The comparison of extraction methods for evaluating
some heavy metals in polluted soils
C. Aydinalp, A.V. Katkat
Faculty of Agriculture, Uludag University, Bursa, Turkey
ABSTRACT
The extractability of Cd, Co, Cr, Ni and Pb was evaluated using ammonium chloride, calcium chloride, strontium 
chloride and DTPA extractants in this research. The eight surface soils were used to assess plant available metals with 
different extraction methods. The amounts of metal extracted were related to pseudototal contents, determined a�er 
microwave digestion using HNO3. Quantification of dissolved metals was used by ICP-MS matrix-matched stan-
dards. The obtained results indicated a high variability of metal extraction depending on extraction procedure, source 
of pollution, and nature of the soil. The results showed that the extractability for calcareous soils was best determined 
by DTPA. In comparison of chloride salts, a higher efficiency of extraction with ammonium chloride for these soils 
was found.
Keywords: ICP-MS; heavy metals; polluted soils; pollution; extraction methods; mobility
Many soils in developing and industrialized designated to determine the extractability of some 
countries are affected by acid deposition, mine heavy metals in polluted soils.
waste disposal, utilization of organic refuses such as 
sewage sludge, industrialization and urbanization 
that could be inputs of pollutants and especially MATERIAL AND METHODS
heavy metals to the soil. The heavy metals may 
be retained by soil components, may exchange or In this research, eight surface soils (0–25 cm) 
precipitate or coprecipitate as sulfides, carbonate were selected on the basis of the diversity of pol-
and/or Fe or Mn oxides or hydroxides. In arid lution sources (industrial and agricultural activi-
zone soils, the presence of carbonate minerals ef- ties). The soils were classified as non-calcareous 
fectively immobilizes heavy metals by providing (soils 1 to 3) and calcareous (soils 4 to 8). The soil 
an absorbing surface and by buffering pH at high samples were taken from the Bursa plain, Turkey. 
values where precipitation takes place. Soil 1 was polluted by acidic industrial wastewater. 
The chemical forms of heavy metals in the solid Soils 2 and 3 are typical sewage sludge amended 
phase can strongly influence their behavior, such as soil. Soils 4 to 6 are sewage sludge amended calcar-
their mobility, toxicity, bioavailability and chemical eous soils irrigated by water with increasing level 
interactions, since metal speciation has determinis- of Cd contamination (25, 85 and 210 mg/l). Soils 
tic control over physical, chemical, and biological 7 and 8 represented typical calcareous agricultural 
processes that lead to transport and transformation soils without anthropogenic pollution.
of heavy metals among environmental compart- The following soil characteristics were evalu-
ments (air, water, biota, soil/sediment) in complex ated: Particle size distribution determined by the 
ecosystems (Allen et al. 1995). hydrometer method (Gee and Bauder 1982), pH in 
Mobility of heavy metals relates to their capacity a 1:2 soil:water ratio (McLean 1982), organic carbon 
to pass from one soil compartment to another where (Nelson and Sommers 1982), calcium carbonate 
the elements is bound more or less energetically, the (Nelson 1982), electrical conductivity (SCS 1972) 
ultimate mobile compartment being the soil solution and CEC (Rhoades 1982). Pseudototal contents 
which determines the bioavailability of heavy met- of metals were determined following microwave 
als. Extraction of heavy metals by unbuffered salt digestion using concentrated HNO3 and analysed 
solutions (CaCl2, NH4Cl, etc.) is a rapid and simple by ICP-MS (Krishnamurti et al. 1994).
way to evaluate their phytoavailability (Beckett Four different extraction methods were used to 
1989). However, in some cases, salt solutions do establish the extractability and predict the phytoa-
not reflect the plant available metals (Gupta and vailability of heavy metals for these soils (Table 1). 
