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TEM-EDX investigation on Zn- and Pb-contaminated soils
Applied Geochemistry 16 (2001) 1165±1177
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TEM-EDX investigation on Zn- and Pb-contaminated soils Martine D. Buatier a,*, Sophie Sobanska b, FrancËoise Elsass c a
DeÂpartement GeÂosciences, Universite de Franche ComteÂ, 16 Route de Gray, 25030 BesancËon, France Micro and Trace Analysis Center, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Belgium c Institut National de la Recherche Agronomique (INRA), Sciences du Sol, Route de Saint-Cyr, 78026 Versailles, France b
Received 14 December 1999; accepted 27 September 2000 Editorial handling by R. Fuge
Abstract Lead and zinc contaminated soils from a smelter area in the northern part of France have been studied by transmission electron microscopy (TEM). This study was carried out with 4 dierent soils contaminated by Pb and Zn but with dierent chemical and physical characteristics. Two soils are tilled and have a neutral or slightly basic pH, one is a wooded soil and the last one is a meadow soil with acidic pH and high total organic content . TEM images of the soil samples have been coupled with focused energy dispersive X-ray (EDX) analyses and chemical mapping on a few micron-sized windows. This study demonstrates that TEM is a particularly ecient method to investigate metal speciation in the ®ne fractions of the contaminated soils. Zinc could be detected locally in sulphide minerals probably coming from the smelter emissions, but the major phases retaining Zn are Fe-oxyhydroxides and smectites. Lead could be detected in small aggregates which were characterized by EDX and selected area electron diraction. Their structural formulae correspond to a pyromorphite-like mineral in which Pb is partly substituted by Ca and Na. Pyromorphite is present only in the wooded and meadow soils where it forms partly from amorphous Si-rich phases (slags) coming from the smelter. These results are compared with data previously obtained by spectroscopic methods on the same samples. # 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Speciation of heavy metals in soils is an important factor governing their mobility, toxicity and bioavailability. Many recent studies have focused on the characterization of metal speciation in soils by indirect approaches such as selective chemical extraction (Pickering, 1981; Schuman, 1985; Gruebel et al., 1988; Gommy, 1997) or by direct in situ analyses such as extended X-ray absorption ®ne structure (EXAFS ) spectroscopy (CotterHowells et al., 1994; Manceau et al., 1996; Morin et al., 1999; Juillot et al., in press). These methods are particularly useful for determining the main speciation of heavy metals in soils. However, metal speciation in contaminated soils can take various forms and it is important to use complementary methods at dierent scales of investigation, in order to characterize all the solid phases * Corresponding author. Fax: +33-0381-666558. E-mail address: martine.buatier@univ- fcomte.fr (M.D. Buatier).
that can retain heavy metals to be able to understand the reactivity of these pollutants in a given environment. In this study, the authors present a TEM-EDX investigation on contaminated soils that have been previously studied by the methods speci®ed above. TEM-EDX is a particularly ecient method for direct in situ characterization of all phases, either crystallized or amorphous, that could contain heavy metals. This permits a better knowledge of the heavy metal distribution in the ®ne solid fraction of soils. In the present paper, TEM-EDX analyses are interpreted to establish the speciation of Zn and Pb in contaminated soils from the northern part of France, in a smelter area. 2. Sample selection The studied soils come from one of the 896 polluted sites recognized in 1996 by the French Ministry of Environment. They are located in the northern part of France at Evin-Malmaison, 40 km south of Lille, in the
0883-2927/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(01)00015-4
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largest Pb and Zn processing area in Europe. The contamination of the soils is mainly related to atmospheric emissions of Pb- and Zn-bearing particles scattered around the central smelter. A detailed characterization of the dust emissions from the smelter suggests that Pb and Zn sul®de together with Pb sulfate and oxysulfate are the major Pb and Zn bearing solid phases (Sobanska, 1999). The slag particles also contain heavy metal-rich particles formed during the smelter processing. These by-products are generally accumulated in a waste dump. They are mostly composed of amorphous phases containing Ca, Fe, Si, Pb (0.5±3%) and Zn (10±12%). Zn±Fe-oxides and metallic Pb were main forms identi®ed (Sobanska et al., 1999). All these particles have been transported by the wind and pollute the surrounding soils. Consequently, the concentrations of heavy metals now exceed 200 ppm throughout a 40 km2 area (Godin et al., 1985). The studied samples come from super®cial horizons from tilled, wooded and meadow soils. All samples come from a highly polluted area located downwind in the dominant wind direction, 1000 m away from the smelter but they have dierent metal concentrations, dierent pH and total organic C (TOC). (Fig. 1). The wooded soil has not been disturbed for about 50 a, one of the tilled soils has not been disturbed for about 30 a (T1) the other one (T2) is still exploited. The natural meadow has never been tilled. Because heavy metals are concentrated in surface horizons (Gommy, 1997; Juillot, 1998; Sobanska, 1999;
Sterckeman et al., 1996, 2000), only surface horizons (0± 20 cm) were investigated in this study (Table 1). Bulk concentrations of Zn are 570±1865 ppm in surface horizons and show an exponential decrease with depth. Lead concentrations display the same distribution with depth, with a maximum value in the surface horizons ranging between 460 and 2670 ppm. The pH of soils is naturally slightly acidic in organic-rich horizons (Baize, 1988); pH values are below 6 for the natural wooded and meadow soils, while it is 7.5 and 8.5 in the tilled soils, because of the lime amendment (Table 1). Previous investigations on the studied soils showed that Pb and Zn are preferentially concentrated in the <2 mm fraction (70 and 45% in the tilled soils and wooded soil surface horizons respectively). The clay fraction is composed of smectite, kaolinite and mica. Smectite is the major phase present in the clay assemblages (HargeÂ, 1997; Juillot, 1998; Sobanska, 1999). In the dense fractions (>2.89 g/cm3) Fe oxides, a Si-rich amorphous phase and locally, Zn-sul®de were detected (Juillot, 1998; Sobanska, 1999). 3. TEM-sample preparation and analytical procedures The samples were impregnated following the procedure established by Tessier (1984) and Elsass et al. (1998): this procedure consists of progressive exchanges of the interstitial water with alcohol, solvent and then Spur resin.
Fig. 1. Map location of the studied samples from the Evin-Malmaison site (France) (modi®ed after Sobanska, 1999).
M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177 Table 1 Chemical characterization of the 4 studied soils. Site of EvinMalmaison (France) (after Juillot, 1998 and Sobanska, 1999) Samples
pH
TOC
Zn
Pb
Fe
Ca
(0±20 cm) Wooded Tilled (1) Tilled (2) Meadow
5.5 8.5 7.5 5.8
(%) 6.5 4.2 1.5 13
(ppm) 1400 965 570 1865
(ppm) 2000 1060 460 2670
(ppm) 36,700 51,300 50,000 21,839
(ppm) 5800 24,400 11,600 10,840
Ultra-thin sections were obtained by ultramicrotomy using a diamond knife and sedimented on Cu- or Ti-grids. The most detailed TEM investigation was focused on the surface horizon from the wooded soil. Polished thin sections were prepared from bulk samples that were previously impregnated following the method described above. Clayrich selected areas were then removed from the glass, drilled, stuck on a 2 mm Cu-disk and Ar ion-milled. TEM investigations were carried out on a Philips CM30 at 300 kV (Centre commun de Microscopie, Universite de Lille). TEM images were complimented by selected area diraction pattern (SAED) in order to distinguished amorphous and crystallized phases. EDX analyses were performed with a Ge detector and careful calibration using standards permitted quantitative analyses of major elements and of Pb and Zn with a detection limit of about 100 ppm. Analyses can be carried out with a spot size of 3 nm, however, EDX analyses were carried out in STEM mode using a square window of about 20 nm for the clay mineral analyses, which are easily destroyed by the beam. EDX spectra were recorded in a 20 eV channel in order to detect all the Pb peaks. In order to examine the possible correlations between major elements and heavy metals in the ®ne fraction of the studied soils, element maps were produced with a Philips STEM420 microscope operated at 120 kV (Sciences du sol, INRA). Element mapping was performed on several areas from dierent samples from the wooded soil. EDX analyses were performed with a windowless detector and an Oxford Link AN10000 system. Being aware of the superposition of S-Ka peak at 2.3 keV with Pb-Ma peak, the Pb-L peak at 10.55 keV was also recorded. The scanned areas were about 24 mm2 at 12,500 magni®cation and 96 mm2 at 6400 magni®cation. Analytical conditions were chosen in order to optimize the detection of minor elements versus the resolution: a spot size of 30 nm and an acquisition time of 100 ms/step were selected. A rectangular grid was scanned. For a resolution of 128 lines128 columns, each pixel represents a mesh 25 nm wide over 16 nm long at magni®cation 25000, and double at magni®cation 12500. Data from element maps were treated statistically in order to calculate the inter-element correlation coecients. The peak intensity measurement was corrected for each pixel after substracting the background. Background was selected
1167
in a window corresponding to the Cr Ka peak because this element is absent in the studied samples. Correlation coecients (r) were then calculated from the analytical point having intensity higher than zero. 4. Results and discussion 4.1. Low magni®cation, TEM images and chemical mapping The soils display a typical silt texture with large grains of quartz in a clay-rich matrix (Fig. 2). The bulk sample of the surface horizon of the wooded soil is composed of quartz, clays and small grains of about 100 nm with a strong contrast. Rounded aggregates composed of amorphous particles were also locally observed. The clays are mostly smectite or smectite-illite mixed-layers, but mica and more rarely chlorite have also been observed. The small aggregates are generally located in the clay matrix. The element maps in Fig. 3a correspond to the area presented in Fig. 2. Each square displays the spatial distribution of one element. Since the background has been subtracted from each element map, pixel per pixel, these maps ®gure out the net intensities of the signals recorded in energy windows characteristic of the speci®ed elements. The intensity of each peak is related to the presence of the speci®ed element and to its relative concentration. The correlation should not be aected by the latter but the element concentration must be signi®cant to be taken into account in the correlation coecient calculation. In both selected maps, the presence of alumino-silicates (mostly clay minerals) in the ®ne fraction of the studied soil is con®rmed by the O, Si and Al rich area (and ocasionally K and Fe). Zinc and Pb concentrations are signi®cant. The S distribution map is compared with the Pb-L distribution in order to discriminate the contributions of S and Pb in the S map. Fig. 3a shows that S is concentrated in a relatively large area (>1 mm), where Zn is also detected but not Pb-L. This area is also O poor, suggesting the presence of a Zn sul®de species rather than a sulfate. ZnS particles were detected in small quantities by SEM and XRD analyses in the dense fractions of the wooded and tilled soils by Sobanska (1999) and Juillot (1998). These particles correspond to atmospheric dust deposition, originating from smelter emission. Similar results were obtained in other areas from the same sample. In the element maps presented (Fig. 3b), ZnS is also detected on the right side of the map. Phosphorus-rich phases mostly concentrate Pb with Cl and Ca. Besides the phases bearing these elements, the matrix is composed of silicates, mainly clay minerals. Silicates are detected from their high concentrations of Al, Si, O and Fe. The calculated correlation coecient r between elements con®rms these inferences. Table 2 presents the
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Fig. 2. Low magni®cation TEM image of the wooded soil (microtome sample) from the Evin-Malmaison site (microtome sample ultra-thin section).
