Article
pubs.acs.org/est
Coagulation Behavior of Graphene Oxide on Nanocrystallined Mg/Al
Layered Double Hydroxides: Batch Experimental and Theoretical
Calculation Study
Yidong Zou,†,‡ Xiangxue Wang,†,§ Yuejie Ai,*,† Yunhai Liu,‡ Jiaxing Li,*,§,⊥ Yongfei Ji,∥
and Xiangke Wang*,†,#
†
School of Environment and Chemical Engineering, North China Electric Power University, Beijing 102206, P. R. China
School of Chemistry, Biological and Materials Sciences, East China Institute of Technology, Nanchang, 330013, P. R. China
§
Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Science, P.O. Box 1126, Hefei,
230031, P.R. China
∥
Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, Roslagstullsbacken 15, 10691
Stockholm, Sweden
⊥
Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions and School for Radiological and
Interdisciplinary Sciences, Soochow University, 215123, Suzhou, P.R. China
#
NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
‡
S Supporting Information
*
ABSTRACT: Graphene oxide (GO) has attracted considerable attention
because of its remarkable enhanced adsorption and multifunctional properties.
However, the toxic properties of GO nanosheets released into the
environment could lead to the instability of biological system. In aqueous
phase, GO may interact with fine mineral particles, such as chloridion
intercalated nanocrystallined Mg/Al layered double hydroxides (LDH−Cl)
and nanocrystallined Mg/Al LDHs (LDH−CO3), which are considered as
coagulant molecules for the coagulation and removal of GO from aqueous
solutions. Herein the coagulation of GO on LDHs were studied as a function
of solution pH, ionic strength, contact time, temperature and coagulant
concentration. The presence of LDH−Cl and LDH−CO3 improved the
coagulation of GO in solution efficiently, which was mainly attributed to the
surface oxygen-containing functional groups of LDH−Cl and LDH−CO3
occupying the binding sites of GO. The coagulation of GO by LDH−Cl and
LDH−CO3 was strongly dependent on pH and ionic strength. Results of theoretical DFT calculations indicated that the
coagulation of GO on LDHs was energetically favored by electrostatic interactions and hydrogen bonds, which was further
evidenced by FTIR and XPS analysis. By integrating the experimental results, it was clear that LDH−Cl could be potentially used
as a cost-effective coagulant for the elimination of GO from aqueous solutions, which could efficiently decrease the potential
toxicity of GO in the natural environment.
■
INTRODUCTION
Over the past decade, there has been an intense focusing on the
graphene family nanomaterials (GFNs) (i.e., few-layered
graphenes (FLGs), graphene nanosheets (GNS), reduced
graphene oxide (rGO), and graphene oxide (GO)).1−3 The
GFNs have grown in importance as their unique physical,
chemical, and biocompatibility properties.3,4 Compared with
the carbon nanotubes, GFNs can provide a lower aspect ratio,
larger surface area, and better dispersibility in most solvents.4
Along with the rapid development of engineered nanomaterials
and the widespread utilization of GFNs, there will be potential
risk in the toxicity and high dispersion of graphene-related
nanomaterials, whose widespread contamination in the
environment has become one of the most forefront environ© 2016 American Chemical Society
mental problems. Given the potential threats of long
persistence in the environment, long-distance transformation
and accumulation in the food chain, these materials have been
proposed to have potential toxic effects on cells, microorganisms, animals, and environment.5,6 Among the GFNs, GO
is less hydrophobic and also tends to aggregate due to the van
der Waals interactions between neighboring sheets in water.7
Due to its excellent electronic, catalytic, mechanical, optical and
magnetic properties, GO has been proven to exhibit great
Received:
Revised:
Accepted:
Published:
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January 17, 2016
February 25, 2016
March 15, 2016
March 15, 2016
DOI: 10.1021/acs.est.6b00255
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promise for potential applications in the fields of hybrid
materials, environmental pollutant removal, sensors, nanoelectronics, batteries, and hydrogen storage, etc.,8−11 because of
large amounts of oxygen-containing functional groups on its
basal planes and edges. These abundant oxygen-containing
functional groups can either assist the dispersion of GO in
water or as active sites for functionalization and hybridization
with other nanomaterials.14,15 For instance, due to the
formation of hydrogen bonds between the polar functional
groups on GO surface and water molecules, a stable GO
colloidal suspension can be attained as compared with other
carbon-related materials. Thereby, GO has received extensive
attentions as adsorbents for the efficient removal of organic and
inorganic pollutants from aqueous solutions.12−16 However, on
the other hand, with the dramatic development and
commercialization of GO in recent years, it has increased the
possibility of human exposure to engineered graphene-based
nanomaterials mainly through inhalation, ingestion, dermal
penetration, and injection or implantation.3,4,14 Therefore, the
understanding of the physicochemical behavior of GO in the
natural environment is crucial to evaluate its potential toxicity
to human beings.
