Determination of yttrium, scandium and other rare earth elements in uranium-rich geological materials by ICP–AES

Talanta 46 (1998) 533 – 540

Determination of yttrium, scandium and other rare earth elements in uranium-rich geological materials by ICP–AES G.V. Ramanaiah * Chemical Laboratory, Atomic Minerals Di6ision, Department of Atomic Energy, Begumpet, Hyderabad-500 016, India Received 6 March 1997; received in revised form 25 August 1997; accepted 28 August 1997

Abstract A rapid method is described for the determination of yttrium, scandium, and other rare earth elements (REEs) in uranium-rich geological samples (containing more than 0.1% U) and in pitch blende type of samples by inductively coupled plasma atomic emission spectrometry (ICP – AES) after separation of uranium by selective precipitation of the analytes as hydroxides using H2O2/NaOH in the presence of iron as carrier. Uranium goes into solution as soluble peruranate complex. The precipitated rare earth hydroxides (including Y and Sc) are filtered and dissolved in hydrochloric acid prior to their aspiration into plasma for their individual estimation after selecting interference free REE emission lines. The method has also been applied to some international reference standards like SY-2 and SY-3 (by doping a known amount of uranium) along with one in-house pitch blende sample and the REE values were found to be in agreement with the most usable values, offering an R.S.D. of 1 – 8.8% for all the REEs’, Y and Sc. The method compared well, with the well- established cation exchange separation procedure. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Rare earth elements; Yttrium; Scandium; Geological materials; Inductively coupled plasma atomic emission spectrometry

1. Introduction The determination of the rare earth elements in silicate rocks and their constituent minerals is of fundamental importance to modern petrological studies on the origin of igneous, metamorphic and sedimentary rocks. The processes by which rocks have formed have been greatly helped, by a knowledge of the rare earth elements (REE) con* Tel.: +91 40 844101; [email protected]

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tents, and accurate determination of the concentrations of these elements in a wide variety of rocks has formed the basis for much research effort by geochemists in recent years. The comparative behaviour of the rare earths together with major elements, trace elements and selected isotopic concentrations are the most widely used data recorded in the recent geochemical literature. The use of U, Th in nuclear reactors places stringent demands on the levels of impurities and about 35 trace elements are listed in the C787 [1] specifications issued by ASTM for UF6 and high

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purity ThO2. Therefore, some of the REEs’ like Sm, Gd, Dy, Tb, Er, and Tm, if present in uranium are harmful to the efficient functioning of a nuclear reactor, even in sub microgram level due to their large neutron absorption cross-section. Furthermore, the determination of REEs’ is important due to an increasing demand in the industrial and metallurgical fields. Accurate determination of the REEs’ in uranium- rich geological materials, as well as in nuclear grade uranium, is therefore, essential prior to its use as a nuclear fuel at different stages. In case of high uranium-bearing geological samples a preliminary separation of the REEs’ is to be carried out to avoid spectral, as well as the matrix interferences of uranium on Sm, Gd, Tb, Er and Tm. Furthermore, uranium being a line rich emitter in inductively coupled plasma (ICP)source, emits more than 1500 lines [2] thereby interfering on almost all emission lines of the studied elements. Therefore, uranium has to be separated from the trace REE as soluble peruranate complex by treating the sample solution with H2O2 in the presence of NaOH prior to their quantification by inductively coupled plasma atomic emission spectrometry (ICP – AES). Iron is used as carrier substrate for trace rare earth hydroxides during the separation of uranium. A number of instrumental methods are now available for accurate determination of REEs’ at various concentration levels viz. instrumental neutron activation analysis (INAA) [3], atomic absorption spectrometry (AAS) [4], spectrophotometry [5], X-ray fluorescence spectrometry (XRF) [6], DC arc emission spectrography [7], electron microprobe analysis (EMPA) [8], ICP–AES [9 – 13] and inductively coupled plasma mass spectrometry (ICP – MS) [14]. ICP – AES has now been established as a powerful analytical technique for the estimation of REEs’ in the presence of high amounts of matrix elements preferably after separation of major elements by cation exchange column chromatography [15,16], solvent extraction [13] from uranium before quantification. Several workers have studied the applications of ICP – AES for the determination of REEs’ in high uranium bearing samples [13,17– 19] after chemical separation.

