Alkoxy-Group-Functionalized UiO-66 as Highly Efficient Adsorbents for Hydrogen Chloride Removal from Aqueous Solution

Article abstract of DOI:10.1021/acs.jced.8b00802

A series of alkoxy group-functionalized UiO-66 were designed for hydrogen chloride adsorption from aqueous solution, which were characterized by various methods to verify the structures and study the adsorption mechanism. A volcano-shaped change of adsorp

Full text of DOI:10.1021/acs.jced.8b00802

Article
pubs.acs.org/jced
Cite This: J. Chem. Eng. Data 2019, 64, 286−295
Alkoxy-Group-Functionalized UiO-66 as Highly Efficient Adsorbents
for Hydrogen Chloride Removal from Aqueous Solution
Hongxu Liu, Xiaoyu Lan, Peng Bai, and Xianghai Guo*
Department of Pharmaceutical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350,
P.R. China
Key Laboratory of Systems Bioengineering (Ministry of Education), Tianjin University, Tianjin, 300350, P.R. China
S
* Supporting Information
ABSTRACT: A series of alkoxy group-functionalized UiO-66
were designed for hydrogen chloride adsorption from aqueous
solution, which were characterized by various methods to
verify the structures and study the adsorption mechanism. A
volcano-shaped change of adsorption capacity was discovered.
UiO-66-C2 exhibited the highest adsorption capacity and
excellent structure stability. The adsorption kinetics, iso-
therms, and thermodynamics were studied. It was found that
the pseudo-second-order kinetic model fit the experimental
data well in kinetic studies. Meanwhile, both Henry and Freundlich models fitted the isotherm experiment data well. The
thermodynamics parameters further confirmed that hydrogen chloride adsorption on UiO-66-C2 was a spontaneous and
exothermic physisorption process. Regeneration experiment demonstrated that UiO-66-C2 has high regeneration efficiency and
no significant change in adsorption capacity was confirmed after recycles. All the results substantiated that UiO-66-C2 could be
an efficient solution to treatment of acidic wastewater.
distillation,15 and membrane separation16 have been reported
to remove hydrogen chloride. Although these methods show
1. INTRODUCTION
Annually, a huge amount of acidic wastewater is produced in
good separation selectivity, some disadvantages, including
pharmaceutical, chemical, and iron and steel industries.1−3
secondary pollution, high cost, and energy consumption, make
Acidic wastewater could cause serious corrosion of steel pipes
them commercially unfeasible. For example, neutralization is
and misbalance of acid and base in soil, and also represent a
commonly used in industrial applications, while it could
significant hazard to the health and safety of people and
environment. Therefore, acidic wastewater needs to be treated,
disposed, or recycled effectively to satisfy environment
requirements.
Among these acidic wastewaters, wastewater containing
hydrochloric acid is one of the major and urgent concerns.
Vast amounts of hydrochloric acid wastewater are produced
produce a large volume of sludge that requires further
disposal.17 Adsorption is considered as a potential way to
deal with dilute hydrochloric acid (≤5 wt %), and adsorption
technique possesses higher efficiency and flexibility in design
and operation compared with the above-mentioned meth-
ods.18,19 After adsorption, ideally, the adsorbent can be
regenerated, so it renders significant cost savings and no
from the stainless steel pickling process and electroplating
industry every year.4−6 Chloride ion is the main acidified ion
secondary pollution. Although widely used adsorbents, such as
activated carbon20 and zeolite,21,22 have been tested to adsorb
present in coal plant emissions. HCl is a greater driver of
hydrogen chloride, disadvantages of poor adsorption perform-
pollution and environmental damage than previously thought,
ance and low recyclability rate greatly hinder their further
influencing water and terrestrial ecosystems alike. Due to the
high mobility of Cl, HCl can effectively acidify both the soil
and surface water. The increase in acidity in the soil prevents
some organisms from growing, which could have a knock-on
effect on the entire ecosystem. Moreover, the effects of HCl
deposition may extend beyond acidification to include a
industrial applications.
Metal−organic frameworks (MOFs) are porous coordina-
tion polymers composed of organic linkers and metal ions or
clusters to form three-dimensional structures.23,24 Since they
can be potentially used for gas storage and separation,25
sensing,26 catalysis27 and energy storage,28 MOFs have
broader range of ecosystem processes by affecting the carbon
cycle.7 Therefore, there is great demand to develop highly
stimulated significant interest. Particularly, MOFs can be
used to adsorb contaminants from wastewater.29−31 However,
efficient separation techniques to remove hydrogen chloride
from wastewater. However, this kind of wastewater often
contains only 5−10 wt % free hydrogen chloride, which is
extremely difficult to remove.8 Several techniques including
neutralization,9 roasting,10 extraction,11−13 evaporation,14
Received: September 6, 2018
Accepted: December 6, 2018
Published: December 13, 2018
© 2018 American Chemical Society
286
DOI: 10.1021/acs.jced.8b00802
J. Chem. Eng. Data 2019, 64, 286−295

Journal of Chemical & Engineering Data
Article
Scheme 1. Synthetic Method of Organic Linkers
because of the water-unstable coordination bond, most of
metal−organic frameworks have limited applications in
industry. Fortunately, an increasing number of water-stable
and even acid-stable MOFs have been reported as promising
agents for removing pollutants, such as ZIFs,32,33 UiOs,34 and
MILs.35
2.2. Synthesis of the Organic Linkers. The organic
linkers were prepared according to the reported methods.36,37
For the synthesis of H2BDC-C1 (H2BDC = terephthalic acid),
H2BDC-C1 was obtained through potassium permanganate
oxidation of 2,5-dimethylanisole. As for the synthesis of
H2BDC-C2, H2BDC-C3, and H2BDC-C4, these linkers were
obtained via nucleophilic substitution of 2-hydroxyterephthalic
acid and bromoalkane (Scheme 1). Detailed synthetic methods
are depicted in the Supporting Information. Structures of the
organic linkers were confirmed by 1H NMR. 1H NMR spectra
(Figure S1) of the compounds in dimethyl sulfoxide-d6
(DMSO-d6) were measured with a BRUKER spectrometer.
2.3. Synthesis of the UiO-66-CX. According to the
synthetic method of UiO-66 reported in the literature, UiO-66-
CX were synthesized by substituting the H2BDC linker with
alkoxy functionalized linkers.38 In the Supporting Information,
the detailed synthetic procedure is depicted.
