Environ. Sci. Technol. 2009, 43, 1947–1951
Ionic Liquids for Simultaneous Preconcentration of Some Lanthanoids Using Dispersive Liquid-Liquid Microextraction Technique in Uranium Dioxide Powder M O H A M M A D H . M A L L A H , †,‡ F A R Z A N E H S H E M I R A N I , * ,† A N D MOHAMMAD G. MARAGHEH‡ Department of Analytical Chemistry, Faculty of Chemistry, Group of Science, University of Tehran, P.O. Box 14155-64555, Tehran, Iran, and Jaber Ibn Hayan Research Laboratories, Nuclear Fuel Cycle Research School, Nuclear Science & Technology Research Institute, Atomic Energy Organization of Iran, End of North Karegar Ave. P.O. Box 14395-836, Tehran, Iran
Received October 29, 2008. Revised manuscript received December 23, 2008. Accepted January 14, 2009.
Ionic liquids in a dispersive liquid-liquid microextraction technique were used for determination of lanthanoids such as samarium, europium, gadolinium, and dysprosium in uranium dioxide powder. In this process, an appropriate mixture of extraction solvent and disperser is rapidly injected into an aqueous sample containing samarium, europium, gadolinium, and dysprosium ions complexes with 1-hydroxy-2, 5-pyrrolidinedione, and consequently a cloudy solution is formed. It consists of fine droplets of extraction solvent which are dispersed entirely into the aqueous phase. After centrifugation of this solution, the whole enriched phase was determined by inductively coupled plasma optical emission spectrometry. In the present work, the preconcentration factor, limit of detection, and relative standard deviation were investigated for samarium, europium, gadolinium, and dysprosium in uranium dioxide powder.
Introduction The continued use of organic solvents as liquid media for chemical reaction, extraction, and formulation is a major concern in today’s chemical processing industry. The perceived deleterious effects of these materials on human health, safety, and the environment combined with their volatility and flammability has to increase pressure for minimizing their use from both a public relations and a cost perspective. Ionic liquids as new solvents are recently interesting for many researchers. Some of the properties which make the ionic liquids attractive as media for various applications are the wide liquid range, nonvolatility (negligible vapor pressure), nonflammable nature, lower reactivity, and strong ability to dissolve a large variety of organic and inorganic substances including even polymer materials in high concentration. Many of these properties have made ionic liquids a nature* Corresponding author fax: +982188221113; e-mail: shemiran@ khayam.ut.ac.ir. † University of Tehran. ‡ Atomic Energy Organization of Iran. 10.1021/es8030566 CCC: $40.75
Published on Web 02/12/2009
2009 American Chemical Society
friendly “green solvent” that can be applied in liquid-liquid solvent extraction processes for the separation of metal ions (1-6). In recent years, the lanthanoids, often called rare earth elements (REE), have widely been used in high technology applications, such as in superconductors, lasers, and other products in industry. The amounts of the used lanthanoids have increased considerably in the modern societies. Consequently, the emission of lanthanoids into the environment has also been increasing. These circumstances have resulted in an increase in our exposure to lanthanoids and an increase in our dietary intake of lanthanoids (7). Under these circumstances, the concentration of lanthanides may be increasing even in natural waters; hence, monitoring techniques for lanthanoids such as samarium (Sm), europium (Eu), gadolinium (Gd), and dysprosium (Dy) are required for environmental protection. Furthermore, the analysis and measurement of trace lanthanoids has been increased in order to elucidate their biological functions and toxicities (8, 9). The quantification of these lanthanoids is extremely difficult because they are a group of closely related elements having similar physical and chemical properties. Neutron activation analysis (NAA), inductively coupled plasma mass spectrometry (ICP-MS), X-ray fluorescence (XRF), spectrophotometers with arsenazo III and II, and inductively coupled plasma optical emission spectrometers (ICP-OES) are techniques allowing the individual determination of these lanthanoids with good sensitivity by applying a sample preparation method as a prior step to its determination (10-13). Recently, these methods have been extended to include novel and powerful supercritical fluid (SCF) extraction, SCF chromatographic, and new combined dispersive liquid-liquid microextraction (DLLME) method with ICP-OES. In this method an appropriate mixture of extraction and disperser solvent is injected into the aqueous sample, which forms a cloudy solution. The cloudy state results from the formation of fine droplets of the extraction solvent which are dispersed in the sample solution. The cloudy solution can be centrifuged, and fine droplets are sedimented at the bottom of a conical test tube. Determination of analytes in the remaining phase can be performed by instrumental techniques. In this extraction method, every component in the solution, directly or indirectly after derivatization, interacts with the fine droplets of the extraction solvent and is consequently extracted from the initial solution and concentrated to a small volume in the remained phase. The performance of DLLME was illustrated with the determination of polycyclic aromatic hydrocarbons (PAHS), organphosphorus pesticides, chlorobenzenes, trihalomethanes, chlorophenols, and metals ions in aqueous samples (14-21). The authors intended to eliminate extraction organic solvent in DLLME to enhance the level of environmental protection and equivalent boron content calculation of Sm, Eu, Gd, and Dy (Figure 1) using ICP-OES. Therefore, in this method ionic liquids were used instead of the organic solvent, using the 1-hydroxy-2, 5-pyrrolidinedione (HYD) ligand which showed a significant increase in the preconcentration factor of Sm, Gd, and Dy ions, not Eu, in comparison with organic solvents.
