1. No category

Applied Clay Science 85 (2013) 74–79
Contents lists available at ScienceDirect
Applied Clay Science
journal homepage: www.elsevier.com/locate/clay
Research paper
Adsorption and diffusion of Pb(II) on the kaolinite(001) surface:
A density-functional theory study
Man-Chao He a, Jian Zhao a,⁎, Shuang-Xi Wang b
a
b
State Key Laboratory of Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Beijing 100083, China
State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
a r t i c l e
i n f o
Article history:
Received 8 November 2012
Received in revised form 15 August 2013
Accepted 30 August 2013
Available online 6 October 2013
Keywords:
First-principles calculations
Kaolinite
Pb(II)
Adsorption
Diffusion
a b s t r a c t
The adsorption and diffusion of Pb(II) atom on the hydroxylated (001) surface of kaolinite were investigated
using density-functional theory within the generalized gradient approximation and a supercell approach. The
coverage dependence of the adsorption structures and energetics was systematically studied for a wide range
of coverage Θ [from 0.11 to 1.0 monolayers (ML)] and adsorption sites. The most stable among all possible
adsorption sites was the two-fold bridge site followed by the one-fold top site, and the adsorption energy
increased with the coverage, thus indicating the higher stability of surface adsorption and a tendency to
the formation of Pb(II) islands (clusters) with increasing coverage. Moreover, the energy barrier for diffusion of Pb(II) atom between the one-fold top and the two-fold bridge adsorption sites on kaolinite(001)
surface was 0.23 (0.31) eV, implying that the Pb(II) atom is prone to diffusing on kaolinite(001) surface.
The other properties of the Pb(II)/kaolinite(001) system including the different charge distribution, the lattice relaxation, and the electronic density of states were also studied and discussed in detail.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Heavy metal pollution is currently of great concern because it has
been recognized as a potential threat to air, water, and soil (Jiang
et al., 2010; Zhang and Hou, 2008). The toxicity of heavy metals is
enhanced through accumulation in living tissues and consequent biomagnification in food chain, thus finally brought serious impacts on
human health. Lead (Pb) is one kind of heavy metal elements with
widespread availability, including the use of leaded gasoline, industrial
sources such as lead mining, smelting and coal combustion, and the use
of lead-based paint and lead-containing pipes in water supply systems.
Acute lead poisoning usually affects the gastrointestinal track, or the
nervous system, and sometimes both. Therefore, experimentally many
investigators have studied the removal of heavy metal Pb(II) using
chemical precipitation or physical treatment, such as ion exchange,
solvent extraction, reverse osmosis, and adsorption (Kömider, 2010;
Zhao et al., 2011). Specially, natural clay minerals as the adsorbent
with a low cost have received much attention on heavy metals adsorption from contaminated water and soil (Churchman et al., 2006; Gu and
Evans, 2008; Gu et al., 2010; Gupta and Bhattacharyya, 2005). Unfortunately, however, the microscopic adsorption and diffusion mechanism
of Pb(II) atom on natural clay mineral surface have not yet been derived
(Coles and Yong, 2002; Du et al., 2011). It remains in experiment
unclear, for example, what the adsorbed Pb(II) bonding properties
⁎ Corresponding author.
E-mail address: [email protected] (J. Zhao).
0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.clay.2013.08.045
and charge density are on clay mineral surface. Computer simulation
based on the density-functional theory (DFT) has been proven to be a
powerful and reliable tool to study Pb(II)–solid interfaces at the molecular level. Kaolinite is one of the more highly weathered clay minerals. It
is common in tropical soils and is the second most abundant clay mineral in ocean sediments (Coles and Yong, 2002; Grim, 1968). Greater
insight into the Pb(II) atom adsorption on kaolinite surface, through
detailed first-principles calculations, is needed.
