Fe3O4@ZrO2 nanoparticles magnetic solid phase extraction coupled with flame atomic absorption spectrometry for chromium(III) speciation in environmental and biological samples

Applied Surface Science 258 (2012) 6772–6776

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Fe3 O4 @ZrO2 nanoparticles magnetic solid phase extraction coupled with flame atomic absorption spectrometry for chromium(III) speciation in environmental and biological samples Yi-Wei Wu a,b,∗ , Jing Zhang a , Jun-Feng Liu c , Lin Chen a , Zhen-Li Deng a , Mu-Xian Han a , Xiao-Shu Wei a , Ai-Min Yu a , Hai-Li Zhang a a

Department of Chemistry and Environmental Engineering, Hubei Key Laboratory of Pollutant Analysis and Reuse Technique, Hubei Normal University, Huangshi 435002, PR China Key Laboratory of Analytical Chemistry for Biology and Medicine (Wuhan University) 430072, Ministry of Education, PR China c Department of Clinical Laboratory, the Second Hospital of Huangshi, Huangshi 435002, PR China b

a r t i c l e

i n f o

Article history: Received 15 July 2011 Received in revised form 13 January 2012 Accepted 12 March 2012 Available online 3 April 2012 Keywords: Chromium speciation Environmental and biological samples FAAS Magnetic solid phase extraction Magnetic nanoparticles

a b s t r a c t A new method for Cr(III) speciation in seven kinds of environmental and biological samples by Fe3 O4 @ZrO2 nanoparticles magnetic solid phase extraction (MSPE) and flame atomic absorption spectrometry (FAAS) has been developed. Fe3 O4 @ZrO2 nanoparticles were simply prepared by sol–gel method, and the adsorptive behaviors of Cr(III) and Cr(VI) on Fe3 O4 @ZrO2 nanoparticles were assessed. At pH 8.0–9.0, Fe3 O4 @ZrO2 nanoparticles were selective towards Cr(III) but hardly Cr(VI). The retained Cr(III) was subsequently eluted with 3.0 mL of 0.5 mol L−1 HNO3 followed by magnetic decantation. Total chromium was determined after reduction of Cr(VI) to Cr(III) by ascorbic acid. Various parameters affecting Fe3 O4 @ZrO2 nanoparticles MSPE were optimized systematically. Under the optimum conditions, the adsorption capacity of Fe3 O4 @ZrO2 nanoparticles for Cr(III) is 24.5 mg g−1 . With an enrichment factor (EF) of 25, detection limit of Cr(III) was 0.69 ng mL−1 , and the proposed method has been successfully applied for Cr(III) speciation in seven kinds of environmental and biological samples with satisfactory results. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Chemical and biological properties of an element species depend very much on its oxidation state, and an accurate determination of different species of a given element is important for evaluating the comprehension of its biological and physiological functions, as well as potential toxicity [1]. Chromium usually exists in form of Cr(VI) or Cr(III) species in natural media., Cr(VI) is more toxic than Cr(III) owing to its high mobility across biological cell membrane and oxidizing potential [2]. Therefore, differentiation and quantification of chromium in the two oxidation states are important [3]. However, the extreme low concentration, estimated at the level of ng mL−1 , and severe matrix interference existed in actual samples, even sensitive analytical techniques such as FAAS [4], stripping voltammetry [5], ICP-OES [6] and ICP-MS [7] are usually insufficient for chromium. An effective separation and

∗ Corresponding author at: Department of Chemistry and Environmental Engineering, Hubei Key Laboratory of Pollutant Analysis and Reuse Technique, Hubei Normal University, Huangshi 435002, PR China. Tel.: +86 7146515602; fax: +86 7146573832. E-mail address: [email protected] (Y.-W. Wu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.03.057

preconcentration procedure is usually indispensable to differentiate between Cr(VI) and Cr(III). The most widely used techniques for separation and preconcentration of chromium, include cloud point extraction (CPE) [8], membrane extraction (ME) [2,9], dispersive liquid–liquid microextraction [10,11], solid phase extraction (SPE) [12,13], coprecipitation [14], and ion-exchange separation [15,16], have been carried out to concentrate, purify and differentiate between Cr(III) and Cr(VI). Among these techniques, SPE using ion exchange resins or nanometer sorbent have proved to be especially effective. Methods for speciation of Cr(III) and Cr(VI) have been developed by using crosslinked chitosan-bound FeC nanoparticles [1], smallsized thin solid phase column resin reactors [17], amberlite XAD-16 resin [18], and ambersorb 563 resin [19]. In SPE technique, selection of an appropriate sorbent is an important strategy in the elaboration of analytical procedure, and nanometer sorbents have proved to be especially effective as a result of its high specific surface areas, highly active surface sites and the absence of internal diffusion resistance in the separation process [20]. Magnetic nanoparticles (MNPs), as a new kind of nanometer material, have gained more attention in analytical atomic spectroscopy for trace analysis and speciation research owing to its suitable for bulk solution, easy control and fast magnetic separation under an extra magnetic field [21,22]. It should be

