Extraction of U(Ⅱ)from Nitric Acid Medium by N,N,N′,N′-Tetra-2-ethylhexyldiglycolamide in Sulfonated Kerosene

CAI Xu-CanCUI YuYANG Xiao-FengLI Ye-XinSUN Guo-Xin

(School of Chemistry and Chemical Engineering,University of Jinan,Jinan 250022,China)

Extraction of U(Ⅱ)from Nitric Acid Medium by N,N,N′,N′-Tetra-2-ethylhexyldiglycolamide in Sulfonated Kerosene

CAI Xu-CanCUI YuYANG Xiao-FengLI Ye-XinSUN Guo-Xin＊

(School of Chemistry and Chemical Engineering,University of Jinan,Jinan 250022,China)

The extractive properties of N,N,N′,N′-tetra-2-ethylhexyldiglycolamide(T2EHDGA)were examined for the extraction of uranium(Ⅱ)from nitric acid medium,in which sulfonated kerosene was used as diluent.There′s no third phase formation under the studied conditions.The number of ligand molecules present in the stoichiometry of the extracted species of U(Ⅱ)was found to be two.Thermodynamics studies suggested that the extraction is exothermic.The extraction influence of nitric acid concentration and T2EHDGA concentration as well as salting-out agent and temperature on the extraction distribution ratio was investigated.The extraction mechanism was established and the stoichiometry of the main extracted species was confirmed to be UO2(NO3)2·2T2EHDGA.The IR spectra of the loaded organic phase were also recorded.

extraction;U(Ⅱ);N,N,N′,N′-tetra-2-ethylhexyldiglycolamide;sulfonated kerosene

Uranium(Ⅱ)is both the major constituent of nuclear fuel and one of the key nuclides in spent fuel reprocessing[1].Separating uranium(Ⅱ)from High Level Waste(HLW)solution generated during the reprocessing of the spent nuclear fuel is useful both for the storage of nuclear waste and for recycling of nuclear fuel[2-3].Solvent extraction,in combination with chelation chemistry,has been one of important method in separation and recovery of uranium(Ⅱ)[4-5].Designing appropriate chelation agents is a crucial task in the application of innovative solvent extraction techniques to the efficient separation and recovery of uranium(Ⅱ) from HLW solution or raw materials[6-7].Recently,new class of diamide extractants,diglycolamides have beenthe focus of many extraction groups working on actinide partitioning in view of offering several advantages like high radiolytic stability,innocuous nature of degradation products and complete incinerability as well as the easy of synthesis[8-14].This class of extractants exhibit higher extraction of actinide ions from acidic solutions as compared to malonamides due to containing ether linkage between two amide groups[9,15-16].

The stereochemical hindrance around the potential binding sites of diglycolamides is likely to play important roles on the metal ion extraction and separation performances,study on the relationships between ligand structures and metal extraction are stillimportant.Amongstvariousderivativesof diglycolamides,N,N,N′,N′-tetraoctyldiglycolamide (TODGA)has attracted considerable attention and has been investigated intensively[17-20].However,studies on its homologue,viz.T2EHDGA having ethyl branched chain are scarce[21-24].Sharma et al.investigated the extraction behavior of uranium,thorium and nitric acid for TEHDGA/isodecyl alcohol/n-dodecane solvent system,they found the extracted species of uranium and thorium in the organic phase were UO2(NO3)2· 2TEHDGAand Th(NO3)4·2TEHDGA.A workable separation factor(DTh/DU)of the order of 300 was observedbetweenthoriumanduranium,which indicated that TEHDGA solvent system could be a potential candidate for separation of thorium from uranium[23].

As we all know,diluents play an important role in the solvent extraction techniques[11,25].In this paper, sulfonatedkerosene,adiluentwidelyusedin industrial scale extraction process,was selected as solvent considering its high flash point and low cost comparedwithisodecylalcohol/n-dodecane.The extraction properties of uranium(Ⅱ)by T2EHDGA in sulfonatedkerosenefromnitricacidmediawas investigatedindetailandtheresultshasgreat reference value for practical application.

