Environ. Sci. Technol. 2003, 37, 4417-4424
Magnetic Response of Soils and Vegetation to Heavy Metal PollutionsA Case Study N E L I V . J O R D A N O V A , * ,† DIANA V. JORDANOVA,† LUDMILA VENEVA,† KITKA YOROVA,‡ AND EDUARD PETROVSKY§ Geophysical Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl.3, 1113 Sofia, Bulgaria, University of Forestry, Sofia, Bulgaria, and Geophysical Institute, ASCR, Bocni II/1401, 141 31 Prague 4, Czech Republic
Fast and cost-effective detection of industrial pollution can significantly promote its ecological, economic, and social assessment. A magnetometric method, used for qualitative determination of anthropogenic contamination, meets these requirements but needs further development in more quantitative terms. It could be used successfully in numerous cases when the heavy metals coexist with strongly magnetic iron oxide particles in the source dust. We present an integrated magnetic and geochemical study that examines the utility of magnetometric techniques for rapid, qualitative detection of metallic pollutants in soils and vegetation. The new aspect of our approach, in comparison with previously published articles on this subject, is the combined investigation (magnetic and geochemical) of both soils and vegetation, thus using an additional medium for employing the magnetometry as a pollution proxy at a site. The study area is a small (∼3 km2) region in the suburbs of Sofia (Bulgaria), with the main pollution source being a metallurgical factory. Soil samples have been taken from the topmost 20 cm from private gardens, located at different distances from the factory. Vegetation samples were taken from ryegrass (both leaves and roots) and leaves from two kinds of deciduous trees (maple and acacia). The results show that both vegetation and soils are characterized by enhanced magnetic properties, compared to background material, which is due to the presence of magnetite particles of anthropogenic origin accompanying heavy metal emissions. SEM images and microprobe analyses reveal the presence of a significant amount of particles, containing heavy metals (including iron) in vegetation samples taken close to the main pollution source. Correlation analyses show a statistically significant link (correlation coefficients ranging from 0.6 to 0.7) between magnetic susceptibility and the main heavy metals (Cu, Zn, Pb) in soil samples, indicating that the magnetic susceptibility can provide a proxy method for identifying the relative contribution of industrial pollution
in soils and vegetation, that is reliable, inexpensive, and less time-consuming than standard chemical analyses.
Introduction Environmental pollution caused by different industrial activities has direct influence on the quality of life and health risks. A recently developed magnetometric method (magnetometry) for mapping anthropogenic heavy metal pollution (for a review see ref 1) uses one easily measurable physical parameter of the solid matter - magnetic susceptibility (χ). Generally magnetic susceptibility reflects the concentration of strongly magnetic iron oxide particles in a material. The application of magnetometry as a proxy method for the detection of environmental contamination is based on the fact that heavy metal pollution in many cases is accompanied by emissions of ferromagnetic particles (2-4) because of the abundant presence of Fe in natural resource materials. Many mineralogical and geochemical studies have proven the presence of a significant amount of iron oxides (magnetite, maghemite, hematite) together with different heavy metals in fly ash and waste products from the steel industry (1-7). The magnetometric method, which uses field and/or laboratory measurements of magnetic susceptibility, is a fast and inexpensive technique, compared to classical chemical methods. The instruments are commercially available and possess high sensitivity. Furthermore, a large number of samples can be processed within a short time (e.g., when using samples, a single magnetic susceptibility measurement takes approximately 3 min). It is also possible to perform a fast qualitative assessment of the relative pollution impact directly in the field. Further refinement and calibration of the methodology would allow the technique to be used as semiquantitative tool for field mapping. This requires systematic laboratory studies of different magnetic characteristics and independently measured contents of the main heavy metals in sets of companion samples. Soils and sediments are most often the subject of the magnetic studies applied to environmental problems (e.g. refs 2, 3, and 5-8), and only a few results for needles (9, 10) and tree leaves (11) are reported. In our study we present quantitative data on the heavy metal content and magnetic characteristics obtained for soils and vegetation (grass and tree leaves) as well as source material (slag from production kilns) from a small industrially polluted area in Bulgaria. In this aspect, our study could be considered a “first step” investigation, which elucidates the physical background of the magnetic susceptibility method as a proxy for environmental pollution and shows its potential for fast screening and monitoring of environmental quality. Magnetic susceptibility of a soil is a function of several natural factors that vary with depth along a soil profile. The absolute values of magnetic susceptibility of a natural nonpolluted soil depend on five major soilforming factors (time, parent material, climate, geomorphology, vegetation) (32, 33). Thus, background susceptibility values should always be determined in an unimpacted soil of similar genesis in order to evaluate the relative enhancement due to anthropogenic influence.
