Effect of soil properties and sample preparation on extractable and soluble Pb and Cd fractions in soils

doi:10.4236/as.2010.13015

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Vol.1, No.3, 119-130 (2010) Agricultural Sciences

doi:10.4236/as.2010.13015

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.

ABSTRACT

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-

cessary.

Keywords: Lead; Cadmium; Contaminated Soils;

Extractability; Soil Solution; Speciation

1. INTRODUCTION

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

120

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. MATERIAL AND METHODS

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],

6）extraction 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 6C 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

mmol.kg-1

TOC b

DOMc

mg.kg-1

Cd (total)

mg.kg-1

Pb (total)

mg.kg-1

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

121

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

J. Száková et al. / Agricultural Sciences 1 (2010) 119-130

122

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  =

0.05.

3. RESULTS AND DISCUSSION

3.1. Comparison of Strong Soil Extraction

Procedures

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

123

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

Hora.

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

used.

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

Methods

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,

J. Száková et al. / Agricultural Sciences 1 (2010) 119-130

124

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

J. Száková et al. / Agricultural Sciences 1 (2010) 119-130

125

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).

J. Száková et al. / Agricultural Sciences 1 (2010) 119-130

126

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).

J. Száková et al. / Agricultural Sciences 1 (2010) 119-130

127

Figure 5. Comparison of the methods of soil solution sampling (% of total element content in soil)

J. Száková et al. / Agricultural Sciences 1 (2010) 119-130

128

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

129

[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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