Speciation of chromium in environmental samples by dual electromembrane extraction system followed by high performance liquid chromatography

Analytica Chimica Acta 789 (2013) 58–64

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Speciation of chromium in environmental samples by dual electromembrane extraction system followed by high performance liquid chromatography Meysam Safari, Saeed Nojavan ∗ , Saied Saeed Hosseiny Davarani ∗ , Amin Morteza-Najarian Faculty of Chemistry, Shahid Beheshti University, G. C., 1983963113 Evin, Tehran, Iran

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Speciation of chromium in different water samples.

• Chelating of Cr species with APDC and quantification using HPLC.

• Application of dual electromembrane extraction system.

• Simultaneous extraction of anionic and cationic species of Cr.

• Quantification of Cr(VI) and Cr(III) in tap, mineral and river water samples.

a r t i c l e

i n f o

Article history: Received 12 March 2013 Received in revised form 17 June 2013 Accepted 17 June 2013 Available online 21 June 2013 Keywords: Chromium speciation Dual electromembrane extraction Environmental samples Liquid phase microextraction

a b s t r a c t This study proposes the dual electromembrane extraction followed by high performance liquid chromatography for selective separation-preconcentration of Cr(VI) and Cr(III) in different environmental samples. The method was based on the electrokinetic migration of chromium species toward the electrodes with opposite charge into the two different hollow fibers. The extractant was then complexed with ammonium pyrrolidinedithiocarbamate for HPLC analysis. The effects of analytical parameters including pH, type of organic solvent, sample volume, stirring rate, time of extraction and applied voltage were investigated. The results showed that Cr(III) and Cr(VI) could be simultaneously extracted into the two different hollow fibers. Under optimized conditions, the analytes were quantified by HPLC instrument, with acceptable linearity ranging from 20 to 500 ␮g L−1 (R2 values ≥ 0.9979), and repeatability (RSD) ranging between 9.8% and 13.7% (n = 5). Also, preconcentration factors of 21.8–33 that corresponded to recoveries ranging from 31.1% to 47.2% were achieved for Cr(III) and Cr(VI), respectively. The estimated detection limits (S/N ratio of 3:1) were less than 5.4 ␮g L−1 . Finally, the proposed method was successfully applied to determine Cr(III) and Cr(VI) species in some real water samples. © 2013 Elsevier B.V. All rights reserved.

1. Introduction

Abbreviations: APDC, ammonium pyrrolidinedithiocarbamate; CPE, cloud point extraction; DEME, dual electromembrane extraction; FAAS, flame atomic absorption spectroscopy; HF, hollow fiber; ICP, inductively coupled plasma; NPOE, 2-nitrophenyl octyl ether. ∗ Corresponding authors. Tel.: +98 21 22431667; fax: +98 21 22431663. E-mail addresses: s [email protected] (S. Nojavan), [email protected], s [email protected] (S.S.H. Davarani). 0003-2670/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aca.2013.06.023

Contamination of heavy metals in the environment is a global concern because of their toxicity and threat to human life and environment. In response to this growing problem, determination of trace amounts of these heavy metals in different environmental samples plays a decisive role. According to the World Health Organization (WHO), the most toxic metals are aluminum, chromium, iron, cobalt, nickel, copper, zinc, cadmium, mercury and lead [1]. It has been widely recognized that the impact of detrimental heavy