correlation factors calculated from Fig. 3a. The good correlation coecient obtained for O, Al, Si and K con®rm the presence of clay minerals. The Zn and S correlation coecient is close to 1, con®rming the preferential occurrence of Zn in sul®de form in the analyzed samples. Good correlations are also observed between Pb and P, P and Ca, Pb and Cl, P and Cl. The Pb-S correlation can not be related to mineralogical occurences because the S Ka peak intensity may have been modi®ed by the Pb-Ma contribution. In the results presented here, the Pb-S coecient is relatively low because the Pb-Ma contribution is lower than the S-Ka peak. It is noticable that the correlation coecient between major elements like Si or Al with Pb or Zn is low. In the most of the maps acquired for this study, Zn is detected only in ZnS grains. The possible correlation of Zn with clay particles can not be detected here because of the low concentration of Zn in the clay matrix. In order to determine more precisely the speciation of Pb and Zn in the ®ne fraction of the 4 soil samples, TEMEDX analyses were pinpointed on individual metalbearing particles. 4.2. Particle characterization 4.2.1. Zn speciation With the exception of small crystals of Zn sul®de, the detection of Zn by EDX analyses is generally related to the presence of clay minerals. Iron-rich particles with or
without Zn are also present in the ®ne fractions of the 4 studied soils. These particles are observed in aggregates with a strong contrast. These iron-rich phases (probably Fe-rich oxy-hydroxides) are commonly associated with clay minerals (Fig. 4) and it was impossible to obtain any analysis of isolated particles. For this reason, Table 3 presents EDX data on smectite particles and on smectite-oxy-hydroxide mixtures. In many analyses focused on smectite particles, a low concentration of Zn is detected (Fig. 5). XRD analyses on the clay fractions indicated that most smectites are of dioctahedral type (Juillot, 1998). This is con®rmed here by the EDX data. All smectites are Al- and Fe-rich (Fe is supposed to be Fe3+) with a total octahedral occupancy limited to 2 atoms per half cell. One can notice that Mg or Mn-rich smectites are not detected. Some analyses displaying a higher octahedral content probably correspond to clay particles having Fe coatings (Table 3). In the smectite and oxide analyses, Zn is present in various quantities with a maximum of 1.5 at.%, its concentration being unrelated to the abundance of Fe. These data suggest clearly that Zn is both retained by clay minerals and/or Fe-rich particles; the latter ones having the peculiarity to occur either as individual grains or as coatings on clay mineral surfaces. The speciation of Zn in clay minerals has been the object of many experimental studies. Several authors have described exchange reactions between soluble Zn and smectite exchangeable cations (Tiller et al., 1984; Brigatti
M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177
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Fig. 3. EDX element maps of the wooded soil from the Evin-Malmaison site. Window size 4.06.4 mm at magni®cation 12500 on ultrathin section (a); window size 812.8 mm at magni®cation 6400 (b). Map (a) corresponds to the area presented Fig 2. All elements were recorded from their Ka line except Pb which was recorded from its La line.
et al., 1996; Ma and Uren, 1998). The maximum adsorption of Zn on smectite was observed for a swelling clay having a high speci®c surface (190 m2/g) at pH ranging between 6.5 and 8 (Tiller et al., 1984). According to Brigatti et al. (1996), the adsorption of Zn is enhanced when the total layer charge is due to an octahedral
charge de®cit. Ma and Uren (1998) showed that Zn is not simply exchanged with K and Na but forms Zn(OH)+ complexes in the hexagonal tetrahedral cavities. Direct precipitation of metal-rich smectites (Zn, Co, Ni, Cu) have been obtained in laboratory experiments at low temperatures (less than 100 C) (Decarreau, 1981).
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M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177
Table 2 Cross-correlation coecients from EDX mapping data of the wooded soil from the Evin-Malmaison site (see Fig. 3a). Statistical treatment of net intensities obtained after subtraction of the background. The signi®cant coecients (>0.5) are given in bold Al Al C Ca Cd Cl Fe K Mn O P Pb S Si Ti Zn
1.00 0.24 0.06 0.29 0.08 0.38 0.71 0.12 0.80 0.14 0.80 0.03
C 1.00 0.03 0.08 0.11 0.21 0.16 0.04 0.21 0.07 0.05 0.17 0.22 0.18
Ca
Cd
Cl
Fe
1.00 0.22 0.24 0.14 0.10 0.11 0.13 0.65 0.29 0.15 0.05 0.03 0.02
1.00 0.45 0.22 0.40 0.29 0.34 0.46 0.32 0.46 0.26 0.08 0.35
1.00 0.18 0.13 0.22 0.17 0.58 0.64 0.48 0.09 0.04 0.24
1.00 0.22 0.15 0.47 0.38 0.30 0.12 0.36 0.04
K
1.00 0.14 0.56 0.16 0.11 0.58 0.02 0.10
Data concerning the speciation of Zn in contaminated soils are less abundant. In contaminated soils from the same area (south of Lille), Harge (1997), using EXAFS spectroscopy detected the presence of Zn silicates and a Zn trioctahedral smectite which was supposed to have formed in situ in the soil. The EDX analyses presented here indicate that all smectites (with or without detectable Zn) have similar compositions suggesting that neoformation of Zn-rich smectite does not occur in these soils. The EXAFS spectroscopic analyses obtained by Juillot et al. (in press) for two soils studied here (wooded and tilled soil II), demonstrated that the major speciation of Zn is its adsorption on Fe oxy-hydroxide or its incorporation in phyllosilicates. TEM-EDX analyses are
Fig. 4. General aspect of smectite particles and associated Ferich phases (probably Fe-oxyhydroxides) on the tilled soil sample from the Evin-Malmaison site.
Mn
1.00 0.15 0.19 0.11 0.14 0.13 0.05 0.10
O
1.00 0.32 0.08 0.05 0.88 0.16 0.03
P
1.00 0.68 0.48 0.17 0.06 0.21
Pb
1.00 0.39 0.02 0.02
S
1.00 0.03 0.90
Si
1.00 0.04 0.04
TI
1.00
Zn
1.00
compatible with these interpretations but more detailed investigations by HRTEM on the smectite particles are needed to support this assumption. 4.2.2. Pb speciation According to EDX analyses, Pb occurs in Fe±Mn rich particles always in association with Zn which probably correspond to the oxy-hydroxides described above. This result con®rms previous investigations carried out on the same samples with dierent techniques. Indeed, these studies indicated that Pb can be adsorbed on Mn and Fe oxy-hydroxides (Gommy, 1997, Juillot, 1998; Morin et al., 1999; Sobanska, 1999). TEM analyses suggest that Fe-oxyhydroxide are particularly abundant in the tilled soils. In wooded and meadow soils, distinct Pb-rich grains associated with Ca, Fe, P and Na are detected in amorphous Si-rich phases (Fig. 6). It is noted that amorphous Si-rich phases are commonly observed in the studied samples but not always with Pb. Finally, Pb-grain aggregates of about 0.2 mm diameter are also common in both organic rich soils. The EDX spectra of small particles revealed high concentrations of Pb with P, Cl, Ca, O and sometimes Fe, Zn (Fig. 7). The P±Pb, P±Ca, P±Cl and Pb±Cl correlations observed from the element map data are con®rmed by these particle analyses. Data obtained from the elemental map con®med that Cl is present in the Pb-rich aggregate because the Cl-Ka map displays signi®cantly lower Cl concentration in the matrix and the spurr resin compared with the small Pb and P-rich aggregates. These element associations are compatible with the presence of a pyromorphite phase (Figs. 6 and 7). They correspond to small aggregates of about 0.2 mm with a large amount of Pb with P, Ca, Na and Cl (Figs. 6 and 7). Iron and Zn detection could be
M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177
due to the presence of mixtures of pyromorphite grains with Fe±Zn oxides or to a pyromorphite with small amounts of Fe and Zn as described by Cotter-Howells et al. (1994). A structural formula can be calculated for the pyromorphite-like phase from EDX analyses: Pb4(Ca)y, Na x, (PO4)3Cl, with y=2±2x. This formulae is typical for an apatite group mineral. It does not correspond to the pure pyromorphite end-member, but rather to a solid solution with Ca, Na and Pb substitutions.
