Journal of Environmental Quality 30:934-939 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
Heavy Metals in the Environment
Availability of Arsenic, Copper, Lead, Thallium, and Zinc to Various Vegetables Grown in Slag-Contaminated Soils
K. Bunzla,
M. Trautmannsheimerc,
P. Schramelb and
W. Reifenhäuserc
a Institute of Radiation Protection, GSF-National Research Center for Environment and Health, 85764 Neuherberg, Germany
b Institute of Ecological Chemistry, GSF-National Research Center for Environment and Health, 85764 Neuherberg, Germany
c State Environmental Protection Agency of Bavaria, Augsburg, Germany
Corresponding author (bunzl{at}gsf.de)
Received for publication March 13, 2000.
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ABSTRACT
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To anticipate a possible hazard resulting from the plant uptake of metals from slag-contaminated soils, it is useful to study whether vegetables exist that are able to mobilize a given metal in the slag to a larger proportion than in an uncontaminated control soil. For this purpose, we studied the soil to plant transfer of arsenic, copper, lead, thallium, and zinc by the vegetables bean (Phaseolus vulgaris L. ‘dwarf bean Modus’), kohlrabi (Brassica oleracea var. gongylodes L.), mangold (Beta vulgaris var. macrorhiza), lettuce (Lactuca sativa L. ‘American gathering brown’), carrot (Daucus carota L. ‘Rotin’, ‘Sperlings’s'), and celery [Apium graveiolus var. dulce (Mill.) Pers.] from a control soil (Ap horizon of a Entisol) and from a contaminated soil (1:1 soil–slag mixtures). Two types of slags were used: an iron-rich residue from pyrite (FeS2) roasting and a residue from coal firing. The metal concentrations in the slags, soils, and plants were used to calculate for each metal and soil–slag mixture the plant–soil fractional concentration ratio (CRfractional,slag), that is, the concentration ratio of the metal that results only from the slag in the soil. With the exception of Tl, the resulting values obtained for this quantity for As, Cu, Pb, and Zn and for all vegetables were significantly smaller than the corresponding plant–soil concentration ratios (CRcontrol soil) for the uncontaminated soil. The results demonstrate quantitatively that the ability of a plant to accumulate a given metal as observed for a control soil might not exist for a soil–slag mixture, and vice versa.
Abbreviations: CR, plant–soil concentration ratio • RMA, relative metal availability with respect to the control soil
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INTRODUCTION
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SEVERAL types of slags are not only used in concretes but also for constructing farming and forestry roads and for landfill. The main components of slags are mixed oxides of elements such as silicon, sulfur, phosphorus, and aluminum. If the phosphorus content of the slags is sufficiently high, they can be also used as fertilizers. Most slags contain impurities of toxic elements, such as As, Cu, Pb, Cd, Co, Cr, Ni, or Tl. Since these substances can be leached to some extent from the slags, possible environmental hazards cannot always be excluded. For this reason, several investigations of water, soil, and plant pollution by slags are available (e.g., Koenig et al., 1990; Mench et al., 1994; Manz, 1995; Wegelin et al., 1995; Wilke et al., 1997; Gimmler et al., 1998; McLaughlin et al., 1998; Meuser et al., 1998; Trautmannsheimer et al., 1998; Tossavainen and Frossberg, 1999). A systematic, comparative investigation of the plant uptake of several metals by typical garden vegetables from different types of slags is lacking. To estimate the availability of toxic elements for root uptake from slag-contaminated soils, leaching experiments using various extractants are frequently used. Since a redistribution of the extracted elements during the leaching process can occur (Bunzl et al., 1999), it is difficult to obtain realistic information on the available fraction of a given trace element in the soil in this way. In addition, plant specific effects, such as the release of organic compounds by the roots, or the presence of mycorrhizal fungi to mobilize plant-essential elements in the soil, cannot be assessed in this way. Therefore, the most realistic information on the availability of toxic metals in slags for root uptake can only be obtained by an analysis of the plants grown on slag-contaminated soils.