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metals on the ecological system, biological organisms, as well as human health, not only depends on the total amount of the element, but also depends significantly on its chemical forms [2–4]. Chromium occurs naturally in the earth crust, but its extensive use in various industrial processes such as mining, leather tanning, textile dyeing, electroplating and wood preservatives has led to widespread chromium contamination in the environment. World Health Organization (WHO) recommended maximum allowable concentration in drinking water for chromium (VI) is 0.05 mg L−1 [5]. It is well known that the toxicological, as well as biological properties of chromium, are strongly dependent on its chemical forms [6–8]. Cr(III) is considered as an essential trace element for the proper functioning of living organisms. It is an essential nutrient that potentiates insulin action and thus influences carbohydrate, lipid and protein metabolism [7,9]. Chromium in its (VI) oxidation state is a health hazard as it may be involved in the pathogenesis of some diseases like liver, kidney, lung and gastrointestinal cancers. It can induce carcinogenesis because of its ability to penetrate biological membranes and react with protein components and nucleic acids inside the cell [10,11]. Total chromium does not provide sufficient information to understand its toxicity, bioavailability, biotransformation and ways of circulation. In view of these facts, the development of speciation techniques with high sensitivity and sufficient selectivity is a challenge for analytical chemists [12,13]. The available methods for total chromium determination include flame atomic absorption spectrometry (FAAS), inductively coupled plasma-atomic emission spectrometry (ICP-AES), ICP-mass spectrometry (ICP-MS), electrothermal-AAS (ET-AAS), electrochemistry, fluorometry and chemiluminescence [14–21]. These methods can easily detect the total amount of chromium but cannot distinguish between Cr(III) and Cr(VI). The speciation of chromium calls for efficient separation technique which can provide reliable results as well as high preconcentration factors (PFs). The separation techniques include chemical co-precipitation [22], liquid–liquid extraction [23], single-drop microextraction [24], cloud point extraction [8,15,25], solid-phase microextraction [26], emulsion liquid membrane [27], solid phase extraction [28,29] and temperature controlled microextraction [30]. Electromembrane extraction (EME) as an alternative to hollow fiber-liquid phase microextraction (HF-LPME) was introduced by Pedersen-Bjergaard in the year 2006 [31]. In this method, the applied electrical potential across the supported liquid membrane (SLM) can be acted as a powerful driving force for migration of charged specious toward the electrode of opposite charge in the acceptor phase [32,33]. In a study, EME coupled with capillary electrophoresis (CE) and ultraviolet (UV) detection was developed for determining lead (Pb) ions [34]. In this study ethylenediaminetetraacetic (EDTA) was used as complexing agent. In another work, EME was used as an off-line sample pre-treatment method for the determination of heavy metal cations in aqueous samples using CE with capacitively coupled contactless conductivity detection [35]. Also, the same authors used EME procedure for simultaneous sample cleanup and preconcentration of lithium from untreated human body fluids [36]. First time, Basher et al. reported simultaneous extraction of positive and negative species using EME procedure [37]. However, recently a new setup named as dual electromembrane extraction (DEME) was introduced and applied for simultaneous extraction of basic and acidic compounds [38–40]. In DEME procedure, both electrodes (anode and cathode) were placed into the separate hollow fibers. In this work, a simultaneous extraction and speciation of chromium species has been carried out using DEME technique. This setup enables the extraction of positive and negative charged

