Cent. Eur. J. Chem. • 10(1) • 2012 • 27-46
DOI: 10.2478/s11532-011-0134-3
Central European Journal of Chemistry
Removal of heavy metals from wastewater using
micellar enhanced ultrafiltration technique:
a review
Review Article
Alka A. Mungray*, Shrirang V. Kulkarni, Arvind K. Mungray**
Department of Chemical Engineering,
Sardar Vallabhbhai National Institute of Technology, Surat 395007, India
Received 2 July 2011; Accepted 19 October 2011
Abstract: Application of Micellar enhanced ultrafiltration (MEUF) for the removal of different heavy metals has been reviewed. It is considered
an economical alternative available to the conventional membrane separation process, because it reduces the requirement of higher
pressure and high membrane costs. MEUF is a separation processes which uses surfactants and ultrafiltration membranes to remove
multivalent ions from wastewater with high percent rejection using electrostatic attraction between metals and micelles.
This review seeks to define the effect of the operating parameters, i.e., applied pressure, surfactant concentration,
feed temperature, metal ion concentration, feed flow rate, operating time etc. on the removal of metal ions. Emphasis is given
to the application of MEUF for the removal of single metal ions, multiple metal ions and different metals along with other organic
materials. Also, this review focuses on studies related to micelle formation, attraction between metal ions and micelles, and recovery
of surfactants for future research.
Keywords: Heavy metal ions • Micellar enhanced ultrafiltration • Removal • Operating parameters • Rejection
© Versita Sp. z o.o.
1. Introduction
Heavy metals present in the wastewaters discharged
from industries are a subject of major concern for the
environmental issues. The Environmental Protection
Agency (EPA) has assessed the hazards caused by the
various heavy metals. Table 1 shows the discharge limits
of various heavy metals, their potential health effects on
humans along with their sources as per EPA [1].
When these metal ions present at excessive levels in
an aqueous discharge, the stream remains unusable due
to the adverse effects associated with consumption [2].
These metal ions are highly toxic and if they are directly
discharged can cause environmental imbalance and can
damage the subsequent treatments associated in the
wastewater purification plants. Therefore, inexpensive
and efficient methods of wastewater purification or
improvements in the existing methods will have to be
made to adjust the new requirements [3].
The classical methods such as adsorption, ion
exchange, chemical precipitation and evaporation have
been used for the removal of metal ions from aqueous
* E-mail: [email protected]
** E-mail: [email protected]
effluents [4-6]. But these methods are not capable
of reducing toxic level of the metals considerably.
Membrane separation technique is found as an easily
achievable and better technique for the separation of
toxic metals from wastewater which is used frequently
for the separation process [7]. Several membrane
based separation techniques including nanofiltration
and reverse osmosis have been developed to remove
undesired constituents from aqueous streams [8-10].
These processes are carried out in homogeneous
solution. The usual processes such as reverse osmosis,
nanofiltration seems to be highly expensive due the
requirement of higher pressures and high membrane
cost. Micellar enhanced ultrafiltration (MEUF) can be
considered as a good alternative to remove heavy metals
from wastewater using micelle. It was first introduced by
Scamehorn for the removal of both dissolved organic
compounds and heavy metal ions from waste stream
[3,7,11]. Number of investigations has been carried out for
the removal of metal ions from wastewater using MEUF
technique. The present paper attempts to review those
investigations. The influence of various parameters such
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Removal of heavy metals from wastewater
using micellar enhanced ultrafiltration technique: a review
as applied pressure, temperature, feed flow rate, feed
ion as well as surfactant concentration, operating time
etc. are considered for the review. Emphasis is given to
review MEUF technique for the removal of single, and
multiple metals along with other organic materials.
1.1. Micellar Enhanced Ultrafiltration (MEUF)
Micellar enhanced ultrafiltration is a membrane based
separation technique for metal ions, organic pollutants
or inorganic compounds from aqueous streams. In this
process, surfactants are added into the aqueous stream
at levels equal to or higher than their critical micelle
concentrations (CMCs). The minimum concentration at
which micellarization occurs is called the critical micellar
concentration (CMC). At this particular surfactant
concentration, surfactant monomers will assemble
and form aggregates called micelles. Metal ions and
organic compounds tend to be soluble in the micelles by
electrostatic or Van der Waals force. This micelle solution
is then filtered through an ultrafiltration membrane
with an appropriate molecular weight cut-off (MWCO)
size. The micelles containing the solubilized pollutants
can thus be removed by the ultrafiltration membrane
[7]. Generally, in MEUF, as the concentration of the
surfactant increases up to CMC, the retention coefficient
of the removing metals also increases [11]. MEUF is
having certain advantages such as: low operating cost
and high removal efficiency, high permeate volume flux
etc. In short, this technique combines the high selectivity
of reverse osmosis and the high flux of ultrafiltration.
