ARTICLE pubs.acs.org/est
Multielementary (Cd, Cu, Pb, Zn, Ni) Stable Isotopic Exchange Kinetic (SIEK) Method To Characterize Polymetallic Contaminations Yann Sivry,*,†,‡ Jean Riotte,† Valerie Sappin-Didier,§ Marguerite Munoz,† Paul-Olivier Redon,§ Laurence Denaix,§ and Bernard Dupre† †
Laboratoire des Mecanismes et Transferts en Geologie, Universite de Toulouse-CNRS-IRD-OMP, Avenue Edouard Belin, 31400 Toulouse, France ‡ Univ Paris Diderot, Sorbonne Paris Cite, Institut de Physique du Globe de Paris, UMR 7154 CNRS, F-75013 Paris, France § UMR 1220 TCEM, INRA Bordeaux-Aquitaine, av. E. Bourlaux, BP 81, 33883 Villenave d’OrnonFrance
bS Supporting Information ABSTRACT: A new method is proposed to precisely and simultaneously quantify the exchangeable pool of metals in soils and to describe its reactivity at short- and long-term. It is based on multielementary Stable Isotopic Exchange Kinetics (multi-SIEK), first validated by a comparison between two monoelementary radioactive (109Cd*, 65Zn*) IEK experiments, a mono- (106Cd) and multi- (62Ni, 65 Cu, 67Zn, 106Cd, 204Pb) SIEK. These experiments were performed on a polluted soil located near the Zn smelter plant of Viviez (Lot watershed, France). The IEK results obtained for Cd and Zn were consistent across the experiments. 109Cd*, 65 Zn* IEK, and multi-SIEK were then applied on 3 non- and moderate impacted soils that also provided consistent results for Cd and Zn. Within these experimental conditions, it can be concluded that no competition occurs between Cd, Zn, and the other metals during SIEK. Multi-SIEK results indicate that the isotopically exchangeable pool of Ni, Zn, and Cu are small (ENi, EZn, and ECu values up to 17%) whatever the pollution degree of the soils considered in this study and whatever the duration of the interaction. On the contrary, Cd displays the highest E values (from 35% to 61% after 1 week), and EPb displays a maximum value of 26% after 1 week. The multi-SIEK provides useful information on metal sources and reactivity relationship. Ni would be located in stable pedogenic phases according to its very low enrichment factor. The low EZn and ECu are consistent with location of Zn and Cu in stable phases coming from tailings erosion. Though Pb enrichments in soils may also be attributed to tailings particles, its larger exchangeable pool suggests that the Pb-bearing phases are more labile than those containing Zn and Cu. The high mobility of Cd in upstream soils indicates that it has been mostly emitted as reactive atmospheric particles during high temperature ore-treatment.
’ INTRODUCTION Quantification of the mobility, bioavailability, and fate of metal contaminants in environmental systems is a key issue for assessing health hazards and metal remediation in ecosystems. Various approaches have been applied to characterize the trace metal fraction in soil which contributes to the dynamics of metal ions between the soil and the soil solution.1 Many techniques involve single or multiple extraction steps that selectively leach the labile metal pool.2 These approaches attempt to define the metal location/availability in the soil. However, though sequential extractions are relatively easy to apply and may give useful information on the metals associations in the soil, interpretation in terms of ion availability is not obvious.3 Moreover, they are plagued with a number of problems such as reprecipitation.4 Microphysical characterization methods offer the possibility of comprehensive speciation analysis of solid phases but are expensive, and thus access to equipment (e.g., EXAFS) is often restricted. Such solid-phase techniques can also suffer from detection limit issues and are not yet well developed in some cases. r 2011 American Chemical Society
