Journal of Radioanalytical and Nuclear Chemistry, Vol. 266, No. 2 (2005) 251–257

Synthesis of xanthate functionalized silica gel and its application for the preconcentration and separation of uranium(VI) from inorganic components P. Gopi Krishna,1 J. M. Gladis,2, K. S. Rao,3 T. Prasada Rao,2 G. R. K. Naidu1* 1 Department

of Environmental Sciences, S.V. University, Tirupati – 517 502, India Research Laboratory, (CSIR), Trivandrum – 695 019, India 3 Institute for Advance Energy, Kyoto University, Gokasho, Uji, Japan

2 Regional

(Received December 14, 2004)

A new chelating solid extractant prepared by the chemical immobilization of xanthate on silica gel was characterized by Fourier transform infra red spectrometry (FTIR), thermogravimetric analysis (TGA) and microanalysis and used for the preconcentration and separation of uranyl ion prior to its determination by Arsenazo-III. The effect of pH, weight of the solid extractant, volume of the aqueous phase and the interference of neutral electrolytes, cations and anions on the determination of uranium, have been studied in detail to optimize the conditions for trace determination of uranium(VI). The accuracy of the developed procedure was tested by analyzing marine sediment (MESS-3) and soil (IAEA-SOIL-7) reference materials. The results obtained on analysis of soil and sediment samples are comparable to standard ICP-MS values.

Introduction In spite of the improvements in the sensitivity of analytical systems, preconcentration and separation of analytes from complex matrices are still necessary to obtain reliable results. Liquid-liquid extraction (LLE) is a classical method for preconcentrating metal ions and/or for matrix removal. Solid phase extraction (SPE) is another approach that offers a number of advantages over LLE such as (1) higher enrichment factors, (2) absence of emulsion formation, (3) minimal costs due to low consumption of reagents, (4) minimal waste generation, (5) flexibility, (6) reusability and (7) ecofriendliness.1,2 Hence, SPE is now routinely used in different, chromatographic, clinical, pharmaceutical, environmental, industrial and agricultural analytical studies.3 SPE materials such as ion-exchange and chelating polymeric resins4–7 have been widely employed for the preconcentration of metal ions from aqueous solution. However, slow kinetics, irreversible adsorption of organics, swelling, sensitivity towards many chemical environments and loss of mechanical stability in modular operation are the main disadvantages typically exhibited by the above resins. On the other hand, the use of suitable inorganic supports may have several advantages such as (1) good selectivity, (2) no swelling, (3) rapid sorption of metal ions and (4) good mechanical stability.8 Silicagel is an amorphous inorganic polymer composed of inert siloxane groups (Si–O–Si) with silanol groups (Si–OH) distributed on the surface.9,10 After activation of the silica gel surface,11,12 three approaches are frequently used to prepare SPE sorbents. These include (1) use of nascent sorbent, (2) sorption of chelating ligand onto a matrix and (3) chemical

immobilization via covalent bonding of a ligand with activated silica gel. The latter methodology renders rugged systems free from ligand leaching problems.13 Hence, chemically modified silica gel finds increasing application in different areas of chemistry, which includes metal ion preconcentration.14,15 The intense current interest in uranium analysis arises from its known toxicity and the possibility of human exposure to it. Severe exposure to uranium compounds can cause acute renal failure resulting in fatality and also minor damage to liver.16 The safe limit for uranium in drinking water samples fixed by WHO is ~5 ng/ml. Table 1 summarizes the preconcentration procedures developed since 1990 for U(VI) using silica gel.17–24 As seen from the table, barring one flow injection procedure reported by HAVEL et al.,24 all other procedures are in offline mode (batch or column). Among these either unmodified silica gel17–21 or chelate impregnated silica gel22 sorbents were used, but there were no chemically immobilized silica gel sorbents. PURI et al.25 adsorbed U(VI) as its ion-pair complex with trifluoroethyl xanthate and cetyl trimethyl ammonium bromide onto microcrystalline naphthalene. Unlike most of the procedures mentioned in Table 1, this procedure was successful in analyzing real samples like alloys, coal fly ash, etc. In our previous works ethyl xanthate sorbed on naphthalene or benzophenone was employed for the preconcentration of cobalt and nickel,26 chromium(VI) and (III)27 and copper, cadmium and lead28 and subsequent determination by flame atomic absorption spectrometry. These analytical procedures were successfully tested for their applicability to hair,26 tannery effluents27 and soils,28 respectively.

