Tao Yang,Jing Fu,Qingde Chen,Xinghai Shen*

Beijing National Laboratory for Molecular Sciences(BNLMS),Fundamental Science on Radiochemistry and Radiation Chemistry Laboratory,College of Chemistry and Molecular Engineering,Peking University,Beijing 100871,China

1.Introduction

The long lifetime strontium ion is one of the main high heat load,highly radioactive fission products in high level liquid wastes(HLLW)during the reprocessing of spent nuclear fuel.Many advantages were shown to remove Sr(II)from HLLW before the final geological disposal[1].So far,solvent extraction has been a main method of separating Sr(II)from HLLW,and the ionic liquids(ILs),holding numerous excellent properties and becoming environmentally benign “green solvents”alternative to volatile organic solvents[2–7],have been employed in the extraction of Sr(II).

Daiet al.[8] first used ILs as solvents and DCH18C6 as extract ant to extract Sr(II)from the aqueous phase.The distribution coefficient(D)reached 1.1×104,which was much higher than those of the conventional extraction systems.For example,theDvalues of the extraction systems with C6H5CH3,CHCl3,2-octanol and 1,1,2,2-tetrachloroethane as diluents were only 0.76[8],0.77[8],6.5[9]and 10.8[10],respectively.Dietz and Dzielawa[11]proposed that the extraction of Sr(II)by IL systems was based on a cation exchange mechanism.Our research group studied the effect of the structure of ionic liquid,the acidity and the addition of inorganic salt in the aqueous phase on the extraction efficiency of Sr(II)by the DCH18C6–CnmimNTf2(n=2,4,6)system,and further confirmed the cation exchange mechanism[1].In addition,we found that theγ-radiation stabilities of I Ls were excel lent during the extraction of Sr(II)with DCH18C6[12,13],which is conducive to fit to the high irradiation environment in the spent nuclear fuel reprocessing.

Although IL shave lots of advantages towards the extraction of metal ions,the stripping,which is as important as the extraction process in the application,still remains a challenge.Our research group tried to study the stripping of Sr(II)with K2SO4and the stripping efficiency reached 99%[1].But there are two serious obstacles:cross contamination between two phases and loss of ILs.Electrode position is another method to recover metal ions from the IL phase[14,15].However,this method applies at many strict conditions,i.e.,only for highly electro positive elements,and requiring the electrochemical window of an IL matching the reduction potential of a metal ion.

Supercritical CO2(sc-CO2)is considered as another “green solvent”,because CO2can be recycled and generates no wastes.Sc-CO2has many advantages including enhanced diffusivity,chemical inertness,non- flammability and low cost[16–18].Sc-CO2has been employed to extract various metal ions such as lanthanides,actinides,alkali metals from aqueous or solid matrices[19–32].Due to the limited mutual solubility between sc-CO2and IL,that is,the insolubility of IL in sc-CO2and the solubility of sc-CO2in IL[33–35],a feasible method to recover metal ions from IL phase by sc-CO2has been put forward.Some metal ions were stripped effectively by sc-CO2from IL phase[36–40].However,the mechanism of stripping by sc-CO2has been rarely reported.Our research group recently used ESI-MS to study the mechanism of stripping U(VI)from the CMPO–C2mimNTf2system by TOPO-modified sc-CO2[40].

To the best of our knowledge,although DCH18C6 as the extractant in C2mimNTf2performs excellently for the extraction of Sr(II),there have been no effective methods so far to strip it from the DCH18C6–C2mimNTf2system with high stripping efficiency,no crosscontamination,and no loss of ILs.

In this study,we aim to use sc-CO2to strip Sr(II)from the DCH18C6–C2mimNTf2system.Due to the strong polarity of DCH18C6·Sr2+but nonpolarity of CO2,the solubility of crown ether complexes in sc-CO2is very small.It is well known that fluorinated metal chelates are CO2-philic[41],thus utilizing fluorinated anion to combine crown ether–metal cation as ion-pair is a favorable method.Waiet al.[23]successfully extracted Sr(II)from aqueous media into sc-CO2employing DCH18C6 and a fluorinated anion,i.e.,CF3(CF2)6(PFOA-)orCF3(CF2)6CF2(PFOSA-).Another method to improve the solubility of crown ether complexes is adding cosolvent to sc-CO2,such as acetonitrile,methanol,and acetone to enhance its polarity.Considering the two aspects in favor of the solubility of crown ether complexes in sc-CO2,we chose a fluorinated metal chelate hexafluoroacetylacetone(HFAA)diluted in acetonitrile as the modifier.