Isotopic dilution (ID) technique, also called spiking method, provides an alternative approach for determining the solutionsolid-phase partitioning coefficient (Kd) and isotopically exchangeable concentration (E value5,6 and references therein). It was first used to estimate availability (E values) of phosphorus, an essential nutrient for plants.7 This technique was later applied to many others elements such as Al, As, Cd, Ca, Cu, Fe, F, K, Mg, Mn, N, Ni, Pb, S, Se, and Zn, in order to assess the nutrient supplying ability of soils or to study their sorption properties in relation with mineralogy, pH, or other parameters.5,6,815 Calculations of E values at several time steps following addition of tracers to the soil suspension are used to characterize the size of operationally defined element pools. This technique is known as the ‘isotopic exchange kinetic’ (IEK) and allows a description of the evolution of soil-solution transfers by taking Received: November 29, 2009 Accepted: July 5, 2011 Revised: June 27, 2011 Published: July 05, 2011 6247
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Environmental Science & Technology into account kinetics and concentration of elements in solution. IEK are of particular interest to assess the soil capacity to supply with elements a soil solution depleted by plant roots uptake, assuming that the plant does not significantly modify the soil chemical equilibrium.5,16 Also first developed for phosphorus study,16,17 this technique is commonly applied to measure the amount of exchangeable Cd at different time steps in soils.18,19 IEK experiments are often necessary, since the E values would hardly be estimated on the basis of short-term experiment (typically 1 to 100 min20). In most cases, the tracer used for IEK determination is a radioactive isotope, which presents the advantage of being detected in very low amounts. Nevertheless, the inherent hazard related to radioactivity accumulates with the difficulties of tracing short-lived isotopes (e.g., 64Cu, 12.4 h) and of conducting the experiments in the field. Furthermore, the simultaneous tracing of several elements is restricted to cases where emission energies are different from each other.9,11,21 The ICP-MS improvements during the past decade allow for a wide range of isotopic ratios22 to be measured at the same time, which makes their use possible for E value determination with precisions equivalent to those of radioactive tracers.3,13,14,23 This method not only addresses the behavior of elements which lack suitable radioisotopes24 but also avoids both radioactivity constraints and reactant selectivity or elements reprecipitation inherent to sequential extractions. Some E values determined by stable ID were successfully compared to the pool assimilated by plants (L value, 14,21) and to the sorbed amounts of metals determined by ethylenediamine tetra-acetic acid (EDTA) extractions.10 Nevertheless, very few studies have compared stable and radioactive isotopic dilutions,13,23 and the simultaneous exchange kinetics determination for various contaminants by multielementary isotopic dilution (MID) is not yet validated. The aim of this study was to consider both multielementary and kinetic aspects of stable isotopic dilution for the study of a multimetallic pollution. To achieve this objective (i) the Stable Isotopic Exchange Kinetic method (SIEK method) was validated in a polluted soil by comparing both stable (106Cd) and radioactive (109Cd*) monoelementary IEK in the case of Cd spike alone; (ii) the multielementary SIEK method was validated for Cd and Zn by performing the same experiments with two radioactive spike solutions (109Cd* and 65Zn*) and a spike solution enriched in 62Ni, 65Cu, 67Zn, 106Cd, and 204Pb; and (iii) the multielementary SIEK method was applied on four soils contaminated at various degrees by heavy metals, emphasizing the usefulness of this method for the study of multimetallic pollutions.
