Relationships between heavy metals content and soil properties in minesoils

Analytica Chimica Acta 524 (2004) 141–150

Relationships between heavy metals content and soil properties in minesoils F.A. Vega∗ , E.F. Covelo, M.L. Andrade, P. Marcet Departamento de Biolog´ıa Vegetal y Ciencia del suelo, Universidad de Vigo, As Lagoas, Marcosende, Vigo 36200, Espa˜na Received 1 December 2003; accepted 22 June 2004 Available online 10 August 2004

Abstract Mining can be important causes of environmental degradation. Opencast mines produce a large amount of waste because the ore is a small fraction of the total volume of the mined material. In mine spoils, the formed soils have severe physical, chemical and biological limitations. Minesoils usually have high heavy metal levels because wastes contain metallic minerals. High trace elements levels, mainly in available form, will impede the revegetation to stabilize the tailings. Twenty-five soils (Anthropic Regosols) were selected and characterized. Minesoils come from two mine tailings (Touro: copper mine and Meirama: lignite mine) located in Galicia (Spain). Total, DTPA-extractable and total dissolved contents of Cd, Cr, Cu, Ni, Pb and Zn were determined. Minesoils characteristics and heavy metal availability were related to establish the dependency among them. Total heavy metal dissolved and DTPA–extractable contents are low in all the soils except the Cu-dissolved content in soils from copper mine spoils. These soils have a total Cu content higher than the intervention limit of diverse reference guides. In Touro minesoils, Cr, Ni and Zn total contents come from the parent matter of these soils. The correlation established between the Cr, Ni, and Zn total content suggests its common origin through the soils parent matter. Heavy metal contents of Meirama minesoils, furthermore, come from the fertilizers, and animal manure. Iron and manganese oxides, humified organic matter, and clay minerals, like gibbsite, chlorite, smectite and goethite, are the soil components with greater effect in the decrease of heavy metal availability. © 2004 Elsevier B.V. All rights reserved. Keywords: Tailings; Soil characteristics; Heavy metals; Availability

1. Introduction Opencast mining causes serious environmental impact like the destruction of natural soils and the extraction of important volumes of materials. This causes the formation of new soils, on the accumulated wastes of the mine known as Anthropic Regosols. Minesoils are very young soils developed on unstable materials and characterized by instability and scarce cohesion; these properties easily expose the minesoils to water and air erosion. Minesoils have low contents of nutrients and organic matter. Their texture and structure are unfavourable. Elevated ∗

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levels of trace elements and the acidic drainage due to the oxidation of sulphide are also frequently common characteristics of the most mine tailings [1]. Therefore, there are severe limitations for the implantation of vegetation in minesoils [2]. The scarce vegetation leads to acidification of adjacent soils by leaching the mine spoils [3], giving rise to high environmental contamination. The extraction of metal ores causes generally a multielemental contamination of the environment [3]. In addition, the natural metal content of the soils will be increased if materials with heavy metals are added. Adding organic material has been used as a means for ameliorating minesoils and improving their quality. These supplements can increase the contamination because they contain heavy metals [4].

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The recovery of an ecosystem and mine soil quality depends on vegetation for improving the soil’s biological, chemical, and physical conditions of disturbed sites. The recovery the mine spoils, with potentially toxic metal levels, is very difficult because the trace elements are frequently present in high concentrations and impede the revegetation. The heavy metals diminish the absorption of water and nutrients by plants and the breathing of the root, and they inhibit mitosis in the meristematic root regions [5]. Metal absorption by the plants can be affected by several factors such as the soil heavy metal content, the varieties and the age of the plants and by properties like pH, the cation exchange capacity (CEC) and the organic matter content [6]. The supplements added to minesoils can increase [4] their natural heavy metal content. In addition, the heavy metals in mine spoils with a hyperacidic and hyperoxidaizing environment can be mobilized through drainage and run-off waters. This implies contamination risk of the adjacent zones and underground waters. The soil heavy metal content can impede the mine spoil’s revegetation if the available contents are toxic for the vegetation. For that reason it is necessary to know not only the total content, but also the available and dissolved contents. It is also necessary to know the minesoil’s characteristics, such as clay minerals, oxides and organic matter contents, the cation exchange capacity and pH because [2] the soil properties affect the available total heavy metal content. The soil properties can determine the degree of toxicity the vegetal growth. The CEC of the clay minerals varies with the crystal lattice structure and with the degree of isomorphic substitution so it is necessary to identify the minesoil clay minerals, their diversity and their abundance. The different clay minerals will

