NATIONAL ACADEMY OF SCIENCES OF UKRAfNE v. VERNADSKYI NATIONAL LIBRARY OF UKRAINE

MACEDONIAN ACADEMY OF SCIENCES AND ARTS

UKRAINIAN-MACEDONIAN SCIENTIFIC COLLECTION

Issue 5

Kyiv 2011

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ISBN 978-966-02-6028-3

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© HauioHarlhHa 6i6JlioTeKa YKpai'Hli ISBN 978-966~02-6028-3 iMeHi B. 1. BepH8,D,CbKOrO, 2011

Trajce STAFILOY, Institute of Chemistry, Faculty of Science, Ss. Cyril and Mcthodius University (Skopje, Republic of Macedonia)

DETERMINATION OF TRACE ELEMENTS IN MINERALS BY ATONfIC ABSORPTION

AND EMISSION SPECfROMETRY

1. Introduction

Minerals are natural occurring inorganic substances with a relatively constant chemical composition and well-defined physical properties. Analytical chemistry of minerals is a velY important part of mineralogy, because all properties of minerals, including crystal structure, and the [mal identication are dependent on the composition and atomic and molecular arrangement. Although minerals are considered to be of constant chemical composition, this does not mean they are chemically pure substances. During long geologically periods it is not possible to obtain absolutely pure minerals without any contamination, which means that most minerals contain extraneous substances that change some of their characteristics. Often, these small amounts of extraneous substances give an economic value to many deposits, such as silver, gold or other rare metals in sulfide minerals or vanadium, chromium and titanium in iron minerals, cobalt in nickel minerals etc.

There are a number ofelements that are quite easily interchangeable, with the result that one mineral may grade into another. Many ofthese types ofsubstances could be considered as a mixture oftwo or more minerals. If the amount of one ofthe minerals is too small, that mineral is considered an impurity. Therefore, there are many reasons to analyze trace element content in different minerals: to determine the purity ofminerals, to determine the presence ofvery rare and important elements which could be extracted and used to obtain data which give very important information on the geology of the mines and mineral localities. Also, these results some times give an explanation about background radiation ofthe localities, etc.

In some cases, analysis of trace elements in minerals plays an important role in obtaining knowledge from fundamental investigations as it is in the case of the possibility to use thallium mineral lorandite (TIAsS ) from Allchar mine in Macedonia for solar neutrino

2

detection [1] through the determination o[2osPb obtained in the nuclear reaction between

340

205TI and neutrino. Because of the low cross-section of the 20ST1(v,eyosPb reaction, a high concentration ofTJ atoms is necessary to produce measurable amounts opo5Pb. For this pw-pose, a thallium-rich mineral is required as a target. One candidate is lorandite, T1AsS .zThe Sb-As-TI deposit at Allchar in the Republic ofMacedonia is a unique mineral deposit in the world and lorandite is the most wide spread TI mineral in the deposit. In this case, it is very important to anal sis the purity of lorandite because these minerals, under the condition that they are co-genetic, can be used as monitors for detennining the contribution ofbackground reactions, which originate from natural radioactivity.

Electrothennal atomic absorption spectrometry (ETAA) is one of the most commonly used techniques which can be employed for trace element analyses in minerals [2]. However, very often, due to matrix interferences, some additional operations in the preparation of mineral samples are necessary. In order 0 avoid these interferences and because of the very low concentration of trace elements in the samples investigated, it is necessary to separate and concentrate those elements in the samples. To achieve this goal, different methods could be used: extraction, ion exchange, precipitation, etc. In this work, a review of the results obtained by the group working in atomic spectroscopy at the Institute of Chemistry, Faculty of Science in Skopje, in the methods development and the possibilities ofthe application ofETAAS for trace element detennination in different minerals is giv n.

2. Matrix interferences and matrix modification

Matrix interferences occur especially in the detennination of trace elements in minerals by ETAAS [2]. Matrix elements as a main constituent of the minerals have very high concentrations in the solutions after dissolution of the mineral samples. There are a lot of data about matrix interferences in trace element analysis in minerals. In some cases, depending on the matrix and matrix interferences it is possible to determine trace elements in minerals directly from the solution. Thus, a direct determination ofmany trace elements (Au, Pb, Be, B", Cd, Co, Pb, 1'1) in mineral samples (silicates, iron matrices, sulfide minerals) is suggested [3-13]. Direct determination of trace elements by ETAAS was also perfonned using direct introduction ofsolid mineral samples in to a graphite furnace (14-18].