Due to these properties MEUF is used for the removal
of heavy metals [12]. The MEUF process can be carried
out by two types of mechanisms namely:
Dead end filtration: It is a conventional form of
filtration, mostly used in flat sheet
membranes, but
also widely used in hollow fibre applications [13-15].
Cross flow filtration: In cross flow filtration, feed
flows parallel to the membrane surface. Solids available
in the feed are trapped in the membrane, and the filtrate
is released at the bottom of the membrane. Cross-flow
filtration gets its name because the majority of the feed
flow travels across the filter surface, rather than into the
filter [16]. It is having advantages over tangential flow, for
example: retentate is substantially washed away during
the filtration process, increasing the length of time that
a filter unit can be operational. It can be a continuous
process, unlike batch-wise dead-end filtration.
2. Effect of operating parameters
on MEUF
2.1. Effect of applied pressure
The permeate flux in presence of constant surfactant
concentration varies linearly with applied pressure.
This may be due to the fact that the operating pressure
between retentate and permeate was the effective
driving force for process. The increase of this could
overcome the osmotic pressure and the resistance
(micelle aggregation layer (MAL)), thereby forcing more
solution to filter through the membrane and leading to a
higher permeate flux [17-19]. At CMC the concentration
of micelles near the membrane surface increases.
Therefore, more sites are available for the attachment
of metal ions, which increases rejection. The pressure
should be varied according to the capacity of membrane
to withstand [20,21].
2.2. Effect of surfactant concentration in feed
solution
Concentration below CMC, micelles does not appear
and surfactant remains present in the form of monomer.
These monomers form complex with the metal ions
which can easily pass through the membrane pores.
At smaller concentrations, due to the membrane effect,
monomer attracts towards membrane and either
adsorbed in pores or lie on the membrane surface to
form gel layers [22]. When surfactant concentration
increased up to CMC, the micelle formation takes place
to provide sites for metal ions to attach and the rejection
of metal ion increases [23]. On further increase in the
concentration of surfactant, formed micelles break into
smaller molecules and forms surfactant aggregates of
smaller size. These aggregates then can bind with the
metal ions and easily pass through the membrane to
increase the concentration of the metal in permeate
[24,25].
2.3. Effect of feed temperature
Figure1.
Schematic diagram of the mechanism of MEUF.
The permeate flux varies linearly with increase in
temperature; this statement is true for distilled water
as well as for surfactant solution. As the temperature
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A. A. Mungray, S.V. Kulkarni, A. K. Mungray
increases, permeate flux also increases due to thermal
expansion of membrane material and lower viscosity of
the solution. But this increased flux resulted in higher
concentration polarization [26-29].
For MEUF, temperature is the most important
parameter because; CMC of surfactant is a function
of temperature. CMC of the surfactant increased with
the increase of temperature due to the de-micellazation
process because of the disruption of the palisade layer
of the micelle. Thus, surfactant ions start detaching
from micellar bulks. For example, CMC values of
sodium dodecyl sulphate (SDS) are 2.257, 2.445
and 2.706 mg L-1, at the temperature of 25, 40 and
45°C respectively. As CMC values increase, Kraft point
also increases, which results in the increase resistance
of surfactant. Researchers investigated that at the high
temperature the Cetyl pyridinium chloride (CPC) micelle
gets easy to dissociate and decreases micelle number
and its size. Because of this reason, the passage of
more CPC monomers in the permeate. And one more
reason of passage of CPC in the permeate was may be
due to the thermal expansion of membrane [27].
2.4. Effect of metal ion concentration in feed
Without surfactant and at high concentration of metal
ions, as metal ion concentration in feed increases, the
permeate flux decreases. This may be due to the increase
of concentration difference across the membrane and
subsequent increase in the osmotic pressure opposes
the permeate flux [30]. It is well known that the increase
of metal cation concentration releases the repulsive
forces between the head groups, and the formation of
micelles become easier [31,32].
The increase of metal concentration promoted more
surfactant molecule present in micelle form, thereby
resulted in the increase of surfactant retention. As the
feed metal ions concentration increased, metal ions
retention increased firstly, and then decreased quickly
because the micelles became more and more saturated.
As the feed metal concentration increases, the number
of free metal ions in the solution also increases
proportionally because of equilibrium between the
adsorbed ions and free metal ions in the bulk, thus the
metal ion concentration in the retentive also increases.
Another reason is that when increasing the heavy
metal feed concentration, the zeta potential of the
micelles increases, which results in decrease the
surface charge density. So, the reduction in retention at
higher feed concentration might be due to the lack of
available binding sites. Mostly, MEUF is more efficient in
solutions with diluted metal concentrations, in compare
with conventional techniques such as precipitation,
which are inefficient at dilute streams [33].
There was no significant influence on the permeate
flux when the feed metal ions concentration increases.