Atten 1993), so that must be used other extractants Each extraction method was performed in duplicate 
based on DTPA or hydroxylamine that are more in 50 ml polycarbonate centrifuge tubes. All the 
predictive of phytoavailability. This research was extracts were centrifuged for 10 min at 3000 rpm, 
212 PLANT SOIL ENVIRON., 50, 2004 (5): 212–217 PLANT SOIL ENVIRON., 50, 2004 (5): 212–217 213
Table 1. Summary of the extraction methods used for soils
Extractant Ratio Time Temperature(w/v) (h) (°C) Pool Reference
1M NH Cl 1:6 16 20 neutral salt,4 soluble/exchange Krishnamurti et al. (1995)
0.01M CaCl 1:5 16 20 neutral salt,2 soluble/exchange Whitten and Ritchie (1991)
0.01M SrCl 1:7 2 20 neutral salt,2 soluble/exchange Ahnstrom and Parker (1999)
0.005M DTPA 1:2 2 20 chelating extraavailable pool Lindsay and Norvell (1978)
and the supernatant filtered into volumetric flasks industrial wastewater. Electrical conductivity val-
containing 10 µg In/Rh/l. All the extracts and stand- ues ranged from 0.29 to 3.40 dS/m. The highest 
ard solution were acidified to 1% HNO3. Inductively EC value occurred in the soil 1 that was polluted 
coupled plasma mass spectrometry (ICP-MS) of by acidic wastewater. CEC varied from 9.2 to 
Ultramass 700 Varian Model determinations were 32.5 cmol(+)/kg. Organic carbon content ranged 
obtained with matrix-matched standards. from 0.54 to 2.46% with the highest level for the 
sewage sludge amended soils.
The results of extractable Cd, Co, Cr, Ni and 
RESULTS AND DISCUSSION Pb for the different extractants are presented in 
Table 3. The values are expressed as percentages 
The physical and chemical properties of the stud- of the pseudototal content in these soils.
ied soils were presented in Table 2. These soils 
showed a wide range of physical and chemical 
properties. Clay content of soils ranged from 5.1 Cadmium
to 35.0%. The silt and sand contents varied from 
14.1 to 69.1% and from 17.2 to 80.4%, respectively. Nakhone and Young (1993) stated that the lability 
The pH values of soils ranged from 3.1 to 8.73. The of Cd ranged from 3 to 102% of total Cd (HNO3 
low pH of soil 1 is due to acidic pollution from extractable digest). Cd was the most extractable 
Table 2. The chemical and physical properties of the experimental soils
Non-calcareous Calcareous
Features
soil 1 soil 2 soil 3 soil 4 soil 5 soil 6 soil 7 soil 8
Sand (%) 29.1 60.4 80.4 18.1 17.2 18.4 33.4 32.2
Silt (%) 60.3 21.5 14.1 67.7 69.1 71.5 61.5 32.8
Clay (%) 10.6 18.1 5.5 14.2 13.7 10.1 5.0 35.0
pH (1:2) 3.1 6.43 6.54 8.09 8.16 7.95 7.83 8.73
EC (dS/m) 3.4 0.39 0.73 0.74 0.63 1.01 1.52 0.29
Total CaCO3 (%) 2.0 3.6 3.7 52.1 50.5 49.9 37.3 43.6
Organic C (%) 1.15 0.54 0.71 2.07 1.97 2.46 1.11 0.63
CEC [cmol(+)/kg] 36.9 14.3 9.2 32.0 32.5 31.9 26.7 14.6
Total Cd (mg/kg) 14.0 7.0 215.0 76.0 100.0 191.0 4.8 3.4
Total Co (mg/kg) 8.4 6.0 22.0 11.0 10.3 11.1 7.5 7.8
Total Cr (mg/kg) 28.4 17.9 5.6 28.7 31.1 33.8 22.8 20.7
Total Ni (mg/kg) 17.5 8.4 43.1 37.5 38.2 56.2 30.2 27.4
Total Pb (mg/kg) 282.0 29.0 41.0 221 35.0 36.0 26.0 18.0
212 PLANT SOIL ENVIRON., 50, 2004 (5): 212–217 PLANT SOIL ENVIRON., 50, 2004 (5): 212–217 213
Table 3. Percentage of total metal content released by each extraction procedure
Non-calcareous Calcareous
Metal Extractant
soil 1 soil 2 soil 3 soil 4 soil 5 soil 6 soil 7 soil 8
NH4Cl 3.90 0.47a 1.94 11.7 15.6 23.6 2.28 0.63
CaCl2 2.97a 0.10b 0.33 0.54 0.41 0.31 0.22 0.22a
Cd SrCl2 3.28a 0.20b 0.57a 0.91 1.46 2.65 3.30 0.13a
DTPA 2.45 0.51a 0.57a 56.3 95.3 81.5 9.11 1.63
