The process and mechanism for cesium and rubidium extraction with saponified 4-tert-butyl-2-(α-methylbenzyl) phenol

Dengke Pang ,Zhihong Zhang ,Yongquan Zhou,*,Zhenhai Fu ,Quan Li ,Yongming Zhang ,Guangguo Wang,Zhuanfang Jing

1 Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources,Qinghai Institute of Salt Lakes,Chinese Academy of Sciences,Xining 810008,China

2 University of Chinese Academy of Sciences,Beijing 100049,China

Keywords:t-BAMBP Optimization Separation Solvent extraction Cesium Rubidium

ABSTRACT Cesium (Cs) and rubidium (Rb) separation from brine is an important and application-oriented topic.4-tert-butyl-2-(α-methylbenzyl) phenol (t-BAMBP) has been used for Cs and Rb extraction.However,the traditional extraction technology is base and acid consumed.In the present work,an innovative process for Cs and Rb extraction with t-BAMBP is developed,which consists of saponification,extraction,scrubbing and stripping.Both infrared spectrum and electrostatic potential analysis indicate the hydrogen of phenolic hydroxyl is dissociated from t-BAMBP during saponification and the oxygen of phenolic hydroxyl is the binding site for alkali metal ions.Saponified organic phase shows an excellent extraction effect for Cs+and Rb+.The extraction reaches equilibrium in 5 min,with 99.5% Cs+and 46.7% Rb+are loaded into the organic phase in the single-stage extraction.Slope method indicates the structure of the extraction complex is MOR?3ROH (M=Cs+,Rb+,K+),where the electrostatic attraction between M+ and the oxygen of phenolic hydroxyl is dominant,and the cation–π interaction has a significant effect also.The extraction complex of MOR?3ROH dissociates in the acid environment while scrubbing and stripping is completed.The Cs+ and Rb+ are separated from the mixture phase,the proton H bonds to the phenolic hydroxyl group,and the extractant is regenerated.

1.Introduction

Cesium(Cs)and rubidium(Rb)are widely used for new materials,new energy,medicine,aeronautics,fiber optic telecommunication systems and precision instruments,etc.[1–8].The Cs and Rb enormously reserve in the salt lake brine.Exploiting of Cs+and Rb+from the salt lake brine has a promising application prospect.Cs+and Rb+extraction from nuclear waste and salt lake brine has been widely studied [4–8].However,considering the effects of high concentrations of K+(has highly similar physical and chemical property to Rb+and Cs+) and other complex compositions (Na+,Ca2+,and Mg2+etc.) in the salt lake brines [5,9].It’s a great challenge to effectively separate and extract trace Cs+and Rb+from salt lake brine.

Substituted phenols such as the 4-sec-butyl-2-(αmethylbenzyl) phenol (s-BAMBP) and 4-tert-butyl-2-(α-methylbenzyl) phenol (t-BAMBP) [10–12] has been used in trace amount of Cs+and Rb+extraction.Horneret al.[13,14] found thes-BAMBP(Fig.1(a))performs well for Cs+and Rb+extraction.t-BAMBP(Fig.1(b)) is an isomers ofs-BAMBP that performs similarly ass-BAMBP in terms of Cs+and Rb+extraction and has a higher synthesis yields[5,11].Although a high extraction rate of Cs+and Rb+can be achieved,the interferences of the coexist K+has not been effectively solved [15,16].The previous studies show that Cs+and Rb+could be efficiently separated from K+solution employing multistage continuous counter current extracting and multistage stripping [17].The extraction selectivity oft-BAMBP to the alkalis metal ions is Cs+>Rb+>K+>Na+>Li+in the strong alkaline environment [3].Eganet al.[11] suggested the extraction mechanism could be attributed to the cations exchange between Cs+or Rb+and the hydrogen of the phenolic hydroxyl in thet-BAMBP.Rydberget al.[18] and Dyrssenet al.[19] obtain the equilibrium constant of the extraction reaction.They found the dimer dominantly in the organic phase while alkali metal ions are loaded intot-BAMBP [20,21].However,the structure of the extraction complex and it’s extraction mechanism at molecular level are unclear.