The present investigation reports separation of REE from U-dominant geological samples and subsequent estimation using ICP–AES. Uranium is separated as soluble peruranate complex formed using NaOH and H2O2 while REEs’ are removed as insoluble hydroxides in the presence of ‘Iron’ as carrier. Furthermore, the proposed separation technique is very simple, rapid and without the need of costlier ion exchange resins. The method can be applied on a routine basis for such type of samples without adhering to any stringent operating conditions of acidity, elution rate, aqueous-to-organic volume ratio, etc. with more sample throughput, unlike in other cases of separation methods like ion exchange/solvent extraction technique.

2. Experimental

2.1. Instrumentation All measurements were made using a ICP–AES model-8410 (GBC, Australia) plasmascan spectrometer, employing a computer controlled rapid scanning monochromator with the operating parameters presented in Table 1. The readings were obtained using a single point calibration method by calibrating the instrument with 1 mg/ ml for, Sc, Eu, Yb, and Lu and 10 mg/ml for Y and the rest of the REE. The detection limits obtained under the above optimised conditions Table 1 ICP – AES instrumental parameters and operating conditions R.F. generator Forward power Reflected power Observation height PMT voltage Integration time Entrance slit Exit slit Sample flush time Argon gas flow rates: Coolant Auxiliary Sample Solution uptake rate

27.12 MHz, crystal controlled 1200 W B5 W 14 mm above load coil 1000 V 3 s (n =3) 20 mm and 3 mm height (fixed) 40 mm 10 s 14 l/min 1.0 l/min 0.8 l/min 3.0 ml/min

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Table 2 Analytical results of REEs Y, Sc in in-house pitch blende type of sample as per procedure (data mg/g) Element

Sc Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Wavelength (nm)

361.384 371.030 333.749 418.660 422.293 430.358 442.434 381.967 364.619 350.917 353.170 345.600 349.910 346.220 328.937 261.542

Pitch blende a (P.M.)

b (% R.S.D.)

c (I.E)

5.2 447 740 2582 422 1640 398 39 301 14 100 9 11 B5 6 1.7

5.8 1.1 1.2 1.2 1.1 1.2 1.0 3.8 1.7 5.7 1.5 4.5 4.0 — 6.3 8.8

6.0 440 750 2572 425 1677 398 39 304 15 102 11 10 B5 8 B5

(a) P.M, present method (average of five values). (b) % R.S.D. by the present method. (c) I.E, ion exchange method [15].

are in the range of 0.00014 mg/ml in case of Lu (261.542 nm) to 0.0099 mg/ml for Pr (422.293 nm), as reported [20] in an earlier publication from our laboratory.

2.2. Chemicals All single element REE, Y, Sc (as oxides) 1.0 mg/ml standard stock solutions are made by dissolving Spec Pure rare earth oxides in hydrochloric acid, except CeO2 (dissolved in a mixture of HNO3 and H2O2 and later converted to HCl medium. Single element working standards were prepared (10 mg/ml) by dilution of the stock solution with 0.5 M HCl. Acids and all other chemicals used are of analytical reagent grade.

2.3. Procedure 2.3.1. Uranium rich samples (pitch blende) and other geological samples (U-content 0.1% and abo6e) Rock sample, 1.0 g, was weighed into 100 ml beaker and treated with 10 ml of 1:2 (HNO3:HCl)