F-anova ** ** *** *** *** *** *** ***
NH4Cl 20.4a 0.31a 1.52 0.38a 0.33a 0.35a 0.40a 0.39
CaCl2 22.0a 0.15a 1.05a 1.26 0.18a 0.15a 0.10 0.08a
Co SrCl2 21.1a 0.78 0.82a 0.22a 0.22a 0.24a 0.26a 0.14a
DTPA 19.0a 2.09 1.06a 8.21 9.37 7.56 5.49 6.47
F-anova ns ** * *** *** *** *** ***
NH4Cl 0.93 1.20b 0.95a 3.28 1.39 1.91 2.09 5.93
CaCl2 0.02a 0.02a 0.04 0.09a 0.05a 0.05a 0.05a 0.03a
Cr SrCl2 1.53 0.81b 3.65 1.06 0.83 1.35 1.45 1.28
DTPA 0.08 0.07a 0.81a 0.04a 0.09a 0.07a 0.05a 0.03a
F-anova ** ** *** *** *** ** ** ***
NH4Cl 11.4 1.31 3.04 0.61a 0.43a 0.40 0.57 0.30
CaCl2 7.91a 0.04 1.40a 0.71a 0.10b 0.06a 0.10a 0.08a
Ni SrCl2 7.15a 0.85 0.84b 0.26 0.20ab 0.21a 0.26a 0.02a
DTPA 4.00 9.30 1.21ab 1.90 1.98 1.28 3.07 1.84
F-anova *** *** ** ** * ** ** **
NH4Cl 0.49 0.48 0.16a 0.04a 0.31a 0.05a 0.04a 0.03a
CaCl2 0.04a 0.05a 0.07 3.67 0.10 0.04a 0.04a 0.05a
Pb SrCl2 0.02a 0.03a 0.24ab 0.22a 0.31a 0.34 0.14a 0.21
DTPA 0.02a 5.14 0.33b 9.37 8.47 8.41 6.11 10.50
F-anova ** *** * *** *** *** *** ***
*, **, *** indicate significant differences at p = 0.05, 0.01, 0.001, respectively
Figures within vertical columns followed by the same letter are not statistically different (p = 0.05)
element with DTPA from the contaminated calcar- pears that high extraction percentages of Cd with 
eous soils, which corresponds to the findings of DTPA are closely related to the chelating capacity 
Lindsay and Norvell (1978). Young et al. (2000) in of the Lindsay-Norvell solution while maintaining 
comparison of labile Cd with Cd dissolved in four the soil pH at alkaline values to optimize cation 
extractants showed that weak extractants like CaCl2 extraction. It was observed that an increase of the 
and KNO3 consistently underestimated labile Cd. In exchangeable Cd fraction with increasing organic 
this research, very high recovery of total Cd (57 to matter in the soil (from soil 4 to 6), which is a con-
96%) was determined with DTPA for calcareous sistent result with He and Singh (1993).
soils, especially for soils 4 to 6, where Cd pollu- The ammonium was the most efficient chloride 
tion was due to irrigation with polluted water and salt for extracting Cd for the non-calcareous soils. 
organic amendments. Mahler (1988) stated that This could be due to pH of the soil-NH4Cl sus-
both native and sludge-derived Cd was mainly pension being similar to soil pH, and also to the 
in the carbonate fraction in calcareous soils. It ap- similar order of magnitude of the stability con-
214 PLANT SOIL ENVIRON., 50, 2004 (5): 212–217 PLANT SOIL ENVIRON., 50, 2004 (5): 212–217 215
stant of Cd-chloride complex and Cd-organic acid soils) of Cr in the soluble-exchangeable pool. DTPA 
complex for many of the low molecular-weight extracted less Cr than the chloride salts. Comparing 
organic acids that exist in the rhizosphere (Smith these salts, we can observe that ammonium chlo-
and Martell 1977). The influence of the cation in ride and in lesser extent strontium chloride were 
the chloride salts was minimal in the very acidic more efficient extracting agent for Cr than calcium 
conditions (soil 1). chloride. The ammonium chloride seems to be the 
Simple linear correlation analysis shows a highly best extractant for evaluating the extractability of 
significant relationship for Cd-NH4Cl and Cd-DTPA Cr for calcareous soils.
extracts with soil organic carbon content between Cd 
in the different extracts and selected soil properties 
(Table 4). The obtained results indicated that cadmi- Cobalt
um was the most easily extractable metal studied.
