Speciation analysis of trace-level arsenic and selenium in soil using ultrasonic slurry sampling hydride generation-double channel atomic fluorescence spectrometry

Liang Shuxuan¹, Lv Tianfeng¹, Wen Chunhui¹, Sun Hanwen¹, Liu Aichang²
(¹College of Chemistry and Environmental Science, Hebei University, Key Laboratory of Analytical Science and Technology of Heibei Province, Baoding 071002; ²China Metallurgy Perambulation Design Academe of limited company, Bao Ding, 071069, China)

1. INTRODUCTION
The levels of heavy metals circulating in the environment have been seriously increased during the last decades due to human and industrial activities, but at the same time, it has been growing interest to study the soil, since the soil is a very important abiotic element for all terrestrial ecosystems. It provides the habitat for many invertebrates, which is a source of many nutrients, controls of the circulation of matter in the land environment and the composition of soil inform about the activity of all other phases of land environment. Also the chemical composition of soil is a reliable indication of environmental pollution that quick changes in response to environmental changes^[1]. Accumulation of pollutants in soil can result in a high risk for plants and animals, and eventually human health^[2]. In this sense, there is an urgent need for analytical methods applied to the determination of trace elements in soil.
    The health effects of trace elements are often dependent upon both the quantities and chemical forms of the element. Arsenic and selenium are potential toxic elements that are found in the environment^[3,4]. For arsenic, the two most toxic species are arsenite (As(III)) and arsenate (As(V)), respectively, which represent the main forms of arsenic present in soil^[5]. In the case of selenium, it is an essential nutrient at low levels of intake and produce toxic symptoms when it is ingested at levels higher than those required for adequate nutrition^[6]. The narrow concentration range between the two opposing effects requires accurate and precise knowledge of selenium species present in the environment. In natural soil, selenium exists predominantly in two inorganic forms which are more toxic than organic species^[7]. Studies over the years on arsenic and selenium speciation have clearly demonstrated the importance of chemical speciation.
    The traditional pretreatment of solid samples to dissolve the metals present is an acid digestion procedure. However, this procedure is time consuming, require adequate laboratory conditions and make use of large quantities of glassware and reagents, increasing the risks of analyte contamination or loss. As the elements exist in different oxidation states, the acid digestion makes them to their highest oxidation states. Therefore, acid digestion is used for the determination of total element content, not for the speciation analysis of elements. An alternative for the pretreatment of solid samples is the slurry formation by using the power and direct analysis of the solid suspension. Compared with traditional sample preparation methods, slurry formation combines the benefits of the solid and liquid sampling, avoiding the contamination and the loss of volatile elements and reduced sample amount needed. Slurry sampling can keep the oxidation states of elements unchanged, so this method is used for the speciation analysis of different elements. This mode has been extensively used in AAS^[8-10], ICP-MS^[11,12]and AFS^[13-15].
    Atomic fluorescence spectrometry (AFS) has been used for the determination of hydride-forming elements because of its high sensitivity, wide linear dynamic range, speed of analysis, ease of use, and low cost^[16,17]. A literature survey revealed that most of such work concentrated on single element speciation analysis, whereas papers dealing with multielement species determination by hydride generation-double channel AFS are rare.
    The main goal of this work is to develop a new analytical method based on the use of ultrasound aided slurry sampling for fast and accurate simultaneous determination of total As, As(III), total Se and Se(IV) in soil by HG-AFS. In this study, the instrument operating parameters, slurry preparation and determination conditions have been optimized. Finally, data for different soil samples were compared with those found after acid digestion, demonstrating that the traditional acid digestion of the samples can be avoided by ultrasonic slurry sampling. This method was applied to the speciation analysis of As and Se in reference material and some soil samples.

2. EXPERIMENTAL
2.1 Instrumentation
A hydride generation-double channel atomic fluorescence spectrometry AFS-230 (Beijing Haiguang Instrumental Co. Ltd., Beijing, China) equipped with hollow cathode lamps(Beijing Vacuum Electronics Research Institute, Beijing, China) were employed for determination of total As, As(III), total Se and Se(IV) throughout this work and the schematic diagram of the system is shown in Fig.1.

