Determination of trace elements in some copper minerals by k0-neutron activation analysis

Applied Radiation and Isotopes 70 (2012) 35–39

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Determination of trace elements in some copper minerals by k0-neutron activation analysis M. Taseska a, R. Jac´imovic´ b,n, V. Stibilj b, T. Stafilov a, P. Makreski a, G. Jovanovski a a b

Institute of Chemistry, Faculty of Science, SS. Cyril and Methodius University, P.O. Box 162, MK-1001 Skopje, Macedonia Department of Environmental Sciences, Jozˇef Stefan Institute, P.O. Box 3000, SI-1001 Ljubljana, Slovenia

a r t i c l e i n f o

abstract

Article history: Received 27 January 2011 Received in revised form 10 July 2011 Accepted 14 July 2011 Available online 23 July 2011

Trace element contents in two copper minerals [brochantite [Cu4SO4(OH)6] and native Cu] using k0-NAA were determined before and after quantitative removal of copper. The distribution of 44 elements in the studied minerals was investigated. An important advantage of the proposed method is the possibility to determine the content of several elements (Al, Dy, Mg, Mn and V) via their short-lived nuclides after the electrolytic removal of Cu due to the elimination of matrix interferences. & 2011 Elsevier Ltd. All rights reserved.

Keywords: Copper minerals Trace elements Electrolysis k0-NAA

1. Introduction Copper was one of the earliest metals used by man, dating from about 8000 BC. Its abundance, attractive colour, ease of working and resistance to corrosion made it an ideal material for tools, utensils and weapons, especially when alloyed with tin as bronze. Copper minerals are common and modern civilization is heavily dependent on copper and its products. Copper ores can be found in large deposits, relatively close to the surface, and amenable to relatively low cost bulk (open cast) mining methods. Copper occurs occasionally as native Cu and in many minerals, particularly as sulphides (chalcopyrite, bornite, chalcocite and covellite), sulphosalts (enargite), sulphates (chalcanthite, brochantite), oxides (cuprite) and carbonates (malachite and azurite) (Christie and Brathwaite, 2010). Copper is a chalcophile element, and most ores consist of sulphides, usually associated with sulphides of lead and zinc. Native copper is a relatively rare mineral, being mostly found either in minerals associated with sulphur or in their oxidized products. There are a limited number of studies concerning the determination of elements in copper-based geological samples by different methods, such as atomic absorption spectrometry (AAS) (Benzaazoua et al., 2002; Taseska et al., 2005; Tanaka et al., 1993), atomic emission spectrometry with inductively coupled plasma (ICP-AES) (Benzaazoua et al., 2002; Li et al., 2005) or laser

n

Corresponding author. Tel.: þ386 1 5885 353; fax: þ 386 1 5885 346. E-mail address: [email protected] (R. Jac´imovic´).

0969-8043/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2011.07.009

ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) (Huminicki et al., 2005). Non-destructive instrumental methods (INAA, XRF, PIXE) are rarely used to determine trace elements in minerals due to matrix and interelement interferences and background effects (Varadi et al., 1986; Todorov, 1991; Frantz et al., 1994; Raimbault et al., 1997; Sharara et al., 1999; Li et al., 1997; 2005; Jac´imovic´ et al., 2002, 2003, 2005, 2007, 2008; Necˇemer et al., 2003; Safilov et al., 2005; Zhou et al., 2005; Makreski et al., 2008). In this work, the k0-method of neutron activation analysis (k0-NAA) was used for direct and simultaneous determination of trace elements in both powdered minerals and dissolved samples after Cu removal by electrolysis. In this investigation, two copper minerals originating from the Republic of Macedonia (brochantite and native Cu) were analysed.

