A stable extractant-impregnated resin (EIR) containing Chrome Azurol B was prepared using Amberlite XAD-2010 as a porous polymeric support. The new EIR was employed for trace separation and preconcentration of U(VI) ion followed by spectrophotometric determination with the arsenazo III procedure. CAB/XAD-2010 exhibited excellent selectivity for U(VI) ion over coexisting ions. Experimental parameters including pH, contact time, shaking speed, and ionic strength were investigated by batch extraction methods. Maximum sorption of U(VI) ions occurred at pH 4.3–6.9. The capacity of EIR was found to be 0.632 mmol·g−1. Equilibrium was reached in 25 min and the loading half-time,
Nowadays, a great attention is paid to the analytical monitoring of uranium in environmental samples due to its serious toxic effects even at low concentrations [
Solid-phase extraction (SPE) has come to the forefront compared with other preconcentration techniques because of the development of solid adsorbents, including chelating polymeric supports, and the advantages in the use of these adsorbents in metal ions preconcentration. SPE offers several important advantages such as [ higher enrichment factors, absence of emulsion, safety with respect to hazardous samples, minimal costs due to low consumption of reagents, flexibility, ease of automation.
In addition, several properties such as selectivity, simplicity of equipment, ease of operation, and the possibility of using adsorbents for many separation and preconcentration cycles without losses in the metal ion sorption capacity have made their use popular.
Extractant-impregnated resins, EIRs, have recently been developed for designing chelating polymeric supports and separating transition metal ions from aqueous media, because the preparation of chelating polymeric ion exchangers with chelating ligand connected to the polymer matrix by chemical bonds is usually very complex, time consuming, and costly. The preparation of chelating polymeric ion exchangers by the impregnation methods is exceedingly easy to perform, merely requiring stirring of an adequate extractant and the polymeric support. In addition, there is a wide choice of reagents for desired selectivity [
In the followup of our group researches on the EIRs applications [
Deionized water was used to prepare all solutions. Unless stated, all solvents and reagents used were analytical reagent grade and purchased from Merck (Darmstadt, Germany). Stock standard solution of U(VI) was prepared by dissolving the appropriate amounts of uranyl nitrate in deionized water, acidified with a small amount of HNO3. The solutions of uranium(VI) were standardized gravimetrically (as U3O8). Buffer solutions of pH 1–3, 4–6, and 7–9 were prepared by mixing appropriate ratios of 0.1 M HCl and KCl, 0.5 M acetic acid and ammonium acetate, and 0.5 M ammonia and NH4Cl solutions, respectively. Chrome Azurol B (Chromazurol B; Mordant Blue 1; CI Mordant Blue 1; CI Mordant Blue 1, free acid; Eriochrome Blue SBB; Eriochrome Azurol B free form; Eriochrome Azurol 6B free form; EINECS 239-098-7), CAB (4-[(3-Carboxy-5-methyl-4-oxo-1-cyclohexa-2,5-dienylidene)(2,6-dichlorophenyl)methyl]-2-hydroxy-3-ethylbenzoic acid), and Amberlite XAD-2010 (surface area of 660 m2 g−1, pore diameter 28.0 nm, and bead size 20–60 mesh) were obtained from Sigma Chem. Co., St. Louis. The surface area, pore diameter, and mesh size of the resin were quoted by the supplier.
