2. 2
| OU et al.
et al., 2017). Heavy metal pollution would result in a sig-
nificant impact on ecosystems and human beings, especially
through the food chain effect even at trace levels (Yin et al.,
2018).
Heavy metals have characteristics of persistent and non-
biodegradable, and they are found to be present in environ-
ment in various forms. Moreover, different types of heavy
metal ions can also co-
exist, and thus, the remediation tasks
of heavy metal-
contained environmental media become more
difficult (Han et al., 2016; Tóth et al., 2016). Chromium (Cr)
is a harmful and commonly found heavy metal in environ-
ment (Markiewicz et al., 2015). Chromium compounds are
usually in the forms of chromium metal, Cr(III) (trivalent
chromium), and Cr(VI) (hexavalent chromium). Chromate is
the major form of Cr(VI), which has mutagenic and cytoge-
netic effects because it has a higher oxidative potential and it
is able to penetrate cell membranes (Al-
Senani & Al-
Fawzan,
2018; Hu et al., 2018a). Cr(VI) needs to be removed from the
environment or it needs to be reduced to Cr(III) because it is
less toxic to human health and environment (Enniya et al.,
2018; Hamilton et al., 2018).
Technologies such as membrane filtration, ion exchange,
electrochemical process, adsorption, and chemical precipita-
tion have been applied to cleanup heavy metal-
contaminated
waters (Efome et al., 2018; Tran et al., 2017). Adsorption is an
applicable technology to treat heavy metal-
contained water.
When adsorption process is applied for water treatment, se-
lection of feasible adsorbent is an important task to achieve an
acceptable treatment efficiency (Hong et al., 2019; Wu et al.,
2019). Activated carbon and biochar, which have micropo-
rous structures, are commonly used adsorption materials to
remove heavy metals (Xu et al., 2019; Zhou et al., 2016).
However, these adsorbents have uncontrollable morpholo-
gies and pore sizes, which are adverse to Cr(VI) adsorption
(Di Natale et al., 2015). Thus, the materials with controllable
morphologies and pore sizes would have wider applicability
for Cr(VI) adsorption (Cao et al., 2018; Dong et al., 2018).
Metal-
organic frameworks (MOFs) are new adsorbents
with porous crystalline, and they have attracted significant
attention in their application for the adsorption of different
gases (Vaitsis et al., 2018; Yu et al., 2017). MOFs are com-
posed of metal cations or metal cation aggregates, which
are linked by organic ligands (Hu et al., 2018b; Wang et al.,
2018b). MOFs with various morphologies can be synthesized
via methods including microwave synthesis, in situ hydro-
thermal synthesis, seed crystal growth, and stirring at room
temperature (Kumar et al., 2019; Shah et al., 2019). MOF
can be used as an adsorbent for contaminant removal because
it has the following characteristics: tenability, tunable pore
structures, high specific surface areas, and large pore sizes
(Chen et al., 2019; Wu et al., 2020).
Adsorptive removal or separation is one of the most
important applications of MOFs for water or wastewater
treatment. MOF shows high porosity and necessary chem-
ical activities for the adsorptive interactions of adsorbates
(Ahmed & Jhung, 2017; Li et al., 2018; Pan et al., 2019;
Safaei et al., 2019). Researchers have reported that MOFs
could be applied for the removal of different hazardous ma-
terials (e.g., antibiotics, biological compounds, toxic pollu-
tions) from aqueous or non-
aqueous media (Hasan & Jhung,
2015; Sun et al., 2019).
Zeolitic imidazolate frameworks (ZIFs) are composed
of organic ligands with imidazole backbone and a transition
metal ion (Co2+
or Zn2+
) located in the center (Ling & Chen,
2019), and they inherit qualities from zeolite and MOFs.
ZIF is a sub-
group of MOF, and it has a stable character-
istic. Because the crystalline framework of ZIFs is similar
to zeolites, they have higher chemical and thermal stability
compared to other MOFs. This allows them to have a wider
range in chemical processes and temperatures for the field
application (Bensiradj et al., 2019).
The advantages of ZIF-
8 include stable thermal and
chemical characteristics, high surface area, ease of synthe-
sis, and adjustable pore structure (Abdel-
Magied et al., 2019;
Geng et al., 2018; Liu et al., 2015). Researchers reported that
ZIF-
8 could be synthesized in aqueous phase, and the pro-
duced ZIF-
8 particles could effectively adsorb trivalent arse-
nic (As3+
) in water. The nanocrystals of ZIF-
8 demonstrated
a high sorption capability of copper (Cu2+
) with an efficient
adsorption time (<30 min) (Zhang et al., 2016). The findings
from Shahrak et al. (2017) demonstrated that ZIF-
8 could be
produced greenly and cost-
effectively, and it could be used
for Cr(VI) separation (Shahrak et al., 2017).
