Mudpsr13 by Dr DPS RATHORE

Exploration and Research for Atomic Minerals Vol. 23, 2013, pp. 137-144

DETERMINATION OF TOTAL CONCENTRATION OF URANIUM IN BOREHOLE CORE SAMPLES: COMPARATIVE STUDIES USING DIFFERENTIAL TECHNIQUE IN LASER-INDUCED FLUORIMETRY AND ICPOES P. Murugesan1, S. K. Jain1, Manjeet Kumar2, P. K. Tarafder3 and D. P. S. Rathore1 Chemistry Laboratory, Atomic Minerals Directorate for Exploration and Research Department of Atomic Energy, 1Nagpur-440001, 2New Delhi-110066, 3Jamshedpur-831002. E-mail: dpsrathore.amd@gov.in

Abstract In the present work, Inductively Coupled Plasma Optical Emission Spectrometer ( ICPOES) has been used for the direct, rapid and accurate measurement of total concentration of uranium in borehole core samples. Under standard operating conditions of the instrument, the spectral line at 409.014 nm shows better signal-to-background intensity ratio and is free from spectral interferences by matrix elements present in silicate borehole core and ore samples with a detection limit of 0.025 µg ml-1 U3O8. Under the optimized conditions, the reproducibility of the proposed method was checked by performing nine replicate determinations of 0.597, 0.140, 0.062 and 0.005 %U3O8 over a period of nine consecutive days. The relative standard deviation (RSD) of the method was 0.79, 0.80, 2.04 and 8.10 % respectively. The proposed ICPOES method is suitable for the determination of more than 0.001 %U3O8 in silicate rock samples. The method has been applied for the determination of uranium in international standard samples, SY-2, SY-3 and uranium ore, BL-2a, IAEA-low grade uranium ores and borehole core samples. “t”-test for paired data has been applied for comparing the results obtained in the same borehole sample by ICP-OES method with those obtained by using ‘differential technique in laser-induced fluorimetry method’(DT-LIF) as a reference measurement procedure. The calculated value for “t” is 0.52, which is less than the tabulated value of “t” for 20 borehole sample results (t=2.09 at 95% confidence level for n=20 results). This shows that the results of the measurements carried out without any chemical preparation or extraction by both the ICP-OES and ‘differential technique in laser-induced fluorimetry’ are not significantly different. The results of uranium content compare favorably with those obtained by using conventional fluorimetry. Keywords: Uranium Determination, ICP-OES, Differential Technique, Laser-Induced Fluorimetry, Core Samples

INTRODUCTION Selection of the most appropriate method for the determination of uranium depends on many parameters: on the purpose of analysis, nature of the sample, the concentration of uranium, the presence and concentration of the other elements in the matrix, the methodology available, the accuracy required, simplicity,

rapidity, easy calibration and optimization of the instrument, minimum generation of analytical waste, etc [1, 2]. There are some reviews already present in the current literature about various analytical methods available for the determination of uranium in various matrices. Rathore [2],

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reviewed the advances in technologies for the determination of uranium in diverse matrices. Special emphasis is placed on the wide utility of laser fluorimetry/spectrofluorimetry. Hou and Roos [3] reviewed the critical comparison of radiometric and mass spectrometric methods for the determination of uranium and other radionuclides in biological, environmental and waste samples. In this article, different radiometric methods, such as gamma ( γ )spectrometry, alpha (α)-spectrometry, and beta (β)-counting, and mass spectrometric methods, such as ICP-MS, accelerator mass spectrometry (AMS), thermal ionization mass spectrometry (TIMS), resonance ionization mass spectrometry (RIMS), secondary ion mass spectrometry (SIMS) and glow discharge mass spectrometry (GDMS) and their application for the determination of radionuclides are compared. The application of on-line methods (flow injection/sequential injection) for separation of radionuclides and automated determination of radionuclides is also discussed. Santos et al. [4], presented an overview on uranium determination using atomic spectrometric techniques. More recently, Shrivastava et al. [5] presented a review article on all electroanalytical techniques for the determination of uranium and its compounds in various matrices. These review papers will be helpful to the analysts and researchers to select the most suitable techniques as well as to improve upon the analytical capabilities for such applications.

both these techniques are based on emission phenomenon. Laser-induced fluorimetry technique is based on the measurement of ‘fluorescence emission’ from uranyl ion using appropriate fluorescence enhancing reagents, while ICP-OES is based on ‘plasma emission’ of uranium. The instrument, ULTIMA-2 is a Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) for the determination of concentration of elements in a sample [10]. Samples are mainly introduced in the instrument in a liquid form. As reported in the literature, ICP-OES was used earlier for the determination of uranium but only after solvent extractive separation of uranium from the matrix [11]. A rapid method for direct and accurate assay of uranium in mineralized rock samples, such as core, feed and ores is necessary for replacing the time consuming solvent-extractive conventional fluorimetric procedure. In the present work, ICP-OES has been used for the analysis of uranium in mineralized borehole core and ore samples.