2. Experimental 2.1. Samples The brochantite specimen originated from the Sasa mine, whereas native copper was collected from Bucˇim (both are active mines in the Republic of Macedonia). Mineral specimens were carefully hand-picked under an optical microscope from the ore samples and ground to powder. CuSO4  5H2O (chemical no. 102790, p.a., Merck, Darmstadt, Germany) and multielement standard sample (no. 109487, p.a., Merck, Darmstadt, Germany) containing 100 mg/L (1 L¼ 1.0313 kg)

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M. Taseska et al. / Applied Radiation and Isotopes 70 (2012) 35–39

1.2

2.2. Sample preparation (Cu removal)

1.1

Brochantite: The powdered sample (0.1 g) was dissolved in 10 mL redistilled water in a 250 mL glass beaker. Afterwards, 2 mL of concentrated H2SO4, 1 mL concentrated HNO3 and redistilled water up to 100 mL were added before electrolysis. Native Cu: The natural sample (0.1 g) was dissolved in 2 mL of concentrated HNO3 by heating on a hot plate. The solution was evaporated to dryness and the residue redissolved in 2 mL of concentrated H2SO4,and 1 mL concentrated HNO3 and made up with redistilled water to 100 mL before electrolysis. A Pt electrode was employed for copper removal by electrolysis for 45 min at 2 V and 3–4 A (Taseska et al., 2005). Afterwards, the solution was evaporated to about 2 mL, transferred to a polyethylene ampoule by adding 2 mL redistilled water and the ampoule was sealed. 2.3. k0-neutron activation analysis (k0-NAA) About 100 mg of powdered sample was sealed into a pure polyethylene ampoule (SPRONK system; Lexmond, The Netherlands). A sample and a standard (Al–0.1%Au IRMM-530R disc 6 mm in diameter and 0.2 mm high) were stacked together and fixed in the polyethylene ampoule in sandwich form and irradiated for 20 h in the carousel facility (CF) of the Institute’s TRIGA reactor at a thermal neutron flux of 1.1  1012 cm  2 s  1. The gamma activities of irradiated powdered samples were measured after 6, 14 and 40 days cooling time on absolutely calibrated HPGe detectors (Ortec and Canberra, USA; Smodiˇs et al., 1988). Measurements were performed at such distances that the dead time was kept below 10% with negligible random coincidences. An HPGe detector with 40% relative efficiency was connected to a CANBERRA S100 multichannel analyzer, while a detector with 45% relative efficiency was connected to an EG&G ORTEC Spectrum Master high-rate multichannel analyzer. The electrolysed solution (about 4 mL, see above) in a polyethylene ampoule and a standard were fixed together and irradiated for 15 s (short irradiation) in the pneumatic tube (PT) of the TRIGA reactor at a thermal neutron flux of 3.5  1012 cm  2 s  1. After irradiation the polyethylene ampoules were measured twice after 2 min and after 2 h cooling time on the absolutely calibrated HPGe detectors. This allowed determination of elements via radionuclides with relatively short/intermediate half-lives (a few minutes up to some hours). Then, after an appropriate cooling time (8–10 days) the same ampoules were re-irradiated for 20 h (long irradiation) in the CF of the TRIGA reactor. After this long irradiation the ampoules were opened and the solutions were transferred to clean polyethylene ampoules and measured on the absolutely calibrated HPGe detectors after 2–3, 8–10 and 30 days cooling times. The HyperLab (HyperLab 2002 System, 2002) program was used for net peak area evaluation. The ’’Cd-ratio’’ method for multimonitor was applied to determine f (thermal to epithermal flux ratio) and a (epithermal flux deviation from the ideal 1/E distribution) parameters (Jac´imovic´ et al., 2003). The values f¼28.6 and a ¼–0.001 were used to calculate the element concentrations for samples irradiated in the CF and values f¼28.0 and a ¼ –0.015 for samples irradiated in the PT. The software package Kayzero for Windows (User’s Manual Kayzero for Windows, 2005) was employed for elemental concentrations and effective solid angle calculations. The k0-NAA technique was validated by certified reference materials and reference materials with inorganic matrices and good agreement was found between the obtained and certified

Ratio

of trace elements (As, Cd, Cr, Co, Cu, Fe, Mn, Mo, Sb, Se, Sr, Ti, V and Zn) were used for method optimization.