In this study, the ZIF-
8 was prepared through bridging
Zn2+
with 2-
methylimidazole ligands, and it is an original
type (Zn-
cored ZIF) of ZIFs. The framework of Zn-
cored
ZIF is created by linking Zn2+
cores to the N-
atoms in the
five-
membered imidazolate ring at 1,3-
positions, producing
SOD zeolite structure (type material: Sodalite) (Sann et al.,
2018). ZIF-
8 (Zn-
cored ZIF) was produced in the normal
temperature pressure, and its feasibility, effectiveness, and
mechanisms on Cr(VI) removal from Cr(VI)-
contained water
were evaluated under different treatment conditions. The ki-
netics of Cr(VI) sorption and removal from the water phase
using ZIF-
8 as the adsorbent were also studied. The main
tasks of this study included the following: (1) synthesis of
ZIF-
8 (Zn-
cored ZIF) using the ultrasonic and drying meth-
ods, (2) analysis of produced ZIF-
8 characteristics including
crystal phase and appearance, stability, and pore properties,
(3) evaluation of the effectiveness, mechanisms, and optimal
operational conditions of Cr(VI) removal from the Cr(VI)-
contained water using ZIF-
8 as the adsorbent, (4) conduction
of the ZIF-
8 regeneration experiment for ZIF-
8 reuse, and (5)
determination of the sorption kinetics using pseudo-
first and
pseudo-
second models and Langmuir and Freundlich adsorp-
tion models.
3.
| 3
WATER ENVIRONMENT RESEARCH
METHODOLOGY
ZIF-8 production
The main chemical compounds used in this study included
ammonium hydroxide (CAS Number: 1336–
21–
6, J. T.
Baker, NH3 28–
30% aqueous solution), 2-
methylimidazole
(CAS Number: 693–
98–
1, Sigma, 99%), and zinc nitrate
hexahydrate (CAS Number: 10196–
18–
6, Baker, 99%).
The main synthetic procedures are described in He et al.
(2014) and Tran et al. (2019). A total amount of 2.1 g of
2-
methylimidazole (Hmim) was dissolved in 90 ml of deion-
ized (DI) water to prepare Hmim solution, and zinc nitrate
hexahydrate solution was prepared by dissolving 3.9 g of
chemical in 50 ml of ammonium hydroxide solution. The
magnetic stirrer (rotation speed = 1000 rpm) was used dur-
ing the mixing of the solution, and the mixed solution had
a Zn:Hmim:NH4
+
:H2O molar composition of 1:2:30:550.
After mixing, a milk-
like suspension was formed during the
mixing process. The solution was stirred sonically for 10 to
20 min (at 25°C) until the completion of crystallization. After
the filtration process, the products were collected by washing
off the deionized water from the filter and then the washed
products were heat dried at 105°C or freeze-
drying at −80℃
for 24 hr. The dried products (white powder) were produced.
Characteristic analyses of ZIF-
8
Effects of ultrasonic time and drying methods on the par-
ticle shapes of synthesized ZIF-
8 were evaluated using
the environmental scanning electron microscope (ESEM,
FEI, Quanta 200, AUT). The X-
ray diffractometer (XRD)
(Siemens D5000, KS Analytical Systems) was applied to as-
sess the influence of treatment conditions on crystal stability.
The patterns of XRD were studied using the Cu-
Kα radiation
(40 mA and 40 kV) source on a θ/2θ diffractometer between
5° and 50° (scan rate = 5°/min, step size = 0.1°). Nitrogen
isotherm measurements were obtained via a specific surface
area analyzer (Micromeritics) for evaluating adsorption be-
haviors and pore properties of ZIF-
8 particles. The X-
ray
photoelectron spectroscopy (XPS) (Thermo Scientific Theta
Probe, UK) was applied for surface analyses of all catalysts
and examination of the charge valence state of Cr(VI) ad-
sorbed on ZIF-
8 particles.
Adsorption experiments using ZIF-
8
as the adsorbent
Batch tests were conducted to assess the adsorption ability
of Cr(VI) by ZIF-
8 adsorbent. Cr(VI)-
contained aqueous
solution was prepared with deionized water and potassium
dichromate (CAS Number: 7778–
50–
9, Hanawa, 99.5%).
The ZIF-
8 particles were dispersed in water with orbital
shaker (TS-
500, Yihder Co) and ultrasonic cleaner (5510
DTH, Branson Ultrasonics). Oscillation speed of orbital
shaker was 200 rpm, and frequency of ultrasonic cleaner
was 40 KHz.
The following three groups of the adsorption ex-
periments were conducted: (1) effects of ZIF-
8 dosage
(0.005, 0.01, 0.02, 0.04, and 0.08 g) on Cr(VI) removal,
(2) effects of initial concentrations of Cr(VI) (20, 50,
100 and mg/L) on Cr(VI) removal, and (3) effects of
aqueous solution pH (pH = 5, 7, 9, and 11) on Cr(VI)
removal. For the ZIF-
8 dosage test (group 1), the ini-
tial Cr(VI) concentration was 50 mg/L (volume = 40 ml
and experimental time = 45 min). For the test of var-
ied initial Cr(VI) concentration (group 2), the ZIF-
8
dosage was 0.02 g (volume = 40 ml and experimen-
tal time = 120 min). For the varied pH test (group 3),
the initial concentration of Cr(VI) was 50 mg/L (vol-
ume = 40 ml, ZIF-
8 dosage = 0.02 g, and experimental
time = 45 min).
