Structural characterization of dodecyltrimethylammonium (DTMA) bromide modified sepiolite and its adsorption isotherm studies

Article abstract of DOI:10.1016/j.molstruc.2011.09.044

In this study, dodecyltrimethylammonium (DTMA) bromide was used to modify natural sepiolite via an ion exchange reaction to form DTMA-sepiolite. Sepiolite and DTMA-sepiolite were then characterized by using Brunauer-Emmett-Teller (BET), elemental analysis, XRD, FT-IR, thermogravimetric (TG) and zeta potential analysis techniques. The BET surface area of sepiolite significantly decreased from 152.14 m² g^-1 to 88.63 m² g^-1, after the modification, due to the coverage of the pores of sepiolite. DTMA was located onto sepiolite according to the differential thermogravimetric (dTG) peaks of DTMA-sepiolite. XRD results confirmed the interaction between DTMA ⁺ cations and sepiolite. FT-IR spectra indicated the existence of DTMA functional groups on sepiolite surface. After the characterization was accomplished, adsorption isotherm studies of naphthalene, which is the first member of the polycyclic aromatic hydrocarbons (PAHs), were carried out. The maximum adsorption capacity of DTMA-sepiolite for naphthalene was determined from Langmuir isotherm equation at pH 6 and 20 °C as 1.88 × 10 ^-4 mol g^-1 or 24.09 mg g^-1.

Full text of DOI:10.1016/j.molstruc.2011.09.044

Journal of Molecular Structure 1007 (2012) 36–44
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Structural characterization of dodecyltrimethylammonium (DTMA) bromide
modified sepiolite and its adsorption isotherm studies
⇑
A. Safa Özcan , Özer Gök
Department of Chemistry, Faculty of Science, Anadolu University, Yunusemre Campus, 26470 Eskißsehir, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, dodecyltrimethylammonium (DTMA) bromide was used to modify natural sepiolite via an
ion exchange reaction to form DTMA-sepiolite. Sepiolite and DTMA-sepiolite were then characterized by
using Brunauer–Emmett–Teller (BET), elemental analysis, XRD, FT-IR, thermogravimetric (TG) and zeta
potential analysis techniques. The BET surface area of sepiolite significantly decreased from
152.14 m2 g–1 to 88.63 m2 g–1, after the modification, due to the coverage of the pores of sepiolite. DTMA
was located onto sepiolite according to the differential thermogravimetric (dTG) peaks of DTMA-sepio-
lite. XRD results confirmed the interaction between DTMA+ cations and sepiolite. FT-IR spectra indicated
the existence of DTMA functional groups on sepiolite surface. After the characterization was accom-
plished, adsorption isotherm studies of naphthalene, which is the first member of the polycyclic aromatic
hydrocarbons (PAHs), were carried out. The maximum adsorption capacity of DTMA-sepiolite for naph-
thalene was determined from Langmuir isotherm equation at pH 6 and 20 °C as 1.88 ꢀ 10–4 mol gꢁ1 or
Received 18 July 2011
Received in revised form 20 September
2011
Accepted 22 September 2011
Available online 29 September 2011
Keywords:
Sepiolite
Modification
Characterization
Naphthalene
Surfactant
24.09 mg gꢁ1
.
Adsorption
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
which have cationic head groups with long chain hydrocarbon
molecule forming the surfactant tail, thus it can be effectively used
to remove hydrophobic nonionic organic compounds from aqueous
solutions [4,6,7].
Sepiolite is a fibrous hydrated magnesium silicate and a natural
clay mineral with a unit cell formula [Si12Mg8O30(OH)4(H2O)4ꢂ
8H2O] and a general structure formed by the alternation of blocks
and tunnels which grow up in the fiber direction. Each block con-
sists of two tetrahedral silica sheets enclosing a central magnesia
sheet. However, the silica sheets are discontinued and inversion
of these silica sheets gives rise to tunnels in the structure. These
characteristics of sepiolite make it a powerful adsorbent for organ-
ic molecules. In addition, some isomorphic substitutions in the tet-
rahedral sheets of the lattice of sepiolite, such as Al3+ instead of Si4+
form negative adsorption sites. Such sites are occupied by
exchangeable cations that compensate for the electrical charge
[1,2].
Natural sepiolite and modified sepiolite can be characterized by
Brunauer–Emmett–Teller (BET), elemental analysis, XRD, FT-IR and
thermogravimetric (TG) analysis techniques. The BET surface area
significantly decreases, when natural sepiolite was modified with
a surfactant due to the coverage of the pores of sepiolite.
