Cloud point extraction for Co and Ni determination in water samples by flame atomic absorption spectrometry

Separation and Purification Technology 54 (2007) 349–354

Cloud point extraction for Co and Ni determination in water samples by flame atomic absorption spectrometry Valfredo Azevedo Lemos ∗ , Robson Silva da Franc¸a, Bruno Oliveira Moreira Laborat´orio de Qu´ımica Anal´ıtica (LQA), Universidade Estadual do Sudoeste da Bahia, Campus de Jequi´e, Jequi´e-BA 45200-000, Brazil Received 25 July 2006; received in revised form 10 October 2006; accepted 11 October 2006

Abstract Cloud point methodology was successfully employed for preconcentration of trace cobalt and nickel prior to their determination by flame atomic absorption spectrometry (FAAS). The metals react with 2-[2 -(6-methyl-benzothiazolylazo)]-4-bromophenol (Me-BTABr) in a surfactant Triton X-114 medium. Dilution of the surfactant-rich phase with acidified methanol was performed after phase separation, and the cobalt and nickel content was measured by FAAS. The proposed procedure allowed the determination of cobalt and nickel with detection limits of 0.9 and 1.1 ␮g L−1 , respectively. The method was applied to metal determination in water samples. The validation of the procedure was carried out by analysis of a certified reference biological material, NIST 1570a Spinach Leaves. © 2006 Elsevier B.V. All rights reserved. Keywords: Me-BTABr; Cloud point extraction; Cobalt; Nickel; FAAS

1. Introduction Cobalt is a naturally occurring element found in rocks, soil, water, plants, and animals. It is an essential micronutrient required for the growth of both plants and animals. Cobalt can be beneficial for humans because it is part of Vitamin B12 . However, the metal can also be harmful, because exposure to high levels of cobalt can result in lung and heart effects and dermatitis. Nickel is a very abundant natural element and it can be combined with other metals, such as iron, copper, chromium, and zinc, to form alloys. Many people are highly sensitive to nickel. The most common harmful health effect of nickel in humans is an allergic reaction. People can become sensitive to nickel when jewellery or other things containing it are in direct contact with the skin for a long time. Studies for cobalt and nickel determination in water and biological matrices are very important because it is a good tool for environmental and toxicological monitoring [1–3]. The determination of extremely low concentration of cobalt and nickel is generally associated to extraction steps due to insufficient sensitivity or matrix interference [4–6]. Cloud point

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1383-5866/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2006.10.004

extraction (CPE) can be an alternative to conventional solvent extraction due to a number of possible advantages, such as reducing of the consumption of a solvent, disposal costs, and extraction time [7,8]. The solubility of several non-ionic and zwitterionic surfactants in water decreases as temperature increases, with materials that are fully soluble at room temperature becoming partially insoluble forming separate phases at higher temperatures. The temperature at which this substances and water separate is known as the cloud point temperature [9]. The concentration of the surfactant-rich phase is the critical micellar concentration. CPE methodologies are based on this property. The use of CPE in procedures for separation and preconcentration of metal ions has been centered on the extraction of these metallic substances as sparingly water-soluble chelate complexes. Several CPE methods for cobalt and nickel determination have been described in the literature. Ligands such as dithizone [10], 1-(2-pyridylazo)-2-naphthol (PAN) [11], ammonium pyrrolidinedithiocarbamate (APDC) [12], 1-nitroso-2-naphthol [13], and 2-amino-cyclopentene-1-dithiocarboxylic acid (ACDA) [7] have been employed for cloud point extraction in several procedures for Co and Ni. These ligands are widely employed due several advantages obtained, such as capacity to form complexes with a large variety of metals and low solubility in water. The hydrophobicity of ligands and complexes are the

