ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2011, том 66, № 1, с. 13-17
ОРИГИНАЛЬНЫЕ СТАТЬИ
УДК 543
SOLID PHASE EXTRACTION OF TRACE AMOUNTS OF URANIUM(VI) FROM WATER SAMPLES USING 8-HYDROXYQUINOLINE IMMOBILIZED
ON SURFACTANT-COATED ALUMINA © 2011 г. M. Kazeraninejada, A. M. Haji Shabania, S. Dadfarniaa, S. H. Ahmadib
a Department of Chemistry, Yazd University 89195-741, Yazd, Iran
b Department of Analytical Chemistry, Chemistry and Chemical Engineering Research Center of Iran
14335-186, Tehran, Iran Received 28.12.2009; in final form 29.05.2010
A simple and reliable method has been developed for the determination of uranium(VI). The method is based on the separation and preconcentration of uranium(VI) using a column packed with 8-hydroxyquinoline immobilized on surfactant coated alumina prior to its spectrophotometric determination with arsenazo III. The effect of pH, sample flow rate and volume, elution conditions, and foreign ions on the sorption of urani-um(VI) has been investigated. A preconcentration factor of 200 was achieved by passing 1000 mL of sample through the column. The relative standard deviation for 10 replicate analyses at the 100 ng/mL level of ura-nium(VI) was 2.1% and the detection limit was 0.12 ng/mL. The method was successfully applied to the determination of uranium in natural water samples. The accuracy was assessed through recovery experiments and the analysis of a certified reference material.
Keywords: uranium(VI), solid phase extraction, modified alumina, 8-hydroxyquinoline.
Uranium is a radionuclide naturally occurring in granite and other mineral deposits. It enters local water and food supplies in various concentrations through leaching from natural deposits, release from mill tailings, emissions from nuclear industry, dissolution in phosphate fertilizers and combustion of coal and other fuels. The greatest health risk from large intakes of uranium is its damage to the kidneys, because in addition to being radioactive, uranium is a toxic metal. Uranium exposure also increases the risk ofgetting cancer due to its radioactivity. Uranium is concentrated in some specific parts of the body, so the risks of bone cancer, liver cancer, and blood disease (such as leukemia) are high [1, 2].
Several techniques have been used for the determination of uranium including inductively coupled plasma mass spectrometry [3], potentiometry [4], neutron activation analysis [5], fluorimetry [6] and spectrophotometry [7]. Some of these methods are often not sufficiently sensitive for the direct determination ofuranium in water samples. Therefore, a separation and/or preconcentra-tion step is necessary.
Various procedures for the separation and preconcentration of trace amounts of uranyl ions have been developed, including flotation [8], liquid—liquid extraction [9, 10], cloud point extraction [11, 12], ion exchange [13], and solid phase extraction [14—18].
The preconcentration based on solid phase extraction is simple, rapid and usually helps to eliminate the interference from the matrix elements. The preconcentration
factor is higher than in liquid-liquid extraction, the solvent consumption is minimal and the target species are collected on the solid surface in a more stable chemical form.
8-Hydroxyquinoline (oxine) forms stable complexes with many metal ions under proper conditions and is used as reagent for complexation with a wide variety of metal cations in solvent extraction, photometric extraction, and precipitation methods. The selectivity can be improved by choosing a proper pH and masking agents. Oxine has been immobilized on a number of supports through chemical or physical procedures and used for the preconcentration of trace metals [19—26]. Some of these adsorbents are fairly effective but the methods of preparation are lengthy and require strict control of the experimental conditions [22—26].
Surfactant molecules form self-aggregates called "hemi-micelles" or "ad-micelles" on alumina surface. The hydrocarbon cores of these micelles have the unique ability to solubilize hydrophobic organic compounds, which are otherwise sparingly soluble in water [27—29]. The traditional chelating agents such as oxine can be rapidly trapped in the hemi-micelles on alumina. This sor-bent has been used for the preconcentration of trace amounts of chromium and thallium in aqueous samples [30, 31], but to the best of our knowledge no such studies have been directed at uranium.
In this work, the use of oxine immobilized on surfactant coated alumina for the preconcentration of urani-
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Recovery, % 120
100
80 60 40 20
10
12 pH
Fig. 1. Effect of pH on the sorption of uranium(VI). Concentration, 8 p,g/mL; sample volume, 50 mL.
um(VI) from aqueous solution, followed by its spectrophotometry determination using arsenazo III as chro-mogenic reagent, is described.
EXPERIMENTAL
Reagents. Doubly distilled water and analytical reagent-grade chemicals (Merck, Darmstadt, Germany) were used throughout the experiments. All glassware was soaked in 5 M nitric acid for at least 24 h, washed with distilled water, and dried in a dust-free environment.
