Heavy Metals in Agricultural Soils From Piedmont, Italy. Distribution, Speciation and Chemometric Data Treatment

8/17/2019 Heavy Metals in Agricultural Soils From Piedmont, Italy. Distribution, Speciation and Chemometric Data Treatment

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Heavy metals in agricultural soils from Piedmont, Italy.

Distribution, speciation and chemometric data treatment

O. Abollino a, M. Aceto b, M. Malandrino a, E. Mentasti a,*, C. Sarzanini a,F. Petrella c

a Department of Analytical Chemistry, University of Torino, Via Giuria 5, 10125 Torino, Italyb Department of Sciences and Advanced Technologies, University of East Piedmont, Spalto Marengo 33, 15100 Alessandria, Italy

c Institute for Wood, Plant and Environment (I.P.L.A.), Corso Casale 476, 10132 Torino, Italy

Received 8 February 2002; received in revised form 18 June 2002; accepted 28 June 2002

Abstract

The distribution and speciation of heavy metals in five agricultural soils of Piedmont Region (north-western Italy)

were investigated. Ten metals, namely Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Ti and Zn were considered. Analytical deter-

minations were performed by atomic emission or atomic absorption spectroscopy after microwave sample dissolution in

acid solution. Total metal concentrations fit in the typical concentration ranges for unpolluted soils, with the exception

of cadmium and lead content in some horizons. The effect of sampling depth on concentrations was discussed. Spe-

ciation studies were carried out by applying Tessiers procedure, which allows to subdivide the total metal content into

five fractions, representing portions bound to different components of the soil. Moreover, the element labilities in two

soils were evaluated by extraction with EDTA. Correlations among the variables and/or similarities among the sam-

pling points were identified by principal component analysis and hierarchical cluster analysis.

2002 Elsevier Science Ltd. All rights reserved.

Keywords: Metals; Speciation; Agricultural soils; Pattern recognition; Piedmont; Italy

1. Introduction

The concentrations of heavy metals in soils are as-

sociated with biological and geochemical cycles and are

influenced by anthropogenic activities, such as agricul-tural practices, transport, industrial activities, waste

disposal (Lund, 1990). The knowledge of both the total

concentration and chemical speciation is necessary to

characterise the behaviour of heavy metals in soils. In

fact it is well known that metals are present in soils in

different chemical forms, which influence their reactivity

and hence their mobility and bioavailability. Speciation

studies are usually carried out by single or sequential

extractions with reagents having different chemical prop-

erties (Rauret, 1998). These procedures partition metals

into fractions bound to different soil components. The

effect of the extractants simulates the influence of dif-fering environmental conditions, such as acid rain, on

the hypothetical release of metals from soil. Tessier et al.

developed an extraction scheme which allows the divi-

sion of the total metal content into five fractions: ex-

changeable, carbonate bound, iron/manganese oxide

bound, organic bound and residual fraction (Tessier

et al., 1979, 1980). The scheme was developed for sedi-

ments, but it can be applied to soils as well. Similar

procedures mainly differ in the extraction conditions

used. For example, F€oorstners method (Calmano et al.,

1982) is based on conditions that allow the iron/man-

ganeseoxide fraction to be subdivided into three fractions

Chemosphere 49 (2002) 545–557

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* Corresponding author. Tel.: +39-011-6707625; fax: +39-

011-6707615.

E-mail address: [email protected] (E. Mentasti).

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(easily reducible, moderately reducible and nonsilicate

iron). Another commonly used speciation scheme is the

one developed by Community Bureau of Reference

(BCR), in which three fractions are identified: (I) ex-

changeable, water and acid soluble; (II) reducible; (III)

oxidisable species (Davidson et al., 1999).

The results obtained with extraction procedures areof course operationally defined, because redistribution

and adsorption phenomena usually take place during

the treatments. Anyway the information obtained from

this kind of speciation study is of great value in order to

assess metal reactivity and hence their availability to the

environment and their potentially harmful effects.

In this study the total content of heavy metals and

their speciation were investigated in five agricultural

soils from Piedmont, Italy. The following elements were

determined: Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Ti, Zn.

Tessiers extraction scheme was used in the investigation

of the mobility and transport of the metals. Leachingtests with Ethylenediaminetetraacetic acid (EDTA) were

applied to two soils. The results obtained were elabo-

rated with pattern recognition techniques.