* E-mail: [email protected] 0236–5731/USD 20.00 © 2005 Akadémiai Kiadó, Budapest

Akadémiai Kiadó, Budapest Springer, Dordrecht

P. GOPI KRISHNA et al.: SYNTHESIS OF XANTHATE FUNCTIONALIZED SILICA GEL AND ITS APPLICATION

Table 1. Summary of preconcentration procedures for uranium(VI) using silica gel as sorbent or support Sample No. 1 2 3 4 5 6 7 8 9

Chelating ligand, sorbent Activated silica gel17 Hydrous SiO2 (silica gel)18 Colloidal silica19 Silica gel20 Silica gel21 Tri-n-octyl phosphine oxide, silica gel22 Aliquat-336, silica gel23 Activated silica gel24 Xanthate, silica gel (present method)

Analytical technique Fluorimetry

Mode of preconcentration Off-line, column

–

Detection limit in initial solution, ng/ml 1.4

–

Off-line, batch

–

–

Luminescence – – K-spectrometry

– Off-line, column – Off-line, column

– – 4.0 –

0.2 – 2.10–5M –

–

Off-line, column

1.25–4.0M HCl

–

Fluorimetry

Online, flow injection

–

–

Spectrophotometry

Offline, batch

5.0–9.0

4.0

Since xanthate functionalized silica gel sorbents have not been described for the preconcentration of U(VI) so far, it was felt worthwhile to prepare and characterize xanthate functionalized silica gel materials and test their usefulness for the analysis of various geological materials using a simple and readily available instrument viz. UV-visible spectrophotometer. Experimental Apparatus Absorbances were measured using Hitachi 220 microprocessor controlled double beam spectrophotometer (Hitachi, Japan). LI-120 digital pH meter (ELICO, India) was used for pH measurements. Fourier transform infrared (FTIR) spectra were recorded using a MAGNA IR-560 spectrometer (Nicolet, USA). Thermogravimetric analysis (TGA) was carried out using TGA-50H (Shimadzu, Japan) instrument. Elemental analysis was carried out by using CHNS/O analyzer 2400 (Perkin Elmer, USA). A Varian ultra mass 700 inductively coupled plasma mass spectrometer (Varian, USA) was used for the analysis of soil and sediment samples. Reagents A stock solution of U(VI) was prepared by dissolving an appropriate amount of uranyl nitrate hexahydrate [UO2(NO3)2.6H2O] (Aldrich, USA) in deionized water. Concentrated HNO3 (5 ml) was added to 100 ml of solution to suppress hydrolysis. ArsenazoIII (Aldrich, USA) solution was prepared by dissolving 0.10 g of the reagent in 100 ml of deionized water. Sodium acetate–acetic acid buffer (1.0M) was prepared to maintain the pH of the aqueous phase. Silica gel (Devisil Grade 710, 4–20 µm, 60° A) and carbon

252

pH

Application Brines and iron(III) oxides – – – – Environmental samples – Waste and natural waters Soils, sediments and monazite sand

disulphide were obtained from Aldrich, USA. All other chemicals including neutral electrolytes, cations and anions were of analytical reagent grade. The standard reference material, MESS-3, (Marine sediment reference material) and IAEA Soil-7 (reference material for soil) certified for trace elements are used for quality assurance. Preparation of xanthate functionalized silica gel (XSG) Preparation: Silica gel (5 g) was taken in a 100 ml round bottom flask and stirred for 3 hours with 20 ml of 1 : 1 of (v/v) of HCl for activation. The activated silica gel was filtered and dried (yield 100%). The dried material was stirred for 18 hours in the presence of 10 ml of 2M KOH; filtered, washed with water and dried (yield 100%). To this 10 ml of carbondisulphide was added dropwise and stirred for 18 hours at room temperature. The resulting material was filtered, washed with water, ethanol and air dried (yield 49%). Characterization: The FTIR spectrum of xanthate functionalized silica gel (XSG) exhibits a new absorption band (weak) at 678 cm–1 corresponding to C-S stretching vibration that does not appear in the spectrum of bare silica gel. TGA studies showed a weight loss of 5% in the temperature range 30–110 °C of XSG soaked in water for 4 hours (which was filtered and air dried before analysis) compared to unsoaked material. This data shows better water retaining capacity of the xanthate functionalized material and thus offer a faster metal ion-exchange. Furthermore, a 10% weight loss in the temperature range 120–800 °C indicates the functionalization of silica gel with xanthate to around 10%. The microanalysis data for the carbon and sulphur are 1.2 and 6.0% which compared well with the calculated values of 1.3 and 6.9% (calculated on the basis of 10% functionalization), respectively.

