Vol.1, No.3, 119-130 (2010) Agricultural Sciences
Copyright © 2010 SciRes. Openly accessible at http:// www.scirp.org/journal/AS/
Effect of soil properties and sample preparation on
extractable and soluble Pb and Cd fractions in soils
Jiřina Száková1*, Daniela Miholová1, Pavel Tlustoš1, Ivana Šestáková2, Zuzana Frková1
1Czech University of Life Sciences Prague, Faculty of Agrobiology, Food and Natural Resources, Prague, Czech Republic;
Corresponding Author: szakova@af.czu.cz
2J. Heyrovský Institute of Physical Chemistry of ASCR, Prague, Czech Republic
Received 22 June 2010; revised 30 July 2010; accepted 5 August 2010.
The effect of soil extraction procedures and/or
sample pretreatment (drying, freezing of the soil
sample) on the extractability of cadmium and
lead was tested in a model experiment, with an
employment of optical emission and atomic ab-
sorption spectrometry methods. In the first part,
6 extraction procedures were compared: 2 mol l-1
HNO3, 0.43 mol l-1 CH3COOH, 0.05 mol l-1 EDTA,
Mehlich III extraction procedure (0.2 mol l-1
CH3COOH + 0.25 mol l-1 NH4NO3 + 0.013 mol.l-1
HNO3 + 0.015 mol.l-1 NH4F + 0.001 mol.l-1 EDTA),
0.01 mol.l-1 CaCl2, and deionised water. Addi-
tionally, two methods of soil solution sampling
were compared, and the centrifugation of satu-
rated soil and the use of suction cups and dif-
ferential pulse anodic stripping voltametry was
applied to assess free and complexed metals
portions. The results showed that different soil
sample extraction methods and/or sample pre-
treatments including soil solution sampling can
lead to different absolute values of mobile ca-
dmium and lead content in soils. However, the
interpretation of the data can lead to similar
conclusions as are apparent from the compari-
son of the soil solution sampling methods
where fairly good correlation was observed (for
Cd r = 0.76, and for Pb r = 0.74). The ambiguous
results were reported for voltammetric deter-
minations of free and complex portions of Cd
and Pb where a different behavior was observed
for water extracts of soil and soil solution ob-
tained using suction cups. Moreover, a chang-
ing extent of lead complexation was determined
with prolonged storage of the samples. The re-
sults confirmed that soil and/or soil solution
sampling under immediate soil conditions and
limitations of pre-extraction operations are ne-
Keywords: Lead; Cadmium; Contaminated Soils;
Extractability; Soil Solution; Speciation
Soil properties, as well as root exudates, substantially
affect plant-availability of potentially toxic elements in
soil and their mobility and chemical forms in soil solu-
tion. Therefore, the composition of soil solution include-
ing organic compounds, complexes and elements bound
to individual components of the soil solution are fre-
quently investigated, including the determination of
validation parameters of individual analytical methods
for soil solution samples. Soil solution represents essen-
tial electrolytic water solution containing dissolved or-
ganic and inorganic compounds (coloids, complexes,
free salts and ions of these salts), atmospheric gases and
exudates of plant roots and microorganisms. The know-
ledge of the composition of a soil solution is substantial
for the elucidation of element uptake processes by plants,
as well as for plant growth. Leaching, evaporation, and
plant transpiration can affect the contents of trace metals
and metalloids to a greater extent than mere changes in
the contents of main ions [1,2]. Mechanisms of released
organic acids, such as malate, citrate and oxalate, and
their activity in soil (sorption, complex formation, de-
composition by soil microorganisms) represent complex
and yet to be fully elucidated processes [3,4].
Various soil extraction procedures were developed and
tested for the determination of plant-available, mobile,
and potentially mobilizable pools of trace elements in
soils. This was carried out without a general consensus
by the authors on which extractant would be the most
suitable in this case. Moreover, the effectiveness of indi-
vidual extractants to determine the plant-available ele-
ments depends on soil physicochemical parameters, the
source of contamination, and contamination levels [5-7].
J. Száková et al. / Agricultural Sciences 1 (2010) 119-130
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The sample pre-treatment and/or storage before extrac-
tion can affect the composition of a soil solution which
in turn affects element mobility in soils [8,9]. Various
methods were developed for the collection of soil solu-
tions [10] such as centrifugation, suction cups or lysi-
meters and in situ sampling is considered to better rep-
resent the soil solution composition and the physical
structure of the sample remains intact.