Experimental Section Ionic liquids such as 1-butyl-3-methylimidazolium hexafluorophosphate (1-B) and 1-hexyl-3-methylimidazolium hexafluorophosphate (1-H), methanol (for spectroscopy), HNO3 (65%, suprapur), uranium dioxide, sodium dodecyl sulfate VOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Chemical structure of 1-hydroxy-2, 5-pyrrolidinedione (HYD).
TABLE 1. Instrumental Parameters of ICP-OES parameters
values
RF generator power (W) plasma gas flow rate (L min-1) auxiliary gas flow rate (L min-1) nebulizer pressure (kPa) torch mode (minitorch) pump flow rate (mL min-1) wavelength of samarium (nm) wavelength of europium (nm) wavelength of gadolinium (nm) wavelength of dysprosium (nm)
1100 15 0.8 200 axial 1 359.160 381.967 376.838 353.170
(SDS), lanthanoids (in oxide form) and 1-hydroxy-2, 5-pyrrolidinedione (HYD) (analytical grade), di-natriumtetraborate-10-hydrate, Cadmium-acetate-2-hydrate were purchased from Merck Co. (Germany). The stock solutions of samarium, europium, gadolinium, and dysprosium were prepared at a concentration of 5 × 10-2 mol/L of ions by dissolving an appropriate amount of Sm, Gd, and Dy oxides in 20 mL of 12.8 mol/L HNO3 solution, and the solutions were then evaporated carefully at low temperature (not above 60 °C) to dry. The residues were dissolved in 26.4 mL of 7.6 mol/L nitric acid solution and diluted to 200 mL. All solutions were prepared using double-distilled water. An aqueous ammonium solution (1% w/w) was used for adjusting the pH of a 10-3 mmol/mL solution of 1-hydroxy2, 5-pyrolidinedione salt. All vessels used for trace analysis were kept in a 1 mol/L HNO3 solution at least 24 h and subsequently washed twice with double-distilled water before using. A simultaneous inductively coupled plasma optical emission spectrometer (Optima 2100 DV) equipped with a minitorch and equipped with a segmented-array chargecoupled device detector and peristaltic pump was used. The operating conditions and analytical wavelength are summarized in Table 1. The pH values were measured with a Schoct pH-meter (CG 841) supplied with a glass-combined electrode. Phase separation was assisted using a centrifuge
FIGURE 2. Relationship between preconcentration factor and pH (Cen: 3500 rpm, methanol: 8 mL, Ex: 1-hexyl-3-methylimidazolium hexafluorophosphate (600 µL), t: 6 min, HYD: 1 mL, T: 160 ((5) °C, surfactant: 0.02% w/v SDS). 1948
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FIGURE 3. Relationship between preconcentration factor and extraction solvent type (Cen: 3500 rpm, methanol: 8 mL, Ex: 600 µL, t: 6 min, pH: 9.5, HYD: 1 mL, T: 160 ((5) °C, surfactant: 0.02% w/v SDS).
FIGURE 4. Relationship between preconcentration factor and extraction solvent volume (Cen: 3500 rpm, methanol:8 mL, Ex: 1-hexyl-3-methylimidazolium hexafluorophosphate, t: 6 min, pH: 9.5, HYD: 1 mL, T: 160 ((5) °C, surfactant: 0.02% w/v SDS).