+ 10 %

1.0

0.9

- 10 % BC-R320R

k0-NAA

0.8 As

Co

Cr

Fe

Hg

Sc

Th

U

Zn

Elements Fig. 1. Comparison of k0-NAA data to certified values for BCR-320R Channel Sediment (error bars for k¼ 2).

values (Jac´imovic´ at al., 2003; Makreski et al., 2009). Additionally, in this work for QA/QC purposes for k0-NAA certified reference material BCR-320 R Channel Sediment was used and results are presented in Fig. 1. In this case k0-NAA data for As, Co, Cr, Fe, Hg, Sc, Th, U and Zn were inside the 95% confidence interval (k¼2) of certified values and also inside 710%.

3. Results and discussion The copper radionuclides 64Cu (T1/2 ¼12.7 h with gamma energies Eg ¼ 511.0 keV and Eg ¼1345.9 keV) and 66Cu (T1/2 ¼ 5.12 min with gamma energy Eg ¼1039.2 keV) are present in the gamma spectrum (De Corte and Simonits, 2003) in the case when INAA is applied to copper matrix element samples. Since the Cu nuclides are moderately activated, they interfere with the determination of many trace elements. In order to avoid these interferences, the elimination of copper from the matrix and separation of trace elements before their determination by k0-NAA is essential. Therefore, the method of electrolysis to remove Cu was used in order to avoid interferences in k0-NAA determination of trace elements in the selected copper minerals. To check which other elements undergo electrolysis (besides Cu being effectively removed, 499%), dissolved copper(II) sulfate [Merck, (CuSO4  5H2O, p.a.)] was spiked with a multielemental standard solution containing 13 trace elements (As, Cd, Cr, Co, Fe, Mn, Mo, Sb, Se, Sr, Ti, V and Zn) and these elements were analyzed after electrolysis (Table 1). Three different amounts of the multielement standard were spiked (50, 100 and 200 mg of each element). As can be seen from the results given in Table 1 all investigated trace elements remain in solution after electrolysis, except for selenium, even though their concentrations are higher than those usually present in minerals. Losses of Se may be expected during evaporation of the solution to 2 mL because the dissolved sample was heated to about 80 1C. Additionally it is evident that the solution was contaminated by Zn during electrolysis, not allowing determination of low contents of this element in the sample analysed. The results for the chemical composition of the copper minerals obtained by k0-NAA with their combined standard uncertainty with a coverage factor k¼1 (considering net peak area, nuclear data for a particular nuclide, neutron flux parameters, full-energy peak detection efficiency, etc.) are given in Tables 2 and 3. The power of k0-NAA combined with the reduced copper activity produced by the electrolysis enabled the determination of thirtyfive elements in brochantite and thirty-two in native copper (powder and solution).

M. Taseska et al. / Applied Radiation and Isotopes 70 (2012) 35–39

Table 1 Results obtained by k0-NAA after Cu removal by electrolysis from a CuSO4  5H2O (chemical no. 102790, p.a. Merck, Germany) solution with addition of multielement standard solution (no. 109487, p.a. Merck, Germany). El.

As Cd Co Cr Fe Mn Mo Sb Se Sr Ti V Zn

Recovery range (%) (Added 50 mg)

n

(Added 100 mg)

n

(Added 200 mg)

n

110, 112, 107 116, 119, 107 99, 101,93 98, 99, 92 100, 102, 85 91, 88 114, 114, 105 101, 103, 94 11, 12 97, 111, 111 n.d. 92, 92 240, 109, 99

3 3 3 3 3 2 3 3 2 3

109, 102 98, 101 99, 92 98, 91 111, 104 88 116, 103 96, 86 8, 5 135, 135 n.d. 91 105, 91

2 2 2 2 2 1 2 2 2 2

100, 93, 103 103, 96, 112 91, 89, 110 75, 88, 110 104, 96, 122 84, 90, 84 103, 96, 112 79, 90, 88 1, 15, 6 119, 98, 97 95, 97, 92 89, 90, 94 102, 106, 128

3 3 3 3 3 3 3 3 3 3 3 3 3

2 3

1 2

n—Number of replicate electrolyses, n.d.—No data.