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species at the same time. Finally, the proposed method was successfully applied for speciation of chromium in different environmental samples. 2. Experimental 2.1. Instrumentals and chemicals The DC power supply used was an EPS-600Z model (Paya Pajohesh Pars, Tehran, Iran) with programmable voltage in the range of 0–600 V, providing currents in the range of 0–0.5 A. Platinum wires with a diameter of 0.25 mm were obtained from Pars Platin (Tehran, Iran). The utilized hollow fibers were PP Q3/2 polypropylene hollow fiber (Membrana, Wuppertal, Germany) with an internal diameter of 1.2 mm, 300 ␮m wall thickness, and 0.2 ␮m pores. All chemicals were of analytical grade, and water used to prepare sample solution was freshly deionized by Milli-Q (ultra-pure) water purification system (Bedford, MA, USA). Solutions of ammonium pyrrolidinedithiocarbamate (APDC, 20 g L−1 ) were prepared every day by dissolving 1.0 g of APDC (Shanghai Reagent Factory, China) in 50.0 mL of ultrapure water. The pH values were adjusted by the addition of 0.1 mol L−1 ammonia or hydrochloric acid solutions before use. 1-Hexanol, 2-nitrophenyl octyl ether (NPOE) and 1-octanol were purchased from Sigma–Fluka (Buchs, Switzerland) whereas potassium hydroxide (KOH), hydrochloric acid (HCl), Cr(NO3 )3 , K2 Cr2 O7 , dodecanol, toluene, nitrobenzene were produced by Merck (Darmstadt, Germany). 2.2. Chromatographic analysis The chromatographic system used was an Agilent Technologies 1200 series system consisting of a solvent degasser (G1322A), a quaternary pump (G1311A), a manual injection valve (G1328B) equipped with a 50 ␮L injection loop and a variable wavelength UV-detector (G1314B). Separations were carried out on an Eclipse XDB-C18 HPLC column (150 mm × 4.6 mm, 5 ␮m) (Agilent Technologies, CA, USA). Data acquisition was performed by using Chemstation software (Agilent Technologies). In this work, a mixture of methanol–water (75:25, v/v) was used as the mobile phase at a flow rate of 1.0 mL min−1 and the wavelength used for UV detector was 254 nm. 2.3. Standard solutions and samples Stock solutions of Cr(III) and Cr(VI) at a concentration of 1000 mg L−1 were prepared from Cr(NO3 )3 and K2 Cr2 O7 , respectively. Working standard solutions were prepared by stepwise diluting the stock solutions just before use. Sample solutions were also prepared daily by dilution of the stock solutions to a final concentration of 1 ␮g mL−1 . The pH values were adjusted by the addition of 0.1 mol L−1 ammonia or hydrochloric acid solutions before use. 2.4. Dual EME procedure As sample compartment, 2.4 mL homemade glass vial was used with a height of 50 mm and internal diameter of 8 mm. Hollow fibers were cut into 31 mm pieces, and then were dipped into the organic solvent for 10 s to impregnate the pores. The lumen of each hollow fiber was filled with 30 ␮L of acceptor phase (aqueous solution) using a HPLC syringe. The end of the hollow fiber was sealed using a pair of hot flat-tip pliers. Both electrodes (anode and cathode) were placed into the lumen of HF, separately. 2.1 mL sample solution was injected into the sample vial and the hollow fibers and electrodes were placed into the vial. The predetermined voltage

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was turned on, and extraction was performed. During the extraction, sample solution was stirred at 750 rpm. After predetermined time, the power supply was cut off, and the fibers were taken out. Acceptor phases were collected using a HPLC syringe and placed into a microvial. Subsequently, 50 ␮L of APDC solution was successively added to a test tube with a plug, and the mixture was then incubated for 5 min in a thermo stated water bath maintained at the desired temperatures used for equilibration temperature experiments (65 ◦ C). Finally, 100 ␮L of the solution was collected and injected to 50 ␮L injection port of HPLC to run the detection. 3. Results and discussion 3.1. Complexation of chromium species A complexation step is necessary for analysis of inorganic cations by HPLC that contain UV detection. Ammonium pyrrolidinedithiocarbamate (APDC) is most widely used complexation reagent for quantification of inorganic cations, such as Cr, Cd, Pb, Ni, Co, etc [41]. Usually, cation–APDC complexes were measured by UV–vis spectrophotometry [42] and flame atomic absorption spectrometry [43]. It is well known that both Cr(VI) and Cr(III) react with APDC and form different types of complexes [44]. Actually, Cr(III) reacts directly with APDC to give as Cr(III)-PDC. On the other hand, APDC reduces Cr(VI) to Cr(III) and leads to form two different complexes: the bis-[pyrrolidine-1-dithioato-S,S ][pyrrolidine-1-peroxodithioato-O,S]-Cr(III) denoted as Cr(VI)-MP, which is the main product, and the byproduct tris-[pyrrolidine1-dithioato-S,S]-Cr(III), denoted as Cr(VI)-BP [45]. In this study, only the complexes of Cr(III)-PDC and Cr(VI)-MP were considered for determination of Cr(III) and Cr(VI), respectively, neglecting the existence of Cr(VI)-BP. The optimized conditions for this complexation were reported in several works [46]. The focus of this work is on the extraction procedure, and complexation was done after extraction. Thus, fully description of optimization for complexation step is neglected. 3.2. Optimization of dual EME procedure As it is well known, EME efficiency can be affected by several working parameters, including SLM solvent, sample solution volume, sample solution pH, extraction time, acceptor solution pH, stirring rate as well as applied voltage. The effects of above mentioned experimental parameters were investigated by modifying one at a time while keeping the remaining constant. All the experiments were performed in triplicate, and the mean of chromatographic peak areas was the parameter used to evaluate the influence of those variables on the extraction efficiency of EME technique. The aim of this work is simultaneously extraction of Cr(III) and Cr(VI) at the same time. In some parameters, the optimized values for extraction of two species were different. Consideration that Cr(VI) is toxic form of Cr and detrimental to health [47], the quantification of this species of chromium is more important than Cr(III). Consequently, the optimized values for maximum extraction recovery of Cr(VI) oxidation state were chosen for the work. 3.2.1. Selection of SLM solvent In conventional liquid–liquid–liquid microextraction (LLLME) techniques, the SLM, as a medium between the donor and acceptor solutions, prevents from the mixing of two aqueous phases (acceptor and donor) and transfer the analytes from donor to acceptor phase. This same approach was also adopted in EME procedure [32]. Regarding the EME process, the selection of a suitable organic solvent is limited by several characteristics that are necessary for enabling the electrokinetic migration and phase transfer of analytes

