Journal of Geochemical Exploration 116–117 (2012) 51–59
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Comparison of EDTA, HCl and sequential extraction procedures, for selected metals (Cu, Mn, Pb, Zn), in soils, riverine and marine sediments Lydia Leleyter ⁎, Christelle Rousseau 1, Laetitia Biree 1, Fabienne Baraud 1 Université de Caen Basse-Normandie, Equipe de Recherche en Physico-Chimie et Biotechnologies (ERPCB), EA 3914, Bd du Mal Juin, Campus 2, 14032 CAEN Cedex, France
a r t i c l e
i n f o
Article history: Received 7 July 2011 Accepted 22 March 2012 Available online 29 March 2012 Keywords: Chemical extraction Mobility evaluation Environmental risk
a b s t r a c t Knowledge of the total concentration of metals in soils and sediments is frequently insufficient to ascertain environmental risk. Simple and sequential extractions are useful tools for estimating the mobility of metals. Many chemical extraction procedures have been proposed in the literature. This study compares the efficiency of three chemical extractions (two single procedures, using EDTA or HCl as reactant, and a sequential chemical extraction) on soils, riverine, estuarine and marine sediments. In the case of riverine sediments and soils, similar results are observed with 0.05 mol.L − 1 EDTA or 0.2 mol.L − 1 HCl extractions, whereas 0.2 mol.L − 1 HCl is inefficient for marine or estuarine samples. Comparison of the results obtained for the various samples, suggests that it is necessary to use a unique procedure for all the samples. The use of 0.05 mol.L − 1 EDTA rather than 0.2 mol.L− 1 HCl, as reactant for the single extractions is recommended. The applied sequential extraction procedure is more aggressive than EDTA (except to evaluate Pb mobility for some samples). Assuming that the metal enrichments are mainly of anthropogenic origin and that these metals are of higher mobility, compared to native metals, it is concluded that, for estimation of metal mobility, EDTA leaching is better adapted for Pb, whereas the sequential extraction procedure is better suited for Zn and Cu. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The occurrence of metals in soils or sediments results from natural weathering processes affecting soils and rocks, also potentially additional anthropogenic inputs. The fate of these metals in the environment depends on several factors, such as soils or sediments properties, (e.g., metal source, loading rate, soil pH, redox potential, texture, organic matter and mineral composition), as well as external factors, such as chemical and biological processes. The metals can be bound in various ways. For example, they may be adsorbed on clay surfaces, or iron and manganese oxyhydroxides, and or, also present in the lattice of residual primary mineral phases (e.g., silicates) and or, secondary mineral phases, such as carbonates, sulphates and oxides. The metals may also be bound in amorphous materials, such as iron and manganese oxyhydroxides, or complexed with organic matter (Gismera et al., 2004; Tessier et al., 1979). Depending on the partitioning of the metal, the labile fraction may be dissolved, due to changes in environmental physico-chemical conditions, or reactions resulting from biological activities, related to microorganisms or plants roots (Abollino et al., 2002; Bourg, 1995; Forstner, 1993;
⁎ Corresponding author. Tel.: + 33 2 31 56 72 18; fax: + 33 2 31 56 73 03. E-mail addresses:
[email protected] (L. Leleyter),
[email protected] (C. Rousseau),
[email protected] (L. Biree),
[email protected] (F. Baraud). 1 Tel.: + 33 2 31 56 72 18; fax: + 33 2 31 56 73 03. 0375-6742/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2012.03.006
Forstner and Kersten, 1988; Leleyter and Probst, 1999). That means that the mobile metals (in sediments) is concerned metals which could be solubilise due to changes in the physico-chemical properties of the aquatic environment and, on the contrary, residual (or no mobile) metals are associated with very stable fractions of sediments, which can never be solubilised in the natural environment. Hence, knowledge of total metal concentration is not sufficient to evaluate elemental mobility. Determination of their solid fractionation is known to be useful to predict their actual mobility and their fate in the environment (Giancoli Barreto et al., 2004). Many chemical extraction (leaches) procedures have been proposed in the literature, to estimate the mobility of metals in soil and sediments, or their bioavailability defined as, the capacity of an element to be transferred from a soil fraction, to a living organism, regardless of mechanism (Baize, 1997). Despite the fact that the transfer of metals from soils or sediments to plants represents a major pathway of human exposure to contamination, there is still no agreement as to which extractant most accurately estimates the lability, or the bioavailability, of the metals. The various extractions procedures described in the literature mainly differ by the number of steps of operation. The single leaches (one step) provide inexpensive and rapid assessment methods. Depending on the nature of the reactant used, they fall into three categories (Lebourg et al., 1996; Sutherland, 2002). Firstly, those which employ salts as CaCl2 or Ca(NO3)2 (e.g., An and Kampbell, 2003; Fang et al., 2007), in order to leach cations adsorbed onto solid materials, due to permanent