Meanwhile,the influences of temperature,pressure,static stripping time,dynamic stripping time,and the concentration of HFAA on the stripping efficiency of Sr(II)by sc-CO2were investigated systematically.Due to the difficulty in selecting the optimized conditions,the orthogonal array design(OAD)as an artful method was used to evaluate objectively the whole conditions in the sc-CO2stripping progress.The results of OAD experiments were investigated by the analysis of variance(ANOVA)[42,43].

2.Experimental

2.1.Materials

C2mimNTf2was synthesized and purified according to the reported methods in the literature[44–46].The purity was above 99%as determined by elemental analyses and1H NMR.Dicyclohexano-18-crown-6(DCH18C6)was received from INET,Tsinghua University and purified by recrystallization inn-heptane.The purity of final product was above 98%.2,2-Dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedione(HFOD,99%,J&K),hexa fluoroacetylacetone(HFAA,99%,J&K),thenoyltrifluoroacetone(HTTA,99%,J&K)and perfluorooctanoic acid(HPFOA,98%,J&K)were used as received.The chemical structures of C2mimNTf2,DCH18C6,HFAA,HTTA,HPFOA and HFOD are shown in Fig.1.Sr(NO3)2and other reagents were of analytical grade.CO2(99.95%)from Reijiye in China was used in all experiments.

2.2.Instruments

All stripping experiments were performed in a lab supercritical fluid extraction apparatus(100 ml,SEPAREX,France).The concentration of Sr(II)was analyzed by an Inductively Coupled Plasma-Atomic Emission Spectrometer(ICP-AES,Leeman,USA).The high-resolution mass spectrometry used with electrospray ionization(ESI-HRMS)was done on a Fourier transform ion cyclotron resonance mass spectrometer,APEX IV(Bruker,USA).Microwave digestion was completed in the instrument of CEM MARS6(USA).

2.3.Methods

The stripping samples were obtained after extracting Sr(II)from the aqueous phase by DCH18C6–C2mimNTf2.The concentration of Sr(II)was about 0.025 mol·L-1in the C2mimNTf2phase.0.35 ml C2mimNTf2solution containing 0.025 mol·L-1of strontium was added in a 10 ml glass-basket,which was fixed in the high-pressure extraction vessel with stirring.The stripping process was allowed to take place under static and dynamic extraction conditions.Understatic extraction condition,5 ml modifier was injected in vessel and kept for some time.The flow rate of CO2was fixed at 50 g·min-1,and the modifier was the 0.5 ml·min-1of HFAA–acetonitrile solution under dynamic extraction condition.

Fig.1.Molecular structures of C2mimNTf2(a),DCH18C6(b),HFAA(c),HFOD(d),HTTA(e),and HPFOA(f).

When the stripping progress was completed,the system was allowed to slowly depressurize and the IL sample was then removed from the cell and analyzed.The stripping efficiency was calculated based on the contents of Sr(II)in the initial and the residual IL phases,which were analyzed by ICP-AES after microwave digestion.The mass spectra of the stripping product and the IL phases were measured with ESI-MS.

3.Results and Discussion

3.1.Experimental design and data analysis

As known,the density of sc-CO2decreases with raising the temperature and reducing the pressure.Generally,the solubility of the solids in the sc-CO2mainly depends on the density of the solvent.However,the extraction efficiency does not only depend on the density of sc-CO2[43,47].For example,the rising of temperature in the course of stripping could increase the volatility of the analytes,accelerate the complexation reactions and desorption of the analytes from the IL phase,which increased the extraction efficiency[43,47].With respect to the pressure,the interaction between sc-CO2and IL was reduced because of the decrease of the diffusion coefficient at a higher density,which can reduce the extraction efficiency[43,47].Therefore,the temperature(T)and pressure(P)should be optimized.In addition,static(ts)and dynamic(td)extraction times as well as the concentration of HFAA(C)are also important factors on the stripping efficiency.Thus,a five-factor, fivelevelOAD withan OA25(55)matrix was designed to evaluate the effects of the following factors:T,P,ts,tdandC.In order to obtain the best condition for the stripping of Sr(II),25 experiments were performed.The factors and their corresponding levels as well as the results of the stripping experiments are reported in Table 1.

Table 1Results of OA25(55)experimental design for the extraction of Sr(II)

The mean value of each stripping efficiency for the respective factors and at each level was calculated according to the assignment of the experiment(Table 1).For example,the stripping efficiency of the five trials at 308 K was evaluated as mean value of the corresponding five runs(Trial number 1-5).The mean values of the five-levels of each factor reveal how the stripping efficiency changes when changing the level of that factor.Fig.2 shows stripping efficiency as a function of levels of the studied factors.In all instances,it should be noted that each calculated stripping efficiency is the average of five measurements,and in each of which the interested parameter was kept constant,and all the other parameters were changed(Table 1).