This is in accordance with the result of micelle size.
In order to find an efficient retention of heavy metals, the
surfactant feed concentration has to be high enough to
create micelles and to have enough available binding
sites. But, in order to find the maximum retention in the
process optimisation, it is essential to find the optimum
S/M ratio [34].
2.5. Effect of salt concentration
If salts concentration is increased in feed, CMC is
decreased for ionic surfactants. This may be due to the
electrostatic shielding effect, i.e., the repulsive forces
between the head groups are normally fighting against
the aggregation, which becomes easier in the presence
of electrolyte. Therefore, micelles formation can take
place at less than CMC. An empirical law has been
proposed to take into account the salt effect observed
with different kinds of surfactants;
where Cc is the total counter ion concentration and α, β are
constants for a particular ionic head group. An increased
salt concentration leads to decrease in rejection.
This may be because of two reasons. First, while adding
salt, the electrical double layer would compress because
of increased electrostatic concentration. Because of this
reason, reduction is possible in electrostatic attraction
between ions and the micelles. Another reason is that
as salt concentration increased the competitors of metal
ions also increased in the feed solution. Therefore,
decreases the attachment of metal ions with micelles
in presence of salts. The relative flux slightly decreased
with the concentration of salt. This may be due to lower
surfactant leakage [35,36].
2.6. Effect of feed flow rate
The increase in feed flow rate in tangential system
shows increase in the rejection up to certain point and
then it starts decreasing. This point obtained can be
considered as optimal flow rate. On further increase in
flow rate, some of the micelles may get forcibly pumped
through the pores along with unbounded metal ions
which lead to decrease the rejection [37]. The increase in
the flow rate causes increase in velocity and turbulence
near the membrane surface. Increase in turbulence
causes increase in mass transfer across the membrane
surface results in increase in the permeate flux as well
as rejection [38].
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Removal of heavy metals from wastewater
using micellar enhanced ultrafiltration technique: a review
2.7. Effect of operating time
With increase in time, the permeate flux decreases
slightly but with further increase, it remains constant
in tangential system. The concentration polarization
occurring causes the flux to decrease slightly. Initially
the micelles deposit on the membrane surface and
after reaching high concentration, gel layer formation
occurs. The micelles present on the membrane surface
may block the membrane pores and causes resistance
to flow, so permeate flux decreases. But on further
increase in time the thickness of the gel layer deposition
at the surface remain constant so the permeate flux
remains constant. Similar results for rejection are also
observed [37].
2.8. Effect of the ratio of surfactant ion to
metal ion concentration (S/M)
Surfactant is often used in the selective separation of
metal ions from wastewater [39-41]. With increase
in S/M ratio, the rejection of the metal ion increases
because of more surfactant micelles availability for the
metal ions to bind. It is observed that at certain ratio,
maximum rejection of metal ions is obtained which is
nothing but the optimal ratio. The dead end system is
observed to have optimal ratio of 10 while cross flow
system observed to have a ratio of 7 [21,38]. The
difference is because of turbulence improvement upon
membrane surface [42].
2.9. Effect of ratio of chelating agent to metal
ion concentration (C/M)
The chelating agent is used often for the separation of
mixture of metals from wastewater. Chelating agents
used which form complex with the target metals and
help in the separation [43-45]. At low concentration of
chelating agent, complexation bond between metal and
chelating agent is weaker for remaining metals except
to the target metal. So the remaining metals bind with
the surfactant which increases rejection. On further
increase in concentration interaction between surfactant
and chelating agent increases, to form complex between
them and the rejection decreases [46].
2.10. Effect of surfactant types
In MEUF, use of surfactant can be reduced by reducing
the CMC. One way of it is by using non- ionic surfactant.
In the mixture of surfactant, hydrophilic parts of nonionic surfactant counter balance the charge of ionic
hydrophilic groups [47]. It results in decrease of charge
density at the surface of micelle leading to diminish of the
electrical potential. Eventually, it enhances the formation
of micelle at the lower CMC but the retention of metal
ions reduced slightly [47,48]. Permeate flux decreased
by non-ionic surfactant in micelle solution; it was mainly
due to the higher viscosity of non ionic surfactant [48].
One more reason is that the decrease of permeate flux
was mainly due to the transition of micelle configuration
from spherical to cylindrical or lamellar.
2.11. Effect of ultrafiltration membrane types
Metal rejection can vary depending upon the type
of ultrafiltration(UF)membrane whether they are
hydrophilic or hydrophobic. Hydrophilic part of micelle or
monomer tries to adsorb on the hydrophilic membrane
surface compared to hydrophobic membrane surface
[36]. The type of membrane such as hydrophilicity or
hydrophobicity has higher effect on permeates flux
than by transmembrane pressure (TMP). Jonsson and
Jonsson [49] indicated that flux reduction was much
higher in the hydrophobic membranes than that in
hydrophilic membranes.