From the literature survey, the environmental fate and
biosafety of GO have been studied extensively,2−4,17,18 and the
results demonstrated that the GO presents a significant
cytotoxicity to cells and human.18 To reduce the toxicity of
GO in the environment, the coagulation of GO to form large
agglomerates is the efficient way to decrease the GO
concentration in solutions, which can be further separated by
aggregation, filtration or centrifugation easily. Coagulation has
been used in many industrial processes such as water treatment
and sludge dewatering.19−22 Addition of a coagulant can
depress the electric double layer of GO nanosheets and hence
causes the destabilization and coagulation.
Layered double hydroxides (LDHs) is a family of twodimensional anionic clays with the general formula
[MII1−xMIIIx(OH)2]x+(An‑)x/n·yH2O, where MII represents a
divalent metal cation, MIII is a trivalent metal cation and An‑ is
an interlayer anion, x is defined as the molar ratio of MII/
(MII+MIII).23−25 LDHs exhibit attractive physical and chemical
properties including effective dispersion, high specific surface
areas, high anion exchange capacities, which make them ideal
adsorbents for many cations and anions.24,26 LDHs as an
important clay mineral, which has been reported as secondary
minerals in mine areas where it can play a key role in
geochemical cycling of pollutants. It can be easily produced
using various industrial wastes and hence it could be costeffective. Moreover, it is highly expandable and as a result
exhibits excellent ability to capture inorganic anionic contaminants in natural systems. Compared with conventional
flocculants, LDHs could product from natural environment and
could aggregate with nanomaterials (i.e., GO), which could
form steady composites with target in the aqueous solutions
and form new minerals (secondary minerals). For example, the
Mg/Al LDHs can form composites with GO in natural
environments through the coagulation of GO on LDHs, which
is benefit for the migration of GO or other materials.23−26
However, to the best of our knowledge, no literature is available
to study the interaction between GO and LDHs in aqueous
solutions. Such knowledge is important to improve surface
coagulation/precipitation models and to better evaluate the
environmental fate of toxic GO nanomaterials. To further
understand the GO coagulation mechanism, herein chloridion
intercalated nanocrystallined Mg/Al LDHs (LDH−-Cl) and
nanocrystallined Mg/Al layered double hydroxides (LDH−
CO3) were used as coagulants for the GO coagulation in LDH
suspensions.
The objectives of the current study were (1) to synthesize
LDH−CO3 and LDH−Cl by a facile hydrothermal progress
and apply them as coagulants for the removal of GO from
aqueous solutions; (2) to investigate the effect of pH, ionic
strength, coagulation time, temperature and LDH contents on
GO coagulation onto LDHs; (3) to characterize the microscopic and macroscopic surface properties before and after
coagulation of GO on LDHs by using scanning electron
microscopy (SEM), transmission electron microscopy (TEM),
Fourier transformed infrared spectroscopy (FT-IR) and X-ray
photoelectron spectroscopy (XPS); (4) to discuss the
coagulation mechanism of GO on LDHs by theoretical
calculations. It is a highlight to demonstrate the coagulation
mechanism of GO on LDHs by using experiments and density
functional theory (DFT) calculations. In addition, the
coagulation behavior provides new insights of GO coagulation
by LDHs, which can further understand the behavior of GO in
natural environment.
