ATOMIC ABSORPTION ANALYSIS ACCOMPANIED BY TEMPERATURE CONTROL OF ELEMENT RELEASE: APPLICATION TO HEAVY METAL TRIAD Hg-Cd-Pb

Vladimir L. Tauson, Vitaliy I. Men'shikov, Irina Yu. Parkhomenko, Vladimir F. Gelety

A.P.Vinogradov Institute of Geochemistry, Siberian Branch of Russian Academy of Sciences, Irkutsk, 664033, P.O.Box 4019, Russia. E-mail: vltauson@igc.irk.ru

Abstract

Taking as an example the triad of relatively volatile heavy metals Hg-Cd-Pb, the thermal behavior of different binding forms of the elements is considered. The method of combined thermal and atomic absorption analyses is utilized. The temperature parameters of element release in the triad correlate with the element volatility and thermal stability of species depending upon the bond strength in the particular binding form. Possible applications of the method are discussed. It is shown that Cd and Hg mainly occur as sorbed species in meteoritic matter that may indicate lower primary cosmochemical abundance of these elements. The study of Hg speciation in environmental objects gives valuable information on the origin and sources of Hg contamination. The data obtained show sufficient contribution (25-100 %) of sorbed Cd form to contamination of Baikal Lake sediments with cadmium. The concentrations of physisorbed Hg in the sediments are usually lower than 0.1 ppm but some layers of sediments show higher contents (up to 2 ppm) and discernible contribution of a more high-temperature binding form. This may reflect a periodic igneous activity or washdown of Hg-containing material from the area of mineral deposit .

Introduction

The method of combined thermal and atomic absorption analyses is based upon the technique of a simultaneous registration of temperature of a sample and the absorbance signal from spectrometer during analytical experiment. The temperature parameters of the release of elements are dependent upon the features of species in which these elements occur in mineral matter (element binding forms). These parameters are pre-estimated during the calibration of the method mainly using the synthetic minerals. The compositions of minerals and element speciation are known from other methods or theoretical considerations. Primarily, the method has been developed for the analysis of mercury forms in mineral matter (Tauson et al., 1996). Subsequently, the method has been expanding to broader assortment of elements – Cd and Pb. The present communication is devoted to the brief analysis of this experience.

Methods

Analytical Technique

Element speciation studies are generally based on different versions of the selective extraction procedure (Companella et al., 1995; Biester & Scholz, 1997). Along with it, the data on element release under continual or stepwise sample heating can give an important information about the state of element in the specimens of minerals, rocks, soils, and sediments (Azzaria & Aftabi, 1991; Tauson et al., 1996; Biester & Scholz, 1997). However, reliable data on element speciation can be obtained only using reference samples. For studying speciation and measuring different element binding forms contents the combined technique of thermal and atomic absorption analyses is used. The basic design of analytical device can be found elsewhere (Tauson et al., 1995; 1996). The Perkin-Elmer Model 503 atomic absorption spectrometer equipped with deuterium background corrector, and Perkin-Elmer HGA-72 graphite furnace were applied. The sample, typically 20-30 mg, is contained in a platinum boat. The sample boat is positioned in the furnace very close to the hot spot of Pt-Pt/Rh thermocouple. The furnace is operated under the argon flow conditions. The heating rate is preset with a Perkin-Elmer heating programm regulator which allows this parameter to vary over a wide range and provides a steady temperature elevation. Within the most important interval 200-1200 ^oC, the value of 7 degrees per second has been recommended as optimum (Tauson et al.,1995). In principle, this scheme can be used for measuring both element concentration and temperature of release, although the latter may be determined more precisely by means of a special device where Pt boat combined with Pt-Pt/Rh thermocouple is placed in a graphite furnace and held in position by a special holder (Tauson et al., 1996). The system calibration is carried out with standard synthetic minerals containing different binding forms of the element under investigation.

Determination of temperature parameters of element release

The element binding forms can be defined as follows. The physically sorbed form (P) is the result of physical adsorption caused by the forces of physical nature as dispersion attractive forces, short-range repulsive forces, and electrostatic (Coulomb) forces. The chemisorbed form (C) is the result of chemical interaction between the element or complex and adsorbent which give rise to the formation of surface chemical compound or surface solid solution. The chemisorbed species may be weakly or strongly bound (Bargar et al.,1996). The difference between these species is reflected in the activation energy and temperature of their desorption. The isomorphous form (I) is the result of isomorphic exchange between the element and sorbent matrix ions giving rise to the incorporation of element into the crystal structure of sorbent. In a broader sense, the structurally bound form of the element is concerned. For diagnostic purposes we use such parameters as the temperature of maximum release (T_m), temperature of the peak end (T_e), and their difference (T_e-T_m) specifying the width of temperature interval of element release. The study of synthetic minerals supplemented with the data on few natural minerals and reagent grade chemicals allowed these parameters to be evaluated, and hence the main inorganic forms of the element to be distinguished. The sulfide, oxide and silicate minerals are primarily considered. Synthetic minerals were prepared hydrothermally.