Existing experimental (Adams, 1983; Benco et al., 2001; Bish, 1993)
and theoretical data (Hayashi, 1997; Hess and Saunders, 1992; Hobbs
et al., 1997; Hu and Angelos, 2008; Plançon et al., 1997; Teppen et al.,
1997) for the kaolinite Al2Si2O5(OH)4 surface are often rationalized by
modeling two surfaces as almost perfect 1:1 layer structures consisting
of two different surfaces of aluminosilicate. One side consists of a
gibbsite type sheet where Al ions are coordinated octahedrally by
oxygen ions and hydroxyl groups; the other side of the layer consists
of a silica sheet in which Si ions are coordinated tetrahedrally by only
oxygen ions. Quantitative estimates indicate that there is a certain
degree of van der Waals attraction and hydrogen bonding between
the hydroxyl groups of the gibbsite sheet and the oxygen atoms of
the adjoining silica sheet (Sato et al., 2005). While the silica-sheet
side is saturated and hydrophobic, the hydroxyl at the Al (oxyhydroxide)
side is hydrophilic. The (001) (basal) surface of kaolinite is that which is
mainly exposed. In particular, the hydroxylated (001) surface is of primary interest in adsorption and diffusion studies. In this paper, the
first-principles DFT investigation of Pb(II) adsorption and diffusion on
the (001) surface of kaolinite was performed.
M.-C. He et al. / Applied Clay Science 85 (2013) 74–79
2. Method of calculations
The calculations were performed using the local-density approximation (LDA) as implemented in the Vienna ab-initio simulation
package (Kresse and Furthmuller, 1996). Projector augmented
wave pseudopotentials (Blöchl, 1994) and plane waves were used.
The energy cutoff for the plane-wave basis was 400 eV, which is
large enough to make the error from calculation of the adsorption
energies and activation barriers below 0.02 eV. The so-called repeated slab geometries were applied. The kaolinite(001) surface was
modeled by a slab composing of six atomic layers and a vacuum
region of 20 Å, which was found to be sufficiently convergent. The
Pb(II) atom was adsorbed and diffused on one side of the slab.
During geometry optimizations, all the hydrogen, oxygen and aluminum atoms in the outermost three layer (octahedral aluminum
oxide surface), as well as the Pb(II) atoms, were allowed to relax
while the rest three atomic layers (including the middle oxygen
and hydrogen atoms, the bottom silica and oxygen atoms) of the
slab were kept fixed at their calculated bulk positions. If not mentioned
differently, a (3 × 3 × 1) k-point grid for p (2 × 2) and p (3 × 3)
surface cell with Monkhorst–Pack scheme was used. A Fermi broadening of 0.02 eV/Å was chosen to smear the occupation of the bands
around EF by a finite-T Fermi function and extrapolate to T = 0 K. In
the present paper, the calculations for Pb(II) atoms in the twelve adsorption sites including three onefold top sites (T1–T3), three twofold
bridge sites (B1–B3), and six threefold hollow sites (H1–H6) depicted
in Fig. 1 have been performed for coverage ranging from 0.11 to
1.0 ML. Specially, the Pb(II) coverage of 0.11 and 0.33 ML was calculated
using p (3 × 3) surface unit cell, while the coverage of 0.25, 0.5, 0.75, and
1.0 ML was calculated in the p (2 × 2) surface cell. The calculated lattice
parameters a = 5.155 Å, b = 5.155 Å, c = 7.405 Å, α = 75.14°, β =
84.12°, and γ = 60.18° were used throughout the study, in good agreement with the experimental lattice parameters reported previously
(Brigatti et al., 2006; Kresse and Joubert, 1999).
3. Results
One central quantity tailored for the present study was the adsorption energy of the Pb(II) atom on kaolinite substrate,
which is defined
as Eads ðΘÞ ¼ N1Pb NPb EPb þ Esubstrate −EPb=substrate . Here NPb is the total
number of Pb(II) adatoms present in the supercell at the considered
coverage Θ (Θ was defined as the ratio of the number of adsorbed
atoms to the number of atoms in an ideal substrate layer). EPb/substrate, Esubstrate, and EPb are the total energies of the slabs containing Pb(II) of
the corresponding clean kaolinite surface, and of a free Pb(II) atom,
75
respectively. According to this definition, a positive value of Eads indicated that the adsorption was exothermic (stable) with respect to a free
Pb(II) atom and a negative value indicated endothermic (unstable) reaction. The coverage regime of Pb(II) atom adsorption on kaolinite(001)
was 0 b Θ ≤ 1.0. All the three kinds of high-symmetry adsorption sites
on (001) surface were considered. It turned out that upon optimization,
the Pb(II) atom that was initially put on the threefold hollow sites
would relax to the neighboring top or bridge sites, which have proven
to be the stable sites for Pb(II) atom adsorption on kaolinite(001)
surface as shown in Fig. 2. During relaxation, the hydroxyls of surface
neighboring Pb(II) atom turned to parallel the surface from being perpendicular or having an angle with the surface initially. The adsorption
on top (T1–T3) and bridge (B1–B3) sites were energetically preferred
within observable energies about 1.58 and 1.89 eV/Pb, respectively.
