. Distribution ratio (D) is defined as the ratio of concentrations of metal ion in the organic and aqueous phases. The loaded organic phase containing REEs is used to analyze the molecular structure by spectrums.
complexing agent, thus hindering the extraction by masking effect. Within our group, the extraction and separation of rare earths were attempted utilizing complexing agent such as lactic acid or citric acid to avoid saponification of extractants, highlighting improved extraction capacity and separation selectivity [16–18]. Furthermore, the complexing agent is reserved in the raffinate after the extraction process, which could be extracted and recycled in a new extraction process, and this recycling is beneficial to reducing industrial pollution also caters the demand of “green chemistry”. Additionally the advantages of microstructured devices over macro scale reactors, are being highlighted due to high interface to volume ratio and short diffusion path, especially for microfluidic solvent extraction (μSX) of rare earth elements. Nishihama et al. [19] have reported micro solvent extraction system to separate lanthanides in Pr/Nd and Pr/Sm binary solutions, wherein it was stated that both lanthanides are extracted together first, and then the lighter lanthanide extracted in the organic phase alternatively while the heavier one in the aqueous phase attaining extraction equilibrium. Kubota et al. [20] have studied the extraction of rare earths with PC88A on a microreactor, and reported unhindered flow of two phases while keeping an aqueous-organic interface. Leblebici et al. [21] have reported successful recycling of rare earths from lamp phosphors using a milli-channel mixer. Chen et al. [22] have introduced gas into a simple one-step microfluidic device to enhance the extraction and enrichment of REEs with a low concentration (less than 100 ppm) from waste water at a high phase ratio (greater than 50:1). Hou et al. [23–25] have reported high efficiency of extraction of La, Ce and Pr with EHEHPA using membrane dispersion micro-extractor, with short residence time. Kolar et al. [26] have studied the microfluidic solvent extraction of rare earth elements from a mixed oxide concentrate using Cyanex 572 solvent, found good selectivity. Thus, compared with the batch solvent extraction, μSX presents the advantages of precise control of contact time of two phases, leading to high speed and high performance separation without mechanical stirring, mixing or shaking. However, the application of microfluidic reactors to liquid–liquid extraction of rare earths in the presence of complexing agent (lactic acid), as well as extraction mechanism in microchannel has not been reported. In this work, the extraction mechanism of rare earths from chloride solution in the presence of complexing agent lactic acid in microchannel, with P507/sulfonated kerosene has been assessed, and the extracted complex has been characterized using spectroscopic techniques including FT-IR, UV–Vis, NMR and MS spectroscopies, to understand the extraction performance of μSX.
D=
Cin−Cout Cout
(1)
where Cin and Cout are the aqueous phase concentrations at the inlet and outlet, respectively. 2.3. Characterization of extracted complexes FT-IR analysis: Fourier transform infrared (FT-IR) spectrum of the extracted complexes is recorded at room temperature in the range of 4000–400 cm−1 with an infrared spectrometer (Nicolet-740, USA) using KBr pellet technique, is used to understand the molecular structure. UV–Vis analysis: The extracted complexes are placed into quartz cell to measure the absorption peaks with the sulfonated kerosene as the reference solution in a UV–Vis spectrophotometer (Optizen 2120UV, Meacsys. Co., Ltd), and UV–Vis spectra are recorded within the wavelength range from 280 nm to 800 nm with a scan speed of 300 nm/min. NMR analysis: NMR experiments are performed in CDCl3 on a Bruker AV-600 spectrometer equipped with a TXI cryoprobe (Bruker, Fällanden, Switzerland). The chemical shifts of 1H NMR and 31P NMR spectra are referenced to the internal standard 10% tetramethylsilane (TMS) and external standard 85% phosphoric acid. MS analysis: Firstly, 0.5 mL sample is dissolved in acetonitrile, and uniformly mixed by ultrasonic oscillation. Secondly, the ionization mode such as electrospray ionization (ESI) is chosen. MALDI-HRMS are analyzed with an Axima Performance Maldi-TOF mass spectrometer (Shimadzu Biotech, Japan). 3. Results and discussions 3.1. Extraction process
2. Materials and methods
In principle, the ion exchange mechanism is responsible for the extraction of rare earths using P507 (abbreviated as H2A2) at lower acidic range; usually the transfer of a rare earth ion is accompanied by release of three hydrogen ions from the organic phase under these conditions, as shown in Eq. (2).
