REVIEW
The role of nanomaterials as effective adsorbents and their
applications in wastewater treatment
Hamidreza Sadegh
1
•
Gomaa A. M. Ali
2,3,4
•
Vinod Kumar Gupta
5
•
Abdel Salam Hamdy Makhlouf
6
•
Ramin Shahryari-ghoshekandi
1
•
Mallikarjuna N. Nadagouda
7
•
Mika Sillanpa
¨
a
¨
8,9
•
El
_
zbieta Megiel
10
Received: 21 November 2016 / Accepted: 10 January 2017
Ó The Author(s) 2017. This article is published with open access at Springerlink.com
Abstract Nanomaterials have been extensively studied for
heavy metal ions and dye removals from wastewater. This
article reviews the role of nanomaterials as effective
adsorbents for wastewater purification. In recent years,
numerous novel nanomaterial adsorbents have been
developed for enhancing the efficiency and adsorption
capacities of removing contaminants from wastewater. The
innovation, forthcoming development, and challenges of
cost-effective and environmentally acceptable nanomateri-
als for water purification are discussed and reviewed in this
article. This review concludes that nanomaterials have
many unique morphological and structural properties that
qualify them to be used as effective adsorbents to solve
several environmental problems.
Graphical Abstract
& Hamidreza Sadegh
h.sadegh@chemist.com; hamidreza.sadegh@srbiau.ac.ir
& Gomaa A. M. Ali
gomaasanad@azhar.edu.eg; gomaasanad@gmail.com
& Vinod Kumar Gupta
vinodfcy@gmail.com
& Mika Sillanpa
¨
a
¨
mika.sillanpaa@lut.fi
1
Department of Chemistry, Science and Research Branch,
Islamic Azad University, Tehran, Iran
2
Chemistry Department, Faculty of Science, Al-Azhar
University, Assiut 71524, Egypt
3
Al-Azhar Center of Nanoscience and Applications (ACNA),
Al-Azhar University, Assiut 71524, Egypt
4
Faculty of Industrial Sciences and Technology, Universiti
Malaysia Pahang, 26300 Gambang, Kuantan, Malaysia
5
Department of Applied Chemistry, University of
Johannesburg, Johannesburg, South Africa
6
Department of Manufacturing and Industrial Engineering,
College of Engineering and Computer Science, University of
Texas Rio Grande Valley, 1201 West University Dr.,
Edinburg, TX 78541-2999, USA
7
Department of Mechanical and Materials Engineering,
Wright State University, Dayton, OH, USA
8
Laboratory of Green Chemistry, Lappeenranta University of
Technology, Sammonkatu 12, FI-50130 Mikkeli, Finland
9
Department of Civil and Environmental Engineering, Florida
International University, Miami, FL 33174, USA
10
University of Warsaw, Faculty of Chemistry, Pasteura 1,
02-093 Warsaw, Poland
123
J Nanostruct Chem
DOI 10.1007/s40097-017-0219-4
Keywords Nanomaterials Heavy metal ions Dyes
Adsorption Wastewater treatment
Introduction
Treatment processes for wastewater as well as drinking
water are one of the major prerequisites for developing,
growing the economy as well as health maintaining.
Therefore, it is crucial to develop and implement innova-
tive technologies for treating water at high efficiencies and
low energy consumption. On a global scale, waterborne
diseases are still a major cause of death in developing
countries where access to safe drinking water is often
limited. With the introduction of disinfection processes
(mainly using chlorine), waterborne infectious dise ases
have been significantly reduced. However, it is known that
the application of disinfection agents such as chlorine,
chlorine dioxide or ozone is associated with the formation
of disinfection by-products (e.g., trihalomethanes, halo-
phenols, ketones, aldehydes) with a high mutagenic and/or
carcinogenic potential [
1–4]. Chlorination also affects the
taste and odor of drinking water. Therefore, the reduction/
elimination of toxic by-products formation resulting from
disinfection processes is necessary. Further, many toxic
materials such as heavy metal ions and azo dyes in
wastewaters cannot be completely removed during
wastewater treatment processes that are common ly used on
a large scale [
5–10]. Thus, these toxic materials are per-
manently introduced into rivers and streams by wastewater
discharges, while diffuse sources such as runoff from
agricultural fields are possible, but frequently contribute to
a much smaller extent to the overall pollution [
7, 11, 12].