2. Experimental

2.1. Sampling

Soil samples were collected in five sites in Piedmont

Region (north-western Italy). The sites are located in the

small towns of Poirino (Province of Asti), Vigone (twosites, hereafter called Vigone 1 and Vigone 2), S. Mau-

rizio Canavese and Carmagnola (Province of Torino).

Table 1 shows their taxonomic classification (Soil Con-

servation Service USDA, 1984; Soil Survey Staff, 1990;

Soil Survey Staff, 1999).

All soils were used as permanent meadows, with the

exception of the one from Carmagnola, cultivated to

maize. Vigone soils have an alluvial origin, whereas the

others come from fluvial terraces. The samples were

collected at different depths, corresponding to the hori-

zons shown in Table 2. A soil layer in the central part of

each horizon was sampled, e.g. sample PA for horizon

Ap (0–20 cm) corresponds to the 5–15 cm layer. The

depth profiles of Poirino, S. Maurizio Canavese and

Carmagnola consisted of three samples, whereas thoseof Vigone 1 and Vigone 2 consisted respectively of seven

and five samples. The layer between 120 and 160 cm in

Vigone 1 is made of two bands, coloured red and grey

respectively; probably the first one (sample V1E) has a

greater content of Fe(III) compounds and the other

(sample V1F) of Fe(II) and/or Cr(III) compounds. It is

therefore interesting to analyse them separately. The

code used to identify each sample refers to its origin (C

for Carmagnola, M for San Maurizio Canavese, P for

Poirino, V1 and V2 for Vigone 1 and Vigone 2) and to

the sampling depth (A for the first horizon, B for the

second and so on).Samples were collected with plastic tools and stored

in polyethylene bags; in-depth samples were obtained

from holes dug with a motor-driven excavator.

2.2. Apparatus and reagents

Metal determinations were performed by atomic

emission or atomic absorption spectroscopy depending

on the concentration level. In both cases calibrations

were performed with standard solutions prepared in

aliquots of sample blanks.

Higher concentrations were determined with a Var-ian Liberty 100 Inductively Coupled Plasma-Atomic

Emission Spectrometer (ICP-AES). Metal concentra-

tions close to or lower than the detection limits for ICP-

AES were determined with a Perkin Elmer 5100

Graphite Furnace Absorption Spectrometer (GF-AAS)

equipped with a Zeeman-effect background correction.

H2O2 present in the fourth fraction of Tessiers spe-

ciation scheme can damage the graphite furnace, there-

Table 1

Soil identification and description

Code Position Soil phase USDA taxonomy

C Carmagnola Mezzi Po, coarse-loamy, typical phase Typic Udifluvent, coarse-loamy, mixed,

mesic, calcareous

M S. Maurizio Canavese Foglizzo, coarse-loamy over sandy-skeletal,

little deep phase

Dystric Eutrudept, mixed, nonacid, mesic

P Poirino Poirino, fine-loamy, phase redoximorphic Oxyaquic, maplustept fine-loamy, mixed,

nonacid, mesic

V1 Vigone 1 Quintanello, coarse-loamy, typical phase Dystric Eutrudept, coarse-loamy, mixed,

nonacid, mesic

V2 Vigone 2 Fontanette, coarse-loamy, typical phase Aquic Dystrudept, coarse-loamy, mixed,

nonacid, mesic

546 O. Abollino et al. / Chemosphere 49 (2002) 545–557


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fore the extract was pretreated (1:1) with a solution of

ascorbic acid to eliminate H2O2 in excess. The addition

of the reducing agent did not cause a contamination of

the extract.

Cadmium and lead in extracts from Tessiers second

fraction were determined by adsorptive cathodic strip-

ping voltammetry (ACSV) with an Autolab PGSTAT10

voltammetric analyser connected to a Metrohm 663VA

stand equipped with a hanging mercury drop electrode.

A Milestone MLS-1200 Mega microwave laboratory

unit was used for the dissolution of the samples. High

purity water (HPW) produced with a Millipore Milli-Q

system was used throughout. All the reagents used were

of analytical grade. Standard metal solutions were pre-

pared from concentrated stock solutions (Merck Titr-

isol).

2.3. Procedures

The granulometric analysis was executed according

to standard procedures (Gee and Bauder, 1986; Sheld-

rick and Wang, 1993). All analyses were performed in

triplicate. Blanks were simultaneously run. Soil samples

were air-dried, passed through a 2-mm stainless steel

sieve and ground in a centrifugal ball mill in order to

homogenise them.

