Highly Efficient and Selective Removal of Lead Ions from Aqueous Solutions by Conjugated Microporous Polymers with Functionalized Heterogeneous Pores

Article abstract of DOI:10.1021/acs.cgd.9b01265

The development of viable absorbents for highly efficient and selective removal of lead (Pb) ions, which might exist in various waste discharges related to human activities, is considered a long-standing challenge. Current scientific interests reside in a

Full text of DOI:10.1021/acs.cgd.9b01265

Article
pubs.acs.org/crystal
Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX
Highly Efficient and Selective Removal of Lead Ions from Aqueous
Solutions by Conjugated Microporous Polymers with Functionalized
Heterogeneous Pores
Published as part of a Crystal Growth and Design virtual special issue on Crystalline Functional Materials in
Honor of Professor Xin-Tao Wu
Xing-Xing Qiao,† Gui-Fang Liu,† Jun-Tao Wang,† Yu-Qing Zhang,† and Jian Lu
̈
*
,†,‡
†Fujian Provincial Key Laboratory of Soil Environmental Health and Regulation, College of Resources and Environment, Fujian
Agriculture and Forestry University, Fuzhou 350002, P.R. China
‡State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of
Sciences, Fuzhou 350002, P.R. China
S
* Supporting Information
ABSTRACT: The development of viable absorbents for highly efficient and selective
removal of lead (Pb) ions, which might exist in various waste discharges related to
human activities, is considered a long-standing challenge. Current scientific interests
reside in achieving material absorbents with high porosity and desired functionality
that respectively relate to generally higher Pb(II) uptake and selectivity. In this work,
two-dimensional conjugated microporous polymers (CMP-2 and CMP-3) were
rationally designed in which the heterogeneous (large and narrow) pores of these
materials were chemically functionalized with cyano and pyridyl groups, respectively.
As expected, the activated CMP-2a and CMP-3a displayed a high Pb(II) adsorption
capacity of 62.7 and 93.2 mg g−1, respectively. Of note is that the CMP-3a could
selectively uptake Pb(II) (84%) over Cu(II) (40%) and Ni(II) (18.6%) in simulated
wastewater. It was demonstrated that the formation of functionalized and heterogeneous pore structures in CMP-3a accounted
for its highly efficient and selective adsorption property toward Pb2+. In addition, CMP-2a and CMP-3a could be easily
recovered and regenerated for Pb2+ removal from aqueous solutions and exhibited nondetectable cytotoxicity, which were
favorable for their applications as viable and safe absorbents for Pb(II) removal in aqueous phases.
and removal of toxic heavy metal ions from aqueous solutions
has always been a challenging task.
INTRODUCTION
■
There is an urgent need in the development of advanced
materials for the high-performance purification of wastewater
which is produced from industrial and agricultural water
discharge, as well as the burning of fossil fuels in recent
years.1−4 It is also well-known that wastewater greatly affects
soil and drinking water resources, and damages ecosystems
with trace amounts of heavy metal contamination and further
results in severe harm to terrestrial and aquatic organisms and
even humans.5,6 Of special note, the effective removal of lead
ions, as a main source of heavy metal pollutants, from aqueous
system represents one of the most desirable techniques that
claims the highest priority in wastewater treatment. Traditional
materials used in water purification include active carbon,
mesoporous silica, inorganic polymers, ceramic materials,
etc.7−9 During the past decade, various methods have been
developed for the removal of toxic metal ions from
wastewater,10−13 and most of these processes are yet to be
improved, owing to either high cost, low efficiency, or
cytotoxicity.14−16 Therefore, the quest for new chemistry
materials with high efficiency and selectivity for the adsorption
Conjugated microporous polymers (CMPs) define emerging
framework materials synthesized from conventional organic
coupling reactions.17−21 In comparison with the well-known
metal organic frameworks (MOFs) analogues,22−24 CMPs
have become attractive due to the low framework density, low
toxicity, as well as controllable assemblies taking advantage of
the crystal engineering concepts.25−29 These materials have
thus shown great potentials in the fields of materials science
such as catalysis, medicine, as well as molecular storage and
separation.17,20 CMP materials possess well-defined network
structures, high specific surface area, and easily regulated
functionality at the molecular level. In addition, the micro-
environment inside pores/channels of CMP materials can be
tuned by means of modification with various functional groups,
as either active or inert docks for heavy metal ions, which is
Received: September 25, 2019
Revised: November 10, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.cgd.9b01265
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Scheme 1. Proposed Synthetic Pathway for CMP-2 and CMP-3
very promising for enhanced adsorption capacity and
selectivity.