■
EXPERIMENTAL SECTION
Materials. GO nanosheets were obtained from the natural
flake graphite (average particle diameter of 20 mm, 99.95%
purity, Qingdao Tianhe Graphite Co. Ltd., China) by using the
modified Hummers method.24,27 Typically, flake graphite and
sodium nitrate (NaNO3) were added into concentrated H2SO4
under ultrasonication and ice bath conditions. KMnO4 was
added slowly into the suspension, and the excess MnO4− anions
were eliminated by adding H2O2 (30 wt %). The desired
products were rinsed with Milli-Q water. LDH−CO3 and
LDH−Cl were prepared based on previous report with
modification.24 See detailed processes were listed in the
Supporting Information.
All the chemicals used in the experiments were purchased in
analytic purity without any further purification. Aluminum
chloride hexahydrate (AlCl3·6H2O), magnesium chloride
hexahydrate (MgCl 2 ·6H 2 O), hexamethylene tetramine
(HMT), sodium nitrate (NaNO 3 ), sodium hydroxide
(NaOH), and nitric acid (HNO3) were purchased from
Sinopharm Chemical Reagent Co., Ltd.Milli-Q water (18.2
MΩ·cm−1) was used in all the experiments.
Characterization of GO and LDHs. GO and LDHs were
characterized by the scanning electron microscopic (SEM), the
transmission electron microscopy (TEM), the Fourier transformed infrared (FT-IR) and X-ray photoelectron spectroscopy
(XPS). The SEM images were obtained on a field emission
scanning electron microscope (FEI-JSM 6320F) operated at
the beam energy of 15.0 kV. The TEM images were employed
on a JEQL transmission electron microscope (JEM-2010,
Japan) with an accelerating voltage of 200 kV. The FT-IR
spectra were mounted by using a Nicolet Magana-IR 750
spectrometer over a range from 400 to 4000 cm−1. The XPS
measurements were conducted with a Thermo Escalab 250
electron spectrometer using Al Kα radiation source (1486.6
eV) at 10 kV and 5 mA under 10−8 Pa residual pressure. The
zeta potentials of the samples were measured as a function of
pH by using a Nanosizer ZS instrument (Malvern Instrument
Co., UK) at 25 °C.
Batch Coagulation Experiments. All the coagulation
experiments were carried out in the polyethylene tubes at 25 ±
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Figure 1. Characterization of the samples. TEM images of as-prepared LDH−CO3 sample (a), LDH−Cl sample (b), GO coagulated LDH−CO3
sample (c), GO coagulated LDH−Cl sample (d); FT-IR spectra (e); Wide scan XPS of LDHs before and after GO coagulation (f); C 1s before and
after GO coagulation for LDH−CO3 and LDH−Cl respectively (g).
0.1 °C by batch technique.28 LDH−CO3 and LDH−-Cl were
selected as coagulants in the static experiments, and the samples
were settled down for 6 h. The desired pH of the suspensions
in each tube was adjusted in the range of 3.0−10.0 by adding
negligible volumes of 0.10 or 0.05 mol·L−1 HCl or NaOH.
Meanwhile, the UV−vis spectrophotometer (UV-2550, PerkinElmer) was employed to monitor the coagulation of GO and
LDHs by testing the absorbance at the wavelength of 201 nm
(Figure S1). All the experimental data were the average of
duplicate determinations, and the relative errors were about 5%.
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Figure 2. Coagulation of GO on LDH−Cl and LDH−CO3. (A) Effect of pH in the presence of 0.01 mol·L−1 NaCl, C(GO)initial = 60 mg·L−1, m/V =
1.0 g·L−1 and T = 293 K; (B) Zeta potentials of GO, LDH−Cl and LDH−CO3 as a function of pH in the presence of 0.01 mol·L−1 NaCl; Effect of
ionic strength on the removal of GO to LDH−Cl (C) and to LDH−CO3 (D), C(GO)initial = 60 mg·L−1, m/V = 1.0 g·L−1 and T = 293 K.
with GO, indicating the formation of Cl− intercalated LDH
sample.25,30−32 The peaks at 786 and 685 cm−1 are attributed to
the “Mg−O” lattice vibrations, while the peak at 553 cm−1 is
dominated by the lattice vibration of “Al−O”. The CC
stretching vibration of GO exhibits band in the energy range of
∼1630,33,34 which is also found in the spectra of LDH−Cl−GO
and LDH−CO3−GO, indicating that GO nanosheets are
coagulated successfully on the surface of LDHs.
