2.2.1. elemental analysis - university of ioannina
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WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 1 8 9 7 – 1 9 0 61898
et al., 2006). Consequently, research in the development of
effective and low-cost heavy metals and radionuclide sor-
bents is not only scientifically and economically attractive but
also urgent for environmental and health protection.
Naturally occurring clay minerals have been extensively
studied as sorbents for the removal of various pollutants from
wastewater and aqueous solutions (Al-Qunaibit et al., 2005).
Montmorillonite, coated and intercalated by aluminum
hydroxides, exhibits much higher adsorption capacity for
some heavy metal ions than that of natural montmorillonite
(Lothenbach et al., 1997). High-temperature calcination of
intercalated clays results in ‘‘pillared’’ materials, where the
polyhydroxo-cationic species are irreversibly fixed to the
layers. Pillared montmorillonites, first tested as cracking
catalyst in view of their acidic properties, have recently been
studied as sorbents for the removal of hazardous inorganic
elements (Karamanis et al., 1997; Cooper et al., 2002; Jiang and
Zeng, 2003; Manohar et al., 2005, 2006). However, very little is
still known about the removal efficiency and the sorption
mechanisms of metals by pillared clays (Manohar et al., 2005,
2006). Therefore, the present work investigated the efficiency
of aluminum-pillared-layered montmorillonites (PILMs) for
the removal of copper and cesium from aqueous solutions
under kinetic and equilibrium conditions. In order to identify
the mechanisms of copper and cesium sorption on pillared
montmorillonites, the pillaring conditions were varied and
different forms of PILMs were prepared and tested in sorption
experiments.
2. Materials and methods
2.1. Preparation of PILMs
Bentonite was provided by Silver & Baryte Min. Co., Athens.
The montmorillonite fraction of the starting bentonite was
brought to its Na-homoionic form (Na-montmorillonite,
NaM). Specifically, bentonite was repeatedly contacted with
1 M NaCl solutions and washed with deionized water until Cl�
free (AgNO3 test). Then, a 1 wt% aqueous suspension of the
treated bentonite was prepared and left to stand for 24 h. The
upper 80% of the colloid column was then separated,
centrifuge-washed and finally gently ground after drying at
room temperature.
An aluminum-pillared montmorillonite, coded Al1MFD3,
was prepared according to the method described by Karama-
nis et al. (1997). Briefly, the Naþ form of montmorillonite was
added to the pillaring solution (a 0.2 M solution of AlCl3 with a
0.2 M solution of NaOH up to a molar ratio OH/Al of 1) at a
ratio of 3 mmol Al per gram clay and left to react for 1 h under
vigorous stirring at 70 �C. Once the exchange process was
completed, the PILM precursor was repeatedly washed and
then freeze-dried. Heating the PILM precursor in air at 500 �C
for 4 h in order to create the rigid, non-swelling, three-
dimensional zeolite-like structure (Varma, 2002), delivered
the final PILM in powder form. Its cation-exchange capacity
was restored by contacting the powder with an ammonia
atmosphere and then with a NaCl solution at pH 10
(Karamanis et al., 1997). The PILM material was recovered by
centrifugation and was subsequently centrifuge-washed
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several times with deionized water; it was finally dried at80 �C. PILMs coded as AlXMFDY were prepared accordingly by
varying the OH/Al ratio (X ¼ 0:125, 1 or 2) and the ratio of
added mmol Al per gram of clay (Y ¼ 3, 5, 15).
2.2. Materials characterization
2.2.1. Elemental analysisThe concentration of each element in the prepared materials
was determined with the spectrometric methods of proton
induced gamma-ray emission (PIGE) and X-ray fluorescence
(XRF) (Karamanis et al., 2001). PIGE measurements were
carried out at the 5.5 mV terminal voltage TN11 Tandem
accelerator of the National Center for Scientific Research
‘‘Demokritos’’ in Athens. A proton beam with energy of Ep ¼
4:0 MeV was used and the emitted g-radiation was detected
with an 80% high purity germanium (HPGe) detector. Char-
acteristic g-rays emitted from the deexcitation of the residual
nuclei following (p,p0g) reactions, were used for the determi-
nation of light elements as Al (1014 keV), Si (1779 keV), Mg
(585 keV) and Na (440 keV). For the normalization and extrac-
tion of weight percentages, samples of the certified reference
material IAEA Soil-7 and of powder graphite mixed with
cellulose and a compound containing the element under
investigation (in its natural isotopic abundance), were used as
standards.
