Determination of Lead and Heavy Metals in Soil Samples Around a Lead Smelter Plant and Environs in East St. Louis, Missouri by Wavelength Dispersive X-Ray Flourescence

and
S. Landsberger
Department of Nuclear Engineering
University of Illinois
Urbana, Illinois

Wavelength Dispersive X-Ray Flourescence (WDXRF) technique was used for the preliminary determination of lead and heavy metals in soil samples around a lead smelter plant and environs in East St. Louis Missouri. Out of a hundred samples, 6 samples close to a smelter and 12 samples were analyzed. The results show relatively high concentrations of lead from 61 to 824 ug/g dry weight. Other heavy metals such as Cd, Sn, Zn, also show significantly high concentrations. Analytical accuracy was found to be in good agreement with the certified standard reference material for all elements.

Introduction

Wavelength Dispersive X-Ray Flourescence (WDXRF) analysis has been applied for the preliminary determination of lead (Pb) and other trace elements in soil samples. The samples were taken around the vicinity of a lead smelter plant in St. Louis, Missouri. The main objective of this study is to assess the results of the preliminary analysis for further and extensive research in East St. Louis.

Over the past 20 years the focus of concern about toxicity of lead to humans has shifted from industrial to environmental exposure. The danger to health from lead residues in soil and dust is now widely accepted. Lead is one of the most serious environmental health hazards than any other element [Chan and Wong, 1984]. It is especially hazardous to children. Excessive lead exposure results in permanent neurological damage to developing central nervous system of young children [Tolgyessy and Khler, 1987]. Primary contamination of lead has been the alkylead compounds used in gasoline as antiknock agents. Combustion of coal and smelting operation has also contributed greatly to lead pollution [Tacket, 1986].

Apart from Neutron Activation Analysis and Protect Induced X-Ray Emission flourescence analysis is one of the convenient methods for the multielemental determination of trace elements. The method is especially useful in the study of environmental pollution by heavy metals such as Pb [Das,H.A., Faanhof A. and Van der Sloot, H.A. 1983]. The method is used almost exclusively for routine analyses where the need is for fast and accurate analysis. The flexibility and range of the various types of XRF spectrometers, coupled to a detector with high sensitivity and good inherent precision make them ideal for quantitative analysis [Jenkins, R., 1988]. Recent developments have allowed sensitivities that are quite sufficient to the sample masses typically encountered in the analysis of trace metals in air, water and soil samples. XRF methods find increasing application in this area [Dzubay, T.G., 1977]. A good example of the wide applicability of this method are in the paper industry [Kochman, V. Foley, L. and Woodger, S.C. 1986]; geology [Adler, 1966]; archeometry [Cox, G.A. and Gillies, K.J.S. 1986]; medicine [Cesareo, R. 1982]; and metallurgy and art [Hertzglotz, H.K. and Birks, L.S., 1978].

Basic Theory and Principle of X-Ray Flourescence

The fundamental theory of x-ray flourescence analysis involve the bombardment of a sample with radiation of sufficient energy to eject inner shell electron from the atom in the sample. The ejected electrons are immediately replaced by electrons from outer atomic shells, with simultaneous emission of x-rays and with energies characterestics of the electron transition responsible for their emissions. Variations of this basic process primarily lie in the nature of the bombarding radiation, the method of detection and energy descrimination of the emitted x-rays [Jenkins,R. 1988]. The basic principle of an x- ray spectrometric analysis is that the x-ray tube produces the primary x-ray beam. The primary x-ray beam irradiates the specimen and excites each chemical element to emit its characterestic x-ray spectrum. The intensity of each line is related to the concentration of the emitting element. The collimator renders the secondary x-rays parallel to the specimen. The analyzer crystal spacely disperses the secondary x-rays, causing each analyte line to go in a different direction. The detector then receives each wavelength diffracted by the crystal and for each x-ray photon absorbed produces an output pulse of elctric current having an amplitude proportional to the incident x-ray photon energy.

The preamplifier and amplifier amplify these pulses linearly, that is, without changing their relative amplitudes. The pulse-height selector passes all pulses having a preset mean photon energy. The ratemeter displays the rate of arrival of pulses from the pulse-height selector on a meter or strip chart recorder. The scaler counts these pulses digitally.

