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gels
Article
Synthesis of Oxidant Functionalised Cationic Polymer
Hydrogel for Enhanced Removal of Arsenic (III)
Yu Song , Takehiko Gotoh * and Satoshi Nakai
Citation: Song, Y.; Gotoh, T.; Nakai,
S. Synthesis of Oxidant
Functionalised Cationic Polymer
Hydrogel for Enhanced Removal of
Arsenic (III). Gels 2021,7, 197.
https://doi.org/10.3390/gels7040197
Academic Editors: Maria Valentina
Dinu and Gaio Paradossi
Received: 14 September 2021
Accepted: 31 October 2021
Published: 4 November 2021
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Attribution (CC BY) license (https://
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4.0/).
Laboratory of Polymer Engineering, Department of Chemical Engineering, Faculty of Engineering,
Hiroshima University, Hiroshima 7398527, Japan; d215240@hiroshima-u.ac.jp (Y.S.);
sn4247621@hiroshima-u.ac.jp (S.N.)
*Correspondence: tgoto@hiroshima-u.ac.jp
Abstract:
A cationic polymer gel (N-[3-(dimethylamino)propyl]acrylamide, methyl chloride quaternary)
(DMAPAA-Q gel)-supported oxidising agent (KMnO
4
or K
2
Cr
2
O
7
) was proposed to remove As from
water. The gel could adsorb arsenite, As(III), and arsenate, As(V), through the ion exchange method,
where the oxidising agent oxidised As(III) to As(V). theoretically speaking, the amount of oxidant
in the gels can reach 73.7 Mol%. The maximal adsorption capacity of the D-Mn gel (DMAPAA-Q
gel carrying MnO
4−
) and D-Cr gel (DMAPAA-Q gel carrying Cr
2
O
72−
) for As(III) could reach
200 mg g
−1
and 263 mg g
−1
, respectively; moreover, the As(III) removal rate of the gels could still
be maintained above 85% in a neutral or weak acid aquatic solution. Studies on the kinetic and
adsorption isotherms indicated that the As adsorption by the D-Mn and D-Cr gels was dominated by
chemisorption. The thermodynamic parameters of adsorption confirmed that the adsorption was an
endothermic process. The removal of As is influenced by the co-existing high-valence anions. Based
on these results, the gels were found to be efficient for the As(III) adsorption and could be employed
for the As(III) removal from the industrial wastewater.
Keywords: hydrogel; oxidant; arsenic removal; Langmuir model; kinetics; co-existing ions
1. Introduction
In the recent years, the reports of As contamination exceeding the environmental and
wastewater standards in industrial wastewater have been increasing worldwide [
1
–
6
]. Ac-
cording to the latest fact sheets from WHO [
7
], tens of millions of people have consumed
high-concentration As-contaminated groundwater [
8
,
9
]; health problems such as skin can-
cer caused by the As contaminants have become a major social problem [
2
,
10
,
11
]. Therefore,
the development of advanced materials or technologies for the efficient treatment of As
wastewater continues to be a global research priority. Various removal methods have been
developed and applied to treat the As-contaminated wastewater, such as co-precipitation
for the removal of arsenous acid using Fe(III) and Mn oxide, adsorption methods using
various adsorbents and minerals, and ion exchange methods [
4
,
5
,
12
–
14
]. Among them,
adsorption is widely used because of its technological simplicity, high efficiency, and
low secondary pollution risk [
15
]. However, some conventional adsorbents have many
drawbacks, such as high cost, few adsorbent sites, and small absorption capacity, which
limits their application in the water treatment. Moreover, some conventional adsorbents
have a low removal efficiency towards the highly toxic As(III), which further limits their
application in water treatment [16].
Among the many adsorbents developed and used for As(III) removal, the applica-
tion of polymer sorbents has been the most popular approach for As(III) removal from
industrial wastewater, owing to their environmental safety, superior chemical modifiability,
many adsorbent sites, and outstanding renewability [
17
–
19
]. There are two main types of
polymer adsorbents, one of which adsorbs via electrostatic interactions through its cationic
functional groups, while the other adsorbs by a functional group that has a high affinity for
Gels 2021,7, 197. https://doi.org/10.3390/gels7040197 https://www.mdpi.com/journal/gels
Gels 2021,7, 197 2 of 13
As. Among the cationic polymers [
20
], the polymers with amino groups are particularly
well known. For example, N,N-dimethylamino propylacrylamide(DMAPAAQ) is a well-
known quaternary ammonium salt-type cationic monomer. The quaternary amino group
of DMAPAAQ is positively charged, which could attract counter anion Cl
−
to maintain the
charge balance [
21
]. Therefore, it is possible to either load the necessary anions or remove
the harmful anions by exchanging the Cl−ions with them.
The main forms of As in the groundwater are arsenic acid (H
3
AsO
4
) and arsenous acid
(H
3
AsO
3
), which exist as molecules and exhibit negatively charged ionic states [
22
]. The As
in groundwater often exists as arsenous acid as groundwater is in a reducing environment.
It is known that As(III) has a lower adsorption capacity than As(V). For example, activated
alumina (Al
2
O
3
), Fe compounds, such as Fe hydroxide, and Ce oxide [
23
] are known as As
removers. However, the As ions are hardly adsorbed on the activated Al
2
O
3
. In addition,
it is generally considered that the trivalent As (H
3
AsO
3
) is more toxic than the pentavalent
As (H
3
AsO
4
) form. As illustrated in Figure 1, the anions of H
3
AsO
4
exist over a wide pH
range, whereas H
3
AsO
3
does not dissociate in the acidic range and its anions exist only
above pH 7, owing to which, it is less adsorbed. Therefore, it is necessary to oxidise the
arsenite into arsenate in order to increase the removal efficiency. In other words, to increase
the removal efficiency of the trivalent As, both an increase in the number of adsorption
sites and the oxidation of As is essential.