Table 1 shows the temperature parameters of the elements release from minerals with different binding forms of Hg, Cd and Pb. It is relevant to note that only in the case of Pb the temperature intervals of the element release from different forms overlap. Therefore, it seems to be difficult to recognize structurally bound lead.

Quantitative analysis

The calibration curve for mercury determination was obtained using sphalerite (ZnS) and galena (PbS) crystals both containing P and I mercury forms (Tauson et al., 1996). Galena was found to be more suitable because of symmetrical peak and lower temperature of isomorphous Hg release compared to sphalerite. The peak area represents the integrated absorbance due to particular form. Special investigation gives evidence that galena (as well as other minerals) adsorbed some Hg from air, so the use of isomorphous form for calibration is preferable.The calibration curve for Hg may well be approximated with a straight line over the interval 1-100 ng of Hg amount (Tauson et al., 1996). The calibration curve for Cd was obtained using hydrothermally synthesized galena crystals containing 0.02 wt.% of isomorphous Cd and minor amount of chemisorbed form. The calibration curve may be represented by a straight line at least over the interval of 0.5 -10 ng Cd. Unfortunately, we have no reliable calibration for Pb. This is due to the character of Pb release possibly accompanied by its reduction to metal form in mineral structures.

Results and Discussion

Regularities in the triad

Lead is the least volatile element in the triad and this is reflected in the temperature parameters of Pb release from all binding forms studied (Table 1). It can be seen that the parameters under consideration are growing regularly in the sequence Hg-Cd-Pb as well as in the sequence physisorbed - chemisorbed - mineral - isomorphous form. Therefore, the temperature parameters of element release correlate with the element volatility and thermal stability of species depending upon the bond strength of the element in the particular binding form.

Cosmochemical abundance of the elements

The study of a small collection of meteorites and tectites provides evidence on predominant Cd presence as chemisorbed species indicating lower primary abundance of Cd in the meteoritic matter as compared with the recently published data (Schmitt et al., 1963; Anders and Grevesse, 1989). As seen in Table 2, Cd contents in chemisorbed form are from ~2 to ~6 times higher than in mineral form. Tauson et al. (1998) showed that Cd can be accumulated on mineral surfaces at room temperature. Therefore, it might be adsorbed from environment during the Earth life-period of meteorite existence. This effect is well-known for mercury (Stakheev et al.,1975). Table 2 supports the opinion that the mercury admixture is Earth-born. Mercury is determined as physisorbed form only; there were no other forms defined at the detection limit 10 ppb that is the minimum possible value for Hg determination by the method considered. Therefore, the basis for qualification of meteoritic substance with elevated Hg and Cd contents as a low-temperature material where the elements proportions are equal to their primary cosmic abundances should be revised. Additional information is to be obtained on the forms of the elements which are used as “cosmothermometers”.

Environmental research

The study of mercury speciation in different environmental objects gives a valuable information about the origin and sources of Hg contamination and provides a rough estimate of the proportion of anthropogenic contribution (Tauson et al., 1995; 1996). The product of the frequency (f_i) and average Hg concentration () may be considered as a possible measure of abundance of particular mercury form i. Physisorbed Hg (i=P) is common for both types of pollution, whereas the mineral (mainly sulfide) (i=S) and isomorphous (i=I) forms are specific for natural contamination. Therefore, we can use the ratio as a measure of natural contribution. We have evaluated R value change from 0 for purely anthropogenic to ~1 for heavy natural pollution observed within the mercury ore objects. A low-level anthropogenic contamination with Hg is found for the bottom sediments of Baikal Lake and Irkutsk man-made reservoir.

Although the concentrations of physisorbed Hg in Baikal sediments are usually lower than 0.1 ppm, the data obtained under studying the drill cores from submarine Academicheskiy Ridge (Central basin of Lake Baikal) show the elevated Hg contents (up to ~2 ppm) in the local parts of the hole attributed to geologically short periods of time of the sediment accumulation (~250 ky). It is important to emphasize that, in addition to physisorbed, the chemisorbed and more high-temperature forms were observed. The latter may be interpreted as isomorphous Hg belonging to the mineral with a relatively low speсific energy of atomization (as realgar or antimonite). Therefore, mercury displays a nonuniform distribution throughout the bottom sediments. This feature might indicate Hg release in igneous processes like volcanic eruption or tectonic event accompanied by hydrothermal activity. Also a periodic washdown of Hg-containing material from the area of mineral deposit cannot be excluded.