Fig. 1. Effect of SLM solvent on the extraction efficiency; analyte concentration: 100 ␮g L−1 ; voltage: 50 V; pH of sample solution: 7; pH of acceptor solution: 7; sample solution volume: 2.1 mL; extraction time: 10 min, and stirring rate: 750 rpm. Error bars were obtained based on four replicates.

in the presence of electrical potential. Some of these characteristics are relatively good stability along the extraction time and low water solubility. Furthermore, because of using applied voltage, it is crucial for the organic solvents to have sufficient electrical conductance to allow a continuous electric field in the entire system. Based on these considerations and other experiences, five common organic solvents including 1-hexanol, 1-octanol, dodecanol, toluene and NPOE were examined in the preliminary experiments. As it is illustrated in Fig. 1, the results revealed that 1-octanol had the highest extraction efficiency compared to the other tested solvents. Therefore, 1-octanol was selected for further examinations. 3.2.2. Effect of pH in the sample and acceptor solutions The pH of sample solution determines the existing form of the chromium species in the solution. Therefore, pH of sample solution plays a decisive role in extraction selectivity and efficiency of extraction. Depending on the solution pH values, Cr(VI) species may be in the form of dichromate (Cr2 O7 2− ), hydrochromate (HCrO4 − ), or chromate (CrO4 2− ). Cr(III) species may take the form of hydrated trivalent chromium, Cr(H2 O)6 3+ , and chromium hydroxide complexes, Cr(OH)(H2 O)5 2+ or Cr(OH)2 (H2 O)4 + . All forms of Cr(VI) species have the negative charge, and all forms of Cr(III) species have the positive charge. Consequently, in the presence of applied voltage in the solution, Cr(VI) species migrated toward the positive electrode and Cr(III) species migrated toward the negative electrode. It is difficult to predict which form of these species in the solution could extract better than others because the form with the higher number of charges feels stronger electrical potential, but the diffusion to the SLM is more difficult for that forms. Based on these considerations, the pH of sample solution should be optimized experimentally. In this section, the pH of both donor and acceptor solutions were optimized for maximum extraction recovery. For this purpose, pH of acceptor solution was constant at 7 when the pH of sample solution was optimized (4–7). The solutions of HCl (0.1 M) and ammonia (0.1 M) were employed to adjust the pH of donor solutions to 4, 5, 6 and 7. As indicated by Fig. 2A, the best extraction results were achieved at pH 6. It seems that Cr(VI) has anionic form as HCrO4 − in pH 6 and this form could extract easily into the SLM. Also, the presence of other ions (such as H+ ) in acidic pHs could affect the thickness of double layer and it can affect the extraction efficiency. Similar experiments were performed to investigate the effect of

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Fig. 2. Effect of (A) sample solution pH and (B) acceptor solution pH on the extraction efficiency; analyte concentration: 100 ␮g L−1 ; SLM solvent: 1-octanol; voltage: 50 V; sample solution volume: 2.1 mL; extraction time: 10 min, and stirring rate: 750 rpm; acceptor solution pH for (A) was 7 and donor solution pH for (B) was 6. Error bars were obtained based on four replicates.