P. GOPI KRISHNA et al.: SYNTHESIS OF XANTHATE FUNCTIONALIZED SILICA GEL AND ITS APPLICATION

General procedure An aliquot of solution containing 2–100 µg of U(VI) was diluted to 500 ml and pH was adjusted to ~5.0 after adding 10 ml of 1.0M sodium acetate–acetic acid buffer and transferred to a 1000-ml beaker. To the above solution, 0.1 g of XSG was added and stirred for 5 minutes. The preconcentrated uranium(VI) ions were eluted with 5.0 ml of 1.0M HCl by stirring for 5 minutes and treated with 5.0 ml of 11M HCl and 1.0 ml of 0.10% Arsenazo-III solution and diluted to 25 ml. The absorbance of Arsenazo-III complex of uranium(VI) was measured at 656 nm.29

concentration and aqueous phase volume were systematically varied in the preconcentration of 25 µg of U(VI) present in 500 ml of solution. The results are summarized in Table 2. Thus, quantitative preconcentration of U(VI) was observed with 0.05 g of XSG, preconcentration and elution times of 5 minutes and as low as 5 ml of 0.5M HCl is enough for quantitative recovery of U(VI). Furthermore, up to an aqueous volume of 500 ml, quantitative enrichment of U(VI) was observed.

Procedure for analysis of marine sediment reference material (MESS-3), soil (IAEA Soil-7) and soils and sediments 0.5 g of sample was fumed with 5 ml of HF and 1 ml of 18M H2SO4 at 150 °C. The process was repeated twice. The residue was cooled and fused with 2 g of KHSO4 at 800 °C in an electric oven for 30 minutes. Then the melt was cooled, dissolved in 50 ml of water and diluted to 100 ml. The preconcentration, elution and determination of U(VI) were carried out as described above. Procedure for analysis of synthetic samples of standard minerals or alloys Synthetic sample corresponding to standard monazite sand mineral was prepared in water in the presence of a few ml of 11M HCl and subjected to determination of uranium by following the procedure described above. Results and discussion As a preliminary experiment, the preconcentration of 25 µg of U(VI) present in 500 ml of aqueous solution whose pH was adjusted to 5.0 in presence of 10 ml of 1.0M sodium acetate–acetic acid buffer onto 0.10 g of xanthate functionalized silica gel (XSG) was carried out. The enrichment of uranium(VI) under these conditions was found to be quantitative. Hence, XSG was used for subsequent preconcentration studies. Effect of pH: The effect of pH on the preconcentration of 25 µg of U(VI) present in 500 ml of solution was studied under the conditions mentioned in Experimental. The preconcentration of U(VI) onto XSG is quantitative in the pH range 5–9 (Fig. 1). In all subsequent work, the pH was adjusted to ~5 [to avoid the hydrolysis of U(VI)] after the addition of 10 ml of 1.0M sodium acetate–acetic acid buffer. Reaction conditions: Other conditions, viz. weight of XSG, preconcentration time, elution time, eluent

Fig. 1. Effect of pH on the preconcentration of 25 µg of uranium(VI) present in 0.5 l of aqueous phase onto 0.10 g of XSG. (Preconcentration and elution times 5 minutes, eluent: 5 ml of 0.5M HCl)

Table 2. Influence of various parameters on the percent enrichment/extraction (%E) of uranium onto xanthate functionalized silica gel %E Weight of XSG, gram 0.02 0.05 0.10 0.20 Preconcentration time, minutes 5 10 20 Desorption time, minutes 5 10 20 Desorption agent (HCl), M 0.1 0.5 1.0 Aqueous phase volume, ml 25 50 100 500 1000

Chosen condition

56.0 >99 >99 >99

0.10 g

>99 >99 >99

5 min

>99 >99 >99

5 min

7.7 >99 >99

1.0M

>99 >99 >99 >99 73.1

500 ml

Conditions: U(VI) = 25 µg, pH 5.0.

