Intercalating negatively charged pillars into graphene oxide sheets to enhance sulfonamide pharmaceutical removal from water

Abstract

Herein, novel composite materials were prepared by intercalating functional pillars, i.e., pentafluorobenzene (PFB) and sodium 2,3,4,5,6-pentafluorobenzoate (PFBS), into graphene oxide (GO) sheets. It led to forming size hives and increased availability of intrinsic area of GO. The synthesized materials (GO-PFB and GO-PFBS) were investigated as adsorbents to eliminate sulfadiazine (SD) from aqueous solutions. The adsorption capacities of GO-PFBS (1002.21 μmol/g) and GO-PFB (564.17 μmol/g) were 6.37 and 3.59 times higher than that of GO (157.21 μmol/g), respectively. The adsorption of SD onto GO-PFBS decreased with increasing solution pH. Density functional theory (DFT) results revealed that the SD adsorption onto the adsorbents was exothermic, and the introduction of the carboxylate groups showed lower binding energy. It was found that hydrophobic interaction fully participates in the adsorption process, and the electrostatic complementation of hydrogen bonding further enhances the SD adsorption. Obtained results showed that intercalating functional rigid molecules as pillars to support GO sheets could improve its adsorption behavior.

Full text

Vol.:(0123456789)
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https://doi.org/10.1007/s11356-022-20949-w
RESEARCH ARTICLE
Intercalating negatively charged pillars intographene oxide sheets
toenhance sulfonamide pharmaceutical removal fromwater
WeiWang1· ShiyiWang1· MohammadtaghiVakili2· YanWang1· ChangSun1· HaoruYang3· GuotaoXiao1·
MinjuanGong1· ShuangxiZhou1
Received: 31 March 2022 / Accepted: 16 May 2022
© The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2022
Abstract
Herein, novel composite materials were prepared by intercalating functional pillars, i.e., pentafluorobenzene (PFB) and
sodium 2,3,4,5,6-pentafluorobenzoate (PFBS), into graphene oxide (GO) sheets. It led to forming size hives and increased
availability of intrinsic area of GO. The synthesized materials (GO-PFB and GO-PFBS) were investigated as adsorbents to
eliminate sulfadiazine (SD) from aqueous solutions. The adsorption capacities of GO-PFBS (1002.21μmol/g) and GO-PFB
(564.17μmol/g) were 6.37 and 3.59 times higher than that of GO (157.21μmol/g), respectively. The adsorption of SD onto
GO-PFBS decreased with increasing solution pH. Density functional theory (DFT) results revealed that the SD adsorption
onto the adsorbents was exothermic, and the introduction of the carboxylate groups showed lower binding energy. It was
found that hydrophobic interaction fully participates in the adsorption process, and the electrostatic complementation of
hydrogen bonding further enhances the SD adsorption. Obtained results showed that intercalating functional rigid molecules
as pillars to support GO sheets could improve its adsorption behavior.
Keywords Graphene oxide· Rigid molecules· Sulfadiazine· Density functional theory· PPCPs
Introduction
Antibiotics are one of the widely applied PPCPs in humans
and animals to treat/prevent infections (Li etal. 2018; Sar-
mah etal. 2006). Sulfonamides (SNs) are one of the com-
monly used antibiotics (Serna-Carrizales etal. 2021; Wei
etal. 2019). In recent years, massive utilization has led to
the presence of SNs in the environment and gradual accu-
mulation in living organisms, which can threaten human
health and environmental safety (Szymańska etal. 2019).
Traditional wastewater treatment plants cannot remove the
SNs completely, and thus these compounds could appear
in natural waters. Different methods have been developed
to eliminate SNs from aqueous solutions such as biodegra-
dation, electrochemical oxidation, membrane filtration, and
photocatalysis (Zhuang etal. 2020). Low manufacturing
cost, simple operating requirements, high selectivity, envi-
ronmentally friendly, and easy control make the adsorption
more advantageous. However, it is necessary to develop an
effective adsorbent that is inexpensive, easy to prepare, and
regenerable.
