CHAPTERS
Calcium oxide nanoparticles
as an adsorbent for the removal of fluoride
• Abstract
5.1 Introduction
5.2 Experimental
5.2.1 Materials
5.2.2 Apparatus
5.2.3 Preparation ofCaO nanoparticles
5.2.4 Defluorination
5.3 Results and discussion
5.3.1 Sol-gel method
5.3 .2 Characterisation
5.3.3 Adsorption of fluoride on calcium oxide nanoparticles
5.3 .4 Effect of adsorbent dose
5.3.5 The Effect of contact time
5.3.6 Adsorption kinetics
5.3. 7 Effect of temperature
5.3.8 Effect of initial fluoride concentration
5.3 .9 Adsorption isotherm
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --
5.3.10 Mechanism of surface adsorption
5.3.11 Effect of pH
5.3 .12 Effect of competitive ions
5.3 .13 Desorption study
5.3.14 Analytical application
5.3.4 Conclusion
• Figures and tables
• References
Chapter 5
Calcium oxide nanoparticles
ABSTRACT
Calcium oxide (CaO) nanoparticles were synthesized by the sol-gel method. The
particles were characterized by FT-IR, TGA, DTA, TEM and DLS analysis. The ability
of the CaO nanoparticles for removal of fluoride from aqueous solution through
adsorption has been investigated. All the experiments were carried out by batch mode.
The effect of various parameters viz. contact time, pH effect (pH 2-1 0), adsorbent dose
(0.01-0.1 g/100 ml), initial fluoride concentration (10-100 mg L- 1) and competitive ions
has been investigated to determine the adsorption capacity of CaO nanoparticles. Almost
complete removal (98 %) of fluoride was obtained within 30 min at an optimum
adsorbent dose of 0.6 g L- 1 for initial fluoride concentration of I 00 mg L- 1• Adsorption
isotherms were also studied to find the nature of adsorbate- adsorbent interaction.
122
Chapter 5
Calcium oxide nanoparticles
5.1 INTRODUCTION
Recently, metal oxides have attracted much attention 1 because of their wideranging potential applications in catalysis, adsorption and nanotechnology. Materials with
nanodimension have gained considerable attention because of their usefulness as sensors,
as coating materials and for the miniaturization of devices. Nanocrystalline alkaline earth
metal oxides have gained significant attention as effective chemisorbents for toxic gases,
HCl, chlorinated and phosphorus containing compounds.Z-5
Fluoride-related health hazards are a major environmental problem in many
regions of the world. As per the WHO standards for drinking water recommended a
1
desirable limit of 1.0 mg L- 1 and a permissible limit of fluoride is 1.5 mg L- . When the
fluoride concentration is more than the permissible limit, it causes dental and skeletal
fluorosis, bone diseases, mottling of teeth, lesions of the thyroid, liver and other organs.
6
Various treatment and technologies, based on the principle of precipitation, ion exchange,
electrolysis, membrane and adsorption have been proposed and is tested for removal of
excess of fluoride from drinking water as well as from industrial effluent.
?-IO
Among
these methods, adsorption is the most widely used method for the removal of fluoride
from aqueous solution. Several materials have been utilized for removal of fluoride from
aqueous solution by adsorption and precipitation processes, such as activated alumina
amorphous alumina, 12 activated carbon, 13 zeolite, 14 hydrotalcite like compounds,
15
11
,
etc. In
recent years, considerable attention has been given to the study of different types of lowcost materials such as calcite, clay charcoal, tree bark, saw dust, rice husk and ground nut
husk. 16- 19 However, the lowest limit for fluoride reduction by most of the adsorbents is
greater than 2 mg L- 1 with low adsorption capacity and high equilibration time. Such
123
Chapter 5
Calcium oxide nanoparticles
adsorbents are not suitable for the drinking water treatment purpose, especially as some
of them can work only at an extreme pH value, such as activated carbon which is only
effective for fluoride removal at pH less than 3.0. 20
Calcium metal has a good affinity to fluoride ion. Many researches have been
done for the removal of fluoride by different calcium salts.Z 1-25 However the adsorption
capacities of the reported compounds are not satisfactory. Therefore a new method for
fluoride removal by calcium metal is highly desirable. Since, nanoparticles have unique
adsorption properties due to their small size and large surface area, in the present study,
an attempt has been made to prepare CaO nanoparticles, through a simple and effective
sol-gel route, which could be a cheap and efficient adsorbent for fluoride removal. The
present study deals with the kinetic and mechanistic aspects of fluoride removal using
CaO nanoparticles.