3. Removal of metals by MEUF
technique
MEUF is used to remove different heavy metals from
wastewater containing single metal, mixture of metals,
feed mixtures containing metals as well as organic
material etc. Below data is compiled from the best of
authors knowledge from various literatures for the
removal of metals on the above mentioned scenarios
and accordingly five tables (Tables 2-6) are prepared for
ready reference.
3.1. Removal of single metals
Table 2 is prepared from the data taken from various
literatures only for the removal of single metal by MEUF
process. Many research papers were found for the
removal of a particular metal and accordingly variable
rejections were found. In MEUF, mostly polymeric
membranes are used. That is why, the type of surfactant
used and the parameters studied for the rejection are
also compiled.
3.1.1. Removal of copper ions
The metal removal by simple MEUF was first reported
in 1986 by Scamehorn, in which a divalent copper
was removed and the observed rejection was 99.8%.
It was observed that purity of the permeate decreases
as concentration of the metal in the feed increases.
UF membrane of 20kD was used in this study [2]. With
the application of ionic exchange model, prediction of
the influence of positive metal ion complexation upon
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A. A. Mungray, S.V. Kulkarni, A. K. Mungray
Table 1. Discharge limit of various heavy metals, their potential health effects and common sources as per EPA.
Heavy metal
Permissible
discharge limit in
wastewater body
(mg L-1)
Potential health effects from
long-term exposure above the
MCL*
Common source of contamination
Antimony
0.02
Increase in blood cholesterol, decrease in
blood sugar
Discharge from petroleum refineries, fire
retardants, ceramics, electronics solder
Arsenic
0.2
Skin damage or problems with circulatory
systems and may have increased risk of
getting cancer
Erosion of natural deposits; runoff from
orchards; runoff from glass and electronics
production wastes
Cadmium
2.0
Kidney damage
Corrosion of galvanized pipes; erosion of natural
deposits; discharge from metal refineries, runoff
from waste batteries
Hexavalent
chromium
0.1
Allergic dermatitis
Discharge from chemical and agricultural
chemical factories
Copper
3.0
Short term exposure: gastrointestinal distress.
Long term exposure: liver or kidney damage
Corrosion of household plumbing systems;
erosion of natural deposits
Lead
0.1
Infants and children: delays in physical or
mental development; adults kidney problems;
high blood pressure
Corrosion of household plumbing systems;
erosion of natural deposits
Iron
3.0
Risk of lung cancer
Naturally occurring in groundwater and corroded
water system pipes
Mercury
0.01
Kidney damage
Erosion of natural deposits; discharge from
refineries and factories; runoff from landfills
Selenium
0.05
Hair or fingernail loss; numbness in fingers or
toes; circulatory problems
Discharge from petroleum and metal refineries;
erosion of natural deposits; discharge from
mines
Thallium
0.002
Hair loss; changes in blood; kidney, intestine
or liver problems
Leaching from ore-processing sites; discharge
from electronics, glass and drug factories
Zinc
Manganese
Nickel
5
2.0
3.0
Nausea, vomiting, loss of appetite, abdominal from corrosion of galvanized pipes by soft, acidic
cramps, diarrhea, and headaches
water and from fertilizer company effluent
Increased ferroportin protein expression in
human embryonic kidney cells
Disease such as pulmonary fibrosis, renal
edema, skin dermatitis, and gastrointestinal
distress
production or processing of manganese alloys
Industries such as paint formulation,
electroplating, nonferrous metal, mineral
processing, steam-electric power plants,
porcelain enameling, and copper sulfate
manufacture
MCL*: Maximum containable limit
MEUF was investigated [50]. The utilization of a single
ionic surfactant makes it possible to remove organic
and inorganic compound simultaneously. But, the CMC
of ionic surfactants, however, is quite higher than that
of a nonionic surfactant. The monomeric surfactants
permeating through the membrane cause the secondary
pollution in aquatic environment. In addition, the cost of
cationic surfactant is approximately as triple as that of a
nonionic surfactant. Therefore, mixed surfactant system
might be an alternative to solve this problem. Three
different systems were used for the application, i.e., SDS
– ethylene di-amine tetra acetic acid (EDTA) - copper;
SDS -Tartrate ion - copper and the hydrolysis of the
uranyl ion - SDS. Out of these three systems the EDTA
system shows maximum rejection of 98.0%. Accurate
prediction of the influence of metal ion complexation
on MEUF was reported by this model [50]. Effect of
the mixture of surfactant was investigated [51], which
shows better results (99.9%) for the removal of copper.