Samples and Methods * Corresponding author phone: ++359 2 9793958; fax: ++359 2 9713005; e-mail:
[email protected]. † Bulgarian Academy of Sciences. ‡ University of Forestry. § Geophysical Institute, ASCR. 10.1021/es0200645 CCC: $25.00 Published on Web 09/05/2003
2003 American Chemical Society
The study area covers about 3 km2 of Novi Iskar, a suburb of Sofia, which is a district characterized by a significant concentration of industrial activities. The main pollution source is a recent metallurgical plant for Al-melting and production of different Al-alloys. The site was formerly a VOL. 37, NO. 19, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Magnetic Susceptibility and Heavy Metal Content for Vegetation Samplesa
vegetation
sample
magnetic susceptibility χ (×10-9 m3/kg)
ryegrass ryegrass tree maple acacia
roots leaves
926.3 188.1
leaves leaves
117.3 78.1
ryegrass
roots leaves leaves leaves
145.3 38.5 76.0 33.0
maple ryegrass a
Pb (mg/kg) Polluted Samples 74.0 19.0 24.0 20.0
Background Samples 6.5 11.0 13.2
Cu (mg/kg)
Zn (mg/kg)
Mn (mg/kg)
28.5 12.0
146.0 74.5
641.5 140.0
10.0 8.0
36.5 28.5
107.0 78.0
9.0 11.0 n.d.
47.5 43.5 22.0
49.5 190.5 188.0
Heavy metal analyses are performed on single representative samples (see “sample preparation” in the text); nd - not detected.
Pb-melting factory. The smelting process in the kilns is accomplished with addition of Cu, Mn, Si, Fe, Ni, Zn, Pb, and Ti. Atmospheric emissions from the factory include dust, ash, SO2, and NO2. The main sources of particulate pollution are the kilns, which are not well equipped with the necessary filtering systems. A slag sample was obtained from the smelting kilns and investigated for heavy metal concentration and magnetic susceptibility, thus serving for a direct characterization of the wastes coming from this pollution source. To compare the impact of industrial heavy metal pollution both on soils and vegetation, two types of samples have been investigated: (1) leaves of deciduous trees and leaves and roots of a species of grass (ryegrass) and (2) soil samples taken from the surface of private gardens. The vegetation samples were taken from an area at about 500 m away from the main pollution source. The materials examined are roots and leaves of ryegrass (Lolium perenne) and leaves from two broad-leaf tree speciessmaple (Acer platanoides) and acacia (Robinia pseudoacacia). For the evaluation of the “background” signal in vegetation and soils, samples from ryegrass (roots and leaves) and from the soil profile were taken from a site situated about 3 km away from the pollution source, supposed to be the least affected by the pollution. Because broad-leaf species were not available in this area, a background leaf signal was evaluated from maple tree leaves taken at a nearby mountain (ca. 20 km from the study area) National Reserve from a locality situated at 1000 m above sea level. Soil samples were collected at different distances (501000 m) from the main pollution source. They are representative of the uppermost 0-20 cm of the solum. Two soil typessChromic Luvisols and Fluvisols (according to the FAO-UNESCO classification scheme) developed on Quaternary sediments (colluvium, alluvial clays, sand, gravels) are found in the area. The vertical soil profile of the Chromic Luvisol, representing the background signal, was sampled at 5 cm intervals, over a total thickness of 60 cm.
