Bachelor Degree of Chemical Engineering Final Year Project

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1
Kinetic and Thermodynamic Study of Fluoride
Removal by Using GO/Eggshell Adsorbent
Amirul Izan Mansor,1a*
, Dr Norhusna Mohamad Nor2b*
1
Faculty of Chemical Engineering, University Teknologi Mara (UiTM),Pulau Pinang, Malaysia
1
amirulizan.017@gmail.com , 2
norhusna8711@ppinang.edu.my
ARTICLE HISTORY ABSTRACT
Received
201X
Received in revised form
201X
Accepted
201X
Fluoride contamination in water bodies has been identified as
one of the major problems worldwide imposing serious threat
not only to human, but also to plant, animal and environment
generally. Hence, finding a sustainable and cost effective
adsorbent to reduce fluoride from water has become a major
challenge for researchers. The present work demonstrates one
of the new types of adsorbent materials, which is a modification
of graphene oxide (GO) with egg shell. Graphene oxide
impregnated with egg shell (GO/ES) was synthesized via the
Modified Hummer’s method prior the impregnation method. A
batch adsorption experiment was conducted to investigate the
adsorption of fluoride from aqueous solution by GO/ES
adsorbent. Parameters such as contact time and temperature
were evaluated. The results showed that GO/ES is an effective
fluoride adsorbent with an adsorption capacity of up to 28.12
mg/g at 25°C and 36.08 mg/g at 120 mins of contact time. The
adsorption kinetic analysis indicated that the adsorption data
can be well described by Pseudo-second-order kinetic model
which indicate chemisorption process, while thermodynamic
studies revealed that the adsorption reaction was an exothermic
process. The FTIR analysis was done to determine the
functional group responsible in fluoride adsorption.
Keywords: Graphene oxide, Egg shell, Fluoride, Adsorption, Kinetic and
Thermodynamic.
1.INTRODUCTION
Most of sewage and industrial effluents are disposed or discharged into water bodies
and on land causing serious pollution problem. Organic and inorganic substance contained in
the wastewater which sometime could be toxic are usually discharged without any appropriate
treatment into water body such as oceans, rivers, streams and lakes. One of the most common
pollutant identified in these water bodies is fluoride [1].
Fluoride is an ion of the chemical element fluorine whereas fluorine is the pure
elemental form typically found as F2. Fluorine belongs to the halogen group and it is the most
electronegative and reactive of all the elements. Fluoride in the other hand, can be found in
almost every water sources [2]. In the groundwater, the main source of fluoride is from the

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geological formation and natural sources such as weathering process and volcanic activity.
There are also minerals that is formed from the reaction between fluoride ion and existing
substance or components in nature. As a result, inorganic minerals such as hydrogen fluoride
(HF), calcium fluoride (CaF2), sodium fluoride (NaF), and sulfur hexafluoride (SF6) are formed
[3].
Use of phosphatic fertilizers for agricultural purposes and industrial activities the usage
of clays in ceramic industry or burning of coals can also contribute to high fluoride
concentrations in ground water [4]. However, our everyday needs such as the tooth pastes,
chewing gums, cosmetics, drugs, mouthwashes, and etc. also contain certain amount of fluoride.
If amount of fluoride is within the permissible limits, it has beneficial effects towards human
health where it can encourage healthy bone formation in human body. However, fluoride is
considered to be excessive if the concentration is more than 1.5 mg L-1.
This can cause negative
effect such as dental and skeletal fluorosis are observed at human body [5]. This limit which is
fixed by the WHO must be obeyed in order to prevent harmful consequent due to excessive
fluoride. Excessive fluoride in human body could lead to osteoporosis, thyroid disorder and
brittle bones [6]. Hence, it is very important to come out with an effective and systematic
methods to reduce the fluoride levels to the acceptable limit.
The adsorption phenomenon is probably the most widely used in separation method.
Variety of adsorbents are known for the removal of fluoride from drinking water or solutions.
One of the most emerging adsorbent used to remove fluoride is graphene [7]. Graphene, a new
carbon nanomaterial, has unique physical, chemical, electrical and mechanical properties.
Graphene oxide (GO) can be considered as an appealing member of carbon family due to the
presence of various functional groups such as hydroxyl, epoxy groups and carboxylic groups
and high surface area. GO has also proven to be an effective adsorbent for the adsorption of F-
ion in aqueous solution with a very good and proven adsorption capacity [8].
Generally, GO possess a good bio-compatibility and high electron transfer ability. It
also has high surface to volume ratio. Because of these special features of GO it can be easily
impregnated with various polymers and nanoparticles to enhance their adsorption capability
[9]. Due to these enhanced abilities between GO and nanoparticles, GO impregnated with
nanoparticles such as natural calcium carbonate offer great potential for various applications
including energy storage, energy conversion devices and adsorption [10].
2. EXPERIMENTAL
2.1 Materials
The GO is prepared by using the modified Hummers method. 2.5g of graphite powder
and 1.25 g of sodium nitrate (NaNO3) were mixed with 60 ml of sulphuric acid (H2SO4) and
was kept in an ice bath (0-6°C) while continuously stirred for duration of 2 hours. 7.5 g of
potassium permanganate (KMnO4) was added slowly into the mixture, then, the mixture was
continuously stirred. At this time the temperature of the mixture must be lower than 10°C. Then,
the ice bath was removed while the mixture’s temperature was maintained at 30°C for 2 hours
until it become pasty brownish in color. Then, 130 ml of DI water was added to the solution
slowly as exothermic reaction occurred and the color of the mixture changed to brownish as the