In order to scrutinize the surface interactions of GO on
LDH−Cl and LDH−CO3, the XPS spectra were analyzed to
give further explains of the state of functional groups before and
after GO coagulation, as presented in Figure 1f. Various strong
peaks, such as Al 2p, Mg 2p, Cl 2p, Cl 1s, O 1s, and C 1s, could
be obviously observed, indicating the magnesium, aluminum
and oxygen are the predominant elements on the surface of the
samples.24 The peak centered at about 50 eV is assigned to Mg
2p from the magnesium hydroxide structure in LDH layers.35
Compared with the C 1s spectra before GO coagulation, a
noticeable increase of C 1s intensities was observed after GO
coagulation. From the high-resolution spectra of C 1s before
and after GO coagulation (Figure 1g), the C 1s spectrum can
be deconvoluted into four components at about 284.8, 286.2,
287.9, and 289.0 eV, respectively.23 The first component at
binding energy of ∼284.8 eV is attributed to the nonoxygenated ring carbon, and the other three components are
attributed to the carbon in C−O (∼286.2 eV), the carbonyl
carbon (CO, ∼ 287.9 eV) and the carboxylate carbon (O−
CO, ∼289.0 eV), respectively.36,37 Interestingly, it can be
found that compared with the C 1s spectrum of LDH−CO3
before GO coagulation, the main oxygen-containing group after
From the blank tests, the coagulation of GO on the
polyethylene tube wall was negligible. The amount of GO
coagulated on LDHs was calculated from the difference
between the initial concentration (C0) and the equilibrium
one (Ce). Removal percentage (R) was calculated as R (%) =
100% × (1 − Ce/C0), and the amount of GO coagulated on
LDHs can be expressed as Cs = (C0 - Ce)/m × V, where V is the
volume of the suspension (L), and m is the mass of LDHs (g).
■
RESULTS AND DISCUSSION
Characterization of GO and LDHs. The morphologies
and microstructures of as-prepared GO and LDHs were
characterized by SEM and TEM images. The TEM images
(Figure 1) show that large amounts of GO are coagulated on
the surface of LDHs,29 although it does not estimate the layers
of GO nanosheets exactly. As shown in Figure S2, the SEM
images of GO show that the structure of the GO agglomerate is
multilayered with the lateral size ranging from several to scores
of nanometers. According to the SEM images of LDHs before
and after GO coagulation, it is clear that the few-layered GO
nanosheets are well coagulated on the surface of LDHs.24 The
LDHs exhibit excellent coagulation capacity for the deposition
of GO in aqueous suspension.
As can be seen from the FT-IR spectra of GO and LDHs in
Figure 1e, the broad band at 3440 cm−1 is attributed to the
stretching mode of O−H group involved in hydrogen bonds or
from interlayer water molecules.22 For LDH−CO3 systems, the
strong peak at 1351 cm−1 is attributed to the vibration mode of
CO32− ions embedded in the interlayer of Mg/Al LDH−CO3,
which disappears in the spectra of LDH−Cl and its composite
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Figure 3. (A) Concentrations of the residual GO nanosheets in the supernatant as a function of LDH−Cl or LDH−CO3 concentration. C(GO)initial =
60 mg·L−1, I = 0.01 mol·L−1 NaCl, m/V = 1.0 g·L−1, pH 7.0 and T = 293 K; (B) Effect of the contact time on GO coagulation on LDH−Cl and
LDH−CO3, C(GO)initial = 60 mg·L−1, I = 0.01 mol·L−1 NaCl, m/V = 1.0 g·L−1, pH 7.0 and T = 293 K.
electrostatic attraction between the negatively charged GO
and the positively charged LDH−Cl at the same pH values.41,42
At pH < pHpzc, the surface charge of LDH−Cl (pH < 9.6) and
LDH−CO3 (pH < 9.2) are positively charged, which is
favorable for the binding of negative charged GO on LDHs,
whereas at pH > pHpzc, that is, pH > 9.6 for LDH−Cl or pH >
9.2 for LDH−CO3, the surface charge of LDHs becomes
negative and repulsive to the GO via the deprotonation of the
surface hydroxyl groups.20,23,38 With pH increasing, the
concentration of OH− in solution increases, which can compete
strongly with GO to form surface complexes on LDH−Cl or
LDH−CO3.23,43−45 As a result, the negative charged GO is
difficult to be coagulated on the negative charged surface of
LDHs at high pH values because of the electrostatic repulsion.