The preliminary data show that Cd contents in Baikal sediments vary from 190 up to 550 ppb with the contribution of sorbed forms varying over the interval 25-100 per cent. The origin of sorbed Cd species still remain to be solved.

Acknowledgement - The support from Russian Foundation for Basic Research under grants Nos. 99-05-64240 and 00-05-64635 are greatly appreciated.

References

Anders E.,Grevesse N. (1989), Geochim. Cosmochim. Acta. 53: 197-214.

Azzaria L.M., Aftabi A. (1991),Water, Air and Soil Pollut. 56: 203-217.

Bargar J.R., Towle S.N., Brown G.E.,Jr., Parks G.A. (1996), Geochim. Cosmochim. Acta. 60: 3541-3547.

Biester H., Scholz Ch. (1997), Environ. Sci. Technol. 31: 233-239.

Companella L., D’Orazio D., Petronio B.M., Pietrantonio E. (1995), Analyt. Chim. Acta. 309: 387-393.

Schmitt R.A., Smith R.H., Olehy D.A. (1963),Geochim.Cosmochim. Acta. 27: 1077-1088.

Stakheev Yu.I., Lavrukhina A.K., Stakheeva S.A. (1975),Geokhimia. 9: 1390-1398 (in Russian).

Tauson V.L., Gelety V.F., Men’shikov V.I. (1995), Chem. Sustain. Develop. 3: 137-144.

Tauson V.L., Gelety V.F., Men’shikov V.I. (1996), In: Global and Regional Mercury Cycles: Sources, Fluxes and Mass Balances (W. Baeyens et al, Eds.), Dordrecht et al., Kluwer Acad.Publ., pp.441-452.

Tauson V.L., Parkhomenko I.Yu., Men’shikov V.I. (1998), Russian Geol. Geophys. 39:474-479.

Table 1. Temperature parameters (^oC) of element release from minerals with

different metal binding forms

Element Binding Form		Tm	Te	Te-Tm
Physisorbed (P)	Hg:	250-290	290-360	40-90
	Cd:	360-480	380-510	20-100
	Pb:	560-610	610-810	50-200
Chemisorbed I (CI)-	Hg:	310-320	380-400	70-80
weakly bound	Cd:	440-670	570-810	100-150
	Pb:	690-850	770-960	40-160
Chemisorbed II (CII)-	Pb:	1000-1060	1080-1240	80-180
strongly bound
Mineral (M)	Hg:	350-410	390-480	40-70
	Cd:	650-740	870-1030	170-380
	Pb:	1020-1220	1150-1400	50-380
Isomorphous (I)	Hg:	500->1000	500->1100	80-100
	Cd:	770-860	880-1200	90-340
	Pb:	1180-1350	1330-1470	120-180

Table 2. Cadmium and mercury speciation in some meteorite and tectite samples

	C a d m i u m					M e r c u r y
Object	Tm	Te	Te-Tm	Spec.	Average content (ppb)	Tm	Te	Te-Tm	Spec.	Average content (ppm)
Chondrite“Tzarev”,	550	630	80	C	58	260	330	70	P	0.11
L5-6	670	750	80	M	~10
Chondrite	500	630	130	C(+P?)	60	250	300	50	P	0.19
“Markovka”, H4	700	810	110	M	10
Pallasite	550	640	90	C	20	270	320	50	P	0.11
“Omolon”*
Iron meteorite	600	750	150	C	97	270^i	320^i	50^i	P	9.8^i
“Sikhote-Alin”						280^s	350^s	70^s	P	120^s
Iron meteorite	540	630	90	C	20	260	320	60	P	7.2
“Elga”**	670	750	80	M	~10
Iron meteorite	550	650	100	C	70	270	310	40	P	1.5
“Darinskoe”
Tectites***	550	720	170	C	80	270	320	50	P	0.16

Note: i – inner, s – surface parts of the sample; species: C – chemisorbed, P – physisorbed, M – mineral form.

*Olivine mineral fraction.

**Contains up to 15% of silicate minerals and troilite inclusions, and strongly inhomogeneous Cd distribution.

***Zhamanshinites from Kazakhstan.