Analytica Chimica Acta 577 (2006) 126–133
Extraction and determination of elemental selenium in sediments—A comparative study Yu-Wei Chen a,∗ , Lu Li a , Alessandro D’Ulivo b , Nelson Belzile a a
Department of Chemistry and Biochemistry, Laurentian University, Ramsey Lake Road, Sudbury P3E 2C6, Canada b C. N. R., Institute for Chemical and Physical Processes, Laboratory of Instrumental Analytical Chemistry, Area della Ricerca, Via G. Moruzzi, 1, 56124 Pisa, Italy Received 16 February 2006; received in revised form 9 June 2006; accepted 10 June 2006 Available online 16 June 2006
Abstract This paper proposes a new technique to extract elemental Se from soil and sediment samples. In this study, we have identified that the purchased red elemental selenium standard (PF-Se) was impure and rather consisted of a mixture of CS2 soluble amorphous elemental Se (ca. 10%, w/w), water soluble oxidized Se (ca. 15–17%, w/w) and, CS2 insoluble red monoclinic elemental Se. In more recent studies, a slow oxidation and a mineral phase transition of this sample was also observed. The solubility of the amorphous elemental Se in CS2 was at least 0.64 mg L−1 . The black elemental Se purchased from Sigma–Aldrich had a much lower solubility in CS2 (7.2 g mL−1 ) compared to that given in the literature. Any selenium compounds with electrical charge and polar nature is insoluble in CS2 . In a sodium sulphite solution, PF-Se was completely dissolved thus giving a clear indication of the lack of selectivity in that extraction system. Other comparative studies also demonstrated that over extraction did occur with the Na2 SO3 method. Compared to Na2 SO3 , CS2 extraction of elemental Se is not only much simpler, straightforward and with higher analytical precision, but also much more selective and accurate. With HG-AFS, the detection limit can reach as low as 1.0 ng g−1 in sediment sample owing to a low reagent blank of CS2 solvent. © 2006 Published by Elsevier B.V. Keywords: Elemental selenium; Extraction; Speciation; Carbon disulphide; Solubility; Sediment; Hydride generation-atomic fluorescence spectrometry
1. Introduction Selenium is an element that has triggered scientific interest in a large variety of disciplines. There is an abundant literature related to selenium, describing various aspects of its chemistry, biochemistry, geology, medicine, pharmaceutics, toxicology and industrial applications [1]. Selenium possesses many complex chemical and biochemical properties. It has four different possible chemical oxidation states, Se(IV) and Se(VI), present as SeO3 2− and SeO4 2− in the aqueous phase, Se(0) present in solid phase or in a colloidal form and Se(−II), occurring as selenide in minerals under reducing conditions or in organic and biochemical compounds. With the later chemical valence, many chemical compounds can be formed and the great majority of them exist in organic forms or in proteins [2].
∗
Corresponding author. Tel.: +1 705 675 1151; fax: +1 705 675 4844. E-mail address:
[email protected] (Y.-W. Chen).
0003-2670/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.aca.2006.06.020
Se(0) is a relatively biochemically inert species, being much less bioavailable compared to Se(IV), Se(VI) and some other forms of organic compounds [3]. The formation of Se(0) in natural environments is usually through a biotic process involving the reduction of selenate [4] or selenite [5] by bacteria. The abiotic reduction has also been reported [6,7]. Possible transformations of Se(0) include its oxidation to Se(IV) or Se(VI) [8] or its incorporation into iron sulfides or selenides [9–11]. As Se(0) sets between the higher chemical valence of Se(IV) and Se(VI), and the lower valence of Se(−II), it could be transformed to either direction depending on the redox potential of the environment and/or the presence of biological activity. The average total selenium concentration in the earth crust is around 0.09 g g−1 (ppm). In non-toxic surface soil, Se concentration varies from 0.01 to 2.0 g g−1 in many parts of the world [12–14]. The accurate determination of Se(0) is a key step in order to understand any process whether it is geological, environmental, or biological. However, finding an appropriate method to identify and measure Se(0) in natural systems such