The calculated adsorption energies Eads of Pb(II) atom on these two
kinds of sites with respect to the free atomic Pb(II) were summarized
in Table 1 for different Pb(II) coverage. One can see that the bridge
site was more stable than the top site and the adsorption energy
increased with Pb(II) coverage for all the two kinds of adsorption
sites, which indicated a prominent attraction among the ad-Pb(II) and
implied a tendency to form Pb(II) islands or clusters on the kaolinite(001) surface. In addition, interestingly, the adsorption energy difference between the top and bridge sites displayed noticeable increases
with Pb(II) coverage, which implied a substrate-induced anisotropy in
the lead–solid chemical bonding. Table 2 presented the calculated
results for the relaxed atomic structure, including the height hPb–H
of Pb(II) above the surface, the Pb(II)–O bond length Ra, and the topmost interlayer relaxation Δd12 for various coverage with Pb(II)
atoms on the top and bridge sites. The Δd12 is calculated from
Δd12 = (d12 − d0)/d0, where d12 and d0 are the depth between the
first and second layer of the relaxed surface and the corresponding
depth between the first and second layer of clean kaolinite(001)
surface, respectively. The adsorption of Pb(II) atoms on kaolinite(001) induced notable changes in the interlayer distance of the
substrate. Interestingly, for the bridge adsorption, the value of
Δd12 was negative and increased with increasing Pb(II) coverage,
which meant that the distance between the topmost two atomic
layers of the kaolinite(001) surface was contracted and became
smaller with increasing Pb(II) coverage. On the contrary, for the
top adsorption, the value of Δd12 was positive from 0.0% to 3.92% in
the coverage regime 0.11 b Θ ≤ 1.0, which meant that the topmost
interlayer was expanded and became larger with increasing Pb(II)
coverage. This fact reflected the strong influence of the Pb(II) adsorbates on the neighboring O atoms and thus results from important
redistribution of the electronic structure. The results verified that
Fig. 1. (a)–(c) Top view of kaolinite(001) surface with three top adsorption sites (T1–T3), three bridge adsorption sites (B1–B3) and six hollow adsorption sites (H1–H6). Here, white
spheres, red spheres, yellow spheres, and purple spheres represent hydrogen, oxygen, aluminum and silicon, respectively.
76
M.-C. He et al. / Applied Clay Science 85 (2013) 74–79
Fig. 2. (a) and (b) for top view of adsorbed Pb(II) atom on the top and bridge sites of kaolinite(001). (c) for side view of adsorbed Pb(II) atom on the bridge site of kaolinite(001). Here
adsorbed Pb(II) atom is colored dimgray for clarity.
the Pb(II) adsorption caused the kaolinite(001) outmost layer separation to relax back to be close to its “ideal” bulk value. Concerning the
Pb(II)–O bond length Ra at different Pb(II) coverage, one can see from
Table 2 that for the top and bridge adsorption, the Pb(II)–O bond length
varied around 2.3 Å and 2.4 Å with increasing Θ, respectively. In particular, the calculated results of Ra by using the p (2 × 2) surface model
varied only within an amplitude of 0.03 Å (0.04 Å) for top (bridge)
site. The short bond length Ra implied a strong interaction between
Pb(II) and neighboring O atoms. It is noted that the value of hPb–H for
bridge was slightly shorter than that for top, which was consistent
with the fact that the bridge was more stable than the top site.