Sepiolite has two types of water, which are magnesium coordi-
nated water and adsorbed water. It is a trioctahedral type mineral
with three divalent magnesium cations filling the three positions
of the octahedron. The dehydration and dehydroxylation of the
palygorskite clays have been examined by thermogravimetric
analysis techniques [8–13].
Most of clay minerals have high surface areas, but they are
hydrophilic and thus they do not have large adsorption capacities
for hydrophobic organic compounds [3]. However, clays such as
sepiolite may be modified by surfactant coating in a manner that
significantly enhances its capability to remove hydrophobic con-
taminants from aqueous solution [4,5]. This material is known as
organoclay since the exchangeable inorganic cations are replaced
by organic cations including quaternary ammonium compounds,
Often, the combination of both the thermogravimetric (TG) and
differential thermogravimetric (dTG) analysis curves may be used
to interpret thermal behavior of natural sepiolite and organosepio-
lite. All mass loss events in natural sepiolite are associated with
the loss of water. Up to 200 °C both hygroscopic and zeolitic water
are lost. The bound water is released from the structure between
250 and 450 °C; more strongly bound water is left the structure in
the temperature range 450–610 °C; and coordinated water is lost
in the temperature range 730–860 °C. For sepiolite, the partial dehy-
dration of bound H2O in the ranges 250–610 and 210–550 °C results
in the formation of sepiolite anhydride. Dehydration of the bound
water in two steps is attributed to the difference in bonding position
⇑
Corresponding author. Tel.: +90 222 3350580/4781; fax: +90 222 3204910.
E-mail addresses: asozcan@anadolu.edu.tr (A.S. Özcan), ogok1@anadolu.edu.tr
(Ö. Gök).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.09.044

A.S. Özcan, Ö. Gök / Journal of Molecular Structure 1007 (2012) 36–44
37
of water in the structure of this mineral. The amount of water uptake
of sepiolite depends on the relative humidity of the environment to
which they are exposed. Organo-cations can be located on the sur-
face of sepiolite according to the differential thermogravimetric
(dTG) analysis curve [14]. X-ray diffraction (XRD) analysis is con-
ducted to evaluate the changes that occur in the interlamellar dis-
tance of sepiolite as a result of their interaction with hydrophobic
organic compounds. Intercalation of a larger size of the surfactant
cations is compared with that of the replaced cations among layers
of clays not only changes the surface properties from hydrophilic
to hydrophobic, but also greatly increases the interlamellar distance
(basal spacing) of the layers, thus easing the attraction of hydropho-
bic organic molecules. The alkylammonium cations in modified
sepiolite are arranged in monolayers, bilayers, pseudotrimolecular
arrangements, as well as paraffin-type structures in the interlayer
[15–17]. FT-IR spectra indicate functional groups of natural sepiolite
and surfactant modified sepiolite.
Hydrophobic organic contaminants, e.g. polycyclic aromatic
hydrocarbons (PAHs), are mainly generated during the incomplete
burning of coal, oil, gas, wood, garbage, tobacco and charbroiled
meat. They are well known one of the environmental pollutants
and notable amounts produced and emitted. They are toxic, carcin-
ogenic and mutagenic and can cause both short- and long-term
health problems. Lower molecular mass PAHs e.g. naphthalene
are not carcinogens and can cause short-term toxicity problems
via inhalation and are characterized by high volatility; on the con-
trary, high molecular mass PAHs such as benzo(a)pyrene are
known as human carcinogen. PAHs have also accumulated in the
air, water bodies, soil and foods. Once inside the human body,
PAHs can easily pass through cell membranes and are readily ad-
sorbed into cells and therefore, the immune system converts PAHs
to diolepoxides and epoxide hydrolase, which react to DNA and
block its synthesis [18–20]. PAHs can enter natural water sources
via atmospheric fallout, urban run-off, municipal wastes, industrial
effluents and oil spillage or leakage. Although they also take place
naturally, high levels of PAHs are mainly attributed to the anthro-
pogenic sources, including oil spills, automobile exhaust, industrial
processes, natural-gas consumption, coal refining processes,
domestic heating systems, incinerators and smoke; and natural
sources such as volcanic eruptions and forest fires [20–27].
Naphthalene is a natural constituent of coal tar and generally
used as a wool preservative, moth repellent, and raw material for
the production of fine chemicals. The source of naphthalene is
mainly from the burning of fossil fuels. It, which is the simplest
PAHs, has less toxicity than other PAHs and is easily found in the
environment. In this study, naphthalene is chosen as a target com-
pound to view as the primary method of inquiry dealing with com-
plicated PAHs.