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stressed that pure inorganic magnetic particles (such as Fe3 O4 and Fe2 O3 ) are neither likely to form a large aggregation, difficult to operate, nor target-selective and unsuitable for samples with complex matrices. To overcome these problems, modification of the particles’ surface in different ways is usually needed and has been proven to be one of the most efficient ways [23,24]. The aim of this work is to prepare Fe3 O4 @ZrO2 nanoparticles by sol–gel method and explore the possibility of Fe3 O4 @ZrO2 nanoparticles as an adsorbent for speciation of chromium prior to their determination by FAAS. Factors affecting Fe3 O4 @ZrO2 nanoparticles MSPE were investigated in detail. The developed method was applied to the determination of chromium species in seven kinds of environmental water and biological samples. 2. Experimental 2.1. Instrumentation An AA-6200 atomic absorption spectrometer (Shimadzu, Japan) using an air–acetylene flame was employed for the determination of target element (acetylene:air = 2.8:8 L min−1 ; wavelength: 357.9 nm; lamp current: 6 mA; burner height: 7 mm; slit width: 0.2 nm); The reported pH of solution was carefully measured with a PHS-3C pH-meter (Shanghai Precision & Scientific Instrumental Co., Ltd., Shanghai, China). An ultrasonicator (Kedao instrument Co., Ltd., Shanghai, China) was used to disperse the nanoparticles in solution. Scanning electron micrograph (SEM) of Fe3 O4 @ZrO2 was obtained with a JEOL JSM-6700F scanning electron microscope (Japan) at an acceleration voltage of 25 kV. The Nitrogen sorption experiment was carried out at a Coulter (FL, USA) SA 3100 Plus surface area and pore size analyzer. 2.2. Standard solution and reagents Stock solutions (1.0 g L−1 ) of Cr(III) and Cr(VI) were prepared from high-purity CrCl3 ·6H2 O and K2 Cr2 O7 (The First Reagent Factory, Shanghai, China), respectively. The sizes of high purity Fe3 O4 nanoparticles (Shanghai Jingchun Reagent Co. Ltd., China) are about 20 nm. Analytical grade zirconyl chloride was purchased from Tianjin Kermel Chemical Reagent Co., Ltd. 10.0% (m/v) aqueous ascorbic acid solution was prepared fresh daily. Britton–Robinson (B–R) buffer solution, composed of an appropriate volume of 0.04 mol L−1 each of H3 PO4 , HAc and H3 BO3 and 0.2 mol L−1 NaOH, was used to adjust the pH of sample solution. All other chemicals (the First Reagent Factory, Shanghai, China) are of analytical grade, and doubly distilled water (DDW) was used throughout.

Fig. 1. SEM of Fe3 O4 @ZrO2 (surface view: 30,000×).

external magnet and the supernatants were decanted directly. After removing the magnet, 3.0 mL of 0.5 mol L−1 HNO3 was added as eluent and ultrasonicated again for 5 min. Finally, the magnet was used again to settle the MNPs. The eluate was poured out into a test tube for subsequent FAAS analysis. 2.3.3. Sample pretreatment Water samples collected from two natural lakes (Qingshan Lake and Sanjiao Lake, Huangshi, China), Changjiang River (Huangshi, Hubei, China) and landfill leachate (Daye, Huangshi, Hubei, China) were filtered through a 0.45 ␮m membrane filter (Shanghai Xingya Purifying Materials Factory). Then the filtrates were collected and stored at 4 ◦ C in polyethylene container for subsequent usage. Sediment samples were collected from Changjiang River (Huangshi, Hubei, China). The samples were dried in an oven at 105 ◦ C and homogenized with a sieve. Then a 1.0 g sediment sample was weighed and transferred into conical flask. Sample solution was obtained by leaching the sediment with water in a 1:5 ratio (soil:deionized water) for 2 h in an end-over-end shaker, 4000 rpm for 15 min and filtering through 0.45 ␮m filters [8]. Human serum and urine samples, obtained randomly at the Department of Clinical Laboratory, the Second Hospital, Huangshi, China, were stored at 4 ◦ C in polyethylene container for subsequent usage. 3. Results and discussion 3.1. Characterization of Fe3 O4 @ZrO2