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Table 1 Summary of mineralogical, physical and chemical properties of elemental selenium [15,16] Allotropic form
Common name
Solubility
Density (g cm−3 )
Crystalline, hexagonal
-Se, or grey-Se or black-Se, or metallic Se
4.82
Crystalline, monoclinic red
Crystalline red two forms: ␣-monoclinic, -monoclinic
The most stable form. Soluble: ether, chloroform; CS2 : 2 mg/100 mL Soluble in CS2
Amorphous
May exist as black or red amorphous, or colloidal Se
as soil or sediment is a difficult task due to its low concentration and the complexity of the matrix. An extraction method requires selectivity in the dissolution of Se(0) from a complex sample and accuracy in its determination. One of the most difficult problems analysts are facing in quantitative speciation of environmental samples is the lack of appropriate standard reference materials (SRM). It is particularly problematic for Se(0) because the exact chemical and mineralogical properties of this species in nature are unknown. Elemental selenium exists in several allotropic forms of remarkably different mineralogical, physical and chemical properties. Unstable amorphous elemental selenium is also susceptible to a phase transition to possess a more stable mineral structure. Some information of allotropic elemental Se is summarized in Table 1. It should be kept in mind that though this information can be found in few literatures, the original work on identifying or synthesizing those forms of selenium is very difficult to find and often lacks of details. For instance, the solubility of selenium in various solvents given in those sources is impossible to verify due to the unavailability of those specific selenium forms, which often rises important discrepancies between the literature and our experiments (see Sections 3.1 and 3.3). It is however generally assumed that Se(0) freshly formed in natural sediments is likely present in an amorphous form. Therefore, the chemical and mineralogical properties of red or black amorphous Se(0) would be the closest to elemental Se formed in natural sediments when compared to other types of well crystallized Se(0) which are synthesized under rather unnatural laboratory conditions [17]. Up to now the analytical method in the determination of Se(0) in the aquatic environment and agriculture soil are still scarce. Some promising techniques based on X-rays such as absorption near edge structure (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS) have been used to study the redox transformations of Se in sediments and soils [6], but these methods provide only qualitative or descriptive information. To date, only a few papers have dealt with the quantitative determination of Se(0) in natural systems. The technique more frequently used is the one proposed by Velinsky and Cutter [18], in which an extraction with sodium sulphite is applied at pH 7 to form a water soluble selenosulfate (Na–Se–SO3 ). However, it seems that the proportion of Se(0) in samples extracted with this method are often suspiciously high, ranging from more than 30% to as high as 90% of the total selenium present in the sediments [11,19–21].
␣-Monoclinic: 4.46; -monoclinic: 4.50
Soluble in CS2 , CH2 I2 , benzene or quinoline
Uchida et al. [22] have proposed to determine Se (−II + 0) in water samples by subtracting Se(IV) from the measured values after oxidation by a 3% (v/v) bromine solution. It was believed that with bromine oxidation, Se (−II) plus Se(0) could be selectively determined. With this method, no distinction between Se(0) and Se(−II) could be made. In their paper they mentioned to use carbon disulfide to dissolve elemental selenium. Unfortunately they did not give any detail on the characteristics of the elemental selenium used in the experiment. Yamada et al. [23] and, more recently, Wright et al. [24] mentioned that the sodium sulphite extraction might not be specific for elemental selenium; the later authors speculated a possible overestimation of the percentage of Se(0) by large values due to the solubilization of organic and iron selenide in sodium sulphite. Yamada et al. [23] used carbon disulfide to extract elemental selenium from soil, but their work seriously lacks of necessary detailed studies. In addition the purpose of back extraction of Se(0) into an acetate buffer, through a reaction with KCN to form SeCN− ion, is unclear. Because CS2 is a non-polar solvent, it