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Water Research 37 (2003) 16191627

Adsorption of heavy metals on Na-montmorillonite.

Effect of pH and organic substances

O. Abollinoa, M. Acetob, M. Malandrinoa, C. Sarzaninia, E. Mentastia,*

aDepartment of Analytical Chemistry, University of Torino, Via Giuria 5, 10125 Torino, ItalybDepartment of Sciences and Advanced Technologies, University of East Piedmont, Corso Borsalino 54, 15100 Alessandria, Italy

Received 6 August 2001; received in revised form 3 June 2002; accepted 16 October 2002

Abstract

Clays (especially montmorillonite and bentonite) are widely used as barriers in landfills to prevent contamination of

subsoil and groundwater by leachates containing heavy metals. For this reason it is important to study the adsorption

of metals by these clays. The sorption of seven metals (Cd, Cr, Cu, Mn, Ni, Pb and Zn) on Na-montmorillonite was

studied as a function of pH and in the presence of ligands, forming complexes of different stabilities with the metals of

interest. The continuous column method was used as it better simulates natural conditions. The total capacity of Na-

montmorillonite towards these metals was determined. The pH variations influence to a higher extent the

concentrations of Cu, Pb and Cd in the effluent. Moreover the results suggest that complex formation hinders the

sorption of the metals on the clay, with an increasing influence in the order: Mn p Pbp Cdp Zno Nio Cuo Cr.

The evaluation of the total capacity of Na-montmorillonite shows that this clay is a good sorbent towards all examined

metals.

r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Na-montmorillonite; Heavy metals; Adsorption; Metal complexes; Breakthrough

1. Introduction

The concentration and the mobility of heavy metals in

soils and sediments have been widely studied in the last

decades [1,2]. Although many heavy metals are neces-

sary in small amounts for the normal development of the

biological cycles, most of them become toxic at highconcentrations. Heavy metals are introduced into the

environment through natural phenomena and human

activities, such as agricultural practices, transport,

industrial activities and waste disposal [3].

Clay linings have been used as barriers in landfills to

prevent contamination of groundwater and subsoil by

leachates containing metals. Generally, these linings are

constituted of bentonite and, in particular, montmor-

illonite. These clays are chosen to avoid pollutant release

into the environment owing to their high specific surface

areas, low cost and ubiquitous presence in most soils [4].

Montmorillonite can adsorb heavy metals via two

different mechanisms: (1) cation exchange in the

interlayers resulting from the interactions between ions

and negative permanent charge and (2) formation ofinner-sphere complexes through SiO and AlO

groups at the clay particle edges [57]. Both mechanisms

are pH dependent because in acid conditions (pH o 4)

most silanol and aluminol groups are protonated;

therefore, in particular for the latter, an acidification

can lead to an increase in mobility of metals bound to

soil [8]. For this reason, it is necessary to improve the

knowledge of the effect of pH on the sorption capacity

of montmorillonite in soilsolution system.

The continuous column method, instead of a conven-

tional batch technique, was used to understand the effect

of pH and of the presence of ligands able to originate

*Corresponding author. Tel.:+39-11-6707625; fax: +39-11-

6707615.

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

0043-1354/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 4 3 - 1 3 5 4 ( 0 2 ) 0 0 5 2 4 - 9

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metal complexes on the sorption capability of mon-

tmorillonite. Batch technique, in fact, is very useful to

study the reaction mechanisms happening at the soil/

water interface but has the following limitations:

solubilisation of soil components due to soil sample

agitation and soil/solution ratios very different from

those existing in natural systems [9]. The used technique,not having these disadvantages, better simulates the real

natural conditions. With this method, the total capacity

of montmorillonite towards the considered heavy metals

has been determined under the considered conditions

[10].