After the adsorption experiments, the solid–
liquid sep-
aration was conducted for the analyses of water and solid
phases. Cr(VI) concentration in aqueous solution was deter-
mined using UV–
Vis (ultraviolet–
visible spectrophotometer
(DR6000, Hach, USA) and ICP-
AES (inductively coupled
plasma-
atomic emission spectrometer) (Optima 7000 DV,
Perkin-Elmer, USA).
Equations (1) and (2) were used to calculate the removal
efficiency (RE) of Cr(VI) and the adsorption capacity (AC)
of ZIF-
8, respectively:
where Ce =Cr(VI) concentration at equilibrium (mg/L),
C0 = initial concentration of Cr(VI) (mg/L),
D = adsorbent dosage (g), and.
V = water volume (ml).
Regeneration of ZIF-8
The used ZIF-
8 particles (initial concentration of
Cr(VI) = 50 mg/L, volume = 40 ml, ZIF-
8 dosage = 0.02 g,
and experimental time = 45 min) were separated from
Cr(VI) aqueous solution by centrifuge. For Cr(VI) desorp-
tion of ZIF-
8 particles, the particles were added and stirred in
(1)
RE% =
C0 − Ce
C0
× 100%
(2)
AC% =
(C0 − Ce)V
D
× 100%
4. 4
| OU et al.
different solutions including ethanol (CAS Number: 64–
17–
5, ECHO, 95%), Panreac, 99%), NaCl solution (0.1 M) (CAS
Number: 7647–
14–
5, Panreac, 99%), and deionized water for
2 h and then the ZIF-
8 particles were separated and dried
overnight by oven (at 105°C). Dried ZIF-
8 particles were
tested again on Cr(VI) adsorption using the following experi-
mental conditions: initial concentration of Cr(VI) = 50 mg/L,
volume = 40 mL, ZIF-
8 dosage = 0.02 g, and experimental
time = 45 min.
Cr(VI) adsorption kinetics
The pseudo-
first-and pseudo-
second-
order kinetic mod-
els were used to assess adsorption rates of Cr(VI) by ZIF-
8
(Dubey et al., 2016; Simonin, 2016). The following equa-
tions are used to describe the models:
where qt =Cr(VI) (mg)/ ZIF-
8 (g) at any time,
qe =Cr(VI) (mg)/ ZIF-
8 (g) at equilibrium,
k1 = rate constant (pseudo-
first-
order sorption) (1/min),
k2 = rate constant (pseudo-
second-
order sorption) (1/
min),
ln qe =slope of the straight-
line plot of ln(qe − qt) versus
t, and.
k1 = intercept of the straight-
line plot of ln(qe − qt) versus
t.
The adsorption isotherms were assessed using the
Freundlich and Langmuir adsorption models (Chen, 2015).
The Langmuir isotherm model can be illustrated as follows
(Chung et al., 2015):
where Ce =Cr(VI) concentration at equilibrium (mg/L),
qe = adsorbed Cr(VI) (mg)/ZIF-
8 (g) at equilibrium,
KL = constant (L/mg), and
qmax = the maximum adsorption capacity (mg/g).
The Freundlich model can be applied to illustrate the mul-
tilayer adsorption concerning the energetic surface heteroge-
neity. It can be illustrated as follows:
where KF = constant of the isotherm (L1/n
mg(1−1/n)
/g) and
1/n = Freundlich exponent.
RESULTS AND DISCUSSION
Crystal phase and appearance of ZIF-
8
The effects of ultrasonic time (10 and 20 min) and drying
method for ZIF-
8 synthesis were evaluated. The two drying
methods were heat-drying at 105°C for 24 h and freeze-drying
at −80°C for 24 h. Figure 1a,d presents the ESEM images of
produced ZIF-
8 particles under different synthesized condi-
tions. Results reveal that the sizes of ZIF-
8 particles varied
from 1.8 to 2.2 μm. The synthesized ZIF-
8 particles had a
clear rhombic dodecahedron morphology shape.
Figure 2a,b shows the results of XRD. Results show that
the peaks were observed at 2 Theta (2θ) = 7.3, 10.4, 12.7,
14.8, 16.5, 18.1, 22.2, 24.6, and 26.8, which matched with
the XRD patterns for SOD-
type single crystal data described
(Shahrak et al., 2017; Wang et al., 2015).
With a longer contact time, the peak intensity of ZIF-
8
was relatively stronger and the baseline was also relatively
stable. Compared to the results using 10 min contact time,
the strong and sharp peaks in Figure 2a demonstrate that the
ZIF-
8 particles with a contact time of 20 min had higher crys-
tallinity. Moreover, compared to the particles using freez-
ing as the drying method, results show that the heat-
drying
method could produce ZIF-
8 particles with stronger peak
intensities. It is speculated that the thermal energy provided
by heat could remove the crystal water bound to the surface
of ZIF-
8 more efficiently, and thus, the crystal structure of
ZIF-
8 particles would be more complete (Duan et al., 2016).
