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Send Orders of Reprints at [email protected] 140 Nanoscience
& Nanotechnology-Asia, 2012, 2, 140-150
Application of Nanomaterials for the Removal of Pollutants from
Effluent Streams
Gayathri Gangadhara, Utkarsh Maheshwarib and Suresh Guptac,*
aDepartment of Chemical Engineering, Birla Institute of
Technology and Science (BITS), Pilani- 333031, Rajasthan, India;
bDepartment of Chemical Engineering, Birla Institute of Technology
and Science (BITS), Pilani- 333031, Rajasthan, India; cDepartment
of Chemical Engineering, Birla Institute of Technology and Science
(BITS) Pilani, -333031, Rajasthan, India
Abstract: Rapid industrialization with the increase in the
population leads to the water crisis. The number of industries
using heavy metals such as copper, chromium, nickel, zinc, etc. in
their process is also leaving behind the effluent containing a
large amount of heavy metals which discharged directly to the water
bodies. There are constraints set by the regulatory bodies of
government on the industries to maintain an upper level discharge
limit for each of the metal ion. There are various methods
available for the removal of metal ions which are selected
according to the requirement.
Adsorption is one of the optimal solutions for the removal of
metal ions from industrial effluent streams. It is helpful in
reducing the operational cost and size of equipment along with the
increase recovery of metal ions. Adsorption is a surface phenomenon
so the foremost property required for a perfect adsorbent is the
higher surface area. Nanoparticles are now being preferred to be
used as an adsorbent due to their large surface area which is a
very important characteristic for a desired adsorbent. Development
of nanoparticles has been the subject of enormous interest since
the past decade. They have incredible adsorption properties due to
the presence of high-energy adsorption sites and they also have
excellent binding energies or interaction potentials for
physisorption than traditional adsorbents. This study summarized
the use of nanomaterial for the removal of metal ions from
wastewater streams. It also highlights the various types of
nanomaterials, their fabrication method and characteristics. The
mechanism of metal adsorption onto various nanomaterials is also
described in this study.
Keywords: Adsorption, Binding energy, Heavymetals,
Nanoparticles, Physisorption, Surface area, Wastewater.
INTRODUCTION
As a result of increased industrial activities both flora and
fauna are getting affected by the excessive pollution which is
disturbing their ecological balance. Industries that use hazardous
chemicals have the potential to pollute water resources through the
discharge of the effluent to rivers and other water bodies.
Industrial effluents include different wastes such as organic and
inorganic. Heavy metal ions, aromatic compounds and dyes are often
present in the environment due to industrial pollution [1]. As a
result of the strict environmental regulation, it is required to
remove dyes, heavy metals and organic matter from wastewater before
it is discharged to water bodies as these are toxic and even
carcinogenic in nature [2]. The inorganic waste is mainly composed
of heavy metals such as copper in the form of Cu (II), chromium as
Cr (VI), zinc as Zn (II), arsenic as As (II), cobalt as Co (II),
nickel as Ni (II), lead as Pb (II), cadmium as Cd (II), etc.
Presence of heavy metals in wastewater even at trace levels is
considered to be highly risky for mankind [3]. Thus the demand
for
*Address correspondence to this author at the Department of
Chemical Engineering, Birla Institute of Technology and Science
(BITS) Pilani, -333031, Rajasthan, India; Tel: +91 1596515224; Fax:
+91 1596244183; E-mail:
[email protected],[email protected]
developing technologies leading to an effective removal of these
ions from the effluent streams has become a great challenge.
Nowadays owing to the stringent environmental laws and regulations
various methods of removing the heavy metal ions from effluent
streams have been proposed. The various conventional methods
developed so far are filtration, chemical treatment, UV radiation,
adsorption, distillation, precipitation, ion exchange,
electrochemical technologies etc. The various filtration methods
use biosand filters, ceramic filters, charcoal bed and activated
carbon bed. These filters are ineffective in removing organic
contaminants, unable to handle high turbidity and bacteria growth
on filter media. Filters require regular backwashing which causes
high maintenance cost. In chemical precipitation, removal of metals
is achieved by the addition of coagulants such as alum, lime, iron
salts and other organic polymers. The main drawback of this method
is the production of large amount of sludge containing toxic
compounds [4]. In ion exchange cations are exchanged for H+ or Na+.