EXPERIMENTAL SECTION
■
Synthesis of Amino-[4-(amino-cyano-methyl)-phenyl]-ace-
tonitrile (1). 1 was synthesized with yields of ca. 59% (based on
terephthalonitrile; Supporting Information) from the procedure
documented in the literature.30−32
In this work, two reticular conjugated microporous polymer
materials (CMP-2 and CMP-3) featuring two-dimensional
(2D) network structures with alternate heterogeneous (large
and narrow) pores decorated by cyano and pyridyl groups,
respectively, were developed. These CMP materials were
thermally stable, which was superior for their practical usage as
absorbents in aqueous systems. The activated materials, CMP-
2a and CMP-3a, showed excellent adsorption capacity of 62.7
and 93.2 mg g−1, respectively, toward Pb2+. Of notice is that
the CMP-3a could selectively uptake Pb(II) (84%) over
Cu(II) (40%) and Ni (II) (18.6%) in 50 mL of 10 mg L−1
solutions at a dosage of 10 mg due to the high porosity and
heterogeneously functioned pores of the host framework.
Moreover, CMP-2a and CMP-3a could be recovered and
regenerated for Pb2+ removal from aqueous solutions and
exhibited nondetectable cytotoxicity against Escherichia coli (E.
coli) as discovered by antibacterial plate experiments. These
results suggested that CMP-2a and CMP-3a were recognized
as highly efficient and safe material absorbents for the removal
of Pb(II) ions from wastewater.
Synthesis of Triphenyl-4,4′-dicarbaldehyde (2). 4-Formyl-
phenylboronic acid (300 mg, 2.0 mmol), 1,4-diiodobenzene (330 mg,
1.0 mmol), and K2CO3 (660 mg, 2.0 mmol) were added to 20 mL of
toluene/H2O (10:1) and heated to reflux under inert atmosphere
(Ar). The reaction mixture was then condensed, followed by filtration
and workout with 5:1 hexane/ethyl acetate eluent. Yields: ca. 28%. 1H
NMR (DMSO-d6): 10.1 (s, 2H, aldehyde-H); 8.0 (d, J = 4.2 Hz, 8H,
phenyl-H); 7.9 (d, J = 6.2, 4H, phenyl-H). IR (KBr, υmax, cm−1): 1944
(w), 1867 (w), 1827 (w), 1695 (s), 1625 (m), 1584 (m), 1498 (m),
1428 (w), 1387 (m), 1362 (m), 1332 (w), 1301 (s), 1195 (s), 1089
(w), 1008 (w), 816 (s), 770 (s), 523 (w). Elemental analysis for
C20H14O2 (found/calcd): C, 82.6/83.9; H, 4.4/4.9%.
Syntheses of CMP-2 and CMP-3. In a typical synthetic
procedure,30 1 and biphenyl-4,4′-dicarbaldehyde were added to 10
mL of glacial acetic acid in a 2:1 ratio. The reaction mixture was
purged with nitrogen for 30 min and reacted at 125 °C for 48 h.