2.4. Characterization of the UiO-66-CX. X-ray diffraction
(XRD) analysis (D8-Advance, Bruker AXS, Germany, Cu Kα
radiation, 15 mA and 40 kV) over 2θ = 5−50° was employed
to ascertain the crystal structures of the UiO-66-CX. The
morphologies of microcrystalline were characterized by field
emission scanning electron microscopy (FE-SEM, Hitachi SU-
4080, Japan, S-4800, 3 kV). Transmission electron microscopy
(TEM, JEOL, Japan, JEM-2100F) operated at an accelerating
voltage 100 kV was utilized to observe the morphology of the
nanoparticles. The thermal behaviors of the structures were
confirmed by thermogravimetric analysis (TGA, Q5000IR, TA
Instruments, USA), which was performed from room temper-
ature to 800 °C with an increasing rate of 5 °C ·min−1 at an air
flow rate of 60 mL·min−1. A porosity and surface area analyzer
(Quantatech Co., USA, Autosorb-iQA 3200-) was used to
examine the N2 adsorption isotherms of the materials at 77 K
after evacuating the samples at 100 °C. Fourier transform
infrared spectroscopy (FT-IR, Nicolet 6700, Thermo Fisher,
USA) was utilized to investigate the functional groups of the
UiO-66-CX. X-ray photoelectron spectroscopy (XPS, PHI-
5000C ESCA, Japan) was measured to study the mechanism of
hydrogen chloride adsorption.
In our previous study, the adsorption capacity of hydrogen
chloride on several kinds of well-studied MOFs with water-
stability, such as MIL-53(Cr), MIL-100(Cr, Fe), and UiO-66,
were carefully assessed. As a highly porous MOF with great
chemical and thermodynamic stability, UiO-66 exhibited
excellent adsorption capacity toward hydrogen chloride (up
to 289.81 mg·g−1).18 On that basis, inspired by alkyl ether
extractant of acid, a sequence of UiO-66 MOFs with various
alkoxy functionalized groups (C1−C4) have been designed
and synthesized, abbreviated as UiO-66-C1, UiO-66-C2, UiO-
66-C3, and UiO-66-C4, respectively. All of them showed their
potential for the adsorption capacity of hydrogen chloride from
dilute aqueous solution. Particularly, UiO-66-C2 possessed the
up-to-date highest adsorption capacity. The adsorption
kinetics, isotherms, thermodynamics, adsorptive mechanism,
and regeneration of this material toward hydrogen chloride
were further studied. The results substantiated the potential of
UiO-66-C2 as an effectual hydrogen chloride adsorbent from
aqueous solution.
2. EXPERIMENTS
2.1. Chemicals. Hydrochloric acid (36 wt %, Yuanli,
Tianjin, China), 2-hydroxyterephthalic acid (TCI, Shanghai,
China), 2-bromoethane (Yuanli, Tianjin, China), 2-bromopro-
pane (TCI, Shanghai, China), 2,5-dimethylanisole (Adamas,
Shanghai, China), potassium permanganate (Tianjin Univer-
sity Kewei, Tianjin, China), methanol (Kermel, Tianjin,
China), sulfuric acid (Jiangtian, Tianjin, China), potassium
carbonate (Yuanli, Tianjin, China), N,N-dimethylformamide
(DMF, Concord, Tianjin), ZrCl4 (Strem Chemicals, USA),
ethanol (Jiangtian, Tianjin, China), dichloromethane (Yuanli,
Tianjin, China), sodium hydroxide standard solutions (0.01
mol·L−1, Yuanli, Tianjin, China), and deionized water (Yuanli,
Tianjin, China). All reagents and solvents were obtained from
commercial sources and were used as received.
2.5. Adsorption Experiment. The adsorbents were stored
in a desiccators and dried overnight at 100 °C in vacuum
287
DOI: 10.1021/acs.jced.8b00802
J. Chem. Eng. Data 2019, 64, 286−295

Journal of Chemical & Engineering Data
Article
Figure 1. (a) Simulated XRD pattern of UiO-66 and patterns of synthesized UiO-66-CX. (b) FT-IR spectrum of UiO-66-CX. (c) TGA curves of the
synthesized UiO-66-CX. (d) Left: N2 adsorption−desorption isotherms; right: pore diameter distributions.
before adsorption experiments. During the adsorption experi-
ments, 100 mg ( 0.1 mg) of MOFs was added into 20 mL 0.5
mol·L−1 HCl solutions in a dosage of 5 g·L−1, and continuously
stirred for 24 h at 25 °C. Syringe filters (aqua system, 0.22 μm)
were utilized to filter solid MOFs from hydrochloride solution.
The concentration of hydrochloride solution can be obtained
through titrating with sodium hydroxide standard solution
(0.01 mol·L−1) and the indicator was phenolphthalein. Each
data was obtained by titration at least three times and the
average number, as a final result, was applied to further
analysis. The adsorption capacities of hydrogen chloride on
adsorbents Qt (mg·g−1), was determined by eq 1:
concentration ranging from 0.4 to 1.8 mol·L−1. With a dosage
of 5 g·L−1, the vacuum-dried UiO-66-C2 was added to those
hydrochloride solutions.
2.8. Regeneration of UiO-66-C2. The spent UiO-66-C2
was recycled through a washing process by using deionized
water and anhydrous ethanol three times and then dried in
vacuum at 150 °C overnight. Under the same conditions as
former experiment (dosage: 5 g·L−1; C0: 0.5 mol·L−1; T: 25
°C), an adsorption experiment of recycled UiO-66-C2 was
conducted.
3. RESULTS AND DISCUSSION
3.1. Characterization of UiO-66-CX. The XRD patterns
(Figure 1a) of the synthesized UiO-66-CX showed two main
peaks at 7.4 and 8.5°, which was consistent with the simulated
UiO-66, indicating that the structures of synthesized UiO-66-
CX were basically the same as that of UiO-66.37 However, there
was also some amorphous phase present, and its influence on
adsorption performance was ignored in this study.
Q t = (C0 − Ct)VM × 1000/m
(1)
where V (L) is the volume of hydrochloric acid solution; C0 is
the initial concentration of HCl; Ct is the concentration of HCl
on sampling time t (mol·L−1); m (g) is the weight of spent
adsorbents; and M (g·mol−1) is the relative molecular mass of
hydrogen chloride.