contribute in different ways to the minesoil’s heavy metal retention capacity [7]. Soil clay minerals will affect the minesoil’s capacity to retain their own heavy metals and the trace elements that were added through the supplements. The objectives of this work were to characterize soils coming from the tailings of two opencast mines (in Galicia, Spain) and to determine the total content and the availability of Cd, Cr, Cu, Ni, Pb, and Zn. Knowledge of this content will serve to select the supplements to use in the revegetation, avoiding an increase of the heavy metal content and availability.

2. Materials The superficial horizons of twenty-five soils were sampled (Anthropic Regosols) [8]. They are located in Coru˜na province (Galicia) (Fig. 1) and developed on mine tailings. Ten come from the depleted copper mine of Touro and fifteen from the lignite mine of Meirama. Two zones were selected in the depleted copper mine, now dedicated to the extraction of material for road construction. The tailings are formed fundamentally by oxidized materials, with amphibolites, chalcopyrite, limonite, garnet, and mainly iron and copper sulphides. The minesoils of these spoils (TE) are in an initial period of recovery, brought about by means of the plantation of eucalyptus and the application of fertilizers and diverse types of sewage sludge. The decantation-bank (TB), where the sludge from the copper extraction in the flotation plant was deposited, and was covered with ash coming from a paper mill. The soils of these tailings practically lack vegetation. Meirama minesoils have been formed on the waste materials of the lignite mine. Three zones had been selected in

Fig. 1. Study area.

F.A. Vega et al. / Analytica Chimica Acta 524 (2004) 141–150

these mine spoils. One, in a wetland that was made in a mine spoil (MH). The second is situated in the tailings formed approximately in the year 2000. Their revegetation began that same year. The soils are thin with strong limitations for plant growth (MI). The last selected zone is the mine spoil, 10 years old, with more developed soils (ME) with vegetation of herbaceous plants, legumes and trees (alders and pines). Five soils (Anthropic Regosols) have been selected in each zone (Fig. 1). Sampling was carried out from topsoils (0–30 cm). The samples were collected using an Eijkelkamp sampler. Five samples of each site sample were taken and stored in polyethylene bags. The samples were air dried, passed through a 2 mm sieve and homogenized in a vibratory homogeniser for solid samples (Fritsch Laborette 27 rotary sampler divider) and five sub samples from the composed sample were taken for the analyses.

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passes with a time of 10 s per pass [17]. Total dissolved Cd, Cr, Cu, Ni, Pb and Zn were extracted using acidified calcium chloride solution (0.1 M), according to the method developed by Houba et al. [18]. The available Cd, Cr, Cu, Ni, Pb and Zn content was extracted using the diethylene triamine pentaacetate (DTPA) method developed by Lindsay and Norwell [19]. Total contents were extracted by means of acid digestion using a mixture of concentrated nitric, hydrochloric and hydrofluoric acids (1:3:3 v/v) in PTFE bom placed in a microwave oven [20]. The analysis for Cd, Cr, Cu, Ni, Pb and Zn was carried out by ICP-AES. All the experiments were performed in triplicate. The data obtained in the analytical determinations were treated statistically using the programme SPSS version 10.1 for Windows [21]. Analysis of variance (ANOVA) and test of least significant difference (L.S.D.) were made. The evaluation of the influence of the soil characteristics on the metal adsorption capacity was examined by correlation analysis.

