Determination of Trace Cu (II) in Environmental Water Samples by Ionic Liquid Solvent Flotation and GF-ASS

By Chunhong Ma¹’², Hong Zhu², Liang Wang², Dayu Jiang², Qingwei Wang² and Yongsheng Yan¹
June 2010

  1. Department of Chemistry and Chemical Engineering, Jiangsu University, Jiangsu Zhenjiang 212013, China
  2. College of Chemistry, Jilin Normal University, Jilin Siping 136000, China
Abstract
Ionic liquid solvent flotation was established for detecting trace Cu(II) in environmental water by graphite furnace atomic absorption spectrometry (GF-AAS), with the mixture of Ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF6) and ethyl acetate (EA) (1:1, V/V) as floating agent, tetracycline (TC) as trapping agent. It is a new method. The effects of pH of solution, the composition of the complexes, gas flow rate, floating time, and interference ions were studied. The optimum conditions were ensured. When gas flow rate was 50mL·min-1 and floating time was 50min, enrichment factor (α) of Cu(II) was up to 98 (500mL initial sample/5mL determination liquid). Linear range was 0.08∼0.56mg/L, detection limit was 0.3µg/L. The proposed method was applied to determine Cu(II) in environmental water. Recovery was 91.5%∼103.0%, RSD<3.6%. This method is non-poisonous, low pollution, high enrichment factor. It is fit for analysis trace/ultra-trace Cu(II) of environmental water samples.

Keywords: Ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF6); Ionic liquid gas-solvent sublation; separate/enrich; tetracycline (TC); GF-ASS.

Introduction

Copper is an indispensable micronutrients for human health, it is important for maintaining normal activities. Human body absorbs copper mainly through drinking water and food, so an efficient and sensitive measurement is of a great significance. Atomic absorption spectrometry and spectrophotometry are measurements of trace copper. Spectrophotometry usually uses hydrazine compounds[1], 4,6-dichloro-2- (2-tetrahydrogen imidazole)-amino pyrimiidine[2], ferrozine[3], acetaldehyde-BCO[4] and Tetra(o-chloro-p-sulfophenyl)porphin[5] as color reagents. Tetracycline (TC) is a broad-spectrum antibiotic. Its structure is shown in Figure 1. The existence of electron donor can form complexes with copper[6], magnesium and calcium[7], and other metal ions. TC as colour agent in the determination of trace copper has not been reported. Solvent extraction[8] and SPE[9] are common methods of sample separation, but a large number of organic solvents are necessary, for example 4-methyl-acetophenone, acetonitrile and so on. Ionic liquid solvent flotation is non-toxic, less organic reagents and high enrichment factor. It has been applied to tetracycline antibiotics[10] and Al (III) [11] separation.

Fig 1 · Tetracycline
Fig 1

The mixture of [Bmim]PF6 with low toxicity, inexpensive ethyl acetate (EA) (volume ratio is 1:1) was selected as floating agent in this study. TC and Cu (II) formed a stable 1:1 hydrophobicity complex under acidic conditions, the complex was soluble in flotation agent, and it was shown the maximum absorption at 373nm.The procedure was applied to environmental water samples, and the recovery was 91.5%∼103.0%, RSD<3.6%.

2. Experimental Part

2.1 Instruments and Reagents

UV-2550 UV-vis spectrophotometer (Shimadzu Instruments Co., Ltd.) was used for optimizing the parameters of the solvent sublation. pHS-4 Intelligent pH Meter (Jiangsu Jiangfen Electroanalytical Instrument Co., Ltd.) was used for pH measurements. BN0828 Electronic Analytical Balance (Shanghai Precision Scientific Instrument Co., Ltd. China Bridge) was used for measuring reagents. Self-made solvent flotation tank, Solvent Flotation Device (Figure 2) was used for floating. Trace amounts of Cu(II) were determined by GF-AAS (operating conditions given in Table 1).

Table 1 · Operational Conditions for Cu(II) by GF-AAS
Cu(II)
Wavelength (nm) 324.8
Current (mA) 7.5
Bandwidth (nm) 1.3
Drying (°C) 80-120 (30s)
Charring (°C) 600 (30s)
Atomization (°C) 2700 (10s)
Cleaning (°C) 2800 (3s)
Notice: Don't correct background

Ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF6) was purchased from Shanghai Cheng Jie Chemical Company. Ethyl acetate (EA) was purchased from Shenyang Sinopharm. Tetracycline was purchased from National Institute for Control of Pharmaceutical and Biological Products. Reserve liquid(1.0 × 10-3 mol · L-1) were prepared by dissolving given amounts Cu(SO4)2 and diluting to the mark in a 250mL measuring flask with proper amount of de-ionized water. working solution(1.0×10-4 mol·L-1) were freshly prepared by diluting the stock solutions with de-ionized water before use.Tetracycline solution(1.0×10-4 mol·L-1): a suitable amount of tetracycline was weighed with Electronic Analytical Balance, placed in 50mL beaker, dissolved to the mark in a 250mL measuring flask with proper amount of water. Clark-Lubs buffer solution: 0.2 mol·L-1 H3BO3, 0.2 mol·L-1 KCl and 0.2 mol·L-1 NaOH were mixed according to a certain percentage to form different pH buffer solutions, and calibrated them with pH meter.

