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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;g.walker@qub.ac.uk (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

ARTICLE IN PRESS

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

ARTICLE IN PRESS

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

ARTICLE IN PRESS

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

ARTICLE IN PRESS

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).

ARTICLE IN PRESS

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.

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)

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

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

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

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.

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.

ARTICLE IN PRESS

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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