EXPERIMENTAL Apparatus

Instrumentation: The method developed at this laboratory was studied and evaluated using the instrument, Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, ULTIMA-2 Horiba JY) [10]. The method developed at this laboratory was compared with the “Differential technique in Laser-induced More recently, differential technique in fluorimetry” [6-9] using the instrument, laser-induced fluorimetry [6-8] has been Scintrex UA-3 Uranium Analyzer (Scintrex recommended as a reference measurement Limited, Concord, Ontario, Canada) [12-14]. procedure for the determination of total uranium content in ores and similar matrices [9]. There ICP-OES Measurements: All measurement is a basic similarity between Inductively were made using the instrument, Inductively Coupled Plasma Optical Emission Spectrometer Coupled Plasma Optical Emission Spectrometer (ICP-OES) and Laser-induced fluorimetry, as (ICP-OES ULTIMA-2 Horiba JY). The 138

Determination of Core Samples: Comparative Studies Using Differential Techniques

optimum plasma emission spectrometer parameters and other operating conditions are given in Table 1. The instrument was calibrated using 1, 5 and 10 µg mL -1 U 3 O 8 solutions, prepared by dilution of the standard 1mgmL-1 U3O8 maintaining 10% HNO3. Aqueous standard U 3O8 (1 mg mL -1) stock solution: Aqueous standard stock solution of uranyl ion of 1mgmL-1 was prepared from U3O8 or uranyl nitrate of analytical-reagent grade having 10% HNO 3 .The concentration of uranium in this stock solution was verified using the method of Davies and Gray [15]. Fluorescence-enhancing reagent: An acidic buffer mixture of H3PO4 (1 M) and NH4H2PO4 Table 1. Operating Parameters of ICP-OES ULTIMA2 Horiba JY

Forward Power

1000 W

Reflected Power

<5W

Type of Generator

Solid state

Frequency

40.68 MHz

Coolant Gas Flow Rate

12.0 L/min

Sample Gas Flow Rate

0.91 L/min

Nebulisation Pressure

2.45 bar

Nebuliser

Concentric Glass

Monochromator

Modified CzernyTurner

Focal Length

1.0 m

Grating grooves

4320 gr/mm 2400 gr/mm

Order 1st order resolution

0.005 nm

Solution uptake

1 mL/min

(2.17 M) has been used as a fluorescenceenhancing reagent [6] Reference standards: International samples from CANMET (SY-2, SY-3, uranium ore, BL2a) and IAEA low grade uranium ore, Torbernite (Australia)—S1, Torbernite (Spain)—S2 and Carnotite (USA)—S3 were used as standard reference samples. The solutions of standard reference samples were prepared as per the prescribed procedure. Alternatively, 1, 5 and 10 µg mL -1 U 3 O 8 solutions were prepared by dilution of the standard 1mgmL -1 U 3 O 8 maintaining 10% HNO 3 . These solutions were used for calibration of the instrument. All other chemicals were of analytical-reagent grade. Distilled water was used throughout. Differential technique in laser-induced fluorescence (DT-LIF) procedure [6, 7]: With an Eppendorf micropipettor, take 100 µL aliquots of test samples and at least two reference standard solutions (take at least one higher reference standard containing ~0.01 mg mL -1 U 3O8) separately into 25 mL calibrated flasks (final concentration of U 3O8 up to 40 ng mL-1). Add 10 mL buffer mixture of phosphoric acid and ammonium dihydrogen phosphate as a fluorescence-enhancing reagent and dilute to the mark with distilled water. Adjust the sensitivity control of the instrument to give a scale deflection of 6–8 in the panel meter (i.e. more than half of the scale deflection of panel meter) with reference standard solution containing ~0.01 mg mL -1 U 3 O 8 in the low operational range of the instrument. Then, measure the fluorescence of the solutions prepared as above. Measure the fluorescence of the reagent blank prepared in the similar manner but containing no uranium. Compute the results using the differential technique as described earlier [6, 7] as follows

