Ultrasound-assisted cloud point extraction for speciation and indirect spectrophotometric determination of chromium(III) and (VI) in water samples

Spectrochimica Acta Part A 92 (2012) 189–193

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Ultrasound-assisted cloud point extraction for speciation and indirect spectrophotometric determination of chromium(III) and (VI) in water samples Mahdi Hashemi a , Seyed Mosayeb Daryanavard a,b,∗ a b

Department of Analytical chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran Faculty of science, University of Hormozgan, P.O. Box 3995, Bandar Abbas, Iran

a r t i c l e

i n f o

Article history: Received 23 July 2011 Received in revised form 11 February 2012 Accepted 17 February 2012 Keywords: Chromium Ultrasound-assisted cloud point extraction Speciation Spectrophotometry Electroplating wastewater

a b s t r a c t Ultrasound-assisted cloud point extraction (UACPE) procedure was developed for speciation and indirect spectrophotometric determination of chromium(III) and (VI) in environmental water samples. The method is based on the reduction of Cr(VI) by iodide in acidic media and subsequently formation of I3 − anion. The I3 − formed can further react with cetyltrimethylammonium bromide (CTAB) and induce its clouding due to formation of an ion-association complex. The formed complex was separated from solution and dissolved in ethanol for spectrophotometric measurement. Cerium(IV) ammonium sulphate was chosen as an oxidizing reagent for pre-oxidation step of Cr(III) to Cr(VI) species before the addition of iodide to the system, up to chromium in trivalent can be determined by the procedure. Experimental parameters for both spectrophotometric reaction and extraction procedure have been optimized. Under optimized conditions Cr(VI) can be determined in the range 20–400 ng mL−1 (R2 = 0.999). Detection limit, preconcentration factor and relative standard deviation were 12 ng mL−1 , 20.0 and 2.2% (n = 5), respectively with 10 mL sample volumes. The proposed method has been successfully applied for determination of chromium(V) in spiked water, synthetic seawater and electroplating wastewater samples with average recoveries of 100.1, 99.4 and 99.1%, respectively. © 2012 Elsevier B.V. All rights reserved.

1. Introduction In the environment chromium occurs mainly in the oxidation states (III) and (VI). The Cr(III) is essential for plants and animals at trace concentrations, whereas Cr(VI) is considered to be a more toxic form because of its high oxidizing potential. Recently, extensive use of chromium in industrial processes has led to discharged large quantities of chromium into the environment, leading to serious problems and hazardous risks for the human health. For speciation analysis in environmental studies, the determination of redox species (Cr(VI)/Cr(III)) is very important. This is due to the fact that the toxicity and the environmental impacts of elements are strongly dependent upon their oxidation state. Therefore, to obtain sufficient information on the toxicity and biotransformation of these elements, it is necessary not only to determine the total amount but also to speciate the different oxidation states. To determine trace or ultra-trace inorganic

∗ Corresponding author at: Department of Analytical chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran. Tel.: +98 811 8282807; fax: +98 811 8282807. E-mail addresses: [email protected], [email protected] (S.M. Daryanavard). 1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2012.02.073

chromium species in environmental sample, a chemical separation and preconcentration step is often required prior to analysis [1]. The separation/preconcentration methods for determination of chromium reported in literature include: coprecipitation [2], solvent extraction and solid phase extraction [3,4], ionexchange separation [5,2], electrochemical methods [6], highperformance liquid chromatography [7], ion chromatography [8] and capillary electrophoresis [9]. However, most of the methods mentioned above are often complicated and timeconsuming or have high operation costs. Thus, a simple and efficient separation technique for chromium species is essential. Cloud point extraction (CPE) is a kind of environmentally benign liquid–liquid extraction method and has some advantages such as low cost, safety and a high preconcentration factor. CPE is an extraction technique based on the clouding phenomenon of surfactants which is depended on the phase behavior of non-ionic and ionic surfactants in aqueous solutions, that exhibit phase separation after an increase in temperature or the addition of a salting-out agent [10]. It has been used for separation/preconcentration of trace metals in various samples with complicated matrix. Due to these advantages, CPE was combined with spectrometry methods as preconcentration step for speciation and determination of chromium [11–14].

