Evaluation of fluoride removal from water by hydrotalcite-like compounds synthesized from the kaolinic clay
The present study investigates the fluoride sorption by Mg – Al – CO₃ prepared from a kaolinite as natural source of aluminium using two simple methods. Исследована сорбция фторида на Mg–Al–CO₃, полученном из каолинита как природного источника алюминия с использованием двух простых методов. Дослідже...
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2011
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| Cite this: | Evaluation of fluoride removal from water by hydrotalcite-like compounds synthesized from the kaolinic clay / Khaled Hosni, Ezzeddine Srasra // Химия и технология воды. — 2011. — Т. 33, № 3. — С. 282-302. — Бібліогр.: 46 назв. — англ. |
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Khaled Hosni Ezzeddine Srasra 2018-02-17T19:15:02Z 2018-02-17T19:15:02Z 2011 Evaluation of fluoride removal from water by hydrotalcite-like compounds synthesized from the kaolinic clay / Khaled Hosni, Ezzeddine Srasra // Химия и технология воды. — 2011. — Т. 33, № 3. — С. 282-302. — Бібліогр.: 46 назв. — англ. 0204-3556 https://nasplib.isofts.kiev.ua/handle/123456789/130637 661.183.2+628.16.094.3-926.214 The present study investigates the fluoride sorption by Mg – Al – CO₃ prepared from a kaolinite as natural source of aluminium using two simple methods. Исследована сорбция фторида на Mg–Al–CO₃, полученном из каолинита как природного источника алюминия с использованием двух простых методов. Досліджена сорбція фториду на Mg–Al–CO₃, отриманому з каолініту як природного джерела алюмінію з використанням двох про- стих методів. en Інститут колоїдної хімії та хімії води ім. А.В. Думанського НАН України Химия и технология воды Физическая химия процессов обработки воды Evaluation of fluoride removal from water by hydrotalcite-like compounds synthesized from the kaolinic clay Article published earlier |
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Evaluation of fluoride removal from water by hydrotalcite-like compounds synthesized from the kaolinic clay |
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Evaluation of fluoride removal from water by hydrotalcite-like compounds synthesized from the kaolinic clay Khaled Hosni Ezzeddine Srasra Физическая химия процессов обработки воды |
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Evaluation of fluoride removal from water by hydrotalcite-like compounds synthesized from the kaolinic clay |
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Evaluation of fluoride removal from water by hydrotalcite-like compounds synthesized from the kaolinic clay |
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Evaluation of fluoride removal from water by hydrotalcite-like compounds synthesized from the kaolinic clay |
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Evaluation of fluoride removal from water by hydrotalcite-like compounds synthesized from the kaolinic clay |
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evaluation of fluoride removal from water by hydrotalcite-like compounds synthesized from the kaolinic clay |
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Khaled Hosni Ezzeddine Srasra |
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Khaled Hosni Ezzeddine Srasra |
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Физическая химия процессов обработки воды |
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Физическая химия процессов обработки воды |
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2011 |
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English |
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Химия и технология воды |
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Інститут колоїдної хімії та хімії води ім. А.В. Думанського НАН України |
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The present study investigates the fluoride sorption by Mg – Al – CO₃ prepared from a kaolinite as natural source of aluminium using two simple methods.
Исследована сорбция фторида на Mg–Al–CO₃, полученном из каолинита как природного источника алюминия с использованием двух простых методов.
Досліджена сорбція фториду на Mg–Al–CO₃, отриманому з каолініту як природного джерела алюмінію з використанням двох про- стих методів.
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https://nasplib.isofts.kiev.ua/handle/123456789/130637 |
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Evaluation of fluoride removal from water by hydrotalcite-like compounds synthesized from the kaolinic clay / Khaled Hosni, Ezzeddine Srasra // Химия и технология воды. — 2011. — Т. 33, № 3. — С. 282-302. — Бібліогр.: 46 назв. — англ. |
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AT khaledhosni evaluationoffluorideremovalfromwaterbyhydrotalcitelikecompoundssynthesizedfromthekaolinicclay AT ezzeddinesrasra evaluationoffluorideremovalfromwaterbyhydrotalcitelikecompoundssynthesizedfromthekaolinicclay |
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282 ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3
KHALED HOSNI, EZZEDDINE SRASRA, 2011
UDK 661.183.2+628.16.094.3-926.214
Khaled Hosni, Ezzeddine Srasra
EVALUATION OF FLUORIDE REMOVAL FROM WATER
BY HYDROTALCITE-LIKE COMPOUNDS SYNTHESIZED
FROM THE KAOLINIC CLAY
The present study investigates the fluoride sorption by Mg – Al – CO
3
prepared from
a kaolinite as natural source of aluminium using two simple methods. The first method
uses the kaolinite in natural solid state; the second method uses the filtrate of the
kaolinite after dissolution by acidic solutions. The adsorption characteristics of the
fluoride from synthetic wastewater on calcined LDH samples were evaluated under
laboratory conditions. The anionic clays tested were [K
sd
3P10-T150] synthesized
by method (1) using kaolinitic clay in solid state and [K
liq
3P10] synthesized by method
(2). The equilibrium isotherm showed that the uptake of fluoride ion was consistent
with the Langmuir and Freundlich equations and that the Freundlich model gave a
better fit to the experimental data than the Langmuir model. The maximum adsorption
capacity is 238 and 175 mg/g for K
li q
P10-(500)-LDH and K
sd
P10-(500)-LDH,
respectively, higher than that reported on other adsorbents for fluoride removal A
mechanism f or removal of f luor ide ion has been confirmed by X-ray
diffraction.Overall, the results demonstrate the convenient synthesis of the hydrotalcite
from the kaolinite and the high efficiency of fluoride removal that is promising for
potential applications of calcined K
liq
P10 and K
sd
P10-LDH in the environmental
clean-up and remediation of contaminated water.