The most common toxic materials in wastewaters
responsible for particular problems are heavy metal ions and
azo dyes [
13–18]. Despite the fact that the human body needs
low doses of metal ions such as for example Zn(II) ions, their
excess may cause eminent health problems such as depres-
sion, lethargy, neurological signs and increased thirst. In
addition, exposure to metal ions, often toxic, can cause health
problems such as liver or kidney damage, Wilson disease,
insomnia, cancer, diarrhea, nausea, vomiting, dermatitis,
chronic asthma, coughing and headaches [
19–21].
Removal of toxic materials from wastewa ter is neces-
sary for health and environmental protection. For this
purpose, conventional methods such as reduction, precipi-
tation, adsorption, oxidation and ion exchange are com-
monly used. However, among them the adsorption process
is the most suitable method because of its high efficiency
and economic consideration [
22–26]. Such adsorbents such
as activated carbon (AC), zeolites, biomaterials, polymers,
have been used extensively for wastewater treatment
[
22–31]. However, the adsorption efficiency of these
materials is relatively low [ 26]. Therefore, it has become
essential to find more efficient adsorbents.
Recently, there has been a remarkable potential for the
remediation of environmental problems as a result of
nanoscience and nanotechnology developments [
32, 33]. In
comparison to conventional mater ials, the nanostructured
adsorbents, mainly due to the excep tionally high surface
area, show much higher efficiencies and faster adsorption
rates in water treatment [34–36]. A variety of efficient, low-
cost and eco-friendly nanomaterials with unique function-
alities have been proposed for potential applications in
detoxification of industrial effluents, groundwater, surface
water and drinking water [
34, 37]. An ideal adsorbent for
wastewater treatment purposes should satisfy the following
criteria [
26]: (1) should be environmentally benign; (2)
should demonstrate a high sorption capacity and high
selectivity especially to the pollutants occurring in water at
low concentration; (3) the adsorbed pollutants can be easily
removed from its surface, and (4) should be recyclable. In
recent years, many studies have proved that the nanoma-
terials can satisfy most of these requirements [
38–40
].
It was demonstrated that the nanomaterials such as
carbon nanotubes (CNTs), graphene, ferric oxide (Fe
3
O
4
),
manganese oxide (MnO
2
), titanium oxide (TiO
2
), magne-
sium oxide (MgO) and zinc oxide (ZnO) may play an
important role in the waste water treatment processes
[
41–49]. The nanomaterials may be successfully used as
efficient, cost-effective and environmentally friendly
adsorbents for the removal of various toxic substrates from
wastewater such as heavy metals, azo dyes, etc.
[
2, 5–9, 34, 37 – 46 , 49–51].
Adsorption phenomenon
The adsorption process is a surface phenomenon in which
the adsorbate is accumulated on the adsorbent surface.
When a solution containing absorbable solute comes into
contact with a solid with a highly porous surface structure,
liquid–solid intermolecular forces of attraction cause some
of the solute molecules from the solution to be concen-
trated or deposited on the solid surface [46–48]. In case of
bulk materials, all the bonding requirements (ionic, cova-
lent, or metallic) of the material constituent atoms are filled
by other atoms in the material. However, the atoms on the
surface of the adsorbent are not wholly surrounded by other
adsorbent atoms, therefore they can attract adsorbates
[
48–52]. The exact nature of the bonding depends on the
details of the species involved, but the adsorption process is
generally classified as physisorption (an adsorbate bound to
the surface by weak van der Waals forces), chemisorption
(an adsorbate tethered through covalent bonding [
53]or
due to electrostatic attraction [26]).
