Removal Efficiency of an Aluminum Based Hydrotalcite-Like-Compound on Arsenic

Journal of Science & Technology 118 (2017) 030-035
30
Removal Efficiency of an Aluminum Based Hydrotalcite-Like-Compound
on Arsenic
Dao Thi Hong Nhung, Vo Thi Le Ha, Ly Bich Thuy*
Hanoi University of Science and Technology – No. 1, Dai Co Viet Str., Hai Ba Trung, Ha Noi, Viet Nam
Received: April 03, 2017; accepted: June 9, 2017
Abstract
Hydrotalcite has been considered as a promised adsorbent to remove arsenic. In this study, bench scale
studies were conducted to evaluate the effects of adsorbent dose and initial pH on the efficiency for
removing arsenic (As)(III) and As(V) by adsorption onto an Al-based hydrotalcite (named MA3). Then the
maximum removal adsorption capacity and isothermal models were investigated. The results showed that
the optimum pH values for removing As(III) and As(V) were 10 and 5-6, respectively. The investigated MA3
were capable of removing As(III) and As(V) at maximum capacity of 0.14 and 21.9 mg/g, respectively at
25 oC and optimum pH. Langmuir isotherm model was better fitted to invested data than Freundlich isotherm
model for both As(III) and As(V) with R2 value of 0.93 and 0.99, respectively. It is implied that the adsorption
mechanisms were closed with monolayer adsorption.
Keywords: Hydrotalcite, arsenic removal, As(III), As(V)
1. Introduction*
Arsenic (As) pollution in ground water has been
recognized in several places in Asia including the
Red river delta and the Mekong river delta in
Vietnam [1, 2]. Levels of As in ground water in Red
river delta were as high as 201- 427 μg/L [3].
Sand filtration to remove iron is practically
being applied in almost water treatment plants and
households using ground water in Vietnam. A large
part of arsenic can be co-removed in sand filtration
system. However, the arsenic levels after several sand
filtration systems are still higher than WHO
recommendation and Vietnamese standard for
drinking water (QCVN 01: 2009/BYT) of 10 g/L
[4]. Arsenic extra-treatment step is required to treat
As to meet mentioned standard.
Several adsorption materials have been studied
in Vietnam to remove As, such as latorite, limonite,
mangan oxide, AC with Zn, nano Fe and hydrotalcite
[5-8].
Hydrotalcite (HT) is classified as a layered
double hydroxide (LDH) composed of metal complex
hydroxide: [M2+1-xM3+x(OH)2]x+[(An-)x/n nH2O]x- (x =
0.2 - 0.33), where M2+ and M3+ are divalent and
trivalent metal ions, respectively, and An- is one of
any anionic ion [9]. The structure of HT consists of a
positive charged brucite-like octahedral layer and a
negatively charged interlayer containing anions and
* Corresponding author: Tel.: (+84) 1256981722
Email: thuy.lybich@hust.edu.vn
water molecules [9]. Figure 1 illustrates the structure
of a HT. In HT, the positively charged layer is formed
by partial substitution of a trivalent metal for a
divalent one (Mg). The layers can be stacked, and the
balancing interlayer anions can be exchanged with
other anions. HT has been received huge attentions in
recent years as an ion-exchanger, catalyst and
environmental treatment [10-12]. In Vietnam, a
research on As removal had been done for an
activated HT which had original formula of
[Mg6Al2(OH)16](CO3).4H2O on removal of As(V)
showing the adsorption capacity of 0.3 mg/g [8, 9].
This HT was activated at 450oC as it had better
adsorption ability than inactivated one [9]. In this
research, we aimed at determining absorption
capacity of an untreated HT with similar formula in
hydroxide layer to evaluate the potential to apply this
HT in the practice without any pre-treatment of
material.
Fig. 1. Structure of an Al based HT
Journal of Science & Technology 118 (2017) 030-035
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2. Materials and Methods
2.1. Materials
[Mg6Al2(OH)12]2+[NO3.CO3.zH2O]2- was
obtained from Tomita Pharmaceutical Co., LTD.
Analytical grade chemical were used to prepare
for 1000 mg/L stock solution of As(III) and As(V).
pH adjustment solution, HCl and NaOH 0.1 M,
were prepared from analytical grade solution.
2.2. Batch experiments
2.2.1. Arsenic adsorption
Batch adsorption experiments were carried to
examine the affecting factors including adsorbent
doses (0.05-1 g/L), pH (4-11) on arsenic uptake
performance. The suitable adsorption doses were
chosen to investigate for pH effect. Optimum pH
value was applied for further experiments.