Recently, we have shown that the root uptake of natural radionuclides from a slag-contaminated soil in excess to that from an uncontaminated soil can be characterized by defining a fractional concentration ratio (CRfractional,slag), that is, a plant–soil concentration ratio with respect to the slag fraction in the soil (Bunzl and Trautmannsheimer, 1999). To evaluate this quantity, the plants were grown in a control soil and in a soil–slag mixture containing a given amount of slag. In this way, realistic and quantitative information on the uptake of a metal from the slag by a plant is obtained. For a given metal, the extent of metal accumulation by a plant from a soil is known to be quite plant specific. For this reason, not only does the concentration ratio (CRcontrol soil) of a metal in the control soil depend on the plant species but so will the CRfractional,slag. If a given plant is known to accumulate a metal only slightly (or strongly), one might expect that this tendency is also observable for a slag-contaminated soil. As a result, the ratio CRfractional,slag to CRcontrol soil should depend much less on the plant species. If this ratio exhibits unusually high values for a certain plant, this will suggest that this plant is able to mobilize this metal from the slag to an extent not predicted from its metal accumulation ability in the control soil. The purpose of the present study thus was to quantify the relative availability of As, Cu, Pb, Tl, and Zn from slags for plant uptake by various vegetables by determining the corresponding values for CRfractional,slag and CRcontrol soil. For this purpose, we selected two types of slags, one from coal firing (black slag) and one from pyrite roasting (red slag). These wastes had been used as landfill material, for the construction of farming roads, and to improve the water drainage in the soil around houses in rural areas. Both types of slag contained the above trace elements in substantial quantities. All vegetables were grown in field experiments on a typical uncontaminated agricultural soil (control), as well as on a 1:1 slag–soil mixture.
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MATERIALS AND METHODS
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Slags
The slags, both by-products from a paper mill, were applied to the above soil. The slags were a red slag (an iron-rich residue from pyrite [FeS2] roasting) and a black slag (a residue from coal firing). The slags, which were actually used as landfill material in this area, were collected from a waste heap, where they had been stored in the open for several years. Their characteristics are given Table 1. All slag samples were deliberately not crushed or sieved, but used as distributed from the manufacturer for landfill or road construction purposes. Only the small fraction of particles >5 mm was removed by hand. The various size fractions of the two slags were rather similar.
Soil and Soil–Slag Mixtures
The uncontaminated agricultural soil (control) used was taken from the Ap horizon of an Entisol (Cambisol in the FAO system). Several physico–chemical properties and the concentrations of the trace elements in this soil are given in Table 2. To prepare the soil–slag mixtures, either red or black slag was mixed with the control soil in the ratio 1:1 according to weight and well homogenized. The resulting concentrations of the trace elements in these mixtures as well as some physico–chemical properties of the soil–slag mixtures are given in Table 2.
Vegetables
Beans, kohlrabi, mangold, lettuce, carrots, and celery were grown in field experiments in the open in 18 adjacent plots (5.0 m long and 1.1 m wide), that is, one plot for each vegetable species cultivated on the two soil–slag mixtures and the control soil. For fertilization, 7 g N, 9 g P, and 8 g K were applied per square meter.
Transplanting of the young plants in the open and harvesting occurred for the various vegetables in the following months: celery (May, October); beans (May, July); lettuce (April, June); carrots (April, July), mangold (July, October); kohlrabi (June, October). To protect the plants from snails, a special snail fence was used. All plants received natural precipitation and were harvested in a growth stage usual in trade. The edible parts (in the case of celery only the tuber) were separated and washed as usual for culinary preparation, weighed, dried in an oven at 80°C, reweighed, and homogenized mechanically. Two plant samples were always taken separately from each plot and analyzed. The dry weight (in percent of the fresh weight) was as follows: beans (25), kohlrabi (8), mangold (8), lettuce (6), carrots (9), and celery (9).