Keywords: adsorption; fluoride; hydrotalcite; kaolinite; layered compound;
X-ray diffraction.
1. Introduction
Fluoride is an important micronutrient in human beings which, when
consumed excessively, may lead to various diseases such as fluorosis,
osteoporosis, arthritis, cancer, brain damage, Alzheimer syndrome and thyroid
disorder [1]. The maximum acceptable concentration of fluoride in drinking
water as regulated by World Health Organization is 1.5 mg/L [2]. Presence of
fluoride in drinking water above acceptable concentration increased cases of
fluorosis among the people has been reported from all over the world including
China, India, Pakistan and Thailand [3].
Several defluoridation methods proposed and tested worldwide are mostly
based on the principles of precipitation, ion exchange and adsorption. A wide
variety of adsorbents have been tested for the abatement of fluoride from water.
These include activated and impregnated alumina [4 – 8], cation exchanged
ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3 283
zeolite F-9 [9], activated clay [10], carbonaceous materials [11 – 12], solid
industrial wastes like red mud, fly ash and spent catalysts [13 – 14]. However,
the defluoridation methods developed so far lack viability at the end-user level
due to one or more reasons such as high cost technology, limited efficiency,
unnoticeable break through, deteriorated water quality and taboo limitations [15].
Application of LDHs for defluoridation of water has attracted attention as
LDHs are prepared in aqueous medium from low-cost precursors and can be
easily regenerated. Layered double hydroxides (LDHs), also known as
hydrotalcite-like compounds (HTlc) or anionic clays, consist of brucite-like
hydroxide sheets, where partial substitution of trivalent for divalent cations results
in a positive sheet charge compensated by anions within interlayer galleries [16].
Calcined Mg – Al – CO
3
form of LDHs have been demonstrated to reconstruct
their original layered structure after the adsorption of various anions and are
good ion exchangers/adsorbents for removal of toxic anions from contaminated
water [17 – 21]. In the light of this so-called "memory effect" [22], the removal
of fluoride from aqueous solution by calcined LDHs was studied. In this work, a
detailed study on the synthesis of Mg – Al – CO
3
LDHs, by coprecipitaion and
by mechanochemical synthesis method using kaolinite as aluminium source,
their characterization and the influence of different parameters viz: initial fluoride
concentration, time and solution pH on their performance in the adsorption of
fluoride from water have been presented. The mechanism for removal of fluoride
ion has been confirmed by X-ray diffraction (XRD).
2. Experimental
2.1. Material preparation. The Mg – Al – CO
3
hydrotalcite-like layered
compounds has been synthesized by two methods using kaolinite as a natural
source of trivalent cation and an aqueous solution of Na
2
CO
3
as the precipitant.
Preparation of K
sd
3P10. The sample was synthesized by crushing kaolinite
with the magnesium nitrate hexahydrate (their amounts were so as to have the desired
Mg2+/Al3+ molar ratio), followed by heating at 500°C for 4 h. The product obtained
was dispersed under constant stirring in an aqueous solution containing Na
2
CO
3
(100 ml). The pH of the dispersion was maintained constant at 10 0.1 by adding
NaOH or HNO
3
when necessary. The slurry was subsequently agitated at room
temperature for 24 h, and then aged at 150°C for 24 h. The resulting products were
collected by centrifugal separation and washed thoroughly with deionised water to
eliminate excess Na+ followed by drying overnight at room temperature.
The samples were identified as [K
sd
3P10-150], where K
sd
represents the
trivalent cation source (kaolinite) used to prepare the materials and "sd"
represents the solid state. For example, K
sd
3P10-T150 stands for the product
prepared with kaolinite in solid state, an Mg/Al ratio of 3, an aging temperature
of 150°C, and a pH of synthesis of 10.
284 ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3
Preparation of K
liq
3P10. The sample was synthesized using as aqueous
solution of Na
2
CO
3
as precipitant. The solutions containing Mg(NO
3
)
2
.6H
2
O
and Al3+, cation resulting from dissolution of the purified kaolinite by acidic
attack, (their concentration varied so as to have the Mg/Al molar ratio of 3) were
added dropwise to the aqueous solutions of Na
2
CO
3
with vigorous stirring. The
pH of the dispersion was maintained constant at 10 ± 0.1 by adding NaOH (10%).
The slurry was subsequently agitated at room temperature for three day. The
resulting products were collected by centrifugal separation and washed thoroughly
with deionised water to eliminate excess Na+ followed by drying overnight at
room temperature. The samples were identified as [K
liq
3P10], where K
liq
represents
the trivalent cation source (kaolinite) used to prepare the materials and “liq”
represents its liquid state. For example, K
liq
3P10 stands for the product prepared
with kaolinite in liquid state, an Mg/Al ratio of 3 and a pH of synthesis of 10.