2.3. Experiment procedure 2.3.1. Preparation of Fe3 O4 @ZrO2 nanoparticles Zirconia sol was used to accomplish the modification process, and its preparation was similar to the procedure reported by Wu and coworkers [25]. Briefly, 1.0 g of zirconyl chloride was dissolved in 20 mL of a 5:3 (v/v) mixture of ethanol and water and kept at 343 K for 2 h. And 5.0 g of Fe3 O4 nanoparticles were added into the zirconia sol, then ultrasonicated for 1 h. After 12 h, the mixture was heated at 573 K for 2 h in a muffle furnace. 2.3.2. General Fe3 O4 @ZrO2 nanoparticles MSPE procedure A portion of sample containing the target ions was transferred to a 100.0 mL beaker, the pH value was adjusted to 8.0 with B–R buffer solution and the final volume was diluted to 75.0 mL with DDW. Then, 50.0 mg of Fe3 O4 @ZrO2 nanoparticles were added, and the solution was ultrasonicated for 15 min to facilitate adsorption of the target metal ions onto the MNPs. Subsequently, the magnetic adsorbent was separated conveniently and quickly using an

The adsorption characteristics of a material are related to its physical morphology. Thus, the surface morphology of Fe3 O4 @ZrO2 is an important factor affecting its performance. Fig. 1 shows the morphologies of Fe3 O4 @ZrO2 characterized by SEM (surface view: 30,000×). It displays a roughened, porous structure and the size is about 30 nm. In addition, according to the result of BET measurements, specific surface area and the pore diameter of Fe3 O4 @ZrO2 were 87.5 m2 g−1 and 3–5 nm, respectively. Such physical morphology of Fe3 O4 @ZrO2 should significantly increase the available surface area of Fe3 O4 @ZrO2 , improve the kinetics of the extraction and, therefore, enhance extraction efficiency. 3.2. Effect of pH on adsorption of Cr(III) and Cr(VI) The effect of pH on Fe3 O4 @ZrO2 nanoparticles MSPE of 0.5 mg L−1 Cr(III) and Cr(VI) were investigated at pH 2.0–9.0, and the results are presented in Fig. 2. Fe3 O4 @ZrO2 nanoparticles show different adsorption characteristics towards Cr(III) and Cr(VI) with

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adsorbed Cr(III) to Cr(VI), while Cr(VI) usually exists as anions at acidic medium. As a result, both the acidity and the oxidizing of HNO3 may be all advantageous to stripping Cr(III) from Fe3 O4 @ZrO2 nanoparticles. Effects of HNO3 concentrations (0.1–2.5 mol L−1 ) on Cr(III) recovery were further investigated. It was found that 0.5 mol L−1 HNO3 was sufficient to recover Cr(III), and 0.5 mol L−1 HNO3 was selected as the optimum. Effect of HNO3 volume on 0.5 mg L−1 Cr(III) recovery was also studied. It was found that quantitative recoveries (>90%) was obtained with 3.0 mL of 0.5 mol L−1 HNO3 . Therefore, a volume of 3.0 mL of 0.5 mol L−1 HNO3 was used in the following experiments.

Adsorptive percentage (%)

100 Cr (III) Cr (VI)

80 60 40 20 0

3.4. Effect of sample volume

1

2

3

4

5

6

7

8

9

pH Fig. 2. Effect of pH on the adsorption percentage of Cr(III) and Cr(VI). Cr(III) and Cr(VI): 0.5 mg L−1 .

the pH range. The adsorptive rates of Cr(III) increase with pH, then attain a quantitative platform (>90%) at pH 8.0–9.0. On the contrary, Cr(VI) almost cannot be adsorbed by Fe3 O4 @ZrO2 nanoparticles at pH 2.0–9.0. Consequently, Fe3 O4 @ZrO2 nanoparticles are quantitatively selective towards Cr(III) but hardly Cr(VI) at pH 8.0–9.0, and a pH of 8.0 was selected in the subsequent work. The adsorption mechanism of Cr(III) and Cr(VI) on Fe3 O4 @ZrO2 nanoparticles may be as following. The pH of the solution influences the distribution of active sites on the surface of ZrO2 , and the OH group on the surface provides the ability of a binding cation [25]. Trivalent chromium tends to form cation (Cr(OH)2+ , Cr(OH)2 + ) or electrically neutral products (Cr(OH)3 ), while hexavalent chromium usually exists as anions of HCrO4 − , Cr2 O7 2− and CrO4 2− . As a result, Fe3 O4 @ZrO2 nanoparticles are likely to absorb trivalent chromium.