2. Experimental

2.1. Materials, reagents and instruments

The sodium form of montmorillonite was supplied asmontmorillonite KSF by Aldrich. The element composi-

tion of the clay was determined by ICP-AES (Induc-

tively Coupled Plasma Atomic Emission Spectrometry)

after solubilisation by acid digestion in microwave oven

and is reported in Table 1. The percentages of SiO2 and

Al2O3, principal constituents of montmorillonite, are

48.57 and 34.95, respectively. In order to respect the

natural characteristics of montmorillonite and to eval-

uate the retention behaviour in landfill liners, the clay

was not treated. The surface area is 2040 m2 g1 and the

cation exchange capacity (CEC) is B30 meq/100 g,

therefore, they are lower than those generally reportedfor montmorillonites [11], but this is probably due to the

lack of treatment. In fact, it is possible that, in this way,

N2 molecules (employed to calculate the specific surface

area by BET method) and cations can be introduced less

easily in the interlayer regions as indicated by Barbier

et al. [12].

All the reagents used were of analytical grade.

All metal solutions were prepared from concentrated

stock solutions (Merck Titrisol).

High-purity water (HPW) produced with a Millipore

Milli-Q system was used throughout.

A Delta 320 Mettler Toledo pH meter provided with

incorporated thermal probe was used for pH measure-

ments; two standard buffer solutions at pH 4.0 and 7.0

were employed for calibration.

Metal determinations were performed with a Varian

Liberty 100 model Inductively Coupled Plasma Atomic

Emission Spectrometer (ICP-AES). The instrument is

provided with a Czerny-Turner monochromator, a

Sturman-Masters spray chamber, a V-groove nebuliser

and a radio frequency (RF) generator at 40.68 MHz.

The instrumental conditions were: plasma power

1.0 kW; nebuliser pressure 150 kPa; sample aspiration

rate 15 rpm (B2ml/min1); argon auxiliary flow 1.5 /min1. A Gilson Minipuls 2 multichannel peristaltic

pump was used to drive the sample solutions into the

columns with a constant and reproducible flow rate

(0.5 cm3 min1). This pump was connected to the

columns by polypropylene low-pressure fittings and

PVC tubes (i.d. 1.29mm). The polypropylene columns

(Bio-Rad), 4 cm high and 5 mm i.d., were packed with

1 g of Na-montmorillonite.

A Milestone MLS-1200 Mega microwave laboratory

unit was used for the dissolution of the clay.

3. Procedure

The continuous column method was used to study the

adsorption of heavy metals on Na-montmorillonite. The

apparatus for continuous ion exchange is schematised in

Fig. 1. At the bottom of the column, on the porous

polymer bed support, a 0.45 mm cellulose acetate filter

(Millipore) was placed. The slurry packing technique

was used to obtain a homogenous bed in the column.

Before each experiment the clay packed in the column

was conditioned at the working pH with a buffer

solution. Solutions were driven through the columnswith the aid of a peristaltic pump. Six replicates of each

experiment were performed. Blanks were simultaneously

run in order to take account of possible contaminations

and eventual releases of the investigated metals from the

clay.

In the study of pH and ligand effect, 25 ml of the

eluate was collected. The Cd, Cu, Cr, Mn, Ni, Pb and Zn

concentrations in the influent and effluent solutions were

determined using an atomic emission spectrophotometer

(ICP-AES). The amount of every metal adsorbed on to

the clay was calculated by the difference between the

content of metal in influent solution and that one in

effluent solution, corrected with the blank, and it was

expressed in percentage.

The pH range studied was 2.58.0. The NaAcetate/

Acetic Acid buffer was used in the pH interval between

2.5 and 5.5, while the HEPES-Na [4-(2-hydroxyethyl)-

Table 1

Element composition of the Na-montmorillonite determined by ICP-AES

Element Cd Cr Cu Fe K Mg Mn Na Ni Pb Ti Zn Zr

g/kg o 0.01 0.03 0.01 32.50 9.71 12.64 0.18 1.93 o 0.03 o 0.07 1.78 0.01 0.12

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piperazine-1-ethanesulphonic acid, sodium salt] buffer

was used to obtain pH 8.0. All buffer solutions were

prepared at 1.0 101 M concentration; the concentra-

tion of every metal was always kept to 1.0 104 M in

order to minimise the competitions among the consid-

ered metals towards the adsorption of montmorillonite.

