Inorganic selenium speciation in environmental samples using selective electrodeposition coupled with electrothermal atomic absorption spectrometry

Spectrochimica Acta Part B 65 (2010) 334–339

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Analytical note

Inorganic selenium speciation in environmental samples using selective electrodeposition coupled with electrothermal atomic absorption spectrometry☆ Nahid Mashkouri Najafi ⁎, Shahram Seidi, Reza Alizadeh, Hamed Tavakoli Department of Chemistry, Faculty of Science, Shahid Beheshti University, Evin, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 29 September 2009 Accepted 22 February 2010 Available online 1 March 2010 Keywords: Selenium Speciation Electrodeposition Mercury electrode Electrothermal

a b s t r a c t Speciation analyses are of increasing interest in the environmental, toxicological and analytical fields, because the toxicity and reactivity of trace elements depend strongly on the chemical forms in which they are present. A simple electrodeposition–electrothermal atomic absorption spectrometry method for speciation analysis of some organic and inorganic selenium species in typical environmental water and agricultural soil samples has been developed. The method is based on the selective reduction of watersoluble Se(IV) and selenocystine (Se–Cys) species by an uncontrolled applied potential (1.8 V) on a mercurycoated electrode. In acidic media (1.0 M HCl solution) the only inorganic selenium species electrodeposited was Se(IV), and, of the water-soluble organic selenium species Se–Cys and Se–Met only Se–Cys was electrodeposited onto the mercury electrode surface. The proposed methodology was successfully applied to the speciation and determination of selenium in a few environmental samples. The spiked recovery value varied between 91% and 99%. The suggested method has been shown to have a characteristic mass (m0) of 25 pg, a limit of detection (LOD) of 1.0 μg L− 1 and a relative standard deviation (RSD%) of 3.5% for 6 measurements at a concentration of 100 μg L− 1 Se(VI). © 2010 Elsevier B.V. All rights reserved.

1. Introduction Speciation analyses are of increasing interest in the environmental, toxicological and analytical fields, because the toxicity and reactivity of trace elements depend strongly on the chemical forms in which they are present. Selenium is an essential element for plants, animals and the human body, but at high concentrations it can become toxic. It has been reported that selenium exhibits an anti-cancer effect, by protecting the human body from free radicals, and the recommended intake is between 50 and 200 μg/day [1]. The difference between the concentration range in which selenium is essential and that in which it is toxic is very narrow [2,3]. The toxicity of selenium depends on the particular identity of the inorganic and organic forms found in natural compounds. The most frequent inorganic species are selenite and selenate. Moreover, inorganic forms of selenium are more toxic than organic forms, with the toxicity of Se(VI) begin more severe than Se(IV) for humans and most other mammals [4]. However, in environmental samples, selenium's main species have been identified as Se(IV); Se(VI); dimethylselenide, (CH3)2Se; dimethyldiselenide, (CH3)2Se2 and dimethylselenone, (CH3)2SeO2, while in biological materials sele☆ This paper was presented at the Colloquium Spectroscopicum Internationale XXXVI, held in Budapest, Hungary, August 30–September 3, 2009 and is published in the special issue of Spectrochimica Acta Part B, dedicated to that conference. ⁎ Corresponding author. E-mail address: n-najafi@sbu.ac.ir (N.M. Najafi). 0584-8547/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2010.02.017

noamino acids like selenocystine (Se–Cyst) and selenomethionine (Se–Met) may also appear in low concentrations. Therefore, it is particularly important to develop analytical methods for the separation and preconcentration of selenium in environmental and biological systems [5]. In the majority of environmental matrices, such as natural water, selenium is usually present as Se(IV) and Se (VI) [4,6]. Recently, numerous coupled techniques for separation and detection have been developed to achieve the speciation of elements in inorganic and organic compounds. Some of these analytical methods for the separation and preconcentration of selenium species have been reviewed [4,7,8]. Separation techniques for elemental speciation can be generally classified into chromatographic and non-chromatographic techniques. The chromatographic techniques include high performance liquid chromatography [9], capillary electrophoresis [10], size exclusion chromatography (SEC) [11] and ion-chromatography [12]. Non-chromatographic separation techniques include solvent extraction [13], co-precipitation [14], cloud point extraction [15] and solid phase extraction (SPE) [16–23]. For a simple elemental speciation, especially for different oxidation states of a given element, non-chromatographic methods are more popular than chromatographic techniques. However, these techniques are time consuming and hazardous to the environment and human health, especially in the case of liquid–liquid extraction (LLE) in which, large volumes of solvents are used. Because of the disadvantages of conventional extraction techniques, solvent-free sample preparation methods or those employing fewer organic solvents are becoming increasingly important. Combinations of