In most cases, interferences from the matrix, especially from the matrix elements, which are constitutive elements of the minerals, are very high and direct determination of trace elements is not possible. Thus, the main macroeIements ofminerals which were the subject of our investigation are sulfides of As (realgar and orpirnent), As and 1'1 (lorandite), As (stibnite), Fe (marcasite, pyrite, Pb (galena), Zn (sphalerite), as well as carbonates (dolomite, calcite, aragonite, gypsum). The investigations showed that in the case ofAu determination in As, Sb, Fe, and 1'1 sulfide minerals or in dolomite, the absorbance ofgold decreased [19]. Also, in the case of the detennination of silver we found that the interfering elements especially when they are present in high concentration tend to decrease the absorbance of Ag [20). It was also found [12] that 1'1, As, Fe and Ca interfere on the determination ofPb by decreasing the Pb absorbance. The interferences of 1'1, As, Sb, Fe, Pb and Zn as matrix elements on copper detennination were also studied [21] and the results show the same effect. Typical example ofthe influence ofmatrix element is the effect on decreasing ofthe absorbance ofCu in absence (Fig. la) and presence ofSb (Fig 1b) [21].

The interference ofiron, as a matrix element ofthe minerals studied, on the Pb, Co, N~ Cr, Mn, Tl and Zn determination was also investigated [22,23]. Results show that iron tends to

341

decrease absorbance of Co, Cr and Pb and increase the absorbance of Ni at high concentration (Figs. 2 and 3). These interferences are mainly due to high quantity of iron present in the graphite tube preventing complete atomization of the investigation elements during the atomization step (Co, Cr, Pb) or effecting by spectral interferences overlapping the resonance line ofNi [22] and Tl [23,24] by reach atomic spectrum ofiron. In some cases spectral interference from Fe were solved by previous precipitation of Fe [19] or by mathematical correction [24]. It should be noted that the results showed that background absorption was present and that the use of background correction was necessary.

Fig. I. Signal of copper (meu = 0.2 ng) in absence (a) and presence of (b) antimony. Background . signal (c)(21 J

0.4

0.3 A

0.2

0.1

0 0 50 100 150 200 250

[m(Fe):m(M)] °10-3

Fig. 2. Influence of Fe on the absorbance of Co ( ), Ni (. )" Cr (*) and Pb ( J,.,) measured by ETAAS [22]

342

0,4

0,3 A

0,1

oL---=:::::=:::x::::==:=r::===~::::::J========---

o

[m(M):m(fl)]')O""Z

Fig. 3. Influence afPb (+), Zn ( ), As (A) an Fe (*) as matrix elements on TI absorbance [23]

Interferences on decreasing ofthe absorbance ofmany trace elements were also found in the analysis of calcium based minerals (calcite, aragonite, dolomite gypsum) [25-28] (Figs. 4 and 5).

0,3

A

0,2

A

20 40 60 80 100

0,1

0

• 0 5000 10000 15000 20000

m(Ca)/m(M)

Fig. 4. Influence ofCa on the absorbance orCa (+), Cu (_), Pb (A ) and Ni (e) [25]

In cases when matrix interferences do not give an opportunity for direct determination of trace elements, matrix modification or separation of investigated elements is necessary. Thus, in the case ofthe determination ofNi and Co in silicate minerals, NHl was added as a matrix modifier [29]; (NH4)2HPO4in the determination ofAg [30] or ascorbic acid in Mo determination [31]. In TI determination in some sulfide or carbonate minerals, modification with ~S04 [8, 12] or another matrix modifiers [32], gives satisfactory results in the elimination ofmatrix interferences. Nickel was also used as a matrix modifier in many studies for trace element in ETAAS determination [33,34].