Ion exchange resins are synthetic polymers having active ion group
such as SO3H. Zeolites are the natural materials that can be used
as ion exchange media. Even though some modified zeolites such as
zeocarb and chalcarb have greater affinity for Ni and Pb, the usage
of this method for inorganic effluent treatment is restricted
because of high cost, requirement of pretreatment
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and partial removal of certain ions [5,6]. UV radiation and
chemical treatment do not treat turbidity, chemical contamination,
heavy metals and some protozoa. Distillation is not suitable for
removing chemical contaminants such as pesticides, fertilizers and
volatile carbon compounds as their boiling point is lower than
water. Distillation process is slow and energy consuming (a heating
source is required), making it expensive. It also consumes large
amounts of water if the coolant used in the distillation process is
water. Materials such as iron and copper in plumbing systems can be
corroded by distilled water. The electrochemical technologies such
as the electrochemical oxidation and electrochemical coagulation
have been used for industrial waste water treatment. But
electrochemical process is mainly used as pre-treatment step to
enhance the biodegradability of waste water [7]. Among these
techniques, adsorption is attractive in terms of design, operation
and scale up, high capacity, insensitivity to toxic substances,
ease of regeneration and low cost. It does not use toxic solvents
and minimizes degradation [8, 9]. The quality of the effluent
generated is also better than the rest of the processes because the
adsorbent has a high affinity towards the metal ions. The affinity
may be due to electrostatic forces of the solute to the adsorbent
surface, van der Waals attraction or chemisorptions. It also has an
advantage of reversibility, where the adsorptive bed can be
regenerated when it gets exhausted with metal ions. This process is
mostly preferred as it requires low maintenance cost, has high
metal removal efficiency, easy to operate and uses solid adsorbent
which resists degradation [2, 3, 10]. The main selection parameters
of adsorbents are the high adsorption capacity, fast kinetics, and
low cost. The various adsorbents such as activated alumina,
activated carbon, calcite, rare earth oxides, etc., which are used
for the removal of metal ions and other pollutants from wastewater
streams. The main disadvantage related to commercial activated
carbon is that it is non selective and ineffective against disperse
and vat dyes and it is expensive [2,11]. Regarding the ease of
availability and cost, low cost adsorbents have gained attention in
the present scenario. The various low cost adsorbents are neem
sawdust, timber bark, tree fern, metal hydroxide sludge, red mud,
fly ash, etc. The low voidage value of commercial and low cost
adsorbents causes the decrease in the available sites for
adsorption and results to low adsorption capacity [12]. Considering
the efficiency and capacity as important parameters in the
selection of adsorbents, the researchers have proposed new
particles on nano-meter scale called as the nanoparticle.
WHAT IS NANOPARTICLE?
Nanoscience and nanotechnology are the emerging fields of
science which offer a significant scientific and technological
advancement in different fields such as medical, electrical,
environmental engineering, etc. The word nano comes from the Greek
word nanos meaning dwarf. Nano term refers to something of a scale
of 10-9 m. Nanoscience deals with the study of atoms, molecules,
and objects having size on the nanometer scale. Nanotechnology is
the manipulation of matter for the use in particular applications
through certain chemical and / or physical processes to create
materials having nanosized dimensions in
the range of 1-100 nm with the specific properties. Nano-
science is gaining much importance nowadays as the properties such
as quantum, mechanical and thermo- dynamics, which are not visible
on the macroscale, are accessible on nanometre scale [13]. New
materials with amazing characteristics can be produced by putting
molecules with desired properties together. Nanoparticles have
greater surface area to volume ratios than larger particles. This
causes them to be more reactive than other materials. Nanoparticles
have vast applications in the field of biomedical, electrical,
environmental engineering fields, etc.
NANOADSORBENTS
Nanoadsorbents find wide range of applications in engineering
field as they are efficient biocompatible adsorbents having large
specific surface area, more active sites and low intra-particle
resistances. Nanoadsorbents have nanoscale pores, high selectivity,
high surface area, high permeability, good mechanical stability and
good thermal stability [14, 15]. Nanomaterials could be of four
types [15]. Carbon based materials composed mostly of carbon
and
they are available in the form of hollow spheres, ellipsoids, or
tubes. Spherical and ellipsoidal carbon nanomaterials are referred
to as fullerenes, while cylindrical ones are called nanotubes.
Carbon nanotubes can remove pollutants from industrial waste water
for the fact that they can establish - electrostatic interactions
[16].
Metal based nanomaterials include quantum dots, nanogold,
nanosilver, and metal oxides, such as titanium dioxide.
Dendrimers are nanosized polymers built from branched units. The
surface of a dendrimer has numerous chain ends, which can be
adapted to perform specific chemical functions. This property makes
it useful for catalysis.
Composites, nanoparticles are combined with other nanoparticles.
Nanoparticles, such as nanosized clays, are added to enhance
mechanical, thermal, barrier, and flame-retardant properties.
The various nanoadsorbents proposed are nanotubes, nanomesh,
nano-filtration membranes, nanofibrous alumina filters, magnetic
nanoparticles, nanoporous ceramics and clays, cyclodextrin
nanoporous polymer, polypyrrolecarbon nanotube composite, etc.
[14]. Carbon nanotubes have large specific surface area, small
hollow and layered structures making them promising adsorbents for
various organic pollutants and metal ions removal. They can be
easily modified by chemical treatment to increase their adsorption
capacity [10]. Nanotubes provide faster flow rates despite smaller
pores because of smooth interior of the nanotubes. It saves energy
as it exhibits fast flow rates that reduce the amount of pressure
required to push the water through the tubes. It can be cleaned by
ultrasonification and autoclaving at 121oC for 30 minutes and reuse
with the same filtering efficiency [17]. Gas adsorption in carbon
nanotubes and nanotube bundles is an important issue for both
fundamental research and its technological application. The
adsorption energy and
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charge transfer of H2 in the interstitial and the groove sites
of tube bundle are considerably larger than those on the surface
site and also the pore site is more favourable than the surface
[17]. Thus, the molecule adsorption of the nanotube bundle is
stronger than that on an individual tube. Titanium nanotube is
reported to be an effective adsorbent for removing Cu(II) within a
range of sodium content >7.23 wt%. The removal capacity is found
to be 120 mg/g at pH value of 5 [18]. Nanomesh is prepared by
comprising nanotubes on a flexible porous medium sufficient to
attach at least one functional chemical group to the nanotube.