Ligand stock solutions were prepared in water. The

following ligands were investigated: nitrilotriacetic acid

(NTA), ethylenediaminetetraacetic acid (EDTA), oxalic

acid, malonic acid, succinic acid, glutaric acid, tartaric

acid and citric acid. The effect of metal complexation on

adsorption was studied with sample solutions bufferedat pH 5.5 containing 1.0 103 M ligand and

1.0 104 M metal ions.

The evaluation of the total capacity of Na-montmor-

illonite towards every studied metal was performed

with the breakthrough technique generally used in

chromatography to measure the highest degree of

column utilisation in a given process. The solutions

containing a single investigated metal at a concentration

of 1 103 M were driven, with pH controlled at 5.5,

through the column packed with the clay and successive

aliquots of 25 ml of the effluent were collected and

analyzed.

4. Results and discussion

4.1. Effect of pH

The influence of pH on the adsorption of bivalent and

trivalent metal ions (Cd, Cr, Cu, Mn, Ni and Pb) on Na-

montmorillonite was studied. The results obtained arerepresented in Fig. 2. As expected, the adsorption of

metals decreases with decreasing pH because the

aluminol and silanol groups are more protonated and,

hence, they are less available to retain the investigated

metals. This effect is strongly evident for Cu, Pb and Cd,

and less pronounced for the others. The reason of this

behaviour is that the surface complexation reactions are

influenced also by the electrostatic attraction between

the surface charge and the dissolved ions. In fact, since

cadmium and lead have larger ionic radius (0.97 and

1.20 (A respectively), they have lower charge density and,

therefore, are more affected by the protonation of thesurface groups that determine a reduction of the

adsorption sites on clay. The Cu(II) behaviour, instead,

is probably due to the structure of its aquaion. In fact,

[Cu(H2O)6]2+ has a tetragonal distortion due to the

JahnTeller effect in which the octahedral structure has

been contracted along the x and y axes [13]. There-

fore, the binding of ligands along the x and y axis is

supported whereas the ligands along the zaxis are

shielded from the Cu2+ ion by an extra electron. This

contraction along the x and y axis results in a

structure having four shorter bonds and two longer

bonds and this hinders in part the binding of the copper

(II) aquaion with the clay surface groups. This effect is

more evident when these groups are more protonated

because the binding is supported only along particular

directions (x and y axis). For these reasons, hence, the

adsorption of Cu, Pb and Cd is hindered by cation

exchange mechanism whereas this mechanism domi-

nates the adsorption of Cr, Mn, Ni and Zn ions. These

Na-montmorillonite4 cm

i.d. 5 mm

Inlet

Outlet

porous polymerbed support

0.45 m celluloseacetate filter

polypropylene lowpressure fitting

Fig. 1. Schematic representation of continuous ion exchange

column system.

Fig. 2. Adsorption of Cd, Cr, Cu, Mn, Ni, Pb and Zn on Na-

montmorillonite as a function of pH (initial concentration of

metals 1.0

10

4

M).

O. Abollino et al. / Water Research 37 (2003) 16191627 1621

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factors contribute to a different pH effect on each metal.

At pH p 3.5, the studied metals are increasingly

adsorbed in the following order: Cu2+ o Pb2+ o

Cd2+ o Zn2+ p Mn2+ D Cr3+ D Ni2+.

4.2. Effect of ligands

The soils and the leachates coming from rubbish

dump generally contain also organic substances beyond

heavy metals. For this reason, it is very important to

know the behaviour of heavy metals in the presence of

organic ligands toward their adsorption on Na-mon-

tmorillonite, often used as linings to keep the pollutants.

