Cui Yu1,2, Sun Sixiu, Zhang ZhenweiSun Guoxin
Collage of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100 ; Institute of Chemistry and Chemical Engineering, Jinan University, Jinan, 250022)
Received Feb.16, 2004; Support from the National Natural Science Foundation of China (20301008) and Natural Science Foundation of Shandong Province(Q2003B01)
AbstractExtraction behavior of N,N,N’,N’-tetrabutylmalonamide (abbr.TBMA ) employing toluene as diluent toward Sm(III) and Er(III) from nitrate media has been investigated. The effects of temperature as well as the concentrations of nitric acid, lithium nitrate and extractant on the extraction distribution ratio have been studied. The stoichiometries of the extracted species of Sm(III) and Er(III) were conformed to be Sm(NO· 3TBMA and Er(NO·3TBMA, respectively. The values ofH for the extraction of Sm(III) and Er(III) with TBMA in toluene are -36.31 and-26.71kJ/mol, respectively. The infrared spectrum of the organic phase of TBMA loaded Er(NO was recorded and discussed.
Keywords N,N,N’,N-tetrabutylmalonamide, extraction, Er(III), Sm(III)
The utilization of substituted amides in extraction processes with special regards to their application in the extraction of actinides and some other metals of economic or strategic value, has drawn the attention of separation chemistry in recent years. In addition to their improved extraction behavior toward actinides and fission products, these extractants offer distinct advantages over TBP especially with respect to their complete incinerability (which helps in reducing the bulk of secondary nuclear waste), and the innocuous nature of their radiolytic degradation products (mainly carboxylic acids and amines) that can easily be washed out(1-4).
The separation of the minor actinides from the high level effluent from spent fuel reprocessing wastes is not a simple task, particularly for the trivalent actinides (An(III)), Am(III) and Cm(III), which exhibit chemical properties very similar to those of the trivalent lanthanides (Ln(III)) also present in the wastes, and which account for about one third of the total fission product inventory. There have been several reports on the extraction behavior of Am(III) using malonamides. However, the solvent extraction of the trivalent rare-earth metals (Ln(III), here used to include the lanthanides and yttrium) from nitrate solutions by amide extractants has not been extensively reported in the literature(5-9). Our contribution was aimed at clearly identifying the extraction regime and the extractability of amidic extractants for Ln(III). In the present paper, studies have been carried out to investigate their extraction behavior towards Sm(III) and Er(III) under varying experimental conditions. Stoichiometry of the extracted species has also been evaluated.
TBMA was synthesized by the method reported in our previous paper(10). The other reagents were all AR grade. The initial aqueous solutions of Sm(III) or Er(III) were prepared as described elsewhere(10).
2.2 Solvent extraction procedure
In distribution studies, solutions of desired concentrations of TBMA were prepared in toluene. Equal volumes of the organic and aqueous phases were agitated for 40 min (enough for the attainment of equilibrium) at 25ºC under the desired experimental conditions. The two phases were then centrifuged and assayed by taking known aliquots (0.05–0.1 mL) from the aqueous phases. The concentration of the sample was determined using Arsenazo-III spectrophotometric method and the amount in organic phase was obtained by subtracting the aqueous concentration from the initial aqueous concentration of Sm(III) or Er(III). The distribution ratio (D) was calculated as the ratio of the concentration in the organic phase to that in aqueous phase.
2.3 Measurement of infrared spectra
Equal volumes of 0.5 mol·dm-3 TBMA in toluene and 0.2 mol·dm-3 Er(III) in aqeous solution were agitated for 40min, assayed and then two drops of the organic solution were deposited on KBr flake. The diluent was evaporated before IR recording to eliminate the effect of toluene. FTIR spectra were recorded on an FTS-165 Spectrometer in the range 400-4000cm-1 using air as the background. The scan times were 50 and the resolution is 2 cm-1
3 RESULTS AND DISCUSSION
3.1 Effect of nitric acid concentration on the extraction of Sm(III) and Er(III)
The extraction of Sm(III) or Er(III) from nitric acid solution in the range of 1.0-8.0 mol·dm-3 has been investigated. The results show that the extractability of TBMA for Sm(III) or Er(III) from nitric acid solutions is very poor (D<0.01). When D was plotted against the aqueous nitric acid concentration in the presence of 5.00 mol·dm-3 LiNO, a maximum (D=7.20 for Sm(III) and D=2.85 for Er(III)) was noticed at around=0.50 mol·dm-3 (Fig.1). The decrease in D at higher acid concentrations is attributed to the competition between nitric acid and metal ion for amide (2). All further extraction studies were carried out at 0.50 mol·dm-3 HNO
Fig.1 Effect of Nitric Acid Concentration on Extraction Distribution Ratio of Ln(III)
=5.00 mol·dm-3, =5.00×10-3 mol·dm-3, CTBMA=0.50 mol·dm-3
3.2 Effect of lithium nitrate concentration on extraction of Sm(III) and Er(III)
Fig.2 shows the effect of lithium nitrate concentration on the extraction distribution ratio of Sm(III) or Er(III). The distribution ratio of Sm(III) or Er(III) increases apparently with the increase of LiNO concentration in which the coions effect plays a crucial role.
