fu2018

Review
Hall of Fame Article
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Novel 2D Nanosheets with Potential Applications in Heavy
Metal Purification: A Review
Liangjie Fu, Zhaoli Yan, Qihang Zhao, and Huaming Yang*
cadmium (Cd), mercury (Hg), arsenic
(As), and chromium (Cr) are the main
causes for most heavy metal-related diseases, for which the toxicity mechanisms
and their relationship to the induction of
oxidative stress were reported.[5]
Various techniques, including chemical
precipitation, ion exchange, adsorption,
flotation, membrane filtration, and electrochemical treatment, have been used
for the efficient removal of heavy metals
from water,[3] in which adsorption is one
of the most attractive options. Chemical
precipitation is widely used for high concentration wastewater for the low capital cost and simple process, while ion
exchange resins are limited for large scale
use due to the high cost and secondary
pollution under regeneration. The adsorption method, although highly sensitive on
the type of adsorbents, is one of the most
attractive options.
To date, various novel nanomaterials, such as graphene
family members, carbon nanotubes, carbon nanofibers,
fullerene, metal, and metal oxide nanoparticles, and others
were used for water protection.[4] Active carbons, produced
from various biomaterials or industrial wastes were widely
studied for water purification.[6,7] Low-cost adsorbents, such
as raw and modified lignocellulosic materials,[8] were found
to exhibit high adsorption capacities compared to commercial
activated carbon.[9,10] So far, carbon materials and their composite materials are some most active and hitherto extensively
studied materials.[11–15]
2D nanosheets are an emerging class of nanomaterials
that possess sheet-like morphologies, which are composed by
layer structures with lateral size larger than 100 nm and thickness less than 10 nm. It is well known that the formation of
nanosheets with 2D structures from inorganic compounds will
increase their specific surface area due to the decrease of the
dimension of nanoparticles and meanwhile alter their physical
and chemical properties.[16–19] To date, 2D nanosheets of various inorganic compounds were reported with great diversity
in physicochemical, electronic, and surface chemical properties, which make them highly promising in the field of energy
storage,[17] electronics and catalysis,[16,20,21] as well as in water
purification.[13,22–25] In this review, we discuss the advantages
of the composition and structure of various 2D nanosheets for
heavy metal purification, and then discuss current status of
their applications and future perspectives.
Over the past decades, extensive studies have been carried out for the
design of advanced materials for water purification. Heavy metal pollution
in water is a serious global environmental problem due to their toxicity
and carcinogenicity. With the discovery of graphene, novel 2D nanosheets,
derived from the wide variety of traditional materials, have emerged as some
of the most promising candidates for heavy metal purification. This review
summarizes the recent progress on the novel 2D nanosheets with their
applications in heavy metal purification. First, the authors introduce the
unique advances on 2D nanomaterials, followed by the description of their
composition and crystal structures. Some novel 2D nanosheets with great
success in heavy metal purification field are then summarized, including
insights on their advantages over traditional materials and limitations in
practical applications, along with some advances in interfacial structure
modifications for future work. The functional design of more and more novel
2D interfacial materials with unique properties is still demanding for future
industrial applications.
1. Introduction
Heavy metal pollution in water, especially drinking water, is a
serious global environmental problem due to their toxicity and
carcinogenicity. They are commonly defined as metals with
atomic weights between 63.5 and 200.6 g mol−1 or a density
of more than 5 g cm−3.[1–4] These heavy metals are difficult to
degrade, easy to accumulate in living organisms through the
food chain, and tend to cause greater risk to human health
and environment. Among various heavy metals, lead (Pb),
Dr. L. Fu, Dr. Z. Yan, Dr. Q. Zhao, Prof. H. Yang
Centre for Mineral Materials
School of Minerals Processing and Bioengineering
Central South University
Changsha 410083, China
E-mail: hmyang@csu.edu.cn
Dr. L. Fu, Dr. Z. Yan, Dr. Q. Zhao, Prof. H. Yang
Hunan Key Lab of Mineral Materials and Application
Central South University
Changsha 410083, China
Prof. H. Yang
State Key Lab of Powder Metallurgy
Central South University
Changsha 410083, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/admi.201801094.
DOI: 10.1002/admi.201801094
Adv. Mater. Interfaces 2018, 1801094
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2. Composition and Structure of Heavy Metal
Complex
The molecular form of heavy metal complex can be divided
into cationic and anionic types depending on their charge
state. The heavy metal ions normally existed in the ion form of
Mz+ species in nature water and chelated with other groups in
acid wastewater, while polynuclear species might form under
concentrated level or alkali conditions.[2,26,27] Among the toxic
heavy metal ions, ions like Pb, Cd, Hg, and Cu can simply exist
in the form of Mz+ or bonded with one or more hydroxyl groups
under hydrolysis. These cations exist simply as M2+ at pHs
well below 7, but precipitate to hydroxylated species or soluble
hydroxides.[28]
The aqueous species of Pb(II) and Cd(II) are influenced by
the solution pH and the coexisting anions. The hydroxylated
species and the polynuclear species may also occur at high
pH or concentration level above 0.01 m due to the hydrolysis
of ions.[17] Hg can be Hg+ or Hg2+, and has a strong tendency
to bind with halide ions and form toxic RHg+ or self-perpetuate
to form dimer species. Cr(VI), as a common contaminant in
surface and ground water, was widely used in industries. In
aqueous solution, the toxic Cr(VI) exists in different ionic forms
primarily as HCrO4−, Cr2O72−, CrO42−, and H2CrO4, dependent
on the Cr(VI) concentration and pH value. One preferred
method to purify Cr(VI)-contained wastewater is the photocatalytic/catalytic reduction of Cr(VI) to Cr(III), which is less toxic
and can be easily precipitated and removed.[29] At pH range
1.0–6.5, HCrO4− is the dominant species, while at pH above
6.0 CrO42− species increases. Cr2O72− is the dominant species
at Cr(VI) concentration above 0.01 m and low pH value, while
H2CrO4 predominates at pH value lower than 0.8.[30] Arsenic,
commonly existed as arsenite and arsenate in natural water, are
referred as As(III) and As(V).[31] As(III) favors the bonding with
oxides and nitrides, while As(V) favors sulfides, which are both
sensitive to pH and redox conditions. As(V) species include
H3AsO4, H2AsO4−, HAsO42−, and AsO43− while As(III) species
include As(OH)3, As(OH)4−, AsO2OH2−, and AsO33−. Normally,
for heavy metal ions with multiple valence states, high-valent
species predominate and are stable in oxygen rich aerobic environments, while low-valent species predominate in moderately
reducing anaerobic environments such as groundwater.
3. Composition and Structure of 2D Nanosheets
Decreasing the dimension of nanomaterials could increase the
specific surface area and alter structure crystallinity, which will
affect the surface structure and thermodynamics properties,
as well as chemical reactivity.[4] The composition and crystal
structures of various 2D nanomaterials which can be exfoliated from their corresponding bulk materials are well known,
as well as their synthetic methods. The well-established synthetic methods for 2D nanosheet include micromechanical
cleavage, chemical vapor deposition, wet-chemical syntheses,
and liquid exfoliation under mechanical force, ion intercalation,
ion exchange, oxidation, or selective etching, all of which can
be categorized into two categories: top-down and bottom-up
methods.[19]
Adv. Mater. Interfaces 2018, 1801094
Liangjie Fu is currently
an associate professor at
Central South University,
China, where he received his
Ph.D. in Materials Science
in 2014 under the guidance of Professor Huaming
Yang. He was a postdoctoral
fellow at the Peter A. Rock
Thermochemistry Lab and
NEAT ORU at the University
of California, Davis from
2015–2017. His research mainly focuses on the synthesis
and theoretical study of clay mineral materials and metal
oxides in the areas of catalysis, energy, and drug delivery.
Zhaoli Yan received his
B.S. degree from Henan
Polytechnic University in
2011 and Ph.D. degree from
Central South University in
2018 under the guidance of
Professor Huaming Yang.
He is currently a lecturer
in the College of Chemistry
and Chemical Engineering at
Xinyang Normal University.
His research interests include
2D porous clay-based composites for adsorption and
catalysis.
Huaming Yang is a
professor at the School
of Minerals Processing
and Bioengineering at Central
South University, China.
He completed his Ph.D. at
Central South University
of Technology in 1998, and
went to the University of
Bristol as a visiting professor
from 2008–2009. His research
interests involve the synthesis strategy and application of advanced materials from
natural minerals, and the functional design and interfacial
structure of mineral materials at atomic-molecular level.
To date, since the rise of graphene family, various 2D materials
are synthesized by top-down exfoliation method (Figure 1A) from
their layer structures.[32] 2D materials such as graphene/graphene oxide/reduced graphene oxide (G/GO/RGO), hexa­gonal
boron nitride (h-BN), graphitic carbon nitride (g-C3N4), transition
metal dichalcogenides (TMDs), transition metal carbides/carbonitrides/nitrides (MXenes), clay minerals, layered double hydroxides (LDHs), metal oxide/hydroxide, metal organic frameworks
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Figure 1. A) Schematic description of the main liquid exfoliation mechanisms: ion intercalation, ion exchange, and sonication-assisted exfoliation.
