The fly ash (FA) can be converted to zeolite P by
hydrothermal treatment in NaOH solution. After
treatment by NaOH solution, BET specific surface
area and small pores volume (micropore) of FA are
increased. The FA treated by NaOH solution and
H2SO4 solution has more effective adsorption ability
for Hg2+ and Cd2+ ions than the untreated FA. The
FA treated by NaOH solution has adsorption
capacity for Hg2+ and Cd2+ ions higher than the FA
treated by H2SO4 solution and the untreated FA.
Langmuir model is more suitable than Freundlich
model for the simulation of experimental data and
expressing adsorption isotherm for Hg2+ and Cd2+
ions using FA treated by NaOH solution
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Vietnam Journal of Chemistry, International Edition, 55(2): 196-201, 2017
DOI: 10.15625/2525-2321.2017-00443
196
Using fly ash treated by NaOH and H2SO4 solutions
for Hg2+ and Cd2+ ion adsorption
Nguyen Thuy Chinh
1*
, Tran Thi Mai
1
, Nguyen Thi Thu Trang
1
,
Nguyen Thi Thanh Huong
2
, Thai Hoang
1
1
Institute for Tropical Technology, Vietnam Academy of Science and Technology
2
Hai Duong Medical Technical University
Received 4 July 2016; Accepted for publication 11 April 2017
Abstract
This paper presents the results of adsorption ability of heavy metal ions (Hg
2+
and Cd
2+
) by fly ash (FA) before and
after treatment using NaOH and H2SO4 solutions. Original- and treated FA were characterized by Fourier Transform
Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), and Scanning Electron Microscope (SEM). Specific surface
area of FA before and after treatment was calculated by Brunauer – Emmett – Teller (BET) isotherm equation. The
obtained results indicated that the morphology and specific surface area of FA changed clearly after treatment by acid
or alkaline solutions. Adsorption capacity the Hg
2+
and Cd
2+
ion by FA was determined from data of UV-Vis spectra.
After treatment, the adsorption capacity of ions by FA increased remarkably in comparison with non-treated FA. The
FA treated by NaOH solution has the adsorption capacity higher than FA treated by H2SO4 solution. The maximum
adsorption capacity of the FA treated by NaOH solution for Cd
2+
and Hg
2+
ions at room temperature is 28.97 and 14.60
mg/g, respectively. The equilibrium adsorption data were described by the Langmuir and Freundlich isotherm models.
The results showed that equilibrium data were fitted well to the Langmuir isotherm.
Keywords. Fly ash, treatment, adsorption capacity, heavy metal, Langmuir isotherm.
1. INTRODUCTION
Heavy metals in water pollutants are especially
dangerous for human body due to the potential to
bio-accumulate. Among them, the ions such as
mercury (Hg
2+
) and cadmium (Cd
2+
) have the
highest toxicity. Mercury (Hg) has the ability to
react with aminoacids containing sulfur, the
hemoglobin, and albumin. It also can link the
membrane and make the change in potassium
content balance between the acid and base of the
tissues, causing energy shortages provide neurons.
Cadmium (Cd) entering the body accumulates in the
kidneys and bones, jams activity of some enzymes,
causes hypertension, lung cancer, renal dysfunction,
destroys bone marrow, affects the endocrine, blood,
heart. The common method used to remove toxic
heavy metal from municipal and industrial waste
water are the adsorption of heavy metal ions onto
insoluble compounds and the separation of the
formed sediments [1-4]. Some materials applied for
removing heavy metals in agriculture and forest
wastes were reported such as bagasse fly ash [4],
sugar beet pulp [5], activated carbon derived from
bagasse [6], humus [7], bituminous coal, and
kaolinite [8].
Fly ash (FA) is a solid waste produced from the
combustion of carbon and other fossil fuels from
thermal power plants. It has become an economic
and environmental burden. In the recent years, the
study using FA for adsorption of some heavy metal
ions in waste water has been focused [9-11].
However, FA has not been used for adsorption of
Hg
2+
and Cd
2+
ions. Thus, in this study, FA treated
by H2SO4 and NaOH solutions was selected for this
purpose. The characteristics of non-treated and
treated FA including morphology, structure, and
special surface area were presented. The adsorption
capacity of the Hg
2+
and Cd
2+
ions by non-treated
and treated FA is also discussed.