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L. Leleyter et al. / Journal of Geochemical Exploration 116–117 (2012) 51–59
structural charges (phyllosilicates, phyllomanganates and sometimes organic matter). Secondly, techniques which employ acid solutions, in order to simulate the effect of an acid input (e.g. through acid rain or an accidental spill), because low pH favours the dissociation of the existing complexes. The third category consists of those techniques which employ complexing or reducing agents, such as EDTA, (Alvarez et al., 2006; Chao, 1984; Gismera et al., 2004; Madrid et al., 2007; McCready et al., 2003; Sahuquillo et al., 2003). Dilute HCl is one of the most widely used reagents, in techniques which employ acid solutions to isolate the non-residual phase, in a variety of solid environmental media. HCl is assumed to extract metals on exchange sites, due to its acidic properties, that dissolve calcareous materials, combined to the chelatant property of Cl − which is a strong ligand. The HCl concentration employed differs within separate studies. Giancoli Barreto et al. (2004) study of metal availability in sediments from Lake Ipê, Brazil, employed 0.1 mol.L − 1 HCl. The study concluded that HCl extraction might lead to erroneous conclusions about metal associations and lability. Similarly, Wei et al. (2005) concluded that estimating the level of metals taken-up by plants, with 0.1 mol.L − 1 HCl, can underestimate the intake of lead, because Pb 2 + readily reacts with chloride, to form a PbCl2 precipitate, that is only slightly soluble in dilute acids or water. In a further study, Yu et al. (2004) concluded that 0.1 mol.L − 1 HCl overestimates soil available Cu, because it releases some Fe/Mn oxide-bound Cu that may not be bioavailable under field conditions. Menzies et al. (2007) conclude that 0.1 mol.L − 1 HCl extraction is generally poorly correlated to metals plant uptake. Sutherland (2002) indicates that the dilute 0.5 mol.L − 1 HCl leach is slightly more aggressive than a three steps sequential procedure and that a dilute HCl leach is a valuable, rapid, and cost-effective analytical tool in contamination assessment. Kubová et al. (2008) also propose that 0.5 mol.L − 1 HCl offers a good reactant concentration, to estimate metal phytoavailability on contaminated soil. In a separate study of marine sediments from Antartica, Snape et al. (2004) recommend a 1 mol.L − 1 HCl acid extraction as a standard method for assessing metal contamination. EDTA is one of the most widely used complexing agents because of his high extraction capacity (Sahuquillo et al., 2003). EDTA is assumed to extract metals on exchange sites of both inorganic and organic complexes. Additionally, it can dissolve calcareous materials through complexation of calcium and magnesium (Chao, 1984; Gismera et al., 2004; Sahuquillo et al., 2003). The EDTA leaching seems less questioned than HCl leaching, most of the authors using the same EDTA concentration value (0.05 mol.L − 1 ETDA), even if 0.02 mol.L − 1 EDTA is also sometimes reported (Gismera et al., 2004). Sequential extraction procedures (several steps) do not give direct information about mineralogy but also enable the differentiation of mobile and residual fractions, with the advantage of characterizing the different labile fractions (Leleyter and Baraud, 2006; Leleyter and Probst, 1999; Shuman, 1985; Tessier et al., 1979; Ure et al., 1995). Generally three to height extractants are used in a sequence, the earlier ones are the least aggressive and the more specific, subsequent extractants are progressively more destructive. These sequential extraction procedures are a useful tool for solid speciation of particulate elements, to study the origin, the fate, the biological and physicochemical availability and transport of sorbed elements. As many chemical extraction procedures have been proposed in the literature (Alvarez et al., 2006; Giancoli Barreto et al., 2004; Gismera et al., 2004; Sahuquillo et al., 2003; Sutherland, 2002), the aim of this work is to compare the mobility of Cu, Mn, Zn and Pb, determined by two single leaches (HCl and EDTA) and one sequential extraction procedure (developed by Leleyter and Probst, 1999), applied to soils, riverine, estuarine and marine sediments; thus, to compare three chemical procedures, to estimate the metals mobility, on four surficial deposit types.
