Abstract
A rapid, sensitive, and selective method for determination of thorium based on the complex with ortho-ester tetra-azophenylcalixarene (TEAC) was described. In the presence of pH of 4–6, TEAC-Th(IV) complex is extracted from an acidic aqueous solution into chloroform layer. The absorbance intensity of complex was measured by UV-Vis spectrometer at 525 nm and the molar absorptivity was found to be 2.4 × 104. Beer’s law was obeyed in the range of 1.0 to 25 × 10−5 M thorium(IV). The effects of pH, TEAC concentration, and shaking time were also studied. The tolerance limits for several metal ions were calculated. The proposed method was applied to the determination of thorium in synthetic solution and in the monazite sand samples with good results.
1. Introduction
Thorium is a natural radioactive element which has been paid more attention for energy demands and environment concerns. The main source of thorium is monazite sand mainly associated with a small amount of uranium and other rare earth metals, which are undesirable because of their radioactivity [1, 2]. Therefore, it is important to separate thorium to avoid the environment pollution before monazite processing. Separation of toxic Th(IV) ions is of intense current interest in research and environmental cleanup [3, 4]. The traditional preconcentration and separation methods for Th(IV) are liquid-liquid extraction, coprecipitation, liquid membrane and ion-exchange resins, and so forth [5–8]. These methods often require large amounts of high purity organic solvents, some of which are harmful to health and cause environmental problems. In this case, the solvent extraction processes were the most interesting technique for separation and recovery of thorium [9]. Various extractants have been employed for the extraction of thorium and rare earths, such as organophosphorus reagents [10, 11], amines [12–14], isoxazolones, crown ethers [15], fatty acid [16], and ionic liquids [17, 18].
Calixarenes have attracted much attention due to their distinctive structures. Calixarene was extensively used for solvent extraction of transition metals, lanthanides and actinides [19–22]. The most prominent research belonged to Delegoz team who investigated the ability extraction of azocalixarenes for solvent extraction of various transition metals from the aqueous phase to the organic phase [23–25]. Oueslati and coworkers have synthesized four azocalixarene derivatives bearing bipyridyl units with functional groups such as amide, ester, and pyridine at the phenolic oxygen atoms [26]. The extraction properties towards different metals of these azocalixarenes derivatives using liquid-liquid extraction by atomic absorption spectrometry have been studied and exhibited high selectivity for Ag+. Calixarene semicarbazone derivatives of resin showed the good separating ability with maximum sorption pH between 2.5 and 4.5 for thorium(IV) which was reported recently. The procedure was applied for determination of thorium in monazite sand and some standard geological materials [27]. To the best of our knowledge, however, the TEAC has not been reported as an extractant for extraction of thorium. In this study, we reported the novel method for solvent extraction of thorium using 5,11,17,23-tetra[(2-ethyl acetoethoxyphenyl)(azo)phenyl]calixarene(TEAC) as an extractant. The effects of various experimental parameters such as pH, contact time, and extractant concentration were investigated.
2. Experimental Section
2.1. Reagents and Instruments
All chemicals and solvents were of analytical grade and used without further purification unless otherwise mentioned. Double distilled water was degasified before experiment. The preparation and characterization of 5,11,17,23-tetra[(2-ethyl acetoethoxyphenyl)(azo)phenyl]calixarene (TEAC) was reported elsewhere [28, 29]. The structure of TEAC is shown in Figure 1. Certified reference materials (soil: IAEA-SOIL-7, sediment IAEA-SL-1) have been used for the standardization of this study.
Standard solutions of TEAC 10−3 M and Th(IV) 10−3 M were prepared by dissolving TEAC in CH3OH and Th(NO3)4 in double distillated water. All work solutions were obtained by appropriate dilution of the standardized stock solutions. pH values of the solutions were adjusted by the addition of HNO3 and NaOH solutions. Perkin Elmer’s Lambda 25 UV/Vis Scanning Spectrophotometer was used to record the absorbance spectra with 1.0 cm path length quartz cell.
Real samples (monazite) were collected from Ha Tinh, Binh Dinh, and Phu Yen Provinces, Vietnam. The monazite samples were dried at 105°C until the weight reached a constant value; they were ground in blender and kept in the clean polyethylene containers for elemental analysis. 0.5 g of monazite and 20 mL of concentrated HF and H2SO4 solution were placed in a platinum crucible and then evaporated. The residue was dissolved with 0.1 N HNO3 and diluted with double distillated water to 25 mL by using volumetric flask. The concentration of thorium in sample was determined as below.