Thus, ZIF-
8 particles were produced using a 20-
min ultra-
sonic time and the heat-
drying method at 105°C for 24 h.
Pore properties of ZIF-
8
Results from the analyses of pore properties of ZIF-
8 particles
demonstrate that the Langmuir surface area was 1925 m2
/g, total
pore volume was 0.7 cm3
/g, mean pore diameter was 2.0 nm, and
specific BET surface area was 1373 m2
/g. Figure 2c shows the
nitrogen adsorption and desorption isotherms of ZIF-
8 particles.
With classification of physisorption isotherms, the microporous
structure of the ZIF-
8 (mean pore diameter below 2.0 nm) be-
longed to the reversible type-
I(b) isotherms (Jian et al., 2015).
Results indicate that ZIF-
8 had an adsorption behavior of typical
microporous materials with an increased adsorption volume at
low relative pressure (Ding et al., 2017; Thommes et al., 2015).
Stability of ZIF-8
XRD analyses were performed to assess effects of water
immersion and pH value on the crystal stability of ZIF-
8
(3)
ln
(
qe − qt
)
= lnqe − k1t
(4)
t
qt
=
1
k2qe2
+
t
qe
(5)
qe = qmaxKL ⋅
Ce
1 + KL ⋅ Ce
(6)
qe = KF ⋅ Ce1∕n
5.
| 5
WATER ENVIRONMENT RESEARCH
particle. Figure 2d,e presents ZIF-
8 XRD patterns of stability
under different (a) water immersion time and (b) pH value.
The micropores of ZIF-
8 particles were filled with molecules
of water with an increase in particle sizes. This also caused
the decreased peak intensities and unstable baselines. It is
worthy to note that the heat-
drying method had a more thor-
ough removal of water molecules, and thus, the release of
the water molecules would not damage the framework struc-
tures, as evidenced from the characteristic of XRD spectra
(Pan et al., 2019; Zhang et al., 2015).
Compared to freshly synthesized ZIF-
8 particles, the peak
intensities became weaker and baseline became unstable in
water with different pH values for 24 h. The weakened peaks
reveal that the synthesized ZIF-
8 particles were partly de-
structed. However, the peak characteristics were still iden-
tifiable, indicating the existence of ZIF-
8 particles in water
phase under acidic or alkaline conditions. The results imply
that ZIF-
8 particles were not stable under acidic or alkaline
conditions, but their crystal structures could still be partly
maintained. This demonstrates that ZIF-
8 particles could
be also applied for water treatment under slightly higher or
lower pH conditions in the range from pH 5 to 11.
Influence of ZIF-
8 dosage on removal
efficiency (RE) and adsorption capacity (AC)
Effects of the amount of ZIF-
8 used on the removal
of Cr(VI) were evaluated. Figure 3 presents (a) RE of
Cr(VI) and (b) AC of Cr(VI) with different dose of ZIF-
8. Results indicate that a significant improvement of
Cr(VI) RE (from 11.8 to 26.0%) was observed when the
adsorbent (ZIF-
8) dosage raised from 0.005 to 0.08 g.
However, AC dropped from 44.3 to 6.3 mg/g with in-
creased ZIF-
8 dosages from 0.005 to 0.08 g. This could
be because the higher amounts of adsorbent could cause
the agglomeration of the particles, and this led to the de-
creased active locations on ZIF-
8 surface. Thus, the ad-
sorbed Cr(VI) mass onto the ZIF-
8 particles decreased
with the increased dosages of ZIF-
8 particles (Ibrahim
et al., 2016; Maheshwari et al., 2015). Thus, in the fol-
lowing Cr(VI) adsorption experiments [effects of ZIF-
8
on Cr(VI) adsorption with different Cr(VI) initial con-
centrations], a middle dosage of ZIF-
8 (0.02 g/40 ml) was
used to prevent the occurrence of AC drop due to the ag-
glomeration effect.
FIGURE 1 ESEM images of ZIF-
8 particles of (a) ultrasonic for 10 min and freeze-
drying, (b) ultrasonic for 20 min and freeze-
drying, (c)
ultrasonic for 10 min and heat-
drying, and (d) ultrasonic for 20 min and heat-
drying
6. 6
| OU et al.
FIGURE 2 (a) XRD images of ZIF-
8 particles of different ultrasonic time and drying method, (b) comparison with results from Wang et al.
(2015), (c) nitrogen adsorption–
desorption isotherms of ZIF-
8 particles, (d) XRD images of ZIF-
8 water stability with different immersion time,
and (e) XRD images of ZIF-
8 water stability with different pH values
7.
| 7
WATER ENVIRONMENT RESEARCH
Influence of Cr(VI) initial concentrations on
RE and AC
In this study, effects of ZIF-
8 particles on Cr(VI) adsorp-
tion with different Cr(VI) initial concentrations (20, 50, and
100 mg/L) were evaluated (mixing time =120 min, ZIF-8 par-
ticle = 0.02 g) using the wave shaker and ultrasonic cleaner
mixing methods. Figure 3c presents the RE of Cr(VI) during
the 120-
min mixing using shaking and ultrasonic methods.