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(1) compute a factor for each of the two comparison standards, factor = concentration of U3O8 (mg mL-1) in comparison standard net fluorescence of comparison standard (2) calculate an average from the two factors, (3) compute the concentration of U3O8 (mg mL-1) in the original sample, U3O8 (mg mL-1) = average factor × net fluorescence reading of sample × dilution factor (4) compute the percent U3O8 in the sample, U3O8(mg mL-1) in the original sample × volume of sample × 100 U3O8 (%) = weight of sample (mg) Sample solution preparation procedure for rock samples: A sample solution of powdered rock sample (0.5 g, 150–200 mesh) is obtained by repeated evaporation with HF–HNO3 and then evaporation with HNO3 (to remove F-). The sample is digested with HNO3 and boiled for 15–20 min to get a clear solution. If the sample solution is not clear, a few drops of H2O2 are added and then boiled vigorously for a further 20–30 min to decompose the excess H2O2. If a little unattacked residue remains, it is filtered off, washed and brought into solution by sintering and fusing with a minimum amount of a mixture of sodium fluoride and potassium pyrosulphate. The melt, after cooling, is dissolved in nitric acid. The two solutions are mixed and made up to 100 mL in a calibrated flask. The final solution is in 10% HNO3.

RESULTS AND DISCUSSION The ranges of major, minor and trace element data reported in international samples from CANMET (SY-2, SY-3, uranium ore, BL-2a), IAEA-low grade uranium ores and the borehole core samples from Devri area, Surguja Distt, Chattisgarh, Vindhyan Surguja Gondwana Investigation, Central Region,AMD, Nagpur, Maharashtra, India have been examined in detail and analysed for their uranium content for the evaluation of the proposed ICP-OES method. Effect of high nitric acid concentration: A 10% nitric acid concentration in original sample solutions is sufficient to keep all metal ions in solution and also for additional dilutions. Selection of spectral emission line wavelengths and effect of foreign elemental interference: There are different emission lines of uranium listed in the library of the instrument [10] and also reported in the literature at, 409.010, 409.014,

Table 2. Detection limits (DLs) and background equivalent concentrations (BECs) of 10 µg/mL U3O8) in 10% (v/v) HNO3 with peak integration Time of 5 s ( n=10). Ip = Peak intensity; Ib = Background intensity.

a b

Wavelength (nm)

BEC (µg/mL)a

DL (µg/mL)b

Ip-Ib/Ib

409.010

860378.90

69242

0.877

0.026

11.42

409.014

790090.10

62113

0.850

0.025

12.62

409.016

864570.00

74862

0.947

0.028

10.54

BEC is calculated from: Concentration (µg/mL) x Ib /Ip-Ib DL is calculated based on three times the standard deviation of the blank at 1% RSD (DL = BEC x 0.03)

140

Determination of Core Samples: Comparative Studies Using Differential Techniques Table 3. Effect of varying amount of zirconium on uranium determination Uranium found, U3O8( µg mL-1) Uran Zir ium conium Added, (µg mL-1) U3O8 ( µg mL-1)

wave length 409.010 nm

RSD* %

% recovery

wave length 409.014 nm

RSD* %

% recovery

wave length 409.016 nm

RSD* % % recovery

5.0 5.0 5.0 5.0 5.0

0 2.738 5.476 10.952 27.391

5.156 5.087 5.000 4.929 5.085

0.84 0.31 1.16 1.25 1.15

103 101 100 98.5 101

5.158 4.996 4.897 4.872 5.059

0.07 2.69 0.06 1.29 0.02

103 99.9 97.9 97.4 101

5.092 4.941 4.842 4.800 5.035

0.03 0.86 0.31 0.12 0.84

102 98.8 96.8 96 100

5.0

52.17

4.583

1.58

91.6

4.566

0.54

91.3

4.566

1.32

89.8

* three replicates

409.016, 411.610, 424.167, 367.007, 385.465, 385.958, 398.579, 406.255 nm were examined [11]. There is no spectral interference of other elements on the emission line of uranium at 409.010 nm wavelength. At 409.014 nm and 409.016 emission wavelengths of uranium, as reported in the library of the spectrometer, there is a spectral interference of zirconium at 409.051 nm, but the differ ence in wavelengths are 0.037nm and 0.035nm, respectively. In the ICP-OES ULTIMA-2 Horiba JY spectrometer, 1 st order resolution of spectrum is 0.005 nm. Based on our

exper imental obser vations, ther e is no spectral interference of zirconium found by adding varying amount of zirconium up to 27 µg mL -1 in the determination of uranium at 5 µg mL -1. The recovery of uranium was found to be 100%, which corresponds to up to 0.54% zirconium concentration in the samples, based on 0.5 g core sample solution make up to 100mL volume basis, Table 2. At emission wavelength of uranium at 411.610 nm, there is spectral interference of thorium at 411.577 nm and vanadium, 411.647 nm. At emission wavelength of uranium at 424.167nm, there is

Table 4. Uranium values obtained by proposed method in some standard reference materials using ICP-OES at (409.014 nm). Sl. No

Samples

Proposed method, Uranium content (% U3O8)

1. 2. 3. 4. 5. 6. 7.