The removal percentages of GO on LDHs as a function of
pH under different NaCl concentrations were shown in Figures
2C, D. It can be seen that the removal percentage and
coagulation of GO slightly increases with the increase of NaCl
concentration. Moreover, compared with the maximum
removal percentage of GO at pH 6, it could be found that
the maximum removal of GO using LDH−Cl (∼100%) is
obviously higher than that of GO using LDH−CO3 (∼80%),
which is consistent with the results of zeta potential analysis
and pH-dependent coagulation. The enhancement of the
coagulation at higher ionic strength may be related to the
salting-out effect of the electrolytes rather than the electrostatic
attraction.32 The ionic strength can affect the thickness and
interface potential of GO and LDHs, influencing the bonding
of the coagulating species, and the electrolyte ions are placed in
the same plane through the formation of outer-sphere surface
complexes.45 The oxygen-containing functional groups on the
surface of LDHs and GO could form strong surface complexes,
which could improve the removal of GO from natural
environment.
GO Coagulation. To gain insight into the coagulation
mechanism of GO in aqueous solutions, the coagulation of GO
at different experimental conditions was investigated. According
to the initial states of the as-prepared samples (Figure S3), the
as-prepared samples show high dispersion stability within a few
hours. Since the concentration of coagulation agent plays an
important role in the coagulation process, the effect of LDH−
Cl and LDH−CO3 concentrations on GO coagulation was
investigated (Figure 3). From Figure 3A, one can see that the
residual concentration of GO decreases from ∼48.0 mg·L−1 to
∼1.0 mg·L−1 for LDH−Cl and ∼49.0 mg·L−1 to ∼8.0 mg·L−1
GO coagulation is O−CO with an intense peak that shifts to
higher binding energy, which is attributed to the intercalated
CO32− in the LDH phase.23 For the carbonyl carbon (CO,
∼287.9 eV), the peak shifts to lower binding energy and the
peak intensity decreases after GO coagulation, indicating that
some interaction occurs between the CO and O−CO due
to the oxygen-containing functional groups of GO.38−40 At the
same time, the peak intensity of CO increases in the C 1s
spectrum of LDH−Cl−GO as compared that of LDH−Cl,
which indicates that GO has been coagulated onto the surface
of LDH−Cl. The relative contents of different groups are
tabulated in Table S1. Based on the above analysis, it shows
that GO can be removed efficiently by LDH−Cl and LDH−
CO3 through the coagulation of GO on the surfaces of LDHs.
Effect of pH and Ionic Strength. The adsorption of GO
on both nonspecific and specific adsorbents is pH-dependent.8,12−16 The surface property of coagulants is affected by
solution pH distinctly, and the removal percentages of GO on
LDH−Cl and LDH−CO3 as a function of pH are shown in
Figure 2A. The zeta potential values of GO, LDH−Cl and
LDH−CO3 are also measured at different pH values (Figure
2B). As illustrated in Figure 2A, the removal percentage of GO
increases quickly with increasing pH at pH < 6, and then
decreases with pH increasing at pH > 6. The maximum removal
percentages of GO by LDH−Cl and LDH−CO3 are 95% and
75%, respectively, at pH ∼ 6, which is attributed to the
electrostatic interactions between the negative charge of GO
and the positive charge of LDHs. At pH < 6.0, the main
interaction was controlled by chemisorption and hydrogen
bonds may also show great effort to the coagulation of GO on
LDHs. However, above pH 6.0, the main interaction was
controlled by physisorption and electrostatic interaction also
controlled the coagulation process of GO on LDHs.20,23,24
From the blank data in the absence of LDHs, it is clear that GO
suspension is very stable and no coagulation is observed.