Different functional groups are attached to the nanotubes so as to
remove different types of contaminants. Multi walled carbon
nanotubes coated with alumina is reported to be an efficient
adsorbent for the removal of Pb (II) from industrial effluent.
Percentage Pb (II) removal is increased with the increase in
effluent pH between 3 and 7, speed of agitation, contact time and
adsorbent dosage [10, 19]. Multi walled carbon nanotubes are mixed
with polyacrylic acid/PVA mixture to generate uniform nanofibres
through electrospinning. Cu(II) ions are effectively removed with
multi walled carbon nano- tubes and reinforced nanofibrous mats
immobilized with zero valent iron nanoparticles [20].
Functionalized nanotubes provide multiple bonding sites to the
organic/inorganic polymer matrix. Carbon nanotubes are
functionalised by adding functional groups such as hydroxyl,
carboxyl and carbonyl. Amino groups attached to the carbon
nanotubes offer high reactivity and can react with many chemicals.
It has been reported that amino functionalised multiwalled carbon
nanotubes offer the best adsorption capacity for Cd(II) [21]. The
steps involved in the development of amino functionalised carbon
naotubes are carboxylation, acylation and amidation which are
represented by Eqs. 1 3 [22].
MWNTH2SO4 /HNO3! "!!!!! MWNT#COOH (1)
MWNTSOCl2! "!!! MWNT#COCl (2)
MWNTDiamine compound (R)
! "!!!!!!! MWNT# CO-R (3) Another interesting adsorbent is the
nanoparticle having magnetic properties. Adavantages of magnetic
nanoparticle is that, they can be easily removed from water under
magnetic field and have small diffusion resistance. A set of
alginate polymers mainly with magnetite, maghenite and jacobsite
are investigated to have the ability to remove heavy metal ions
such as Co (II), Cr (VI), Ni (II), Pb (II), Cu (II) and organic
dyes [13, 23]. Iron oxide nanoparticles synthesized hydrothermally
is reported to be a suitable adsorbing media for purification of
water from As (V) ions even below the maximum concentration limit
of 10 g [24].
FABRICATION OF NANOMATERIALS
There are mainly two approaches for the fabrication of
nanostructures. These are top down approach and bottom up
approaches. Top down synthesis includes mechanical processes such
as mechanical alloying, high energy ball milling, equal channel
angular pressing, high pressure torsion and accumulative roll
bonding. Bottom up method includes colloidal dispersion, inert gas
condensation, electro deposition, spray pyrolysis, high temperature
evaporation,
flame synthesis and plasma synthesis techniques. A hybrid of
these two approaches is lithography which involves the growth of
thin films as well as etching. Both methods of fabrication are
important but bottom up method is mostly preferred. The top down
synthesize method has some disadvantages such as crystallographic
damage to the processed patterns, internal stress and additional
defects may be introduced during the etching steps. In the bottom
up method materials are built up from the bottom like atom by atom,
molecule by molecule, cluster by cluster sequence. Bottom up
approach offers a better chance to get nanostructures with
comparatively less defects, more homogenous chemical composition
and better range of ordering as it is driven mainly by Gibbs free
energy. So the nanostructures produced by bottom up method are in a
state of close to thermodynamic equilibrium [25-27]. The properties
of the nanomaterials vary with the type of fabrication route. The
most significant characteristics of nanomaterials are the grain
size in case of a single phase material; phase size in case of
multiphase materials; composition of the material, distribution of
phases in case of multiphase components. Various methods are
recommended and given below for the synthesis of nanostructured
materials on the basis of starting phase and the nature of the end
product obtained [27]: a) If the starting phase is vapour, the
techniques available are: o Inert gas condensation (IGC), produces
3D product o Physical vapour condensation, produces 1D product o
Evaporation and sputtering o Plasma processing, produces 3D product
o Chemical vapour condensation, 2D and 3D product o Chemical
reactions, 3D product b) If the starting phase is a liquid, the
techniques available are: o Rapid solidification, 3D product o
Electro-deposition, 1D and 3D product o Chemical reactions, 3D
product c) For solid phase, the techniques available are o
Mechanical alloying, 3D product o Spark erosion, 3D product o
Sliding wear, 3D product Out of the above mentioned techniques IGC
technique produces particles of well defined grain size, narrow
size distribution and high purity. IGC is a bottom up approach
consisting of two steps evaporation of the material and rapid
controlled condensation so as to produce the required particle
size.
FABRICATION OF MEMBRANES USING NANO- MATERIALS Polymeric
membrane and ceramic membrane filtration find a lot of importance
in water treatment. The performance of membrane filtration is
limited by fouling. Membrane
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fouling is caused by rejected colloids, chemicals and microbes
which demands high energy, costly clean-up and replacement of
membranes [28]. It has been reported that membrane fouling can be
reduced to satisfactory levels by adding nanomaterials to the
membranes. Nanomaterial also enhances properties like mechanical
strength, selectivity and reactivity. Ceramic membranes can be made
using metal oxides such as iron oxide, Al2O3, ZrO2, TiO2 [29]. The
development of low fouling membranes using nanoparticles offers
both high degree of control over membrane characteristics and the
ability to produce ceramic membranes in the nano-filtration
membrane range. Ceramic membranes derived from alumina have high
permeability, porosity and narrow pore size distribution. The pore
size distribution of ceramic membrane can be controlled by
controlling the size of alumoxane nanoparticles and sintering
conditions. Alumoxane nanoparticles are the aluminium oxide nano-
particles whose surface is functionalised by organic molecules.