2. EXPERIMENTAL
2.1. Materials
Fly ash (FA): FA particles were supplied from
Pha Lai Thermal Power Plant (Vietnam). The total
accumulated weight percentage of SiO2, Fe2O3, and
VJC, 55(2), 2017 Nguyen Thuy Chinh et al.
197
Al2O3 is approximately ca. 86 wt.% whereas the
content of the moisture is about 0.3 wt.%. The
chemical composition of FA is presented in table 1
[12]. Sulfuric acid (H2SO4) 98 % and sodium
hydroxide (NaOH) are the commercial products of
China.
Table 1: The chemical composition of FA [12]
Major oxides [%]
SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 MnO LOI
a
53.32 22.05 8.97 5.24 2.44 2.66 0.63 1.07 0.08 1.58
2.2. Treatment of FA
FA particles were hydrothermally treated by
NaOH solution as follows: 200 ml of NaOH 0.5 M
(pH = 13) solution was added into a flask containing
20g of FA and stirred at 70 °C for 8 hours, then
continuous stirring at room temperature for 24
hours. After that, FA treated by NaOH solution was
filtered and washed with distilled water until filtered
aqueous reached pH 7. The treated FA particles were
dried in an oven at 80 ºC for 12 hours and were
abbreviated N-FA.
The FA particles treated by H2SO4 0.25 M (pH =
1) solution were also carried out similarly to N-FA
and were abbreviated H-FA.
2.3. Adsorption of Hg
2+
and Cd
2+
ions by treated
FA particles
2.3.1. Hg
2+
adsorption by N-FA
300 mg of N-FA were added into a 100 ml of
Hg
2+
solution 200 mg/L. The solution was stirred at
room temperature for 120 mins. After 120 min
stirring, the solution was filtered and 5 ml of
aliquots was withdrawn and the concentration of
Hg
2+
was monitored by UV Spectrophotometer
(CINTRA 40, GBC, USA) at max 218 nm. All
studies were done in triplicate.
2.3.2. Cd
2+
adsorption by N-FA
Cd
2+
ion adsorption by N-FA was also carried
out similarly to Hg
2+
ion adsorption, in which Cd
2+
solution has concentration of 140 mg/L at max 215
nm.
2.3.3. Hg
2+
and Cd
2+
adsorption by H-FA
Experiments for Hg
2+
and Cd
2+
ion adsorption by
H-FA were proceeding similarly to the above ions
adsorption by N-FA.
2.3.4. Adsorption isotherms
For solid–liquid system, adsorption isotherm is
important in description of adsorption behavior. In
this work, two important isotherms, Langmuir and
Freundlich isotherms have been selected.
Langmuir isotherm takes an assumption that the
adsorption occurs at specific homogeneous sites
within the adsorbent. The general form of Langmuir
isotherm equation for Hg(II) and Cd(II) adsorption
can be written as:
is the adsorbent amount of the ions (mol/g), is
the equilibrium concentration of the ions in solution
(M), is the monolayer adsorption capacity
(mol/g) and k is the constant related to the free
energy of adsorption (L/mol).
The Freundlich isotherm is an empirical equation
employed to describe heterogeneous systems. The
Freundlich equation is expressed:
Where k and n are Freundlich adsorption isotherm
constants, being indicative of the extent of the
adsorption and the degree of non-linearity between
solution concentration and adsorption, respectively.
2.4. Characterization
2.4.1. Fourier transforms infrared spectroscopy
(FTIR)
FT-IR spectra were recorded on a Nicolet/Nexus
670 spectrometer (USA) at Institute for Tropical
Technology, VAST at room temperature in the
wavenumbers range from 400 to 4000 cm
-1
by
averaging 16 scans with a resolution of 4 cm
-1
.
2.4.2. Field emission scanning electron microscopy
(FESEM)
FESEM images were obtained with S-4800 SEM
(Hitachi, Japan) at Institute of Materials Science,
VAST to observe the morphology of the FA before
and after treatment.
2.4.3. X-ray diffraction (XRD)
XRD analyses of samples were performed on A
VJC, 55(2), 2017 Using fly ash treated by NaOH and H2SO4
198
Siemens D5000 X-ray Diffractometer (XRD) with CuKα
radiation source at a generator voltage of 40 kV and
a current of 30 mA in the 2θ scan range from 10° to
60° at Institute of Science Materials, VAST.