Fig.1 Schematic diagram of the intermittent flow system

Table 2 Program of intermittent flow

2.2 Reagents
The stock solutions of As(III) and Se(IV) (1.0 g L^-1) were purchased from the National Center for Analysis and Testing of Steel Materials(Beijing, China). The working standards were obtained from the stock solution after dilution with 1%(v/v) HCl prior to use.
    The KBH₄ solutions of 20 g·L^-1were daily prepared by dissolving proper amounts of KBH₄ (Tianjin Institute of Chemical Reagents, Tianjin, China) in 0.5% (m/v) NaOH solution.
    The thiourea and ascorbic acid solution that was used to reduce As(V) to As(III) before reaction with KBH₄ was prepared by dissolving 10.0 g ascorbic acid and 10.0 g thiourea in 100mL ultrapure water.
    The agar solution (2.0g L^-1) was prepared by dissolving 1.0g agar (Haiyang,Guangdong) in 500mL boiled water.
    Aqua regia was prepared by mixing HNO₃ and HCl 1:3 (v/v) from the concentrated solutions.
    HCl, NaOH, HNO₃, HClO₄and H₂SO₄ were of high purity grade. All of the chemicals were of high purity grade available. Ultrapure water (18.3W) was used throughout.
2.3 Experimental procedure
2.3.1 Soil properties
The soils used in this study were obtained from the main agricultural areas in Baoding (China) varying widely in physicochemical properties. The soil samples, collected from the surface horizon (0–15cm), were air dried and mildly ground to pass through a 80mm stainless steel sieve, homogenized and stored for subsequent analysis.
2.3.2 Slurries preparation
Two portions of 0.5g soil were weighed accurately in 50mL glass flasks, and 20mL agar solution (2.0g L^-1) was added to each one, and shaken energetically. The resulting slurries were dispersed with an ultrasonic vibrator for 15 min in order to obtain homogeneous dispersions.
    To one of the portions, which was for total Se and As simultaneous determination, 10 mL conc. HCl and 10mL thiourea-ascorbic acid solution of 10% (m/v) were added, diluted to the mark with ultrapure water and heated in a 95^oC water bath for 30 min.
    To the second slurry, which was for As(III) and Se(IV) determination, 10 mL conc. HCl was added, diluted to the mark with ultrapure water.
    In the two cases 0.5mL of the treated sample were injected into an HCl stream, which is merged with KBH₄ to form the hydrides. The analytes were determined in the experimental conditions indicated in Table 1. AFS data found for samples were interpolated in the calibration curve obtained for each element in the same conditions as the samples.
2.3.3 Wet digestion
For comparison, determinations were also performed with wet digestion of soil samples. Test portions (0.5g) were accurately weighed into borosilicate glass vessels and 4mL aqua regia was added. Then the solution was boiling for 2 hours. 2 mL HClO₄ was added in the solution, which was going on boiling until dried. The digested samples were transferred to volumetric flasks with 10mL 1%(v/v) HCl.
    For total As and Se determination, it is necessary to acidify with 10 mL of conc. HCl and add 10mL of 10% (w/v) thiourea-ascorbic acid solution, diluted to 50 mL with ultrapure water, and then heated at 95^oC for 30 min. Digested sample solutions were analysed by HG-AFS following the procedure indicated before for slurries.