The available analytical methods and/or approaches of
the determination of individual forms of elements and
other compounds in soil solutions were reviewed by
Peijnenburg and Jager [11] and Nolan et al. [10]. Among
them, chelating ion-exchange [12], size exclusion chro-
matography [9], and the Donnan dialysis membrane
technique [13,14] are frequently discussed. Anodic strip-
ing voltametry (ASV) is a very sensitive technique
which is usually used for the detection of ions, and espe-
cially metals. It is also suitable for the determination of
stability constants on a wide pH scale [10]. Differential
pulse anodic stripping voltammetry (DPASV) was ap-
plied for measurement of complexation of heavy metals
with humic and fulvic acids in model soil solutions [15-
17] as well as in extracted soil solutions [18,19]. More-
over, voltammetric methods are used for the study of
changes in metal solubility and speciation in agricultu-
ral soils treated by sewage sludge or other organic matter
such as pig manure [20,21].
In our experiment the extractability of Cd and Pb with
various extracting agents as well as the effect of sample
pretreatment were evaluated in eight soil samples differ-
ing in their physicochemical parameters. The main goal of
the study was to evaluate and compare some of the most
widely used methods of soil extraction and soil solution
collection. Moreover, the effect of sample preparation
methods will be discussed, especially if the abundance of
free ions of Cd and Pb is necessary for an evaluation of
the potential plant-availability of soil heavy metals. Two
sets of soil solutions obtained by different procedures
were analyzed to assess free and complex Cd and Pb us-
ing anodic stripping voltametry on a hanging mercury
drop electrode.
2.1. Extraction Procedures
Eight soil samples differing in physical-chemical prop-
erties and total element contents Table 1 were extracted
by the following extraction procedures:
1) extraction with 2 mol.l-1 solution of HNO3 at a ratio
of 1: 10 (w/v) at 20°C for 6 hours [22],
2) extraction with 0.43 mol.l-1 solution of CH3COOH at
a ratio of 1: 40 (w/v) for 5 hours [23],
3) extraction with 0.05 mol l-1 EDTA aqueous solution
at pH 7 at a ratio of 1:10 (w/v) for 1 hour [23],
4) Mehlich III extraction procedure (0.2 mol.l-1
CH3COOH + 0.25 mol.l-1 NH4NO3 + 0.013 mol.l-1 HNO3
+ 0.015 mol.l-1 NH4F + 0.001mol.l-1 EDTA at a ratio of 1:
10 (w/v) for 10 minutes; [24]),
5) extraction with 0.01 mol.l-1 aqueous CaCl2 solution
at a ratio of 1:10 (w/v) for 6 hours [25],
6extraction with deionized water at a ratio of 1:10
(w/v) overnight [26].
Each extraction was provided in three replicates, all the
chemicals used were of analytical grade purity and were
purchased from Analytika and Lach-Ner Ltd., Czech Re-
public. For the centrifugation of the extracts, the Hettich
Universal 30 RF (Germany) device was used. The reac-
tion mixture was centrifuged at 3000 rpm (i.e. 460 xg) for
10 minutes at the end of each extraction procedure, and
the supernatants were kept at 6C before measurement.
Blank extracts representing 5% of the total number of
extracts were prepared using the same batch of reagents
and the same apparatus. The blank extracts were analyzed
at the same time and in the same way as the soil extracts.
The total concentration of trace elements in the soils
was determined in the digests obtained by the following
decomposition procedure: Aliquots (0.5 g) of air-dried soil
samples were decomposed in a digestion vessel with a
Table 1. Basic characteristics of the experimental soils.