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general separation methods with various detection techniques such as X-ray absorption spectrometry [24–26], neutron activation analysis (NAA) [27–29], inductively coupled plasma optical emission spectrometry (ICP-OES) [10,30–32] and inductively coupled plasmamass spectrometry (ICP-MS) [8,33], have been applied to measure trace selenium and its species in various samples. Although a high sensitivity is reported for these methods, they are very expensive and unavailable in many academic labs. However, atomic absorption spectrometry (AAS), especially electrothermal atomic absorption spectrometry (ETAAS) is superior and more popular than other methods due to its high sensitivity, low detection limit, convenience and availability. To the best of our knowledge, no study involving electrodeposition using a mercury-coated electrode as a separation step prior to quantitation by ETAAS has been reported for inorganic selenium speciation. Electrodeposition is a technique that has simple, rapid and inexpensive characteristics, as well as high ability for preconcentration and selective separation. The use of electrodeposition coupled with ETAAS as a preconcentration and separation technique has already been reported by our laboratory [34–37]. In our recent work, we developed the same technique for speciation and determination of trace inorganic tellurium in water samples [38]. In this report, we present and discuss a simple, rapid, accurate, sensitive and selective method for speciation and determination of Se(IV), Se(VI), Se–Cys and Se–Met species in environmental water and agricultural soil samples using a selective separation step for Se (IV) and Se–Cys by electrodeposition onto a mercury-coated electrode under acidic conditions prior to quantitation by ETAAS. The proposed methodology was successfully applied to the speciation and determination of selenium in the relevant samples. 2. Experimental 2.1. Reagents and standard solutions All chemicals used in this work were of analytical reagent grade and were used without further purification. Furthermore, solutions were prepared with deionized and distilled water. All plastic materials and glassware were cleaned by soaking in dilute HNO3 (1 + 9) and were rinsed with distilled water prior to use. Stock standards (1000 mg L− 1) of Se(IV), Se(VI), Se–Cys and Se–Met were obtained by respectively dissolving appropriate amounts of Na2SeO3 (Merck chemical company; INC.), Na2SeO4 (Aldrich, Milwaukee, WI), Se–Met (97%, Sigma-Aldrich) and Se–Cys (99%, Merck) in water and storing the solutions in a refrigerator at 4 °C. Working solutions were prepared daily by appropriate dilutions of stock solutions. The elemental standard solutions used for calibration were produced by diluting a stock solution of 1000 mg L− 1 of the given elements. A nickel solution of 1000 mg L− 1 was prepared by dissolution of nickel (II) nitrate in hydrochloric acid and diluted with water to an acid concentration of 1 mol L− 1.

Table 1 Summary of operating parameters and optimum conditions of ED-ETAAS. Factor

Optimum

Tash (°C) tatom (s) tash (s) tdry (s) Lamp type Lamp current (mA) Wavelength (nm) Ar flow rate (mL min− 1) Slit setting (nm) Sample volume (mL) Deposition voltage (V) Deposition time (min) Stirring rate (rpm)

500 4 15 20 HCL 15 196.7 0.7 0.7 15 1.8 10–15a 1000

a

Until the deposition current dropped to zero.

and detection of the organic analytes were performed using a KNAUER HPLC containing a K-501 HPLC pump, a six-port Cheminert HPLC valve with a 20-µL sample loop and a KNAUER K-2501 UV–Vis detector. The separations were carried out on an ODS column (250 mm × 4.6 mm, with 5-µm particle size). The mobile phase consisted of water and methanol (20:80). The flow was set to 0.8 mL/min. The injection volume was 20 µL for all the samples, and the detection was performed at a wavelength of 210 nm. 2.3. Electrodeposition procedure The mercury-coated electrode was prepared by using an iridium rod of 2-mm diameter and 10-cm length as the cathode and platinum as the anode. The same procedure as in our recent study was used for mercury-coated-electrode preparation [38]. The electrolysis system comprised the power supply, the mercury-coated iridium cathode and the platinum electrode. The applied electrodeposition apparatus is shown in Fig. 1. The electrodeposition behavior of Se(IV) and Se (VI) was studied in a 1.0 M HCl solution using the above electrolysis system. Aliquots containing 15 mL of acidified water solution in a 20-mL cell spiked with different concentrations of Se(IV) and Se(VI) were electrolyzed at an applied potential of 1.8 V until the deposition