Graphene (an atomic layer of sp2 bonded carbon atoms,
tightly bound in a honeycomb or hexagonal lattice) was
first identified in 2004 through mechanical exfoliation
by Novoselov and Geim (Novoselov etal., 2004). Due to
unique physicochemical properties (e.g., high hydropho-
bicity, optical transmittance, chemical inertness, high cur-
rent density, and high thermal conductivity) (Priyadarsini
etal. 2018), it is widely applied in tissue scaffolds (Nayak
etal. 2011), energy storage (Wu etal. 2021), biosensing (Lu
etal. 2009), drug delivery (Sun etal. 2008), antibacterial
compositions (Perreault etal. 2015), and so on. Also, such
material has attracted growing attention as an effective and
potential adsorbent in environmental remediation because
of its high surface area (2630 m2/g), fast adsorption rate,
Responsible Editor: George Z. Kyzas
* Shuangxi Zhou
15105176897@126.com
1 State Key Laboratory ofPlateau Ecology andAgriculture,
Qinghai University, Xi’ning810016, QinghaiProvince,
China
2 Green Intelligence Environmental School, Yangtze Normal
University, Chongqing408100, China
3 Colorado College, ColoradoSprings, CO80903, USA
/ Published online: 24 May 2022
Environmental Science and Pollution Research (2022) 29:72545–72555

1 3
and high adsorption capacity (Verma etal. 2021). However,
the aggregation and hard dispersion of graphene in water
affect its adsorption performance (Gunes etal. 2021). The
methods, i.e., the introduction of abundant oxygen elements
to prepare graphene oxides (GO), and hybridization with
other nano-materials, could promote its application and
help overcome aforementioned problems. For example, the
GO-MnO2 nanocomposite performed better in methylene
blue and methyl orange removal compared to GO (Verma
etal. 2021). Travlou etal. prepared a chitosan-GO composite
adsorbent and applied it for reactive dye adsorption (Travlou
etal. 2013).
In recent years, connecting GO sheets to excavate its
effective surface area has also been considered a feasible
strategy. For instance, the decafluorobiphenyl (a rigid mol-
ecule) was successfully inserted into hydroxylated carbon
nanotubes by a nucleophilic substitution reaction. The
adsorption capacity increased by 2.1 and 2.7 times com-
pared with the adsorption capacity of CNTs powder for car-
bamazepine and tetracycline, respectively (Shan etal. 2018).
In addition, 1,4-phenylenebisboronic acid was successfully
intercalated into rGO-Br and greatly improved the conduc-
tion of UO2+ (Wang etal. 2021c).
The intercalation of rigid molecules into GO sheets could
enhance the availability of intrinsic area (Shan etal.2017).
There is almost no report that using GO (prepared by inter-
calating pre-designed functional pillars) to remove con-
taminants, e.g., pharmaceutical and personal care products
(PPCPs), for the PPCPs in an aqueous solution could be
seriously affected by the solution pH (Berhane etal. 2015).
The use of pillars (containing carboxylate groups) for the
connection of GO sheets would significantly increase the
adsorption of positively charged PPCPs. Therefore, it is a
practical and feasible strategy to enhance the adsorption
capacity of nano-material by a nucleophilic substitution
reaction between rigid molecules with carboxylates and GO
layers to form functionally layered porous structure. Due to
the typical electron-donating of amino groups on SNs, the
introduction of strong electron-acceptor groups into pillars
would seriously improve the adsorption of SNs.
Therefore, the carboxyl functional group was pre-design-
ing on the pillars, which was intercalated into GO sheets
in this work. The rigid molecules, i.e., pentafluorobenzene
(PFB) and sodium 2,3,4,5,6-pentafluorobenzoate (PFBS,
carbonylated pillar), were reacted with hydroxyl groups on
GO by a nucleophilic substitution reaction to obtain the GO-
based materials with layered porous structutre and named as
GO-PFB and GO-PFBS, respectively. Meanwhile, one of
the SNs, i.e., sulfadiazine (SD), was chosen as an objective
contaminate to estimate the adsorption performance of the
prepared GO-based materials. Comparative adsorption stud-
ies between GO, GO-PFB, and GO-PFBS were conducted
to obtain the optimal nano-materials in SD removal. The
adsorption performance of the adsorbents was investigated
by studying the adsorption kinetics, isotherms, effects of
solution pHs, and ionic strength. Experimental data and DFT
calculation proposed the possible adsorption mechanism.
Additionally, the popularized application of adsorbent for
sulfonamide removal was studied.