For the determination of U(VI) in solutions using arsenazo III procedure, adsorption measurements were recorded on a Shimadzu double-beams UV-Vis (2150-PC, Japan) spectrophotometer equipped with quartz cuvettes of 1 cm thickness. The pH measurements were made on a model PHS-3BW pH-meter (Bel, Italy). A Fine PCR automatic shaker model SH30L-t, Korea, was used for the batch experiments. The flow of sample and eluent solutions through the short column was controlled with a BT100-1L peristaltic pump and a DG-2 head pump (Longer pump, China). A Sartorius membrane filter of pore size 0.45
The dry procedure was used for preparing CAB-impregnated XAD-2010 resin beads [
A batch technique was used to sorb the U(VI) ions at
Five hundred milligrams of new EIR was slurried in water and then packed into a polyethylene column with an internal diameter of 0.4 cm. The ends were fitted with a small amount of glass wool to keep the EIR beads inside of the column and to prevent any loss of the EIR beads during the sample running. The bed length of EIR in the column was about 32 mm. Working solutions containing U(VI) metal ion, with the concentration exceeding the detection limit, prepared in which the pH and ionic strength were, respectively, adjusted to 4.5 and 0.1 M, using the acetic acid and ammonium acetate solutions, and passed through the column at a known flow rate. After this step, stripping experiments were performed. For this purpose, the column was washed with distilled water (5 mL), and then, 5 mL of 0.50 mol L−1 HCl was used to strip U(VI) ions. The desorbed metal ions were analyzed spectrophotometrically by arsenazo III procedure described in Section
The arsenazo III procedure was utilized for the determination of U(VI) in solutions [
Chrome Azurol B (CAB) is one of the triphenylmethane dyes. It is a dicarboxylic acid with molecular formula of C23H16Cl2O6 (structure is shown in Figure
Molecular structure of CAB.
In the current study, CAB was impregnated onto/into Amberlite XAD-2010 which is used for the preconcentration and isolation of organic materials at trace levels. The resin also has been used as a polymeric support for solid-phase extraction and preconcentration of certain metal ions [
To prepare the appropriate form of the EIR, the impregnation process was carried out at various impregnation ratios (g CAB/g dry polymer adsorbent). Figure
Effect of CAB/resin ratio (g CAB/g dry resin) on the EIR preparation at the conditions in which portions of 1 g of the dry polymer beads of Amberlite XAD-2010 were subjected to the impregnation process in 200 mL methanol.
The impregnation of porous matrices leads to the immobilization of the extractant both in pores and in the gel regions of the polymer beads. The impregnating extractant located in the pore volume is weakly retained by the polymer (mainly due to the capillary forces) and can be easily leached out from the freshly prepared EIR samples during the first days of its use (unstable part of EIR capacity). The impregnating extractant taken up by the gel regions of the matrix represents the most stable part of the EIR capacity, which remains practically constant for a long period [
Stabilizing impregnating extractant capacity on the resin weight change after stabilizing extractant content of EIR beads by carrying out sorption-desorption cycles.
The chemical stability of the EIR was examined by sequentially suspending a 0.150-g portion of the EIR in different pHs and shaking for 10 h. After filtering the solutions and rinsing the EIR with distilled water, the released amount of CAB in solution was examined, spectrophotometrically. The EIR demonstrated high chemical stability since no quantity of CAB was released into the solutions.
Initially, these experiments were carried out to select the optimum sorption medium. Buffer solutions of pH 1–8 were used to measure the pH effect on the sorption of U(VI) ions. The concentration and volume of U(VI) solutions used for this study were 100
Effect of pH on the recovery percent of U(VI) using 100 mL of model solutions of 100
The effect of ionic strength on the sorption process was also studied at the presence of sodium nitrate within the concentration range 0.01–0.40 mol·L−1. For this purpose, 100 mL aliquots of U(VI) solution (pH 4.5) having concentration of 100
Sorption of metal ions as a function of shaking speed was studied in the range of 30–200 rpm. The aliquots of 100 mL of U(VI) solution having concentration of 100
The sorption of U(VI) ions onto/into EIR was studied as a function of shaking time. Sorption process was very rapid and
The sorption capacity of the Amberlite XAD-2010 resin impregnated with CAB for the extraction of uranium was also determined. Increasing amounts of uranium were added to 0.150 g of impregnated resin. The sorption curve (not shown) appears to be linear in the range of 1.5 × 10−6–1.5 × 10−4 mol of U(VI) per 150.0 mL and it reaches a plateau at maximum sorption capacity, that is, 0.632 mmol Uranium/g EIR at pH 4.5. To compare the sorption capacity of the EIR with that of nonimpregnated Amberlite XAD-2010, the sorption capacity of the nonimpregnated Amberlite XAD-2010 resin, also, was determined at the above mentioned concentrations. The results showed that the sorption capacity of the nonimpregnated Amberlite XAD-2010 resin was negligible. This indicates that CAB/XAD-2010 resin could be used as a good sorbent for preconcentration of uranium in the trace concentration range.