2.2. Instrumentation A PERKIN-ELMER 503 electrothermal atomic absorption spectrometer with a deuterium lamp background correction and an HGA2100 furnace controller was used for graphite furnace measurements, and argon was used as the inert gas. Pyrolytic-coated graphite tubes (PERKIN-ELMER Part No. B3001264) were used. Data were evaluated using peak height absorbance. The operating parameters for working elements were optimized using a multivariate optimization method. A summary of the optimum conditions for all necessary parameters for ED-EAAS measurements is shown in Table 1. A mercury-coated cathode and a platinum anode were connected to a D.C power supply (0–12 V) via an amperometer (0–12 mA) indicating the deposition current. Electrochemical experiments were performed using a colometer/electrolysis system form the Chemie Company. Separation

335

Fig. 1. Schematic electrodeposition procedure.

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current dropped to zero at a stirring rate of 1000 rpm. Se(IV) is selectively reduced to the elemental state and separated from solution by deposition onto the surface of the mercury-coated electrode [39,40]. The electrode is withdrawn from the solution, and the spent electrolyte containing Se(VI) was analyzed by ETAAS. The deposition time was 10–15 min (until the deposition current dropped to zero). For each experiment, a fresh mercury-coated electrode was used.

Table 2 Determination of inorganic selenium species in real and spiked water and agricultural soil samples (n = 3). Sample

River water

2.4. Sample preparation Caspian sea water

River water samples (Eslamsahahr, Iran), Caspian sea water and tap water samples were collected in pre-washed (by soaking in dilute HNO3 (1 + 9) and deionized distilled water, respectively) Teflon bottles. Immediately after sampling, all water samples were centrifuged at 2000 rpm for 10 min and filtered through a cellulose membrane filter (Millipore) of 0.45-μm pore size in order to remove suspended particles. The same procedure as in our recent study was used for preparation of water samples [38]. For preparation of soil samples, about 1 g of different agricultural soil samples (all of them were collected from different parts of Saleh Abad, one of the suburbs of Tehran, Iran) was dried at 100 °C for 5 h. Cold acidic digestion of the soil samples was performed based on the Tsopelas method [40]. The storage period was kept as short as possible. 3. Result and discussion The selective reduction and deposition of Se(IV) in acidic media onto a mercury-coated electrode are the basis for separation of Se(IV) from Se(VI). The following mechanism for the selective reduction of Se(IV) has been reported in the literature [39,40]. þ

H2 SeO3 þ Hg þ 4e þ 4H →Hg ðSeÞ þ 3H2 O

ð1Þ

As shown by this reaction, the analytical signal from selenium strongly depends on the pH of the medium. It should be emphasized that the presence of Se(VI), has no effect on the reduction of Se(IV) even when it is present at a 100-fold higher concentration than Se(IV) [39,40]. After applying an uncontrolled electrodeposition potential (1.8 V) to the mercury-coated electrode, Se(IV) was reduced and deposited onto the cathode electrode and the Se(VI) species remained in the spent electrolyte. The Se(VI) content of the spent electrolyte was measured using the ETAAS technique, and the Se(IV) content was calculated as the difference between the total measured selenium content and the content of Se(VI). 3.1. Analysis of environmental water and agricultural soil samples The proposed method was applied to determine the speciation of water-soluble species of selenium in different environmental water and agricultural samples. As has already been discussed, the most frequent species in the water samples are Se(IV) and Se(VI); therefore, the experimental plan for analyzing speciation in the water samples is only based on quantitation of these two species. The speciation and quantitation results for Se(IV) and Se(VI) in the environmental water samples obtained using the developed technique are shown in Table 2. The spiked recovery value of 94–99% is very promising. However, in addition to Se(IV) and Se(VI), water-soluble Se–Cys and Se–Met are reported in the soil samples [40]. The acidic digested soils were treated using the liquid–liquid extraction (LLE) method with water and CH2Cl2 as the aqueous and organic solvents, respectively. Using these solvents, aqueous phase contains Se(IV), Se(VI), Se–Cys and Se–Met, which are thus separated from organic phase. For the aqueous phase, two different pathways and procedures are proposed based on the separation of electroactive species