Notably, as the pH increases from 3.0 to 6.0, the removal
percentages of GO by LDH−Cl and LDH−CO3 are increased
simultaneously, and GO is much easier coagulated on LDH−Cl
than on LDH−CO3. The results indicate that the coagulation
behavior of GO in LDH suspensions is affected remarkably by
pH and also the surface properties of LDHs. As shown in
Figure 2B, the pHpzc (point of zero charge) of GO, LDH−Cl
and LDH−CO3 are measured to be ∼3.4, ∼ 9.6 and ∼9.2,
respectively, indicating that the LDH−Cl has a higher efficiency
in GO coagulation than LDH−CO3 due to the strong
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Figure 4. (A) Concentrations of the residual GO nanosheets in the supernatant as a function of temperature; (B) Coagulation capacity of LDH−Cl
and LDH−CO3 as a function of temperature. C(GO)initial = 60 mg·L−1, I = 0.01 mol·L−1 NaCl, m/V = 1.0 g·L−1 and pH 7.0.
LDHs is affected by temperature slightly, the removal
percentages of GO on LDH−Cl and LDH−CO3 are quite
different. According to Figure 4A and B, with the temperature
increasing from 20 to 50 °C, the dependence of temperature on
removal percentages of GO on LDH−Cl and LDH−CO3 were
various and the change of removal percentage of GO on LDH−
CO3 was greater than that of GO on LDH−-Cl, which was
attributed to the various binding process and binding energy of
LDH−Cl and LDH−CO3, suggesting that the coagulation
capacity was dependent on the initial surface properties of the
solid particles.
Interaction Mechanism of GO Coagulation. In the
interest of evaluating the coagulation mechanism and reaction
process, it is necessary to compare the coagulation behavior of
GO between LDHs and other regular coagulants.6,41,47,48 LDHs
as an important clay mineral showed great potentials for the
aggregation and coagulation of GO via to the special structure
and anion exchange capacities. Similarly to Al2O3,41 the
aggregation of GO and its deposition on LDHs depended on
the solution pH and the concentrations of background
electrolytes, and in general the presence of NaH2PO4 and
poly(acrylic acid) (PAA) improved the stability of GO with the
increase in pH values as a result of electrostatic interactions and
steric repulsion. Compared with poly(vinyl alcohol) (PVP),47
LDHs has the similar interaction process, which is dominated
by hydrogen bonding in some pH values, and the different
interaction for LDHs was controlled with electrostatic
interactions. Considering the negative charge of GO, electrostatic repulsion is a major driving force preventing the GO
sheets from aggregating.6 For another clay mineral, such as
hematite and kaolin, the possible mechanism has been reported
recently.48 An electrostatigated patching could influence the
coagulation of GO on positively charged hematite, however,
Lewis acid−base and hydrogen-bonding interaction could
dominated the coagulation of GO on kaolin, among these
materials, LDHs is an special positively charged mineral clay,
which has two-dimensional anionic structure, and the mainly
interaction mechanism of GO coagulation on LDHs is
attributed to the coexisting electrostatic interactions and
hydrogen-bonding. Interestingly, it can be seen that the
optimum pH values of coagulation process are close to 7.0,
indicating that carbon-based materials are favorable at near
neutral environmental conditions. Moreover, LDHs, as an
important minerals in certain environment, show an effective
adsorption capacity to organic compounds of arsenate,
chromate and selenate.49−51 Using LDHs as the coagulant, it
for LDH−CO3 as LDHs concentration increases from 0.2 to
1.5 g·L−1. There is a sharp fall of GO concentration at C[LDHs] <
1.0 g·L−1, and then a flat curve decreases slowly at C[LDHs] > 1.0
g·L−1. The effective polymerization and precipitation between
the LDHs and GO results in the high coagulation of GO on
LDHs at C[LDHs] < 1.0 g·L−1.22,23 At C[LDHs] > 1.0 g·L−1, most
of GO is coagulated on the surface of LDHs. Thereby the
relative low concentration of the residual GO in the suspension
decreases slowly with LDHs concentration increasing.