Gels 2021, 7, x FOR PEER REVIEW 2 of 14
polymer adsorbents, one of which adsorbs via electrostatic interactions through its cati-
onic functional groups, while the other adsorbs by a functional group that has a high af-
finity for As. Among the cationic polymers [20], the polymers with amino groups are par-
ticularly well known. For example, N,N-dimethylamino propylacrylamide(DMAPAAQ)
is a well-known quaternary ammonium salt-type cationic monomer. The quaternary
amino group of DMAPAAQ is positively charged, which could attract counter anion Cl−
to maintain the charge balance [21]. Therefore, it is possible to either load the necessary
anions or remove the harmful anions by exchanging the Cl− ions with them.
The main forms of As in the groundwater are arsenic acid (H3AsO4) and arsenous
acid (H3AsO3), which exist as molecules and exhibit negatively charged ionic states [22].
The As in groundwater often exists as arsenous acid as groundwater is in a reducing en-
vironment. It is known that As(III) has a lower adsorption capacity than As(V). For exam-
ple, activated alumina (Al2O3), Fe compounds, such as Fe hydroxide, and Ce oxide [23]
are known as As removers. However, the As ions are hardly adsorbed on the activated
Al2O3. In addition, it is generally considered that the trivalent As (H3AsO3) is more toxic
than the pentavalent As (H3AsO4) form. As illustrated in Figure 1, the anions of H3AsO4
exist over a wide pH range, whereas H3AsO3 does not dissociate in the acidic range and
its anions exist only above pH 7, owing to which, it is less adsorbed. Therefore, it is nec-
essary to oxidise the arsenite into arsenate in order to increase the removal efficiency. In
other words, to increase the removal efficiency of the trivalent As, both an increase in the
number of adsorption sites and the oxidation of As is essential.
Although the As adsorption method is used to remove As (III), most adsorbents have
a very low As removal efficiency because they have few adsorption sites and a small ab-
sorption capacity. This restricts its application in the water treatment. To resolve these
problems, we propose a polymer gel adsorbent with a dense network structure and many
adsorption sites. In the present work, a polymer hydrogel was prepared by radical
polymerisation of DMAPAA-Q by carrying an oxidising agent (KMnO4, K2Cr2O7) to en-
hance the adsorption capacity of As by changing the valence. The synthesised D-Mn gel
(DMAPAAQ gel carrying MnO4−) and D-Cr gel (DMAPAAQ gel carrying Cr2O72−) were
systematically characterised before and after the As adsorption. Their As (III) removal
performances were evaluated via a batch adsorption experiment. The adsorption kinetics
and isotherms were also investigated in detail.
Figure 1. (a) As(III) and (b) As(V) speciation as a function of the solution pH [24].
Figure 1. (a) As(III) and (b) As(V) speciation as a function of the solution pH [24].
Although the As adsorption method is used to remove As (III), most adsorbents
have a very low As removal efficiency because they have few adsorption sites and a
small absorption capacity. This restricts its application in the water treatment. To resolve
these problems, we propose a polymer gel adsorbent with a dense network structure
and many adsorption sites. In the present work, a polymer hydrogel was prepared by
radical polymerisation of DMAPAA-Q by carrying an oxidising agent (KMnO
4
, K
2
Cr
2
O
7
)
to enhance the adsorption capacity of As by changing the valence. The synthesised D-Mn
gel (DMAPAAQ gel carrying MnO
4−
) and D-Cr gel (DMAPAAQ gel carrying Cr
2
O
72−
)
were systematically characterised before and after the As adsorption. Their As (III) removal
performances were evaluated via a batch adsorption experiment. The adsorption kinetics
and isotherms were also investigated in detail.
2. Results and Discussion
2.1. Effect of pH of the As Solution on the Gel Adsorption
For the adsorption of As(III) in aqueous environments, pH is a crucial parameter
that affects the surface charge of the adsorbent as well as the conversion of the As species
(Figure 1). Generally, different forms of As(III) are present in aqueous environments.
H
3
AsO
3
is dominant under the acidic conditions (pH < 7), while the anions, such as
H
2
AsO
3−
, HAsO
32−
, and AsO
33−
are dominant in alkaline environments [
24
]. To deter-
mine the effect of external pH on the As removal property of the gel, the As(III) adsorption
by the D-Mn and D-Cr gels was first investigated in the pH range of 2
−
6 (Figure 2). As
shown in Figure 2A,B, the DMAPAAQ gel removes a comparatively lesser amount of As
Gels 2021,7, 197 3 of 13
(the removal rate of the DMAPAAQ < ~10%) than that of the D-Mn and D-Cr gels due to
the formation of H
3
AsO
3
under acidic conditions, which could interfere in the ion exchange
between the Cl−and As-based anions. Figure 2shows that the As(III) removal rate of the
D-Mn and D-Cr gels could still be maintained at a high level between pH 4 and 6. The
adsorption process took place inside the gel, on which the effect of the external environment
was minimal. Under acidic conditions, the redox reaction between MnO
4−
/Cr
2
O
72−
and
As(III) in the gel is more likely to occur. Meanwhile a greater number of As(V) anions
are produced by redox reactions, which eases As adsorption via the exchange of Cl
−
and
As(V) anions in the gels. Figure 2A also shows that the As removal rate of the D-Mn gel
was significantly higher than that of the DMAPAAQ gel. This was because the Mn oxide
supported in the gel oxidised As (III) into As (V), and also adsorbed the oxidation product,
i.e., As (V).