In order to gain more insights into the precise nature of chemisorbed
atom state, the electronic partial density of state (PDOS) of the Pb(II)
atom and the neighboring O atoms was calculated. As a typical example,
the PDOS for the two kinds of stable adsorption configuration of B3 and
T3 was plotted. For comparison, correspondingly the PDOS of the free
Pb(II) atom and the neighboring O atoms of clean kaolinite(001) was
also calculated (see Fig. 3). Three-dimensional (3D) electron density difference ρ(r), which was obtained by subtracting the electron densities
of noninteracting component systems, ρkaolinite(001)(r) + ρPb(r), from
the density ρ(r) of the Pb(II)/kaolinite(001) surface, while retaining
the atomic positions of the component system at the same location as
in Pb(II)/kaolinite(001), was also shown in the insets to Fig. 3(b) and
(d), respectively. Positive (blue) ρ(r) indicated accumulation of electron
density upon binding, while a negative (yellow) one corresponded to
electron density depletion. After Pb(II) atom adsorption on top site of
kaolinite(001), the amplitudes of s, p, and d bonding orbitals were
weaker than that in free Pb(II) atom. Especially, the s and p electronic
states of adsorbed Pb(II) expanded in energy comparing with free
Pb(II) (Fig. 3a and b). Furthermore, for adsorbed Pb(II) states, appeared
some new peaks, aligning in energy with the s and p bonding orbital of
adsorbed neighboring O atom of kaolinite(001). One can clearly see
from Fig. 3(c) that after adsorption, the s and p bonding orbitals of
neighboring O atom shifted down in energy by 2.43 eV and 1.96 eV,
respectively. These features are essentially caused by the different electronegativities of kaolinite and Pb(II) atom, which induced charge redistribution and thus built a global electrostatic attraction between the
Pb(II) and neighboring O atom. The result was substantiated by the 3D
electron density difference. A large charge accumulation existed between the adsorbate and substrate, also one ionic bond was formed,
which donate electrons from surface neighboring O atom to the
Pb(II) atom. The PDOS for the stable adsorption configuration of B3
was also calculated, as shown in Fig. 3(d) and (e). After adsorption,
the amplitudes of all bonding orbitals were much weaker than those
in free Pb(II) and even the adsorbed Pb(II) on top site. The overlap
between adsorbed Pb(II) and neighboring O atoms of kaolinite(001) surface electrons in the energy ranged from − 11.28 eV
to − 3.57 eV. From the 3D electron density difference, which was
shown in the inset of Fig. 3(d), a large charge accumulation existed
between the adsorbate and substrate, also two oxygen bonds were
formed, among which two donate electrons from surface of O
atoms to the Pb(II) atom.
Fig. 4(a) and (b) showed the orbital-resolved PDOS of the Pbtop layer
and the neighboring O at Θ = 0.25 and Θ = 1.0, respectively. The
Fermi energy has been set at zero. At low coverage (Θ = 0.25), the
narrow peaks from −9.55 eV to −7.50 eV with large amplitude denoted Pb(II) s state, which, as shown in Fig. 4(a), mainly hybridized with
the p state of the neighboring O atom, whereas, the hybridization
between Pb(II) s and O s states was negligibly small. With increasing
Pb(II) coverage [Fig. 4(b) for Θ = 1.0], three prominent changes involving the Pb(II)\O chemical bonding occurred. (i) The peaks in the Pb(II)
s and p PDOS were broadened and shifted down in energy by 1.02 eV
and 0.82 eV, respectively. These changes for the Pb(II) s and p PDOS
were due to the fact that at high coverage as Θ = 1.0, the Pb(II) adatom
was highly coordinated, which drove the Pb(II) not only the s states but
also the p states to bond with the p state of the O atoms. (ii) Compared
to case of Θ = 0.25, the hybridization of Pb(II) p and O p states was distinctly enhanced in the case of Θ = 1.0. In particular, the main peak
around E = −5.49 eV in the Pb(II) p PDOS in Fig. 4(b) was a result of
the hybridization between Pb(II) p states and O p states. Since the O p
state lies mainly in the interior of the valence band, thus the Pb p state
Table 2
The calculated adsorbate height (hPb–O), the bond length (Ra), and the interlayer relaxation
(Δd12) for different coverages of atomic Pb(II) adsorption on kaolinite(001) surface.
Coverage Θ
Table 1
The calculated adsorption energy Eads (in eV) as function of atomic Pb(II) coverage on the
different sites of kaolinite(001).