Non-ionic surfactant (poly-oxyethylene octyl phenyl
ether (Triton-X)) and an anionic surfactant (SDS) were
used in the study. Results show that the addition of
Triton-X at concentrations greater than its CMC could
reduce the SDS dosage required for effective copper
removal and at the same time minimize the permeate
SDS concentration also. Response surface methodology
(RSM) was successfully applied to study the effect of
the various parameters, i.e., surfactant concentration,
pH, and surfactant to metal molar ratio for optimizing
the process conditions for the maximum removal of
copper from aqueous solutions using MEUF [51,52].
The maximum rejection obtained was 98.4% which is
greater than the EDTA system [50]; this may be the
effect of using mixed surfactant system of non-ionic and
anionic surfactants.
Aqueous solution containing mixture of copper
(cationic) and potassium permanganate (KMnO4)
(inorganic impurity; anionic) was treated by using
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Removal of heavy metals from wastewater
using micellar enhanced ultrafiltration technique: a review
Table2.
Removal of single metal by MEUF process.
Metal
Removed
Surfactant
used
Parameters studied
Observed
rejection
Copper
SDS
Effect of feed and surfactant concentration
99. 8%
Copper
SDS
Effect of complexation and pH
98.0%
[22]
Copper
Triton X-100 and SDS
Effect of adding Triton X-100, permeate SDS
concentration, copper binding capacity of SDS micelles,
membrane fouling
>92.0%
[23]
Copper
SDS
Effect of surfactant concentration, pH and S/M ratio
98.4%
[24]
SDS, CPC
Effect of trans-membrane pressure drop and cross flow
rate
90.0 to 100%
Copper
SDS
Effect of applied pressure, pH
>90.0%
[26]
Copper
CPC
Effect of operating parameters
99.2%
[4]
Copper
CTAB
Effect of pH, concentration of copper, calcium, surfactant
and ligand
>99.0%
[27]
Copper
CPC
Effect of concentrations of copper, ligand, calcium
surfactant and NaCl
up to 99.8%
Copper
SDS
Effect of the type and concentration of ligands
99.0%
[15]
Chromate
CPC
Effect of operating parameters
99.9%
[3]
Chromate
CTAB and CPC
Effect of pH, pressure, feed chromate and surfactant
concentrations, temperature
99.0%
[14]
Chromate
CTAB and CPC
Effect of ionic strength, pH and salt
>99.0%
[17]
Chromate
DDAB
Effect of DDAB/chromate concentration ratio
>90.0%
[32]
Copper
References
[1]
[25]
[28]
Chromate
CPC
Effect of operating parameters
99.9%
[29]
Chromate
CPC
Effect of surfactant
98.0%
[30]
Chromate
CPC
Effect of surfactant concentration, MWCO operating time
up to 98.0%
[19]
Chromate
SDS and NPE
Effect of surfactant concentration
99.5%
[31]
Chromate
CTAB
Effect of trans-membrane pressure drop (ΔP) and crossflow velocity, CTAB concentrations
98.0%
[11]
Chromium
CTAB
Effect of pH, feed concentration and temperature
99.0%
[13]
Chromium
CPC
Effect of pH, Metal ion concentration, surfactant
concentration
<99.0%
[33]
Chromium
SDS-NPE
Effect of feed concentration
Up to 99.5%
[34]
Zinc
SDS
Effect of surfactant concentration
----
[2]
Zinc
SDS
Effect of the ratio of SDS to zinc ions
97.0%
[35]
Zinc
SDS
Effect of SDS concentration
97.5%
[36]
Zinc
SDS
Effect of pressure, NMWCO, zinc feed concentration and
SDS feed concentration
Up to 99.0%
[37]
Zinc
SDS
Brij35
Effect of S/M and pH
97.94%
[66]
Cadmium
Laurylsulphate
Natrium
Effect of pH, concentration of
Surfactant
>90.0%
[38]
Cadmium
SDS
Effects operating time, concentration of SDS, transmembrane pressure, pH, concentration of feed, electrolyte
and the mixture of SDS
99.0%
[39]
Cadmium
SDBS and SDS
Effect of surfactant species, surfactant concentration,
operating time, trans-membrane pressure, addition of
electrolyte, solution pH
97.8%
[40]
Cadmium
SDS,
Triton X-100
Effect of feed surfactant concentration, cadmium
concentration, the molar ratio of non-ionic surfactants to
SDS
85.0% to 90.0%
[16]
Cadmium
Brij 35 and
Triton X-100
Effect of non-ionic surfactant
97.0%
[20]
CPC
Effect of pressure, pH, feed concentration
100%
[41]
CPC
Effect of co-occurring
inorganic solutes
Almost 100%
[42]
Arsenic
Arsenic
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A. A. Mungray, S.V. Kulkarni, A. K. Mungray
Table2.
Continued
Metal
Removed
Removal of single metal by MEUF process.