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fundamental factors which regulate the extraction efficiency [14]. The aim of this work was to develop a cloud point extraction method by the use of 2-[2 -(6-methyl-benzothiazolylazo)]-4bromophenol (Me-BTABr) reagent as a complexing agent prior to flame atomic absorption spectrometric determination of cobalt and nickel. This complexing reagent was firstly synthesized by our research group and it has been used in preconcentration methods for zinc [15] and copper [16] determination. 2. Experimental 2.1. Apparatus Absorbance measurements were performed using a flame atomic absorption spectrometer of the Perkin-Elmer Instruments (Shelton, USA) model AAnalyst 200. It was equipped with hollow cathode lamps and an air–acetylene burner. The instrumental parameters were as follows: wavelength 240.7 nm (Co) and 232.0 nm (Ni) and lamp current 4 mA (Co and Ni). Nebulizer flow rate was 5.0 mL min−1 . Deuterium lamp background correction was also used. All pH measurements were made by Digimed DM 20 (Santo Amaro, Brazil) pH meter. Cloud point preconcentration experiments were performed using a thermostated bath (Soc. Fabbe LTDA, S˜ao Paulo, Brazil), maintained at the desired temperature and phase separation was assisted using a BIO ENG model BE 5000 centrifuge (S˜ao Paulo, Brazil) in 20 mL centrifuge tubes. Digestion of certified reference material was carried out in a Parr Instrument 4749 (Moline, IL, USA) Acid Digestion Bomb enclosing a chemically inert Teflon sample cup of 23 mL. 2.2. Reagents All reagents used were of analytical grade. Deionized water was used to prepare all solutions. The laboratory glassware was soaked in a 5% (v/v) nitric acid solution for 24 h and rinsed with high purity water at least three times prior to use. The stock solutions of 1000 ␮g mL−1 cobalt or nickel were obtained from Merck (Darmstadt, Germany). Working standard solutions were prepared daily by appropriate dilution of the stock standard solution with 5% (v/v) nitric acid. Acetate, borate, and ammoniacal buffers were used to adjust the sample pH in the range of 3.7–6.5, 7.0–8.0, and 9.2, respectively. The non-ionic surfactant Triton X-114 purchased from Sigma–Aldrich (Milwaukee, USA) was used in this work. A 7.0 × 10−4 mol L−1 Me-BTABr solution was prepared by dissolving 50 mg of 2-[2 -(6-methylbenzothiazolylazo)]-4-bromophenol laboratory-prepared [15] in 200 mL of absolute ethanol. More dilute solutions were prepared by appropriate stepwise dilution. Nitric acid solutions were prepared by direct dilution with deionized water from the concentrated solutions. Methanol, acetone, and ethanol (Merck) were used to decrease the viscosity of surfactant-rich phase. The accuracy of the method was assessed by analysing the certified reference biological material NIST 1570a Spinach Leaves from the National Institute of Standards and Technology (Gaithersburg, MD, USA).

2.3. Procedure for cloud point extraction To 10 mL of the standard or sample solution containing cobalt or nickel, 2 mL of a convenient buffer solution was added for adjustment of pH. Me-BTABr (4.2 × 10−5 mol L−1 ) and Triton X-114 (1.0%, v/v) solutions were added. To reach the cloud point temperature, the system was allowed to stand for about 30 min into a thermostated bath at 40 ◦ C. To settle the produced micelles, the mixture was centrifuged for 15 min at 3500 rpm and afterwards it was cooled down in an ice bath. After cooling, the viscosity of the surfactant-rich phase has been increased. The supernatant was then decantated by inverting the tube. The surfactant-rich phase was dissolved with 200 ␮L of methanol solution containing 1.0 mol L−1 HNO3 to decrease the viscosity and facilitate introduction in FAAS nebulizer. The content of nickel or cobalt in the solution was then measured by flame atomic absorption spectrometry (FAAS) under the operating conditions described in Section 2.1. 2.4. Sample preparation A certified reference material (CRMs) furnished by the National Institute of Standards and Technology, NIST 1570a Spinach Leaves has been analyzed. Approximately 0.20 g of this material was weighed accurately into a Teflon cup. In order to decompose the sample, about 4.0 mL of 1:1 (v/v) nitric acid solution were added, and the acid digestion bomb was heated in a stove at 150 ◦ C for 5 h [17]. After cooling at room temperature the bomb was opened carefully in a fume cupboard. Sodium hydroxide and appropriate buffer solution were used to adjust the pH of the final digests. The mixture was finally diluted to 25 mL by double deionized water. These solutions were analyzed immediately after preparation. Water samples were collected from the city of Jequi´e, Bahia state in Brazil. The only pretreatment was acidification to pH 2.0 with nitric acid, which was performed immediately after collection, in order to prevent adsorption of the metal ions on the flask walls. Samples were filtered before analysis. At least one blank solution was run for each sample in order to evaluate the metal contamination by the reagents used. 3. Results and discussion The variables were optimized by applying the cloud point extraction procedure described in Section 2.3 to a 50 ␮g L−1 cobalt or nickel solution. 3.1. Optimization of variables In order to study the influence of Me-BTABr on the analytical response for cobalt and nickel, different concentrations of the chelating reagent in the range of 3.5 × 10−6 to 1.4 × 10−4 mol L−1 were used, and the general procedure was applied. These solutions were prepared by dilution of the MeBTABr solution described in Section 2.2. The results (Fig. 1) showed that the signal is maximum when the concentration of Me-BTABr is in the ranges of 1.7 × 10−5 to 5.2 × 10−5 and

V.A. Lemos et al. / Separation and Purification Technology 54 (2007) 349–354

Fig. 1. Effect of Me-BTABr concentration on the cloud point extraction for cobalt and nickel determination.