Reproduced with permission.[32] Copyright 2013, Science. B) The pristine and modified atomic structures of graphene, BN, g-C3N4, and MoS2
nanosheets. C) The pristine and modified atomic structures of MXene, MgO, kaolinite, montmorillonite, and LDH nanosheets.
(MOFs), covalent organic frameworks (COFs), and other novel
2D hybrids or nanocomposites were widely studied for heavy
metal purification, among which the first four are graphene-like
2D nanomaterials that exhibit versatile properties similar to graphene. For these layered materials, the atoms between each layer
structures are weakly bonded to each other via van der Waals
forces, while the atoms in each layer are strongly bonded via chemical bonds. To date, the adsorption properties of above four layer
materials were improved by oxidation (O, OH, COOH groups),
reduction (NH2, CN groups), or grafting of some functional
groups (cetyltrimethylammonium bromide (CTAB), ethylene
Adv. Mater. Interfaces 2018, 1801094
diamine tetraacetic acid (EDTA)) and nanoparticles (zero-valent
iron, Fe3O4, MnFe2O4) on surfaces of these nanosheet materials.
It should be noted that the corresponding zeta-potential of these
2D nanosheet materials is determined mainly by the charge
states of the functional groups. It is well known that for materials
with similar structure and chemical components, some slight
change in the synthesis details might change adsorption capacity
and other properties significantly. For example, the bond lengths
of surface bonds at the surface structure of nanosheet are generally slightly longer than that in bulk structures, which are more
reactive and accessible for interface modifications.
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Graphene is a monolayer graphite, in which each C atom is
covalently bonded to three neighboring ones through σ-bond
(bond distance ≈ 1.42 Å) and forming hexagonal 2D network
with a layer distance of about 3.4 Å (Figure 1B). h-BN and
g-C3N4, with similar hexagonal structure as graphene, can be
regarded as doped graphene by B and N atoms, although the
latter possesses lots of defective sites. Interestingly, TMDs
also have a hexagonal surface structure as graphene similar
to graphene, only that the in-plane anion atoms in each layer
are splitted into two identical layers. Generally, there are some
changes of physicochemical properties accompanied with the
exfoliation process. First, the transformation from multilayer
structure of bulk layer materials into few-layered or monolayer
structure of nanosheet materials will greatly increase the surface area (≈2630 m2 g−1 for graphene), thermal, and electrical
conductivities. Second, the surface modification using various
inorganic or organic functional groups will introduce novel
adsorption properties which vary with the specific functional
groups.
The structure of MXenes, clay minerals, LDHs, metal oxides,
and other nanosheet materials are much more complicated,
for which the surface modifications can be more versatile
(Figure 1C). MXenes are a new class of 2D nanostructured
materials with general formula Mn+1XnTx (n = 1–3), where M
is transition metal (Ti, Ta, Nb, Zr, Hf, V, Cr, Mo, Sc, etc.), X
is carbon and/or nitrogen, T is surface terminations anion
(O, OH, F). The variation of the valence states of M, X, and
T atoms would render the surface charge distributions on
MXenes and thereby change the heavy metal adsorption properties. The size and shape of nanoparticles are two important
factors to affect the adsorption performance. The novel metal
oxide nanomaterials with shape-controlled, highly stable, and
monodisperse properties have been widely studied during the
last decade.[33] 2D metal (hydr)oxide nanosheets are formed via
chemical bonding in three dimensions,[19] for example, TiO2,[34]
MoO3,[35] NiO,[36] MgO,[37–39] Fe2O3,[40] AlOOH,[41] FeOOH,[42]
and Mg(OH)2.[43] A wide variety of cation-exchangeable layered
transition metal oxides and anion-exchangeable layered hydro­
xides, have been exfoliated into individual nanosheets.[44] They
are classified as some promising ones for heavy metals removal
from aqueous systems, partly because of their increased surface
areas and their active sites under size quantization effect.[45]
Clay minerals are nature raw material, connected by tetrahedral silicate layers and octahedral hydroxide layers.[46]
Depending on the physiochemical properties of the clay minerals, they have received widespread attribution in remediation
of wastewater. Normally, nature clay can be classified as 1:1
type (kaolinite) which consist of tetrahedral layer linked with an
octahedral layer, and 2:1 type (montmorillonite) which consist
of one octahedral layer between two tetrahedral layers. Kaolinite
[A12Si2O5(OH)4] is composed of a tetrahedral silica layer and a
dioctahedral alumina layer.[47] The continuous sheet structure
is bound hydrogen bonding of the hydroxyl face of one flake
to the oxygen face of the adjacent flake. The permanent negative charge can be existed on the surface of kaolinite due to the
isomorphic replacement in the silica tetrahedral sheet or in the
alumina octahedral sheet.[48] In addition, the protonation and
deprotonation of hydroxyl groups led to variable charge in the
alumina layer and the edge sites. Montmorillonite (MMT) has a
Adv. Mater. Interfaces 2018, 1801094
layered crystal structure and one of the most abundant natural
clay.[49] It is a promising adsorption material due to the large
specific surface area, potential for ion exchange, and specific
surface properties.
LDHs, also known as anionic clays or hydrotalcite-like
compounds, are often described by the general formula,
[M2+1−xM3+x(OH)2]x+An−x/n.yH2O, where M is metal ion and A
represents an interlayer anion.[50,51] The structure of the LDHs
is analogous to 2D sheets with edge-sharing octahedra form.
The trivalent metal ions can be substituted by divalent metal
ions, which introducing a net positive charge to the interlayer
gallery balanced by the anions.[52] Similarly, LDHs can also be
intercalated by various types of anions.[53] The excellent adsorption capacity for heavy metals is attributed to the special structure and high anionic exchange ability of these materials.[54,55]
In order to obtain efficient, selective, and stable adsorbents, the
LDHs can be modified by various methods, including intercalation, surface functionalization, exfoliation, engineering
nanocomposites.[56–60]
Recently, Jeon et al.[61] used a nanocrystal-seeded growth
method triggered by a single rotational intergrowth to synthesize high-aspect-ratio zeolite nanosheets with a thickness of
5 nm. These high-aspect-ratio nanosheets allow the fabrication
of thin and defect-free coatings that can be intergrown to produce high-flux and ultraselective zeolite ZSM-5 (MFI) membranes that surpass other MFI membranes prepared from exfoliated nanosheets or nanocrystals.
4. 2D Nanosheets Applied for Heavy Metal
Purification
Various adsorbents such as activated carbon, zeolites, clay
minerals, molecular sieves, and other inorganic nanomaterials
have been extensively studied for removing heavy metal pollutants from aqueous solutions. However, these adsorbents usually suffer from low removal capacities, slow capture kinetics,
difficult separation, secondary environmental pollution, or
unsatisfactory recycling ability for their practical applications.
Therefore, there is an urgent demand to develop novel adsorbents with higher efficiency and reusability in order to remove
rapidly and efficiently toxic heavy metal ions for water remediation applications.
4.1. Graphene Families and Their Composites
Pristine graphene is a super-hydrophobic material, which is
difficult to disperse in water for the pollutant purification.[62]
The graphene and graphene-based composites for the adsorption of heavy metal ions are listed in Table 1, along with some
other novel 2D nanosheet materials. The adsorption capacities of pristine few-layered graphene for heavy metal adsorption varied upon exfoliation method, calcination temperature,
and layer number of the graphene used.[63,64] For doped graphene, Liu et al.[22] reported highly porous N-doped graphene
nanosheets for removal of multiple heavy metal ions (Pb2+,
Cd2+, Cu2+, Fe2+, etc.) in water with a wide range of concentrations (0.05–200 ppm). The as-synthesized nanosheet exhibited
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exchange, while the electrostatic interaction
could also contribute to the whole interaction.