FT-IR patterns (Figure 1b) displayed bands ranging from
3000 to 2840 cm−1 and 1275 to 1200 cm−1, which could be
credited to C-Hst (st: stretching vibration) and arC-O-alC (ar:
aromatic; al: alkane), clearly proving the presence of alkoxy
groups in the structures. TGA was performed to examine the
three-dimensional framework stability and structural integrity
of the synthesized UiO-66-CX. As is shown in the TGA curve
(Figure 1c), the removal of H2O molecules resulted in slight
weight loss of all UiO-66-CX powders from 30 to 150 °C.
Then, the removal of DMF molecules leads to a weight loss
from 150 to 350 °C. Decomposition occurred when the
temperature was higher than 350 °C, and the organic linker
2.6. Adsorption Kinetics. The impact of adsorption time
on the adsorption of hydrogen chloride on UiO-66-C2 was
carefully studied in the investigation of adsorption kinetics. An
amount of 100 mg ( 0.1 mg) of UiO-66-C2 was put in 20 mL
hydrochloride solutions at a dose ratio of 5 g·L−1 with initial
HCl concentration range of 0.6−1.4 mol·L−1 at 25 °C.
Approximately 0.3 mL of samples was extracted at different
times to calculate the amount of unadsorbed HCl for kinetic
analysis.
2.7. Adsorption Isotherms and Adsorption Thermo-
dynamics. Batch adsorption isotherms experiments were
carried out at 25, 35, 45, and 55 °C with the initial HCl
288
DOI: 10.1021/acs.jced.8b00802
J. Chem. Eng. Data 2019, 64, 286−295

Journal of Chemical & Engineering Data
Article
molecules lost gradually. The decomposition temperature was
quite lower than the typical value of 500 °C of pristine UiO-66
mentioned in literature.39,40
The N2 adsorption isotherms of pristine UiO-66 (Figure S2)
and the prepared UiO-66-CX (Figure 1d) at 77 K were in
accord with type I isotherm. BET surface area, pore diameter,
and total pore volume of pristine UiO-66 and UiO-66-CX are
listed in Table S1. BET surface areas of UiO-66-C1 and UiO-
66-C2 were 922 and 930 m2·g−1, respectively, which were
consistent with the data of UiO-66 in literature.41,42 However,
the addition of propoxy and butoxy group on the ligand, which
were linked to UiO-66-C3 and UiO-66-C4, had an obvious
negative influence on the BET surface areas, which were 791
and 549 m2·g−1, respectively. Since the propoxy or butoxy
group on the ligand had larger volume, porous channel was
blocked by these bulky substituents. Consequently, surface
area was significantly reduced. It may explain the decreased
adsorptive capacity of UiO-66-C3 and UiO-66-C4. The pore
volume, estimated at P/P0 = 0.99 was 0.51 cm3·g−1 for UiO-66-
C1, 0.55 cm3·g−1 for UiO-66-C2, 0.53 cm3·g−1 for UiO-66-C3,
and 0.61 cm3·g−1 for UiO-66-C4. Two pore diameters of 16.1
and 19.1 Å were found in pore size distribution calculated by
DFT method, which were accordance with the pore diameters
of tetrahedron and octahedron in UiO-66 respectively.43
SEM (Figure 2) showed that the sizes of UiO-66-CX
nanocrystals were approximately located in the range of
Figure 3. Adsorption capacities of hydrogen chloride on alkoxy
group-functionalized UiO-66 (C0: 0.5 mol·L−1; T: 25 °C, dosage: 5 g·
L−1).
functionalized UiO-66 is possiblely related to the introduction
of alkoxy group. By introducing the alkoxy group, the structure
of alkoxy group-functionalized UiO-66 was modified in terms
of BET surface area and pore size. The hydrogen chloride
adsorption capacity represented a volcano-shaped variation
with increasing carbon number of alkoxy groups. UiO-66-C2
presented the greatest adsorption capacity of 398.0 mg·g−1. To
the best of found knowledge, its capacity was 1.4 times that of
pristine UiO-66. Additionally, compared to other adsorbents
reported in the literature (Table S2), UiO-66-C2 not only
exhibited excellent adsorption capacity, but it can be recycled
after adsorption, which is an important property for
adsorbents. The surface area, pore diameter, total pore volume,
and HCl adsorption capacity of the synthesized UiO-66-CX are
listed in Table S1. The substituted groups played an important
role in the HCl adsorption. Therefore, UiO-66-C2 showed the
highest adsorption capacity because it had more adsorbed sites
provided by ethoxy groups and still kept a high surface area.
3.3. Adsorption Kinetics. Adsorption kinetics were
measured to study the impact of adsorption time of hydrogen
chloride on UiO-66-C2 (Figure 4a). The adsorption capacity of
hydrogen chloride grew quickly in the first and second hours,
then increased slowly and eventually achieved equilibrium in 4
h, which suggested that UiO-66-C2 can remove hydrogen
chloride faster than other adsorbents reported in literature
(Table S2). Three adsorption kinetics models, namely, pseudo-
second-order, intraparticle diffusion, and Elovich models, were
harnessed in the analysis of adsorption kinetics results, and the
results are summarized in Table 1 and Figure 4.
Figure 2. SEM images of the synthesized UiO-66-CX.