XRF measurements were performed at the XRF Unit of the
University of Ioannina. A vertical Si(Li) detector was used and
the exciting radiation was provided by a ring shaped radio-
isotope source (109Cd or 241Am). Samples in the form of pellets
were placed at the top of the assembly in a pi geometry
between the exciting radiation and the samples. Induced
X-rays were detected through a small hole in the shielding
material. Normalization was performed with direct compar-
ison with the reference IAEA material Soil-7 and matrix
effects were corrected through simulation.
2.2.2. X-ray diffraction (XRD)XRD patterns of PILM materials were collected on a Bruker
AXS D8 Advance Bragg–Brentano geometry with Cu sealed-
tube radiation source (l ¼ 1:54178 A) plus a secondary beam
graphite monochromator. A step of 0:02� and a time of 3 s
step�1 were selected.
2.3. Sorption experiments
2.3.1. Cesium sorption studiesThe uptake of Csþ was investigated via equilibration and
exchange kinetic measurements. The conventional batch
technique was employed for the equilibration measurements
in which a known amount of material was contacted with a
solution containing chloride salts of Csþ, traced with 137Cs.
After attaining equilibrium, the two phases were separated by
centrifugation and the g activity of the supernatant was
measured with a 22% HPGe detector. In addition to batch
kinetic measurements, a dynamic dialysis method was also
used (Karamanis et al., 1997). Periodically removing the
dialysis bag and measuring the remaining solution activity
determined the rate of cation uptake.
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WAT E R R E S E A R C H 41 (2007) 1897– 1906 1899
2.3.2. Copper sorption studiesThe aqueous solutions of Cu2þ were obtained from dilutions
of stock solutions prepared from dissolving CuCl2. All
sorption experiments were conducted in mechanically stirred
glass reactors equipped with a thermometer and a pH
electrode to measure variations in temperature and pH. At
equilibrium, PILMs were filtered through a 0:47mm millipore
filter. Sorption kinetics were studied with the batch or the
dialysis method. In each experiment, duplicate samples of
1 mL from the sorption solution were analyzed by XRF prior to
materials immersion in the solution and till equilibrium
attainment. The volume of 1 mL of solution was pipetted onto
a 12 mm Whatman No. 42 filter (held in a mylar film of 6:3mm
thick) under continuous drying with an IR-lamp and was
directly XRF measured. The quantity of metal ion sorbed on
each material was determined by the difference between the
initial metal concentration and the remaining concentration
at equilibrium. XRF was calibrated with standards, which
were prepared by diluting known volumes of the standard
stock solutions in deionized water. Metal concentrations of
1.0–100:0 mg L�1 were used and four replicates were prepared
of each. The calibration curve was linear in all the studied
region and with a minimum detection limit of 0:09mg of
copper on filter.
Fig. 1 – XRD patterns of sodium montmorillonite (NaM) and
the prepared pillared samples (01MFD3 is the sample with
OH/Al ratio of 0.125 and Al/clay ratio of 3 mmol g�1, 1MFD3
3. Results and discussion
3.1. PILMs characterization
Elemental analysis of NaM and PILMs is shown in Table 1. The
increase in aluminum weight percentage between NaM and
the PILM derivatives was dependent on the initial aluminum
availability per gram of clay. It is known that the Al/clay ratio
has a significant effect on the accessibility properties in Al-
pillared clays. With an Al/clay ratio of higher than 5 mmol g�1,
a bimodal micropore distribution has been observed
with peak dimensions at 0.4 and 0.55 nm (Hutson et al.,
1999). Moreover, higher initial aluminum availability leads
to a higher pillar density (Gil and Montes, 1995) and
therefore higher fraction of pores with an opening of less
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Table 1 – Elemental analysis of Na-montmorillonite (NaM) and(%) NaM Al1aMFD3b A
SiO2c 55.9 52.2
Al2O3c 18.6 24.3
Fe2O3d 2.9 2.9
MgOd 2.9 2.7
CaOc 0.1 0.1
K2Od 0.5 1.0
Na2Oc 2.5 2.2
a Molar OH/Al ratio.b Al/clay ratio (mmol g�1Þ.c PIGE.d XRF.
than 0.45 nm. In this way, the effect of the pillar density or the
interpillar distance on cesium or copper sorption was further
studied.