The goniometer , a collimator-crsytal-detector system, is set at 2q (l) for a specified analyte line. The intensity is measured from each sample and several standards of known concentration see Figures 1 and 2 [Jenkins, R., 1988]. The calibration curve, intensity versus concentration, is established from the standard data. The intensity from each sample is applied to the curved to derive analytical concentration.

Figure 1. Schematic diagram of a Wavelength Dispersive X-Ray Flourescence Spectrometer [Jenkins,R., 1988].

Figure 2. Parts of the Wavelength Dispersive X-Ray Flouresence Spectrometer [Jenkins, R. 1988].

Experimental

Sample Preparation:

Soil samples of about 50 to 100g were air dried overnight. The samples were sieved using a 2000µ or 2mm siever by the U.S. Standard Sieve Series. Particles with less ² 2mm were collected, weighed and ground for 2 minutes using an alumina ceramic container in an enclosed shatterbox. The grounded particles were kept in 250 ml pre-ceaned glass bottle. About 4g of the grounded samples were weighed into a clean dry platinum crucible and dried in an oven at 110¡C for 4 to 6 hours. The samples were cooled in a desiccator for 10 to 15 minutes and then reweighed to record the weight of the dry sample. The samples were then ashed for 10 to 12 hours at 500¡C. The samples were allowed to cool in a desiccator for 15 to 20 minutes and then reweighed to get the percent ashed.

About 5.2 of dried x-ray fusion mix and 2.5 of previously dried sample (500¡C ashed) were transferred into the mixing polystyrene vial and the weight of the mixture. The mixture was initially mixed with a platinum spatula before mixing it in a Spex 8000 mixer mill with methacrylate mixing balls for 2 minutes. The sample from the vial were then transferred into a preweighted Pt-Au crucible. The weight of the sample and crucible was recorded and fused in the furnace at 1000¡C for 20 minutes. The crucible was removed from the furnace and allowed to cool for 2 to 5 minutes with Pt lid, then cooled rapidly in a beaker of deionized water.

The crucible was cooled for 5 to 10 minutes before recording the total weight of the crucible and the sample. The fusion product was then transferred to an envelope and the empty crucible was reweighed empty. It was then grounded in a mixer mill for 4 minutes using an assembled tungsten carbide vial with two tungsten carbide balls. A 5% by weight of the fused product of Bakelite plastic resin was added and remixed for 2 minutes in a mixer mill. The mixed samples was transferred into a cleaned Spex 31 mm die set and padded down lightly with plexiglass rod. The die was set in hydraulic press and pressed at a pressure gauge reading of 40,000 psi for 2 minutes and the pressed pellet was placed in an oven at 110¡C for 20 minutes. It was then cooled and placed in an envelope until analyzed with Rigaku x-ray spectrometer.

Instrumentation:

A Rigaku Model 3371 wavelength dispersive x-ray spectrometer equipped with an end window Rh-target x-ray tube an automatic sample changer with 6 positions and associated software was used.

Calibration:

Based on natural and artificial standards about 25 major, minor and trace elements were calculated by the spectrometer's using a fundamental parameter calibration curves. A set of calibration curves based on the intensity ration (IE1 / IRhC) where IRhC is the intensity of the Rh Compton peak has been prepared for the elements Pb, Zn, and Cu.

QA/QC:

A certified Standard Reference Material (SRM) 2711 (Montanoa Soil) obtained from National Institute of Standards and Technology was used. It is a moderately contaminated soil that was oven dried, sieved, and blended to achieve a high degree of homogeneity. It was analyzed in the same procedure as the true sample to assess accuracy and reproducibility.

Results and Discussions

A total of 18 soil samples from about a hundred samples were analyzed randomly. The six samples as shown in Table 1, were taken within the vicinity of the smelter plant, while the 12 samples as shown in Table 2, were from the nearby areas.