It is hypothesized that the availability of unoccupied sites on
the surface of the adsorbent plays a determinant role on
adsorption rate in pseudo-second-order model.48 The pseudo-
second-order model can be described as eq 2:
100−150 nm. TEM images of UiO-66-C2 are shown in Figure
S3a, the results observed by TEM were in accordance with
SEM, and no obvious impurities could be observed in the
surface of UiO-66-C2.44,45
dQ /dt = k2(Q − Q )2
(2)
t
e
t
3.2. Adsorption Capacity. The adsorption capacities of
the MOFs are shown in Figure 3. Apparently, except for UiO-
66-C4, alkoxy group-functionalized UiO-66 displayed a higher
adsorption capacity of hydrogen chloride than that of original
UiO-66. The weaker adsorption capacity of HCl on UiO-66-C4
can be attributed to its smaller surface area and larger pore
diameter than the pristine UiO-66. The reduced adsorption
site concentration on the surface can be observed with an
increase of pore size and BET surface area reduction.46,47
Consequently, the adsorption capacity of UiO-66-C4 de-
creased. The BET surface area decrease of alkoxy group-
In which Qt and Qe (mg·g−1) are hydrogen chloride adsorption
capacities per unit weight of the used MOFs at time t (h) and
at equilibrium. k2 (g·mg−1·h−1) is the rate constant. The
equation for the boundary range from t = 0 to t and Qt = 0 to
Qt are integrated and rearranged as eq 3:
2
t/Q t = 1/(k2Q e ) + 1/Q et
(3)
With the presence of high adj. R2, the kinetic results were
fitted with pseudo-second-order model quite well, and
measured Qe values were almost equal to the calculated
289
DOI: 10.1021/acs.jced.8b00802
J. Chem. Eng. Data 2019, 64, 286−295

Journal of Chemical & Engineering Data
Article
Figure 4. (a) Relationship between time and adsorption capacity of HCl adsorption on UiO-66-C2. (b) Pseudo-second-order kinetics. (C)
Intraparticle diffusion kinetics. (d) Elovich model of hydrogen chloride adsorption with UiO-66-C2 (C0: 0.6, 1.0, and 1.4 mol·L−1; T: 25 °C;
dosage: 5 g·L−1).
Table 1. Kinetics Parameters of HCl Adsorption on UiO-66-C2
pseudo-second-order model
intraparticle diffusion model
Elovich model
C0
Qe (expt)
Qe (calcd)
k2
kip (mg·g−1
·
106×α
(mol·L−1
)
(mg·g−1
)
(mg·g−1
)
(g·mL−1·h−1
)
adj. R2
h−0.5
)
c
adj. R2
(mg·g−1·h−1
)
β (g·m·g−1
)
adj. R2
0.6
1.0
1.4
284.15
398.00
429.53
282.49
400.00
432.90
0.0515
0.0230
0.0223
0.9998
0.9999
0.9998
151.87
110.32
122.88
121.29
220.71
224.23
1.0000
0.9386
0.9985
2.5313
1.8030
1.3376
0.0489
0.0330
0.0296
0.8246
0.8406
0.8200
ones, which meant the adsorption of hydrogen chloride on
UiO-66-C2 was controlled by two elements. Since the pseudo-
second-order model was an empirical formula, it cannot
determine whether hydrogen chloride adsorption was mono-
layer or multilayer.49,50
where α is the adsorption rate and β is the coefficient of
desorption. The values of α and β can be acquired from the
slope and intercept of the straight line of Qt versus ln t. The
adj. R2 values of the Elovich model were less than those of
pseudo-second-order model.
3.4. Adsorption Isotherms. Batch adsorption isotherms
experiments were conducted at 25, 35, 45, and 55 °C with the
initial HCl concentration ranging from 0.4 to 1.8 mol·L−1. Just
as is demonstrated in the Figure 5a, the HCl adsorption
capacity increased as the initial HCl concentration increased.
But the increasing rate was slow at the beginning, and then
became faster, which can be explained by the hydrophobicity
of the MOFs. Finally, the rate became low again, indicating
that the adsorption process reached equilibrium. The
equilibrium adsorption capacity Qe decreased as temperature
increased, which meant the adsorption process was exothermal.
Three different models were utilized to analyze the results of
adsorption isotherms.
The equation of intraparticle diffusion model can be
expressed as follows eq 4:
Q = kipt0.5 + c
(4)
t
where c is the constant of diffusivity; kip is the constant
referring to thickness and boundary layer. It means that
intraparticle diffusion is controlled by a single rate, when the
plots Qt against t0.5 are straight line and pass origin. As shown
in Figure 4c, they were three-phase nonlinear diagrams, which
meant that adsorption of hydrogen chloride on UiO-66-C2 was
a continuous but sectionalized process, in which the first phase
was linear adsorption related to surface diffusion, the second
phase was intraparticle diffusion process, and the third phase
was the dynamic equilibrium process of adsorption and
desorption.51
Langmuir isotherm model is an ideal model, hypothesizing
that adsorption is a monomolecular layer and it happens in the
surface of the adsorbent whose adsorption sites are uniformly
distributed.52 The Langmuir isotherm equation (eq 6) is
described as follows:
The liner equation of Elovich model can be given as eq 5:
Q t = 1/β × ln(αβ) + 1/β × ln t
Q e = KLQ mCe/(1 + KLCe)
(5)
(6)
290
DOI: 10.1021/acs.jced.8b00802
J. Chem. Eng. Data 2019, 64, 286−295

Journal of Chemical & Engineering Data
Article
Figure 5. (a) Adsorption isotherms of hydrogen chloride on UiO-66-C2. (b) Langmuir isotherms, (c) Freundlich isotherms, and (d) Henry
isotherms of hydrogen chloride adsorption on UiO-66-C2 (C0: 0.4−1.8 mol·L−1; T: 25, 35, 45, and 55 °C; dosage: 5g·L−1).
Table 2. Isotherms and Thermodynamic Parameters of Hydrogen Chloride Adsorption with UiO-66-C2
Langmuir model
KL
Freundlich model
Henry model
1/Qm
KH
ΔG
ΔH
ΔS
T (°C)
(mol·g−1
)
(L·mol−1
)
adj. R2
1/n
KF
adj. R2
(mL·g−1
)
adj. R2
(KJ·mol−1
)
(KJ·mol−1
)
(J·mol−1·K−1
)
25
35
45
55
43.96
34.75
14.80
13.55
0.58
0.32
0.09
0.07
0.9533
0.8453
0.0512
0.1273
0.68
0.79
0.96
0.97
0.0081
0.0068
0.0055
0.1060
0.9667
0.9807
0.9665
0.9688
7.17
6.22
5.32
4.59
0.9706
0.9841
0.9884
0.9885
−4.90
−4.66
−4.42
−4.17
−12.12
−24.21
where KL (L·mol−1) is the constant of adsorption equilibrium;
Qe (mol·g−1) is adsorption capacity of HCl at equilibrium; Qm
(mol·g−1) is the maximum adsorption capacity of the
adsorbent; and Ce (mol·L−1) is the liquid-phase concentrations
of HCl at equilibrium. The equation can be linearized as eq 7:
degree of adsorption intensity related to the surface
heterogeneity. If 1/n = 0 to 1, it is a favorable physisorption
process (n > 1); if 1/n = 1, it is a homogeneous adsorption
without interaction among the adsorbates (n = 1); and if 1/n >
1, it is an unfavorable chemisorption process (n < 1).