In this study, initially, natural sepiolite and dodecyltrimethyl-
ammonium (DTMA) bromide modified sepiolite were character-
ized by using various characterization methods. After that, the
adsorption isotherm studies from aqueous solutions were accom-
plished. The experimental data were fitted into the Langmuir, Fre-
undlich and Dubinin Radushkevich (D-R) isotherm equations to
determine which isotherm gives the best correlation with the
experimental data. The calculated thermodynamic parameters
from the Langmuir isotherm constant (KL) were also used to ex-
plain the nature of adsorption.
2. Materials and methods
2.1. Materials
Naphthalene was obtained from Merck and was used in the
adsorption experiments. The adsorbent, which is sepiolite, was
provided from Eskisßehir-Turkey. It was crushed, ground and sieved
through a 63 lm size sieve. Then the samples were collected from
under the sieve and dried in an oven at 110 °C for 2 h before use.
2.2. Material characterization
Natural sepiolite was characterized with respect to its cation
change capacity (CEC) by the methylene blue method [30]. The
BET surface area of natural sepiolite was determined from N2
adsorption isotherm with a surface area analyzer (Quantachrome
Instruments, Nova 2200e). The chemical analysis of natural sepio-
lite was conducted using an energy dispersive X-ray spectrometer
(EDX-LINK ISIS 300) attached to a scanning electron microscope
(SEM-Cam Scan S4). The crystalline phases present in sepiolite
were determined via X-ray diffractometry (XRD-Rigaku Rint
2000) using Cu Ka radiation.
Natural sepiolite and DTMA-sepiolite were analyzed by X-ray
powder diffraction equipment (XRD-Bruker D8 Advance), before
and after the modification procedure to observe the intercalation
of surfactant into the sepiolite layers.
FT-IR spectra were recorded on transmission mode by using KBr
pellets with a Jasco FT/IR-300E model Fourier transform infrared
spectrometer to see functional groups of natural sepiolite and
DTMA-sepiolite. Infrared spectroscopy with attenuated total
reflection (ATR) technique was also performed to observe possible
interactions between KBr and DTMA-sepiolite by using Bruker IFS
66v/S Model Infrared Spectrophotometer.
The elemental analysis (Vario EL III Elemental Analyzer, Hanau,
Germany) of DTMA-sepiolite was performed to determine C/N ra-
tio in DTMA-sepiolite. Thermal analysis (Setaram) was done to ob-
serve the modification of DTMA+ cations onto sepiolite. The
analyses for natural sepiolite and DTMA-sepiolite were carried
out in the temperature range 25–1000 °C and at a heating rate of
10 °C min–1. Zeta potentials for natural sepiolite and DTMA-sepio-
lite in the presence of naphthalene were measured using Malvern
Zetasizer Nano Series (UK) instrument.
Various physical, chemical and biological techniques such as
adsorption, microbial biodegradation, liquid-liquid extraction,
steam stripping, and chemical oxidation have been attempt the re-
moval of PAHs [28]. Liquid-liquid extraction is not feasible because
it involves the introduction of another solvent in the waste stream.
Microbial biodegradation is also not suitable for highly refractory
organic compounds. Steam stripping is especially not practiced
for compounds with boiling points higher than water. One of the
most popular methods, which is namely adsorption, can be used
the removal of PAHs from aqueous solutions. Although the most
promising adsorbent for the adsorption method is activated car-
bon, which has a high surface area and high adsorption capacity,
it is expensive and the regeneration stage alone can contribute to
80% of the operating cost, which limits their usage [28,29]. In this
manner, there is a need to find the low-cost, locally available
adsorbents, which can be an alternative to the activated carbon.
Clays such as sepiolite, zeolite and montmorillonite are being con-
sidered alternative low-cost adsorbents for this purpose.
2.3. Preparation of DTMA-sepiolite
The Na-exchanged form of sepiolite was prepared by stirring
samples for 24 h with a 1 M aqueous solution of NaCl. Excess NaCl
and other exchangeable cations were removed from the exchanged
sepiolite by washing the latter several times with deionized water.
Sepiolite was then re-suspended and filtered until a negative chlo-
ride test was obtained with 0.1 M AgNO3.
The Na-saturated sepiolite (30 g) was dispersed in 0.8 dm3 of
deionized water and DTMA-sepiolite prepared by adding
DTMA-bromide at twice the cation-exchange capacity of sepiolite

38
A.S. Özcan, Ö. Gök / Journal of Molecular Structure 1007 (2012) 36–44
ln qe ¼ ln KF þ 1=n ln Ce
ð3Þ
and stirring for 24 h. After the treatment procedure, DTMA-sepio-
lite was washed with deionized water until free of salts and a neg-
ative bromide test had been obtained with 0.1 M AgNO3 [31–35]. It
was further used for naphthalene adsorption experiments.
where n (dimensionless) is the heterogeneity factor which has a
lower value for more heterogeneous surfaces. The fit of the experi-
mental data to the Freundlich model was explored by plotting ln Ce
versus ln qe to generate the values of KF and n.