Eads
Site
0.11 ML
0.22 ML
0.25 ML
0.33 ML
0.50 ML
0.75 ML
1.0 ML
Top
Bridge
1.57
1.80
1.59
1.87
1.58
1.89
1.65
2.0
1.92
2.28
2.06
2.41
2.21
2.54
0.11 ML
0.22 ML
0.25 ML
0.33 ML
0.50 ML
0.75 ML
1.0 ML
hPb–O
Δd12 (%)
Ra (Å)
Top
Bridge
Top
Bridge
Top
Bridge
1.34
1.33
1.32
1.30
1.26
1.23
0.22
1.13
1.12
1.10
1.14
1.09
1.07
1.06
2.31
2.31
2.31
2.32
2.34
2.32
2.34
2.41
2.42
2.41
2.39
2.43
2.40
2.38
0.0
1.06
1.06
2.13
2.33
2.86
3.92
−3.19
−2.43
−2.31
−2.26
−1.38
−1.35
−0.75
M.-C. He et al. / Applied Clay Science 85 (2013) 74–79
77
Fig. 3. The PDOS for the Pb(II) atom and the neighboring O atoms bonded to Pb(II), at the stable one-fold top and two-fold bridge adsorption site on surface: (a), (b) and (d) for free,
adsorbed Pb(II) atom on T3 and B3, respectively, where the inset shows the side view of electron density difference for Pb(II) atom at the stable top and bridge adsorption site. (c) and
(e) for the clean and adsorbed kaolinite(001) surface. The Fermi energy is set at zero.
had to shift down in energy to overlap with the O p state. (iii) In fact, one
can see from Fig. 4(b) that in the energy interval of 0.57 eV b E b 1.49 eV
there is a large filling of the Pb p states, which was empty in this energy
region in the case of free [Fig. 3(a)] and low coverage [Fig. 4(a)] Pb
atom. Obviously, this state which transferred toward a lower energy in
increasing adsorption coverage will gain the energy, which overcompensated the energy that was caused by the formation of bonding states between Pb(II) p and O p atomic orbitals. The orbitalresolved PDOS for the on surface Pbbridge layer and the neighboring
O at Θ = 0.25 and Θ = 1.0 were also shown in Fig. 4(c) and (d),
respectively. By comparing with the case of Pbtop, the Pb(II) and O
orbital-resolved PDOS had similar changes in the case of Θ = 1.0.
The peaks in Pb(II) s and p PDOS were broadened and shifted
down in energy by 1.02 eV and 0.82 eV, while the peaks in O s
and p PDOS shifted down in energy 0.30 eV and 1.02 eV, respectively. Some new peaks appeared in Pb(II) p state from − 7.30 eV
to − 4.54 eV in Fig. 4(d), aligning in energy with the s and p bonding orbital of neighboring O atoms. All of the above results implied
that the adsorption of the surface Pbbridge and Pbtop at Θ = 1.0 was
more stable than at Θ = 0.25, respectively, and the adsorption of
Pbbridge was more stable than Pbtop at each corresponding adsorption coverage.
Finally, the diffusion energetics of Pb(II) on kaolinite(001) surface
were investigated and plotted in Fig. 5. Here, the calculation of the diffusion barriers was performed using the nudged elastic band method. The
method is used for calculating the diffusion barrier between two known
minimum-energy sites by optimization of a number of intermediate
images or snapshots of the adatom along the diffusing path (Jonsson
et al., 1998; Monkhorst and Pack, 1976). The difference between the
highest energy and that of the initial binding site is taken as the diffusion barrier (Michaelides et al., 2004; Roehl et al., 2010). The diffusion
path between two neighboring top and bridge sites was modeled
using seven images. The route was investigated by moving a single
Pb(II) from its initial top site to its final bridge site and five linearly interpolation between the initial and final positions. Given a Pb(II) atom at a
top site then sought to investigate how it would negotiate its way across
the (001) surface. The activation energies Ea determined from the
diffusion energy path are investigated. The Ea for a given route was
the energy difference between the highest and lowest energy points
along that route. The activation energy for the route was calculated
to be 0.23 (0.31) eV. It is clear, therefore, that the diffusion path encountered a little bit different activation barriers for Pb(II) to move
from top site to bridge site. Also, it can be seen that the energy profile
displayed in Fig. 5, which was displayed was reasonably symmetric
78
M.-C. He et al. / Applied Clay Science 85 (2013) 74–79
Fig. 4. The PDOS for the top site Pb(II) atoms and the neighboring O atoms bonded to Pb(II) at Θ = 0.25 ML (a) and Θ = 1.0 ML (b), and for the bridge site Pb(II) atoms and the neighboring O atoms bonded to Pb(II) at Θ = 0.25 (c) and Θ = 1.0 (d). The Fermi level is set at zero.
about the highest energy points, which tend to be located at bridge
sites.