The ANOVA results for calculated models are shown in Table 2.The ANOVA indicates that the temperature and pressure of sc-CO2play key roles and the dynamic time plays important role in the stripping of Sr(II)from the IL phase,whereas,in the selected range the static time and the concentration of HFAA do not have significant effects on the stripping of Sr(II).

3.2.Effect of extraction temperature

The variation of temperature during stripping affects the density of sc-CO2,the volatility of the analytes and desorption of the analytes from the IL phase.At higher temperatures,the density of sc-CO2decreases and analytes become more volatile leading to lower efficiency as shown in Fig.2(a).

Table 2ANOVA of the Sr(II)extractions

The ANOVA of the results shows that the temperature plays a key role in the extraction of Sr(II).Therefore,we ought to select a lower extraction temperature(308 K)as the optimum temperature.

3.3.Effect of extraction pressure

Solubility of a solute in sc-CO2depends on a balance between sc-CO2density and solute vapor pressure,both of which are controlled by the pressure of sc-CO2.As can be learnt from the ANOVA of the results,the pressure of the sc-CO2also plays a key role in the stripping of Sr(II)from the IL phase.This means that the stripping efficiency is enhanced by an increase in the pressure.As shown in Fig.2(b),the optimized pressure is 30 MPa.

3.4.Effect of static and dynamic extraction times

In order to achieve high stripping efficiency,the static extraction can make the penetration of sc-CO2in the IL phase better than the dynamic extraction.The dynamic extraction follows static extraction to enhance the solubility of analyte in sc-CO2.

Based on the ANOVA calculations,the dynamic time plays an import ant role in the stripping of Sr(II)from the IL phase.According to Fig.2(d),when the dynamic time is increased,the stripping efficiency of Sr(II) firstly increases and then decreases,and the best dynamic time is 40 min.On the contrary,the static time has no significant effects on the stripping efficiency.For a rapid extraction of Sr(II)from the IL phase and on the basis of economy principle,one can select a shorter static time(15 min).

3.5.Effect of the concentration of HFAA

Based on the ANOVA calculations,the concentration of HFAA has no significant effects on the stripping of Sr(II)from the IL phase in the selected range(60–140 mmol·L-1).Therefore,we select 60 mmol·L-1as the optimum concentration of HFAA according to the economy principle.

Fig.2.Effect of temperature(a),pressure(b),static(c)and dynamic(d)extraction times and the concentration of HFAA(e)on the stripping of Sr(II)from DCH18C6–C2mimNTf2.

On the basis of the above results,the optimum values of the selected factors(temperature,pressure,static and dynamic extraction times and concentration of HFAA)for strip ping of Sr(II)are308 K,30 MPa,15 min,40 min and 60 mmol·L-1,respectively.Further experiment was performed under the proposed conditions and the stripping efficiency was about 100%.

In order to obtain the better conditions,we further optimized static stripping time and concentration of HFAA.When the static time was 15,10,5 and 0 min respectively at 308 K,30 MPa,40 min of dynamic stripping time and 60 mmol?L-1HFAA in acetonitrile,the stripping efficiency of Sr(II)always reached 100%.The effect of the concentration of HFAA is shown in Fig.3.The stripping efficiency is reduced when the concentration of HFAA decreases.So we can obtain the optimized conditions,namely,no static stripping time and the concentration of HFAA is 60 mmol·L-1at 308 K,30 MPa,40 min of dynamic stripping time.

Fig.3.Effect of concentration of HFAA on the stripping efficiency of Sr((II)at308K,30 MPa,40 min of dynamic stripping time from C2mimNTf2.

3.6.Stripping mechanism

The above results demonstrate that Sr(II)can be effectively stripped from the IL phase into the sc-CO2phase with HFAA–acetonitrile solution as the modifier.Acetonitrile as a co-solvent can enhance the polarity of CO2to increase the solubility of the complexes of Sr(II)in the modified sc-CO2.

The stripping efficiency of Sr(II)from the IL phase can reach 100%and less than 5%of IL is lost during the stripping experiment under the optimal condition:308 K,30 MPa,40 min of dynamic stripping time and 60 mmol·L-1HFAA in acetonitrile.

We investigated the original IL phase,the recovered IL phase,and the stripping product by ESI-MS(Fig.4(a-c)),respectively.The compositions of the fragments and their corresponding complexes are listed in Table 3.