The grain size of all the prepared PILMs was measured to be
lower than 45mm. The variation of the OH/Al ratio resulted in
different d001-spacing of the pillared samples as deduced from
the PILMs XRD patterns (Fig. 1). The d001 peak was narrower
and more intense in Al2MFD15 (1.75 nm) and Al2MFD5
(1.74 nm) than in the other two pillared samples Al2MFD3
(1.72 nm) and Al1MFD3 (1.67), indicating a more ordered or
higher crystallinity in these samples. The OH/Al ratio of 0.125
did not result in a pillared sample and thus, the sample was
not used in the subsequent sorption tests. All other samples
were tested in sorption experiments and the influence of
PILMs interlayer spacing in their sorption ability was further
investigated.
its aluminum pillared products
l2aMFD3b Al2aMFD5b Al2aMFD15b
52.7 52.3 52.4
24.5 25.3 28.5
2.9 2.9 2.9
2.6 2.6 2.4
0.1 0.1 0.1
1.3 1.1 1.2
2.2 2.2 2.4
is Al1MFD3 with an OH/Al ratio of 1 and Al/clay ratio of
3 mmol g�1, etc.).
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Fig. 3 – Effect of pH on cesium sorption (1:7� 10�11 M) on
Al2MFD15 (0:15 g L�1).
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3.2. Cesium sorption experiments
3.2.1. Cesium sorption on different PILMsEven in the case of a nuclear accident, the concentration of
cesium ions in the surface waters is extremely low (less than
1 ppb). For example, 1 MBq L�1 137Cs is equivalent to
6:8� 10�9 M. Therefore, the prepared PILMs were tested for
their ability to sorb cesium in a solution of natural mineral
water (0.2 L) with cationic concentrations of Ca: 1� 10�3 M,
Mg: 0:65� 10�4 M, Na: 0:75� 10�4 M, K: 0:2� 10�4 M and a
small added radiocesium concentration (1:7� 10�11 M). In
these experiments, the cesium removal for a contact time of
1.5 h was measured to be around 75� 2% for all the four
pillared samples (20 mg). Therefore, either the d001 interlayer
spacing or the interpillar distance had no effect in cesium
sorption from dilute concentrations.
3.2.2. Cesium sorption kineticsDifferent kinetic models such as the Lagergen’s pseudo-first-
order equation, second-order equation and Elovich equation
were tested to find out which model is in agreement with the
experimental results of the kinetic study. On comparison, the
pseudo-first-order rate equation yielded the best results for
cesium sorption on PILMs. It can be represented as
dqdt¼ k1 � ðqe � qÞ. (1)
Integrating Eq. (1) for the boundary condition qt ¼ 0 at t ¼ 0,
the equation becomes
qt ¼ qe � ð1� e�k1�tÞ. (2)
The rate constant k1 (min�1Þ and the equilibrium capacity,
qe ðmmol g�1Þ were determined as free parameters by a ‘‘best-
fit’’ minimization of the corresponding w2 function. The
kinetic isotherms and their corresponding fitting results in
Fig. 2 showed that Csþ sorption in PILMs is a fast process and
is accomplished in less than 30 min. The rate constant was
the same for the different PILMs while the equilibrium
capacity was higher for the Al2MFD15 material due to a
higher restoration of the cation-exchange capacity of the
initial NaM material. A similar behavior was also observed in
kinetics conducted in the presence of the competing cations
of Kþ or Ca2þ.
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Fig. 2 – Cesium (1 mM) sorption kinetics by Al1MFD3 andAl2MFD15 (0:5 g L�1).
3.2.3. Effect of pHThe influence of pH on the sorption of cesium was examined
in the pH range from 3.0 to 8.0. As shown in Fig. 3, the
percentage of sorbed cesium increased with increasing pH
with a plateau region from around 4.5 to 8.0. The decrease in
the Cs uptake observed at low pH values can be partly
attributed to the positive charge, which develops on the PILM
edges according to the process noted by Avena et al. (1990)
SO� þHþ ! SOH and SOHþHþ ! SOHþ2 where S stands for
any surface site.