37

A comparison of the contents of the investigated elements showed that the contents of As, Fe, K, Na, Se, U and rare-earth elements (REEs) in brochantite in powder form are higher compared to other investigated trace elements (Table 2). In the case of native Cu, most of the investigated elements (except Fe) are present in the order of a few mg kg  1 (Table 3). Comparison between the results obtained by k0-NAA after electrolysis and in powdered samples of brochantite and native copper is presented in Tables 2 and 3. After complete dissolution of the minerals, the removal of Cu was more than 99%. It was shown that the content of REEs and some others elements (Ca, Mg, Mn, V) in brochantite is lower than in chalcanthite (CuSO4  5H2O), even though both minerals are sulphates (Taseska et al., 2010). The ratio of the content of the REEs (Ce, Eu, La, Nd, Sm, Tb and Yb) in the powdered sample to that after copper removal in brochantite (Fig. 2) is near 0.9, indicating that these elements are partly removed by electrolysis. This result was not evident for the native copper sample, because the content of REEs is very small (Table 3). It is also important to point out that after the electrolysis of Cu,

Table 2 Results obtained by k0-NAA for powdered and for dissolved samples of brochantite (Cu4SO4(OH)6) from Sasa. All results are in mg kg  1. El.

Powder

Electrolysis Aliquot 1

Ag Al As Au Ba Br Ca Cd Ce Cl Co Cr Cs Cu Dy Eu Fe Ga Hf Hg K La Mg Mn Mo Na Nd Pt Rb Sb Sc Se Sm Sr Ta Tb Th Ti U V W Yb Zn Zr

Content

Unc.c

Content

0.71 n.a. 70.7 0.187 o7 2.54 o 448 o3 7.82 n.a. 1.08 6.68 0.36 457317 n.a. 0.95 963 o 24 0.112 o 2.7 2010 5.94 n.a. n.a. o2 106 9.25 n.a. 11.8 0.28 0.16 17.9 6.08 o 17 o 0.02 1.20 2.05 n.a. 126 n.a. 2.62 1.94 2.73 o 14

0.04

o 0.52 10313 61.9 0.017d o 23 0.57d o 312 o 4.7 5.51 67.0 1.70d 4.30 0.49 6242 5.79 0.74 931 7.12 o 0.14 o 0.84 2577 5.76 o 3048 4.60 o 5.1 389d 7.79 16.4d 16.1 4.94d 0.23 o 2.5 5.09 o 68 o 0.05 1.05 2.15 o 1056 114 o 4.3 2.76 1.53 204d o 28

2.6 0.007 0.23

0.59 0.04 0.39 0.01 17328 0.04 34 0.008 402 1.13

5 0.50 0.5 0.012 0.006 0.7 0.30

0.04 0.07 5 0.49 0.07 0.18

Aliquot 2 Unc.c

363 2.2 0.001 0.07

0.42 6.2 0.06 0.53 0.02 236 0.24 0.05 34 0.83

101 0.94 0.18 17 0.78 0.9 0.6 0.19 0.01 0.19

0.04 0.09 4 0.13 0.06 7

Content

Unc.c

0.63 12293 65.4 0.319d o 13 1.78d o 329 o 3.8 5.15 o 90 0.63d 4.17 0.75 8290 4.88 0.77 980 o 3.3 13.8 o 5.6 3991 5.45 o 1878 3.58 o 3.3 5539d 8.97 153d 18.0 1.98d 0.37 37.4 5.12 o 37 o 0.04 1.06 2.28 o 1073 111 3.31 2.64 2.03 126d 532