2.2. General Procedure
To a solution of 2 mL Th(IV) 10−4 M in aqueous solution and 5 mL 5 × 10−4 M TEAC in MeOH was added 5 mL acetate buffer solution. The pH of the resulting solution was adjusted to pH 4/5 by HNO3 and NaOH solutions. The complex solution was diluted by adding CH3OH + H2O (7 : 3 v/v) to a final volume of 25 mL and mixed with 5 mL organic solution and then shaken for 5 min at 25°C. After splitting of organic and aqueous phases, the red-coloured organic phase was separated and 1 g of anhydrous Na2SO4 was added to remove contaminated water. The content of Th(IV) was determined spectrophotometrically at 525 nm using TEAC as the reference. The extraction percentage () and distribution ratios () were calculated as follows:where is the initial metal ion concentration in the aqueous phase before extraction and is the equilibrium concentration of metal ion in the aqueous phase after extraction, is volume of the aqueous phase, is volume of the solvent extraction, and is determined by INAA method.
3. Results and Discussion
3.1. Extraction of Th(IV) with TEAC
The preparation and characterization of 5,11,17,23-tetra[(2-ethyl acetoethoxyphenyl)(azo)phenyl]calixarene (TEAC) was reported elsewhere. In the previous works, the complex of TEAC with Th(IV) and its property were also investigated [30]. The FT-IR of TEAC and complex of TEAC with thorium exhibited 1512 cm−1 and 1639 cm−1 for N=N groups and –C=N, respectively. The new bands at 1639 cm−1 (–C=N) and 576 cm−1 (Th-N) provided the evidence for the bonding of nitrogen to the metal ion at the azo region. The 1H NMR spectrum of TEAC-Th(IV) showed a small downfield shift in the δ values of the peaks that occurred in the aromatic region and the signal of protons in –OH groups disappeared. Additionally, a new peak which appeared at 405 cm−1 was attributed to the bonds of Th(IV) with nitrogen atoms of TEAC in the Raman spectra of TEAC-Th(IV). The ESI-MS of TEAC-Th(IV) was recorded in solution and showed the fragment ion TEAC-Th () in the MS spectra.
The complexation ratio between the TEAC reagent and metal ions was carried out by varying the concentration of both host and metal ions by Job’s plot experiment. The maximum absorbance was observed at a mole fraction of [TEAC]/([TEAC]+[Th(IV)]) of about 0.5 corresponding to a TEAC-Th(IV) complex ratio of 1 : 1. The stability constant was estimated by monitoring the decrease in the intensity of the absorbance at the peak with the data reduction being effected using Benesi-Hildebrand plots and the stability constant () was calculated to be 6.2 × 104 mol−1·L by this method.
Different types of solvents were examined (toluene, isoamyl alcohol, benzene, carbon tetrachloride, and chloroform). Absorption spectra and the extraction efficiency were demonstrated in Figure 2 and Table 1. The absorption peak of TEAC is at 385 nm, corresponding to transition of the –N=N– bond, whereas the absorption peak of the TEAC-Th(IV) complex is located at 525 nm for chloroform, which is in accordance with typical diazo spectra [28]. However, for methanol and a mixture of methanol : water, the maximum absorption peak appeared at 520 nm; the difference in maximum absorption may be explained by effect of the polar solvent. Additionally, the extracted species was stable for several hours. Results showed that the optimal solvent for the extraction of thorium is chloroform. Therefore, chloroform was selected for further studies. In this work, the extraction of TEAC-Th(IV) complex was examined and the spectrophotometric determination procedure of thorium after extraction process was proposed.
3.2. Effect of pH on Extraction
The optimum pH for extraction of Th(IV) was determined by carrying out the extraction with pH in the range of 1 to 10. As can be seen in Figures 3 and 4, the extraction (%) and the distribution ratio of Th(IV) reached maxima at the pH of 4–6. The lower extraction at higher pH values may be attributed to the hydrolysis of the Th(IV) ions which indirectly promotes competing equilibrium with the formation of complex. The pH range of 4–6 is recommended to control pH, as the use of sodium acetate and acetic acid buffer solution was found to give a maximum and constant absorbance.