Materials andmethods
Materials andreagents
Graphene oxide (GO, TNGO-3) was purchased from
Chengdu Organic Chemicals Co. Ltd. Pentafluorobenzene
(PFB, 98%) and sodium 2,3,4,5,6-pentafluorobenzoate
(PFBS, 95%) were obtained from Shanghai Aladdin Bio-
chemical Technology Co., Ltd. and J&K Scientific Co., Ltd.,
respectively. Sulfonamides, i.e., sulfadiazine (SD, 98%), sul-
famethoxazole (SMZ, 96%), sulfamethazine (STZ, 99%),
sulfapyridine (SPD, 98%), and sulfachloropyridazine (SPZ,
98%), were supplied by Aladdin Biochemical Technology.
Potassium carbonate (K2CO3), N,N-dimethylformamide
(DMF), methanol (99.9%), and acetonitrile (99.9%) were
purchased from Sinopharm Co., Ltd.
Preparation ofGO‑PFB andGO‑PFBS
The nucleophilic substitution reaction was referred to in the
previous research (Shan etal. 2018). Initially, 0.1g GO was
fully dispersed in 50mL DMF assisted by ultrasonic for
15min in an ice bath. Then, 20mmol/L of PFB or PFBS and
0.60g of K2CO3 were added into the dispersion liquid and
ultrasonicated for 10min in an ice bath. Next, the gas circuit
filled with N2 was penetrated into the mixture for 10min
to crow out air. And then, the glass vial was kept at 65°C
and stirred at 500rpm for 48h. In the next step, the cooled
mixture was washed with HCl (1M), H2O, methanol, and
CH2Cl2 in turns. Finally, the solid was vacuumed in a liquid
nitrogen bath for 10min and then placed into a vacuum
drying oven under 50°C for 48h. The obtained materials
were denoted as GO-PFB and GO-PFBS for the different
intercalations of steel molecules. The synthetic scheme of
GO-PFB and GO-PFBS is shown in Fig.1.
Characterization
The surface morphology of the materials was observed
by a field emission scanning electron microscope (FE-
SEM, JSM-7900F, Japan). The pore size distribution and
Brunauer–Emmett–Teller (BET) surface area were recorded
by an adsorption instrument (Micromeritics, USA) using
nitrogen gas at 77K. Functional groups on the prepared
adsorbents were characterized by an FTIR spectrometer
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(PerkinElmer, USA). Raman spectra were obtained with
an excitation line at 532nm (Horiba, France). The zeta
potential was measured at different pH values (1–9) using a
Zetasizer instrument (Brookhaven, USA) and the point zero
charge was calculated by the curve between pH vs surface
electrical property.
Adsorption experiments andanalysis
The kinetic experiments were carried out in 150-mL glass
vials, where 0.1g/L of the prepared materials were mixed
with 90mL of pharmaceutical solution (0.2mmol/L) at pH
5 with a shaking rate of 180rpm under 25°C for 30h. The
sample was taken out by a syringe with a filter membrane
(PES, 0.45μm) at interval time. The isotherm experiments
were performed by mixing 0.1g/L of the adsorbents and sul-
fonamide solutions (0.02–0.44mmol/L) at pH 5 180rpm for
24h. The favorable adsorption conditions were obtained by
investigating the effects of solution pH (1–11) adjusted with
1M HCl or NaOH and the effect of ion strength by varying
NaCl/CaCl2 concentration ranging from 20 to 80mg/L.
The concentration of sulfonamides was measured by
HPLC (Shimadzu LC-20, Japan). SD was separated by a
C18 column (4.6 × 250mm, Shimadzu, Japan). The injected
volume and the mobile phase flow rate, i.e., methanol/0.1%
formic acid (50:50, v/v) under 40°C, were 10 µL and
1.0mL/min, respectively.
Computational method
The optimal structures of SD and adsorbents were mod-
eled by Gaussian 09 program (Frisch etal. 2010). B3LYP
hybrid functional at the 6-311G level was used to optimize
the frequency calculation of the optimal structures (Wang
etal. 2021a). The polarizable continuum model based on
the integral equation formalism variant (IEFPCM) (Tomasi
etal. 1991) was adopted, and water was used as the solvent
in the system. Ebinding values between GO-PFBS and SD
were calculated using Eq.(1):
To further explore the interaction between SD and the
prepared materials, electrostatic potential (ESP) was calcu-
lated by the Multiwfn program (Lu and Chen 2012) and the
VMD 1.9 software package (Humphrey etal. 1996).