For the colloidal stability and aggregation of
GO
during the heavy metal removal process,
Adsorbents
Solution
Sorption capacity Equilibrium
SBET [m2 g−1]
it
is
also observed that heavy metal cations
−1
time [min]
condition
[mg g ]
Cr(III), Pb(II), Cu(II), Cd(II), and Ag(I) would
[63]
22.42
–
Graphene nanosheets
≈400
30 °C, pH = 4
destabilize GO suspension more aggressively
120
842
–
Few-layered GO[64]
20 °C, pH = 6
than common cations Ca(II), Mg(II), Na(I),
–
400
–
2- or 3-layered graphene[64]
20 °C, pH = 6
and K(I) (Figure 2B).[70] Heavy metal cations
[23]
can easily cross electric double-layer suppres430
328
15
GO
25 ± 2 °C, pH = 6.8
sion, bind to GO surface, and then change
623
479
15
EDTA-GO[23]
25 ± 2 °C, pH = 6.8
the surface potential, which would lead to the
EDTA-RGO[23]
730
204
–
25 ± 2 °C, pH = 6.8
transformation of GO nanosheets to 1D tube–
673
20
GO-MnFe2O4[74]
25 °C, pH = 5
like carbon material, 2D multiple overlapped
–
644
30
GO plane, or 3D sphere-like particles during
25 °C, pH = 6
3D graphene/δ-MnO2[84]
[185]
adsorption. Kinetics study showed that the
–
305.28
360
g-C3N4
30 °C, pH = 6.3
destabilizing capability of metal cations in
[101]
–
137
30
Fe3O4 and g-C3N4
25 °C, pH = 6
line with their adsorption affinity with GO is
2078
225
–
Activated BN[94]
pH = 6
controlled by their electronegativity and hydra196.5
536.7
720
Nanosheet-structured BN spheres[97]
pH = 5.5
tion shell thickness. Sun et al. investigated the
interaction mechanism between Eu(III) and
–
2
2D layered alk-MXene[13]
30 °C, pH = 6.5
≈120
GO nanosheets (Figure 2C).[71] The maximum
[103]
–
Room temperature,
288
–
Flowerlike WSe2
adsorption
capacity of Eu(III) on graphene oxide
pH = 4–5
nanosheets
was 175.44 mg g−1. The thermodyWS2 microspheres[103]
–
Room temperature,
386
–
namic study suggested that Eu(III) adsorppH = 4–5
tion process on graphene oxide nanosheets
MoS2/CeO2[107]
–
333
30
25 °C, pH = 2
(GONS) was endothermic and spontaneous,
168.33
479
27
DPA-LDH[186]
25 °C, pH = 6
and extended X-ray absorption fine structure
145.5
Room temperature,
124.22
5
Fower-like γ-AlOOH[41]
(EXAFS) results indicated the formation of
pH not adjusted
inner sphere surface complexes for Eu(III)
adsorption with 6–7 O atoms bonded at a disFlowerlike MgO[38]
72
Room temperature,
1980
30
pH = 7
tance of ≈2.44 Å in the first coordination shell.
Furthermore, numerous efforts have been
Functionalized 2D clay derivative[48]
259
184
5
25 °C, pH = 5.8–6.0
made to further functionalize graphene
derivatives. Metal/metal oxides/metal hydroxides (especially magnetic materials, e.g., Fe0,[72] Fe3O4,[12,73] and
a high removal efficiency (90–100%), fast removal (30 min),
high capacity (>500 mg g−1), and good recycling performance
MnFe2O4[74]) and organic functional groups (sulfonic acid, poly2+
2+
(10 cycles, 99% retention) to remove Pb and Cd ions.
acrylic acid poly-dopamine, and carboxylic acid)[12,22,75–81] have
To improve the solubility of graphene, the graphene derivabeen attached to the carbon backbone of GO/RGO nanosheets
tives, e.g., GO and RGO, have been developed and synthesized,
by various methods.[62] Magnetically modified GO nanosheets,
which can form the homogeneous aqueous suspension. GO,
formed by the deposition of magnetite (Fe3O4) on GO, were
the oxidized form of graphene, which is usually prepared by the
used for the efficient regenerative removal of heavy metals
Hummers method,[65] has a large surface area with various reacfrom aqueous solutions. The main strategies using such materials as adsorbents for the removal of metal ions from aqueous
tive oxygen functional groups (e.g., epoxy, hydroxyl, carbonyl, and
solutions are schemed in Figure 3A.[82] Recently, Zhao et al.[83]
carboxyl groups). GO holds great promise in adsorption-based
remediation approaches because of its extremely high surface
reported a generalized and facile strategy to synthesize largearea and abundant oxygen-containing functional groups.[66,67]
size 2D metal oxide nanosheets with controllable thickness and
unique surface chemical state on GO (Figure 3B). The thickness
The high dispersion stability of graphene oxide makes it an
(less than 5 nm) of metal oxide (MO) nanosheets synthesized
excellent adsorbent for heavy metal removal (Figure 2A).[64] The
by this method can be controlled by the concentration of metal
maximum adsorption capacity of Pb(II) ions on few-layered grasalts, which can have great impact on the improvement of the
phene oxide (FGO) was calculated as high as 842 mg g−1 at 293 K
adsorption abilities of various MO nanosheets.
from the Langmuir model, which is much higher than pristine
Kumar et al. reported that MnFe2O4 loaded single-layer gragraphene. Besides, the hydrazine reduced GO could stably disperse in ammonia solution through electrostatic stabilization,
phene oxide nanosheet (Figure 3C) had exceptional adsorpdue to the residual oxygen content and carboxylic groups.[68]
tion performance for efficient removal of Pb(II), As(III), and
As(V), due to the combination of the layered nature (large
The presence of these hydrophilic functional groups not only
surface area) of the nanosheet, the easy magnetic separaincreased the dispersion of GO in water but also made GO a
tion of MnFe2O4 nanoparticles (Figure 3D), and the excellent
superior adsorbent for various heavy metal ions.[69] The main
strength of adsorption was the chemical adsorption under ion
adsorption abilities of both the MnFe2O4 nanoparticles and GO
Table 1. Summary of previous studies on the adsorption of Pb(II) ions by various 2D
nanosheet materials. The surface area, solution condition, maximum sorption capacity, and
equilibrium time are listed.
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Figure 2. A) The dispersion status of graphite and FGO in solutions after 5 months aging. The adsorption isotherms of Pb(II) on FGO and graphene
at T = 293, 313, and 333 K. Reproduced with permission.[64] Copyright 2011, Royal Society of Chemistry. B) Bulk flocculation of GO in aqueous solution, isotherms of Pb(II), Cu(II), and Cd(II) on GO, and the interactions of heavy metal cations with GO nanosheets. Reproduced with permission.[70]
Copyright 2016, American Chemical Society. C) The interaction mechanism between Eu(III) and graphene oxide nanosheets, and EXAFS results for the
samples. C) Reproduced with permission.[71] Copyright 2012, American Chemical Society.
nanosheet (Figure 3E).[74] Thermodynamic results indicated
a spontaneous and endothermic process in which the adsorption kinetics increased with the temperature. The highly reactive and corrosive nanoscale zero-valent iron particles were
supported on reduced graphene oxides (NZVI/RGOs) using a
plasma reduction method to improve the reactivity (50 min),
stability, and capacity (425.72 mg g−1) of NZVI toward Cd(II)
adsorption.[72] The NZVI/RGOs maintained high removal performance after four cycles of regeneration using plasma reduction technique. Fe3O4 nanoparticles were also supported on
RGO nanosheets using a facile hydrothermal self-assembly
method to enhance the removal of various heavy metal ions.[73]
The dispersed Fe3O4 nanoparticles in RGO/Fe3O4 composite
with plenty of active adsorption sites were effectively protected
against oxidation in wastewater, which also added a superparamagnetic property to the RGO nanosheets for fast magnetic recycling. Similar phenomena were also found for MnO2.
Adv. Mater. Interfaces 2018, 1801094
Birnessite MnO2 nanosheets were homogenously deposited
on the GO surface via an in situ solution-phase deposition
method.[84] While δ-MnO2 loaded graphene nanosheet exhibited maximum adsorption capacity for Pb(II) ions of 781 mmol
g−1,[85] graphene/δ-MnO2 showed adsorption capacity as
large as 643.62 mg g−1 for Pb(II), much larger than the corresponding pristine 3D graphene and δ-MnO2 nanosheets.[84] The
nanocomposite exhibited fast kinetics, large capacity, and excellent reusability toward heavy metal purification.
Madadrang et al. successfully grafted the EDTA, a wellknown functional group which can form stable chelated with
metal ions, onto GO nanosheet (Figure 4A).[23] The nanocomposite exhibited higher adsorption capacity (479 mg g−1) than
pure GO (328 mg g−1), activated carbons and CNTs for Pb(II)
adsorption, as well as higher removal efficiency (equilibrium
time 20 min, equilibrium concentration 0.5–5 ppb). This
phenomena can be explained by the higher ion exchange ability
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Figure 3. A) Main strategies to apply graphene-based materials as adsorbents for heavy metal purification. Reproduced with permission.[82] Copyright
2015, Royal Society of Chemistry. B) The strategy to synthesize large-size ultrathin 2D MO nanosheets on GO. Reproduced with permission.[83]
Copyright 2017, Royal Society of Chemistry. C) Atomic force microscopy (AFM) images of GO and GO−MnFe2O4 hybrid (GONH). D) Hysteresis curves
and E) adsorption capacities of MnFe2O4 nanoparticles (NP) and GONH for Pb(II), As(V), and As(III) ions. D,E) Reproduced with permission.[74]
Copyright 2014, American Chemical Society.
of EDTA compared to that of OH and the higher stability constant of Pb(II)–EDTA complex on EDTA-grafted GO compared
to that of Pb(II)–OH complex on GO.