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temperature increased. Finally, oxidizer, which is 25 mL of hydrogen peroxide (H2O2) was
added in order to terminate the mixture. By repeatedly using 400 ml of hydrochloric acid (HCl)
and DI water, the mixture was filtered and washed prior dried in oven at 60°C for 1 day. Then,
5g of egg shell was mixed with 1 g GO in 300 mL of distilled water. Wait until the mixture
turns homogeneous dark yellow. Then, the resulting suspension will be sonicated for 5 minutes
and promote the intercalation of Ca2+
on the GO surface by stirring the mixture for 24 hours.
The suspension will be filtered and washed by using distilled water several times. Centrifuge
the filtrate at 3000 rpm and freeze-dried the filtrate. The Ca2+
ion will dispersed on the sheet of
graphene oxide.
2.2 Characterization of materials
The characterization study of GO/ES adsorbent was conducted by using the Fourier
Transform Infrared Spectroscopy (FTIR). FTIR spectrum of graphene was recorded with a
FTIR Spectrometer over the wave number range from 4000 to 400 cm-1
. The FTIR analysis is
done to determine the existences of functional groups in both GO and GO/ES adsorbent so that
the active functional groups that is responsible in adsorbing the fluoride ion could be identified.
2.3 Batch adsorption experiment
Batch adsorption experiments were conducted to determine the fluoride adsorption
equilibrium, kinetic and thermodynamic properties of the adsorption process. Fluoride stock
solution (1000 mg/L) was prepared by dissolving sodium fluoride (NaF) in deionized water and
further diluted to the required concentrations for the batch adsorption experiment.
The study on the effect of contact time were conducted by varying it from 30, 60, 90 to
120 minutes. 0.05g of GO/ES adsorbent will be added respectively into 3 conical flasks
containing 100mL of sodium fluoride each and will be stirred by using the orbital shaker at 200
rpm at ambient temperature. The recording time was started when GO/ES adsorbent was added
into the flasks. In the other hand, To determine the effect of temperature, adsorption
experiments will be carried out at 25, 45 and 65°C, respectively. 0.05g of GO/ES adsorbent
dosage are added into 3 conical flasks containing 100 mL of sodium fluoride solution each.
Then, the mixtures will be stirred by using the orbital shaker at 200rpm for 120 minutes and the
amount of removal of fluoride for each flask/sample will be measured
3 RESULT AND DISCUSSION
3.1 Characterization of materials
3.1.1 Fourier Transform Infrared Spectroscopy (FTIR)
The functional groups of both GO and GO/ES adsorbent were determined by using the
FTIR. Figure As seen in figure 1(a), medium and broad band where the peak is at 3203.88 cm-
1
indicates oxygen containing functional group which is hydroxyl. The next absorption peak
observed is at 1720.11 cm-1
and 1618.06 cm-1
which has medium intensity but sharp shape
indicating C=O stretching of carboxylic or/and carbonyl functional groups [11]. The next