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Some papers described techniques used for the indirect spectrophotometric determination of chromium [15–18]. In these investigations, the method based on the reduction of chromium from its hexavalent state to its trivalent state in the presence of a known excess of reducing agent in acidic medium, followed by formation of the complex that its spectrophotometric characteristics measured for determination of chromium, is followed. Use of I− /I3 − system as a reducing agent for the determination of chromium is well known [16]. Triiodide ions formed in this system usually in the presence of cationic compounds such as dyes [19,20] or cationic surfactants [15], form ion-association complexes that their absorption characteristics can be used for the determination of chromium in trace level. Because of slow reaction between I− and Cr(VI), to achieve shortest procedure time, it is necessary to accelerate this reaction. Ultrasound is a type of energy which can help analytical chemists in almost all their laboratory tasks, from cleaning to detection. The different steps of the analytical process can be expedited and (or) improved by the use of ultrasound energy [21]. We used ultrasonic waves in our procedure and the reactions and clouding phenomena were accelerated. In this paper, a simple and rapid method for indirect spectrophotometric speciation of chromium in environmental water samples is proposed by using CPE with cationic surfactant CTAB as the extractant in the presence ultrasonic waves. To the best of our knowledge, this study may be first report describing the application of UACPE procedure for determination of an inorganic species. Under the optimal conditions, the complex [CTAB][I3 − ] formed was separated from aqueous phase. Following preconcentration, the analyte in surfactant rich phase was determined by UV–Vis spectrophotometry measurement. The developed method was applied to the speciation of chromium in tap water, sea water and electroplating wastewater with satisfactory results.

3.0 g of ion exchanger resin in 25 mL double distilled water was prepared and the column was packed to a height of 4 cm. Glass wool was placed at the bottom of the column for allowing the resin to settle properly. Before the experiments, the resin was pretreated by repeated washings with 1 mol L−1 HCl solution to remove solvents and other chemicals. The last step of the conditioning consisted in percolating a solution of HCl through the column in order to convert the resins to H+ form. The excess of Cl− groups were removed from the resins by rinsing with deionized water. 2.4. UACPE procedure An aliquot of a chromium synthetic mixture or sample solution was transferred to 10 mL volumetric balloon and 1.0 mL of 1 mol L−1 HCl and 1.0 mL of 0.005 mol L−1 cerium(IV) ammonium sulphate acidic solutions were added. Then the solution was taken up to the mark with double distilled water and passed through a resin column at 0.5 mL min−1 . Then 1.0 mL of collected solution at the outlet column was transferred to a 10 mL centrifuge tube and 1.0 mL HCl/glycine buffer solution was added. This was followed by the addition of 1.0 mL of 4.0 × 10−3 mol L−1 surfactant CTAB solution and 1.0 mL of 8.0 × 10−3 mol L−1 of KI solution. The solution was diluted to the mark with double distilled water and allowed to stand for 2 min in ultrasonic bath for accelerating reaction and best phase separation [21]. Separation of the aqueous and surfactant-rich phase was accomplished by centrifugation for 10 min at 3400 rpm. Then, the aqueous phase could be separated by inverting the tube. The surfactant rich phase of this procedure was dissolved and diluted to 0.5 mL with the ethanol and transferred into a 0.5 mL quartz cell. The absorbance of the solution was measured at 360 nm. 3. Results and discussion

2. Experimental 2.1. Apparatus A Perkin-Elmer Lambda 45 UV/Vis spectrometer was used for recording absorbance spectra with 1-cm quartz cell (0.5 mL). A Metrohm pH meter (model 744) with a combined glass electrode was used for pH measurements. A Pars Nahand ultrasonic bath (model parsonic mini 40 kHz) with temperature control and a centrifuge with 10 mL calibrated centrifuge tubes (Isolab, Germany) were used to accelerate the reactions and phase separation process.

The analytical approach for the determination of chromium depends on two reactions which must be quantitative. First, chromium in hexavalent state must oxidize an equivalent amount of iodide ions. It is known that Cr(VI) and iodide ion react quantitatively in an acidic medium as follows; 2HCrO4 − + 6I− + 14H+ → 2Cr3+ + 3I2 + 8H2 O In the presence of an excess of iodide ions, the librated iodine forms triiodide ions, which react with a cationic surfactant (CTAB) to form ion-association complex.