Using again the continuous column method, the

simultaneous adsorption of Cd, Cr, Cu, Mn, Ni, Pb

and Zn in the presence of ligands having different

complexation constants was evaluated. The equilibrium

constants of these ligands are reported in Table 2. The

concentration of ligands was chosen so as to be in excess

with respect to that of metals and the experiments were

executed at pH 5.5 to assure the formation of Meligand

complexes and to minimise the one of hydroxo-species.

The results are reported in Fig. 3 and Table 3. Theequilibrium constants are not all known in the literature

[14,20,21]. From the results it is evident that the presence

of ligands having high complexation constants, that is

EDTA and NTA, strongly hinders the adsorption of all

metals on the clay. In fact the adsorption percentages

vary between 0.1% for NiNTA complex and 36% for

MnNTA complex. This behaviour, moreover, indicates

that the adsorption of the free metal ion is supported

with respect to that of MeNTA/EDTA complex. This

Table 2

Equilibrium constants of the considered ligands [14,20,21]a,b

Cd Cr Cu Mn Ni Pb Zn

EDTA A 16:54 A 12:8 A 18:7 A 14:05 A 18:4 A 18:0 A 16:44

pka1=2.00; pka2=2.69;

pka3=6.13; pka4=10.19

B 9:07 B 6:1 B 11:91 B 5:47 B 11:56 B 9:68 B 9:00

C 6:70 C 6:22

NTA A 9:4 A 13:1 A 11:26 A 11:4 A 10:0

pka1=1.9; pka2=2.48;

pka3=9.65

B 4:9 B 3:39 B 3:99 B 3:5

Meso-tartatic acid A 1:30 A 3:15 A 2:49c A 3:01 A 3:09 A 2:2c

pka1=2.97; pka2=4.49 B 0:80 B 2:06 B 1:21c B 2:03 B 2:09 B 1:0c

C 0:

76Oxalic acid A 2:78 A 4:49 A 2:15 A 3:83 A 4:16

pka1=1.04; pka2=3.81 B 1:22 B 3:92 B 1:90 B 3:23 B 1:43 A 3:48

C 0:90 D 9:54 C 1:75 D 6:6 D 6:33 B 2:00

D 4:1 C 1:60

E 5:1

Malonic acid A 2:64 A 5:04 A 2:5 A 3:29 A 2:60 A 2:95

pka1=2.63; pka2=5.28 B 1:49 B 2:08 C 1:24 B 1:04 B 1:02 B 0:99

C 1:04 C 7:8 C 0:70

D 4:20

E 4:16

Succinic acid A 1:47 A 2:7 A 1:26 A 1:6 A 2:40

pka1=4.00; pka2=5.24 B 0:82 B 1:85 B 1:33 A 2:33

C 0:45 C 0:38 B 2:14

D 2:29 D 3:99

E 2:74 E 3:89

Glutaric acid A 2:0 A 2:4 A 1:13 A 1:6 A 2:48 A 1:6

pka1=4.13; pka2=5.01 B 0:97

C 0:45

D 3:77

Citric acid A 3:65 A 5:90 A 3:79 A 5:35 A 4:44 A 4:76

pka1=2.90; pka2=4.35; B 2:19 B 3:7 B 2:22 B 3:25 B 2:98 B 2:78

pka3=5.30 C 1:1 C 2:26 C 1:5 C 1:75 C 1:70 C 1:3

D 4:54 D 5:92 D 5:90

aKa=acid dissociation constant; b=cumulative stability constant.bA=log b (ML); B=logb (MHL); C=logb (MH2L); D=logb (ML2); E=logb (ML3).c

Cumulative stability constant of D-tartaric acid.