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The presence of pyromorphite was con®rmed by the SA electron diraction pattern. SAED on isolated grains of pyromorphite reveals that they are poorly crystallized; however, a SAED pattern was obtained in aggregates which showed the most intense re¯ection at 2.98 AÊ corresponding to the (211) and (112) re¯ections (Fig. 8). The other re¯ections could not be clearly identi®ed probably because of the poorly crystallized pyromorphite grains. Pyromorphite seems to be abundant in the wooded soil
Table 3 Chemical analyses of clay particles and oxy-hydroxyde-clay mixtures from the Evin-Malmaison site (all soils, ultra-thin sections and ion-milled microdrillings). (A) Clay mineral structural formulae are calculated from EDX punctual analyses (in atm.%) on the basis of 22 negative charges. Total O corresponds to the number of cations in octahedral site. (B) Mixtures of smectites and oxy-hydroxides analyses are given in atm.% A
Wooded soil
No.
1
2
3
4
Si AlT CT
3.66 0.34 0.34
3.90 0.10 -0.10
3.09 0.91 -0.91
3.45 0.55 -0.55
3.37 0.63 -0.63
3.58 0.42 -0.42
3.48 0.52 -0.52
3.37 0.63 -0.63
AlO Fe Mg
1.12 0.67 0.30
1.54 0.30 0.18
0.50 0.91 0.52
0.95 0.92 0.22
0.77 1.00 0.20
1.01 0.75 0.27
1.12 0.67 0.17
Total O CO
2.09 0.04
2.01 0.14
1.93 0.73
2.09 0.05
1.97 0.28
2.03 0.18
Zn
0.03
0.01
0.08
0.02
0.03
Na K Ca Total I
0.10 0.12 0.04 0.25
0.06 0.08 0.02 0.17
0.22 0.07 0.11 0.40
0.18 0.23 0.03 0.44
0.41 0.17 0.14 0.72
Fe/AlTot.
0.46
0.18
0.64
0.61
0.71
Meadow soil No.
15
16
5
6
7
8
9
10
11
12
13
14
3.47 0.53 -0.53
3.62 0.38 -0.38
3.54 0.46 -0.46
3.60 0.40 -0.40
3.64 0.36 -0.36
3.54 0.46 -0.46
0.71 0.83 0.46
0.75 0.87 0.44
0.85 0.76 0.57
1.41 0.55 0.13
1.26 0.55 0.25
0.98 0.82 0.26
1.19 0.58 0.22
1.96 0.28
1.99 0.49
2.06 0.25
2.19 0.01
2.09 0.14
2.05 0.09
2.05 0.10
2.00 0.22
0.03
0.04
0.02
0.02
0.03
0.01
0.01
0.02
0.02
0.27 0.17 0.05 0.49
0.31 0.29 0.06 0.66
0.40 0.20 0.23 0.83
0.43 0.26 0.15 0.85
0.00 0.05 0.14 0.19
0.00 0.14 0.09 0.22
0.00 0.29 0.09 0.39
0.00 0.18 0.11 0.30
0.00 0.19 0.23 0.42
0.53
0.41
0.62
0.68
0.62
0.29
0.33
0.61
0.35
Cultivated soil 17
18
19
20
22
23
24
25
26
Si AlT CT
3.45 0.55 0.55
3.03 0.97 0.97
3.43 0.57 0.57
3.55 0.45 0.45
3.30 0.70 0.70
3.42 0.58 0.58
3.51 0.49 0.49
3.55 0.45 0.45
3.78 0.22 0.22
3.61 0.39 0.39
3.67 0.33 0.33
AlO Fe Mg
1.89 0.06 0.08
0.65 1.22 0.36
1.35 0.27 0.27
1.39 0.51 0.18
1.60 0.31 0.21
1.74 0.22 0.16
1.42 0.62 0.13
1.54 0.44 0.05
1.25 0.67 0.16
1.14 0.83 0.13
1.17 0.79 0.15
Total O CO
2.03 0.01
2.22 0.30
1.88 0.62
2.08 0.07
2.12 0.16
2.11 0.17
2.18 0.40
2.03 0.06
2.08 0.07
2.10 0.17
2.11 0.19
Zn
0.01
0.03
0.05
0.02
0.07
0.01
0.00
0.00
0.02
0.02
0.01
Na K Ca Total I
0.02 0.37 0.06 0.46
0.12 0.33 0.08 0.53
0.00 0.64 0.23 0.87
0.04 0.19 0.06 0.29
0.07 0.14 0.10 0.31
0.04 0.33 0.01 0.39
0.00 0.03 0.03 0.05
0.00 0.34 0.02 0.36
0.00 0.08 0.02 0.10
0.00 0.13 0.03 0.16
0.00 0.09 0.02 0.11
Fe/AlTot.
0.03
0.75
0.14
0.28
0.14
0.09
0.33
0.22
0.46
0.54
0.53 (continued on next page)
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M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177
Table 3 (continued) B
Wooded soil
No
27
28
29
30
31
32
33
34
35
36
37
38
39
40
Si Al Fe Mg
16.64 6.6 8.1 3.2
14.1 5.86 15.09 0.92
17.92 7.89 6.27 2.93
14.74 7.62 9.33 3.35
14.7 7.6 9.3 3.38
15.42 7.75 8.78 2.65
17.18 6.47 7.76 4.25
11.81 7.96 13.06 3.68
11.21 7.88 13.85 3.66
11.7 9.11 12.33 4.13
5.71 5.15 26.36 0.42
16.16 9.88 7.05 2.49
13.77 10.16 9.4 2.57
11.7 11.77 9.11 9.61 12.33 11.72 4.13 3.59
Zn
0.23
0.12
1.03
1.24
1.24
1.18
0.48
1.16
1.26
0.42
0.52
0.43
0.96
0.42
0.75
Na K Ca
0.68 3.48 0.12
0.05 0.72 1.04
0.59 0.55 0.6
0.57 1.01 0.93
0.68 1.01 0.93
0.67 1.11 1.05
0.37 1.12 0.59
0.36 0.73 0.35
0.86 0.33 0.21
0.65 0.32 0.37
0.54 0.23 0.52
0.38 1.4 0.34
0.58 0.46 0.57
0.65 0.32 0.37
0.65 0.54 0.45
60.95 1.23
62.09 2.58
62.21 0.79
61.21 1.22
61.15 1.22
61.4 1.13
61.78 1.20
60.89 1.64
60.74 1.76
60.47 1.35
60.54 5.12
61.87 0.71
61.52 0.93
O Fe/Al
Meadow soil 41
60.47 60.92 1.35 1.22
Cultivated soil No.