2.2. Reagents

I2 + I− → I3 −

All chemicals used in this study were obtained from Merck (Darmstadt, Germany). The surfactant CTAB was used without further purification. Stock solution of chromium at a concentration of 1000 ␮g mL−1 was prepared by dissolving appropriate amount of K2 Cr2 O7 and Cr(NO3 )3 ·9H2 O salt in double distilled water. Working standard solutions were obtained by appropriate dilution of the stock solution. Stock Ce(IV) solution was prepared by dissolving appropriate amount of cerium(IV) ammonium sulphate in sufficient concentrated sulphuric acid to prevent hydrolysis of Ce(IV) and obtain a final concentration of 5.0 × 10−2 mol L−1 on completing the volume to 500 mL. This solution is stable for several weeks [9].

CTAB + I3 − → [CTAB][I3 − ]

2.3. Preparation of column A strong acidic ion exchanger type I, Merck (Darmstadt, Germany) was used in this study. A glass column with 1.0 cm diameter and 10 cm length was used for column preparation. A slurry of

In order to find the optimum conditions for the formation and extraction of ion-association complex, the effect of pH, iodide ions concentration, CTAB concentration, sonication and centrifugation time on the absorbance of the complex was investigated. 3.1. Effect of pH An acid medium is necessary for Cr(VI) to oxidize I− to form I3 − , but too excessive amount of H+ is disadvantageous to the association reactions between I3 − anion and CTAB [22]. The effect of pH on the absorbance at a constant concentration of complex in surfactant-rich phase was investigated in the range 1.0–7.0. The absorbance of the [CTAB][I3 − ] system at 360 nm in surfactant-rich phase was studied against the reagent blank. The experiments show (Fig. 1) that with the increase in the pH range, the absorbance of complex decreases strongly, therefore the optimum pH was used

0.7

0.6

0.6

0.5

0.5

0.4

Absorbance

Absorbance

M. Hashemi, S.M. Daryanavard / Spectrochimica Acta Part A 92 (2012) 189–193

0.4 0.3 0.2

0.3 0.2 0.1

0.1 0

191

0 0

1

2

3

4

5

6

7

0

1

2

8

pH

as pH 1.0. So in the experiment, HCl/glycine buffer solution of pH 1.0 is added. 3.2. Effect of sonication time Traditionally, the scientific community engaged in work on ultrasound has been divided into those who use it to induce physical or chemical effects in a medium (by using high-power, low-frequency ultrasound from 20 kHz to 2 MHz in sonochemistry) and those who use it for measurement without altering the medium (e.g. use of high-frequency (5 MHz to several GHz), low-power ultrasound for nondestructive testing). Analytical chemists fall largely in the former group; thus although an increasing number of analytical processes is being facilitated or improved by use of ultrasound, analytical chemists have contributed to the development of new, interesting modes of ultrasound-based and ultrasound-assisted detection [21]. Oxidation of I− by Cr(VI) is relatively slow. This reaction can be accelerated by ultrasonic waves and experiments show the enhancement of the absorbance in the presence of ultrasonic waves. The exposure time of the reaction mixture to ultrasonic waves was also investigated. It was found that an exposure time of 2 min is enough for the best sensitivity. 3.3. Effect of CTAB concentration As it is expected, surfactant solutions enhance the absorption intensity at concentrations well above their CMC [6,7]. The surfactant type, the chemical structure of the reagent and the reaction mechanism are some of the parameters that control the enhancement factor [23]. As in the literature, the CMC value for CTAB reported about 8.0 × 10−4 mol L−1 [24]. The concentration of the cationic surfactant CTAB was studied from less to more than CMC value of CTAB, in the concentration range of 5.0 × 10−5 to 1.0 × 10−3 mol L−1 , observing quantitative extraction for a CTAB concentration higher than 3.0 × 10−4 mol L−1 (Fig. 2). In order to achieve adequate conditions to obtain a high preconcentration factor and sensitivity, 5.0 × 10−4 mol L−1 was selected as the most appropriate concentration of CTAB.