2. Experimental 2.1. Sampling Soils and sediments were collected from various localities in France (Fig. 1). The sampled materials fall into four categories: riverine sediments (RS), estuarine sediments (ES), marine sediments (MS) and soils (S). All the samples were air-dried, then passed through a 2 mm sieve before analysis and stored at 4 °C in polypropylene bottles. The use of air dried materials in this study, does not present a problem, because the sediment samples were collected in oxic conditions (Kersten and Forstner, 1986; Rapin et al., 1986). 2.1.1. Riverine and estuarine sediments The Vire and Orne rivers and their tributaries drain an area of 3500 km 2 each, (Fig. 1). Five river sediment (RS) samples were collected from the upper basins (water depth about 30 cm). Three estuarine sediments were collected, from the Orne (ES1), Aure (ES2) and Vire (ES3) (water depth about 30 cm). 2.1.2. Marine sediments One marine sediment (MS-1) obtained from the seawater settling (49°18 N; 00°21 W), was collected in Luc-sur-Mer (Normandie) in the North of France (Fig. 1). Two marine sediments were collected in the Thau lagoon (water depth 8 m) (Fig. 1). This Mediterranean shallow coastal lagoon, a total surface 75 km 2, is a site of an intense shell fish production. The two sampling stations were chosen in contrasting areas. MS-2 was sampled in the middle of the lagoon (43°24 N; 3°36E), MS-3 was sampled from the oyster bank zone (43°25 N, 3°39E). 2.1.3. Soils Two local soils (S1, S2) submitted to different anthropic pressure were selected. Soil S1 (48°48 N; 00°54 W) was a private garden soil from a rural area. S2 (49°18 N; 00°35 W) was an agricultural soil, periodically amended and/or treated. These surface soils samples were collected from the top 10 cm. 2.2. Extractions procedures 2.2.1. Single extractions To evaluate the mobile fraction, representative aliquots of each sample were leached by two different chemical reagents. 1 g of dry sediment was leached with 10 mL of extractant solution 0.2 mol.L − 1 HCl (Kuo et al., 2006) or 0.05 mol.L − 1 EDTA (Chao, 1984; Sahuquillo et al., 2003), at room temperature and shaken for 1 h. The resulting mixture was filtered at 0.45 μm and the filtrate was analysed using ICP-AES. 2.2.2. Sequential extraction The samples were also leached by an optimized sequential chemical extraction procedure. This method was chosen among several procedures because it was checked for selectivity, reproducibility, and repeatability of the different steps (for details, see Leleyter, 1998, and Leleyter and Probst, 1999) and it was commonly used in literature (Bur et al., 2009; Cecchi et al., 2008; N'guessan et al., 2009; Salvarredy-Aranguren et al., 2008). This procedure selectively and efficiently dissolves all the chemical constituents of the sediments, in the following fractions [Fx]. The water soluble fraction [F0] is released by ultrapure water. The [F1] exchangeable fraction is extracted with a magnesium nitrate solution. The acido-soluble [F2] fraction is leached by an acid/acetate buffer. The reducible fraction [F3] is extracted with hydroxyl ammonium, oxalic acid and ascorbic acid. The oxidable fraction [F4] is released by hydrogen peroxide–
L. Leleyter et al. / Journal of Geochemical Exploration 116–117 (2012) 51–59
53
Sampling stations English ES3 ES2
Channe l
MS1
Vire
Bayeux
CAEN Saint Lo
Or ne
Aure
ES1
on May sur Orne Od
RS1
20km Vire
So Clecy ul eu vr e
N
Laize
RS4
RS2 RS3
Vène BOUZIGUES
RS5
BALARUC MEZE
A
Canal du Rhône à Sète
C5 MS3
SETE
B
C4 MS2
Canaux de Sète
C Mediterranean Sea MARSEILLAN
0
Canal du Midi
Grau de Pisses-Saumes
5 km
Sampling stations A,B,C : Oyster bank zones
Fig. 1. Location map of the study area with the sediments sampling stations.
ammonium acetate extraction. [F5] is the sum of all labile fractions ([F5] = Σ[F0] to [F4]). Total concentrations are determined after acid digestion (aqua regia) in a microwave oven. After cooling, separation of the digests from the solid residue was achieved by filtration on 0.45 μm filter and then diluted to 50 mL with Milli-Q water. Metals extractability is expressed by percentage values that correspond to the ratio between the metal concentrations analysed in the leachate and the pseudototal content. 2.3. Chemical analysis Elemental analyses of the digests are carried out using inductively coupled plasma atomic emission spectrometry (ICP-AES; Varian Vista-MPX). The quality control of ICP-AES analyses was assessed by the analysis of blank reagents and calibration standards, prepared with commercially available solutions (Varian standard solution). Accuracy of ICP-AES measurements was determined with various certified reference materials (Rousseau et al., 2009). 3. Results and discussion 3.1. Soils and sediments composition Table 1 presents the total concentration of some major and trace elements in the studied samples. Also presented are average values reported by Zwolsman and Van Eck (1999) for marine suspended matter of the Scheldt estuarine (Stevenson, 2001) and world average values for soils reported by Allègre and Michard (1973), to provide baseline reference values. Metal concentrations are a result of the dispersal of both natural and anthropogenic particulate discharges that finally mix into the sediments. To discriminate these sources, and hence to determine with confidence the degree of enrichment/contamination of a given metal M, a common practice is to calculate enrichment factors (EFs), which can be defined as the ratio between the sample and the natural
background normalized concentrations, where C is a conservative element that is strongly associated with the finest sediment fraction (clays): EF ¼ ð½M=½C Þsample =ð½M=½C Þreference
ð1Þ
Hence it corrects the values between samples with variable grainsize distributions (Roussiez et al., 2006). The choice of the natural background reference is an important step because it can strongly influence decision making, as to whether a given soil or sediment has been impacted by anthropogenic activity. In general, authors refer to world average shales and/or crust composition, even if these data are not strictly representative of the local lithology, potentially leading to erroneous interpretations. The most widely used conservative element (C) is Sc. However, Sc can substitute for Al (Hernandez et al., 2003). In this work, Al is used as the conservative element, because it is an abundant element in the Earth's crust, thereby, Table 1 Chemical composition in g.kg− 1 of *marine sediment around Scotland (Stevenson, 2001), **suspended matter of the Scheldt estuarine (NDL) (Zwolsman and Van Eck, 1999), ***of world average soils (Allègre and Michard, 1973) and studied samples.