Increase in sintering temperature (>1000oC) tends to increase
the pore size of the membrane as the alumoxane nanoparticles
transform to -alumina particles. Earlier studies stated that
fabricated ceramic membranes with alumoxane nanoparticles have pore
size in the range of 10 to 20 nm under the sintering temperatures
in the range of 600 to 1000oC [30]. The most popular deposition
techniques for sol gel coating are dip-coating and spin-coating
technique. The traditional method of coating is dip coating. As
this method is time consuming and uses lot of solution, alternative
methods are developed. The wet-spraying technique can be a good
alternative to this technique when thickness uniformity is not
important. In this method the polyelectrolyte solution is sprayed
on a vertical surface from an atomiser producing thin, smooth and
stratified films. The unique properties of this method are lower
consumption of solution and time [31]. Spin coating method can be
used to produce multilayered coating. Wet spraying has the
advantages in comparison with a spin-coating process such as high
speed of deposition, high automation potential and the flexibility
in the shape of the substrate (tubular and planar) [29]. Spraying
on a spinning substrate is an appropriate method for the formation
of multilayered coating on nanoparticle with uniform thickness.
This may be due to the fact that the conventional spraying method
solution drains along the substrate by gravity but in spinning
draining is promoted by centrifugal force [31, 32]. For the
preparation of modified Al2O3 membrane, nano sized - Al2O3 grains
are coated on the membrane. The dried membrane is immersed in a
solution containing aluminium isopropoxide and dimethyl benzene.
Aluminium isopropoxide gets adsorbed on the Al2O3 membrane surface.
The thickness of the adsorbed layer depends on the temperature, the
concentration of aluminium isopropoxide and the soaking time [33].
Tubular ceramic absorbers are developed by depositing nanoscale
iron particles on porous alumina tubes for the removal of As(V) (an
extremely toxic contaminant even in low concentrations). Batch
experiment for the removal of As (V) is conducted at room
temperature by adding a constant mass of iron oxide (0.5 g/L) to a
solution of changing initial As(V) concentration. MES buffer
(C6H13NO4S) is used to set
the pH 5. Samples are centrifuged and filtered using a 0.22 mm
filter after 48 hrs. All samples are acidified using 0.3% HNO3 and
the arsenic content in the supernatant solution is analyzed by
Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES)
[34].
SYNTHESIS OF NANOTUBES AND NANO- COMPOSITES
The synthesis of nanotubes can be done by different techniques
such as arc evaporation, sputtering, chemical vapour evaporation,
chemical vapour deposition and plasma enhanced chemical vapour
deposition (PECVD). The most commonly used method for the
fabrication of carbon nanotubes is electric arc discharge method.
The arc occurs in the gas filled space between two conductive
electrodes. At an average temperature of 4000 K, the nanotubes are
formed in the plasma. Co, Ni, or mixtures of certain other metals
are the catalysts which are added to the evaporated single shell
carbon nanotubes. During the arc discharge method, web like
structures are formed around the electrodes having 10-100 single
shell nanotubes. The catalyst addition leads to impurities as it is
added along with graphite to the system. Fabrication process is
followed by purification step as around 33% of the carbon clusters
formed does not contain nanotubes with desired tube like structure
[35]. A new method has been reported in the literature for the
fabrication of carbon nanotube field emitter by attaching single
wall carbon nanotube with high graphitization on Sn or Ni layered
glass substrate. 100 nm thicknesses of nanotubes are deposited on
Sn layer and annealed at 300oC [36]. For the fabrication of titania
nanotubes (TNT), three methods have been reported, which are
template assisted method, electrochemical method and hydrothermal
method. TNT was first synthesized using template assisted method
[37]. Electrochemical anodic oxidation and hydrothermal treatment
succeeded the template assisted method. The template assisted
method increases complexity of the process as TNTs are vulnerable
to get damaged during the fabrication process. Electrochemical
anodic oxidation method does not have the potential to produce
heaps of TNTs and cost of the process is also high. The limitations
of other methods are overcome by the hydrothermal treatment, as it
is an easily modifiable method and produces powdered TNTs in random
alignments. The important factors in the fabrication of TNTs are
applied temperature, treatment time, type of alkali solution used
and Ti precursor [38]. Modifications can be made on the TNTs
produced by this method to obtain different nanostructures such as
single nano wire, branched nano wire, nano rods and nano belts so
as to improve the properties of TNTs. Different concentrations of
NaOH can be used to fabricate modified nanostructures from TNT
[39]. The quantity and length of TNTs increase with an increase in
the temperature (100-200oC). A larger inner diameter of TNTs
emerged at a synthesis temperature of 150oC [40]. Enhanced physical
and mechanical properties can be obtained by using nano sized
materials as reinforcement in titanium matrix composite [41]. TNTs
can be used in the removal of Co2+, Cu2+, Ni2+, NH4+ [H] and NO2
[42]. The adsorption capacity of metal ions by TNT is found out by
immersion method. In this method 0.1g TNT is
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added to 100 mL of desired concentration of Cu (II). Equilibrium
concentration is obtained by keeping the sample in a reciprocating
shaker with isothermal conditions. Centrifugation at 8000 rpm
separates the liquid and solid phases. The resultant supernatant
can be used for analyzing Cu (II) ions by atomic absorption
spectrometry [43]. The adsorption capacity of Cu (II) is found to
be low at acidic conditions and increases sharply by increasing the
pH value up to 5. This may be due to the fact that TNT is
negatively charged above the pH value 3 [44]. pH value above 5 is
not recommended as copper will precipitate at high pH [45,46].