Laboratory Measurements Sample Preparation. Vegetation Samples. Each maple or acacia leaf sample represents an average sample of leaves, taken from 3 to 5 trees at 1-3 m height from the southern part of the tree. Tree leaves for one sample were collected from trees found in an area of about 6-8 m2. The leaves were air-dried in the laboratory. Preparation procedure for Atomic Absorption Spectroscopy (AAS) analyses was as follows: (1) leaves were milled to obtain a homogeneous leaf powder; (2) leaf powder was burnt in a muffle furnace at 550 °C; and (3) 10 g of the leaf ash was treated with 10 mL of HCl acid. The solution was filtered through a standard filter until no plant residue remained. Then it was digested with distilled deionized water. 4418
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The grass species were taken with roots. The average grass sample represents material from 3 to 5 single plants of the respective grass species with roots. A single grass sample was collected from an area of about 1 m2. In the laboratory roots were removed from the stems. The roots were washed with distilled water for removal of all adhering particles. The grass samples were air-dried. The leaves and roots were milled separately to obtain homogeneous material. Soil Samples. Soil samples were taken from private gardens, used mainly for agriculture. Each sample is a mixture of 20 single soil samples, taken along the two diagonals of a square shaped area (1 × 1 m) in the gardens at 20 cm depth. In the laboratory soil samples were air-dried and sieved through 1 mm sieve. The procedure for determination of the heavy metals in soils was the following: (1) 5 g of the sieved soil was treated with 10 mL of perchloric acid and 30 mL of nitric acid and left on a sand bath for decomposition of soil to simple components and (2) the obtained soil solution was filtered through a filter with a standard diameter of pores and digested with distilled deionized water. The same methodology for heavy metal determination was applied for the slag material. The content of Pb, Zn, Cu, and Mn was determined by flame AAS using a Perkin-Elmer M-5000. Scanning electron microscopy (SEM) was performed using a JEOL JSM 35 CF, and X-ray microprobe analyses were made with a TRACOR NORTHERN TN-2000 EDS microanalyzer (analytical conditions of 25 kV of accelerating voltage and 2 × 10-9 A of beam current) for the most heavily polluted samples of ryegrass roots. Samples were covered by a 30 nm thick carbon layer. SEM observations and microprobe analyses of the interior of the roots were done on a cross-cut of the root’s interior (perpendicular to the root’s diameter). Bulk magnetic susceptibility was measured with a Kappabridge KLY-2 (AGICO, Brno). Frequency-dependent susceptibility (χfd%), made using a Bartington (Bartington Ltd.) MS2D probe, and hysteresis measurements, using a Variable Field Translation Balance at room temperature, were done at the Institut fur Geophysik in Munich. High- and lowtemperature behavior of magnetic susceptibility was measured with a Kappabridge KLY-3 (AGICO, Brno).