Solvent extraction Cs and Rb from nuclear waste and salt lake brine witht-BMBP has been widely studied [2–5,8].The main drawbacks of the traditional extraction and stripping process consume a lot of base and acid [12,22].In the present work,a saponification process is introduced into the experiment to reduce the consumption of alkali solution.The optimum conditions of extraction,scrubbing,and stripping are checked.Meanwhile,slope method,Infrared spectroscopy and density functional theory(DFT) [23] are used to study the extraction mechanism and structure of extraction complex.These results are helpful to further understand of the extraction and separation of Cs+and Rb+from the salt lake brine or nuclear waste in the practical industry production.

2.Experimental and Calculation Methods

2.1.Materials and reagents

The feed solution (pH=8.031) is prepared by dissolving AR grade CsCl,RbCl and KCl in distilled water,which contains Cs+,Rb+and K+of 5 mmol?L–1(0.842 g?L–1Cs+,0.598 g?L–1Rb+and 0.373 g?L–1K+,respectively).The commercialt-BAMBP(＞90% ,Beijing Ruilekang Separation Technology Co.,Ltd.,China)was used as the extractant.Sulfonated kerosene (SK) and cyclohexane (C6H12)were used as diluent.All other chemical reagents (AR) were used without further purification unless otherwise stated.

2.2.Experiment processes

Fig.2.The extraction process for Cs+,Rb+ and K+ separation.

The Cs+,Rb+and K+separation procedure shows in Fig.2,the process includes saponification,extraction,scrubbing and stripping stages[24].Alkaline solutions are employed as saponification reagents to improve the extracting ability oft-BAMBP/SK before extraction,and the alkali solution can be recycled.Then a certain amount of feed solution and saponificationt-BAMBP/SK were mixed for sufficient reaction.After the phase separation,the aqueous phase was used for multi-stage extraction[25].Pure water was employed to scrub the K+from the loaded organic phase.Most of the K+would be removed by the repeated scrubbing.Stripping the Rb+(less-extractable metal) for purification and the Cs+(more-extractable metal) can be stripped finally.

During the experimental progress,the aqueous phase is detached for ion content analysis,and the ion content in organic phase are gotten through minusing.The extraction rate (E),scrubbing rate (Eˊ),stripping rate (E′′),distribution ratio (D),separation factor (β) are defined as follows [25,26]:

whereC0,C1,C2,andC3refer to the concentration of alkali metal ions in the feed solution,raffinate,scrubbing and stripping solutions,respectively.V0,V1,V2,andV3refer to the volume of the feed solution,raffinate,scrubbing and stripping solution,respectively.DxandDyrepresent the distribution ratio of different alkali metal ions.The extraction was carried out in 50 ml centrifuge tube and shaken in an incubator shaker at the 150 r?min-1under different conditions if there are no special instructions.

The infrared spectrometer (IR,Thermo Electron,Nexus 670,America)is applied to analyze the organic phase.The organic samples are stripped using acid to obtain the loaded strip liquor.The raffinate and loaded strip liquor containing Cs+and Rb+are diluted to appropriate concentrations and analyzed with atomic absorption spectroscopy (AAS,PerkinElmer 700/800,America),with the analytical accuracy of 1.0% and 1.5% for Rb+and Cs+,respectively[12].K+and Na+are also analyzed by inductively coupled plasma optimal emission spectroscopy(ICP-OES,Agilent Model 735,America),with the analytical accuracy of 1.0% [27].The pH of the organic phase and aqueous phase are measured by the model Seven Excellence pH meter (Mettler Toledo,Switzerland).

2.3.DFT calculations

The structure,stability,and IR spectra of ROH(t-BAMBP),RO(t-BAMBP with the hydrogen of phenolic hydroxyl dissociated) and the extraction complexes were investigated by using the ωB97XD method proposed by Head-Gordon and coworkers [28],which involves dispersion and features excellent geometry optimisation performance.The coulomb-fitting basis set def2-tzvp,was employed for all the atoms [29].Vibrational frequency analysis was performed at the same basis level to ascertain the nature of the stationary point(there was no virtual frequency).All the geometry optimizations and frequency analysis were performed with the Gaussian [30].The electrostatic potential (ESP) and the weak interactions [31] in reduced density gradient (RDG) [32] of the extraction complex were studied by wave function analysis with Multiwfn(Multifunctional wavefunction analyzer)[33]and visualized with VMD (visual molecular dynamics) [34].

3.Results and Discussion

3.1.t-BAMBP saponification

Cs+and Rb+can be effectively extracted witht-BAMBP in high alkalinity solutions by the traditional way[8],where a lot of alkali and acid are consumed.It is unfriendly for the practical production and environment.In the present method,thet-BAMBP is saponified with the alkali liquor before extraction,which sharply reduce alkali solution consumption.