3.5 × 10−5 to 7.0 × 10−5 mol L−1 for cobalt and nickel, respectively. Therefore, a 4.2 × 10−5 mol L−1 Me-BTABr solution was selected as optimal for both metal. The effect of pH on the cloud point extraction of nickel and cobalt was investigated because this parameter plays an important role in metal-chelate formation. For this study, a pH range of 3.7–9.2 was used. As can be seen in Fig. 2, maximum extraction of metals occurred at pH ranges of 7.0–8.0 (Co) and 7.0–7.5 (Ni). Borate buffer pH 7.3 was chosen as the optimum for subsequent experiments. Triton X-114 was chosen as surfactant due owing to its low cloud point temperature and high density of the surfactant-rich phase, which facilitates phase separation by centrifugation. The effect of surfactant concentration on the extraction of cobalt and nickel was examined within the Triton X-114 concentration range from 0.1 to 2.0% (v/v). As shown in Fig. 3, the signal is maximum as the surfactant concentration was 0.5–1.0% (v/v) for both metals. The analytical signal decreased at concentrations higher than 1.0% (v/v) due to the increase of the surfactant volume, deteriorating the FAAS signal. At concentrations below this value, the extraction efficiency of complexes

351

Fig. 3. Effect of surfactant concentration on the cloud point extraction for cobalt and nickel determination.

was low because there is few molecules of the surfactant to entrap the Me-BTABr complexes quantitatively. A Triton X-114 concentration of 1.0% (v/v) was selected for subsequent studies. Two important parameters in cloud point extraction of cobalt and nickel are incubation time and equilibration temperature. The effect of the equilibration temperature (30 to 60 ◦ C) on the cloud point temperature was also investigated. It was found that the extraction efficiency reach maximum in the range of 35–45 ◦ C, for both metals. So, an equilibration temperature of 40 ◦ C was used. The incubation time was also studied. Maximum extraction efficiency was observed at 40 ◦ C from 10 to 20 min for Co and Ni. When incubation time above 20 min was used, a significant decrease of the efficiency was observed, probably due to instability of the complexes. Accordingly, an incubation time of 15 min was chosen for use in next experiments. For the sample introduction in the FAAS nebulizer, it was necessary to decrease the surfactant-rich viscosity. Several synthetic mixtures of varying composition with respect to organic solvents and their acid mixtures were investigated, to taking into account that these solvents increase the analytical signal of the FAAS [18]. Solvents tested include acetone, ethanol and methanol. Best results were obtained using methanol. A 0.5 mol L−1 nitric acid solution was added (1:1) to methanol and 200 ␮L of this solution was used as diluent. In these conditions, the analytical signals were at a maximum. The analytical characteristics were not affected by other parameters such as centrifugation time for phase separation and ionic strength. 3.2. Analytical figures of merit

Fig. 2. Effect of pH on the cloud point extraction using Me-BTABr reagent for cobalt and nickel determination.

The calibration graphs were linear in the range 0.9– 100.0 ␮g L−1 cobalt and 1.1–100.0 ␮g L−1 nickel under the optimum conditions of the general procedure. The regression equations for cobalt and nickel determination were A = 1.12 × 10−3 + 2.63 × 10−3 C and A = 3.21 × 10−3 + 1.98 × 10−3 C, respectively, where A is the absorbance and C is the metal concentration in solution (␮g L−1 ). By using direct aspiration in FAAS without the preconcentration procedure,

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Table 1 Analytical performance of the cloud point extraction method using Me-BTABr reagent for cobalt and nickel determination

Table 2 Tolerance limit of foreign ions on cobalt and nickel (50 ␮g L−1 ) determination by proposed procedure

Element

Cobalt

Nickel

Substance

Enrichment factor Sample volume (mL) Consumptive index (mL) Limit of detection (␮g L−1 ) Limit of quantification (␮g L−1 ) Precision (50 ␮g L−1 , n = 7) (%RSD) Linear range (␮g L−1 ) Calibration function