Results Vegetation. Mass-specific values of magnetic susceptibility and the total content of Pb, Zn, Cu, and Mn for the vegetation species are shown in Table 1. The measured high concentrations of heavy metals and the maximum magnetic susceptibility values in polluted roots of the ryegrass point to the presence of a considerable amount of magnetic particles in the material accompanying the heavy metal emission. For identification of these ferromagnetic minerals responsible for the enhanced susceptibility values, we used low-tem-
FIGURE 1. Low-temperature behavior of magnetic susceptibility for leaves of maple and acacia. Vertical dashed line denotes the Verwey transition temperature. perature magnetic measurement techniques. High-temperature magnetic methods are not applied because of the problems with strong mineral transformations due to the presence of organic matter. The low-temperature measurements of susceptibility (χ) versus temperature were carried out from -200 °C to 0 °C. This experiment is nondestructive for the naturally present minerals and is useful for detection of the strongly magnetic iron oxide magnetite (FeIII2 FeIIO4). It is detected through the structural transformation at ∼ -153 °C (Verwey transition) (17). For all leaf samples as well as roots we observed an indication of the Verwey transition, expressed as a sharp increase of susceptibility during heating to room temperature. Typical examples are shown in Figure 1. The presence of a ferromagnetic fraction in the vegetation samples studied is also indicated by the measured hysteresis (nonreversible behavior in a progressively changing magnetic field), characteristic of strong ferrimagnetic minerals (18). The obtained
FIGURE 2. SEM pictures for (a) polluted ryegrass root surface, scale bar: 100 µm; (b) interior of polluted ryegrass root; scale bar: 10 µm. A large polymineral particle (denoted in a frame) was found inside the root. hysteresis parameters (after correction for the diamagnetic contribution) are listed in Table 2. Geochemical data for the vegetation samples, supplied by the SEM technique, are combined with microprobe analyses. The photographs of the root’s surface (Figure 2a) and interior (Figure 2b) show the presence of significant amounts of Fe and other heavy metals in the ryegrass sample, visible as bright particles in a regime of backscattered electrons. Adsorbed metallic particles on the root’s surface were identified, as can be seen from the photograph of the root’s surface (Figure 2a). Microprobe analyses of several particles observed on the surface and inside the roots (Table
TABLE 2. Percent Frequency-Dependent Magnetic Susceptibility (χfd%) and Magnetic Hysteresis Parameters: Coercive Force (Hc), Coercivity of Remanence (Hcr) and Ratios: Saturation Remanence/Saturation Magnetization (Jrs/Js) and Hcr/Hc for Polluted Vegetation and Soil Samples samples
χfd%
Hc (mT)
Hcr (mT)
Jrs/Js
Hcr/Hc
Vegetation ryegrass roots ryegrass leaves tree leaves acacia maple
1.3 0.0
10.13 13.34
32.78 35.63
0.144 0.191
3.224 2.671
3.4 1.5
11.19 17.50
35.16 46.36
0.160 0.238
3.140 2.649
no. 11 no. 14 no. 2 no. 17
2.7 6.1 1.2 1.9
30.83 20.77 33.54 32.39
0.133 0.152 0.112 0.153
3.452 3.275 3.800 2.721
26-40 16-55 16-39 >40 29 56.4
0.2-0.3 0.10.02 0.17-0.02
1.8-5.0 2.8-7.5
Soil Samples 8.93 6.34 8.82 11.90
Literature Data grown magnetite crystalsa natural magnetite crushed grainsb natural magnetite crushed grainsc polluted birch leavesd polluted pine needlese power plant fly ashf
10-22 3-30 2-14
1.3
6.7 10.8
0.08 0.12
4.3 5.2
a Literature data are given in the last section for comparison: Dunlop, D. J. Geophys. Res. 1986, 91, 9569-9584. b Day, R.; Fuller, M.; Schmidt, V. Phys. Earth Planet. Inter. 1977, 13, 260-267. c Hartstra, R. Geophys. J. Roy. Astron. Soc. 1982, 71, 477-495. d Matzka, J.; Maher, B. Atmos. Environ. 1999, 33, 4565-4569. e Knab, M., Ph.D., 2001. f Kapicka, A.; Jordanova, N.; Petrovsky, E.; Ustjak, S.; J. Appl. Geophys. 2001, 48, 93-102.