FIGURE 5. Relationship between preconcentration factor and ligand volume of HYD with 10-3 mol L-1 (Cen: 3500 rpm, methanol: 8 mL, Ex: 1-hexyl-3-methylimidazolium hexafluorophosphate (600 µL), t: 6 min, pH: 9.5, T: 160 ((5) °C, surfactant: 0.02% w/ v SDS). (Mistral 1000, MSB 100/CE 1.4). Meanwhile, a furnace (EHRET) was used for drying the precipitate phase. An 80 mL solution containing Sm, Eu, Gd, and Dy in equal amounts at a concentration of 5 × 10-5 mol/L ion, and 1 mol/L HNO3, the pH being maintained by an aqueous ammonium solution (0.1mol/L) and SDS 0.02 w/v, was added to a 100 mL screw-cap glass test tube with a conical bottom. Eight milliliters of methanol as disperser solvent, which contains 600 µL of ionic liquid as extraction solvent (Ex) and 1 mL solution of HYD 1 mmol/L as chelating agent, was rapidly injected into a sample solution. A cloudy solution was formed in the test tube. In this step, Sm, Eu, Gd, and Dy ions reacted with chelating reagent and were extracted into the fine droplets of the ionic liquid. The mixture was then centrifuged (Cen) for 6 min at 3500 rpm. After this process, the dispersed fine droplets of ionic liquid were precipitated at the bottom of the conical test tube (0.5 mL). This precipitate
FIGURE 6. Relationship between preconcentration factor and surfactant concentration (Cen: 3500 rpm, methanol: 8 mL, Ex: 1-hexyl-3-methylimidazolium hexafluorophosphate (600 µL), t: 6 min, pH: 9.5, HYD: 1 mL, T: 160 ((5) °C). phase which is usually mixed with drops of dispersive solvent or an excess amount of HYD was dried by a furnace in 160 ((5) °C. Then it was diluted to 0.5 mL by adding solution 1 mol/L HNO3. The sample was introduced into the inductively coupled plasma optical emission spectrometer by a peristaltic pump. This procedure was repeated for uranium dioxide using the same method.
FIGURE 7. Relationship between preconcentration factor and salt concentration (Cen:3500 rpm, methanol: 8 mL, Ex: 1-hexyl3-methylimidazolium hexafluorophosphate (600 µL), t: 6 min, pH: 9.5, HYD: 1 mL, T: 160 ((5) °C, surfactant: 0.02% w/v SDS).
Results and Discussion Preconcentration factor was calculated using the following equation: PF ) Cpp/C0 where PF, Cpp, and C0 are respectively preconcentration factor, concentration of the analyte in the precipitate phase, and initial concentration of the analyte in the aqueous sample. They were determined by ICP-OES. The analyte concentration in the precipitate phase was defined from the direct calibration curve. This study considered the application of ionic liquids in a dispersive liquid-liquid microextraction technique with a higher preconcentration factor rather than with organic solvents. In addition to the same effects, some other factors were reported in a previous paper, such as the type of dispersive solvent and volume, the extraction time, and the centrifuge speed (22). Nevertheless, there are some other factors which can affect the simultaneous preconcentration of samarium, europium, gadolinium, and dysprosium in the dispersive liquid-liquid microextraction technique when ionic liquids solvents are used as an extraction solvent and a new ligand of HYD. Therefore, the operating conditions should be optimized. For this reason, the one-variable-ata-time optimization was used. Figure 2 shows the effect of pH on the extraction of Sm, Eu, Gd, and Dy complexes. It can be seen that the preconcentration factor increased with the increase of pH up to 9.5. At lower pH values, the ligand is protonated. The preconcentration of metal ions by dispersive liquid-liquid microextraction involved a prior formation of a complex with sufficient hydrophobicity to be extracted into the small volume of the precipitate phase; thus, for obtaining the desired preconcentration, pH played a unique role in metal-ligand formation and subsequent extraction of Sm, Eu, Gd, and Dy ions. This parameter was fixed by aqueous ammonium solution (0.1 mol/L). The type of extraction solvent used in DLLME is an essential consideration for efficient simultaneous preconcentration. Generally, there are two requirements during selection of extraction solvent. First, it should have higher density than water. Second, it should have a low solubility in water and be nonvolatile to prevent solvent loss during
FIGURE 8. Relationship between preconcentration factor and drying temperatures of the samples (Cen: 3500 rpm, methanol: 8 mL, Ex: 1-hexyl-3-methylimidazolium hexafluorophosphate (600 µL), t: 6 min, pH: 9.5, HYD: 1 mL, surfactant: 0.02% w/v SDS). extraction. For this purpose, the performance of two kinds of ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate and 1-hexyl-3-methylimidazolium hexafluorophosphate) was compared to an organic solvent (carbon tetrachloride). A series of sample solutions was studied using 8 mL of methanol containing 1 mL of HYD and different types and volumes of the extraction solvent to achieve 0.5 mL of the precipitate phase. The solubility of the extraction solvents in water was different. Therefore, for obtaining 0.5 mL of the precipitate phase at the bottom of the test tube, it was necessary to add an excess amount to account the solubility. According to Figures 3 and 4, 600 µL of 1-hexyl3-methylimidazolium hexafluorophosphate showed the best result. It can be found that the ionic liquids with a longer chain improved the preconcentration factor in the