4

5

6

7

8

9

10

Fig. 2. Effect of CTAB concentration on the amount of complex formed [CTAB][I3 ] on wavelength 359 nm. Conditions: chromium(VI), 0.1 ␮g mL−1 ; KI, 5.0 × 10−4 mol L−1 ; pH 1.0; in 0.5 mL ethanol.

ion-association reaction, the absorption intensity is also low. If the concentration of iodide is beyond the optimum range, absorption intensity decreases due to the competitive reaction of I− and I3 − with CTAB. Also, effect of centrifugation time on the reaction and on the CPE procedure was investigated. The results showed that 10 min centrifugation at 3400 rpm is enough for successful CPE. Because the surfactant-rich phase was precipitate, different solvents were examined in order to select the one producing the optimal results regarding sensitivity. Among chloroform, ethanol, dimethyl formamide (DMF), and acetonitrile, ethanol was selected because the best results of absorption and sensitivity were attained at ethanol media. In order to have appropriate amount of sample for transferring and measurement of the absorbance and also a suitable preconcentration factor, surfactant-rich phase was dissolved in 0.5 mL of ethanol. The pre-concentration factor (PF), which was defined as the ratio between the sample volume (VS = 10.0 mL) and volume of organic phase (VO = 0.5 mL) was used to evaluate the extraction efficiency under different experimental conditions, is obtained about 20 for the system. 3.5. Analytical figure of merits Under the optimum experimental conditions, the relative standard deviation (R.S.D.) and relative error for five replicate measurements of 100.0 ng mL−1 of chromium(VI) were 2.2% and 1.8%, respectively. Because the amount of chromium in 10 mL of sample solution is measured after preconcentration by CPE in a final volume of 0.5 mL ethanol, the solution is concentrated by a factor of 20. The detection limit, defined as LOD = 3SB /m (where LOD, SB , and 0.6 0.5

Absorbance

Fig. 1. Effect of pH on the amount of complex formed [CTAB][I3 ] on wavelength 359 nm. Conditions: chromium(VI), 0.1 ␮g mL−1 ; KI, 5.0 × 10−4 mol L−1 ; CTAB, 2.0 × 10−4 mol L−1 ; in 0.5 mL ethanol.

3

CTAB concentration (×10 -4 M)

0.4 0.3 0.2 0.1

3.4. Effect of iodide concentration 0

The effect of iodide concentration on absorption intensities has been studied in range 2.5 × 10−4 to 1.5 × 10−3 mol L−1 . The results show (Fig. 3) that the optimum concentration of iodide is 8.0 × 10−4 mol L−1 for best complex formation. If the concentration of iodide is too low, due to the incompleteness of the

0

2

4

6

8

10

12

14

16

KI concentration (×10-4 M) Fig. 3. Effect of KI concentration on the amount of complex formed [CTAB][I3 ] on wavelength 359 nm. Conditions: chromium(VI), 0.1 ␮g mL−1 ; CTAB, 2.0 × 10−4 mol L−1 ; in 0.5 mL ethanol.

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Table 1 Analytical features of the proposed method. Regression equation (n = 14) Linear range (ng mL−1 ) Limit of detection (3SB /m blank, ng mL−1 ) (n = 5) Preconcentration factorb

A = 0.0026Ca + 0.0685, R2 = 0.999 20.0–400.0 12.0 20.0

Concentration in ng mL−1 . Defined as the ratio between the volume of organic phase and the sample volume. a

b

Table 2 Tolerance ratios of diverse ions on the determination of 0.1 ␮g mL−1 of chromium(VI). Ion

Tolerance limit (w/w)

F− , Cl− , Br− , NO3 − , CH3 COO− , SO4 2− Zn2+ , Cu2+ , Ni2+ , Co2+ , Fe3+ , Fe2+ Ca2+ , Mg2+ , Na+ , K+ , NH4 + Mn2+ NO2 −

1000 1000 1000 50 5 (500)a

a

In the presence of 5.0 × 10−3 mol L−1 Ce(III).

Table 4 Results obtained in electroplating bath wastewater sample applying the proposed USA-CPE procedure for Cr(VI) determination. Sample

Present (ng mL−1 )

Founda (ng mL−1 )

Recovery (%)

Electroplating bath wastewaterb

100.0 200.0

97.1 ± 0.3 203.5 ± 0.3

97.1 101.7

Mean ± SD (n = 3). Composition of sample (g L−1 ): Cr6+ (42.0), Cu2+ (8.0), Zn2+ (3.2), Ni2+ (14.0), Ca2+ (78.0), Na+ (182.0), K+ (10.0) and Na2 SO4 (74.1). a

b

Table 5 Comparison of UACPE with some spectrophotometry reported procedures. Method

LODa (ng mL−1 )

LRb (ng mL−1 )

Refs.

