- Email: [email protected]
Speciation of scandium and gallium in soil
Chemosphere 73 (2008) 572–579
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Speciation of scandium and gallium in soil Justyna Połedniok * Department of Analytical Chemistry, University of Silesia, 9 Szkolna Street, 40-006 Katowice, Poland
a r t i c l e
i n f o
Article history: Received 27 February 2008 Received in revised form 19 May 2008 Accepted 5 June 2008 Available online 23 July 2008 Keywords: Gallium Scandium Soil Speciation Sequential extraction Spectrophotometry
a b s t r a c t A method for the speciation of scandium and gallium in soil has been developed. The sequential extraction scheme of Tessier et al. for heavy metals was examined for the scandium and gallium separation. The regents proposed by Tessier were used for the extraction, and only for the residual fraction the HClO4 was replaced with H2SO4. The optimum conditions for leaching scandium and gallium from the soil were chosen for each fraction. Very sensitive, spectrophotometric methods based on the mixed complexes of Sc(III) and Ga(III) with Chrome Azurol S and benzyldodecyldimethylammonium bromide were applied for the scandium and gallium determination after their separation by solvent extraction. 100% mesityl oxide and a 0.5 M solution of 2-thenoyltrifluoroacetone in xylene were chosen for the extraction of scandium and butyl acetate was selected for gallium. Soil samples from two different regions of Poland were the object of this research. The content of scandium and gallium found in the individual fractions of Upper Silesia soil (industrial region) was [in lg g 1] Sc: I, 1.52; II, 0.53; III, 7.78; IV, 1.79; V, 0.20; Ga: I, 24.7; III, 29.2; IV, 35.4; V, 6.9. In Podlasie soil (agricultural region), the content of both elements was clearly lower. The total content of scandium and gallium in the five soil fractions was in good correlation with the total content of these elements in the soils found after HF–H2SO4 digestion. Analysis using the ICP-OES method gave comparable results. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Among the many chemical constituents, trace elements play an important role in the biological environment, showing a positive or toxic impact on the natural environment depending on their concentration. As a result of human economic and technological activity, particularly in industrialised areas, the natural circulation of trace elements in the environment has been subject to interference. Each change in the chemical balance causes not only disturbances in the plant or animal world, but may also have an adverse influence on human health. One of the environmental components from which trace elements transfer to living organisms is soil: the principal place of accumulation of many chemical substances, polluted by atmospheric dusts, solid waste dumps, plant protection agents, mineral fertilisers and others (Kabata-Pendias and Pendias, 1992, 1999). Gallium is a very widespread trace element. Its content in the lithosphere amounts to 10 g/t. Gallium is a cryptomorphic element whose average clarkes (geochemical density indicators) in the lithosphere are relatively high, but due to the practical inability to form its own minerals in natural conditions, it is considered quite rare. The gallium content in soil varies between 5 and 300 ppm (usually 5–70 ppm). The lowest gallium concentration can be * Tel.: +48 32 3591642; fax: +48 32 2599978. E-mail address: [email protected] 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.06.012
observed in light sandy and limestone soils. The presence of gallium in the soil is connected with the occurrence of silty minerals. Moreover, it is sorbed by Fe(III) and Mn(III) oxides and by an organic substance. Despite quite a large clarke (11 ppm), scandium is also considered a rare element, hard to concentrate or separate, which is connected with a lack of specific reagents reacting with this element. Similar to gallium, scandium is accumulated in soil in silty minerals and accompanies iron and magnesium minerals. The average scandium content in soil amounts to 2–12 ppm. The smallest content of scandium can be observed in podzols whereas in brown soil and black-earth it is the greatest. The soil around industrial plants exploiting the natural energy resources contains an increased amount of the element (Kabata-Pendias and Pendias, 1999). Scandium and gallium are the components of almost all plants. The average scandium content is between 0.008 and several tenths of a ppm, whereas the gallium content varies between 0.05 and 5 ppm. The biological role of Sc and Ga in the development and functioning of plants has not been specifically defined so far. No information on their toxicity has been reported, either. However, there are hardly any publications concerning this question. The topic still remains open, up-to-date and raised by researchers in various parts of the globe. The latest studies have proven that some plants (lichen, the phylum Bryophyta, and also papilionaceous plants) show the ability to accumulate gallium, even at the 60 ppm concentration level, and the larger values of Ga/Al ratio
J. Połedniok / Chemosphere 73 (2008) 572–579
in land plants confirm the selective absorption of gallium. Moreover, it has been proven that Sc absorbed by plants is accumulated in the roots, seeds and old leaves, while the accumulation is scarce in the young leaf explants (Shtangeeva, 2005). The scandium content in the leaves of various plants and in the soil of the Brazilian forests was compared, and a distinct correlation was observed between the amount of contaminants present and the content of scandium in the soil. It was found that the scandium content growth in the soil could affect its concentration in the roots of plants and the absorbability of other significant nutrients (Ferrari et al., 2006). The effect of scandium accumulation in the wheat seeds on the absorbability of the macro- and micro-components from the soils was examined, and it was found that the bioaccumulation of scandium had significantly affected the changes in the concentration of the elements that are important for the plants (Na, K, Zn) (Shtangeeva et al., 2004). In view of the above, considering the growing pollution of soils with trace elements, including Sc and Ga, the determination of the functional speciation of the elements in the soil seems to be advisable and helpful for further studies on the correlations between the soil and the plant, particularly in areas subjected to various anthropogenic influences. In the last 15 years there have been few publications concerning scandium and gallium determination in soils. Certified materials and real samples were analysed and the total content or concentration of these elements in soil profiles was determined. Scandium was determined using the following analytical methods: NAA (Schulze et al., 1997; Minowa and Ebihara, 2003; Riberio et al., 2005), ICP-MS (Tyler, 2004), ICP-OES (Buhl and Połedniok, 1999, 2000; Tyler, 2004) and spectrophotometry (Buhl and Połedniok, 1999, 2000). As far as gallium is concerned, the most frequently used techniques were: AAS (Langodegard and Wibetoe, 2002; Lopez-Garcia et al., 2004) and XRF (Saini et al., 2002; Zambello and Enzweiler, 2002; Liang et al., 2004; Yu et al., 2006), but also AES (Sun et al., 2004), NAA (El-Ghawi et al., 2005), ICP-MS (Chandrajith et al., 2005) and spectrophotometry (Buhl and Połedniok, 1999, 2000). Currently, speciation is the approach more and more frequently mentioned in scientific publications. An increased interest in speciation results from the fact that the total content of a given element does not provide satisfactory information to describe its environmental behaviour (e.g. toxicity, bioavailability, migration, and accumulation). Most publications on speciation are devoted to heavy metals due to their impact on human health (Solecki and Chibowski, 2000; Sobczyn´ski and Siepak, 2001; Jarosz and Mirzyn´ska, 2006). Research so far has been carried out for Cr, Cd, Co, Cu, Fe, Pb, Ni, As, U, Hg, Sn, Sb, Zn, Mn and V. Various instrumental methods have been used, e.g. ICP-OES, ICP-MS, AAS, FIA, HPLC, XRF and spectrophotometry. The research covered: brown coal (Vogt, 1994), marine sediment (Marin et al., 1997), soil (Arunachalam et al., 1996; Połedniok and Buhl, 2003) and water (Plessow and Heinrichs, 2000; Hirose, 2006). Gallium speciation has not been analysed so far but an article on scandium speciation was published (Marin et al., 1997), covering the determination of scandium in a marine sediment (Standard Reference Material SRM 1646a) using a sequential extraction scheme recommended by BCR and the ICP-MS technique. The purpose of the study was to determine the speciation of scandium and gallium in real soil samples from two different regions of Poland: Upper Silesia (an industrial region) and Podlasie (an agricultural region) based on the sequential extraction scheme developed by Tessier et al. (1979) for heavy metals (Cd, Co, Cu, Ni, Pb, Zn, Fe and Mn). For the determination of scandium and gallium, very sensitive, spectrophotometric methods developed in the Department of Analytical Chemistry of the University of Silesia were used. The methods are based on the mixed complexes Sc(III) and Ga(III) with Chrome Azurol S (CAS) and benzyldodecyldimethylammonium bromide (ST) (eSc = 1.37 105 and eGa =
573
1.10 105 L mol 1 cm 1) (Kwapulin´ska et al., 1993; Kwapulin´ska, 1995). The scandium determination was preceded by Sc(III) extraction with 100% mesityl oxide (TM) from an ammonium thiocyanate solution (Kwapulin´ska et al., 1993) or a 0.5 M solution of 2-thenoyltrifluoroacetone (TTA) in xylene (Buhl and Połedniok, 1999). The gallium determination was preceded by Ga(III) extraction with butyl acetate (Buhl and Połedniok, 1999).
2. Experimental 2.1. Apparatus A DR/2010 (Hach, USA) VIS spectrophotometer was used for the research. The spectrometric measurements were carried out in a Spectroflame-ICP type M (Spectro Analytical Instruments, Germany) spectrometer. The operating conditions were as follows: power 1.1 kW, frequency 27.12 MHz, nebuliser gas Ar 1.0 L min 1, coolant gas 14.0 L min 1, auxiliary gas 0.5 L min 1, amount of dosed sample 1 mL min 1, viewing height over induction coil 11 mm, dispersion of grating in 1st reciprocal order 0.55 nm mm 1. pH-meter type N-517 (Mera Elwro, Poland). Universal Shaker type 327 (Premed, Poland). 2.2. Reagents and standard solutions All reagents used in the study were analytically pure reagents. Doubly distilled water obtained from an ELIX3 (Millipore, Molsheim, France) was used during the experiments. Scandium(III) standard solution: 1 mg mL 1 (Promochem, Poland). The working solutions were prepared by the appropriate dilution of the scandium(III) standard solution with the 1 10 2 M HCl. Gallium(III) standard solution: 1 mg mL 1 (Wzormat, Warsaw, Poland). The working solutions were prepared by the dilution of the gallium (III) standard solution with 1 10 3 M HCl. Chrome Azurol S (CAS) was purchased from Fluka (Buchs, Switzerland): 1.43 10 3 M and 5 10 4 M aqueous solutions were used for the research. Benzyldodecyldimethylammonium bromide: 10% commercial product Sterinol (ST) was from Polfa (Cracow, Poland): 1.43 10 2 M and 5 10 2 M aqueous solutions were used in the studies. TM: 100% mesityl oxide (Fluka, Buchs, Switzerland). TTA: a 0.5 M solution in xylene was made from 2-thenoyltrifluoroacetone (Fluka, Buchs, Switzerland). Xylene (POCh Gliwice, Poland). Butyl acetate (POCh Gliwice, Poland). AA: a 50% solution in chloroform was made from acetylacetone (Ubichem, Hampshire, England). Chloroform (POCh Gliwice, Poland). Ascorbic acid (Polfa, Cracow, Poland): aqueous solutions were prepared directly before using. Magnesium chloride (MgCl2 6H2O), sodium acetate and ammonium thiocyanate were from (POCh Gliwice, Poland). Hydroxylamine hydrochloride and ammonium acetate (POCh Gliwice, Poland): a 0.04 M solution in 25% CH3COOH and a 3.2 M solution in 20% HNO3, respectively, were used for the research. Hydrogen peroxide: 30% (POCh Gliwice, Poland). Hydrochloric acid (POCh Gliwice, Poland): 35– 38%. Hydrofluoric acid (POCh Gliwice, Poland): 40%. Nitric acid (Fluka, Buchs, Switzerland): 65%. Sulfuric acid (Hetalab, USA): 96%. Acetic acid (POCh Gliwice, Poland): 99.5%. 2.3. Sampling and sample preparation Soil samples were collected from the arable layer (0–20 cm) of two Polish fields located in Upper Silesia (Da˛brówka Wielka) and Podlasie (Biała Podlaska) and prepared according to the procedure described in Polish Standard PN-R-04031 (1997). Next, the samples were dried at room temperature for 7 days and after division by
574
J. Połedniok / Chemosphere 73 (2008) 572–579
the coning and quartering method the samples were ground in an agate mortar. The grain size was less than 200 lm. Soil from Upper Silesia is brown soil developed in weakly structured clayey sands and loams. Soil from Podlasie is white soil. Brown soils in most cases originate from rocks abundant with plant nutrition components and calcium. They are characterised by a well-formed accumulation level, gray or brown coloured, gradually turning into an intensive weathering, gray-brown coloured strata (Bac et al., 1999). The reaction of the soils is slightly acid or neutral, i.e. the best for crop growth. The intensive biological circulation of the mineral components prevents their washing into the depth of the soil and facilitates their accumulation on the upper levels. The fast mineralisation of the forest rainfall and favourable direction of its humification prevent the accumulation of acid overlay humus and production of more fulvic acids that would enable the movement of soil mineral weathering product into the depth of the soil (Bednarek and Prusinkiewicz, 1997). White soils are formed from vernacular formations with low nutrient content. The soils have an acid or slightly acid reaction, high hydrolytic and exchangeable acidity and a very low basic cation saturation ratio, which makes some of the nutrients unavailable to the plants. The moving humus acids, washed from the overlay humus, with their complexation abilities, play the most important role in the genesis of white soils. The humus acids (mainly the fulvic acid fraction) create easily soluble complex connections with iron and aluminium ions in the upper part of the profile (Bednarek and Prusinkiewicz, 1997).