Regeneration experiment
Regeneration experiment was carried out by mixing
0.1g/L GO-PFBS and 0.2mmol/L SD solution at pH 5 for
24h. Then the adsorbent was separated by a centrifuge at
8000rpm for 10min. The collected saturated GO-PFBS was
immersed in the mixture of CH3OH and NaOH (with a mass
ratio of 10:1) and stirred at 180rpm for 5h at 25°C (Meng
etal. 2020). The eluent was abandoned, and the humid GO-
PFBS was dried at 60°C overnight for the next cycle.
Results anddiscussion
Characterization
The SEM images of GO, GO-PFB, and GO-PFBS are pre-
sented in Fig.2a–c. Compared with GO, the combined, shrill
wrinkled sheets (similar to a broken tree trunk) can be seen
in the complete view of GO-PFB and GO-PFBS. This might
be caused by the intercalation of pillars into GO sheets. The
(1)
Ebinding =Ebindingcomplex −ESD −Eadsorbent
Fig. 1 Synthetic scheme of GO-
PFB and GO-PFBS
72547Environmental Science and Pollution Research (2022) 29:72545–72555

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GO sheets, connected by the rigid molecules through the
nucleophilic substitution reaction, are attached and formed
numerous pores, which would enhance the adsorption capac-
ity of GO.
The prepared materials’ specific surface area and pore size
distribution are shown in Table1 and Fig.2d, respectively.
It was apparent that both of the modified adsorbents dis-
played a relatively high BET specific surface area of 40.23
m2/g (GO-PFB) and 26.10 m2/g (GO-PFBS) compared with
that of GO (7.94 m2/g). Also, the total pore volume showed
similar results, and it indicated that reactions between rigid
molecules and GO effectively increased the granulation and
decreased the aggregation of GO sheets (Shan etal. 2018).
In addition, although the mesoporous volume of GO-PFB
and GO-PFBS was dramatically increased, GO-PFBS
showed relatively concentrated pore sizes within 3–35nm,
and GO-PFB displayed a wide range from 1.2 to 185nm.
This difference might be attributed to the inadequate reac-
tion between rigid molecules and GO or the formed tilt pillar
by the vacancy in the benzene ring of PFB compared to fully
filled with fluorine atoms in PFBS.
The FTIR spectra of chemicals and related prepared
materials are shown in Fig.2e. For GO, the peak at
3434 cm−1 corresponded to the stretching vibration of
Fig. 2 The SEM images of GO
(a), GO-PFB (b), and GO-PFBS
(c). Pore size distribution (d),
the FTIR spectra (e), and the
Raman spectra (f) of GO, GO-
PFB, and GO-PFBS
Table 1 Specific surface area and pore volume of GO, GO-PFB, and
GO-PFBS
* Pore volume was calculated by the DFT method
Materials BET surface area (m.2/g) Total pore
volume
(cm.3/g)
GO 7.94 0.010
GO-PFB 40.23 0.101
GO-PFBS 26.10 0.047
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hydroxyl groups, and the peak at 1626 cm−1 might accord
to the skeletal stretching vibration of the aromatic ring
and 1726 cm−1 assigned to the stretching vibration of
O-C = O (Han et al. 2021; Li etal. 2020; Singh etal.
2009). In case of PFB and PFBS molecules, the peaks
at 945 cm−1, 1099 cm−1, 987 cm−1, and 1118 cm−1 are
due to the C-F vibration (Singh etal. 2009). The peaks at
1641 cm−1 and 1656 cm−1 are assigned to the stretching
vibrate of C = C in the benzene ring. Compared to PFB,
some new peaks at 3480 cm−1, 1475 cm−1, 1295 cm−1,
1391 cm−1, and 1603 cm−1 were observed in PFBS (due to
the presence of carboxyl groups in PFBS structure) (Han
etal. 2021; Li etal. 2020; Singh etal. 2009). Moreover,
the disappearance of the hydroxyl group’s peak at
3434 cm−1 on GO and appearance of weakly characteristic
peaks of C-F, i.e., 1045 cm−1 and 987 cm−1 for GO-PFB
and 1053 cm−1 and 983 cm−1 for GO-PFBS, show the
successfully executed intercalation of rigid molecules to
the GO structure.