Fig.2 Effect of Lithium Nitrate Concentration on Extraction Distribution Ratio of Ln(III)
=0.50 mol·dm-3, =5.00×10-3 mol·dm-3, CTBMA=0.50 mol·dm-3
3.3 Effect of TBMA concentration on extraction distribution ratio of Sm(III) and Er(III)
In order to ascertain the nature of the extracted species, the D valuese for both Sm(III) and Er(III) were detrermined as a function of TBMA concentration. The result was shown in Fig.3.
Fig.3 Effect of TBMA concentration on the extraction of Ln(III)
=0.50mol.dm-3, =5.00×10-3mol.dm-3, =5.00mol.dm-3
The plots of logD vs.logCTBMA gave straight lines having slopes of 3.21 and 3.26 for Er(III) and Sm(III), respectively. This shows that Sm(III) and Er(III) are extracted as trisolvate and the stoichiometry of extracted species are Sm(NO·3TBMA and Er(NO·3TBMA, respectively. Then the extraction reaction can be represented by
Ln3++3NO+3TBMA= Ln(NO·3TBMA (1)
where Ln is Er or Sm, subscript (o) refers to the species in the organic phase. The eqilibrium constant, Kex, is
The total Ln3+ concentration in aqeous solution, , can be defined as
where is the stability constant between Ln3+ and NO, the othervalues are so small that may be neglected. The distribution ratio D can be expressed as
The values of activity coefficients under the studied conditions was considered to be constant, we have
where=0.74 for Sm(III) amd 0.11 for Er(III), respectively(11). The values of K, calculated from equation (6), are 1.93±0.56 mol-6.dm for Sm(III) and 0.22±0.06mol-6.dm for Er(III), respectively.
3.4 Distribution studies as a function of temperature
The effect of temperature on the extraction equilibrium is shown in Fig.4 which indicates that the distribution decreases with the increase in the temperature.
Fig. 4 Effect of temperature on the extraction equilibrium
log D was plotted as a founction of 1/T and straight lines were obtained in the temperature range studied. The data were subjected to least-squares analysis using a computer program. From the values of the slopes obtained, the change in enthalpy,H in each case was evaluated using Van’t Hoff’s equation:
The values ofH for the extraction of Sm(III) or Er(III) with TBMA in toluene, were -36.31 and -26.71kJ/mol, respectively.
3.5 IR spectra analysis
Infrared spectra of TBMA and the organic extracted species have been recorded to clarify the nature of the extracted complex. The IR spectra of pure TBMA and the organic phase loaded TBMA and Er(III) are shown in Fig.5 (a) and (b),respectively. The peak at 1030 cm-1 in (b) is attributed to N-O stretching vibrations of the coordinated nitrate groups. A band corresponding to NO stretching of free nitrate anions is observed at 1378.2 cm-1 (12). The results indicate the presence of coordinated nitrate anions and free nitrate anions in the extracted species molecules. We found that the 1650cm-1 peak decreases and shift to about 1620 cm-1 , which agrees with the data of extraction.
Fig.5 IR Spectra of TBMA (a) and loaded Organic Solution(TBMA/Er(III) =50/6 , mol/mol) (b)
Table 1 curve fitting result and(C=O) bands assignment
To elucidate the coordination of the C=O group in extracted species, we used a theoretical fitting of Gauss-Lorentzian curves to the experimental profile of(C=O) band in the range of 1720-1540 cm-1. The result was shown in Fig.6. The three complements that appear in Fig.6 can be assigned: the 1653.2 cm-1 band was assigned to(C=O) of the free TBMA molecules, the lower frequency bands were assigned to(C=O) the coordinated TBMA molecules. These assignments were shown in Table 1. The splitting of stretching vibrations of bidentate coordinated C=O groups is consistent with previously published data(13) but is still unexplained.
Fig.6 Theoretical Fitting of Gauss-Lorentzian Curves to the experimental profile of(C=O) band
Sm(III) and Er(III) can be effectively extracted by TBMA in the presence of LiNO. The extraction distribution of Ln(III) with TBMA in toluene at the experimental conditions is in the order of Sm(III) > Er(III). The stoichiometries of the extracted species of Sm(III) and Er(III) are conformed to be Sm(NO·3TBMA and Er(NO·3TBMA, respectively. The values ofH for the extraction of Sm(III) or Er(III) with TBMA in toluene are -36.31 and -26.71kJ/mol respectively. The extraction decreases with the increase in the temperature. There are coordinated nitrate anions and free nitrate anions in the extracted species molecules.