Magnetite (Fe3O4) nanoparticles can be synthesized by the thermal
decomposition method without using toxic organic concentration of
surfactants and solvents. The various methods available for the
synthesizing of magnetite are co-precipitation, thermal
decomposition, micro emulsion route, hydrothermal synthesis route
and continuous flow techniques. Thermal decomposition is selected
over other methods as it offers excellent particle size control of
Fe3O4 particles. Generally thermal decomposition of iron
pentacarbonyl (FeCO5) is used to synthesize nano disperse -Fe2O3
with average diameter of 4 to 16 nm. Poly ethylene oxide (PEO) is
used instead of iron pentacarbonyl because it is costly and toxic.
PEO is considered as a green solvent due to its low toxicity and
high boiling point. PEO functions both as solvent and surfactant to
synthesize Fe3O4. The synthesis is carried out by using iron
acrylonitrate Fe(acac)3. It is observed that as the volume of PEO
increases, concentration of Fe(acac)3 precursor decreases which
results in the production of Fe3O4 nanoparticles having small sizes
[47]. The adsorption properties of Fe3O4 are similar to that of
nano-composites powdered activated carbon. It is fabricated by
modified impregnation method using HNO3 as the carbon modifying
agent. They are excellent in the removal of methyl orange by
adsorption [48]. The batch experiment is carried out by shaking 5
ml of organic dyes solution with magnetic composites at 250 rpm at
room temperature with an initial pH value of 0.1. Measurement of
concentration of dye after the removal of nanoparticles is done
using UV-vis spectrophotometer at a wavelength of 450 nm for methyl
orange [49], 663 nm for methylene blue and 433 nm for cresol red
[50]. The magnetic nanoparticles can be regenerated for reuse by
treating the nanoparticles with methanolic solution of acetic acid
as eluent or using water with a pH > 8. Methanolic solution is
excellent option for the desorption of methylene blue [51]. A
summarized version of particle size of Fe3O4 nanoparticles
synthesized at various concentration of Fe (acac)3 precursor is
shown in the Table 1 [50]. Carbon nanocages (CNC), synthesized
using a supercritical fluid deposition method, are reported as an
excellent adsorbent than commercially available activated carbons
for the removal of Pb (II) ions from aqueous solutions. The
catalysts used are Co/Mo/MgO, which are heated at a rate of 5oC/min
in a reaction cell in the presence of argon. The flow rate of argon
is maintained at 200 ml/min for reducing the catalysts. p-xylene
(3mL) is placed in the front part of the delivery cell separated by
a piston, followed by the charging of CO2 to maintain a constant
pressure. The catalyst is agitated in 3 M HNO3 solution for 4 h and
is
removed after the completion of reaction. Scanning electron
microscopy (SEM) images show the high porosity of CNCs [52].
Average pore diameter is obtained in the range of 19-24 nm. The
iron/graphite core-shell nanocages are prepared by the pyrolysis of
acetylene with iron carbonyl. Heat treatment is given to the
nanoparticles in the presence of iodine to remove the metallic core
so as to obtain CNCs with good graphitization and high purity [53].
The batch experiment is carried out by preparing 1000 mg/dm3 of
stock solution of Pb2+ in deionised water by dissolving Pb(NO3)2.
This stock solution is used to make 25 mg/dm3 of Pb2+ solution. pH
of the solution is adjusted with 0.01 M NaOH. 10mg of adsorbent is
added to 20 ml of the 25mg/dm3 solution and stirred for 4 h to
achieve adsorption equilibrium [52,54]. As there is an increase in
the adsorption of Pb2+ ions with increase in pH, ion exchange
mechanism exists on the surface of adsorbent between the H+ ions
and metal ions. Pb hydroxide complexes are produced at pH > 6
[55]. Magnetic hydroxyapatite nanoparticles (MNHAP) have been
synthesized by coprecipitation method [56]. Appropriate amount of
FeCl24H2O (1.85 mmol) and FeCl36H2O (3.7 mmol) are dissolved in
30mL of deoxy- genated water under a nitrogen atmosphere at room
temperature. 10 mL of 25% NH4OH solution is added to the resulting
solution under vigorous mechanical agitation (300 rpm) which forms
a black precipitate. After 15 min, an amount of 50 mL of Ca
(NO3)24H2O (33.7 mmol) and 50mL of (NH4).2HPO4 (20 mmol) at pH
value 11 are added dropwise to precipitate with simultaneous
agitation for 30 min. The resulting suspension is heated at 90oC
for 2 hrs followed by cooling to obtain the precipitate which is
separated by magnet followed by washing with deionised water and
drying at 90oC. The washed precipitate is grinded to form the MNHAP
adsorbents. This method is reported to produce MNHAP with spherical
shape with a diameter of about 28 nm. MNHAP adsorbents have been
reported to be effective for the removal of Cd (II) and Zn (II)
ions from aqueous solutions. The properties of this magnetic
adsorbent such as size and morphology (porosity), adsorption
efficiency towards Cd (II) and Zn (II), surface area (142.5m2/g),
strong magnetic response (59.4 emug1), structure and zeta potential
of suspensions are characterized by SEM, energy dispersive analysis