is unlikely that any other charged or polar bonded Se species could be extracted into this solvent. Besides, potassium cyanide is an extremely dangerous chemical, therefore, its use should be avoided as much as possible. The purpose of this paper was to find possible solvents that could be appropriate to extract Se(0) from sediment samples. In this paper the mineralogical and chemical nature of a purchased red elemental selenium sample was carefully investigated. A comprehensive examination of CS2 as a solvent was made, and a comparison between extractions using sodium sulphite and carbon disulphide as solvents was also performed with freshly collected lake sediment samples. 2. Experimental 2.1. Reagents A red elemental selenium standard claimed as 90–100% purity was purchased from Pfaltz & Bauer Inc. in February 1999 and was always stored in a vacuum desiccator. It will be referred to as PF-Se later in the text. Black elemental selenium standard powder (Black Se) 99.8%, selenium dioxide (SeO2 ), selenious acid (H2 SeO3 ) 99.999% and sodium tetrahydroborate (NaBH4 ) 98% were purchased from Sigma–Aldrich. CS2 was Certified Class B1 from Fisher Scientific. All other organic solvents were of analytical grade or equivalent. Organic selenium compounds, seleno-dl-cystine
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C6 H12 N2 O4 Se2 ; selenium-l-methionine C5 H11 NO2 Se and Se(methyl) selenocysteine hydrochloride C4 H9 NO2 Se·HCl purchased from Sigma–Aldrich were at least 95% in purity. All other chemicals were of analytical grade or higher. 2.2. Instruments A PSA 10.055 Millennium Excalibur equipped with a continuous flow hydride generation system and atomic fluorescence spectrometer (HG-AFS) was used for Se determination. A Labconco freeze dryer system (Model 7750) was used for sample preparation. An ultrasonic bath (FS60, 100 W 42 kHz ± 6%, Fisher Scientific) and an ultrasonic probe (CE300, Johns Scientific Inc.) were employed in extraction with sodium sulphite. Infrared spectroscopic studies were carried out with a Fourier Transform Infrared Spectrometer (MB102, BOMEN Hartman & Braun Inc.). X-ray diffraction (XRD) was performed with an Enraf Nonius X-ray generator using Debye–Scherrer cameras. The data were collected on an image plate and digitized to produce diffractograms. The SEM-EDS (scanning electron microscope-energy dispersive X-ray spectroscopy) analysis was carried out on Jeol JSM-6400 with an Oxford Instruments Inca system for EDS using an accelerating voltage of 20 kV and a beam current of 1 nA. 2.3. Preparation of Se(0) standard solutions from the PF-Se and standardization Around 50.0 mg samples of the PF-Se standard was precisely weighed and transferred into a Teflon tube containing 7.0 mL of CS2 . The cap was tightened and the extraction was carried out with a wrist-arm shaker for 8 h. After the extraction, the sample was filtered through a 0.22 m Nylon membrane. The filtrate was then transferred into a clean glass flask with a ground glass stopper. The solution ought to be sealed well and stored in a refrigerator to avoid evaporation. In order to standardize this Se(0) solution, 100.0 L of the solution was precisely transferred, using a glass micro-pipette, into a 50 mL glass beaker and the solvent was evaporated completely below 40 ◦ C (the boiling point of CS2 is 46.3 ◦ C and it is very flammable). The evaporation must be done under efficient ventilation as CS2 is a toxic compound. Then 5.0 mL of concentrated HNO3 or HCl (Trace Metal Grade) and 2.0 mL of 0.5 M Br− + 0.1 M BrO3 − solution were added to the residue and the sample was digested for 3 h covered with a watch glass. The small aliquot of DDW was added and the sample was refluxed at boiling temperature for 20 min. After digestion, a few drops of 5.0% (w/v) NH2 OH·HCl were added to destroy the remaining Br2 . The digested solution was then transferred into a 100.0 mL volumetric flask and fixed to volume with double deionized water (DDW). The concentration of Se in the solution was determined by HG-AFS after an appropriate dilution with 3.0 M HCl. The standard concentration of Se(0) in the solution was calculated. A comparative study with graphite atomic absorption spectrometry showed that under the above digestion conditions, Se was in the form of Se(IV), therefore, no pre-reduction was needed before the analysis by HG-AFS.