FIAc Cr-DPCd complexation with UV–Vis SIWFEe Online FIA Sensitized – UV–Vis USCPE coupled UV–Vis

500 –

up to 20,000 50–600

[25] [26]

up to 100 10–50 100–1370 20–400

[27] [28] [29] This work

a b

m are the limit of detection, standard deviation of the blank, and slope of the calibration graph, respectively) is sufficiently low and lies around 12.0 ng mL−1 . The results are shown in Table 1. 3.6. Effect of foreign ions The effect of foreign ions on the signal intensity of Cr(VI) was tested. Different amounts of common cations and anions were added to the test solution containing 0.1 ␮g mL−1 of Cr(VI) and then same procedure as before was applied. The tolerance limits were determined for a maximum error of 5% and the results from these studies are collected in Table 2. As could be seen, the tolerance limits for NO2 − and Mn(II) were 5 and 50, and for most cations such as Ni(II), Fe(III), Cu(II), Co(III) and Zn(II) were 1000 times of Cr(VI). Moreover, due to use of strong acidic ion exchanger resin, most of the cations retain at resin column and by this treatment, it was found that the common coexisting ions did not have significant effect on the separation and determination of Cr(VI) (Table 2). 3.7. Water sample and electroplating wastewater analysis The proposed method was applied to the speciation of Cr(III) and Cr(VI) in tap water, synthetic seawater and determination of Cr(VI) in electroplating bath wastewater. The proposed method was successfully applied for the preconcentration and speciation of trace amounts of chromium in water samples, and the results are shown in Table 3. The relative error was lower than 1.8% for Cr(VI) and the total chromium. The proposed method was applied to study the recovery of chromium(VI) in electroplating bath wastewater Table 3 Analytical results for speciation of Cr(III) and Cr(VI) in water of different origins.

c d e

4. Conclusions In this study, ultrasound-assisted cloud point extraction coupled with UV–Vis spectrophotometry was successfully applied for the determination of chromium species in environmental water samples with low detection limit, accuracy and good precision. With the use of an ultrasound source, both of spectrophotometric reaction and clouding phenomena were accelerated. The recoveries for Cr(VI) spiked in the natural water samples indicate no interference encountered from these sample matrices. Compared with other chromium spectrophotometric determination methods (Table 5), the analytical technique offers advantages such as simplicity, ease of operation, short analysis time, and lower consumption of organic solvent. A high sensitivity is also obtained as result of the enrichment factor (20-fold). The quantitative recovery of chromium (VI) with a relative standard deviation of 2.2% reflects the validity and accuracy of the method when applied to spiked water and electroplating wastewater samples.

Founded (ng mL−1 )a

Recovery (%)

Cr(III)

Cr(VI)

Cr(III)

Cr(VI)

Cr(III)

Cr(VI)

Tap water

– 50.0 75.0

150.0 100.0 75.0

n.d.b 49.2 ± 0.3 75.4 ± 0.3

151.5 ± 0.2 97.8 ± 0.3 73.9 ± 0.2

– 98.4 100.5

101.0 97.8 98.5

Synthetic seawaterc

– 50.0 75.0

150.0 100.0 75.0

n.d. 47.6 ± 0.2 71.3 ± 0.2

147.9 ± 0.2 102.3 ± 0.2 74.6 ± 0.2

[1] [2] [3] [4]

– 95.2 95.0

98.6 102.3 99.4

[5]

Mean ± SD (n = 3). Not detected. c Composition of 1.0 L synthetic seawater (mol/kg): CaCl2 (0.0107), KCl (0.0105), Na2 SO4 (0.0292), NaCl (0.4266), MgCl2 (0.0551) [8]. a

b

Limit of detection. Linear range. Flow injection analysis. 1,5-Diphenylcarbazide. Sequential injection wetting film extraction.

sample. The electroplating wastewater sample had the following characteristics: pH 2.2, chromium(VI): 42 g L−1 , copper(II): 8 g L−1 , Zn(II): 3.2 g L−1 , Ni(II): 14 g L−1 , calcium: 78 g L−1 , sodium: 182 g L−1 , potassium: 10 g L−1 and sodium sulphate: 74.1 g L−1 collected from Hamedan industrial estate. The wastewater sample was diluted to the required concentration and the preconcentration procedure was applied as mentioned above. The recovery of chromium was found to be quantitative and the results are presented in Table 4.

Added (ng mL−1 )

Sample

2.0 3.0 0.8 12.0

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