2-imino-4-thiobiuret-partially reduced graphene oxide
(IT-PRGO) by chemical modification of GO showed exceptional
selectivity for Hg(II) with a capacity of 624 mg g−1 (Figure 4B),[86]
along with the capacities of 101.5, 63.0, and 37.0 mg g−1 for
Pb(II), Cr(VI), and Cu(II), respectively. Desorption studies
demonstrated the easy regeneration of IT-PRGO, while the sorption kinetics suggested a chemisorption between amidinothiourea groups and metal ions. The adsorption capacities for Cu(II),
Cd(II), Pb(II), Hg(II) were found 87, 106, 197, and 110 mg g−1,
respectively. Furthermore, GO nanosheets modified with thiol
groups by diazonium chemistry were reported to adsorb sixfold
higher concentration of Hg(II) than GO nanosheets due to the
strong adsorption properties of functional groups (Figure 4C).[87]
PEI-PD/GO composite nanosheets, which were produced by
the grafting of polyethylenimine (PEI) brushes onto polydopamine (PD) coated GO (Figure 4D), exhibited an improved performance for adsorption of heavy metal ions as compared to
PEI-coated GO and pure GO.[88] Besides, the covalent functionalization involves the reaction of functional molecules with the
oxygenated groups on GO/RGO surface and noncovalent functionalization involves the introduction of van der Waals force or
Adv. Mater. Interfaces 2018, 1801094
the π–π interaction between RGO and stabilizers (e.g., singlestrand DNA (ssDNA)[89] or 1-pyrenebyturate (PB−)[90]), which
will change their solubility and electronic properties.
4.2. h-BN and g-C3N4 Nanosheets
h-BN and g-C3N4 were widely studied for the removal of
organic pollutants and heavy metal ions from water due to
their polarity, versatility, and high surface area similar to graphene.[91,92] Recently, Chengchun and co-workers have prepared porous BN whiskers,[93] activated BN whiskers,[94] 2D
activated BN sheets,[95] and 3D carbon-BN[96] for heavy metal
purification. The 2D fluorinated activated BN nanosheets,
with low equilibrium Cr(III) concentration (0.052 mg L−1)
and high uptake capacity (up to 387 mg g−1), were promising
candidate for heavy metal purification, which can be attri­
buted to the various surface defects and oxygen-containing
functional groups on the BN sheets as revealed by theoretical
calculations (Figure 5A). Liu at el.[97] also prepared nanosheetstructured BN spheres by a simple catalyzing thermal
evaporation approach. The adsorption capacities of the BN
nanosheet for Cu(II), Pb(II), and Cd(II) are up to 678.7,
536.7, and 107.0 mg g−1, respectively. Novel O-doped BN
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Figure 4. A) The structure of EDTA-grafted GO. The maximum Pb2+ adsorption capacity and pH dependent capacity. Reproduced with permission.[23]
Copyright 2012, American Chemical Society. B) Efficient removal of heavy metals from water with high selectivity for Hg(II) by 2-imino-4-thiobiuret−partially
reduced graphene oxide (IT-PRGO). Reproduced with permission.[86] Copyright 2017, American Chemical Society. C) Thiol-functionalized graphite oxide
(GO-SH) for Hg(II) purification: schematic of functionalization chemistry on GO, and corresponding nuclear magnetic resonance (NMR) analysis.
Reproduced with permission.[87] Copyright 2011, American Chemical Society. D) Polydopamine-mediated surface-functionalization of graphene oxide
(PEI-PD/GO). Reproduced with permission.[88] Copyright 2015, Elsevier.
(BNO) nanosheets with more active sites and increased conductivity were prepared via the direct reaction of CuB23 with
NOCl at room temperature in ionic liquids (Figure 5B).[98]
The obtained BNO demonstrated excellent performance as a
capacitive deionization electrode to remove Cd(II) in a wide
range of concentrations (0.05–600 ppm) with a high adsorption capacity (2281 mg g−1). The unique structure and the
NO− groups promoted the electrosorption of Cd(II), and
simultaneously removed the heavy metal ions (Cd(II), Pb(II),
Ni(II), Co(II), Cu(II)) within 20 min.
Graphitic-C3N4 (g-C3N4) nanosheets prepared by a simple
thermal and ultrasonic method showed high adsorption
capacities of 137.4 mg g−1 for Co(II), 136.9 mg g−1 for Ni(II),
134.1 mg g−1 for Cu(II), and 138.0 mg g−1 for Zn(II), respectively (Figure 5C).[99] The adsorption mechanism was confirmed mainly dominated by inner sphere complexation,
which was an endothermic and spontaneous process. The
g-C3N4 nanosheet also exhibited a maximum adsorption
capacity of 94.4 mg g−1 toward Cd(II) in aqueous solution.[100]
Adv. Mater. Interfaces 2018, 1801094
The adsorption occurred under the coordination of the unoccupied 5s orbital of Cd(II) with the lone-pair electron of the
triazine ring units of g-C3N4.
Similar with the case of graphenes, the magnetic nanoparticles were loaded on g-C3N4 nanosheet to solve the problem of
separation and recovery in industrial applications. The adsorption capacity of Fe3O4/g-C3N4 nanocomposite maintained
88.9% after five cycles of regeneration by EDTA, suggesting
a potential adsorbent for Zn(II), Pb(II), and Cd(II) ions.[101]
The loading of α-Fe2O3 on g-C3N4 nanosheet by a facile
hydrothermal method resulted in a stable α-Fe2O3/g-C3N4
nanocomposite with higher visible-light photocatalytic activity
than pure g-C3N4 and α-Fe2O3 for Cr(VI) reduction.[102] The
enhanced photocatalytic activity was attributed to the wellmatched band structure and interfacial bonding between
g-C3N4 and α-Fe2O3 (Figure 5D), which increased the charge
transfer and separation of the photogenerated carriers, as confirmed by the lower photoluminescence intensity and higher
photocurrent density.
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Figure 5. A) Cr(III) adsorption by fluorinated activated boron nitride nanosheets. Reproduced with permission.[95] Copyright 2014, Royal Society
of Chemistry. B) O-doped BN nanosheets for efficient removal of heavy metal ions. Reproduced with permission.[98] Copyright 2017, Royal Society
of Chemistry. C) g-C3N4 nanosheets for the highly efficient scavenging of heavy metals. Reproduced with permission.[99] Copyright 2018, Elsevier.
D) α-Fe2O3/g-C3N4 composite for efficient photocatalytic reduction of Cr(VI) under visible light. Reproduced with permission.[102] Copyright 2015,
Elsevier.
4.3. TMD Nanosheets
Due to the strong affinity of the chalcogen to heavy metal
ions and the capability of the semiconducting metal dichalcogenides to harvest light in the visible and short-wavelength
near-infrared regions, TMDs have been used in the adsorption
of heavy metals or photocatalytic reduction of Cr(VI).[103,104]
Flowerlike WSe2 and WS2 microspheres, prepared by a facile
and scalable one-pot solvothermal method, exhibited remarkable uptake capacities for Pb2+ (288 mg g−1 for WSe2 and
386 mg g−1 for WS2) and Hg2+ (1512 mg g−1 for WSe2 and
1954 mg g−1 for WS2), due to the abundant chalcogen ligands
with innate reactivity toward soft heavy metal ions Hg(II) and
Pb(II).[103]
Ai at el.[105] prepared a novel MoS2 nanosheets with widened
interlayer spacing (W-DR-N-MoS2), which showed an extremely
high Hg(II) adsorption capacity close to the theoretically
capacity (2506 mg g−1) and an ultrahigh distribution coefficient
value (3.53 × 108 mL g−1). Remarkably, this modified MoS2
could efficiently reduce the Hg(II) concentration of industrial
wastewater (126 ppb) to an extremely low value (0.055 ppb)
under the presence of plenty of competitive cations (Figure 6).
The difference in Pb (Pb2+ and Pb4+) adsorption using the bulk
and nanosheet forms of MoS2 was investigated at room temperature.[106] While the bulk MoS2 did not show any reactivity, the
2D nanosheet MoS2 showed quite rapid kinetics that resulted
in the formation of identical products PbMoO4 with different
morphologies within a few seconds. Furthermore, the unusual
reactivity of MoS2 nanosheets was retained in its supported
form, which can lead to the effective and fast adsorption of
toxic lead from water.
Adv. Mater. Interfaces 2018, 1801094
Ultrathin molybdenum disulfide nanosheets loaded with
cerium oxide nanoparticles (MoS2/CeO2), synthesized using
a two-step hydrothermal reaction, exhibited a high removal
capacity (333 mg g−1 at pH 2.0), excellent selectivity, and reusability for Pb(II) purification.[107] The new composite material
combined the unique adsorption ability of MoS2 nanosheets
and CeO2 nanoparticles to synergistically enhance the uptake
capacity and selectivity toward Pb(II). Zhang at el. synthesized
the SnS2/SnO2 nanoheterojunctions by a simple and costeffective one-step hydrothermal method. Compared to SnS2
nanosheets,[108,109] SnS2/SnO2 with 70 mol% SnS2 displayed
the highest photocatalytic activity for the reduction of aqueous
Cr(VI) under visible-light irradiation. The improved photocatalytic efficiency of SnS2/SnO2 nanoheterojunctions can be
largely ascribed to the enhanced separation of photogenerated
electrons and holes through interfacial charge transfer.