Table 1a presents the results of Cr(VI) adsorption with differ-
ent initial concentrations using shaking and ultrasonic meth-
ods for mixing.
Results show that the shaking method could effectively
improve the Cr(VI) adsorption reaction between Cr(VI) and
ZIF-
8 particles in water by quickly dispersing ZIF-
8 particles
during the initial 10-
min operation (initial Cr(VI) concentra-
tion = 50 mg/L). However, a remarkable raise of Cr(VI) AE
was detected and higher adsorption capacity was obtained
after a 120-
min mixing using the ultrasonic method for mix-
ing. The shaking method could only improve the dispersion
of particles in the aqueous solution. However, the ultrasonic
method could improve the mass transfer and accelerate the
absorption process by enhancing the reactivity between ad-
sorbates and absorbents. After the ZIF-
8 particles obtain suf-
ficient energy through the ultrasonic method, the adsorption
and mass transfer of Cr(VI) on the particle surface were en-
hanced (Chemat et al., 2017; Hasankola et al., 2019).
Figure 4a shows the AC with different reaction time and
initial Cr(VI) concentrations. Results indicate that a signifi-
cantly increased adsorption efficiency was observed during
the earlier operational period. However, the AC slowly lev-
elled off after a significant initial increment. Results also
imply that increased adsorption efficiency was observed with
increased initial concentrations of Cr(VI). The initial concen-
trations had a significant influence on AC and equilibrium
time.
FIGURE 3 (a) Removal efficiency of Cr(VI), (b) Cr(VI) adsorption capacity with different dose of ZIF-
8 [initial Cr(VI)
concentration = 50 mg/L and contact time = 45 min], and (c) removal efficiency of Cr(VI) at 120 min in different conditions
0
10
20
30
40
50
60
70
0 0.02 0.04 0.06 0.08
Adsorption
capacity
(mg/g)
Weight of ZIF-8 (g)
(b)
(a)
(c)
8. 8
| OU et al.
Kinetic model analyses
Figure 4b,c shows the pseudo-
first-
order kinetic model and
pseudo-
second-
order kinetic modeling results for Cr(VI) ad-
sorption, respectively. Table 1b presents the calculated rate
constant and equilibrium concentration from the kinetic mod-
els of Cr(VI) adsorption. The pseudo-
second-
order model
had a better fit with the results obtained from the experiments
[a high correlation coefficient value (R2
) > 0.94 is observed
in Figure 4b]. According to the pseudo-
second-
order model
theory (based on the solid phase adsorption capacity), the re-
moval rate of adsorbate [Cr(VI)] was dependent on Cr(VI)
adsorbed on ZIF-
8 particles and the amount of Cr(VI) ad-
sorbed on the surface of particles at equilibrium. Thus, the
rate-
limiting step might be the chemisorption process (Xiao
et al., 2017; Zanin et al., 2017). Results confirmed the hy-
pothesis that the initial concentration of Cr(VI) had a positive
influence on the Cr(VI) adsorption capacity, and increased
initial concentration resulted in increased adsorption capac-
ity. In this test, the dosage of the adsorbent was fixed, and
thus, the effect of agglomeration on the adsorption capacity
of Cr(VI) was not considered.
Adsorption mechanisms of Cr(VI)
Figure 4d,e presents the results of the Freundlich and
Langmuir model analyses of Cr(VI) adsorption, respectively.
The Langmuir isotherm model had a better fit with the ad-
sorption results (R2
= 0.99). The Langmuir model has a con-
ception that the adsorbent surface is homogeneous, which
results in monolayer adsorption (Abbasi et al., 2016). This
matches with the result of nitrogen adsorption–
desorption
isotherm of ZIF-
8 particles. It implies that the micropores,
which had adsorption capacities, might exist on ZIF-
8 sur-
face, and they might have contributions to Cr(VI) adsorption
(Luan Tran et al., 2019).
Moreover, XPS was applied to demonstrate the effective-
ness and chemisorption mechanisms of Cr(VI) adsorption using
ZIF-
8 particle as the adsorbent. Figure 5a,d presents the XPS
spectra showing Cr(VI) adsorption. ZIF-
8 particles were com-
posed of N, C, Zn, and elements. After Cr(VI) adsorption, Zn2p
peaks had a slight shift to a range with higher binding energy,
which assigned a new signal of Cr appearing in XPS spectra
from 576.0 to 597.0 eV (Figure 5a). Figure 5b shows O1 s spec-
trum of ZIF-
8 before adsorption. The broad peak could be de-
convoluted into two peaks referred to Zn-
OH and H2O bonds
at approximately 531.7 and 532.8 eV, respectively. After Cr(VI)
adsorption, the peak area corresponding to Zn–
OH had a dra-
matic decrease and the new peak corresponding to Zn-
O-
Cr
appeared at 530.1 eV (Figure 5c). The results indicate that the
hydroxyl group (OH−
) was involved in the chemisorption pro-
cess of Cr(VI) adsorption (Li et al., 2015, 2017). Results from
T
A
B
L
E
1
(a)
Results
of
Cr(VI)
adsorption
with
different
initial
concentrations
using
shaking
and
ultrasonic
methods
for
mixing.