2.1. Materials The organic phase is prepared by dissolving 2-ethylhexyl phosphonic acid mono-(2-ethylhexyl) ester (HEH/EHP, P507; Luoyang Zhongda Chemical Industry Co., Ltd.) in sulfonated kerosene. The aqueous phase is prepared by dissolving rare earth oxide (99.95%, Guosheng Rare Earth Co., Ltd., Jiangsu) in hydrochloric acid and lactic acid solutions. All chemicals are of reagent grade received from Sinopharm Chemical Reagent Co. Ltd., China.
RE3(a)+ + 3H2 A2(o) ↔ REA3·3HA (o) + 3H+(a)
(2)
The hydrogen ions affect negatively the rare earth ions extraction due to the increase of acidity in aqueous phase. So it is important to control the pH to keep the equilibrium, and two different methods are commonly used for this propose. As for the first method, the pH in the aqueous phase is adjusted (e.g. by buffering or continuously adding NaOH or NH3·H2O) which proceeds based on the Eq. (2). In the second method, using saponification procedure by neutralizing acidic extractant with aqueous ammonia, the acidic extractant can be converted to ammonium salt (Eq. 3), accordingly, the hydrogen bonds in dimers are broken. Although saponification can enhance the extraction capacities of acidic extractants, the release of ammonium ion to aqueous phase causes serious pollution (Eq. 4).
2.2. Microfluidic setup and extraction The schematic of microfluidic extraction of rare earth ions is shown in Fig. 1. The microchannel has an extraction length (L) of 12.5 cm, a width (W) of 300 μm and a depth (D) of 100 μm with the rectangular section (made of polydimethylsiloxane (PDMS) by pouring and composed of three stages: inlet, channel and outlet plates). The aqueous and organic phases are introduced using two syringes connected to the microfluidic device with PEEK capillary tubing on a double syringe pump (LSP02-1B, Baoding Longer Precision Pump Co., Ltd.), fed into a Y-junction microreactor at equal flow rate (5.6 × 10−10 to 454
NH3·H2 O(a) + H2 A2(o) ↔ NH 4 ·HA2(o) + H2 O(a)
(3)
RE3(a)+
(4)
+ 3NH 4 ·HA2(o) ↔ RE(HA2)3(o) +
3NH+4(a)
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Fig. 1. Schematic diagram of the experimental setup.
In order to overcome the environmental pollution, the present work proposes a complex extraction method, that is, lactic acid (HLac) and hydrochloric acid to dissolve rare earth oxides (Eq. 5). During extraction, rare earth complexes dissociate at the interface due to the larger stability constant of the complex between RE3+ and extractant compared with RE3+ and ligand; at the same time, the released hydrogen ions combine with Lac− to form HLac which buffers the effect of higher acidy change on the extraction [27,28].
D=
[RE3 +](o) [RELac n H(m+ n− 3) (HA2)m ](o) = 3 [RE3 +]t [RE3 +] + ∑i [RE(Lac)3i − i ]
(11)
Taking Eq. (7) into Eq. (11), then
D=
K [H2 A2(o) ]m [Lac (a) ]n Y ·[H+]3 − n
(12)
Taking logarithms of Eq. (12),
log D + log Y = log K + m log[H2 A2](o) + n log[Lac−]−(3−n) log[H+]
3RE2 O3 + 12H+ + 6HLac = 2RE3(a)+ + 2RE(Lac)+2 + 2RE(Lac)2 + + 9H2 O
(13)
(5)
3+
The stoichiometry of complex formation reaction between RE and extractant H2A2 is assessed by studying the dependence of logD + logY with respect to pHeq, log [H2A2] and log [Lac] at constant extraction concentration (Fig. 2). As shown in Fig. 2a, the straight lines with slope of about 3.0 for rare earths, suggests that rare earths extraction releases three hydrogen ions. Fig. 2b shows a linear relationship with a slope around 3.0, suggests that one rare earth ion is extracted with three extractant molecules. At the same time, it is confirmed that lactic acid does not take part in the extraction reaction (Fig. 2c) as the slope less than 0.5, also indicates that lactic acid is not extracted into the organic phase. Based on the slope analysis, rare earth ions extraction using complexing method with P507 can also be expressed as Eq. (2).