The ratio between D band and G band (R = ID/IG) in
Raman spectra was used to estimate the degree of defects for
carbon materials, where the D band represents the structural
defects and the G band shows the first-order scattering of
E2g vibrational model (Verma etal. 2021; Stankovich etal.
2007). As shown in Fig.2f, the prepared materials (GO-
PFB and GO-PFBS) exhibited a higher R value than that of
GO, indicating that the disorder of prepared materials was
enhanced (Shan etal. 2018). This demonstrated that rigid
molecules were successfully intercalated into GO sheets and
formed porous structure, which was advantageous to capture
contaminants.
Comparison oftheprepared materials
Adsorption kinetic
To investigate the adsorption mechanism and process of SD
onto GO, GO-PFB, and GO-PFBS, kinetic experiments were
conducted. The values of kinetic constants were obtained
using Webber-Morris intra-particle, pseudo-first-order,
pseudo-second-order, and Elovich models (Fig.3a, Fig.S1,
and TableS1). The removal of SD by the GO-PFB and GO-
PFBS increased sharply in the initial 12h of the adsorption
process, and then all reached the adsorption equilibrium
after 24h, which was faster than the intercalation of rigid
molecule DFB to CNT-OH in CBZ and TC removal (Shan
etal. 2018). It was apparent that the SD adsorption capacity
of GO was significantly enhanced after modification. These
findings suggest that the introduction of rigid molecules
has successfully connected GO sheets and formed cubicles.
Moreover, the adsorption amount of SD onto GO-PFBS
(536.40μmol/g) was 1.41 times higher than that of GO-
PFB (379.93μmol/g) (Fig.3a and TableS1). It could be
due to introducing the rigid molecule containing carboxyl
groups to the GO sheets, significantly improving its adsorp-
tion capacity.
As shown in Fig.S1, the intra-particle curves could be
roughly divided into three stages. None of the fitting lines
pass through the origin, indicating that intra-particle dif-
fusion was not the dominantly rate-limiting factor and the
adsorption process was comprehensively affected by bound-
ary layer and internal pore structures (Ma etal. 2019). On
the other hand, the experimental data fitted well with the
pseudo-second-order model based on the R2 value (> 0.98).
This indicated that the adsorption process could be involved
in chemisorption, which can be proved by the high R2 value
0510 15 20 25 30
0
100
200
300
400
500
600
GO
GO-PFB
GO-PFBS
First-order model
Second-order model
yticapacnoitprosdA (
μ
g/lom )
Time (h)
(a)
0100 200300 400
0
200
400
600
800
1000
GO
GO-PFB
GO-PFBS
Langmuir model
Freundlich model
yticapacnoitprosd
A(
μ
g/lo
m)
C
e
(
μ
mol/L)
(b)
Fig. 3 Pseudo-first-order and pseudo-second-order kinetic models (a), Langmuir and Freundlich isotherm models (b) for SD adsorption onto the
GO, GO-PFB, and GO-PFBS
72549Environmental Science and Pollution Research (2022) 29:72545–72555

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of the Elovich model (0.99). The SD adsorption capacity
of GO, GO-PFB, and GO-PFBS were found to be 52.53,
379.92, and 536.40μmol/g, respectively. Therefore, the rigid
molecule PFB has successfully built cubicles in GO sheets
promoting SD diffusion. The carboxylic acid-functionalized
rigid molecule (PFBS) promoted SD adsorption by the spe-
cific electronic complementation hydrogen bonding affinity.
Adsorption isotherm
To determine the maximum adsorption capacity of the
adsorbents and provide information on the interaction of
SD with GO, GO-PFB, and GO-PFBS, relevant experimen-
tal data were fitted to the Langmuir and Freundlich adsorp-
tion isotherm models, and the results are shown in Fig.3b
and Table2. Compared to the Langmuir model (R2 ≥ 0.92),
the equilibrium adsorption data was described better by the
Freundlich model (≥ 0.97), indicating that the SD adsorp-
tion occurred on heterogeneous surfaces (He etal. 2017).
In addition, the n values for SD adsorption onto the GO-
PFB and GO-PFBS were within 2.19–3.38, confirming
the favorability of the adsorption process (Ai etal. 2019).
According to the Langmuir model, the maximum adsorp-
tion capacity of GO-PFB (1002.21μmol/g) and GO-PFBS
(564.17μmol/g) was 6.37 and 3.59 times higher than that of
GO (157.12μmol/g), respectively.