Yellow precipitate (CMP-2) formed during the reaction was filtered
and washed with hot acetic acid, water, and ethanol, respectively, and
dried in air. Yield: ca. 46%. CMP-3 was synthesized in a procedure
B
DOI: 10.1021/acs.cgd.9b01265
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Figure 1. Optimized crystal structures of CMP-2 (a) and CMP-3 (b); SEM images of bulk CMP-2 (c) and CMP-3 (d).
similar to that of CMP-2 using 2 in replacement of biphenyl-4,4′-
dicarbaldehyde. Yield: ca. 32%. Bulk samples of CMP-2 and CMP-3
were heated separately with dimethylformamide (DMF) in pressure
tubes at 90 °C to realize partial oxidative dehydrogenation of
dihydropyridyl groups in the materials, so that the structures of CMP-
2 and CMP-3 were more robust and less deformable.31,32 However,
irregular cross-linking and disordered interpenetration might exist in
the polymer structures, in particular, for CMP-3 which was
constructed from longer dialdehyde building units. CMP-2: IR
(KBr, υmax, cm−1): 2193 (s), 1687 (m), 1632 (s), 1588 (s), 1503 (m),
1445 (m), 1358 (w), 1287 (w), 1203 (m), 1036 (w), 1018 (w), 987
(w), 855 (m), 811 (m), 775 (m), 686 (m), 601 (w), 534 (w).
Elemental analysis for C19H8N3 (found/calcd): C, 81.2/82.0; H, 3.2/
2.9; N, 14.4/15.1%. CMP-3: IR (KBr, υmax, cm−1): 2196 (m), 1671
(m), 1574 (w), 1534 (s), 1488 (s), 1439 (w), 1337 (m), 1271 (s),
1210 (m), 1068 (w), 1019 (w), 998 (w), 963 (w), 927 (w), 896 (w),
852 (m), 780 (w), 755 (w), 714 (w), 659 (w), 624 (w), 548 (w), 507
(w). Elemental analysis for C21H10N3 (found/calcd): C, 81.7/82.9; H,
3.8/3.1; N, 13.5/13.8.
Heavy Metal Ion Removal. In a typical procedure, 10 mg of
activated CMP-2a or CMP-3a (ESI) was added to 50 mL solution of
heavy metal ions of various concentrations under magnetic stirring.
Sample solutions were extracted at different time points and tested
using ICP measurements to determine metal ion concentrations.
Heavy metal adsorption on CMP-2a or CMP-3a was also measured
using ICP for solid samples isolated from solutions during recovery
tests by centrifuge and dried overnight in oven.
RESULTS AND DISCUSSION
■
The design and syntheses of CMP-2 and CMP-3 are illustrated
in Scheme 1. The as-prepared porous CMP materials were
functionalized with cyano (in large pores; lp) and pyridyl (in
narrow pores; np) groups, which are potential active sites for
the adsorption of heavy metal ions. Moreover, CMP-2 and
CMP-3 can be recovered and regenerated for reuse, which is
important in applications as viable solid absorbents for heavy
metal removal. The porous 2D network structural units of
CMP-2 and CMP-3 were rationalized and shown in Figure
1a,b; however, as an important feature of porous 2D nets,
versatile packing modes and structural interpenetration and/or
polycatenation can be extremely difficult to predict.33 The
optimized 2D layer structures of CMP-2 and CMP-3 consist of
heterogeneous hexagonal lp and square np windows with
dimensions of ca. 14.5 Å × 16.3 Å/2.9 Å × 3.6 Å (for CMP-2;
Figure 1a) and ca. 19.1 Å × 19.3 Å/2.9 Å × 3.6 Å (for CMP-3;
Figure 1b), respectively.