42
43
44
45
46
47
48
Si Al Fe Mg Zn
12.33 3.75 20.13 0.69 0.13
10.09 4.38 22 0.87 0.31
8.41 3.58 25.28 0.64 0.18
10.96 4.33 21.28 0.53 0.1
9.08 4.68 22.76 0.94 0.23
8.52 4.8 24.04 0.62 0.19
17.3 8.39 8.73 1.26 0.14
Na K Ca O
0.04 0.4 0.5 62.03
0.14 0.06 0.56 61.59
0.03 0.3 0.25 61.33
0.05 0.24 0.71 61.81
0.25 0.23 0.55 61.28
0 0.16 0.25 61.43
0.15 0.79 0.54 62.7
5.37
5.02
7.06
4.91
4.86
5.01
1.04
Fe/Al
samples. Pyromorphite-like phases with very similar composition were identi®ed in the wooded soil and in the meadow, Pb is the major cation but Ca and Na are present in variying amounts. This mineral was not identi®ed in the tilled samples. Intermediate phases between amorphous Si rich-phase with Pb and Ca (Fig. 6) and crystallized aggregates of pyromorphite (Fig. 7) are observed only in the wooded soil. This observation suggests that a part of the pyromorphite grains could have formed by weathering of glassy-like particles with Si, Ca, and Pb coming from slag materials or from in situ Si-rich amorphous phases and P and Cl from the soil itself. Lead-phosphates are a common product of soil alteration. They are generally found as Ca enriched Pb phosphates and not as a pure end-member (Cotter-Howells et al., 1994). Pyromorphite was identi®ed in mine taillings from Leadville, USA (Morin et al., 1999), and soils contaminated by mine wastes in Derbyshire, UK (Cotter-Howells et al., 1994). In these two examples, pyromorphite was identi®ed by EXAF spectroscopy suggesting that it was one of the major Pb species in these soils. Lead phosphates (pyromorphite and plumbogummite) are considered as some
of the most insoluble Pb (II)-solids known to form under surface conditions (Nriagu, 1974, 1984)), and so the precipitation of pyromorphite constitutes an ecient mechanism to decrease the Pb availability. Laboratory experiments (Ma et al., 1993; Valsami-Jones et al., 1998; Zhang et al., 1997; Zhang and Ryan, 1999) demonstrated that pyromorphite or hydroxypyromorphite is the main compound formed from solid Pb-bearing forms such as PbS, PbSO4 and PbCO3 on reaction with hydroxyapatite. This reaction occurs at pH ranging from 3 to 7. These experiments demonstrated that addition of phosphate amendment could increase the immobilization of Pb in contaminated soils. Laperche et al. (1996) showed the importance of pH by examining the interactions of selected Pb minerals and a contaminated soil with apatite. These authors showed that the extent of reactions were pH dependant with more hydroxypyromorphite formation at pH 5 than at pH 6 or 7. The major Pb compounds found in dust emissions deposited in the studied soils are sul®de and sulfate; furthermore, the soils in which pyromorphite formed have pH values around 6 suggesting that the mechan-
M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177
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Fig. 5. Typical features of soil smectite particles from the Evin-Malmaison site: example of the wooded soil. TEM image (a); EDX spectrum (b). Cu is in italic because this element is not present in smectite but its presence is related to the Cu TEM grid supporting the sample. The arrow identicates the EDX analytical area.
isms described below for the formation of pyromorphite could have been active in the studied soils. However, in the studied soil, slag particles containing Si-rich amorphous phases with Pb are also a potential precursor for pyromorphite. The eect of apatite amendments on Pb concentration in roots was studied by Traina and
Laperche, (1999). These authors showed that the presence of rhizosphere created local acidity which may have enhanced the local dissolution of apatite grains and precipitation of pyromorphite on the root surface. The association of pyromorphite and roots and the contribution of root exudates in pyromorphite forma-
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Fig. 6. Pyromorphite grains in a Si-rich amorphous matrix. TEM image (a); EDX spectrum (b). Ion-milled microdrilling of the wooded soil from the Evin-Malmaison site.
M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177
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Fig. 7. Pyromorphite grains associated with Fe±Zn oxy-hydroxydes. TEM image (a); EDX spectrum (b). Py: pyromorphite grains. Cu is in italic because this element is not present in smectite but its presence is related to the Cu TEM grid supporting the sample.
tion were found by Cotter-Howells and Caporn (1996) and Cotter-Howells et al. (1999). A greater amount of roots as suggested by the TOC (Table 1) in the meadow and wooded soils compared with the tilled soils could explain the pyromorphite detection by TEM only in these soils. Previous EXAFS spectroscopy analyses on the studied tilled II and wooded soils suggested that Pb was adsor-
bed to humic acids as well as Mn and Fe-oxy-hydroxides, but pyromorphite could not be identi®ed with this technique (Morin et al., 1999). The Morin et al. (1999) results suggest that pyromorphite was not the major Pb-containing phase in these soils, the bulk of the Pb being in the adsorbed phases. However, the identi®cation of pyromorphite by chemical mapping, TEM and EDX analyses implies that there are various species of Pb in
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tion and alteration of amorphous Si-rich phases with Ca, Pb, and Na. This study demonstrates that TEM can be a particularly ecient method for the investigation of metal speciation in soils. The results also show the complexity of the soil system and the in¯uence of its physical and chemical properties on metal speciation. Acknowledgements
Fig. 8. SAED pattern of pyromorphite grains. The most intense re¯ection corresponding to (211) and (112) re¯ections at 2.98 AÊ is showed by an arrow.
these soils. Pyromorphite appears to be the main solid phase that is able to concentrate Pb, and this species is very stable in the soil. The limited amount of pyromorphite here could be related to the relatively low amount of available P in the soils. The P% measured in the studied soils is only about 0.03% (Juillot, 1998) whereas it reaches 0.2% in the contaminated soils studied by Cotter-Howell et al. (1994) where pyromorphite was identi®ed as the major constituent. The fraction of Pb adsorbed onto mineral and organic matter surfaces in the studied soils needs to be quanti®ed as this type of speciation involves could be important in the bioavailability of Pb in these soils. 5. Conclusion The combination of element maps with focused EDX analyses allow the determination of the speciation of Zn in the surface horizons of the studied soils and the identi®cation of lead phosphate compounds in a wooded soil and a meadow. Fe oxy-hydroxides enriched in Zn and Pb are easily identi®ed by TEM, and are particularly abundant in samples taken from tilled soils. Zinc detected in smectite particles is generally present in small amounts (less than 2%). The TEM and EDX analyses suggest that direct neoformation of Zn-rich smectite does not occur in these soils. However, the location of Zn in the phyllosilicate structure cannot be established from these data. A Pb-apatite (pyromorphite) is identi®ed only in samples having high COT and slightly acidic pH, i.e. the wooded soil and the meadow. Such phosphates seem to be formed within the studied soils by in situ precipita-
We dedicate this paper to the late Philippe Ildefonse who indirectly participated in this paper through various discussions and a critical review of the data. Philippe was an active promoter of fundamental research on alteration of minerals and on metal speciation in natural soils. This work was supported by the PROSE97 CNRS/INSU Program and the PRC program. We thank an anonymous reviewer for a critical and very helpful review.
References Baize, D., 1988. Guide des Analyses Courantes en PeÂdologie. INRA eÂditeur. Brigatti, M.F., Campana, G., Medici, L., Poppi, L., 1996. The in¯uence of layer charge on Zn2+ and Pb2+ sorption by smectites. Clay Mineral 31, 477±483. Cotter-Howells, J.D., Champness, P.E., Charnoch, J.M., Pattrick, R.A.D., 1994. Identi®cation of pyromorphite in minewaste contaminated soils by ATEM and EXAFS. Eur. J. Soil Sci. 45, 393±402. Cotter-Howells, J.D., Caporn, S., 1996. Remediation of contaminated land by formation of heavy metal phosphates. Appl. Geochem. 11, 335±342. Cotter-Howells, J.D., Champness, P.E., Charnock, J.M., 1999. Mineralogy of lead-phosphorus grains in the roots of Agrostis capillaris L by ATEM and EXAFS. Min. Mag 63, 777± 789. Decarreau, A., 1981. CristallogeneÁse aÁ basse tempeÂrature de smectites trioctaeÂdriques par vieillissement de copreÂcipiteÂs silicomeÂtalliques. C. R. Acad. Sci., Paris 292, 61±64. Elsass, F., Beaumont, A., Pernes, M., Jaunet, A.M., Tessier, D., 1998. Changes in layer organization of Na and Ca exchanged smectite during solvent exchange for embedment in resin. Can. Mineral 36, 1325±1333. Godin, P., Feinberg, M., Ducauze, C., 1985. Modelling of soil contamination by airborne lead and cadmium around several emission sources. Environ. Pollut., Series B 10, 97±114. Gommy, C., 1997. Optimisation d'un ScheÂma de SpeÂciation des MeÂtaux Pb, Zn, Cd et Cu: Application aÁ des Sols PollueÂes du Nord de la France. TheÁse de l'Univ. Technologique de CompieÁgne. Gruebel, K.A., Davis, J.A., Leckie, J.O., 1988. The feasibility of using sequential extraction techniques for arsenic and selenium in soils and sediments. Soil Sci. Soc. Am. J. 52, 390± 397.
M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177 HargeÂ, J.C., 1997. SpeÂciation compareÂe du Zinc du Plomb et du ManganeÁse dans les Sols ContamineÂs. TheÁse de l'Univ. Grenoble I. Juillot, F., 1998. Localisation et SpeÂciation de l'Arsenic, du Plomb, et du Zinc dans des Sites et Sols ContamineÂs. Comparaison avec un Sol DeÂveloppe sur une Anomalie GeÂochimique Naturelle en Plomb. TheÁse de l'Univ. Paris VII. Juillot, F., Morin, G., Ildefonse, P., Trainor, T., Benedetti, M., Galoisy, L., Calas, G., Brown, Jr., G.E. (in press) Assessment of zinc speciation in smelter-impacted soils (France) by a combination of XAFS data and selective chemical extractions. Environ. Sci. Technol. Laperche, V., Traina, S.J., Gaddam, P., Logan, T.J., 1996. Chemical, and mineralogical characterizations of Pb in a contaminated soil: reaction with synthetic apatite. Environ. Sci. Technol. 27, 3321±3326. Ma, Y.B., Uren, V.A., 1998. Dehydration, diusion and entrapment of zinc in bentonite. Clays Clay Mineral 46, 132±138. Ma, Q.Y., Traina, T.J., Logan, T.J., Ryan, J.A., 1993. In situ lead immobilization by apatite. Environ. Sci. Technol 27, 1803±1810. Manceau, A., Boisset, M.C., Sarret, G., Hazemann, J.L., Mench, M., Cambier, P., Prost, R., 1996. Direct determination of lead speciation in contaminated soils by EXAFS spectroscopy. Environ. Sci. Technol. 30, 1540±1552. Morin, G., Ostergren, J.D., Juillot, F., Ildefonse, P., Calas, G., Brown, Jr., G.E. 1999. XAFS determination of the chemical form of lead in smelter-contaminated soils and mine tailings: importance of adsorption processes. Am. Mineral. 84. Nriagu, J.O., 1974. Lead orthophosphates Ð IV. Formation and stability in the environment. Geochim. Cosmochim. Acta 38, 887±898. Nriagu, J.O., 1984. Formation and stability of base metal phosphates in soils and sediments. In: Nriagu, J.O., Moore, P.B. (Eds.), Phosphate Minerals. Springer-Verlag, London, pp. 318±329. Pickering, W.F., 1981. Selective chemical extraction of soil components and bound metals species. Anal. Chem. 12, 233±266.