However, it has previously been reported (Delgado et al.,
1986) that the clay sheets of montmorillonites remain
negatively charged at pH values down to 3, since the positive
charge of the edges does not exceed the charge of the
sheets. It can therefore be concluded that the Cs uptake at
acidic pH values is suppressed, because the remaining
negative charge of the PILM is preferentially compensated
by H3Oþ ions.
Hydronium ions, although monovalent, behave mostly
as di or trivalent ion. The same behavior is also usually
observed in non-pillared exchanging clays (Grim, 1968). In
contrast, the observed increase of cesium sorption with
increasing pH can be attributed to the decrease of the
competition of the hydronium ions for PILMs’ sites at higher
pH values.
3.2.4. Cesium sorption isothermsEquilibrium sorption studies were performed to determine
the maximum cesium sorption capacity of PILMs in the
availability range of 0.4–10 mmol cesium per gram of PILMs in
200 mL aqueous solution.
Cesium uptake was quantitatively evaluated using the one
or two-site Langmuir model:
qe ¼Xmi¼1
Ki � Ce
1þ Ki � CeQi, (3)
where qe is the adsorption capacity at equilibrium (mg g�1Þ, Ce
is the equilibrium concentration of metal ions in the solution
(mg L�1Þ, m is the number of energetically different sorption
sites, Qi and Ki are the adsorption capacities and binding
strengths of the adsorbed cations, respectively.
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Table 2 – Freundlich and Langmuir isotherm values andparameters for the sorption of Csþ onto Al2MFD15sample
Values and parameters Magnitude
Freundlich
KF (L g�1Þ 1:40� 0:08
1=n 0:23� 0:04
w2 40.53
One– site Langmuir
Q ðmg g�1Þ 82:64� 2:05
K ðL mg�1Þ 0:10� 0:01
w2 5.381
SD(%) 5.68
Two-site Langmuir
Q1 ðmg g�1Þ 18:56� 26:30
K1 ðL mg�1Þ 0:74� 0:85
Q2 ðmg g�1Þ 66:12� 25:32
K2 ðL P mg�1Þ 0:066� 0:034
w2 4.95
SD(%) 2.52
Fig. 4 – Comparison of the Langmuir and Freundlich
isotherms for the sorption of cesium onto NaM and
Al2MFD15 (qe is the adsorption capacity at equilibrium
(mg g�1) and Ce is the equilibrium concentration of metal in
the solution (mg L�1)).
WAT E R R E S E A R C H 41 (2007) 1897– 1906 1901
The Freundlich model was also used as
log qe ¼ log KF þ1n
log Ce, (4)
where qe and Ce as described in the Langmuir model, b and n
are constants related to the energy of adsorption and KF is a
constant related to sorption capacity.
The values of normalized standard deviation (SD(%)) were
calculated using the equation
SDð%Þ ¼ 100�
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiP½ðqexp
e � qcale Þ=q
expe �
2
N� 1
s, (5)
where the superscripts ‘‘exp’’ and ‘‘cal’’ are the experimental
and calculated values with the fitted parameters and N is the
number of measurements.
The values of the isotherm constants in Table 2 were
determined by fitting the above equations to the experimental
data. To assess the different isotherms and their validity to
correlate experimental results, the experimental data and the
theoretical plots for each isotherm are shown in Fig. 4. The
one-site Langmuir represented better the cesium sorption
data than the Freundlich isotherm (Table 2), suggesting the
monolayer sorption, mainly due to ion exchange. The w2 and
(SD(%)) values for the one-site Langmuir model were very low,
indicating a very good mathematical fit. The cesium sorption
capacity of PILMs was determined to be less than that of the
starting clay and this can be attributed to the remaining
charge of aluminum pillars and/or the incomplete cation-
exchange capacity restoration. As it was observed in kinetic
isotherms, the maximum cesium sorption capacity of the
PILM materials was obtained with sample Al2MFD15.