2.4. Leaching procedure Fraction 1: The soil sample (scandium: 2 g; gallium: 1 g for Upper Silesia soil and 2 g for Podlasie soil) was extracted at room temperature with, respectively, Sc: 10 mL and Ga: 8 mL of 1 M MgCl2 (pH 7) with continuous agitation. Next, the mixture was filtered using a filter with medium wide pores (Spezialpapierfabrik Niederschlag, Germany; yellow label filter no. 389) and the filtrate was diluted to 50 mL with water. Fraction 2: The solid residue from fraction 1 was leached at room temperature with continuous agitation with 1 M sodium acetate adjusted to pH 5 with acetic acid. The 8 mL of CH3COONa was used in the case of gallium and 10 mL was applied in the case of scandium. After that, the mixture was filtered according to the procedure described above. Fraction 3: The solid residue from fraction 2 was extracted with 20 mL of 0.04 M NH2OH HCl in 25% acetic acid at 96 ± 3 °C with occasional agitation. Next, the mixture was filtered and the filtrate was evaporated to dryness. The dry residue was dissolved and diluted to 50 mL with 0.1 M HCl. Fraction 4: The following solutions were added to the solid residue from fraction 3: 3 mL of 0.02 M HNO3 and 5 mL of 30% H2O2 adjusted to pH 2 with HNO3, and the mixture was heated to 85 ± 2 °C with occasional agitation (time 1). Next, 3 mL of 30% H2O2 (pH 2 with HNO3) was added and the sample was heated again to 85 ± 2 °C with occasional agitation (time 2). After cooling, 2 mL of 3.2 M ammonium acetate in 20% HNO3 was added, and the sample was diluted to 20 mL with water and agitated continuously for 30 min. The mixture was filtered and the filtrate was evaporated to dryness. The dry residue was dissolved and diluted to 50 mL with 0.1 M HCl. Fraction 5: The residue from fraction 4 was digested in a platinum crucible with conc. HF–H2SO4 mixture. For 1 g of the sample, 1 mL of H2SO4 and 10 mL of HF were used for the first time and 7 mL of HF was used for the second time. The obtained solution was evaporated to dryness and the residue was dissolved and diluted to 25 mL with 0.5 M HCl.
2.5. Scandium separation and determination Fractions 1, 2, 4 or 5. The following solutions were placed in 25 mL beakers: 5 mL of the solution obtained after the leaching of fractions: 1, 2 or 4; 2 mL of the solution obtained after the digestion of fraction 5 and 5 mL of ammonium thiocyanate solution containing 1.425 g NH4SCN. Next, the pH was adjusted to 1 with HCl using a pH-meter and water was added to make the volume of the solutions about 12.5 mL. The samples were then transferred into 50 mL separatory funnels and scandium was extracted with 5 mL of 100% mesityl oxide for 2 min. The water phase was rejected and Sc was reextracted twice from the organic phase with 10 mL and 5 mL of 0.2 M HCl for 2 min. The reextracts (15 mL) were place in a 100 mL beaker and evaporated to dryness. The residue was dissolved in water and transferred quantitatively into a measuring flask. Then 1 mL of a pH 5 acetate buffer solution, 4 mL of 5 10 4 M CAS and 0.66 mL of 5 10 2 M ST were added subsequently to the flask and the mixture was diluted to 25 mL with water. The absorbance was measured in 1 cm long glass cells at k = 610 nm against a reagent blank. Fraction 3: The following were placed in a 50 mL separatory funnel: 3 mL of the solution obtained after the leaching of fraction 3, 0.5 mL of 0.5 M HCl and 7 mL of water. Scandium was extracted twice with 5 mL of a TTA solution in xylene for 5 min. The water phase was rejected and the both organic phases were washed with 5 mL of 0.03 M HCl (10 s). After that, the scandium was reextracted with 10 mL of 2 M HCl for 5 min. The reextract was washed with 5 mL of xylene (10 s), transferred into a 100 mL beaker and evaporated to dryness. Next, 1 mL of conc. HNO3 was added to the beaker and the sample was evaporated to dryness again. Finally, the residue was dissolved in water and transferred into a 25 mL measuring flask. Then the colour reaction with CAS and ST was developed and the measurements were carried out according to the procedure described above. The scandium concentration was found from the calibration graphs (obtained after extraction) in the range: 0.04–0.50 lg mL 1 (a = 0.0898, b = 0.0298, r = 0.9996) for 100% TM and 0.04– 0.40 lg mL 1 (a = 0.117, b = 0.044, r = 0.9998) for a TTA solution in xylene. 2.6. Gallium separation and determination Fractions 1, 2, 3, 4 or 5. The following solutions were placed in 25 mL beakers: 3 mL of the solution obtained after the leaching of fractions 1 or 4; 5 mL of the solution obtained after the leaching of fractions 2 or 3; 5 mL of the solution obtained after the digestion of fraction 5. Next, the pH was adjusted to about 1.5–2.0 with HCl using a pH-meter. The samples were then transferred into 100 mL separatory funnels and ascorbic acid was added: 2 mL of a 1% ascorbic acid solution was used for the reduction of Fe(III) contained in fractions 1 and 2, 2 mL of a 5% solution was used for fractions 3 and 4, whereas 5 mL of a 25% solution was used for fraction 5. After 10 min., the solutions were diluted to 10 mL with water. Next, 10 mL of conc. HCl and 20 mL of butyl acetate were added and gallium was extracted for 1 min. Then the water phase was rejected and the organic phase was washed twice with 2.5 mL of a 6 M HCl solution. After that, gallium was reextracted twice for 1 min. with 10 mL of water. The reextracts (20 mL) were evaporated in 100 mL beakers until the solutions reached a pH of 3.7–4.0. Then the solutions were quantitatively carried to 25 mL measuring flasks and 1 mL of a pH 4.5 acetate buffer solution, 2 mL of 1.43 10 3 M CAS and 2 mL of 1.43 10 2 M ST were added. Finally, the mixtures were diluted with water up to the mark. The absorbance was measured in 1 cm long glass cells at k = 620 nm against a reagent blank. The content of gallium was
575
J. Połedniok / Chemosphere 73 (2008) 572–579
found from the calibration graph obtained after extraction in the range: 0.12–0.40 lg mL 1. The linear regression coefficients were: a = 0.049, b = 0.011, r = 0.9995. 3. Results and discussion 3.1. Optimisation of scandium and gallium leaching Weight of samples. Preliminary studies showed that the total content of gallium and scandium obtained after digestion of the samples with the HF–H2SO4 mixture was, respectively [in lg g 1] – Upper Silesia soil: 94.7 and 11.1; Podlasie soil: 41.6 and 2.2. Therefore, the weight of soil samples chosen for the research was: scandium: 2 g; gallium: 1 g for Upper Silesia soil and 2 g for Podlasie soil. Effect of extractants: The reagents proposed by Tessier et al. (1979) were applied for the partitioning of scandium and gallium into the five soil fractions. The NH2OH HCl solution was chosen for the leaching of fraction 3. Other extractants used by Tessier et al. for this fraction interfered in the CAS-ST determination. In order to improve the safety of work, conc. HClO4 was replaced with conc. H2SO4 and the residual fraction was digested using the HF– H2SO4 mixture. The maximum concentration of gallium in all soil extracts and scandium in fractions 3–5 were obtained following the Tessier method. In the case of fractions 1 and 2, the optimum volume of reagents for scandium extraction was, respectively: 10 mL of 1 M MgCl2 and 10 mL of 1 M CH3COONa. Effect of temperature: The temperatures of extraction suggested by Tessier et al. (1979) were optimum for scandium and gallium leaching from the soil: room temperature for fractions 1 and 2, 96 ± 3 °C for fraction 3 and 85 ± 2 °C for fraction 4. Effect of time: Figs. 1–3 show the influence of time on the efficiency of scandium and gallium extraction from the soil. The concentration of Sc(III) and Ga(III) in the MgCl2 (pH 7) leaching solution (fraction 1) reached the maximum value after 0.5 h of extraction and it was stable for the next 1 h (Fig. 1). The concentration of Sc(III) in the CH3COONa (pH 5) leaching solution used for the separation of fraction 2 reached the maximum value after 1 h and then slowly reduced with time, whereas the concentration of Ga(III) – for both examined soils – was less than the detection limit of the applied method even after 5 h of extraction. It means that
gallium does not form compounds with carbonates in soil. Fig. 2 indicates that content of scandium and gallium in NH2OH HCl solution applied for the isolation of fraction 3 was at the maximum level after 0.5 h of extraction and it was stable for 1 h and 1.5 h, respectively. Next, the maximum value was clearly decreasing with time. Probably the elements from the leaching solution were adsorbed on the surface of the soil. Fig. 3 shows that the optimum time for scandium and gallium leaching from the fraction 4 was the combination: 1.5 h + 1.5 h (Sc) and 2 h + 2 h (Ga). Gallium concentration in the HNO3–H2O2 mixture was comparable for all times that were examined, whereas scandium concentration after the optimum time was clearly lower. Fraction 5 (residual) was digested with conc. HF and conc. H2SO4 until the white fumes of SO3 appeared. Next, the second part of HF was added, then the mixture was evaporated to dryness and the solid residue was dissolved in the HCl solution. The suitable time of dissolving was 40 min. The optimum time of soil leaching chosen for further research was the following, scandium: 1 h for fractions: 1, 2, 3 and combination (1.5 + 1.5) h for fraction 4; gallium: 1 h for fractions: 1, 3 and a combination (2 + 2) h for fraction 4. The optimum time of dissolving of fraction 5 for both elements was 40 min. 3.2. Selectivity of separation The methods of scandium and gallium separation by the solvent extraction were elaborated using model solutions containing macro- and micro-components of soil and reagents proposed by Tessier for the leaching. The criterion for interference was the content of scandium and gallium varying by more than 5% from the expected value for these elements alone. Scandium. It was shown that 100% mesityl oxide separated 5 lg Sc(III) from 100 mg Ca(II), 30 mg Mg(II), 5 mg Mn(II) and Cr(III), 2 mg Al(III), 0.5 mg Zn(II) and In(III), 150 lg Fe(III), 50 lg Ga(III), Cu(II) and Mo(VI), and 500 mg acetate (Table 1). NH2OH HCl interfered in the CAS-ST scandium determination after extraction with mesityl oxide: absorbance of the complex was clearly lower. The solution of TTA in xylene separated 5 lg Sc(III) from 100 mg Mg(II), 75 mg Ca(II), 5 mg Cr(III) and Zn(II), 0.5 mg Mo(VI), 100 lg Al(III), 50 lg V(V), 50 lg Ga(III), 10 lg In(III), 5 lg Fe(III) and 5.5 mg
Concentration of scandium and gallium [ μg L-1]
1000 900
Gallium in Upper Silesia soil Scandium in Upper Silesia soil
800
Gallium in Podlasie soil
700 600 500 400 300 200 100 0 0.5
1
1.5
2
3
5
Time [h] Fig. 1. Effect of leaching time on scandium and gallium concentration in the MgCl2 extracting solution (pH 7); error bars (±SD).
