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WAT E R R E S E A R C H 4 0 ( 2 0 0 6 ) 2 6 4 5 – 2 6 5 8
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�Corresponding auE-mail address:
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Sorption of Zn(II), Pb(II), and Co(II) using natural sorbents:Equilibrium and kinetic studies
Yahya S. Al-Degsa, Musa I. El-Barghouthia, Ayman A. Issaa,Majeda A. Khraishehb, Gavin M. Walkerc,�
aChemistry Department, The Hashemite University, P.O. Box 150459, Zarqa, JordanbDepartment of Civil and Environmental Engineering, University College of London, Gower Street, London WCIE 6BT, UKcSchool of Chemistry and Chemical Engineering, Queen’s University Belfast, David Keir Building, Stranmillis Road, Belfast,
Northern Ireland, UK
a r t i c l e i n f o
Article history:
Received 12 April 2005
Received in revised form
15 May 2006
Accepted 18 May 2006
Keywords:
Natural adsorbent
Calcite
Clay minerals
Heavy metals
Adsorption
Kinetics
nt matter & 2006 Elsevie.2006.05.018
thor. Tel.: 44 28 90274172;[email protected] (G.M
A B S T R A C T
Natural Jordanian sorbent (consisting of primary minerals, i.e., quartz and aluminosilicates
and secondary minerals, i.e., calcite and dolomite) was shown to be effective for removing
Zn(II), Pb(II) and Co(II) from aqueous solution. The major mineral constitutions of the
sorbent are calcite and quartz. Dolomite was present as minor mineral and palygorskite
was present as trace mineral. The sorbent has microporous structure with a modest
surface area of 14.4 m2 g�1. pHzpc (pH of zero point charge) of the sorbent was estimated by
alkaline–titration methods and a value of 9.5 was obtained. The sorption capacities of the
metals were: 2.860, 0.320, 0.076 mmol cation g�1 for Zn(II), Pb(II) and Co(II) at pH 6.5, 4.5 and
7.0, respectively. The shape of the experimental isotherm of Zn(II) was of a ‘‘L2’’ type, while
that of Pb(II) and Co(II) was of a ‘‘L1’’ type according to Giles classification for isotherms.
Sorption data of metals were described by Langmuir and Freundlich models over the entire
concentration range. It was found that the mechanism of metal sorption was mainly due to
precipitation of metal carbonate complexes. The overall sorption capacity decreased after
acid treatment, as this decreased the extent of precipitation on calcite and dolomite. The
effect of Zn(II) ions concentration on sorption kinetics was investigated. Kinetic data were
accurately fitted to pseudo-first order and external diffusion models which indicated that
sorption of Zn(II) occurred on the exterior surface of the sorbent and the contribution of
internal diffusion mechanism was insignificant. Furthermore, the sorption rate of Zn(II)
was found to be slow, where only 10–20% of the maximum capacity was utilized in the first
30 min of interaction.
& 2006 Elsevier Ltd. All rights reserved.
1. Introduction
The existence of heavy metals in the aquatic system can be
detrimental to a variety of living species. Many industrial
processes discharge aqueous effluents containing heavy
metals (Allen and Brown, 1995). Heavy metals are non
biodegradable and tend to accumulate in living organisms,
r Ltd. All rights reserved.
fax: 44 28 90974627.. Walker).
causing various disorders for living organisms. Accordingly,
improved and innovative methods of water and wastewater
treatment are continuously being developed to treat water-
containing metals (Bailey et al., 1999). Precipitation and ion-
exchange are the most widely used methods for cleaning
water contaminated with metal pollutants. However these
methods are unable to achieve the standards which are
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Nomenclature
A/V the external sorption area to the total solution
volume, cm2 cm�3
b energy related constant in Langmuir equation,
dm3 mmol�1
C0 the initial concentration of solute in solution,
mmol dm�3
Ce concentration of solute in solution, mmol dm�3
Ct concentration of solute in solution at time, t,
mmol dm�3
k1 the rate constant of pseudo-first order reaction, 1/
min
k2 the rate constant of pseudo-first order reaction, 1/
min g mmol/min
kd, internal diffusion constant, mmol g�1 min�1/2
kf external diffusion constant, cm s�1
kF Ferundlich constant, mmol g�1
m mass to volume ratio, g dm�3
n Ferundlich constant
qe sorption capacity at equilibrium, mmol g�1
qt sorption capacity at time t, mmol g�1
q300 min sorption capacity at 300 min, mmol g�1
Qmax the maximum sorption value, mmol g�1
r.p.m revolution per minute
SSE the sum of square errors squared
WAT E R R E S E A R C H 4 0 ( 2 0 0 6 ) 2 6 4 5 – 2 6 5 82646
recommended by international water standards bodies (Gar-
cia–Sanchez et al., 1999). In recent years, many natural
adsorbents have been investigated for the removal of heavy
metals from water. A review of more than 70 natural and
synthetic adsorbents and their potential uses for metal
removal has been reported (Bailey et al., 1999). Calcite,
magnesite and dolomite (which are classed as carbonate
minerals or secondary soil minerals) were investigated for use
as potential adsorbents for the removal of heavy metals and
radio-active metals from solution (Brady et al., 1999; Zachara
et al., 1991; Papadopoulos and Rowell, 1988; Papadopoulos
and Rowell, 1989). Carbonate minerals are effective in
removing heavy metals with the mechanism of interaction
found to be a combination between ion-exchange and
precipitation on the carbonate surface (Garcia-Sanchez and
Alvarez-Ayuso, 2002; Comans and Middelburg, 1987. Brady
et al., 1999) have effectively applied carbonate minerals for
the removal of Ca2+ Mg2+, UO22+, Am3+, NpO2
2+, and PuO22+ from
solution (Brady et al., 1999). Natural silicate minerals, of
which clays are a typical form, have been also investigated as
potential low-cost sorbents for removing toxic heavy metals
(Bailey et al., 1999; Al-Degs et al., 2003; Yavuz et al., 2003;
Sheta et al., 2003; Coles and Yong, 2002; Sanchez et al., 1999).
In addition to high mechanical strength, natural clay miner-
als have good porosity and high surface area (Sanchez et al.,
1999). Natural adsorbents; silicate minerals, carbonate miner-
als and metal oxides (like SiO2, Fe2O3 and Al2O3) are not
present in soil as pure components, but are usually found in
nature as admixtures (Hajjaji et al., 2001; Frimmel and Huber,
1996). Therefore, the determination of the exact type of
mechanism for metal sorption by complex-adsorbents like
natural adsorbents is a complex procedure (Frimmel and
Huber, 1996).
In Jordan, large deposits of clay minerals have been found
in several locations. These deposits however, are not homo-
geneous. The predominant clay types in these deposits are:
ilite, montmorillonite, muscovite, kaolinite, plygorskite, with
quartz, calcite and dolomite found as minor constituents (Al-
Degs et al., 2003; Mahmoud et al., 2003). In this work the
chemical, mineralogical, and textural characteristics of a new
discovered natural clay deposit were investigated. The
feasibility of Jordanian natural silicate minerals—as low cost
sorbents—for removing Zn(II), Pb(II) and Co(II) from solution
was assessed in this study. Furthermore, the effect of acid
washing the adsorbents on metal sorption was assessed and
the mechanism of metal sorption determined. The sorption
kinetics and the rate limiting step(s) of zinc sorption were
also investigated.