Moreover, the two-site Langmuir model fitted the experi-
mental results even better, indicating the existence of
heterogeneity. The two-site Langmuir constants for the
sorption of Csþ showed that high-affinity sites exist with a
1
low sorption maximum (20%) and low-affinity sites exist witha high sorption maximum (80%). However, the error in the
estimated parameters was high and the exact amount of
different sites could not be safely concluded. Therefore, the
surface heterogeneity of PILMs was further investigated by
varying the initial cesium concentration in the presence of
competitive cations.
3.2.5. PILMs selectivity for cesium sorptionCesium selectivity of the PILM materials was studied for the
binary systems of Csþ=Naþ, Csþ=Kþ and Csþ=Ca2þin several
ionic concentrations from dilute up to highly concentrated
solutions of cesium and competitive cations. In all systems,
materials were initially transformed in the form of the
competing cation.
Assuming the general exchange reaction
zst;AAþ zst;bB 2 zst;AAþ zst;BB (6)
the cesium selectivity was studied through:
A.
Plots of the equivalent fractions of cesium sorbed in PILMs(xCsÞ vs. their equivalent fraction in the solution (xCsÞ at
equilibrium for constant solution ionic strength (INÞ.
B.
Variation of the distribution coefficient Kd defined as theratio of concentrations of Cs sorbed xc, (mol kg�1Þ and in
the solution [Cs] (mol L�1Þ.
C.
Variation of the selectivity coefficient Kc defined asKc ¼ðYAÞ
zst;A
ðYBÞzst;B�ðMBÞ
zst;B
ðMAÞzst;A�ðgBÞ
zst;B
ðgAÞzst;A
, (7)
where YA are the equivalent fraction of ions in the solid,
MA the molality of ions in the solution and gA the activity
coefficients in the liquid phase (Ioannidis et al., 2000).
These were calculated with the extended Debye-Huckel
equation.
Starting from the first representation, the results in Fig. 5
clearly demonstrate that PILMs exhibit a high selectivity for
Csþ sorption over Kþ and much higher over Naþ. By varying
the initial concentration of Cs in the solution (10�8210�4 MÞ, a
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WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 1 8 9 7 – 1 9 0 61902
decrease of the distribution coefficient Kd was observed and
shown in Fig. 6. The variation of Kd with the initial Cs
concentration shows that the adsorption isotherm is not
linear (Staunton and Roubaud, 1997). The decrease of the Kd
coefficient was much higher in the system Cs–Na-PILM than
the Cs–K-PILM and this order was observed at all levels of Csþ,
Naþ or Kþ concentrations. In the same figure, the variation of
the distribution coefficient for two constant cesium concen-
trations as a function of the concentration of the competitive
Naþ or Kþ (2� 10�420:5� 10�1 M) is also included. The slope
of the Kd reduction was greater for K-PILM than Na-PILM for
the two constant cesium concentrations. However, the Kd
parameter is not a sensitive measure of the relative sensitiv-
ities of a material for various cations since the activity
coefficients and the stoichiometry of the sorption process are
not included in the Kd definition (Staunton and Roubaud,
1997). To overcome this weakness, the variation of the
selectivity coefficient Kc as a function of the sorbed cesium
fraction (zCs) in PILMs was studied. As shown in Fig. 7 for the
Fig. 5 – Cs–Na and Cs–K selectivity isotherms on PILMs in
aqueous solutions of total ionic strength of 10�3.
Fig. 6 – Variation of the distribution coefficient Kd as a fun
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sodium form of pillared clays, values of the selectivitycoefficient were high for 1–2% of site occupancy and started
to decrease with increasing Cs concentration and higher
sorption. Similar results were also observed with potassium
as the interlayer exchangeable cation. These results indicate
that energetically less favorable sites become involved with
increasing cesium concentration in the aqueous solution.
Such behavior is typical for heterogeneous materials with
sites of different selectivity (Zachara et al., 2002). In PILMs, the
selectivity values for this small fraction of sites are much
higher than those of the starting NaM material and compar-
able to those found in materials with high cesium selectivity
as ferrocyanides (Lin et al., 2001), a small fraction of the total
sites of illite (Staunton and Roubaud, 1997; Zachara et al.,
2002) and titanosilicates (Solbra et al., 2001). The fraction of
the highly selective sites of PILMs for cesium sorption can be
attributed either to micropores with a suitable opening for
cesium uptake (around 0.4 nm according to Hutson et al.,
1998) or to collapse of the aluminosilicate sheets and creation
of sites similar to illite.
ction of cesium, sodium or potassium concentration.