4.4. Transition Metal Carbide/Carbonitride/Nitride (MXene)
Nanosheets
Ti3C2Tx (T = OH or F), a reactive member of MXene family,
was proved to be good heavy metal adsorbent due to the unique
layer structure with large surface area and abundant Ti-OH
groups with high sorption affinity toward heavy metals (denoted
as [TiO]H+).[110] The sorption selectivity was attributed to the
strong metal–ligand interaction with heavy metal ions.
Recently, a 2D layered alk-MXene (Ti3C2(OH/ONa)xF2−x)
material prepared by chemical exfoliation is followed by alkalization intercalation of MXene (Ti3C2Tx), which exhibits excellent
sorption behavior for Pb(II) ions when competing cations Ca(II)
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Figure 6. A) Structural characterization of MoS2: a–d) W-DR-N-MoS2, e–h) commercial MoS2 powder, i–l) DF-N-MoS2, m–p) DR-N-MoS2. B) Hg(II)
adsorption isotherm and C) distribution coefficient value of W-DR-N-MoS2, compared with commercial MoS2 powder, DF-N-MoS2, and DR-N-MoS2.
D) Hg(II) sorption kinetics W-DR-N-MoS2, inset is the purification of natural water samples through a purification column filled with W-DR-N-MoS2.
Reproduced with permission.[105]
and Mg(II) coexisted at high levels (Figure 7A).[13] However,
it seems that the 2D layered structure of above MXene or alkMXene materials could not effectively avoid the pore blockage
after Pb(II) adsorption, which might require a full exfoliation.
2D MXene (Ti3C2Tx) nanosheets were also investigated for Cu
removal in aqueous media (Figure 7B).[111] Delaminated Ti3C2Tx
exhibited excellent Cu removal ability, and oxygenated surface
functional groups of MXene facilitated the reductive adsorption
of Cu(II). Compared to multilayer Ti3C2Tx, delaminated Ti3C2Tx
exhibited a higher and faster Cu uptake. 2D MXene (Ti3C2Tx)
nanosheets, obtained by a similar method, also exhibited excellent removal capacity (250 mg g−1) for Cr(VI) (Figure 7C,D).[112]
X-ray photoelectron spectroscopy (XPS) study indicated that
Cr(VI) ions were reduced under the strong interaction between
Cr and TiO when Cr(VI) adsorbed on Ti3C2Tx surfaces
(Figure 7E–G). A later work on Ti3C2Tx nanosheets, in which
Ti3AlC2 powder was etched by hydrogen fluoride (HF) solution,
showed a very high selectivity toward barium removal in multimetal solution, and the optimum removal pH was found around
6–7.[113] A modified MXenes, with urchin-like rutile TiO2C
structure was prepared by in situ solvothermal alcoholysis of
MXene (Ti3C2(OH)0.8F1.2) under FeCl3 conditions.[114] The higher
Cr(VI) adsorption capacity (≈225 mg g−1) than raw MXene was
attributed to the inhibition of H2O adsorption by bridging oxogroups based on density functional theory (DFT) calculations.
4.5. Clay Nanosheets
Clays and modified clays have been excellent adsorbents for
the removal of heavy metal ions from water solution due to the
Adv. Mater. Interfaces 2018, 1801094
existence of active sites on the surface. The use of various clay
minerals as adsorbent has some advantages over other commercially available adsorbents, such as low-cost, abundant,
nontoxic, and excellent adsorption properties.[26] Sodic-montmorillonite (122 mg g−1) and turkish illitic clay (239 mg g−1) were
reported the best adsorbents for Pb(II) adsorption among several clay minerals. Furthermore, these layered silicates can be
intercalated, swelled, and delaminated into silicate nanosheets
with much larger surface area and adsorption sites, depending
on the swelling agent, pH, and temperature of the solution.[3]
Kaolinite was modified with phosphate and sulphate anions
for adsorption of metal ions (Pb2+, Cd2+, Zn2+, and Cu2+).[115]
The phosphate-modified kaolinite displayed stronger affinity
for Pb2+ with adsorption capacity of 93.28%. Moreover, Srivastava et al. reported adsorption of four metal ions (Cd2+, Cu2+,
Pb2+, and Zn2+) onto kaolinite.[28] The different sequence can be
found between the single-element systems (Cu < Zn < Pb < Cd)
and in the multielement system (Pb < Cu < Zn < Cd). Adsorption onto permanent charge sites can be occurred at higher
pH by generating inner sphere complexes with alumina layers
and crystal edges by forming bidentate complexes. In order to
boost the adsorption capacities of kaolinite, the different modification methods were reported.[116–118] Nanoscale zero-valent
iron modified kaolinite was applied to remove heavy metal ion
(Pb2+) from aqueous solution, in which 90.1% adsorption efficient was achieved within 60 min using 5 g L−1 sample, and
other coexistent ions (Ni2+, Cd2+) had little effect on the adsorption process for removal of Pb2+ ions.[119] Various techniques
were used to characterize nanoscale zero-valent iron modified
kaolinite, in which Pb2+ was adsorbed by kaolinite and reduced
by iron components.[120] Besides, the organic or inorganic
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Figure 7. A) Lead adsorption behavior of activated hydroxyl groups in 2D layered alk-MXene. Reproduced with permission.[13] Copyright 2014,
American Chemical Society. B) 2D Ti3C2Tx MXene nanosheets for efficient copper removal from water. Reproduced with permission.[111] Copyright 2017,
American Chemical Society. C,D) 2D Ti3C2Tx nanosheets for efficiently reductive removal of Cr(VI), E) Cr 2p XPS spectrum of the Cr-loading Ti3C2Tx-10%
nanosheets, F) chemical shift of binding energy of Ti 2p before and after immersing in Cr(VI) solution for 72 h, and G) schematic illustration of the
removal mechanism. C–G) Reproduced with permission.[112] Copyright 2015, American Chemical Society.
modified kaolinite were also reported to enhance the adsorption capacities, though the enhancement of adsorption selectivity, capacities, and regeneration ability still needs further
effort.[121–123]
In recent years, the montmorillonite-base materials have
been widely studied in the adsorption of heavy metals.[26,124–126]
The substitution of lower-valency cations within the interlayer
will generate negative charge in MMT, which can increase the
adsorption of interlayer cations under charge compensation
effect.[127] Previous research has shown that the low molar mass
organic acid is one of the important factors to affect the adsorption of Cd2+ and Pb2+ by montmorillonite.[128] The sodium montmorillonite was intercalated by dodecylamine as a potential
adsorbent for Cr(VI) ions, due to the electrostatic interaction of
HCrO4− with the protonated amine (Figure 8A).[129] The adsorption behavior of montmorillonite was modified by a facile solidstate NaOH treatment,[130] in which the Na-MMT exhibited
excellent sorption capacities of 184.8 and 290.7 mg g−1 for Sr2+
and Cs2+, respectively. The enhancement of adsorption capacity
was contributed to a large Brunauer−Emmett−Teller (BET) surface area and abundant functional groups (SiONa).
The adsorption behavior of raw montmorillonite for heavy
metals was connected to its surface properties. The mechanism
of adsorption is related to the permanent charges on MMT
Adv. Mater. Interfaces 2018, 1801094
surface and edge, which has strong interactions with metal
ions.[131] The kinetics and isotherms of vanadium (V) adsorption on MMT confirmed as the pseudo second-order and Langmuir model.[132] In addition, the pH and ionic-strength have
an obvious effect on the adsorption capacity resulted from the
surface complexation and electrostatic interaction, respectively.
Benefiting from the cation-exchange capacity of montmorillonite, the different MMT with interlayer cations (Ca2+, Mg2+,
and Fe3+) were prepared by the immersion method. The
maximal Pb2+ adsorption capacity (48 mmol per 100 g) was
achieved by Ca-MMT, due to the lower binding energy between
interlayer cations and MMT layers.[133] Moreover, Na-MMT
provided much higher adsorption potential for heavy metals
(Pb(II), Cu(II), Co(II), Cd(II), Zn(II)) than Ca-MMT, which
mainly depended on the ion exchange (Figure 8B).[49]
Montmorillonite based nanocomposites bonded with other
organic/inorganic components have received extensive attention for heavy metal removal.[26,134–137] Organo (2-(3-(2-aminoethylthio)propylthio)ethanamine (AEPE), hexadecyltrimethylammonium (HDTMA), and hexamethlenediamine (DA)) and
inorgano–organo modified clays were developed for remediation of various heavy metal ions contaminated water.[32] Adraa
et al. reported that the cysteine as chelating agents integrated
with montmorillonite and used for the chelation of different
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Figure 8. A) Illustration of the interaction of Cr(VI) complex and protonated amine in MMT. Reproduced with permission.[129] Copyright 2012, Elsevier.