The different behaviors were observed for 
Co extraction depending on soil properties for 
Chromium the non-calcareous soils. The soil 1 induced a very 
high level of soluble Co compared with soil 3 with 
The most stable form in soil is Cr3+ and at slightly its near neutral pH. The influence of soil pH on 
acidic to alkaline pH values, ionic Cr3+ species soluble-exchangeable Co was very high for almost 
precipitate with precipitation being favoured by all the extractants and especially for the neutral 
the presence of Fe (Cifuentes et al. 1996). This low chloride salts. The extractable Co pool was closely 
solubility is consistent with the very low amounts related to the electrical conductivity (i.e. salinity) of 
(from 0.04 to 5.95% compared to total content in the soil. Extractability of Co was very low, except 
Table 4. Pearson correlation coefficients between selected soil properties and different extracting methods
Some soil properties
Metal Extractant
sand silt clay pH CaCO3 OC CEC
CaCl2 ns ns ns –0.87** ns ns 0.92**
DTPA ns 0.72* ns ns 0.75** 0.92** ns
Cd
NH4Cl ns 0.72* ns ns ns 0.95*** ns
SrCl2 ns ns ns ns 0.75* ns 0.78**
CaCl2 ns ns ns –0.92** ns ns 0.92**
DTPA ns ns ns ns ns ns 0.82*
Co
NH4Cl ns ns ns –0.91** ns ns 0.93**
SrCl2 ns ns ns –0.92** ns ns 0.92**
CaCl2 0.80* ns ns ns ns ns ns
DTPA 0.80* ns ns ns ns ns ns
Cr
NH4Cl ns 0.82* ns ns ns ns ns
SrCl2 0.71 ns ns ns ns ns ns
CaCl2 ns ns ns –0.92** ns ns 0.91**
DTPA ns ns ns ns ns ns ns
Ni
NH4Cl ns ns ns –0.96** ns ns 0.89**
SrCl2 ns ns ns –0.95*** ns ns 0.91**
CaCl2 ns ns ns ns ns ns ns
DTPA ns ns ns ns ns ns ns
Pb
NH4Cl ns ns ns ns ns ns ns
SrCl2 ns ns ns ns 0.74* ns ns
*, **, *** indicate significant differences at p = 0.05, 0.01, 0.001, respectively, and ns = non significant
214 PLANT SOIL ENVIRON., 50, 2004 (5): 212–217 PLANT SOIL ENVIRON., 50, 2004 (5): 212–217 215
for the DTPA extraction from calcareous soils. This CONCLUSION
result could mainly reflect the low levels of pollu-
tion by this metal and also the high proportion of Generally, the extractability of heavy metals is best 
soil Co present in non-available pools compared determined with DTPA for calcareous soils. This 
with Co pseudo total concentration in soils. may reflect the chelating capacity of the Lindsay-
Norvell solution while maintaining soil pH at 
alkaline values to optimize cation extraction. For 
Lead the non-calcareous soils, smaller differences were 
observed between extractants although ammonium 
The soluble-exchangeable pool of lead was gen- chloride was the most effective extractant under 
erally low due to the high percentage of insoluble acid conditions for heavy metals. The influence 
forms in the soils. DTPA seems to be a better extract- of soil pH and electrical conductivity on soluble-
ant for the majority of the soils, probably due to exchangeable heavy metals was noticeable for Cd, 
its capacity to dissolve Pb precipitates (Schalscha Co and Ni for all the extractants and especially for 
et al. 1980). Shuman (1988) stated that addition of chloride salts.
organic matter to soil could decrease Pb lability 
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Received on June 16, 2003
ABSTRAKT
Porovnání extrakčních metod pro hodnocení vybraných těžkých kovů ve znečištěných půdách
Byla hodnocena extrahovatelnost Cd, Co, Cr, Ni a Pb z půd roztokem chloridu amonného, chloridu vápenatého, 
chloridu strontnatého a DTPA. Pro hodnocení rostlinou přijatelného podílu kovů v půdách jednotlivými extraktanty 
bylo vybráno osm vzorků kontaminovaných půd. Extrahovatelné obsahy prvků byly vztaženy k pseudototálnímu 
obsahu prvků stanovenému po mikrovlnné digesci vzorků v HNO3. Pro kvantifikaci jednotlivých prvků v roztocích 
bylo použito metody ICP-MS s kalibrací pomocí standardů s modifikovanou matricí vzorku. Výsledky naznačily 
významnou variabilitu extrahovatelnosti prvků v závislosti na metodě extrakce, zdroji kontaminace a původu ana-
lyzované půdy a ukázaly nejlepší extrahovatelnost půd s vysokým obsahem vápníku pomocí roztoku DTPA. Při 
porovnání chloridových solí byla u těchto půd zaznamenána nejvyšší účinnost extrakce v případě roztoku chloridu 
amonného.
Klíčová slova: ICP-MS; těžké kovy; znečištěné půdy; kontaminace; extrakční metody; mobilita
Corresponding author:
Dr. Cumhur Aydinalp, Uludag University, Faculty of Agriculture, Department of Soil Science,
16059 Görükle-Bursa, Turkey
e-mail: cumhur@uludag.edu.tr
216 PLANT SOIL ENVIRON., 50, 2004 (5): 212–217 PLANT SOIL ENVIRON., 50, 2004 (5): 212–217 217