J Nanostruct Chem
123
The equilibrium stage of adsorption betwee n the solu-
tion and adsorbent is attained (where the adsorption of
solute from the bulk onto the adsorbent is minimum) and
the adsorption amount (q
e
, mmol g
-1
) of the molecules at
the equilibrium could be calcul ated according to the fol-
lowing equation [
48, 53]:
q
e
¼
VC
0
C
e
ðÞ
m
; ð1Þ
where V is the solution volume (L); m is the mass of
adsorbents (g); and C
0
and C
e
are the initial and equilib-
rium adsorbate concentrations, respectively. In addition,
adsorption maybe defined as the mass transfer process by
which a substance is transferred from the liquid phase to
the surface of a solid, and becomes bound by physical and/
or chemical interactions [
54]. It is worth pointing out that
the large surface area of adsorbent allows achieving a high
adsorption capacity and surface reactivity [
54].
Adsorption isotherm models
The adsorption isotherm models present of the amount of
solute adsorbed per unit weight of adsorbent as a function
of the equilibrium concentration in the bulk solution at
constant temperature [
52, 54–56 ]. There are many isotherm
models such as: Langmuir and Freundlich, Temkin, Har-
kin–Jura and Dubinin–Radushkevich. Among of them,
Langmuir and Freundlich models are commonly used for
the description of adsorption data [48, 55–57].
The Langmuir equat ion is expressed as [
48, 56]:
C
e
q
e
¼
1
Q
max
K
l
þ
1
Q
max
C
e
; ð2Þ
where C
e
is the equilibrium concentration (mg L
-1
), q
e
is
the amount of adsorbate adsorbed per unit mass of adsor-
bent (mg g
-1
), and Q
max
and K
l
are Langmuir constants
related to monolayer adsorption capacity and affinity of
adsorbent toward adsorbate, resp ectively.
On the other hand, Freundlich isotherm describes
heterogeneous surface adsorption. The energy distribution
for adsorptive sites (in Freundlich isotherm) follows an
exponential type function which is close to the real situa-
tion. The rate of adsorption/desorption varies with the
strength of the energy at the adsorptive sites. The Fre-
undlich equation is expressed as [
48]:
log q
e
¼ log K
F
þ
1
n
log C
e
; ð3Þ
where k (mg g
-1
) and 1/n are the constant characteristics of
the system [
56, 58]. An example of the linear relation of
Freundlich and Langmuir isotherms is displayed in Fig.
1
for MB adsorption on Co
3
O
4
/SiO
2
nanocomposites [57].
Kinetic models
An applicable kinetic model is necessary to analy ze the
rate and the mechanism of adsorption processes (e.g., mass
transfer and chemical reaction). Several kinetic models
such as simple-first-order, pseudo-first-order, pseudo-sec-
ond-order and intra-particle diffusion models
[
48, 53, 57, 59, 60] have been applied to disclose the
adsorbate-adsorption phenomenon.
The simpl e-first-order and pseudo-first-order rate equa-
tions are given by Eqs. (
4) and (5), respectively [48, 56, 57]:
log q
t
¼
k
s
2:303
t þ log q
e
ð4Þ
logðq
e
q
t
Þ¼log q
e
k
1
2:303
t; ð5Þ
where q
e
and q
t
are the amounts of adsorbate (mg g
-1
)at
equilibrium and at time t, respectively. k
s
and k
1
are the rate
constants (h
-1
).
Fig. 1 Freundlich (left) and
Langmuir (right) isotherms for
MB adsorption on Co
3
O
4
/SiO
2
nanocomposite, The solid lines
are the linear fits (copied form
Ref. [
57])
J Nanostruct Chem
123
On the other hand, the pseudo-second-order rate formula
is as following [
48, 56, 57, 60 ]:
t
q
t
¼
1
k
2
q
2
e
þ
1
q
e
t; ð6Þ
where k
2
is the equilibrium rate constant (g mg
-1
h
-1
).
The slopes and intercepts t/q vers us t plot are used to
calculate k
2
.
In addition, intraparticle diffusion model which can be
described as follows [
56, 57]:
q
t
¼ k
p
t
1=2
þ C; ð7Þ
where C is the intercept and k
p
is the int ra-particle-diffu-
sion rate constant (mg g
-1
h
1/2
), which can be evaluated
from the slope of the linear plot of q
t
versus t
1/2
.