B) Scanning electron microscopy (SEM) images of Na–Mt and Ca–Mt. Reproduced with permission.[49] Copyright 2015, Elsevier. C) Schematic illustration for the modification process of MMT with hydroxyiron (III) species. Reproduced with permission.[139] Copyright 2018, Elsevier. D) Adsorption
of phenol and Cu(II) onto cationic and zwitterionic surfactant modified montmorillonite. Reproduced with permission.[143] Copyright 2016, Elsevier.
heavy metal ions (Cd2+, Hg2+, Pb2+, Co2+, and Zn2+).[138] A strong
interaction between the cysteine and heavy metal cations was
achieved, which indicated the chelation effect of amino group,
as confirmed by theoretical calculations. Most recently, the
hydroxyiron-modified montmorillonite nanoclay was prepared
by a simple wet chemical synthesis method, which showed an
extraordinary adsorption removal process for heavy metal As
(III) within the first 30 s of the adsorption (Figure 8C). The fast
adsorption kinetics was attributed to the combined effect of
both outer sphere (physisorption) and inner sphere complexes
(chemisorption).[139] In addition, the reduced graphene oxide
and montmorillonite composite (RGO–MMT) exhibited 94.87%
removal efficiencies for Cr(VI).[140]
The surfactant modification of clay minerals is another
important strategy to enhance the interaction between the
adsorbents and heavy metals.[141] The montmorillonite was
modified by anionic surfactant sodium dodecylsulfate (SDS)
that showed a higher adsorption capacity for heavy metal ions,
in which SDS was intercalated into MMT and clay nanosheets
with a more negative Zeta potential expanded in c-axis.[142] Xi
and co-workers found that the intercalation of MMT using
cationic surfactant (C16) and zwitterionic surfactant (Z16)
also increased the adsorption capacity toward heavy metal
ions (Figure 8D), and the adsorption equilibrium and kinetic
data were better fitted with Langmuir isotherm and pseudosecond-order model, respectively.[143] Moreover, chitosan and
surfactant (HDTMA) comodified montmorillonite prepared in
liquid phase system were used as multifunctional adsorbents
for purifying wastewater containing heavy metals.[144] Based
on structural characteristics, the chitosan and HDTMA were
intercalated into the interlayers of MMT and the arrangement
of HDTMA on MMT can be affected by chitosan. The specific
functional groups (OH, NH2) contributed to the adsorption
of heavy metal (Cd2+) from aqueous solutions. Liao and coworkers found that the intercalation of well dispersed NZVI in
the interlayer of MMT could significantly increase the removal
of Cr(VI), and make NZVI/MMT more stable than NZVI and
remain higher reactivity even after exposed in air for 140 h.[145]
Adv. Mater. Interfaces 2018, 1801094
4.6. LDHs Nanosheets
The layered double hydroxides are an ideal inorganic matrix,
which can be applied to the synthesis of various hybrid materials.[146] The LDHs have excellent intercalation and ionexchange capability, which has played an important role in
adsorption.[50] Thus, incorporating organic and inorganic
components by grafting method was expected to enhance the
adsorption properties. Moreover, the structure and chemical
properties of LDHs can be modulated by intercalated foreign
molecule.[147] Some works were performed on chloridion intercalated Mg/Al LDHs (Cl/LDHs) for the removal of As(V) from
contaminated ground-water.[148] The maximum adsorption
capacities of As(V) on Cl/LDHs were 88.3 mg g−1, and more
than 98% of As was removed from natural groundwater. The
MoS42− intercalated Mg/Al layered double hydroxide (MoS4LDH) has highly efficient removal of heavy metal ions of Hg2+,
Cu2+, and Cd2+,[149] which have very high removal rates (>93%)
of As from complex solutions within 1 min (Figure 9A).[150]
Adsorption kinetics follows a pseudo-second-order model,
which is consistent with chemisorption through AsS bonding.
It is reported that the assembling of nitrogen and sulfur codecorated carbon dots with abundant oxygen-containing functional
groups on LDH resulted in the high selectivity and adsorption
capacity for Hg2+ and Ag+ (Figure 9B).[151] By contrast, thiofunctionalized layered double hydroxide also exhibited enormous saturated uptake capacities (594, 564, and 357 mg g−1)
for Hg2+, Ag+, and Pb2+.[152] Polyaniline (PANI), which has
plenty of amine and imine functional groups,[153] were used to
prepare PANI/LDHs nanocomposite with adsorption capacity
of 393.701 mg g−1 for Cr(VI)).[154] Further thermodynamic
study from adsorption isotherms indicated a spontaneous and
endothermic process. In addition, fulvic acid modified layered double hydroxides showed excellent adsorption capacities for Cu2+, Pb2+, Ni2+, and Cd2+ with an adsorption order of
Cu2+ > Pb2+ > Ni2+ > Cd2+.[155]
Recently, Gong et al. reported efficient adsorption of heavy
metal from wastewater with a self-assembly method.[156] The
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Figure 9. A) SEM images of the samples after MoS4-LDH adsorbed of (a, a′) 10 ppm As(III), (b, b′) 300 ppm As(III), and the dominant phases and
possible binding modes of MoS42− with HAsO42− in the LDH Gallery. Reproduced with permission.[150] Copyright 2017, American Chemical Society.
B) Schematic view of Hg2+ adsorption on N,S-CDs-LDH. Reproduced with permission.[151] Copyright 2018, Elsevier.
adsorbent was synthesized by introducing LDHs onto the
surface of carbon nanospheres (LDH-NCs@CNs). The as-prepared LDH-NCs@CNs exhibited a high efficiency for the Cu2+
(19.93 mg g−1) with the suitable initial concentration (10 mg L−1),
which was higher than LDHs or carbon nanospheres (CNs)
alone. The possible removal mechanism was mainly attributed to mass transfer, isomorphic substitution, and physical/
chemical adsorption processes. Additionally, the hydroxyapatite on magnetic CaAl-layered double hydroxides adsorbent
also showed efficient removal of uranium ions from aqueous
solution.[157] The adsorption/desorption cycle in practice
studies maintains sufficient stable and the adsorption capacity
decreased from 88.6% to 78.8% after four cycles.
GO with multiple oxygenic function groups can form nanocomposites with other 2D sheet-like structures, which have
received great attention for the effective removal of heavy metal
ions from aqueous wastewater.[158] The LDHs/GO nanocomposites were later prepared by a remarkably simple and efficient method.[159] The adsorption performance of the LDHs/
GO nanocomposites containing 6.0% GO showed a increased
adsorption capacity of 183.11 mg g−1 due to the higher surface area and ion exchange ability. Nanocomposites comprised
of 3D Mg-Al LDH/partially reduced GO were tested for the
removal of Pb2+, which exhibited a higher adsorption capacity
due to the unique porous network and functional groups.[160]
Very recently, Huang et al. reported a simple one-pot solvothermal synthesis of magnetic Fe3O4/GO/MgAl-LDH nanocomposite which could be easily separated by extra magnetic
field. The ternary composites can efficiently and rapidly adsorb
heavy ions (Pb2+, Cu2+, and Cd2+).[161] MoS2 coated Mg/Al LDH
(LDHs@MoS2) with excellent chemical stability were applied
for the adsorption of Cr (VI) due to the electrostatic attraction
and outer sphere surface complexation.[162] The LDHs-based
Adv. Mater. Interfaces 2018, 1801094
material is a promising candidate for fast and selective remediation of water polluted with heavy metal ions in environmental
pollution cleanup.
4.7. 2D Metal (Hydr)oxide Nanosheets
The purification of heavy metals in aqueous solutions by nanosized metal oxides was previously reviewed, among which the
most widely used nanosized metal oxides include iron oxides,
manganese oxides, aluminum oxides, and titanium oxides.[45]
Flowerlike MgO nanosheet with high surface area was reported
to show superb adsorption capacities of 1980 and 1500 mg g−1
for Pb(II) and Cd(II), respectively, which were significantly
higher than other nanomaterials due to the solid–liquid interfacial cation exchange.[38] However, the blocking of the newly
formed PbO on MgO nanosheet for further reaction was also
observed. A layered protonic titanate of lepidocrocite-type,
HxTi2−x/4□x/4O4•H2O(x ≈ 0.7; □: vacancy), can be exfoliated and stabled in aqueous solution of tetrabutylammonium
hydroxide.[163] The TiO6 octahedra are combined via edge
sharing to produce a 2D sheet of composition HxTi2−x/4□x/4O4−,
in which the Ti site vacancies are compensated by interlayer
hydronium ions. The loading of Cr2O3 on titanate nanosheets
by one-step hydrothermal method enhanced the photocatalytic activity for K2Cr2O7 reduction under visible-light, due to
the charge recombination and electron transfer from Cr2O3 to
tatanate nanosheets.[164] Zhu and co-workers studied the coadsorption of Cd(II) with sulfate/phosphate on ferrihydrite and
found that in situ Fourier transform infrared spectroscopy
(FTIR) and 2D correlation spectroscopic analysis can be an
efficient tool to analyze the coadsorption mechanisms of heavy
metal cations and anions on iron (oxyhydr)oxides.[165]
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Figure 10. A) Top view of the atomic structure for TAPB-BMTTPA-COF (yellow, S; blue, N; gray, C; white, hydrogen). B) Field emission scanning
electron microscope (FE-SEM) and high resolution-transmission electron microscopy (HR-TEM) (inset) images of TAPB-BMTTPA-COF. C) 13C crosspolarization magic-angle spinning (CP/MAS) NMR spectra of TAPB-BMTTPA-COF (black curve) and Hg(II)-adsorbed TAPB-BMTTPA-COF (blue curve).