Fig. 3. Effect of (A) voltage and (B) stirring speed on the extraction efficiency; analyte concentration: 100 ␮g L−1 ; SLM solvent: 1-octanol; acceptor solution pH: 7; donor solution pH: 6; sample solution volume: 2.1 mL; extraction time: 10 min; stirring speed for (A) was 750 rpm and voltage for (B) was 30 V. Error bars were obtained based on four replicates.

pH in the acceptor solution (3–8). In this experiment, pH of sample solution was adjusted at 6. The results are summarized in Fig. 2B. For basic compounds, the pH of acceptor solution should be sufficiently low to maintain analytes in the ionized form and prevent back extraction of analytes into the organic phase. But, inorganic cations and anions usually have ionic forms in the wide range of pHs. Thus, the optimized pH should be obtained experimentally. Also, a gradual pH increase during extraction may occur because of hydronium ions reduction on the surface of anode in the acceptor phase. Thus, this subject should be considered. High extraction recoveries were obtained at pH 7. In the other hand, extraction efficiency of analytes can be affected by competition among other existing ions in the sample solution such as proton ions resulted from acidic solutions with analytes ions.

heating) of the whole system especially in the HF and this can lead to loss of the organic solvent impregnated in the wall of HF [37]. Also, increasing the voltage may cause to produce faradic current and occurring unwanted electrolytic reaction on the surface of electrodes. As it is shown in Fig. 3A, the effect of applied voltage on the extraction efficiency was investigated. The highest efficiency for both chromium species was obtained in the level of 30 V. In that case, 30 V of applied voltage was chosen for the subsequent analyses. Also, it was found that without application of voltage, extraction was not possible.

3.2.3. Effect of applied voltage In EME, the main driving force for migration, diffusion and phase transfer of analytes is the applied electric potential across the HF. Obviously, the applied voltage determines the strength of electrical field which the analytes sense in the solution. Thus, applied voltage is one of the most important affecting parameters that should be optimized regarding to reach a maximum performance. However, high level of applied voltage increases the temperature (Joule

3.2.4. Effect of stirring speed It is well-known that extraction can be accelerated by stirring aqueous solution. Good stirring permits the continuous exposure of the extraction surface to fresh aqueous sample and also reduces the thickness of double layer around the SLM [48]. Moreover, the possibility of organic solvent losing increases at higher stirring rates which can be attributed to increasing the possibility of its dissolution into the sample solution at higher speeds. Moreover, such this high stirring speed leads to generate excess air bubbles and intense vortex in the solution. To evaluate the effect of stirring rate on the extraction efficiency of chromium species, sample solutions were continuously stirred at different stirring rates (0–1000 rpm)

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Cr(VI) was 200 times for Mg2+ and 50 times for Cu2+ and Zn2+ . This demonstrates that the common coexisting ions did not have a significant effect on the separation and determination of Cr(III) and Cr(VI), and the developed method was free from interferences. 3.3. Validation of proposed method

Fig. 4. Effect of time on the extraction efficiency; analyte concentration: 100 ␮g L−1 ; SLM solvent: 1-octanol; acceptor solution pH: 7; donor solution pH: 6; sample solution volume: 2.1 mL; stirring speed: 750 rpm, and voltage: 30 V. Error bars were obtained based on four replicates.

and the signal intensities of target analytes were obtained in the acceptor phase. As seen from Fig. 3B, stirring enhanced the diffusion of the analytes into the organic phase and the amount of analytes extracted reached its highest value at 500 rpm. Higher stirring rates showed lower efficiencies and also precision was poorer. Higher stirring rates lead to leakage of the SLM to the sample and acceptor solutions, and finally the evacuation of the wall of HF from the organic solvent. Therefore, 500 rpm was selected on the basis of these observations. 3.2.5. Extraction time In this work, the effect of the extraction time was examined in the range of 3–15 min with constant experimental conditions. As it is illustrated in Fig. 4, extraction efficiency of Cr(III) was almost constant in the range of 6–9 min. However, there was a sharp increase in analytical signals of Cr(VI) up to 9 min, but after that, a sharp decrease of signals was observed. The effect of extraction time increasing was same as the high applied voltage. As it is described before, it may be due to the organic solvent evaporation (Joule heating), dissolution of the organic phase in the sample solution and also occurring unwanted electrolytic reactions at the surface of electrodes [37,49,50]. Subsequently, 9 min of extraction time was selected as an optimum value for the analyses. 3.2.6. Interference studies Under the optimized conditions, interference studies were carried out by individually spiking increasing amounts of foreign metal ions into the test standard solutions containing two chromium species (each at 100 ␮g L−1 ) before they were subjected to the extraction. A deviation greater than ±8% from the optimized signals was used as the criterion for interference. It was found that the tolerable concentration ratio of foreign ions to 100 ␮g L−1 Cr(III) or