Determination of the Metals
Several hundred grams of the soil–slags were ground in an agate mill and well homogenized. Total digestion of the soil–slag samples, including the silicates, was achieved according to Schramel et al. (1996), using ca. 300 mg of the homogenized material. The plant materials were homogenized in an agate mill and digested in quartz vessels (single digestion of 200–300 mg plant material each, 4 mL HNO3 conc. + 0.5 mL HCl [30%]; microwave-assisted high pressure digestion [Multiwave; A. Paar, Graz, Austria]). All chemicals used were of suprapure grade. In addition, any contacts of the soil or slag during sampling, storage, or processing of the samples with materials that could release these elements were avoided. Only ultrapure water (Milli-Q; Millipore, Eschborn, Germany) was used. Possible trace metal contamination was checked in blank experiments. The metals were determined by high-resolution inductively coupled plasma (ICP) mass spectrometry. For quality assurance, Certified Standard Reference Materials are analyzed regularly, and the laboratory has participated in the Standard-, Measurement-, and Testing Program (former BCA) of the European Union.
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RESULTS AND DISCUSSION
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Concentrations of the Metals in the Vegetables
The addition of slags to the soil increased the metal content of all vegetables considerably (Table 3). The data may be compared with the published threshold values considered excessive or toxic for mature leaf tissue (Kabata-Pendias and Pendias, 1984). Compared with the limiting values we observed similar or greater concentrations for several vegetables. For As, the values considered as toxic are listed as 5 to 10 mg kg-1 dry weight, which is similar to the values observed for lettuce (11 mg kg-1) or mangold (13 mg kg-1) grown on the red slag–soil mixture (see Table 3). For Cu, the listed limiting values 20 to 100 mg kg-1 were again similar to the values found here for lettuce and mangold cultivated using either type of slag. For Cu, the limiting values are 20 to 100 mg kg-1, for Pb 30 to 300 mg kg-1, and for Zn 100 to 400 mg kg-1. Again, lettuce and mangold exhibited values within this range when grown on the slag-contaminated soils. For Tl, the limiting value of 20 mg kg-1 is only observed for kohlrabi cultivated on the red slag soil mixture.
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Table 3. Concentrations of the metals in the various vegetables (in mg kg-1 dry weight) cultivated on the control soil, the red slag–soil mixture 1:1, and the black slag–soil mixture 1:1. The coefficient of variation (n = 2) for mean values was between 10 and 20%.
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Concentration Ratios for the Control Soil
The conventional concentration ratio (CR) is usually defined as the concentration of the metal in the plant (mg kg-1 dry weight) with respect to the corresponding concentration in the soil (mg kg-1 dry soil). In Fig. 1, we show first the conventional CRcontrol soil of the metals as observed for the vegetables grown on the control soil. On average over all vegetables, the CRcontrol soil increases in the order As < Pb < Tl < Zn < Cu (Fig. 1). For the various metals the dependence of CRcontrol soil on the vegetable species is to some extent similar. Thus, for As and Pb, the lowest CRcontrol soil values were found for kohlrabi, carrots, and beans, followed by somewhat higher values for celery, mangold, and lettuce. For Cu, the lowest CRcontrol soil is observed for kohlrabi, followed by substantially higher values for carrots, beans, and celery, and especially for mangold and lettuce. For Zn, the CRcontrol soil is lowest again for kohlrabi and carrots, but the CRcontrol soil for the other vegetables is higher only by a factor of about two. As far as comparable data are available, these results are generally in agreement with earlier observations. Low availability of As to plants compared with Pb, Cu, and Zn, and the comparatively high uptake of the metals by lettuce has been observed before (Kabata-Pendias and Pendias, 1984; Godzik et al., 1995). Thallium shows a different trend of accumulation than the other metals because it exhibits unusually high values of the CRcontrol soil for kohlrabi when compared with the other vegetables. The reason for this behavior is not known, but Schoer (1984) also observed much higher Tl concentrations in kohlrabi, savoy, and white cabbage than in other vegetables in an area contaminated by emissions of a cement plant.
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Fig. 1. Concentration ratios (CRcontrol soil) of As, Cu, Pb, Tl, and Zn (in mg kg-1 dry plant per mg kg-1 air dry soil) for various vegetables grown on the control soil.