5.2 EXPERIMENTAL
5.2.1 Materials
All the starting materials for the synthesis used were of analytical grade. Calcium
nitrate tetrahydrate was obtained from Pfizer. Analytical reagent-grade ethylene glycol,
citric acid, HCl and NaOH were obtained from Merck. Solutions of metals were prepared
in deionized water and standardized by the reported methods wherever necessary. Water
was purified by Millipore system and was used in all experiments.
5.2.2 Apparatus
The FT-IR of the gel and calcinated powders were recorded from 4000 to
400 cm· 1 by the KBr pellet method using FT-IR Bruker Tensor 27 model. The TGA and
124
Chapter 5
Calcium oxide nanoparticles
DTA curves of dried gel were recorded usmg TGA-50 and DTA-60 Shimadzu
respectively, up to I 000°C, at a heating rate of I 0°C/min. TEM was obtained using a
ZEISS EM-900 at the 85,000 magnification. DLS were performed on Nanotrac NPA
I50/250.
5.2.3 Preparation of CaO nanoparticles
Accurately weighed I.I8 g calcium nitrate tetrahydrate (O.I M) was dissolved in
50 ml water. 0.3 M aqueous solution of ethylene glycol (50 ml) was added. Then, 50 ml
citric acid (0.1 M) solution in water was added drop wise with continuous stirring in the
above mixture. The mixture was heated gently at 60°- 80°C for 2 h under stirring and was
then further heated at I20°C to yield the polymeric gel. The gel precursor so formed was
decomposed at 350 °C for 3 h and subsequently calcinated at 400°, 550°, 700°, 800° and
900 °C for 2 h in air and slowly cooled in furnace to obtain white CaO nanoparticles.
5.2.4 Defluorination
Stock solution of fluoride was prepared by dissolving 2.2I g of sodium fluoride
(Merck, Germany) in I L deionized water (Millipore). The measuring cylinder,
volumetric flask and conical flask used were of PVC. pH measurements were made by
Systronics digital pH meter 335 using a combined glass electrode. The pH meter was
calibrated with Merck Standard buffers before any measurement. pH of the solution was
adjusted with either dilute HCJ or NaOH. The required concentration of fluoride solution
was prepared by serial dilution of I 000 mg L- 1 fluoride solution. The fluoride adsorption
experiments, from its aqueous solution, on CaO nanoparticles were carried out using
standard I 00 mg L- 1 fluoride solution in absence of any other competing ions. The
adsorption experiments were carried out by adding O.OI-O.I 0 g of adsorbent in I 00 ml of
125
Chapter 5
Calcium oxide nanoparticles
100 mg L- 1 fluoride solution in 250 ml PVC flask covered with a glass dish. Glass dish
was provided to avoid change in concentration due to evaporation. All the experiments
were carried out at ambient temperature (25 ± I °C). After continuous stirring using
magnetic stirrer (Remi Equipments, Mumbai, India) at about 400 rpm for a
predetermined time interval, the solid was separated by centrifugation (model no. R-4C,
Remi laboratory centrifuge) (3000 rpm) and filtered through IC Millex-LG filter
(Millipore) and the unadsorbed fluoride was estimated by Dionex ion chromatography
systemICS 1000. Operating conditions were as follows: column, IonPac® AS-11 HC (4 x
250 mm); suppressor conductivity ASRS® ULTRA II 4-mm Anion Self- regenerating
Suppressor (ASRS); eluent, 20 mM sodium hydroxide (Sigma-Aldrich); column
temperature 25±2°C; flow rate 1.5 ml min- 1; injection volume, 25 111; system backpressure
1920 psi; and background conductivity: 22 JlS. Fluoride peak was observed at the
retention time of 1.73 min. Each removal experiment was conducted twice to obtain
reproducible results with an error of less than 5%, more test runs were conducted as and
when required. The analytical results could be reproduced with an accuracy greater than
95%. The experimental parameters studied are adsorbent concentration, contact time,
initial fluoride concentration, pH, temperature, and competitive anion concentrations
(solutions of chloride, nitrate, sulphate, and phosphate).