2.4. Lake description and sediment sample preparation Sediment samples were collected in August 2002 from Clearwater Lake (46◦ 22 N, 81◦ 03 W, Zmax = 7.3 m, Zaverage = 8.4 m) and McFarlane Lake (46◦ 26 N, 81◦ 57 W, Zmax = 20.0 m, Zaverage = 7.3 m) by a diver using a Plexiglas corer. Both lakes are located in Sudbury, Canada. The two lakes are located closely to each other about 5 km in distance. Although the atmospheric loadings of trace elements in these two lakes are very similar, they possess very different nature of sedimentary environment. The former has been recovered from a pH of 4.2 in the early 1980s to a present pH around 6.3. Clearwater L. has a well oxygenated sediment–water interface all year around whereas McFarlane L. has always remained as a circumneutral lake characterized by an anoxic sediment–water interface in some portions of the lake during the late summer period [11,25]. Once the sediment sample was retrieved from the location, it was extruded from the Plexiglas corer and sub-sliced into 1.0 cm fractions under nitrogen atmosphere. The subdivided samples were freeze-dried, carefully ground under nitrogen atmospheric conditions and stored at −80 ◦ C. Sample homogenization is very important as it was found that Se(0) was not evenly distributed in sediment samples, therefore, poor grinding would lead to irreproducible results. For samples that were used for method development and technical studies, a combined sediment sample from different depths was homogenized to obtain a larger quantity of material. 2.5. Extraction and analysis of Se(0) in sediment samples The extraction of Se(0) with carbon disulfide (CS2 ) was done according to the following steps. About 0.5 g of freeze-dried sediment sample was precisely weighed and transferred into a Teflon tube; 5.0 mL of CS2 was then added. Each sample was submitted to an 8 h wrist-shaking extraction followed by a quantitative filtration (0.22 m). The filtrate was directly collected into a 50 mL glass flask. The beaker was put on a heat plate to evaporate CS2 until completely dried. Then, 1.0 mL of 3.0 M HCl and 0.5 mL of 0.5 M Br− + 0.1 M BrO3 − were added and the samples were digested as mentioned in Section 2.3. The digested solution was transferred into a 25.0 mL volumetric flask and fixed to volume with 3.0 M HCl after destroying Br2 by the NH2 OH·HCl solution. Selenium was determined by HG-AFS. Extraction of Se(0) with sodium sulphite (Na2 SO3 ) was done according to the Velinsky and Cutter’s method [18]. Briefly, about 0.5 g of sediment sample was precisely weighed and transferred into a Teflon tube to which 5.0 mL of 1.0 M Na2 SO3 (pH 7.0) was added. The sample was disrupted with an ultrasonic probe (2 kHz) for 2 min. Then the sample was placed into an ultrasonic bath for an 8-h extraction. The extracted sample was then subjected to a centrifugation at 10,000 rpm for 10 min. The sediment pellet was washed with 1.0 mL of the sodium sulphite solution three times. The resulting supernatants were combined and filtered (0.45 m) directly into a 50 mL beaker. The filtrate was undergone a 1-h reflux at 90 ◦ C in the presence of 1.0 mL of concentrated HNO3 . The solution was allowed to almost dryness, and then residue was dissolved twice with 1.0 mL DDW.
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Finally 15.0 mL of 3.0 M HCl was added and the sample was refluxed for about 20 min under boiling temperature. The digest was transferred into a graduated 50 mL polypropylene tube and fixed to 50.0 mL with 3.0 M HCl. The working solution should be in 3.0 M HCl for HG-AFS analysis as required by the instrument. 2.6. Total Se digestion and determination
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Table 2 Mass balance of PF-Se in CS2 extraction experiment Red Se0 (mg)
Dissolved in CS2 (mg/%)
Not diss. (mg/%)
Total Se found (%)
Avg, S.D., R.S.D. (%)
28.7 29.1 29.8 28.6
1.7/6.0 2.0/6.8 1.9/6.5 1.4/4.8
19.8/68.9 21.1/72.5 21.3/71.4 22.7/79.4
74.9 79.3 77.9 84.2
79.1, 3.9, 4.9
Note: In the last column, the values are statistical data of the four replicates.
An approximately 0.25 g of sediment sample was precisely weighed and transferred into a pre-cleaned Pyrex glass tube where 2.5 mL concentrated HNO3 and 3.0 mL of concentrated HCl were added. After a cold digestion over night, the sample was subjected to a 3-h digestion on a hotplate at 130 ◦ C. The filtrate (0.22 m) was collected directly in a 50 mL glass beaker and the solution was carefully evaporated to near dryness. Then 3.0 mL of 3.0 M HCl and 1.5 mL of 0.5 M Br− + 0.1 M BrO3 − were added and the solution was refluxed for about 30 min under this medium and the final digest was treated with hydroxylamine and fixed to 50.0 mL with DDW. The total Se was determined by HG-AFS. It was also noticed that in the Br− /BrO3 − /HCl matrix, there was no remarkable loss of Se in sample refluxing step as a nearly 100% recovery of the standard reference material MESS-3 was obtained using this protocol. 