Gels 2021, 7, x FOR PEER REVIEW 3 of 14
2. Results and Discussion
2.1. Effect of pH of the As Solution on the Gel Adsorption
For the adsorption of As(III) in aqueous environments, pH is a crucial parameter that
affects the surface charge of the adsorbent as well as the conversion of the As species (Fig-
ure 1). Generally, different forms of As(III) are present in aqueous environments. H3AsO3
is dominant under the acidic conditions (pH < 7), while the anions, such as H2AsO3−,
HAsO32−, and AsO33− are dominant in alkaline environments [24]. To determine the effect
of external pH on the As removal property of the gel, the As(III) adsorption by the D-Mn
and D-Cr gels was first investigated in the pH range of 2−6 (Figure 2). As shown in Figure
2A,B, the DMAPAAQ gel removes a comparatively lesser amount of As (the removal rate
of the DMAPAAQ < ~10%) than that of the D-Mn and D-Cr gels due to the formation of
H3AsO3 under acidic conditions, which could interfere in the ion exchange between the
Cl− and As-based anions. Figure 2 shows that the As(III) removal rate of the D-Mn and D-
Cr gels could still be maintained at a high level between pH 4 and 6. The adsorption pro-
cess took place inside the gel, on which the effect of the external environment was mini-
mal. Under acidic conditions, the redox reaction between MnO4−/Cr2O72− and As(III) in the
gel is more likely to occur. Meanwhile a greater number of As(V) anions are produced by
redox reactions, which eases As adsorption via the exchange of Cl− and As(V) anions in
the gels. Figure 2A also shows that the As removal rate of the D-Mn gel was significantly
higher than that of the DMAPAAQ gel. This was because the Mn oxide supported in the
gel oxidised As (III) into As (V), and also adsorbed the oxidation product, i.e., As (V).
(A) (B)
Figure 2. Effect of pH on the As(III) removal rate of the (A) D-Mn and (B) D-Cr gels.
In addition, it was confirmed that the removal rate increased remarkably with the
increasing pH, becoming nearly constant for pH > 3. However, when pH < 2, the As re-
moval rate decreased significantly. This phenomenon could be attributed to two reasons:
(1) large amounts of Cl− ions derived from the HCl were used to adjust the pH, suppress-
ing the formation of the oxidation product As(V) on the gel, as shown in Scheme 1; (2)
when a large amount of H+ ions suppressed the dissociation of H3AsO4 and H3AsO3, the
exchange of Cl− and As (III) and As (V) was inhibited, thereby reducing the adsorption
removal rate, as shown in Scheme 2.
0
10
20
30
40
50
60
70
80
90
100
23456
Removal Rate (%)
pH
D-Mn gel
DMAPAAQ
gel
0
10
20
30
40
50
60
70
80
90
100
23456
Removal Rate (%)
pH
D-Cr gel
DMAPAAQ
gel
Figure 2. Effect of pH on the As(III) removal rate of the (A) D-Mn and (B) D-Cr gels.
In addition, it was confirmed that the removal rate increased remarkably with the
increasing pH, becoming nearly constant for pH > 3. However, when pH < 2, the As
removal rate decreased significantly. This phenomenon could be attributed to two reasons:
(1) large amounts of Cl
−
ions derived from the HCl were used to adjust the pH, suppressing
the formation of the oxidation product As(V) on the gel, as shown in Scheme 1; (2) when a
large amount of H
+
ions suppressed the dissociation of H
3
AsO
4
and H
3
AsO
3
, the exchange
of Cl
−
and As (III) and As (V) was inhibited, thereby reducing the adsorption removal rate,
as shown in Scheme 2.
Gels 2021, 7, x FOR PEER REVIEW 4 of 14
Scheme 1. Schematic representation of the effect of high Cl
−
ion concentration on the As adsorption
of the D-Mn gel.
Scheme 2. Schematic representation of the effect of high H
+
ion concentration on the As adsorption
of the D-Mn gel.
2.2. Adsorption Isotherm
Figure 3 shows the isotherms of As(III) adsorption by D-Mn and D-Cr gels at 10 °C
and 40 °C, respectively, for 24 h (24 h is enough to ensure that the adsorption can reach an
equilibrium state). The maximum amount of As adsorbed in the gel increased with the
increasing temperature, as did the amount of As adsorbed in general. This enhancement
in the adsorption process could be attributed to the progress of the redox reaction facili-
tated by the increase in temperature. The maximum adsorbed amount in the gel and the
equilibrium adsorption constant could be calculated from the slope of the straight line and
the intercept, respectively, according to the Langmuir isotherm adsorption Formula (2),
as shown in Figure 4A,B.
Scheme 1.
Schematic representation of the effect of high Cl
−
ion concentration on the As adsorption
of the D-Mn gel.
Gels 2021,7, 197 4 of 13
Gels 2021, 7, x FOR PEER REVIEW 4 of 14
Scheme 1. Schematic representation of the effect of high Cl
−
ion concentration on the As adsorption
of the D-Mn gel.
Scheme 2. Schematic representation of the effect of high H
+
ion concentration on the As adsorption
of the D-Mn gel.
2.2. Adsorption Isotherm
Figure 3 shows the isotherms of As(III) adsorption by D-Mn and D-Cr gels at 10 °C
and 40 °C, respectively, for 24 h (24 h is enough to ensure that the adsorption can reach an
equilibrium state). The maximum amount of As adsorbed in the gel increased with the
increasing temperature, as did the amount of As adsorbed in general. This enhancement
in the adsorption process could be attributed to the progress of the redox reaction facili-
tated by the increase in temperature. The maximum adsorbed amount in the gel and the
equilibrium adsorption constant could be calculated from the slope of the straight line and
the intercept, respectively, according to the Langmuir isotherm adsorption Formula (2),
as shown in Figure 4A,B.
Scheme 2.
Schematic representation of the effect of high H
+
ion concentration on the As adsorption
of the D-Mn gel.