126
Chapter 5
Calcium oxide nanoparticles
5.3 RESULTS AND DISCUSSIONS
5.3.1 Sol-gel method
As described m chapter I, there are several routes to prepare metal oxide
nanoparticles of the various techniques, the sol-gel method is well known to provide
finely pure and homogeneous particles. The sol-gel method is based on inorganic
polymerization reactions. The sol-gel process includes four steps: hydrolysis,
polycondensation, drying and thermal- decomposition. Considerable efforts have been
made for the development of synthetic approaches for the preparation metal oxide
nanoparticles during the last decade. 1'
45
'
In the present work, very simple and effective
sol-gel route is proposed for synthesis of homogeneous and narrow size distributed
calcium oxide nanoparticles using ethylene glycol and citric acid without any critical
conditions like pH adjustment, surfactants, controlled heating rate etc. The characteristics
of the prepared CaO crystalline powder samples have been investigated using
Transmission Electron Microscope (TEM), Dynamic Light Scattering (DLS), FT-IR
transmission and thermal analysis {TG-DTA).
5.3.2 Characterisation
Drying of gels is considered as very important intermediate stage in synthesizing
oxide nanoparticles.
Characterization of precursor gel
allows controlling and
understanding better the whole process of synthesis.
FT-IR
Fig. 1 shows the FTIR spectra associated with stretching and bending vibrations
from 4000-400 cm- 1 of the gel precursor and its decomposition products at various
127
Chapter 5
Calcium oxide nanoparticles
temperatures. The band at 3400 cm- 1 and 880 cm- 1 in gel (Fig. I a) which is due to OHand H20 vibration disappears after calcination of the gel at 900° C. The bands at 1385
cm- 1 and 1490 cm- 1 in gel corresponds to the presence of N03-2 ions, which also
disappear at higher temperature. A sharp band around 600 cm- 1 (Fig. ld), related to the
metal-oxygen stretching, appeared which sharpens at higher sintering temperature.
Thermal analysis
To have more understanding of the CaO forming mechanism from gel to the
oxide nanoparticles obtained by chelate gel route, the gel was thermally analyzed and the
resultant TGA and DTA curves are shown in Fig. 2 (a) and (b), respectively. During heat
treatment of the gel, several processes such as dehydration, oxidation of the residual
organic groups, decomposition and sintering took place. Fig. 2 shows the TG and DTA
curves of CaO dried gel precursor. In the DTA curve we observed an endothermic peak at
72 °C, which is due to evaporation of water and an exothermic peak at 860°C due to the
combustion of organic substances. The TGA plot for the gel precursor obtained from the
sol-gel process developed is shown in Fig. 2(a). TGA curve shows constant weight loss
due to decomposition of the gel and combustion of organic substances.26•
27
This leads to
homogeneous distributed oxides.
Transmission electron microscope
TEM samples were prepared by depositing a drop of dispersed CaO nanoparticles
in ethanol on the Formvar-coated copper grid (200 mesh), and the electron microscope
was operated at 100 keY. Fig. 3 shows transmission electron micrograph of CaO
nanoparticles. The averaged particle diameter obtained from TEM electro micrographs is
less than 15.0 nm.
128
Chapter 5
Calcium oxide nanoparticles
Dynamic light scattering
DLS samples were prepared by dipping a probe of DLS in the dispersed CaO
nanoparticles in ethanol. Fig. 4 shows dynamic light scattering histogram of CaO
nanoparticles. The mean particle diameter obtained from DLS graphs is 0.0146J.1m which
supports the TEM results.