Various mineral acids were studied as eluent to investigate their efficiency for desorbing and separating U(VI) ions from the EIR, using different volumes and concentrations of each eluent. The results were summarized in Table
Effect of type and concentration of eluting acid on recovery of U(VI) ions (
Eluent | Volume (mL) | Concentration (M) | Recovery (%) |
---|---|---|---|
HNO3 | 10 | 0.5 | 83.1 |
HNO3 | 10 | 1.0 | 87.5 |
HCl | 10 | 1.0 | 98.9 |
HCl | 10 | 0.5 | 98.9 |
HCl | 5 | 1.0 | 98.8 |
HCl | 5 | 0.5 | 98.9 |
HCl | 5 | 0.4 | 94.6 |
H2SO4 | 10 | 0.5 | 92.3 |
H2SO4 | 5 | 1.0 | 94.0 |
H2SO4 | 5 | 2.0 | 93.8 |
HAC | 10 | 0.5 | 38.1 |
HAC | 10 | 1.0 | 40.8 |
Equilibrium properties of adsorption systems are usually expressed as adsorption isotherms. Adsorption isotherm is a function that correlates the amount of metal ion adsorbed per unit weight of the adsorbent,
Langmuir isotherm:
The linear form of the Langmuir, Freundlich and Tempkin isotherms can be expressed by (
The isotherm plots were drawn using (
Parameters of the Langmuir and Freundlich isotherm models for the adsorption of U(II) ion by CAB-impregnated XAD-2010.
Isotherm model | Magnitude |
---|---|
Langmuir | |
|
156.2 |
|
|
|
0.9998 |
Freundlich | |
|
51.02 |
|
3.323 |
|
0.8181 |
Tempkin | |
|
22.41 |
|
16.013 |
|
0.9641 |
Langmuir (a), Freundlich (b), and Tempkin (c) adsorption isotherms for U(VI) adsorption by EIR (pH 4.5 and temperature 298 K).
The Langmuir adsorption isotherm is based on the assumption that all adsorption sites are equivalent and that adsorption in an active site is independent of whether the adjacent sites are occupied or not. The fact that the Langmuir isotherm fits the experimental data very well may be due to homogenous distribution of extractant molecules, or active chelating sites, on the polymeric surface, since the Langmuir equation assumes that the adsorbent surface is homogenous [
The effect of column flow rate on the sorption of U(VI) was separately studied in the range of 1–18 mL min−1 using 1 L of solution with U(VI) concentration of 10
Effect of sample and eluent flow rates on the recovery of U(VI) using 1 L of model solution of 10
Since the concentrations of uranium in real samples are low, the amounts of uranium in these samples should be taken into smaller volumes for high preconcentration factor. Therefore, the limit of preconcentration was determined by increasing the dilution of the U(VI) ion in solution keeping the total amount of loaded U(VI) ion at 10
In order to determine the detection limit of the proposed dynamic method, 1500 mL aliquots of blank solutions were adjusted to optimized conditions and passed through the column. After eluting the column and determining uranium, the detection limit of the procedure (based on
The performance of the technique for the quantitative separation and preconcentration of U(VI) ions in the presence of foreign cations and anions was investigated by measuring the recovery (%) of U(VI) under optimized conditions. For this purpose, 1500 mL aliquots of U(VI) solution with concentration of 7.0
Effect of foreign ions on the determination of 7.0
Foreign ion | Tolerance ratio |
---|---|
Na+, K+, Mg2+, Ca2+, Ba2+, Zn2+, |
5000 |
|
3000 |
|
1000 |
Ag+ | 500 |
Ni2+ | 300a |
Ce3+ | 200 |
Th4+ | 200b |
Cu2+ | 150a |
Zr4+, La3+ | 100 |
Fe3+ | 80 |
Ni2+ | 5 |
Cu2+ | 2 |
Th4+ | 1 |
bIn the presence of EDTA (
The proposed method was applied to the determination of U(VI) in natural water samples including spring, well, and river water samples that they were isokinetically collected in polyethylene bottles from different areas of Kashmar, a city in Iran, Khorasan Razavi province. These water samples were passed through a membrane filter with a pore size of 0.45 mm to remove their particulates and then pH was brought to 4.5. By adding sufficient amounts of potassium cyanide and EDTA, as masking agents, their concentration reached up to 1.0 × 10−3 M. Then, 1500 mL aliquots of the samples were subjected to the recommended dynamic procedure. A recovery test was also performed by determining the spiked amounts of U(VI) to the samples. The obtained results are summarized in Table
Application of the proposed method for the determination of U(VI) in 1500 aliquots of environmental water samples.