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functional group determined is the C-O functional group where two medium and sharp
adsorption peak at about 1223.73 cm-1
and 1040.48 cm-1
are observed.
Figure 1: FTIR Spectrum of: (a) GO and (b) GO/ES adsorbent
GO/ES adsorbent spectrum showed correlation with GO spectrum. For instance, bands
that are already observed in GO spectrum were observed in GO/ES adsorbent spectrum, such
as strong and broad peak at 3203.48 cm-1
can be attributed to the appearance of O-H group. While
the bands at 1618.32 cm-1
indicate the existence of asymmetric and symmetric stretching vibration
of C=O groups. The new functional group that appear after impregnation is between 1600 cm-1
until
600 cm-1
where two sharp and strong peak, at about 1396.04 cm-1
and 871.95 cm-1,
correspond
to the stretching vibration of CaCO3 group. It justifies that the functional peak of CaCO3 and
the region on GO where CaCO3 components are attached [12].
3.2 Effect of contact time
As illustrated in Figure 2(a), the adsorption of fluoride increased with rise in contact
time upto 120 min and further increase in contact time did not enhance the fluoride adsorption
process. The adsorption process attained equilibrium after 100 min with 36.08mg/g adsorption
capacity achieved . The fast adsorption rate at the initial stage may be explained by an increased
availability in the number of active binding sites on the adsorbent surface [13]. The adsorption
rapidly occurs and normally controlled by the diffusion process from the bulk to the surface.
Figure 2: Effect of: (a) Contact time and (b) Temperature on the adsorption of fluoride
4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 515.0
28.4
30
35
40
45
50
55
60
65
70
75
80
85
86.6
cm-1
%T
3203.88
1618.06
1223.73
1040.48
4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 515.0
76.5
78
80
82
84
86
88
90
92
94
96
98
100.4
cm-1
%T
3203.48
1396.04
871.95
711.72
0
10
20
30
40
50
60
0 50 100 150
FluorideRemoval(%)
Contact time (minutes)
0
10
20
30
40
50
60
0 50 100
FluorideRemoval(%)
Temperature
a b
(a) (b)

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3.2 Effect of temperature
Figure 2(b) indicates that the adsorption rate or precipitation as CaF2 decreased with
increase in temperature where the highest adsorption capacity obtained, 28.12mg/g, is at the
lowest experimented temperature which is 25°C. This result indicates low temperature favors
the removal of fluoride molecules by adsorption onto GO/ES adsorbent. The decreasing of
removal may be due to at high temperature thethickness of the boundary layer decreases due to
increased tendency of the molecules toescape from the adsorbent surface to the solution phase,
which results in a decrease in theadsorption capacity as temperature is increased [14].
3.3 Adsorption kinetic study
In this research the kinetic analysis of fluoride adsorption by using GO/ES adsorbent
were analyzed using pseudo-first-order and pseudo-second-order kinetic models. The pseudo-
first-order equation of Lagregren is generally expressed as below [15],
303.2
log)log( 1tK
qqq ee  (1)
where qe and q are the adsorption capacities at equilibrium and at certain time of adsorption
respectively while K1 is the rate constant of pseudo-first-order adsorption. The pseudo-first-
order rate constant, K1, can be obtained from the slope of the graph of log(qe-q) versus time t
as shown in figure 3 below. The calculated K1 values and corresponding linear regression
correlation coefficient values are shown in Table 1.
Figure 3: Pseudo-first-order adsorption kinetic model
The pseudo-second-order adsorption kinetic model is expressed in equation below
which is also in linear form,
t
qqKq
t
ee







11
2
2
(2)
y = -0.015x + 1.815
R² = 0.9234
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 50 100 150
log(qe-q)
Time (minutes)