The analytical accuracy of the procedure utilised to

determine the total metal concentrations in investigated

soils, was confirmed by analysis of a standard reference

material (‘‘Estuarine Sediment’’ CRM 277, BCR UE

Brussels); instead, the accuracy of the sequential ex-

traction procedure was obtained by comparing, for a

few samples, the total metal content with the sum of the

metal percentages extracted in the five fractions after

digestion (HF/aqua regia) and analysis of the fifth

fraction.

2.3.1. Sample dissolution

For the determination of the total metal concentra-

tions acid digestion in a microwave oven was chosen as

the dissolution procedure.

Sample aliquots of 100 mg were treated with a mix-

ture of 5 ml of aqua regia and 2 ml of hydrofluoric acid

in tetrafluoromethoxyl (TFM) bombs. Four heating

steps of 5 min each (250, 400, 600, 250 W power re-

spectively), followed by a ventilation step of 25 min,

were applied. Then 0.7 g of boric acid were added, and

the bombs were further heated for 5 min at 250 W and

Table 2

The general characteristics of the investigated soils

Soil parameter

Horizon Particle size distribution pH % C Organic matter CEC

% sand

(2–0.05 mm)

% silt

(0.005–0.002 mm)

% clay

(


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again cooled by a ventilation step of 15 min. At the end

of the full treatment, the samples appeared completely

dissolved. Finally the resulting solutions were diluted to

100 ml with HPW. The solutions were directly employed

for the ICP-AES and GF-AAS analysis.

2.3.2. Measurement of pH, organic carbon, organicmatter, cation exchange capacity

pH, organic carbon, organic matter and cation ex-

change capacity (CEC) were determined according to

the official methods of soil analysis of the Italian legis-

lation issued in 1992, as described below. While research

was in progress, new official methods were issued in

which a different procedure for pH measurement was

fixed, whereas the methods for organic carbon, organic

matter and CEC determination were not changed.

A 1:2.5 soil–water suspension (8 g of soil for 20 ml of

HPW) was prepared and left standing overnight for pH

measurement.The organic carbon was determined by Walkley–

Black method (Soltner, 1988). It was oxidised to carbon

dioxide with potassium dichromate in the presence of

sulphuric acid. The unreacted potassium dichromate

was titrated with iron(II) sulphate.

Considering that the average content of carbon in

soil organic matter is equal to 58%, the conversion fac-

tor 1.724 was used to calculate the percentage of organic

matter from the content of organic carbon.

The CEC was determined with barium chloride

(Bartels, 1996). The samples of soil were saturated with

Ba by treatment with barium chloride solution buffered

to pH 8.2. The addition of magnesium sulphate caused

the formation of insoluble barium sulphate and, there-

fore, the exchange Ba/Mg. The barium in excess left in

solution was determined by complexometric titration

with EDTA.

2.3.3. Sequential extraction procedure

Extractions were carried out on 1.0 g aliquots of soil

and involved the five following steps (Tessier et al., 1979,

Tessier et al., 1980):

1. Exchangeable fraction: the sample was placed in con-

tact with a high ionic strength solution, in order torelease the so-called exchangeable fraction of metal

traces by altering the sorption–desorption superficial

processes. In this step 8 ml of 1 N MgCl 2 were added

to the sample and the suspension was shaken for 1 h.

2. Fraction bound to carbonates: the fraction of metal

traces bound to carbonates, present in the sample,

may be selectively labilised by varying the pH of

the sample itself with a slightly acidic extraction solu-

tion. 8 ml of 1 M of CH3COONa, plus CH3COOH

(pH 5) were added to the residue obtained from the

first extraction, and the suspension was shaken for

5 h.

3. Fraction bound to iron and manganese oxides: this

fraction may be labilised in anoxic reducing condi-

tions. Therefore, 20 ml of 0.04 M NH2OH HCl in25% CH3COOH were added to the residue and the

suspension was shaken for 6 h at the temperature

of 96 3 C.

4. Fraction bound to organic matter and to sulphides:this fraction can be released by treating the sample

with an oxidising agent. Three ml of 0.02 N HNO3and 5 ml of 30% H2O2 were added to the residue ob-

tained from the third extraction, and the suspension

was shaken for 5 h at the temperature of 85 2 C.After cooling, 5 ml of 3.2 M CH3COONH4 were

added to the suspension, which was diluted to 20

ml with HPW and shaken for 30 min.