* ** *** ES1 ES2 ES3 RS1 RS2 RS3 RS4 RS5 MS1 MS2 MS3 S1 S2
Al
Ca
Cu
Fe
K
Mg
Mn
Na
P
Pb
S
Sr
Zn
45 45 81 10 23 43 41 54 53 50 56 10 19 11 49 35
108 99 36 147 136 117 25 3 3 2 3 134 75 135 10 7
0.004 0.03 0.06 0.01 0.03 0.02 0.02 0.03 0.01 0.02 0.01 0.02 0.08 0.09 0.03 0.01
18 48 50 27 13 22 30 28 23 16 18 20 26 19 26 20
12 16 26 6 7 10 11 15 18 12 13 4 7 5 15 12
10 10 21 3 4 7 3 4 3 3 3 6 8 7 4 3
0.4 1 1.0 0.5 0.2 0.2 1.6 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.6 0.6
/ 6 28 6 4 5 7 11 13 10 10 19 36 35 10 8
0.6 2.0 1.1 0.3 0.4 0.6 0.7 0.3 0.2 0.4 0.3 1.0 1.0 1.3 0.8 0.3
0.02 0.06 0.01 0.01 0.03 0.03 0.03 0.02 0.02 0.03 0.03 0.04 0.05 0.07 0.04 0.02
/ 5.0 0.3 1.1 2.5 4.3 0.6 0.6 0.0 0.1 0.2 3.7 9.4 12.5 0.3 0.1
0.6 0.4 0.4 0.7 0.4 0.4 0.1 0.1 0.1 0.0 0.1 0.4 0.2 0.4 0.1 0.1
0.04 0.2 0.07 0.05 0.05 0.10 0.16 0.09 0.11 0.07 0.05 0.13 0.18 0.24 0.10 0.05
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minimising analytical errors. Moreover, Al levels are rarely influenced by anthropogenic inputs (except in harbours with steel structures, employing sacrificial aluminium anodes). Ideally, the background values would be estimated from the corresponding area, but in the absence of these data, earth crust values are sometimes substituted (Feng et al., 2011; Zhu et al., 2011). Here as no local common background is available (too different studied samples), general world average values for earth crust (reported by Allègre and Michard, 1973) were preferred as reference values for all the studied samples, to allow comparison between all samples. Then, the enrichment factor (EF) is calculated as follows:
EF ¼ ð½M =½AlÞsample =ð½M =½AlÞworld average values reported by Alle`gre and Michardð1973Þ
ð2Þ
Table 2 exhibits enrichment factors for all the studied samples. It is evident that the sediments under marine influence (ES and MS) present Ca and S enrichments, attributed to the higher Ca 2 + and SO42 − concentrations in marine waters (400 and 2650 mg.L − 1), for estuarine and marine settings respectively (Meybeck, 1984). Values generally observed in riverine waters, range from 13.2 to 8.6 mg.L − 1 (Meybeck, 1984). Moreover, as already reported by Leleyter and Baraud (2005), increasing upstream enrichments in Pb and Zn, occur in the Vire river sediments (ES1, ES2, RS4 and RS5), probably due to anthropogenic influences. Similar upstream enrichment is also evident in the Orne basin samples. Sample MS1 (Rousseau et al., 2009) shows a high level of calcium carbonate, most likely attributable to the erosion of marine shells and can be considered as moderately polluted by Cu, Zn and Pb. The impact of the oyster colony on sediment composition is evident when the two marine samples MS2 (collected in the middle of the lagoon) and MS3 (collected from the oyster bank zone) are compared. Indeed enrichment factors in MS2 and MS3 are respectively 7 and 12 for Cu, 16 and 39 for Pb and 11 and 24 for Zn. The two soils samples (S1 and S2) contain few carbonates (less than 1%) and present some enrichment factors suggesting Pb and Zn anthropic input (Baraud and Leleyter, 2006). Despite some chemical similarities, i.e. Pb and Zn enrichments, the composition of the soils and sediments are sufficiently distinct from each other, to get a significant estimation of the extraction procedures efficiency and accuracy on samples of different nature and origin.
Table 2 Enrichment factor (EF = [X] / [Al]sample / [X] / [Al]***) of *marine sediment around Scotland (Stevenson, 2001), **suspended matter of the Scheldt estuarine (NDL) (Zwolsman and Van Eck, 1999), ***world average soils (Allègre and Michard, 1973) and studied samples.
