Abstract. This study aimed to evaluate the ability of magnesium chloride modified carbonized
rice hull (MCRH) material for ammonium removal from synthetic and domestic wastewater. The
MCRH material was prepared by using waste rice hull from a household rice-processing factory
and magnesium chloride salt via a simple mixing and annealing method. The material was then
characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The
effects of rice hull modification by magnesium chloride and experimental conditions such as
initial ammonium concentration (20 - 100 mg/L), amount of adsorbent (0.8 - 2.0 g/L), and
adsorption time (0 - 32 h) on the ammonium removal efficiency and adsorption capacity were
investigated. Adsorption kinetic and isotherms were also studied for MCRH material. Results
showed that magnesium was successfully added on carbonized rice hull with Mg/C molar ratio
of 0.22. Ammonium adsorption isotherm fitted well to Langmuir model with maximum
adsorption capacity of 65.36 mg/g. The adsorption was physical process and adsorption kinetic
was best described by intra-particle diffusion model with the correlation coefficients ranged
from 0.942 - 0.979. Ammonium adsorption feasibility of MCRH was proved through the
treatment of domestic wastewater containing 80.7 ± 1.6 mg/L initial ammonium concentration
with removal efficiency of 86.8 %. Ammonium concentration of the effluent met the allowable
value (10 mg/L) as given by QCVN 14: 2008/BTNMT (column B) - National technical
regulation on domestic wastewater. In conclusion, MCRH has a potential for being an
economical and abundant material in Vietnam. The post adsorption material that accumulated
ammonium would be potentially used for soil quality improvement.
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Vietnam Journal of Science and Technology 58 (3A) (2020) 113-123
doi:10.15625/2525-2518/58/3A/14322
STUDY ON THE REMOVAL OF AMMONIUM IN WASTEWATER
USING ADSORBENT PREPARED FROM RICE HULL AND
MAGNESIUM CHLORIDE
Nguyen Thi Thuy
1
, Tran Dang Lan Van
2
, Nguyen Xuan Hoan
1
, Le Thi Bich Son
1
,
Tran Thi Ngoc Mai
1
, Dang Van Thanh
3
, Nguyen Nhat Huy
2, *
1
Ho Chi Minh City University of Food Industry, 140 Le Trong Tan, Tan Phu District,
Ho Chi Minh City, Viet Nam
2
Faculty of Environment and Natural Resources, Ho Chi Minh City University of Technology,
VNU-HCM, 268 Ly Thuong Kiet Street, District 10, Ho Chi Minh City, Viet Nam
3
Faculty of Basic Sciences, TNU-University of Medicine and Pharmacy, 284 Luong Ngoc
Quyen, Quang Trung Ward, Thai Nguyen City, Thai Nguyen Province, Viet Nam
*
Email: nnhuy@hcmut.edu.vn
Received: 23 August 2019; Accepted for publication: 23 October 2020
Abstract. This study aimed to evaluate the ability of magnesium chloride modified carbonized
rice hull (MCRH) material for ammonium removal from synthetic and domestic wastewater. The
MCRH material was prepared by using waste rice hull from a household rice-processing factory
and magnesium chloride salt via a simple mixing and annealing method. The material was then
characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The
effects of rice hull modification by magnesium chloride and experimental conditions such as
initial ammonium concentration (20 - 100 mg/L), amount of adsorbent (0.8 - 2.0 g/L), and
adsorption time (0 - 32 h) on the ammonium removal efficiency and adsorption capacity were
investigated. Adsorption kinetic and isotherms were also studied for MCRH material. Results
showed that magnesium was successfully added on carbonized rice hull with Mg/C molar ratio
of 0.22. Ammonium adsorption isotherm fitted well to Langmuir model with maximum
adsorption capacity of 65.36 mg/g. The adsorption was physical process and adsorption kinetic
was best described by intra-particle diffusion model with the correlation coefficients ranged
from 0.942 - 0.979. Ammonium adsorption feasibility of MCRH was proved through the
treatment of domestic wastewater containing 80.7 ± 1.6 mg/L initial ammonium concentration
with removal efficiency of 86.8 %. Ammonium concentration of the effluent met the allowable
value (10 mg/L) as given by QCVN 14: 2008/BTNMT (column B) - National technical
regulation on domestic wastewater. In conclusion, MCRH has a potential for being an
economical and abundant material in Vietnam. The post adsorption material that accumulated
ammonium would be potentially used for soil quality improvement.