5. Residual fraction: it is the metal fraction present as

scatter within the crystal lattice of the rocks and min-

erals that constitute the soil. It was calculated from

the difference between the concentration of totalmetal and the sum of the first four fractions.

After each extraction the suspension was subjected

to centrifugation for 20 min at 4000 rpm. The solution

was separated, while the precipitate was washed with

10 ml of HPW and centrifuged again for 5 min. The

washing water then was added to the surnatant, while

the precipitate was used for the subsequent extractions.

The extracts were diluted to 50 (first two fractions) or

100 (next two ones) ml, stabilised by addition of 50 or

100 ll of concentrated nitric acid respectively and

analysed.

2.3.4. EDTA-extractable fraction

This fraction was determined according to Lakanen–

Ervi€oo procedure (Lakanen and Ervi€oo, 1971). Twenty-

five ml of extracting solution (0.02 M EDTA in 0.5 M

CH3COONH4) were added to aliquots of 5 g of soil; the

suspensions, as obtained, were shaken for 30 min, fil-

tered and analysed.

2.3.5. Voltammetric analysis

Aliquots of 10 ml of the samples were added with a

2 103 M solution of 8-hydroxyquinoline. The depo-sition potential was )0.30 V for Pb and )1.10 V (start

potential )0.10 V) for Cd and the deposition time was

120 s.

2.3.6. Chemometrics

PCA and HCA were performed with XLStat 4.4

software package, used as a Microsoft Excel plug-in.

When concentrations were below the detection limit, a

random value between zero and that limit was inserted

in order to be able to thoroughly apply PCA and HCA

without losing any case.



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3. Results and discussion

3.1. Granulometry and general characteristics

Table 2 reports particle size distribution and some

general characteristics, namely pH, percentages of or-

ganic carbon, organic matter and CEC, of the soils ex-amined in this study. In general, all soils have a low

content of clay and organic matter, so they tend to be

permeable. As it can be seen, the percentage of sand is

very high for Vigone 2B, whereas the soil from Car-

magnola has a large silt fraction. The soils in S. Mau-

rizio Canavese, Vigone 1 and Vigone 2 become more

sandy with increasing depth. The highest percentages of

clay were found in samples PB and PC and in two layers

(V1C and V1D) of Vigone 1.

pH values generally increase with depth, more pro-

nouncedly for Vigone 1 and Vigone 2, and slightly for

the other soils. The content of organic carbon and or-ganic matter always decreases with increasing depth.

Also CEC follows this trend, with only one exception.

If the top soil is considered, pH increases in the

order S. Maurizio Canavese < Vigone2 < Vigone1 <

Poirino < Carmagnola; San Maurizio Canavese and

Poirino are the richest in organic matter and have the

highest CEC, whereas samples V1A and V2A have the

lowest values of these parameters.

3.2. Total metal concentrations

Table 3 reports the total metal concentrations in the

investigated soils and, for comparison, their typical

concentration ranges in soils (Alloway, 1990). The ex-

amined soils can be considered unpolluted, since their

concentrations fit in the typical ranges, with the excep-

tion of cadmium in Poirino, S. Maurizio Canavese and

Carmagnola and lead in some horizons of S. Maurizio

Canavese, Vigone 1 and Vigone 2.

The soil from S. Maurizio Canavese has the highest

content of chromium and nickel; probably because of the nature of the parent material, which consists of ul-

tramafic rocks (Alloway, 1990; McGrath and Smith,

Table 3

Total metal concentrations in considered soils, typical literature ranges and most common values, average abundance in the earth s

crust (values in mg/kg unless otherwise stated)