2. Experimental method and procedures
2.1. The sorbent
The sorbent was obtained from the Natural Resources
Authority (Amman, Jordan). The sample was washed with
deionized water several times with constant stirring, to
remove soluble inorganic salts and any adhering materials.
It was observed that the supernatant showed little turbidity
even after leaving the solution overnight. In order to keep the
fine clay particles, the supernatant solution was filtered and
the obtained solid material returned to the original sample.
After drying at 105 1C for 24 h, the sample was crushed, and
separated into different particle size ranges. Sorption tests
were conducted on a size range of o100mm. Most sorption
studies conducted on clays start by the removal of non-clay
minerals (like carbonate minerals and quartz) in order to
concentrate the clay minerals and improve the sorption
properties of the adsorbent. These procedures are employed
either by collecting the small fractions down to 2 mm of the
sample (Hajjaji et al., 2001) or by washing the sample with a
strong acid (Li et al., 2001). In this work, the sorbent was
washed with HNO3 to leach the non-clay materials as follows:
A 12.0-g sample of dried clay deposit was added to 250 ml
distilled water. The mixture was heated to 90 1C, acid
(250.0 cm3 of 0.1 M HNO3) was gradually added to the clay
solution. The mixture was stirred for 2 h. After completion of
acid treatment, the sorbent was thoroughly washed with
deionized water until the pH of the washings remained
constant.
2.2. Physical and chemical characteristics of the adsorbent
The mineral constitution of the adsorbents (raw and treated
samples) was determined using X-ray diffraction techniques
(PANalytical, ExpertPro, equipped with Xlerator detector).
X-ray fluorescence analysis for the adsorbents is shown in
Table 1. The IR-spectra were recorded in the range
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Table 1 – Sorbent characterization
Constituent Naturalsorbent % by
weight
Treatedadsorbent %
by weight
SiO2 15.00 49.75
Al2O3 3.16 9.79
Fe2O3 4.11 8.35
TiO2 0.37 1.06
P2O5 0.22 0.09
CaO 46.85 1.39
MgO 6.26 5.68
K2O 2.00 2.31
Loss on ignition (L.O.I) 22.11 21.58
Density (g cm�3) 0.91 0.67
Surface area (m2 g�1) 14.4 38.2
CEC cmol(+) kg�1 17.8a 40.0a
Total pore volume
(cm3 g�1)
0.10 0.27
Micropore volume
(cm3 g�1)
0.07 0.16
pHZPC 9.5 4.0
a Ammonium displacement method Tan (1995).
WAT E R R E S E A R C H 40 (2006) 2645– 2658 2647
(400–4000 cm�1) for the adsorbents by means of FTIR-spectro-
photometer (Perkin-Elmer, Niclet model). The textural char-
acteristics like surface area (BET method), total pore volume,
micropore volume, and pore size distribution (BJH method)
were all determined using N2-sorption techniques (Nova
4200e, Surface Area and Pore Size Analyser). pHzpc (pH of
zero point of charge) was determined according to standard
alkaline-titration procedures (Coles and Yong, 2002; Babic
et al., 1999). The textural characteristics and the pHzpc are
presented in Table 1.
2.3. Sorption isotherm experiments
A 0.0500 (70.0001) gram of untreated adsorbent of particle size
o100mm was weighted and put into 50.0 cm3 glass bottles.
Metal ion solutions of different concentrations were prepared
from their pure metal salts (ZnSO4, PbNO3.2H2O and CoSO4.7-
H2O) and then transferred into the sample bottles. The
sorption isotherms of Zn(II), Pb (II) and Co (II) were measured,
respectively, at concentration ranges: 0–0.0153 mol dm�3,
0–4.83�10�3 mol dm�3 and 0–2.56�10�3 mol dm�3. The sam-
ple bottles were placed in a shaker bath for 24 h at 2571 1C.
Blank solutions containing no clay were also included. To
prevent the precipitation of metal cations from solution, the
pH of the solution was maintained at pH less than the
pHprecipitation (pHppt) for each single metal (see Table 3). pH
adjustment was employed by adding dilute HNO3 solution.
After attainment of equilibrium, about 10 cm3 sample was
withdrawn and filtered for analysis. The initial and equili-
brium concentration of metal ions were determined by
inductively coupled plasma atomic emission spectrophoto-
metry (Perkin-Elmer 400 series).
The measured liquid phase concentrations were then used
to calculate the sorption capacity, qe (mol g�1 or mmol g�1) of
the sorbent using the following mass balance equation:
qeðmol g�1Þ ¼ ½C0 � Ce�V=m.
where C0, Ce, V, and m are initial metal concentration
(mol dm�3), metal concentration at equilibrium (mol dm�3),
total volume (dm3) and weight (g) of sorbent, respectively. All
the reagents used in this research were of analytical reagent
grade and supplied from Riedel–de Haen chemicals.
2.4. Verification of sorption mechanism
The following sorption tests were employed to verify the
nature of the sorption mechanism. Initially, an exact weight
(0.050070.0001 g) of the dried sorbent (treated and untreated)
was agitated with 50.0 cm3 of metal solution. The initial
concentrations of Zn(II), Pb(II), and Co(II) were 0.01, 0.004, and
0.002 mol dm�3. The pH of the solution was adjusted to pH 5–6
after adding the sorbent using dilute HNO3 solution. The test
bottles were placed in a temperature controlled shaker bath
for 24 h at 2571 1C to attain equilibrium. Blank solutions
containing no clay were also included. The content of
remaining metal was analyzed by atomic absorption spectro-
scopy (Perkin-Elmer 400 series) and Ca(II) by flame emission
spectrometry (AFP Flamephotometer).
2.5. Kinetic experiments
Kinetic studies were carried out in an agitated batch sorption
system, which consisted of a 2.0 dm3 glass vessel, of diameter
0.13 m, filled with 1.7 dm3 of aqueous solution. A six-flat-
blade impeller driven by an electric motor (Heidolph-motors,
Germany) with a speed adjustable from 100 to 700 revolu-
tions/min (r.p.m) was used to achieve homogeneity within the
reactor. This kinetic sorption vessel design has been used by
previous investigators (Cheung et al., 2000).
The effect of initial concentration of Zn(II) on sorption rate
was studied at the following conditions: Initial Zn(II) con-
centration: 11.47, 10.10, 7.20, and 4.05 mmol dm�3. Mass of
sorbent: 3.400 g. Volume of solution: 1.7 dm3. Stirring speed:
500 rpm pH: 7 and particle size: o100mm.
3. Theoretical background
3.1. Sorption isotherms of heavy metals
Two of the most commonly used isotherm theories have been
adopted in this work, namely, the Langmuir and Freundlich
equilibrium isotherm theories. The form of Langmuir equa-
tion can be represented by the following Eq. (1):
qe ¼bQmaxCe
1þ bCe(1)
or in the linear from
Ce
qe
¼1
bQmaxþ
CeQmax
, (2)
where Ce is the equilibrium concentration of remaining metal
in the solution (mmol dm�3). qe is the amount of a metal
adsorbed per mass unit of sorbent at equilibrium (mmol g�1).