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Fractional Concentration Ratios
In the case of a plant growing in a soil–slag mixture, the conventional CR yields only the total uptake of the metal by the plant with respect to the contaminated soil and not quantitative information on the availability of this metal for the root uptake from the slag fraction only. As we have shown recently for the uptake of some natural radionuclides by plants from soil–slag mixtures (Bunzl and Trautmannsheimer, 1999), such a quantity can, however, be obtained if we define a fractional concentration ratio, CRfractional,slag, as:
 | [1] |
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where for the metal of interest A = metal concentration in the plant grown on the slag-contaminated soil (mg kg-1) and B = metal concentration in the plant grown on the control soil (mg kg-1).
If the weight fraction of slag in the contaminated soil is denoted as fslag, one obtains: S = (total metal concentration of the contaminated soil [mg kg-1]) - (1 - fslag) x (metal concentration of the control soil [mg kg-1]) = (fslag) x metal concentration of the pure slag (mg kg-1).
The quantity S thus can be obtained either from a knowledge of fslag, the metal concentration of the contaminated soil and that of the control soil, or from fslag and the metal concentration of the pure slag. Note that the calculation of CRfractional,slag from Eq. [2] is only valid if the slag does not release any elements that either inhibit or enhance root activity, and hence the uptake of background metals from the soil. One possibility to study the presence of these processes would be radioactive labeling of the soilborne metals. In the present evaluation, however, a modified uptake of soilborne metals as a result of the addition of slags was assumed to be negligible as compared with the uptake of the metals released by the slags.
Equation [1] can also be used in the case of an additional airborne metal deposition on the plant surface, because the amount of this deposition will be the same for a plant grown on a control soil and on a slag-contaminated soil, and, therefore, cancels in the numerator of Eq. [1]. The value of CRfractional,slag obtained in the above way is, however, not independent from soil properties (such as the pH) because redistribution processes of the metal within the slag–soil mixture during weathering will also affect the availability of the metals released from the slag. For this reason, determination of CRfractional,slag values for various values of fslag, or for a given fslag of the same slag in different soils, will yield interesting insights in these processes. In addition, long-term effects, which were not studied here, can also be quantified. In the present case, where various plants were grown on a control soil mixed with either red slag or black slag at a ratio of 1:1, the corresponding CRfractional,slag values will allow a direct quantitative comparison of the availability of the metals from these slags for the various vegetables.
In Fig. 2, the values for CRfractional,slag for As, Cu, Pb, Tl, and Zn and the six vegetables grown on the soil contaminated with both slags at a soil to slag ratio 1:1 (fslag = 0.5) are presented. For comparison, the conventional concentration ratios (CRcontrol soil) observed for these metals on the control soil are also given. From this comparison it is apparent that CRfractional,slag of As, Cu, Pb, and Zn for all vegetables is always significantly smaller than the corresponding CRcontrol soil. This illustrates clearly that the availability of these metals in the slag is smaller than in the control soil. An exception to this behavior was observed for Tl. As evident from Fig. 2, CRfractional,slag is for several vegetables about as large or even higher than the corresponding CR for the control soil. This is especially the case for the uptake of Tl by kohlrabi, where values for the CRfractional,slag >0.5 were found compared with a CRcontrol soil of 0.3. A similar behavior was also observed by Makridis and Amberger (1989), who found that Tl in cement dust was more available for rape, bush beans, and rye grass than soilborne Tl. In contaminated alluvial soils of the Neckar River in Germany, Lehn and Schoer (1987) also reported such behavior for winter rape.
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Fig. 2. Concentration ratios (CRfractional,slag) of As, Cu, Pb, Tl, and Zn (in mg kg-1 dry plant per mg kg-1 slag in mixture) for various vegetables grown on a red slag–soil mixture 1:1 (filled gray bars) and a black slag–soil mixture 1:1 (filled black bars) as compared with the CRcontrol soil of the metals in the control soil (hatched bars). Error bars are standard deviation (n = 2).