Therefore, the values of 1/n (1/n < 1) demonstrated that
the HCl adsorption was favored, and physical force, rather than
chemical interactions, contributed more to adsorption.53,54
The Henry isotherm is suitable for adsorption from dilute
solution. The equation of the Henry isotherm is expressed as
eq 10:
Ce/Q e = 1/KLQ m + 1/Q mCe
(7)
All the parameters are listed in Table 2. The Langmuir model
cannot fit isotherm results with low adj. R2.
The Freundlich isotherm model is an experiential model
which can explain heterogeneous adsorption system. The
equation of Freundlich isotherm is presented as eq 8:
Q e = KPCe
(10)
Q = KFCe1/n
(8)
e
where KP is a constant of the Henry model (mL·g−1). The high
adj. R2 of the Henry model indicated that adsorption capacity
of hydrochloric acid was linearly related to equilibrium
concentration.
where KF (mol·g−1(L·mol−1)1/n) is the Freundlich constant. 1/
n is a measure of adsorption intensity. The linear equation of
Freundlich isotherm model can be displayed as
The thermodynamic parameters like enthalpy change (ΔH,
kJ·mol−1), entropy change (ΔS, J·mol−1·K−1) and Gibbs free
energy change (ΔG, kJ·mol−1) were calculated based on
equations as follows:
log Q e = log KF + 1/n log Ce
(9)
The Freundlich model could elucidate the results of
adsorption isotherms with high value of adj. R2, which
describes multilayer adsorption and nonideal adsorption on
heterogeneous surfaces. The Freundlich parameter 1/n is a
ΔG = −RT ln KP
(11)
291
DOI: 10.1021/acs.jced.8b00802
J. Chem. Eng. Data 2019, 64, 286−295

Journal of Chemical & Engineering Data
Article
ln KP = ΔS/R − ΔH/RT
hydrogen chloride. Alkoxy group probably had an interaction
with H+ because of its alkalinity.
(12)
where R is the universal gas constant, and its value is 8.314 J·
mol−1·K−1. T (K) is the Kelvin temperature. KP is the constant
of the Henry model. The van ’t Hoff plot was obtained by
plotting ln KP against 1/T as illustrated in Figure S4. The
values of ΔS, ΔH, and ΔG were obtained from the van ’t Hoff
plot (Table 2).
The values of thermodynamic parameters are given in Table
2. The negative values of △G suggested that the hydrogen
chloride adsorption on UiO-66-C2 was a spontaneous and
thermodynamically favorable process, which was in accordance
with the isotherm results. Also, the values of ΔG was in the
range of −20 and 0 kJ·mol−1, which suggested that the process
was mainly physical adsorption.55 The negative value of ΔH
and ΔS illustrated that the HCl adsorption process was an
exothermic and entropy decreasing process.
XPS was conducted to explore the hydrogen chloride
adsorption mechanism of UiO-66-C2. The peaks correspond-
ing to O 1s, Cl 2p, and Zr 3d can be clearly identified in the
XPS spectra (Figure 7a). As is demonstrated in Figure 7b, the
variation of binding energy at Zr 3d and the expansion of peak
intensity at Cl 2p indicated the interactions between Zr4+ and
Cl− during the adsorption process and Cl− were adsorbed in
the UiO-66-C2. The increased binding energy of Zr 3d
suggested that an electron transfer in the valence band between
Cl− and Zr4+. Moreover, The relative contents of elements C
1s:O 1s:N 1s:Zr 3d:Cl 2p before and after adsorption showed
that before adsorption: 59.66:33.02:1:4.41:1.9; after adsorp-
tion: 56.7:33.95:0.68:4.65:4.03. The relative contents of Cl
increased distinctly, further substantiating that hydrogen
chloride was truly adsorbed on UiO-66-C2.
3.5. Adsorption Mechanism. The variations of chemical
bonds between before and after adsorption of UiO-66-C2 was
analyzed by FT-IR (Figure 6). The appearance of adsorption
3.6. Recycle of UiO-66-C2. Recyclability is an important
factor for adsorbents. As is illustrated in Figure 8, after three
Figure 8. Changes of HCl adsorption capacities among original and
regenerated UiO-66-C2 (C0: 0.5 mol·L−1; T: 25 °C; dosage: 5g·L−1).
Figure 6. FT-IR spectra of prepared and adsorbed UiO-66-C2.
recycles, UiO-66-C2 still exhibited excellent adsorptive
performance and the adsorption capacity remained statistically
stable, which meant UiO-66-C2 could be recycled easily.
However, there was some decrease in hydrogen chloride
adsorption capacity during regeneration. The decrease of
adsorption capacity might be caused by insufficient activation
as the number of regenerations increases. In addition, the
washing process requires centrifugation to separate the solute
from the solvent. During the process of pouring the solvent,
bands 3417 and 3391 cm−1 can be ascribed to the OH of
−COOH or H2O. The enhanced band after adsorption may
indicated that water entered the adsorbent during adsorption.
The adsorption bands 1699 and 1700 cm−1, 1498 and 1498
cm−1, and 1241 and 1306 cm−1 represented CO st, benzene
ring, and arC-O-alC. According to Hooke’s law, the enhance-
ment of the stretching vibration of groups indicated carboxyl,
benzene ring, and alkoxy group might be adsorption sites of
Figure 7. (a) Original XPS patterns. (b) Partially magnified XPS patterns of prepared and adsorbed UiO-66-C2.