Qmax is the amount of adsorbate at complete monolayer
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Inte
nsity
P P C Q
C
P
D
D C CCCC
Q
P
Angle (2θ)
5 10 15 20 25 30 35 40 45 50 55 60 65 70
5 10 15 20 25 30 35 40 45 50 55 60 65 70
Inte
nsity
Q
P
P
P
PQ
Angle (2θ)
(a)
(b)
Fig. 1 – X-ray diffraction (XRD) scans of the sorbent (a) and
acid-treated sorbent (b). C: calcite, Q: quartz. P: palygorskite,
D: dolomite.
WAT E R R E S E A R C H 4 0 ( 2 0 0 6 ) 2 6 4 5 – 2 6 5 82648
coverage (mmol g�1). b (dm3 mmol�1) is a constant that relates
to the heat of adsorption. Freundlich isotherm model has the
following form (Allen and Brown, 1995; Sanchez et al., 1999):
qe ¼ kFCne (3)
or in the linear from
log qe ¼ log kF þ n log Ce (4)
kF (mmol1�n g�1 Ln) represents the sorption capacity when
metal equilibrium concentration equals to 1, and n represents
the degree of dependence of sorption with equilibrium
concentration.
3.2. Sorption kinetic Models
Sorption kinetic models can be divided into two main types:
reaction-based models and diffusion-based models (HO et al.,
2000).
3.3. Reaction-based models
A simple kinetic analysis of Zn sorption can be employed
using pseudo-first-order equation (HO and McKay, 1998):
logðqe � qtÞ ¼ log qe �k1
2:303t. (5)
In addition, a pseudo-second-order equation based on sorp-
tion equilibrium capacity may be written in the form (HO and
McKay, 1998; Chiron et al., 2003):
1qe � qt
¼1qe
þ k2t, (6)
where, qe, qt, k1, k2, and t are the surface concentration
at equilibrium (mmol g�1), surface concentration at time
t (mmol g�1), pseudo-first-order rate constant (1/min), pseu-
do-second-order rate constant (g mmol min�1), and time of
reaction (min). Eqs. (5) and (6) have been applied to many
sorption systems (HO et al., 2000).
3.4. Diffusion-based models
3.4.1. External diffusion modelIf external-diffusion of metal cations (within the diffuse
layers outside the sorbent) is the rate-limiting step then it has
been shown that Eq. (7) can be fitted into sorption data with
some success (Lee et al., 1999):
lnCt
C0¼ �kf
AV
t, (7)
where C0, Ct, A/V, and t are the initial metal concentration,
concentration at time t, the external sorption area to the total
solution volume, and sorption time, respectively. The external
diffusion coefficient kf (cm s�1) can be calculated from the
slope of the straight line obtained from Eq. (7).
3.4.2. Internal diffusion modelWhen the diffusion (internal surface and pore diffusion) of
metal cations inside the sorbent is the rate-limiting step, then
sorption data can be presented by the following equation (HO
et al., 2000):
qt ¼ kdt1=2, (8)
where qt, kd, t are surface concentration of adsorbate at time t
(mmol g�1), internal diffusion coefficient (mmol g�1 min�1/2),
and sorption time (min). The kinetic models outlined above
were applied to kinetic data of Zn(II) sorption on the natural
sorbent.
4. Results and discussion
4.1. Characterization of adsorbent
X-ray diffraction (XRD) analysis confirmed the presence of
calcite 2[CaCO3] and quartz [SiO2] as major minerals as
indicated from their high intensities (Fig. 1a). Dolomite
[CaMg(CO3)2] is present as minor mineral while palygorskite
2[(Mg,Al)5(Si,Al)8O20(OH)2 � 8H2O] is present as trace mineral. A
similar mineral composition was found in Moroccan natural
clay deposits (Hajjaji et al., 2001). The non–clay minerals
(calcite and dolomite) were eliminated after acid treatment as
indicated in Fig. (1b). The removal of non-clay components
increased the concentration of clay minerals like palygors-
kite, muscovite (4[KAl3Si3O10(OH)2) and quartz as indicated in
Fig. (1b). A reduction of 60% was observed in sorbent weight
after acid treatment, which indicates that the percentage of
clay mineral and quartz is about 40% (w/w) in the sample.
Acid treatment of the sorbent has no effect on silica. Silica
has a poor capacity for heavy metals sorption and elimination
of this material requires more complex procedures (Vanloon
and Duffy, 2000).
A chemical assay of the sorbents is shown in Table 1. The
results indicate that the natural sorbent contains a large
fraction of Ca-minerals (which is mainly calcite, from XRD). In
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WAT E R R E S E A R C H 40 (2006) 2645– 2658 2649
is known that CaCO3 minerals are usually present in most
ground and surface soils (Fuller and Davis, 1987). Hajjaji et al.
(2001) reported that carbonate minerals (calcite 38% and
dolomite 12%) are the major minerals present with the
natural Moroccan clay (Hajjaji et al., 2001). The chemical
composition was significantly affected upon acid treatment.
The data indicate that the percentage of silica increased from
15.00% to 49.75% (w/w), with a significant increase the mass
fraction of aluminum and iron. The data also indicate a
reduction in the mass fraction of CaO after acid treatment,
decreasing from 46.85 % to 1.39% (w/w). These data were
expected, as calcium present as CaCO3 or CaMg(CO3)2 is easily
removed by acid washing. Previous research has indicated
that Al(III) ions are easier to release than Si(IV) ions from
silicate adsorbents after acid washing (Suraji et al., 1998).
Therefore, to determine the extent of Al(III) release, an
estimation of Si/Al (w/w) ratio is essential. The Si/Al ratio
for the raw sorbent was found to be 4.2 while that for treated
sorbent was 4.5. Thus it can be concluded that relatively small
amounts of Al(III) ions have released from the sorbent by the
action of the nitric acid. In a similar study, (Flessner et al.,
2001) reported an increase in Si/Al ratio from 7 to 16 after a
long acid treatment of the sample (Flessner et al., 2001).
The acid–base characteristics of the adsorbents and the
pHzpc are reported in Fig. (2). The pHzpc of the natural clay
deposit and after acid treated sample are 9.5 and 4.0
respectively. The pHzpc was determined using a standard
technique (Babic et al., 1999). The pHzpc of natural clay
reported in this work is unusually high when compared to the
literature data (Garcia–Sanchez et al., 1999; Coles and Yong,
2002; Mellah and Chegrouche, 1997). The pHzpc of treated clay
is close to that of natural clay samples free of carbonate
minerals (Garcia–Sanchez et al., 1999). The high shift in pHzpc
after acid treatment is mainly due to the elimination of
alkaline carbonate minerals like calcite and dolomite (Coles
and Yong, 2002).
Infrared techniques have been used by previous researchers
for identification of soil and clay minerals (Gadsden, 1975).