3. RESULTS AND DISCUSSIONS
3.1 Pre-reduction conditions
As and Se can exist in two different oxidation states: As(III) and As(V), Se(IV) and Se(VI). In their highest oxidation states, Se(VI) was unable to form hydrides. As(V) was not rapidly deoxidized into its hydride quantitatively. Therefore, pre-reduction of samples is necessary for the determination of total As and Se content.
    For deoxidizing Se(VI) to Se(IV), it has been suggested in the literature^[18,19] that heated the solution in presence of hydrochloric acid medium which was not lower than 2.0 mol L^-1 for about 30 min. To deoxidizing As(V) to As(III), thiourea and ascorbic acid, potassium iodide, hydrochloric hydroxylamine and L-cysteine were commonly used as the prereducing agent. However, there was serious interference to determine Se, it brought about that the signal intensity of Se was usually decreased by a degree of 20–50% than the present method. This aftermath was not acceptable for simultaneous determination of As and Se from a medium except that it was not necessary to pre-deoxidize As(V) to As(III) that could produce much error to determine total As^[20]. So Se and As were usually determined from different medium respectively. The investigation was shown that thiourea and ascorbic acid was the much suitable prereducer in the present method. With the presence of it not only As(V)was deoxidized into As(III), but also the signal intensity was not interfered for Se when simultaneous determination of As and Se. 10% (m/v) thiourea and ascorbic acid was employed to all of the determinations.
3.2 Effect of ultrasonic agitation
Ultrasonic agitation has been used as an effective system of homogenizing slurries for HG-AFS and can be used in combination with both manual and automated introduction of the slurry. This sample preparation is more profitable than magnetic agitation and vortex mixing. Hence, in the present work, ultrasonic agitation was used for the slurry preparation.
3.3 Choice of slurry medium
Slurry preparation in aqueous solution is seldom suitable because most powdered materials undergo rapid sedimentation. This sedimentation of suspended material usually occurs after mixing the slurry. The sedimentation rate depends on the densities of the diluent and solid material, the viscosity of the diluent medium and the radius of the sample particles. The slurry can be stabilized using a highly viscous liquid medium. Thus far, agar, glycerol, enthanol and Triton-x100 have been used as slurry stabilizing agents. In this study, the results showed that the viscosity of agar kept different types of particles in suspension for a sufficient time. Furthermore, it has been stated that the optimum concentration of agar was 0.08 %(m/v) for the homogenization of slurry soil samples. Therefore, in the present work, 2.0g L^-1 agar solution was used as the slurry stabilizing agent.
3.4 Acidity effects
The generation of nascent hydrogen, which is the actual reducing agent from tetrahydroborate requires an acidic reaction medium. The mechanism of hydride generation that used borohydride as reducing agent under acid conditions was complicated, and the mechanism were related with the characteristics of determined solution and the reagents^[21]. HCl, H₃PO₄, HNO₃ and H₂SO₄ were investigated. As the results of the tests, the signal intensity from hydrochloric acid was higher than those of the other acids.
    The signal intensity of of Se(10 mg L^-1), As(10 mg L^-1) would gradually increased along with increasing hydrochloric acid concentration, and it reached maximal when the concentration of hydrochloric acid was 20% (v/v) for Se and As. The results were presented in Fig.2.

3.5 Interference studies
If the determination procedure established is to have any practical utility, it is essential that it can tolerate the presence of pertinent ions. Thus the influences of common interferents on the fluorescence signals were investigated. The tolerance content, defined as the interferent concentration (by wt.) reducing the analyte signal by 10%, are as follows: 5000-fold K⁺ and Na⁺, 4000-fold Ca²⁺ and Mg²⁺, 2500-fold Al³⁺ and V(III), 1000-fold Fe³⁺, Zn²⁺ Cu²⁺, Mn²⁺, 500-fold Ag⁺ and Ni²⁺, 200-fold Pb²⁺, 50-fold Bi³⁺ have no obvious influence on the determination of 20 mg L^-1 As and Se, respectively. Furthermore 100-fold Sb(III) to As(III) ratios caused serious increases of the signals. However, the content ratio of Sb(III) and As(III) in real samples was much lower, and there were no mutual interferences.
3.6 Analytical performance
The calibration graphs were constructed via determined a set of four element species solutions with concentration equal to 0.0,2.0, 4.0, 6.0, 8.0, 10.0mg·L^-1 for each one. Each solution was treated via the same method with the sample solution in step 2.3. Finally, the obtained calibration graph were of very good linearity in linear ranges which were found to be linear up to at least 310mg L^-1 for As(III), 320mg L^-1 for total As, 35mg L^-1 for Se(IV) and 40mg L^-1 for total Se, with the correlation coefficient 0.9994,0.9995,0.9992 and 0.9998, respectively.
   The detection limits (DL) for four elements species were evaluated on the basis of the standard deviation (S.D.) of the measured signals (11 times) for the blank solution, and were calculated by the formulate DL=3S.D.K^-1 (here K is the slopes of the calibration graph). As the results, the detection limits were 8.5ng L^-1 for total As, 10.5ng L^-1 for As(III), 3.4ng L^-1 for total Se and 6.1ng L^-1 for Se(IV).
    The feasibility of the method was evaluated by analyzing the standard reference material of GBW07411. The results listed in Table 3 reveal that this method is of high accuracy and precision.

Table 3 Analysis of standard reference materials (n=7)

*The number of replicate measurements was five.

4. CONCLUSION
The developed procedure offers an accurate alternative to those based on a previous complete digestion of samples for the determination of total As, As(III), total Se and Se(IV) in soil. The method, based on the excellent sensitivity attainable by AFS and on a soft room-temperature treatment of the samples, is a safe and easy methodology for operators, which reduces the reagent consumption and time of analysis and offers a possibility to explore the presence of total As, As(III), total Se and Se(IV) in soil. The developed method is simple and accurate, so it is promising for speciation analysis of trace of As and Se in other more complex materials.

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