Al Cd Cr Cu Fe Mn Ni Pb Ti Zn

CA 67 900 5.4 162 30 34 000 833 102 215 3550 94

CB 77 800 6.4 174 44 39 500 1080 141 291 4160 103

CC 74 700 3.4 146 28 36 300 899 132 229 3760 86

MA 73 500 4.9 588 34 46 100 1130 675 270 5520 104MB 72 600 4.3 562 28 44 600 965 535 99 5250 73

MC 57 900 2.1 598 30 41 800 802 510 750 6150 73

PA 47 400 2.7 124 19 22 900 525 32 141 3070 82

PB 81 500 4.3 170 46 38 600 1050 126 181 4270 92

PC 77 800 5.1 143 36 36 200 900 94 149 3850 93

V1A 58 600 1.3 116 24 31 600 585 74 374 4210 73

V1B 69 300 1.1 162 27 24 400 670 96 350 4580 74

V1C 72 100 0.9 156 36 42 500 1410 104 582 4280 80

V1D 66 200 0.4 137 34 40 600 1030 113 354 3980 75

V1E 56 600 1.3 138 30 63 700 1260 130 447 3800 69

V1F 67 600 0.3 1580 34 33 800 360 111 392 4060 79

V2A 82 100 0.8 140 45 51 200 877 134 524 4100 135

V2B 75 300 1.6 150 33 40 900 897 123 302 3930 81

V2C 76 200 0.7 115 24 34 500 639 95 213 3690 71

V2D 75 500 0.9 118 17 37 800 623 82 204 3630 61

V2E 63 800 0.7 105 16 32 200 485 81 168 2710 57

Typical

range

0.01–2.0 5–1500 2–250 7000–

42000

20–

10000

2–750 2–300 1–900

Common

value

0.1–1 70–100 20–30 1000 50 10–30a 50

a These values are for rural soils. The common values for lead in urban soils are reported as 30–100. Carmagnola: CA (0–40 cm); CB

(40–90 cm); CC (90–120 cm); S. Maurizio Canavese: MA (10–20 cm); MB (40–50 cm); MC (60–80 cm); Poirino: IA (depth 5–15 cm); IB

(depth 40–50 cm); IC (depth 100–120 cm); Vigone 1: V1A (0–30 cm); V1B (30–50 cm); V1C (50–70 cm); V1D (70–120 cm); V1E (120–

160 cm); V1F (120–160 cm); Vigone 2: V2A (0–35 cm); V2B (35–75 cm); V2C (75–120 cm); V2D (120–175 cm); V2E (175–195 cm).

O. Abollino et al. / Chemosphere 49 (2002) 545–557 549


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1990). Since Vigone soils are in open country, far from

main roads, traffic cannot directly explain the high lead

concentrations found, which could be derived from

the parent material or, especially in the case of Vigone

2, to atmospheric deposition deriving from a distant

source.

As to the vertical profiles, a decreasing trend in theconcentrations of all elements with sampling depth

can be observed in Vigone 2. All metals are depleted in

the top layers of Poirino and Carmagnola soils; this

behaviour can be explained for Carmagnola with the

uptake of metals from maize plants. A rather high con-

centration of chromium was found in the grey coloured

band in Vigone 1; this result confirms the hypothesis

made when samples were collected. No other remark-

able features or trends can be observed in the metal

contents of the five soils.

3.3. Speciation

3.3.1. Tessier’s extraction

Metal speciation according to Tessiers scheme was

performed on all samples. The analyte percentages in the

five fractions are shown in Tables 4–8. It must be

pointed out that the low percentages reported for some

elements correspond to meaningful measured concen-

trations: for instance a percentage of 0.003% of Al ex-

tracted from sample PC in the first fraction corresponds

to 2.24 mg/kg. As remarked in the Introduction, this

scheme, like almost all existing speciation procedures,

gives operationally defined results.

The first fraction contains negligible concentrations

of almost all of the metals considered, except Al, Mn

and sometimes Fe and Ti. In Carmagnola and in most of

the Vigone 2 samples also the exchangeable concentra-tions of Al are undetectable. The extracted percentages

of Al, and especially Mn, in this fraction, are higher in

Poirino and S. Maurizio Canavese than in the other

soils. The extracted fractions of Al and Mn decrease

with increasing depth in most cases.

Most of the considered metals are present in the sec-

ond fraction, even if at low percentages. The concen-

trations of the extracted metals in Poirino and

S. Maurizio Canavese decrease with increasing depth,

with a few exceptions. Chromium was detected only in

S. Maurizio Canavese and in a few other samples

whereas its concentration in the other soils was lowerthan the detection limit. The percentages of major ele-

ments (Al, Fe and Ti) are lower in Vigone 1 than in the

other soils.

Relatively high extracted percentages of most metals

were found in the third fraction. The behaviour of the

elements differs from soil to soil: for instance, copper

was more extensively extracted from Carmagnola and

Vigone1, iron from Poirino and S. Maurizio Canavese

and nickel from S. Maurizio Canavese soil. No regular

trend as a function of depth can be observed.