Soil CECa
Cd (total)
Pb (total)
Píšťany 201 ± 4 2.6 ± 0.1 218 ± 2 6.8 ± 0.5 1.66 ± 0.05 110 ± 29
Mikulov 99.1 ± 8.2 4.2 ± 0.1 436 ± 11 4.2 ± 0.2 1.39 ± 0.01 131 ± 0
Pramenáč 157 ± 1 3.9 ± 0.2 541 ± 9 3.5 ± 0.1 1.52 ± 0.09 81.4 ± 4.5
Příbram meadow 166 ± 20 3.6 ± 0.4 428 ± 9 5.2 ± 0.1 5.86 ± 0.06 1621 ± 27
Kbely 299 ± 4 1.9 ± 0.1 126 ± 2 7.2 ± 0.4 16.1 ± 1.0 50.3 ± 6.8
Příbram arable 151 ± 2 2.1 ± 0.1 252 ± 9 6.0 ± 0.2 4.48 ± 0.61 816 ± 51
Litavka 134 ± 3 1.9 ± 0.2 119 ± 10 4.6 ± 0.1 22.3 ± 2.9 3662 ± 486
Kutná Hora 295 ± 13 2.9 ± 0.1 246 ± 12 7.1 ± 0.2 3.62 ± 0.28 63.3 ± 3
a. cation exchange capacity, b. total organic carbon, c. dissolved organic matter
J. Száková et al. / Agricultural Sciences 1 (2010) 119-130
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mixture of 8 ml concentrated nitric acid, 5 ml of hydro-
chloric acid, and 2 ml of concentrated hydrofluoric acid.
The mixture was heated in an Ethos 1 (MLS GmbH,
Germany) microwave assisted wet digestion system for 33
min at 210°C. After cooling, the digest was quantitatively
transferred into a 50 ml Teflon® vessel and evaporated to
dryness at 160°C. The digest was then dissolved in a 3 ml
nitric and hydrochloric acid mixture (1:3), transferred into
a 25 ml glass tube, filled with deionised water, and kept at
laboratory temperature until measurement. A certified
reference material RM 7001 Light Sandy Soil was applied
for the quality assurance of analytical data.
2.2. Soil Sample Pre-Treatment
Prior to extraction and total element content determina-
tion, the soil samples were air-dried at 20°C, ground in a
mortar and passed through a 2-mm plastic sieve. Alterna-
tively, aliquots of the samples were extracted as moist
samples immediately after soil sample collection while
dry mass of the soils was determined separately. Finally,
aliquots of the samples were frozen at 18°C for 14 days
before being air dried, ground, and sieved. The effect of
sample pre-treatment was tested for 0.05 mol l-1 EDTA,
0.01 mol l-1 CaCl2 and water extracts.
2.3. Soil Solution
Element concentrations in soil solution are related to
the element contents in plant biomass [27]. However,
there are different approaches to soil solution collection.
We tested 1) the centrifugation of fully saturated soil at
10000 rpm for 10 minutes and 2) the application of suc-
tion cups, where specialized plastic suction cups (DI
Gottfried Wieshammer, Wien, Austria) were applied to
pots containing roughly 350 g of the soil at the beginning
of the experiment to get a soil solution. The pots with the
suction cups were filled to capacity with deionised water
one day before suction and left for 24 hours to equilibrate.
10 ml of soil solution was regulerly sampled from each
pot and immediately analyzed for Cd and Pb concentra-
tions. The collection of the soil solution was repeated
twice (14 and 28 days into the experiment). The design of
the suction cup application and soil solution sampling was
described in detail by Jaklová-Dytrtová et al. [28].
2.4. Analytical Methods
The total contents of Cd and Pb in soil digests and ex-
tracts were determined by optical emission spectroscopy
with inductively coupled plasma (ICP-OES) with axial
plasma configuration, Varian, VistaPro (Australia). Cali-
bration solutions were prepared in the corresponding ex-
traction agents as follows, 100-500 µg l-1 for Cd, 100-
1000 µg l-1 Pb. The operating measurement wave-lengths
for ICP-AES were 214.4 nm for Cd, and 220.4 nm for Pb.
The measurement conditions for all lines were as follows:
power 1.2 kW, plasma flow 15.0 l min-1, auxillary flow
0.75 l min-1, nebulizer flow 0.9 l min-1. For the determina-
tion of low concentrations of Cd and Pb in soil solutions,
water and 0.01 mol.l-1 CaCl2 extracts electrothermal
atomic absorption spectrometry (ET-AAS) using the in-
strument VARIAN AA280Z (Varian, Australia) equipped
with a GTA120 graphite tube atomizer was applied. The
operation conditions are summarized in Table 2.
Water extracts [26] and soil solutions obtained with
suction cups (first collection) were analyzed with differ-
ential pulse anodic stripping voltametry (DPASV) on a
hanging mercury drop electrode. Voltammetric measure-
ments were carried out by means of a computer-controlled
polarographic/voltammetric analyzer PC-ETP (Polaro-
Sensors, Prague, Czech Republic). Experiments were per-
formed using a three-electrode configuration, with Ag/
AgCl/KCl (3 mol.L-1) as a reference, and a Pt wire as an
auxiliary electrode. 10 ml of 0.001 M sodium perchlorate
was used as a supporting electrolyte. After degassing with
Table 2. Operational conditions for ET-AAS determination of Cd and Pb in soil extracts and soil solutions.