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TABLE 3. Results from the Microprobe Analyses of Ryegrass Roots from the Polluted Site elements wt %
particle 1 surface
particle 2 surface
particle 3 surface
particle 4 surface
particle 5 surface
particle 6 interior
particle 7 interior
nda nda nda nda 9.7 0.5 89.6 nda nda nda nda nda nda
nda 4.4 nda nda nda nda nda nda nda nda 9.0 nda 86.7
nda nda nda nda 22.2 0.9 76.9 nda nda nda nda nda nda
4.8 9.0 nda nda 1.8 nda 64.5 nda 14.4 3.0 nda 2.5 nda
nda 5.1 nda nda nda nda 65.1 29.8 nda nda nda nda nda
nda 3.7 8.1 5.1 2.0 2.1 17.4 nda 7.8 53.8 nda nda nda
nda 5.7 7.3 3.9 nda nda 26.1 nda 18.5 38.5 nda nda nda
Al Si S Ca Cr Mn Fe Ni Cu Zn Sr Sb La a
nd - not detected.
FIGURE 3. Magnetic susceptibility variations vertically through a background soil profile. Recognized soil horizons denoted: Ap s plough horizon; B s illuvial horizon. 3) show the presence of a significant amount of iron as well as other elements (e.g. Cu, Zn, Sr, La), which are not typical for natural soil composition. Thus we suppose that these are anthropogenic particles. According to the microprobe analyses (Table 3), the large particle in the center of the photograph in Figure 2b is a polymineral, with Zn, Fe, and Cu having the highest weight-percent contributions. This suggests that particles with sizes of up to 6-8 µm can accumulate within the root tissues. Soil Samples. Background data necessary for interpreting results from soil samples are obtained from a soil profile of an uncontaminated Chromic Luvisol. Measured massspecific magnetic susceptibility variations along the soil profile shows magnetic enhancement of the top ∼25 cm (Figure 3). All trace element concentrations in the background sample are low (Pb ) 47 mg/kg; Zn ) 46 mg/kg; Cu ) 27.5 mg/kg) and characteristic of nonpolluted soils, according to the international standards (21) and compared also with the results for background heavy metal content for Bulgarian soils (34). The average magnetic susceptibility (χ) for the uppermost 20 cm of the solum is 333.1 ( 67 × 10-9 m3/kg, which is much lower than the signal from all strongly polluted soil samples. As a comparison, magnetic susceptibility of polluted soils varies from 830 to 2560 × 10-9 m3/kg. Source material (slag from kilns) shows extremely high concentrations of the studied heavy metals: Pb ) 24 750 mg/kg; Zn ) 23 325 mg/kg; Cu ) 12 863 mg/kg; and Mn ) 300 mg/kg as well as high values of magnetic susceptibility: χ ) 7847 × 10-9 m3/kg. High-temperature behavior of susceptibility (heating and cooling measurements from 20-700 °C), revealing the Curie 4420
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point (Tc) of the magnetic minerals (15), was used to identify the ferrimagnetic iron oxides responsible for the increased magnetic susceptibility of soils. Heating runs for a magnetic separate of the slag material, two soil samples taken at different distances from the pollution source, and the “background sample” are shown in Figure 4a. Two ferrimagnetic phases in soil samples could be detected from the change in slope and significant drop of the signal during heatingsone with a low Curie pointsTc ) (320-350) °C (maghemite or pyrrhotite), and the second having a Tc of ∼580 °C (magnetite). These two phases have different relative contributions to the total signal in the samples, as observed from the different heights of the high-temperature peak on the heating curves in Figure 4a. The magnetite phase is best expressed and comprises a larger part of the magnetic susceptibility in polluted soils, while in the background sample its contribution is very small. Heating to temperatures above 500 °C produces a large amount of secondary magnetite (28), which is responsible for the large