Development of nanomaterials as adsorbent
for wastewater treatment
The most widely studied nanomaterials for wastewater
treatment are AC, CNTs, graphene, Fe
3
O
4
, MnO
2
,Co
3
O
4
,
TiO
2
, MgO and ZnO, etc. [22, 43, 46, 52, 57, 61–68]. They
may be prepared in different morphological forms such as
particles, tubes and sheets [
26].
Hereby we review recent advances in heavy metals and
dye removal from wastewater using nanomaterials as
effective adsorbents and perspectives in this area of
research.
Carbon-based nanomaterials
Different types of carbon-based nanomaterials have been
used widely for heavy metals and dye removal in recent
decades due to their nontoxicity, abundance, ease of
preparation, high surface area and porosity, stable structure
and high sorption capacities [
38, 41, 52, 69 –71].
Activated carbon (AC)
AC was used initially as sorbents; however, due to the
difficulties associated with heavy metals and dye removal
at ppb levels, CNTs, fullerenes, and graphene were used as
nanosorbents to overcome this difficulty. AC typically has
high porosity, high surface area, and can be prepared from
readily available carbonaceous precursors such as coal,
wood, coconut shells and agricultural wastes [
72–75]. AC
is extensively used for the removal of inorganic and
organic pollutants from effluent streams and in water
treatment [
22]. In addition, it possesses a significantly
weak acidic ion exchange character, enabling it to remove
metal contaminants and to adsorb pollutants from
wastewater [
22]. The sorption of pentavalent arsenic on
granular activated carbon (GAC) was experimentally
studied [
75]. AC prepared from coconut tree sawdust was
used as an adsorbent for the removal of Cr(VI) from
aqueous solution [
76]. Sorption and stability of mercury on
AC for emission control were also reported [
72]. Powdered
activated carbon (PAC) prepared from Eucalyptus cam al-
dulensis Dehn bark was studied and showed a sorption
capacities (q
m
)at60°C, of 0.85 and 0.89 mmol g
-1
for
Cu(II) and Pb(II), respectively [
73]. A novel sodium
polyacrylate grafted AC was produced using gamma radi-
ation to increase the numb er of functional groups on the
surface which increased the efficiency of metal ions sorp-
tion by AC [
74]. Their high sorption ability and low price
make AC promising materials for heavy metals and dye
removal.
Carbon nanotubes (CNT s)
CNTs (Fig.
2), which were first developed by Iijima [77],
have a unique structural, electronic, op toelectronic, and
semiconductor, as well as mechanical, chemical and
physical properties [
22, 23]. CNTs have been applied
widely to remove heavy metals and dyes in wastewater
treatment [
6, 7, 9, 11–16 , 41, 71, 78–82].
CNTs are considered to be one of the most promising
adsorbents for wastewater treatment because of their large
adsorption capacity for synthetic dyes [
15]. Multi-walled
carbon nanotubes (MWCNTs) have been shown to surpass
cadmium hydroxide nanowire-loaded AC (Cd(OH)
2
–NW–
AC) with respect to their efficient removal of safranin O
(SO) from wastewater [
83]. However, only few studies
were reported on the application of CNTs for dye removal
from aqueous solution [
41, 69, 76, 84–86]. Moreover,
CNTs were typically used directly without further treat-
ment [
41, 69, 85]. Therefore, CNT funct ionalization has
been initiated to introduce various functional groups that
provide new adsorption sites [
15]. Among such modifica-
tions, oxidation is an easy method for introducing hydroxyl
and carbonyl groups to the sidewalls of CNTs. Oxidized
MWCNTs were found to be effective in the removal of
methylene red (MR) and methylene blue (MB) from
aqueous solutions [
87, 88]. Yao et al. [89] reported an
adsorption capacity of 41.63 mg g
-1
at 333 K for the
removal of MB onto CNTs. Shahryari et al. [
90] performed
the same batch of experiment s on MWCNTs having a
higher surface area of 280 m
2
g
-1
as compared to that of
CNTs (160 m
2
g
-1
) used by Yao et al. and reported a
higher MB adsorption of 132.6 mg g
-1
at 310 K. In
addition, cellulose grafted with soy protein isolate/hy-
droxyapatite rod-like nanocrystals showed a high MB
adsorption capacity of 454 mg g
-1
[91].