3.1.1.Optimal conditions for saponification

Previous researches have shown the increasing of solution alkalinity is beneficial for the Cs+and Rb+extraction[27].In the present work,NaOH was selected as the optimal saponifiers (Table S1,in Supplementary Material),1 h is selected as optimum saponification time (Fig.S1).The effect of saponifier concentration on the extraction shows in Fig.3.The extraction rate of Cs+,Rb+,and K+increased along with the increase of NaOH concentration.When the concentration of NaOH exceeded 1.2 mol?L–1,over 97% Cs+and 45% Rb+are extracted,respectively.Besides,approximately 10% K+co-extracted,and K+extraction is relative insensitive to alkalinity.

Thet-BAMBP has a high extraction yield for Cs+and Rb+and a lower extraction rate for K+along with the NaOH concentration increase.Therefore,the separation factors(βCs/K,βCs/Rb,βRb/K)climb up and then decline.The highest separation factor of βCs/K=548.8,βCs/Rb=68.76 and βRb/K=17.99 with 1.2 mol?L–1NaOH as the saponifier,respectively.Hence,1.2 mol?L–1NaOH is chosen as the optimal concentration fort-BAMBP saponification.When the presaponatedt-BAMBP was used as the extractant,where the pH adjustment of Cs+solution is not required any more.The more,the alkaline solution used for the saponification can be reused also.It means the saponification technology will sharply reduce the alkali solution consumption (by over 90% according our estimation) comparing with the traditional method [6].

3.1.2.Saponification mechanism

As Fig.4 shows,the infrared spectrum oft-BAMBP obviously varies with the concentration of NaOH increasing.The intermolecular hydrogen bond of phenols can be observed in the range of 3400–3700 cm-1.A sharp band characteristic could be defined as non-associated O-H in the monomer at 3615 cm-1,the wider band characteristic can be well-defined as hydrogen-bonded of O-H in the dimer at 3545 cm-1.With the concentration of NaOH increasing,more Na+load into the organic phase and form sodium phenolate,and the amount of dimer increased.Thus,the ratio intensities of the bonded O-H band to the dimer band increased significantly.Meanwhile,both the alkalinity increase of the organic phase after saponification and the alkalinity decrease of the organic phase after extraction (Table 1),indicating a part of NaOH solutions are enshrouded in the organic phase (Fig.S2).

Table 1 The Na+ concentrations and the pH of the organic phase at different separation process

The IR spectra of ROH and RO from DFT calculations and the experimental IR oft-BAMBP before or after sapoinfied show in Fig.5(a).The caculation IR of ROH and RO are very similar with the only signigficent difference in the range for O-H stretching vibration.In ROH we can find a very strong peak at 3534 cm-1,while in RO such a peak is inexistent.The same for experimental spectra fort-BMBP and thesaponifiedt-BAMBP,where much weaker absorpation can be found in the saponifiedt-BAMBP,which means the ROH reacted with NaOH producing salt of RONa.Fig.5(b) shows the electrostatic potential distribution on the surface of ROH and RO.The different colors represent the electrostatic potential distribution.The red represent the ESP is positive,the blue represent the ESP is negative.The extreme point of ESP were plotted also.The yellow ball represent the maximum point of ESP,the green ball represent the minimum point of ESP.When ROH loses the proton hydrogen on the phenolic hydroxyl group,the negative ESP of phenolic hydroxyl oxygen sharply increases from-20.05 to -165.11 kJ?mol-1.The phenolic hydroxyl oxygen of RO should have a stronger attraction to positive ions comparing with ROH.When phenolic hydroxyl hydrogen is dissociated fromt-BAMBP during saponification,thus the alkali metal ions are easier to loaded into the extractant.Further evidents can be found through interaction of Cs+between the ROH and RO (Fig.S3).

Fig.3.t-BAMBP saponified with different concentrations of NaOH:(a) extraction rate of Cs+,Rb+ and K+,(b) separation factor of Cs/K,Cs/Rb and Rb/K.Conditions: Ct-BAMBP=1 mol?L–1, CNaOH=0.1–1.6 mol?L–1,saponification time=1 h, Cfeedsolution=5 mmol?L–1,O/A=1:1, T=288.15 K,extraction time=5 min.