increase of susceptibility on the cooling run (Figure 4a). To prove the original presence of magnetite in our samples, we need to exclude the possibility that the “magnetite” Curie point on the heating curve indicates formation of this magnetic phase during the laboratory experiment. The low-temperature behavior of the saturation remanence (SIRM) of the most polluted sample (no. 11) shows a very well expressed Verwey transition (Figure 4b), which confirms the presence of coarse-grained magnetite in the contaminated soil. The high-temperature thermomagnetic analysis of slag material (Figure 4a) confirms that magnetite with Tc ∼ 580 °C is the dominant iron oxide in the source material, which contributes to the susceptibility signal in polluted soils, depending on the distance of the site from the source. Measurements of frequency-dependent magnetic susceptibility (χfd%) (31) provide information on the relative contribution of submicron superparamagnetic (SP) magnetite grains in the material. We obtained values of up to 10-12%, which are characteristic of well-drained soils with pedogenic magnetic enhancement of superparamagnetic magnetite (32). Surface Distribution of Magnetic Susceptibility and Heavy Metals in Soils. The highest values for all of the measured parameters (heavy metal content and susceptibility) are found at point no. 11, close to the main pollution source (Figure 5a). The magnetic shift of the maximum degree of pollution at sampling point 11 (northeast of the main pollution source) agrees with one of the prevailing wind directions (NE), deduced from the wind-rose diagram. A lack of a magnetic susceptibility maximum along the westerly direction, which is one of the prevailing wind directions, could be due to the influence of local relief factors (note that
FIGURE 4. (a) High-temperature behavior of magnetic susceptibility for two polluted sites (at 200 and 250 m from the pollution source), the background site and the slag material from production kilns. Thick arrows indicate heating runs; thin arrows denote cooling runs for the corresponding sample. Note the dual y-axis for cooling run of soil samples. (b) Low-temperature behavior of saturation remanence (SIRM) for a representative polluted sample. Dashed line indicates the Verwey transition.
TABLE 4. Pearson Product Correlation between the Heavy Metals and Magnetic Susceptibility at 95% Significance Levela
a
variables
magnetic susceptibility
Cu
Pb
Zn
Mn
Cu Pb Zn Mn χfd%
0.71 (0.0003) 0.69 (0.0006) 0.61 (0.0034) 0.56 (0.0090) ns
0.60 (0.0037) 0.55 (0.0103) ns ns
0.68 (0.0007) 0.45 (0.0388) ns
ns ns
0.48 (0.0310)
Number of soil samples included in the analysis - 21; ns - not significant.
the meteorological data used are from a station situated some 14 km northwest of the study site). Another maximum in susceptibility is measured south of the main pollution source (Figure 5a). The most probable cause for this is discussed in the next section. Correlation analysis was carried out for a quantitative estimate of the similarity of magnetic susceptibility distribution and heavy metal content in the studied sites. The results are shown in Figure 5b and Table 4. All of the values were standardized by subtracting their means and dividing by their standard deviations (35). A significant correlation between χ and all of the elements was obtained within a 95% confidence level. As can be seen from Figure 5b, the highest correlation coefficient is found between χ and Cu (0.71) and the lowest was obtained between χ and Mn (0.56). The only significant correlation between percent frequency dependent susceptibility (χfd%) and the amount of different heavy metals is observed for the χfd% - Mn pair (Table 4) which we interpret to be connected with the pedogenic origin of Mn.