576
J. Połedniok / Chemosphere 73 (2008) 572–579
Concentration of scandium and gallium [μg L-1]
800 Gallium in Upper Silesia soil
700
Scandium in Upper Silesia soil Gallium in Podlasie soil
600
500
400
300
200
100
0 0.5
1
1.5
2
3
5
Time [h] Fig. 2. Effect of leaching time on scandium and gallium concentration in the NH2OH HCl extracting solution (pH 2); error bars (±SD).
Concentration of scandium and gallium [µg L-1]
1400
Scandium in Upper Silesia soil
1200
Gallium in Upper Silesia soil Gallium in Podlasie soil
1000
800
600
400
200
0 (1+1)
(1.5+1.5)
(2+2)
(2+3)
Time [h] Fig. 3. Effect of leaching time on scandium and gallium concentration in the HNO3 + H2O2 extracting solution (pH 2).
NH2OH HCl (Table 1). Therefore, 100% mesityl oxide was chosen for the scandium separation from fractions: 1, 2, 4 and 5, and the solution of TTA in xylene was applied for scandium separation from fraction 3 instead. Before scandium extraction using TTA and mesityl oxide (from fractions: 3, 4 and 5) Fe(III) was previously separated by solvent extraction with 20 mL of a 50% solution of acetylacetone in CHCl3 at pH 1. The optimum time of extraction was 5 min. The number of extractions was chosen experimentally for each fraction and it was: 5 for fractions 3 and 5; 3 for fraction 4. Ions of Al(III) were separated simultaneously with Fe(III). Gallium. It was pointed out that butyl acetate separated 10 lg Ga(III) from 60 mg Al(III), 22 mg Ca(II), 13 mg Mg(II), 100 lg In(III), Cu(II), Zn(II) and Mo(VI), 75 lg Mn(II), 50 lg Cr(III), 10 lg Sc(III), 57 mg acetate and 5.5 mg NH2OH HCl (Table 2). The coextraction of Ga(III) and Fe(III) was observed: the absorbance was clearly above normal. Therefore, Fe(III) was reduced to Fe(II) with ascorbic acid (t = 10 min) before the gallium extraction. Fe(II) is separated
from Ga(III) using butyl acetate. The concentration and volume of ascorbic acid was chosen experimentally for each fraction and it is presented in Section 2.6. 3.3. Determination of scandium and gallium in synthetic mixture Scandium and gallium were found in a synthetic mixture using the CAS-ST method after extraction with mesityl oxide, TTA and butyl acetate, respectively. The synthetic mixture shows the average composition of the soil (in a sample with the mass of m = 0.1 g), i.e. the content of macro- and micro-elements: 1 mg Ca(II), 0.5 mg Mg(II), 2 mg Al(III), 2 mg Fe(III), 10 lg Cu(II), 60 lg Mn(II), 5.5 lg Cr(III), 0.25 lg Mo(VI), 15 lg Zn(II), 10 lg Pb, 10 lg V(V) and 1 lg In(III) (Wa˛chalewski, 1997). Before the analysis, 5 lg of Sc(III) and 10 lg of Ga(III) were added to each sample. The results obtained are shown in Tables 1 and 2. The extraction with mesityl oxide,
577
J. Połedniok / Chemosphere 73 (2008) 572–579 Table 1 Recovery of scandium in presence of various soil compounds and reagents applying TM and TTA solvent extraction Extractant
Foreign ions, reagents
TM
Ca(II) Mg(II) Mn(II) Cr(III) Al(III) Zn(II) In(III) Fe(III) Cu(II) Mo(VI) Ga(III) Acetate Synthetic mixtureb
TTA
Mg(II) Ca(II) Zn(II) Cr(III) Mo(VI) Al(III) V(V) Ga(III) In(III) Fe(III) NH2OH HCl Synthetic mixtureb
a b
Sc added (lg)
Sc founda (lg)
RSD (%)
Recovery (%)
100 mg 30 mg 5 mg 5 mg 2 mg 0.5 mg 0.5 mg 150 lg 50 lg 50 lg 50 lg 500 mg
5 5 5 5 5 5 5 5 5 5 5 5 5
4.78 ± 0.24 4.79 ± 0.44 5.00 ± 0.59 5.14 ± 0.42 4.83 ± 0.36 5.18 ± 0.21 5.20 ± 0.65 4.98 ± 0.31 5.00 ± 0.43 5.10 ± 0.38 5.14 ± 0.67 4.80 ± 0.32 5.08 ± 0.43
2.0 3.7 4.8 3.3 3.0 1.6 5.1 2.5 3.5 3.1 5.3 2.7 3.4
95.6 95.8 100.0 102.8 96.6 103.6 104.0 99.6 100.0 102.0 102.8 96.0 101.6
100 mg 75 mg 5 mg 5 mg 0.5 mg 100 lg 50 lg 50 lg 10 lg 5 lg 5.5 mg
5 5 5 5 5 5 5 5 5 5 5 5
5.25 ± 0.51 5.00 ± 0.47 4.83 ± 0.32 5.11 ± 0.28 5.09 ± 0.24 5.15 ± 0.51 4.90 ± 0.39 5.10 ± 0.44 4.85 ± 0.48 5.15 ± 0.34 4.87 ± 0.26 4.79 ± 0.43
3.9 3.8 2.7 2.2 1.9 4.0 3.2 3.5 4.0 2.7 2.3 3.6
105.0 100.0 96.6 102.2 101.8 103.0 98.0 102.0 97.0 103.0 97.4 95.8
Average of three replicates of each sample: x3 ± t0.95 SD – confidence interval at a probability level of 95%; t0.95 – student’s coefficient. Synthetic mixture with average content of macro- and micro-components of soil in a sample with the mass of 0.1 g.
Table 2 Recovery of gallium in presence of various soil compounds and reagents applying butyl acetate solvent extraction Extractant
Foreign ions, reagents
Butyl acetate
Al(III) Ca(II) Mg(II) In(III) Cu(II) Zn(II) Mo(VI) Mn(II) Cr(III) Sc(III) Acetate NH2OH HCl Synthetic mixtureb
a b
60 mg 22 mg 13 mg 100 lg 100 lg 100 lg 100 lg 75 lg 50 lg 10 lg 57 mg 5.5 mg
Ga added (lg)
Ga founda (lg)
RSD (%)
Recovery (%)
10 10 10 10 10 10 10 10 10 10 10 10 10
10.30 ± 0.73 9.96 ± 0.30 10.00 ± 0.35 10.10 ± 0.85 9.86 ± 0.86 9.65 ± 0.98 9.70 ± 0.65 10.10 ± 0.86 10.40 ± 0.82 10.10 ± 0.73 10.00 ± 1.24 10.10 ± 0.39 10.20 ± 0.77
2.8 1.2 1.4 3.4 3.3 4.1 2.7 3.5 3.2 3.0 5.0 1.6 3.1
103.0 99.6 100.0 101.0 98.6 96.5 97.0 101.0 104.0 101.0 100.0 101.0 102.0
Average of three replicates of each sample: x3 ± t0.95 SD – confidence interval at a probability level of 95%; t0.95 – student’s coefficient. Synthetic mixture with average content of macro- and micro-components of soil in a sample with the mass of 0.1 g.
TTA and butyl acetate makes it possible to separate 5 lg of Sc(III) and 10 lg of Ga(III) from the elements present in 0.1 g of soil, as well as to determine scandium and gallium in a reliable way. It points to the possibility of using the extractants in the analysis of real samples. 3.4. Speciation of scandium and gallium in soil The content of gallium in individual fractions of soil from Upper Silesia was [in lg g 1]: fraction 1, 24.7; fraction 3, 29.2; fraction 4, 35.4; fraction 5, 6.9 and it was the highest in the organic fraction, indicating of input by anthropogenic activities (due to the complexation properties of natural organic matter and the accumulation of trace metals in living organisms (Tessier et al. (1979)). The organic fraction constitutes an important source of the potentially available gallium and the information to estimate the pollution of the region. While evaluating the anthropogenic activity, the properties of the soil in question must certainly be considered (including the mineral content in the soil, pH, organic matter type or rather the quantitative
share of its various forms), which also affect the metal content in this fraction. The content of scandium in Upper Silesia soil was following [in lg g 1]: fraction 1, 1.52; fraction 2, 0.53; fraction 3, 7.78; fraction 4, 1.79; fraction 5, 0.20. The highest concentration of scandium was found in fraction Fe–Mn oxides. In Podlasie soil, the content of gallium was clearly lower than in Upper Silesia soil and it was [in lg g 1]: fraction 1, 20.0; fraction 3, 10.3; fraction 4, 12.0. The concentration of scandium in all fractions and gallium (fraction 2 and 5) was less than detection limit of the applied method. The precision of the results was satisfactory. The relative standard deviation (RSD, n = 6) for scandium determination was in the range from 4.3% to 11.4% and it depended on scandium content in the fraction. In the case of gallium, the RSD was higher; between 7.8% and 15% (Table 3). In order to assess the reliability of the results obtained and the accuracy of the methods developed, the total content of Sc and Ga was determined in both soils examined, following the procedure given for fraction 5. The recovery of the Sc and Ga standards (introduced in the samples of the soils at the beginning of the analysis) was examined, following the same way as in the determination of the total content of the ele-
578
J. Połedniok / Chemosphere 73 (2008) 572–579
Table 3 Content of scandium and gallium in particular fractions of Upper Silesia and Podlasie soils determined by spectrophotometric and ICP-OES methods Region
Fraction
Upper Silesia
1 2 3 4 5 P
ICP-OES method x6 ± t0.95 SD, lg g
Ga
Ga
Sc
24.7 ± 2.33 – 29.2 ± 3.06 35.4 ± 4.08 6.9 ± 1.08 96.2 94.7 ± 3.58 20.0 ± 1.60 – 10.3 ± 0.97 12.0 ± 1.37 – 42.3 41.6 ± 3.19
Totala 1 2 3 4 5 P
Podlasie
Spectrophotometry CAS-ST method x6 ± t0.95 SD, lg g 1; RSD (%)
Totala
(9.0) (10.0) (11.0) (15.0) (3.6) (7.8) (9.0) (10.9)
(7.3)
1.52 ± 0.14 0.53 ± 0.06 7.78 ± 0.35 1.79 ± 0.10 0.20 ± 0.02 11.82 11.10 ± 0.46 – – – – – – 2.2 ± 0.18
(8.6) (11.4) (4.3) (5.5) (11.2) (4.0)
(7.8)
20.1 ± 0.95 – 29.7 ± 1.88 34.6 ± 2.62 6.7 ± 0.58 90.1 93.2 ± 7.14 19.6 ± 1.29 – 10.8 ± 0.96 11.6 ± 1.22 – 42.0 38.3 ± 3.75
1
; RSD (%) Sc
(4.5) (6.0) (7.2) (8.3) (7.3) (6.2) (8.5) (10.0)
(9.3)
1.63 ± 0.18 0.42 ± 0.06 7.04 ± 0.16 1.95 ± 0.16 0.23 ± 0.33 11.27 12.70 ± 0.11 – – – – – – 2.0 ± 0.20
(10.5) (12.7) (2.5) (7.6) (13.5) (0.8)
(9.2)
All numbers in this table are mean values. Standard deviation (SD) and relative standard deviation (RSD) based on six replicate analysis; x6 ± t0.95 SD – confidence interval at a P probability level of 95%; t0.95 – student’s coefficient; – summary content (1–5). a Total content found after digestion of soil with concentrated HF–H2SO4 mixture.