’ METHODS Soil Location, Sampling, and Analysis. Four fluvisols (FL) were sampled in the riverbanks of the Riou Mort, Riou Viou, and Lot rivers. These rivers drain the Viviez-Decazeville basin (Aveyron, SW France) and are affected by heavy metal pollution arising from an industrial Zn-ore smelting plant active from 1842 to 1987.25 Soils subject to flooding were collected upstream and downstream of the metallurgic site, still containing tailings, on the Riou-Mort river flood terraces (V0 and V1, respectively), and upstream and downstream of the Lot and Riou Mort rivers confluence (L0 and B2, respectively, see the SI and ref 25 for precise soils location). Given its location, L0 is the less impacted by the multimetallic contamination with only atmospheric emissions.2528
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The soils V0, V1, and B2 surround the plant and were impacted by flood events and possibly by tailing erosion/leaching and high temperature atmospheric emissions.25,28 The 20 cm topsoils were sampled on a vegetation-cleaned 1 m2 surface, homogenized and quartered, then air-dried, and sieved at 2 mm. Samples were characterized for pH, organic matter (OM), and cation exchange capacity (CEC, Table 1). Subsamples of soils were powdered and homogenized in agate mortar and then aciddigested with a mixture of ultra pure HNO3HF-HCl after removal of organic matter with ultrapure H2O2. Trace elements contents were determined using a Perkin-Elmer SCIEX quadrupolar ICP-MS Elan 6000 at LMTG (University of Toulouse, France). Detection limit ranged between 0.01 and 5 μg L1 depending on the element and external precision was (5%. Certified values of metal concentrations measured in the SRM 2711 contaminated soil standard (NIST) were recovered at more than 95%. Spiking Solutions. Solution #1 is the spiking solution with the radioactive isotope 109Cd*. It was prepared by dilution of the source (CUS-1, Amersham Biosciences, as CdCl2 in 0.1 M HCl) to obtain a solution of 32 kBq mL1. Solution #2 is the spiking solution with the radioactive isotope 65Zn*. It was prepared by dilution of the source (NEZ-111, Perkin-Elmer, as ZnCl2 in 0.5 M HCl) to obtain a solution of 2.9 kBq mL1. Solution #3 is the spiking solution with the stable isotope 106Cd (79.013% atom abundance), and solution #4 is the multielement spiking solution enriched in stable 62Ni, 65Cu, 67Zn, 106Cd, and 204Pb (96.632%, 99.610%, 94.600%, 79.013%, and 69.293% atom abundances, respectively). Solutions #3 and #4 were prepared from Spectrascan stock solutions of ∼10 mg L1 in HNO3 0.4 M matrix. Each stock solution concentration was first calibrated by isotopic dilution with international standards (Inorganic Venture). Solution #3 contained 10 mg L1 106Cd in HNO3 0.4 M, and solution #4 was obtained by mixing all stock solutions before gentle evaporation followed by spot dissolution in HNO3 15.1 M and dilution to obtain 10 mg L1 for 62Ni, 65Cu, 106Cd, and 204Pb and 100 mg L1 for 67Zn in HNO3 0.4 M. Higher Zn concentration was required to minimize the statistical error on 67Zn/64Zn ratio measurements 29 in the most concentrated soils (Table 1). Isotopic Exchange Kinetic Protocol. The experimental protocol used was the same for all four added spike solutions (radioactives, stable mono- or multielement). Each soil sample was divided into 24 suspensions containing 1 g of soil, mixed with 10 mL of ultrapure water, and then placed on an end-over-end shaker for 16 h at 2023 °C, in order to obtain stationary metal concentrations in solution. At t = 0, from 10 to 100 μL of spike solution were injected in each soil suspension. Samples were then kept on the roller until filtration. The amount of exchanged metal was measured at eight time steps, from 1 min to 1 week (Table 1). For each time step, 2 mL of three spiked soil suspensions were collected and filtered separately through 0.2 μm syringe filters (Minisart Sartorius) to assess the reproducibility of the method. Three filtration blanks were performed to check the filter and reagent contribution. For the Radioactive IEK experiments, a reference flask of the soil suspension at equilibrium was prepared containing 32 kBq of 109Cd* or 2.9 kBq of 65Zn* in 10 mL of soil suspension filtrate. After 420 min of shaking, 1 mL of solution was collected to measure the total radioactivity introduced (R). All filtrates, filtration blanks, and multielementary spiking solution were then weight-diluted in ultrapure HNO3 2% for ICP-MS analysis. 6248
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60
11
OM(g/kg)b
CEC (cmol+/kg) c 1.03 1.26 4.10 4.46 3.79 26
60 min
180 min
420 min
1410 min
10000 min
bulk soil
cone.