Tap water

Soil 1

Soil 2

Added (µg L− 1)

Found (µg L− 1)

Recovery (%)

Se(IV)

Se(VI)

Se(IV) ± SD

Se(VI) ± SD

Se(IV)

Se(VI)

– 15 – 15 – 15 – 15 – 15 – 15 – – 10 – – 10

– – 15 15 – – 15 15 – – 15 15 – 10 10 – 10 10

b LOD 14.7 ± 1.1 b LOD 14.3 ± 1.1 b LOD 14.6 ± 1.2 b LOD 14.1 ± 1.2 b LOD 14.1 ± 1.0 b LOD 14.3 ± 1.1 25.1 ± 1.3 26.3 ± 1.2 34.3 ± 1.1 20.4 ± 1.1 19.8 ± 1.2 30.0 ± 1.1

b LOD b LOD 14.8 ± 1.2 14.4 ± 1.1 b LOD b LOD 14.4 ± 1.1 14.6 ± 1.1 b LOD b LOD 14.6 ± 1.1 14.4 ± 1.1 16.3 ± 1.1 25.7 ± 1.2 25.8 ± 1.2 13.7 ± 1.1 22.8 ± 1.1 23.3 ± 1.2

– 98

–

95 – 97 94 – 94 95 – 92 – 96

99 96 – 96 97 – 97 96 – 94 95 – 91 96

containing Se(IV) and Se–Cys and electroinactive species such as Se (VI) and Se–Met; these are illustrated as a block diagram in Fig. 2. In acidic media (1.0 M HCl solution), only inorganic Se(IV) and organic Se–Cys are electroactive and were electrodeposited onto the surface of the mercury cathode [40]. According to the procedure in Pathway 1 shown in Fig. 2, the proposed electrodeposition technique has been used for separation of Se(VI) and Se–Met in the spent electrolyte after removal of the electrode onto which Se(IV) and organic Se–Cys were deposited, from the electrolysis cell. The spent electrolyte containing Se(VI) and Se–Met was analyzed by ETAAS under optimal conditions. Se(VI) was chemically reduced to Se(IV) by adding 25 mL of concentrated HCl at 90 °C for 15 min, followed by subsequent electroreduction of Se(IV) to elemental selenium and deposition onto the electrode. Se–Met was measured in the spent electrolyte by ETAAS after removal of electrode. In order to measure Se–Cys and Se(IV), as proposed by Pathway 2 in Fig. 2, ammonium pyrolidine dithiocarbamate (APDC) at pH 2 is added to coordinate Se(IV); Se(VI), Se–Cys and Se–Met are inert to this ligand under these conditions according to reports in the literature [20]. The separation and detection of organic selenium species (Se–Met and Se–Cys) and Se(IV)–APDC were performed by HPLC at 210 nm. The chromatogram in Fig. 3 shows only the Se(IV)– APDC complex and extra APDC ligand in the solution, which indicates that no Se–Cys and Se–Met were detected in this solution. Therefore, based on the HPLC results shown in Fig. 3, it can be concluded that, in the agricultural soil samples, the concentration of organic species is below the detection limit of this technique. Hence, only the levels of Se(IV) and Se(VI) were determined in the soil samples, the results are shown in Table 2.

4. Analytical performance The analytical performance of the proposed technique for speciation and quantitation of inorganic selenium was evaluated using optimized parameters for solutions of Se(IV) and Se(VI) at concentrations of 1, 2, 3 and 4 mg L− 1 concentrations and the mixture of both species in water samples. ETAAS measurements using optimized parameters for these solutions were carried out both before and after the electrodeposition process. The results show that, when there is only Se(IV) in the solution, the absorbance signal is nearly zero (Fig. 4a). This is due to reduction and electrodeposition nearly all Se(IV) onto the cathode, followed by withdrawal of the

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Fig. 2. General pathway for separation and determination of organic and inorganic selenium species in soil samples.

electrode, from the electrolyte solution, before quantitation of the spent electrolyte by ETAAS. The results shown in Fig. 4b, which are for the measurements of solutions containing only Se(VI) species after the electrodeposition step, indicate the electrochemical inactivity of this compound, which resulted in the lack of loss of this element form bulk of solution during the electrolysis procedure. Moreover, the results of ETAAS measurements of the spent electrolyte for the

Fig. 3. Chromatogram of a typical agricultural soil sample after Se(IV)–APDC complex formation. (a) APDC and (b) Se(IV)–APDC complex.

solutions of mixtures of both selenium species after the electrolysis step, as illustrated in Fig. 4c, are a combination of the findings in both (a) and (b). Therefore, the assumption that Se(IV) is strongly

Fig. 4. Response functions for demonstration of speciation performance using developed technique: a) only Se(IV) solutions, b) only Se(VI) solutions, (c) mixture solutions of Se(IV) and Se(VI), and (d) mixture solutions of both species by conventional ETAAS measurements. Uncertainties are standard deviations (± SD, n = 3).