In order to explore the possibility of enriching low concentration of analyte from large volume, the maximum applicable sample volume must be determined. For this purpose, 10.0, 25.0, 50.0, 75.0 and 100.0 mL of sample volume containing 2.0 ␮g Cr(III) were tested. The results indicated that recoveries were quantitative (over 90%) within 10.0–75.0 mL. As described previously, 3.0 mL of 0.5 mol L−1 HNO3 is enough to elute the analyte from Fe3 O4 @ZrO2 nanoparticles, enrichment factor (EF) of 25 could be obtained. This trait gives the feasibility of determination of the samples with different analyte concentration levels. 3.5. Effect of ultrasonic time Ultrasonic time for adsorption and elution was also optimized in order to minimize the time required for sample processing. Different time for adsorption (7, 12, 17, 22 min) and elution (3, 5, 8, 10 min) were investigated, and the results revealed that quantitative recovery of Cr(III) (over 90%) was achieved when the ultrasonication time was greater than 7 min for adsorption and greater than 5 min for elution. Therefore, 7 min and 5 min were used for adsorption and elution in the subsequent experiments, respectively.

3.3. Effect of elution condition With respect to stripping of Cr(III) from Fe3 O4 @ZrO2 nanoparticles, HNO3 , HCl, thiourea in 0.1 mol L−1 HCl and EDTA were selected and examined as the possible elution solvents, and the results are displayed in Fig. 3. As seen, only HNO3 has the best elution effect, and HNO3 was adopted as the eluent. The difference between HNO3 and HCl is the oxidation capability of HNO3. HNO3 may oxidize the

100

HNO3

Recovery (%)

80

Conventional static SPE usually requires filtration or centrifugation to separate the adsorbent from aqueous solutions, which makes the method complex and time-consuming. While in MSPE procedure, the adsorbent could be separated rapidly from the sample solutions using an external magnetic field due to the superparamagnetism of the nanoparticles. Effect of sediment time on the Cr(III) recovery was investigated, and no significant effect was found when the sedimentation time was greater than 2 min. Therefore, 2.5 min sediment time was enough for this work. 3.7. Effect of coexisting ions

60

40

HCl 20

3.6. Effect of sediment time

EDTA

thiourea

0

eluent Fig. 3. Effect of various eluents on the recovery of Cr(III). Cr(III): 0.5 mg L−1 , HNO3 : 0.5 mol L−1 , HCl: 2.0 mol L−1 , thiourea in 0.1 mol L−1 HCl: 60 g L−1 , EDTA: 0.02 mol L−1 .

Effects of common coexisting ions on Fe3 O4 @ZrO2 nanoparticles MSPE of Cr(III) were investigated under the optimum conditions. In these experiments, the target analyte solutions containing the added other ions were treated according to the recommended procedure. Effect is expressed as Cr(III) recovery in the presence of interfering ions relative to the interferencefree response. The results showed that Cr(III) recoveries were between 90% and 110% even in the presence of the following ions: 10.0 mg mL−1 Na+ , 8.0 mg mL−1 K+ , 150.0 ␮g mL−1 Mg2+ , 100.0 ␮g mL−1 Ca2+ , 60.0 ␮g mL−1 Al3+ , 60.0 ␮g mL−1 Cu2+ , 4.0 mg mL−1 Cl− , 4.1 mg mL−1 PO4 3− , 50.0 ␮g mL−1 SO4 2− and 145 ␮g mL−1 NO3 − , indicating the method has a good tolerance to matrix interference with a potential to be applied to analyze real samples.

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Table 1 Figures of merits of methods for speciation of Cr(III) and Cr (VI). Matrix

Preconcentration technique and EF

LOD for Cr(III)

Detection technique

Ref.

Environmental and biological samples Water Sediment

Fe3 O4 @ZrO2 nanoparticles MSPE EF: 25 for Cr(III)

0.69 ␮g L−1

FAAS

This work

SPE (FeC nanoparticles) Cloud point extraction EF: 45 for Cr(III) and 40 for Cr(VI).