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is probably due to the large sizes of these complexes that

hinder their introduction in the interlayers of montmor-

illonite. Considering four dicarboxylic acids (oxalic acid,

malonic acid, succinic acid and glutaric acid) having

gradually longer hydrocarbon chains, it can be seen that

the adsorption of metal ions generally increases with the

increasing of hydrocarbon chain length of these acids

and with the decreasing of complex stabilities. In fact the

complexes characterised by rings with five and six atoms

are more stable than those formed by more than six

atoms. The behaviour of Pb and Cd ions is differentbecause their adsorption on clay decreases in the

presence of succinic acid and glutaric acid. This

probably occurs because these ions have larger ionic

sizes, form more stable and larger complexes with

ligands of longer hydrocarbon chain.

The adsorption of oxalic acid (OA) on Na-montmor-

illonite was investigated in order to study the sorption

capability of the clay towards an organic acid of small

size and establish whether the surface structure SMe

OA is possible at the pH of 5.5. Oxalic acid was chosen

for this study because it is the smallest considered ligand

and, therefore, could enter into the spaces between the

layers of montmorillonite. Besides it better differentiates

the adsorption of every metal ion on the clay. The

amount of adsorbed acid was determined by redox

titration with potassium permanganate both in the

influent and in the effluent solutions of blanks and

samples. The percentage of adsorbed oxalic acid is equal

to 46% for the blanks, and to 42% for the samples

containing also the studied metals. It is evident, there-

fore, that oxalic acid is adsorbed on the clay by cation

bridging, water bridging and H-bond complexation [15].

Its adsorption, however, is partially hindered by thepresence of the metal ions in solution; in fact, at pH 5.5,

oxalic acid is deprotonated and, then, it preferentially

forms complexes with bivalent and trivalent ions in

solution. Moreover, the MeOA complex is introduced

in the interlayers with greater difficulty than the free

metal ions because it is characterised by a chelating ring

and, then, it has larger size.

From the results it is evident that the behaviour of

citric acid is similar to that one of glutaric acid; in fact,

also in the presence of citric acid all metals were

adsorbed in high percentages on montmorillonite. This

is probably because, at pH 5.5, only two carboxylic

0

20

40

60

80

100

120

Ni Mn Cr Zn Cu Pb Cd

Metal

Adsorption

(%)

EDTA

NTA

Oxalic

MalonicSuccinic

Glutaric

Citric

Tartaric

Fig. 3. Metal adsorption on montmorillonite in the presence of different ligands (pH 5.5; metal concentrations 1.0104 M; ligands

concentrations 1.0103 M).

Table 3

Percentages of adsorbed metals on Na-montmorillonite in the presence of a ligand

EDTA NTA Tartaric acid Oxalic acid Malonic acid Succinic acid Glutaric acid Citric acid

Cd 9.9 7.1 91.5 90.8 86.1 79.9 83.6 96.4

Cr 24.7 26.7 29.8 25.5 35.2 96.9 97.4 97.3

Cu 9.9 5.4 94.9 14.7 24.7 84.5 93.4 91.5

Mn 10.3 35.9 98.6 99.5 100 96.6 99.8 95.0

Ni 0.3 0.1 49.4 63.9 81.2 97.4 99.3 94.4

Pb 9.8 10.0 99.1 97.7 100 85.5 89.3 96.5

Zn 8.5 5.9 65.3 93.9 100 96.3 98.6 95.3


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groups of the acid are deprotonated and, then, it forms

complexes similar in size and stability to the ones made

by glutaric acid.

A chemometric study on analytical data was made

applying the principal component analysis (PCA) and

hierarchical cluster analysis (HCA), in order to obtain a

visual representation of the existing correlations amongpercentages of metal adsorbed on to montmorillonite

and ligands present in solution and to evidence groups

of ligands and of metal ions having similar character-

istics (an introduction to these topics may be found in

Kellner et al. [16]; while a more advanced discussion on

this chemometric approach may be found in Massart

et al. [17]; Otto [18]; Abollino et al. [19]).