* ** *** ES1 ES2 ES3 RS1 RS2 RS3 RS4 RS5 MS1 MS2 MS3 S1 S2
Al
Ca
Cu
Fe
K
Mg
Mn
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
5 5 1 33 13 6 1 0 0 0 0 30 9 27 0 0
0 1 1 2 2 1 1 1 0 0 0 4 7 12 1 1
1 2 1 4 1 1 1 1 1 1 1 3 2 3 1 1
1 1 1 2 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 0 0 0 0 0 3 2 2 0 0
1 2 1 4 1 0 3 0 1 0 0 3 1 1 1 1
Na
P
Pb
0 1 2 1 0 0 1 1 1 0 5 5 9 1 1
1 3 1 3 1 1 1 0 0 1 0 8 4 9 1 1
3 8 1 4 9 5 4 3 3 3 4 24 16 39 5 3
S
Sr
Zn
35 1 36 33 31 5 3 0 1 1 116 158 347 2 1
3 2 1 15 4 2 0 0 0 0 0 10 3 8 0 0
1 5 1 5 2 3 4 2 2 2 1 15 11 24 2 2
3.2. Comparison of the 3 different extraction procedures 3.2.1. Comparison of the 2 single extraction procedures HCl and EDTA are supposed to dissolve metals weakly adsorbed on both inorganic and organic complexes and metals associated to carbonates. Moreover, as the sediments samples were not centrifuged before air-dried, this means that metals in interstitial river or sea-waters are included into the extracts. Thus EDTA and HCl single leaches usually gave similar results, even if 0.05 mol.L− 1 EDTA extraction results are usually weaker than those obtained by 1 mol.L− 1 HCl extraction in harbour sediments (McCready et al., 2003), by 0.5 mol.L− 1 HCl extraction in urban soils (Madrid et al., 2007) or by 0.2 mol.L− 1 HCl extraction in riverine sediments (Leleyter and Baraud, 2005). Figs. 2 and 3 report the percentage values of leachable metals after HCl or EDTA single extractions, for the various studied samples. Two main sets of results are drawn from these figures. The first one gathered the soils and the sediments that are not under marine influence (RS and S samples). For these riverine sediments and soils, similar results are observed with EDTA and HCl extractions. Similar relative ordering of elements, in terms of their environmental mobility, is observed from the least to the most mobile: Zn ~ Cu b Mn b Pb. These results are in good agreement with the literature (Baize, 1997; Chao, 1984; Juste, 1989; Shuman, 1985). Contrary to some previous studies (e.g., Leleyter and Baraud, 2005; Madrid et al., 2007; McCready et al., 2003), HCl is not more aggressive than EDTA, i.e. results show that S2 and RS2 samples, EDTA is more efficient than HCl, in extraction of Cu, Pb and Mn. The second group of results gathered from the sediments that are under marine influence (ES and MS samples) show that 0.2 mol.L − 1 HCl is less efficient at solubilizing metals compared to the EDTA extraction, except for Mn. It appears that for all the estuarine and marine samples (S content ≥ 1 g.kg − 1), Pb 0.2 mol.L − 1 HCl extractability is equal to 0%, Cu or Zn 0.2 mol.L − 1 HCl extractability is inferior to 6%, whereas the results obtained with EDTA show that 31 to 82% of the total Pb, 5 to 34% of the total Zn, and 4 to 15% of the total Cu are mobile, meaning that these metals are potentially available and can constitute an environmental threat. As the decrease in efficiency of HCl extraction correlates with the ‘marine’ nature of the samples, the specific properties of the marine settings which account for this behaviour require investigation. Fig. 4 shows Mn, Cu, Zn and Pb–HCl extractability, versus the total S and Ca content, for all the samples studied. It is evident that the 0.2 mol.L − 1 HCl extraction efficiency decreases as the initial sample S content increases. In the case of marine or estuarine sediments (with high S content), 0.2 mol.L − 1 HCl extraction is inefficient. Potentially causing erroneous conclusions, regarding Pb, Zn and Cu extraction, if 0.2 mol.L − 1 HCl extraction is the only test performed, it would be concluded that Pb, Cu and Zn are poorly available, and mainly associated to the residual fraction. This would lead to the erroneous conclusion that they present no or a low environmental risk, despite the fact that all these samples show Pb and Zn-enrichments. Three hypotheses can be formulated to explain such a result. These are outlined below. 1- The acidity provided by 0.2 mol.L − 1 HCl might have been totally neutralised by reaction with some minerals, such as carbonates, that are present at high level in marine environment. Then 0.2 mol.L − 1 HCl would not be concentrated enough to additionally react with metal bound to other solid phase fractions. This argument is generally proposed to justify the use of 1 or 6 mol.L − 1 HCl to extract metals from marine sediments. However, we observe that Mn is significantly extracted by 0.2 mol.L − 1 HCl. This could be explained by the fact that the manganese is bound to carbonates (thus co-solubilized with the carbonates), whereas the other metals (Cu, Pb and Zn) are scavenged in the other mobile fractions which cannot be solubilised, once 0.2 mol.L − 1 HCl has been neutralised by the carbonates phase(s).