0.052 ␮g L−1 7.50 ␮g L−1 for Cr(III)

FAAS HPLC

[1] [8]

3.50 ␮g L−1 for Cr(VI) 1.00 ␮g L−1

FAAS

[14]

ICP-AES

[18]

0.04 ␮g L−1 for Cr(VI) 45 ␮g L−1 for Cr(VI) 2.70 ␮g L−1 for Cr(VI)

FAAS Spectrophotometric

[19] [20]

0.043 ␮g L−1 for Cr

ICP-OES

[22]

0.66 ␮g L−1

FAAS

[23]

0.07 ␮g L−1 for Cr(VI)

ETAAS

[26]

2 ␮g L−1

GC-FPD

[27]

Natural water

Coprecipitation of Cr(III) on EPHBAT without carrier element EF: 50 for Cr(III) SPE (small-sized thin solid phase (STSP) column resin)

Fresh water

−1

0.02 ␮g L

SPE (amberlite XAD-16 resin) EF: 25 for Cr(III) SPE (ambersorb 563 resin)

Water Tannery waste water and sediment River and lake water

Bismuthiol-II-immobilized magnetic nanoparticles MSPE EF: 96 for Cr AEAPS-SCMNPs nanoparticles MSPE EF: 100 for Cr(III) Ultrasonic probe-assisted ionic liquid dispersive liquid–liquid microextraction EF: 300 for Cr(VI) SPME

Environmental water and human serum s Lake water and tap water Water

3.8. Adsorption capacity Adsorption capacity of Fe3 O4 @ZrO2 nanoparticles is an important factor because it will reflect how much Fe3 O4 @ZrO2 nanoparticles is required to quantitatively concentrate Cr(III) from a given solution. Therefore, 50.0 mg of Fe3 O4 @ZrO2 nanoparticles was added into 10.0 mL of 20.0, 50.0, 100.0, 150.0 and 200.0 mg L−1 Cr(III), respectively, and the results showed the static adsorption capacity of Cr(III) was 24.5 mg g−1 . It is higher than other MNPs [1,21,22]. 3.9. Analytical performance The linear range of this method was 4.0–400.0 ng mL−1 . Under the optimum conditions, the detection limit (evaluated as the concentration corresponding to three times the standard deviation of 9 runs of the blank solution) of this method for Cr(III) with an EF

for Cr(III)

of 25 is 0.69 ng mL−1 , and the relative standard deviation (RSD) is 2.1% (n = 11, c = 20 ng mL−1 ). A comparison of this method with others for speciation of Cr(III) or Cr(VI) is shown in Table 1. As can be seen, the LOD obtained by this method is superior to other methods [8,14,19,20,27].

3.10. Analytical application The proposed method has been applied to the determination of Cr(III) speciation in environmental water and sediment samples (15.0 mL of the filtrates was diluted to 25.0 mL with B–R buffer solutions at pH 8.0), human serum samples (0.5 mL of the human serum was directly diluted to 5.0 mL with the same buffer solutions) and urine samples (5.0 mL of the urine samples was directly diluted to 10.0 mL with the same buffer solutions). The results of seven kinds of samples in Table 2 show that the recoveries of Cr(III) and Cr(VI)

Table 2 Determination of Cr(III) speciation in environmental and biological samples (mg L−1 ). Samples

Qingshan Lake Sanjiao Lake Changjiang River Sediment Landfill leachate Serumc Serumd Urinee Urinef

Foundeda

Added Cr(III)

Cr(VI)

Cr(III)

Cr(VI)

Total

0 4.0 0 4.0 0 3.3 0 3.3 0 3.3 0 1.0 0 1.0 – 3.3 – 3.3

0 4.0 0 4.0 0 3.3 0 3.3 0 3.3 0 1.0 0 1.0 0 3.3 0 3.3

– 4.03 ± – 4.29 ± – 3.23 ± – 3.15 ± – 3.13 ± 0.23 ± 1.21 ± 0.61 ± 1.63 ± – 3.21 ± – 3.34 ±

– 4.23 ± – 3.76 ± – 3.37 ± – 3.35 ± – 3.41 ± – 1.02 ± – 0.99 ± – 3.40 ± – 3.1 ±

– 8.25 ± – 8.03 ± – 6.58 ± – 6.49 ±

Mean ± average deviation, n = 5. b Calculated value. c Serum sample of patient with in type 2 diabetes mellitus. d Serum sample of healthy person. e Urine samples of patient with in type 2 diabetes mellitus. f Urine samples of healthy person. –: No detected. a