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Schuman, L.M., 1985. Fractionation method for soil microelements. Soil Sci. 140, 11±12. Sobanska, S., 1999. Etude de la SpeÂciation du Plomb et du Zinc dans des PoussieÁres Industrielles et dans un Sol ContamineÂ. Approche par MeÂthodes Spectroscopiques. TheÁse de l'Univ. Lille 1. Sobanska, S., Ricq, N., Laboudigue, A., Guillermo, R., Wignacourt, J.P., Merlin, J.C., Laureyns, J., BreÂmard, C., 1999. Micro-chemical investigations of dust from lead foundries. Environ. Sci. Technol. 33, 1334±1339. Sterckeman, T., Proix, N., Douay, F., Fourrier, H., des Ligneris, L., Schvartz C., 1996. Typologie des sols Ð inventaire des polluants mineÂraux Ð eÂtude de la migration verticale de Cd, Cu, Pb et Zn rapport PRC. Sterckeman, T., Proix, N., Douay, F., Fourrier, H., 2000. Vertical distribution of Cd, Pb and Zn in soils near smelters in the North of France. Environ. Poll. 107, 377±389. Tessier, D., 1984. Hydratation, Gon¯ement et Structuration des MateÂriaux Argileux au cours de la Dessication et la ReÂhumectation.TheÁse de doctorat d'Etat, Univ. Paris, INRA Versailles. Tiller, K.G., Gerth, J., BruÈmmer, G., 1984. The sorption of Cd, Zn and Ni by clay fractions: procedures for partition of bound forms and their interpretation. Geoderma 34, 1± 16. Traina, S.J., Laperche, V., 1999. Contaminant bioavaibility in soils, sediments, and aquatic environments. J. Biol. Chem. 96 (7), 3365±3371. Valsami-Jones, E., Ragnarsdottir, K.V., Putnis, A., Bosbach, D., Kemp, A.J., Cressey, G., 1998. The dissolution of apatite in the presence of aqueous metal cations at pH 2±7. Chem. Geol. 151, 215±233. Zhang, P., Ryan, J.A., Bryndzia, L.T., 1997. Pyromorphite formation from goethite adsorbed lead. Environ. Sci. Technol. 31, 2673±2678. Zhang, P., Ryan, J.A., 1999. K Formation of chloropyromorphite from galena (PbS) in the presence of hydroxyapatite. Environ. Sci. Technol. 33, 618±624.
www.elsevier.com/locate/apgeochem
TEM-EDX investigation on Zn- and Pb-contaminated soils Martine D. Buatier a,*, Sophie Sobanska b, FrancËoise Elsass c a
DeÂpartement GeÂosciences, Universite de Franche ComteÂ, 16 Route de Gray, 25030 BesancËon, France Micro and Trace Analysis Center, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Belgium c Institut National de la Recherche Agronomique (INRA), Sciences du Sol, Route de Saint-Cyr, 78026 Versailles, France b
Received 14 December 1999; accepted 27 September 2000 Editorial handling by R. Fuge
Abstract Lead and zinc contaminated soils from a smelter area in the northern part of France have been studied by transmission electron microscopy (TEM). This study was carried out with 4 dierent soils contaminated by Pb and Zn but with dierent chemical and physical characteristics. Two soils are tilled and have a neutral or slightly basic pH, one is a wooded soil and the last one is a meadow soil with acidic pH and high total organic content . TEM images of the soil samples have been coupled with focused energy dispersive X-ray (EDX) analyses and chemical mapping on a few micron-sized windows. This study demonstrates that TEM is a particularly ecient method to investigate metal speciation in the ®ne fractions of the contaminated soils. Zinc could be detected locally in sulphide minerals probably coming from the smelter emissions, but the major phases retaining Zn are Fe-oxyhydroxides and smectites. Lead could be detected in small aggregates which were characterized by EDX and selected area electron diraction. Their structural formulae correspond to a pyromorphite-like mineral in which Pb is partly substituted by Ca and Na. Pyromorphite is present only in the wooded and meadow soils where it forms partly from amorphous Si-rich phases (slags) coming from the smelter. These results are compared with data previously obtained by spectroscopic methods on the same samples. # 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Speciation of heavy metals in soils is an important factor governing their mobility, toxicity and bioavailability. Many recent studies have focused on the characterization of metal speciation in soils by indirect approaches such as selective chemical extraction (Pickering, 1981; Schuman, 1985; Gruebel et al., 1988; Gommy, 1997) or by direct in situ analyses such as extended X-ray absorption ®ne structure (EXAFS ) spectroscopy (CotterHowells et al., 1994; Manceau et al., 1996; Morin et al., 1999; Juillot et al., in press). These methods are particularly useful for determining the main speciation of heavy metals in soils. However, metal speciation in contaminated soils can take various forms and it is important to use complementary methods at dierent scales of investigation, in order to characterize all the solid phases * Corresponding author. Fax: +33-0381-666558. E-mail address: martine.buatier@univ- fcomte.fr (M.D. Buatier).
that can retain heavy metals to be able to understand the reactivity of these pollutants in a given environment. In this study, the authors present a TEM-EDX investigation on contaminated soils that have been previously studied by the methods speci®ed above. TEM-EDX is a particularly ecient method for direct in situ characterization of all phases, either crystallized or amorphous, that could contain heavy metals. This permits a better knowledge of the heavy metal distribution in the ®ne solid fraction of soils. In the present paper, TEM-EDX analyses are interpreted to establish the speciation of Zn and Pb in contaminated soils from the northern part of France, in a smelter area. 2. Sample selection The studied soils come from one of the 896 polluted sites recognized in 1996 by the French Ministry of Environment. They are located in the northern part of France at Evin-Malmaison, 40 km south of Lille, in the
0883-2927/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(01)00015-4
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largest Pb and Zn processing area in Europe. The contamination of the soils is mainly related to atmospheric emissions of Pb- and Zn-bearing particles scattered around the central smelter. A detailed characterization of the dust emissions from the smelter suggests that Pb and Zn sul®de together with Pb sulfate and oxysulfate are the major Pb and Zn bearing solid phases (Sobanska, 1999). The slag particles also contain heavy metal-rich particles formed during the smelter processing. These by-products are generally accumulated in a waste dump. They are mostly composed of amorphous phases containing Ca, Fe, Si, Pb (0.5±3%) and Zn (10±12%). Zn±Fe-oxides and metallic Pb were main forms identi®ed (Sobanska et al., 1999). All these particles have been transported by the wind and pollute the surrounding soils. Consequently, the concentrations of heavy metals now exceed 200 ppm throughout a 40 km2 area (Godin et al., 1985). The studied samples come from super®cial horizons from tilled, wooded and meadow soils. All samples come from a highly polluted area located downwind in the dominant wind direction, 1000 m away from the smelter but they have dierent metal concentrations, dierent pH and total organic C (TOC). (Fig. 1). The wooded soil has not been disturbed for about 50 a, one of the tilled soils has not been disturbed for about 30 a (T1) the other one (T2) is still exploited. The natural meadow has never been tilled. Because heavy metals are concentrated in surface horizons (Gommy, 1997; Juillot, 1998; Sobanska, 1999;
Sterckeman et al., 1996, 2000), only surface horizons (0± 20 cm) were investigated in this study (Table 1). Bulk concentrations of Zn are 570±1865 ppm in surface horizons and show an exponential decrease with depth. Lead concentrations display the same distribution with depth, with a maximum value in the surface horizons ranging between 460 and 2670 ppm. The pH of soils is naturally slightly acidic in organic-rich horizons (Baize, 1988); pH values are below 6 for the natural wooded and meadow soils, while it is 7.5 and 8.5 in the tilled soils, because of the lime amendment (Table 1). Previous investigations on the studied soils showed that Pb and Zn are preferentially concentrated in the <2 mm fraction (70 and 45% in the tilled soils and wooded soil surface horizons respectively). The clay fraction is composed of smectite, kaolinite and mica. Smectite is the major phase present in the clay assemblages (HargeÂ, 1997; Juillot, 1998; Sobanska, 1999). In the dense fractions (>2.89 g/cm3) Fe oxides, a Si-rich amorphous phase and locally, Zn-sul®de were detected (Juillot, 1998; Sobanska, 1999). 3. TEM-sample preparation and analytical procedures The samples were impregnated following the procedure established by Tessier (1984) and Elsass et al. (1998): this procedure consists of progressive exchanges of the interstitial water with alcohol, solvent and then Spur resin.