Cd Pb
calibration mode calibration curve standard addition
wavelenght (nm) 228.8 (0.5) 283.3 (0.5)
background correction Zeeman Zeeman
signal integration peak area peak area
matrix modifier no (NH4)2HPO4
pyrolysis temp. 350°C 850°C
atomization temp. 2100°C 2400°C
concentration of bulk standard 3 µg/L 60 µg/L
volume injected on platform 30 µL 30 µL
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nitrogen, 1 ml of soil extract or soil solution was added.
Voltammogram of solution with complexed metals was
recorded and then 100 µl of concentrated nitric acid was
added (change to pH 2) and voltammograms of released
metal ions at pH 2 was recorded. A similar procedure was
used earlier for the estimation of soil solution from pots
with experimentally grown plants [28]. Differential pulse
voltametry was applied with a pulse height of 50 mV, a
pulse width 100 ms and a scan rate of 10 mV s-1. Before
the measurement, electrolysis at 1100 mV (per-chlorate
solution) or 850 mV (acidified solution) with stirring of
the solution was applied for 360 s. Measurements were
performed in a nitrogen atmosphere. All chemicals were
of analytical purity grade, deionised water from Milli-
Q-Gradient (Milipore, USA) was used.
2.5. Statistics
Due to the of the inhomogeneity of variance of the data,
non-parametric Kruskal-Wallis and Wilcoxon’s tests were
applied at the significance level = 0.05. Extreme values
were tested by Dixon’s test for the identification of out-
liers. Statgraphics 5.1plus for Windows (Manugistics,
Inc., Rockville, USA) and Microsoft Excel for Windows
XP were applied for the evaluation of the significance of
confidence intervals and correlation coefficients at =
3.1. Comparison of Strong Soil Extraction
As evident from Table 1, all the soils are cadmium
contaminated with Cd concentrations ranging from 1.4 to
22.3 mg.kg-1. In the case of lead, the total soil concentra-
tions varied between 50 and 3662 mg.kg-1. The highest
total element content was determined in the soil from Li-
tavka which represented serious anthropogenic contami-
nation. Fluvisol from the alluvium of the Litavka River,
Czech Republic was heavily polluted by wastes from
smelter setting pits [29]. Figure 1 summarizes the cad-
mium and lead portions extractable by strong extraction
procedures where significant differences were identified
Figure 1. Comparison of the set of strong extraction procedures (%
of total element content in soil).
J. Száková et al. / Agricultural Sciences 1 (2010) 119-130
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among all the extractants tested ( = 0.05). HNO3
(2 mol l-1) was able to extract cadmium portions ranging
between 25 and 100% while a higher extractability was
observed for highly contaminated soils confirming an
anthropogenic origin of the soil pollution. Diluted nitric
acid is able to dissolve the element portion comparable to
the sum of labile soil element fractions [22], and in the
case of cadmium, it can be accepted as the approxima-
tion of the total element content in soil, especially in an-
thropogenically contaminated soils. Surprisingly, the low-
est extractability was reported for the acidic soil Pra-
menáč where the high content of silicious matrix most
likely plays an important role. For the exact evaluation of
the effect of organic matter, a more detailed description of
soil organic matter composition will be necessary in this
case. The percentage of lead extractable by 2 mol l-1
HNO3 varied between 35 and 80% of the total lead con-
tent while a higher extractability was observed in the an-
thropogenically contaminated soils Příbram and Litavka.
These observations were similar in the case of cadmium.
Contrarily, low extractability was observed in the soils
with high sorption capacity, i.e. Píšťany, Kbely and Kutná
With the exception of soils with the highest content of
organic matter, extraction of soil by 0.05 mol l-1 EDTA
showed comparable portions of extractable cadmium and
lead in comparison to extraction with 2 mol l-1 HNO3. A
high affinity of lead to organic matter in soil as well as
low portions of mobile Pb in soil supporting lowplant-
availability of this element was also confirmed by other
authors [30]. A Similar efficiency of lead extraction was
also observed by Sastre et al. [7]. In the case of element
portions extracted with 0.05 mol l-1 EDTA, the effect of
sample pretreatment was tested Figure 2, and significant
differences ( = 0.05) were obtained among air-dried,
frozen and subsequently air-dried samples. This was also
the case among fresh samples. In all the soils, the fresh
samples showed lower extractability of elements com-
pared to the air-dried samples. The effect of air-drying or
sample storage on the mobility of various elements was
already described. During the drying and storage of the
samples, changes (especially in soil organic matter) were
described when soil samples were air-dried and/or stored
at the laboratory temperature. It can result in significant
changes in element distribution to individual soil fractions
[31,32]. Similar problems were observed, for example Cu
distribution in individual fractions of manure samples [33].