Keywords: ammonium, rice hull, adsorption, wastewater treatment, magnesium chloride.
Classification numbers: 2.4.2, 3.7.3, 3.3.2.
Nguyen Thi Thuy, et al.
114
1. INTRODUCTION
Domestic wastewater is a prominent source together with agricultural runoff contribution
for eutrophication around the globe [1]. Together with organic materials and phosphorous,
ammonium is one of the main contaminants presented in the domestic wastewater. Ammonium
contributes to the surface water eutrophication, acidification of soil, vegetation fertilization, and
changes in ecosystem. In addition, this component can be converted to nitrate in aerobic
conditions which eventually leached to groundwater [2]. Hence, ammonium is a contaminant
that needs to be removed from wastewater before discharging into the environment or water
reuse. Several technologies have been developed to remove ammonium in wastewater, including
biological treatment, ammonia stripping, chemical precipitation, and ion exchange, in which
biological treatment is considered as the most common process used for ammonium removal
from domestic wastewater. The advantages and disadvantages of these methods can be found
elsewhere [3].
Bio-charcoal is a carbon-rich, fine-grained, and porous material produced from
carbonization of biomass. This material can be prepared from various feedstocks (e.g., wood
bark, dairy manure, sugar beet tailing, pinewood), which are considered as a promising
adsorbent as cost-effective and environmentally sustainable products for integrated waste
management [2]. Though many types of bio-charcoal have been synthesized using different
feedstocks [2, 4, 5] and modified rice hull for ammonium removal has been reported [6-8], it is
the first time that rice hull modified by magnesium chloride was applied for ammonium
adsorption in domestic wastewater.
In this study, magnesium chloride modified carbonized rice hull (MCRH) were synthesized
using impregnation via simple mixing and annealing method. Since the adsorption treatment was
developed to apply for municipal wastewater, synthetic wastewater was prepared with initial
ammonium concentration ranged from 20 - 100 mg/L. Effect of magnesium chloride
modification, initial ammonium concentration, adsorbent amount, and adsorption time on
ammonium removal efficiency and adsorption capacity were investigated. Adsorption isotherms
and kinetics studies were further conducted. In final step, domestic wastewater was collected and
treated by MCRH to find the feasibility of the material in real application.
2. MATERIALS AND METHODS
2.1. Materials
Rice hull was obtained from a household rice mill, dried at 55
o
C overnight, and then used
as the feedstock biomass. To prepare synthetic wastewater containing NH4
+
at different
concentrations, a desired amount of ammonium chloride salt (NH4Cl) was dissolved in distilled
water. Domestic wastewater was collected from the campus of Ho Chi Minh City University of
Food Industry (Ho Chi Minh City, Vietnam) during May and June 2019. MgCl2.7H2O was used
to prepare 20 % w/w MgCl2 solution. All chemicals used in this study were of analytical grade.
2.2. Adsorbent preparation
The porous MCRH adsorbent was produced from rice hull following the modified
adsorption and pyrolysis method used by Li et al. [5]. At first, 200 mL of 20% MgCl2 solution
was mixed with 10 g of rice hull in Erlenmeyer flask and the mixture was shaken by an
oscillator (LabTech, LMS-2, Korea) at 120 rpm under room temperature (~ 26
o
C in a
Study on the removal of ammonium in wastewater using adsorbent prepared from rice
115
conditioned room) for 24 h. The mixture was subsequently heated at 80
o
C until the water
evaporated and the remained material was dried at 105
o
C in an oven (Memmert, UN55,
Germany) for 6 h. The dried mixture was finally transferred into a porcelain container and
placed in a furnace (Lenton, EF11/8B, England) for pyrolysis at 550
o
C for 1 h. This MCRH
material was then characterized by scanning electron microscopy (SEM) and energy-dispersive
X-ray spectroscopy (EDS) (JOEL, JSM-IT200, Japan). Sole carbonized rice hull (CRH) was also
prepared with the similar procedure but without the addition of MgCl2.
2.3. Adsorption performance tests
General adsorption procedure: 1.0 g of MCRH were added into 1 L of ammonium
synthesized sample. This sample was kept shaking at 120 rpm under room temperature and
pressure. 10 mL of the treated sample was then filtered through membrane filter (0.22 µm, nylon
cellulose, GE) and ammonium concentration was analyzed according to SMEWW 4500 –
NH3.B&D, 2012 using an automatic analyzer (Kjeltec 2300).