Characterizations. In Fourier transform infrared spectros-
copy (FT-IR; Figure S1) of CMP-2 and CMP-3, the
disappearance of CO characteristics and formation of strong
stretching vibration bands for cyano groups at 2193/2196
cm−1 were main indications for the coupling reactions. 13C CP-
MAS NMR spectra of CMP-2 and CMP-3 were recorded and
shown in Figure S2. The intense peaks at ca. 130 ppm were
characteristic of aromatic carbons, and the shoulder peaks
around 90 ppm were assigned to cyano groups. Peaks beyond
200 ppm belonged to guest molecules (CH3COOH) being
trapped in the frameworks. Other characteristics were assigned
to possible oligomeric cross-linking fragments. PXRD patterns
for CMP-2 and CMP-3 within 2θ range of 5−50° showed
dispersion peaks (Figure S3), which were the characters of
Recovery and Regeneration of Material Absorbents. CMP-
3a after each adsorption experiment was first centrifuged, followed by
ion exchange with saturated NaCl solutions. Further separation of the
CMP-3a was performed by centrifugation, washing three times with
distilled water, and drying in air, giving the recovered and regenerated
materials for further adsorption experiments.
C
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Figure 2. (a) The Pb2+ adsorption kinetics and linear fitting plots of CMP-2a and CMP-3a; the Pb2+ adsorption kinetics of the CMP-3a at different
temperatures (b) and pH (c); (d) zeta potential plot of CMP-3a; (e) selective Pb2+ adsorption against Cu2+ and Ni2+ for CMP-3a; and (f) the
adsorption efficiency of heavy metals in solutions with various concentrations.
amorphous structures. These results suggested the extended
network structures of CMPs materials as proposed in Scheme
1, Figure 1a,b. SEM images indicated that both CMP-2 and
CMP-3 had dispersive sphere morphology with particle sizes
around several microns (Figure 1c,d).
Thermal gravimetric analysis (TGA) revealed that CMP
materials were stable up to ca. 300 °C, and decomposed upon
further heating (Figure S4). Activation of the samples was
performed via solvent exchanges with acetone that further
afforded desolvated samples CMP-2a and CMP-3a under mild
heating (Figure S4). The activation of CMP-2 was more
difficult than the CMP-3, possibly due to the smaller pores
present in the structure, which generated greater interactions
to guest molecules. Porosity of the activated CMP-2a and
CMP-3a was demonstrated by gas adsorption studies. N2
adsorption isotherms of CMP-2a and CMP-3a at 77 K
(Figure S5) displayed restricted adsorption behaviors,34−36
with the Brunauer−Emmett−Teller surface area of 118 m2 g−1
(CMP-2a) and 168 m2 g−1 (CMP-3a), respectively. Of special
note, the BET of CMP-3a was comparable to some reported
porous organic materials showing great potential as viable
organic absorbents.25,29
Heavy Metal Ion Adsorption Property of CMP
Materials. Porous molecular materials are popular candidates
as absorbents for the adsorption of heavy metal contents from
wastewater resources and the efficient and selective removal
capability is even more desirable and significant.37−40 Because
of the presence of heterogeneous pores in host frameworks of
CMP-2 and CMP-3 that might provide distinguishable
microenvironment for heavy metal adsorption, the activated
samples CMP-2a and CMP-3a were tested for Pb(II), Cu(II),
and Ni(II) adsorption in simulated wastewater. These two
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materials displayed similar adsorption behaviors of high Pb(II)
adsorption originated from their reticular structural features of
large porosity as well as pore geometry and functionality,
which were unique among materials for selective heavy metal
adsorption.39,40 Especially, the adsorption ability of CMP-3a
was notably higher than that of the CMP-2a due to a higher
BET surface area related to the larger pore channels (Figure
1a,b). Detailed heavy metal ion adsorption and selectivity are
extensively discussed below.