0.86
0.72
0.43
0.31 0.50 0.61 0.75 0.92 1.02 1.17 1.24 18
0.5 0.7 0.9 1.1 1.2 1.2 1.4 1.7 44
0.29 0.42 0.52 0.52 0.60 0.67 0.75 0.80 27
30 min
10 min
1 min
1 min 10 min 30 min 60 min 180 min 420 min 1410 min 10000 min bulk soil cone.
1 min 10 min 30 min 60 min 180 min 420 min 1410 min 10000 min bulk soil cone.
1 min 10 min 30 min 60 min 180 min 420 min 1410 min 10000 min bulk soil cone.
(mg.kg-1)
0.38
0.22
0.49
0.14
0.02
0.01
0.03
0.03
0.04 0.05 0.01 0.02 0.01 0.08 0.08 0.14
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
0.01 0.01 0.03 0.05 0.02 0.03 0.05 0.05
15
17
16
4.8
4.0
3.3
2.8
1.7
1.7 2.7 3.3 4.1 5.0 5.5 6.4 6.7
1.1 1.7 2.0 2.5 2.6 2.7 3.2 3.9
1.1 1.5 1.9 1.9 2.2 2.5 2.8 3.0
42
5.94
6.97
3.74
3.52
2.31
1.71
1.34
0.78
0.29 0.43 0.52 0.64 0.85 1.01 1.31 1.50 19
2.2 4.0 6.4 7.7 9.5 12 18 22 147
0.85 1.34 1.71 1.72 2.30 2.97 3.19 4.00 46
(mg.kg-1)
0.92
0.00
0.36
0.32
0.10
0.10
0.08
0.05
0.03 0.05 0.01 0.02 0.02 0.10 0.08 0.06
0.3 0.3 1.4 0.9 1.0 1.3 1.3 0.0
0.07 0.06 0.05 0.18 0.17 0.26 0.12 0.15
σ
(%)
σ
14
17
8.9
8.4
5.5
4.1
3.2
1.9
1.5 2.2 2.7 3.3 4.4 5.3 6.9 7.9
1.5 2.7 4.3 5.2 6.4 8.1 12 15
1.8 2.9 3.7 3.7 5.0 6.4 6.9 8.6
(%)
109
15
11
9.0
6.6
4.5
3.7
2.2
1.0
0.23 0.47 0.67 1.02 1.31 2.01 2.61 2.71 35
16 35 56 56 56 87 87 150 851
13 22 30 34 37 64 64 64 244
(mg.kg-1)
2.4
1.4
1.5
1.2
0.5
1.0
13.4
10.1
8.3
6.0
4.1
3.4
2.0
0.9 0.3
B2 0.1
1.9 4.1 6.6 6.6 6.6 10.3 10.3 17.6
V1 3 0.1 0.1 0.1 12 54 0.1 0.1
0.7 1.3 1.9 2.9 3.7 5.7 7.4 7.7
5.2 9.2 12.4 13.9 15.4 26.4 26.4 26.4
V0 1 3 6 6 0.1 0.1 0.1 0.1
L0 0.02 0.01 0.04 0.23 0.04 0.08 0.41 0.18
(%)
σ
E208Pb
639
57
65
65
54
51
42
33
20
5.5 9.2 11 12 12 13 13 13 82
147 230 236 266 277 284 308 330 2983
50 68 71 76 78 81 84 90 877
(mg.kg-1)
5.5
8.0
8.0
5.5
0.0
3.0
5.3
1.0
0.6 0.3 0.8 0.7 0.3 0.2 0.1 0.2
7 6 13 15 9 9 9 20
2.8 4.9 0.1 5.1 1.5 3.7 0.3 0.1
σ
E67Zn
9.0
10
10
8.5
8.0
6.6
5.1
3.2
6.7 11 13 14 15 16 16 16
4.9 7.7 7.9 8.9 9.3 9.5 10 11
5.7 7.7 8.1 8.6 8.9 9.3 9.6 10
(%)
639
74
59
65
57
49
44
31
14
3.7 7.4 9.5 11 13 14 15 16 82
177 230 259 266 290 315 337 347 2983
57 77 84 91 100 109 108 118 877
(mg.kg-1
3.5
2.3
2.3
0.4
3.5
0.2