292
DOI: 10.1021/acs.jced.8b00802
J. Chem. Eng. Data 2019, 64, 286−295

Journal of Chemical & Engineering Data
Article
some nanoparticles that are not separated from the solvent
may be poured out, and the lost nanoparticles may have strong
adsorption capacity, which may lead to the decrease of
adsorption capacity. The XRD patterns of regenerated UiO-66-
C2 were compared with that of as-synthesized UiO-66-C2. As
we can see in Figure S5, the locations and intensities of the
peaks in the XRD patterns of regenerated UiO-66-C2 remained
unchanged. On the other hand, SEM (Figure S6) was
performed to measure the crystallite morphology of the
regenerated UiO-66-C2. Distinctive changes cannot be found
in the regenerated UiO-66-C2 in terms of particle size and
crystal morphology. Meanwhile, the UiO-66-C2 was treated by
different concentrations of hydrochloric acid for 2 days, that is,
0.1, 0.5, 1.0, and 2.0 mol/L. TEM and BET were then
performed to test its stability. The results of TEM (Figure S3)
demonstrated that the surface morphology of UiO-66-C2
before and after treatment retained unchanged. The adsorption
isotherms (Figure S7) of treated UiO-66-C2 also did not
deviate much from untreated UiO-66-C2 and the BET surface
area of treated sample kept stable in Table S3. The above
characterization results illustrated the excellent stability of
UiO-66-C2 in acidic solution.
distributions of pristine UiO-66-C2 and UiO-66-C2
treated by 0.1, 0.5, 1.0, and 2.0 mol/L HCl solutions
for 2 days; surface area, pore volume and pore diameter
of UiO-66 and UiO-66-CX; summary of hydrogen
chloride adsorbents; surface area, pore volume, and
pore diameter of pristine UiO-66-C2 and UiO-66-C2
treated by 0.1, 0.5, 1.0, and 2.0 mol/L HCl solutions for
2 days (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: guoxh@tju.edu.cn.
ORCID
Hongxu Liu: 0000-0003-0372-4656
Xianghai Guo: 0000-0002-1519-4050
Funding
This work was supported by the National Natural Science
Foundation of China (No. 21202116), Independent Innova-
tion Foundation of Tianjin University (2017XZY-0052) and
Natural Science Foundation of Tianjin (16JCYBJC20300).
Notes
4. CONCLUSION
The authors declare no competing financial interest.
In summary, we synthesized a sequence of UiO-66 analogues
with various alkoxy groups, which exhibited high adsorption
capacity of hydrogen chloride from aqueous solution. The
adsorption capacity of hydrogen chloride represented a
volcano-shaped variation with increasing carbon number of
alkoxy groups. First, the introduction of alkoxy groups can
enhance the performance of HCl adsorption for the addition of
adsorption sites. Meanwhile, surface area had a significant
influence on the HCl adsorption. UiO-66-C2 showed the
highest adsorption capacity of 398.0 mg·g−1 because it had
more adsorption sites provided by ethoxy and large surface
area. The kinetic, isotherm, thermodynamics, adsorptive
mechanism, and regeneration performances of UiO-66-C2
were studied. The results demonstrated that the adsorption
process is a spontaneous and exthermic physisorption process.
It was anticipated that the carboxyl group, benzene ring, alkoxy
group, and Zr4+ were responsible for HCl sorption. According
to XPS results, it was supposed that Zr4+ interacted with Cl−.
The regeneration of UiO-66-C2 was feasible and easy, and
there was no obvious change in the structure and adsorption
capacity after recycles. All the results illustrated that UiO-66-
C2 could be an efficient solution toward disposal of acidic
wastewater. The performance of HCl adsorption for UiO-66
can be improved via introducing alkoxy functional groups
without significant decrease of surface area.
REFERENCES
■
(1) Goyal, A.; Srivastava, V. C. Treatment of highly acidic
wastewater containing high energetic compounds using dimensionally
stable anode. Chem. Eng. J. 2017, 325, 289−299.
(2) Kong, L.; Peng, X.; Hu, X. Mechanisms of UV-Light Promoted
Removal of As(V) by Sulfide from Strongly Acidic Wastewater.
Environ. Sci. Technol. 2017, 51, 12583−12591.
(3) Shapira, B.; Avraham, E.; Aurbach, D. The Feasibility of Energy
Extraction from Acidic Wastewater by Capacitive Mixing with a
Molecular-Sieving Carbon Electrode. ChemSusChem 2016, 9, 3426−
3433.
́
́
(4) Galvez, J. L.; Dufour, J.; Negro, C.; Lopez-Mateos, F.
Determination of iron and chromium fluorides solubility for the
treatment of wastes from stainless steel mills. Chem. Eng. J. 2008, 136,
116−125.
(5) Li, L.-F.; Caenen, P.; Daerden, M.; Vaes, D.; Meers, G.; Dhondt,
C.; Celis, J.-P. Mechanism of single and multiple step pickling of 304
stainless steel in acid electrolytes. Corros. Sci. 2005, 47, 1307−1324.
(6) Wang, Y.; Fang, Z.; Liang, B.; Tsang, E. P. Remediation of
hexavalent chromium contaminated soil by stabilized nanoscale zero-
valent iron prepared from steel pickling waste liquor. Chem. Eng. J.
2014, 247, 283−290.
(7) Evans, C. D.; Monteith, D. T.; Fowler, D.; Cape, J. N.; Brayshaw,
S. Hydrochloric acid: an overlooked driver of environmental change.
Environ. Sci. Technol. 2011, 45, 1887−94.
(8) Agrawal, A.; Sahu, K. K. Value Addition to Sulfate Waste Pickle
Liquor of Steel Industry Using Hydrometallurgical Processes. Metall.
Mater. Trans. B 2009, 40, 877−885.
ASSOCIATED CONTENT
* Supporting Information
■
(9) Nabeshima, R.; Kusumoto, N.; Katoh, S.; Shiomi, N.
Neutralization of acidified water by use of Rhizopus delemar
immobilized with a cellulose membrane. Biochem. Eng. J. 2008, 41,
188−192.
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jced.8b00802.
1H NMR spectrograms of UiO-66-CX; N2 adsorption−
desorption isotherm and pore diameter distribution of
nonmodified/pristine UiO-66; TEM images of pristine
UiO-66-C2 and UiO-66-C2 treated by 0.1, 0.5, 1.0, and
2.0 mol/L HCl solutions for 2 days; van ’t Hoff plot;
XRD patterns of synthesized and regenerated UiO-66-
C2; SEM images of regenerated UiO-66-C2; Left: N2
adsorption−desorption isotherms; right: pore diameter
(10) Sargsyan, L. Y.; Hovhannisyan, A. M. Roasting of the sulfide-
zinc concentrate with predominant obtaining of sulfate roast for
effective leaching. Russ. J. Non-Ferr. Met. 2010, 51, 212−216.
(11) Sarangi, K.; Padhan, E.; Sarma, P. V. R. B.; Park, K. H.; Das, R.