In order to evaluate the practical applicability of the proposed EME technique for speciation of chromium, the optimized extraction conditions were used to determine the method detection limit, linearity and precision (inter-day and intra-day). The performance of the method is summarized in Table 1. The linearity of the method was tested at five different concentration levels, ranging from 20 to 500 ␮g L−1 . Repeatability (intra-day precision) of the EME-HPLC measurements, reported as RSD values (n = 5) in peak areas, was 10.7% and 13.1% for Cr(VI) and Cr(III), respectively. Reproducibility (inter-day precision) of proposed method reported as RSD values (n = 5 day) in peak areas, was 11.8% and 13.7% for Cr(III) and Cr(VI), respectively. External calibration plots were constructed and excellent linearity were obtained with correlation coefficients of 0.9979 and 0.9981 for Cr(III) and Cr(VI), respectively. The enrichment factor (EF) or preconcentration factor (PF) and extraction recovery (R) were calculated based on the following equations:



R=

na,final nd,initial

 EF =



Ca,final Cd,initial

× 100 =

 V   C a a,final Vd

×

Cd,initial

× 100%

(1)

 (2)

where nd,initial and na,final are the analyte moles initially present in the sample solution and the analyte moles collected in the acceptor solution at the end, respectively. Va is the volume of acceptor solution, Vd is the sample volume, Ca,final is the final concentration of analyte in the acceptor solution obtained from the calibration curve of the analyte after the extraction and Cd,initial is the concentration of analyte in donor solution. EFs of 21.8–33 that corresponded to recoveries ranging from 31.1% to 47.2% were achieved. The limit of detection (LOD) of each analyte was calculated experimentally by considering the HPLC signal to be distinctly discerned at a signal to noise (S/N) ratio of 3 at the final lowest concentration. Limit of detections (LODs) were found to be 2.8 and 5.4 ␮g L−1 for Cr(VI) and Cr(III), respectively. 3.4. Environmental samples analyses The proposed method was applied to the speciation of Cr(III) and Cr(VI) in tap, river and mineral water and their spiked water samples. The results are shown in Table 2. Before any treatment, the pH of each real sample was adjusted to 6. The water samples were analyzed according to the obtained optimum extraction conditions. No Cr(III) and Cr(VI) were detected in the tap and river water. The concentration of Cr(VI) in mineral water was determined as 11.1 ␮g L−1 . Subsequently, the real water samples were spiked at 100 ␮g L−1 level of chromium species, to evaluate the

Table 1 Quantitative performance of proposed method (concentrations are based on ␮g L−1 ). Analyte

Cr(III) Cr(VI) a b c

Equation

Y = (11.3 ± 0.37)X + (24.2 ± 1.09) Y = (17.5 ± 0.51)X + (14.1 ± 0.59)

Extraction recovery (%)

31.1 47.2

PFa

21.8 33.0

Preconcentration factor (PF) was obtained for 100 ␮g L−1 of each analyte. Intra-day precision was calculated for five replicates at the concentration of 100 ␮g L−1 . Inter-day precision was obtained for five days at the concentration of 100 ␮g L−1 .