(b)
The
calculated
rate
constant
and
equilibrium
concentration
from
the
kinetic
models
of
Cr(VI)
adsorption
(a)
Initial
Cr(VI)
concentration
(mg/L)
Shaker
Ultrasonic
Residual
Cr(VI)
concentration
(mg/L)
Removal
efficiency
(%)
Adsorption
capacity
(mg/g)
Residual
Cr(VI)
concentration
(mg/L)
Removal
efficiency
(%)
Adsorption
capacity
(mg/g)
20
15.2
±
0.2
24.1
9.6
11.3
±
0.1
43.6
17.4
50
37.6
±
0.3
24.8
24.8
34.8
±
0.1
30.3
30.3
100
87.0
±
0.5
13.0
26.0
77.2
±
0.3
22.8
45.6
(b)
Initial
Cr(VI)
concentration
(mg/L)
Pseudo-
f
irst-
o
rder
kinetic
model
Pseudo-
s
econd-
o
rder
kinetic
model
Rate
constant
(k
1
)(1/min)
Correlation
coefficient
(R
2
)
Equilibrium
concentration
(q
e
)(mg/g)
Rate
constant
(k
2
)
Correlation
coefficient
(R
2
)
Equilibrium
concentration
(q
e
)
(mg/g)
20
1.47
×
10
−2
0.96
16.0
1.06
×
10
−3
0.95
20.0
50
1.05
×
10
−2
0.95
31.4
2.33
×
10
−3
0.99
33.1
100
1.87
×
10
−2
0.85
41.7
1.81
×
10
−3
0.99
44.8
9.
| 9
WATER ENVIRONMENT RESEARCH
Figure 5d demonstrate that the Cr2p XPS spectrum of the sur-
face of ZIF-
8 particles and two peaks of Cr(VI) and Cr(III) could
be recognized. This phenomenon indicates that the reducing
process of sorbed Cr(VI) to Cr(III) occurred after adsorption.
The reduction of Cr(VI) was ascribed to the nitrogen atom group
on the organic linkers (2-
methylimidazole) of ZIF-
8, which was
FIGURE 4 (a) Adsorption capacity of ZIF-
8 with different initial Cr(VI) concentrations, (b) results of the pseudo-
first-
order kinetic modeling,
(c) results of the pseudo-
second-
order kinetic modeling, (d) results of the Langmuir modeling, and (e) results of the Freundlich modeling
(a) (b)
(c)
(d)
(e)
10. 10
| OU et al.
used as the reducing agent (Cardoso et al., 2018). It exhibited a
behavior of light irradiation, which was similar to the function of
semiconductor, and it subsequently activated the metal sites, and
partial Cr(VI) reduction was achieved by electron transfer (Hu
et al., 2019; Reddy et al., 2016; Zhu et al., 2017).
In summary, ZIF-
8 could serve as an adsorption agent for
Cr(VI) adsorption and could also serve as a reducing agent
for partial Cr(VI) reduction. Thus, Cr(VI) migration in the
environment can be prevented and Cr(VI) toxicity can be re-
duced when ZIF-
8 particles are used to treat Cr(VI)-
polluted
water. During the Cr(VI) reduction process, electron was
transferred from ZIF-
8 to Cr(VI). However, the oxidation
product was not further studied in this test.
Influence of pH of aqueous solution on ZIF-
8
water stability
The initial pH of adsorbate solution usually results in a nota-
ble influence on adsorption performance. Effects of aqueous
solution pH (pH ranged from 5 to 11) on Cr(VI) adsorption
were evaluated (contact time = 45 min, Cr(VI) = 50 mg/L,
and ZIF-
8 = 0.02 g). Figure 6a shows influences of aqueous
solution pH on Cr(VI) adsorption by ZIF-
8.
Results show that the ZIF-
8 could perform Cr(VI) adsorption
over a broad range of pH (pH = 5–
11). The maximum Cr(VI)
AC appeared at pH 5 (34.4 mg/g). A significant drop of the
Cr(VI) AC was observed when the initial pH was higher than 7
(7 to 11) (AC was below 13.7 mg/g). The possible cause was that
Cr(VI) occurred as an anion in water. The dominant Cr(VI) form
was Cr2O7
2−
in acidic aqueous environments, while the domi-
nant Cr(VI) form was CrO4
2−
in basic or slightly acidic solutions
(Gifford et al., 2017; Lytras et al., 2017). The number of hydroxyl
ion (OH−
) increased when the pH values increased in the aque-
ous solution. This resulted in a competitive reaction between
OH−
and CrO4
2−
for adsorption sites on the surface of ZIF-
8
particles (Chen et al., 2014; Gupta et al., 2016; Lian et al., 2019).