mixture and evaporated to dryness on water bath (90°C). The process was repeated, and digested with 50 ml of 0.5 M HCl and filtered. The residue, after ignition in a platinum crucible, was treated with HF and then with HCl, finally fused with 1 g of Na2CO3 and dissolved in 0.5 M HCl solution. Both filtrate and residue solutions were combined. Ammonium chloride/ammonium hydroxide precipitation was carried out and if no precipitate was observed, 5 mg of iron solution was added as carrier. The resultant precipitate, after filtration, dissolved in a minimum quantity of HNO3 and evaporated to dryness in a 100 ml beaker. The residue was digested with 5 ml of 1:4 HNO3 and treated with 5 ml of H2O2 (100 vol) and 20 ml of NaOH (12.5 M) solution and digested in the cold with another 30 ml of distilled water and kept aside for 1 h to allow the precipitate to settle. The precipitate was filtered and washed with 1 M NaOH and 1% (v/v) H2O2 solution to make it free from uranium. The process repeated again to decontaminate uranium from the rare earth hydroxides. Finally, the precipitate was transferred from filter paper into a beaker with a few drops of

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Table 3 Analytical data for Y, Sc, and REEs’ in CCRMP standard reference sample SY-2, (Syenite) (data in mg/g) Element

Sc Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Wavelength (nm)

361.384 371.030 333.749 418.660 422.293 430.358 442.434 381.967 364.619 350.917 353.170 345.600 349.910 346.220 328.937 261.542

SY-2 a (P.M)

b (% R.S.D.)

c (I.E)

d (R.V)

e (R.V)

7.31 120 80 166 19 75 18 3 15 4 19 3.3 14 3.3 18 3.3

5.5 1.25 1.25 1.20 6.3 1.3 6.1 7.3 6.6 6.2 6.3 6.1 7.8 6.1 5.5 6.1

7.0 116 75 164 19 74 18 2.7 14 3.8 21.8 4.3 14.2 2.6 18.8 2.8

7.0 128 75 175 18.8 73 16.1 2.42 17 2.5 18 3.8 12.4 2.1 17 2.7

— 126 91 73 91 164 92 21 90.5 80 92 16.5 90.5 2.6 90.1 17 91 3 90.5 21.3 90.3 5 90.2 15.4 90.4 2.6 90.2 18.4 90.5 3.1 90.1

Sample solutions were doped with 200 mg of uranium and the decontamination for U was effected using the recommended procedure. (a) P.M, present method (mg/g) (average of five values). (b) % R.S.D., by the present method. (c) I.E, ion exchange method (mg/g) [13]. (d) R.V, recommended value [23]. (e) R.V, recommended value [24].

concentrated HCl and distilled water and dissolved. The solution was made up to 100 ml in 0.5 M HCl. The estimation of REEs’, Y, Sc was carried out at the wavelengths listed in Table 2, Table 3, Table 4, and Table 5 using operating parameters presented in Table 1. The above separation procedure was tested by adding a known amount of U (200 mg) to CCRMP standards like SY-2 and SY-3. The method was applied to pitch blende sample and to geological samples. The geological sample solutions were doped with 200 mg of uranium and the decontamination of U was effected using the above recommended procedure. The results obtained along with their % R.S.D. are presented in Table 2, Table 3, Table 4 and Table 5 to validate the efficacy of the proposed procedure. The major matrix element composition of the inhouse pitch blende sample is presented in Table 6.

3. Results and discussion Calcium, if present, in the sample solutions was found to adsorb uranium while separating uranium as peruranate with NaOH and H2O2 and incomplete separation of uranium was observed. For this reason calcium was separated at the stage of solution preparation by carrying out NH4Cl/ NH4OH precipitation. This also removes other matrix elements, e.g. Mg including Na from the Na2CO3 flux. Fig. 1(a) shows the chondrite normalized (C1) plot in respect of REEs’ present in the pitch blende sample. X(CN) represents the REE value in the sample divided by the respective REE value of chondrite [21]. The ratios of individual REEs’ were plotted against the respective at. no. of the element. REE concentrations in rocks or minerals are usually normalized to a common reference standard, which most commonly comprises the values for chondritic meteorites. Chondritic mete-

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Table 4 Analytical data for Y, Sc and REEs’ in CCRMP standard reference sample SY-3 (Syenite) (data in mg/g) Element

Sc Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Wavelength (nm)

