This paper presents the results of adsorption ability of heavy metal ions (Hg2+ and Cd2+) 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 Hg2+ and Cd2+ 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 Cd2+ and Hg2+ 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

Chinh, Nguyen Thuy

Hoang, Thai

Mai, Tran Thi

Thanh Huong, Nguyen Thi

Thu Trang, Nguyen Thi

English

Vietnam Academy of Science and Technology: Journals Online

  
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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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. 
 


Using fly ash treated by NaOH and H2SO4 solutions for Hg2+ and Cd2+ ion adsorption.

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Using fly ash treated by NaOH and H2SO4 solutions for Hg2+ and Cd2+ ion adsorption.

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