4. Summary
In summary, the adsorption of atomic Pb(II) on kaolinite(001)
surface, as well as the energy barriers for atomic Pb(II) diffusion
in these systems through first-principles DFT-LDA calculations
was systematically investigated. A wide range of coverage from
0.11 to 1.0 ML by using different surface models [i.e., p (3 × 3)
and p (2 × 2) surface unit cells] for adsorption in the surface top
and bridge sites were considered. In the coverage range of 0 b Θ
≤ 1, the most stable among all possible pure adsorbed sites as
well as coadsorbed sites is the bridge site, followed by the top
site. The atomic geometry, the charge density distribution, and
the electronic structure upon which the Pb(II) adsorption has also
Fig. 5. Variation in Pb(II) atom adsorption energy from the original top site for the Pb(II) atom diffusion routes on kaolinite(001). The structure of the initial, transition and final states of
pathway is also shown. The solid lines connecting the data points are guides to the eye.
M.-C. He et al. / Applied Clay Science 85 (2013) 74–79
been studied, consistently show the fundamental influence by the
ionic as well as covalent bonding between the Pb(II) adatom and
surface O atoms. Remarkably, this influence in the energetics increased with increasing the Pb(II) coverage, which is highly interesting. The increase in the Pb(II) adsorption energy for the top or
bridge site with Θ in the coverage range (0 b Θ ≤ 1) implies the effective attraction between the Pb(II) adsorbates, which will make it
favorable for the formation of the Pb(II) island or cluster at this
coverage. Furthermore, the surface diffusion path energetics was
calculated. It has been found that the top and bridge sites are the
two local minima. The activation energy for Pb(II) diffusion was
determined to be 0.23 (0.31) eV. The present calculated results
may help understand the microscopic adsorption and diffusion
mechanism of Pb(II) atom on kaolinite(001) surface.
Acknowledgments
This research was supported by the state 973 Program (No2006
CB202200), the Program for Changjiang Scholars and Innovative Research Team in University of China under Grant No. IRT0656, and
the National Natural Science Foundation of China Nos 40972196
and 41172263.
References
Adams, J.M., 1983. Hydrogen atom position kaolinite by neutron profile refinement. Clays
Clay Minerals 31, 352–358.
Benco, L., Tunega, D., Hafner, J., Lischka, H., 2001. Orientation of OH groups in kaolinite
and dickite: ab initio molecular dynamics study. Am. Mineral. 86, 1057–1065.
Bish, D.L., 1993. Rietveld refinement of the kaolinite structure at 1.5 K. Clays Clay Miner.
41, 738–744.
Blöchl, P.E., 1994. Projector augmented-wave method. Phys. Rev. B 50, 17953–17979.
Brigatti, M.F., Galan, E., Theng, B.K.G., 2006. In: Bergaya, F., Theng, B.K.G., Lagaly, G. (Eds.),
General Introduction: Clays, Clay Minerals, and Clay Science of Handbook of Clay Science. Elsevier Ltd, pp. 27–30 (Chapter 1).
Churchman, G.J., Gates, W.P., Theng, B.K.G., Yuan, G., 2006. In: Bergaya, F., Theng, B.K.G.,
Lagaly, G. (Eds.), Clays and Clay Minerals for Pollution Control of Handbook of Clay
Science. Elsevier Ltd, pp. 630–633 (Chapter 11.1).
Coles, C.A., Yong, R.N., 2002. Aspects of kaolinite characterization and retention of Pb and
Cd. Appl. Clay Sci. 22, 39–45.
79
Du, E.L., Yu, S.M., Zuo, L.M., Zhang, J.S., Huang, X.Q., Wang, Y., 2011. Pb(II) sorption on molecular sieve analogues of MCM-41 synthesized from kaolinite and montmorillonite.
Appl. Clay Sci. 51, 94–101.
Grim, R.E., 1968. Clay Mineralogy, 2nd edn. McGraw-Hill, New York.