The concentrations of the metals in the fourth frac-

tion are not higher than in the third one, and in many

cases are lower. Therefore a relatively small fraction

of the metals is bound to organic matter and the high-

est percentages are associated to the mineral matrix of

the soil. As expected, the samples with higher organic

matter contents have higher levels of most metals in

this fraction. The concentrations of major elements (Al,

Fe and Mn) in Vigone 1 and Vigone 2 are frequently

lower than in the other investigated soils. The metal

content in the fourth fraction generally decreases

with increasing depth, probably because organic sub-

stances are less present in the deep levels, because the

biological activities mainly occur in the superficial ho-

rizons.The majority of the metals present in the soils is

contained in the fifth fraction, i.e. they are strongly

bound to the soil matrix. An exception to this trend is

represented by Mn, which is also present at high per-

centages in the third fraction. The most inert element

was found to be Ti.

The presence of ferric and ferrous compounds re-

spectively in E and F levels of Vigone 1 soil does not

seem to influence the extractability of iron, or of other

elements, since there are not remarkable differences be-

tween the percentages found in the two levels, whose

behaviour is similar to the one of the other horizons.

Table 4

The percentages of the extracted metals in the first fraction of

Tessiers speciation scheme

Al Fe Mn Ti

CA


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3.3.2. Extraction with EDTA

The total pool of potentially available species in soils

can be estimated from the EDTA-extractable fraction

(Lund, 1990). Bioavailable concentrations are proba-

bly more correlated to the effect of milder extractants

such as neutral salts (Hani, 1990; Rauret, 1998). The

Table 5

The percentages of the extracted metals in the second fraction of Tessier s speciation scheme


CA 0.03 3.30


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Lakanen–Ervi€oo procedure is usually applied only to

soils that have a pH value lower than 6.5. Anyway, it

was used also for two samples from Vigone 1 with a

slightly higher pH (6.70 and 6.76). The results obtained

are reported in Table 9. As expected, the percentages of

extracted metals are higher than those obtained with the

Table 7

The percentages of the extracted metals in the fourth fraction of Tessier s speciation scheme


CA 0.97


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leaching test with water because EDTA is a strong li-

gand that is able to react not only with the more mobileforms of the metal, but also with those more strongly

bound to organic or inorganic soil constituents. In fact

the percentages of the extracted metals found with this

procedure are comparable to those obtained in the

third and fourth fraction of the Tessiers procedure. The

order of extractability was Mn > Cu > Ni > Zn > Pb >

Fe > Cr > Al > Ti and does not seem to be tightly re-

lated to the values of the formation constants with

EDTA, which decrease in the order Fe(III) > Cu >

Ni > Pb > Zn > Al > FeðIIÞ > Mn (Silleen and Martell,1979). Extractability is probably dependent on the

chemical form of the metal in soil and therefore to its

mobility. The extracted percentages range from 0.017

to 68.59 in Vigone 1 soil and from 0.007 to 39.79 in

Vigone 2.

3.4. Principal components analysis and cluster analysis

A chemometric study on the analytical data was ex-

ecuted in order to obtain a visual representation of the

main characteristics and of metal distribution in the soils

and to find out similarities and correlations among

variables which would be more difficult to detect just

observing the numbers in the tables.Principal components analysis (PCA) is a unsuper-

vised multivariate technique in which new variables are

calculated as linear combinations of the old ones (metal

concentrations or percentages, pH and particle size dis-

tribution, unless otherwise stated, in our case); the new

variables, called principal components (PC), have two

main features: (i) they are uncorrelated between them-

selves; (ii) the first PCs keep the main part of the vari-

ance of the original data set. In this way, it is possible to

show a great part of information by plotting the first two

or three PCs (in this case, the first three PCs were always

computed and the two most meaningful ones, usually

PC1 and PC2, were considered to represent all examined

objects and variables). The combined plot of scores(coordinates of the objects on the new variables) and

loadings (weights of original variables on the linear

combination PCs are built from) allow us to recognise

groups of samples with similar behaviour and the ex-

isting correlation among the original variables.

In Cluster Analysis, samples are considered as objects

in an n-dimensional hyperspace (with n ¼ number of variables), described by n-components vectors. The ag-

glomerative hierarchical clustering procedure was used

to evaluate similarities among samples. In this way, a

dendrogram was obtained, in which correlations among

samples can readily be seen. This technique is comple-

mentary with PCA; the whole information is shown in

the dendrogram.

Only some PC and dendrogram plots will be shown

in this paper, and the other ones are available on request

from the authors.