ments in the soil and in fraction 5. Simultaneously, the parallel analysis of the samples was carried out by another analytical method – the ICP-OES technique. The samples for the analysis were prepared in the same way as for the spectrophotometric methods, only the Fe(III) reduction, Fe(III) extraction and colour reaction steps were omitted. Measurements were carried out at the wavelength k = 294.364 nm in the case of gallium and k = 361.384 nm in the case of scandium. The content of the elements was found from the calibration graphs in the range: 0.12–2.0 lg mL 1 for gallium and 0.05–2.0 lg mL 1 for scandium. The results obtained are presented in Tables 3 and 4. The comparison of the total content to the sumP mary content of Sc and Ga in the particular fractions ( ) reaching the probability level of 95% confirms the reliability of the results. The recovery of the Sc and Ga standards (Table 4) is satisfactory, being at the level of 92–95%, which suggests good accuracy for both methods. The sample analysis with the use of the ICP-OES technique gave comparable results, which also indicates the good accuracy of the said methods and the possibility to use them for the analysis of soils in terms of functional speciation. 3.5. Comparison of spectrophotometric methods with the ICP-OES technique The limit of quantification (LOQ = 3 LOD; limit of detection) indicated for the determination of Sc with the use of the spectro-
Table 4 Determination of the total content of scandium and gallium in soil: checking the standard recovery Region
Element
Addeda (lg)
Foundb x3 ± t0.95 SD (lg)
Average standard recovery (%)
Upper Silesia
Ga
– 4.8 – 2.6
3.79 ± 0.17 8.28 ± 0.35 2.76 ± 0.14 5.16 ± 0.25
93.5
– 3.2 – 1.0
1.67 ± 0.17 4.67 ± 0.47 0.55 ± 0.05 1.50 ± 0.13
93.8
Sc Podlasie
Ga Sc
92.3
95.0
a Per analysed sample: 0.5 g of soil was digested with conc. HF–H2SO4 mixture and diluted to 25 mL (Ga) or 10 mL (Sc) with HCl; 2 or 5 mL of the solutions, respectively, were taken to the analysis. b Average of three replicates of each sample: x 3 ± t0.95 SD – confidence interval at a probability level of 95%; t0.95 – student’s coefficient.
photometric method and the ICP-OES technique is comparable and amounts to [lg mL 1]: 0.0195 and 0.0200, respectively. For gallium, however, the LOQ of the spectrophotometric method is significantly lower (0.0232) than the ICP-OES techniques (0.0500), which indicates its higher sensitivity and predisposes it for determination of lower concentrations of this element in soils. The linear working range is more extensive in the case of the ICPOES technique; however, when determining Sc and Ga in soils, due to their low concentration level, it is practically insignificant. The precision of both methods depends on the concentration of the elements. The precision of the ICP-OES is better (Table 3) for determining Sc occurring in higher concentrations (total content, fraction 3 of Upper Silesia soil). While determining very low Sc concentrations (fractions 1, 2, 4 and 5 of Upper Silesia soils) the RSD values calculated for the spectrophotometric method are significantly lower (Table 3), which indicates the better precision of this particular method. Due to the low contents of scandium in the soils, spectrophotometry seems to be a more suitable method of analysing this material than the ICP-OES technique. In the case of gallium, spectrophotometry is more precise for the determination of higher concentrations, while the ICP-OES technique is more suitable for the lower concentrations of this element (Table 3). Considering the economic aspect (the cost and availability of apparatus) the spectrophotometric methods presented can be recommended for the determination of the Sc and Ga content in soils for practically any laboratory with basic analytical equipment. 4. Conclusions The sequential extraction procedure developed by Tessier et al. (1979) for heavy metals was examined for the speciation of scandium and gallium in soil. The soil from industrial and agricultural regions of Poland was the object of this research. The results obtained indicate that gallium content in the individual fractions of both soils clearly depends on the region (except exchangeable and carbonate fractions) and the total concentration of gallium. The content of gallium was the highest in the organic fraction of Upper Silesia soil (industrial region). In Podlasie soil (agricultural region) the content of gallium was clearly lower. The absence of gallium in fraction 2 of Upper Silesia and Podlasie soils shows that gallium does not form compounds with carbonates. The content of gallium found in the exchangeable fraction was similar for both soils. It indicates the constant amount of Ga
J. Połedniok / Chemosphere 73 (2008) 572–579
accessible for plants, which does not depend on the total concentration of this element in the soil. The speciation of scandium was determined only in Upper Silesia soil. The concentration of Sc in individual fractions of Podlasie soil was less than the detection limit of the applied methods. The content of scandium in Upper Silesia soil was the highest in Fe– Mn oxides fraction. It was in good correlation with literature data concerning the occurrence of scandium in soils (Kabata-Pendias and Pendias, 1999). The content of scandium found in the organic fraction of the soil was considerably lower than in the case of gallium and near to the content of Sc in the exchangeable fraction. The low content of scandium and gallium in fraction 5 confirms the inability of these elements to form their own minerals. The sequential extraction scheme of Tessier et al. and spectrophotometric methods based on the mixed complexes Sc(III) and Ga(III) with Chrome Azurol S and benzyldodecyldimethylammonium bromide can be recommended for gallium speciation in soils from industrial and agricultural areas and for scandium speciation in soils with high content of this element. References Arunachalam, J., Emons, H., Krasnodebska, B., Mohl, C., 1996. Sequential extraction studies on homogenized forest soil samples. Sci. Total Environ. 181, 147–159. Bac, S., Gonel, Z., Krymuski, J., 1999. Foundations of Crop Production. Pan´stwowe Wydawnictwo Rolnicze i Les´ne, Warsaw. Bednarek, R., Prusinkiewicz, Z., 1997. Geography of Soils. PWN, Warsaw. Buhl, F., Połedniok, J., 1999. Content of scandium, gallium and vanadium in Upper Silesia soil. Arch. Environ. Prot. 25, 117–123. Buhl, F., Połedniok, J., 2000. The comparison of gallium, vanadium and scandium content in soil from industrial and agricultural regions. Roczniki gleboznawcze 51, 67–75. Chandrajith, R., Dissanayake, C.B., Tobschall, H.J., 2005. The abundances of rare trace elements in paddy (rice) soils of Sri Lanka. Chemosphere 58, 1415–1420. El-Ghawi, U.M., Bejey, M.M., Al-Fakhri, S.M., Al-Sadeq, A.A., Doubali, K.K., 2005. Analysis of Libyan arable soils by means of thermal and epithermal NAA. Arab. J. Sci. Eng. 30, 147–163. Ferrari, A.A., Franca, E.J., Fernandes, E.A.N., Bacchi, M.A., 2006. Surface contamination effects on leaf chemical composition in the Atlantic Forest. J. Radioanal. Nucl. Chem. 270, 69–73. Hirose, K., 2006. Chemical speciation of trace metals in seawater: a review. Anal. Sci. 22, 1055–1063. Jarosz, M., Mirzyn´ska, B., 2006. Some Problems of Environmental Speciation Analysis of Heavy Metals. Publishing House of Warsaw Technical University, Warsaw. Kabata-Pendias, A., Pendias, H., 1992. Trace Elements in Soil and Plants. CRC Press, Boca Raton. Kabata-Pendias, A., Pendias, H., 1999. Biogeochemistry of Trace Elements. PWN, Warsaw. Kwapulin´ska, G., 1995. A sensitive spectrophotometric method of determination of gallium(III) using Chrome Azurol S and benzyldimethyllaurylammonium bromide. Chem. Anal. Warsaw 40, 783–789.
579
Kwapulin´ska, G., Buhl, F., Połedniok, J., 1993. Spectrophotomertic studies on the system Sc – Chrome Azurol S – benzyldimethyllaurylammonium bromide (ST) and its application in chemical analysis. Chem. Anal. Warsaw 38, 201–210. Langodegard, M., Wibetoe, G., 2002. Determination of gallium in soil by slurry sampling GFAAS. Anal. Bioanal. Chem. 373, 820–826. Liang, S., Liu, Y., Hu, H., 2004. Determination of C, N and other 36 elements in soil samples by XRF. Yankuang Ceshi 23, 102–108. Lopez-Garcia, I., Campillo, N., Arnau-Jerez, I., Hernadez-Cordoba, M., 2004. ETAAS determination of gallium in soils using slurry sampling. J. Anal. Atom. Spectrom. 19, 935–937. Marin, B., Valladon, M., Polve, M., Monaco, A., 1997. Reproducibility testing of a sequential extraction scheme for the determination of trace metal speciation in a marine reference sediment by inductively coupled plasma-mass spectrometry. Anal. Chim. Acta 342, 91–112. Minowa, H., Ebihara, M., 2003. Separation of rare earth elements from scandium by extraction chromatography. Application to radiochemical neutron activation analysis for trace rare earth elements in geological samples. Anal. Chim. Acta 489, 25–37. Plessow, A., Heinrichs, H., 2000. Speciation of trace elements in acidic pore waters from waste rock dumps by ultrafiltration and ion exchange combined with ICPMS and ICP-OES. Aquat. Geochem. 6, 347–366. Połedniok, J., Buhl, F., 2003. Speciation of vanadium in soil. Talanta 59, 1–8. Polish Standard PN-R-04031, 1997. Chemical and Agricultural Analysis of the Soil. Sampling. Riberio, A.P., Fiqueiredo, A.M.G., Sigolo, J.B., 2005. Determination of heavy metals and other trace elements in lake sediments from a sewage treatment plant by neutron activation analysis. J. Radioanal. Nucl. Chem. 263, 645–651. Saini, N.K., Mukherjee, P.K., Rathi, M.S., Khanna, P.P., Purohit, K.K.J., 2002. Trace element estimation in soils: an appraisal of ED-XRF technique using group analysis scheme. J. Trace Microprobe Tech. 20, 539–551. Schulze, D., Krüger, A., Kupsch, H., Segebade, C., Gawlik, D., 1997. Enrichment and distribution of elements in flood-plain soils of the Bitterfeld industrial area studied by neutron activation analysis. Sci. Total Environ. 206, 227–248. Shtangeeva, I., 2005. Trace and Ultratrace Elements in Plants and Soil. WIT Press, Southampton. Shtangeeva, I., Ayrault, S., Jain, J., 2004. Scandium bioaccumulation and its effect on uptake of macro- and trace elements during initial phases of plant growth. Soil Sci. Plant Nutr. 50, 877–883. Sobczyn´ski, T., Siepak, J., 2001. Speciation of heavy metals in bottom sediments of lakes in the area of Wielkopolski National Park. Pol. J. Environ. Stud. 10, 463– 474. Solecki, J., Chibowski, S., 2000. Examination of trace amounts of some heavy metals in bottom sediments of selected lakes of South-Eastern Poland. Pol. J. Environ. Stud. 9, 203–308. Sun, Z., Zhang, Z., Mao, Y., Chen, Z., 2004. Determination of trace amount of silver, tin, lead, boron and gallium in geochemical exploration samples by electric arc distillation- emission spectrometry. Yankuang Ceshi 23, 153–156. Tessier, A., Campbell, P.G.C., Bisson, M., 1979. Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem. 51, 844–851. Tyler, G., 2004. Vertical distribution of major, minor and rare elements in a Haplic Podzol. Geoderma 119, 277–290. Vogt, C., 1994. Speciation of the inorganic components in brown coal. Fresen. J. Anal. Chem. 350, 89–92. Wa˛chalewski, T., 1997. Items of Environmental Chemistry. AGH Publishers, Cracow. Yu, B., Yan, Z., Yang, L., Wang, R., Li, X., 2006. Determination of 36 major, minor and trace elements in soil and stream sediment samples by X-ray florescence spectrometry. Yankuang Ceshi 25, 74–78. Zambello, F.R., Enzweiler, J., 2002. Multi-element analysis of soils and sediments by wavelength-dispersive X-ray fluorescence spectrometry. J. Soils Sediments 2, 29–36.