For rare earth elements (La, Ce, Pr, Nd) the air-drying
process increased the element contents in water soluble,
exchangeable, carbonate bound, and Fe-Mn oxide bound
fractions, whereas the fraction bound on organic matter
decreased [34]. Evidently, in the case of Cd and Pb, the
relatively strong extractant is able to highlight the poten-
tial effect of slightly changing physicochemical parame-
ters of the soils due to different pretreatments of the soil
samples. Similarly Meers et al. [5] documented that no or
little differences in Cd extractability between acidic and
alkaline soils were observed when the more aggressive
extractants such chelate based (DTPA, EDTA) and acid
based (acetic acid, HCl, HNO3) procedures, as well as
stronger extractions used to estimate exchangeable Cd in
the soil (ammonium acetate, magnesium chloride) were
Diluted acetic acid is characterized as the extractant re-
leasing element fraction specifically sorbed on soil clay
minerals. Therefore, this extractant is recommended as a
suitable test to predict the changes in element mobility in
soil amended by sludge originating from a mining exploit-
tation spill [7]. In our experiment, this agent extracted
from 9 to 65% of Cd and from 1.5 to 14% of Pb in ar-
eas/soils where a better extractability of heavy metals
from the highly contaminated soils was confirmed. A
similar extractability (median value 45%) of cadmium
was also observed in our previous experiments [35]. For
the Mehlich III extraction procedure (compulsory for soil
testing in the Czech Republic), a slightly lower extracta-
bility of Cd compared to a 0.43 mol.l-1 solution of
CH3COOH was recorded, whereas an opposite pattern
was reported for Pb. Most likely, the better extractability
of Pb was caused by the presence of EDTA in the Mehlich
III extraction mixture.
3.2. Comparison of Mild Soil Extraction
Procedures/Soil Solution Preparation
Single soil extraction procedures were recently evalu-
ated by Menzies et al. [6] and Meers et al. [5]. They con-
cluded that acid extractants and complexing reagents are
not correlated with the plant-available concentration of
heavy metals. They also confirmed the suitability of neu-
tral salt extractants (0.01 mol l-1 CaCl2, 0.1 mol l-1 NaNO3)
for the assessment of the available pool of elements in soil,
where the extractability is affected by soil pH, total ele-
ment content and cation exchange capacity. According to
previous experiments most of the mild extracting proce-
dures, including pure water, were developed predomi-
nantly for a wide range of elements, especially heavy
metals [26]. The 0.01 mol l-1 CaCl2 extractable cadmium
portions varied from 0.2 to 30% of total cadmium in the
case of the air-dried soil sample Figure 3, although indi-
vidual sample pretreatment methods did not differ sig-
nificantly at = 0.05. Similarly, as in the case of strong
extraction procedures, the heavily contaminated soil
at/from Litavka demonstrated the highest extractability of
cadmium whereas the lowest levels were observed for the
neutral soils Píšťany, Kbely and Kutná Hora. Evidently,
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Figure 2. The effect of sample pre-treatment on 0.05 mol l-1 EDTA extractable
portion of elements in soils (% of total element content in soil).
soil pH seems to be the soil factor significantly determine-
ing the extractability of cadmium (r = 0.56, significant at
= 0.05). For lead, the 0.01 mol l-1 CaCl2 extractable
portion did not exceed 0.4% and no unambiguous effect
of individual soil properties and/or soil contamination
level was observed. The water extractable portion of both
elements Figure 4 exhibited a completely different pat-
tern, and statistical differences ( = 0.05) were not con-
firmed in this case. For Cd, the extractable portions of this
element varied between 0.4% (soil Kutná Hora) and 1.7%
(soil Litavka), and for Pb the levels were between 0.2%
(soil Kbely) and 0.9 % (soil Příbram arable), which cor-
respond in principle to the results given by Svete et al.