2.4.4. Brunauer-Emmett-Teller (BET) Isotherm
Equation
The surface area of fly ash before and after
treatment was determined by nitrogen sorption
method BET on Micromeritics Tristar 3000 devices
at Faculty of Chemistry, Hanoi National University
of Education.
3. RESULTS AND DISCUSSION
3.1. FTIR analysis
FTIR spectra of FA, FA treated by H2SO4 - and
NaOH solutions are shown in Fig. 1. It can be seen
that the FTIR spectra of FA, N-FA and H-FA are
relatively similar. They exhibit the peaks
characterized for Si-O, Al-O, Si-OH group in FA.
For instance, peak at 555 cm
-1
was corresponding to
bending vibration of Al-O-Al while the asymmetric
stretching vibration of Si-O-Si and Si-O-Al was
attributed by the absorption peak at 790 cm
-1
and
1066 cm
-1
. The hydroxyl group in FA was found in
1629 cm
-1
and 3438 cm
-1
[4]. The non-appearance of
any new peaks in FTIR spectra of treated FA can be
suggested that the treatment FA by NaOH or H2SO4
does not cause the change in chemical structure.
Figure 1: FTIR spectra of FA and FA treated by
H2SO4 - and NaOH solutions
3.2. Morphology analysis
Figure 2 presents FESEM images of FA, FA
treated by H2SO4 and NaOH solutions. The FA
particles have the spherical shape with size in the
range 1 to 5 µm.
(a) (b)
(c)
Figure 2: FESEM images of FA (a), FA treated by H2SO4 solution (b) and NaOH solution (c)
The untreated FA particles have slippery and
smooth surface. After the acidic treatment (H2SO4
solution), there are no changes in the morphology of
H-FA in the comparison with untreated FA. After
treatment by NaOH solution, N-FA particles have
the rough and scabrous surface. This suggests that
the FA treated by NaOH solution has the adsorption
ability of heavy metal ions better than untreated FA
and H-FA.
3.3. XRD analysis
The crystalline phases of FA and treated FA
determined by XRD analyses are performed in Fig.
3. It indicates that the untreated FA is composed of
quartz, mullite, and hematite. The XRD pattern of
FA treated by H2SO4 solution shows similarly to that
of the untreated FA. Interestingly, there is a new
phase as zeolite P (Na6Al6Si10O32.12H2O) which was
appeared on the XRD pattern of FA treated by
NaOH solution. This can be caused by the effect of
NaOH solution on the conversion of alumino-silicate
materials by the reaction between NaOH with SiO2
and Al2O3 in FA. It can make change the electric
charge between the Al–O and Si–O bonds resulting
VJC, 55(2), 2017 Nguyen Thuy Chinh et al.
199
in the polarization of the chemical bonds and the
enhancement of their chemically-active centers (of
positive and negative charge) in the frame of N-FA.
Thus, terminal groups such as Si–OH, Si–ONa,
Si–O– and ( Si–O)3Al–O– are developed along
with treating by NaOH agent [13]. This is proved by
the appearance of some new peaks at 2θ = 13, 2θ =
16, 2θ = 27, 2θ = 55 on the XRD pattern of N-FA. It
confirms that chemical restructuring has occurred
within FA treated by NaOH solution.
Figure 3: XRD patterns of FA (a), FA treated by
H2SO4 solution (b) and NaOH solution (c)
3.4. Specific surface area
The specific surface area and pores volume of
untreated FA and FA treated by NaOH solution are
showed in table 2.
Table 2: Specific surface area and pores volume of
untreated FA and FA treated by NaOH solution
Sample
Specific surface
area BET
(m
2
/g)
Volume of
pores (cm
3
/g)
FA 2.1376 0.003
N-FA 3.5178 0.006
From the table 2, it can see that FA treated by
alkaline solution can enhance to create multiple
holes micro (micropore), leading to specific surface
area and the pores volume of N-FA are increased in
comparison with the untreated FA. This is consistent
with the results of morphological analysis.