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Speciation of scandium and gallium in soil Justyna Połedniok * Department of Analytical Chemistry, University of Silesia, 9 Szkolna Street, 40-006 Katowice, Poland
a r t i c l e
i n f o
Article history: Received 27 February 2008 Received in revised form 19 May 2008 Accepted 5 June 2008 Available online 23 July 2008 Keywords: Gallium Scandium Soil Speciation Sequential extraction Spectrophotometry
a b s t r a c t A method for the speciation of scandium and gallium in soil has been developed. The sequential extraction scheme of Tessier et al. for heavy metals was examined for the scandium and gallium separation. The regents proposed by Tessier were used for the extraction, and only for the residual fraction the HClO4 was replaced with H2SO4. The optimum conditions for leaching scandium and gallium from the soil were chosen for each fraction. Very sensitive, spectrophotometric methods based on the mixed complexes of Sc(III) and Ga(III) with Chrome Azurol S and benzyldodecyldimethylammonium bromide were applied for the scandium and gallium determination after their separation by solvent extraction. 100% mesityl oxide and a 0.5 M solution of 2-thenoyltrifluoroacetone in xylene were chosen for the extraction of scandium and butyl acetate was selected for gallium. Soil samples from two different regions of Poland were the object of this research. The content of scandium and gallium found in the individual fractions of Upper Silesia soil (industrial region) was [in lg g 1] Sc: I, 1.52; II, 0.53; III, 7.78; IV, 1.79; V, 0.20; Ga: I, 24.7; III, 29.2; IV, 35.4; V, 6.9. In Podlasie soil (agricultural region), the content of both elements was clearly lower. The total content of scandium and gallium in the five soil fractions was in good correlation with the total content of these elements in the soils found after HF–H2SO4 digestion. Analysis using the ICP-OES method gave comparable results. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Among the many chemical constituents, trace elements play an important role in the biological environment, showing a positive or toxic impact on the natural environment depending on their concentration. As a result of human economic and technological activity, particularly in industrialised areas, the natural circulation of trace elements in the environment has been subject to interference. Each change in the chemical balance causes not only disturbances in the plant or animal world, but may also have an adverse influence on human health. One of the environmental components from which trace elements transfer to living organisms is soil: the principal place of accumulation of many chemical substances, polluted by atmospheric dusts, solid waste dumps, plant protection agents, mineral fertilisers and others (Kabata-Pendias and Pendias, 1992, 1999). Gallium is a very widespread trace element. Its content in the lithosphere amounts to 10 g/t. Gallium is a cryptomorphic element whose average clarkes (geochemical density indicators) in the lithosphere are relatively high, but due to the practical inability to form its own minerals in natural conditions, it is considered quite rare. The gallium content in soil varies between 5 and 300 ppm (usually 5–70 ppm). The lowest gallium concentration can be * Tel.: +48 32 3591642; fax: +48 32 2599978. E-mail address: [email protected] 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.06.012
observed in light sandy and limestone soils. The presence of gallium in the soil is connected with the occurrence of silty minerals. Moreover, it is sorbed by Fe(III) and Mn(III) oxides and by an organic substance. Despite quite a large clarke (11 ppm), scandium is also considered a rare element, hard to concentrate or separate, which is connected with a lack of specific reagents reacting with this element. Similar to gallium, scandium is accumulated in soil in silty minerals and accompanies iron and magnesium minerals. The average scandium content in soil amounts to 2–12 ppm. The smallest content of scandium can be observed in podzols whereas in brown soil and black-earth it is the greatest. The soil around industrial plants exploiting the natural energy resources contains an increased amount of the element (Kabata-Pendias and Pendias, 1999). Scandium and gallium are the components of almost all plants. The average scandium content is between 0.008 and several tenths of a ppm, whereas the gallium content varies between 0.05 and 5 ppm. The biological role of Sc and Ga in the development and functioning of plants has not been specifically defined so far. No information on their toxicity has been reported, either. However, there are hardly any publications concerning this question. The topic still remains open, up-to-date and raised by researchers in various parts of the globe. The latest studies have proven that some plants (lichen, the phylum Bryophyta, and also papilionaceous plants) show the ability to accumulate gallium, even at the 60 ppm concentration level, and the larger values of Ga/Al ratio
J. Połedniok / Chemosphere 73 (2008) 572–579
in land plants confirm the selective absorption of gallium. Moreover, it has been proven that Sc absorbed by plants is accumulated in the roots, seeds and old leaves, while the accumulation is scarce in the young leaf explants (Shtangeeva, 2005). The scandium content in the leaves of various plants and in the soil of the Brazilian forests was compared, and a distinct correlation was observed between the amount of contaminants present and the content of scandium in the soil. It was found that the scandium content growth in the soil could affect its concentration in the roots of plants and the absorbability of other significant nutrients (Ferrari et al., 2006). The effect of scandium accumulation in the wheat seeds on the absorbability of the macro- and micro-components from the soils was examined, and it was found that the bioaccumulation of scandium had significantly affected the changes in the concentration of the elements that are important for the plants (Na, K, Zn) (Shtangeeva et al., 2004). In view of the above, considering the growing pollution of soils with trace elements, including Sc and Ga, the determination of the functional speciation of the elements in the soil seems to be advisable and helpful for further studies on the correlations between the soil and the plant, particularly in areas subjected to various anthropogenic influences. In the last 15 years there have been few publications concerning scandium and gallium determination in soils. Certified materials and real samples were analysed and the total content or concentration of these elements in soil profiles was determined. Scandium was determined using the following analytical methods: NAA (Schulze et al., 1997; Minowa and Ebihara, 2003; Riberio et al., 2005), ICP-MS (Tyler, 2004), ICP-OES (Buhl and Połedniok, 1999, 2000; Tyler, 2004) and spectrophotometry (Buhl and Połedniok, 1999, 2000). As far as gallium is concerned, the most frequently used techniques were: AAS (Langodegard and Wibetoe, 2002; Lopez-Garcia et al., 2004) and XRF (Saini et al., 2002; Zambello and Enzweiler, 2002; Liang et al., 2004; Yu et al., 2006), but also AES (Sun et al., 2004), NAA (El-Ghawi et al., 2005), ICP-MS (Chandrajith et al., 2005) and spectrophotometry (Buhl and Połedniok, 1999, 2000). Currently, speciation is the approach more and more frequently mentioned in scientific publications. An increased interest in speciation results from the fact that the total content of a given element does not provide satisfactory information to describe its environmental behaviour (e.g. toxicity, bioavailability, migration, and accumulation). Most publications on speciation are devoted to heavy metals due to their impact on human health (Solecki and Chibowski, 2000; Sobczyn´ski and Siepak, 2001; Jarosz and Mirzyn´ska, 2006). Research so far has been carried out for Cr, Cd, Co, Cu, Fe, Pb, Ni, As, U, Hg, Sn, Sb, Zn, Mn and V. Various instrumental methods have been used, e.g. ICP-OES, ICP-MS, AAS, FIA, HPLC, XRF and spectrophotometry. The research covered: brown coal (Vogt, 1994), marine sediment (Marin et al., 1997), soil (Arunachalam et al., 1996; Połedniok and Buhl, 2003) and water (Plessow and Heinrichs, 2000; Hirose, 2006). Gallium speciation has not been analysed so far but an article on scandium speciation was published (Marin et al., 1997), covering the determination of scandium in a marine sediment (Standard Reference Material SRM 1646a) using a sequential extraction scheme recommended by BCR and the ICP-MS technique. The purpose of the study was to determine the speciation of scandium and gallium in real soil samples from two different regions of Poland: Upper Silesia (an industrial region) and Podlasie (an agricultural region) based on the sequential extraction scheme developed by Tessier et al. (1979) for heavy metals (Cd, Co, Cu, Ni, Pb, Zn, Fe and Mn). For the determination of scandium and gallium, very sensitive, spectrophotometric methods developed in the Department of Analytical Chemistry of the University of Silesia were used. The methods are based on the mixed complexes Sc(III) and Ga(III) with Chrome Azurol S (CAS) and benzyldodecyldimethylammonium bromide (ST) (eSc = 1.37 105 and eGa =
573
1.10 105 L mol 1 cm 1) (Kwapulin´ska et al., 1993; Kwapulin´ska, 1995). The scandium determination was preceded by Sc(III) extraction with 100% mesityl oxide (TM) from an ammonium thiocyanate solution (Kwapulin´ska et al., 1993) or a 0.5 M solution of 2-thenoyltrifluoroacetone (TTA) in xylene (Buhl and Połedniok, 1999). The gallium determination was preceded by Ga(III) extraction with butyl acetate (Buhl and Połedniok, 1999).
2. Experimental 2.1. Apparatus A DR/2010 (Hach, USA) VIS spectrophotometer was used for the research. The spectrometric measurements were carried out in a Spectroflame-ICP type M (Spectro Analytical Instruments, Germany) spectrometer. The operating conditions were as follows: power 1.1 kW, frequency 27.12 MHz, nebuliser gas Ar 1.0 L min 1, coolant gas 14.0 L min 1, auxiliary gas 0.5 L min 1, amount of dosed sample 1 mL min 1, viewing height over induction coil 11 mm, dispersion of grating in 1st reciprocal order 0.55 nm mm 1. pH-meter type N-517 (Mera Elwro, Poland). Universal Shaker type 327 (Premed, Poland). 2.2. Reagents and standard solutions All reagents used in the study were analytically pure reagents. Doubly distilled water obtained from an ELIX3 (Millipore, Molsheim, France) was used during the experiments. Scandium(III) standard solution: 1 mg mL 1 (Promochem, Poland). The working solutions were prepared by the appropriate dilution of the scandium(III) standard solution with the 1 10 2 M HCl. Gallium(III) standard solution: 1 mg mL 1 (Wzormat, Warsaw, Poland). The working solutions were prepared by the dilution of the gallium (III) standard solution with 1 10 3 M HCl. Chrome Azurol S (CAS) was purchased from Fluka (Buchs, Switzerland): 1.43 10 3 M and 5 10 4 M aqueous solutions were used for the research. Benzyldodecyldimethylammonium bromide: 10% commercial product Sterinol (ST) was from Polfa (Cracow, Poland): 1.43 10 2 M and 5 10 2 M aqueous solutions were used in the studies. TM: 100% mesityl oxide (Fluka, Buchs, Switzerland). TTA: a 0.5 M solution in xylene was made from 2-thenoyltrifluoroacetone (Fluka, Buchs, Switzerland). Xylene (POCh Gliwice, Poland). Butyl acetate (POCh Gliwice, Poland). AA: a 50% solution in chloroform was made from acetylacetone (Ubichem, Hampshire, England). Chloroform (POCh Gliwice, Poland). Ascorbic acid (Polfa, Cracow, Poland): aqueous solutions were prepared directly before using. Magnesium chloride (MgCl2 6H2O), sodium acetate and ammonium thiocyanate were from (POCh Gliwice, Poland). Hydroxylamine hydrochloride and ammonium acetate (POCh Gliwice, Poland): a 0.04 M solution in 25% CH3COOH and a 3.2 M solution in 20% HNO3, respectively, were used for the research. Hydrogen peroxide: 30% (POCh Gliwice, Poland). Hydrochloric acid (POCh Gliwice, Poland): 35– 38%. Hydrofluoric acid (POCh Gliwice, Poland): 40%. Nitric acid (Fluka, Buchs, Switzerland): 65%. Sulfuric acid (Hetalab, USA): 96%. Acetic acid (POCh Gliwice, Poland): 99.5%. 2.3. Sampling and sample preparation Soil samples were collected from the arable layer (0–20 cm) of two Polish fields located in Upper Silesia (Da˛brówka Wielka) and Podlasie (Biała Podlaska) and prepared according to the procedure described in Polish Standard PN-R-04031 (1997). Next, the samples were dried at room temperature for 7 days and after division by
574
J. Połedniok / Chemosphere 73 (2008) 572–579
the coning and quartering method the samples were ground in an agate mortar. The grain size was less than 200 lm. Soil from Upper Silesia is brown soil developed in weakly structured clayey sands and loams. Soil from Podlasie is white soil. Brown soils in most cases originate from rocks abundant with plant nutrition components and calcium. They are characterised by a well-formed accumulation level, gray or brown coloured, gradually turning into an intensive weathering, gray-brown coloured strata (Bac et al., 1999). The reaction of the soils is slightly acid or neutral, i.e. the best for crop growth. The intensive biological circulation of the mineral components prevents their washing into the depth of the soil and facilitates their accumulation on the upper levels. The fast mineralisation of the forest rainfall and favourable direction of its humification prevent the accumulation of acid overlay humus and production of more fulvic acids that would enable the movement of soil mineral weathering product into the depth of the soil (Bednarek and Prusinkiewicz, 1997). White soils are formed from vernacular formations with low nutrient content. The soils have an acid or slightly acid reaction, high hydrolytic and exchangeable acidity and a very low basic cation saturation ratio, which makes some of the nutrients unavailable to the plants. The moving humus acids, washed from the overlay humus, with their complexation abilities, play the most important role in the genesis of white soils. The humus acids (mainly the fulvic acid fraction) create easily soluble complex connections with iron and aluminium ions in the upper part of the profile (Bednarek and Prusinkiewicz, 1997).