361.384 371.030 333.749 418.660 422.293 430.358 442.434 381.967 364.619 350.917 353.170 345.600 349.910 346.220 328.937 261.542

SY-3 a (P.M)

b (% R.S.D.)

c (I.E)

d (R.V)

e (R.V)

7 710 1326 2239 233 696 146 18 100 17 130 18 82 12.7 63.4 9.7

5.5 1.1 1.1 1.3 1.3 1.1 1.4 5.5 1.5 6.5 1.1 5.4 1.5 8.7 1.6 5.2

— 698 1360 2237 238 764 145 19.6 112 24 131.6 27.8 85 12.5 68.7 8.3

6.8 718 1340 2230 223 670 109 17 105 18 118 29.5 68 11.6 62 7.9

— 715 94 1350 2250 940 239 760 9 4 134 9 2 18.3 9 0.4 121 91 15 90.2 138 92 29 9 0.5 88 91 12.5 90.5 71 91 8.6 90.2

Sample solutions were doped with 200 mg of uranium and the decontamination of U was effected using the recommended procedure. (a) P.M, present method (average of five values) (mg/g). (b) % R.S.D., by the present method. (c) I.E, ion exchange method (mg/g) [13]. (d) R.V, recommended value [23]. (e) R.V, recommended value [24].

orites were chosen because they are thought to be relatively unfractionated samples of the solar system dating from the original nucleosynthesis. However, the concentrations of REE in the solar system are very variable because of the different stabilities of the atomic nuclei. REE with even atomic numbers are more stable and, therefore, more abundant than the REE with odd atomic numbers, producing a zig-zag pattern on a composition abundance diagram. Therefore, chondrite normalization (i.e. the REE value in the sample divided by the respective REE value of chondrite, X(CN)) has two important functions: 1. it eliminates the abundance variation between odd and even atomic number elements; and 2. it allows any fractionation of the REE group relative to chondritic meteorites to be identified like in the case of the Eu- or Ceanomaly.

The anomaly is positive if (X(CN) } X* (CN))\1 where X* (CN) is the extrapolated value between the two successive points in the chondrite- normalised plot, and the anomaly, is negative, if the ratio B 1, indicating oxidizing or reducing conditions or the environment, respectively, during the paragenesis of various crystallising phases. The value of Y(CN) was plotted between those for Dy(CN) and Ho(CN) because of the similarity in properties and ionic radii of the three elements. The crustal abundance [22] of yttrium, Fig. 1(b) shows an excellent agreement with the interpolated value. The REE pattern with a striking negative Euanomaly indicates that the studied pitch blende was formed either from Eu-depleted solutions or under a reducing environment during its paragenesis from the solidifying magma. In geological samples containing high uranium (\ 1000 ppm) due to its line rich emission spectra interferes on all the REE ion emission lines and although alternate (interference free) lines could

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Table 5 Analytical results for REEs, Y, Sc and REEs’ in geological samples (data mg/g) Sample 4595 a b c 4596 a b c 4597 a b c 4598 a b c 4605 a b c 4606 a b c