[12]. However, no relation was found between 0.01 mol l-1
CaCl2 extractable portions of these elements and/or the
individual soil properties.
For the assessment of the differences between frozen
and subsequently dried soil and other tested treatments
(air-dried and fresh samples), the possible effect of soil
microflora and/or the presence of individual element
complexes should be taken into account Figures 3 and 4.
Additionally, soil solution metal concentrations are af-
fected by, among others, hydrous oxides of iron and man-
ganese, although it is not the single dominant factor [36].
The effect of soil homogeneity and sil particle structure
has already been described. Tawinteung et al. [37] showed
three factors that influenced Pb removal by the extraction
techniques: 1) initial Pb concentrations, 2) Pb partition-
ing within soil, and 3) particle size of soil matrix. Air-
drying leads to a decrease in Cu, Ni, and Zn concentra-
tions in soil solution, as well as a decrease in soluble or-
ganic carbon [8]. Freezing of the sample can result in a
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Figure 3. The effect of sample pre-treatment on 0.01 mol l-1 CaCl2 extractable
portion of elements in soils (% of total element content in soil).
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Figure 4. The effect of sample pre-treatment on the water extractable portion of
elements in soils (% of total element content in soil).
possible release of elements from soil microbial popula-
tions, although subsequent immobilization of released
elements on soil particles can also occur. For cadmium,
the results tended to lower the extractability of Cd from
fresh samples compared to air-dried ones whereas for Pb,
no such trend occurred. Similarly, in pre-dried, sandy, and
acidic or poorly buffered soils, soil solution Cd concentra-
tions were between 2 and 40 times higher than in the cor-
responding reference soils that had been kept at field ca-
pacity at all times [36]. Soil solution metal concentrations
at any given moment may significantly depend on previ-
ous soil moisture conditions. In contrast to findings sum-
marized by Nolan et al. [10], suction cups showed higher
Cd concentrations compared to the centrifugation of satu-
rated soil whereas the converse was observed for Pb Fig-
ure 5 without an unambiguous confirmation of the results
by Wilcoxon’s test at = 0.05. No significant ( = 0.05)
trends in element extractability according to time de-
pendent samplings Figure 6 of soil solution using suction
cups (24 h, 14 days and 28 days after beginning of the
experiment) were observed, as well.
The results showed that the widely accepted extraction
technique using 0.01 mol l-1 CaCl2 [6] does not reflect the
behavior of Cd, and especially Pb, in soil solution. Wang
et al. [34] and Gray and McLaren [8] recommended ap-
plying the extraction and/or soil solution sampling of
field-moist soil samples, which reflect in situ conditions
more intensively, for better correlations with plantavail-
able element portions. As documented by our results, dif-
ferent sample pretreatment and/or different mild soil ex-
traction procedures can lead to different absolute values of
mobile cadmium and lead content in soils. However, the
interpretation of the data can lead to similar conclusions
when comparing the individual soils; this is apparent from
the comparison of the soil solution sampling methods
where fairly good correlation was observed (r = 0.75, sig-
nificant at α = 0.05). However, the results also show dif-
ferences between the anthropogenically highly contami-
nated soils with a higher mobility of heavy metals, and
less contaminated soils with a lower mobility of the ele-
ments. Evidently, the soil sample pretreatment can sig-
nificantly affect the element portions extractable from the
soil samples, soil, and/or soil solution sampling under
immediate soil conditions, and limitations of pre-extrac-
tion operations are necessary.
3.3. Cd and Pb Speciation in Soil
Extracts/Soil Solution
Using differential pulse anodic stripping voltametry in a
perchlorate solution with aliquots of water extracts, volt-
ammetric peaks were observed at potentials corresponding
to complexed cadmium and lead. The Cd peak was within
a potential range of 567 mV to 645 mV and its height
did not correlate significantly (α = 0.05) with the total Cd
concentration found in water extracts by AAS ranging
from 0.97 to 20.32 µg.L-1. A substantial increase was ob-
served only for extracts from Litavka, where Cd concen-
tration was about 40 µg.L-1. The peak Pb values in the
sodium perchlorate solution with water extract aliquots
was mainly within a potential range of 420 mV to 479
mV, which confirms complexed Pb(II). The height of the
observed Pb peak was not in correlation with Pb concen-
trations found in extracts by AAS ( = 0.05).