The Dubinin-Radushkevich (D-R) isotherm is more general than
the Langmuir isotherm since it does not assume a homogeneous
surface or constant adsorption potential. It was applied to distin-
guish between the physical and chemical adsorption of adsorbate.
The linear form of D-R isotherm equation is
2.4. Adsorption experiments
The pH experiments were conducted by mixing 50 ml of a
10 mg dmꢁ3 aqueous naphthalene solution with 1 g dmꢁ3 of
DTMA-sepiolite concentration at 20 °C and various pH values in
the range of 1.5–7.0. The solution pH was carefully adjusted by
adding a small amount of HCl or NaOH solution and measured
using a pH-meter (Fisher Accumet AB15), while naphthalene solu-
tions contained in 100 ml Erlenmeyer flasks closed with glass stop-
pers to avoid evaporation, then they were stirred using a magnetic
stirrer. The blank experiments were also carried out to observe the
effect of vaporization of naphthalene. The amount of vaporization
during the experiments was subtracted from the experimental
data. Once the optimum pH had been attained as 6.0, contact time
was determined at this pH value for the increasing periods of time,
until no more naphthalene was removed from the aqueous phase
and the equilibrium had been achieved. After such time (60 min),
the samples were filtered and the equilibrium concentrations were
ascertained by spectrophotometer (Shimadzu UV-2101PC) at the
respective kmax value, which is 219 nm for naphthalene. The
amount of naphthalene adsorbed onto DTMA-sepiolite was deter-
mined by the difference between the initial and remaining concen-
trations of naphthalene solution. The adsorption capacity was
determined by using the following equation taking into the con-
centration differences of the solution at the beginning and equilib-
rium accounts:
ln qe ¼ ln qm ꢁ be2
ð4Þ
where b is a constant related to the mean free energy of adsorption
per mole of the adsorbate (mol2 kJ–2), qm is the theoretical satura-
tion capacity, and
e is the Polanyi potential, which is equal to
RT ln (1 + 1/Ce), where R (kJ mol–1 K–1) is the gas constant, and
T (K) is the absolute temperature. Hence by plotting ln qe versus
e2 it is possible to obtain the value of qm (mol g–1) from the inter-
cept, and the value of b from the slope.
3. Results and discussion
3.1. Characterization of adsorbents
In this study, sepiolite and DTMA-sepiolite were characterized
by using Brunauer–Emmett–Teller (BET), elemental analysis,
XRD, FT-IR, thermogravimetric (TG) analysis and zeta potential
techniques.
3.1.1. Cation exchange capacity and surface area of adsorbents
Cation exchange capacity (CEC) of sepiolite was found to be as
544.3 mmol kgꢁ1. The BET surface areas of natural sepiolite and
DTMA-sepiolite were 152.14 m2 g–1 and 88.63 m2 gꢁ1, respectively.
Since the interparticle pores of sepiolite are covered and the inter-
lamellar spaces are blocked leading to the inhibition of the passage
of N2 molecules, therefore the BET surface area has decreased
nearly two times with the modification.
qe ¼ ðCi ꢁ CeÞV=m
ð1Þ
where Ci and Ce are the initial and the equilibrium naphthalene con-
centrations (mg dm–3), V is the volume of solution (dm3) and m is
the amount of adsorbent used (g).
In order to study the adsorption isotherms a 0.05 g of DTMA-
sepiolite were kept in contact with 50 ml of naphthalene solution
at various concentrations to allow attainment of the equilibrium
at constant temperatures of 10, 15 and 20 °C.
3.1.2. Elemental analysis
The ratio of C/N for DTMA-sepiolite from elemental analysis re-
sults is 12.40 and the calculated value of C/N ratio is 12.86. These
results confirm that the modification of sepiolite with DTMA-bro-
mide was accomplished.
The adsorption data were analyzed to see whether the isotherm
obeyed the Langmuir [36], Freundlich [37] and Dubinin-Radushke-
vich (D-R) [38] isotherm models. The Langmuir isotherm theory is
based on the assumption that adsorption on a homogeneous sur-
face possessing identical sites equally available for adsorption
and with equal energies of adsorption involves with the adsorbent
being saturated after one layer of adsorbate molecules has formed
on the surface. The linear form of the Langmuir isotherm equation
is represented by the following equation:
3.1.3. Chemical composition of sepiolite
X-ray diffraction (XRD) analysis together with the chemical
analysis of sepiolite (29.3% SiO2, 12.4% MgO, 9.09% CaO, and
34.99% CaCO3) indicates that sepiolite and dolomite are the major
components along with traces of Al2O3 (0.175%), K2O (0.037%) and
Fe2O3 (0.069%) in the form of impurities and loss of ignition
(13.94%).