3.4. Adsorption of Hg
2+
and Cd
2+
ions by
untreated FA and treated FA
3.4.1. Hg
2+
and Cd
2+
ions adsorption
Fig. 4 displays equilibrium adsorption capacity
of Hg
2+
and Cd
2+
ions by untreated FA and treated
FA. The adsorption capacity of the FA after treated
by NaOH solution for Hg
2+
and Cd
2+
ions is higher
than that of the FA untreated and FA after treated by
H2SO4 solution. The adsorption capacities of the FA
before and after treated by H2SO4 - and NaOH
solutions for Hg
2+
are 4.13, 8.23 and 28.97 mg/g,
respectively. Similarly, the adsorption capacities for
Cd
2+
ions using FA before and after treated by
H2SO4 - and NaOH solutions are 3.08, 4.05 and
14.60 mg/g, respectively. Thus, the FA treated by
NaOH solution is the most appropriate for Hg
2+
and
Cd
2+
ions adsorption.
0
10
20
30
40
50
60
70
0
1
2
3
4
A
b
so
rp
ti
o
n
c
ap
ac
it
y
(
m
g
/g
)
Hg
2+
Cd2+
Figure 4: Adsorption capacity of Hg
2+
and Cd
2+
ions
by FA and treated FA, 1: FA, 2: H-FA, 3: N-FA
3.4.2. Adsorption isotherms
The Langmuir isotherm parameter Q0 indicates
the maximum adsorption capacity of the material, in
other words, the adsorption of metal ions at high
concentration. Langmuir parameter K indicates the
bond energy of the complexation reaction of the
material with the metal ion.
The Freundlich isotherm parameter k indicates
the adsorption capacity when the concentration of
the metal ion in equilibrium is unitary, in our case 1
L/mol. This parameter is useful in the evaluation of
the adsorption capacity of metal ions in dilute
solutions, a case closer to the characteristics of
industrial effluents.
To describe the adsorption isothermal, the
experimental data are matched in turn with
Langmuir and Freundlich equation. The appropriate
levels of the equation are evaluated through
regression coefficients R
2
.
Figs 5 and 6 present Hg
2+
- and Cd
2+
ions adsorption isotherm by FA treated by
VJC, 55(2), 2017 Using fly ash treated by NaOH and H2SO4
200
NaOH solution according to the Langmuir and
Freundlich models. High regression coefficients of
linearized Langmuir and Freundlich equations
indicate that these models can explain metal ion
adsorption by the chosen materials.
Figure 5: Hg
2+
adsorption isotherm using FA treated by NaOH (pH = 13)
solution: Langmuir model (a) and Freundlich model (b)
Figure 6: Cd
2+
adsorption isotherm using FA treated by NaOH (pH = 13)
solution: Langmuir model (a) and Freundlich model (b)
It is clear that the Langmuir model has
regression coefficients (R
2
1) higher than
Freundlich model Therefore, the Langmuir model is
more suitable than the Freundlich model for the
simulation of experimental data. This means the
adsorption centers on the surface of FA have the
same energy and the existence of a maximum
absorbance value can correspond to the creation of a
saturation single layer of heavy metal ions. For the
adsorption of heavy metals including Hg
2+
and Cd
2+
ions, the treated FA has effective adsorption
capacity for Cd
2+
ion higher than Hg
2+
ion due to that
Hg has atomic radius larger than Cd [14].
4. CONCLUSIONS
The fly ash (FA) can be converted to zeolite P by
hydrothermal treatment in NaOH solution. After
treatment by NaOH solution, BET specific surface
area and small pores volume (micropore) of FA are
increased. The FA treated by NaOH solution and
H2SO4 solution has more effective adsorption ability
for Hg
2+
and Cd
2+
ions than the untreated FA. The
FA treated by NaOH solution has adsorption
capacity for Hg
2+
and Cd
2+
ions higher than the FA
treated by H2SO4 solution and the untreated FA.
Langmuir model is more suitable than Freundlich
model for the simulation of experimental data and
expressing adsorption isotherm for Hg
2+
and Cd
2+
ions using FA treated by NaOH solution.
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90.00
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Corresponding author: Nguyen Thuy Chinh
Institute for Tropical Technology
Vietnam Academy of Science and Technology
No. 18, Hoang Quoc Viet Str., Cau Giay Dist., Hanoi
E-mail: thuychinhhn@gmail.com.
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