Effect of different factors on ammonium removal efficiency were investigated. (i) Effects
of magnesium chloride modification (20 %) used in material synthesis on adsorption efficiency
of 50 mg/L ammonia solution were firstly tested. The adsorption process was conducted for 24 h
(followed a published literature, [5]) using MCRH and then followed similar procedure as
mentioned above. For comparison purpose, the material without MgCl2 modification
(carbonized rice hull, CRH) was also prepared and tested with ammonia adsorption under the
same conditon with MCRH material. (ii) To test the effects of initial ammonia concentration and
time of treatment, ammonia concentrations were prepared from 20 to 100 mg/L. (iii) To test the
effect of time on the adsorption, experiments were conducted for ammonium concentration of 50
mg/L with a total adsorption time of 32 h. In this experiment, ammonium was measured every
one hour. For the (i), (ii), and (iii) experiments, adsorbent concentration used were 1.0 g/L. (iv)
Effect of adsorbent concentration (0.8 – 2.0 g/L) on the ammonia removal efficiency of 81.3
mg/L synthetic wastewater was tested for 27 h (obtained from experiment No. iii). All
experiments were repeated at least 3 times and the average values were reported.
Removal efficiency was calculated as followed equation (1):
Removal efficiency (%) = (1)
where Co is initial ammonia concentration (mg/L) and Ct is ammonia concentration at time t
(mg/L).
The data from experiment No. (iv) was then applied for Langmuir and Freundlich isotherm
models, as given by Equations (2) and (3) [5].
Langmuir model:
qe = KqmaxCe/(1+KCe) (2)
Freundlich model:
qe = KfCe
1/ n
(3)
where qe is the equilibrium adsorption capacity (mg/g); K is the Langmuir bonding term
correlated to interaction energy (L/mg) and Kf is the Freundlich affinity coefficient (mg
(1-n)
L
n
g
-1
);
qmax represents the Langmuir maximum capacity (mg/g); Ce is equilibrium solution
concentration (mg/L) of ammonia; n is the Freundlich linearity constant.
Nguyen Thi Thuy, et al.
116
A linearized form of the Dubinin–Radushkevich isotherm is given in Equation (4) [6]:
ln(qe) = ln(qs)−(Kadε
2
) (4)
where qs is theoretical isotherm saturation capacity (mg/g), Kad is Dubinin–Radushkevich
isotherm constant (mol
2
/kJ
2
), and ε is Polanyi potential, which is expressed in Equation (5):
ε = RTln(1+1/Ce) (5)
where R is universal gas constant (8.314 J/mol K) and T is absolute temperature (K). Kad is the
constant in Dubinin–Radushkevich isotherm indicating of the mean adsorption energy (E)
(kJ/mol) of adsorption/mole of the adsorbate. It can be calculated by Equation (6):
E=1/ (6)
(v) To investigate the kinetic of the adsorption, initial ammonium concentration was
prepared from 20 -100 mg/L. Adsorption was conducted for 27 h and ammonium concentration
was measured for each four hours. Equations (7), (8) and (9) express pseudo-first-order, pseudo-
second-order kinetic and intra particle diffusion models, respectively, which were applied to fit
the adsorption kinetics experiment data [5].
Pseudo-first order kinetic model:
ln (qe – qt) = ln qe – k1t (7)
Pseudo-second order kinetic model:
t/qt = 1/(k2qe
2
) + t/qe (8)
Intra-particle diffusion model:
qt = kpt
1/2
+ C (9)
where k1 and k2 are the first-order and second-order apparent adsorption rate constants (1/h and
g/mg/h), respectively; kp is the intra-particle diffusion rate constant (mg/g/h
1/2
); C is the
thickness of the boundary layer; qt and qe are the amount of ammonia adsorbed (mg/g) at time t
and at equilibrium, respectively.
To evaluate the potential of the adsorbent material for domestic wastewater treatment,
adsorption experiment was conducted by mixing MCRH with domestic wastewater taken from
the campus of Ho Chi Minh City University of Food Industry. The procedure was similar to the
general adsorption procedure mentioned above with MCRH concentration was the value gained
from the experiment No. (iv) as mentioned above.