28 10 0.36

23 10 0.43

0.9

1.1

3.0

3.7

2.9

3.4

0.9–100.00 A = 1.12 × 10−3 + 2.63 × 10−3 C

1.1–100.00 A = 3.21 × 10−3 + 1.98 × 10−3 C

the linear equations were A = 4.33 × 10−3 + 9.47 × 10−5 C (Co) and A = 3.02 × 10−3 + 8.73 × 10−5 C (Ni). Some factors that characterize preconcentration systems, such as enrichment factor and consumptive index (CI), were also determined [18]. Enrichment factors were calculated as the ratio of the slopes of the calibration graphs with preconcentration and direct aspiration, respectively. The consumptive index is defined as the sample volume, in millilitres, consumed to reach an unit of enrichment factor (EF): CI = Vs (mL)/EF, where Vs is the sample volume. The precision of the procedure was determined as the relative standard deviation of seven independent measurements carried out in solutions containing 50.0 ␮g L−1 cobalt or nickel. The limit of detection (LOD), defined as the metal concentration that gives a response equivalent to three times the standard deviation (σ) of the blank (n = 11), was also determined. Characteristic data under the optimum conditions by preconcentrating 10 mL of cobalt or nickel solution were determined and are given in Table 1. 3.3. Interference studies The effect of foreign ions on the signal intensity of cobalt and nickel was tested. Different amounts of common cations were

Maximum tolerable amount

Al3+ (mg L−1 ) Ca2+ (g L−1 ) Cd2+ (mg L−1 ) Cl− (g L−1 ) Co2+ (mg L−1 ) Cu2+ (mg L−1 ) Fe3+ (mg L−1 ) K+ (g L−1 ) Mg2+ (g L−1 ) Na+ (g L−1 ) Ni2+ (mg L−1 ) NO3 − (g L−1 ) Pb2+ (mg L−1 ) SO4 2− (g L−1 ) Zn2+ (mg L−1 )

Cobalt

Nickel

50.0 5 2.5 20.0 – 15 3.0 10.0 5 20.0 1.0 10.0 10.0 0.5 5

50.0 1.0 0.5 10.0 0.5 10.0 5.0 10.0 0.1 10.0 – 10.0 2.0 0.5 10.0

Table 3 Metal determination in certified reference materials using proposed methodology (n = 4); Confidence interval 95%. NIST: National Institute of Standards & Technology, USA Sample

NIST 1570a Spinach Leaves

Cobalt amount (␮g g−1 )

Nickel amount (␮g g−1 )

Found

Certified

Found

Certified

0.43 ± 0.09

0.39 ± 0.05

2.23 ± 0.15

2.14 ± 0.10

added to the test solution containing 50.0 ␮g L−1 of cobalt or nickel and the developed procedure was applied. The tolerance limits were determined for a maximum error of 10% and the results from these studies are given in Table 2. These results demonstrate that the common coexisting ions did not have significant effect on the separation and determination of the metals. The effects of other ions at given concentrations are negligible. Also, large amounts of alkali and alkaline earth metals have a negligible effect on the preconcentration of cobalt and nickel.

Table 4 Results obtained for metal determination in water samples (n = 4); confidence interval 95%; LOD: limit of detection Sample

Cobalt amount (␮g L−1 ) Added

Found

River water

0.0 5.0 10.0

5.4 ± 0.2 10.1 ± 0.1 14.9 ± 0.2

Tap water

0.0 5.0 10.0

Well water

0.0 5.0 10.0

Recovery (%)

Nickel amount (␮g L−1 )

Recovery (%)

Added

Found

– 94 95

0.0 5.0 10.0


– 96 105


– 106 101

0.0 5.0 10.0


– 96 99


– 104 106

0.0 5.0 10.0


– 104 103

V.A. Lemos et al. / Separation and Purification Technology 54 (2007) 349–354

353

Table 5 Procedures using cloud point extraction prior cobalt and nickel determination; EF: enrichment factor; CI: consumptive index; LOD: limit of detection; PAN: 1(2-pyridylazo)-2-naphthol; 1,2-N,N: 1-nitroso-2-naphthol; PONPE 7.5: polyethylene glycol-p-nonylphenylether; APDC: ammonium pyrrolidinedithiocarbamate; ACDA: 2-amino-cyclopentene-1-dithiocarboxylic acid; SDS: sodium dodecyl sulfate; 5-Br-PADAP: 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol; Me-BTABr: 2-[2 -(6-methyl-benzothiazolylazo)]-4-bromophenol; SP: spectrophotometry; FAAS: flame atomic absorption spectrometry; GFAAS: graphite furnace atomic absorption spectrometry; TS-FF-AAS: thermospray flame furnace atomic absorption spectrometry; a : in presence of hydrochloric acid; b : in presence of sodium chloride Reagent