system of X-ray (EDAX), BET surface area measurements,
magnetization curves, X-ray powder diffraction (XRD) analysis,
magnetization curves and zeta meter respectively. In the batch
experiment 0.002 g adsorbent and 20 ml metal solution with a pH
value of 50.1
Table 1. Summary of Particle Size of Fe3O4 Nanoparticles
Synthesized at Various Concentration of Fe(acac)3
Concentration of Fe(acac)3, ppm mmol Particle sizes (nm)
0.1 2
1 4
2 5
4 6
8 7
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are taken in conical flasks and kept on a constant temperature
bath oscillator at room temperature. MNHAP are separated by using
permanent magnet [56]. Fe3O4 coated polypyrrole (PPy) magnetic
nanocomposite fabricated by in situ polymerization of pyrrole
monomer. This is reported to be an efficient adsorbent for the
removal of highly toxic Cr (VI). Structure and morphology of the
prepared nanocomposite are characterized by total reflectance
Fourier transform infrared spectroscopy (ATR-FTIR), X-ray
diffraction pattern, Field emission scanning electron microscopy
(FE-SEM) and high resolution transmission electron microscopy
(HR-TEM). Magnetic nature of the nanocomposite is confirmed by
Electron spin resonance (ESR) studies. 100% adsorption is achieved
at a pH value 2 with 200 mg/L Cr (VI) aqueous solution. ATR-FTIR
and X-ray photoelectron spectroscopy (XPS) confirms adsorption of
Cr (VI) on the surface of the adsorbent and also proposed the
possible mechanism of adsorptions as ion exchange and reduction on
the surface of the nanocomposite [57]. The batch experiments are
performed by mixing 50 mL of Cr (VI) and the adsorbent followed by
agitation in thermostatic shaker at 200 rpm for 24 h. Removal Cr
(VI) ions by nanocomposites decreases from 100% (at pH = 2.0) to
57% (at pH = 11.0) [58]. Adsorption experiments for Pb (II), Cd
(II) and Ni (II) ions are carried out using the batch technique at
25-55C. It is reported that an average of 120 mg of wet magnetic
nanoadsorbents are added to 10 mL of Pb (II), Cd (II) and Ni (II)
solution of various concentrations (from 50 mg L-1 to 400 mg L-1)
and shaken in a thermostatic water-bath shaker operated at 230 rpm.
The magnetic nanoadsorbents are removed after equilibrium is
reached using a permanent Nd-Fe-B magnet. The concentrations of Pb
(II), Cd (II) and Ni (II) ions are measured using Inductive Couple
Plasma Mass Spectrometry. For the kinetic experiments, samples are
collected at various interval and the concentrations of heavy
metals are determined [59]. Nano iron oxide impregnated granular
activated carbon (nFe-GAC) is an effective adsorbent for the
removal of phosphate from aqueous solutions. The batch process
steps include addition of nFe-GAC to a centrifuge containing
polypropylene and phosphate solution at various concentrations
mainly 250, 500 and 1000 mg/L. Samples are kept in a water bath
shaker for 13 days at 25oC, supernatant is collected at regular
intervals and the phosphate concentration is measured using
spectrophotometer. The kinetics of phosphate adsorption by nFe-GAC
involves a fast initial sorption followed by a much slower
adsorption process [60, 61].
Nano-crystalline calcium hydroxyapatite (HAp) has the potential
to remove Ni (II) from aqueous solutions. Solution precipitation
method is used to synthesize Nano-crystalline hydroxyapatite
adsorbents. (NH4)2HPO4 and Ca(NO3)24H2O are the starting materials
and pH is adjusted by using ammonia solution. Ca (NO3)24H2O
suspension is made by continous agitation at a temperature
maintained at 25oC and a solution of (NH4)2HPO4 is slowly added
dropwise to it. Centrifugation at a rotation speed of 3000 rpm is
used to remove the HAp precipitate and the resulting powder is
dried at 100 C. The particles produced are characterized by
Transmission electron microscopy (TEM). The crystalline
shapes and sizes are characterized by diffraction (amplitude)
contrast and, for crystalline materials, by high resolution (phase
contrast) imaging, BET method for specific surface area and crystal
phase by X-ray [56,62]. TEM micrograph of the HAp powder shows that
the HAp crystalline structure after drying is more or less needle
shaped, with size in the range 2030 nm [62]. Batch experiments are
performed in a stirred tank reactor (300 rpm) by filling it with
nickel sulfate solution and 0.4g of nano-HAP at 201oC. The solution
has initial pH of 6.6. Residual metal ion concentration is measured
by taking samples at constant time intervals. The same procedure is
followed for the removal of Sn2+ [63]. The equilibrium sorption of
these ions increases with increase in temperature due to increase
in the mobility of the ions. A swelling effect is produced within
the internal structure of the nano-Hap when the temperature is
increased for enabling metal ions to penetrate more [64]. Nano zero
valent iron (nZVI) particles are used for the removal of Cd (II),
Cr (IV), Ni (II) and Pb (II). The usual method reported for the
synthesis of nZVI is the bottom-up approach and also called as
liquid phase reduction by the drop wise addition of NaBH4 to FeCl3
or Fecl2.4H2O provided continuous stirring. The reaction occurring
during the process is given by Eq. 4.