3. Analytical methods The samples were analysed for particle size distribution, pH, nitrogen, organic carbon, effective cation exchange capacity, exchangeable cations, and iron, aluminium and manganese contents. A mineralogical analysis of the fraction <2 ␮m has also been performed. Soil reaction was determined with a pH electrode in 2:1 water to soil extracts [9]. The particle size distribution was conducted after the oxidation of organic matter with hydrogen peroxide, separating the upper fraction to 50 mm by means of sieves, and the lower fraction using the international procedure [10]. The Organic carbon content was determined by the Walkey and Black method [11]. Organic matter was fractionated using the humid sieving procedure proposed by Andriulo et al. [12] and Galantini et al. [13], which facilitates obtaining humified organic matter and non-humified organic matter contents. Total Kjeldahl-N was determined according to Bruemmer and Mulvaney [14]. Soil CEC and exchangeable cations (Ca2+ , Mg2+ , K+ and Na+ ) were extracted with 1 M ammonium acetate solution, buffered to pH 7.0, and concentrations were determined by inductively coupled plasma atomic emission (ICP-AES) spectrometry. Exchangeable acidity was determined using a 1 M KCl replacing solution and titration to a phenolphthalein endpoint [15]. Al, Ca, K, Mg and Na were extracted by BaCl2 solution and concentrations determined by ICP-AES (Perkin-Elmer Optima 4300 DV). The Mehra and Jackson method [16] was used to determine the oxides content, shaking the samples with a solution of sodium hydrogen carbonate and sodium citrate. Fe, Al and Mn were determined in the extract by ICP-AES. Mineralogical analysis of the clay fraction was made by X-ray diffraction, of crystalline powder in a Siemens D-5000 diffractometer. To measure the samples, q-2q a configuration was used (Bragg–Brentano system) with a Cu anode and 0.05

4. Results and discussion 4.1. Minesoils characterization The characteristics of the soils (Table 1) show that there are important and significant differences, mainly between those that can affect heavy metal adsorption, mobility and, consequently, retention by the soil. Minesoil pH ranges from levels close to neutrality (6.11, the wetland Meirama soils, MH) to strong acidity (3.62, Touro minesoils, TE) (Table 1). These differences can influence heavy metal adsorption capacity since these trace elements are very mobile in acidic conditions and their mobility and availability decreases as the pH approaches neutrality [22]. Organic matter contents are low in all the soils and range from 15.04 g kg−1 (TB soils) to 0.80 g kg−1 (ME soils) (Table 1). The organic matter is a component of great importance because it tends to form soluble or insoluble complexes with the heavy metals, the reason why they can migrate throughout the profile [23] or be retained in the soil. The fractionation of the organic matter indicates the adsorption capacity of this component in diverse soils due to the different adsorption capacities, of the humified (stable) and non-humified (fresh) organic matter. The results show that the humified organic matter content, with greater capacity of complexation and retention, is very low in MH soils (0.52 g kg−1 ) and a the high content appears in TB soils (13.83 g kg−1 ) (Table 1). In all the soils, the humified organic matter content is at least 75% of the total organic matter content. The CEC was low for all soils and ranged from 6 cmol kg−1 (TB and MI soils) to 9.47 cmol kg−1 (TE soils) (Table 1). Several authors [7,24,25] have demonstrated that oxides, mainly Fe and Mn oxides, have a high capacity to adsorb heavy metals in the soil mineral–water interphase.