Fig. 3 depicts the IR-spectra of the natural adsorbent. Fig. 3(a)
shows the characteristic bands of calcite at 1428.76, 873.66,
and 712.68 cm�1. The IR peaks appearing at 1798.41 and
2513.52 cm�1 are also an indication of the presence of calcite
and dolomite (Gadsden, 1975). The high intensity of the peak
0123456789
101112
0 4 8 10 12
pH initial
pH f
inal
natural sorbentacid-treated sorbent
2 6
Fig. 2 – Determination of pHzpc of clay and acid treated clay
in 0.1 M NaCl solution.
appearing at 1428.76 cm�1 is an indication of the high content
of calcite in the sample. The strong band at 1029.22 cm�1 (due
to Si–O stretching) is the main characteristic band for quartz.
Quartz also gives two other characteristic bands at 800 and
780 cm�1 (Gadsden, 1975). However, these two bands did not
appear in the spectra. The bands at 515.71 and 468.97 cm�1
are assigned, respectively, to Si–O–Al and Si–O–Si bending
vibrations (Madejova, 2003). These functional groups are
present in silicate minerals like palygorskite which is
identified by XRD in the natural sample (Fig. 1a). The band
at 3424.21 cm�1 is assigned to stretching vibrations of
adsorbed water molecules. Another characteristics band for
bending vibrations of adsorbed water usually appears at
1650–1600 cm�1 as a medium band (Gadsden, 1975). This band
is overlapped by the very strong absorption band of calcite (at
1428.76 cm�1). The stretching vibrations of the surface
hydroxyl groups (Si–Si–OH, or Al–Al–OH) are found at
3543.57 and 3614.29 cm�1 (Hajjaji et al., 2001; Madejova,
2003). The short bands appearing at 2920.00 cm�1 are mainly
attributed to the C–H stretching vibrations of natural organic
matters present in the sample (Hajjaji et al., 2001). Fig. 3(b),
depicts the IR-spectra of the sorbent after treatment with
HNO3 solution. It is obvious that the characteristic bands of
calcite and dolomite have completely disappeared. The
frequency of most remaining bands have changed slightly,
this is attributed to the action of the acid. This also indicates
that the acid treatment had negligible affect on the structure
of silicate materials present in the adsorbent. Fig. 3b indicates
a new band in the spectra, 1630.34 cm�1, which can be
attributed to the bending vibrations of adsorbed water. It is
to be expected that the porosity and surface area have
increased after treatment, therefore the amount of adsorbed
water is also likely to have increased.
4.2. Sorption characteristics of N2 sorption on the sorbent
The textural characteristics and pHzpc of the sorbent are given
in Table 1. The sorbent has a relatively low surface area when
compared to the surface areas of pure clay adsorbents
(Sanchez et al., 1999). It was noted that that the sorbent is
mainly microporous in structure with microporosity account-
ing for 70% of the total pore volume (Table 1). Fig. 4 shows the
pore size distribution of the natural sorbent and the acid
treated adsorbent. The data indicate that the dv/dr ratio (ratio
of the change in volume to the change in radius) doubles
(from 0.04 to 0.08 cm3 g�1 nm�1 at about 2 nm pore radius)
after acid treatment. It is evident form Fig. 4 that both
adsorbents are microporous with the average pore radius for
both approximately 2.0 nm.
4.3. Sorption isotherms of heavy metals
Sorption isotherms of Zn(II), Pb(II) and Co(II) ions are shown
in Fig. 5(a–c). The sorption data were described using the
Langmuir and Ferundlich isotherm models. The results of
these analyses, using linear regression procedures, are shown
in Table 2. The shape of Zn(II) isotherm is of ‘‘L2’’ type, while
that of Pb(II) and Co(II) is ‘‘L1’’ type according to Giles
classification for isotherms (Giles and Smith, 1974).
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Fig. 3 – Infrared spectra. (a) natural sorbent; (b) acid-washed sorbent; (c) the sorbent after Zn(II) sorption.
WAT E R R E S E A R C H 4 0 ( 2 0 0 6 ) 2 6 4 5 – 2 6 5 82650
The shape of the L2 type isotherm (Fig. 5a) for the sorption
of Zn(II) indicated that the data have reached a maximum
value, resulting in the presence of the plateau. This was not
the case for the L1 type curves, of Pb(II) and Co(II), where only
the initial part of the isotherm is present and the plateau is
not entirely represented (Fig. 5d and c). Another important
difference between these isotherms is that the slope of L2
isotherm is steeper than that of L1 isotherm (Giles and Smith,
1974). L-isotherm type (or Langmuir isotherm type) is usually
associated with ionic substrates (e.g., metal cations) sorption
with weak competition from the solvent molecules (Giles and
Smith, 1974).
4.4. Langmuir isotherm
Table 2 indicates that Langmuir model has a limited applica-
tion for Pb(II) sorption with a regression coefficient, r2¼ 0.819.
A better description for Co(II) sorption data was evident
(r2¼ 0.910). The best fit to the Langmuir model was obtained
for Zn(II) adsorption, with a correlation coefficient, r2¼ 0.984.
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ARTICLE IN PRESS
0
0.02
0.04
0.06
0.08
0.1
0 10
Pore Radius (nm)
dv/d
r
Acid-treated adsorbentNatural adsorbent
1 2 3 4 5 6 7 8 9
Fig. 4 – Pore size distribution (BJH method) of the natural
sorbent and the acid-treated adsorbent.
0
1
2
3
0 4 10 12
Ce (mmol/dm3)
qe (
mm
ol/g
)
Zn(II)
0
0.1
0.2
0.3
0 3
Ce (mmol/dm3)
qe (
mm
ol/g
)
Pb(II)
0
0.03
0.06
0.09
0 0.5 1.5 2
Ce (mmol/dm3)
qe (
mm
ol/g
)
Co(II)
1 2 4 5
2 6 8
1
(a)
(b)
(c)
Fig. 5 – Sorption isotherms of metals at 25 1C: (a) Zn(II); (b)
Pb(II); and (c) Co(II).
WAT E R R E S E A R C H 40 (2006) 2645– 2658 2651
Using the Langmuir model, the maximum sorption capacity
for the metals can be estimated as: Zn (2.860 mmol g�1
or 187.0 mg g�1)4Pb (0.320 mmol g�1 or 66.2 mg g�1)4Co
(0.076 mmol g�1 or 4.5 mg g�1).
4.5. Freundlich isotherm
Table 2 indicates that there is a slight deviation from linearity
using the Freundlich isotherm model for describing Pb(II) and
Co(II) sorption (r2¼ 0.937 and 0.984 for Pb(II) and Co(II),
respectively). The model gives a poor presentation for Zn(II)
sorption behavior (r2¼ 0.850). Freundlich parameters (kF and
n) indicate whether the nature of sorption is either favorable
or unfavorable (Frimmel and Huber, 1996). The intercept is an
indicator of sorption capacity and the slope is an indicator of
sorption intensity. A relatively slight slope n� 1 indicates
that sorption intensity is good (or favorable) over the entire
range of concentrations studied, while a steep slope (n41)
means that sorption intensity is good (or favorable) at high
concentrations but much less at lower concentrations (Frim-
mel and Huber, 1996; McKay, 1980). A high value of the
intercept, KF, is indicative of a high sorption capacity (McKay,
1980). In the three sorption systems, n values are all less than
unity which indicates that sorption intensity is good (or
favorable) over the entire range of concentrations studied.