253

P. GOPI KRISHNA et al.: SYNTHESIS OF XANTHATE FUNCTIONALIZED SILICA GEL AND ITS APPLICATION

Retention or sorption capacity of the sorbent: The retention capacity of the sorbent for weighable U amounts/concentrations was established with 100 ml of solution containing increasing amounts of uranium and 0.1 g of XSG mixture with 10 ml of 1.0M (pH 5) sodium acetate–acetic acid buffer and stirred for 2 hours and filtered through a filter paper. The amount of U(VI) sorbed on the XSG was determined after elution (30 minutes) with 25 ml of 1.0M HCl by using the Arsenazo-III procedure. As seen from Fig. 2, the retention capacity of XSG increases with the increase in initial U(VI) concentration and reaches a constant and maximum value of 64.5 mg of U(VI) per g of xanthate functionalized silica gel. Furthermore, a comparison of retention/sorption capacity obtained by the present method is compared with that of other chelate functionalized sorbents synthesized so far by various researchers (Table 3). Barring bicine functionalized Amberlite XAD-4, the present method seems to offer higher retention capacity when compared to other SPE’s.

washing and elution cycle. During these experiments, preconcentration was carried out with 25 µg of U(VI) at pH 5.0. Elution of uranium(VI) was done with 1.0M HCl solutions. The U(VI) content removed per gram of xanthate modified silica gel remains unchanged and recovery of U(VI) is quantitative even after 10 cycles of experiments with the same material.

Stability and reusability The stability of the xanthate functionalized silica gel was determined before and after each preconcentration,

Fig. 2. Uranyl ion retention/sorption capacity of the XSG (pH 5.0, 0.1 g of XSG). (Other conditions are the same as in Fig. 1)

Table 3. Comparison of retention/sorption capacities of various chelate functionalized sorbents during preconcentration of U(VI) Sample No. 1

Sorbent

Chelating agent 5,7-dichloroquinoline-8-ol 1-(2-pyridylazo)-2-naphthol

2.34

31

3

Naphthalene Naphthalene and benzophenone Activated carbon

Retention/sorption capacity, mg per g of sorbent 1.88

18.35

32

4

Merrifield peptide resin

Diarylazobisphenol 11,23-di-semicarbazo 26,28-n-dipropoxy-25,27dihydroxycalix[4] arene

3.09

33

Thenoyltrifluoroacetone

33.32

34

62.50

35

2.74 2.89 12.30

36 37 38

64.26

39

90.44 6.71

40 41

4.03

42

2

5 6 7

Merrifield chloromethylated resin Merrifield chloromethylated resin Amberlite XAD-4 Amberlite XAD-4 Amberlite XAD-4 Amberlite XAD-4

8 9 10

254

Amberlite XAD-4 Amberlite XAD-2 Octadecyl silica membrane discs Silica gel

Di-bis(2-ethylhexyl) malonamide Quinoline-8-ol o-Vanillin semicarbazone Succinic acid Octa-carboxymethyl1-methyl calix[4] resorcinarene Bicine Pyrogallol Tri-n-octylphosphineoxide and catechol Xanthate

64.5

Reference 30

Present method

P. GOPI KRISHNA et al.: SYNTHESIS OF XANTHATE FUNCTIONALIZED SILICA GEL AND ITS APPLICATION

Statistical and calibration parameters Under the optimum conditions described above, the calibration curve is linear over 2–100 µg of U(VI) concentration range in 5 ml of eluent. Ten replicate determinations of 25 µg of U(VI) ion present in 500 ml of solution gave a mean absorbance of 0.115 with a relative standard deviation of 2.12%. The detection limit corresponding to three times the standard deviation of the blank was found to be 4 ng /ml. The linear equation with regression is: Y = 0.00024 + 0.0025X with the correlation coefficient of 0.9999 where Y is the absorbance and X is the concentration in ng/ml. All the statistical calculations are based on the average of triplicate readings for each standard solution in the given range. Effect of neutral electrolytes and diverse ions The effect of various neutral electrolytes and salts of cations and anions present in 500 ml of solution on the preconcentration of 5 µg of U(VI) and subsequent spectrophotometric determination was studied individually by following the procedure described above. Neutral electrolytes (0.1M) and various cations (5 mg/0.5 l) and anions (5 mg/500 ml) listed in Table 4