L. Leleyter et al. / Journal of Geochemical Exploration 116–117 (2012) 51–59
100
EDTA
% leached Pb
HCl
80
F5
100 80 60
40
40
20
20
100
ES1 ES2 ES3 MS1 MS2 MS3 RS1 RS2 RS3 RS4 RS5 S1 EDTA
% leached Mn
HCl
80
S2
0
100
% leached Cu
F5
ES1 ES2 ES3 MS1 MS2 MS3 RS1 RS2 RS3 RS4 RS5 S1
EDTA
80 60
40
40
20
20
ES1 ES2 ES3 MS1 MS2 MS3 RS1 RS2 RS3 RS4 RS5 S1
S2
0
S2
% leached Zn
HCl
F5
60
0
EDTA HCl
60
0
55
F5
ES1 ES2 ES3 MS1 MS2 MS3 RS1 RS2 RS3 RS4 RS5 S1
S2
Fig. 2. Percentage of metal leached by 0.2 N HCl, EDTA or sequential extraction (F5) for all the studied samples.
2- Cu, Zn and Pb are associated to a mineralogical fraction that is not attacked at all by 0.2 mol.L − 1 HCl, but is solubilized by EDTA. This scenario would mean that these metals are not associated to any carbonate phase(s), or exchangeable fraction, but may only be present as highly complexed forms, sorbed on
minerals. Additionally, the proposed complexes would have to be less stable than the complexes they can form with EDTA, but more stable than the ones they can form with Cl −. 3- Cu, Zn and Pb are partly or totally associated to fractions that are known to be dissolved by HCl, but during the experimental
ES
RS S
40
ES
70
MS
% Mn extracted by HCl
% Zn extracted by HCl
50
30 20 10
MS RS
60
S
50 40 30 20 10
0
0 0
10
20
30
40
0
50
10
% Zn extracted by EDTA ES
100
% Cu extracted by HCl
RS
% Pb extracted by HCl
30
40
50
60
70
35
40
ES
40
MS
S
80
20
% Mn extracted by EDTA
60 40 20
MS RS
35
S
30 25 20 15 10 5 0
0 0
20
40
60
80
% Pb extracted by EDTA
100
0
5
10
15
20
25
30
% Cu extracted by EDTA
Fig. 3. Relationship between the amounts of a specific metal (Zn, Mn, Pb and Cu) extracted by HCl and by EDTA for all studied samples.
L. Leleyter et al. / Journal of Geochemical Exploration 116–117 (2012) 51–59 Zn Pb Cu Mn
% HCl leached
100 80 60 40 20
20
40
60
80
100
120
140
80 60 40 20 0 0,0
0 0
Zn Pb Cu Mn
100
% HCl leached
56
160
2,0
4,0
6,0
8,0
10,0
12,0
14,0
S content (mg/g)
Ca content (mg/g)
Fig. 4. Zn, Pb, Cu and Mn-HCl extractability versus S or Ca content for all studied sediments and soils.
% Zn extracted by sequential extraction
Pb re-adsorption and redistribution processes could be explained by Pb-chloride precipitation. Indeed PbCl2 precipitation phenomenon has already been observed by Wei et al. (2005), by XAS analysis. They report that, during 0.1 mol.L − 1 HCl extraction, the main part of Pb is converted into precipitated PbCl2. They conclude that estimating the level of metals up-take by plant material using the 0.1 mol.L − 1 HCl extraction method, can underestimate the Pb up-take, because lead cations readily react with chloride to form PbCl2 precipitate, that is only slightly soluble in dilute acids or water. However, such an interpretation cannot explain the efficiency of Pb (and other metals) HCl extraction on riverine or soils samples. Alternately, metal readsorption and redistribution processes could be explained by
100
80
60
40 ES
20
MS RS
0
x 0
20
40
60
S
80
100
metals-sulfide precipitation. In order to understand these seemingly contradictory results, additional tests were performed. Single extractions were realised, using more concentrated HCl solutions, and replacing HCl by HNO3, to estimate the possible influence of acidity. These additional tests show, for example, that 69% of Pb is solubilized from MS1 when using HCl 1 mol.L − 1 (whereas 0% is leached with HCl 0.2 mol.L − 1), but 0% of lead is leached when replacing 0.2 mol.L − 1 HCl by 0.2 mol.L − 1 HNO3. Thus these additional tests confirm that metals-Cl reprecipitation cannot explain the obtained results. In order to characterise the different mobile fractions for each metal, all the samples were leached according a sequential extraction procedure, proposed by Leleyter and Probst (1999). 3.2.2. Sequential extraction procedure In addition to reporting results obtained by EDTA and HCl single extractions, Fig. 2 also reports the percentage values obtained after sequential extraction (Leleyter and Baraud, 2006; Leleyter and Probst,
% Mn extracted by sequential extraction
process, for these specific marine and estuarine samples, Cu, Zn and Pb is leached by HCl, but then re-adsorbed or re-precipitated onto the solid, thereby, accounting for why they are not detected in the corresponding filtrate.