Figure 6b presents the XRD image of ZIF-
8 particles in solu-
tion with different pH values. When the initial pH was 5, the peak
intensity decreased and the peak characteristic of ZIF-
8 was not
FIGURE 5 (a) XPS spectra of the ZIF-
8 particles before and after Cr(VI) adsorbed, (b) O1 s spectra of ZIF-
8 particle before Cr(VI) adsorbed,
(c) O1 s spectra of ZIF-
8 particle after Cr(VI) adsorbed, and (d) Cr2p XPS spectra of on the surface of ZIF-
8 particles
11.
| 11
WATER ENVIRONMENT RESEARCH
obvious. The concentration of Zn2+
was found to be 0.235 mg/L
by ICP-
AES, but the Zn2+
ion in the aqueous solution was not
found in the pH range from 7 to 11. Cr(VI) adsorption process
under acidic pH conditions could cause the dissolution of ZIF-
8
and partial collapse of the ZIF-
8 structure. That partial collapse
of ZIF-
8 could be because of the anion adsorption on ZIF-
8 sur-
face. Researchers have reported that anions in aqueous solution
could result in a remarkable impact on solution stability (Sheng
et al., 2017; Wang et al., 2018a). Overall, XRD results in different
solution pH range matched with patterns of XRD. The structural
defects on ZIF-
8 particles might cause the increased adsorption
capacity for Cr(VI). However, the hypothesis and possible practi-
cal application need to be confirmed in the future study.
Regeneration of ZIF-8
The feasibility of ZIF-
8 regeneration for its reuse on Cr(VI)
removal could be a very important feature in water treat-
ment. Figure 6c presents the Cr(VI) concentration in different
desorptionsolutions.ResultsshowthattheconcentrationofCr(VI)
(0.45 mg/L) in NaCl solution (0.1 M) was higher than that in eth-
anol (0.30 mg/L) and DI water (0.10 mg/L). Because NaCl could
provide Cl−
anion to replace the adsorbed Cr(VI), this resulted
in the desorption of Cr(VI). Figure 6d presents the AC of ZIF-
8
particles after the regeneration process using different solutions.
Compared with ZIF-
8 particles regenerated by NaCl solution and
water, results show the regenerated ZIF-
8 particles by ethanol
had a higher adsorption capacity (14.4 mg/g). Results indicate
that a 64% AC (dropped from 22.5 to 14.4 mg/g) was obtained
after a regeneration process with ethanol solution. This indicates
that the regeneration process could be an option for the reuse and
sustainable application of ZIF-
8 particles for water treatment.
CONCLUSION
The effectiveness and mechanisms of ZIF-
8 application on
the treatment of Cr(VI)-
polluted water were evaluated in this
feasibility study. Results indicate that ZIF-
8 particle had a
FIGURE 6 (a) Effects of initial solution pH on Cr(VI) adsorption by ZIF-
8, (b) XRD images of ZIF-
8 after Cr(VI) adsorption in solution with
different pH, (c) concentration of Cr(VI) in different desorption solutions, and (d) adsorption capacity of ZIF-
8 particles after the regeneration
process with different solutions
12. 12
| OU et al.
clear rhombic dodecahedron morphology shape and a high
crystallinity with an ultrasonic time of 20 min and a heat-
drying at 105°C for 24 h. The produced ZIF-
8 particles had
good water stability with a water immersion time for 14 days
and a pH range from 5 to 11. Thus, ZIF-
8 particles had good
applicability on water treatment. Results also demonstrate that
the ZIF-
8 particle could perform significant Cr(VI) adsorp-
tion. The AC of Cr(VI) was 30.3 mg Cr(VI)/g ZIF-
8 particle
[Cr(VI) = 50 mg/L]. Because of the competitive interaction
between CrO4
2−
and OH−
, the capacity for Cr(VI) adsorption
at pH 5 was higher than those when pH ranged from 7 to 11.
The capacity for Cr(VI) adsorption raised to 34.3 mg/g at pH
5 when Cr(VI) concentration was 50 mg/L. Results reveal
that the Langmuir and pseudo-
second-
order models could be
used for the description of Cr(VI) adsorption behaviors when
ZIF-
8 particle was used as the adsorbent, and increased AC
was corresponded to the increased Cr(VI) concentration. XPS
analyses demonstrate that the chemical reduction mechanism
was also responsible for Cr(VI) adsorption onto ZIF-
8 parti-
cles. The hydroxyl group was engaged during the process of
Cr(VI) chemisorption on ZIF-
8 particles. In addition, nitro-
gen atom groups on 2-
methylimidazole of ZIF-
8 participated
in the Cr(VI) reduction by electron transfer. Results from this
study conclude that an effective Cr(VI) removal can be ob-
tained and Cr(VI) toxicity can be reduced when ZIF-
8 parti-
cles are used for Cr(VI)-
polluted water treatment, and thus,
ZIF-
8 particles have the potential to be used as the adsorption
and reduction agents for water treatment.