did not interfere. However, thorium is tolerated only up to 10 µg/500 ml of solution during the determination of 5 µg of U(VI). Application to the analysis of marine sediment and soil reference materials The accuracy of the preconcentration procedure developed for U(VI) was tested by analyzing standard reference materials of marine sediment (MESS-3) supplied by the National Research Council, Canada and soil (IAEA Soil-7) supplied by IAEA, Vienna. The sediment and soil samples were brought into solution by following the procedure described above. Uranium(VI) present in the samples was preconcentrated using 0.10 g of xanthate functionalized silica gel by following the procedure described above. The results obtained are shown in Table 5, from which it is clear that the developed U(VI) preconcentration procedure and spectrophotometric determination agree well with the certified values. Furthermore, known amounts of U(VI) ions were added to standard reference materials before dissolution prior to preconcentration, elution and determination procedure. The U(VI) values found as expected indicated the suitability of the developed procedure for the determination of U(VI) in soil and sediment samples.

Table 4. Effect of neutral electrolytes and diverse ions on the preconcentration of uranium and subsequent determination by Arsenazo-III Salt or ion added NaCl, NaNO3, Na2 SO4, NaSCN, Na2C2O4, NaF, KCl, KBr, KI, KNO3, NH4Cl, and Na2EDTA Li+, Ca2+, Ni2+, Co2+, Mn2+, Cu2+ Zn2+, Cd2+, Pb2+, Fe3+, Sb3+, Al3+, VO3–, Cr2O72– and AsO43– Th4+

(1)

(2)

(3)

Remarks No interference at 0.1M

No interference at 5 mg/500 ml

No interference at 10 mg/500 ml

Conditions: U = 5µg, 0,1g of XSG, preconcentration time = 10 min, elution time = 5 min, aqueous phase volume = 500 ml, eluent concentration = 1.0M HCl, eluent volume = 5 ml. Table 5. Determination of U(VI) in certified reference materials Sample Marine sediment MESS-3

Soil IAEA Soil-7

a b

Certified composition, µg/g

Uranium(VI) found,a µg/g

U: 4.0,b Al: 8.59%, Ca: 1.47% Fe: 4.34%, Li: 73.6, Mn: 324 Ni: 46.9, Sr : 129, V: 234 Zn: 159, Cr:105, Cu: 33.9 Pb: 21.1, As: 21.2

4.05 ± 0.15

U: 2.60; Al: 47000;Ca: 163,000; Fe: 25,700; Mg:11,300; Si:180,000, Ti: 3,000; Ba: 159; Pb: 60; V:66; Zn: 104; Zr: 185; Y: 21

2.60 ± 0.10

Average of 3 determinations ± standard deviation. Information value.

255

P. GOPI KRISHNA et al.: SYNTHESIS OF XANTHATE FUNCTIONALIZED SILICA GEL AND ITS APPLICATION

Analysis of synthetic samples of minerals and alloys Synthetic samples prepared according to the composition of monazite mineral were subjected to U(VI) determination by following the procedure described above. The results obtained on the determination of U(VI) in these complex mixtures are shown in Table 6, from which it is clear that the developed U(VI) preconcentration procedure and spectrophotometric determination can be reliably used for analysis of minerals and alloys described above. Analysis of soil and sediment samples Samples of soil, and river and marine sediments collected from Trivandrum, India were dissolved using the procedure described above. These samples were subjected to preconcentration and determination of U(VI) using our general procedure and by using an inductively coupled plasma mass spectrometer (ICPMS). The results obtained with the present method compare favorably well with the ICP-MS results (Table 7).