0.8
1.0
0.1 0.4 0.1 0.4 0.3 0.7 0.2 0.2
6.0 2.6 11 9.2 1.8 10 8.1 7.6
1.9 1.4 4.0 1.0 0.3 0.8 4.0 0.8
σ
E65Zn*
11.5
9.2
10
8.9
7.7
6.9
4.9
2.3
4.6 9.0 12 13 15 18 19 20
5.9 7.7 8.7 8.9 9.7 11 11 12
6.5 8.8 9.6 10 11 12 12 14
(%)
11
5.1
5.4
4.7
4.6
3.8
3.3
2.8
1.3
0.06 0.15 0.17 0.20 0.24 0.26 0.29 0.29 0.83
6.9 13 15 17 21 22 24 29 47
2.1 3.1 3.7 3.8 4.5 4.8 5.1 5.2 8.8
(mg.kg-1)
0.2
0.3
0.1
0.4
0.1
0.1
0.1
0.1
0.01 0.03 0.01 0.01 0.01 0.02 0.01 0.01
1.3 0.6 0.1 1.4 0.6 0.1 0.8 0.1
0.1 0.1 0.1 0.3 0.2 0.1 0.2 0.2
σ
E108Cd
47
51
43
42
35
31
26
13
7.4 18 20 24 29 31 35 35
14 27 33 37 44 46 50 61
23 35 42 43 51 54 58 59
(%)
11
6.1
6.3
5.7
5.2
4.3
3.8
2.9
1.3
0.05 0.14 0.19 0.28 0.29 0.36 0.41 0.36 0.83
7.6 12 14 16 19 21 25 28 47
2.1 3.3 3.8 4.2 4.6 5.0 5.7 6.4 8.8
(mg.kg-1)
0.2
0.3
0.1
0.2
0.1
0.1
0.1
0.1
0.01 0.02 0.02 0.03 0.03 0.03 0.02 0.02
0.2 0.3 0.5 0.2 0.8 1.0 1.5 0.8
0.1 0.1 0.1 0.1 0.1 0.2 0.3 0.1
σ
E109Cd*
57
59
54
48
40
35
27
12
6.2 17 23 33 35 43 50 44
16 25 30 34 40 45 53 59
24 38 43 48 52 57 64 72
(%)
pH: standard NF ISO 10390. b Organic carbon determined by sulfochromic oxidation (standard NF X 31-109) . c CEC: cationic exchange capacity (standard NF X 31-130). d The E values determined for each time step are the averages of the experimental triplicates and incertitude (σ) takes into account the analytical SD and the triplicates measurement SD.
a
6.2
pHa
6.3 34 11
5.3 83 6.7
pHa OM (g/kg)b CEC (cmol+/kg) c
pHa OM(g/kg)b CEC (cmol+/kg) c
5.5 42 8.3
pHa OM (g/kg)b CEC (cmol+/kg) c
exchangeable pool
E65Cu
E62Ni
Table 1. Selected Characteristics of Soils V0, L0, B2, and V1 and Their Respective E Values of Ni, Cu, Pb, Zn, and Cd Measured by Stable IEK and by Radioactive (*) IEK for Cd and Znd
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dx.doi.org/10.1021/es2006644 |Environ. Sci. Technol. 2011, 45, 6247–6253
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Figure 1. ECd (Figure 1a) and EZn (Figure 1b) values measured by radioactive IEK versus multielementary SIEK (mg kg1 of soil) in soils L0, B2, V0, and V1. The regression (solid) and 1:1 (dashed) lines are indicated.