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separated from Se(VI) during the electrolysis step is confirmed. The results of ETAAS measurements of the solutions of mixtures of both species without the electrolysis step shown in Fig. 4d also indicate the agreement of all finding with the assumption. Determination of the figures of merit of this method, including limit of detection (LOD), absolute sensitivity (m0) and the recovery test for the electrodeposition of the inorganic species in aqueous solution, was carried out under the optimized conditions. The response functions for selenium species were linear in the range of (0.01–0.2 mg L− 1) in aqueous 1.0 M HCl media, with linear regression correlation coefficients greater than 0.984. The calculated limit of detection (LOD) is equal to 1.0 μg L− 1 by the described procedure, and a characteristic mass (m0) nearly 25 pg is obtained for Se(VI). The relative standard deviation (RSD%) for 6 measurements was 3.5% at a Se(VI) concentration of 100 µg L− 1. Almost all cations which exist in the solutions are electrodeposited and removed from the solution during the electrolysis step by the high applied uncontrolled potential (1.8 V). To investigate the interference effect of different cations on total reduction and deposition of Se(IV), 15-mL aliquots of a solution containing 100 µg L− 1 of the inorganic selenium species as well as the major contaminating cations in environmental water samples (Li+, Na+, K+, Ag+, Ca2+, Mg2+, Co2+, Ni2+, Mn2+, Cd2+, Zn2+, Cu2+, Pb2+, Cr3+, Fe3+ and Te4+) in different interference-to-analyte ratios were subjected to the electrodeposition procedure. The obtained results showed that, under the conditions specified in the procedure, the major cations in the water samples had no obvious influence on electrodeposition and separation of the target ions. However, the remaining solution was analyzed for Se(VI) content using the ETAAS technique, which is a specific detection technique. 5. Conclusions We have concluded that the developed technique is suitable for the speciation and quantitation of organic and inorganic selenium species in environmental samples, such as water and soil, after an appropriate initial electrolysis step. High selectivity for Se(VI) is obtained due to the selective and nearly complete elimination of Se (IV) in the presence of a mercury-coated coated electrode in acidic media. Under optimal conditions, the calculated limit of detection (LOD) was equal to 1.0 μg L− 1 and the working range of proposed method encompassed 0.01–0.2 mg L− 1 (r2 N 0.984) in aqueous 1.0 M HCl media. The developed technique has the advantages of low cost, reproducibility, accuracy, lack of interference from the matrix and rapidity. Moreover, the very clean and neat sample preparation steps lead to shortening of the duration of the major steps and reduction of both polluting manipulations and the overall number of steps. In addition, selective separation of organic and inorganic selenium species is achieved using a very simple, inexpensive and solvent-free technique. The developed procedure was successfully applied for speciation of trace organic and inorganic seleniums in environmental samples. References [1] P.J. Craig (Ed.), Organometallic Compounds in the Environment, Longman, London, 1986. [2] A.J. Niimi, Q.N. LaHam, Relative toxicity of organic and inorganic compounds of selenium to newly hatched zebrafish (Brachydanio rerio), Can. J. Zool. 54 (1976) 501–509. [3] E. Nuevo, R.D. Ben-Shlomo, B. Lavie, Mercury selection of allozymes in marine organisms: prediction and verification in nature, Proc. Natl Acad. Sci. U. S. A. 81 (1984) 1258–1259. [4] C. Casiot, M. Carmen, B. Alonso, J. Boisson, O.F.X. Donarda, M.P. Gautiera, Simultaneous speciation of arsenic, selenium, antimony and tellurium species in waters and soil extract by capillary electrophoresis and UV detection, Analyst 123 (1998) 2887–2893. [5] C.K. Jain, I. Ali, Arsenic: occurrence, toxicity and speciation techniques, Water Res. 34 (2000) 4304–4312.

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