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Table 1 Some physicochemical characteristics of the soils (mean values: three replicates of each soil and five soils from each zone) Characteristic

TE Mine spoil (Touro)

TB Decantation-bank (Touro)

MH Wetland (Meirama)

MI Inner mine spoil (Meirama)

ME Outer mine spoil (Meirama)

pH H2 O O.M. (g kg−1 )a O.M.nh (g kg−1 )b O.M.h (g kg−1 )c N (g kg−1 ) C/N Sand (%) Silt (%) Clay (%) Al2 O3 (g kg−1 ) Fe2 O3 (g kg−1 ) MnO (g kg−1 ) CECed

3.62 e 0.83 c 0.11 c 0.71 c 0.16 b 2.97 c 75 a 17 d 8c 2.57 b 2.44 c 0.1 b 9.47 a

4.78 c 15.04 a 1.21 a 13.83 a 0.71 a 10.51 a 43 d 40 a 17 b 21.1 a 3.62 a 1.20 a 6.02 d

6.11 a 0.64 c 0.12 c 0.52 c 0.14 b 2.72 c 49b c 34 b 17 b 1.70 d 1.43 d 0.03 d 6.84 b

4.66 d 4.08 b 0.73 b 3.35 b 0.29 b 8.16 b 48 c 35 b 17 b 1.52 d 3.15 b 0.07 c 6.03 d

5.71 b 0.80 c 0.15 c 0.66 c 0.26 b 1.82 c 52 b 27 c 21 a 2.11 c 1.61 d 0.02 d 5.34 c

u.l.: undetectable level. Values of the same parameters followed by different letters (a–e) are significantly different (P < 0.05). a Organic matter. b Organic matter not humified. c Organic matter humified. d Effective cationic exchange capacity.

The studied minesoils have different Mn, Fe and Al oxide contents. TB soils display the greatest content of these oxides: Al2 O3 (21.1 g k−1 ), Fe2 O3 (3.62 g k−1 ), and MnO (1.20 g k−1 ) (Table 1). The particle size distribution shows (Table 1) that the soils from TE contain the lowest clay content. TE soils are very young, the tailings in which they were formed are the most recent and, therefore, the time that has been available for the mine spoils alteration is short. These soils lack vegetation almost totally, which exposes the very fine particles to hydrolytic erosion and being removed by run-off water. The soils from ME spoils contain the greatest clay content. The ME area contains the oldest and most developed soils with vegetation already installed. The soils from TB, MH and MI spoils present very similar clay contents, which can be explained by the similar age of the mine spoil that allowed at least partial modification of the parent matter. The erosion risk is high because the installed vegetation is very scarce.

The different clay minerals properties, like the CEC, specific surface and charge density [7], influence decisively the adsorption capacity. For that reason a semi-quantitative mineralogical analysis of the soils clay fraction was made (Figs. 2 and 3, Table 2). The results show that kaolinite is the most abundant mineral in all the studied soils, mainly in the MH zone; 95% of the total clay content of the MH soils is kaolinite which only contains, in addition, a small content of smectite and mica. In the other soils, the proportion of kaolinite ranges between 40.8% in TB soils to 90.6% in ME soils. The soils from the Touro mine spoils contain a smaller kaolinite content (Table 2). MH, MI and ME soils, coming from Meirama mine spoils, contain smectite, with a great CEC and greater capacity of heavy metal adsorption [7,26]. The soils from the copper mine spoils do not contain smectite (Table 2). The soils from TE, TB and MI contain gibbsite, but MH and ME soils lack this mineral; the greater content appears in

Fig. 2. Mineralogical analysis of the minesoils clay fraction from Touro (mean values: three replicates of each soil and five soils from each zone).

F.A. Vega et al. / Analytica Chimica Acta 524 (2004) 141–150

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Fig. 3. Mineralogical analysis of the minesoils clay fraction from Meirama.