BL-2a SY-2 SY-3 IAEA S1 IAEA S2 IAEA S3 IAEA S4

0.5012 0.0332 0.0764 0.470 0.312 0.417 0.373

DT-LIF method(6,7,9) using Scintrex UA-3 Uranium Analyser (% U3O8) 0.5023 0.034 0.077 0.470 0.312 0.416 0.372 141

Certified value Uranium content (% U3O8) 0.5022 0.034 0.077 0.471 0.313 0.418 0.375

P. Murugesan and others

spectral interference of praseodymium, 424.130 nm and zirconium, 424.169 nm. At emission wavelength of uranium at 367.007nm, there is spectral interference of niobium, 367.005nm, thorium, 367.006 nm and nickel, 367.043 nm. At emission wavelength of uranium at 385.465 nm, there is spectral interference of cerium, 385.432 nm, thorium, 385.451 nm and lanthanum, 385.491 nm. At emission wavelength of uranium at 385.958 nm there is spectral interference of vanadium, 385.934 nm, thorium, 385.983 nm and lanthanum, 385.991nm. At emission wavelength of uranium at 398.579 nm there is spectral interference of holmium, 398.571 nm and lithium, 398.579 nm. At emission wavelength of uranium at 406.255 nm there is spectral interference of cerium, 406.222nm, gadolinium, 406.259 nm, copper, 406.270 nm, praseodymium, 406.282 nm, hafnium, 406.284 nm and cerium, 406.294 nm. In view of the above spectral interferences, uranium and other elemental concentrations, the analytical performance of the ICP-OES method has been evaluated for the determination of uranium at the following spectral line wavelengths at: 409.01, 409.014 and 409.016 nm. Operating parameters of plasma emission spectrometer (ICP-OES, ULTIMA-2 Horiba JY) are given in Table 2. Under standard operating conditions of the instrument, the spectral line at 409.014 nm shows better signal-to-background intensity ratio and is free from spectral interferences by matrix elements with a detection limit of 0.025 µg mL-1 U3O8. Reproducibility of the method: Under the optimized conditions, the reproducibility of the proposed method was checked by performing nine replicate determinations of 0.597, 0.140, 0.062 and 0.005 % U3O8 over a period of nine consecutive days. The relative standard deviation (RSD) of the method was 0.79, 0.80,

2.04 and 8.10 % respectively. Hence, the method appears to be reliable. Determination of uranium: In order to check the validity of the proposed method, the composition of mineralised silicate rock samples of diverse matrices were found to be in the ranges as given below: SiO2 49.95–83.71, TiO2 0.34–2.8, Al2O3 6.87 -13.43, Fe2O3 0.50– 4.45, FeO 0.20–10.73, MnO 0.01–0.24, MgO 0.07–3.57, CaO 0.10–6.17, Na 2O 0.13–2.91, K2O 0.96–5.90, P2O5 0.05–0.35 %, and in µg g1 Zr 125–5400 Sr 62–141, Rb 41–102, Ba 245– 1001, Y 20–25, Ni 41–60, Co 37–56, Cr 21–48, V 205–285, Cu 53–118, Pb 102–160, ThO 2 <150. These samples were analysed directly in a 0.5 g sample after 100 mL dilution by ICPOES after calibration of the instrument. The proposed ICP-OES method is suitable for the determination of more than 0.001 %U 3O 8 in silicate rock samples. Comparison of the proposed ICP-OES method with Differential technique in Laserinduced fluorimetric method (DT-LIF): More recently, differential technique in laserinduced fluorimetry has been recommended as a reference measurement procedure for the determination of total uranium content in ores and similar matrices [9]. Differential technique in laser-induced fluorimetry [6, 7, 9] and conventional fluorimetry were also applied to verify and compare the uranium content analysed using ICP-OES. A large number of samples have been analysed by these techniques. As two different techniques have been used to measure uranium content present in the same borehole samples, “t”-test for paired data [16] has been applied for comparing the results obtained by ICP-OES method on uranium emission line wavelength at 409.014 nm with those obtained by using ‘differential technique in laser-induced fluorimetry method’(DT-LIF) as a reference measurement procedure. The calculated value for “t” is 0.52,

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Determination of Core Samples: Comparative Studies Using Differential Techniques Table 5. Comparison of uranium content in borehole core samples.