In order to explore the effect of the contact time on GO
coagulation on LDHs, individual coagulation process using an
optimized LDHs concentration (1.0 g·L−1) after different
contact time are performed (see Figure 3B and the digital
pictures are shown in Figure S4). The sorption curves show
clearly that the deposition/coagulation of GO on LDHs
increases rapidly within short contract time, reaching the
equilibrium after 60 min, and then remains the steady-state
with increasing contact time. Within the contact time increasing
from 5 to 60 min, the removal percentage of GO increases from
∼20% to ∼95% for LDH−Cl and from ∼10% to ∼75% for
LDH−CO3. At the initial contact time, more active sites and
functional groups on LDHs are available for the binding of GO,
the interaction of GO with LDHs is easily and quickly, and
thereby decreases the concentration of GO in aqueous
solutions. The fast coagulation velocity shows that strong
chemisorption or strong surface complexation contributes to
the coagulation of GO onto the surface of LDHs,45 which is
important for the application of the LDHs to remove GO from
natural environment in real work.
To further understand the migration and aggregation
process, batch experiments of GO coagulation onto LDHs as
a function of temperature were carried out (Figure 4). A slight
increase was found in the coagulation capacities of GO with the
increase of temperature gradually from 20 to 30 °C, whereas
above that range the GO coagulation decreases slightly when
the temperature increases, suggesting the optimal temperature
range is 20−30 °C for GO coagulation. At higher temperature,
the mobility of GO is higher and thereby results in the decrease
of GO coagulation. Modest temperature increase may make up
the energy loss during the binding process and this positive
promoting effect of temperature is beneficial for the coagulation
of GO on LDHs.45 However, if the temperature is too high, for
instance, at T > 30 °C, the Brownian motion of nanoparticles
has aggravated, which will reduce the stability of the binding
process and results in the slightly decreasing of GO
coagulation.46 Moreover, although the coagulation of GO on
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Figure 5. Optimized geometrical structures and the coagulation processes of GO on LDHs.
targets. A high adsorption energy (Ead) between GO and LDHs
(∼5.20 eV) indicates that chemisorption is the main interaction
mechanism of GO and LDHs, at the same time, physisorption
should also be considered in the explanation of coagulation
behavior. Moreover, the positive and negative of adsorption
energy could also express various coagulation behavior, for
example, a positive value of Ead shows that the coagulation
process is exothermic and the reaction system is stable,
however, a negative of Ead indicates that the coagulation process
is endothermic and the reaction system is unstable. A positive
value of Ead indicates that the coagulation process of GO on
LDHs is exothermic and the graphene composite system is
stable.32,39 The Ead values and computational models of GO
and LDHs are also summarized in Table S2 and Figures S5−S7,
respectively. The results show that the total adsorption energy
is 5.20 eV and it is 0.03 eV/ Å2 when the adsorption area is
considered. Figure 5 also shows the complexation interactions
between −OH groups of LDHs and −O− groups on the basal
plane of GO, and it demonstrates that the binding dinstances of
oxygen-containing groups of GO and LDHs are all below 3.0 Å,
indicating high chemical reactivity of −O− or −OH groups
which are easily interacted with LDHs in a natural environment.22,23,31 The comparison of the interaction values reported
previously between various molecules and GO or other carbonbased materials by DFT calculations is summarized in Table 1.
According to the results, it shows the minimum bonding
could coexistence with GO and organic pollutants of high
toxicity, trapping the carbon materials in stable minerals
permanently. Through this research, we thus firmly believe
that LDH−Cl and LDH−CO3 with exceptionally high
coagulation capacities for GO is expected to have potential
applications for the control of GO behavior and the efficient
elimination of GO in the practical GO environmental pollution
cleanup.