Fig. 7 – Variation of the selectivity coefficient Kc as a function
of the cesium fraction in PILMs for sodium solution of
1� 10�3 M.
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WAT E R R E S E A R C H 41 (2007) 1897– 1906 1903
In order to clarify the origin of PILMs heterogeneity,
different blocking agents were used to isolate the PILMs’
group sites. Lithium ions were used to block any sites
associated with the clay’s hexagonal cavities. Silver thiourea
ions (AgTUþÞ, formed from a mixture of SCðNH2Þ2 and AgNO3
in a molar ratio of 3:1, were used to mask the regular
interlamellar (planar) sites (Cremers et al., 1988). Moreover, it
has been observed that the crystal violet (CV) cation is
strongly attached at the surface of PILMs since its diffusion
is hindered in PILMs interlammelar space due to steric
reasons (Mishael et al., 1999). Therefore, CV was used to
block the surface and edge sites. Prior to the experiments
with blocking agents all pillared samples were saturated and
transformed in the form of the blocking agent in concentra-
tions of at least 10-fold of PILMs cesium sorption capacity.
Cesium sorption isotherms were measured by equilibrating
the agent-saturated PILMs with cesium solutions of varying
initial concentration in a background ion concentration of
1� 10�3 M.
As seen in Fig. 8, the values of the selectivity coefficient
Cs–Na for the NaM montmorillonite are in agreement with
those usually observed (Staunton and Roubaud, 1997). The
values of the selectivity coefficient Cs–Na for the PILMs
material are much higher than those of the starting clay in
all the cesium concentration range. The values of the
selectivity coefficient Cs–Li follow the same behavior as those
of the Cs–Na selectivity and are almost the same. Therefore,
the high-affinity sites are not related with the hexagonal
Fig. 8 – Variation of the selectivity coefficient Kc as a function
of the cesium fraction in Al1MFD3 for different blocking
agents of 1� 10�3 M.
Table 3 – Copper sorption from aqueous solutions by different
NaM
Cu sorbed (%) 60:5� 2:3
Cu sorbed (meq g�1)/Cs sorption capacityb (meq g�1Þ 0.75
The solution/solid ratio ðV=MÞ ¼ 1 L g�1. pH ¼ 4:8.a Al2MFD15-Un is the sample that the cation-exchange capacity was nob Cs sorption capacity is assumed to be equal to the cation-exchange ca
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cavities of the clay structure. The values of the selectivitycoefficient Cs–Ca for 1–2% of site occupancy are comparable
to the very high values observed in a 0.1% fraction of illite
sites. The silver thiourea cation does not affect the high
affinity sites for cesium sorption but reduces the low affinity
sites in the same manner as in illite (Zachara et al., 2002). The
CV cation reduces all sites by strongly limiting the high-
affinity sites but also blocking (due to the formation of
molecular aggregates on the outer surface of the clay (Mishael
et al., 1999)) the pores entrance and thus reducing the low-
affinity sites as well.
Concluding, the results of the present study provide direct
evidence that the high selectivity of PILMs for cesium sorption
is rather related to their surface and edge sites than to
micropores of less than 0.4 nm opening. This conclusion is
corroborated by the fact that if the selectivity was due to
micropores, the high-affinity sites should account for more
than 15% of the total sites and in accordance to the ratio of
the micropore volume to the total volume (Hutson et al.,
1998). Moreover, an increase in the micropore volume of less
than 0.4 nm opening has been observed with increasing ionic
radius of the post exchange cation (e.g. cesium) (Hutson et al.,
1998). In this case, the cesium loading should lead to higher
amount of pores with opening less than 0.4 nm and subse-
quently higher selectivity. This result was not observed in the
present study. Finally, increasing the pillaring density and
therefore the fraction of micropores, did not result in higher
selectivity coefficients.