2.2. Adsorption Isotherm
Figure 3shows the isotherms of As(III) adsorption by D-Mn and D-Cr gels at 10
◦
C
and 40
◦
C, respectively, for 24 h (24 h is enough to ensure that the adsorption can reach
an equilibrium state). The maximum amount of As adsorbed in the gel increased with the
increasing temperature, as did the amount of As adsorbed in general. This enhancement in
the adsorption process could be attributed to the progress of the redox reaction facilitated by
the increase in temperature. The maximum adsorbed amount in the gel and the equilibrium
adsorption constant could be calculated from the slope of the straight line and the intercept,
respectively, according to the Langmuir isotherm adsorption Formula (2), as shown in
Figure 4A,B.
Gels 2021, 7, x FOR PEER REVIEW 5 of 14
(A) (B)
Figure 3. Isotherms of the As(III) adsorption in the (A) D-Mn and (B) D-Cr gels at different temperatures.
Figure 4 shows the corresponding simulated results. The adsorption isothermal data
were simulated using the Langmuir models. The parameters are listed in Tables 1 and 2.
(A) (B)
Figure 4. Langmuir isotherm plots for the As(III) adsorption in (A) D-Mn and (B) D-Cr gels at different temperatures.
Table 1. Langmuir model parameters for As(III) adsorption in D-Mn gel.
Temperature(°C) Q
max
(mg g
−1
-gel) K
L
(L mg
−1
) R
2
10 89 0.0005 0.9866
40 200 0.0002 0.9534
Table 2. Langmuir model parameters for the adsorption of As(III) in D-Cr gel.
Temperature(°C) Q
max
(mg g
−1
-gel) K
L
(L mg
−1
) R
2
10 137 0.0004 0.9855
40 263 0.0004 0.9758
Figure 3.
Isotherms of the As(III) adsorption in the (
A
) D-Mn and (
B
) D-Cr gels at different temperatures.
Gels 2021, 7, x FOR PEER REVIEW 5 of 14
(A) (B)
Figure 3. Isotherms of the As(III) adsorption in the (A) D-Mn and (B) D-Cr gels at different temperatures.
Figure 4 shows the corresponding simulated results. The adsorption isothermal data
were simulated using the Langmuir models. The parameters are listed in Tables 1 and 2.
(A) (B)
Figure 4. Langmuir isotherm plots for the As(III) adsorption in (A) D-Mn and (B) D-Cr gels at different temperatures.
Table 1. Langmuir model parameters for As(III) adsorption in D-Mn gel.
Temperature(°C) Q
max
(mg g
−1
-gel) K
L
(L mg
−1
) R
2
10 89 0.0005 0.9866
40 200 0.0002 0.9534
Table 2. Langmuir model parameters for the adsorption of As(III) in D-Cr gel.
Temperature(°C) Q
max
(mg g
−1
-gel) K
L
(L mg
−1
) R
2
10 137 0.0004 0.9855
40 263 0.0004 0.9758
Figure 4.
Langmuir isotherm plots for the As(III) adsorption in (
A
) D-Mn and (
B
) D-Cr gels at
different temperatures.
Gels 2021,7, 197 5 of 13
Figure 4shows the corresponding simulated results. The adsorption isothermal data
were simulated using the Langmuir models. The parameters are listed in Tables 1and 2.
Table 1. Langmuir model parameters for As(III) adsorption in D-Mn gel.
Temperature(◦C) Qmax (mg g−1-gel) KL(L mg−1)R2
10 89 0.0005 0.9866
40 200 0.0002 0.9534
Table 2. Langmuir model parameters for the adsorption of As(III) in D-Cr gel.
Temperature(◦C) Qmax (mg g−1-gel) KL(L mg−1)R2
10 137 0.0004 0.9855
40 263 0.0004 0.9758
As mentioned above, the maximum amounts of As adsorbed by the two gels are
200 mg g
−1
-gel and 263 mg g
−1
-gel, respectively, which are considerably better than the
maximum adsorbed amounts reported in previous studies, as listed in Table 3.
Table 3. Comparison of the maximum As(III) removal capacity of related adsorbents.
Adsorbent
Initial As
Concentration Range
(mg L−1)
Adsorbent Dosage
(g L−1)
Max. As(III)
Adsorption Capacity
(mg g−1)
References
D-Mn gel 1000−10 2 200 This work
D-Cr gel 1000−10 2 263 This work
Fe(III)/La(III)-chitosan 1−0.05 - 109 [19]
ZrPACM-43 100−10 13 41.48 [25]
Ceria-GO composite 200–0.01 0.5 185 [26]
Hydrous Cerium Oxide 100–1 0.5 170 [27]
CuO nanoparticles 400−200 0.08 39 [28]
TiO2nanoparticles 90−5 1 31.35 [29]
Fe3O4-graphene composite 1–0.1 2 0.313 [11]
2.3. Adsorption Kinetics
The adsorption kinetics of D-Mn and D-Cr gels was investigated using both pseudo-
first-order and pseudo-second-order kinetic models.
The initial concentrations for the kinetic studies were set at 20 mg L
−1
, 50 mg L
−1
,
and 100 mg L
−1
. The adsorbent dosage was set to 2 g L
−1
, and the contact time range
was set at 30–1650 min. Figure 5A shows that the reaction reached equilibrium after
20 h for the initial concentrations of 20 mg L
−1
and 50 mg L
−1
. However, at an initial
concentration of 100 mg L
−1
, the adsorbed amount increased rapidly in the first hour, and
the reaction reached equilibrium within the following 3 h. The figure suggests that the
reaction rate increased significantly with the increasing initial concentration. Therefore,
the MnO
4−
ions were converted into MnO
2
at low As(III) concentrations, and MnO
2
could itself continuously adsorb As(III) after the redox reaction. Meanwhile, the MnO
4−
ions underwent a vigorous redox reaction and were directly reduced to Mn
4+
in a highly
concentrated As solution. This is because the oxidised trivalent As became pentavalent
and was rapidly adsorbed by the amino group of the gel at that time.