From the plot of PC1 vs PC2 and the dendrogram

obtained considering the ligands as objects and the

metal ions as variables, reported in Fig. 4, it was possible

to identify the following groups of ligands: (i) strong

ligands: EDTA and NTA; (ii) ligands of intermediate

strength: oxalic acid, malonic acid and tartaric acid; (iii)

weak ligands: succinic acid, glutaric acid and citric acid.By this approach it was possible to understand that the

behaviour of tartaric acid is similar to the one of oxalic

acid and malonic acid. Instead, additional information

was not obtained from PCA plots and dendrogram

computed considering the metal ions as objects and the

ligands as variables. Altogether it is possible to deduce

that the influence of ligands on the adsorption of

EDTA

NTA

Oxalic Acid

Malonic Acid

Succinic Acid

Glutaric Acid

CitricAcid

Tartaric Acid

Ni

Mn

Cr

Zn

Cu

PbCd

-2

-1

0

1

-4 2

PC1 (80%)

PC2(15%)

Tartaric Acid

Citric Acid

Glutaric Acid

Succinic Acid

Malonic Acid

Oxalic Acid

NTA

EDTA

0 2 3 4 5index

1

-2 0

(a)

(b)

Fig. 4. Combined plot of scores and loadings on PC1 vs PC2 (a) and dendrogram (b) computed considering the ligands as objects and

the metal ions as variables.


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investigated metals on Na-montmorillonite increases in

the following order: Mn p Pbp Cdp Zno Nio Cu

o Cr so that the metals are adsorbed in increasing

extent in the opposite order. Therefore, it is evident that

the affinity of metal ions towards Na-montmorillonite

strongly depends not only on pH, but also on the

presence of ligands in solution. In fact, for example,

copper is much more adsorbed as aquaion than as

complex.

4.3. Total capacity

By the use of the continuous column method, it was

possible to determine also the total capacity of Na-

montmorillonite towards every considered metal by the

breakthrough technique. This procedure is generally

used to measure the highest degree of utilisation of a

chromatographic column. When the effluent solution

concentration results to be equal to the one of influent

solution, that is, when the column inlet-to-outlet

concentration ratio (C/C0) attains a value very close to

unity, the saturation point is achieved. In this conditionall surface adsorption sites of the packing material, in

this case Na-montmorillonite, are occupied by the

considered ion. If the C/C0 ratio is reported as a

function of effluent volume, a curve known as the

breakthrough curve is obtained. The model of break-

through curve is reported in Fig. 5, where B indicates the

breakthrough point of the examined ion, that is when

the ion first appears in the effluent solution, and C

indicates the saturation point. Calculating the area

ABCD, the total capacity is obtained. By this approach

the breakthrough curves for all considered metals were

obtained.

One must consider also that the uptake yield of a

metal ion by the investigated clay should depend not

only on thermodynamic reasons (extent of the uptake

equilibrium towards the substrate) but also on kineticeffects given by the possible low rate of metal uptake in

concomitance to the continuous advancement through

the column. The almost quantitative uptake for the

investigated metals at pHsX5.5 (see Fig. 2) points out,

however, that in the present cases kinetic effects should

play a very minor role.

The breakthrough curves for the adsorption of the

seven metals examined on Na-montmorillonite are

reported in Fig. 6. Calculating the area ABCD for each

experimental curve, the total capacity of Na-montmor-

illonite towards each considered metal ion was deter-

mined and the results are given in Table 4.

ECD

0 A B

1.0

0.5C/C0

Volume (ml)

H+

Men+

Fig. 5. Model of breakthrough curve [10].

0

0.2

0.4

0.6

0.8

1.0

1.2

0 25 50 75 100 125 150 175 200 225

Volume (ml)

C/C0

Cd

Cr

Cu

Mn

Ni

Pb

Zn

Fig. 6. Breakthrough curves for the adsorption of Cd, Cr, Cu,

Mn, Ni, Pb and Zn. The metal concentration was 1.0103 M.

The pH was controlled at 5.5.