The XRD patterns of natural sepiolite and DTMA-sepiolite were
recorded (Fig. 1) and their basal spaces were observed at 11.92 and
11.97 Å, respectively. The expansion in the basal spacing of the
natural sepiolite due to the modification of sepiolite was calculated
1=qe ¼ 1=qmax þ ð1=qmaxKLÞ1=Ce
ð2Þ
where qe is the equilibrium naphthalene concentration on the
adsorbent (mol g–1), Ce is the equilibrium naphthalene concentra-
tion in the solution (mol dm–3), qmax is the monolayer capacity of
the adsorbent (mol g–1), and KL is the Langmuir constant
(dm3 mol–1) and related to the free energy of adsorption. The plots
of 1/qe versus 1/Ce for the adsorption of naphthalene onto DTMA-
as
D
d = d ꢁ 11.92 Å, where d is the basal spacing of the DTMA-sepi-
olite and 11.92 Å is the thickness of a clay mineral layer.
D
d is
sepiolite give a straight line of slope 1/qmax KL and intercept 1/qmax
.
found to be 0.05 Å. This result suggests that the expansion in the
basal spacing of natural sepiolite with DTMA+ cations was not ex-
actly observed and therefore it can be said that DTMA+ cations at-
tached to the edges of sepiolite.
The Freundlich isotherm describes adsorption when the adsor-
bate has a heterogeneous surface with adsorption sites that have
different energies of adsorption. One limitation of the Freundlich
model is that the amount of adsorbed solute increases indefinitely
with the concentration of solute in the solution. The energy of
adsorption varies as a function of the surface coverage (qe) and is
represented by the Freundlich constant KF (dm3 g–1) in the follow-
ing equation:
3.1.4. FT-IR analysis
FT-IR spectra of natural sepiolite and DTMA-sepiolite were
illustrated in Fig. 2. The band at 3688 cmꢁ1 that corresponds to

A.S. Özcan, Ö. Gök / Journal of Molecular Structure 1007 (2012) 36–44
39
and 19.5% in Frost et al. [14] study. The higher mass loss event was
observed in this study.
The mass loss 1.897% at 25–150 °C related to the loss of ad-
sorbed water (zeolitic water, which is in channels) according to
the following reaction [14].
Si12Mg8O30ðOHÞ4ðH2OÞ4 ꢂ 8H2O ! Si12Mg8O30ðOHÞ4ðH2OÞ4 þ 8H2O
The second mass loss 0.462% at 300–350 °C connected to the
loss of hydration of water according to the following reaction.
Si12Mg8O30ðOHÞ4ðH2OÞ4 ! Si12Mg8O30ðOHÞ4ðH2OÞ2 þ 2H2O
The thirth mass loss 5.268% at 480–585 °C connected to the loss
of coordination water according to the following reaction.
Si12Mg8O30ðOHÞ4ðH2OÞ2 ! Si12Mg8O30ðOHÞ4 þ 2H2O
The fourth mass loss 22.46% at 600–822 °C connected to the loss
of water through dehydroxylation according to the following
reaction.
Fig. 1. XRD patterns of: natural sepiolite (a) and DTMA-sepiolite (b).
Si12Mg8O30ðOHÞ4 ! 8MgSiO3 þ 4SiO2 þ 2H2O
stretching (mOH) vibrations of hydroxyl groups (belong to Mg3OH)
attached to octahedral Mg2+ ions located in the interior blocks of
natural sepiolite and DTMA-sepiolite. The band at 3619 cmꢁ1 indi-
cates that assigned to Al–OH or Mg–OH stretching vibrations in
both of samples. The broad band at 3418 cmꢁ1, observed each sam-
ple, is due to H–O–H vibrations of strongly hydrogen bonded [39].
A pair of bands at 2857 and 2929 cmꢁ1 was only observed with
DTMA-sepiolite can be assigned to the symmetric and asymmetric
stretching vibrations of the methylene group and their bending
vibrations at around 1456 cmꢁ1, supporting the modification of
functional groups of surfactant (DTMA+) cation, but these stretch-
ing bands are not observed in natural sepiolite [39,40]. The band at
1652 cmꢁ1 corresponds to the OHꢁ deformation of water.
The Si–O coordination bands at 1210, 1017, and 983 cmꢁ1 are
observed as a result of the Si–O vibrations. The intense band at
1017 cmꢁ1 represents the stretching of Si–O in the Si–O–Si groups
of the tetrahedral sheet and two peaks at around 689 cmꢁ1 are rep-
resent the bending vibration of Mg3OH in natural sepiolite and
DTMA-sepiolite. The bands observed at 502 and 480 cmꢁ1 for nat-
ural sepiolite and DTMA-sepiolite related to Si–O–Al (octahedral)
and Si–O–Si bending vibrations respectively. These results are also
consistent with above elemental analysis results.