D) Cycle performance for Hg(II) removal and E) capture efficiency in removing metal ions of TAPB-BMTTPA-COF. Reproduced with permission.[171]
Copyright 2017, American Chemical Society.
4.8. MOF/COF Nanosheets
Recently, highly porous MOF based materials, with abundant
functional groups, have been reviewed for heavy metal purification.[1] The interaction mechanisms between heavy metal ions
and MOF-based materials were systematically studied. As an
extensive class of crystalline materials with ultrahigh porosity
(up to 90% free volume) and internal surface areas (ranging from
1000 to 10 000 m2 g−1), MOFs and MOF-based materials showed
attractive characteristics, such as fast adsorption kinetics, high
adsorption capacity, and excellent selectivity, which can be further increased by introducing functional groups. The photocatalytic performance of MOFs can be modulated via incorporation
of organic or inorganic nanoparticles while their high surface
area can ensure the fast transport and good accommodation of
targeted ions, as reported in Cr(VI) reduction in MOFs.[29]
In the test of a series of zirconium-based MOFs for selenate
and selenite anions purification, NU-1000 showed the highest
adsorption capacity and fastest uptake rates.[166] Using the
Langmuir model, the maximum adsorption capacities for selenite and selenate are 95 and 85 mg g−1, respectively. At ion
concentrations of 1–3 selenite or selenate anions per Zr6 node,
maximum adsorption within 1 min is achieved under fast
anion exchange with the surface ligands. The Ni-based MOF
[Ni(3-bpd)2(NCS)2]n was found to selectively adsorb Hg(II) in
aqueous medium via the soft center of S atom.[109]
By contrast, due to the intrinsic structure of MOFs, to date,
2D MOF nanosheets are rarely used for the heavy metal purification.[167–169] There are two strategies, i.e., top-down and
bottom-up methods, to prepare 2D MOF nanosheets.[170] The
former one involves the delamination of bulk MOFs under
sonication in various solutions, while the latter one involves
directly synthesized 2D MOF nanosheets in high yield. However, the exfoliated 2D MOF nanosheets prepared by top-down
method are rather unstable to their restacking.
Recently, the 2D MOFs (FIR-53 and Ag-3) with large nanotubular channels were used for Cr2O72− purification, in which the
Adv. Mater. Interfaces 2018, 1801094
FIR-53 showed excellent regeneration ability, fast adsorption
kinetics (10 min), and high selectivity for Cr2O72− over Cl−, Br−,
and NO3− ions, through the weak CH⋅⋅⋅O bond with FIR-53.
Unfortunately, the selectivity significantly reduced under the
influence of other coexisting ions.
Owing to the structural diversity of skeletons and pore walls,
COFs offer a platform for designing high-performance materials
for heavy metal removal from water. Huang at el.[171] designed a
novel TAPB-BMTTPA-COF for Hg(II) removal (Figure 10A,B).
The framework contains larger mesoporous channels than
most MOFs and the dense methyl sulfide functional termini
(Figure 10C) on the pore walls are active sites to adsorb Hg(II).
This porous material was stable under strong acid and base condition, and achieved a benchmark for the capacity, efficiency, reusability (Figure 10D), and selectivity (Figure 10E). These results suggested the great potential of COFs for heavy metal purification.
More and more water stable MOFs and COFs are developed
by the introduction of high-valence metal ions, metal azolate
frameworks, or special functional groups.[172] A porous Zn(II)based MOF decorated with O− groups was reported to exhibit
ultrahigh Pb2+ capacity (616.64 mg g−1), fast adsorption (10 min),
high selectivity against competitive ions, such as Ca2+ or Mg2+
(>99.27%).[173] However, there still remain some issues: 1) The
long-term stability and regeneration in practical applications is
still troubled by the structural degradation under water, redox
conditions, acids/bases, and solution conditions such as pH,
ionic strength, organics, and ion types, and concentrations. 2)
The slow adsorption kinetics and the blocking of internal pores
due to the microporous nature of MOFs. 3) The expensive
ligands used for the synthesis of most MOFs and COFs.
4.9. 2D Composite Nanosheets
Since most of exfoliated nanosheets possess distinct surface
charge, they can be assembled to form novel nanohybrids and
nanocomposites under the electrostatic forces.[17] Some of the
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Figure 11. A) Illustration of the formation of the Mg(OH)2/GO composite, typical SEM image of Mg(OH)2/GO nanoflower, and the capacity and
removal efficiency of heavy metal ion adsorption by Mg(OH)2/GO from synthetic wastewater. Reproduced with permission.[174] Copyright 2016, Royal
Society of Chemistry. B) The photocatalytic kinetic parameter of Cr(VI) with different catalysts and the diagram of photocatalytic reduction mechanism
of Cr(VI). Reproduced with permission.[175] Copyright 2018, Elsevier. C) SEM image of GO/g-C3N4/MoS2 nanocomposites (GCM-7), photocatalytic
reduction curves of the Cr(VI) using different samples under visible-light illumination, and the schematic drawing of the photocatalytic process
and charge transfer mechanisms in the GO/g-C3N4/MoS2 composites. Reproduced with permission.[176] Copyright 2018, Elsevier. D) The proposed
photocatalytic mechanism of C-CN-GR ternary nanocomposite. Reproduced with permission.[177] Copyright 2017, Elsevier. E) The TEM image, energy
dispersive spectroscopy (EDS) maps, and interface structure of TP-SiNSs nanosheet for Pb adsorption. Reproduced with permission.[48]
new nanosheet composites using GO as functional support
materials were already mentioned in above sections, hence here
we do not intend to elaborate on them. A layered Mg(OH)2/
GO nanosheet composite was in situ synthesized by laser ablation method for the removal of heavy metal ions from water
(Figure 11A).[174] The GO nanosheet served as a heterogeneous
nucleation and growth site for sheet-like Mg(OH)2 nanocrystals, which were crystallized under the strong reaction between
laser-ablated Mg species and water molecules. The resulting
porous Mg(OH)2/GO nanosheet composite exhibited a maximum adsorption capacity of over 300 mg g−1 for heavy metal
ions Zn(II) and Pb(II).
g-C3N4/SnS2/SnO2 nanocomposites, synthesized by solvothermal method, were designed for the photocatalytic reduction of aqueous Cr(VI) under visible light.[175] The reaction
rate constant of Cr(VI) reduction on g-C3N4/SnS2/SnO2 nanocomposite with the mass ratio of 1:3 can be improved 41.7
and 4.0 times than pure g-C3N4 and SnS2/SnO2, respectively
(Figure 11B). The diffusion of oxygen atoms from SnO2 to
g-C3N4 during the synthesis process made a good combination between SnS2/SnO2 and g-C3N4 nanosheet via SnOC
bond, and enhanced the photocatalytic efficiency of g-C3N4/
SnS2/SnO2. In addition, the formation of oxygen vacancies on
g-C3N4/SnS2/SnO2 surface by ultrasonic assisted solvothermal
reaction could also lift the valence band edge. Ternary-layered
Adv. Mater. Interfaces 2018, 1801094
GO/g-C3N4/MoS2 nanosheets with flower-like morphology
were prepared for photocatalytic reactions for environmental
purification (Figure 11C).[176] The nanocomposite showed good
reproducibility and stability for the visible light photocatalytic
reduction of Cr(VI). While GO acted as a fast transport hole
due to its high conductivity in the ternary nanosheet structure, the collection of electrons in MoS2 and holes in g-C3N4
was effectively improved, along with the reduced recombination of photogenerated electron carriers. A similar ternarylayered graphene/g-C3N4/carbon dots (C-CN-GR) nanocomposites were prepared via a facile hydrothermal method, in
which the loading of a small amount of graphene and carbon
dots improved the photocatalytic performance of g-C3N4 for
Cr(VI) reduction.[177] The nanocomposite with enhanced photocatalytic activity could remove over 83.6% of Cr(VI) within
120 min. And the mechanism of Cr(VI) reduction is shown
below (Figure 11D). Cr(VI) as an electron acceptor would promote the transfer of photogenerated electrons from conduction
band (CB) of g-C3N4. Recently, the grafting of highly dispersed
titanium hydroxyl groups on acid leached clay mineral derivatives was proved to be an efficient route to stabilize functionalgroups and enhance their activities for heavy metal adsorption (Figure 11E).[48] The as-synthesized 2D hierarchical 2D
nanosheet shows balanced hydrophobicity–hydrophilicity, large
maximum capacity, fast kinetic, and excellent selectivity and
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Figure 12. Typical components and strategies for the interface design of 2D nanosheet materials for heavy metal adsorption.
renewability. The higher adsorption selectivity was attributed to
the strong bonding between Pb(II) and the spatially distributed
titanium hydroxyl groups on nanosheet surface, which indicates the great potential of clay mineral derivatives toward environmental remediation.