Touro minesoils (Table 2). These soils also have high goethite content. The TE soils have significant chlorite and talc (4.2%) contents (Table 2). The mineralogical analysis (Fig. 2) indicated a remarkable non-crystalline mineral presence in TE and TB soils; this amorphous material has great affinity to adsorb different heavy metals, according to various authors [27–30]. 4.2. Heavy metals Figs. 4–6 show the total dissolved, the DTPA-extractable and the total contents of Cd, Cr, Cu, Ni, Pb and Zn (mg kg−1 ) in the studied soils. Dissolved Cd has not been found in any

soil. The total dissolved contents of the other heavy metals are low (<2 mg kg−1 ) except for the dissolved Cu content in TE soils (12.97 mg kg−1 ) (Fig. 4). DTPA-extractable heavy metals are relatively low (<5 mg kg−1 ), except DTPA-extractable Cu in the soils from Touro mine spoils (TE: 26.33 mg kg−1 and TB: 67.7 mg kg−1 , Fig. 5). The total heavy metal soils content is slightly high, although only the Cu total content (1217 mg kg−1 ) in TB soils exceeds the intervention limit of diverse reference guides [31,32]. The Cr total content (151 mg kg−1 ) in TB soils and total Zn content (111 mg kg−1 ) in TE soils (Fig. 6) are high; the toxicity risk will depend on the soil characteristics, on

Table 2 Semi-quantitative mineralogical analysis (%) of the clay fraction (mean values: three replicates of each soil and five soils from each zone) Clay minerals

TE Mine spoil (Touro)

TB Decantation-bank (Touro)

MH Wetland (Meirama)

MI Inner mine spoil (Meirama)

ME Outer mine spoil (Meirama)

Kaolinite Chlorite Quartz Smectite Gibbsite Goethite Mica Talc

59.6 d – – – 5.3 b 6.2 b 24.6 a 4.2 a

40.8 e 21.1 a 13.8 a – 7.8 a 7.4 a 8.9 c –

94.9 a – – 2.8 c – – 2.4 e –

70.8 c – – 10.6 a 1.8 c – 16.9 b –

90.6 b – – 4.3 b – – 5.1 d –

Semi-quantitative analysis: the results are expressed in percentage relative to the sum of the intensities of the tips of the basal planes of each identified mineral. For each parameter, different letters (a–d) in each file indicate significant differences (P < 0.05).

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Fig. 4. Total dissolved−metal content (mean values: three replicates of each soil and five soils from each zone). u.l.: undetectable level. For each element bars with different letter are statistically different using Duncan’s Multiple Range test (P < 0.05).

each metal, on environmental conditions and on soil handling, according to Dudka and Domy [3]. All can increase Cr and Zn mobilization. Table 3 shows the proportion of each element extractable with DTPA, which gives information about the heavy metal availability. For most metals, this is 10% of the total content. The proportion of DTPA-extractable Cd is 11.48% in MH soils (Table 3) although the total content is low (<5 mg kg−1 ) (Fig. 6). DTPA-extractable Cd has not been found in Touro minesoils, whereas in the Meirama minesoils (MI and ME) it does not reach 3% of the total content (Table 3).

The relation established between the soils clay content and the DTPA-extractable Cd (Table 4) shows that the clay is one of the most strongly involved soil components in Cd availability, therefore, the greater the amount of clay, the greater the amount of available Cd, which agrees with the results of other authors [33–35]. In all the studied soils, proportion of the DTPA-extractable Cr is very low (Fig. 5, Table 3), which indicates that most of Cr is strongly bound in soil primary minerals. The proportion of DTPA-extractable Cu is 5% in all soils (Table 3). The TB soils display the smallest proportion (5.6%) and MI the highest one (16.1%), whereas the DTPA-

Fig. 5. DTPA-extractable metal content (mean values: three replicates of each soil and five soils from each zone). u.l.: undetectable level. For each element bars with different letter are statistically different using Duncan’s Multiple Range test (P < 0.05).