Unlike the D-Mn gel, there was no significant change in the adsorption rate of the
D-Cr gel at the different initial As(III) concentrations. This suggests that an increase in the
initial concentration of the As solution did not change the reaction rate of the D-Cr gel.
To elucidate the mechanism of As adsorption by the D-Mn and D-Cr gels, the adsorption
behaviour was analysed using the pseudo-first-order and pseudo-second-order dynamics
equations, as shown in Figure 5C–F. Since both the D-Mn and D-Cr gels showed higher
Gels 2021,7, 197 6 of 13
correlations with the pseudo-second-order dynamics model than with the pseudo-first-
order model, As adsorption by the D-Mn and D-Cr gels was considered to be dominated
by chemisorption. The parameters are listed in Tables 4and 5.
Gels 2021, 7, x FOR PEER REVIEW 7 of 14
Figure 5. Adsorption kinetics of (A) D-Mn and (B) D-Cr gels; plots of the pseudo-first-order dynamic model of (C) D-Mn
and (D) D-Cr gels; plots of the pseudo-second-order dynamic model of (E) D-Mn and (F) D-Cr gels.
A B
C D
F
E
Figure 5.
Adsorption kinetics of (
A
) D-Mn and (
B
) D-Cr gels; plots of the pseudo-first-order dynamic model of (
C
) D-Mn
and (D) D-Cr gels; plots of the pseudo-second-order dynamic model of (E) D-Mn and (F) D-Cr gels.
Gels 2021,7, 197 7 of 13
Table 4. Kinetic parameters of As(III) adsorption by the D-Mn gel.
Initial Concentration Pseudo-First-Order Equation Pseudo-Second-Order Equation
k1(min−1)qe(mg g−1)R2k2(g mg−1min−1)qe(mg g−1)R2
20 mg L−10.0027 5.45 0.96586 0.00097 8.98 0.99830
50 mg L−10.0027 10.91 0.97547 0.00056 20.75 0.99822
100 mg L−1− − − 0.00680 21.79 0.9999
Table 5. Kinetic parameters of As(III) adsorption by the D-Cr gel.
Initial Concentration Pseudo-First-Order Equation Pseudo-Second-Order Equation
k1(min−1)qe(mg g−1)R2k2(g mg−1min−1)qe(mg g−1)R2
20 mg L−10.0030 7.23 0.98977 0.00043 9.08 0.99546
50 mg L−10.0039 16.27 0.99419 0.00029 20.20 0.99681
100 mg L−10.0044 27.46 0.99267 0.00023 34.01 0.99683
Figure 6shows the changes in the amount of As adsorbed on the gel with time at
initial concentrations of 20 mg L
−1
and 100 mg L
−1
. This figure shows that the adsorption
performance of the D-Mn gel is superior to that of the D-Cr gel at the low As concen-
trations, which could be attributed to the ability of the MnO
4−
ions to form MnO
2
after
the redox reaction, and continuously adsorb As even at low As concentrations. Mean-
while, the adsorption capacity of the D-Cr gel is much higher than that of the D-Mn gel,
but the rate at which the latter reaches equilibrium is higher in the high-concentration
As solutions. This is because the oxidising property of the MnO
4−
ion is higher than
that of the Cr
2
O
72−
ion in a neutral environment (standard oxidation reduction potential:
KMnO4(+1.51 V) > K2Cr2O7(+1.35 V)).
Gels 2021, 7, x FOR PEER REVIEW 8 of 14
Table 4. Kinetic parameters of As(III) adsorption by the D-Mn gel.
Initial Concentration Pseudo-First-Order Equation Pseudo-Second-Order Equation
k
1
(min
−1
) q
e
(mg g
−1
) R
2
k
2
(g mg
−1
min
−1
) q
e
(mg g
−1
) R
2
20 mg L
−1
0.0027 5.45 0.96586 0.00097 8.98 0.99830
50 mg L
−1
0.0027 10.91 0.97547 0.00056 20.75 0.99822
100 mg L
−1
− − − 0.00680 21.79 0.9999
Table 5. Kinetic parameters of As(III) adsorption by the D-Cr gel.
Initial Concentration Pseudo-First-Order Equation Pseudo-Second-Order Equation
k
1
(min
−1
) q
e
(mg g
−1
) R
2
k
2
(g mg
−1
min
−1
) q
e
(mg g
−1
) R
2
20 mg L
−1
0.0030 7.23 0.98977 0.00043 9.08 0.99546
50 mg L
−1
0.0039 16.27 0.99419 0.00029 20.20 0.99681
100 mg L
−1
0.0044 27.46 0.99267 0.00023 34.01 0.99683
Figure 6 shows the changes in the amount of As adsorbed on the gel with time at
initial concentrations of 20 mg L
−1
and 100 mg L
−1
. This figure shows that the adsorption
performance of the D-Mn gel is superior to that of the D-Cr gel at the low As concentra-
tions, which could be attributed to the ability of the MnO
4−
ions to form MnO
2
after the
redox reaction, and continuously adsorb As even at low As concentrations. Meanwhile,
the adsorption capacity of the D-Cr gel is much higher than that of the D-Mn gel, but the
rate at which the latter reaches equilibrium is higher in the high-concentration As solu-
tions. This is because the oxidising property of the MnO
4−
ion is higher than that of the
Cr
2
O
72−
ion in a neutral environment (standard oxidation reduction potential: KMnO
4
(+1.51 V) > K
2
Cr
2
O
7
(+1.35 V)).