1 Experimental

1.1 Reagents

T2EHDGA,yellow oil,98%purity,was synthesized by a three-step process as reported in Manoha′s paper[26].However,thionyl chloride was used to convert diglycolic acid to diglycolyl chloride[9]rather than phosphorus pentachloride.T2EHDGA was characterized by IR,1H NMR and13C NMR.The corresponding data are given as follows:IR(KBr,cm-1)νmax: 2 959,2 874(C-H),1 658(C=O),1 463,1 456(-CH3), 1 117(C-O),728(-CH2-)；1H NMR(400 MHz,CDCl3, TMS)4.43(d,J=7.8Hz,,4H,2-OCH2-),3.07～3.35(m, 8H,4-N-CH2-),1.55～1.66(s,4H,4-CH-),1.21～1.29 (s,32H,16-CH2-),0.84～0.90(d,J=7.0,3.5 Hz,24H, 4-CH3);13C NMR(400 MHz,CDCl3,TMS)δ:169.80, 69.35,50.18,48.04,38.06,36.82,30.69,30.63,28.96, 28.85,23.91,23.23,14.22,14.17,11.08,10.76.

Other reagents employed in this work were all AR grades.

1.2 Extraction and analytical procedures

Equal volumes of T2EHDGA in sulfonated kerosene and UO2(NO3)2solution were shaken mechanically for 30 min(to obtain equilibrium)at(25±0.2)℃. The two phases were then centrifuged and assayed by taking known aliquots(0.05～0.10 mL)from the aqueous phases.The analytical procedures were just like those described in our previous paper[27].

1.3 Preparation and characterization of the extracted species

Theextractedspecieswereprepared.The extractant solutions were shaken with a concentrated solution of UO2(NO3)2and centrifuged.The organic phase separated was dropped on a potassium bromide plate and the diluent was removed by evaporation. FT-IR spectra of the residuals were recorded on Perkin Elmer-Spectrum One in the range of 400～4 000 cm-1.The scan times and the resolution were 60 and 2 cm-1,respectively.

2 Results and discussion

2.1 Effect of nitric acid concentration on the extraction of U(Ⅱ)

The extraction of uranium(Ⅱ)with 0.20 mol·dm-3 T2EHDGA in sulfonated kerosene from nitric acid solutions has been investigated.As shown in Fig.1, in the range of 1.00～5.00 mol·dm-3nitric acidconcentrations,the distribution ratio ofincreases with increasing concentration of nitric acid,which can mainly be attributed to co-ion effect ofand, possibly,also to classic salting out effect[28].It also indicates that no obvious competing extraction of nitric acid appeared in all the range of concentrations tested in the proposed condition.This phenomenon is somewhat similar to that reported with N,N,N′,N′-tetrahexyl-4-oxa-heptanediamide in nitric acid medium systems[19].There′s no third phase formation under the studied conditions.

Fig.1Effect of HNO3concentration on the extraction of

2.2 Effect of extractant concentration on the extraction of U(Ⅱ)

The logarithm distribution ratio is plotted as a function of the logarithm concentration of T2EHDGA in Fig.2.The distribution ratio increases linearly with an increase in the T2EHDGA concentration.The calculated slope of the line is about two.Hence,the reaction equilibrium of UO22+from nitric acid solution by T2EHDGA can be described as follows:

where subscripts(a)and(o)refer to aqueous and organic phase,respectively.

Fig.2Effect of T2EHDGA concentration on the extraction of

As we known,the coordination number of uranium in UO22+manner is 6[29].Thus,the present result suggests T2EHDGA functions as a tridentate ligand, which is in good agreement with Sharma et al.[23]. From Fig.2,there are two molecules of T2EHDGA in one extracted species molecule.However,Panja et al.[30]reported a species of the type UO2(NO3)2·T2EHDGA(o)in n-dodecane with 30%iso-decanol as the phase modifier,which is in sharp contrast to the species reported by us.This is probably connected with the diluents that have a major impact on the extraction due to their polarity[31].Moreover,Panja et al mentioned that at higher nitric acid concentration the ligand dependence is close to 1,while at lower nitric acid concentration which is close to 2 with T2EHDGA[32]. Inthisexperiment,nitricacidconcentrationis controlled at 0.01 mol·dm-3,it is exactly at low nitric acid concentration range,and the result is consistent with that reported by Panja et al.[32].

The conditional equilibrium constant is

The value of Kexis calculated to be(2.18±0.13) mol-4·dm12.We had reported the extraction of UO22+with N,N,N′,N′-tetraoctyl diglycolamide under the same conditions and found that the value of Kexis (26.45±1.05)mol-4·dm12[33].These results show the steric hindrance of ethyl branched chain.

2.3 Temperature effect on the extraction of U(Ⅱ)

The effect of the temperature on the extraction equilibrium of uranium(Ⅱ)was studied at different temperatures(15～55℃),and shown in Fig.3.It wasfound the distribution ratio decreases with increasing temperature,which demonstrates that the extraction reaction is an exothermic process and it is disadvantageous for the extraction reaction at high temperature. The change in enthalpy,ΔH,associated with Eq.(1), evaluated by means of the Van′t Hoff equation[34],is -(30.0±1.27)kJ·mol-1.