2.2. Characterization of materials. The dried precipitates were
characterized by XRD in order to determine the species present and their degree
of crystallinity. Diffractograms were obtained by using a ‘PANalytical X’Pert
HighScore Plus’ diffractometer using monochromated CuK radiation.
Nitrogen adsorption measurements were performed at -196°C with an Autosorb-
1 unit (Quantachrome, USA) for the determination of sample textural properties
using the multipoint Brunaner-Emmet-Teller (BET) method. The samples were
out gassed at 120°C under a vacuum at 10-3 mm Hg for 3.5 h. Fourier-
transformed infrared (FT-IR) spectra were recorded as KBr pellets using a
Perkin-Elmer FT-IR (model 783) instrument. KBr pellets were prepared by
mixing 5 wt % anionic clay with 95 wt % KBr and pressing.
2.3. Fluoride sorption experiments. The samples used in the sorption
experiments were identified as K
sd
3P10-(500) and K
liq
3P10-(500). These two
adsorbents were obtained by the heating of K
sd
3P10 and K
liq
3P10 in a muffle
furnace at 500°C for 4 h. All solutions were prepared and stocked in polyethylene
flasks. The adsorption experiments were carried out in 50 ml polyethylene tubes. Fluoride
stock solutions were prepared from NaF. Most of the experiments were carried out at
the room temperature (around 25 °C). Analysis of F concentrations in the samples was
done using F ion selective electrode (FISE) Model 9609BN [23]. FISE was calibrated
using standard F solutions in the concentration range 5 – 50 mg/L — a range in which
the electrode exhibited true Nernstian response. The results were plotted as fluoride
concentration (mg/L) versus potential (mV). The observed selectivity ratio of the
electrode was < 10-3 for all the ions, except OH-. For F estimation, 10 ml aliquots were
mixed with 10 ml of total ionic strength adjusting buffer (TISAB). This buffer contains
a chelate, which forms complexes with other ions, such as iron and aluminum that
could interfere in the determination. Each litre of TISAB contained 58 g NaCl, 57 ml
glacial acetic acid, 0.3 g tri-sodium citrate and sufficient NaOH to yield a pH of 5.3.
The time-dependent sorption of fluoride on calcined K
sd
3P10-LDH and
K
liq
3P10-LDH was carried out with 100 mg of the adsorbent and 20 ml of
ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3 285
1000 mg·L-1 of fluoride solution. The mixture was stirred at low speed (~100
mg·L-1) for different time intervals (0.5 – 6 h).
pH-dependent experiments were carried out by adjusting the initial pH (in
the range 5 – 12) using 0.1 M HNO
3
or 0.1 M NaOH solutions. One hundred
milligrams of K
sd
3P10-LDH and K
liq
3P10-LDH were weighed into polyethylene
tubes and 20 mL of the stock solution were added and stirred for 4 h. Then, the
mixture was centrifuged and supernatant fluoride concentration was determined.
The adsorption isotherms were obtained by the batch equilibrium technique.
One hundred milligrams of K
sd
3P10-LDH and K
liq
3P10-LDH were weighed into
polyethylene tubes and then these were filled with 20 ml of aqueous solutions of
NaF (the pH of mixture was adjusted at pH~6) ranging in concentration from 0
to 1000 mg F/L-1. The mixtures were stirred for 4 h in room temperature,
centrifuged and the supernatant fluoride concentrations were determined.
3. Results and discussion
3.1. Characterization of the clay. The sample selected for this study is
Tabarka clay (Tunisian clay).
XRD: the nature of the impurities was determined by examining the crude
sample. Quartz (reflection at 3.35
o
A ) is the major impurity. The diffractogramme
of purified sample (Fig.1, a) shows the reflections at d = 7.21
o
A and 10.05
o
A
characteristic of the kaolinite and illite respectively [24].
5 10 15 20 25 30 35 40 45 50 55 60 65 70
a
2/°
(H
) (H
)
(H
)
(H)
(H)
(H
)
(H
)
(Q)°3.35A
(K)
d
(006)
=3.89A
°d
(003)
=7.82A
d
c
b
°
d (0
0
6)
=
3.
8
7A°
d
(0
03
)=
7.
81
A°
°
3.
5
9A
°7.21A°
10
.0
5A
(
I)
°
1.
5
2
A °
1.
5A
°
4.
5A
in
te
ns
it
y
(u
.a
)
Fig. 1. X-ray patterns of clay sample: (a) purif ied clay; (b) Clay +
Mg(NO
3
).6H
2
O and heated at 500°C; (c) K
sd
3P10-T150; (d) K
liq
3P10.[(K)
Kaolinite, (I) Illite, (Q)Quartz,(H) hydrotalcite
286 ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3
Fig.1, b shows the powder XRD pattern of the mixture of kaolinitic clay
and the magnesium nitrate after heating at 500°C. Due to the absence of the
peak corresponding of the d-spacing of 7.21
o
A and attributed of the kaolin in
the XRD pattern of the product shows that the structure of the original clay is
completely destroyed and indicates the metal oxide peaks, suggesting an almost
total decomposition of the original clay. This observation is consistent with
the result given by the study of the thermal stability for the clay sample.
Infrared spectra: Fig. 2, a shows the Infrared spectra of purified clay over
the wavenumber range of 4000 – 400 cm-1. The figure shows that purified
sample contains quartz (800 cm-1). The spectrum exhibited the characteristic
band at 3697 cm-1 confirming the dominant presence of kaolinite. The band, at
1637 and 3450 cm-1 corresponds to the bonding modes of absorbed water.