Figure 6. Comparison of the adsorption kinetics of the D-Mn and D-Cr gels at the initial As(III)
concentration of (A) 20 mg L
−1
and (B) 100 mg L
−1
, respectively.
2.4. Adsorption Thermodynamics
The mechanisms of the As(III) adsorption on the gels were investigated by calculat-
ing the thermodynamic parameters, such as adsorption enthalpy (ΔH), adsorption free
energy (ΔG), and adsorption entropy (ΔS).
Thermodynamic analyses of the adsorption experiments were conducted to elucidate
the associated adsorption mechanisms. Experiments were conducted at the temperatures
of 10, 40, and 60 °C to confirm the effect of temperature on the amount of As adsorbed in
A B
Figure 6.
Comparison of the adsorption kinetics of the D-Mn and D-Cr gels at the initial As(III)
concentration of (A) 20 mg L−1and (B) 100 mg L−1, respectively.
2.4. Adsorption Thermodynamics
The mechanisms of the As(III) adsorption on the gels were investigated by calculating
the thermodynamic parameters, such as adsorption enthalpy (
∆
H), adsorption free energy
(∆G), and adsorption entropy (∆S).
Thermodynamic analyses of the adsorption experiments were conducted to elucidate
the associated adsorption mechanisms. Experiments were conducted at the temperatures
of 10, 40, and 60
◦
C to confirm the effect of temperature on the amount of As adsorbed
in the gel (Figure 7). The figure was obtained by plotting the results calculated by the
thermodynamic equation, and then, using other thermodynamic equations, the
∆
Hand
∆
S
values can be calculated. The results of the calculations are listed in Tables 6and 7, wherein
Gels 2021,7, 197 8 of 13
∆
G< 0 suggests that the adsorption process could be performed naturally. The higher
the reaction temperature, the greater is the decrease in
∆
G. Therefore, as the temperature
increased, the spontaneous reaction was facilitated to a certain extent. Since both
∆
Hand
∆Sare > 0, the adsorption process could be interpreted as an endothermic chemisorption.
Gels 2021, 7, x FOR PEER REVIEW 9 of 14
the gel (Figure 7). The figure was obtained by plotting the results calculated by the ther-
modynamic equation, and then, using other thermodynamic equations, the ΔH and ΔS
values can be calculated. The results of the calculations are listed in Tables 6 and 7,
wherein ΔG < 0 suggests that the adsorption process could be performed naturally. The
higher the reaction temperature, the greater is the decrease in ΔG. Therefore, as the tem-
perature increased, the spontaneous reaction was facilitated to a certain extent. Since both
ΔH and ΔS are > 0, the adsorption process could be interpreted as an endothermic chem-
isorption.
Figure 7. Effect of temperature on the distribution of the adsorption coefficient of the D-Mn (trian-
gles) and D-Cr gels (circles).
Table 6. Thermodynamic parameters of As adsorption in the D-Mn gel.
T(K) 𝒍𝒏𝑲𝒅 ∆𝑮 (kJ mol
−1
) ∆𝑯 (kJ mol
−1
) ∆𝑺 (kJ mol
−1
K
−1
)
283.15 0.93258781 −1.5815 29.565 0.110
313.15 1.35287955 −4.8815 29.565 0.110
333.15 2.97647589 −7.0815 29.565 0.110
Table 7. Thermodynamic parameters of As adsorption in the D-Cr gel.
T(K) 𝒍𝒏𝑲𝒅 ∆𝑮 (kJ mol
−1
) ∆𝑯 (kJ mol
−1
) ∆𝑺 (kJ mol
−1
K
−1
)
283.15 0.2777234 −0.3872 35.856 0.128
313.15 1.46240344 −4.2272 35.856 0.128
333.15 2.61940407 −6.7872 35.856 0.128
2.5. Effect of Co-Existing Ions on the As Removal Property of the D-Mn and D-Cr Gels
In wastewater treatment, various anions like HCO
3−
, SO
42−
, PO
43−
, and Cl
−
are
present
in aqueous solutions. To determine the effect of the coexistence of other anions on the As
adsorption, a coexisting ion adsorption experiment was conducted at an initial As(III) con-
centration of 20 mg L
−1
.
Figure 8 shows that the amount of As adsorbed by the D-Mn gel varies considerably
depending on the type of the co-existing ions. The influence on the As adsorption in-
creased in the following order: PO
43−
> SO
42−
> HCO
3−
; therefore, the valence of the co-ex-
isting anions is considered to be strongly related to the amount of adsorbed As. Figure 9
shows that the adsorption capacity decreased with increasing valence of the co-existing
Figure 7.
Effect of temperature on the distribution of the adsorption coefficient of the D-Mn (triangles)
and D-Cr gels (circles).
Table 6. Thermodynamic parameters of As adsorption in the D-Mn gel.
T(K) lnKd∆G(kJ mol−1)∆H(kJ mol−1)∆S(kJ mol−1K−1)
283.15 0.93258781 −1.5815 29.565 0.110
313.15 1.35287955 −4.8815 29.565 0.110
333.15 2.97647589 −7.0815 29.565 0.110
Table 7. Thermodynamic parameters of As adsorption in the D-Cr gel.
T(K) lnKd∆G(kJ mol−1)∆H(kJ mol−1)∆S(kJ mol−1K−1)
283.15 0.2777234 −0.3872 35.856 0.128
313.15 1.46240344 −4.2272 35.856 0.128
333.15 2.61940407 −6.7872 35.856 0.128
2.5. Effect of Co-Existing Ions on the As Removal Property of the D-Mn and D-Cr Gels
In wastewater treatment, various anions like HCO
3−
, SO
42−
, PO
43−
, and Cl
−
are
present in aqueous solutions. To determine the effect of the coexistence of other anions
on the As adsorption, a coexisting ion adsorption experiment was conducted at an initial
As(III) concentration of 20 mg L−1.