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Fig. 6. Total metal content (mean values: three replicates of each soil and five soils from each zone). u.l.: undetectable level. For each element bars with different letter are statistically different using Duncan’s Multiple Range test (P < 0.05).

extractable Cu is 15% in TE and MI soils. Although in TB soils the proportion of DTPA-extractable Cu is low, the total content of Cu is very high (Fig. 6), therefore the available content is also high. The relation established (Table 4) between the soil organic matter content, and the humified organic matter, with the total, DTPA-extractable and soil dissolved Cu content, shows that the humified organic matter is the fraction involved in the formation of soluble complexes and in the electrostatic adsorption of Cu. This demonstrates the capacity of the organic matter to establish not only soluble complexes but also insoluble compounds with Cu, which has been found by other authors [36–38]. A positive correlation has also been established between the DTPA-extractable Cu and the Fe and Mn oxides content, which probably indicates that the Cu2+ can be, partly, adsorbed by oxides. The positive correlation (Table 4) established between the DTPA-extractable Cu content with the gibbsite, goethite and chlorite contents demonstrates that these minerals, have a

high capacity to adsorb Cu, which agrees with the results obtained by Schwertmann and Taylor [39]. The proportion of DTPA-extractable Ni in MI soils is 7.5% (Table 3). In the other studied soils this proportion is 2%, which indicates that Ni is probably strongly bound in the soil primary minerals. A positive correlation has been established (Table 4) between iron oxides, micas and talc contents with the DTPA-extractable Ni, which indicates the electrostatic combination of the Ni2+ and these soil components. Several authors [33] found that Ni is mainly adsorbed by soil iron oxides. In MI soils the proportion of DTPA-extractable Pb is 13.1%, whereas in TE soils it is 5.2% and in TB, MH and ME soils it is close the 2% (Table 3). A positive correlation has been established (Table 4) between the smectite content and the DTPA-extractable Pb content [36]. In spite of the high total content of Zn (Fig. 6), mainly in copper minesoils, the proportion of DTPA-extractable Zn in TE and TB soils does not reach 2% and in lignite

Table 3 Proportion of DTPA-extractable heavy metal (%) TE Mine spoil (Touro)

TB Decantation-bank (Touro)

MH Wetland (Meirama)

MI Inner mine spoil (Meirama)

ME Outer mine spoil (Meirama)

Cd Cr Cu Ni Pb Zn

0 0 15.3 1.81 5.18 1.80

0 0.01 5.56 0.94 2.17 1.08

11.5 0.01 8.02 0.79 1.87 3.99

2.34 0 16.1 7.52 13.11 4.23

2.41 0.02 13.3 1.17 2.06 4.09

Totala

24.0

9.8

26.2

43.3

23.1

Mean values: three replicates of each soil and five soils from each zone.
n+ a Total-extractable heavy metal species ( M ) (% of total content).

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Table 4 Pearson’s correlation matrix Dissolved

Total Cd Cr Cu Ni Pb Zn a ∗ ∗∗

Total

Cd

Cr

Cu

Ni

Pb

Zn

Cd

Cr

Cu

Ni

Pb

Zn

Cd

Cr

Cu

Ni

Pb

Zn

a

−0.925∗∗ 0.320 0.313 0.550∗ 0.710∗∗ 0.256 0.410 −0.735∗∗ 0.866∗∗ −0.776∗∗ −0.102 0.724∗∗ 0.659∗∗ 0.605∗ 0.240

−0.810∗∗ −0.196 −0.186 0.932∗∗ 0.082 −0.091 0.101 −0.922∗∗ 0.801∗∗ −0.397 −0.519∗ 0.487 0.618∗ 0.992∗∗ −0.131

−0.933∗∗ −0.155 −0.159 0.820∗∗ 0.330 −0.178 0.014 −0.887∗∗ 0.981∗∗ −0.461 −0.103 0.449 0.472 0.882∗∗ −0.204

0.106 −0.155 −0.159 −0.208 −0.046 −0.218 −0.256 0.201 −0.046 0.209 0.336 −0.270 −0.308 −0.164 −0.211

−0.855∗∗ −0.310 −0.317 0.760∗∗ 0.237 −0.371 −0.192 −0.815∗∗ 0.966∗∗ −0.291 0.103 0.250 0.257 0.815∗∗ −0.391