4Fe3+
+ 3B!H4+ 9H
2O" 4Fe0 # +3H
2B!O3+12H
++ 6H
2$ (4)
nZVI particles are then washed with iso-propanol or absolute
ethanol to prevent oxidation. nZVI synthesis is done in an
anaerobic chamber by purging with O2-free Ar (95% Ar : 5% H2). The
lifetime and using efficiency of nZVI are affected by metal
concentration, pH, and temperature. The properties of the nZVI are
characterized using XPS, XRD, HR-TEM and SEM/EDX [65-70]. SEM
images of fresh nZVI particles show that the particles are composed
of separate spherical particles in the size range of 20-200 nm in
the form of aggregates and chains [65]. The temperature effect on
adsorption of Cd2+ is carried out at different temperatures. 0.5g/L
nZVI is added to 60 mL of 112.5mg/L Cd2+ solution. The samples are
kept on a temperature controlled shaker. Results show that a rise
in the temperature results in increasing cadmium adsorption rate
which indicates that the process is endothermic [65].
CuFe2O4/sawdust nano magnetic composite with a mass ratio of 1:10,
synthesized by chemical co-precipitation method is reported to be
used for the removal of cyanine acid blue (CAB) from aqueous
solution [70, 71]. The properties of the nanocomposite are
characterized by Fourier transform infrared spectroscopy (FTIR) and
SEM. SEM image shows that the composite has a spherical shape with
50 nm particle size. The FTIR spectroscopic analysis of sawdust
nanomagnetic composite indicates that broad bands are at 34063556
cm1. The co-precipitation method involves the dissolution of
sawdust in a 50ml solution containing Cu (II) chloride (0.1ml) and
ferric chloride (0.2ml) at 60oC. Suspension pH is raised to 10 by
the addition of NaOH solution with vigorous agitation. The magnetic
precipitate formed is washed and dried at 105oC [72,73]. Adsorption
affinity for Cu(II) [74], Pb(II) [75], and Cd(II) is increased by
grafting amino groups on the silica surface of the nanomaterial
[70,76]. The experimental procedure remains
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146 Nanoscience & Nanotechnology-Asia, 2012, Vol. 2, No. 2
Gangadhar et al.
the same as other magnetic nanocomposites. The rotary shaker
speed is maintained at 150 rpm and experiments are carried out at
25oC. Nano structure alumina is reported as a good adsorbent for
the removal of heavy metals such as Pb(II), Ni(II), Zn(II) and
Cu(II) [77]. The adsorption process follows pseudo-second-order
reaction kinetics. Equilibrium data are well fitted by the Langmuir
and Freundlich adsorption isotherms. The adsorption capacity of
nano structure alumina for Pb(II), Ni(II) and Zn(II) are 125, 83.33
and 58.82 mg g1 respectively. Batch experimental studies are
carried out in Erlenmeyer flasks which contain nanostructured
-Al2O3 and metal ion solution at pH value of 4. The samples are
mixed well. After 3 hrs, solid-liquid phases are separated by
centrifugation. The properties of nano structure alumina are
characterised by SEM, BET and FTIR [78, 79]. The SEM and TEM images
indicate that the alumina nanoparticle has a mean diameter of 53 nm
and the modified alumina nanoparticle has a mean diameter of 75 nm
[79]. The modified anodic aluminium oxide membrane is able to
remove heavy metal ions from aqueous solution as polyrhodanine has
the ability to form coordinate compounds with specific metal ions.
Polyrhodanine is deposited onto the inner surface of anodic
aluminium oxide (AAO) membrane by vapour deposition polymerization
method. Polyrhodanine is fabricated by adding iron chloride to
rhodanine aqueous solution which is forming coordinate compounds.
Magnetic nanoparticles are formed after injecting sodium
borohydride. Fe ions induce oxidation of rhodanine monomers
initiating the polymerization rhodanine. Polyrhodanine magnetic
nanoparticles are obtained at the end of polymerization [80].
The fabrication process of polyrhodanine magnetic nano-
particles is shown in Fig. (1) [80]. TEM images indicate that the
PR-MNPs have an average diameter of 10 nm. BET surface area is
obtained as 94.65 m2/g. The modified anodic aluminium oxide
membrane is able to remove heavy metal ions from aqueous solution
as polyrhodanine has the ability to form coordinate compounds with
specific metal ions. Polyrhodanine is deposited onto the inner
surface of anodic aluminium oxide (AAO) membrane by vapour
deposition polymerization method. The heavy metal removal
capability of the AAO-polyrhodanine mem- brane in form of catridge
is investigated. Aqueous solution of heavy metal ions (10 ml) is
passed through the membrane at 2 ml/h (Fig. 2) and final metal ion
concentration is measured. Kinetic study is carried out by
collecting aliquots of filtered solution as a function of time
[81].
MECHANISM OF HEAVY METAL REMOVAL USING NANOADSORBENTS
Fe3O4 magnetic nanoparticles (MNPs) modified with
3-aminopropyltriethoxysilane (APS), copolymers of acrylic acid (AA)
and crotonic acid (CA) are good adsorbents for removing heavy metal
ions such as Cd (II), Zn (II), Pb (II) and Cu (II) from aqueous
solutions. The surface of the adsorbents is in carboxyl form and
has less adsorption of heavy metal ions when pH is less than pH of
zero point (between 3 and 4). The carboxyls turn into carboxylate
anions and the adsorption increases gradually until pH > pHpzc
which is due to increase in alkalinity. Finally, carboxyls
completely turn into carboxylate anions. The probable adsorption
mechanism is shown in Fig. (3). Chelation
Fig. (1). Schemcatic illustration of the fabrication process of
polyrhodanine encapsulated magnetic nanoparticles [80].