Figure 8shows that the amount of As adsorbed by the D-Mn gel varies considerably
depending on the type of the co-existing ions. The influence on the As adsorption increased
in the following order: PO43−> SO42−> HCO3−; therefore, the valence of the co-existing
anions is considered to be strongly related to the amount of adsorbed As. Figure 9shows
that the adsorption capacity decreased with increasing valence of the co-existing anions.
The gel also exhibited good stability with negligible change in adsorption at anion concen-
trations of 0, 0.1, and 1 mM. Therefore, this gel appears to be suitable for removing the
trivalent As in the presence of the monovalent anions.
Gels 2021,7, 197 9 of 13
Gels 2021, 7, x FOR PEER REVIEW 10 of 14
anions. The gel also exhibited good stability with negligible change in adsorption at anion
concentrations of 0, 0.1, and 1 mM. Therefore, this gel appears to be suitable for removing
the trivalent As in the presence of the monovalent anions.
Figure 8. Effect of co-existing ions on the As removal property of the D-Mn gel.
Figure 9. Effect of co-existing ions on As removal by the D-Cr gel.
HCO3- SO
42- PO
43- Cl
-
HCO3- SO
42- PO
43- Cl
-
Figure 8. Effect of co-existing ions on the As removal property of the D-Mn gel.
Gels 2021, 7, x FOR PEER REVIEW 10 of 14
anions. The gel also exhibited good stability with negligible change in adsorption at anion
concentrations of 0, 0.1, and 1 mM. Therefore, this gel appears to be suitable for removing
the trivalent As in the presence of the monovalent anions.
Figure 8. Effect of co-existing ions on the As removal property of the D-Mn gel.
Figure 9. Effect of co-existing ions on As removal by the D-Cr gel.
HCO3- SO
42- PO
43- Cl
-
HCO3- SO
42- PO
43- Cl
-
Figure 9. Effect of co-existing ions on As removal by the D-Cr gel.
Similar to Figure 8, Figure 9shows that the As adsorption capacity was significantly
different in the co-existence of different ions. The valence of the co-existing anions appears
to be closely related to the amount of As adsorbed: the adsorption capacity of the gel
decreased with increasing valence of the co-existing anions. However, unlike that of the
D-Mn gel, the adsorption capacity of the D-Cr gel gradually decreased with the increasing
concentration of the co-existing ions, indicating that its stability is inferior to that of the
D-Mn gel. However, in the presence of divalent anions, the D-Cr gel showed a larger
Gels 2021,7, 197 10 of 13
adsorption capacity than the D-Mn gel. Therefore, the D-Cr gel is considered suitable for
removing the trivalent As in the presence of the monovalent or divalent anions.
3. Conclusions
In summary, the polymer gel adsorbents (DMAPAAQ) carrying an oxidant (KMnO
4
/
K
2
Cr
2
O
7
) were successfully synthesised (D-Mn and D-Cr gels). We demonstrated that a
DMAPAAQ gel carrying an oxidant could significantly enhance the adsorption of As(III).
Owing to the redox reaction with MnO
4−
and Cr
2
O
72−
inside the gel, As(III) was oxidised
to As(V), which increased the adsorption efficiency. As was also adsorbed onto MnO
2
in the gel. The maximum adsorption capacities for As(III) of D-Mn and D-Cr gels reach
200 and 263 mg g
−1
-gel, respectively. The maximum adsorption capacities of the D-Mn
and D-Cr gels were also larger than those of the other inorganic As adsorbents. The As
adsorption in the D-Mn and D-Cr gels was dominated by chemisorption, as revealed by
the adsorption kinetic analysis. The D-Mn gel appears to be suitable for removing As(III) in
the presence of monovalent anions, while the D-Cr gel is considered suitable for removing
As(III) in the presence of monovalent or divalent anions.
4. Materials and Methods
4.1. Reagents
N,N-dimethylamino propylacrylamide monomer (DMAPAAQ) was obtained by KJ
Chemicals Co., Tokyo, Japan. Potassium dichromate (K
2
Cr
2
O
7
) and N,N,N’,N’- tetram-
ethylethylenediamine (TEMED) were obtained from Nacalai Tesque, Inc., Kyoto, Japan,
N,N’-methylenebisacrylamide (MBAA), ammoniumperoxodisulfate (APS), and potassium
permanganate (KMnO
4
) were obtained from Sigma Aldrich Co., St. Louis, MO, USA. All
the reagents were reagent grade and used as received. Aqueous solutions were prepared
using distilled water (DW).
4.2. Synthesis of DMAPAAQ Hydrogel
In a 10 mL volumetric flask, 4.1342 g of DMAPAAQ (monomer), 0.1156 g of MBAA
(cross-linking agent), and 0.0349 g of TEMED (accelerator) were dissolved in distilled water.
APS (initiator, 0.0685 g) was dissolved in distilled water in a 5 mL volumetric flask (as
shown in Table 8). N
2
purge was performed for 30 min on each solution and the device
to remove O
2
to prevent the inhibition of radical polymerisation in the flask containing
distilled water. After N
2
purging, the initiator solution and monomer solution were mixed
and stirred for 20 s. The obtained mixture was then injected into a gel-plate formation kit
(AE-6401 1-mm Dual Mini Gel Cast, ATTO Corp., Tokyo, Japan). DMAPAA-Q and MBAA
were polymerised at 25
◦
C for 24 h, following which, the gel was peeled off from the glass
plate and cut into a 10 mm
×
10 mm
×
1 mm plate. The gel was washed with methanol
for 24 h using a Soxhlet extractor (Asahi Glassplant Inc., Arao-city, Japan) to remove the
unreacted monomers. After washing, the gel was dried at 25
◦
C for several days and then
thoroughly dried in an oven at 50 ◦C.