DLLME technique. It also depends on the cation type in the ionic liquids. The effect of the chelating agent volume (10-3 mol/L) of HYD on preconcentration factor of Sm, Eu, Gd, and Dy ions was also studied. As can be seen in Figure 5, the preconcentration factor of Sm, Eu, Gd, and Dy ions increased up to a known volume of HYD and then decreased. Therefore, a volume of 1 mL was chosen for the subsequent experiments. After the centrifugation step in DLLME, a precipitate of ionic liquid solvents was observed on the test tube walls which caused a loss of repeatability and reduced precision in the measurements. To solve this problem, a surfactant was used because this material surrounds ionic liquid fine droplets and reduces its adhesiveness. The preconcentration factors of Sm, Eu, Gd, and Dy ions as a function of the concentration of three kinds of the surfactants, that is, Triton X-114, Triton X-11, and SDS, in the range of 0-0.09% (w/v) were investigated. The results showed that SDS was the most effective surfactant. As can be seen in Figure 6, the preconVOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Optimum Operating Conditions operating conditions
results
type and amount of extraction solvent
type and amount of extraction solvent
1-hexyl-3-methylimidazolium hexafluorophosphate (600 ( 0.001 µL) Methanol (8 ( 0.001 mL)
centration factor slightly increased with an increase in the concentration of the surfactant and reached a maximum at a concentration of 0.02% (w/v) SDS due to the enhancement of extraction and then remained almost constant over the selected range. This value was employed as the optimum concentration for the subsequent experiments. Salt addition is frequently used to adjust the ionic strength. It improves the extraction efficiency and reduces the limit of detection. The effect of the ionic strength on the preconcentration factor was tested in the concentration range of 0-0.5 mol/L NaNO3. As observed in Figure 7, the salt addition did not have a significant effect on the preconcentration factor of Sm, Eu, Gd, and Dy ions. On this basis, all the extraction experiments were carried out without salt addition. According to Figure 8, the preconcentration factor increased with an increase in the drying temperature of the samples and reached a constant value which was selected as drying temperature of the samples (up to 150 °C). At this temperature, the volume of ionic liquid solvent in samples decreased and its adverse effects on the instrument measurement was also eliminated. Also, the effect of various ions on the preconcentration factor of Sm, Eu, Gd, and Dy ions has been investigated. Among the ions tested B3+ and Cd2+ were important in that they did not show interference at the concentration 50 times higher than that of the Sm, Eu, Gd, and Dy concentration. A calibration curve was obtained by simultaneous preconcentration of 80 mL of sample standard solutions containing known amounts of Sm, Eu, Gd, and Dy ions under optimum conditions. The preconcentration factor is defined as the ratio of the calibration curves slopes with or without preconcentration. To confirm the optimum method, the process evaluation was carried out by establishing the basic analytical requirements of the performance which was appropriate for quantitative determination of Sm, Eu, Gd, and Dy in aqueous samples. The limit of detection (LOD) was calculated as the lowest concentration required for producing a signal which is three times higher than the standard deviation of a matrix blank signal. Eight replicated analyses of the aqueous sample were previously chosen without any Sm, Eu, Gd, and Dy, which were used for estimating the matrix blank signal standard deviation. In addition, eight replicated measurements of a sample containing of 200 µg L-1, 20 µg L-1, 100 µg L-1, and 50 µg L-1 Sm, Eu, Gd, and Dy were used for the calculation of relative standard deviation (RSD). The optimum conditions of extraction are listed in Table 2. Furthermore, the obtained values of preconcentration factor, LOD, and RSD for Sm, Eu, Gd, and Dy ions by ionic liquid solvent of 1-hexyl-3-methylimidazolium hexafluorophosphate with HYD reagent are presented in Table 3. This study presents the DLLME technique that results under the best operating conditions. ICP-OES results show that the best operating condition for DLLME was suitable for the simultaneous preconcentration of Sm, Eu, Gd, and Dy with ionic liquid solvents using a HYD reagent. This result suggests that the hexafluorophosphate anions with a partial ion exchange mechanism in ionic liquid solvents play a key role in the electrical neutralization of the lanthanoid complex 1950
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volume of the chelating reagent centrifuge drying temperature extraction time (HYD) speed of the samples
6 min
1 ( 0.001 mL
3500 rpm
160 ((5) °C
TABLE 3. Preconcentration Factor, LOD, and RSD target ions
PF
LOD (µg L-1)
RSD (%)
Sm Eu Gd Dy
84.04 19.34 86.04 67.92
1.29 0.66 0.59 0.34
2 0.8 2 1.5
in DLLME. This is the first report to successfully demonstrate application of ionic liquids in a DLLME method for determination of Sm, Eu, Gd, and Dy in uranium dioxide powders. Therefore, it can be referred as a new advanced technique for practical determination of equivalent boron contents of nuclear materials.
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