The isothermal experiments were also carried
out at optimum pH and at initial concentration of
As(III) and As(V) ranged of 10 µg/L - 10 mg/L and
of 20 µg/l - 230 mg/L, respectively.
All these experiments were performed by using
100 mL glass conical flasks with 50 ml As solution
shaken at 150 rpm within 30 min at 25 oC. The
experiment period of 30 min was chosen from
previous study that the equilibrium state could be
fully obtained. 1M HCl or 1M NaOH solution was
used to adjust pH. After the shaken period, solution
was separated from material by filtration. All samples
were prepared and analyzed in triplicate.
The pHpzc determination [13]
Batch equilibrium method was employed to
measure pHpzc of MA3. The initial pH values were
adjusted within the range of 2-12 using 0.1 M and
0.01 M NaCl, the dosage of MA3 was given 1 g/L for
As(III) and 0.25 g/L for As(V). The mixtures were
equilibrated for 24 hours, before the final pH values
of solution were measured.
The removal efficiency was calculated using
equation (1):
( )(%) 100o t
o
C C
Efficiency
C
 (1)
Where Co was the initial concentration arsenic
in the feed and Ct was the concentration of arsenic at
time t.
2.3. Analytic method
Concentrations of As were analyzed by ICP-MS
Perkin Elmer.
3. Results and disscution
3. 1. Effects of adsorbent dose on As(III) and As(V)
removal rate
The effects of adsorbent doses on As(III) and
As(V) removal capability were examined. The results
were presented in Figure 2. As(III) removal rate
increased slowly and approached 17 % at the highest
investigated dose of 1 g/L. As(V) removal efficiency
increased rapidly with the increasing of MA3 dose to
the criterial point around dose of 0.2 – 0.25 g/L that
removal rate reached to 90 %. Thereafter a very
slowly increasing was observed with the increasing of
adsorbent dose. The up-trends can be explained by
the fact that the availability of more sorption sites
with an increment in the sorbent mass resulting on the
increase in the As uptake. MA3 yielded low As(III)
removal capacity, but it had a good affinity toward
As(V). As(V) can be treated by MA3 from 50 μg/L to
5 μg/L, fully meet QCVN 01- 2009/BYT.
The adsorbent doses of 0.25 and 1 g/L were
used for further experiments for adsorbing As(V) and
As(III), respectively. Those doses were chosen that
the removal rate is high enough that the difference of
removal rate corresponding with different pH can be
well observed. Besides, the dose should not be so
high after the criterial point that the unfavorite pH
condition could be overcome by a high dose of
adsorbent.
Fig. 2. Effect of adsorbent dose on:
a) As(III) removal (pH= 7, Co= 100 μg/L)
b) As(V) removal (pH=7, Co = 50 μg/L)
Journal of Science & Technology 118 (2017) 030-035
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3.2. Effects of pH on As(III) and As(V) removal rate
The effects of pH on As(III) and As(V) removal
rates were presented in Figure 3. The removal rate of
As(III) was more or less the same in the range of pH
from 4 to 9. A peak of removal rate was obtained at
pH from 9 to 11 with the tip at pH of 10. The removal
rate at pH of 10 was more than 3 times higher than
removal rate at plateaus area approaching 22 %.
As(V) removal rate was varied moderately in the
range of 60 % to 90 %. The removal efficiency of
As(V) achieved the maximum percentage of 90 % at
pH of 5-6. The highest removal percentage of
activated HT for the investigated pH range of 6-11
was 11 [8]. The difference of optimum pH between
inactivated and activated HT can mainly be explained
because of the structure change of HT when be
activated.
Fig. 3. Effect of pH on:
a) As(III) removal (adsorption dose = 1 mg/L,
Co= 100 μg/L)
b) As(V) removals (adsorption dose = 0.25 mg/L,
Co = 50 μg/L)
Regarding to the pHpzc values, the point of zero
charge (PZC) of MA3 adsorbent was ranged from 6.5
to 7. When pH value was lower than the pHpzc, the
adsorbent surface was more positively charged. In
that pH range, H2AsO4- was the predominant As(V)
species [7]. Therefore, a more amount of As(V) was
adsorbed on the surface of MA3 due to the
electrostatic attraction of surface and As(V). On the
contrary, when the pH values were higher than pHpzc,
the adsorbent surface was prevailing negatively
charged. In that condition, a more amount of As(V)
remained in the solution because of electrostatic
repulsive forces between the negatively charged
surface and As(V) compounds. Therefore As(V)
removal rate was lower. These findings could be used
to explain for the trend of adsorbate removal under
the effect of pH.