3. Results and discussion 3.1. Search for appropriate solvents Various solvents, including potassium thiocyanide, methyl iodide, cyclohexene, quinoline and carbon disulphide, were tested to dissolve the PF-Se sample. Only carbon disulphide could partially dissolve PF-Se, which resulted in a remarkable yellowish color in the solution. It was also noted that only a limited fraction of PF-Se could be dissolved in CS2 . However, all the PF-Se was dissolved using the Na2 SO3 method. The huge solubility difference in both solvents and the apparent low solubility of PF-Se in CS2 stimulated our interests in further investigating on the nature of the PF-Se sample and in the comparison of these two techniques of Se(0) extraction in sediment samples. It should also be pointed out that although some literatures commented the capacity of quinoline to dissolve amorphous elemental selenium, we did not observe the phenomenon with PF-Se. In our early studies, some attempts to dissolve Sigma–Aldrich black Se(0) with chloroform, carbon tetrachloride and benzene were also done; however, no signs of dissolution were at all observed. 3.2. The nature of PF-Se standard sample According to several sources [16,22,23] including the CRC Handbook of Chemistry and Physics, amorphous red elemental selenium should be soluble in CS2 , but there is no information on its real molar solubility value. The PF-Se was claimed to be 90–100% in purity. However, the source or method to synthesize the product, its mineralogical information and the shelf time before sold are unknown. In the Merck Index (10th Edi-
tion), it is mentioned that amorphous elemental selenium can be formed by reducing selenious acid in water; on the other hand the freshly form elemental Se can react with water and form selenious acid and hydrogen at 50 ◦ C. This suggests that some selenium oxide could be formed during or after synthesis. In addition, some freshly formed amorphous elemental selenium could also undergo through a phase transition to become better crystallized form, such as red monoclinic Se and gain a higher stability. In either case, the solubility of the PF-Se in CS2 would be significantly reduced. To be able to better understand the nature of PF-Se, a mass balance study was first performed. The PF-Se sample was ground carefully under nitrogen atmosphere to avoid oxidation and to obtain good sample homogeneity. About 20.0 mg of sample was first submitted to a 10.0 mL CS2 extraction. The supernatant and residue obtained after centrifugation were subjected a total digestion with 5.0 mL of concentrated HNO3 and 2.5 mL of 0.5 M Br− + 0.1 M BrO3 − . Se was determined in the two fractions. A reagent blank was carried along in each analytical process. Se was non-detectable in both reagent blank fractions. The detailed analytical protocols were similar to that described in Section 2.3. The analytical results are presented in Table 2. It is noticed in this experiment that about 7% of the PF-Se sample was extracted into CS2 and the majority of PF-Se was insoluble in CS2 . The most remarkable fact was that about 20% of sample mass was missing. According to the possible synthesis conditions, only oxygen and hydrogen can be counted for the missing weight in the sample. To further study the nature of the PF-Se standard, 25.0 mg of the sample was precisely weighed in quadruplicates and transferred into Teflon tubes; then 10.0 mL of DDW was added and a 2 and 8 h extractions were performed. It was found that the percentages of selenium dissolved in DDW were 15.7 ± 3.9 and 16.6 ± 1.4% (w/w), respectively. If this PF-Se standard was indeed pure elemental selenium, then it should not be soluble in a strong polar solvent such as water. This water soluble selenium could be present as selenious acid or/and selenium dioxide. However, there was still a large fraction of PF-Se remaining insoluble in either CS2 or in water, which implies that there was significant amount of selenium existing in some undefined forms, possibly better crystallized elemental selenium and other selenium oxyoanionic intermediates. Contrary to PF-Se, black Se is very stable. The mass balance study showed that black Se stilled remained as elemental Se after stored in a refrigerator for nearly 10 years. The samples and potassium bromide (KBr) used for FTIR were previously dried at their respective appropriate
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Fig. 2. Comparison of X-ray diffraction of black Se and PF-Se. Their remarkable X-ray diffraction indicates that neither of them is pure amorphous sample. The higher background of PF-Se suggests that it is generally poorer crystallized.
Fig. 1. FTIR spectra of high purity blank elemental selenium powder, selenious oxide and PF-Se purchased from Sigma–Aldrich and Pfaltz & Bauer Inc., respectively.