3. RESULTS AND DISCUSSION
3.1. Magnesium chloride modified carbonized rice hull characterization
In this experiment, MCRH prepared using 20 % MgCl2 solution was characterized by EDS
analysis. Figure 1 showed the presence of C, O, Mg, and Cl as major components of MCRH
material. In the inset of Figure 1, mapping result of MCRH proved the successful adding and
distribution of Mg element on rice hull support. The data from Table 1 identified that Mg
contributed 13.75 % (w/w) and the Mg/C molar ratio of MCRH material was calculated to be
0.22.
Study on the removal of ammonium in wastewater using adsorbent prepared from rice
117
Figure 1. EDS result of MCRH (inset is mapping result).
Table 1. Element composition of MCRH.
3.2. Comparison of ammonium adsorption between MCRH and CRH
In this experiment, we compared ammonium removal ability between MCRH and the sole
carbonized rice hull (CRH) to find whether modification of rice hull with MgCl2 positively
changed the treatment efficiency.
As can be seen in Figure 2, MCRH provided significantly higher ammonium removal
efficiency and capacity (90.3 % and 45.9 mg/g, respectively) than CRH material (21.6 % and 8.7
mg/g, respectively). According to Li et al. [5], material (sugarcane residue) modified by MgCl2
provided MgO at the surface of the modified material which helped to increase the removal
efficiency of pollutants such as ammonium. Hence, we also expected the addition of MgO on
MCRH which enhanced the adsorption ability of this adsorbent.
CRH MCRH
R
e
m
o
v
a
l e
ff
ic
ie
n
c
y
(
%
),
a
d
s
o
rp
tio
n
c
a
p
a
c
tit
y
q
e
(
m
g
/g
)
0
20
40
60
80
100
Removal ef f iciency
qe
Figure 2. Removal efficiency and adsorption capacity of CRH and MCRH
(initial concentration of 50 mg/L).
3.3. Effect of initial ammonium concentration
Element Mass % Atom %
C 30.22 ± 0.51 44.36 ± 0.75
O 29.44 ± 0.58 32.44 ± 0.64
Mg 13.75 ± 0.38 9.97 ± 0.27
Cl 26.60 ± 0.71 13.23 ± 0.35
Total 100.00 100.00
Nguyen Thi Thuy, et al.
118
Ammonia concentration in domestic wastewater can vary from different studies, e.g. 17 -
48 mg/L [1], 20 - 75 mg/L [9], and 33.4 mg/L [10]. In this experiment, initial ammonia
concentration was prepared from 20 to 100 mg/L and the adsorption was conducted for 24 h
with MCRH concentration of 1.0 g/L. As can be seen from Figure 3, increasing ammonia
concentration from 20 to 100 mg/L resulted in the significant increase of adsorption capacity,
which reached 64.4 mg/g at initial ammonium concentration of 100 mg/L. This trend was similar
to the one found by Sarkhot et al. [4] when NH4
+
concentrations were in the low concentration
range in pure solution (0-100 mg/L). Accordingly, further increase of ammonium concentration
(100-1000 mg/L) also increased adsorption capacity but at a lower rate.
As observed in Figure 3, the change of ammonium concentration in the effluent suggested
that the wastewater with initial ammonium concentration less than or equal to 60 mg/L treated
by 1.0 g/L of adsorbent can produce the effluent ammonium concentration lower than the
allowable value for domestic wastewater (10 mg/L, QCVN 14 : 2008/BTNMT, column B). For
initial ammonium concentrations higher than 60 mg/L, the applied adsorbent concentration of
1.0 g/L may not be sufficient for the treatment.
Initial concentration (mg/L)
20 40 60 80 100
C
e
(
m
g
/L
)
0
10
20
30
40
50
q
e
(
m
g
/g
),
r
e
m
o
va
l
e
ffi
c
ie
n
c
y
(
%
)
0
20
40
60
80
100
Ce
qe
Removal efficiency
Figure 3. Ammonium effluent concentration, removal efficiency and adsorption capacity of MCRH at
various initial ammonium concentrations of 20 - 100 mg/L.
3.4. Effect of adsorption time
In this experiment, the effect of adsorption time was evaluated by conducting the treatment
for 32 h and ammonium concentration was measured and recorded for each hour. As can be seen
from Figure 4, ammonium concentration decreased sharply for the first 4 h, then continuously
decreased but at slower speeds. After 13 h of treatment, ammonium concentration in the effluent
was approximate 10 mg/L. Further treatment of the solution leaded to the decrease of
ammonium concentration to 1.6 mg/L after 27 hours, and then remained unchanged for 3 h later.