DFT Calculations. In order to explain the coagulation
mechanism of GO on LDHs, the interaction of GO with LDHs
was calculated by Plane-wave-based DFT calculations. The
optimized stable structures of GO coagulated on LDHs were
shown in Figure 5. Interestingly, the minimum and maximum
hydrogen bond distance between GO and LDHs are 1.693 and
2.848 Å, respectively, which indicates that strong hydrogen
bonds are formed between the functional oxygen-containing
groups of GO and the hydroxyl groups on the surface of
LDHs.32,39 The adsorption energy Ead was calculated as the
following:
Ead = EGO + E LDHs − E LDHs ‐ GO
where EGO, ELDHs, and ELDHs‑GO correspond to the total
energies of the coagulation GO, LDHs, and the combined
LDHs-GO system, respectively. Adsorption energy described
the basic energy to reach the reaction process and it could show
the possible interaction mechanism between coagulants and
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Environmental Science & Technology
Table 1. Comparison of the Interaction between Various
Molecules and GO or Carbon-Based Materials by DFT
Calculations
adsorbent
adsorbate
minimum binding distance (Å)
ref
rGO
rGO
graphene
graphene
SWCNTs
graphene
graphite
LDH
BPA
4-n-NP
ozone
Cr
Co
Pd
benzene (K+)
GO
2.708
2.677
2.800
2.518
2.028
2.020
2.800
1.693
32
32
52
53
53
54
55
this study
■
and analysis of FT-IR and XPS spectra. More detailed
information on DFT calculation (PDF)
AUTHOR INFORMATION
Corresponding Authors
*(X.K.W.) Phone: 86-10-61772890; fax: 86-10-61772890; email: [email protected] or [email protected].
*(Y.A.) E-mail: [email protected].
*(J.L.) E-mail: [email protected].
Author Contributions
Y.Z. and X.W. contributed samely to this manuscript.
Notes
The authors declare no competing financial interest.
■
distance is shorter than that of other chemicals on GO or
carbon-based materials, which indicates that the strong
interaction exists between GO and LDHs,32 and LDHs are
important coagulants for the removal of GO from natural
environment. Comparison of the results of FT-IR, XPS and
theoretical calculations, it demonstrates that hydrogen bonds
and electrostatic interactions dominate the coagulation of GO
on the surface of LDHs from aqueous solutions.
Environmental Implications. With the rapid development
and extensive applications of GO, its release into the
environment is inevitable.2 Therefore, the rapid coagulation
of GO appears particularly important and urgent due to its high
activity and toxicity in the environment. Generally, since the
coagulation behavior strongly influences the toxicity, aggregation, adhesion and migration of GO in the environment, this
research is of high significance for the evaluation of the
behavior of GO in the natural environment.4,5,41 Batch
experimental results prove that the coagulation of GO is highly
dependent on solution chemistry. The pH values of the aquatic
environment range from 5.0 to 9.0, and the notable changes in
GO removal suggest that the coagulation behavior is pHdependent.49,56 The coagulation and aggregation of GO are
attributed mainly to the changes of the surface properties of
solid particles. These findings provide crucial insight regarding
the fate and coagulation of GO in the natural environment and
would partly allow us to assess its environmental impact. At the
same time, LDH−Cl is expected to be an efficient coagulant for
GO and carbon-based materials, which could provide us a
simple method for the efficient elimination of GO from
aqueous solutions.
In summary, LDH materials present high coagulation
capacity for GO in aqueous solutions, indicating that LDH
materials, especially LDH−Cl, can be a promising material to
remove GO efficiently from aqueous solutions by using a
simple and rapid chemical coagulation. Therefore, the results of
this work might facilitate a better understanding of the
coagulation behaviors of GO and other carbon-based materials
in both natural and engineered aqueous systems, which is
crucial for the elimination of GO in aqueous solutions and
reduces the environmental toxicity of GO in the natural
environment.
■
ACKNOWLEDGMENTS
Financial support from NSFC (21225730, 91326202,
21403064, 21577032, 21377132, 21307135, 41273134), the
Fundamental Research Funds for the Central Universities
(JB2015001), the Kunlun scholarship of Qinghai province, the
Jiangsu Provincial Key Laboratory of Radiation Medicine and
Protection and the Priority Academic Program Development of
Jiangsu Higher Education Institutions are acknowledged. The
Swedish National Infrastructure for Computing (SNIC) is
acknowledged for computer time.
■
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S Supporting Information
*
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ACS Publications website at DOI: 10.1021/acs.est.6b00255.
Preparation of graphene oxide nanosheets and LDHs.
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