temperature. About 2.5–3.5 mg of sample was weighed and ground with 0.20 g of KBr in an agate mortar. The pellet was pressed at 10 tonnes of pressure over a 0.64 cm die for 4 min. FTIR spectra of the black Se, PF-Se, selenious acid and selenium dioxide were thus obtained. Black Se possesses a distinct FTIR spectrum of a non-polar compound, except at 697.63, 1090.63 and 1400.9 cm−1 which could be caused by some minor vibration motions. Selenious acid and selenium dioxide produced almost identical spectra with a wide double peak at 897/863 and 670/645 cm−1 (H2 SeO3 /SeO2 ) and a single broad peak between 1240 and 1059 cm−1 . By comparing FTIR spectral database of other Se oxyoanions, the above strong IR peaks of H2 SeO3 and SeO2 could be probably assigned as characteristic Se–O stretching. PF-Se sample produced several infrared peaks which are clear indications of the existence of some polar compounds (Fig. 1). In the PF-Se spectrum, a wide single peak between 1323 and 1176 cm−1 looks similar to that of H2 SeO3 and SeO2 at 1240–1061 cm−1 , but slightly left shifted by 115 cm−1 . At its lower wave number range, two sharp doublets at 884/851 and 613/578 cm−1 are overlapped with the double peaks of H2 SeO3 and SeO2 . The similarity in their FTIR spectra strongly suggests that some selenium oxide exists in PF-Se, whereas the weaker IR absorption peaks indicates that the Se–O concentration in PF-Se is much smaller compared to that in pure selenious acid. In our recent study, it was found that both black Se and PFSe gave a strong and similar X-ray diffraction pattern in a short exposure time of 3 h, indicating that they both contain crystalline forms of Se. By space group fitting analysis of XRD, it was iden-
tified that black Se presents dominantly in hexagonal form and with a rather simple mineral phase whereas PF-Se is a much more complex material in which red monoclinic Se coexists with various types of selenium oxides and some unidentified substances. A remarkably higher background of PF-Se suggests that this sample is less crystallized than black Se (Fig. 2). According to the mineralogical studies, it is highly possible that synthesized PF-Se, although initially containing relative high percentage of amorphous Se, has been slowly transformed to more stable red monoclinic elemental selenium and, at the same time it has also been undergoing through a slow oxidation. A SEM-EDS analysis on the sample composition also confirmed that black Se is pure Se(0) sample whereas PF-Se has an average of 14% oxygen atoms on a few spots of the analyzed sample. From the above evidences, we can conclude that (1) the PF-Se standard contains a small portion of CS2 soluble amorphous elemental selenium (10% (w/w) in 2002); (2) in this sample about 15–17% (w/w) selenium is in water soluble forms (in 2002), which could be in form of H2 SeO3 or similar compounds. The missing mass is probably mainly due to oxygen and hydrogen; (3) both black Se and PF-Se contain some crystalline structures producing similar and strong X-ray diffraction patterns and it is probably that the initially less stable amorphous elemental selenium in PF-Se has been undergoing through a slow phase transition to become more stable and CS2 insoluble red monoclinic Se. 3.3. The properties of carbon disulphide as a solvent for Se(0) extraction It had been found that the purchased standard PF-Se was not pure amorphous elemental selenium, therefore, a complete dissolution of PF-Se is impossible. The study on its solubility in CS2 was done by adding an increasing amount of the PF-Se sample in a fixed volume (7.0 mL) of the solvent. In solubility
Y.-W. Chen et al. / Analytica Chimica Acta 577 (2006) 126–133 Table 3 Solubility study of PF-Se in CS2 Sample weight (mg)
Extracted Se in CS2 (mg)
Se(0) in CS2 (mg mL−1 )
14.8 25.7 49.8 74.6 99.9
1.49 1.59 2.32 3.03 4.44
0.21 0.23 0.33 0.45 0.64
calculations, the exact volume was calculated from the weight and specific gravity of CS2 . There was no detectable amount of Se in the reagent blank. Results showed that the concentration of dissolved Se(0) in CS2 increased with an increasing amount of added PF-Se (Table 3). The highest dissolution of PF-Se was found in the lowest sample mass (ca. 10%, w/w). The solubility of amorphous Se(0) in CS2 can reach at least as high as 0.64 mg mL−1 . No further tests were carried out, as the concentration of Se(0) in natural sediment sample is expected to be much lower. It was found in our recent studies that the solubility of PF-Se in CS2 was reduced by approximately 50% after 2 more years of storage. It is probably due to its further oxidation and slow transformation during this period. This hypothesis is also supported by a changing FTIR spectrum of PF-Se sample. It was noticed that its IR spectrum is now more similar to the H2 SeO3 spectrum than it was 2 years ago even though the sample had been stored in a vacuum desiccator. In a recent study, 50.8 mg of black Se was also extracted with 7.00 