S.No

ICP-OES 409.010 (U3O8%)

ICP-OES

DT-LIF as a

Conventional

409.014 nm

409.016 nm

reference

(U3O8%)

measurement procedure [6, 7, 9] using Scintrex UA-3 Uanium Analyser (U3O8%)

Pellet Fluorimetry (U3O8%)

0.043

0.044

0.043

0.045

0.043

0.0278

0.028

0.027

0.048

0.049

0.048

0.049

0.048

0.059

0.058

0.060

0.058

0.025

0.023

0.024

0.043

0.046

0.043

0.050

0.052

0.051

0.052

0.051

0.010

0.009

0.010

0.007

0.006

0.007

0.006

0.007

0.008

0.007

0.008

12.

0.597

0.596

0.600

0.611

13.

0.017

0.015

0.016

14.

0.011

0.012

0.010

0.011

15.

0.140

0.141

0.142

0.143

16.

0.029

0.028

0.029

0.032

0.029

17.

0.031

0.032

0.031

18.

0.045

0.046

0.045

19.

0.019

0.018

0.017

20.

0.066

0.067

0.066

0.065

which is less than the tabulated value of “t” for 20 borehole sample results (t=2.09 at 95% confidence level for n=20 results). This shows that the results of the measurements carried out without any chemical preparation or extraction by both the ICP-OES and ‘differential technique in laser-induced fluorimetry’ are not significantly different. The results of uranium content in these samples compare favorably with those obtained by using conventional fluorimetry.

Based on our investigations and keeping in view of the reported data in the literature, the ICP-OES measurement at 409.014 nm spectral line wavelength has been recommended for the accurate and rapid analysis of uranium in borehole core samples, ores and similar matrices without any chemical preparation or extraction. The details of our investigation results are summarized in Table 1- 5.

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CONCLUSION

6. Shrivastava A., Sharma J. and Soni V. (2013). Bulletin Facult Pharmacy Cairo Univ, v. 51, The simplicity, rapidity, freedom from pp. 113-129. matrix effects, no separation steps, minimum generation of radioactive analytical waste, 7. Rathore D. P. S., Tarafder P. K., Kayal M. and maximum throughput, and high metrological Kumar M., (2001). Analytica Chimica Acta, quality are the significant features of the proposed v. 434, pp. 201-208. alternative ICP-OES instrumental method for the 8. Rathore D. P. S. and Kumar M., (2004). Talanta, analysis of uranium at 409.014 nm spectral line v. 62, pp. 343-349. wavelength for mineralized silicate rocks, ores, the borehole core samples, beneficiation products, 9. Rathore D. P. S. (2007). EARFAM, v. 17, pp.145-149. certification of reference materials and other diverse applications in nuclear fuel cycle. The ICP- 10. Rathore D. P. S., Kumar M. and Tarafder P. OES results obtained for the measurement of K., (2012). Accred. Qual. Assur., v. 17, pp. uranium in these diverse geological samples 75-84. compare favorably by using differential technique in laser-induced fluorimetry, as a ‘reference 11. ULTIMA-2 User Manual, HORIBA Jobin Yvon plasma emission spectrometer (ICPmeasurement procedure’. OES).

ACKNOWLEDGEMENT Authors thank Mr. P. S. Parihar, Director, AMD, for his kind permission to publish this work.

REFERENCES

12. Satyanarayana K. and Nayeem M. A. (2002)., Atomic Spectroscopy, v. 23, pp. 77-85. 13. Robbins J. C., (1978). CIM Bull. v. 71, pp. 61-67.

1. Analytical Techniques in Uranium Exploration 14. Robbins J. C. and Kinrade J. D., (1980). United and Ore Processing, Technical Reports Series States Patent, patent no. 4, 239, 964. no. 341, IAEA, Vienna, 1992, Chapters 3-4 15. Robbins J. C., Castledine C. and Kostiak W., and references cited there in. (1985). Analytical Procedures for UA-3 2. Rathore D. P. S. (2008), Talanta, v. 77, pp. 9Uranium Analysis - Applications Manual, 20. Scintrex Ltd., Ontario, Canada. 3. Hou X. and Roos P. (2008), Analytica Chimica 16. Davies W. and Gray W., (1964). Talanta v. 11, Acta, v. 608, pp. 105-139. pp.1203-1211. 4. Santos J. S., Teixeira L. S., dos Santos W. N., Lemos V. A., Godoy J. M. and Ferreira S. L. 17. Vogel’s Textbook of Quantitative Chemical Analysis, London, 6th edn., (2004). pp. 131(2010). Analytica Chimica Acta, v. 674 pp. 132. 143-156.

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