Differential thermogravimetric (dTG) analysis of natural sepio-
lite and DTMA-sepiolite were illustrated in Fig. 3a and b to observe
the modification. The intensity of the differential thermogravimet-
ric (dTG) peaks for DTMA-sepiolite in the temperature region of
25–100 °C was lower than natural sepiolite due to the hydrophob-
icities of this sample as it can be seen from Fig. 3a and b. The dTG
peak between 300 and 400 °C, centered at 365 °C, was observed in
DTMA-sepiolite and it confirms the bonding of the surfactant with
sepiolite, but this peak is not observed in natural sepiolite.
3.1.6. Zeta potential measurements
The zeta potential graphs for natural sepiolite and DTMA-sepi-
olite in the presence of naphthalene were depicted in Fig. 4, as a
function of suspension pH. As shown in Fig. 4, natural sepiolite
has a point of zero charge (pHpzc) at 4.33. The zeta potential is
important in the case of DTMA-sepiolite in naphthalene solution
and dependent on pH (Fig. 4). The point of zero charge was mea-
sured as 4.62 for DTMA-sepiolite in naphthalene solution.
3.2. Adsorption variables
Infrared spectroscopy with attenuated total reflection (ATR)
attachment was used for the observation of possible interactions
between KBr and DTMA-sepiolite. Petit et al. [41] have indicated
that the use of the KBr pressed pellet technique can affect NH4þ
exchangeability in NHþ4 -smectites. During pellet preparation NH4þ
can be exchanged with K+ from KBr. In this manner, ATR-FTIR spec-
trum was recorded in this study for DTMA-sepiolite and the pres-
ence of the band near 1435 cm–1 shows that ammonium ions in the
interlayers of the clay mineral did not exchange with potassium
from KBr [41,42].
The effects of pH, contact time, adsorption isotherms and ther-
modynamics were discussed in detail in this part.
3.2.1. Effect of pH
The effect of pH on the removal of naphthalene onto DTMA-
sepiolite from aqueous solution was illustrated in Fig. 5. As it can
be seen from Fig. 5, the maximum naphthalene removal was
observed at around neutral pH values. Naphthalene adsorption de-
creases at pH 1.5, when it is compared with adsorption at neutral
pH values. Adsorption increases with the increasing pH value at
around 3 and there is almost no further increase in the adsorption
up to pH 6 and all experiments were carried out at pH 6.
3.1.5. Thermogravimetric analysis
Thermal analysis using thermogravimetric techniques enables
the mass loss steps and the mechanism for the mass loss to be
determined. The thermogravimetric (TG) analysis curve related to
natural sepiolite exhibited mass losses by 1.897%, 0.462%, 5.268%
and 22.46% in temperature ranges 25–150 °C, 300–350 °C, 480–
585 °C and 600–822 °C, respectively. Frost et al. [14] found that
to natural sepiolite exhibited mass losses by 10.9%, 3.5%, 1.8%,
2.9% and 0.4% in temperature ranges 25–800 °C. The comparison
this result with our results, total mass loss was 30.09% in this study
3.2.2. Effect of contact time
The influence of contact time on the amount of naphthalene ad-
sorbed was investigated at 20 °C as shown in Fig. 6. It can be seen
that the amount of adsorption increased with increasing contact
time. Maximum adsorption was observed after 75 min, beyond
which there was almost no further increase in the adsorption. This
was therefore fixed as the equilibrium contact time.

40
A.S. Özcan, Ö. Gök / Journal of Molecular Structure 1007 (2012) 36–44
Fig. 2. FTIR spectra of: natural sepiolite (a) and DTMA-sepiolite (b).
3.2.3. Adsorption isotherms
Langmuir constant (KL) for naphthalene adsorbed onto DTMA-sepi-
olite increased from 2.13 ꢀ 103 to 2.57 ꢀ 103 dm3 molꢁ1 as the
temperature increased from 10 to 20 °C indicating that the affinity
of binding sites for naphthalene increased with increasing
temperature.
The Langmuir, Freundlich and Dubinin-Radushkevich (D-R) iso-
therm graphs are illustrated in Figs. 7–9 and their calculated
parameters for the adsorption of naphthalene onto DTMA-sepiolite
are being listed in Table 1. It is evident from these data that the
adsorption of naphthalene onto DTMA-sepiolite is fitted well to
all of the isotherm models as indicated by the r2 values in Table 1.