2.4. Leaching procedure Fraction 1: The soil sample (scandium: 2 g; gallium: 1 g for Upper Silesia soil and 2 g for Podlasie soil) was extracted at room temperature with, respectively, Sc: 10 mL and Ga: 8 mL of 1 M MgCl2 (pH 7) with continuous agitation. Next, the mixture was filtered using a filter with medium wide pores (Spezialpapierfabrik Niederschlag, Germany; yellow label filter no. 389) and the filtrate was diluted to 50 mL with water. Fraction 2: The solid residue from fraction 1 was leached at room temperature with continuous agitation with 1 M sodium acetate adjusted to pH 5 with acetic acid. The 8 mL of CH3COONa was used in the case of gallium and 10 mL was applied in the case of scandium. After that, the mixture was filtered according to the procedure described above. Fraction 3: The solid residue from fraction 2 was extracted with 20 mL of 0.04 M NH2OH HCl in 25% acetic acid at 96 ± 3 °C with occasional agitation. Next, the mixture was filtered and the filtrate was evaporated to dryness. The dry residue was dissolved and diluted to 50 mL with 0.1 M HCl. Fraction 4: The following solutions were added to the solid residue from fraction 3: 3 mL of 0.02 M HNO3 and 5 mL of 30% H2O2 adjusted to pH 2 with HNO3, and the mixture was heated to 85 ± 2 °C with occasional agitation (time 1). Next, 3 mL of 30% H2O2 (pH 2 with HNO3) was added and the sample was heated again to 85 ± 2 °C with occasional agitation (time 2). After cooling, 2 mL of 3.2 M ammonium acetate in 20% HNO3 was added, and the sample was diluted to 20 mL with water and agitated continuously for 30 min. The mixture was filtered and the filtrate was evaporated to dryness. The dry residue was dissolved and diluted to 50 mL with 0.1 M HCl. Fraction 5: The residue from fraction 4 was digested in a platinum crucible with conc. HF–H2SO4 mixture. For 1 g of the sample, 1 mL of H2SO4 and 10 mL of HF were used for the first time and 7 mL of HF was used for the second time. The obtained solution was evaporated to dryness and the residue was dissolved and diluted to 25 mL with 0.5 M HCl.
2.5. Scandium separation and determination Fractions 1, 2, 4 or 5. The following solutions were placed in 25 mL beakers: 5 mL of the solution obtained after the leaching of fractions: 1, 2 or 4; 2 mL of the solution obtained after the digestion of fraction 5 and 5 mL of ammonium thiocyanate solution containing 1.425 g NH4SCN. Next, the pH was adjusted to 1 with HCl using a pH-meter and water was added to make the volume of the solutions about 12.5 mL. The samples were then transferred into 50 mL separatory funnels and scandium was extracted with 5 mL of 100% mesityl oxide for 2 min. The water phase was rejected and Sc was reextracted twice from the organic phase with 10 mL and 5 mL of 0.2 M HCl for 2 min. The reextracts (15 mL) were place in a 100 mL beaker and evaporated to dryness. The residue was dissolved in water and transferred quantitatively into a measuring flask. Then 1 mL of a pH 5 acetate buffer solution, 4 mL of 5 10 4 M CAS and 0.66 mL of 5 10 2 M ST were added subsequently to the flask and the mixture was diluted to 25 mL with water. The absorbance was measured in 1 cm long glass cells at k = 610 nm against a reagent blank. Fraction 3: The following were placed in a 50 mL separatory funnel: 3 mL of the solution obtained after the leaching of fraction 3, 0.5 mL of 0.5 M HCl and 7 mL of water. Scandium was extracted twice with 5 mL of a TTA solution in xylene for 5 min. The water phase was rejected and the both organic phases were washed with 5 mL of 0.03 M HCl (10 s). After that, the scandium was reextracted with 10 mL of 2 M HCl for 5 min. The reextract was washed with 5 mL of xylene (10 s), transferred into a 100 mL beaker and evaporated to dryness. Next, 1 mL of conc. HNO3 was added to the beaker and the sample was evaporated to dryness again. Finally, the residue was dissolved in water and transferred into a 25 mL measuring flask. Then the colour reaction with CAS and ST was developed and the measurements were carried out according to the procedure described above. The scandium concentration was found from the calibration graphs (obtained after extraction) in the range: 0.04–0.50 lg mL 1 (a = 0.0898, b = 0.0298, r = 0.9996) for 100% TM and 0.04– 0.40 lg mL 1 (a = 0.117, b = 0.044, r = 0.9998) for a TTA solution in xylene. 2.6. Gallium separation and determination Fractions 1, 2, 3, 4 or 5. The following solutions were placed in 25 mL beakers: 3 mL of the solution obtained after the leaching of fractions 1 or 4; 5 mL of the solution obtained after the leaching of fractions 2 or 3; 5 mL of the solution obtained after the digestion of fraction 5. Next, the pH was adjusted to about 1.5–2.0 with HCl using a pH-meter. The samples were then transferred into 100 mL separatory funnels and ascorbic acid was added: 2 mL of a 1% ascorbic acid solution was used for the reduction of Fe(III) contained in fractions 1 and 2, 2 mL of a 5% solution was used for fractions 3 and 4, whereas 5 mL of a 25% solution was used for fraction 5. After 10 min., the solutions were diluted to 10 mL with water. Next, 10 mL of conc. HCl and 20 mL of butyl acetate were added and gallium was extracted for 1 min. Then the water phase was rejected and the organic phase was washed twice with 2.5 mL of a 6 M HCl solution. After that, gallium was reextracted twice for 1 min. with 10 mL of water. The reextracts (20 mL) were evaporated in 100 mL beakers until the solutions reached a pH of 3.7–4.0. Then the solutions were quantitatively carried to 25 mL measuring flasks and 1 mL of a pH 4.5 acetate buffer solution, 2 mL of 1.43 10 3 M CAS and 2 mL of 1.43 10 2 M ST were added. Finally, the mixtures were diluted with water up to the mark. The absorbance was measured in 1 cm long glass cells at k = 620 nm against a reagent blank. The content of gallium was
575
J. Połedniok / Chemosphere 73 (2008) 572–579
found from the calibration graph obtained after extraction in the range: 0.12–0.40 lg mL 1. The linear regression coefficients were: a = 0.049, b = 0.011, r = 0.9995. 3. Results and discussion 3.1. Optimisation of scandium and gallium leaching Weight of samples. Preliminary studies showed that the total content of gallium and scandium obtained after digestion of the samples with the HF–H2SO4 mixture was, respectively [in lg g 1] – Upper Silesia soil: 94.7 and 11.1; Podlasie soil: 41.6 and 2.2. Therefore, the weight of soil samples chosen for the research was: scandium: 2 g; gallium: 1 g for Upper Silesia soil and 2 g for Podlasie soil. Effect of extractants: The reagents proposed by Tessier et al. (1979) were applied for the partitioning of scandium and gallium into the five soil fractions. The NH2OH HCl solution was chosen for the leaching of fraction 3. Other extractants used by Tessier et al. for this fraction interfered in the CAS-ST determination. In order to improve the safety of work, conc. HClO4 was replaced with conc. H2SO4 and the residual fraction was digested using the HF– H2SO4 mixture. The maximum concentration of gallium in all soil extracts and scandium in fractions 3–5 were obtained following the Tessier method. In the case of fractions 1 and 2, the optimum volume of reagents for scandium extraction was, respectively: 10 mL of 1 M MgCl2 and 10 mL of 1 M CH3COONa. Effect of temperature: The temperatures of extraction suggested by Tessier et al. (1979) were optimum for scandium and gallium leaching from the soil: room temperature for fractions 1 and 2, 96 ± 3 °C for fraction 3 and 85 ± 2 °C for fraction 4. Effect of time: Figs. 1–3 show the influence of time on the efficiency of scandium and gallium extraction from the soil. The concentration of Sc(III) and Ga(III) in the MgCl2 (pH 7) leaching solution (fraction 1) reached the maximum value after 0.5 h of extraction and it was stable for the next 1 h (Fig. 1). The concentration of Sc(III) in the CH3COONa (pH 5) leaching solution used for the separation of fraction 2 reached the maximum value after 1 h and then slowly reduced with time, whereas the concentration of Ga(III) – for both examined soils – was less than the detection limit of the applied method even after 5 h of extraction. It means that
gallium does not form compounds with carbonates in soil. Fig. 2 indicates that content of scandium and gallium in NH2OH HCl solution applied for the isolation of fraction 3 was at the maximum level after 0.5 h of extraction and it was stable for 1 h and 1.5 h, respectively. Next, the maximum value was clearly decreasing with time. Probably the elements from the leaching solution were adsorbed on the surface of the soil. Fig. 3 shows that the optimum time for scandium and gallium leaching from the fraction 4 was the combination: 1.5 h + 1.5 h (Sc) and 2 h + 2 h (Ga). Gallium concentration in the HNO3–H2O2 mixture was comparable for all times that were examined, whereas scandium concentration after the optimum time was clearly lower. Fraction 5 (residual) was digested with conc. HF and conc. H2SO4 until the white fumes of SO3 appeared. Next, the second part of HF was added, then the mixture was evaporated to dryness and the solid residue was dissolved in the HCl solution. The suitable time of dissolving was 40 min. The optimum time of soil leaching chosen for further research was the following, scandium: 1 h for fractions: 1, 2, 3 and combination (1.5 + 1.5) h for fraction 4; gallium: 1 h for fractions: 1, 3 and a combination (2 + 2) h for fraction 4. The optimum time of dissolving of fraction 5 for both elements was 40 min. 3.2. Selectivity of separation The methods of scandium and gallium separation by the solvent extraction were elaborated using model solutions containing macro- and micro-components of soil and reagents proposed by Tessier for the leaching. The criterion for interference was the content of scandium and gallium varying by more than 5% from the expected value for these elements alone. Scandium. It was shown that 100% mesityl oxide separated 5 lg Sc(III) from 100 mg Ca(II), 30 mg Mg(II), 5 mg Mn(II) and Cr(III), 2 mg Al(III), 0.5 mg Zn(II) and In(III), 150 lg Fe(III), 50 lg Ga(III), Cu(II) and Mo(VI), and 500 mg acetate (Table 1). NH2OH HCl interfered in the CAS-ST scandium determination after extraction with mesityl oxide: absorbance of the complex was clearly lower. The solution of TTA in xylene separated 5 lg Sc(III) from 100 mg Mg(II), 75 mg Ca(II), 5 mg Cr(III) and Zn(II), 0.5 mg Mo(VI), 100 lg Al(III), 50 lg V(V), 50 lg Ga(III), 10 lg In(III), 5 lg Fe(III) and 5.5 mg
Concentration of scandium and gallium [ μg L-1]
1000 900
Gallium in Upper Silesia soil Scandium in Upper Silesia soil
800
Gallium in Podlasie soil
700 600 500 400 300 200 100 0 0.5
1
1.5
2
3
5
Time [h] Fig. 1. Effect of leaching time on scandium and gallium concentration in the MgCl2 extracting solution (pH 7); error bars (±SD).