100
80
60
40 ES
20
MS RS
0
x 0
20
80
60
40 ES
20
MS RS
x 20
40
60
% Pb extracted by EDTA
80
S
100
% Cu extracted by sequential extraction
% Pb extracted by sequential extraction
100
0
60
S
80
100
% Mn extracted by EDTA
% Zn extracted by EDTA
0
40
100
80
60
40 ES
20
MS RS
x 0
0
20
40
60
80
S
100
% Cu extracted by EDTA
Fig. 5. Relationship between the amounts of a specific metal (Zn, Mn, Pb and Cu) extracted by EDTA and by sequential extraction for all studied samples.
L. Leleyter et al. / Journal of Geochemical Exploration 116–117 (2012) 51–59
1999). Results show that the applied sequential extraction procedure is more aggressive than the EDTA single leachate, at removing Mn, Zn and Cu from all the studied samples, with the exception of Mn in sample RS2 (Figs. 2 and 5). Such a result is in full agreement with Madrid et al. (2007), who show that only a small proportion of the metals accessible to the 3-step standardized sequential procedure (BCR) reagents are likely to be soluble in EDTA. On the other hand, the results contradict the results obtained for Cu and Mn in other studies. Giancoli Barreto et al. (2004) noted that application of HCl 0.1 mol.L − 1 produced superior extraction of Cu and Mn relative to Tessier's sequential extraction. Sutherland (2002) concluded that the 0.5 mol.L − 1 HCl single leach is slightly more aggressive than the BCR to extract Cu and Mn from soils. The results for Pb are more contradictory, as the EDTA extraction is more aggressive than the sequential extraction on marine and estuarine sediments (except ES1). Whereas, sequential extraction is more aggressive than the EDTA or HCl single leaches on soils and riverine samples (except RS4 and RS5). Contrary to this, Sutherland (2002) observed no significant differences between Pb and Zn concentrations, liberated by 0.5 mol.L − 1 HCl or BCR extraction. Giancoli Barreto et al. (2004) also noted a higher efficiency of the Tessier's sequential extraction to remove Pb, compared to 0.1 mol.L − 1 HCl extraction. Fig. 6 shows the percentage values obtained by the three extractions, versus the enrichment factor calculated for each element for the samples with EF > 2.0. Metals of anthropic origin are generally assumed as more mobile than metals of natural origin. Therefore, there should be a positive correlation between the quantity chemically leached and the initial enrichment factor, where EF > 2.0. The best correlation is observed for Pb employing EDTA (Fig. 6), whereas for Zn or Cu, the best correlation is obtained with the sequential extraction procedure [F5]. As only 3 samples show some 100
% EDTA
Mn enrichment, no conclusion about the best adapted method to evaluate Mn mobility can be drawn. As the samples with the strongest enrichment factor are the marine and estuarine samples, results show that the HCl extraction is unsuitable for these samples. EDTA leaching appears as the best method to estimate Pb mobility, whereas sequential extraction appears more appropriate to estimate Zn or Cu mobility. This conclusion agrees with the results obtained by Menzies et al., 2007, who show that 0.05 mol.L − 1 EDTA provided a poor prediction of Cu and Zn-phytoavailability, and the results of Kubová et al. (2008), who showed that 0.5 M-HCl single extraction has the low selectivity, considering the Cu phytoavailability assessment, for various soils. This is also in agreement with Weimin et al. (1994) who show that accumulated metals in the soldier crab Mictyris longicarpus are mostly derived from sediments, rather than from the water column. The correlation between Pb extracted by 0.05 mol.L − 1 EDTA and Pb in the crabs are significant, and superior to the correlation with 0.05 mol.L − 1 HCl. However this conclusion disagrees with Menzies et al., 2007 who showed that 0.05 mol.L − 1 EDTA provided a poor prediction of Pb-phytoavailability in soils. In Fig. 7, metals fractionation is shown. The two first fractions ([F0] and [F1]) are negligible for all the studied metals in all samples. The sequential extraction procedure distinguishes, among the mobile elements, those highly affected by reduction processes (e.g. Cu scavenged preferentially in reducible fraction [F3]), those released by some reduction and/or acidification processes (e.g. Mn scavenged preferentially in acido-soluble or reducible fractions [F2], [F3]), and those affected by reduction and/or acidification processes and/or oxidation of organic matter or sulfur (e.g. Pb, Zn scavenged preferentially in [F2], [F3] or [F4] fractions). These results show that, for the marine (MS1 to 3) and estuarine sediments (ES1 to 3), significant amount of Pb and Zn are associated with the acido-soluble fraction. The weak extractability with 80
Cu
57
% EDTA
Mn
% HCl
% HCl
80
% F5
60
% F5
60 40 40 20
20
0
0 0
2
4
6
8
10
12
14
1
2
3
EF 100
4
EF % EDTA
Pb
100
Zn
% HCl
80
% F5
80
% EDTA % HCl
60
60 % F5
40
40
20
20
0
1
11
21
31
EF
41
51
0
1
11
21
EF
Fig. 6. Correlation between each metal percentage leached and the initial enrichment factor.