3.3. Effect of Contact Time and Stability
To determine the equilibrium time, effect of contact time on the distribution ratios was evaluated from extraction of TEAC-Th(IV) with chloroform. The shaking time for the extraction was varied from 30 to 350 s. The highest distribution ratios for extraction of TEAC-Th(IV) with chloroform were observed within 60 s at °C. Therefore, to ensure the complete equilibrium, the contact time was chosen as 60 s in all further extraction experiments. Additionally the results also indicated that the absorbance of TEAC-Th(IV) complex in chloroform is stable for at least 5 h at wavelength of 525 nm.
3.4. Effect of Extractant Concentration
The effects of extractant concentration on the extraction of Th4+ were investigated with varying the concentration of TEAC within 0.1–20 × 10−5 M. As shown in Figure 5, the extraction (%) of thorium ions increased with rising TEAC concentration from 0.1 to 2.5 × 10−5 M. However the extraction was constant when the TEAC concentration was higher than 2.5 × 10−5 M. The maximum extraction efficiency of thorium ions occurred at extractant concentration of 2.5 × 10−5 M and reached 98%.
3.5. Calibration Curve
A calibration curve for the determination of thorium was prepared under optimum conditions. A good linear relationship was obtained over the concentration range of 1 × 10−6 to 2.5 × 10−5 M of Th(IV) ions per 5 mL of chloroform. The molar absorption coefficients of the extracted species were approximately 2.8 × 104 at 525 nm. This value is about 1.2 times as high as that obtained by direct reaction in aqueous solution [30].
3.6. Effect of Foreign Ions
In order to study the effect of various ions on the determination of the TEAC-Th(IV) complex, a fixed amount 2 × 10−5 M of Th(IV) was taken with different amounts of foreign ions and the recommended procedure was above. The tolerance limits are summarized in Table 2. The results showed that most common ions do not interfere with the determination suggesting the high selectivity of the proposed method. Only some elements interfered which can be eliminated by masking with 1,10-phenanthroline and EDTA solutions.
3.7. Application of Synthetic Mixtures
The proposed method has been successfully applied to the determination of Th(IV) in the synthetic mixture. The analytical results for the synthetic mixture are given in Table 3. Results indicated a clear separation of Th(IV) ions from Al3+, Ni2+, Hf4+, Zr4+, , Ce4+, and Fe3+ ions and left these metal ions in the aqueous phase. After extraction of Th(IV) ions, the concentrations of Al3+, Ni2+, Hf4+, Zr4+, , Ce4+, and Fe3+ ions were determined by INAA.
3.8. Application of the Method to Standard Samples and Real Samples
3.8.1. Standard Samples
Two standard samples SOIL-7 and SL-1 were also analyzed by this proposed method. To prepare a solution of these samples, 0.5 g of the sample was weighed and dissolved in concentrated HNO3 and HClO4. The solution was evaporated nearly to dryness, extracted with 10 mL of 0.05 M HCl, filtered, and diluted to 100 mL with distilled water. An aliquot of this solution was analyzed for Th(IV) ions by the proposed method. The result was shown in Table 4. The results showed that certificated values and found values had good agreement. The difference of the proposed method and INAA method is not significant (Fisher standard). Therefore, the proposed method presents the potential application for determination of thorium ions in real samples.
3.8.2. Real Samples
The results of Th(IV) ions estimated by this proposed method were compared with that found by previous method [30]. Additionally, as shown in Table 5, the results were in agreement with those determined by neutron activation analysis method (INAA). Therefore, the proposed method provides a simple, rapid, and selective determination for thorium ions in real samples.
4. Conclusion
The extraction of Th(IV) ions using 5,11,17,23-tetra[(2-ethyl acetoethoxyphenyl)(azo)phenyl]calixarene at pH of 4–6 was studied. The results showed that the extraction efficiency was 98% and the complex of TEAC-Th(IV) was stable. The foreign ions had no significant effect on the extraction of thorium ions. The results of this study clearly showed the potential and versatility of calixarene derivative for separation of Th(IV) from rare earths. This method also increased the sensitivity of the analysis and can be applied to the analysis of thorium ions in real samples.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Authors’ Contribution
The authors have contributed equally to this work.
Acknowledgments
The research is funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under Grant no 104.07-2012.47. The authors also appreciate Professor J. S. Kim from Korea University and Professor H. S. Kim from Seoul National University for their help on TEAC preparations.