5.3.3 Adsorption of fluoride on calcium oxide nanoparticles
The main objective was to study the different aspects of adsorption of fluoride ion
on CaO nanoparticles. Effect of adsorbent dose, effect of contact time, optimization of
the membrane composition, adsorption Kinetics, effect of temperature, effect of initial
fluoride concentration, selectivity, adsorption
isotherm, mechanism of Surface
Adsorption, effect of pH, effect of competitive ions and desorption study of fluoride from
synthetic solutions have been investigated in this study.
5.3.4 Effect of adsorbent dose
The effect of adsorbent dose on the removal of fluoride was studied at ambient
temperature (25±2°C) and contact time of 30 min for initial fluoride concentration of
100 mg L- 1• The results are presented in Fig.5. It is evident from the Fig. 5 that the
1
removal of fluoride increased from 21.2% to 98.0% for 0.01- 0.10 g L- of CaO.
Therefore the capacity of the adsorbent is 163333 mg Kg- 1• The distribution coefficient
Ko which reflects the binding ability of the surface for fluoride mainly depends on the
type of adsorbent surface. The distribution coefficient K0 for fluoride was calculated
according to the following equation:28
129
Chapter 5
Ko
Calcium oxide nanoparticles
(l)
CJCw (Lig)
where, Cs is the concentration of fluoride in the solid particles (mg g- 1) and Cw is fluoride
concentration in aqueous solution (mg L- 1). The concentration of fluoride in the solid
phase was calculated based on measurement of fluoride in the water before and after the
adsorption of fluoride on the solid phase. It is seen from the results in Table I that the
distribution coefficient (Ko) increases with the increase in nanoparticles concentration.
5.3.5 Effect of contact time
The percentage of fluoride adsorbed for initial fluoride concentration of
I 00 mg L- 1 keeping other parameter constant, is presented in Fig. 6. It is evident that the
adsorption process is very fast and most of the fluoride ions are adsorbed in the first I5
min and equilibrium was established, almost after 30 min. The change in the rate of
removal might be due to the fact that initially all adsorbent sites were vacant and the
solute concentration gradient was high. Later, the fluoride uptake rate by adsorbent was
decreased significantly, due to the decrease in number of adsorption sites as well as
fluoride concentration. Decreased removal rate, particularly, towards the end of
experiment, and the smooth removal curve indicates the possible monolayer formation of
fluoride ion on the outer surface of adsorbent.
5.3.6 Adsorption Kinetics
Adsorption of fluoride ion was rapid for the first I5 min and its rate gradually
slowed down as the equilibrium approached. The rate constant
Kad
for sorption of
fluoride was studied by the pseudo-first order rate expression, popularly known as the
130
Chapter 5
Calcium oxide nanoparticles
Lagergren equation, is generally described by the following equation (2) (Lagergren,
I898): 28
(2)
log (qe-qt) =log qe- Kad (t/2.303)
where, qe is the amount of fluoride adsorbed at equilibrium per unit weight of adsorbent
(mg g- 1); q1 is the amount of fluoride adsorbed at any time (mg g- 1). The straight line plot
of log (qe- q1) versus't' with r2 > 0.99, indicates the validity of Lagergren equation of
first order kinetics (Fig. 7). The adsorption rate constant (Kad) is calculated from the slope
ofthe above plot and was found to be 5.5 X 10-2 min- 1 for initial fluoride concentration
of I 00 mg L- 1 under experimental condition.
5.3.7 Effect oftemperature
The effect of temperature on the adsorption of fluoride with the initial
concentration of I 00 mg L- 1 was studied using optimum adsorbent dose (0.06 gil 00 mL).
A little adverse effect was observed on the adsorption of fluoride. The results are
represented as percentage removal of fluoride versus temperature (Fig. 8). The
percentage removal of fluoride with initial concentration decreased from 98.0% to 89.0%
for 298-353 K temperature, even though the fluoride adsorption at a given temperature
increased with time. This may be happening because the rise in temperature increases the
escaping tendency of the molecules from interface and thereby diminishes the extent of
adsorption. However it is seen from observations that the decrease in percentage removal
is not that large.