Fig.4.Infrared absorption of saponified t-BAMBP in the O-H region with various concentrations of NaOH as the saponifier.Conditions:Ct-BAMBP=1 mol?L–1,saponification time=2 h,O/A=1:1,T=288.15 K.

3.2.Extraction

In the extraction process,the saponified organic phase was directly applied to alkali metal ions extraction.Feeding solution(pH=8.031,without pH adjustment) was extracted and used as eluent also,the change of alkalinity and Na+content suggest most of the Na+and NaOH were eluted into aqueous phase from the organic phase (Table 1).

3.2.1.The effect of phase ratio

The effect of the phase ratio(O/A)on the alkali metal ion extraction shows in Fig.6.The optimal concentration oft-BAMBP is determined as 1.0 mol?L–1(Fig.S4).The extraction yield of Cs+,Rb+and K+and separation factor (βCs/K,βCs/Rb,βRb/K) apparently increase along with the O/A increasing.More than 92% Cs+are extracted with the O/A of 1:1.When the O/A is 5:1,99.6% Cs+,96.4% Rb+and 46.1% K+are extracted,respectively.The maximum separation factor of βCs/K=120.3,βCs/Rb=56.9 and βRb/K=37.6.Thus,the optimum O/A for extraction was selected as 1:1.When the extraction reaction reaches equilibrium in 5 min,the maximum extraction quantities for Cs+,Rb+and K+are 4.70,0.59 and 0.11 mg?g-1,respectively (Figs.S5 and S6).

3.2.2.The effect of temperature

The effect of temperature on thet-BAMBP extraction alkali metal ions was investigated in the range of 278.15–323.15 K,saponified 1 mol?L–1t-BAMBP for 1 h with 1.2 mol?L–1NaOH,extraction in 5 min at O/A 1:1.The result of the temperature effect is listed in Table 2.The distribution ratio (DCs,DRb,DK) decreased with the temperature increase.The enthalpy (ΔH) of Cs+,Rb+,and K+can be gotten employ the slope method [35,36].

The data of Table 2 were fitting with the Eq.(6),theRis a fixed value of 8.314 in the ideal conditions.

Table 2 The effect of temperature on the distribution ratio

The dependence of the distribution ratioDon theT-1(on the logarithmic scale) is linear.

The differential of Eq.(6) could get the following formula:

Fig.5.(a) Infrared spectrogram and (b) electrostatic potential distribution of ROH and RO from experiment and calculated at ωB97XD/def2-tzvp level.

Fig.6.Effect of the phase ratio (O/A) on the alkali metal ion extraction:(a) extraction rate of Cs+,Rb+ and K+,(b) separation factor of Cs/K,Cs/Rb and Rb/K.Conditions: Ct-BAMBP=1.0 mol?L–1, Cfeedsolution=5 mmol?L–1,O/A=1:5–5:1, T=288.15 K,Time=5 min.

The dlgD/dT-1represents the slope of the line.The Fig.7 are equal to 1.872,0.943 and 0.860 for Cs+,Rb+and K+,respectively.The ΔHwas given the following:

ΔHCs=-35.84 kJ?mol-1,

ΔHRb=-18.09 kJ?mol-1,

ΔHK=–16.47 kJ?mol-1.

The negative enthalpy indicates that extraction Cs+,Rb+and K+byt-BAMBP is an exothermic reaction.The low temperature is favorable to the extraction and separation of Cs+and Rb+from K+solution.However,the larger viscosity at temperature means of a longer time for kinetic equilibrium,the cost of operation will increase.Therefore,the room temperature is appropriate.The extraction equilibrium reaches in 5 min and conforms pseudosecond-order kinetic model (Fig.S7 and Table S2).

3.2.3.The structure and weak interactions of extraction complex

The studies have shown thatt-BAMBP extracted Cs+,Rb+,K+is a cation exchange reaction(Fig.S8 and Table S3)and the structure of extraction complex is MOR?3ROH(Fig.S9 and Table S4)[5,11].DFT caculations show that the extraction complex CsOR?3ROH exists in the form of bis-dimer in the organic phase (Fig.8(b)),where two ROH molecules from a dimer,a ROH and a RO form another one.The average distance between the oxygen atoms of the ROH and Cs+is 3.32 nm and the distance between RO and Cs+is 3.01 nm.It’s indicates that all the oxygen atoms have electrostatic attraction with the central Cs+,especially for the RO has a stronger attraction to the Cs+in the bis-dimer.In addition,the minimum range of Cs+and benzene ring is 3.44 nm,the cation–π interaction may play important roles in the extraction [37].