Fig. 2. Infrared spectra of: (a) purified clay; (b) K
sd
3P10-T150; (c) K
liq
3P10
3.2. Characterization of the adsorbent K
liq
3P10-LDH and K
sd
3P10-
LDH. Powder XRD. Fig.1, c and Fig.1, d show the XRD patterns for the
precipitates obtained by method 1 and method 2 respectively. It is shown that
the K
sd
3P10-LDH and K
liq
3P10-LDH samples patterns were comparable to
ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3 287
that pattern of the sample prepared by the conventional method. The K
liq
3P10-
LDH sample showed a layered structure as observed from the peaks at 7.82,
3.89 and 2.61
o
A , corresponding to planes (003), (006) and (009) for a layered
hydrotalcite-like material, respectively [25]. The K
sd
3P10-LDH sample display
very weak and broad peaks at a 2 value of 11° compared to the sample prepared
by coprecipitation at the same conditions (pH = 10 and Mg2+/Al3+ = 3). K
sd
3P10-
LDH shows a structure different from the previous samples; it was an ill-
defined hydrotalcite contaminated with argillaceous phase (d = 4.5
o
A ).
IR spectroscopy. The FT-IR spectra of the K
sd
3P10-LDH and K
liq
3P10-
LDH were presented in Fig. 2, b and Fig. 2, c. It shows a broad band around
3470 cm-1 due to the stretching mode of the structural – OH groups in the
metal hydroxide. However, a small shoulder at 2900 – 3000 cm-1 suggests the
presence of a second type of – OH stretching vibration (possibly due to hydrogen
bonding with carbonate in the interlayer spacing [26].
The two spectra show: (i) A shoulder at 1638 cm-1 is ascribed to the bending
mode of the interlayer water molecules [27].
(ii) The three characteristic bands of carbonate in hydrotalcite at around
1384 cm-1 (н
3
), 877 cm-1 (н
2
) and ~1020 cm-1 (н
1
) [28 – 29].
(iii) The bands around 420 and 668 cm-1, which are ascribed to the bending
mode Al – O and Mg–O.
The infrared spectrum of the K
liq
3P10-LDH shows additional bands
appearing at 1193 and 1100 cm-1 which could not be identified.
Surface area and N
2
adsorption-desorption studies. The N
2
adsorption-
desorption isotherm is of type II for all samples, which is typical of
mesoporous materials (Fig. 3). All of the materials possessed zero micropore
volume. Adsorption isotherms of this type are represented by mesoporous
materials with no micropores and strong interactions between adsorbent and
adsorbate molecules. This type of hysteresis loop is formed when the
adsorption and desorption curves do not coincide and is caused physically
by the phenomenon of capillary condensation in the mesopores.
From Fig. 3, it was determined that all samples show a horizontal course
of the hysteresis branch over an appreciable range of gas uptake (P/P
0
= 0.6),
while it is vertical above this ratio. This type of hysteresis loop is often observed
with aggregates of plate-like particles that give rise to slit-shaped pores.
Specific surface areas of the K
sd
3P10-(500)-LDH and K
liq
3P10-(500)-LDH
were determined by the single-point BET method (Table 1) and were found to
be 179 and 165 m2g-1 respectively, much greater than the 80 and 78 m2g-1 values
obtained for their precursors at the precursor temperature at 150°C. It was
suggested that a porous system was developed in the calcined sample during
the transformation of CO
3
2- to CO
2
[30].
288 ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0
0
50
100
150
200
250
300
350
400
V,cm
3
/g
P/P
0
Desorption
Adsorption
b
a
Fig. 3. N
2
adsorption-desorption isotherms: (a) K
sd
3P10-LDH; (b) K
liq
3P10-
LDH
Table 1. Textural properties for various sorbent samples
KsdP10 KliqP10
SBET, m
2/g
Vmacropore,
cm
3
/g
SBET, m
2/g
Vmacropore,
cm
3
/g
Before alcination 80 0.2044 78 0.6052
After calcination 178 – 165 –
PZC determination. Batch equilibrium method for determination of the
point of zero charge (PZC) was proposed by [31]. Accordingly, the samples
of calcined hydrotalcite (0.1g) were shaken in PVC vials, for 24 h, with 30
ml of 0.01M KCl, at different pH values. Initial pH values were obtained by
adding a certain amount of KOH or HCl solution so that to keep the ionic
strength constant. Experimental results of the pH
pzc
determination are given
in Fig. 4. They are presented as pH values of filtered solutions equilibration
(pH
f
) with calcined HT as a function of initial pH value (pH
i
). It can be seen
that PZC is at pH = 8.6 and 8.5 (pH
f
level, where common plateau is obtained)
for K
sd
3P10-(500)-LDH and K
liq
3P10-(500)-LDH respectively.
ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3 289
Fig. 4. Determination of pH
PZC
of (n) K
sd
3P10-(500)-LDH and (%) K
liq
3P10-
(500)-LDH in KCl solution
3.3. The optimisation of the operating conditions of adsorption
3.3.1. The effect of pH. Generally, the pH is an important variable; it
controls the adsorption at water-adsorbent interfaces. Therefore, the adsorption
of fluoride on the CLDH was examined at different pH values ranging from
5.0 to 10.0 and relevant data are presented in Fig. 5.