The 1/n values for the three systems studied fall in the
range: 2–10, which again indicates a favorable sorption
process (McKay, 1980). The kF values, reported in Table 2,
can be used to indicate the relative sorption capacity of the
system (Mohan and Singh, 2002). It was noted that kF values
show the same trend as that of Qmax for the metals studied,
see Table 2.
4.6. Interaction of metals with Calcite (CaCO3)
The sorbent used in this study has a number of constituents,
the presence of which, may explain the variation in the metal
adsorptive capacities. In terms of electronegativity, conven-
tional theory states that metals of higher electronegativity
should adsorb more readily (McBride, 1994). This convention
is partially observed where Zn(II) and Pb(II) show a higher
adsorptive capacity than Co(II). However, Zn(II) shows a
higher adsorptive capacity than Pb, which possesses a greater
electronegativity (electronegativities of metals are presented
in Table 3). Furthermore, the conventional sorption theory on
solid surfaces would suggest that metals of higher hydrolysis
constants have increased adsorptive capacity (McBride, 1994).
The reverse of this convention was observed in this study,
Pb(II) adsorbed less than Zn(II) (hydrolysis constants are
presented in Table 3).
As calcite is a principal component of the adsorbent, the
differences between metal sorption capacities may be due to
their affinity to the surface of calcite.
The theoretical analysis of metal sorption on calcite surface
is complex. However, the following generalisations can be
considered as guidelines for prediction of metal sorption on
calcite surface (Papadopoulos and Rowell, 1988; Papadopoulos
and Rowell, 1989): (a) ionic radius of M(II) cations; metals of
ionic radius close to that of Ca(II) adsorb stronger (i.e.,
stronger displacement) than other metals; (b) solubility of
the carbonate complexes formed after sorption of M(II);
metals that form less soluble complexes with carbonate
adsorb stronger than metals which form more soluble
complexes. The first convention can give a reasonable
explanation for the higher capacity of Zn(II) and Pb(II)
Page 8
ARTICLE IN PRESS
Table 2 – Langmuir and Freundlich model parameters
Metal b dm3 mmol�1 Qmax mmol g�1 Qmax mg g�1 R2 kF n 1/n r2
Zn(II) 0.785 2.860 187.0 0.984 1.05 0.50 2.0 0.850
Pb(II) 0.622 0.320 66.2 0.819 0.12 0.50 2.0 0.937
Co(II) 1.650 0.076 4.5 0.910 0.04 0.47 2.1 0.984
Note: Unit of kF is mmol1�n g�1 Ln Chen et al. (1999).
WAT E R R E S E A R C H 4 0 ( 2 0 0 6 ) 2 6 4 5 – 2 6 5 82652
compared to Co(II). The ionic radius for Co(II) is much smaller
than Ca, while the ionic radii of Zn(II) and Pb(II) are much
closer to that of Ca(II) (ionic radii are presented in Table 3).
However, the convention does not explain the preferential
sorption of Zn(II) compared to Pb(II) where the latter has a
radius more similar to Ca(II). According to the second
convention, Pb(II) should have the highest sorption capacity
of the metals studied, however, this was not found to be case,
where Zn(II) of higher solubility was adsorbed to a greater
extent than Pb(II) which possesses lower solubility constant
(see Table 3 for metals–carbonate solubility constants). The
data, however, do show some correlation with convention
indicting a high affinity of Pb(II) compared to Co(II) (Table 3).
In general the poor correlation with conventional theories for
the sorption of metals on calcite may be attributed to the
heterogeneous nature of the adsorbent.
4.7. Mechanisms of metals sorption
It was found that the sorbent had a significant ion exchange
capacity as indicated in Table 1. However, the calculated CEC
of the sorbent is 17.8 cmol(+)/kg (or 0.178 mmol(+)/g), which
indicates that the intrinsic ion exchange capacity of the
sorbent, was not high enough to remove large amounts of the
heavy metals via ion-exchange mechanism. As indicated in
Table 4, the sorption value of Zn(II) is 2.75 mmol Zn2+/g. If one
assumed that all Zn(II) ions were removed via ion-exchange
mechanism, then the CEC value should be 5.50 mmol(+)/g
(2.75�2) or more to facilitate the removal of all Zn(II) ions
from solution. This analysis suggests that only 3% of
adsorbed Zn(II) was removed via an ion-exchange mechan-
ism. For the Pb(II) system, the amount of metal removed
(0.30 mmol Pb2+/g or 0.60 mmol(+)/g) is higher than the CEC
value (0.178 mmol(+)/g) which would indicate that precipita-
tion is the predominant removal pathway. However, ion-
exchange mechanisms are also involved in a range of
sorption processes. Even though the retention capacity of
Co(II) (0.08 mmol Co2+/g or 0.16 mmol(+)/g) is relatively close
to CEC value (0.178 mmol(+)/g), the entire sorption process
may involve a precipitation mechanism. It has been shown
that the removal of heavy metals from solution by calcite
surfaces can occur via precipitation mechanisms at high
metal concentrations, and can occur via ion exchange at very
low metal concentrations (Zachara et al., 1991; Papadopoulos
and Rowell, 1989; McBride, 1980) showed that Cadmium Cd(II)
is the metal cation which replaces more easily Ca(II), due to
the similitude of their ionic radii, for the rest of metal cations
ion exchange will occur in much lesser extent. Therefore,
most of the amount of Zn(II), Pb(II) and Co(II) retained by
calcite may be due to a precipitation process. The precipita-
tion of metals on the surface of sorbents or even in solution is
probable considering the dissolution of calcite into solution
under the conditions of this study. Ksp of calcite is 4.5�10�9 at
25.0 1C (Harris, 1995). Based on this, the carbonate (CO32�)
content in solution is equal toffiffiffiffiffiffiffiffiKsp
p¼ 6:71� 10�5 mol L�1.
Also, the initial concentration of Zn(II), Pb(II), and Co(II) are
0.01, 0.004, and 0.002 mol L�1 are, respectively. At these
concentrations, the precipitation of ZnCO3, PbCO3, and CoCO3
is possible where the concentration product ([M2+][CO32�]) is
much higher than the corresponding solubility product
constants Ksp (Table 3). It can be calculated that the formation
of metal hydroxycarbonates (M2(OH)2CO3) is not possible for
these metals under the experimental conditions of this study,
as follows.
Hydrolysis of Zn(II) ions by water can be presented as
(Wulfsberg, 1987):
Zn2þðaqÞH2OðLÞ Ð ZnðOHÞþðaqÞ þHþðaqÞ
pKa1 ¼ 9:6 or Ka1 ¼ 2:5� 10�10� �.
The equilibrium expression can be expressed as:
Ka1 ¼½Hþ�½ZnðOHÞþ�
½Zn2þ�
.
The value of [H+] at equilibrium can be calculated from
pHequi. For Zn(II) sorption system, pHequi ¼ 6.5 or
[H+] ¼ 3.2�10�7 M. Using the values of [H+] and pKa1 in the
equilibrium expression one can obtain
2:5� 10�10¼½3:2� 10�7
�½ZnðOHÞþ�
½Zn2þ�.
The ratio: ½Zn2þ�=ð½ZnðOHÞþ�Þ can be calculated from the above
equation:
½Zn2þ�
½ZnðOHÞþ�¼ 1280.