Hence, we considered the adsorption process reached its equilibrium after 27 h of treatment.
After 27 h of adsorption, 97.0 % of ammonium was removed, corresponding to the
adsorption capacity of 50.8 mg/g. This adsorption capacity is significantly higher than the values
gained from Sarkhot et al. [4] (i.e. about 0.75 mg/g), who used the material made by hardwood
shavings to treat 50 mg/L NH4
+
solution for 24 h, and the capacity (i.e. 20 mg/g) found by Li et
al. [5] who synthesized MgO impregnated material of sugarcane crop harvest residue using
MgCl2 solutions with mass concentration from 2 to 20 % to treat swine wastewater.
Study on the removal of ammonium in wastewater using adsorbent prepared from rice
119
t (h)
0 5 10 15 20 25 30
C
t
(m
g
/L
)
0
10
20
30
40
50
q
t
(m
g
/g
),
r
e
m
o
va
l e
ff
ic
ie
n
c
y
(%
)
0
20
40
60
80
100
Ct
qt
Removal efficiency
QCVN 14 : 2008/BTNMT, column B
Figure 4. Changes of ammonium concentration (Ct), adsorption capacity (qt) and removal efficiency by
time (initial concentration of 50 mg/L).
3.5. Effect of adsorbent concentration
As mentioned above, using 1.0 g/L of adsorbent to treat the solution with initial ammonium
concentration less than or equal to 60 mg/L would result in the effluent meeting well the
allowable value of ammonium. For the wastewater containing higher initial ammonium
concentrations, effect of adsorbent concentration on the treatment efficiency need to be
investigated to find the suitable adsorbent amount. Since initial ammonium concentration in the
domestic wastewater taken from the dormitory were 80.7 ± 1.6 mg/L (n = 3), we tested the effect
of MCRH concentration on the treatment of synthetic wastewater prepared at initial
concentration of 81.3 mg/L. As can be seen from Figure 5, increase of adsorbent led to the
increase of ammonium removal, significantly from 0.8 to 1.5 g/L, slightly from 1.5 - 1.8 g/L,
and almost unchanged after that. After 27 h of treatment, using 1.8 g/L or higher concentration
of adsorbent resulted in the ammonium concentration in the effluent below 10 mg/L. The result
from this experiment hence suggests the ratio of adsorbent and initial ammonium concentration
was 22.5 g/g.
Biochar concentration (g/L)
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
C
e
(
m
g
/L
),
q
e
(
m
g
/g
),
r
e
m
o
va
l e
ff
ic
ie
n
c
y
(%
)
0
20
40
60
80
100
qe
Ce
Removal efficiency
QCVN 14 : 2008/BTNMT, column B
3.6. Adsorption isotherms and kinetics
Ammonium adsorption data were fitted to Langmuir, Freundlich, and Dubinin–
Radushkevich isotherm models as given in Figure 6 and values of parameters for these three
models are provided in Table 2.
Figure 5. Effect of adsorbent dose on
effluent ammonium concentration,
removal efficiency and adsorption
capacity of MCRH (initial concentration
of 81.3 mg/L).
Nguyen Thi Thuy, et al.
120
Obviously, Langmuir and Dubinin–Radushkevich isotherm models described ammonium
adsorption better than Freundlich isotherm model, which is consistent to the results from the
literature [2, 5]. However, several previous studies found that the fitness of Freundlich isotherm
model was similar (for rice husk biochar) [6] or slightly better than Langmuir isotherm model [7].
Langmuir isotherms
Ce(mg/L)
0 10 20 30 40
C
e
/q
e
(
g
/L
)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
y = 0.0138x+0.0753
R
2
= 0.976
Freundlich isotherm
log (Ce)
0.6 0.8 1.0 1.2 1.4 1.6 1.8
lo
g
(
q
e
)
1.0
1.2
1.4
1.6
1.8
2.0
y = 0.209x+1.473
R
2
= 0.695
Dubinin–Radushkevich isotherm
ε2
1000 2000 3000 4000 5000 6000
L
n
(
q
e
)
3.4
3.5
3.6
3.7
3.8
3.9
4.0
4.1
4.2
y = -0.0002x + 4.376
R
2
= 0.967
Figure 6. Langmuir, Freundlich, and Dubinin–Radushkevich isotherms.
Table 2. Isotherm model parameters from adsorpti