Fig.3Effect of temperature on the extraction of UO22+

2.4 Effect of salting-out agent concentration

Fig.4 shows the influence of the salting-out agent (LiNO3and NaNO3)concentration on the distribution ratio of uranium(Ⅱ).The distribution ratio of uranium(Ⅱ) increases sharply with increasing LiNO3concentration. Here nitrate plays an important role not only as a salting-out agent which increases the LiNO3activity owing to hydration of cation but also as a homo-ion solution,shifting the extraction reaction equilibrium to the right[35].However,the distribution ratio changes little with NaNO3concentration.That is say,the extractant in LiNO3system shows a stronger extraction propensity to uranium(Ⅱ)than that in NaNO3solution. Thisbehavioroflithiumhassometimesbeen explained by invoking the small size and hydration properties of Li+[36].

Fig.4Effect of salting-out agent concentration on the extraction of

2.5 IR spectra of extracted species

The IR spectra of the organic phase loading extracted species(Fig.5)have been studied to give insight into the structure of the extracted species.The wave number of the stretching frequency of C=O band was shifted to lower numbers:1 658 to 1 608 cm-1. Sasaki reported that the C=O band of TODGA was shifted to 1 614 cm-1after complexation with Eu(Ⅱ)[37]. The IR stretching frequency of the ether group was shifted from 1 117 to 1 130 cm-1.The above result indicates that the three oxygen atoms of T2EHDGA molecule are all coordinated with UO22+ion in the extractedspecies,inotherwords,T2EHDGAis tridentate ligand.The experimental bands assigned to N-O asymmetric and symmetric stretching modes of the coordinated nitrate anion are observed at 1 275 and 1 024 cm-1.The band at 932 cm-1is correspond to asymmetric O-U-O uranyl stretching vibration,which is similar to the IR data of U(Ⅱ)nitrate monoamides complexes[38].It is worthy to note that there′s a strong band at 1 352 cm-1.The non-metal bonded nitrate ions is of D3hsymmetry and are easily recognized by the very strong band usually found at 1 380～1 350 cm-1[39-40]. Then the band at 1 352 cm-1is assigned to ν3of not coordinated.

Fig.5IR spectra of T2EHDGA and loading U(Ⅱ)

3 Conclusions

Uranium(Ⅱ)can be extracted effectively from nitric acid medium by T2EHDGA in sulfonated kerosene.The values of distribution ratio UO22+increases with increasing concentration of nitric acid,extractant and salting-out agent.The stoichiometry of the extracted species is confirmed to be UO2(NO3)2·2T2EHDGA and the value of Kexis calculated to be(2.18±0.13)mol-4· dm12.In addition,the extraction reaction is exothermic and it is disadvantageous for the extraction reaction at high temperature.The extractant in LiNO3system shows a stronger extraction propensity to uranium(Ⅱ) than that in NaNO3solution.

Acknowledgements:The authors are grateful for the support from the National Nature Science Foundation of China (Grant No.21171069 and 21077044).

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磺化煤油為稀釋劑N，N，N′，N′-四(2-乙基己基)-3-氧戊二酰胺從硝酸中萃取U(Ⅱ)

蔡緒燦崔玉楊小鳳李業(yè)新孫國(guó)新＊
(濟(jì)南大學(xué)化學(xué)化工學(xué)院，濟(jì)南250022)

本文研究了以磺化煤油為稀釋劑，N，N，N′，N′-四(2-乙基己基)-3-氧戊二酰胺(T2EHDGA)從硝酸中對(duì)U(Ⅱ)的萃取性能?？疾炝薍NO3濃度、T2EHDGA濃度、鹽析劑濃度及溫度對(duì)萃取性能的影響。該萃取過程為一放熱過程，在所研究的條件下沒有三相的形成。給出了萃取機(jī)理，確定由2個(gè)萃取劑分子參與U(Ⅱ)配位，其萃合物組成為UO2(NO3)2·2T2EHDGA。通過紅外光譜確定了由羰基及醚氧鍵參與配位。

萃?。烩?Ⅱ)；N，N，N′，N′-四(2-乙基己基)-3-氧戊二酰胺；磺化煤油

O615.11

A

1001-4861(2016)06-1089-06

2015-11-23。收修改稿日期：2016-02-20。

10.11862/CJIC.2016.140

國(guó)家自然科學(xué)基金(No.21171069，21077044)資助項(xiàng)目。

＊通信聯(lián)系人。E-mail：sun-guo-xin@hotmail.com