Recovery (%) b

0.04 0.03 0.02 0.02 0.02 0.002 0.03 0.002 0.01 0.02 0.02

0.03 0.02 0.01 0.03 0.01 0.02 0.02 0.02 0.02

6.55 ± 0.22 ± 2.22 ± 0.68 ± 2.60 ± – 6.64 ± – 6.42 ±

Cr(III)

Cr(VI)

0.05

100.8

105.8

0.03

107.2

94.0

0.02

97.9

102.1

0.02

95.4

101.5

0.01 0.003 0.03 0.002 0.02

94.8

103.3

98.0

102.0

102.0

99.0

0.02

97.3

103.0

0.01

101.2

93.9

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are acceptable, between 93.9 and 105.8%, indicating the method is potentially useful for speciation of chromium in various matrices. To verify the validity of the method, total chromium in GBW07605 certified reference Tea Leaves (obtained from Geophysical and Geochemical Prospecting, Langfang, P.R. China) was analyzed. A 0.12 g amount was weighed and dissolved in 12.0 mL of HNO3 –HClO4 (4:2, v/v) on a hot plate under mild heating, then vaporized to near dryness, and dissolved in the B–R buffer solution containing 1.0 mL of 10% (m/v) aqueous ascorbic acid solution to reduce Cr(VI) to Cr(III). Total chromium concentration was determined according to the recommended procedure. The value determined by this method (0.81 ± 0.015 ␮g g−1 ) was in good agreement with the certified value (0.80 ± 0.02 ␮g g−1 ).

[8]

[9]

[10]

[11]

[12]

4. Conclusions [13]

A new magnetic sorbent of Fe3 O4 @ZrO2 nanoparticles has been simply prepared by sol–gel method and used to separate/preconcentrate chromium species in seven kinds of environmental and biological samples. Fe3 O4 @ZrO2 nanoparticles are highly monodisperse and magnetically separable. A simple, selective, and sensitive MSPE–FAAS method, resulting in a 25-fold enhancement in sensitivity, has been established for separation and determination of chromium species in various matrices with satisfactory results.

[14]

[15] [16] [17]

Acknowledgements This project was financially supported by National Nature Science Foundation of China (No. 20975031), Education Committee of Hubei Province (Q20102506), Key Laboratory of Analytical Chemistry for Biology and Medicine (Wuhan University), Ministry of Education (ACBM2010004), Young and Middle-Aged Elitists’ Scientific and Technological Innovation Team Project of Hubei Education Department of China under Grant (T201007). References [1] Y.W. Wu, Y.Y. Jiang, D.Y. Han, F. Wang, J.X. Zhu, Speciation of chromium in water using crosslinked chitosan-bound FeC nanoparticles as solid-phase extractant, and determination by flame atomic absorption spectrometry, Microchim. Acta 159 (2007) 333–339. [2] R.A. Kumbasar, Selective extraction of chromium(VI) from multicomponent acidic solutions by emulsion liquid membranes using tributhylphosphate as carrier, J. Hazard. Mater. 178 (2010) 875–882. [3] M.F. Bergamini, D.P. Dos Santos, M.V.B. Zanoni, Development of a voltammetric sensor for chromium(VI) determination in wastewater sample, Sens. Actuators B: Chem. 123 (2007) 902–908. [4] P. Hemmatkhah, A. Bidari, S. Jafarvand, Speciation of chromium in water samples using dispersive liquid–liquid microextraction and flame atomic absorption spectrometry, Microchim. Acta 166 (2009) 69–75. [5] E.O. Jorge, M.M. Rocha, I.T.E. Fonseca, M.M.M. Neto, Studies on the stripping voltammetric determination and speciation of chromium at a rotating-disc bismuth film electrode, Talanta 81 (2010) 556–564. [6] Y.J. Li, B. Hu, Z.C. Jiang, Speciation of chromium in water samples by cloud point extraction combined with low temperature electrothermal vaporization ICP-OES, Anal. Lett. 39 (2006) 809–822. [7] W.L. Hu, F. Zheng, B. Hu, Simultaneous separation and speciation of inorganic As(III)/As(V) and Cr(III)/Cr(VI) in natural waters utilizing capillary

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