synthesis after activation is easier to prepare than many other chelate functionalized sorbents described up to now. This material allows a rapid (<5 minutes of preconcentration and elution times) and reliable off-line preconcentration of traces of U(VI). In addition to 100 fold enrichment, the developed procedure is useful in separation of traces of U(VI) form 0.1M levels of various neutral electrolytes and 5 mg amounts of a number of inorganic species. The developed preconcentration procedure is precise as the relative standard deviation is 2.12% during the enrichment and determination of 25 µg of U(VI) present in 500 ml of solution. Furthermore, the detection limit corresponding to 3 times the standard deviation of the blank was found to be 4 ng/ml. Moreover, XSG material offers a very high retention capacity of 64.5 mg of U(VI) per g. The accuracy of the developed preconcentration procedure was tested by analyzing U(VI) present in certified marine sediment and soil reference materials. Unlike most of the preconcentration procedures developed for U(VI), the present enrichment procedure allows a reliable and rapid analysis of U(VI) in soils and sediments using a simple and readily available instrument like spectrophotometer.

Conclusions The XSG material synthesized in this paper via chemical modification of silica gel by a two step

Table 6. Determination of uranium(VI) in synthetic samples corresponding to monazite sand mineral Sample with composition, % Ce (49–74); Y (1–4); ThO2 (5–12) and U in traces

Amount of metal taken, µg/100 ml U:5.0; Ce:40.0; Nd:20.0; Gd:10.0; Th: 10.0; Y: 4.0; Sm :16.0

Uranium(VI) found,* µg/100 ml 4.8 ± 0.10

* Average of 3 determinations ± standard deviation.

Table 7. Analysis of soil and sediment samples Sample No.

Description

1

Soil sample from station I

2

Soil sample from station II

3

River sediment, (Karamana river) Trivandrum Marine sediment (Arabian sea, Trivandrum)

4

* Average of 3 determinations ± standard deviation.

256

Added – 6.00 12.00 – 8.00 16.00 – 8.00 16.00 – 5.00 10.00

Uranium(VI), µg/g Found Present method* 6.90 ± 0.15 12.85 ± 0.30 18.90 ± 0.50 8.00 ± 0.20 15.95 ± 0.40 23.95 ± 0.50 8.25 ± 0.20 16.25 ± 0.35 24.30 ± 0.55 5.00 ± 0.10 10.00 ± 0.20 15.00 ± 0.30

ICP-MS 6.96 ± 0.11

7.96 ± 0.14

8.22 ± 0.15

4.96 ± 0.08

Recovery, % – 97.5 100.0 – 99.4 99.7 – 100.0 100.3 – 100.0 100.0

P. GOPI KRISHNA et al.: SYNTHESIS OF XANTHATE FUNCTIONALIZED SILICA GEL AND ITS APPLICATION

* One of the authors, (P.G.K.) thanks UGC, New Delhi for the award of fellowship while carrying out this work.

References 1. T. PRASADA RAO, J. M. GLADIS, Rev. Anal. Chem., 20 (2001) 145. 2. C. R. PREETHA, J. M. GLADIS, T. PRASADA RAO, Talanta, 58 (2002) 701. 3. J. S. FRITZ, Analytical Solid Phase Extraction, 1 st ed., WileyVCH, New York, 1999. 4. P. K. JAL, S. PATEL, B. K. MISHRA, Talanta, 62 (2004) 1005. 5. C. KANTIPULY, S. KATRAGADDA, A. CHOW, H. D. GESSER, Talanta, 37 (1990) 491. 6. G. V. MYASOEDOVA, S. B. SAVVIN, Crit. Rev. Anal. Chem., 17 (1986) 1. 7. R. S. S. MURTHY, J. HOLZBECHER, D. E. RYAN, Rev. Anal. Chem., 6 (1982) 113. 8. N. V. DEORKAR, L. L. TAVALRIDES, Ind. Eng. Chem. Res., 36 (1997) 399. 9. A. R. CESTARI, C. AIROLDI, J. Colloid Interface Sci., 195 (1997) 246. 10. L. N. H. ARAKAKI, C. AIROLDI, Quim. Nova, 22 (1999) 246. 11. H. H. WEETALL, in: Methods in Enzymology, Vol. 135, J. N. ABELSON, M. I. SIMMON (Eds), Academic Press, San Diego, CA, 1987. 12. K. K. UNGER, Packing and Stationary Phases in Chromatographic Techniques, Marcel Dekker, New York, 1990. 13. R. SAXENA, A. K. SINGH, Anal. Chim. Acta, 340 (1997) 285. 14. P. M. PADILHA, L. A. DE MELO GOMES, C. C. F. PADILHA, J. C. MOREIRA, N. L. DIAS FILHO, Anal. Lett., 32 (1999) 1807. 15. V. CAMEL, Spectrochim. Acta, B58 (2003) 1177. 16. H. G. SEILER, H. SIGEL, A. SIGEL, Handbook on Toxicity of Inorganic Compounds, Marcel Dekker, New York, 1988. 17. J. DEPABLO, L. DURO, J. JIMÉNEZ, J. HAVEL, M. E. TORRERO, I. KASAS, Anal. Chim. Acta, 264 (1992) 115. 18. K. H. LEISER, S. QUANDTKLENK, S. THYBUSCH, Radiochim. Acta, 57 (1992) 45.