To date, novel 2D nanosheet materials, due to their unique
physicochemical properties and surface properties, have been
developed on the basis of various nanotechnologies (Figure 12).
To sum up, the functional design of novel 2D nanosheet materials with special interface structures can be divided into five
cases: 1) 2D nanosheet composites which are combined by
two or more different types of layer structures which can be
monolayer or few layer structures. In this case, graphene-like
or graphitic nanosheet materials (G/GO/RGO, h-BN, g-C3N4,
TMDs) with better charge transfer and hole–electron seperation ability can be used to support other layer materials (metal
oxide/hydroxide, TMDs, MXenes, clays, LDHs, MOFs, etc.) and
reduce negatively charged heavy metal ions such as Cr(VI) and
As(V) species. 2) Nanosheet composites which are combined
by one layer material with other nanoparticles which are not
in their layer morphologies. In this case, the size of nanoparticles must be small enough to enhance their surface reactivity
and increase their adsorption capacity and rate. Meanwhile, the
layer support material should be able to inhibit the corrosion
and instability of the supported nanoparticles (such as NZVI).
3) Hybrid nanosheet materials which are produced by grafting
organic or inorganic functional groups on the surface of one
layer material. In this case, the stability of these functional
Adv. Mater. Interfaces 2018, 1801094
groups and their dynamics properties should be examined to
exclude unstable combinations of two components. 4) Intercalated nanosheet materials which are some layer materials
(mainly clay and LDH) intercalated by cations or anions. 5)
The modification of one pristine layer material or complicated
nanocomposites by element doping, redox method, or other
advanced physicochemical methods.
5. Adsorption Mechanisms by Theoretical
Calculations
The reactions at the mineral–aqueous solution interface have
attracted much attention during recent years.[178] The detailed
atomic level understanding of the adsorption of heavy metal
ions on hydrated oxide surface is still unclear, due to the hydrolysis of metal ions under different pH values. To date, DFT
calculations have only been applied to the adsorption configurations of Pb(II) on some oxides surface, such as hydrated alumina, hematite surfaces,[179] and α-Al2O3–water interfaces.[180]
The most important advantage of DFT calculation is that it can
simulate the interfacial structure and the interactions between
various ion species and microstructures on the nanosheet surface, as well as clarify the adsorption mechanism at atomic level
for the complicated experimental phenomena observed, as also
demonstrated in the study of boron nitride-based materials.[181]
DFT calculations were performed for LDH used as a slow
release fertilizer or a controlled release fertilizer for iron
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Figure 13. A) (110) and (−110) plane ELF slices of pristine Ti3C2(OH)2 and Ti3C2(O2H2−2mPbm) for 1/9 ML coverage. Reproduced with permission.[13] Copyright 2014, American Chemical Society. B) Three different models of M2X(OH)2/MXene, and the adsorption of Pb on M2X(OH)2/MXene. C) The formation
energies of M2X(O2H2−2x)Pbx for different models when Pb coverage is 1/9 ML. b,c) Reproduced with permission.[183] Copyright 2016, Elsevier. D) The
dehydration energy as a function of Ti number. E) The adsorption energy of Pb, Ca, and Mg on SiNSs and TP-SiNSs as a function of pH value and ion
concentration. F) Partial charge density corresponding to the electron states at valence band and conduction band edges. Reproduced with permission.[48]
release.[182] The thermodynamic parameters such as Gibbs
energy, enthalpy, and entropy of these exchange reactions were
calculated at room temperature to show the nature of the adsorption process. The interactions at the interface were explored to
identify the acidity and alkalinity sites for each structure.
Recently, the adsorption behavior of Pb(II) species on 2D
alk-MXene was elucidated by DFT calculations.[13] Different
Pb coverages are considered for the possible Pb adsorption
on exposed titanium surfaces after exfoliation process, which
is terminated by OH, ONa, or F group. The valence electron
localization function (ELF) provides the bonding information
by measuring electron localization density, which disclosed the
Ti-OH terminal as key functional groups for the adsorption at
Pb/MXene interface (Figure 13A). To understand the stability
of Pb occupation in alk-MXene, the formation energies of some
possible chemical reactions were calculated to evaluate the stability of Pb adsorption on alk-MXene. Later, MXenes with the
highest valuable applied structure of M2X(OH)2 (M = Sc, Ti,
V, Cr, Zr, Nb, Mo, Hf, Ta, and X = C or N) were systemically
investigated by DFT calculations (Figure 13B).[183] The results
demonstrated that the N doping in X site is more effective for
Pb(II) adsorption than C doping (Figure 13C), due to the lower
formation energies of M2N(O2H2−2xPbx) than those of M2C(O2
H2−2xPbx).
The molecular structure of Pb(II) species and chemical
properties of metal oxide surfaces are influenced by the solution pH and ion concentration,[184] which further complicated
the adsorption reactions at the interface structure. The adsorption mechanism was previously studied by using the adsorption
energies of Pb(OH)2[180] or Pb(NO3)2[13] molecule in literatures.
Recently, based on DFT calculations, a simplified method was
Adv. Mater. Interfaces 2018, 1801094
proposed to calculate the adsorption mechanism as a function
of pH value and ion concentration (Figure 13D,E).[48] The higher
adsorption selectivity was attributed to the strong bonding
between Pb(II) and the spatially distributed titanium hydroxyl
groups, which was confirmed by the intense hybridization of Pb
5d orbitals with the surface orbitals of adsorbent (Figure 13F).
6. Summary and Outlook
Novel 2D nanosheet materials, developed on the basis of various nanotechnologies, are promising alternative to the traditional adsorbents for more efficient heavy metal purification,
due to their unique physical, mechanical, chemical properties
and most importantly their modified surface properties. Many
works have been done to the adsorption capacity, rate, and
mechanism of 2D nanosheet materials for heavy metal purification, in which the parameters of adsorption isotherms, kinetics,
thermodynamics, etc., have been mostly studied. Although
more and more researchers performed selectivity, desorption,
and regeneration test of their samples for future industry applications, the rational design of novel materials should focus on
novel nanosheet materials which can selectively adsorb heavy
metal ions from real water and possess long-term stability.
Generally, the adsorption behavior varies with surface
properties of adsorbents, which are dominated by the grafted
surface organic/inorganic functional groups or the loaded
nanoparticles on each nanosheet structure. Although some
nanosheet materials such as MoS2, MgO have extra high
adsorption capacity for heavy metal ions via cation exchange
process, the second pollution or surface blocking problem still
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needs to be solved. The long term stability of organic components on various nanosheet materials as well as their low selectivity also needs to be concerned for industry applications. And,
to date, structure investigations of ion/adsorbent interface at
atomic level remain scarce. The most important advantage of
DFT calculation is that it can simulate the interactions between
various ion species and nanosheet adsorbent surface, as well
as local structure evolution during the adsorption process. For
future development of novel nanomaterials for heavy metal
purification, there is a growing demand for the comprehensive
understanding of the interaction between metal ion and adsorbent surface as well as their interface structure by combining
experimental approaches and DFT calculations. The systematic
study of the adsorption mechanisms and their thermodynamics
as a function of temperature, pH values, ion types, and concentrations would be necessary to understand and clarify the
adsorption behaviors of various nanomaterials for heavy metal
purification.
Acknowledgements
L.F. and Z.Y. contributed equally to this work. This work was supported
by the National Natural Science Foundation of China (Grant No.
41572036), the National Science Fund for distinguished Young Scholars
(Grant No. 51225403), the Strategic Priority Research Program of Central
South University (Grant No. ZLXD2017005), the Innovation Driven Plan
of Central South University (Grant No. 2018CX018), the National "Ten
Thousand Talents Program" in China (Grant No. 2016-37), the State Key
Lab of Powder Metallurgy, Central South University (2015–19), the Hunan
Provincial Science and Technology Project (Grant Nos. 2016RS2004 and
2015TP1006), and the Central South University Graduate Independent
Exploration Innovation Program (Grant No. 2017zzts107). This article
is part of the Advanced Materials Interfaces Hall of Fame article series,
which highlights the work of top interface and surface scientists.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
2D nanosheets,
purification
adsorption,
heavy
metal,
interfacial
structure,
Received: July 18, 2018
Revised: September 29, 2018
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