P. Removal/recovery of hydrochloric acid using Alamine 336, Aliquat
336, TBP and Cyanex 923. Hydrometallurgy 2006, 84, 125−129.
(12) Schunk, A.; Maurer, G. Distribution of hydrochloric, nitric, and
sulfuric acid between water and organic solutions of tri-n-octylamine.
Fluid Phase Equilib. 2003, 211, 189−209.
293
DOI: 10.1021/acs.jced.8b00802
J. Chem. Eng. Data 2019, 64, 286−295

Journal of Chemical & Engineering Data
Article
(13) Schunk, A.; Maurer, G. Distribution of hydrochloric, nitric, and
sulfuric acids between water and organic solutions of tri-n-octylamine.
Fluid Phase Equilib. 2003, 207, 1−21.
(33) Zahmakiran, M. Iridium nanoparticles stabilized by metal
organic frameworks (IrNPs@ZIF-8): synthesis, structural properties
and catalytic performance. Dalton Trans. 2012, 41, 12690−6.
(34) Zhang, H.; Wang, Y.; Bai, P.; Guo, X.; Ni, X. Adsorptive
Separation of Acetic Acid from Dilute Aqueous Solutions: Adsorption
Kinetic, Isotherms, and Thermodynamic Studies. J. Chem. Eng. Data
2016, 61, 213−219.
(35) Luo, X.; Ding, L.; Luo, J. Adsorptive Removal of Pb(II) Ions
from Aqueous Samples with Amino-Functionalization of Metal−
Organic Frameworks MIL-101(Cr). J. Chem. Eng. Data 2015, 60,
1732−1743.
(14) Di Somma, I.; Marotta, R.; Andreozzi, R.; Caprio, V. Nitric acid
decomposition kinetics in mixed acid and their use in the modeling of
aromatic nitration. Chem. Eng. J. 2013, 228, 366−373.
(15) Wu, B.; Li, X.; Li, Y.; Zhu, J.; Wang, J. Hydrolysis Reaction
Tendency of Low-Boiling Organic Chlorides To Generate Hydrogen
Chloride in Crude Oil Distillation. Energy Fuels 2016, 30, 1524−1530.
(16) Susanto, H. Towards practical implementations of membrane
distillation. Chem. Eng. Process. 2011, 50, 139−150.
(17) Devi, A.; Singhal, A.; Gupta, R.; Panzade, P. A study on
treatment methods of spent pickling liquor generated by pickling
process of steel. Clean Technol. Environ. Policy 2014, 16, 1515−1527.
(18) Lan, X.; Zhang, H.; Bai, P.; Guo, X. Investigation of metal
organic frameworks for the adsorptive removal of hydrochloride from
dilute aqueous solution. Microporous Mesoporous Mater. 2016, 231,
40−46.
(36) Xiao, C.; Goh, T. W.; Brashler, K.; Pei, Y.; Guo, Z.; Huang, W.
In situ X-ray absorption spectroscopy studies of kinetic interaction
between platinum(II) ions and UiO-66 series metal-organic frame-
works. J. Phys. Chem. B 2014, 118, 14168−76.
(37) Hu, Z.; Peng, Y.; Kang, Z.; Qian, Y.; Zhao, D. A Modulated
Hydrothermal (MHT) Approach for the Facile Synthesis of UiO-66-
Type MOFs. Inorg. Chem. 2015, 54, 4862−8.
́
(19) Wawrzkiewicz, M.; Wisniewska, M.; Wołowicz, A.; Gun’ko, V.
(38) Vandichel, M.; Hajek, J.; Vermoortele, F.; Waroquier, M.; De
Vos, D. E.; Van Speybroeck, V. Active site engineering in UiO-66 type
metal−organic frameworks by intentional creation of defects: a
theoretical rationalization. CrystEngComm 2015, 17, 395−406.
(39) Bon, V.; Senkovska, I.; Weiss, M. S.; Kaskel, S. Tailoring of
network dimensionality and porosity adjustment in Zr- and Hf-based
MOFs. CrystEngComm 2013, 15, 9572.
M.; Zarko, V. I. Mixed silica-alumina oxide as sorbent for dyes and
metal ions removal from aqueous solutions and wastewaters.
Microporous Mesoporous Mater. 2017, 250, 128−147.
(20) Wang, Z.; Liu, H.; Zhou, K.; Fu, P.; Zeng, H.; Qiu, J. Effect of
surface oxygen/nitrogen groups on hydrogen chloride removal using
modified viscose-based activated carbon fibers. RSC Adv. 2015, 5,
86006−86012.
(21) Kim, J.-Y.; Kyung, D.-H.; Park, Y. C.; Jo, S.-H.; Ryu, H.-J.; Park,
Y. S.; Moon, J.-H. The Adsorption and Desorption Breakthrough
Behavior of Hydrogen Chloride Gas Mixture on Zeolite 13X Pellet in
a Fixed Bed Reactor. J. Chem. Eng. Jpn. 2015, 48, 202−211.
(22) Micoli, L.; Bagnasco, G.; Turco, M. HCl removal from biogas
for feeding MCFCs: Adsorption on microporous materials. Int. J.
Hydrogen Energy 2013, 38, 447−452.
(40) Garibay, S. J.; Cohen, S. M. Isoreticular synthesis and
modification of frameworks with the UiO-66 topology. Chem.
Commun. (Cambridge, U. K.) 2010, 46, 7700−2.
(41) Jasuja, H.; Walton, K. S. Experimental Study of CO2, CH4, and
Water Vapor Adsorption on a Dimethyl-Functionalized UiO-66
Framework. J. Phys. Chem. C 2013, 117, 7062−7068.
(42) Shen, J.; Liu, G.; Huang, K.; Li, Q.; Guan, K.; Li, Y.; Jin, W.
UiO-66-polyether block amide mixed matrix membranes for CO2
separation. J. Membr. Sci. 2016, 513, 155−165.
(23) Eddaoudi, M.; Sava, D. F.; Eubank, J. F.; Adil, K.; Guillerm, V.
Zeolite-like metal-organic frameworks (ZMOFs): design, synthesis,
and properties. Chem. Soc. Rev. 2015, 44, 228−49.
(24) Zhou, H. C.; Kitagawa, S. Metal-organic frameworks (MOFs).
Chem. Soc. Rev. 2014, 43, 5415−8.