3.4.1. Total metal

Fig. 1a shows the combined plot obtained by PCA

for total metal concentrations. In general, even if no

neat grouping is present, most samples from the same

site are in the same area of the plot. S. Maurizio Ca-

navese soil and some horizons of the other soils (e.g.V2A and V1C) are characterised by higher metal con-

centrations than the other samples. It is interesting to

note that the position of the surface horizon of Poirino

soil is quite far from the ones of the lower layers, re-

flecting a different composition: in fact it is poor in clay

and rich in organic matter. The V2C, V2D and V2E

samples are characterised by low concentrations of

metals, low percentages of clay and silt and high per-

centages of sand: this is exactly as would be expected

since sandy matrices have normally low heavy metals

sorption capacities. Besides all the horizons of Car-

magnola soil are in the same area of the plot mainly

Table 9

The percentages of metals extracted with EDTA

Al Cr Cu Fe Mn Ni Pb Ti Zn

V1A 0.15 0.20 16.12 1.20 63.59 6.39 1.17 0.10 4.84

V1B 0.15 0.07 4.22 0.75 31.79 5.76 0.93 0.07 0.98

V1C 0.08 0.12 2.54 0.25 9.99 1.94 0.54 0.05 0.76

V1D 0.10 0.10 1.74 0.38 18.74 2.21 0.99 0.03 0.59

V1E 0.08 0.13 1.96 0.17 33.09 5.48 0.54 0.02 1.05

V1F 0.07 0.01 1.06 0.21 7.81 0.50 0.84 0.02 0.41

V2A 0.12 0.25 9.09 0.65 42.99 3.35 0.78 0.04 3.47

V2B 0.09 0.10 5.76 0.52 8.53 2.06 0.97 0.04 1.94

V2C 0.04 0.06 2.92 0.22 6.59 0.53 1.05 0.01 0.97

V2D 0.02 0.04 1.45 0.11 6.90 0.62 0.65


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because they contain low percentages of sand. The

dendrogram, reported in Fig. 1b, confirms the differ-

ences and similarities visible in the PCA plot, and in

particular allows us to distinguish Carmagnola and, in

smaller entity, S, Maurizio Canavese soils from the other

ones. Vigone 1, Vigone 2 and Porino do not form sep-

arate clusters.

Sand and pH are the only parameters with negative

loadings on PC1. A clear anticorrelation between sand

and silt percentages is present. The correlations among

metals, in particular for Mn–Cu, Fe–Ni–Ti, Al–Zn,

cannot be clearly explained in terms of chemical pro-perties, origin or biological function.

We performed the PCA considering also CEC and

the percentage of organic matter as variables, and the

samples for which these data are available as object. In

the plot of PC1 vs. PC2 (not shown) obtained from these

data we note that organic matter is correlated to CEC,

suggesting that the binding sites on organic matter, such

as the negatively charged humic acids, contribute to the

retention of cations in soil. These parameters are anti-

correlated to the percentage of clay, probably because

the soils from the investigated areas mainly contain

kaolinite, which has a low CEC. Finally, CEC is anti-

correlated or not related to metal concentrations, prob-

ably because such metals are mainly bound to the soil

matrix and are present in the exchangeable form only in

very low percentages, as shown by the results obtained

with Tessiers speciation protocol.

3.4.2. Tessier’s fractions

In the plot of PC1 vs. PC2 obtained from the per-

centages of extracted metals in the first fraction (Fig.

2a), it is possible to note one group formed by S.

Maurizio Canavese and Poirino soils, sample MA being

distinguished from the others because it is characterised

by a lower pH value. The samples from Carmagnola are

also close to each other. Vigone 1 and Vigone 2 samples

are partially overlapped.

Fig. 2b shows the results of PCA for the data ob-tained in the second step of Tessiers extraction. It is

possible to identify the following groups: (i) MB, MC

and PB, PC samples, with sample MC partially apart

because of its higher sand content; the topsoils from S.

Maurizio Canavese and Poirino, i.e. samples MA and

PA, clearly differ from the corresponding subsoils; (ii) all

horizons of Vigone 1, (iii) all horizons of Vigone 2, (iv)

all horizons of Carmagnola soil. It is evident that the

samples of the first two groups are characterised by high

percentages of clay and low percentages of extracted Al,

Fe and Mn; the samples from Carmagnola are distin-

guished from the others because they have the lowest

Fig. 1. Total metal concentrations: (a) combined plot of scores

and loadings on PC1–PC2, and (b) dendrogram obtained by

cluster analysis.

Fig. 2. Combined plot of scores and loadings on PC1–PC2 for

the percentages of the extracted metals: (a) in the first extraction

of Tessiers procedure, and (b) in the second extraction.