Discussion Vegetation. Iron is one of the elements essential for many physiological processes in plants (21). It is suggested in the literature (12, 13) that in nonpolluted leaves Fe exists in the form of plant phytoferritin, which is similar to ferrihydrite according to their Mossbauer spectra (13). Recently published work on the presence of botanical magnetite in leaf and stem clippings from grass species (16, 36) suggests also a natural biogenic pathway for magnetic enhancement with ferromagnetic particles (mainly magnetite) or aggregates of up to ∼4 µm in size in plants. Other studies (13) suggest that no
Fe2+ exists in vegetation leaves (13). Ferrihydrate is paramagnetic at room temperature (14), while pure organic matter is diamagnetic. Thus, the magnetic response (magnetization (M)) of nonpolluted vegetation samples to an external magnetic field (H) is a linear M-H dependence (15). This diamagnetic signal in our study was evidenced by the highfield portion of the hysteresis curves (magnetic fields higher than 0.5 T), where they show a negative slope. However, a clear indication of ferromagnetic response was observed in the lower magnetic fields, and the corresponding hysteresis parameters were calculated (Table 2). Our results show significantly higher susceptibility for polluted ryegrass leaves, compared to the background sample (Table 1), suggesting an enhanced iron oxide content. The same is also true for maple leaves, although the contrast in the magnetic susceptibility values for polluted and nonpolluted leaves is not large (Table 1). This magnetic enhancement is not likely to be due solely to the variation of the biogenic magnetite production (if it is present in leaves). In our opinion, this magnetite is of anthropogenic origin and had adhered as particles to the leaf’s surface. Polluted tree leaves from acacia and maple also show measurable ferromagnetic hysteresis (the calculated parameters are shown in Table 2) and magnetic susceptibility, comparable to that of the ryegrass leaves (Table 1). We hypothesize that in our case the measured high magnetic susceptibility reflects an atmospheric deposition of strongly magnetic particles on the leaf’s surface. As shown also by Matzka and Maher (11) in their study of birch leaves, between 60 and 80% of the magnetization signal was removed after cleaning the leaves. VOL. 37, NO. 19, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. (a) Surface map of magnetic susceptibility created using the Kriging method. The yearly wind rose for the study area is shown on the right. Numbers indicate the sampling sites; the white cross symbol represents the main pollution source. (b) Correlation plots for magnetic susceptibility and Pb, Zn, Cu, Mn. Solid line represents the linear approximation, dashed lines denote the 95% confidence level. Data for the source material are not included because of artificially increased correlation coefficients due to the extremely high values of the heavy metal contents and susceptibility. It is well-known that roots accumulate greater amounts of heavy metals (and also Fe) in comparison with leaves (21). In contrast to leaves, roots have been shown to contain Fe2+ (13, 22) as well. However, because of the very low Fe concentrations, the nature of this iron oxide is not reliably identified. Magnetic susceptibility (Table 1) and hysteresis measurements carried out in our study suggest that root samples contain a significantly higher amount of ferrimagnetic particles, as compared to leaves, but probably of the same grain size, as deduced from the obtained similar values of the coercivity parameters and ratios (Table 2). Because magnetite is clearly identified as the major ferromagnetic phase in the vegetation samples it is possible to interpret hysteresis parameters in terms of effective magnetic grain-sizes (16). The data for coercive force (Hc), coercivity of remanence (Hcr), and the ratios Jrs/Js and Hcr/ Hc (Table 2) compared with typical values for synthetic magnetite (Table 2 (19, 20)) suggest that magnetite particles in vegetation are probably in the range of 1-3 µm in size. Relatively high values of Hcr are consistent with the published data on atmospherically deposited dusts in urban areas (10, 4422
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11). The interpretation of the exact mode of occurrence of the ferromagnetic particles in roots is complicated by the difficulties in removing all adhering particles. As seen in Figure 2a, there is a significant amount of adsorbed metallic particles to the root surface. There is no quantitative relationship between the degree of magnetic enhancement and the content of heavy metals in root or leaf samples (Table 1). However, clearly the more magnetic the vegetation sample is, the higher the probability for stronger heavy metal pollution. Taking into account the effectiveness and speed of the magnetometric method, even this qualitative relation benefits a rapid determination of the areas subjected to increased atmospheric fallout of anthropogenic particles. The main advantage in using vegetation together with soil samples is that it offers the possibility of distinguishing contributions from atmospheric deposition of fly ash particles (leaf data) and those from direct pollutant input to the soil (i.e., via industrial discharge and the subsequent incorporation within vegetation tissues through the root system).