Analytical reagents and second distilled water were used in the experiment.

2.2 Experimental methods

A 250 mL water sample was transferred to a 500mL beaker and a certain amount of 1.0×10-4 mol·L-1 TC solution was added. Then the pH of the solution was adjusted to 5.8 with a small amount of Clark-Lubs buffer solution. The solution was held still for 10 minutes and 70mL NaCl solution (30%) was added, then the mixed solution was transferred to 500mL flotation cell (Fig.2). 30% NaCl solution was increased to the scale A (500mL) and mixed intensively. 5mL mixture of [Bmim]PF6 and EA (1:1, V/V) was added on the surface of sample solutionon. The system was passed into N2, stopped ventilation after 50min, kept still for a moment. When there was no micro-bubble in flotation cell, The complexes were pre-concentrated in the [Bmim]PF6 -EA layer and this was used to determine analytes by GF-AAS directly.

Fig 2 · Solvent Sublation Apparatus
Fig 2
  1. Nitrogen cylinder
  2. Cushion bottle
  3. Rotameter
  4. Solvent sublation column

3. Results and Discussion

3.1 Absorption Spectrum

Fig 3 · Absorption Spectrum of TC- Cu(II)
Fig 3

In this study, UV-vis spectrometry was used to optimize the parameters of the flotation, because it was more economical, rapid, simple and convenient than GF-AAS. But Cu(II) in the real samples as TC-Cu(II) complexes were simultaneously floated into ionic liquid phase, so it was more suitable to determine them by GF-AAS than by UV-vis spectrometry.

Figure 3 shows absorption spectrums that are measured TC-Cu(II) complexes with blank reagent as reference. As can be seen, TC appears two absorption peaks at 275 nm and 356 nm. After forming TC- Cu(II), absorption peak of TC shift from 275 nm to 277 nm, another absorption peak of TC shift from 356 nm to 373 nm. Changes in absorption spectrum indicate that TC could form a stable complex with Cu(II).

Figure 4 shows absorption spectrums that TC-Cu(II) complexes in aqueous solution before and after flotation, blank ionic liquid phase and ionic liquid phase after flotation. As can be seen, absorbance changes of complex in aqueous phase before and after flotation are large. After flotation, absorbance of complexes in aqueous phase reduced greatly. Blank ionic liquid phase don’t appear maximum absorption at 300-700 nm, while ionic liquid phase appear maximum absorption at 373 nm after flotation. This shows that TC-Cu(II) complexes have floated to the ionic liquid phase. The maximum absorption wavelength of 373nm was chosen as optimized flotation condition.

Fig 4 · Absorption Spectrum of Flotation
Fig 4
  1. Absorption spectrum of aqueous phase before flotation
  2. Absorption spectrum of aqueous phase after flotation
  3. Absorption spectrum of [Bmim]PF6-EA phase before flotation
  4. Absorption spectrum of [Bmim]PF6-EA phase after flotation

3.2 The Influence of the pH

Fig 5 · Effect of pH
Fig 5

1mL 1.0×10-4 mol·L-1 Cu(SO4)2 was added by an excessive amount of TC. The pH of liquid was adjusted with Clark-Lubs buffer solution and calibrated aqueous with pH meter. The absorbance of system was detected with UV. Different pH values impacted on the formation of TC-Cu(II) complexes were shown in Figure 5. The results showed that, absorbance increase gradually below pH 5.8, and absorbance reached the maximum value when pH was 5.8; Absorbance decreased significantly when pH was over 5.8. The experimental results showed that tetracycline was easy to form complex in the weak acidic conditions. The most optimum acidity for forming complexes was pH 5.8. So pH 5.8 was chosen in the following experimental system.

3.3 The Composition of the Complexes

The ratio of TC-Cu (II) complexes was determined by equimolar continuous varied method and mobile equilibrium method in this experiment. The results of two methods are same. When the concentration of Cu(II) below the concentration of TC, complex that ratio (L:M) is 1:1 can be formed; When the concentration of Cu(II) is higher than the concentration of TC, complexes that ratio (L:M) is 1:2 can be formed. Experiment is in line with literature values[12]. This study analyzed the content of Cu, and TC is excessive, so the type of generating complexes is 1:1.