3.2. Stoichiometry of the complex formation reaction It is assumed that Lac− is involved in the extraction process, and the overall extraction reaction is expressed by Eq. (6). K
RE3(a+) + mH2 A 2(o) + nLac−(a)⟷RELacn H (m + n − 3) (HA2)m (o) + (3−n)H+(a) (6) The equilibrium constant (K) for the above reaction is given by
[RELacn H(m + n − 3) (HA2)m (o) ][H+]3 − n
K=
[RE3(a+) ][H2 A 2(o) ]m [Lac−(a)]n
Lac
RE3 +
−
+
3+
can complex with RE
iLac−
β
→
RE(Lac)i3 − i
(7)
in the aqueous phase, namely:
0⩽i⩽3
3.3. UV–Vis analysis (8)
A comprehensive UV–Vis spectral analysis was performed to confirm coordination between RE3+ and extractant after extraction. The organic phase and extracted complex UV spectra are shown in Fig. 3 at the absorbance scale of 200–800 nm. As can be seen, the peaks are similar, indicating that the energy of the extracted complex is mainly from the ligands and the ligands structure have not changed when rare earth ions are extracted by P507. In addition, one characteristic absorption band for the extractant P507 is visible in the UV–Vis spectrum at 308 nm (corresponds to the transfer of π-π* electrons in the P]O). After extraction, the position of the band shifted to high wavelength namely “red shift”, meaning that extractant P507 has stronger coordination ability to rare earth ions and more stable extraction complex are formed. Especially, after coordination between P]O and rare earth ions, the electron cloud is shifted to the empty orbital of the rare earth
where β is the cumulative stability constant. Then the total concentration of RE3+ in the aqueous solution is given by 3
[RE3 +]t = [RE3 +] +
∑ [RE(Lac)i3−i] i
(9)
Introducing the definition of complexing degree (Y) and distribution ratio (D), listed as follows:
Y=
[RE3 +]t [RE3 +]
=
[RE3 +] + ∑3i [RE[Lac]3i − i ] [RE3 +] 3
Y = 1 + ∑i βi [Lac−]i
(10) 455
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Fig. 3. UV–Vis spectra of the extractant P507 and RE-P507 extracted compounds. Extraction conditions: [RECl3] = 0.1 mol/L, [P507] = 1.0 mol/L, [HLac] = 0.6 mol/L, pH = 2.5.
3.4. FT-IR spectra analysis In order to further confirm the composition of extracted complexes, infrared spectrum measurement was conducted based on the unsaturated extraction (Fig. 4). By comparing the IR spectra of the rare earth complex with the parent compound P507, some obvious differences are seen: (1) the broad absorption bands (2301–2350 cm−1, and 1666–1697 cm−1) corresponding to the P(O)OH vibrations characteristic in the parent P507 still existed in the complex after extraction [29]; (2) the bands at 1164 cm−1 and 985 cm−1 are assigned to the P]O and PeO vibrations of P507, after extraction, the peak of P]O disappeared whilst the peak of PeO shifted to low wave number, with the formation of P-O-RE groups; (3) in the complex (Fig. 4b), the asymmetric absorption (1192 cm−1) and the symmetric absorption (1160–1174 cm−1) of
occur [30]. The above analysis indicates
that the ligand and rare earth ions are bonded through P]O and PeO− to form complex. 3.5. 1H NMR and
31
P NMR analysis
In order to confirm the conclusion from the FT-IR analysis, 1H NMR and 31P NMR were conducted. Firstly, 1H NMR was used to identify types of hydrogen atoms present in a molecule [31]. The fact of analysis lies in the integrated area under a peak in a particular range of chemical shift (δ) is directly proportional to the relative abundance of a particular type of H attached to a functional group [32]. Areas under the peaks are fitted with the deconvolution software DMfit providing a relative contribution (%) of each functional group designated by the chemical shift regions. Fig. 5 shows the 1H NMR spectra of the extractant P507 and P507-Nd extracted compounds. Spectra of the extractant P507 are divided into 6 regions: 0.713–0.851 (m, 12H, CH3), 1.202–1.392 (m, 16H, CH2), 1.536–1.549 (m, 2H, CH), 1.677–1.739 (m, 2H, CH2P), 3.876–3.920 (m, 2H, CH2O), 11.428 (s, 1H, PeOeH), where the signal of 7.28 ppm is considered to be the signal pointing to solvent CDCl3. In the Nd-P507 extracted compound, the signal at 11.428 ppm disappears, suggesting that the hydrogen in PeOeH is replaced by rare earth ion. 31 P NMR measurements were obtained on the same NMR spectrometer used for the chemical shift determinations. All the 31P chemical shifts reported are relative to 85% phosphoric acid (δ = 0 ppm). Fig. 6 shows the 31P NMR spectra of P507 before and after Nd extraction. As shown in Fig. 6a, the 31P NMR spectra are characterized by a strong,
Fig. 2. Plots of logD + logY versus pHeq (a), log [H2A2] (b), and log [Lac−] (c). Extraction conditions: [RECl3] = 0.003 mol/L.
ions, making the conjugated system improved. Thus, π electron cloud of the centered ion is enhanced, and the degree of delocalization of conjugated π electron of the ligand is also improved, resulting in a decrease in the energy level difference of π-π* electrons, and the “red shift” occurs. Based on the analysis of UV–Vis spectra, one can state that the extractant P507 can coordinate with rare earth ions after extraction.