131
Chapter 5
Calcium oxide nanoparticles
This was further supported by calculating thermodynamic parameters such as
change in free energy (L\G), enthalpy (L\H) and entropy (L\S) of adsorption were
calculated using the following equations. 29
lnKo
L\S/R -
L\H/RT
L\G
L\H
TL\S
(3)
where, L\S and L\H are the changes in entropy and enthalpy of adsorption, respectively,
which was evaluated from the slope and intercept of van't Hoff plots (lnK 0 vs. 1/T)
respectively and is represented in Table 2. The positive value of entropy (L\S) indicates
increase in randomness of the ongoing process and hence a good affinity of fluoride for
CaO nanoparticles. Negative value of L\G at each temperature indicates the feasibility and
spontaneity of ongoing adsorption.
5.3.8 Effect of initial fluoride concentration
The adsorption of fluoride onto CaO nanoparticles was studied by varying initial
fluoride concentration using optimum adsorbent dose (0.06 g/100 mL) at ambient
temperature (25±2°C) for a contact time of 30 min. The results are represented in
graphical form as percentage removal versus initial fluoride concentration in Fig. 9. The
initial fluoride concentration was increased from 100 to 500 mg L- 1 and the
corresponding removal achieved a maximum value of98.0% (initial concentration of 100
mg L- 1) and then it gradually decreased to 55.8% (initial concentration of 500 mg L- 1).
However, it is clear from the Fig. 9 that, maximum removal takes place when the initial
132
Chapter 5
Calcium oxide nanoparticles
concentration is low (i.e. 100 mg L- 1) and removal is very less at high concentrations.
This result suggests the need of high adsorbent dose to reduce the residual fluoride to
permissible level.
5.3.9 Adsorption isotherm
The adsorption isotherm is the very first and significant parameter of adsorption
studies because the shape of the adsorption isotherm not only provides a quantitative
relationship between the adsorbed amount and equilibrium concentration of adsorbate,
but also reflects the manner in which the adsorbate molecules are adsorbed on the surface
of the adsorbent.
To model the present process, the equilibrium data was fitted to Fruendlich,
Langmuir, and Dubinin-Radushkevich (D-R) isotherm. The linearized Freundlich
adsorption isotherm can be written as:
logKr +
(4)
1/n logCe
where, qe is the adsorbed fluoride at equilibrium per unit mass of adsorbents (mg g-
1
),
Kr
is the minimum sorption capacity (mg g- 1) and 1/n is the adsorption intensity. Ce is the
equilibrium concentration of fluoride (mg L- 1). Fig. 10 shows a plot of log qe versus log
Ce. The constants 1/n and log Kr values are obtained from the slope and intercept
2
respectively. It is found that the related correlation coefficient R value for the Freundlich
model is 0.999. Freundlich isotherm with minimum adsorption capacity of 21.33 mg g- 1
of the adsorbent and adsorption intensity is 0.999. The intensity value <I indicates the
133
Chapter 5
Calcium oxide nanoparticles
favorable situation. The experimental data fit well to the Freundlich isotherm model and
validate the adsorption occurring on the homogeneous surfaces. 28
The linearized Langmuir equation, which is valid for monolayer sorption onto a
surface with finite number of identical sites, is written as: 29
+
(5)
where, q0 is the maximum amount of the fluoride ion per weight of CaO nanoparticles to
form a complete monolayer on the surface (adsorption capacity), Ce denotes equilibrium
adsorbate concentration in solution, qe is the amount adsorbed per· unit mass of adsorbent,
and b is the binding energy constant. The linear plot of liCe versus 1/qe (Fig. 11)
indicates the applicability of Langmuir adsorption isotherm. The values of Langmuir
parameters, q0 and bare 163.3 mg g- 1 and 0.043 L mg- 1, respectively.