The adsorption capacity of DTMA-sepiolite increased with
increasing in the temperature. The highest value of qmax obtained
at 20 °C is 1.88 ꢀ 10–4 mol gꢁ1 or 24.09 mg gꢁ1 (Table 1) with the
r2L values of 0.975. The qmax values of naphthalene adsorbed onto
DTMA-sepiolite increased as temperature increased, suggesting
that the adsorption of naphthalene may be endothermic. The
In this study, the maximum adsorption capacity of DTMA-sepi-
olite obtained for naphthalene was 24.09 mg g–1 from the Lang-
muir adsorption isotherm model. It was comparable to data in
the literature with Triton X-100 impregnated chitosan hydrogel
beads (TCB) (32.39 mg g–1
hydrogel beads (CCB) (31.77 mg g–1
chitosan hydrogel beads (SCB) (28.66 mg g–1) [22], petroleum
coke-derived porous activated carbon (0.0463 mg g–1
[27],
)
[22], CTAB impregnated chitosan
[22], SDS impregnated
)
)

A.S. Özcan, Ö. Gök / Journal of Molecular Structure 1007 (2012) 36–44
41
Fig. 3. Differential thermogravimetric (dTG) curves of: natural sepiolite (a) and DTMA-sepiolite (b).
HDTMA-montmorillonite (0.342 mg g–1) [43], BDTDA-montmoril-
lonite (3.888 mg g–1) [43], TPMA-montmorillonite (5.141 mg g–1
[43], HDTMA-kaolinite (21.02 mg g–1
[44], HDTMA-halloysite
(28.45 mg g–1
[44] and particulate organic matter (POM)
onto DTMA-sepiolite lie between 0.689 and 0.728; therefore, its
adsorption is favorable.
)
The values of KF derived from the Freundlich model at all tem-
peratures lie at a range of 0.087–0.115 dm3 gꢁ1. The numerical val-
ues of n at all temperatures change from 1.139 to 1.185 and are
greater than unity, indicating that naphthalene is favorably ad-
sorbed by DTMA-sepiolite at all the studied temperatures.
)
)
(0.0272 mg g–1) [45]. As it can be seen from above results, the max-
imum adsorption capacity of DTMA-sepiolite was found to be high-
er than that of many corresponding adsorbents reported in the
literature.
For the D-R isotherm model, the qm values were 1.39 ꢀ 10ꢁ3
,
1.43 ꢀ 10ꢁ3 and 1.84 ꢀ 10ꢁ3 mol gꢁ1 and the
b values were
The effect of isotherm shape has been discussed [46] with a
view to predict whether an adsorption system is favorable or unfa-
vorable. The essential feature of the Langmuir isotherm can be ex-
pressed by means of ‘R0L’, a dimensionless constant referred to as
separation factor or equilibrium parameter. RL is calculated using
the following equation.
8.39 ꢀ 10ꢁ3, 7.76 ꢀ 10ꢁ3 and 7.77 ꢀ 10ꢁ3 mol2 kJꢁ2 at 10, 15 and
20 °C, respectively.
3.2.4. Thermodynamic parameters
In any adsorption process, both energy and entropy consider-
ations must be taken into account in order to determine what
the process occurs spontaneously. Values of thermodynamic
parameters are the actual indicators for practical application of a
process. The amount of naphthalene adsorbed at equilibrium at
various temperatures (10, 15 and 20 °C) has been examined to ob-
tain thermodynamic parameters for the adsorption system.
RL ¼ 1=ð1 þ KLCoÞ
ð5Þ
where Co is the initial naphthalene concentration (mol dmꢁ3). The
calculated values of RL as above equation are incorporated in Table 1.
As the RL values lie between 0 and 1, the on-going adsorption pro-
cess is favorable [46]. In this study, the RL values for naphthalene

42
A.S. Özcan, Ö. Gök / Journal of Molecular Structure 1007 (2012) 36–44
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
20
40
60
80
100
120
140
160
t (min)
Fig. 6. Effect of contact time for the adsorption of naphthalene onto DTMA-sepiolite
at 20 °C.
80000
70000
60000
50000
40000
10oC
30000
15oC
20oC
20000
Fig. 4. The zeta potentials of natural sepiolite and DTMA-sepiolite in the presence
of naphthalene.
10000
6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000
1/Ce (dm3 mol−1)
Fig. 7. Langmuir plots for the adsorption of naphthalene onto DTMA-sepiolite at
various temperatures.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-9.8
10oC
-10.0
15oC
20oC
-10.2
-10.4
-10.6
-10.8
-11.0
-11.2
-11.4
1
2
3
4
5
6
7
8
pH
Fig. 5. Effect of pH for the adsorption of naphthalene onto DTMA-sepiolite at 20 °C.