This indicates that the concentration of free divalent Zn(II) is
much higher than Zn(OH)+ at pH ¼ 6.5. At pHinitial (5.5), the
ratio ½Zn2þ�=ð½ZnðOHÞþ�Þ
� �is much higher and becomes 12,800.
The ratio becomes unity (i.e. 50% of metal concentration is
present as Zn(II) and the remaining is present as Zn(OH)+) at
pH ¼ 9.6. At solution pH higher than 9.6 the concentration of
Zn(OH)+ becomes more than Zn(II) in solution. In light of the
above analysis, the formation of Zn hydroxycarbonate com-
plexes is not possible at pHp6.5. Therefore, the following
reactions are not possible under the experimental conditions
employed due to the concentration of Zn(OH)+ being insig-
nificant:
ZnðOHÞþðaqÞ þ CO2�3 ðaqÞ Ð ZnðOHÞCO�3 ðaqÞ
Page 9
ARTICLE IN PRESS
Ta
ble
3–
Ion
icp
rop
ert
ies,
solu
bil
ity,
an
da
cid
ity
con
sta
nts
for
the
stu
die
dm
eta
ls
Meta
lIo
nic
rad
ius
(pm
)aE
lect
ron
ega
tiv
ity
(Pa
uli
ng)a
pK
sp
bfo
rM
CO
3
pK
sp
cfo
rM
(OH
) 2
pH
pp
tdp
Ka
1e
pH
eq
uif
Hy
dra
tio
nen
tha
lpy
(kJm
ol�
1)g
Sec
on
dio
niz
ati
on
en
tha
lpy
(kJm
ol�
1)h
Zn
(II)
137
1.6
10.0
15.5
7.1
9.6
6.5
�2044
1734
Pb
(II)
175
1.9
13.1
15.2
7.5
7.0
4.5
�1480
1450
Co
(II)
125
1.0
10.0
14.9
7.8
9.6
7.0
�2054
1644
Ca
(II)
197
–8.3
––
––
––
aD
ata
were
tak
en
fro
mM
cMu
rry
an
dFa
y(1
995).
pm
:p
ico
mete
r.b
For
the
rea
ctio
n:
MC
O3ðsÞÐ
M2þða
qÞþ
CO
3�ða
qÞ
Ha
rris
(1995).
cFo
rth
ere
act
ion
:MðO
HÞ 2ðsÞÐ
M2þða
qÞþ
2O
H�ða
qÞ
Ha
rris
(1995).
dp
Ha
fter
wh
ich
ap
reci
pit
ati
on
by
OH�
ion
so
ccu
rs.
Th
ese
va
lues
were
calc
ula
ted
fro
mth
efo
llo
win
geq
ua
tio
n:
pH
pp
t¼
14�
log
ffiffiffiffiffiffiffiffiffiffi
M2þ
Ksp
s
Ksp
are
giv
en
inth
eta
ble
(co
lum
n5),
M2+
are
the
meta
lsco
nce
ntr
ati
on
s(i
nm
old
m�
3).
pH
pp
tva
lues
of
Zn
(II)
an
dP
b(I
I)w
ere
calc
ula
ted
at
init
ial
con
cen
tra
tio
ns
of
0.0
153
an
d4.8
3�
10�
3m
olL�
1.
For
Co
(II)
ion
s,p
Hp
pt
wa
sca
lcu
late
da
t2.5
6�
10�
3m
olL�
1.
eFo
rth
ere
act
ion
:M
2þða
qÞþ
H2OðLÞÐ
MðO
HÞþða
qÞþ
Hþða
qÞ
Wu
lfsb
erg
(1987).
fp
Hva
lues
aft
er
est
ab
lish
men
to
feq
uil
ibri
um
.p
Ho
fm
eta
lso
luti
on
befo
rea
dd
ing
the
sorb
en
t(p
Hin
itia
l)a
re5.5
,3.5
,a
nd
6.0
for
Zn
(II)
,P
b(I
I),
an
dC
o(I
I),
resp
ect
ively
.g
Da
tao
bta
ined
fro
mW
ulf
sberg
(1987).
hD
ata
ob
tain
ed
fro
mW
ulf
sberg
(1987).
WAT E R R E S E A R C H 40 (2006) 2645– 2658 2653
Page 10
ARTICLE IN PRESS
Ta
ble
4–
Va
lues
of
(M/C
a)
rati
os
for
sorp
tio
nsy
stem
sa
Sy
stem
Zn
(II)
sorb
ed
(mm
olg�
1)
Ca
(II)
—re
lea
sed
aft
erZ
nso
rpti
on
(mm
olg�
1)
Zn
/Ca
rati
oP
b(I
I)so
rbed
(mm
olg�
1)
Ca
(II)
—re
lea
sed
aft
er
Pb
sorp
tio
n(m
mo
lg�
1)
Pb
/Ca
rati
oC
o(I
I)so
rbed
(mm
olg�
1)
Ca
(II)
—re
lea
sed
aft
erC
oso
rpti
on
(mm
olg�
1)
Co
/Ca
rati
o
Na
tura
lA
dso
rben
t2.7
51.8
01.5
30.3
00.2
71.1
10.0
80
0.0
90.9
Tre
ate
da
dso
rben
t0.5
00.0
150
zero
nd
bn
dze
ron
dn
d
aA
llth
ere
sult
sre
po
rted
inth
eta
ble
are
avera
ged
of
thre
eex
peri
men
ts(7
10
RS
Din
ea
chre
sult
).b
No
td
ete
rmin
ed
.
WAT E R R E S E A R C H 4 0 ( 2 0 0 6 ) 2 6 4 5 – 2 6 5 82654
or
2ZnðOHÞþðaqÞ þ CO2�3 ðaqÞ Ð Zn2ðOHÞ2CO3ðsÞ.
The concentration of Zn(II) is much higher in comparison
with Zn(OH)+ concentration, therefore, the following reaction
is more likely to occur:
Zn2þðaqÞ þ CO2�
3 ðaqÞ Ð ZnCO3ðsÞ.
Adopting a similar analysis for Pb(II) and Co(II) ions, the
following ratios can be determined at equilibrium pH:
½Pb2þ�
½PbðOHÞþ�¼ 320,
½Co2þ�
½CoðOHþÞ�¼ 400.
The formation of metal hydroxycarbonates complexes of
Pb(II) [Pb2(OH)2CO3] or Co [Co2(OH)2CO3] is not possible under
the experimental conditions in this study.
Based on the above analysis, the sorption mechanism of
metals by the sorbent can be represented by the following
surface reactions:
Dissolution of calcite that is present in the sorbent:
�S� CaCO3ðsÞ Ð Ca2þðaqÞ þ CO2�
3 ðaqÞ. (9)
Interaction between the free metal (M2+) in solution and CO32�
in solution:
M2þðaqÞ þ CO2�
3 ðaqÞ Ð MCO3ðsÞ. (10)
Deposition of MCO3 on the sorbent surface:
�S2CaCO3ðsÞ þMCO3ðsÞ ! ½2S2CaCO32MCO3�ðsÞ, (11)
where M2+ represents the free metal cation in solution.
–S–CaCO3 represents the surface active cites for adsorption.
–S–CaCO3–MCO3 represents the metal–carbonate complex on
the sorbent surface.