343

A

o.os

0.f..............,f-'-'-'-'+...........~-1....L.J~i:t::I:=F====t=--"""+ ............-'+'..........'-l-'-............~

o 10 20 30 40 SO 60 70 80 90 100

m(Ca)/m(M}10-3

Fig. 5. Influence ofCa on Ag (+), Cd (.), Cr (.A.) and TI (e) absorbance [26]

3. Separation and concentration of trace elements

In order to avoid matrix interferences and because of the very low concentration of trace elements in the samples investigated, it is necessary to separate and concentrate trace elements in the samples [2]. There are few papers dealing with concentrations ofsome trace elements by ion exchange separation, such as determination ofsome rare elements in silicate minerals [35] or using ofresin colunm [36,27]. Some authors suggest the possibility of a concentration of trace elements noble metals from mineral samples by coprecipitation [38] or adsorption [39].

However, the most applied methods for trace element separation and concentration include the extraction ofmetal complexes of trace elements into organic solvent. There are cases of direct extraction ofchloride, bromide or iodide complexes of trace elements in to some organic solvents, as for TI [3,40], Te [41 ],Au [6, 19,41,42], Pt [43], Pd [43], Se and Sb [44]. Also, there are different methods for trace element separation and concentration from mineral matrix followed by ETAAS determination, applying extraction of complexes with some organic ligands into different organic solvents or other systems for preconcentration and separation oftrace elements [12,20,21,24,45-58].

The results from our investigation in the development of the methods for trace elements detelTIlination by ETAAS are given bellow. Beside the proposed methods for trace elements determination, data about their content in numerous minerals originated from different localities from the Republic of Macedonia were also obtained.

3.1. Liquid-liquid extraction

3.1.1. Extraction of chloride complexes of Au from arsenic and antimony ores and minerals

To overcome the interference from the matrix in arsenic and antimony ores and minerals, gold was extracted by methyl isobutyl ketone (MIBK) in hydrochloric acid solution [19]. It was found that iron interferes with the detelTIlination ofAu as well even after the extraction.

344

Separation of iron is based on precipitation by ammonium hydroxide before the extraction. The limit of detection (LaD) of the method was found to be 0.1 ng g-t. The method was validated by standard additions and by its application to the standard reference materials.

3.1.2. The application of diethyldithiocarbamate complexes

Sodiwn diethyldithiocarbamate was used for the extraction of different trace elements in their determination in various minerals [59]. For this purpose a method for the extraction was optimized depending of the type of minerals and of the analyzed trace element and solvent used for the extraction. Thus, Pb was extracted from the solution of antimonite at pH value ofabou! 11.5 with the addition ofKCN and CCl

4 as solvent [59]. Similar conditions

were optimized in Pb determination in minerals from Allchar mine, lorandite (T AsS ) and 2

marcasite (FeS) [12]. In the case of marcasite ir n was extracted previously by isoamyl acetate and then Pb was determined directly from the aqueous phase. Lead diethyldithiocarbamate complex was used also for the determination ofPb in dolomite by Zeeman ETAAS [48J with MIBK as solvent at pH from 6.0 to 10.0. The detection limit was found to be 1.5 ng g-I. Beside Pb, this complexing agent was applied for copper separation from dolomite at pH from 11-12 by different organic solvent (CCI

4, CHCl or MIBK) [49J

3 achieving the limit of detection 0 Cu in dolomite of 5 ng g-I.

Similar method was optimized also for the determination of copper traces in sulfide minerals (lorandite, realgar, orpiment, marcasite, stibnite, galenite, and sphalerite) [21]. The procedure was validated by its application to reference standard ore samples. The method for Co, Cu, Ni and Pb in some arsenic minerals (realgar, orpil'nent) was also developed by the extraction ofdithiocarbamate complexes into MIBK and CCl [60] with a LaD of2 ng g.l.

4

Optimization ofthe method for the extraction ofdithiocarbamate complexes of Co, Cu, Pb and Ni from gypsum and aragonite at pH=6 into MIBK, was optimized as well (Fig. 6) [61, 62J. The efficiency oftne extraction could be seen from Fig. 7 were the absorption signals of Co in aqueous (Fig. 7-a ) and in organic phase (Fig. 7-a ) are almost the same and the

1 2

background signal is close to zero.