LOD

5.4 2.8

Linearity

20–500 10–500

R2

0.9979 0.9981

Precision Intra-dayb

Inter-dayc

13.1 10.7

13.7 11.8

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Table 2 Speciation of Cr(III) and Cr(VI) in environmental samples. Sample

Added (␮g L−1 )

Found (␮g L−1 )

Relative recovery (%)

RSD (%)

Cr(III)

Cr(VI)

Cr(III)

Cr(VI)

– 91.4 93.4

– 93.8 99.1

– 12.4 13.4

– 12.7 10.1

n.d 45.7 95.6

– 87.8 91.5

– 91.4 95.6

– 11.7 12.4

– 10.9 9.8

11.1 59.4 110.9

– 103.6 83.0

– 96.6 99.8

– 13.7 12.9

13.4 12.6 11.3

Cr(III)

Cr(VI)

Cr(III)

Cr(VI)

Tap water

– 50 100

– 50 100

n.d 45.7 93.4

n.d 46.9 99.1

River water

– 50 100

– 50 100

n.d 43.9 91.5

Mineral water

– 50 100

– 50 100

n.d 51.8 83.0

n.d, not detected. Table 3 Comparison of the proposed method with other methods applied for the speciation and determination of chromium. Method

Sample volume (mL)

PF

LOD (␮g L−1 )

RSD%

Matrix

References

CPE-HPLC LLE-UV SPE-FAAS CPE-HPLC SPE-FAAS HF-LPME-FAAS IL-HPLCa SPE-FAAS SPE-ICP-MS SPE-spectrophotometry DEME-HPLC

10 25 250 10 15 100 10 150 20 100 2.1

40 5 25 19 24 175 – 75 20 – 33

3.5 7.5 45 5.2 2.3 0.7 1.0 7.7 0.15 50 2.8

2.7 7.5 – 0.6 3.0 4.9 1.8 5.7 <10 1.7 9.8–13.7

Sediment Natural water Natural water Wastewater Natural water Natural water Wastewater Tap and mineral water and Red lentil Lake, mineral and ground water Natural water Tap, mineral and river water

[8] [51] [52] [53] [54] [55] [56] [57] [58] [59] This work

a

IL, ionic liquid.

matrices effect on the extraction efficiency. The results indicated that the matrices of the examined real samples have no considerable effects on the speciation and determination of both chromium species (Table 2). A typical chromatogram of extraction from mineral water is shown in Fig. 5. The comparison between the proposed method and other methods for speciation of chromium is given in Table 3. The results indicated that dual EME in terms of extraction time, PF and especially required volume of sample solution (2.1 mL) is highly efficient compared to previously reported methodology. Though, the repeatability of the proposed method (9.8–13.7) is not very acceptable. Also, the simplicity of the proposed method in this work was more than the other methods such ICP-MS.

4. Conclusions A new methodology of chromium speciation has been developed, based on the use of dual electromembrane extraction followed by complexation with APDC before the quantification by HPLC. Proposed microextraction methodology is environmentally friendly, fast and easy to handle. LOD values obtained satisfy the consumer requirements for these analytes on environmental samples. The proposed method has been successfully employed for the determination of Cr species in tap, mineral and river water. However, some matrices had a significant effect on the extraction performance and relative recoveries were more than 83%. In addition, a high sample throughput is attained since the whole analytical process, including sample preparation and determination, is performed in about 30 min. Conflicts of interest There are no financial or commercial conflicts of interest. Acknowledgment The financial support from the Research Affairs of Shahid Beheshti University is gratefully acknowledged. References

Fig. 5. HPLC chromatograms obtained after extraction from mineral water under optimal conditions: (a) unspiked sample, (b) spiked sample (concentration 50 ␮g L−1 ), (c) spiked sample (concentration 100 ␮g L−1 ); peak identification: (1) Cr(VI) and (2) Cr(III).

[1] IPCS (International Programme on Chemical Safety), Environmental Health Criteria 61, World Health Organization, Geneva, 1988. [2] G. Darrie, in: L. Ebdon, L. Pitts, R. Cornelis, H. Crews, O.F.X. Donard, Ph. Quevauviller (Eds.), Trace Element Speciation for Environment, Food and Health, Royal Society of Chemistry, Cambridge, 2001, pp. 315–328. [3] J. Kotas, Z. Stasicka, Environ. Pollut. 107 (2000) 263–283. [4] A. Kot, J. Namiesnik, Trends Anal. Chem. 19 (2000) 69–79. [5] World Health Organization (WHO), Guidelines for Drinking-Water Quality, Recommendations, vol. 3/e, WHO, Geneva, 2004, pp. 334.

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