0.05 432 2.3 0.011

Average

Unc.a

Ratiob

0.63 11303 63.7

0.05 562 3.2

0.89

0.51 6.2

0.68

4.24 0.62 7266 5.34 0.76 956 7.12 13.8 o 5.6 3284 5.61 o 3048 4.09 o 5.1

0.61 0.04 385 0.31 0.06 49 0.83 0.5

0.63 1.72 0.02

123

193 1.31

1.63 0.94

8.38

1.01

0.91

17.0

0.9

1.44

0.30 37.4 5.11 o 68 o 0.05 1.06 2.22 o 1073 113 3.31 2.70 1.78

0.02 1.3 0.27

1.88 2.09 0.84

0.06 0.13

0.88 1.08

6 0.43 0.26 0.09

0.89

532

21

0.90

o 23 0.1

0.29 0.04 0.31 0.03 308 0.20 0.04 36 0.5 174 0.91 0.18 196 0.61 6 0.7 0.07 0.01 1.3 0.19

0.04 0.09 4 0.43 0.22 0.07 5 21

o 329 o 4.7 5.33 67.0

0.79 0.99

0.26

1.03 0.92

pffiffiffi n.a.—not analyzed; o—Limit of detection (LD) with a confidence interval of 95% calculated as LD ¼ 2:706 þ 4:653 B, where B is the background (in counts) of the expected peak. rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2  2ffi a u1 Average uncertainty calculated as Average þ uC22 , where u1, C1 and u2, C2 are the uncertainties and concentrations of Aliquots 1 and 2, respectively. C1 b

Ratio of dissolved to powdered sample. Combined standard uncertainty of the method used with a coverage factor k ¼1. d Not used for calculation of average value due to the inhomogeneity of the mineral, contamination after dissolution/electrolysis or losses of volatile elements (dissolution/electrolysis/irradiation). c

38

M. Taseska et al. / Applied Radiation and Isotopes 70 (2012) 35–39

Table 3 Results obtained by k0-NAA in powdered and for dissolved samples of native copper from Bucˇim. All results are in mg kg  1. El.

Powder

Electrolysis Aliquot 1

Ag Al As Au Ba Br Ca Cd Ce Cl Co Cr Cs Cu Dy Eu Fe Ga Hf Hg K La Mg Mn Mo Na Nd Pt Rb Sb Sc Se Sm Sr Ta Tb Th Ti U V W Yb Zn Zr

Content

Unc.c

Content

0.16 n.a. o 0.76 0.0060 o1 1.04 o 85 o 0.3 0.17 n.a. 1.42 5.28 0.012 894516 n.a. 0.042 135 o 30 o 0.003 0.062 o 2577 o 0.03 n.a. n.a. o 0.2 26.4 0.32 n.a. o 0.14 0.016 0.050 0.18 0.15 o3 o 0.005 0.096 0.69 n.a. 1.30 n.a. o 2.4 0.76 3.05 o3

0.01

o 0.24 156 0.73d o 0.004 o 25 0.41 o 1410 o5 o 2.6 o 11 0.047 o 2.3 o 0.1 1269 0.39 o 0.05 277d o1 o 0.12 o 0.9 60.2d 0.04 o 319 o 0.5 o 5.7 100

0.0002 0.06

0.02 0.05 0.28 0.002 33101 0.005 5

0.004

4 0.07

14.7 o 1.3 0.28 0.032 o 1.7 0.139 o 59 o 0.035 0.062 0.45 o 139 0.75 1.27 o 0.2 0.49 62.8d o 45

0.001 0.002 0.01 0.01

0.003 0.02 0.12

0.03 0.12

Aliquot 2 Unc.c

6 0.03

0.02

0.007

48 0.02 11

6.2 0.01

5 1.4 0.02 0.002 0.007

0.004 0.02 0.06 0.10 0.03 2.3

Content o0.48 136 0.34d o0.004 o25 0.34 565 o8 o2.5 52.9 o0.046 o2.3 o0.1 4982 o0.18 o0.01 32.6d o1 o0.12 o0.9 121d o0.06 o439 0.68 o6.9 104 10.4 o1.4 0.18 0.021 o0.9 0.087 o58 o0.036 o0.031 0.26 o167 0.65 2.61 o0.2 0.30 338d o46