3.4 The Influence of Gas Flow Rate

The system was floated according to experimental method. Gas flow rate were different for each flotation, they were 10, 20, 30, 40, 50mL·min-1. Then [Bmim]PF6-EA phase was removed and measured their absorbance after the flotation. The results showed that 50mL·min-1 was the best.

3.5 The Influence of Floating Time

Floating time was changed from 20min to 70min and the impacts of flotation efficiency with time were examined. The results showed that flotation efficiency increased with the flotation time within a certain range, flotation efficiency was the highest when the floating time was 50 min; when floating time was increase sequentially, flotation efficiency was no significant change. So 50 min was the best.

3.6 The Impact of Coexisting Substances

Under the optimal experimental conditions, a variety of coexistence components of 5 mL 1.0 × 10-4 mol·L-1 TC were inspected in the 500 mL solution (relative error is not exceeding 5%). For example, Na+, Cl-, K+, Cd2+, Pb2+, SO42-, Mn2+(700); Fe3+, Mg2+(110); Ca2+, Al3+, Zn2+(60); chloramphenicol, penicillin, gentamicin, erythromycin(1000), and so on. Experiment results showed that coexistence substances did not affect the formation of complexes, it also did not disturb flotation behavior and determination.

3.7 The Linear Range and Detection Limit

In the optimal experimental conditions, the standard solution of serial concentration were floated and measured, the results were shown in Table 2. TC-Cu(II) complexes were shown a good linear relationship in 0.08 ~ 0.56 mg·L-1. Blank reagent were measured 11 times under the same conditions, detection limit of Cu(II) is calculated by 3σ/κ. It is 0.3µg·L-1. This satisfies the determination of real samples. Detection limit of Cu(II) is far below the national standard of the maximum limit of Cu(II) that is required in water by this method. It is also lower than detection limit by the national standard method.

Table 2 · Regression equation, correlation coefficient and detection limit of Cu(II)
System Regression equation Correlation Coefficient(r) Linear range(mg·L-1)
TC-Cu(II) 0.082ρ(mg/L)+ 0.0038 0.9996 0.08∼0.56

3.8 Flotation Effects

Experimental optimum floating conditions was that 5mL 1.0×10-4molL-1 CuSO4, added 5mL 1.0×10-4 TC, adjusted pH 5.8 with Clark-Lubs buffer solution. The concentration of NaCl solution was 30%, solvent flotation was [Bmim]PF6-EA (V/V=1:1), gas flow rate was 50mL·min-1, floating time was 50min. Under these conditions, the experiment was done with gas-solvent sublation. Flotation efficiency and enrichment factor were calculated using the following formula. Flotation rate (E) is 98%, enrichment factor (α) is 98.

Flotation rate: E = CtVt/C0Vb; enrichment factor: α=Ct/C0.

Wherein C0 is the total concentration of CuSO4; Ct is the concentration of CuSO4 in [Bmim]PF6-EA phase after flotation; Cb is the concentration of CuSO4 in aqueous phase after flotation; Vt is volume of [Bmim]PF6-EA phase; Vb is volume of aqueous phase.

4. Recovery and Sample Determination

Drinking water samples in three different regions were determined by experimental methods. Cu(II) was detected in two samples, and measured values were 0.8 mg·L-1 and 0.2 mg·L-1. They were in the limited of national requirements. The drinking water sample that was not detected Cu (II) was measured parallel 5 times by standard addition method. Standard addition experiments of two concentration levels were done. Recoveries and RSD were calculated, the results were shown in Table 3. Recoveries were 91.5 %∼103.0 %, RSD≤3.6 %.The recovery and reproducibility were good.

Table 3 · Determination results of Cu(II) in water samples by solvent sublation GF-AAS
Sample Spiked
(mg·L-1)
Recovered
(mg·L-1)
Recovery
(%)
RSD
(%, n=5)
1 1.00 1.03 103.0 3.6
2.00 1.85 92.5 2.4
2 1.00 0.94 94.0 2.1
2.00 2.01 100.5 1.5
3 1.00 0.95 95 3.1
2.00 1.83 91.5 1.5

5. Conclusion

The trace of the Cu(II) in environmental water samples was determined by ionic liquid solvent flotation and GF-AAS in this experiment. The method is non-toxic, low pollution, and high enrichment factor. The results are satisfactory. A new approach for analysing trace/ultra-trace Cu(II) in environmental water samples was provided.

Acknowledgement

This work was financially supported by the National Natural Science Foundation (No. 20777029), Jilin Province Education Foundation (No. 2009195) and Jiangsu University Ph.D. Innovative Projects (No. CX09B_199Z).

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