(43) Abid, H. R.; Tian, H.; Ang, H.-M.; Tade, M. O.; Buckley, C. E.;
Wang, S. Nanosize Zr-metal organic framework (UiO-66) for
hydrogen and carbon dioxide storage. Chem. Eng. J. 2012, 187,
415−420.
(25) Sahu, D.; Mishra, P.; Edubilli, S.; Verma, A.; Gumma, S.
Hydrogen Adsorption on Zn-BDC, Cr-BDC, Ni-DABCO, and Mg-
DOBDC Metal−Organic Frameworks. J. Chem. Eng. Data 2013, 58,
3096−3101.
(26) Cheng, Y.; Zhang, H.; Yang, B.; Wu, J.; Wang, Y.; Ding, B.;
Huo, J.; Li, Y. Highly efficient fluorescence sensing of phosphate by
dual-emissive lanthanide MOFs. Dalton Trans. 2018, 47, 12273−
12283.
(27) Xu, G.; Xu, G. C.; Ban, J. J.; Zhang, L.; Lin, H.; Qi, C. L.; Sun,
Z. P.; Jia, D. Z. Cobalt and cobalt oxides N-codoped porous carbon
derived from metal-organic framework as bifunctional catalyst for
oxygen reduction and oxygen evolution reactions. J. Colloid Interface
Sci. 2018, 521, 141−149.
(28) Salunkhe, R. R.; Kaneti, Y. V.; Kim, J.; Kim, J. H.; Yamauchi, Y.
Nanoarchitectures for Metal-Organic Framework-Derived Nano-
porous Carbons toward Supercapacitor Applications. Acc. Chem. Res.
2016, 49, 2796−2806.
(29) Akpinar, I.; Yazaydin, A. O. Adsorption of Atrazine from Water
in Metal−Organic Framework Materials. J. Chem. Eng. Data 2018, 63,
2368−2375.
(30) Han, P.; Xia, Y. Thiol-functionalized metal-organic framework
for highly efficient removal of bromate from water. J. Environ. Chem.
Eng. 2018, 6, 3384−3391.
(44) Zeng, L.; Xiao, L.; Long, Y.; Shi, X. Trichloroacetic acid-
modulated synthesis of polyoxometalate@UiO-66 for selective
adsorption of cationic dyes. J. Colloid Interface Sci. 2018, 516, 274−
283.
(45) Ouyang, W.; Zhao, D.; Wang, Y.; Balu, A. M.; Len, C.; Luque,
R. Continuous Flow Conversion of Biomass-Derived Methyl
Levulinate into γ-Valerolactone Using Functional Metal Organic
Frameworks. ACS Sustainable Chem. Eng. 2018, 6, 6746−6752.
(46) Marikutsa, A. V.; Rumyantseva, M. N.; Konstantinova, E. A.;
Shatalova, T. B.; Gaskov, A. M. Active Sites on Nanocrystalline Tin
Dioxide Surface: Effect of Palladium and Ruthenium Oxides Clusters.
J. Phys. Chem. C 2014, 118, 21541−21549.
(47) Wurster, D. E.; Oh, E.; Wang, J. C. Determination of the
mechanism for the decrease in zinc oxide surface area upon high-
temperature drying. J. Pharm. Sci. 1995, 84, 1301−7.
(48) Azhar, M. R.; Abid, H. R.; Sun, H.; Periasamy, V.; Tade, M. O.;
Wang, S. Excellent performance of copper based metal organic
framework in adsorptive removal of toxic sulfonamide antibiotics from
wastewater. J. Colloid Interface Sci. 2016, 478, 344−52.
(49) Amarasinghe, B. M. W. P. K.; Williams, R. A. Tea waste as a low
cost adsorbent for the removal of Cu and Pb from wastewater. Chem.
Eng. J. 2007, 132, 299−309.
(50) Ho, Y. S.; McKay, G. Kinetic Models for the Sorption of Dye
from Aqueous Solution by Wood. Process Saf. Environ. Prot. 1998, 76,
183−191.
(51) Liu, Q.-S.; Zheng, T.; Wang, P.; Jiang, J.-P.; Li, N. Adsorption
isotherm, kinetic and mechanism studies of some substituted phenols
on activated carbon fibers. Chem. Eng. J. 2010, 157, 348−356.
(52) Wang, Z.; Huang, Y.; Yang, J.; Li, Y.; Zhuang, Q.; Gu, J. The
water-based synthesis of chemically stable Zr-based MOFs using
(31) Wei, C.; Feng, D.; Xia, Y. Fast adsorption and removal of 2-
methyl-4-chlorophenoxy acetic acid from aqueous solution with
amine functionalized zirconium metal−organic framework. RSC Adv.
2016, 6, 96339−96346.
(32) Sarker, M.; Ahmed, I.; Jhung, S. H. Adsorptive removal of
herbicides from water over nitrogen-doped carbon obtained from
ionic liquid@ZIF-8. Chem. Eng. J. 2017, 323, 203−211.
294
DOI: 10.1021/acs.jced.8b00802
J. Chem. Eng. Data 2019, 64, 286−295

Journal of Chemical & Engineering Data
Article
pyridine-containing ligands and their exceptionally high adsorption
capacity for iodine. Dalton Trans. 2017, 46, 7412−7420.
(53) Kiruba, U. P.; Kumar, P. S.; Prabhakaran, C.; Aditya, V.
Characteristics of thermodynamic, isotherm, kinetic, mechanism and
design equations for the analysis of adsorption in Cd(II) ions-surface
modified Eucalyptus seeds system. J. Taiwan Inst. Chem. Eng. 2014,
45, 2957−2968.
(54) Wang, M.; Yan, W.; Kong, W.; Wu, Z.; An, X.; Wang, Z.; Hao,
X.; Guan, G. An Electroactive and Regenerable Fe3O4@Polypyrrole
Nanocomposite: Fabrication and Its Defluorination in an Electro-
magnetic Coupling System. Ind. Eng. Chem. Res. 2017, 56, 12738−
12744.
(55) Lin, K. A.; Liu, Y. T.; Chen, S. Y. Adsorption of fluoride to
UiO-66-NH2 in water: Stability, kinetic, isotherm and thermody-
namic studies. J. Colloid Interface Sci. 2016, 461, 79−87.
295
DOI: 10.1021/acs.jced.8b00802
J. Chem. Eng. Data 2019, 64, 286−295