Fig. 1. Map location of the studied samples from the Evin-Malmaison site (France) (modi®ed after Sobanska, 1999).
M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177 Table 1 Chemical characterization of the 4 studied soils. Site of EvinMalmaison (France) (after Juillot, 1998 and Sobanska, 1999) Samples
pH
TOC
Zn
Pb
Fe
Ca
(0±20 cm) Wooded Tilled (1) Tilled (2) Meadow
5.5 8.5 7.5 5.8
(%) 6.5 4.2 1.5 13
(ppm) 1400 965 570 1865
(ppm) 2000 1060 460 2670
(ppm) 36,700 51,300 50,000 21,839
(ppm) 5800 24,400 11,600 10,840
Ultra-thin sections were obtained by ultramicrotomy using a diamond knife and sedimented on Cu- or Ti-grids. The most detailed TEM investigation was focused on the surface horizon from the wooded soil. Polished thin sections were prepared from bulk samples that were previously impregnated following the method described above. Clayrich selected areas were then removed from the glass, drilled, stuck on a 2 mm Cu-disk and Ar ion-milled. TEM investigations were carried out on a Philips CM30 at 300 kV (Centre commun de Microscopie, Universite de Lille). TEM images were complimented by selected area diraction pattern (SAED) in order to distinguished amorphous and crystallized phases. EDX analyses were performed with a Ge detector and careful calibration using standards permitted quantitative analyses of major elements and of Pb and Zn with a detection limit of about 100 ppm. Analyses can be carried out with a spot size of 3 nm, however, EDX analyses were carried out in STEM mode using a square window of about 20 nm for the clay mineral analyses, which are easily destroyed by the beam. EDX spectra were recorded in a 20 eV channel in order to detect all the Pb peaks. In order to examine the possible correlations between major elements and heavy metals in the ®ne fraction of the studied soils, element maps were produced with a Philips STEM420 microscope operated at 120 kV (Sciences du sol, INRA). Element mapping was performed on several areas from dierent samples from the wooded soil. EDX analyses were performed with a windowless detector and an Oxford Link AN10000 system. Being aware of the superposition of S-Ka peak at 2.3 keV with Pb-Ma peak, the Pb-L peak at 10.55 keV was also recorded. The scanned areas were about 24 mm2 at 12,500 magni®cation and 96 mm2 at 6400 magni®cation. Analytical conditions were chosen in order to optimize the detection of minor elements versus the resolution: a spot size of 30 nm and an acquisition time of 100 ms/step were selected. A rectangular grid was scanned. For a resolution of 128 lines128 columns, each pixel represents a mesh 25 nm wide over 16 nm long at magni®cation 25000, and double at magni®cation 12500. Data from element maps were treated statistically in order to calculate the inter-element correlation coecients. The peak intensity measurement was corrected for each pixel after substracting the background. Background was selected
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in a window corresponding to the Cr Ka peak because this element is absent in the studied samples. Correlation coecients (r) were then calculated from the analytical point having intensity higher than zero. 4. Results and discussion 4.1. Low magni®cation, TEM images and chemical mapping The soils display a typical silt texture with large grains of quartz in a clay-rich matrix (Fig. 2). The bulk sample of the surface horizon of the wooded soil is composed of quartz, clays and small grains of about 100 nm with a strong contrast. Rounded aggregates composed of amorphous particles were also locally observed. The clays are mostly smectite or smectite-illite mixed-layers, but mica and more rarely chlorite have also been observed. The small aggregates are generally located in the clay matrix. The element maps in Fig. 3a correspond to the area presented in Fig. 2. Each square displays the spatial distribution of one element. Since the background has been subtracted from each element map, pixel per pixel, these maps ®gure out the net intensities of the signals recorded in energy windows characteristic of the speci®ed elements. The intensity of each peak is related to the presence of the speci®ed element and to its relative concentration. The correlation should not be aected by the latter but the element concentration must be signi®cant to be taken into account in the correlation coecient calculation. In both selected maps, the presence of alumino-silicates (mostly clay minerals) in the ®ne fraction of the studied soil is con®rmed by the O, Si and Al rich area (and ocasionally K and Fe). Zinc and Pb concentrations are signi®cant. The S distribution map is compared with the Pb-L distribution in order to discriminate the contributions of S and Pb in the S map. Fig. 3a shows that S is concentrated in a relatively large area (>1 mm), where Zn is also detected but not Pb-L. This area is also O poor, suggesting the presence of a Zn sul®de species rather than a sulfate. ZnS particles were detected in small quantities by SEM and XRD analyses in the dense fractions of the wooded and tilled soils by Sobanska (1999) and Juillot (1998). These particles correspond to atmospheric dust deposition, originating from smelter emission. Similar results were obtained in other areas from the same sample. In the element maps presented (Fig. 3b), ZnS is also detected on the right side of the map. Phosphorus-rich phases mostly concentrate Pb with Cl and Ca. Besides the phases bearing these elements, the matrix is composed of silicates, mainly clay minerals. Silicates are detected from their high concentrations of Al, Si, O and Fe. The calculated correlation coecient r between elements con®rms these inferences. Table 2 presents the
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Fig. 2. Low magni®cation TEM image of the wooded soil (microtome sample) from the Evin-Malmaison site (microtome sample ultra-thin section).
correlation factors calculated from Fig. 3a. The good correlation coecient obtained for O, Al, Si and K con®rm the presence of clay minerals. The Zn and S correlation coecient is close to 1, con®rming the preferential occurrence of Zn in sul®de form in the analyzed samples. Good correlations are also observed between Pb and P, P and Ca, Pb and Cl, P and Cl. The Pb-S correlation can not be related to mineralogical occurences because the S Ka peak intensity may have been modi®ed by the Pb-Ma contribution. In the results presented here, the Pb-S coecient is relatively low because the Pb-Ma contribution is lower than the S-Ka peak. It is noticable that the correlation coecient between major elements like Si or Al with Pb or Zn is low. In the most of the maps acquired for this study, Zn is detected only in ZnS grains. The possible correlation of Zn with clay particles can not be detected here because of the low concentration of Zn in the clay matrix. In order to determine more precisely the speciation of Pb and Zn in the ®ne fraction of the 4 soil samples, TEMEDX analyses were pinpointed on individual metalbearing particles. 4.2. Particle characterization 4.2.1. Zn speciation With the exception of small crystals of Zn sul®de, the detection of Zn by EDX analyses is generally related to the presence of clay minerals. Iron-rich particles with or
without Zn are also present in the ®ne fractions of the 4 studied soils. These particles are observed in aggregates with a strong contrast. These iron-rich phases (probably Fe-rich oxy-hydroxides) are commonly associated with clay minerals (Fig. 4) and it was impossible to obtain any analysis of isolated particles. For this reason, Table 3 presents EDX data on smectite particles and on smectite-oxy-hydroxide mixtures. In many analyses focused on smectite particles, a low concentration of Zn is detected (Fig. 5). XRD analyses on the clay fractions indicated that most smectites are of dioctahedral type (Juillot, 1998). This is con®rmed here by the EDX data. All smectites are Al- and Fe-rich (Fe is supposed to be Fe3+) with a total octahedral occupancy limited to 2 atoms per half cell. One can notice that Mg or Mn-rich smectites are not detected. Some analyses displaying a higher octahedral content probably correspond to clay particles having Fe coatings (Table 3). In the smectite and oxide analyses, Zn is present in various quantities with a maximum of 1.5 at.%, its concentration being unrelated to the abundance of Fe. These data suggest clearly that Zn is both retained by clay minerals and/or Fe-rich particles; the latter ones having the peculiarity to occur either as individual grains or as coatings on clay mineral surfaces. The speciation of Zn in clay minerals has been the object of many experimental studies. Several authors have described exchange reactions between soluble Zn and smectite exchangeable cations (Tiller et al., 1984; Brigatti
M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177
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Fig. 3. EDX element maps of the wooded soil from the Evin-Malmaison site. Window size 4.06.4 mm at magni®cation 12500 on ultrathin section (a); window size 812.8 mm at magni®cation 6400 (b). Map (a) corresponds to the area presented Fig 2. All elements were recorded from their Ka line except Pb which was recorded from its La line.
et al., 1996; Ma and Uren, 1998). The maximum adsorption of Zn on smectite was observed for a swelling clay having a high speci®c surface (190 m2/g) at pH ranging between 6.5 and 8 (Tiller et al., 1984). According to Brigatti et al. (1996), the adsorption of Zn is enhanced when the total layer charge is due to an octahedral
charge de®cit. Ma and Uren (1998) showed that Zn is not simply exchanged with K and Na but forms Zn(OH)+ complexes in the hexagonal tetrahedral cavities. Direct precipitation of metal-rich smectites (Zn, Co, Ni, Cu) have been obtained in laboratory experiments at low temperatures (less than 100 C) (Decarreau, 1981).