Fig. 5 shows that the adsorption value increases with the increase in pH and
reaches maximum (70% and 80% for K
sd
3P10-(500)-LDH and K
liq
3P10-(500)-
LDH, respectively) at the pH of 6.3 before decreasing to 40% at pH 12.5. However,
at low pH (pH < 6), the adsorption capacity was low, probably due to the protonation
of the F ions and dissolution of the layered materials in acid medium, this observation
has been confirmed by the analysis of Mg, which presents in the resulting solution
after the adsorption assisted by atomic absorption spectrometry. Therefore, an
optimal pH value should be near that of 6.0. This is important inference, since the
pH values of most water streams vary from neutral to weakly basic range except
for the acid drainages. Similar observations have been made on the pH dependence
of fluoride adsorption with ZnAl-Cl-LDH, in which the fluoride uptake decreased
with the increasing pH [32].
As the pH approached that corresponding to the PZC of calcined LDH,
the surface charge decreased resulting in reduced F LDH interactions leading
to low F uptake. An increase in the concentration of the competing OH anions
at pH above 7 might also be responsible for the observed decrease in the
290 ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3
adsorption capacity at higher pH. Similar changes of the adsorption capacity
with solution pH have been reported by Das et al. [33].
3 4 5 6 7 8 9 10 11 12 13
0
10
20
30
40
50
60
70
80
90
100
pH
b
a
%
A
ds
or
pt
io
n
Fig. 5. The percentage of fluoride removal by (a) K
sd
P10-(500) and (b) K
liq
P10-
(500) as a function of pH variation
3.3.2. Effect of contact time. The variation of fluoride adsorption as a
function of time is shown in Fig. 6. The kinetic curves show that the equilibrium
time and adsorption capacity of K
sd
P10-(500) and K
liq
P10-(500) are similar.We
observed that adsorption of fluoride is a fast process on HT-like compounds.
This behaviour has already been reported in the adsorption of anionic species
on HT-like compounds [34].
For K
liq
3P10-(500)-LDH used as a sorbent, the plot of the removed amount
of fluoride gives an initial steep portion (~30 mn) followed by the slow increase
for approximately 1h. The yield of fluoride removal was of 65%. For K
sd
3P10-
(500)-LDH, the period of 3 h was necessary for reaching equilibrium and the
maximum percentage of fluoride removal was of 65%. It is well known that
HT- like compounds interacts strongly with anionic species due to the existence
of positive charges on the lamellae and also over their external surface [35].
This property makes possible the occurrence of two different mechanisms of
anion removal: (i) adsorption on external surface (fast process) and (ii) anion
exchange (slow process) [16].
ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3 291
0 50 100 150 200 250 300 350 400
0
10
20
30
40
50
60
70
80
Time,mn
%
A
ds
or
pt
io
n
K
sd
3P10
K
liq
3P10
Fig. 6. Effect of time on fluoride adsorption on (%) K
sd
3P10-(500)-LDH and
(%) K
liq
3P10-(500)-LDH (C
0
= 600 mgF·L-1 and pH 6)
The adsorption process depends on the variable charge density on the
surface as well as on the surface area of the adsorbent. The ion exchange
depends on the nature of the interlayer anions represented in the sample
[36, 37]. Generally, the adsorption process is faster than the anion exchange
because of the strong interaction between negative ions and the positive
external surface and due to the high exposition of the external surface of
LDH, while ion exchange is a diffusion process. Moreover, in this study,
the fluoride removal is realized by sorption process followed by a rebuilding
of the initial structure with fluoride anion in the interlayer space.
Generally, three steps are involved in the process of sorption by porous
particles [19] (i) external mass transfer (by mechanism of boundary layer or
film diffusion) between the external surface of the sorbent particles and
surrounding fluid phase; (ii) inner transport within the particle; and (iii)
chemisorption (reaction kinetics at phase boundaries), where the rate of
adsorption is generally controlled by the kinetics of bond formation.
Kinetic modeling not only allows estimation of sorption rates but also
leads to suitable rate expressions characteristic possible react ion
mechanisms. In this respect, several kinetic models including the pseudo-
first-order (Eq.(1)), pseudo-second-order equation (Eq.(2)) and intraparticle
diffusion model (Eq.(3)) [38] were tested.
292 ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3
,
111 1
mmt QtQ
k
Q
(1)
,
1
2
2 mmt Q
t
QkQ
t
(2)
,2/1 CtkQ pt (3)
here is the amount of fluoride sorbed (mg/g) at a given time, the maximum
adsorption capacity (mg/g), and the pseudo-first-order and pseudo-second-
order rate constant, respectively, the intraparticle diffusion rate constant and
C is the intercept. The calculated kinetic parameters for fluoride sorption by
calcined K
sd
P10 and K
liq
P10 are given in table 2.
Table 2. Kinetic parameters for sorption of fluoride by K
sd
P10-(500) and
K
liq
P10-(500)
k1
(h-1)
Q1
(mg/g)
2
1R
k2
(g/mg h)
Q2
(mg/g)
2
2R
kp
(g/mg h)
C
(mg/g)
2
pR
KsdP10-
(500)
0.111 92.59 0.99 0.118 91.74 1.0 7.55 73.71 0.76
KliqP10-
(500)
0.065 93.45 0.91 0.023 91.74 1.0 4.55 81.492 0.58
It seems that, of the three kinetic equations tested, the pseudo-second-
order best described the kinetic data for fluoride sorption by calcined LDH
(Fig. 7), based on the correction coefficient R2. The pseudo-second-order
equation is based on the adsorption loading of the solid phase and is in
agreement with a chemisorption being the rate determining step mechanism.
ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3 293
0 1 2 3 4 5 6 7
0,00
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0,09
t/q
t
, h g/mg
Fig. 7. Pseudo-second-order kinetics model. Plot of (t/q
t
) as a function for
time () K
sd
P10-(500)-LDH (R2 = 1.0) and () K
liq
P10-(500)-LDH (R2 =1.0)
3.3.3. Effect of the adsorbent dosage. The fraction of fluoride removal
increases with the increase of the adsorbent dose from 0.5 to 4.0 g/L at a fixed
fluoride concentration (600 mg F/L), pH 6 and temperature of 303 K (Fig. 8).
This is consistent with the argument that surface sites of oxide systems are
heterogeneous [39, 40]. According to this model; there is a spectrum of binding
energies of the adsorption sites. At a low fraction of adsorbent, all types of
sites are fully exposed to the interaction with the adsorbate, and the surface
saturation is reached rapidly. For higher particle concentrations, the availability
of sites with high energy decreases, while a larger fraction of low energy sites
are occupied. That is, the number of active adsorption sites is larger at a fixed
adsorbate concentration. The given plot shows a maximum of fluoride removal
at an adsorbent concentration of 3 g/L, while Liang et al. 2006 [41] were found
that 1.1 g/L is sufficient to have a maximum of fluoride removal using Mg –
Al – CO
3
– LDH synthesized by conventional method.
Time, h
294 ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3
0 1 2 3 4
0
10
20
30
40
50
60
70
80
90
100
b
a
%
A
d
so
rp
ti
o
n
Adsorbent quantity, g L
-1
Fig. 8. Effect of adsorbent dose on the fluoride removal at adsorbate con-
centration 600 mg/L, pH ~ 6.3, and temperature 303 K: (a) K
sd
P10-500-LDH
and (b) K
liq
P10-(500)-LDH
3.3.4. Effect of ionic strength. The effect of ionic strength on fluoride
anion adsorption was studied by conducting batch experiments at varied ionic
strength of 0.001, 0.01 and 0.1 M KCl. The results are shown in Fig. 9. The
fluoride uptake varies depending on the salinity solution due to the presence
of competing anions. The percentage of fluoride uptake decreases steeply from
60 to 38 for K
sd
P10-(500)-LDH and 80 to 50 for K
liq
P10-(500)-LDH with an
increase in the ionic strength from 0.001 to 0.01 M and decreases slowly to 35
and 45, for K
sd
P10-(500)-LDH and for K
liq
P10-(500)-LDH, respectively, for
an ionic strength of 0.1 M KCl. The influence of ionic strength is more
pronounced in the region of low values. This suggests the presence of two
types of adsorption sites in the samples; (a) non-specific sites only capable of
weak interactions, and (b) specific ones that strongly interact with fluoride
ions. The non-specific sites may be sensitive to the coexisting anions, and
therefore the adsorbed fluoride may be easily exchanged with Cl– ions from
solution even at low concentration. The fluoride ions adsorbed on the strong
specific sites are rarely substituted on the surface even in a solution with a
large excessive amount of coexisting ions. However, we cannot distinguish
the strong specific sites and weak non-specific ones due to the nearly amorphous
structure of the samples. Similar observations have been made on the ionic
strength dependence of fluoride adsorption with calcined Mg – Al – CO
3
–
LDH in aqueous solution, in which the fluoride uptake decreased where Cl-
exists in solution [41].
ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3 295
0,00 0,02 0,04 0,06 0,08 0,10
0
10
20
30
40
50
60
70
80
90
100
b
a
%
A
d
so
rp
tio
n
Ionic strenght, M
Fig. 9. Effect of ionic strength on the sorption of fluoride by (a) K
sd
P10-(500)-
LDH and (b) K
liq
P10-(500)-LDH (C
0
=600 mg/L and pH 6)
3.4. Adsorption isotherms. Equilibrium adsorption isotherms are of
fundamental importance in the study of adsorption systems since they indicate
how the anionic species partition themselves between the medium and liquid
phase with increasing concentration at equilibrium. When medium and anionic
species solutions are contacted, the concentration of the anions on the medium
will increase until a dynamic equilibrium is reached, at which point there is a
defined distribution of anionic species between the solid and liquid phases. The
adsorption isotherms were constructed using the batch equilibrium technique
where a fixed mass of adsorbent (100 mg) was agitated with fluoride solution of
various concentrations (0–800 mg/L) for a sufficient length of time to ensure
equilibrium had been achieved. An agitation time of 4h and equilibrium
temperature of 25°C were used.
Sorption isotherms of fluoride retention by calcined LDH are shown in
Fig. 10. It is evident that sorption capacity of K
liq
P10-(500) is much larger than
these of K
sd
P10-(500)-LDH. For example, at a similar fluoride concentration
of 100 mg/L sorption capacity of K
liq
P10-(500) was greater three times than
that of K
sd
P10-(500)-LDH. These results can be explained by the fact that
K
sd
P10-(500)-LDH contains an argillaceous phase as impurity. Indeed, the
PZC of silica, which coming from the argillaceous phase, is ~ 2 thus at pH 6,
the surface of silica is charged negatively what disadvantages the adsorption
of the ions fluoride.