E-Value Determination. The isotopic exchangeable amount of metals (E values) deduced from both radioactive and stable IEK corresponds to the sum of isotopically exchangeable elements in both solid (Ee) and solution phases (Cs). This pool, defined as the “Ea” value by Hamon et al.,5 is the total isotopically exchangeable pool. Radioactive Tracers. The E109Cd* and E65Zn* values were determined by isotopic exchange kinetic following a procedure adapted from ref 17. After addition of 109Cd* or 65Zn* into a soil solution system at steady state, the radioactivity in solution decreases with time (t). The E value (mg kg1) is expressed as follows17,18
EðtÞ ¼
V Medis rt M R
ð1Þ
with R representing the total introduced radioactivity (Bq kg1 soil), and rt representing the radioactivity remaining in solution at t (Bq kg1 soil). rt/R was calculated according to ref 18. V is the volume of sample (10 mL) and M is the mass of the sample (1 g). Medis is the dissolved (0.2 μm filtered) metal (Cd or Zn) concentration in solution (mg L1). Stable Tracers. E values are calculated for each element in each filtrate using the classical isotopic dilution formula, adapted from ref 30 TI TI NI NI M A STD S S F ð2Þ E ¼ QS TI TI MS ASTD NI F NI STD with QS representing the amount of added spike (in μg) per gram of sample, MSTD and MS representing the atomic masses of element (in g mol1) in the standard and the spike solutions, respectively, and ASTD and AS representing the abundance percentage of 114Cd, 64Zn, 58Ni, 63Cu, or 208Pb in the standard and spike solutions, respectively. TI/NI refers to the tracerdominant isotope over the natural-dominant isotope ratio (i.e. 62 Ni/58Ni, 65Cu/63Cu, 67Zn/64Zn, 106Cd/114Cd, 204Pb/208Pb) measured in the spike (subscript “S”), the filtrate (“F”), and the ICP standard solutions (“STD”, international standards Inorganic Venture solutions), respectively. Note: natural isotope abundances were initially determined in bulk soil samples and were found to be consistent with those of the standard solutions used (i.e., typically within statistical uncertainty of the ICP-MS
analyses), hence the isotopic ratios within the standards were used to represent ‘natural abundances’ in the E value calculations. 109 Analysis Radioactive Tracers. The activity (rt) of Cd* or 65 Zn* in the filtrates and the total radioactivity introduced (R) were measured by liquid scintillation (Fluoran Safe, Scintran) with a liquid scintillation counter (Tri-Carb 2100TR, Packard). The time of counting for each sample was 10 min maximum. The total stable metal concentrations in the solution (Medis) were measured after 60, 420, 10000 min and 180, 1410, 10000 min for Cd and Zn, respectively, using an atomic absorption spectrophotometer (flameless AAS, Solaar Thermo Elemental). Stable Tracers. The 62Ni/58Ni, 65Cu/63Cu, 67Zn/64Zn, 106 Cd/114Cd, and 204Pb/208Pb isotopic ratios were analyzed in the filtrate, the spike, and the standard solutions using quadrupolar ICP-MS (Perkin-Elmer SCIEX Elan 6000 at LMTG (Toulouse) and Thermo-Finnegan HR-ICP-MS at IPGP (Paris). The isotopes 60Ni, 105Pd, 118Sn, 57Fe, and 202Hg were also analyzed to correct possible isobaric interferences. Each isotopic ratio corresponds to the average of 25 blocks of 100 replicates allowing an internal analytical reproducibility with a standard error better than 0.05%. Within session, reproducibility of the inhouse multielement isotopic reference material was checked every 10 samples and yielded less than 0.5% shift from the certified values. Most of the subprocedural variation was found to be within the method’s stated overall external reproducibility (SE), determined on the three experimental replicates. The simultaneous analysis of the isotopic ratios of filtrate, spike, and standard solutions prevented any need of correction of instrumental mass bias. For both radioactive and stable IEK, the E value for each time step corresponds to the average of the triplicate measurements. The incertitude takes into account the standard deviations on each ICP-MS analysis and on triplicate’s average.