The above pathway indicates that the formation of MCO3 in
solution (Eq. (10)) will decrease the amount of [CO32�] in
solution and as a result the dissolution of calcite will increase
(Eq. (9)) according to Le Chatelier principle and hence more
metals sorption would be predicted. The new surface created
in Zn(II) sorption system [–S–CaCO3–ZnCO3] was identified by
using IR techniques.
Fig. 3(c) shows the IR spectra of the sorbent after sorption of
Zn(II). The characteristic bands of calcite were greatly affected
after zinc sorption, in that the band at 712.68 cm�1 shifted to
708.42 cm�1 and the bands at 873.60 and 1798.41 cm�1 were
completely removed. The evidence of formation of a new
material on the surface was recognized from the significant
split in the band at 1428.76 cm�1 and the formation of a new
band at 835.10 cm�1. The 1428.76 cm�1 band was split into
1509.41 and 1385.30 cm�1. It was noted that only one
characteristic band for ZnCO3 was detected which is
835.10 cm�1; the other new bands cannot be attributed to
ZnCO3 (Gadsden, 1975). Several studies have indicated that
the sorption of zinc on pure calcite occurs by formation of
hydroxo, and carbonate complexes of zinc within the crystal
structure of calcite (Papadopoulos and Rowell, 1989; Garcia-
Sanchez and Alvarez-Ayuso, 2002). In this work, it has
been shown that Zn(II) sorption on calcite occurs by forma-
tion of ZnCO3 complexes. Regarding Pb(II) and Co(II) sorption
Page 11
ARTICLE IN PRESS
WAT E R R E S E A R C H 40 (2006) 2645– 2658 2655
mechanisms, the obtained IR spectra (not shown) were
identical to the spectra of the sorbent (Fig. 3a) and no new
bands were observed. This would indicate that amount of
new precipitated phases (PbCO3 and CoCO3) are very low with
respect to the amount of mineral sample, and therefore the
intensity of the corresponding bands is not high enough to be
detected.
The data presented in Table 4 suggest that the Zn/Ca ratio is
very high (E50) in the case of the treated adsorbent. This
indicates that other cations such as Mg(II) and K(I) may
exchange with Zn(II) ions. Other adsorption processes, such
as retention by means of terminal OH groups of clays, may
also be involved. The precipitation mechanism is unlikely in
this case due to the absence of calcite. It is proposed that the
primary silicate minerals (muscovite, palygorskite) are re-
sponsible for Zn(II) sorption from solution. Table 4 indicates
the importance of primary minerals (calcite and dolomite) for
metal adsorption. Sorption of Zn(II) has been significantly
affected by the acid treatment process, as the treated sorbent
was ineffective for Pb(II) and Co(II) removal from solution.
4.8. Kinetics of Zn(II) sorption by natural sorbent
As has been shown the metal with the highest adsorptive
capacity was Zn(II), in light of this, kinetic studies and
modeling were undertaken with this particular sorbate. The
assessment of the employed models for fitting the sorption
data was made by calculating the sum of square errors
squared (SSE). Lower values of SSE show better fit to sorption
data and can give an indication of the sorption mechanism
(Cheung et al., 2000; HO et al., 2000):
SSE ¼Xðqt;exp � qt;theoÞ
2, (12)
where qt, exp and qt, theo are the experimental sorption capacity
of Zn(II) (mmol g�1) at time t and the corresponding value
which is obtained from the kinetic models.
4.9. Pseudo-first-order and second-order models
Kinetic data of Zn(II) sorption were analyzed using pseudo–-
first order and second order models (Eqs. (4) and (5)). In order
to use these models, the equilibrium capacity (qe) needs to be
determined. Some researchers (Cheung et al., 2001) have used
a trial and error procedure to determine the value of (qe) that
Table 5 – The results of the solution of Eq. (17) for some sorpt
Initial Zn(II)concentrationmmol dm�3 (Ce)
mass ofadsorbent
(g)
m (g dm�3) q3
mm
11.47 3.400 2.0
10.10 3.400 2.0
7.20 3.400 2.0
4.05 3.400 2.0
q300 min is the sorption capacity at 300 min from the start of the sorption
solutions of Eq. (17).a qe is the capacity assuming that all Zn(II) ions were removed from the
best describes the kinetic data, however, other researchers
have used Langmuir isotherm to determine the value of qe
(Chiron et al., 2003). In this work, the values of qe were
calculated from Langmuir equation with modification of
Eq. (1) shown below.
The equilibrium concentration (Ce) can be presented as in
the following equation:
Ce ¼ C0 � Csurface, (13)
where, C0 and Csurface are, respectively, the initial concentra-
tion (mol dm�3) and the surface concentration at equilibrium
(mol g�1). The ratio of the sorbent mass to the volume of the
solution can be presented as m:
m ¼massðgÞ
volumeðdm3Þ. (14)
The surface concentration (Csurface) is equal to ¼ mqe, accord-
ingly, Eq. (14) becomes:
Ce ¼ C0 �mqe. (15)
Combining Eqs. (1) and (15), Eq. (16) can be obtained:
qe ¼bQmaxðC0 �mqeÞ
1þ bðC0 �mqeÞ. (16)
By rearranging Eq. (16), Eq. (17) is obtained:
bmq2e � ½1þ bC0 þmbQmax�qe þ bQmaxC0 ¼ 0. (17)
Eq. (17) was applied to determine the values of (qe) at the
experimental conditions used in this study. In these systems,
the solution to Eq. (17) is possible and gives reasonable values
for qe. The use of Eq. (17) is novel to this study, however,
similar expressions have been derived for estimating the
equilibrium capacity (qe) for sorption of heavy metals on
grafted silica (Chiron et al., 2003). Table 5 summarizes the
solution (of qe values) obtained from Eq. (17) for this system.
It is noted in Table 5 that sorption mass transfer is required
to cease before the attainment of an equilibrium state, where
qe14q300 min; taking into account that (qe1) is the true
equilibrium capacity. As shown in Table 5, the values of qe2
were higher than the capacities calculated (assuming that all
Zn(II) ions were removed from solution), indicating that qe2
does not represent the true equilibrium capacity. In the
following discussion qe1 is simply referred to as qe.
The effect of Zn(II) ion concentration on sorption rate is
depicted in Fig. 6. The parameters of pseudo–first order model
ion systems
00 min
ol g�1qe1
mmol g�1qe2
mmol g�1qe
a
mmol g�1
0.74 2.40 6.83 5.74
1.26 2.32 6.23 5.05
1.26 2.03 5.06 3.60
0.91 1.41 4.11 2.03
test. qe1 and qe2 are the equilibrium capacities which represent the
solution.