576
J. Połedniok / Chemosphere 73 (2008) 572–579
Concentration of scandium and gallium [μg L-1]
800 Gallium in Upper Silesia soil
700
Scandium in Upper Silesia soil Gallium in Podlasie soil
600
500
400
300
200
100
0 0.5
1
1.5
2
3
5
Time [h] Fig. 2. Effect of leaching time on scandium and gallium concentration in the NH2OH HCl extracting solution (pH 2); error bars (±SD).
Concentration of scandium and gallium [µg L-1]
1400
Scandium in Upper Silesia soil
1200
Gallium in Upper Silesia soil Gallium in Podlasie soil
1000
800
600
400
200
0 (1+1)
(1.5+1.5)
(2+2)
(2+3)
Time [h] Fig. 3. Effect of leaching time on scandium and gallium concentration in the HNO3 + H2O2 extracting solution (pH 2).
NH2OH HCl (Table 1). Therefore, 100% mesityl oxide was chosen for the scandium separation from fractions: 1, 2, 4 and 5, and the solution of TTA in xylene was applied for scandium separation from fraction 3 instead. Before scandium extraction using TTA and mesityl oxide (from fractions: 3, 4 and 5) Fe(III) was previously separated by solvent extraction with 20 mL of a 50% solution of acetylacetone in CHCl3 at pH 1. The optimum time of extraction was 5 min. The number of extractions was chosen experimentally for each fraction and it was: 5 for fractions 3 and 5; 3 for fraction 4. Ions of Al(III) were separated simultaneously with Fe(III). Gallium. It was pointed out that butyl acetate separated 10 lg Ga(III) from 60 mg Al(III), 22 mg Ca(II), 13 mg Mg(II), 100 lg In(III), Cu(II), Zn(II) and Mo(VI), 75 lg Mn(II), 50 lg Cr(III), 10 lg Sc(III), 57 mg acetate and 5.5 mg NH2OH HCl (Table 2). The coextraction of Ga(III) and Fe(III) was observed: the absorbance was clearly above normal. Therefore, Fe(III) was reduced to Fe(II) with ascorbic acid (t = 10 min) before the gallium extraction. Fe(II) is separated
from Ga(III) using butyl acetate. The concentration and volume of ascorbic acid was chosen experimentally for each fraction and it is presented in Section 2.6. 3.3. Determination of scandium and gallium in synthetic mixture Scandium and gallium were found in a synthetic mixture using the CAS-ST method after extraction with mesityl oxide, TTA and butyl acetate, respectively. The synthetic mixture shows the average composition of the soil (in a sample with the mass of m = 0.1 g), i.e. the content of macro- and micro-elements: 1 mg Ca(II), 0.5 mg Mg(II), 2 mg Al(III), 2 mg Fe(III), 10 lg Cu(II), 60 lg Mn(II), 5.5 lg Cr(III), 0.25 lg Mo(VI), 15 lg Zn(II), 10 lg Pb, 10 lg V(V) and 1 lg In(III) (Wa˛chalewski, 1997). Before the analysis, 5 lg of Sc(III) and 10 lg of Ga(III) were added to each sample. The results obtained are shown in Tables 1 and 2. The extraction with mesityl oxide,
577
J. Połedniok / Chemosphere 73 (2008) 572–579 Table 1 Recovery of scandium in presence of various soil compounds and reagents applying TM and TTA solvent extraction Extractant
Foreign ions, reagents
TM
Ca(II) Mg(II) Mn(II) Cr(III) Al(III) Zn(II) In(III) Fe(III) Cu(II) Mo(VI) Ga(III) Acetate Synthetic mixtureb
TTA
Mg(II) Ca(II) Zn(II) Cr(III) Mo(VI) Al(III) V(V) Ga(III) In(III) Fe(III) NH2OH HCl Synthetic mixtureb
a b
Sc added (lg)
Sc founda (lg)
RSD (%)
Recovery (%)
100 mg 30 mg 5 mg 5 mg 2 mg 0.5 mg 0.5 mg 150 lg 50 lg 50 lg 50 lg 500 mg
5 5 5 5 5 5 5 5 5 5 5 5 5
4.78 ± 0.24 4.79 ± 0.44 5.00 ± 0.59 5.14 ± 0.42 4.83 ± 0.36 5.18 ± 0.21 5.20 ± 0.65 4.98 ± 0.31 5.00 ± 0.43 5.10 ± 0.38 5.14 ± 0.67 4.80 ± 0.32 5.08 ± 0.43
2.0 3.7 4.8 3.3 3.0 1.6 5.1 2.5 3.5 3.1 5.3 2.7 3.4
95.6 95.8 100.0 102.8 96.6 103.6 104.0 99.6 100.0 102.0 102.8 96.0 101.6
100 mg 75 mg 5 mg 5 mg 0.5 mg 100 lg 50 lg 50 lg 10 lg 5 lg 5.5 mg
5 5 5 5 5 5 5 5 5 5 5 5
5.25 ± 0.51 5.00 ± 0.47 4.83 ± 0.32 5.11 ± 0.28 5.09 ± 0.24 5.15 ± 0.51 4.90 ± 0.39 5.10 ± 0.44 4.85 ± 0.48 5.15 ± 0.34 4.87 ± 0.26 4.79 ± 0.43
3.9 3.8 2.7 2.2 1.9 4.0 3.2 3.5 4.0 2.7 2.3 3.6
105.0 100.0 96.6 102.2 101.8 103.0 98.0 102.0 97.0 103.0 97.4 95.8
Average of three replicates of each sample: x3 ± t0.95 SD – confidence interval at a probability level of 95%; t0.95 – student’s coefficient. Synthetic mixture with average content of macro- and micro-components of soil in a sample with the mass of 0.1 g.
Table 2 Recovery of gallium in presence of various soil compounds and reagents applying butyl acetate solvent extraction Extractant
Foreign ions, reagents
Butyl acetate
Al(III) Ca(II) Mg(II) In(III) Cu(II) Zn(II) Mo(VI) Mn(II) Cr(III) Sc(III) Acetate NH2OH HCl Synthetic mixtureb
a b
60 mg 22 mg 13 mg 100 lg 100 lg 100 lg 100 lg 75 lg 50 lg 10 lg 57 mg 5.5 mg
Ga added (lg)
Ga founda (lg)
RSD (%)
Recovery (%)
10 10 10 10 10 10 10 10 10 10 10 10 10
10.30 ± 0.73 9.96 ± 0.30 10.00 ± 0.35 10.10 ± 0.85 9.86 ± 0.86 9.65 ± 0.98 9.70 ± 0.65 10.10 ± 0.86 10.40 ± 0.82 10.10 ± 0.73 10.00 ± 1.24 10.10 ± 0.39 10.20 ± 0.77
2.8 1.2 1.4 3.4 3.3 4.1 2.7 3.5 3.2 3.0 5.0 1.6 3.1
103.0 99.6 100.0 101.0 98.6 96.5 97.0 101.0 104.0 101.0 100.0 101.0 102.0
Average of three replicates of each sample: x3 ± t0.95 SD – confidence interval at a probability level of 95%; t0.95 – student’s coefficient. Synthetic mixture with average content of macro- and micro-components of soil in a sample with the mass of 0.1 g.
TTA and butyl acetate makes it possible to separate 5 lg of Sc(III) and 10 lg of Ga(III) from the elements present in 0.1 g of soil, as well as to determine scandium and gallium in a reliable way. It points to the possibility of using the extractants in the analysis of real samples. 3.4. Speciation of scandium and gallium in soil The content of gallium in individual fractions of soil from Upper Silesia was [in lg g 1]: fraction 1, 24.7; fraction 3, 29.2; fraction 4, 35.4; fraction 5, 6.9 and it was the highest in the organic fraction, indicating of input by anthropogenic activities (due to the complexation properties of natural organic matter and the accumulation of trace metals in living organisms (Tessier et al. (1979)). The organic fraction constitutes an important source of the potentially available gallium and the information to estimate the pollution of the region. While evaluating the anthropogenic activity, the properties of the soil in question must certainly be considered (including the mineral content in the soil, pH, organic matter type or rather the quantitative
share of its various forms), which also affect the metal content in this fraction. The content of scandium in Upper Silesia soil was following [in lg g 1]: fraction 1, 1.52; fraction 2, 0.53; fraction 3, 7.78; fraction 4, 1.79; fraction 5, 0.20. The highest concentration of scandium was found in fraction Fe–Mn oxides. In Podlasie soil, the content of gallium was clearly lower than in Upper Silesia soil and it was [in lg g 1]: fraction 1, 20.0; fraction 3, 10.3; fraction 4, 12.0. The concentration of scandium in all fractions and gallium (fraction 2 and 5) was less than detection limit of the applied method. The precision of the results was satisfactory. The relative standard deviation (RSD, n = 6) for scandium determination was in the range from 4.3% to 11.4% and it depended on scandium content in the fraction. In the case of gallium, the RSD was higher; between 7.8% and 15% (Table 3). In order to assess the reliability of the results obtained and the accuracy of the methods developed, the total content of Sc and Ga was determined in both soils examined, following the procedure given for fraction 5. The recovery of the Sc and Ga standards (introduced in the samples of the soils at the beginning of the analysis) was examined, following the same way as in the determination of the total content of the ele-
578
J. Połedniok / Chemosphere 73 (2008) 572–579
Table 3 Content of scandium and gallium in particular fractions of Upper Silesia and Podlasie soils determined by spectrophotometric and ICP-OES methods Region
Fraction
Upper Silesia
1 2 3 4 5 P
ICP-OES method x6 ± t0.95 SD, lg g
Ga
Ga
Sc
24.7 ± 2.33 – 29.2 ± 3.06 35.4 ± 4.08 6.9 ± 1.08 96.2 94.7 ± 3.58 20.0 ± 1.60 – 10.3 ± 0.97 12.0 ± 1.37 – 42.3 41.6 ± 3.19
Totala 1 2 3 4 5 P
Podlasie
Spectrophotometry CAS-ST method x6 ± t0.95 SD, lg g 1; RSD (%)
Totala
(9.0) (10.0) (11.0) (15.0) (3.6) (7.8) (9.0) (10.9)
(7.3)
1.52 ± 0.14 0.53 ± 0.06 7.78 ± 0.35 1.79 ± 0.10 0.20 ± 0.02 11.82 11.10 ± 0.46 – – – – – – 2.2 ± 0.18
(8.6) (11.4) (4.3) (5.5) (11.2) (4.0)
(7.8)
20.1 ± 0.95 – 29.7 ± 1.88 34.6 ± 2.62 6.7 ± 0.58 90.1 93.2 ± 7.14 19.6 ± 1.29 – 10.8 ± 0.96 11.6 ± 1.22 – 42.0 38.3 ± 3.75
1
; RSD (%) Sc
(4.5) (6.0) (7.2) (8.3) (7.3) (6.2) (8.5) (10.0)
(9.3)
1.63 ± 0.18 0.42 ± 0.06 7.04 ± 0.16 1.95 ± 0.16 0.23 ± 0.33 11.27 12.70 ± 0.11 – – – – – – 2.0 ± 0.20
(10.5) (12.7) (2.5) (7.6) (13.5) (0.8)
(9.2)
All numbers in this table are mean values. Standard deviation (SD) and relative standard deviation (RSD) based on six replicate analysis; x6 ± t0.95 SD – confidence interval at a P probability level of 95%; t0.95 – student’s coefficient; – summary content (1–5). a Total content found after digestion of soil with concentrated HF–H2SO4 mixture.