Gu, X.Y., Evans, L.J., 2008. Surface complexation modelling of Cd(II), Cu(II), Ni(II), Pb(II)
and Zn(II) adsorption onto kaolinite. Geochim. Cosmochim. Acta 72, 267–276.
Gu, X.Y., Evans, L.J., Barabash, S.J., 2010. Modeling the adsorption of Cd (II), Cu (II), Ni (II),
Pb (II) and Zn (II) onto montmorillonite. Geochim. Cosmochim. Acta 74, 5718–5728.
Gupta, S.S., Bhattacharyya, K.G., 2005. Interaction of metal ions with clays: I. A case study
with Pb(II). Appl. Clay Sci. 30, 199–208.
Hayashi, S., 1997. NMR study of dynamics and evolution of guest molecules in kaolinite/
dimethyl sulfoxide intercalation compound. Clays Clay Miner. 45, 724–732.
Hess, A.C., Saunders, V.R., 1992. Periodic ab initio Hartree–Fock calculation of the lowsymmetry mineral kaolinite. J. Phys. Chem. 11, 4367–4374.
Hobbs, J.D., Cygan, R.T., Nagy, K.L., Schultz, P.A., Sears, M.P., 1997. All-atom ab initio energy
minimization of the kaolinite crystal structure. Am. Mineral. 82, 657–662.
Hu, X.L., Angelos, M., 2008. Water on the hydroxylated (001) surface of kaolinite: from
monomer adsorption to a flat 2D wetting layer. Surf. Sci. 602, 960–974.
Jiang, M.Q., Jin, X.Y., Lu, X.Q., Chen, Z.L., 2010. Adsorption of Pb(II), Cd(II), Ni(II) and Cu(II)
onto natural kaolinite clay. Desalination 252, 33–39.
Jonsson, H., Mills, G., Jacobsen, K.W., 1998. In: Berne, B.J., et al. (Ed.), Classical
and Quantum Dynamics in Condensed Phase Simulations. World Scientific,
Singapore, p. 385.
Kömider, K., 2010. Theoretical study of CO and Pb adsorption on the Ni(1 1 1) and Ni3Al(1
1 1) surfaces. Appl. Surf. Sci. 256, 4806–4812.
Kresse, G., Furthmuller, J., 1996. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 54, 11169–111173.
Kresse, G., Joubert, J., 1999. From ultrasoft pseudopotentials to the projector augmentedwave method. Phys. Rev. B 59, 1758–1762.
Michaelides, A., Ranea, V.A., Andres de, P.L., King, D.A., 2004. First-principles study of H2O
diffusion on a metal surface: H2O on Al{100}. Phys. Rev. B 69, 075409–075413.
Monkhorst, H.J., Pack, J.D., 1976. Special points for Brillouin-zone integrations. Phys. Rev. B
13, 5188–5192.
Plançon, A., Giese Jr., R.F., Snyder, R., Drits, V.A., Bookin, A.S., 1997. Stacking faults in
the kaolinite-group minerals: defect structures of kaolinite. Clays Clay Miner.
37, 195–198.
Roehl, J.L., Kolagatla, A., Ganguri, V.K.K., Khare, S.V., Phaneuf, R.J., 2010. Binding
sites and diffusion barriers of a Ga adatom on the GaAs(001)-c(4 × 4) surface
from first-principles computations. Phys. Rev. B 82, 165335–165340.
Sato, H., Ono, K., Johnston, C.T., Yamagishi, A., 2005. First-principles studies on the elastic
constants of a 1:1 layered kaolinite mineral. Am. Mineral. 90, 1824–1826.
Teppen, B.J., Rasmussen, K., Bertsch, P.M., Miller, D.M., Schäferll, L., 1997. Molecular dynamic modeling of clay minerals. 1. Gibbsite, kaolinite, pyrophyllite, and beidellite.
J. Phys. Chem. B 101, 1579–1587.
Zhang, S.Q., Hou, W.G., 2008. Adsorption behavior of Pb(II) on montmorillonite. Colloids
Surf. A: Physicochem. Eng. Aspect 320, 92–97.
Zhao, L., Sheng, G.D., Hu, J., Chen, C.L., Wang, X.K., 2011. The adsorption of Pb(II) on Mg2Al
layered double hydroxide. Chem. Eng. J. 171, 167–174.