Effects ofsolution pH andionic strength
Effects ofsolution pH
Solution pH is an important controlling factor in the adsorp-
tion process as it can influence the adsorbate’s dissociation
and the adsorbent’s surface charge, thus affecting the adsorp-
tion mechanism (Wu etal. 2020). For understanding the net
surface charge on the adsorbent, the optimum pH value for
effective adsorption, and the adsorption mechanism, evalu-
ating the adsorbent’s pHpzc could be beneficial (Egbedina
etal. 2021). It has been reported that the pH < pHpzc is
favorable for the adsorption of anionic adsorbates where the
adsorbent surface has a positive charge (Yagub etal. 2014).
The pHpzc of GO-PFBS was found to be 1.9 (Fig.S2). At
pH < 1.9, protonation of SD and functional groups on the
adsorbent could prevent the migration of cation SD to GO-
PFBS. Due to the deprotonation of adsorption sites at alka-
line conditions, the SD molecule was in anionic form, and
the surface charge of the GO-PFBS was found to be nega-
tive. Therefore, the strong electrostatic repulsion between
the negative GO-PFBS and SD− could seriously inhibit the
adsorption of SD from aqueous solution onto GO-PFBS,
which significantly decreased the SD adsorption capacity
of the adsorbent (Hu etal. 2019).
The effect of the initial pH value of SD solution on the
adsorption capacity of GO-PFBS is shown in Fig.4a. The
adsorption capacity of SD onto GO-PFBS presented an
upward trend with the increasing solution pH values, and
Table 2 The parameters of Langmuir and Freundlich model for SD
Langmuirmodel ∶ qe=qmCe∕(1∕b+Ce)
Freundlichmodel ∶
q
e
=K
f
C
e
1∕n
Models Parameters GO GO-PFB GO-PFBS
Langmuir model qm (μmol/g) 157.21 564.17 1002.21
R.20.940 0.920 0.960
b (L/μmol) 0.0053 0.037 0.014
Freundlich model Kf (μmol1−1/n L1/n
g.−1)
5.06 98.82 60.88
n1.93 3.38 2.19
R.20.980 0.970 0.970
0
100
200
300
400
500
600
11
9
753
yticapacnoitprosdA (
μ
g/lom )
Initial pH
1
(a)
0
100
200
300
400
500
600
805020
yticapacnoitprosd
A(
μ
g/lo
m)
Na
+
/Ca
2+
concentration (mg/L)
Blank
Na+
Ca2+
0
(b)
Fig. 4 The influence of solution pH (a) and ion strength (b) on the SD removal by GO-PFBS
72550 Environmental Science and Pollution Research (2022) 29:72545–72555

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the maximum adsorption capacity of SD (564.91μmol/g)
occurred at pH 5. It could be attributed that the main species
of SD is in neutral form under this pH value and thus could
be easily attracted via the electron-accepting of carboxylates
on PFBS pillars, hydrogen bond interaction, and electrostatic
interaction.
Effects ofionic strength
Figure4b shows the effects of ionic strength on the SD
removal by GO-PFBS. With an increase in both salt concen-
trations, the SD adsorption capacity of GO-PFBS showed a
decreasing trend. Both the two Na+ and Ca2+ ions weakened
the adsorption capacity of GO-PFBS. The SD adsorption
capacity reached 412.20μmol/g and 393.73μmol/g when
the salt concentration increased to 80mg/L. This indicated
that cations could occupy the adsorption sites on GO-PFBS,
and due to the higher valence, the Ca2+ performed a slightly
stronger restraint/inhibition than Na+. To verify the conclu-
sion, the same concentrations of Na+ and Ca2+ were added
to the mixture of SD and GO-PFB under the same condition,
respectively. As shown in Fig.S3, the adsorption capacity
of SD by GO-PFB was almost no change, indicating that
Na+ and Ca2+ might occupy the adsorption sites supported
by carboxylate groups of GO-PFBS. In addition, the point
zero charges (pHpzc, Fig.S2) of GO-PFBS were calculated
to about pH 1.9, and the strongest negative charge was under
pH 5, demonstrating that the surface of GO-PFBS might be
occupied by cations (Na+ or Ca2+) and thus decrease the
adsorption of SD.