Average

Unc.a

o 0.48 146

8

Ratiob

Unc.c

5 0.02

0.02 158

3.1

178

o 0.004 o 25 0.38 565 o8 o 2.6 52.9 0.047 o 2.3 o 0.1 3126 0.39 o 0.05

0.03 158

3.1 0.007

163 0.02

0.36

0.03

0.003

4.2 o1 o 0.12 o 0.9 9

0.04 5 1.6 0.01 0.001 0.005

0.02 0.06 0.13 0.02 12

0.04 o 439 0.68 o 6.9 102 12.6 o 1.4 0.23 0.027 o 1.7 0.113 o 59 o 0.036 0.062 0.36 o 167 0.70 1.94 o 0.2 0.40

0.01 0.04 7

3.86

2.3 0.02 0.002

14.4 0.53

0.009

0.75

0.004 0.03

0.65 0.51

0.09 0.18

0.54

0.04

0.52

o 46

Ratio: Electrolysed/Powdered

Note: For explanation of letters used, see note under Table 2.

1.4 1.2 1.0 0.8 Powdered

0.6 La

Ce

Electrolysed

Nd Sm Eu Rare Earth Elements

Tb

Yb

Fig. 2. Ratio of the content of REEs in the electrolysed to powdered samples of brochantite obtained by k0-NAA (error bars for k¼ 1 for two repetitions).

several elements could by analysed due to the elimination of the interferences arising from the matrix element (Al, Dy, Mg, Mn, Ti and V). An additional advantage of the proposed method is the

lowering of the limit of detection for numerous elements (As, Ga, K, W) in the electrolysed solution compared to their corresponding values obtained by k0-NAA measurements in the powdered sample (Table 3). However, it should be mentioned that the content of platinum increases during electrolysis because of the use of a Pt-electrode. The results presented in Tables 2 and 3 show that losses of volatile elements like Br, Cr and Se occurred. At the same time, other elements were introduced into the solution either after dissolution (Na, K, Rb and Cs) or after electrolysis (Sb, Pt and Zn) by contamination.

4. Conclusions The trace element contents in two copper minerals (brochantite and native copper) were determined using k0-NAA before and after quantitative removal of copper by electrolysis. The advantage of using the combination of k0-NAA for the powdered

M. Taseska et al. / Applied Radiation and Isotopes 70 (2012) 35–39

mineral and after electrolysis is the possibility of simultaneous study of the distribution of many elements (up to 44 elements) from a complex mineral matrix. Comparison between the results obtained by k0-NAA after removal of Cu by electrolysis and in the powdered mineral showed that (i) Cu was quantitatively removed after complete mineral dissolution and (ii) the REEs and the majority of other analyzed elements remained in the solution. The important advantage of the proposed method is the possibility to determine the content of Al, Dy, Mg, Mn, Ti and V (after Cu electrolysis) via their short-lived radionuclides as the matrix element interference was eliminated. In addition, the limit of detection for several elements (As, Ga, K, W) was lower in the electrolysed solution compared to the corresponding values determined in the powdered mineral. However, it could also be concluded that the large variation of the content of Au, Co, Hf, Se and Zr arises from the inhomogeneity of the mineral samples studied. In the case of Na, K, Rb and Cs after dissolution, even though we used suprapur acids, contamination with these elements from the reagents still caused larger uncertainties. Also for Sb, Pt and Zn, electrolysis resulted in contamination again resulting in greater errors. This work confirmed the results obtained in our previous work, that the content of some trace elements (Na, K, Rb, Cs, Sb, Pt and Zn) was higher after Cu removal in chalcopyrite (CuFeS2; Taseska et al., 2008) and chalcantite (CuSO4  5H2O; Taseska et al., 2010). This means that using electrolysed solutions for determination of these elements (lower than 100 mg kg  1) by other spectrometric methods probably also results in systematically higher results. Additionally, spectrometric methods (AAS, ETAAS, AES-ICP, etc.) are not appropriate for determining volatile elements (Br, Cr, Se) because their losses during Cu removal cannot be determined.

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