0.871∗∗ −0.577∗ −0.576∗ −0.506 −0.778∗∗ −0.583∗ −0.725∗∗ 0.658∗∗ −0.708∗∗ 0.948∗∗ 0.421 −0.947∗∗ −0.910∗∗ −0.561∗ −0.561∗

0.173 0.408 0.393 −0.365 0.150 0.458 0.387 0.413 −0.301 −0.205 −0.157 0.204 0.174 −0.317 0.461

−0.429 0.883∗∗ 0.890∗∗ 0.090 0.724∗∗ 0.945∗∗ 0.990∗∗ −0.225 0.157 −0.909∗∗ −0.605∗ 0.946∗∗ 0.908∗∗ 0.115 0.932∗∗

−0.892∗∗ 0.064 0.051 0.522∗ 0.607∗ −0.069 0.083 −0.668∗∗ 0.940∗∗ −0.554∗ 0.239 0.449 0.354 0.571∗ −0.077

−0.185 −0.121 −0.150 −0.205 0.328 −0.372 −0.375 0.094 0.352 0.052 0.920∗∗ −0.257 −0.452 −0.198 −0.346

0.181 −0.796∗∗ −0.810∗∗ −0.005 −0.467 −0.928∗∗ −0.937∗∗ 0.075 0.108 0.714∗∗ 0.762∗∗ −0.804∗∗ −0.828∗∗ −0.031 −0.911∗∗

−0.153 0.109 0.081 −0.018 0.122 0.113 0.117 0.116 0.092 −0.147 −0.062 0.135 0.139 0.067 0.102

−0.623∗ 0.839∗∗ 0.839∗∗ 0.215 0.847∗∗ 0.828∗∗ 0.912∗∗ −0.377 0.390 −0.963∗∗ −0.461 0.965∗∗ 0.899∗∗ 0.242 0.816∗∗

−0.223 0.958∗∗ 0.964∗∗ −0.134 0.711∗∗ 0.990∗∗ 0.990∗∗ −0.005 −0.056 −0.820∗∗ −0.515∗ 0.842∗∗ 0.778∗∗ −0.126 0.986∗∗

−0.718∗∗ 0.413 0.419 0.554∗ 0.510 0.478 0.614∗ −0.619∗ 0.571∗ −0.759∗∗ −0.571∗ 0.815∗∗ 0.847∗∗ 0.605* 0.453

0.628∗ −0.251 −0.260 −0.446 −0.431 −0.246 −0.365 0.608∗ −0.571∗ 0.574∗ 0.227 −0.573∗ −0.555∗ −0.472 −0.233

−0.823∗∗ 0.177 0.181 0.753∗∗ 0.401 0.227 0.400 −0.791∗∗ 0.742∗∗ −0.647∗∗ −0.511 0.706∗∗ 0.766∗∗ 0.802∗∗ 0.197

0.078 0.682∗∗ 0.331 0.593∗ −0.588∗ 0.701∗∗

0.081 0.351 −0.006 0.686∗∗ −0.516∗ 0.852∗∗

0.066 0.361 −0.089 0.600∗ −0.541∗ 0.793∗∗

0.340 −0.254 −0.233 −0.308 0.159 −0.209

0.027 0.173 −0.288 0.436 −0.492 0.660∗∗

−0.145 −0.896∗∗ −0.647∗∗ −0.835∗∗ 0.649∗∗ −0.816∗∗

0.680∗∗ 0.217 0.416 0.050 0.014 −0.071

0.145 0.938∗∗ 0.965∗∗ 0.694∗∗ −0.423 0.506

0.047 0.424 0.002 0.485 −0.574∗ 0.609*

−0.031 −0.178 −0.372 −0.287 −0.019 −0.201

−0.130 −0.782∗∗ −0.929∗∗ −0.569∗ 0.248 −0.361

1 0.198 0.138 0.187 0.093 0.210

0.198 1 0.887∗∗ 0.766∗∗ −0.490 0.651∗∗

0.138 0.887∗∗ 1 0.545∗ −0.292 0.322

0.187 0.766∗∗ 0.545∗ 1 −0.542∗ 0.926∗∗

0.093 −0.490 −0.292 −0.542∗ 1 −0.485

0.210 0.651∗∗ 0.322 0.926∗∗ −0.485 1

a a a a a a a a a a a a a a

a a a a a a

It is not possible to be calculated because at least one variable is constant. Correlation is significant at level.0.05 (bilateral). Correlation is significant at level.0.01 (bilateral).