296 ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3
0 50 100 150 200 250 300 350 400
0
20
40
60
80
100
120
140
160
180
200
220
Q
e
, mgF/g
b
a
C
e
, mgF/L
Fig. 10. Equilibrium isotherms of fluoride removal on (a) K
sd
P10-(500)-LDH
and (b) K
liq
P10-(500)-LDH
Isotherms for fluoride sorption by K
sd
P10-(500)-LDH and K
liq
P10-(500)-
LDH were modeled by two commonly used isotherm equations, Langmuir
(Eq. 4) [42] and Freundlich (Eq. 5) [43].
e
nFe CKQ (4)
or
eFe CnKQ logloglog (5)
where Q
e
(mg/g) is the amount of fluoride sorbed at equilibrium, Q
m
(mg/g) the
theoretical maximum monolayer sorption capacity, C
e
(mg/L) the equilibrium
concentration of fluoride in solution, and K
F
, n and K
L
are empirical constants. The
calculated Langmuir and Freundlich isotherm constants are given in Table 3. Data
for fluoride sorption by K
sd
P10-(500)-LDH and K
liq
P10-(500)-LDH were fitted
better by Freundlich equation than by the Langmuir equation based on the correction
coefficient R2. The best fit Freundlich parameters are K
F
= 4.21 and 10.26, n =
0.5936 and 0.5962 for K
sd
P10-(500)-LDH and K
liq
P10-(500)-LDH respectively.
The n value in the range of 0.1–1 indicates a favorable adsorption process. The
best fit Langmuir parameters are Q= 175 and 213.22 mg/g, K
L
= 0.008 and 0.017
L/mg for K
sd
P10-(500)-LDH and K
liq
P10-(500)-LDH respectively.
The removal of fluoride by different adsorbents has been studied in recent
years and some of these reports provide Q values. Although these values were
ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3 297
obtained under different ranges of conditions, they can be useful in criterion of
the adsorbent capacity. The Q value obtained in this study is greater than those of
reported for alum sludge (5.394 mg/g) [44], actived alumina (16.34 mg/g) [45],
flyash (20 mg/g) [13], lignite (7.09 mg/g) and bituminous coal (7.44 mg/g) [46].
Table 3. Langmuir and Freundlich isotherm constant for sorption of fluoride
by K
sd
P10-(500) and K
Liq
P10-(500)-LDH
Langmuir Freundlich
Qm
(mg/g)
KL
(L/mg)
R2 n
KF
(L/g)
R2
KsdP10-(500) 175 0.008 0.9023 0.5936 4.21 0.9968
KLiqP10-500) 238 0.017 0.9705 0.5962 10.26 0.9826
R2: Correction coefficient.
The mechanism of removal of fluoride ions by calcined LDH can be
explained as follows.
The regeneration of LDH is due to the ability of calcined LDH to incorporate
anions into its structure by means of the so-called "memory effect". LDH
containing carbonates as the interlayer anion decomposes into magnesium and
aluminum oxides when heated at 500°C. The calcined product M g
1 x
Al
x
O
1+x/2
can be rehydrated and incorporate anions, such as fluoride, to rebuild the initial
layered structure.
[Mg
1 x
Al
x
(OH)
2
](CO
3
)
x/2
Mg
1 x
Al
x
O
1+x/2
+(x/2)CO
2
+H
2
O ; (6)
Mg
1"x
Al
x
O
1+x/2
+(x)F- + (1 + (x/2)+y)H
2
O [Mg
1"x
Al
x
(OH)
2
](F)
x
+xOH-. (7)
OH- is produced during rehydration of CLDH. As a result, controlling the
pH of solution is important in order to maintain a driving force for removal of
fluoride ion by CLDH [42].
Structural reconstruction was confirmed by XRD analysis. The XRD pattern
of K
sd
P10-(500) (Fig. 11, a) and K
liq
P10-(500) (Fig.12, a) are evidence of the
presence of mixed oxide phase, which possesses a typical poorly crystalline
MgO-like structure.
Upon addition of the thermally activated hydrotalcite to an aqueous solution
containing sodium fluoride the so-called “memory” effect or reformation effect
of hydrotalcites comes into play. Hydrotalcites after thermal decomposition
will regain their original structure, providing the compound is not heated to
298 ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3
too high a temperature. The XRD pattern of the fluoride adsorbed HT shows a
d
003
spacing of 7.63 and 7.65 Е for K
sd
P10-(500) and K
liq
P10-(500)-LDH
respectively. These basal spacing values are weaker than of the original samples,
what indicates that fluoride ions are intercalated in the interlayer space, because
F has a radius smaller than carbonate ion.
Fig. 11. XRD powder spectra of the: (a) K
sd
P10-(500) and (b) K
sd
P10-(500)
after a contact with the fluoride solution
Fig. 12. XRD powder spectra of the: (a) K
liq
P10-(500) and (b) K
liq
P10-(500)
after a contact with the fluoride solution
ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3 299
The XRD pattern in Fig.12, b which corresponds to the product obtained
after the interaction of K
liq
P10-(500)-LDH with F- reveals additional reflections
other than of the hydrotalcite structure. These reflections were attributed to the
formation of the aluminium fluoride phase which was formed by precipitation.