Soil Samples. The effectiveness of the magnetometric method as a means of assessing soil pollution depends on the degree to which magnetic as well as other anthropogenic particles are retained in the topmost soil levels. Soil retention strongly depends on the inherent soil properties, which are controlled by the soil-forming processes (33). Measured neutral soil reaction (pH varies between 7.1 and 7.7) and clay content (4.5-13.5%) favors relatively low mobility of heavy metals (21) and Fe-oxides (14). The variation of magnetic susceptibility along the “clean” soil profile (Figure 3) most likely relates to naturally occurring pedogenic processes (32). Maximum values are obtained in the uppermost level, which suggests the absence of significant leaching, podzolization, and illuviation, which would lead to vertical redistribution of both ferromagnetic oxides and the heavy metals (22-27). The fact that a Verwey transition is identified for the nonheated material indicates that magnetite was initially present in the sample and did not form as a result of laboratory heating, since as mentioned above, the low-temperature treatment does not permanently alter the magnetic properties of iron oxide minerals. Therefore, we can conclude that the magnetite phase in soil represents an anthropogenic ferrimagnetic fraction. This is consistent with the published data on the mineralogy of dust and fly ash (1, 29). Nonetheless, soil samples may contain some pedogenic ferrimagnetic particles. Hysteresis measurements on soil samples show that they possess lower coercivities in comparison with the vegetation samples (Table 2). This most probably reflects the presence of a mixture of pedogenic ferrimagnetic particles, which are indicated to be very finegrained (superparamagnetic (SP) (30)), combined with larger anthropogenic ones. The anthropogenic origin of magnetite in soil samples is also supported by the thermomagnetic analysis of the slag material (Figure 4a) in which only magnetite-Tc is present. It is well-known that Mn and Fe oxides are common products of pedogenesis (33). Since high values of the frequency-dependent magnetic susceptibility are usually used as proof of the pedogenic origin of ultrafine iron oxide grains, this magnetic parameter is very helpful in our case for addressing the origin of the second susceptibility maximum found at the site south of the main pollution source (Figure 5a). The lack of correlation between magnetic susceptibility and the percent frequency-dependent susceptibility (Table 4) indicates that the main susceptibility (χ) variations are due to magnetic enhancement as a result of industrial pollution. This is supported also through the significant positive correlation between χ and Cu, Pb, Zn (Table 4). The obtained positive correlation between χFD% and Mn, in contrast to the other heavy metals (Table 4), suggests that their origins are linked. Consequently, higher susceptibility and Mn values found in the area south of the pollution source likely reflect the contributions of a superparamagnetic pedogenic fraction and Mn oxides but not an anthropogenic input. The laboratory magnetic and spectroscopic study of soils and vegetation samples presented here demonstrates the high potential and reliability of the magnetometric method as a fast and inexpensive delineation of environmental pollution in different areas where ferromagnetic particles are brought to the environment together with heavy metals. A clear identification of the major pollution source is evidenced in our study by comparison of the magnetic susceptibility and heavy metal analyses of variably polluted soils and slag material taken from the smelting kilns of the pollution source. Not only soils but also vegetation respond to an increased loading of anthropogenic ferrimagnetic particles, which are linked to heavy metals. Identified polymineral particles on the surface and the interior of roots of ryegrass are also proposed to be of anthropogenic origin.
Our results show that there is a significant correlation between the dominant trace element composition and magnetic susceptibility of soils and vegetation samples in the studied area. This demonstrates that in many cases the magnetic method is a reliable and powerful technique for identification of the relative contribution of industrial pollution. It can be applied as a standard proxy method prior to more laborious and expensive chemical analyses in various environmental situations concerned with both soils and vegetation.
Acknowledgments This study is supported by the Bulgarian Ministry of Science and Education through contract No. MU-F-1201/02 and a bilateral cooperative project between the Bulgarian Academy of Sciences (Geophysical Institute) and the Czech Academy of Sciences (Geophysical Institute). We are grateful to three anonymous reviewers for their useful suggestions and comments, which greatly helped to improve the manuscript. We also highly appreciate valuable corrections made by Dr. J. Glen and Dr. E. Oches.
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Received for review March 18, 2002. Revised manuscript received July 17, 2003. Accepted August 1, 2003. ES0200645