mL of CS2 and it was found that its solubility in CS2 was about 7.0 g mL−1 . This is a much lower solubility than that of PF-Se and it is likely due to the fact that the black Se is better crystallized, therefore, less soluble in CS2 than PF-Se. Carbon disulphide, being a non-polar solvent, should dissolve only species that are non-polar, and any selenium species that contain dipoles in their chemical bonds should not be significantly extracted into CS2 . To be able to confirm that CS2 would not extract any polar selenium compounds that could exist in sediments such as organic selenium compounds and selenium oxoanions, the solubility of three organic selenium compounds, seleno-dl-cystine, seleno-l-methionine and selenium (methyl)selenocysteine was investigated. In this experiment, a small amount (4–5 mg) of a solid organic selenium compound was precisely weighed and transferred into a Teflon tube, and 5.00 mL
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of CS2 was added. After an 8-h extraction, the Se concentration in the filtrate (0.22 m) was determined (Section 2.3). It was found that none of the three tested organic selenium compounds could be extracted into CS2 . However, it was shown in a separate test that the solubility of these organic selenium compounds increased significantly in polar solvent such as ethanol and methanol (Table 4). The reagent blanks of all these solvents contained no detectable Se. The results showed that CS2 could only selectively extract non-polar and amorphous elemental selenium and that any polar inorganic and organic selenium compounds would not be extracted into the solvent. 3.4. Comparison between extraction methods with CS2 and Na2 SO3 A parallel test on the Na2 SO3 extraction method was done with the PF-Se standard. About 30.0 mg of PF-Se was weighed and extracted according to the procedure described in Section 2.5. To prevent a temperature increase of the ultrasonic bath cold water was added periodically to replace warm water and maintain the temperature below 45 ◦ C. It was noticed that the PF-Se sample was completely dissolved under the Na2 SO3 extraction conditions. A complete dissolution of PF-Se in Na2 SO3 extraction suggests that this solvent is not selective because it has extracted not only elemental Se but also all other non-elemental selenium species such as selenium oxides. To compare the CS2 and Na2 SO3 extraction with an environmental sample, a standard addition test was performed on a freeze-dried Clearwater L. sediment sample (collected in 1995) with the two extraction methods. Appropriate volumes of 745 g L−1 secondary standard Se(0), prepared as in Section 2.3 and diluted accordingly, were pipetted into Teflon tubes containing 0.50 g of sediment sample to which 5.00 mL of CS2 or 5.00 mL of 1.0 M NaSO3 was added. The extractions were carried as described previously. A triplicate extraction was performed for each spike condition. The results are presented in Table 5. The extracted Se(0) was remarkably different with the two methods. With the CS2 extraction, the amount of extracted Se(0) was about 40 times lower than that with sodium sulphite. The reproducibility was much better with CS2 , i.e. the standard deviation of triplicate analysis was 20 times lower than those with the Na2 SO3 extraction method. The poor recovery of the Na2 SO3 extraction is likely due to the small spike of Se0 standard
Table 4 Solubility of organic selenium compounds in different organic solvents Solvent
Sample ID
Sample wt. (mg)
Extracted Se (g)
CS2
Se-dl-cystine Se-l-methionine Se-(methyl) selenocyst.
3.7 3.9 3.8
0.0 0.2 0.1
0.0 0.01 0.0
CH3 CH2 OH
Se-dl-cystine Se-l-methionine Se-(methyl) selenocyst.
4.2 4.5 3.7
0.1 81 42
0.0 4.5 3.3
CH3 OH
Se-dl-cystine Se-l-methionine Se-(methyl) selenocyst.
4.2 3.6 3.1
41 466 691
Dissolved (%)
2.1 32.1 64.9
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Table 5 Comparison of extraction methods with CS2 and Na2 SO3 using a sediment sample from Clearwater Lake (1995) Spiked red Se0 (ng) CS2 extraction 0 23.1 36.1 Na2 SO3 extraction 0 36.5 51.0
Measured Se0 (ng)
S.D. (ng)
R.S.D. (%)
Recovery (%)
6.4 29.1 42.6
0.4 0.2 0.3
6.8 0.4 0.5
98.4 100.2
263.9 275.1 289.6
7.4 7.1 6.7
2.8 2.6 2.3
30.7 50.4
relatively to the selenium extracted from the sample and to the large variations associated with the Na2 SO3 extraction. To further investigate and compare these two techniques, sediment cores from Clearwater L. and McFarlane L. were collected freshly in August 2002 and Se(0) was determined with the CS2 extraction method. Total Se was also determined in the two cores. Profiles are presented in Figs. 3 and 4. To be able to display the profiles of Se(0) together with their corresponding total Se profiles, Se(0) values were multiplied by 10. The percentage of Se(0) in total Se of McFarlane L. sediment was about less than 2% at the top 8 cm and the profile was more or less constant all along the sediment column. The Se(0) profile in Clearwater L.