Noncovalent interactions are essential for the extraction.Johnsonet al.[32]propose the reduced density gradient(RDG)to study the noncovalent interactions such as H-bond,electrostatic interaction,van der Waal’s interaction and steric effectetc.The expression of RDG is defined as follow:

Fig.7.Effect of temperature on (bottom) Cs+,(middle) Rb+ and (top) K+ extraction distribution ratio.

Fig.8.(a)Plots of the RDG versus the sign(λ2)ρ and(b)the RDG isosurface map with isovalue of 0.5 for CsOR?3ROH,the value of sign(λ2)ρ in surfaces is represented by filling color according to the color bar in middle.

RDG method can clearly shows the type,location and intensity of the weak interactions at molecular level.The difference colors represent various weak interactions.Blue means attraction,such as Hbond,halogenetc.[32,38].Green means van der Waal’s weak interaction,and red means strong repulsive,such as ring and clathrate.

Here,we use RDG and electron density multiplied by the sign of the second hessian eigenvalue (sign(λ2)ρ) function to reveal the weak interactions in the extraction complex.The blue spikes appearing on the left side of the scatter plot(Fig.8(a))and blue isosurface(Fig.8(b))indicates the electrostatic attraction.It’s attributed to the hydrogen bonding between the ROH and RO.As Fig.8(b)shows there are blue isosurfaces around all the phenolic hydroxyl O and the central Cs+,all the four phenolic hydroxyl O interact with the central Cs+.The green spike around-0.01 of scatter plot(Fig.8(a))indicates the van der Waal’s weak interaction in the extraction complex.Many interactions contribute to the van der Waal’s weak interaction in the extraction complex.We can find same remarkable blue isosurfaces between the benzene rings and the central Cs+(a more intuitive understanding through the RDG of CsOR can be found in Fig.S10),they are contributed to the cation-π interaction in the extraction complex,which is considered as outstanding importance in determining the structure and function of supramolecular assembliesetc.[37].The red spikes (0.01– 0.03)on the right side of the scatter diagram indicate the mutual exclusion (Fig.8(a)).In CsOR?3ROH,the center of Cs+is surrounded by the clathrate structure in the extraction complex,where the steric effect is much more significant than CsOR (Fig.S10).The steric effect is an unstable factor for the extraction complex.However,this clathrate structure sharply reduces the polarity of this extraction complex and makes it more stable in the organic phase.

3.3.Stripping

The loaded organic phase contains 0.807 g?L–1Cs+,0.234 g?L–1Rb+and 0.0219 g?L–1K+after single-stage scrubbing process by pure water (Table S5).HNO3is the optimal stripping reagent(Table S6),and the effect of HNO3concentration,phase ratio,temperature and stripping time were taken into consideration.1.2 mol?L–1HNO3solution is the optimum condition for the Cs+,Rb+and K+stripping(Fig.S11).The stripping is an exothermic reaction and stripping equilibrium reachs in 4 min(Figs.S12 and S13).

The O/A is one of the significant factors for the stripping.As shown in Fig.9(a),the stripping rate decrease along with the increase of O/A.It’s clear that the high stripping yield could be achieved by the single-stage stripping while the O/A lower than 1:2.However,it’s not beneficial to separate Cs+,Rb+and K+with a higher stripping rate.The McCabe-Thiele diagram of Rb+stripping shows in Fig.9(b),where the Rb+content in the loaded organic phase could be reduced to 0.01 g?L–1by three stripping stages at the O/A of 5:1.Almost all of Cs+and Rb+could be striped by three stages stripping.Hence,the higher stripping efficiency of Cs+,Rb+and K+can be obtained employing the multistage stripping at O/A of 5:1.

3.4.The ions transfer in the whole process

The Na+concentrations and the pH of the organic phase at extraction,scrubbing and stripping stage are collected in Table 1.In the saponification stage,2.713 g?L–1Na+were loaded into the organic phase from the aqueous phase.Around 63.2% Na+are leached-out from the extractant while the Cs+,Rb+and K+are extracted.This indicates the extraction is a process of ions exchange,it also indicates Cs+,Rb+and K+are easier to be extracted than Na+.After stripping,almost all of the Na+are eluted from the extractant,and thet-BAMBP regeneration.