Sample | Added ( |
Found ( |
Recovery (%) |
---|---|---|---|
0.0 | 1.4 (2.9)a | — | |
Spring water | 2.0 | 3.3 (2.6) | 97.0 |
(Ghazi Spring) | 5.0 | 6.2 (2.7) | 96.9 |
10.0 | 11.0 (2.6) | 96.5 | |
20.0 | 20.7 (2.6) | 96.7 | |
| |||
0.0 | 5.7 (2.3) | — | |
River water | 2.0 | 7.8 (2.2) | 101.3 |
(Shesh-Taraz) | 10.0 | 15.8 (2.2) | 100.6 |
20.0 | 25.9 (1.9) | 100.7 | |
30.0 | 35.6 (2.0) | 99.7 | |
| |||
0.00 | — | — | |
Well water | 10.00 | 9.8 (1.9) | 98.0 |
(Argha) | 20.00 | 19.6 (1.7) | 98.0 |
30.00 | 29.3 (1.3) | 97.7 | |
40.00 | 39.1 (1.2) | 97.8 |
For testing the accuracy of the measurements, two model solutions were prepared just according to the composition of JG-1a and JR-1 (Geological Survey of Japan reference samples) by precisely following the literature considerations [
Results of the accuracy test of proposed method for U(VI) determination in two geological standard reference materials.
Samples | U(VI) content ( |
Found ( |
Recovery (%) |
---|---|---|---|
JG-1aa | 4.7 | 4.63 (3.1)b | 98.6 |
JR-1a | 8.9 | 8.81 (3.0)b | 99.0 |
aThe treatments were carried out on 0.5 g of the sample.
b RSD of five replicate measurements.
Chrome Azurol B-impregnated XAD-2010 resin beads were prepared and used for solid-phase extraction and preconcentration of U(VI) in various environmental water samples. The proposed procedure for the determination of trace amounts of U(VI) ions combines preconcentration of the analyte with spectrophotometric measurement with the arsenazo III procedure. The sorption equilibrium data were analyzed using widely used isotherm models. The Langmuir isotherm gave the best fit of the experimental equilibrium data. Thus, the distribution of extractant molecules on the polymeric surface is homogenous, and the adsorption process is monolayer. The optimized experimental parameters for the new solid-phase extraction system have been summarized in Table
Optimized experimental parameters for separation and pre-concentration of U(VI) ions by CAB-impregnated XAD-2010.
Experimental parameter | Optimum magnitude |
---|---|
Batch parameters | |
pH | 4.2–4.7 |
Ionic strength | 0.01–0.30 mol |
Shaking speed | 180 rpm |
Contact time | 25 min |
|
6 min |
Capacity | 0.632 mmol |
Concentration of eluent (HCl) | 0.50 M |
Volume of eluent | 5 mL |
| |
Dynamic parameters | |
Sample flow rate | 11 mL |
Eluent flow rate | 0.5 mL |
Sample volume | 1600 mL |
LOD |
|
The authors wish to thank Dr. M. J. Mosavi (the president of Islamic Azad University-Kashmar branch, Kashmar, Iran) for sincere support and Professor M.S. Hosseini (Birjand University, Birjand, Iran) for a great help during the experimental works. Also, they acknowledge the financial and technical support provided by the Research Center of Islamic Azad University-Kashmar branch, Kashmar, Iran.