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From the experimental curves, it is evident that, in

order to saturate the clay, higher volumes of chromium

solution and, on the contrary, smaller volumes of

cadmium and lead solutions are necessary, therefore,the breakthrough curve of chromium ion has a lower

slope than the one of cadmium and lead. Moreover it is

interesting to note that the adsorption behaviour of Zn,

Mn and Ni, having similar course, seems to be

intermediate between that of Cd, Cu and Pb, and the

one of Cr. In the case of Cr, for which Fig. 6 does not

show a complete levelling-off trend, a volume of 300 ml

ensured the attainment of saturation.

From the results it is evident that the total capacity of

Na-montmorillonite towards the investigated metals

increases in order: Pb2+ = Cd2+ o Cu2+ o Zn2+ o

Mn2+ o Ni2+ o Cr3+.

Cadmium and lead ions are adsorbed less on clay even

if they have the same valence as Cu2+, Zn2+, Mn2+ and

Ni2+ because they are characterised by larger ionic radii

and, then, their introduction in interlayers and com-

plexation with surface sites is limited by steric hindrance

and lower electrostatic attraction. As observed in the

study of the pH effect, copper (II) ion is the least

adsorbed among the ions of the first series of transition,

probably because the different geometry of its aquaion

principally hinders the complexation reactions with

surface groups. Chromium (III) ion, instead, has a

higher charge density and, then, the coulombic attrac-

tion towards the superficial sites of the clay is greater.For this reason the total capacity of Na-montmorillonite

towards the trivalent ion considered is the highest.

In general, however, Na-montmorillonite was found

to have a good total capacity towards all considered

metals.

5. Conclusions

The adsorption of metal ions on Na-montmorillonite

decreases with decreasing pH. At low pH values (2.5

3.5), the hydrogen ion competes with the heavy metals

towards the superficial sites and, moreover, the SiO

and AlO groups are less deprotonated and they form

complexes with bivalent and trivalent ions in solution

with greater difficulty. This effect is particularly evident

for Cu2+ that, as aquaion [Cu(H2O)6]2+, has a distorted

geometry and for Pb2+, Cd2+ that have a lower

electrostatic attraction versus the clay because of theirlower charge density. For these reasons, the adsorption

of these ions is unsupported by cation exchange

mechanism and, hence, they are more influenced by

pH variations. Therefore, the pH effect is different on

each metal and at pH p 3.5 the studied metals are

adsorbed in increasing entity in the following order:

Cu2+ o Pb2+ o Cd2+ o Zn2+ p Mn2+ D Cr3+ D

Ni2+.

It was also shown that the presence of ligands in

solution influences the adsorption of heavy metals on

Na-montmorillonite. This effect is a function of the

ligand and metal considered but the formation of Meligand complexes in solution altogether hinders the

adsorption of the metal ions on the clay. In this case the

metal adsorption increases in the following order: Cr3+

o Cu2+ o Ni2+ o Zn2+ p Cd2+ p Pb2+ p Mn2+.

The results evidenced that the sorption capability of

Na-montmorillonite towards each metal ion examined is

different in the various conditions, and it varies both as

a function of pH and of the ligand present in solution. It

is necessary, hence, to consider both these factors to

study a real soil/solution system and effectively predict

the fate of heavy metals in the environment.

Besides, it was possible to determine also the total

capacity of Na-montmorillonite towards the considered

metals by the breakthrough technique. By this approach

it has been verified that the sorption capacity of this clay

is remarkable and increases in the following order: Pb2+

= Cd2+ o Cu2+ o Zn2+ o Mn2+ o Ni2+ o Cr3.

Acknowledgements

We thank the Ministero dellUniversit"a e della

Ricerca Scientifica e Tecnologica (MURST, COFIN

2000, Rome) for financial support.

References

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Table 4

Total capacity of Na-montmorillonite towards Cd, Cr, Cu, Mn,

Ni, Pb and Zn

Total capacity

mg/g meq/100 g

Cd 5.20 9.25

Cr 5.13 29.6

Cu 3.04 9.57

Mn 3.22 11.7

Ni 3.63 12.4

Pb 9.58 9.25

Zn 3.61 11.0


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