International Journal of Hygiene and Environmental Health
Int. J. Hyg. Environ. Health 204, 375 ± 376 (2002) ¹ Urban & Fischer Verlag http: // www.urbanfischer.de/journals/intjhyg
Short Communication Sorption of toxic heavy metals to soil* Priit Alumaaa, Uuve Kirsoa, Valter Petersellb, Eiliv Steinnesc a b c
National Institute of Chemical Physics and Biophysics, Tallinn, Estonia Geological Survey of Estonia, Tallinn, Estonia Norwegian University of Science and Technology, Department of Chemistry, Trondheim, Norway
Abstract The surface soil is a major recipient of pollutants, including heavy metals, through atmospheric deposition, agricultural practices, and waste disposal. In the present work the sorption capacity of different types of soils to toxic heavy metals, i. e. chromium, copper, cadmium and lead has been studied. Experimental adsorption data for metals to the soil obtained by the batch method were fitted by linear isotherm. The various soils showed a very different behaviour in sorption of heavy metals. The distribution coefficient Kd, which is an indication of the adsorbing capacity of the substrate, varies within a wide range, from 57 to 53 000 l kg 1 . Desorption of metals from the solid phase was found to be small, indicating that the soil matrix is affecting the metal mobility by modifying the bonding of pollutants to the soil system consequently affecting the potential for soil remediation processes. Key words: soil ± heavy metals ± sorption ± distribution coefficients
Introduction Interest in heavy metals in soil arises from the fact that soil is the main source of these elements for plants and human nutrition and from the possibility of soil pollution due to industrial wastes. Even though the plants need trace amounts of several heavy metals for growing, some heavy metals such as Cd present a great health and environmental risk, even at low concentrations. Heavy metals in the soil are to a large extent sorbed to soil particles. Adsorbed heavy metals can desorb into soil water, and thus move in dissolved form into plants or move to lower soil horizons and groundwater. The mobility of heavy metals in soil can be
described by a distribution coefficient, defined as the ratio of metal concentration in the solid phase to that in liquid phase at equilibrium (Anderson et al., 1988). Distribution coefficients can be determined from the slopes of the adsorption isotherms. The objective of this study is to report experimental results on sorption of four heavy metals ± Cd, Pb, Cu and Cr ± on different soil samples.
Materials and methods The soil samples were air-dried and ground to pass through a 3 mm sieve. They were subjected to a variety of
* Following a presentation given as part of the International Congress on Environmental Health and the 4th Annual Meeting of the International Society of Environmental Medicine (ISEM), 1 ± 4 October, 2000, Hannover, Germany. Corresponding author: Priit Alumaa, National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, Tallinn 12618, Estonia, Phone: 37 2509 4148, Fax: 37 2639 8393, E-mail:
[email protected]
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chemical and physical tests, e. g. measurement of soil pH, determination of cation exchange capacity, surface area, distribution of pore volumes and organic matter content, etc. (Alumaa et al., 2001). For adsorption measurements two-gram soil samples were shaken in polypropylene centrifuge tubes at room temperature with 10 ml of 0.01 M CaCl2 containing from 1.0 ppb to 30 ppm of metal ions (chloride salts). After 16 h equilibration time the tubes were centrifuged at 3000 rpm for 10 minutes and the supernatant solution analysed. For desorption measurements the soil was resuspended several times in 10 ml CaCl2 and metal concentrations were determined in all dilutions by graphite furnace AAS.
Results and Discussion Studies (Travis et al., 1981) have confirmed that heavy metal sorption to soil can be described by Freundlich or Langmuir isotherms. In the present study isotherms tended to be rectilinear at equilibrium solution concentrations lower than 0.015 ± 0.03 mg/ml, whereas at higher concentrations curvilinear isotherms were obtained. The distribution coefficients (Figure 1) determined from the linear part of the sorption isotherm varied within a wide range, from 57 to 63 000 l kg 1 . Metals can be arranged according to their Kd values, i. e. their affinities for the soil, here resulting in the following relative sequence: Pb > Cr > Cu > Cd. This general sequence tends to vary slightly for different soil types, as it can be seen in Figure 1. The Cd concentration in soil is heavily affected by soil type, being lowest in podzol and highest in rendzina (average 0.30 and 0.54 ppm respectively). A similar pattern is evident for Cu (6.2 ppm in podzol and 13.8 ppm in rendzina). The average Pb and Cr
Table 1. Average concentration of heavy metals in Estonian soil and plant ash (ppm). Metal
Soil
Plant
Ratio plant/soil
Cd Pb Cr Cu
0.5 10 200 20
0.01 10 250 200
0.02 1.0 1.25 10
concentrations in soil tend to be less affected by soil type than that of Cd and Cu. The sorption has great influence on how deep the metal can migrate in the soil profile and to what extent it is taken up by plant roots (bioavailability). Metals with small distribution coefficient values (e. g. Cd) are the most mobile among the metals under investigation. The observed desorption isotherms show that the amounts of heavy metals readily desorbed from the soil were quite small. Desorption of Cd was greater than that of the other metals. The different adsorption-desorption behaviour of the heavy metals generally suggests that metals strongly sorbed to soil can stay in the plant root area for a long period and thus will be potentially available for plant uptake and for soil microorganisms and fungi. More weakly adsorbed metals can migrate down to deeper soil horizons and away from the main root area, but may be transported to the ground water. This is supported by the monitoring data in Table 1 where metals can be divided into three groups: a) metals accumulating heavily in plants; b) metals tending to stay in soil and c) metals accumulating approximately in equal amounts in plants and soil. Copper concentration in plant ash is 10 times higher than in soil. Lead and chromium show similar characteristics, having approximately equal concentrations in plant ash and soil. Cadmium concentration in plant ash is lower than in soil. Acknowledgements. This study was supported by a research grant from the Research Council of Norway, Project No 120400/730, and Estonian Science Foundation, Grant No 2922.
References
Fig. 1.
Affinities of heavy metals to various types of soils.
Alumaa, P., Steinnes, E., Kirso, U., Petersell, V.: Proc. Estonian Academy of Sci. 50, 2 (2001). Anderson, P. R., Christensen, T. H.: J. Soil Sci. 39, 15 (1988). Travis, C. C., Etnier, E. L.: J. Environ. Qual. 10 (1981).