Surfactant

Sample volume (mL)

Element

Dithizone

Triton X-114

10

Ni

39

0.26

12.5

Coa Cob

29 22

5-Br-PADAP

Triton X-100 and SDS

EF

CI (mL)

LOD (␮g L−1 )

Sample

Detection

Reference

1.2

Water

FAAS

[10]

0.43 0.57

1.1 1.6

Pharmaceutical samples

FAAS

[19]

PAN

Triton X-114 Tween 80 Triton X-100

10 – –

Ni Ni Co

25 – 100

0.40 – –

6.0 – 0.003

Water Alloys and water Water

FAAS SP GFAAS

[11] [20] [21]

APDC

Triton X-114

10

Co

130

0.08

2.1

Biological tissues

TS-FF-FAAS

[22]

Co Ni

20 20

0.50 0.50

5.0 11.0

Water

FAAS

[12]

0.37 0.34

Water

FAAS

[13]

7.5 10.0

Water

SP

[7]

0.9 1.1

Water

FAAS

This work

1,2-N,N

PONPE 7.5

10

Co Ni

27 29

ACDA

Triton X-114

10

Co Ni

–

Me-BTABr

Triton X-114

10

Co Ni

28 23

3.4. Accuracy of the method In order to validate the method for accuracy, cobalt and nickel were determined in a certified reference material (NIST 1570a Spinach Leaves). Results are given in Table 3. It was found that there is no significant difference between results obtained by the proposed method and the certified results for both metals. These results indicate the applicability of the developed procedure in cobalt and nickel determination free of interference. 3.5. Determination of cobalt and nickel in water samples The proposed procedure has been applied to the determination of cobalt and nickel content from different water samples. The results are described in Table 4. According this table, the added cobalt and nickel ions can be quantitatively recovered from the water samples by the proposed procedure. Recoveries (R) of spike additions (5.0 or 10.0 ␮g L−1 ) to water samples were quantitative. R was calculated as follows: R (%) = {(Cm − Co )/m} × 100, where Cm is a value of metal in a spiked sample, Co the value of metal in a sample, and m is the amount of metal spiked. These results demonstrate the applicability of the procedure for metal determination in water samples. The recovery of cobalt and nickel added to the samples demonstrates the efficiency of the proposed method. 4. Conclusion The determination of cobalt and nickel in water samples was successfully performed by using cloud point extraction and Me-BTABr reagent. The proposed procedure shows interesting features, such as fastness, simplicity, and sensitivity. The

– 0.36 0.43

1.22 1.09

methodology also offers an eco friendly alternative to other separation preconcentration techniques. In comparison with solvent extraction methods, this procedure employs only a small amount of solvent and surfactant. The procedure is inexpensive, because it consists of much low equipment and running costs, such as FAAS which is available in most laboratories. Table 5 shows a comparison of the proposed method with other CPE procedures for cobalt and nickel using several regents. From the table, it was observed that the proposed procedure presents analytical characteristics comparable to that reported in the literature. Finally, further work is being carried out for the application of thiazolylazo reagents in the determination of several species by cloud point and coprecipitation methodologies in our laboratory. Acknowledgements Authors acknowledge the financial support of Fundac¸a˜ o de Amparo a` Pesquisa do Estado da Bahia (FAPESB), Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol´ogico (CNPq), and Financiadora de Estudos e Projetos (FINEP). References [1] A. Sasmaz, M. Yaman, Distribution of chromium, nickel, and cobalt in different parts of plant species and soil in mining area of Keban, Turkey, Commun. Soil Sci. Plan. 37 (2006) 1845–1857. [2] M. Soylak, L. Elci, M. Dogan, Determination of trace amounts of cobalt in natural water samples as 4-(2-thiazolylazo) recorcinol complex after adsorptive preconcentration, Anal. Lett. 30 (1997) 623–631. [3] M. Soylak, L. Elci, I. Narin, M. Dogan, Application of solid-phase extraction for the preconcentration and separation of trace amounts of cobalt from urine, Trace Elemen. Eletroly. 18 (2001) 26–29.

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