3.3. Copper sorption experiments
3.3.1. Copper sorption on different PILMsConsidering the 0.96 nm layer thickness of montmorillonite
and the 0.54 nm diameter of the hydrated copper cation
CuðH2OÞ2þ6 (Kukkadapu and Kevan, 1988), a minimum d001-
spacing of 1.50 nm is necessary for the existence of the
CuðH2OÞ2þ6 species in PILMs interlayer space. Thus, the
hydrated copper ion can enter the interlammelar space of
all the prepared aluminum-pillared montmorillonites. In this
frame, sorption of copper (0.5 mM) on the prepared PILM
materials was studied in aqueous solutions without or with
the presence of competing cations of Hþ, Kþ or Ca2þ in
concentrations similar to that found in drinking water
(1.5 mM).
As seen in Table 3, the Al2MFD15 sample exhibited the
higher sorption ability of all the prepared materials and its
PILMs (total concentration ¼ 0:5 mM)
Al1MFD3 Al2MFD5 Al2MFD15-Una Al2MFD15
42:8� 1:8 67� 2:5 20:7� 1:6 95:1� 2:9
1.47 1.49 1.61
t restored after pillaring.
pacity (CEC) of the sample.
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Fig. 9 – Copper (0.5 mM) sorption kinetics by NaM and
Al2MFD15.
Fig. 10 – Effect of pH on copper sorption (0.5 mM) on NaM
and Al2MFD15.
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sorption capacity was higher than the cation-exchange
capacity of the starting clay material. Assuming the equality
of the maximum cesium sorption capacity and the cation-
exchange capacity of the PILMs materials (Karamanis et al.,
1997), the second row in Table 3 indicates that the amount of
Cu2þ sorbed on PILMs increased with higher pillar density.
Therefore, copper sorption on PILMs involves both a cation-
exchange process in the surface and interlammelar clay sites
and complexation reactions with the pillar oxides. This
conclusion is further corroborated from the observation that
the PILM sample with unrestored cation-exchange capacity,
sorbed 0.21 meq of copper per gram of material. This amount
is mainly bound to pillar oxides since PILMs’ cation-exchange
capacity is drastically reduced after the pillaring process
(Karamanis et al., 1997).
Furthermore, a reduction of 21.3% and 15.3% of sorbed
copper by Al2MFD15 was observed with the addition of
potassium or calcium ions as competitive cations. Both
reductions can be attributed to the reduction of the sorption
fraction of the interlayer sites group due to PILMs preference
for the sorption of monovalent cations as potassium over
divalent as calcium. This result was also observed in cesium
sorption. Finally, the Al2MFD15 sample with a solution to
solid ratio (V=M) of 1 L g�1, removed 99.7% of an initial copper
concentration of 32 mg L�1 in natural mineral water, reducing
in this way the copper level in a value much lower than the
maximum acceptable level of 2 mg L�1.
3.3.2. Copper sorption kineticsCopper sorption on the PILM material with the highest Al/clay
ratio (Al2MFD15) was fast and was accomplished within
20 min (Fig. 9). After testing different models, the copper
sorption by PILMs was better described by the Lagergren
pseudo-first-order equation. The rate constant k1 (min�1) and
the equilibrium capacity, qe ðmmol g�1Þ determined by fitting
Eq. (2) to the experimental results are included in Fig. 9. The
rate constant of copper sorption on the pillared sample was
higher than the starting montmorillonite. This result can be
attributed to the easiness of pores accessibility due to the
three-dimensional structure of PILMs than the blocking of
pores after the initial sorption of copper within the mon-
tmorillonite interlayer space and the subsequent collapse of
the aluminosilicate clay sheets. Concluding, it is clear from
the kinetic measurements that the velocity of transport of
copper from the liquid phase to solid phase is rapid enough
for PILMs application purposes in the treatment of polluted
aqueous solutions.
3.3.3. Effect of pHThe effect of the pH on the copper sorption by PILMs was
studied in the pH region between 3.0 and 6.0. The pH was
limited to values less than 6 because of the formation of
copper hydroxyl species at higher pH as determined by the
visual MINTEQ code (Gustafsson, 2003). As seen in Fig. 10, the
sorption of copper ions increased around 30% with an
increase in pH of the solution from 3.0 to 4.0 and then
remains constant. As it has been observed in similar studies
with PILMs, the increase of pH decreases the competition
between the protons and the metal ions for surface sites and
results in increased metal uptake by the PILM (Manohar et al.,
1
2006). Finally, copper sorption on the Al2MFD15 sample wasmuch higher than the removal by montmorillonite in all the
studied pH range.