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Table 2 Cross-correlation coecients from EDX mapping data of the wooded soil from the Evin-Malmaison site (see Fig. 3a). Statistical treatment of net intensities obtained after subtraction of the background. The signi®cant coecients (>0.5) are given in bold Al Al C Ca Cd Cl Fe K Mn O P Pb S Si Ti Zn
1.00 0.24 0.06 0.29 0.08 0.38 0.71 0.12 0.80 0.14 0.80 0.03
C 1.00 0.03 0.08 0.11 0.21 0.16 0.04 0.21 0.07 0.05 0.17 0.22 0.18
Ca
Cd
Cl
Fe
1.00 0.22 0.24 0.14 0.10 0.11 0.13 0.65 0.29 0.15 0.05 0.03 0.02
1.00 0.45 0.22 0.40 0.29 0.34 0.46 0.32 0.46 0.26 0.08 0.35
1.00 0.18 0.13 0.22 0.17 0.58 0.64 0.48 0.09 0.04 0.24
1.00 0.22 0.15 0.47 0.38 0.30 0.12 0.36 0.04
K
1.00 0.14 0.56 0.16 0.11 0.58 0.02 0.10
Data concerning the speciation of Zn in contaminated soils are less abundant. In contaminated soils from the same area (south of Lille), Harge (1997), using EXAFS spectroscopy detected the presence of Zn silicates and a Zn trioctahedral smectite which was supposed to have formed in situ in the soil. The EDX analyses presented here indicate that all smectites (with or without detectable Zn) have similar compositions suggesting that neoformation of Zn-rich smectite does not occur in these soils. The EXAFS spectroscopic analyses obtained by Juillot et al. (in press) for two soils studied here (wooded and tilled soil II), demonstrated that the major speciation of Zn is its adsorption on Fe oxy-hydroxide or its incorporation in phyllosilicates. TEM-EDX analyses are
Fig. 4. General aspect of smectite particles and associated Ferich phases (probably Fe-oxyhydroxides) on the tilled soil sample from the Evin-Malmaison site.
Mn
1.00 0.15 0.19 0.11 0.14 0.13 0.05 0.10
O
1.00 0.32 0.08 0.05 0.88 0.16 0.03
P
1.00 0.68 0.48 0.17 0.06 0.21
Pb
1.00 0.39 0.02 0.02
S
1.00 0.03 0.90
Si
1.00 0.04 0.04
TI
1.00
Zn
1.00
compatible with these interpretations but more detailed investigations by HRTEM on the smectite particles are needed to support this assumption. 4.2.2. Pb speciation According to EDX analyses, Pb occurs in Fe±Mn rich particles always in association with Zn which probably correspond to the oxy-hydroxides described above. This result con®rms previous investigations carried out on the same samples with dierent techniques. Indeed, these studies indicated that Pb can be adsorbed on Mn and Fe oxy-hydroxides (Gommy, 1997, Juillot, 1998; Morin et al., 1999; Sobanska, 1999). TEM analyses suggest that Fe-oxyhydroxide are particularly abundant in the tilled soils. In wooded and meadow soils, distinct Pb-rich grains associated with Ca, Fe, P and Na are detected in amorphous Si-rich phases (Fig. 6). It is noted that amorphous Si-rich phases are commonly observed in the studied samples but not always with Pb. Finally, Pb-grain aggregates of about 0.2 mm diameter are also common in both organic rich soils. The EDX spectra of small particles revealed high concentrations of Pb with P, Cl, Ca, O and sometimes Fe, Zn (Fig. 7). The P±Pb, P±Ca, P±Cl and Pb±Cl correlations observed from the element map data are con®rmed by these particle analyses. Data obtained from the elemental map con®med that Cl is present in the Pb-rich aggregate because the Cl-Ka map displays signi®cantly lower Cl concentration in the matrix and the spurr resin compared with the small Pb and P-rich aggregates. These element associations are compatible with the presence of a pyromorphite phase (Figs. 6 and 7). They correspond to small aggregates of about 0.2 mm with a large amount of Pb with P, Ca, Na and Cl (Figs. 6 and 7). Iron and Zn detection could be
M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177
due to the presence of mixtures of pyromorphite grains with Fe±Zn oxides or to a pyromorphite with small amounts of Fe and Zn as described by Cotter-Howells et al. (1994). A structural formula can be calculated for the pyromorphite-like phase from EDX analyses: Pb4(Ca)y, Na x, (PO4)3Cl, with y=2±2x. This formulae is typical for an apatite group mineral. It does not correspond to the pure pyromorphite end-member, but rather to a solid solution with Ca, Na and Pb substitutions.
1171
The presence of pyromorphite was con®rmed by the SA electron diraction pattern. SAED on isolated grains of pyromorphite reveals that they are poorly crystallized; however, a SAED pattern was obtained in aggregates which showed the most intense re¯ection at 2.98 AÊ corresponding to the (211) and (112) re¯ections (Fig. 8). The other re¯ections could not be clearly identi®ed probably because of the poorly crystallized pyromorphite grains. Pyromorphite seems to be abundant in the wooded soil
Table 3 Chemical analyses of clay particles and oxy-hydroxyde-clay mixtures from the Evin-Malmaison site (all soils, ultra-thin sections and ion-milled microdrillings). (A) Clay mineral structural formulae are calculated from EDX punctual analyses (in atm.%) on the basis of 22 negative charges. Total O corresponds to the number of cations in octahedral site. (B) Mixtures of smectites and oxy-hydroxides analyses are given in atm.% A
Wooded soil
No.
1
2
3
4
Si AlT CT
3.66 0.34 0.34
3.90 0.10 -0.10
3.09 0.91 -0.91
3.45 0.55 -0.55
3.37 0.63 -0.63
3.58 0.42 -0.42
3.48 0.52 -0.52
3.37 0.63 -0.63
AlO Fe Mg
1.12 0.67 0.30
1.54 0.30 0.18
0.50 0.91 0.52
0.95 0.92 0.22
0.77 1.00 0.20
1.01 0.75 0.27
1.12 0.67 0.17
Total O CO
2.09 0.04
2.01 0.14
1.93 0.73
2.09 0.05
1.97 0.28
2.03 0.18
Zn
0.03
0.01
0.08
0.02
0.03
Na K Ca Total I
0.10 0.12 0.04 0.25
0.06 0.08 0.02 0.17
0.22 0.07 0.11 0.40
0.18 0.23 0.03 0.44
0.41 0.17 0.14 0.72
Fe/AlTot.
0.46
0.18
0.64
0.61
0.71
Meadow soil No.
15
16
5
6
7
8
9
10
11
12
13
14
3.47 0.53 -0.53
3.62 0.38 -0.38
3.54 0.46 -0.46
3.60 0.40 -0.40
3.64 0.36 -0.36
3.54 0.46 -0.46
0.71 0.83 0.46
0.75 0.87 0.44
0.85 0.76 0.57
1.41 0.55 0.13
1.26 0.55 0.25
0.98 0.82 0.26
1.19 0.58 0.22
1.96 0.28
1.99 0.49
2.06 0.25
2.19 0.01
2.09 0.14
2.05 0.09
2.05 0.10
2.00 0.22
0.03
0.04
0.02
0.02
0.03
0.01
0.01
0.02
0.02
0.27 0.17 0.05 0.49
0.31 0.29 0.06 0.66
0.40 0.20 0.23 0.83
0.43 0.26 0.15 0.85
0.00 0.05 0.14 0.19
0.00 0.14 0.09 0.22
0.00 0.29 0.09 0.39
0.00 0.18 0.11 0.30
0.00 0.19 0.23 0.42
0.53
0.41
0.62
0.68
0.62
0.29
0.33
0.61
0.35
Cultivated soil 17
18
19
20
22
23
24
25
26
Si AlT CT
3.45 0.55 0.55
3.03 0.97 0.97
3.43 0.57 0.57
3.55 0.45 0.45
3.30 0.70 0.70
3.42 0.58 0.58
3.51 0.49 0.49
3.55 0.45 0.45
3.78 0.22 0.22
3.61 0.39 0.39
3.67 0.33 0.33
AlO Fe Mg
1.89 0.06 0.08
0.65 1.22 0.36
1.35 0.27 0.27
1.39 0.51 0.18
1.60 0.31 0.21
1.74 0.22 0.16
1.42 0.62 0.13
1.54 0.44 0.05
1.25 0.67 0.16
1.14 0.83 0.13
1.17 0.79 0.15
Total O CO
2.03 0.01
2.22 0.30
1.88 0.62
2.08 0.07
2.12 0.16
2.11 0.17
2.18 0.40
2.03 0.06
2.08 0.07
2.10 0.17
2.11 0.19
Zn
0.01
0.03
0.05
0.02
0.07
0.01
0.00
0.00
0.02
0.02
0.01
Na K Ca Total I
0.02 0.37 0.06 0.46
0.12 0.33 0.08 0.53
0.00 0.64 0.23 0.87
0.04 0.19 0.06 0.29
0.07 0.14 0.10 0.31
0.04 0.33 0.01 0.39
0.00 0.03 0.03 0.05
0.00 0.34 0.02 0.36
0.00 0.08 0.02 0.10
0.00 0.13 0.03 0.16
0.00 0.09 0.02 0.11
Fe/AlTot.