F.A. Vega et al. / Analytica Chimica Acta 524 (2004) 141–150

pH H2 O O.M. O.M.h CECe Fe2 O3 Al2 O3 MnO Clay Mica Kaolinite Smectite Gibbsite Goethite Talc Chlorite

DTPA-extractable

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minesoils, nearly 4% of the total Zn content is DTPAextractable (Table 3). The low proportion of DTPAextractable Zn indicates that most of the Zn remains in the non-altered parent matter strongly bound in soil primary minerals. The origin of the Zn, Ni, and Cr contents of minesoils has been confirmed by means of the established positive correlation between the total content of these metals (Table 4). This correlation indicates that these metals come from the minerals of the parent matter of copper minesoils (chalcopyrite, amphibolites and limonite). Nevertheless, the Zn, Ni and Cr origin in lignite minesoils is the parent matter, fertilizers, like superphosphate [40], and animal manure [41] that have been added to the soils for several years. In lignite minesoils the contents of DTPA-extractable Zn, Ni and Cr are greater than in copper minesoils (Fig. 5, Table 3) because the metals are weakly bound in the added supplements than in the soil minerals. The total Cu content of TB soils (Fig. 6) is the only one that surpasses the intervention limit indicated in various reference guides [31,32] and these TB soils contains the
smallest proportion of DTPA-extractable metals content ( M n+ = 9.8%) (Table 3). They have, in addition, the highest content of the soil components that have the greatest heavy metals adsorption capacity (Table 1). For this reason they can neutralize, at least in the short term, the toxic effects possible from the liberation of their heavy metal content (Figs. 4–6). The soils that contain the higher proportion of DTPAextractable metal species (TE, MH and ME soils: ≈25% and MI soils: 43.3%) (Table 3), will tend to mobilize these metals. In these soils, the high available heavy metal content will make revegetation difficult and consequently the stabilization of tailings can be very complex and slow due to heavy metals toxicity.

5. Conclusions The total heavy metal dissolved contents are low (<2 mg kg−1 ) in all the studied soils except the content of dissolved Cu in soils from copper mine tailings (13.0 mg kg−1 ). Soil DTPA-extractable heavy metals concentrations are low (<5 mg kg−1 ), except for DTPA-extractable Cu in the soils from Touro mine spoils (TE: 26.3 mg kg−1 and TB: 67.7 mg kg−1 ). The total heavy metal content of the soils is slightly high. In TB soils the total Cu content (1217 mg kg−1 ) exceeds the intervention limit of various reference guides. In Touro minesoils, the Cr, Ni and Zn total contents come from the parent matter of these soils. The correlation established between the total contents of these metals suggests its common origin through the soils parent matter. In Meirama minesoils, the fertilizers, and animal manure which were added for several years, and the parent matter contribution, produce the heavy metal content.

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The toxic effects that heavy metals content could cause can be minimized, at least in the short term, in the minesoils with the higher concentrations of iron and manganese oxides, humified organic matter, and clay minerals, especially chlorite, gibbsite and goethite. The unfavourable properties of the soils will improve, the natural heavy metal contents in available forms will diminish and the stabilization of the tailings will be possible if the organic matter and clay contents were increased.

Acknowledgements The authors acknowledge to the Ministerio de Educaci´on y Ciencia and Xunta de Galicia (Espa˜na) for financial support (Projects: REN2002-0187 and PGIDTI03PXIC3001 PN).

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