Table 8. Synthetic condition of DMAPAAQ hydrogel.
Component Function Molecular Weight (g mol−1) Concentration (mol m−3) Mass (g)
DMAPAAQ Monomer 206.71 1000 4.1342
MBAA Linker 154.17 50 0.1156
TEMED Accelerator 116.21 20 0.0349
APS Initiator 228.19 20 0.0685
4.3. Synthesis of the D-Mn Gel and D-Cr Gel
A 0.01 mol L
−1
solution was prepared by dissolving 0.079 g KMnO
4
in 50 mL distilled
water. The DMAPAA-Q gel (0.2 g) was immersed in the prepared KMnO
4
solution, and
the mixture was kept at 25
◦
C for 24 h. The gel was then washed with ion-exchanged water
Gels 2021,7, 197 11 of 13
for 24 h to remove the excess ions from the surface of the gel. The ion-exchanged water
was then replaced several times in a fixed time interval (every 4~6 h). After washing, the
gel was completely dried in a drying oven at 50 ◦C.
A 0.01 mol L
−1
solution was prepared by dissolving 0.1471 g of K
2
Cr
2
O
7
in 50 mL
distilled water. In the prepared K
2
Cr
2
O
7
solution, 0.2 g of DMAPAAQ gel was immersed,
and the mixture was kept at 25
◦
C for 24 h. The gel was then washed with ion-exchanged
water for 24 h to remove the excess ions on its surface. The ion-exchanged water was
replaced several times at fixed time intervals in 24 h (every 4~6 h). After washing, the gel
was completely dried in an oven at 50 ◦C.
4.4. Batch Adsorption Experiments
The batch adsorption experiments were performed in plastic vials (10 mL) with 2 g L
−1
adsorbent dosage to investigate the effects of pH, reaction time, temperature, co-existing
anions, and the effect of co-existing competing anions on As removal in the synthetic
As(III) solution.
The effect of pH on the As(III) removal was explored at different initial pH levels
(ranging from 2–6). The concentration of As(III) was 10 mg L
−1
, and the dosage of the D-Mn
or D-Cr gels was 2 g L
−1
, respectively. The effect of the co-existing ions was investigated
as follows: 20 mg of a given gel was added to 10 mL of As(III) solution (20 mg L
−1
)
containing the ions HCO
3−
, SO
42−
, and PO
43−
, Cl
−
, wherein the initial concentrations of
the co-existing ions were 0, 0.1, 1, and 10 mM. The mean values of all the data obtained
were calculated from three individual measurements, and the standard deviations were
added to some of the data.
The adsorption isotherms were measured to determine the As adsorption capaci-
ties of the adsorbents in the As(III) solutions with initial concentrations ranging from
10–1000 mg L−1
for 24 h. After reaching equilibrium, the D-Mn and D-Cr gels were sepa-
rated to measure the residual concentration of As. The equilibrium adsorption capacity q
e
was determined as follows:
qe=(C0−Ce)V
M(1)
where C
0
(mg L
−1
) and C
e
(mg L
−1
) are the initial and equilibrium As (III) concentrations,
respectively, V(L) is the solution volume, and M(g) is the weight of the gel.
The Langmuir equations are expressed as follows:
Ce
qe
=Ce
Qm
+1
KLQm(2)
where q
e
and C
e
are the adsorption quantity and equilibrium concentration of As(III) at the
equilibrium state, respectively. Q
m
(mg g
−1
) is the maximum adsorbed amount, and K
L
(L mg−1) is the Langmuir adsorption isotherm constant.
The kinetics of As(III) adsorption was simulated using two mathematical models,
namely the pseudo-first-order and pseudo-second-order kinetic models, which can be
expressed as the following equations:
ln(qe−qt)=lnqe−k1t(3)
t
qt
=1
k2qe2+t
qe(4)
Here, q
e
(mg g
−1
) is the equilibrium adsorption capacity, q
t
(mg g
−1
) is the adsorbed
amount at time t, and k
1
(min
−1
) and k
2
(g mg
−1
min
−1
) are the pseudo-first-order and
pseudo-second-order rate constants, respectively.
Thermodynamic experiments were conducted by varying the initial concentration
from 20 mg L
−1
to 100 mg L
−1
for 24 h at three different initial temperatures (10
◦
C, 25
◦
C,
and 40
◦
C). The thermodynamic parameters for As(III) adsorption were quantified to
explore the degree of spontaneity and heat exchange during the adsorption process. The
Gels 2021,7, 197 12 of 13
standard Gibbs free energy
∆
G(kJ mol
−1
), enthalpy change
∆
H(kJ mol
−1
), and entropy
change ∆S(J mol−1K−1) were calculated as follows:
Kd=qe
Ce(5)
∆G=∆H−T∆S(6)
∆G=−RTlnKd(7)
lnKd=∆S
R−
∆H
RT (8)
where K
d
is the distribution adsorption coefficient, q
e
(mg g
−1
) is the adsorption capacity
per unit mass of the adsorbent, and Ce(mg L−1) is the equilibrium As concentration.
All of the aqueous samples were filtered through a 0.22
µ
m membrane, and the
concentration of the residual As was analysed by the inductively coupled plasma atomic
emission spectroscopy (ICP-AES).
Author Contributions:
Y.S.: investigation, methodology, data curation, writing
−
original draft; T.G.:
conceptualisation, funding acquisition, supervision, writing—reviewing and editing. S.N.: project
administration, resources, writing—reviewing and editing. All authors have read and agreed to the
published version of the manuscript.
Funding:
This research was funded by the Japan Society for the Promotion of Science (JSPS) KAK-
ENHI (grant number 17K06892).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement:
All data included in this study are available upon request by contact
with the corresponding author.
Conflicts of Interest: The authors declare no conflict of interest.
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