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Fig. 5. 1H NMR of the extractant P507 and RE-P507 extracted compounds.
Fig. 4. FT-IR spectra of the extractant P507 and RE-P507 extracted compound: (1) P507-kerosene; (2) La-P507 extracted compound; (3) Ce-P507 extracted compound; (4) Pr-P507 extracted compound; (5) Nd-P507 extracted compound. [P507] = 1.0 mol/L, Extraction conditions: [RECl3] = 0.1 mol/L, [HLac] = 0.6 mol/L, pH = 2.5.
narrow peak at about +34 ppm and −2 ppm. Fig. 6b is an expansion of the spectrum shown in Fig. 7a. The 31P of P507 is located at −1.814 ppm, while it shifts to −2.982 ppm in the Nd-P507 extracted compound, indicating that the coordination between Nd and P507 is through P]O [33]. The 31P peak of P507 after extraction of rare earth ions is shifted to higher magnetic field, most probably due to increased positive shielding from π-electrons of P]O. 3.6. MALDI-MS analysis MALDI-HRMS was used to determine the various compounds in the extracted complex. The positive mass spectra of the P507 and the extracted-Nd compound dissolved in methylene chloride were recorded. In general, the P507 belongs to unstable ester compound, so it is easy to decompose into some smaller molecules on electron impact. MS analysis in the positive ion mode for the P507 extractant is shown in Fig. 7, MS-ESI m/z: calculated for ([P507]+1) m/z = 307.4, is found close to 307.2 corresponding to a monomer P507; calculated for (2[P507]+1) m/z = 613.8, is found close to 613.4 corresponding to a dimer P507; calculated for (3[P507]+1) m/z = 920.2, is found close to 919.7 corresponding to a trimer P507. In addition, some other fragment ions also
Fig. 6.
457
31
P NMR of the extractant P507 and RE-P507 extracted compounds.
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Fig. 7. MS analysis of the extractant P507.
the center of the extracted complex. The possible structure of the extracted complex is shown in Fig. 9.
emerged in this spectrogram. The extracted compound was analyzed by the recorded mass spectra of P507-Nd extracted complex, and shown in Fig. 8. As can be seen, the abundance of the molecular ions is less than that of the fragmented ions, suggesting that the extracted compound is not stable and easy to decompose into fragments. As we know, in the extraction reaction the transfer of a rare earth ion is accompanied by release of three hydrogen ions from the organic phase, at the same time 3 molecules of P507 dimer are involved in the extraction step. Analyses for the P507-Nd complex, calculated for (6[P507] + 144−3 + 1) m/z = 1980.4, is found close to 1980.3 corresponding to a P507–Nd complex. In addition, there are many ion peaks of isotope and many different qualities of fragment ions. Based on the above spectra analysis, the extracted complexes is formed by the cation exchange process between PeOeH and Nd3+, and coordination process between P]O and Nd3+, with Nd3+ located in
4. Conclusions The interaction of rare earth elements with P507 in a microfluidic extraction system has been studied by spectroscopic (FT-IR, UV–Vis, NMR and MS) analysis. The saponification of P507 is avoided by adding complexing agent lactic acid to eliminate environmental pollution, wherein lactic acid plays the role of dissolving rare earth oxides and buffering acidity. The stoichiometry of the complex formation reaction is estimated based on the slope analysis of logD + logY versus pHeq and logD + logY versus log [H2A2], which confirmed that lactic acid does not take part in the extraction reaction. A comprehensive analysis of the molecular interactions between P507 and rare earth ions has been based on the FT-IR, UV–Vis, NMR and MS spectrums, which confirmed
Fig. 8. MS analysis of the P507-Nd extracted compound. 458
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Fig. 9. Structure of extracted complex.
the extracted complexes of rare earth ions located in the center formed by the cation exchange between PeOeH and RE3+, and coordination between P]O and RE3+.
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