-10.2
-10.0
-9.8
-9.6
-9.4
-9.2
-9.0
-8.8
Because KL is the Langmuir constant and its dependence on the
temperature, it can be used to estimate the thermodynamic
ln Ce
parameters, such as the change in the Gibbs free energy (
thalpy ( H°) and entropy ( S°) associated to the adsorption pro-
cess and they were determined by using the following equations:
DG°), en-
Fig. 8. Freundlich plots for the adsorption of naphthalene onto DTMA-sepiolite at
various temperatures.
D
D
D
Gꢃ ¼ ꢁRT ln KL
ð6Þ
ð7Þ
The plot of ln KL as a function of 1/T (Fig. 10) yields a straight
line from which
DH° and DS° were calculated from the slope and
ln KL ¼ ꢁ
D
Gꢃ=RT ¼ ꢁ
D
Hꢃ=RT þ
D
Sꢃ=R
intercept, respectively.

A.S. Özcan, Ö. Gök / Journal of Molecular Structure 1007 (2012) 36–44
43
-9.8
-10.0
-10.2
-10.4
-10.6
-10.8
-11.0
-11.2
-11.4
ꢁ19.13 kJ mol–1 at 20 °C, which were all negative, corresponding
to a spontaneous process of naphthalene adsorption and that the
system does not gain energy from an external source.
10oC
15oC
20oC
The low positive value of the enthalpy change (+7.161 kJ mol–1
)
indicates that the adsorption is physical in nature involving weak
forces of attraction and is also endothermic. The positive entropy
change (D
S°) value (+88.10 J mol–1 K–1) corresponds to an increase
in the degree of freedom of the adsorbed species [47].
4. Conclusions
In this study, natural sepiolite was firstly modified by using a
surfactant, dodecyltrimethylammonium (DTMA) bromide, to ob-
tain DTMA-sepiolite. The BET, XRD, elemental analysis, FT-IR, ther-
mal analysis and zeta potential measurement methods were used
for the characterization of adsorbents. Since the pores of natural
sepiolite are coverage with modification, the BET surface area de-
creases. The XRD results revealed that DTMA+ cations attached to
the edges of sepiolite. TGA and dTG results confirm to the modifi-
cation of sepiolite with DTMA-Br. The functional groups of natural
sepiolite and DTMA-sepiolite were determined by using FT-IR
spectrophotometer. The adsorption dynamics of naphthalene onto
DTMA-sepiolite were then investigated. The experimental data
were fitted well to the Langmuir, Freundlich and D-R adsorption
isotherm models. The maximum adsorption capacity for the re-
moval of naphthalene obtained from Langmuir adsorption iso-
therm model was found to be as 1.88 ꢀ 10–4 mol gꢁ1 or
24.09 mg gꢁ1 at pH 6 and 20 °C.
420 440 460 480 500 520 540 560 580 600 620
ε2 (kJ2 mol-2)
Fig. 9. D-R plots for the adsorption of naphthalene onto DTMA-sepiolite at various
temperatures.
Table 1
Isotherm constants for the adsorption of naphthalene onto DTMA-sepiolite at various
temperatures.
T (°C)
10
15
20
Langmuir
qmax (mol gꢁ1
)
1.52 ꢀ 10ꢁ4
2.13 ꢀ 103
0.976
1.75 ꢀ 10ꢁ4
2.32 ꢀ 103
0.995
1.88 ꢀ 10ꢁ4
2.57 ꢀ 103
0.975
KL (dm3 molꢁ1
)
The enthalpy change (
process indicated that the weak physical forces occurred between
naphthalene molecules and DTMA-sepiolite. The G° values were
negative at all studied temperatures, therefore the adsorption
was spontaneous. The positive value of S° suggests an increase
D
H° = +7.161 kJ mol–1) for the adsorption
rL2
RL
0.728
0.711
0.689
D
Freundlich
n
1.139
0.087
0.975
1.185
0.078
0.991
1.153
0.115
0.982
D
KF (dm3 gꢁ1
)
randomness at the solid/solution interface through the adsorption
of naphthalene onto DTMA-sepiolite. It may be concluded that
DTMA-sepiolite acts a respective adsorbent for the removal of
hydrophobic organic contaminants in aqueous solutions.
rF2
Dubinin–Radushkevich (D–R)
qm (mol gꢁ1
)
)
1.39 ꢀ 10ꢁ3
8.39 ꢀ 10ꢁ3
0.973
1.43 ꢀ 10ꢁ3
7.76 ꢀ 10ꢁ3
0.993
1.84 ꢀ 10ꢁ3
7.77 ꢀ 10ꢁ3
0.980
b (mol2 kJꢁ2
r2D-R
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