3.1.3. Determination ofAg by the extraction with dipheny/ thiocarbamate

A method for determination of Ag in sulfide minerals (realgar, orpiment, lorandite, marcasite and stibnite) as well as dolomite by ETAAS was also proposed [20]. After dissolution of the samples, Ag was extracted by diphenyl thiocarbamate in hydrochloric acid media using MIBK, butyl acetate or toluene as the solvent. The procedure was verified by the method of standard additions and with standard reference samples. The standard deviation (SD) for low contents ofAg was 0.01 ng. The relative standard deviation (RSD) ranged from 3-5 % and LOD calculated as three SDs of the blank, was fmUld to be 1ng gl.

3.1.4. Extraction methodfor TI and Fe with isoamyl acetate

Due to the established matrix interferences in Tl determination by ETAAS in sulfide minerals ofAs, Sb, Fe, Pb, Zn, or Ca minerals (Figs. 3 and 5), the extraction (and separation from the matrix elements) ofthallium was performed by its extraction with isoamyl acetate in

345

0,3 - ..~ .. . \

0,2

A \ ===-0 \\'0 ,0,1 -<~

o '----....~--'------'-. I

2 4 6 8 10 12

pH

Fig. 6. Influence of pH on the recovery of Co (+), Cu (_), Pb (J.) and Ni (e)

j\" az

o.OQ.-i:...'?C•.•.'::":\•. :>-<;:__<>----..,~.....-._~":~~,J" t' .

-0.17 1--........~~~-......,.-~-~..,...."..,........----..,--~----.---,..........,r-1

Fig. 7. Absorption signals of cobalt in aqueous (al) and in organic phase (az)

and background signal (b)

hydrochloric media [22-24]. In the case of iron minerals, a cOlTection of the obtained value for thallium was applied [24] or reextraction of iron in sulfuric acid media (4 moll' i

) from the organic phase [22]. The determination of Tl was performed in the organic phase using palladium as matrix modifier with an atomization temperature of21 00 °C [22]. The detection limi of the method was found to be 50 ng g'l of Tl. The efficiency of the extraction of thallium could be seen from Fig. 8 were its absorption signals in inorganic phase before the extraction (Fig. 8-a) and in isoamy1acetate after the extraction (Fig 8-b).

346

! i

'151.00

c

a

j

//

JI-------------'I

Fig. 8, Absorption signals ofTl in inorganic phase before the extraction (a) and in isoamyl acetate after the extraction (b). (c) background signal

3.2. Flotation metllod/or simulta"eously separation oftrace elementsfrom minerals

It was mentioned before that taking into account that dithiocarbamate ions have the ability to fonn extractable chelate complexes with numerous heavy metals, the extraction of diethyldithocarbamate chelates ofmany trace elements in methylisobutyl ketone prior their ETAAS was applied for different mineral-samples [12, 21,48,49,59-62]. On the other hand, because the physical and chemical properties of Ca and Mg aqueous solutions are similar to those of the natural waters with higher water hardness, a method of flotation is also applied as the second way to eliminate the Ca and Mg matrix effects. Flotation is a well known technique for selective a separation ofvaluable substances from ores and minerals, but today this tech-nique is used mainly in other fields of chemical engineering and more rarely in analytical chemistry also [62]. There are many methods developed in our laboratory for flotation of trace elements from sea or fresh waters for analytical chemistry purposes [63-79], but application of this separation for analysis of trace elements in minerals was suggested by us for the first time [25].

It was found that different dithiocarbamate chelate, like Fe(III) hexa­methylenedithiocarbamate, Fe(HMDTC)3' lead(lI) hexamethylenedithiocarbamate, Pb(HMDTC)2' or Co(IlI) hexamethylenedithiocarbamate, CO(HMDTC)3' are very effective and successful precipitate collectors for flotation ofdiverse microelements from waters [70, 72-77]. Therefore, they has been chosen for the selective separation ofanalytes investigated from aqueous solution ofCa and Mg minerals (dolomite, aragonite, calcite, gypsum) and their detennination by ETAAS [25-28] or atomic emission spectrometry with inductively coupled plasma (ICP-AES) [80, 81]. Many important parameters for satisfactory separation oftrace elements were established (mass ofmetal and collector, pH, induction time, type of surfactant, etc.).

347

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