19. M. LOPEZ, B. J. S. BIRCH, Analyst, 121 (1996) 905. 20. E. GUIBAL, R. LORENZELLI, T. VINCENT, P. LECLOIREC, Environ. Technol., 16 (1995) 101. 21. T. REICH, H. MOLL, T. ARNOLD, M. A. DENECKE, C. HENNING, G. GELPEL, G. BERNHARD, H. NITSCHE, P. G. ALLEN, J. J. BUCHER, N. M. EDELSTEIN, D. K. SHUH, J. Electron. Spectrosc. Related Phenomena, 96 (1998) 237. 22. J. M. GODOY, D. C. LAURIA, L. D. P. GODOY, R. P. CUNHA, J. Radioanal. Nucl. Chem., 182 (1994) 165. 23. K. K. DUTTA, U. S. ROY, J. Indian Chem. Soc., 77 (2000) 499. 24. J. HAVEL, M. VRCHLABSKY, Z. KOHN, Talanta, 39 (1992) 795. 25. B. K. PURI, K. A. LAL, H. BANSAL, Anal. Sci., 18 (2002) 427. 26. P. G. KRISHNA, K. S. RAO, T. P. RAO, G. R. K. NAIDU, Microchim. Acta, 144 (2004) 285. 27. P. G. KRISHNA, J. M. GLADIS, U. RAMBABU, T. P. RAO, G. R. K. NAIDU, Talanta, 63 (2004) 541. 28. P. G. KRISHNA, K. S. RAO, V. M. BIJU, T. P. RAO, G. R. K. NAIDU, Chemia Analit., 49 (2004) 383. 29. F. D. SNELL, Photometric and Fluorimetric Methods of Analysis – Metals, John-Wiley and Sons, New York, 1978. 30. J. M. GLADIS, T. P. RAO, Anal. Lett., 35 (2002) 501. 31. C. R. PREETHA, T. P. RAO, Radiochim. Acta, 91 (2003) 247. 32. A. M. STARVIN, T. P. RAO, Talanta, 63 (2004) 225. 33. V. K. JAIN, A. HANDA, R. PANDYA, P. SHRIVASTAV, Y. K. AGARAWAL, React. Funct. Polym., 51 (2002) 101. 34. D. PRABHAKARAN, M. S. SUBRAMANIAN, Anal. Lett., 36 (2003) 2277. 35. D. PRABHAKARAN, M. S. SUBRAMANIAN, Talanta (in press). 36. J. M. GLADIS, T. P. RAO, Anal. Bioanal. Chem. Commun., 373 (2002) 867. 37. V. K. JAIN, A. HANDA, S. S. SAIT, P. SHRIVASTAV, Y. K. AGARAWAL, Anal. Chim. Acta, 429 (2001) 237. 38. P. METILDA, K. SANGHAMITRA, J. M. GLADIS, G. R. K. NAIDU, T. P. RAO, Talanta, 65 (2005) 192. 39. N. DEMIREL, M. MERDIVAN, N. PIRINCCIOGLU, C. HAMANCI, Anal. Chim. Acta, 485 (2003) 213. 40. K. DEV, R. PATHAK, G. N. RAO, Talanta, 48 (1999) 579. 41. M. KUMAR, D. P. S. RATHORE, A. K. SINGH, Microchim. Acta, 137 (2001) 127. 42. M. SHAMSIPUR, A. R. GHIASVAND, Y. YAMINI, Anal. Chem., 71 (1999) 4892.

257