Scheme 2. (a−c) Presentation for the Adsorption of Various
Heavy Metal Ions in the Pores of CMP-3a
As shown in Figure 2a, the Pb(II) adsorption of CMP-2a
and CMP-3a was rapid in the first 30 min, and the adsorption
equilibrium was generally reached after 12 h. The fast
adsorption was the result of high BET surface areas and
abundant pore paces for physical adsorption, and the slow
adsorption stage was mostly ascribed to the chemical and
irreversible adsorption.41 Furthermore, the pseudo-second-
order model was introduced to describe the adsorption kinetics
(Figure 2a). In comparison with CMP-2a, CMP-3a exhibited a
longer half-life (t1/2) of 22.8 min (versus 13.6 min) and a high
qe of 93.2 mg g−1, which were comparable to some of the best
Pb(II) absorbents documented in the literature.42 Moreover,
the influence of temperature and pH on Pb2+ adsorption of
CMP-3a was further studied. As shown in Figure 2b, the qe
increased slightly with a temperature increment from 92.8 (at
15 °C) to 96.6 mg g−1 (at 45 °C). However, the qe at pH = 2.0
(81.7 mg g−1) was significantly lower than other pH conditions
(91.5−94.8 mg g−1; Figure 2c). Zeta potentials of CMP-2a and
CMP-3a were thus measured to determine surface charges of
these materials at different pH conditions. As shown in Figure
2d, CMP-2a and CMP-3a displayed positive charges at pH =
2.0, whereas surfaces of CMP-2a and CMP-3a were negative
due to the partial deprotonation of dihydropyridyl groups.
These results were consistent with the trend of qe at various
pH and further suggested that CMP-3a generally exhibited
high adsorption capacity toward Pb(II) at relatively wide
temperature and pH ranges, except for the low pH condition
(below 4.0) when surfaces of the materials were positively
charged. Meanwhile, the selective Pb(II) adsorption against
Cu(II) and Ni(II) of CMP-3a was investigated in heavy metal
solutions with various concentrations (Figure 2e). The
removal of Pb(II) reached ca. 84% (versus ca. 40% for Cu(II)
and ca. 18.6% for Ni(II)) in 50 mL 10 mg L−1 solutions at
dosage of 10 mg (Figure 2f), and the high adsorption capacity
of Pb2+ over Cu2+ and Ni2+ was related to the higher
electronegativity value of the Pb(II) (Pauling’s scale 2.33)
compared to that of Cu2+ (1.90) and Ni2+ (1.91).42,43
Moreover, the large amount of delocalized π electrons on
cyano groups was available to donate the vacant 6p orbitals of
Pb(II) ions and thus facilitated more Pb2+ adsorption.
and distorted octahedral [Cu(H2O)6]2+ (4 + 2) in confined
spaces,44,45 due to the Jahn−Teller effect (JTE) on Cu(II)
centers. The presence of coordination unsaturated hydrated
Cu2+ ions increases the coordination interactions between
Cu(II) ions and the pyridyl groups in the np of CMP-3a.
Coupled with physicochemical adsorption in the lp, CMP-3a
exhibited better Cu2+ adsorption against Ni2+, although these
two ions have comparable electronegativity (Scheme 2b); (iii)
in the case of Pb(II) that is known to have flexible
coordination numbers and uneven coordination geometry,
the exposure of lone pair electrons of hydrated Pb2+ ions to the
host framework of CMP-3a can be significant, and thus
coordinative bonds account largely for the fast and high
adsorption of Pb2+ in both np and lp of the CMP-3a. Because
of the multiple and viable host−guest interactions, including
HBs, EAs, and coordination bonds, the highly efficient and
selective Pb2+ adsorption is extremely superior (Scheme 2c).
Reusability. Recovery and regeneration of CMP-3a were
carried out by centrifugation, ion exchange, and washing,
followed by performing Pb(II) ion removal for up to five
consecutive runs. The calculated overall adsorption rate of
recovered CMP-3a preserved more than 80% of the initial
adsorption rate after five times of recovery (Figure 3c). The
rational decrease of adsorption rate might account for the
insignificant loss of materials during centrifugation and
washing, as well as the incomplete ion exchange in the
multiple recovery processes (Figure 3c). Moreover, the high-
angle annular dark-field scanning transmission electron
microscopy (HAADF−STEM) elemental mapping image
from EDX analysis further confirmed the entire overlapping
of C, N, and Pb elements in Pb2+@CMP-3a after the first run
of adsorption (Figure 3a) and the fifth run of adsorption
(Figure 3b), indicating the effective absorption of Pb(II) in the
recovered materials. Furthermore, it is vital to test the effective
dosage, since safe and low toxic materials are expected to be
used in practical applications for adsorption of heavy metal
ions in wastewater treatment. The antibacterial effect of CMP
materials against Escherichia coli (E. coli) was studied to
determine cytotoxicity in vitro (Supporting Information). The
antibacterial experiments were carried out by regrowing the E.