ACKNOWLEDGEMENTS
This study was funded by Taiwan Ministry of Science and
Technology(MOST)(GrantNo.106-2221-E-110-012-MY3).
The authors would like to thank the personnel at MOST for
their direction and help during the research period.
ORCID
Chih-Ming Kao https://orcid.org/0000-0002-6151-7076
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How to cite this article: Ou J-
H, Sheu Y-
T, Chang
BK, Verpoort F, Surampalli RY, Kao C-
M.
Application of zeolitic imidazolate framework for
hexavalent chromium removal: A feasibility and
mechanism study. Water Environ Res. 2021;00:1–15.
https://doi.org/10.1002/wer.1571
![Water Environ Res. 2021;00:1–15. wileyonlinelibrary.com/journal/wer | 1
© 2021 Water Environment Federation
INTRODUCTION
Heavy metal-
polluted water bodies are a severe environ-
mental problem worldwide. Illegal discharge and improper
treatment of wastewaters discharged from different industrial
factories (e.g., mining, steel production, electroplating) could
be major causes of heavy metal pollution of water bodies.
Heavy metals have strong affinities with organic matters,
which allow them to accumulate in soils and sediments
(Ahmed & Ahmaruzzaman, 2016; Chen et al., 2018; Wang
Received: 21 November 2020
| Revised: 13 March 2021
| Accepted: 1 April 2021
DOI: 10.1002/wer.1571
R E S E A R C H A R T I C L E
Application of zeolitic imidazolate framework for hexavalent
chromium removal: A feasibility and mechanism study
Jiun-Hau Ou1
| Yih-
Terng Sheu1
| Bor Kae Chang2
| Francis Verpoort3
|
Rao Y. Surampalli4*
| Chih-
Ming Kao1*
*WEF Members.
1
Institute of Environmental Engr.,
National Sun Yat-
Sen University,
Kaohsiung, Taiwan
2
Department of Chemical and Materials
Engineering, National Central University,
Taoyuan, Taiwan
3
State Key Laboratory of Advanced
Technology for Materials Synthesis
and Processing, Wuhan University of
Technology, Wuhan, China
4
Global Institute for Energy, Environment
and Sustainability, Lenexa, KS, USA
Correspondence
Chih-
Ming Kao, Institute of
Environmental Engineering, National Sun
Yat-
Sen University, Kaohsiung 80424,
Taiwan.
Email: jkao@mail.nsysu.edu.tw
Funding information
Ministry of Science and Technology,
Taiwan, Grant/Award Number: 106-
2221-
E- 110-012-MY3
Abstract
The mechanisms and effectiveness of using zeolitic imidazolate framework (ZIF-
8)
[a sub-
family of metal-
organic framework (MOF)] particles on hexavalent chromium
[Cr(VI)] removal were evaluated. The ultrasonic mixing method was applied for the
preparation of ZIF-
8, and chemicals used for ZIF-8 synthesis included ammonium hy-
droxide, zinc nitrate hexahydrate, and 2-
methylimidazole. ZIF-
8 particle had a clear
rhombic dodecahedron morphology shape and a strong peak intensity with high crys-
tallinity. The adsorption capacity (AC) of ZIF-
8 was 30.3 mg of Cr(VI)/g of ZIF-
8
[Cr(VI) = 50 mg/L]. The AC of Cr(VI) raised to 34.3 mg/g under acidic conditions
(pH = 5), and the AC dropped to below 13.7 mg/g with a pH range from 7 to 11. It
could be because of the competitive effects between CrO4
2−
and hydroxide ions for
adsorption locations of ZIF-
8. Cr(VI) removal relied on the amount of Cr(VI) ad-
sorbed on the particles of ZIF-
8, and the mechanisms of Cr(VI) adsorption by ZIF-
8
included chemical/physical processes and the rate-
limiting step was the chemical ad-
sorption. A fraction of sorbed Cr(VI) was reduced to Cr(III), and thus, ZIF-
8 could
serve as a reducing agent during Cr(VI) reduction. Cr(VI) was removed effectively
from the water phase by ZIF-
8 via adsorption and reduction mechanisms.
Practitioner Points
• ZIF-
8 particles had an adsorption capacity of 30.33 mg of Cr(VI)/g of ZIF-
8.
• Cr(VI) sorption by ZIF-
8 has chemical (rate-
limiting step) and physical
processes.
• ZIF-
8 can serve as a reducing agent for Cr(VI) reduction.
• Cr(VI) can be removed by ZIF-
8 via the adsorption and reduction mechanisms.
K E Y W O R D S
adsorption, hexavalent chromium, metal-
organic framework, zeolitic imidazolate framework](https://image.slidesharecdn.com/2021-applicationofzeoliticimidazolateframeworkforhexavalentchromiumremovalafeasibilityandmechanismst-230711075115-d1c81b59/85/2021-Application-of-zeolitic-imidazolate-framework-for-hexavalent-chromium-removal-A-feasibility-and-mechanism-study-pdf-1-320.jpg)