Fig.9.(a) Effect of phase ratio (O/A) on stripping.(b) McCabe-Thiele diagram for Rb+ stripping.Conditions: CHNO3=1.0 mol?L–1, T=288.15 K,O/A=1:5–5:1,Time=4 min.

Fig.10.Infrared spectra of t-BAMBP changes in the process:(a)1 mol?L–1 t-BAMBP,(b) saponified t-BAMBP,(c) extracted t-BAMBP,(d) t-BAMBP after scrubbing,(e)stripped t-BAMBP.

The alkalinity change of the organic phase closely relates with the intensities of the O-H band (～3545 cm-1and～3615 cm-1)in the different separation steps (Fig.10).After saponification,the alkalinity of organic phase increased from 8.183 to 12.163(Table 2).The increase of pH and the intensity decrease of O-H bonds for the organic phase indicate that the H on the phenolic hydroxyl group is exchanged to the aqueous phase (Fig.10(b)).The increase oft-BAMBP dimers along with the Cs+,Rb+or K+are loaded into the organic phase after extraction (Fig.10(c)).The Na+concentration and pH of the organic phase decrease to 0.999 g?L–1and 11.353,respectively(Table 1).It’s indicate that part of Na+or NaOH is exchanged to the aqueous phase from the loaded organic phase.Thus,the intensity of the bonded O-H band for dimer and non-associated O-H band for monomer decrease in thet-BAMBP.After scrubbing,the alkalinity of the organic phase approaches to the rawt-BAMBP (Table 1).The intensities of associated O-H band and non-associated O-H band reach the minimum (Fig.10(d)).The pH of organic phase is 3.369 after stripping(Table 1),a mass of H+rebounds with the phenolic hydroxyl group and the increase of O-H band intensity in the organic phase(Fig.10(e)).The extraction ability recovers and thet-BAMBP could be reused for extraction.The ions transfer in the whole process can describe as the Fig.11.

Table 3 The extraction rates of Cs+,Rb+ and K+ with recycled saponified extractant

3.5.The extraction and stripping cycles

Fig.11.The alkali metal ion transfer at different stages.

The result of multi-stage countercurrent extraction shows in the Support Information (Fig.S14 and Table S7).The extraction rates of Cs+and Rb+sharply decrease with the number of extraction stages increases.No extraction ability was keeping after five-time countercurrent extractions.Which means the saponification is the crucial process for the extractant recycling.Thus,the extraction and stripping cycles experiments were done by adding the saponification process.Which means the stripped organic phase re-contacts with 1 mol?L–1NaOH.

The extraction rates of Cs+,Rb+and K+with the five-times recycled saponified extractant show in Table 3.The extraction rates of Cs+,Rb+slight decreases with recycled extractant.It indicates that the extractant can be recycled by saponifying.

4.Conclusions

(1) Saponificationt-BAMBP sharply reduces the alkali solution consumption in the whole process,and improves the selectivity of extraction (Table S8).The result of DFT calculations and experiments show the phenolic hydroxyl group O is the binding site of alkali metal ions.A fraction of Na+and NaOH is enshrouded in extractant,the alkalinity of the organic phase is the dominate effect on extraction.

(2) 99.5% Cs+and 46.7% Rb+are loaded into the organic phase,about 10% K+are co-extracted in the single-stage extraction process.The extraction process is an exothermic reaction and the lower temperature is favorable to the extraction.Both the electrostatic attraction between Cs+and the phenolic hydroxyl O,and the cation-π interaction are the important noncovalent interactions in the extraction complex.

(3) Stripping is an exothermic reaction also.The kinetics of stripping indicates that the stripping reaches equilibrium at 4 min,98.6% Cs+,62.6% Rb+and 79.2% K+are stripped from the loaded organic phase.When the dimers are dissociated,the proton H is bonded to the phenolic hydroxyl group,and the extractant is regenerated.

Declaration of Competing Interest

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.

Acknowledgements

Gratefully acknowledge to Hui-Juan Wang and Dr.Jing Wang for their assistance in element analysis.The research is financially supported by the Science and Technology Department of Qinghai Province (2019-ZJ-7001),West Light Foundation of the Chinese Academy of Sciences (Y910041014),Youth Innovation Promotion Association,CAS (2017467) and the Tibet A-Li La-Guo Resources Co.Ltd.,China.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.07.001.