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Application of Nanomaterials for the Removal of Pollutants from
Effluent Nanoscience & Nanotechnology-Asia, 2012, Vol. 2, No. 2
147
between the ions and the carboxylate anion provide the main
interaction between metal ions and adsorbents [82]. Micro-nano
structure poly (ether sulfones)/poly (ethyleneimine) (PES/PEI)
nanofibrous membrane is utilized as an adsorbent for the removal
anionic dyes and heavy metal ions from aqueous solutions. Amino and
imino groups on the PEI macromolecular chains provide the main
adsorption sites. They possess bifunctional properties to adsorb
cationic and anionic target compounds at different pH values in
aqueous solutions [83]. A relatively high concentration of protons
are available to protonate amine and imino groups of PEI chain to
form NH3+ and NH2+ at low pH values. This can lead to strong
electrostatic repulsion of the
cationic metal ions to be adsorbed. At lower proton
concentrations (pH of 5-7) the availability of protons to protonate
NH2 and NH to form NH3+ and NH2+ groups would be less. A metal
complex can be formed when neutral nitrogen of amine and imino
group with lone pair of electrons binds a metal ion [84, 85]. The
adsorption capacity of the anionic dyes decreases with increase in
solution pH value from 1 to 7 and it increases with increase in pH
from 1 to 7 for heavy metal ions. The adsorption of anionic dyes on
PES/PEI nanofibrous membrane is reported to be endothermic and the
adsorption of heavy metal ions on PES/PEI nanofibrous membrane is
exothermic [86]. Amino-functionalized materials are reported to be
efficient for the removal of heavy metals. Electrostatic
interaction [87], ion exchange [88], hydrogen bonding [89], are the
ways by which the removal of anionic metal species can be achieved.
For the removal cationic metal species, coordination interaction
between metal ions and amino groups may be the possible mechanism
[90-92]. For the removal of Cu (II), the chemical reactions
observed are given by Eq. 5 7. Protonation/deprotonation reaction
of the amine groups of the NH2-NMPs in the solution:
!NH2+H
+" NH
3
+ (5)
Formation of surface complexes of Cu2+ with the amine groups
through coordination interactions:
!NH2+Cu
2+" NH
2Cu
2+ (6)
!NH2+OH
+"!NH
2OH- (7)
!NH2OH!+ Cu
2+ (or CuOH
+"!NH2OH ! .......Cu
2+
(or ! NH2OH ! .....CuOH+
)(8)
Low pH values favour the protonation of the amine groups,
causing less number of NH2 sites available on the adsorbents
surface for Cu (II) adsorption through coordination interactions.
This leads to an increase in the electrostatic repulsion between
the Cu (II) ions and the adsorbent surface which causes a decrease
in the Cu (II) adsorption with decrease in pH values. As the pH
increases, the deprotonation reaction is favoured. This is due to
the
Fig. (2). Schematic diagram of removal procedure of heavy metal
ions from wastewater using the fabricated AAO-polyrhodanine
membrane as a filter [81].
Fig. (3). Schematic representations of possible mechanism for
adsorption of metal ions by Fe3O4@APS@AA-co-CA [81].
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148 Nanoscience & Nanotechnology-Asia, 2012, Vol. 2, No. 2
Gangadhar et al.
increase in the number of NH2 sites on the surface of the
adsorbents for Cu (II) adsorption which results in increased
adsorption capacity. At high pH values, adsorption capacity
decreases as adsorption of OH is favoured, but adsorption capacity
increases through electrostatic attraction as indicated in the last
reaction [93].
CONCLUSIONS
Advancements in nanoscience and engineering are giving new
opportunities to develop more cost-effective and environmentally
acceptable water treatment technology. The property of having
higher specific surface area leading to a higher capacity makes the
nanoparticles one of the best adsorbents for the effective removal
of heavy metals from waste water. They also exhibit various
advantages such as fast kinetics, and preferable sorption toward
heavy metals in effluent streams. As discussed, earlier
nanosorbents such as CNTs, TNTs, nanometal oxides, magnetic
nanoparticles etc are reported to be successful in removing various
heavy metal ions such as Pb(II), Ni(II), Zn(II),Cu(II),
Co(II),Cd(II) from industrial wastewater. Some aspects such as the
toxicity and cost effectiveness are to be studied in detail to make
the process economically feasible on an industrial scale.
CONFLICT OF INTEREST
The author(s) confirm that this article content has no conflict
of interest.
ACKNOWLEDGEMENTS
Declared none.
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Received: September 05, 2012 Revised: November 24, 2012
Accepted: November 30, 2012
NN Asia_Contents.docFinal Graphical
Abstract1-Editorial-MS.NNA2-Zhao Graphene-MS3-Dutta
Nanostructured-MS4-Niu Design-MS5-Nuraje and Kudaibergenov.docx6-Du
New Generation-MS7-Gupta Application-MS8-Chopabayeva-MS9-Zhu
Self-MS10-Zhang Sol-gel-MS11-Chaudhary-MS12-Diaz-MS13-Jain-MSList
of Rev NNA 2NNA Asia_BACKNNA 2-2-Spine