Regeneration andreuse
FigureS4 illustrates the reusability of GO-PFBS after 5
adsorption–desorption cycles. The results revealed that the
SD adsorption capacity of the GO-PFBS at the first cycle
(535.46μmol/g) decreased to 328 μmol/g at the second
cycle. This reduction could be attributed to the decrease
in porosity of GO-PFBS, saturation, and unavailability of
free functional groups on the GO-PFBS. With increasing
regeneration cycles, the adsorption capacity of the adsorbent
remained almost stable and did not change much.
Adsorption mechanism
The density functional theory (DFT) model was used to cal-
culate the adsorption configurations and the binding energy
between SD and pillars, i.e., PFB and PFBS (Fig.5). The
values of ΔEbinding for SD adsorbed onto PFB and PFBS
were − 4.68 and − 7.09kJ/mol (Fig.5a and b), respectively,
indicating that the adsorption was an exothermic process and
the geometry of GO-PFB-SD and GO-PFBS-SD was stable.
The ΔEbinding of GO-PFBS-SD was lower than GO-PFB-
SD, suggesting that GO-PFBS performed a stronger affinity
Fig. 5 DFT optimized structures of SD adsorption on GO-PFB (a) and GO-PFBS (b). ESP mapping of GO-PFB (c) and GO-PFBS (d) after SD
adsorption
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for SD than GO-PFB, consistent with their experimental
adsorption capacity (Fig.3b).
As shown in Fig.5c and d, the electrostatic potential
(ESP) of GO-PFB and GO-PFBS after SD adsorption was
marked red and blue, representing nucleophilic and electro-
philic groups, respectively. Compared with PFB, the surface
electrostatic potential of PFBS changed to completely nega-
tive for the introduction of carboxylate and showed a strong
affinity for the electrophilic pollutant. For the adsorption
configurations of GO-PFBS-SD (Fig.5d), the carboxylate of
PFBS becomes the dominant active site for attracting elec-
trophilic amino on SD via the electrostatic complementation
of hydrogen bonding (Wang etal. 2021b), while the surface
electrostatic potential of PFB (Fig.5c) has been affected
by the vacancy on the benzene ring and thus weakened the
electrophilic ability. Hence, assembling carboxylate groups
onto pillars enhanced the nucleophilic ability of GO-PFBS
and made it more feasible to capture objective SD.
It was reported that SD possessed two dissociation con-
stants with pKa1 = 2.0 and pKa1 = 6.8 (He etal. 2021), which
were mainly affected by the groups of amino and sulfonamide
in SD molecules. As shown in Fig.S2, the pHpzc of GO-PFBS
was about 1.9, demonstrating that the surface electrical prop-
erty was weakly positive at pH 1 and negative at alkalinity.
It was evident that GO-PFBS showed low electricity at pH 1
(about − 0.70mV), probably attributed to the protonation of
carboxylate. On the other hand, the adsorption amounts of SD
by GO-PFBS were frequently affected by solution pH, espe-
cially in alkaline conditions for the previous results, indicating
that the electrostatic interaction might dominate the adsorption
process (Wu etal. 2020). DFT results showed that carboxyl
groups on GO-PFBS exhibited a stronger attracting ability for
SD than GO-PFB. Meanwhile, the hydrophobic interaction
would participate in the whole process of SD removal. The
proposed adsorption mechanism is shown in Fig.6.
At low pHs (pH < 2.0), the special groups of amino
(-NH2) and sulfonamide (-SO2NH-) on the aromatic rings
of SD could function as strong π-electron receptors, and
thus the π+-π electron donor–acceptor interaction would
dominate the adsorption of positive SD+ (He etal. 2021;
Peiris etal. 2017; Zhang etal. 2016). The alkaline solution
would promote the deprotonation of -SO2NH- and form the
anionic SD (SD−), and thus, the migration of SD− would be
seriously inhibited by the strongly negative surface of GO-
PFBS, and the hydrophobic interaction force might rarely
participate in the adsorption of SD−. When the solution pH
was adjusted to 5, the exposed affine carboxylates of GO-
PFBS could appeal to the migration of SD molecules for
its hydrogen element of amino (-NH2) via the electrostatic
interaction and hydrogen bond. It was indicated that the elec-
trostatic complementation of hydrogen bonding participated
in the adsorption process, which was proved in our previ-
ous report (Wang etal. 2021b). In addition, the electrostatic
repulsion between SD and negative GO-PFBS disappeared
and thus promoted the diffusion of SD in GO-PFBS. Mean-
while, the hydrophobic interaction and π-π stacking also
participate in the adsorption process.