A stock solution (1000 mg/L) of uranium(VI) was prepared by dissolving an appropriate amount of UO2(NO3)2 • 6H2O in 0.5 M nitric acid. Working solutions were prepared from the stock solution by serial dilutions with distilled water. Alumina (10—50 ^m, y type, chromatographic grade) was purifed by shaking with 5 M nitric acid and washing three times with water. Sodium dodecyl sulfate (SDS) and oxine were used without further purification. A 0.8% solution of oxine was prepared by dissolving 80 mg of oxine in methanol. A 0.05% solution of arsenazo III was prepared by dissolving 0.05 g of Arsenazo III in 100 mL ofwater. The solution was stored in a amber glass bottle.
Apparatus. The absorbance measurements were carried out with a Jasco model 7800 double beam spectrophotometer with 1-cm quartz cuvettes. pH was determined with a model 691 Metrohm digital pH meter with a combined glass-calomel electrode.
Preparation of the sorbent. Purified alumina (1.5 g) was placed in 50 mL of water and mixed with 100 mg of SDS, the suspension was acidified to pH 2—2.5 with 4 M hydrochloric acid and mixed for 10 min with a mechanical shaker. The supernatant was decanted and the SDS-coated alumina was washed thoroughly with several portions of distilled water. Then, 50 mL ofwater and 4 mL of oxine solution were added, and the solution was mixed for 15 min. After removing the supernatant solution, the
oxine-coated alumina was packed into a column (15 mm in height x 5 mm in diameter).
General Procedure. A given volume of aqueous sample (up to 1000 mL) containing 0.4—60 ^g uranium(VI) (pH 5.5) was passed through the column at a flow rate of 6 mL/min with the aid of a suction pump. The sorbed an-alyte was then eluted with 4 mL of 2.5 M HNO3 at an elu-tion rate of 1 mL/min and the eluent was collected in a 5 mL volumetric flask containing 0.5 mL of 0.05% Arsenazo III solution and diluted to 5 mL with distilled water. The uranium(VI) concentration was then determined at 650 nm against a blank solution.
RESULT AND DISCUSSION
Oxine has a nitrogen atom in the heterocyclic ring and a phenolic group adjacent to it. The reagent forms sparingly soluble complexes with several inorganic ions, including uranium(VI). When SDS-coated alumina particles were shaken with the solution of oxine, it was homogeneously trapped on the hemimicelles or admicelles, formed by the surfactant on alumina surfaces, and the color of alumina was changed from white to yellow. The resultant sorbent was characterized by employing Fourier transform infrared spectrometry (FTIR). The characteristic IR bands in cm-1 for immobilized oxine were: 1607.7 (C=N stretching in aromatic ring), 1474.1 and 1387.4 (Skeleton bands of aromatic rings) and 1284.6 (C-O stretching), which proves the trapping of oxine on SDS-coated alumina.
The first group of experiments was designed to investigate whether unmodified alumina and immobilized ox-ine on SDS-coated alumina have any affinity to retain uranium(VI) ions at pH of~ 5. It was found that while the unmodified alumina adsorbed only 12% of uranium(VI) ions, the alumina modified with oxine was capable of retaining 98% of uranium(VI) ions.
Effect of pH on the sorption of uranium(VI) cation. The recovery of uranium(VI) cation with oxine-coated alumina was studied in the pH range 1-10. For this purpose, 50 mL of 8 ^g/mL uranium(VI) solution was passed through the column at different pH values. The effluent was analyzed by spectrophotometric method with arsenazo III. The result in Fig. 1 revealed that urani-um(VI) was quantitatively retained on the sorbent over the pH range of 5.0-6.0. Lowering the pH value of the uranium(VI) solution decreases the uptake capacity of the sorbent due to the electrostatic repulsion of the pro-tonated active sites and the positively charged uranyl species. At higher pH values, the uranyl ion may hydrolyze
to form species such as UO2OH+ and (UO2)2 (OH)^+. Therefore, pH of 5.5 was chosen as the optimum pH for extraction.
Desorption of uranium(VI) from the column. The desorption of uranium(VI) from the oxine-coated alumina was studied by using 4 mL of different eluents such as hydrochloric acid, nitric acid and acetic acid. It was found that the quantitative recovery can be obtained with nitric
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0
2
4
8
Recovery, % 110
100 90 80 70 60 50
0
2 3 4 5
Concentration of nitric acid, M
Fig. 2. Effect of the concentration of nitric acid on the uptake of uranium(VI). Concentration, 0.4 p,g/mL, sample volume, 50 mL; pH 5.5; sample flow rate, 5 mL/min.
Recovery, % 120 г
100 80 60 40
20 0
6 8 10 Flow rate, mL/min
Fig. 3. Effect of the sample flow rate on the recovery of ura-nium(VI). Concentration, 0.4 p,g/mL; sample volume, 50 mL; pH 5.5.
1
0
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acid as eluent. The concentration of nitric acid was then varied from 0.5—4.5 M and, as th
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