The preliminary results of the analysis as shown in Table 1 and 2, indicates a varied concentrations of Pb from 61 to 824 ug/g dry weight. For samples taken within the perimeter of the plant as shown in Table 2, Pb concentrations are from 167 to 595 ug/g dry weight. The high concentration of Pb can be found on the top soil, Top 4" #95. The behavior of Pb, as explained by Ahrens(1964), during weathering and sedimentation is strongly dependent on environmental conditions, a fact which is reflected by the highly variable Pb content of soils. Under reducing conditions Pb forms the highly insoluble sulfide. Under normal oxidizing conditions Pb may be carried in solution in somewhat acidic waters (but not in high concentration because of the low solubility of the sulfate). In neutral or somewhat alkaline water the lead ion becomes hydrolyzed and is readily co-precipitated with hydroxides of more abundant elements or absorbed by clay minerals.

The concentrations of Pb in the nearby areas are also very significant as shown in Table 3. Two areas, Winter St. and Southern Burlington Northern has concentrations of about 800 ug/g dry weight. Both places are said to be within the same area.

A total of 10 elements were determined by XRF. Other elements determined were Zn, Sr, Cu, Cd, V, Cr, Rb, Sr, Zr, and Ba. Some elements has elevated concentrations like Zn (125 to 2,025 ug/g dry weight). Computations of results were based on the formula as shown in table 1. Other elements with apparent elevated concentration include Cd, Zn, and Sn.

Measured values of the reference material used was in good agreement with the certified values. The range of the elemental contents in the reference materials are shown in Table 3.

Conclusion

The preliminary study about Pb and other heavy metal contaminations in soil samples around the smelter and its environs. It has been proven that one of the more appropriate and effective method, for the analysis of Pb and other elements is x-ray flourescence analysis. The determination of other elements are also being analysed by NAA. An in depth study about the area and an extensive analysis of the samples are needed to fully assess the extent of contamination of Pb and heavy metals in the soil.

Acknowledgement

We would like to thank Brian Orland from the Department of landsceng for providing the samples, one of us (L.E) would like to thank Dr. Robert Frost of the Illinois Geological Survey for providing me all the knowledge about WDXRF and for allowing me to used the facility, and my sincerest appreciation also goes to Dr. Joyce Frost for interesting consultations and providing me with needed literature. I would also like to especially acknowledg the International Atomic Energy Agency and United States National Research Council for their full support in the duration of the fellowship.

References

Adler, I. [1966]; X-ray Emissions Spectrography in Geology, Elsevier, Amsterdam
Ahrens, L.H., [1964], "The Significance of the Chemical Bond for Controlling the Geochemical Distributions of the Elements - Pt 1", in ( Ahrens, L.H., et al, editors), "Physics and Chemistry of Earth, Vol. 5, Oxford, Pergamon Press, pp. 1-54.
Bertin, E.P., [1985], X-Ray Spectrometric Analysis, Principles, Instrumentations, Practice and Applications, RCA Laboratories, Princeton, New Jersey
Cesareo, R. [1982], X-Ray Flourescence (XRF and PIXE) in Medicine, Field Education Italia
Chan, Y. K. and Wong, P.T.S. [1984], In Biological Effects of Organolead Compounds, (ed. Grandjean, P. and Grandjean, E.C. CRC Press, Boca Raton, FL., pp. 21-31
Cox, G. A. and Gillies, K.J.S/. [1986] Archeometry 28, 57
Dzubay, E.D. [1977], X-Ray Flourescence Analysis of Environmental Samples, Ann Arbor Service: Ann Arbor, Michigan
Hertzgloz, H.K. and Briks, L.S.[1978], X-Ray Spectrometry, Dekker, New York
Jenkins, R. [1988], X-Ray Flourescence Spectrometry, International Centre for Diffraction Data, Swarthmore, Pennyslvania
Tacket, R. L. [1987], Lead in the Environment: effect of Human Exposure, Am. Lab., 40,32
Tolgyessy, J. and Khler, e. [1987], Nuclear Environmental Chemical Analysis

Appendix 1

Conversion of data from 500¡C Ash Basis to Dry Sample Basis:
Multiply all concentraions by % 500¡C Ash/ 100 For Major and minor elements:
% SiO2 (Ash) x % 500¡C Ash/ 100 = % SiO2 (dry weight) For trace elements:
% Pb (Ash) x % 500¡C Ash / 100 x 1000 = ppm Pb (dry weight)

Document author(s) : LA 437/465 Fall 1995
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East St. Louis Action Research Project