Fig.4.Positive ESI spectra of the original IL phase(a),the recovered IL phase(b),and the stripping product(c).

Cationic complexes[C2mim-DCH18C6-NTf2-Sr-2H2O]2+,[Sr-NO3-H2O]+,[Sr-NO3-FAA-Sr]2+,[DCH18C6-Sr-FAA]+,[C2mim-NTf2-DCH18C6-FAA-Sr]+and[DCH18C6-Sr-NTf2]+are observed in the ESI spectrum of the stripping product(Fig.4(c)),which indicates that various kinds of the complexes of Sr(II)were formed in the stripping process.FAA-not only competes with DCH18C6to associate with Sr2+,but also strips Sr2+with DCH18C6 synergistic ally.According to the above results,one can infer that the affinity of Sr2+with FAA-is larger than that with DCH18C6,and the complexes of Sr(II)associating with FAA-become more soluble in sc-CO2.Therefore,with FAA-as the modifier,sc-CO2can strip strontium from the DCH18C6–C2mimNTf2system effectively.With respect to the recovery of Sr(II)from the chelate,two methods could be applied.If the recovery of complexing agents is not necessary,Sr(NO3)2aqueous solution could be obtained via the digestion of the stripping product.If the recovery of ligands is obligatory,SrSO4precipitate could be collected by washing the organic solution of the stripping product with the aqueous solution containing.

The cationic complexes[DCH18C6-Na]+and[DCH18C6-NH4]+demonstrate the existence of the extract ant DCH18C6 in the stripping product and illustrate that part of DCH18C6 can be brought out by modified sc-CO2.The cationic complex containing[C2mim-NTf2-C2mim]+is also observed in the stripping product(Fig.4(c)),which means that the IL was brought out partly by sc-CO2with HFAA–acetonitrile as the modifier.According to the literature,the dissolving capacity of ILs in sc-CO2can be enhanced significantly,when the sc-CO2phase contains polar organic solvents acting as co-solvents[35].The effect of a cosolvent on the solubility of sc-CO2for the ILs primarily depends on its polarity and concentration in sc-CO2[48,49].Therefore,we believe that the loss of the ILs can be avoided to some extent if the stripping conditions are further optimized.

Table 3The compositions of the fragments and their corresponding complexes of the original IL phase,the recovered IL phase and stripping product

In the ESI spectrum of the recovered IL phase(Fig.4(b)),the signals of the cationic complexes[C2mim-NTf2-C2mim]+and[C2mim-NTf2-C2mim-H]2+are observed,which are consistent with those in the ESI spectrum of the original IL phase(Fig.4(a)).And the cationic complexes[Sr-NO3-H2O]+exist in the recovered IL phase,but the intensity is very weak.This indicates that Sr(II)has been stripped almost completely,which is in agreement with the results of ICP-AES.Thus,highly efficient stripping of Sr(II)from the DCH18C6–C2mimNTf2system was achieved in this work.

Combined with our previous work about the extraction of Sr(II)by DCH18C6–C2mimNTf2[1],a complete cycle including extraction and stripping processes for Sr(II)with both IL and sc-CO2canbe established.The whole process is illustrated in Fig.5.

Finally,we evaluated the other three fluorinated metal chelates HTTA,HPOD,and HFPOA,respectively,diluted in acetonitrile at the optimized conditions of HFAA,and the stripping efficiencies are 66%,87%and 91%(Fig.6).They can also be chosen as modifiers and their stripping efficiencies could be further optimized by OAD.

Fig.5.A diagram showing the extraction–stripping procedure for strontium.

Fig.6.Effect of different modifiers diluted in acetonitrile on the stripping of Sr(II)from DCH18C6–C2mimNTf2.

4.Conclusions

The stripping of Sr(II)from the DCH18C6–C2mimNTf2system using the modified sc-CO2was studied.It was found that HFAA–acetonitrile is an efficient modifier to facilitate the stripping of Sr(II)from the IL phase.By the method of OAD and ANOVA,nearly 100%of Sr(II)could be successfully stripped off under the following conditions:308 K,30 MPa,40 min of dynamic stripping time and 60 mmol·L-1HFAA in acetonitrile.

The present work demonstrates the feasibility of employing sc-CO2to strip and promises a highly efficient extraction–stripping cycle for IL-based Sr(II)recovery process.

Nomenclature

Cconcentration of HFAA,mmol·L-1

Ppressure,MPa

Ttemperature,K

tddynamic extraction time,min

tsstatic extraction time,min

Subscripts

d dynamic extraction

s static extraction

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