’ RESULTS AND DISCUSSION Multi-SIEK Validation. The stable vs radioactive Cd Isotopic Dilution was recently investigated by Sterckeman et al.23 The tests realized in the present study put emphasis on the kinetic aspects of exchanges. The Isotopic Exchange Kinetic was first performed on the most contaminated soil, V1, with the radioactive tracer 109Cd* (10 μL of solution #1) and different amounts of the stable tracer 106Cd (10 and 100 μL of solution #3, i.e. 0.2% and 2% of total Cd, respectively) in order to identify the limits of the method. The calculated values of E109Cd*, E106Cd0.2%, and E106Cd2% are displayed in the SI. In both radioactive and Stable IEK, the E values increase strongly during the first 7 h of 6250
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Figure 2. E values as a function of metal enrichment factors relative to the Upper Continental Crust (UCC) and normalized with Mn concentrations as conservative element,25 after 1 h of interaction (60 min, Figure 2a) and 1 week (10000 min, Figure 2b).
experiment, from a value of 6.97.6 mg kg1 (after 1 min) to 2224 mg kg1 (after 7 h, Figure S2a). After 1 week, a plateau is almost reached, corresponding to 5961% of total Cd. It corresponds to the long-term isotopic equilibrium between the tracers and the pools of element in the soil17 and means that almost 53% of Cd is fast exchangeable Cd (24 h), whereas only 69% is slow exchangeable Cd. This is consistent with values obtained by previous studies.23,31,32 The E106Cd0.2% are closely correlated to the E109Cd* throughout the IEK experiments (R2 = 0.97, Figure S2b) despite both experiments are independent and include the soil sample heterogeneity. E106Cd2% values determined with higher amounts of stable tracers also correlate with E109Cd* after 48 h of interaction. Nevertheless, at the earlier stages of the experiment (124 h) the isotopically exchangeable Cd is overestimated. It coincides with a pH drop in the soil solution, from 5.3 to 4.5, due to addition of the larger amount of spike (0.4 M HNO3) that would have induced Cd desorption. Such interpretation is consistent with Sterckeman et al.23 and previously Hamon et al.5 who observed an increase of E109Cd* values when progressively adding HCl to the soil suspension. Therefore, the good consistency between E109Cd* and E106Cd0.2% indicates that both techniques measure the same exchangeable pool of Cd in a soil and also describe its reactivity, provided that (i) the decrease of pH induced by addition of spike is limited and does not induce a leaching of metal and (ii) the solubility product of the considered element is respected, preventing any precipitation and disturbance of isotopic equilibrium. A competition among metals due to simultaneous addition of several metal tracers may affect the E values determination by IEK, depending on the specificity and the amount of sites involved in exchanges. Such competition was assessed by comparing soil V1 ECd and EZn values measured by multielementary SIEK (10 μL of spiking solution #4) with those determined by monoelementary radioactive IEK. Again, the ECd and EZn values obtained from both methods were consistent and displayed a good correlation (Figure S3). Multielementary SIEK assessment for the less (or not) contaminated V0, L0, and B2 soils samples also provided ECd and EZn results consistent with those obtained from 109Cd* and 65Zn* experiments (Figure 1). Overall, within these experimental conditions that encompass a large range of metal concentrations, no competition between Cd, Zn, and other metals or equilibrium disturbance was detected. Hence, it is proposed that multispiking would not affect the behavior of other divalent metals, making the multielementary SIEK method a useful tool for simultaneously determining E values of several
metals and for studying their reactivity. Future work should confirm this for a wider range of conditions. Multi-SIEK Results. The absolute amounts of isotopically exchangeable Cu, Cd, Pb, and Zn at short-term are closely related to total concentration in each metal in the soil. Ni displays no trend as its concentration is similar in all soils examined, from 18 to 44 mg kg1. Its exchangeable pool is small and slightly lower than in other sites,34 from 0.52 to 1.1 mg kg1 (Table 1). Two metals, Cu and Pb, evolve on the same trend. ECu values range from 0.6 to 7.7 mg kg1 and EPb from 1.0 to 56 mg kg1 after 1 h (Figure 2a). These values compare well with those obtained in dam sediments downstream of the Lot River26 at similar metal concentrations and with ECu obtained by Ma et al.35 in unpolluted soils, from