Surfactant
used
Parameters studied
Observed
rejection
References
Arsenic
CPC, CTAB, and
ODA
Effect of critical micelle concentration
96.0%, 94.0%, and
80.0%
[43]
Arsenic
CPC
Effect of trans-membrane pressure, pH, CPC
concentration, Arsenic concentration and ionic strength
93.0 to 98.0%
[44]
Arsenic
CPC
Effect of membrane pore Size, trans-membrane pressure,
pH, surfactant concentration, arsenic concentration
93.0% to 98.0%
[45]
SDS alkylphenol
polyetoxilate
Effect of non-ionic surfactant
>75.9%
[47]
Nickel
CTAB
Effect of feed metal ion concentration, surfactant
concentration, pH, trans-membrane pressure, S/M ratio
>99%
[7]
Nickel
SLES
Effect of trans-membrane pressure and addition of salt
98.6%
[46]
Lead
SDS
Effect of pressure, surfactant concentration, pH
99.0%
[48]
Lead
SDS Triton X-100 and
NP12
Effect of surfactant concentration
>98.4%
[12]
Lead
SDS
Effect of process variables
91%
[66]
Lead
SDS, TX-100
NP-12
Effect of surfactant concentration, pH, S/M ratio
> 98.4%
[25]
Iron
C12E8
Effect of pH, ligand concentration,
ionic strength
>98.0%
[5]
Uranyl
C12E8
Effect of ligands
94.0%
[49]
Uranyl
Triton X-100
Effect of chelating agents
>90.0%
[50]
PONPE, SDS, CPC
Effect of surfactant concentration
>90.0%
[51]
HS
Effects of HS concentration, pH
Up to 97.5%
[52]
Platinum
CPC
Effect of operating pressure, temperature, surfactant
concentration, concentration and type of electrolyte
present in the feed solution
>90.0%
Palladium
DTAC
Effect of surfactant concentration,
solution pH
>95.0%
[54]
Americium
SDS polyethylene
glycol ether
Effect of NMWCO of the membrane, concentration of
surfactant metal ions, organic ligands
Almost 100%
[55]
Aluminium
CTAB
Lumogallion
Effect of ligand, pH, surfactant concentration
Almost 98.0%
[56]
Nickel
Gold
Cobalt
mixed micellar system consisting of SDS and CPC
with concentrations of 25 and 10 kg m-3. The rejection
observed was 90.0%-100% for copper and 96.0%-99.0%
for KMnO4 (PP) which is similar to that of using single
solute [53]. Flux decline behaviour in MEUF of aqueous
solutions containing copper using SDS had been studied
[54]. It was performed at a solution of pH=3.0-5.0,
S/M=2.5-12.7. It shows greater than 90.0% of rejection
at an S/M ratio of 12.7 and pH 5. The identification of
the flux decline mechanism during the MEUF using the
blocking filtration law was also investigated [54].
Ligand modified MEUF was also carried out in which
an amphiphilic ligand is added to the copper containing
wastewater to enhance the metal rejection [11,32,55,56].
The selection of ligand was based on its capacity to form
complex with the target ion. The complex formed consists
of high fraction of ions attached to the micelles to achieve
maximum rejection. One copper-specific ligand, i.e.,
N-n-dodecyl-imino-diacetic acid was used to investigate
[53]
its performance in MEUF [11]. In another study a ligand
l-phenyl-3-isoheptyl-1, 3-propane dione was used
with a surfactant cetyl tri-methyl ammonium bromide
(CTAB), for the removal of copper. This combination
gave a removal of 99.0% which shows good removal
efficiency of the combination of ligand and surfactant.
Copper was also recycled (>94.0%) from the retentate
by using sulphuric acid [55]. The ligand 4-hexadecyloxybenzyl-imino-diacetic acid was used with CPC
which gave rejection up to 99.7%. This process showed
that the semi-equilibrium dialysis can also be used for
recovery of copper by using acid stripping technique
[56]. Effect of the type and the concentrations of ligands
with surfactants for the rejection of copper were studied
by Liu et al. [32]. Various ligands, i.e., EDTA, citric acid,
nitrilo tri-acetic acid (NTA) were used for the removal of
copper. The ligands showed rejection of copper in the
range citric acid>NTA>EDTA.
33
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Removal of heavy metals from wastewater
using micellar enhanced ultrafiltration technique: a review
Table 3. Removal of mixture of metal ions by MEUF process.
Metal
Removed
Surfactant
used
Parameters
studied
Observed r
ejection
Triton X-100
Effect of pH
>90.0%
of mixture
Lecithin
● In presence of all five heavy metals, the lecithin
showed affinity: copper>cadmium, zinc>nickel
● When only one metal was present, lecithin
exhibited the following affinity nickel>copper,
zinc> cadmium
Cadmium, Copper,
Nickel,
Zinc
SDS
Effect of operating parameters
>96.0% of mixture
[58]
Chromium,
Cobalt, nickel and
magnesium
SDS
Effect of trans-membrane pressure, flow rate,
feed concentration.