In order to predict the adsorption efficiency of the adsorption process, the
dimensionless equilibrium parameter was determined by using the following equation
R
(6)
1/l+bCo
where, b is the Langmuir constant and C0 is the initial adsorbate concentration (mg L- 1),
R value indicate the type of isotherm be irreversible (R = 0), favorable (0 < R < 1), linear
(R =I) or unfavorable (R > 1). The R-value for initial concentration of 100 mg L- 1 was
found to be 0.699. The value indicated a favorable system. 30
134
Chapter 5
Calcium oxide nanopartic/es
It is known that the Langmuir and Freundlich adsorption isotherm constant do not
give any idea about the adsorption mechanism. In order to understand the adsorption
type, equilibrium data were tested with Dubinin-Radushkevich (DR) isotherm. 30 The
linearized DR equation can be written as:
(7)
where,
f:
is the Polanyi potential, and is equal to RT ln(l + 1/Ce), qe the amount of
fluoride adsorbed per unit mass of adsorbent, qm the theoretical adsorption capacity, Ce
the equilibrium concentration of fluoride, K the constant related to adsorption energy, R
the universal gas constant and T is the temperature in K. Fig. 12 shows the plot of lnqe
against fl, which was almost linear with correlation coefficient, R = 0.944. D-R isotherm
constant K was calculated from the slope ofthe plot. The value ofK was found to be -2.0
X
10-3 moe kr2 •
E
==
<-2Krl/2
(8)
It is defined as the free energy change when I mol of ion is transferred to the
surface of the solid from infinity in solution. The value of E was found to be 15.76 kJ
mol- 1• The value of E is very useful in estimating the type of adsorption and if the value
is less than 8 kJ mor 1' then the adsorption is physical in nature and if it is in between 8
and 16 kJ mol-l, then the adsorption is due to exchange of ions.32 The value found in the
present study was in between 8 and 16 kJ mol- 1 so the adsorption can be best explained
as exchange of ions.
135
Chapter 5
Calcium oxide nanopartic/es
5.3.10 Mechanism of Surface Adsorption
When CaO nanoparticles are added to water, it is converted into calcium
hydroxide immediately and the adsorption of fluoride occurs by surface chemical
reaction, in which hydroxide ions of calcium hydroxide are replaced rapidly by fluoride
ions with the formation of CaF2. Both of the above process can be represented by the
following equation:
CaO
+
Ca(OH)2
+
Ca(OH)2
+
CaF2(s)
+
20fr
(9)
Many researchers21 -25 have used various forms of lime [Ca(OH)2, CaO, CaC0 3] or
other calcium salts to increase calcium activity in solution, thereby removing fluoride by
precipitation as fluorite (CaF 2). In comparison all the above adsorbents show lower
adsorbent capacity against CaO nanoparticles which can be attributed to the small particle
size and large surface area of CaO nanopartices. Amount of fluoride removed is depends
on particle size and surface area of adsorbent. 19 Decrease of the particle size to the
nanometer range results in an increase of the dispersion (i.e. exposed surface area), which
in tum, increases the adsorption capacity and diminishes the adsorbent volume. TEM
micrographs ofCaO nanoparticles before and after adsorption are presented in Fig. 13(A)
and (B). The TEM micrograph clearly shows the surface modification.
5.3.11 Effect of pH
The most important single factor that controls the adsorption of ions on the oxide
surface is the pH of the aqueous solution. In basic (>8 pH) adsorbate medium, the
136
Chapter 5
Calcium oxide nanopartic/es
fluoride removal efficiency and capacity of the adsorbent decreased. The progressive
decrease (Fig. 14) of fluoride uptake at pH>8 is possibly due to the electrostatic repulsion
of fluoride ion to the negatively charged surface and the competition for active sites by
excessive amount of hydroxyl ions. 33 It is apparent that the percentage of fluoride
removal remains nearly constant at the lower pH (2-8). Since anion adsorption is coupled
with a release of Oir ion, the adsorption of the fluoride on the particle surface is
probably favored in low pH. The fluoride uptake capacity of the solution is not much
affected in the pH range less than or equal to 8, possibly due to the presence of positively
charged and neutral sites at the surface of the adsorbent.
5.3.12 Effect of competitive ions
Drinking water and waste water contains many anions. Therefore, it was thought
worthwhile to study the effect of competitive ions like chloride, sulphate, nitrate and
phosphate on the adsorption of fluoride. Varying concentration of these solutions was
prepared from their potassium salts (Merck). The initial concentration of fluoride was
fixed at 100 mg L-I while the initial concentrations of other anions varied from 100 to
500 mg L- 1 and added individually and mixed in the solution. The result ofthese studies
is given in table 3 and Fig. 15. It is clear from the result that the adsorption capacity for
fluoride decreased nominal (<2%) in the case of highest concentration 500 mg L- 1 of
other competitive ions. Fig. 15 is the chromatogram of the mixture of ions. This indicates
that the adsorption of fluoride is not adversely affected by the presence of other ions in
solution.