J Nanostruct Chem
123
The adsorption capacity also depends on the experi-
mental conditions, nature and type of adsorbent. The
comparative adsorption of anionic orange II (OII) from
aqueous solution using MWNTs and carbon nanofibers
(CNF) as adsorbents was studied in b atch experiments by
Rodrı
´
guez et al. [
75]. They found that the adsorption of II
(OII) onto MWCNTs was slightly higher than CNF (the
adsorption capacity in case of MWCNTs was
77.83 mg g
-1
, while it was 66.12 mg g
-1
in case of CNF
[
75]). In addition, MWCNTs showed higher adsorption
than PAC for removal of reactive red M-2BE (RRM). The
maximum amounts of RRM uptake were 335.7 and
260.7 mg g
-1
for MWCNTs and PAC, resp ectively [76].
The higher adsorption capacity can be explained on the
basis of higher average pore diameter of MWCNTs, which
was 7.62 nm as compared to 3.52 nm of PAC. It seems that
dye molecules can easily be diffused from the surface to
pores of MWCNTs due to larger pore size.
CNT-impregnated chitosan hydrogel beads (CSBs) have
been developed for the removal of congo red (CR). CSBs
demonstrated a higher maximum adsorption capacity
(450.4 mg g
-1
) than chitosan without impregnation
(200 mg g
-1
) based on Langmuir adsorption modeling
[
72]. A new generation of CSBs has been prepared by
sodium dodecyl sulfate and MWCNTs to improve the
mechanical properties [
73]. The new CSBs have demon-
strated a high maximum adsorption capacity for CR of
375.94 mg g
-1
[73]. Compared to MWCNTs and hybrid
CNTs (HCNTs), single wall carbon nanotubes (SWCNTs)
can demonstrate better adsorption properties for organic
contaminants because of their higher specific surface area.
SWCNTs are more efficient for removing benzene and
toluene and have shown maximum adsorption capacities of
9.98 and 9.96 mg g
-1
, respectively [74]. A maximum
adsorption capacity of 496 mg g
-1
was achieved when a
reactive blue 29 (RB29) has been removed from aqueous
solution by using SWCNTs [
92].
On the other hand, CNTs showed high sorption effi-
ciency of divalent metal ions. The advantages and draw-
backs of Co(II) and Cu(II) removal using AC, CNTs, and
carbon-encapsulated magnetic nanoparticles were reported
by Pyrzyn
´
ska and Bystrzejewski [
80]. The results showed
that the carbon nanomaterials have significantly higher
sorption efficiency compared to commercial AC. Mean-
while, Stafiej and Pyrzynska [
79] found out that the solu-
tion conditions such as pH and metal ion concentrations
could affect the adsorption characteristics of CNTs. Oxi-
dized CNTs have also shown exceptionally high sorption
capacity and efficiency for Pb(II), Cd(II) and Cr(VI) from
water [
78, 93, 94]. CNTs were also reported as good
adsorbents for multi-component sorption of metal ions
[
71]. The sorption mechanisms were reported to be gov-
erned by the surface features, ion exchange process and
electrochemical potential [
95]. The latter plays a significant
role in multi-component sorption where redox reactions,
not only on the adsorbent surface but also among the dif-
ferent adsorbates, are likely to occur. MWCNTs were
found to adsorb
243
Am with extraordinarily high efficiency
by forming very stable complexes [
96]. The sorption
characteristic of Pb(II) from aqueous solution was studied
using oxidized MWCNTs [
81]. The reported results
showed a slope of V/m and intercept of C
o
V/m for the
same initial concentration of Pb(II) and the same content of
oxidized MWCNTs for each experimental data [
81].
Fig. 2 Schematics of SWCNTs
(a) and MWCNTs ( b) (copied
form Ref. [
22])
J Nanostruct Chem
123