What explains why Q
m
(238 mgF-/g) obtained by K
liq
P10-(500)-LDH is large
compared to that is obtained by K
sd
P10-(500) (Q
m
=175 mg/g). The calcined
LDH after removal of fluoride can be regenerated using Na
2
CO
3
aqueous
solution (0.1 mol/L) followed by calcination at 500°C. It can be concluded
from the above observations that the adsorption of fluoride is a reversible
process, thereby facilitating the recyclability of the material for further use.
4. Conclusion
This work describes a new method to remove fluoride from aqueous solutions
that is promising for the treatment of wastewater in industrial processes. The
fluoride removal from water by calcined HT synthesized by co-precipitation
(K
liq
P10-(500)-LDH) and by mechanochemical synthesis method (K
sd
P10-(500)-
LDH) using the kaolinitic clay as trivalent cation resource.
Our results show that the calcined LDH synthesized from the cationic clay
has a marked ability to adsorb fluoride. The adsorption loading is higher for
K
liq
P10-(500)-LDH than that of K
sd
P10-(500)-LDH. It was found that fluoride
sorption occurs with a high extraction rate and the removal of fluoride by
calcined HT strongly depends on the process parameters. The increase of pH
from 5 to 13 results in a reduction of the positive charge of the adsorbent that
leads to a decrease of the amount of adsorbed fluoride. The maximum removal
of fluoride from aqueous solutions occurs at pH 6.3 in 4 h and the retention of
fluoride ions by the CLDH material was ~80% or higher. The increase in the
ionic strength by the addition of KCl results in a large decrease in the amount
of removed fluoride. This effect is probably due to the enhancement of anion
exchange during the adsorption process. Equilibrium results could be fitted by
the Freundlich isotherm and Langmuir isotherm, and the former is a better
model. The Freundlich constant (n) in the range of 0.1–1 indicates a favorable
adsorption process. The maximum adsorption capacity is 238 and 175 mg/g
for K
liq
P10-(500)-LDH and K
sd
P10-(500)-LDH, respectively, higher than that
reported on other adsorbents for fluoride removal. A mechanism of the
adsorption phenomenon has been proposed on the basis of XRD. Overall, the
results demonstrate the convenient synthesis of the hydrotalcite from the
kaolinite and the high efficiency of fluoride removal that is promising for
potential applications of calcined K
liq
P10 and K
sd
P10-LDH in the environmental
clean-up and remediation of contaminated water.
300 ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3
Резюме. Исследована сорбция фторида на Mg–Al–CO
3
,
полученном
из каолинита как природного источника алюминия с использованием двух
простых методов. В первом методе применен каолинит в твердом состо-
янии; во втором – фильтрат каолинита после растворения подкисленны-
ми растворами. В лабораторных условиях оценены характеристики ад-
сорбции фторида из синтетической сточной воды на кальцинированных
образцах LDH. Исследовали анионные глины [K
sd
3P10-T150] (синтези-
рованная по первому методу) и [K
liq
3P10] (синтезированная по второму
методу). Равновесная изотерма показала, что извлечение фторид-иона со-
ответствует уравнениям Ленгмюра и Фрейндлиха и модель Фрейндлиха
лучше соответствует экспериментальным данным, чем модель Ленгмю-
ра. Максимальная адсорбционная емкость составляет 238 и 175 мг/г со-
ответственно для K
liq
P10-(500)-LDH и K
sd
P10-(500)-LDH, что выше, чем
значения, приводившиеся для других адсорбентов, которые применялись
для удаления фторида. Механизм удаления фторид-ионов подтвержден
рентгеновской дифракцией. Результаты демонстрируют эффективный
путь синтеза гидротальцита из каолинита и высокую степень удаления
фторида, которая является многообещающей для применения кальцини-
рованных глин K
liq
P10 и K
sd
P10-LDH в очистке объектов окружающей
среды и загрязненных вод.
Резюме. Досліджена сорбція фториду на Mg–Al–CO
3
, отриманому з
каолініту як природного джерела алюмінію з використанням двох про-
стих методів. У першому методі використовується каолініт в твердому
стані; у другому – фільтрат каолініту після розчинення розчинами, що
підкисляють. У лабораторних умовах оцінені характеристики адсорбції
фториду з синтетичної стічної води на кальцинованих зразках LDH. Дос-
ліджували аніонні глини [K
sd
3P10-T150] (синтезована першим мето-
дом) і [K
liq
3P10] (синтезована другим методом). Рівноважна ізотерма
показала, що витягання фториду-іона відповідає рівнянням Ленгмюра і
Фрейндліха і модель Фрейндліха краще відповідає експериментальним
даним, чим модель Ленгмюра. Максимальна адсорбційна ємність скла-
дає 238 і 175 мг/г відповідно для K
liq
P10-(500)-LDH і K
sd
P10-(500)-LDH,
що вище, ніж значення, які приводилися для інших адсорбентів, що зас-
тосовувалися для видалення фториду. Механізм видалення фторид-іонів
підтверджений рентгенівською дифракцією. Результати демонструють
еффективний спосіб синтезу гидротальцита з каолініту і високу міру ви-
далення фториду, яка є багатообіцяючою для вживання кальцинованих
глин K
liq
P10 і K
sd
P10-LDH в очищенні об’єктів довкілля і забруднених
вод.
ISSN 0204–3556. Химия и технология воды, 2011, т. 33, №3 301
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Centre National des Recherches en Sciences
des Materiaux (CNRSM),
Pole Technologique de Borj Cedria, Tunisia Recieved 17.09.2010
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