3.3.4. Copper sorption isothermsThe equilibrium data for copper sorption covered the
concentration range from 10 to 100 mg L�1 and were subjected
to the sorption isotherms of Langmuir and Freundlich
(Fig. 11). The Langmuir model parameters and the statistical
fits of the sorption data to Eqs. (3) and (4) are given in Table 4.
The one-site Langmuir isotherm appeared to be inadequate to
describe the sorption behavior of PILMs because it under-
estimated sorption at low initial concentrations. The high
SD(%) value of the one-site Langmuir model was dramatically
reduced with the application of the Freundlich model. The
Freundlich model is characterized by 1=n, the heterogeneous
factor, hence it is applicable to sorption on heterogeneous
surfaces, i.e., surface with non-energetically equivalent sites.
The fact that the Freundlich isotherm fits the experimental
data very well can be explained from the heterogeneous
distribution of active sites on the PILM materials. Indeed, the
two-site Langmuir model described copper sorption onto
ARTICLE IN PRESS
Fig. 11 – Comparison of the Langmuir and Freundlich
isotherms for the sorption of copper onto NaM and
Al2MFD15 (qe is the adsorption capacity at equilibrium
(mg g�1) and Ce is the equilibrium concentration of metal in
the solution (mg L�1)).
Table 4 – Freundlich and Langmuir isotherm constants
for the sorption of Cu2þ onto Al2MFD15
Values and parameters Magnitude
Freundlich
KF (L g�1Þ 19:53� 1:02
1=n 0:112� 0:006
R2 0.995
SD(%) 2.78
One-site Langmuir
Q ðmg g�1Þ 32:92� 1:41
K ðL mg�1Þ 0:394� 0:275
R2 0.998
SD(%) 43.84
Two-site Langmuir
Q1 ðmg g�1Þ 23:89� 4:36
K1 ðL mg�1Þ 0:011� 0:004
Q2 ðmg g�1Þ 22:12� 0:50
K2 ðL mg�1Þ 15:97� 1:69
w2 0.07
SD(%) 0.45
WAT E R R E S E A R C H 41 (2007) 1897– 1906 1905
1
PILMs even better. The determined constants showed thathigh and low affinity sites exist with almost the same
capacity and with a value half to the sorption maximum
(0:72 mmol g�1Þ. These two different affinity sites can be
attributed to the pillar oxides and to typical exchange sites
compensating the negative charge of the clay sheets. The last
are restored after the pillaring process. In desorption studies,
the Cu2þ ions were detected in negligible quantities for all
samples when the desorption was carried out using deionized
water. When the supernatant was replaced by CaCl2 or KCl
solutions, very small amounts of Cu2þ were detected in the
solutions. In contrast, by lowering the pH from about 5–6 after
the end of sorption to around 1.8, almost all the sorbed copper
was released in the solution.
4. Conclusion
The results of this study indicate that Al-pillared montmor-
illonites are potential sorbents for the removal of cesium or
copper from aqueous solutions. The most effective pH range
was found to be 4.0–6.0 for the removal of copper and 3.0–8.0
for cesium. The sorption of either cesium or copper follows
the pseudo-first-order sorption reaction while the sorption
isotherms follow the two-site Langmuir model. Sorption
experiments with blocking agents revealed that complemen-
tary to the interlayer clay sites, a small fraction of PILMs sites
exist (1–2%) that is very selective for cesium sorption over
competitive monovalent and divalent inorganic and organic
cations. These sites are related to the surface or edges of the
PILM materials and their fraction is independent on PILMs
d001 spacing or the Al/clay ratio used in their preparation. In
contrast, the Al/clay ratio appears to have a significant effect
on the sorption of copper on PILMs. With increasing Al/clay
ratio, the amount of sorbed copper increases. This indicates
that the sorption of copper involves specific group of high-
affinity sites on the pillar surfaces in addition to the low-
affinity restored clay interlayer sites of the PILM materials.
Therefore, copper sorption should be driven by both a cation-
exchange mechanism and by complexation reactions with
the pillar oxides.
Acknowledgments
This work was partially supported by the Empirikion Founda-
tion. The authors thank Dr. N. Kourkoumelis of the XRD Unit
of the University of Ioannina.
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