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percentage of sand and high percentages of Cd and Mn.

Sample PA is far from the others because it contains the

highest percentages of extracted Cd, Mn, Zn.

It is interesting to note also that the plots, referring to

the percentages of extracted metals in the third, fourth

and fifth fractions (Figs. 3a,b, 4a), show analogous

sample groups, with few differences: for instance, the

subsoil samples from S. Maurizio Canavese and Poirino

form two separate groups; moreover, in the fourth

fraction, V2C, V2D, V2E samples are distinguished

from V2A and V2B because of their high percentages of cadmium and titanium. In all fractions the top horizons

of Poirino and S. Maurizio Canavese markedly differ

from the lower ones and from all other samples. More-

over it is evident that Vigone 2 samples are characterised

by low percentages of most metals in the first four

fractions. Therefore metals result to be less available in

these soils and are mainly present in the fifth fraction.

This behaviour could be associated with the high con-

tent in sand present in these samples.

Therefore the soil origin influences the behaviour of

metals towards extraction. The groups present in PCA

plots for the five fractions are clearly visible also from

the dendrograms; as an example the dendrogram rela-

tive to the fourth fraction is reported in Fig. 4b.

PCA was also performed with all variables. As ex-

pected, CEC was found to be strongly correlated to the

percentages of Mn, Al, Fe extracted in the first fraction,

corresponding to exchangeable metals. On the other

hand, this variable was anticorrelated to the metal per-

centages in the fifth fraction, i.e. to the species more

strongly bound to the soil matrix.

3.4.3. Extraction with EDTA

With regard to these results, the plot of PC1 vs. PC3(Fig. 5) is more interesting than the one of PC1 vs. PC2.

The direction of the pH and percentage of sand vectors

is opposite to the one of the vectors corresponding to

other parameters. This is an evidence that the extract-

ability of the metals actually decreases with increasing

pH, as expected: metals are more easily released from

soils at low pHs because they are more weakly com-

plexed by hydroxy groups and organic ligands. The first

layers of Vigone 1 and Vigone 2 soils are characterised

by higher extraction percentages. The similarity between

these samples (V1A and V2A) is confirmed also by the

dendrogram.


the percentages of the extracted metals: (a) in the third ex-

traction of Tessiers procedure, and (b) in the fifth extraction.

Fig. 4. Percentage of the extracted metals in the fourth ex-

traction of Tessiers procedure: (a) combined plot of scores and

loadings on PC1–PC2, and (b) dendrogram obtained by cluster

analysis.



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The higher availability of metals in the topsoils couldreflect an anthropogenic input, since exogenous metals

are usually more weakly bound to the soil matrix and

therefore more easily released (Li et al., 1995).

4. Conclusions

Total metal concentrations in the five investigated

agricultural soils fit in the typical ranges for unpolluted

soils, with the exception of lead and cadmium in some of

the sites. The concentrations in Vigone 2 samples de-

creased with increasing depth, whereas a surface deple-

tion was observed for Carmagnola and Poirino.

Of course the results of this study apply to only a

limited zone of the considered areas, because of the

restricted number of samples, and cannot be directly

extended to the whole sites. The main goal of the

investigation was understanding metal mobility and

distribution also in connection with main soil charac-

teristics.

For this reason metal speciation and mobility were

studied with Tessiers sequential extraction procedure.

The highest percentages of metals were found to be

strongly bound to soil matrix, i.e. in a form not readily

available for introduction into the food chain. Extract-ability in EDTA was investigated in Vigone 1 and Vi-

gone 2 soils: in all cases metals were found to be more

readily available from topsoil than from subsoil. The

chemometric study enabled us to evidence similarities or

differences among the five soils and correlations or anti-

correlations among variables that were not clearly visi-

ble from an examination of the analytical data in the

tables. For instance, samples from the same site showed

a similar behaviour toward extraction in Tessiers pro-

tocol, and soil pH was found to be anticorrelated to the

percentages of metals extracted by EDTA, owing to the

higher element mobility at low pHs.

Acknowledgements

We thank the Ministero dellUniversitaa e della Ric-

erca Scientifica e Tecnologica (MURST, Rome, COFIN

2000) and the Italian National Research Council

(C.N.R., Rome) for financial support.

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Heavy Metals in Agricultural Soils From Piedmont, Italy. Distribution, Speciation and Chemometric Data Treatment