0.03
0.75
0.14
0.28
0.14
0.09
0.33
0.22
0.46
0.54
0.53 (continued on next page)
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M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177
Table 3 (continued) B
Wooded soil
No
27
28
29
30
31
32
33
34
35
36
37
38
39
40
Si Al Fe Mg
16.64 6.6 8.1 3.2
14.1 5.86 15.09 0.92
17.92 7.89 6.27 2.93
14.74 7.62 9.33 3.35
14.7 7.6 9.3 3.38
15.42 7.75 8.78 2.65
17.18 6.47 7.76 4.25
11.81 7.96 13.06 3.68
11.21 7.88 13.85 3.66
11.7 9.11 12.33 4.13
5.71 5.15 26.36 0.42
16.16 9.88 7.05 2.49
13.77 10.16 9.4 2.57
11.7 11.77 9.11 9.61 12.33 11.72 4.13 3.59
Zn
0.23
0.12
1.03
1.24
1.24
1.18
0.48
1.16
1.26
0.42
0.52
0.43
0.96
0.42
0.75
Na K Ca
0.68 3.48 0.12
0.05 0.72 1.04
0.59 0.55 0.6
0.57 1.01 0.93
0.68 1.01 0.93
0.67 1.11 1.05
0.37 1.12 0.59
0.36 0.73 0.35
0.86 0.33 0.21
0.65 0.32 0.37
0.54 0.23 0.52
0.38 1.4 0.34
0.58 0.46 0.57
0.65 0.32 0.37
0.65 0.54 0.45
60.95 1.23
62.09 2.58
62.21 0.79
61.21 1.22
61.15 1.22
61.4 1.13
61.78 1.20
60.89 1.64
60.74 1.76
60.47 1.35
60.54 5.12
61.87 0.71
61.52 0.93
O Fe/Al
Meadow soil 41
60.47 60.92 1.35 1.22
Cultivated soil No.
42
43
44
45
46
47
48
Si Al Fe Mg Zn
12.33 3.75 20.13 0.69 0.13
10.09 4.38 22 0.87 0.31
8.41 3.58 25.28 0.64 0.18
10.96 4.33 21.28 0.53 0.1
9.08 4.68 22.76 0.94 0.23
8.52 4.8 24.04 0.62 0.19
17.3 8.39 8.73 1.26 0.14
Na K Ca O
0.04 0.4 0.5 62.03
0.14 0.06 0.56 61.59
0.03 0.3 0.25 61.33
0.05 0.24 0.71 61.81
0.25 0.23 0.55 61.28
0 0.16 0.25 61.43
0.15 0.79 0.54 62.7
5.37
5.02
7.06
4.91
4.86
5.01
1.04
Fe/Al
samples. Pyromorphite-like phases with very similar composition were identi®ed in the wooded soil and in the meadow, Pb is the major cation but Ca and Na are present in variying amounts. This mineral was not identi®ed in the tilled samples. Intermediate phases between amorphous Si rich-phase with Pb and Ca (Fig. 6) and crystallized aggregates of pyromorphite (Fig. 7) are observed only in the wooded soil. This observation suggests that a part of the pyromorphite grains could have formed by weathering of glassy-like particles with Si, Ca, and Pb coming from slag materials or from in situ Si-rich amorphous phases and P and Cl from the soil itself. Lead-phosphates are a common product of soil alteration. They are generally found as Ca enriched Pb phosphates and not as a pure end-member (Cotter-Howells et al., 1994). Pyromorphite was identi®ed in mine taillings from Leadville, USA (Morin et al., 1999), and soils contaminated by mine wastes in Derbyshire, UK (Cotter-Howells et al., 1994). In these two examples, pyromorphite was identi®ed by EXAF spectroscopy suggesting that it was one of the major Pb species in these soils. Lead phosphates (pyromorphite and plumbogummite) are considered as some
of the most insoluble Pb (II)-solids known to form under surface conditions (Nriagu, 1974, 1984)), and so the precipitation of pyromorphite constitutes an ecient mechanism to decrease the Pb availability. Laboratory experiments (Ma et al., 1993; Valsami-Jones et al., 1998; Zhang et al., 1997; Zhang and Ryan, 1999) demonstrated that pyromorphite or hydroxypyromorphite is the main compound formed from solid Pb-bearing forms such as PbS, PbSO4 and PbCO3 on reaction with hydroxyapatite. This reaction occurs at pH ranging from 3 to 7. These experiments demonstrated that addition of phosphate amendment could increase the immobilization of Pb in contaminated soils. Laperche et al. (1996) showed the importance of pH by examining the interactions of selected Pb minerals and a contaminated soil with apatite. These authors showed that the extent of reactions were pH dependant with more hydroxypyromorphite formation at pH 5 than at pH 6 or 7. The major Pb compounds found in dust emissions deposited in the studied soils are sul®de and sulfate; furthermore, the soils in which pyromorphite formed have pH values around 6 suggesting that the mechan-
M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177
1173
Fig. 5. Typical features of soil smectite particles from the Evin-Malmaison site: example of the wooded soil. TEM image (a); EDX spectrum (b). Cu is in italic because this element is not present in smectite but its presence is related to the Cu TEM grid supporting the sample. The arrow identicates the EDX analytical area.
isms described below for the formation of pyromorphite could have been active in the studied soils. However, in the studied soil, slag particles containing Si-rich amorphous phases with Pb are also a potential precursor for pyromorphite. The eect of apatite amendments on Pb concentration in roots was studied by Traina and
Laperche, (1999). These authors showed that the presence of rhizosphere created local acidity which may have enhanced the local dissolution of apatite grains and precipitation of pyromorphite on the root surface. The association of pyromorphite and roots and the contribution of root exudates in pyromorphite forma-
1174
M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177
Fig. 6. Pyromorphite grains in a Si-rich amorphous matrix. TEM image (a); EDX spectrum (b). Ion-milled microdrilling of the wooded soil from the Evin-Malmaison site.
M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177
1175
Fig. 7. Pyromorphite grains associated with Fe±Zn oxy-hydroxydes. TEM image (a); EDX spectrum (b). Py: pyromorphite grains. Cu is in italic because this element is not present in smectite but its presence is related to the Cu TEM grid supporting the sample.
tion were found by Cotter-Howells and Caporn (1996) and Cotter-Howells et al. (1999). A greater amount of roots as suggested by the TOC (Table 1) in the meadow and wooded soils compared with the tilled soils could explain the pyromorphite detection by TEM only in these soils. Previous EXAFS spectroscopy analyses on the studied tilled II and wooded soils suggested that Pb was adsor-
bed to humic acids as well as Mn and Fe-oxy-hydroxides, but pyromorphite could not be identi®ed with this technique (Morin et al., 1999). The Morin et al. (1999) results suggest that pyromorphite was not the major Pb-containing phase in these soils, the bulk of the Pb being in the adsorbed phases. However, the identi®cation of pyromorphite by chemical mapping, TEM and EDX analyses implies that there are various species of Pb in
1176
M.D. Buatier et al. / Applied Geochemistry 16 (2001) 1165±1177
tion and alteration of amorphous Si-rich phases with Ca, Pb, and Na. This study demonstrates that TEM can be a particularly ecient method for the investigation of metal speciation in soils. The results also show the complexity of the soil system and the in¯uence of its physical and chemical properties on metal speciation. Acknowledgements
Fig. 8. SAED pattern of pyromorphite grains. The most intense re¯ection corresponding to (211) and (112) re¯ections at 2.98 AÊ is showed by an arrow.
these soils. Pyromorphite appears to be the main solid phase that is able to concentrate Pb, and this species is very stable in the soil. The limited amount of pyromorphite here could be related to the relatively low amount of available P in the soils. The P% measured in the studied soils is only about 0.03% (Juillot, 1998) whereas it reaches 0.2% in the contaminated soils studied by Cotter-Howell et al. (1994) where pyromorphite was identi®ed as the major constituent. The fraction of Pb adsorbed onto mineral and organic matter surfaces in the studied soils needs to be quanti®ed as this type of speciation involves could be important in the bioavailability of Pb in these soils. 5. Conclusion The combination of element maps with focused EDX analyses allow the determination of the speciation of Zn in the surface horizons of the studied soils and the identi®cation of lead phosphate compounds in a wooded soil and a meadow. Fe oxy-hydroxides enriched in Zn and Pb are easily identi®ed by TEM, and are particularly abundant in samples taken from tilled soils. Zinc detected in smectite particles is generally present in small amounts (less than 2%). The TEM and EDX analyses suggest that direct neoformation of Zn-rich smectite does not occur in these soils. However, the location of Zn in the phyllosilicate structure cannot be established from these data. A Pb-apatite (pyromorphite) is identi®ed only in samples having high COT and slightly acidic pH, i.e. the wooded soil and the meadow. Such phosphates seem to be formed within the studied soils by in situ precipita-
We dedicate this paper to the late Philippe Ildefonse who indirectly participated in this paper through various discussions and a critical review of the data. Philippe was an active promoter of fundamental research on alteration of minerals and on metal speciation in natural soils. This work was supported by the PROSE97 CNRS/INSU Program and the PRC program. We thank an anonymous reviewer for a critical and very helpful review.
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