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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:edoardo.mentasti@unito.it (E. Mentasti).
0043-1354/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
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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).
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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.
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Table 4
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