Sc

Y

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

4.8 4.2 4.9

206 1.4 210

277 1.4 264

461 1.1 471

52 1.3 54

206 1.40 207

42 2.4 45

1.8 8.0 2.6

32 1.9 33

6 8.3 6.5

36 3.0 38

6 8.3 7

24 5.8 26

3.2 7.0 3.5

22 3.6 24

2.7 6.5 3.5

1.3 7.1 1.4

366 1.1 379

70 1.4 69

115 1.0 117

9 3.3 10

62 2.2 65

24 4.2 26

1.2 8.6 1.6

28 2.9 29

10 5 9

64 1.9 69

14 4.2 16

58 1.4 61

10 5 9.5

60 1.2 63

8 6.0 8.6

20.3 4.0 20.2

68 1.2 69

111 1.8 106

209 1.0 212

16 6.2 18

84 1.4 86

19 6.3 21

1.7 7.0 2.6

14 3.6 15

8 5 8.5

12 4.2 14

B3

12 4.2 10

3.3 7.2 B3

11 4.5 10

1.7 7.0 1.5

3.3 6.1 3.8

62.5 1.5 65

68 1.6 74

111 0.8 122

10 7.2 12

44 3.2 46

10 6.0 12

B1

6 6.7 B10

B5

10 6 11

B3

6 6.7 7

B3

8.5 5.9 8

1 8 1

3.5 5.7 4.1

53.5 1.6 56

140 1.4 138

216 1.0 217

19 5.8 22

74 1.6 78

15 5.3 12

B1

10 6 11.5

B3

B3

5 6.0 5

B1

B3

4 6.8 5

2.6 6.9 3.5

79 1.4 80

56 2.7 50

49 2.4 51

2.2 9.1 B5

18 6.1 21

6 8.3 B10

B1

11 5.4 13

B3 5.0 B3

8 5.0 9

B3

9 4.7 10

1.3 7.8 1.6

B1

1

B1

2

9 5.6 11

B5

B5 — B5

B5

B5

B5

B3

B3

B3

B3

B3

B1

Wavelength (nm) of elements used for measurements: Sc (361.384), Y (371.030), La (333.749), Ce (418.660), Pr (422.293), Nd (430.358), Sm (442.434), Eu (381.967), Gd (364.619), Tb (350.917), Dy (353.170), Ho (345.600), Er (349.910), Tm (346.220), Yb (328.937), Lu (261.542). Sample solutions were doped with 200 mg of uranium and the decontamination of U was effected using the recommended procedure. (a) Present method (average of five values, mg/g) (b) % R.S.D. by the present method. (c) Ion exchange method [15].

be selected for some of the REE, extreme care has to be exercised in selecting the line for Sm, Tb, Er, and Tm. Since the correction factors due to inter REE interferences on some of the alternate emission lines do not follow the additive principle and Table 6 Major matrix element composition of in-house pitch blende sample (data in wt%) U3O8 PbO SiO2 Total Fe } As Fe2O3 } P2O5

74.10 1.02 15.10 1.18 4.67

the spectral bias by uranium is also not always additive, a separation of the REE from uranium has been carried out by the above procedure. Separation of rare earths from high concentration of matrix elements and trace elements in the studied geological materials, including the pitch blende sample and uranium doped CCRMP certified reference standards SY-2 and SY-3 has also been carried out using the often-recommended cation exchange [15,16] separation method. The results obtained by both the methods are in very good agreement in case of all the studied samples including the certified reference standards, SY-2 and SY-3. The method is simple and rapid and easy to operate on a routine basis. The proposed

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539

of the REE emission lines. In the present system, the U was efficiently complexed by H2O2 in presence of NaOH and all the analytes are precipitated quantitatively as hydroxides by using iron as the carrier before dissolving in hydrochloric acid for quantification by ICP–AES. The proposed separative preconcentration method can be applied successfully to different type of U-rich samples like uraninite, pitch blende, brannerite, etc. on a routine basis.

Acknowledgements The author wishes to thank colleagues of the laboratory and Dr K.K. Dwivedi, Director, AMD for his encouragement and permission to publish the paper.

References

Fig. 1. (a) Chondrite-normalized REE and Y pattern for pitch blende [21]. (b) Chondrite-normalized REE and Y pattern for crustal abundance [22].

method offers an R.S.D. of 1.0% at the 398 mg/g level in the case of Sm and 8.8% at 1.7 mg/g level in the case of Lu.

4. Conclusion The determination of REE, Y, Sc and Th in several U-rich ( \0.1%) geological materials including the pitch blende type of samples has been successfully accomplished by ICP-AES, after separating the analytes from U-matrix by complexing uranium with NaOH/H2O2 system and the values are in very good agreement with the reported results. The method is simple, rapid, precise and accurate and has been applied to the certified reference standards, SY-2 and SY-3 (after doping with huge amount of U). In U-rich geological materials, it is important to separate the analytes from U-matrix before estimating by ICP–AES, due to its several spectral interferences on many

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