>90% of mixture
[59]
PONPE10 and SDS
Effects of surfactant concentration,
applied pressure, salt addition,
membrane geometry
>90.0% of
mixture
[60]
SDS
Effects of pH, S/M ratio, salt addition
>90.0% of mixture
[18]
DSA and
Dodecylamine
Effect of feed concentration
>99.0% and 80.0%
respectively.
[61]
>94.0% of copper
[20]
Nickel, Copper,
Cobalt,
Manganese and
Zinc
Cadmium, Copper,
Nickel, Zinc,
Cobalt Nickel(II)
Cesium, Strontium,
Manganese,
Cobalt, Copper,
Zinc, chromium
Lead and Arsenic
>90.0% of mixture
>90.0% for nickel
References
[21]
[57]
Copper and
Calcium
SDS
Cadmium
Copper
Cobalt
Zinc
SDS
Effect of S/M ratio
>95.0%
[62]
Nickel and
Cobalt
CTAB
Effect of applied pressure, surfactant
concentration, S/M ratio and pH of solution
>99.0%
[9]
Cadmium and Zinc
SDS
Effect of chelating agents, acid agents pH values
and molar concentration ratio of EDTA versus
heavy metal ions
1.Using chelating agents
90.1% for cadmium and
87.1% for zinc
2.Using acid agents 98.0%
for cadmium and 96.1%
for zinc
[63]
Cadmium and Zinc
SDS
Effect of pressure, NMWCO, heavy metal feed
concentration and SDS feed concentration
98.0 ± 0.4% for zinc and
99.0 ± 0.4% for cadmium
[8]
Cadmium and Zinc
SDS
Effect of surfactant and Feed metal ion
concentration
98.0%
[64]
Rhamnolipid
Effect of pressure, surfactant concentration and
temperature
>99.0%
[65]
SDS
Effect of pH, Effect of feed concentration of SDS
98% no phosphorus
80% with
phosphorus
[21]
Copper, Zinc,
Nickel, Lead and
Cadmium
Cadmium and
copper
Removal efficiency of metal by ligand-MEUF
depends on the ligand to metal ratio. As the ligand to
metal ratio kept constant, the complex formation of ligand
with surfactant as well as metal may occur. The quantity
of the complex formed also depends on the valence of
the used compounds used. So, at fixed ligand to metal
ratio, the metal removal efficiency decreases. The
ligand to metal ratio is observed to be safe in between
12 to 60. Beyond this, the concentration of surfactant
in the effluent increases rapidly and caused problems
in the subsequent treatment effluent. The increasing
concentration of surfactant in the effluent might cause
decrease in the metal rejection.
3.1.2. Removal of chromate ions
Removal of chromate ions by MEUF was first reported
using a cationic surfactant (CPC) by Christian et al.
[7]. Chromate ions preferentially adsorb on the outer
surface of the surfactant micelles. Because of the
interaction of cationic surfactant micelles and chromate
ions, permeate solution contained only about 0.1%
of chromate ions with almost 99.9% rejections [7,57].
34
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A. A. Mungray, S.V. Kulkarni, A. K. Mungray
Table 4. Removal of combination of organics and metal by MEUF process.
Element
Removed
Surfactant
Used
Parameters Studied
Observed
Rejection
References
Phenol and
o-Cresol, Zinc and
Nickel
SDS
Effect of the presence of metals on organic removal
99.8% of zinc
[66]
Copper and Phenol
SDS and Triton-X 100
Effect of surfactant concentration
85.0%and 27.0%
respectively
[67]
Ferric cyanide
Chromate
ODA
Effect of the molar ratio of ODA to ferric cyanide
and to chromate
98.0% and
>99.9% respectively
[68]
Ferric cyanide and
Nitrate
CPC
Effects of composition and concentration of mixed
anionic/non-ionic surfactants
>99.9% and 78.0%
respectively
[69]
Chromate and
Ferric cyanide
CPC
Effect of surfactant to ferric cyanide/nitrate molar
ratio
up to 98.0%
[70]
Chromate,
Chlorinated
aromatic
hydrocarbons,
Nitrate
CPC
Effect of nitrate and chloro-benzenes on removal
of chromate
>99.0% of chlorinated
aromatic hydrocarbons
99.5% for Chromate and
98.0% for nitrate
[71]
TCE Chromate
CPC and
Tween-80
Effect of mixed surfactants
Upto 93.7%
[72]
Triton-X
SDS
Effect of adding triton-X and TCE on the copper
removal efficiency
>90.0%
[73]
CTAB and SDS
Effect of feed CTAB and
SDS concentration
almost 100.0% for the
highest CTAB and SDS
concentrations
[74]
(i)Copper, calcium;
(ii) Copper
beta Naphthol
SDS
Effect of the feed composition, trans-membrane
pressure drop, and the cross flow rate
(i) 99.0%to 92.0%
(ii) 82.0%to 84.0%
[75]
Uranyl, DBP,TBP
SDS
Effect of pH
membrane pore size
surfactant concentration
DBP concentration
>90.0%
[76]
Cadmium and
phenol
SDS and mixed
surfactant (Triton
X-100/SDS)
Effect of surfactant concentration & S/M
91.3%
[105]
Cadmium and
methylene blue
SDS
Effect of pH, Influence of Initial SDS level
98.8%
[104]
Copper
TCE
Phenol,
p-Cresol, Xylenol
Cr3+
Various factors like fouling resistance, concentration
polarization resistance, and membrane resistance
were also observed to play vital role in the removal of
metal ions. The presence of electrolytes in feed solution
increases fouling which results in reduction in the flux.