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Where K2 is the rate constant of pseudo-second-order adsorption model. Similar with pseudo-
first-order, the rate constant, K2 can be determined by the slope of the graph of t/q versus time
t as shown in figure 4 below.
Figure 4: Pseudo-second-order adsorption kinetic model
The adsorption capacity equilibrium, qe and the rate constant K for both models are also shown
in table below.
Table 1: Kinetic parameter for the adsorption of fluoride by GO/ES adsorbent
As shown in table 1, R1
2
values for pseudo-second-order kinetic model is higher than pseudo-
first-order, indicates that the pseudo-first-order model can be used to represent the kinetic of
adsorption of fluoride onto GO/ES adsorbent. According to pseudo-second-order theory, the
adsorption of fluoride onto GO/ES adsorbent is a chemisorption process which involve sharing
or exchange of electrons between the adsorbent [16].
3.4 Adsorption thermodynamic study
Thermodynamic parameters, such as change in Gibbs free energy (ΔGo ), enthalpy (ΔHo) and
entropy (ΔSo), were evaluated in this study. Ko, which is the equilibrium adsorption constant
calculated as the ratio between adsorption concentration and equilibrium concentration of
solution as shown in equation 3 below,
y = 0.0153x + 1.375
R² = 0.9386
0
0.5
1
1.5
2
2.5
3
3.5
0 50 100 150
t/q
Time (minutes)
Kinetic Model Parameters Values
Pseudo-first-order Rate Constant (K1) 0.0345
qe (mg/g) 37.36
R1
2
0.9234
Pseudo-second-order Rate Constant (K1) 0.0005
qe (mg/g) 37.36
R1
2
0.9386

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f
ae
o
C
C
K  (3)
The Gibbs free energy (ΔGo
) for adsorption of fluoride onto GO/ES adsorbent at all
temperatures was obtained from equation below where R is the gas constant (8.314 J/mol.K)
and T is the temperature in Kelvin (K).
d
o
KRTG ln (4)
In order to determine the value of ΔSo
and ΔHo
, from the linearized equation below, a graph of
log Ko vs. 1/T has been plotted. The values of ΔHo
and ΔSo
were determined from the slope
and intercept of the graph plotted as shown in figure 4.
RT
H
R
S
LogK
oo
o
303.2303.2




(5)
Figure5: Thermodynamic study of adsorption of fluoride onto GO/ES adsorbent
Table 2: Thermodynamic parameters for the adsorption of fluoride adsorbed by GO/ES
adsorbent
Temper
ature
(K)
Thermodynamics parameters
Ko ΔGo
(kJ/mol) ΔSo
(J/mol.K) ΔHo
(kJ/mol)
298.15 0.47 1.872
-57.78 -15.41318.15 0.336 2.885
333.15 0.244 3.902
-0.328
-0.474
-0.613
y = 804.89x - 3.0178
R² = 0.9899
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.00295 0.003 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034
LogKo
1/T

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As stated in table 2 above, the positive value of ΔGo
indicates that the process is a physic-
sorption process. This is because the values of ΔGo
in between 0 and to 20 kJ/mol indicate a
physic-sorption process, while the values in between −80 to −400 kJ/mol correspond to
chemisorption process [14]. The increment in value of ΔGo
with increase in temperature
suggests that higher temperature makes the adsorption unfavorable [17]. While the negative
value of ΔSo
indicate that the process is enthalpy driven and the negative value of ΔHo
implies
that the adsorption phenomenon is exothermic [15].
4 CONCLUSION
GO/ES adsorbent has considerable potential for the removal of fluoride from aqueous solution.
The presence of high fluoride content in water bodies results in the deterioration of water
quality. So fluoride removal is important in the control of quality of water bodies and adsorption
is tested to be one of the promising approach. GO was synthesized by modified Hummer’s
method before being impregnated with egg shell.
Batch adsorption experiments were carried out to investigate adsorption kinetics and
equilibrium. The characterization of the adsorbent was investigated using FTIR while in the
adsorption test, contact time, temperature, adsorption kinetics and thermodynamics of fluoride
removal were examined. The optima for contact time and temperature parameter were obtained
which are 120 minutes and 25o
C where 36.08mg/g and 28.12mg/g of adsorption capacity were
achieved.
The studies of adsorption of fluoride through the kinetic modeling has been investigated where
the adsorption kinetics is well-suited to pseudo-second-order kinetic model with correlation
coefficient of 0.9386. This indicates that the process is a chemisorption process. The
thermodynamic studies revealed from the value of ΔGo
, ΔHo
and ΔSo
of the adsorption of
fluoride were an exothermic process and decrement in the degree of freedom of the adsorbed
species. Increase in value of ΔGo
with increase in temperature suggests that higher temperature
makes the adsorption unfavorable.
FTIR analysis of both GO and GO/ES adsorbent was done and the FTIR spectrums obtained
confirmed that GO had been successfully synthesized and calcium carbonate had been
composited over the GO surface indicating successful impregnation of egg shell onto the GO.
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