In case of As(III), at pH<pHPzc, MA3 surface
was positive charged. However, at that pH range,
As(III) occurred predominantly in neutral form
(H3AsO3). Electrostatic force including Columbic
attraction of repulsion between active sites and As
species did not appear resulting in low removal rate
of As(III). At pH from pHPzc to 9, the negative charge
surface of MA3 also did not interact well with neutral
form of As(III). However, when pH reached 10,
As(III) in aqueous solution existed mainly in the form
of H2AsO32-, ion-exchange reaction happened
between H2AsO32- and CO32- inside MA3. Therefore,
As(III) was removed the most effectively at the pH of
10. The As(III) removal trend decreased when pH
reached 11. It was likely because of the competitive
reaction between ion OH- and H2AsO32- in
exchanging with ion CO32- leading to the less
favorable adsorption condition for As(III) [14-16].
3.3. Adsorption capacity of MA3 on As
The Langmuir and Freundlich equations were
used to describe the interaction between adsorbate
and adsorbent. Those models can be represented as in
the equation 2 and 3, respectively:
max max
1
.
e e
e
C C
q q b q
 (2)
1ln ln lne eq K C
n
 (3)
where Ce is the equilibrium concentration of
adsorbate in the solution, qe is quantity of As
adsorbed at equilibrium, qmax is maximum adsorption
capacity (monolayer capacity); b is Langmuir
constant related to binding energy; K and 1/n are
Freundlich constants related to adsorption capacity
and adsorption intensity, respectively [17].
In Figure 4 and Figure 5, isothermal curves and
Langmuir and Freudlich graphs of As(III) and As(V)
were presented, respectively. Both Langmuir and
Freundlich models fitted reasonably well with the
experiment data. Langmuir isotherm model had
correlation coefficient (R2) = 0.93 and 0.99 for
Journal of Science & Technology 118 (2017) 030-035
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Fig. 4. Interaction between MA3 and As(III): a)
Isothermal curve, b) Langmuir isothermal equation,
c) Freundlich isothermal equation at pH = 10,
adsorbent dose = 1 mg/L.
As(III) and As(V), respectively, could describe the
isotherm better than the Freundlich isotherm model
(R2 = 0.88 and 0.91). It implied that the adsorption
processes were closer with monolayer adsorption than
non-specific adsorption on herterotrophic surface.
The maximum adsorption capacities of MA3 for
As(III) and As(V) varied from 0.14 to 21.9 mg/g,
higher than adsorption capacities of other materials
for As(V) such as latorite of 1.1 and limonite of 0.9
[6] illite of 0.52, kaolinite of 0.86, montmorillonite
[18], synthesis material from ferric sludge of 0.024
mg/g [19] and activated HT of 0.3 mg/g [8]. The
maximum adsoption capacity of MA3 was much
higher than the activated HT in research of Tho et al.
[8] because the adsorption of activated HT was done
at Co = 330 g/L much lower than the range of As(V)
investigated in this research. Comparing with
adsorption capacity of MA3 at Co=100 g/L,
adsorption dose of 0.05 g/L, pH = 7 a same range
adsorption capacity of 0.38 mg/g was obtained
(Figure 2b).
Fig. 5. Interaction between MA3 and As(V): a)
Isothermal curve, b) Langmuir isotherm equation, c)
Freundlich isotherm equation at pH = 6, adsorbent
dose = 0.25 mg/L
Journal of Science & Technology 118 (2017) 030-035
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Table 1. Langmuir and Freundlich isotherm constants
for MA3
Species Langmuirqmax (mg/g) b R2
As(III) 0.14 0.001 0.93
As(V) 21.9 3.3 10-4 0.99
Freundlich
n K R2
As(III) 0.05 2.1 0.89
As(V) 3.1 2.2 0.95
4. Conclusions
The MA3 material demonstrated a good capacity of
arsenic adsorption (e.g As(V)), highlighting its
potential application for ground water treatment
process. Under the optimum pH of 10, adsorbent dose
of 1 g/L, 30 min contacting time, percentage of
As(III) removal was 22%. Under the optimum pH of
5, 6, adsorbent dose of of 0.05 g/L, 30 min contacting
time, percentage of As(V) removal was 90 %. The
arsenic adsorption curve was fitted well with both
Langmuir and Freudlich isotherm model, in which
Langmuir model describe better than Freundlich
isotherm model. The maximum adsorption capacity
in removing As(V) and As(III) were 21.9 and 0.14
mg/g, respectively.
Acknowledgments
This research is funded by the Hanoi University
of Science and Technology (HUST) under project
number: T2016-PC-136.
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