coli on solid agar plate after exposure with CMP-2a or CMP-
3a. As shown in Figure 3d, no obvious growth inhibition of E.
coli was observed for CMPs at various dosages. Therefore,
CMP-2a and CMP-3a were considered as low-toxic materials
for heavy metal ion removal in wastewater treatment.
Adsorption Mechanisms. The proposed mechanism for
selective Pb(II) adsorption in this current system is as follows:
Pb(II), Cu(II), and Ni(II) present in the main forms of
hydrated ions and thus hydrogen bonds (HBs) and electro-
static attractions (EAs) are possible forces that determine
host−guest interactions. The [Ni(H2O)6]2+ possesses stable
and rigid octahedral geometry which allows mainly hydrogen
bonds (HB) to the host framework of CMP-3a; however,
viable HBs are scarce due to the weak interaction nature of
HBs. Physicochemical adsorption of hydrated Ni(II) ions may
take place mostly in the lp of CMP-3a (Scheme 2a); (ii) in
comparison with the Ni(II), the Cu(II) can vary the
coordination sphere among dominant square planar [Cu-
(H2O)4]2+ and possible square pyramid [Cu(H2O)5]2+ (4 + 1)
CONCLUSIONS
■
In summary, 2D conjugated microporous polymers with
functionalized heterogeneous pores, CMP-2 and CMP-3,
were designed and prepared as metal-free absorbents for the
adsorption of heavy metal ions from simulated wastewater.
Interestingly, activated CMP-3a showed fast and selective
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Figure 3. Elemental mapping images of Pb@CMP-3a after the first run of adsorption (a) and the fifth run of adsorption (b), showing the
distribution of C (red), N (orange), and Pb (cyan); (c) cyclic Pb(II) adsorption/desorption rate in CMP-3a; and (d) antibacterial test of CMP-2a
and CMP-3a on Escherichia coli (E. coli) with different material dosages (5.0 mg L−1, black; 3.0 mg L−1, red).
removal of Pb(II) ions that was related to the formation of
highly ordered pore structure and framework functionality by
creating the appropriate microenvironment for selective Pb2+
adsorption in the molecular level. Moreover, CMP-2a and
CMP-3a exhibited low cytotoxicity in vitro against Escherichia
coli (E. coli), and thus the CMP materials are safe to be
potentially used for wastewater treatment, and minor leakage
of the CMP materials in water is unlikely to introduce hazards
or toxins. Therefore, the CMP materials developed in this work
are promising for potential applications in Pb(II) removal from
wastewater; further the strategy of microenvironment mod-
ification on porous materials will create greater opportunities
in both structural design and functionality optimization of
more advanced materials.
ACKNOWLEDGMENTS
■
The authors acknowledge the International Science and
Technology Cooperation and Exchange Project of Fujian
Agriculture and Forestry University (No. KXGH17010), the
State Key Laboratory of Structural Chemistry, Chinese
Academy of Sciences (No. 20170032), and the New Century
Excellent Talents in Fujian Province University for funding.
DEDICATION
■
This work is dedicated to Prof. Xin-Tao Wu (FJIRSM, CAS)
on the occasion of his 80th birthday.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.cgd.9b01265.
IR spectra, 13C CP-MAS NMR spectra, and PXRD of
CMP-2 and CMP-3; TGA of CMP-2 and CMP-3,
CMP-2a, and CMP-3a; N2 uptake isotherms for CMP-
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AUTHOR INFORMATION
Corresponding Author
*E-mail: jian_lu_fafu@163.com.
ORCID
̈
Jian Lu: 0000-0002-0015-8380
Notes
The authors declare no competing financial interest.
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