Fig. 4. Profiles of total Se and elemental Se in McFarlane L. sediments obtained by CS2 extraction. Triplicates of both total and elemental Se were preformed at depth 5–6, 18–19 and 27–28 cm. Standard deviations are all smaller than 0.04 mg kg−1 .
sediment is remarkably different from that of McFarlane L. and it is well correlated with the total Se profile. The percentage of Se(0) reached as high as 11% of the total Se at a depth of 4–5 cm in the sediment. This difference in Se(0) distribution might be related to the contrasting geochemical conditions existing at the sediment–water interface of the two lakes [11,25]. However, the percentage of Se(0) obtained using the Na2 SO3 extraction was between 40 and 60% of total Se in Clearwater L. sediment and between 40 and 90% in McFarlane L. sediment as previously reported [11]. Furthermore, there were no correlation between the profiles of total Se and Se(0) obtained by the Na2 SO3 extraction and the irregular profile obtained at that time also indicated a large variation involved in the Se(0) extraction by Na2 SO3 system. It is believed that under more reducing conditions existing in deeper sediments, some Se could further be converted into Se(−II) as FeSe or FeSe2 , or incorporated into iron sulfide or pyrite [9–11]. The high percentage of Se0 produced by Na2 SO3 extraction method is probably due to its poor selectivity over a range of different selenium species in the sediment samples, as it has been suspected by Wright et al. [24] and demonstrated in this study. For these reasons, the validity of Na2 SO3 extraction method for elemental Se speciation in sediment and soil samples is highly questionable. 4. Conclusion Fig. 3. Profiles of total and elemental Se in Clearwater L. sediments obtained by CS2 extraction. Triplicates of both total and elemental Se were preformed at depth 5–6, 18–19 and 27–28 cm. Standard deviations are all smaller than 0.05 mg kg−1 .
A thorough investigation on PF-Se has shown that this purchased elemental selenium is a mixed material. It contains about 10% of CS2 soluble amorphous Se(0), 15–17% of water
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dissoluble Se oxides, likely selenious acid or similar compounds at the time of year 2002. Mineralogical studies show that in black Se hexagonal crystalline is a dominant form, whereas PF-Se is a mixture of red monoclinic Se, various selenium oxides and other minor unidentified phases. The data explain well the low solubility of PF-Se in CS2 . The very similar Se–O IR stretching peaks as in H2 SeO3 and the presence of a significant amount of oxygen atom (14%) in PF-Se, as determined by SEM-EDS, further confirmed that PF-Se contains selenium oxide. The fact that Na2 SO3 completely dissolved PF-Se sample clearly demonstrates that it is not a selective solvent for elemental Se extraction. The unstable nature of amorphous elemental selenium observed in the study requires a particular attention during the speciation work. Samples should be freeze-dried, stored under low temperature and analyzed as quickly as possible. Nevertheless, the impurity and instability of PF-Se does not change the fact that carbon disulfide can selectively dissolve newly formed amorphous elemental selenium. The solubility of this kind of elemental selenium in CS2 (at least 0.64 mg mL−1 ) is much higher than its concentration in any environmental samples. The black elemental selenium can also be slightly dissolved in CS2 , but to a much lesser extent (∼7.0 g mL−1 ), suggesting that well crystallized hexagonal elemental selenium is not readily soluble in CS2 . Any other selenium species either electrically charged or polar bonded, will not be extracted into CS2 . As a matter of fact, it is unnecessary to carry a pure amorphous elemental selenium standard sample in a future analytical process because a great deal of systematic work has been done in this paper and a large amount of data has proven that CS2 has an excellent dissolution capacity and a good selectivity for amorphous elemental selenium. Furthermore, the extraction and analytical process is simple and reliable, and the results show an excellent reproducibility. Due to very low reagent blank value of selenium in the CS2 solvent, a detection limit of 1.0 ng g−1 sediment sample could be obtained using HG-AFS. Carbon sulphide has a low flash point and a toxic nature, therefore, evaporation below 40 ◦ C in a fume hood is compulsory. Acknowledgements The authors are grateful for the financial support from the Collaborative Mercury Research Network and the Natural Sciences and Engineering Research Council of Canada. Dr. Skage
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