58
L. Leleyter et al. / Journal of Geochemical Exploration 116–117 (2012) 51–59
100
ES1
80
100
Zn
ES2
80
MS1
60
RS1
MS1 MS3 RS1 RS2
RS3
40
RS4
RS3 RS4
RS5
20
ES3 MS2
MS3 RS2
40
Pb
ES2
ES3 MS2
60
ES1
S1
20
S2
RS5 S1 S2
0
100
F0
F1
F2
ES1
F4
F5
100
Cu
ES2
80
F3
0
80
MS1
60
F4
F5
F3
F4
F5
Mn
ES3 MS1 MS3 RS2
RS3
40
RS4
RS3 RS4
RS5
RS5
20
S1 S2
0
ES1
F3
RS1
RS2
20
F2
MS2
MS3 RS1
40
F1
ES2
ES3 MS2
60
F0
S1 S2
F0
F1
F2
F3
F4
F5
0
F0
F1
F2
Fig. 7. Percentage of leached Zn, Pb, Cu and Mn in the different labile fractions.
0.2 mol.L − 1 HCl cannot be explained, by Mn bound to carbonates, with Cu, Pb and Zn scavenged in the other mobile fractions, which cannot be solubilized because of 0.2 mol.L − 1 HCl neutralization. Nor, can the weak extractability be accounted for by association of the metals with a mineralogical fraction that is not attacked by acid extractant, but solubilized by EDTA. It seems that in the marine or estuarine sediments, a pH value of 0.7, corresponding to HCl or HNO3 0.2 mol.L − 1, is not sufficient to release some elements such as Pb, Cu, Zn, even if they are partly associated to the acido-soluble fraction. This could only be achieved with strong acid conditions (pH near to 0), or when strong ligands are used (EDTA or the acetate ions that are used in the sequential extraction to liberate the [F2] associated elements). The hypothesis that Cu, Zn and Pb are partly associated with fractions dissolved by HCl, but for specific marine and estuarine samples, subsequently precipitate as metals-sulfides could explain the results. Such an interpretation also explains why during EDTA and [F2] extractions, no re-adsorption or redistribution processes occur, i.e., the complexes formed between the metals and EDTA or [F2]-acetate reactant must be stable. For example, pkd values for lead are 18 and 3.3 for Pb-EDTA and Pb-(AcO)2 respectively, which, upon formation, probably ensures enough stability to the Pb complexes, to prevent Pb-sulfide precipitation. The might not be the case for the less stable PbCl2 complexes that can be formed with Cl −, pKd of PbCl2 = 0.82 (Bernard and Busnot, 1978). When more concentrated HCl (1 mol.L − 1) is applied, the equilibrium might be displaced towards soluble Cl/metals forms. Consequently, in case of the estuarine and marine samples, 0.2 mol.L − 1 HCl-extraction might lead to a significant underestimation of the mobility of metals and potentially erroneous conclusions concerning metal associations and mobility. Some authors (e.g., Luoma and Bryan, 1981, 1982; Snape et al., 2004) propose using higher HCl concentrations (1 mol.L − 1 HCl). According to these authors, results prove that 1 mol.L − 1 HCl extraction is efficient to solubilize metals for such samples. However, employing 1 mol.L − 1 HCl may also partially attacks clays and amorphous allophonic materials (e.g., Chao and Zhou, 1983) thereby leading to an overestimation of metals mobility.
4. Conclusion For riverine sediments and soils, a concentration of 0.2 mol.L − 1 HCl extraction is sufficient evaluate metal availability. Similar results are observed with 0.05 mol.L − 1 EDTA or 0.2 mol.L − 1 HCl extractions. In case of marine and estuarine samples, with elevated S content, 0.2 mol.L − 1 HCl only poorly extract metals. Some authors (e.g., Luoma and Bryan, 1981, 1982; Snape et al., 2004) estimate that 1 mol.L − 1 HCl could be a suitable compromise between extraction efficiency and impact on the residual clays or sulpfides However, other studies (Chao and Zhou, 1983) estimate that 1 mol.L − 1 HCl may partially attack the residual materials. Moreover, in order to compare different results for different samples collected from the same basin (as riverine and estuarine sediments), it seems better to use exactly the same extractant (at the same concentration) for all the studied samples. Hence, this study suggests that 0.05 mol.L − 1 EDTA, rather than HCl be used as the reactant for single extractions. The sequential procedures present the advantage of characterizing the different labile fractions, which is important to improve the understanding of the metals mobility and transfers. The applied sequential extraction presented here, is here more aggressive than EDTA for the studied elements. By assuming that metal enrichments are mainly a consequence of anthropogenic pollution, and that anthropic sorbed metals are much more mobile than other metals, it is concluded that EDTA leaching is the best suited method to estimate the Pb mobility, whereas sequential extraction is more adapted to Zn or Cu mobility. However these results have to be confirmed by further study and repeatability tests. Moreover further studies using plants or animals to determine the actual bioavailable fraction have to be performed to determine if EDTA might underestimate metal bioavailability, or if applied sequential extraction overestimates bioavailability. It is suggested that it can only be assumed that the residual fraction, determined by sequential extraction, contains fractions of a given elements concentration that is never bioavailable.
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