Application ofGO‑PFBS
From the above results, the prepared materials via the suc-
cessful intercalation of rigid molecules into GO sheets
exhibited excellent performance in SD removal. It is attrac-
tive to investigate the wider application of GO-PFBS in
other sulfonamide removal. Thus, four typical sulfonamides,
including sulfamethoxazole (SMZ), sulfamethazine (STZ),
sulfapyridine (SPD), and sulfachloropyridazine (SPZ), were
used as the objectives, and the results are shown in Fig.7.
Surprisingly, the adsorption capacity of SPZ, SMZ, SPD,
and STZ onto GO-PFBS has been improved by 18, 10, 3.7,
and 3.4 times compared with that of GO, respectively. The
prepared material’s outstanding properties revealed that
introducing rigid molecules to broaden the inner surface
and create cubicles is a rational design in future applications.
Conclusions
Intercalation of rigid molecules into GO, GO-PFB, and
GO-PFBS through the nucleophilic aromatic substitution
reaction with pillars of PFB and PFBS was performed
successfully. A series of characterization results revealed
Fig. 6 The proposed adsorption
mechanism of SD on GO-PFBS
72552 Environmental Science and Pollution Research (2022) 29:72545–72555

1 3
that the stacked GO sheets were prospectively isolated and
supported. Both of the prepared materials showed an out-
standing performance in SD removal, and the introduction
of carboxylate functional groups improved the adsorption
capacity of GO. According to the Langmuir model, the
adsorption capacity of GO-PFBS, GO-PFB, and GO was
1002.21, 564.17, and 157.21μmol/g, respectively. In addi-
tion, the solution pH played a critical role in SD removal by
altering the surface charges of GO-PFBS and SD. The DFT
calculation revealed that the adsorption process was spon-
taneous and exothermic, and GO-PFBS-SD showed lower
binding energy than GO-PFB-SD. The possible adsorption
mechanism was proposed that electronic complementation
of hydrogen bonding, π+-π electron donor–acceptor interac-
tion, and hydrophobic action participated in SD removal.
Moreover, the prepared GO-PFBS also showed an excellent
removal performance for other four sulfonamide pharma-
ceuticals. In brief, it is a very effective strategy to solve the
low surface area utilization of GO and enhance the func-
tionalization of nano-materials via inlaying functionally
rigid molecules and can be wide application in the future.
Supplementary Information The online version contains supplemen-
tary material available at https:// doi. org/ 10. 1007/ s11356- 022- 20949-w .
Author contribution Wei Wang: conceptualization, writing (original
draft), funding acquisition. Shiyi Wang: investigation. Mohammadtaghi
Vakili: writing (review and editing). Yan Wang: formal analysis. Chang
Sun: methodology. Haoru Yang: writing (review and editing). Guotao
Xiao: data curation. Minjuan Gong: validation. Shuangxi Zhou: con-
ceptualization, supervision, writing (original draft).
Funding This work was financially supported by the Key Youth Foun-
dation of Qinghai University (grant number: 2020-QGY-2).
Data availability All data generated or analyzed during this study are
available from the corresponding author on reasonable request.
Declarations
Ethics approval and consent to participate Not applicable.
Consent for publication Not applicable.
Competing interests The authors declare no competing interests.
0510 15 20 25 30
0
100
200
300
400
500
yticapacnoitprosdA (
µ
g/lom )
Time (h)
GO
GO-PFBS
(a)
0510 15 20 25 30
0
140
280
420
560
700
yticapacnoitprosd
A(
µ
g/lo
m)
Time (h)
GO
GO-PFBS
(b)
0510 15 20 25 30
0
150
300
450
600
yticapacnoitprosdA (
µ
g/lom )
Time (h)
GO
GO-PFBS
(c)
0510 15 20 25 30
0
80
160
240
320
yticapacnoitprosdA(
µ
g/lom)
Time (h)
GO
GO-PFBS
(d)
Fig. 7 The application of GO-PFBS in sulfonamide removal of SPZ (a), SMZ (b), SPD (c), and STZ (d) compared with GO
72553Environmental Science and Pollution Research (2022) 29:72545–72555

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