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Since for a given metal the extent of metal accumulation by a plant is known to be quite plant specific, it is not unexpected that both CRcontrol soil and CRfractional,slag were also found to be plant specific. If, however, a given plant is known to accumulate a given metal only slightly (or strongly), one might expect that this tendency is also observed for a slag-contaminated soil. As a result, the relative metal availability (RMA) of a metal, defined here as RMA = CRfractional,slag/CRcontrol soil, should depend much less on the plant species. Also, values of RMA >1 (or <1) imply that this plant is able to take up the metal from the slag to a larger (or smaller) fraction than from the control soil. For this purpose we calculated this quantity for each metal and each vegetable. The resulting values clearly reveal that for a given metal the RMA can be larger or smaller than unity (Fig. 3). In addition, it still depends to some extent on the vegetable species. In the case of arsenic the RMA for red slag is only 0.12 for kohlrabi, while it is as much as 0.64 for mangold (Fig. 3a). A similar difference is found for Zn, where the RMA for lettuce or mangold from both types of slag is always more alike but quite different to that found for beans. For Tl from red slag, the RMA is as high as 2.5 for carrots, but only 0.5 for beans. For several vegetables, however, the metal uptake from slag with respect to the control soil is reduced by the same factor. This is the case, for example, for the uptake of Pb from red slag by beans and celery, where the same values of 0.12 for the RMA were found (see Fig. 3a), even though the uptake of this metal from the control soil is quite different for these vegetables (see Fig. 1). Also, for many other vegetables, rather similar RMA values were observed. For the uptake of As from red slag this is the case for carrots, beans, and celery; or for the uptake of Cu from red slag it is found for kohlrabi, carrots, celery, mangold, and lettuce. Also, for the uptake of As from black slag, such a behavior is observable for carrots, beans, celery, and mangold. Rather high values for the RMA were found for the uptake of As and Zn from red slag by mangold, and for As and Pb from black slag by lettuce. Values of the RMA above 1.0 were observed only for the uptake of Tl from both types of slag by kohlrabi, carrots, and celery, suggesting that Tl from the slags is more available to these vegetables than soilborne Tl. In both slags, Cu seems to be least available for the uptake by the vegetables (see Fig. 3), even though most vegetables accumulated this metal from the control soil considerably (see Fig. 1).
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Fig. 3. Relative metal availability (RMA) of As, Cu, Pb, Tl, and Zn for root uptake from slag with respect to its uptake from a control soil for various vegetables. (a) red slag, (b) black slag. The RMA is obtained for each metal and vegetable as RMA = CRfractional,slag/CRcontrol soil.
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CONCLUSIONS
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The transfer coefficients for plant uptake of As, Cu, Pb, and Zn from soils contaminated by two slags were considerably smaller compared with an uncontaminated soil. For Tl, this was not the case for all vegetables. The data revealed that for a given type of slag and a given metal not only the concentration ratios, but also the relative availability of a metal in the slag for plant uptake with respect to its uptake from a control soil, depended strongly on the plant species. Thallium from both types of slags was more available for plant uptake by kohlrabi, carrots, and celery than soilborne Tl. For beans, this was not the case. For several vegetables, however, the availability for root uptake from slag with respect to the control soil was reduced by the same factor. The results thus demonstrate that the factor by which the metal uptake of a plant from slag is decreased (or increased) with respect to an uncontaminated soil can be quite plant specific. This suggests that some plants are obviously able to mobilize the metals in the slag to an extent that is difficult to anticipate. Determination of CRfractional,slag and of the metal availability with respect to a control soil (RMA) should be useful to quantify this behavior.
As a result of these plant-specific effects for metal mobilization, it is not surprising that attempts to estimate in the laboratory the availability of a metal for plant uptake from any solid contaminant (e.g., slags, fly ashes, tailing material) by leaching tests with extractants are usually only moderately successful (e.g., Taylor et al., 1992, 1993; Pitchel and Salt, 1998).
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ACKNOWLEDGMENTS
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This work was partially supported by the Bayerisches Staatsministerium für Landesentwicklung und Umweltfragen, München, Germany.
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REFERENCES
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ABSTRACT
INTRODUCTION