137
Chapter 5
Calcium oxide nanoparticles
5.3.13 Desorption Study
To make a cost effective and user-friendly process, the adsorbent should be
regenerated, so as to reuse for further fluoride adsorption. Desorption studies were carried
out by using the fluoride adsorbed CaO nanopartices adsorbent. First the fluorideadsorbed CaO nanoparticles are generated by adsorbing 100 mg L- 1 fluoride solution on
0.06 g/100 ml at pH 6.5. After the equilibration, the residue was filtered and the filtrate
was estimated for fluoride content. Then this fluoride-adsorbed CaO nanoparticles were
subjected for desorption studies by maintaining the pH between 2-12 by addition of0.1M
NaOH or 0.1 M HCI solution in each flask and the suspensions were conditioned for 30
min at 50 rpm. The process of adsorption is predominantly chemical in nature, and hence
the desorption of the adsorbed fluoride on adsorbent was only about 50%.
5.3.14 Comparison with other adsorbents
The capacity of novel CaO nanomaterial was found to be much higher than many
of the adsorbents reported recently (Table 4).
5.3.15 Analytical application
In Gujarat (India), there are many locations where fluoride is present in excess of
acceptable limits (>I mg L- 1) in ground water. The applicability of CaO nanoparticles in
the field condition was tested with water samples taken from a well of Lilia village,
Amareli distict, Gujarat, India. The sorbent reduced the level of fluoride concentration
from 18.0 mg L- 1 to 0.45 mg L- 1 using 0.060 g of CaO nanoparticle for 100 ml of sample
within 30 min at room temperature. Even though the sorbent was influenced by the
138
Chapter 5
Calcium oxide nanoparticles
presence of foreign anions, this factor did not affect lowering the fluoride level to the
tolerance limit in field conditions.
5.4 CONCLUSION
A novel material, CaO nanoparticles was prepared by sol-gel method. The results
of TEM and TGA-DTA indicate that particles are homogeneous and in nanometer size.
The following conclusions may be drawn:
(I) Kinetic study shows that removal of fluoride was found to be very rapid during the
initial period, i.e. most of the fluoride was removed within 15 min and reaches to
maximum of
98 % at 30 min.
(2) The adsorption followed first order rate kinetics, and data fit into linear form of
Fruendlich, Langmuir, and Dubinin-Radushkevich (DR) isotherm models.
(3) Other ions such as sulphate, nitrate and phosphate up to 500 mg L- 1 did not affect the
adsorption of fluoride thereby indicating that the calcium oxide nanoparticles is a
selective adsorbent for fluoride. Higher adsorption capacity of CaO nanoparticles makes
them to have great potential application in fluoride removal from water.
139
Calcium oxide nanopartic/es
Chapter 5
Figures and tables
(d)
(c)
(b)
(a)
4000
S$00
3000
2500
2000
Wavelength
1500
1000
500
cm-1
Fig.1. IR spectra of CaO nanoparticles at various temperatures: prepared by sol gel method (a)
•D
120
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100
2.0
80
0~
t.
4
lD
60
OD
...,
·11>
20
~
·1.0
0
200
400
l(Q
roo
800
1000
Fig. 2 (a) TGA curves of dried gel. {b) DTA curve of gel at the heating rate of 10° C min-
140
1
Chapter 5
Calcium oxide nanoparticles
••• •<
•
•
'
.
•••
r
Fig. 3 Transmission electron microscope image of CaO nanoparticles
100
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•
j
1 ; 1 >P
40
30
20
10
o+-;-rnTmr-~~~
0.000
0.001
om
0.1
10
Size(Microns)
Fig. 4 Dynamic light scattering size histogram of CaO nanoparticles
141
Calcium oxide nanoparticles
Chapter 5
100
ii
~ 80
E
~ 60
0
Q
til
40
..