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Figure 5. Comparison of the methods of soil solution sampling (% of total element content in soil)
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Figure 6. Time dependent changes of element concentrations in soil solution (% of
total element content in soil).
Soil solutions obtained by suction cups exhibited a pH
which was in relation with values found for soils Table 1,
and varied from 3.5 to 7.6. The Cd-peak observed with
DPASV in a perchlorate solution with aliquots of soil so-
lutions obtained by suction cups; it was within a potential
range of 572 to 645 mV (therefore higher than 545
mV which corresponds to free Cd2+ in sodium perchlorate
solution). It can be concluded that cadmium is complexed
in all samples - although there is no correlation between
the Cd-peak height and Cd concentrations determined by
AAS ( = 0.05). On the contrary, Sakurai et al. [38]
showed that more than 90% of cadmium in water extracts
of soil was detected in the cation fraction after fractiona-
tion with ion exchange resin. In most samples, lead was
found to be only slightly complexed, and in sample 3
(Pramenáč), Pb peak potential corresponded to free Pb
(355 mV). This seems to be in accordance with Sauve et
al. [39], which confirm that the percentage of Pb com-
plexes in soil solution decreases in soils with low pH or a
high total Pb content. Under conditions utilized, only one
Pb peak was observed, a potential of which shifts with
increasing complexation from 355 mV up to 421 mV.
The total peak height (complexed and not-complexed
Pb(II)) correlated with the Pb concentration found by
AAS (r = 0.99, significant at = 0.05). Contrary results
were described by McBride et al. [21] and Ge et al. [40]
where a high percentage of complexed Pb and less com
plexed Cd was observed.
A lack of correlation between the height of voltammet-
ric peaks of complexed metals (recorded at equal condi-
tions) and AAS concentrations is not surprising as several
different ligands can occurre in different soils. For lead,
the obtained correlation for suction 1 is mainly due to a
Figure 7. Voltammograms of soil solution (soil Příbram meadow)
at different pH levels. Record of differential pulse anodic strip-
ping voltammetry, accumulation 360s at 1100 mV, pulse 50 mV,
scan 10 mV.s-.
low extent of complexation (only 12 hours of contact).
During prolonged intervals, a decrease of Pb peak occurr-
ed. Similarly Cd and Pb complexes with oxalic (OA) and
citric acid (CA) were detected by Jaklová-Dytrtová et al.
[41] in a model with soil solutions using cyclic and
stripping voltammetry. A mixed complex consisting of Cd,
Pb, and OA was determined, and these complexes must be
potentially taken into account in our case. After the aci-
dification of the solution with a soil solution aliquot,
complexed metals were released and their concentrations,
determined voltammetrically, corresponded with values of
AAS, as was found earlier both for soil ammonium nitrate
extracts [18] or soil solutions obtained with suction cups
J. Száková et al. / Agricultural Sciences 1 (2010) 119-130
Copyright © 2010 SciRes. Openly accessible at http://www.scirp.org/journal/AS/
[28]. In our case, linear dependences of DPASV peak
height at pH 2 (Y) on AAS metal content (X) were veri-
fied. For the peak height of released lead; Y= 0.753 X +
9.397, R2 = 0.975 (suction cup), Y= 0.537 X + 9.614, R2 =
0.999 (water extracts). For cadmium, linear dependence of
the DPASV peak on AAS values was obtained (only for
suction cup solutions): Y = 0.248 X + 0.137, R2 = 0.948;
for water extracts at pH 2, there was no significant corre-
lation ( = 0.05) between the voltammetric peak height of
free Cd and AAS concentration.
Evidently, in contrast to the determination of the total
Cd and Pb concentrations in soil extracts and/or soil solu-
tions where the analyses resulted in the comparable inter-
pretation of the data, the speciation of free and complexed
portions of the elements are more affected by the details
of individual sample treatments and sample preparation
methods. The elucidation of factors affecting element
complexation in soil solution will be necessary, in further
research, to elicit reasonable explanations of available soil
Cd and Pb uptake by plants.
The authors wish to thank the Czech Ministry of
Agriculture and the Ministry of Education, Youth, and
Sports for their financial support of the project MSM
6046070901, Ministry of Agriculture, Czech Republic for
their financial support of the NAZV project No. QH81167,
and the GAAVCR for their financial support of the project
No. IAA400400806. The authors also wish to thank Mr.
Brian Kavalir, Ontario, Canada for correction and im-
provement of language.
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