Page 12
ARTICLE IN PRESS
WAT E R R E S E A R C H 4 0 ( 2 0 0 6 ) 2 6 4 5 – 2 6 5 82656
are shown in Table 6. The model adequately fits the data over
the entire course of the experiment, with high correlation
coefficients obtained (Table 6). The values of rate constants
(k1) should be consistent if the whole process is controlled by
a first order mechanism. However, the value of the first order
rate constant varied in this study. This phenomenon has
attributed to the heterogeneous nature of the sorbent surface
(Sparks, 1989). The sorption capacity after 30 min (q30 min)
of interaction was calculated with the results presented in
Table 6. It appears that the process was slow, with 10–20% of
the available capacity achieved in the first 30 min. Further-
more, 30–100% of the sorbent capacity was utilized after
300 min. The analysis of sorption data were recalculated by
neglecting the initial stage of interaction (0–30 min). The
obtained k1 value was close to those reported in Table 6. This
is an indication that the initial stage of sorption was rapid and
has a slight effect on the process. The high correlation of this
model to the sorption data is an indication that Eq. (17) gives
an accurate estimation for the equilibrium capacity.
0.4
0.5
0.6
0.7
0.8
0.9
1
0 50 100 150 200 250 300 350
Time (min)
Ct/
Co
11.47 mM10.10 mM7.20 mM4.05 mM
Fig. 6 – Effect of Zn(II) initial concentration on sorption rate.
Mass of sorbent: 3.400 g, volume of solution: 1.7 dm3,
stirring speed: 500 rpm, pH: 7 and particle size: o100 lm.
Table 6 – Pseudo first and second order model parameters for
C0
(mmol dm�3)mass
(g)ma
g dm�3k1
(1/min)r2 k
g.mmo
11.47 3.400 2.0 0.00138 0.9331 0.0
10.10 3.400 2.0 0.00276 0.9685 0.0
7.20 3.400 2.0 0.00346 0.9842 0.0
4.05 3.400 2.0 0.00507 0.9277 0.0
a Volume of solution in all experiments is 1.7 dm3.b Calculated from Eq. (17).
Table 7 – External and internal diffusion models parameters f
C0 (mmol dm�3) mass (g) kf cm s�1�10�6
11.47 3.400 1.736
10.10 3.400 2.777
7.20 3.400 4.861
4.05 3.400 9.027
The parameters of second order model are shown in Table
6. This model was not as effective as the pseudo-first order
model in describing the kinetic data, with the correlation
coefficients obtained lower than those obtained from the
pseudo-first order model.
4.10. External diffusion model
It is probable that the sorption of Zn(II) occurred only on the
external surface of the adsorbent, it follows therefore, that an
external diffusion model should describe the sorption data.
The parameters for external diffusion model are shown in
Table 7. The external diffusion model shows excellent
correlation with the sorption data, with high correlation
coefficients obtained. This would indicate that the sorption of
Zn(II) is probably a surface process occurring on the exterior
of the sorbent particle. As shown in Table 7, kf values increase
the sorption of Zn(II)
2
l min�1r2 q30 min
(mmol g�1)q300 min
(mmol g�1)qe
b
(mmol g�1)
009 0.9300 0.20 0.71 2.32
021 0.9498 0.25 1.19 2.00
027 0.9327 0.21 1.31 2.0
051 0.9162 0.20 1.12 1.14
or Zn(II) adsorption
r2 kd (mmol g�1 min�1/2) r2
0.9272 0.0811 0.9673
0.9768 0.0813 0.9788
0.9930 0.0930 0.9809
0.9728 0.0856 0.9622
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 10 15 20
t1/2 (min1/2)
qt(m
mol
/g)
11.47 mmol/L4.05 mmol/L
5
Fig. 7 – Plots of internal diffusion model for Zn(II)
adsorption. Mass of sorbent: 3.400 g, volume of solution:
1.7 dm3, stirring speed: 500 rpm, pH: 7 and particle size:
o100 lm.
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Table 8 – The sum of the squares of the errors (SSE) of kinetic models employed for Zn(II) adsorption
Co
(mmol dm�3)mass (g) Pseudo-first
order modelSecond-order
modelExternaldiffusion
model
Internaldiffusion
model
11.47 3.400 0.04 0.09 0.05 0.21
10.10 3.400 0.10 0.11 0.25 0.58
7.20 3.400 0.06 0.69 0.07 1.30
4.05 3.400 0.17 0.38 0.07 3.74
WAT E R R E S E A R C H 40 (2006) 2645– 2658 2657
with a decrease in the initial Zn(II) concentration. For
example, a 5-fold increase was observed in the kf value when
the zinc concentration decreased from 11.47 to 4.05 mM. The
reason for this behavior can be attributed to the lower
competition for the sorption surface sites at lower concentra-
tion. At higher concentrations, the competition for the
surface active sites will be high and consequently lower
sorption rates are obtained.
4.11. Internal diffusion mechanism
Theory indicates that external diffusion is the dominant
process, if the straight line obtained from Eq. (8) does not pass
through the origin (HO et al., 2000). This is shown to be the
case in Fig. 7, indicating the presence of external diffusion at
the earlier stages of interaction. Due to the heterogeneous
nature of the adsorbent, and the presence of active materials,
i.e., calcite and clay minerals, the molecular movement of
Zn(II) deep inside the sorbent particles is unlikely. However,
the process of migration of Zn(II) ions inside the sorbent can
not be totally excluded. The linear plots shown in Fig. 7
indicate the presence of internal diffusion with this mechan-
ism involved in the process at t425 min. Prior to this, the
process appears to be entirely controlled by an external
diffusion mechanism. The external diffusion model shows
good correlation with the sorption data at t425 min, where
the obtained correlation coefficients for all systems are higher
than 0.95 as shown in Table 7. It is noted that the changes in kf
values, due to variations in experimental conditions, are
much greater than those in kd values. This would indicate
that external diffusion was the controlling mechanism during
Zn(II) adsorption.
4.12. The best-fit model
The empirical models employed in this study (Eqs. (5)–(8))
fitted the experimental adsorption data with a high degree of
correlation. For better model assessment, the sum of the
square errors squared (SSE) values for each model were
compared. It is assumed that the model which gives the
lowest SSE value is the best model for this system and that
the mechanism of sorption can be explained based on that
model (Cheung et al., 2001). The SSE values were summarized
in Table 8. It is interesting to notice that the pseudo-first order
and the external diffusion models are much better than the
other two models for representing the kinetics of Zn(II)
sorption. This in fact gives further evidence that the reduction
in aqueous phase Zn(II) by the adsorbent is a surface process
and that the contribution of internal diffusion is less
significant under the studied experimental conditions.
5. Conclusions
Natural Jordanian sorbent containing silicate and carbonate
minerals is an effective sorbent for removing Zn(II), Pb(II), and
Co(II) ions from solution. The equilibrium sorption capacities
of the metals were: 2.860, 0.320, 0.076 mmol cation g�1 for
Zn(II), Pb(II) and Co(II) at pH 6.5, 4.5 and 7.0, respectively. Acid-
treatment of the sorbent reduces the sorption capacity with
this attributed to elimination of carbonate minerals (calcite
and dolomite) from the adsorbent. It was found that the
mechanism of metal sorption is mainly precipitation as metal
carbonate complexes. Furthermore, the natural sorbent is
especially suited to retaining Zn, as Zn is generally considered
to be more mobile than Pb. The rate of Zn(II) sorption was
found to be slow, with only 10–20% of available sorbent
capacity utilized in the first 30 min of interaction. Kinetic data
showed good correlation to a pseudo-first order and external
diffusion models which indicated that sorption of Zn(II)
occurred on the external surface of the sorbent with internal
diffusion less significant under the experimental systems
investigated.
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