20
"C
0
~
0
a.
0
0
0.02
0.04
0.06
0.08
0.1
0.12
Adsrobent dose in 113ms
Fig. 5 Adsorbent dose vs. percentage removal of fluoride
Table 1 Variation of distribution coefficient with adsorbent dose
Adsorbent dose (g)
Distribution Coefficient (Ko)
0.01
2.69
0.02
3.69
0.03
5.72
0.04
9.70
0.05
16.18
0.06
81.66
142
Calcium oxide nanoparticles
Chapter 5
120
100
iii
J
0
80
60
Q
Ill
c0
..
40
0..
20
u
0
0
0
10
5
15
20
25
30
35
40
Tmein minUtes
Fig. 6 Time vs. percentage removal of fluoride
2.500
2.000
i
1.500
i.3
1.000
0.500
0.000
~--..,.---.,....---.....----.----.----.
0
5
10
15
20
25
Time in minutes (t)
Fig. 7 Adsorption kintetics, time vs. log (Qe- q1)
143
30
Calcium oxide nanoparticles
Chapter 5
100
98
1
I.
96
i
92
-+-After 30 min
.....-After mln
90
-.-Atter90 min
&
tO
~
II>
0.
94
eo
es
86
320
300
360
340
Temparature (K)
Fig. 8 Temperature (K) vs. percentage removal of fluoride
Table 2 Thermodynamic parameters of fluoride adsorption
303
3.324
313
0.009
0.015
3.434
323
3.544
120
100
"li
I!
80
60
1!
el. 40
20
0
0
100
200
300
400
500
600
Initial Concentration In mgll
Fig. 9 Initial fluoride concentration vs. percentage removal
144
Calcium oxide nanoparticles
Chapter 5
2.5
2
•
17
c
.2
1.5
1
0.5
0
0
0.5
1.5
1
2
2.5
log C.
Fig. 10 Linearized Freundlich adsorption isotherm, logCe vs. logqe
0.07
0.06
c 0.05
~ 0.04
• 0.03
l!'
"" 0.02
0.01
0
s
0
0.02
0.04
0.06
0.08
0.1
1JC.(Umg)
Fig. 11 Langmuir adsorption isotherm 1/Ce vs. 1/qe
145
0.12
Chapter 5
Calcium oxide nanoparticles
0
..0.5
20
40
60
so
100
120
-1
,J -1.5
.5 -2
·2.5
-3
-3.5
t2
Fig. 12 DR adsorption isotherm, In qe vs. r.2
B
Fig. 13 TEM images of GaO nanoparticles (A) before and (B) after fluoride adsorption
Note: Some shape variation occurred on GaO nanoparticles after adsorption of fluoride.
146
Chapter 5
Calcium oxide nanoparticles
120
100
ii
I
80
Cll
60
~•
Ill
'E
•
l
40
20
0
0
4
2
8
6
10
14
12
pH
Fig. 14 pH vs. percentage removal of fluoride
Table 3 Effect of competitive ion on fluoride removal
Competitive ion Concentration in ppm
Fluoride Removal (%)
(Chloride, Sulphate, Nitrate and Phosphate)
10
98
20
98
30
97.5
40
97.5
50
97
147
Chapter 5
Calcium oxide nanoparticles
12
10
8
6
I
4
2
o~~~~~~~~~--~~~~~~
-2
x.,__ _...:__...;__..;,.~~:...;;;,..;:~...;_...;_~...;_-..=.._ _;_;r
Time(min)
-BeforeaddingCaOnanoparlldes
-After adding caonanoparlldes
L __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
i
_j
Fig. 15 Chromatogram of the mixture of competitive ions and fluoride
Table 4 Comparison of the present method with other reported methods
Adsorbent
Capacity (mg kg- )
1450
Activated alumina
Amorphous alumina supported
carbon nanotube
Zeolite
14
391
Hydrotalcite like compound
Soil
14900
12
15
1202
16
150
Carbon nanotube
Rare earth oxide
Quicklime
34
3000
35
123
36
8050
Present method (CaO nanoparticles)
148
16333
Chapter 5
Calcium oxide nanopartic/es
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150