ments in the soil and in fraction 5. Simultaneously, the parallel analysis of the samples was carried out by another analytical method – the ICP-OES technique. The samples for the analysis were prepared in the same way as for the spectrophotometric methods, only the Fe(III) reduction, Fe(III) extraction and colour reaction steps were omitted. Measurements were carried out at the wavelength k = 294.364 nm in the case of gallium and k = 361.384 nm in the case of scandium. The content of the elements was found from the calibration graphs in the range: 0.12–2.0 lg mL 1 for gallium and 0.05–2.0 lg mL 1 for scandium. The results obtained are presented in Tables 3 and 4. The comparison of the total content to the sumP mary content of Sc and Ga in the particular fractions ( ) reaching the probability level of 95% confirms the reliability of the results. The recovery of the Sc and Ga standards (Table 4) is satisfactory, being at the level of 92–95%, which suggests good accuracy for both methods. The sample analysis with the use of the ICP-OES technique gave comparable results, which also indicates the good accuracy of the said methods and the possibility to use them for the analysis of soils in terms of functional speciation. 3.5. Comparison of spectrophotometric methods with the ICP-OES technique The limit of quantification (LOQ = 3 LOD; limit of detection) indicated for the determination of Sc with the use of the spectro-
Table 4 Determination of the total content of scandium and gallium in soil: checking the standard recovery Region
Element
Addeda (lg)
Foundb x3 ± t0.95 SD (lg)
Average standard recovery (%)
Upper Silesia
Ga
– 4.8 – 2.6
3.79 ± 0.17 8.28 ± 0.35 2.76 ± 0.14 5.16 ± 0.25
93.5
– 3.2 – 1.0
1.67 ± 0.17 4.67 ± 0.47 0.55 ± 0.05 1.50 ± 0.13
93.8
Sc Podlasie
Ga Sc
92.3
95.0
a Per analysed sample: 0.5 g of soil was digested with conc. HF–H2SO4 mixture and diluted to 25 mL (Ga) or 10 mL (Sc) with HCl; 2 or 5 mL of the solutions, respectively, were taken to the analysis. b Average of three replicates of each sample: x 3 ± t0.95 SD – confidence interval at a probability level of 95%; t0.95 – student’s coefficient.
photometric method and the ICP-OES technique is comparable and amounts to [lg mL 1]: 0.0195 and 0.0200, respectively. For gallium, however, the LOQ of the spectrophotometric method is significantly lower (0.0232) than the ICP-OES techniques (0.0500), which indicates its higher sensitivity and predisposes it for determination of lower concentrations of this element in soils. The linear working range is more extensive in the case of the ICPOES technique; however, when determining Sc and Ga in soils, due to their low concentration level, it is practically insignificant. The precision of both methods depends on the concentration of the elements. The precision of the ICP-OES is better (Table 3) for determining Sc occurring in higher concentrations (total content, fraction 3 of Upper Silesia soil). While determining very low Sc concentrations (fractions 1, 2, 4 and 5 of Upper Silesia soils) the RSD values calculated for the spectrophotometric method are significantly lower (Table 3), which indicates the better precision of this particular method. Due to the low contents of scandium in the soils, spectrophotometry seems to be a more suitable method of analysing this material than the ICP-OES technique. In the case of gallium, spectrophotometry is more precise for the determination of higher concentrations, while the ICP-OES technique is more suitable for the lower concentrations of this element (Table 3). Considering the economic aspect (the cost and availability of apparatus) the spectrophotometric methods presented can be recommended for the determination of the Sc and Ga content in soils for practically any laboratory with basic analytical equipment. 4. Conclusions The sequential extraction procedure developed by Tessier et al. (1979) for heavy metals was examined for the speciation of scandium and gallium in soil. The soil from industrial and agricultural regions of Poland was the object of this research. The results obtained indicate that gallium content in the individual fractions of both soils clearly depends on the region (except exchangeable and carbonate fractions) and the total concentration of gallium. The content of gallium was the highest in the organic fraction of Upper Silesia soil (industrial region). In Podlasie soil (agricultural region) the content of gallium was clearly lower. The absence of gallium in fraction 2 of Upper Silesia and Podlasie soils shows that gallium does not form compounds with carbonates. The content of gallium found in the exchangeable fraction was similar for both soils. It indicates the constant amount of Ga
J. Połedniok / Chemosphere 73 (2008) 572–579
accessible for plants, which does not depend on the total concentration of this element in the soil. The speciation of scandium was determined only in Upper Silesia soil. The concentration of Sc in individual fractions of Podlasie soil was less than the detection limit of the applied methods. The content of scandium in Upper Silesia soil was the highest in Fe– Mn oxides fraction. It was in good correlation with literature data concerning the occurrence of scandium in soils (Kabata-Pendias and Pendias, 1999). The content of scandium found in the organic fraction of the soil was considerably lower than in the case of gallium and near to the content of Sc in the exchangeable fraction. The low content of scandium and gallium in fraction 5 confirms the inability of these elements to form their own minerals. The sequential extraction scheme of Tessier et al. and spectrophotometric methods based on the mixed complexes Sc(III) and Ga(III) with Chrome Azurol S and benzyldodecyldimethylammonium bromide can be recommended for gallium speciation in soils from industrial and agricultural areas and for scandium speciation in soils with high content of this element. References Arunachalam, J., Emons, H., Krasnodebska, B., Mohl, C., 1996. Sequential extraction studies on homogenized forest soil samples. Sci. Total Environ. 181, 147–159. Bac, S., Gonel, Z., Krymuski, J., 1999. Foundations of Crop Production. Pan´stwowe Wydawnictwo Rolnicze i Les´ne, Warsaw. Bednarek, R., Prusinkiewicz, Z., 1997. Geography of Soils. PWN, Warsaw. Buhl, F., Połedniok, J., 1999. Content of scandium, gallium and vanadium in Upper Silesia soil. Arch. Environ. Prot. 25, 117–123. Buhl, F., Połedniok, J., 2000. The comparison of gallium, vanadium and scandium content in soil from industrial and agricultural regions. Roczniki gleboznawcze 51, 67–75. Chandrajith, R., Dissanayake, C.B., Tobschall, H.J., 2005. The abundances of rare trace elements in paddy (rice) soils of Sri Lanka. Chemosphere 58, 1415–1420. El-Ghawi, U.M., Bejey, M.M., Al-Fakhri, S.M., Al-Sadeq, A.A., Doubali, K.K., 2005. Analysis of Libyan arable soils by means of thermal and epithermal NAA. Arab. J. Sci. Eng. 30, 147–163. Ferrari, A.A., Franca, E.J., Fernandes, E.A.N., Bacchi, M.A., 2006. Surface contamination effects on leaf chemical composition in the Atlantic Forest. J. Radioanal. Nucl. Chem. 270, 69–73. Hirose, K., 2006. Chemical speciation of trace metals in seawater: a review. Anal. Sci. 22, 1055–1063. Jarosz, M., Mirzyn´ska, B., 2006. Some Problems of Environmental Speciation Analysis of Heavy Metals. Publishing House of Warsaw Technical University, Warsaw. Kabata-Pendias, A., Pendias, H., 1992. Trace Elements in Soil and Plants. CRC Press, Boca Raton. Kabata-Pendias, A., Pendias, H., 1999. Biogeochemistry of Trace Elements. PWN, Warsaw. Kwapulin´ska, G., 1995. A sensitive spectrophotometric method of determination of gallium(III) using Chrome Azurol S and benzyldimethyllaurylammonium bromide. Chem. Anal. Warsaw 40, 783–789.
579
Kwapulin´ska, G., Buhl, F., Połedniok, J., 1993. Spectrophotomertic studies on the system Sc – Chrome Azurol S – benzyldimethyllaurylammonium bromide (ST) and its application in chemical analysis. Chem. Anal. Warsaw 38, 201–210. Langodegard, M., Wibetoe, G., 2002. Determination of gallium in soil by slurry sampling GFAAS. Anal. Bioanal. Chem. 373, 820–826. Liang, S., Liu, Y., Hu, H., 2004. Determination of C, N and other 36 elements in soil samples by XRF. Yankuang Ceshi 23, 102–108. Lopez-Garcia, I., Campillo, N., Arnau-Jerez, I., Hernadez-Cordoba, M., 2004. ETAAS determination of gallium in soils using slurry sampling. J. Anal. Atom. Spectrom. 19, 935–937. Marin, B., Valladon, M., Polve, M., Monaco, A., 1997. Reproducibility testing of a sequential extraction scheme for the determination of trace metal speciation in a marine reference sediment by inductively coupled plasma-mass spectrometry. Anal. Chim. Acta 342, 91–112. Minowa, H., Ebihara, M., 2003. Separation of rare earth elements from scandium by extraction chromatography. Application to radiochemical neutron activation analysis for trace rare earth elements in geological samples. Anal. Chim. Acta 489, 25–37. Plessow, A., Heinrichs, H., 2000. Speciation of trace elements in acidic pore waters from waste rock dumps by ultrafiltration and ion exchange combined with ICPMS and ICP-OES. Aquat. Geochem. 6, 347–366. Połedniok, J., Buhl, F., 2003. Speciation of vanadium in soil. Talanta 59, 1–8. Polish Standard PN-R-04031, 1997. Chemical and Agricultural Analysis of the Soil. Sampling. Riberio, A.P., Fiqueiredo, A.M.G., Sigolo, J.B., 2005. Determination of heavy metals and other trace elements in lake sediments from a sewage treatment plant by neutron activation analysis. J. Radioanal. Nucl. Chem. 263, 645–651. Saini, N.K., Mukherjee, P.K., Rathi, M.S., Khanna, P.P., Purohit, K.K.J., 2002. Trace element estimation in soils: an appraisal of ED-XRF technique using group analysis scheme. J. Trace Microprobe Tech. 20, 539–551. Schulze, D., Krüger, A., Kupsch, H., Segebade, C., Gawlik, D., 1997. Enrichment and distribution of elements in flood-plain soils of the Bitterfeld industrial area studied by neutron activation analysis. Sci. Total Environ. 206, 227–248. Shtangeeva, I., 2005. Trace and Ultratrace Elements in Plants and Soil. WIT Press, Southampton. Shtangeeva, I., Ayrault, S., Jain, J., 2004. Scandium bioaccumulation and its effect on uptake of macro- and trace elements during initial phases of plant growth. Soil Sci. Plant Nutr. 50, 877–883. Sobczyn´ski, T., Siepak, J., 2001. Speciation of heavy metals in bottom sediments of lakes in the area of Wielkopolski National Park. Pol. J. Environ. Stud. 10, 463– 474. Solecki, J., Chibowski, S., 2000. Examination of trace amounts of some heavy metals in bottom sediments of selected lakes of South-Eastern Poland. Pol. J. Environ. Stud. 9, 203–308. Sun, Z., Zhang, Z., Mao, Y., Chen, Z., 2004. Determination of trace amount of silver, tin, lead, boron and gallium in geochemical exploration samples by electric arc distillation- emission spectrometry. Yankuang Ceshi 23, 153–156. Tessier, A., Campbell, P.G.C., Bisson, M., 1979. Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem. 51, 844–851. Tyler, G., 2004. Vertical distribution of major, minor and rare elements in a Haplic Podzol. Geoderma 119, 277–290. Vogt, C., 1994. Speciation of the inorganic components in brown coal. Fresen. J. Anal. Chem. 350, 89–92. Wa˛chalewski, T., 1997. Items of Environmental Chemistry. AGH Publishers, Cracow. Yu, B., Yan, Z., Yang, L., Wang, R., Li, X., 2006. Determination of 36 major, minor and trace elements in soil and stream sediment samples by X-ray florescence spectrometry. Yankuang Ceshi 25, 74–78. Zambello, F.R., Enzweiler, J., 2002. Multi-element analysis of soils and sediments by wavelength-dispersive X-ray fluorescence spectrometry. J. Soils Sediments 2, 29–36.