Fouling can be reduced by increasing the temperature
and pressure. The MEUF can be effective upto 82.0%
removal of chromate ions from aqueous steams even
in the presence of up to 100 mM NaCl [29]. Removal of
chromate and nitrate was simultaneously investigated
using CPC. Chromate anions bind more to the micelles
than the nitrate ions. So the rejection of chromate ions
obtained was greater than the nitrate ion i.e., 98.0% and
80.0% respectively [58,59]. The surfactant di-decyl dimethyl ammonium bromide (DDAB) was also reported
to remove the chromate ions from the wastewater.
Chromate removal efficiency was observed to increase
with increase in the DDAB to chromate ratio [60].
For the removal of chromate ions using CTAB, it was
observed that for low CTAB concentration the efficiency
of chromate removal increased with increasing trans-
membrane pressure, but decreased with increasing
cross flow velocities. It was also observed that the
effect of cross flow velocities and trans-membrane
pressures on the metal ion rejections decreased at high
CTAB concentration. Fouling of the membranes by
surfactants very rapidly occurs at low cross-flow velocity
and high pressures at higher CTAB concentrations. As a
result, permeate flux decreased with decreasing crossflow velocity and increasing pressure at various CTAB
concentrations [24].
The mixed surfactants are having lower CMC than
single surfactant used, so it shows better rejections.
It was reported that when MEUF was carried out using
two surfactants i.e. CTAB and CPC, the rejection
coefficients were higher than 99.0% obtained at
optimal conditions of pressure and feed concentration.
The rejection rate observed to be dependent on the ionic
strength and pH. With increase in the ionic strength the
retention of chromate ions and the permeate surfactant
concentration decreases [35].
35
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Removal of heavy metals from wastewater
using micellar enhanced ultrafiltration technique: a review
Table 5.
Combination of MEUF and other processes.
Metal
Removed
Parameters studied
% Rejection
Processes
References
Lecithin
Effect of continuous binding
of the ions through membrane
bioreactors
For aqueous wastes
(50.0% to 90.0%) For
non-aqueous wastes
(90.0 to 97.0%)
MEUF and membrane
bioreactor
[77]
Naphthalene and
trichloroethylene
DPDS
Effect of operating parameters
93.0% to 99.0 %
MEUF and air stripping
[78]
Copper
CTAB
Effects of feed surfactant, ligand
and copper concentration
>99.0%
MEUF and conventional solvent extraction
[79]
Metals, Copper
SDS
Effect of surfactant concentration,
current density hydraulic
Retention time, pH
>90.0%
Combined electrolysis
and MEUF
[80]
Copper
SDS
Effect of S/M ratio, operating
retentate pressure
98.0% using both and
85.0%
using two ACF in series
MEUF and ACF
processes
[81]
Chromate
CPC
Effect of initial retentate pressure,
initial permeate flux, initial
chromate concentration, pH,
S/M ratio
Up to 99.9%
MEUF and
ACF
[82]
Cadmium
SDS
Effect of time, air flow rate,
feed flow rate, liquid height,
foam height, feed surfactant
concentration, ethanol
concentration, temperature
99.4%
MEUF and continuous
foam fractionators
[83]
Nickel,
Zinc
SDS
Effect of
pressure, S/M ratio
99.3%, 99.9%
respectively
MEUF and ACF
[84]
Lead
SDS
Effect of M/S ratio, effect of
MWCO of membrane, Effect of
co-existing heavy metal.
>95%
MEUF and ACF hybrid
process
[114]
Different heavy
metals
Surfactant
used
3.1.3. Removal of chromium ions
The chromium removal was first reported using CTAB
by Sadaoui et al. [28] where almost 99.0% rejection
was obtained. Chromium removal was also investigated
using CPC [61]. The maximum rejection was found up to
99.0% at optimal conditions of pressure, feed chromate
and surfactant concentrations, i.e., 4 atm, 0.5 mM,
30 mM respectively. It was found that the capacity of
chromium adsorption on surfactant micelle increases
with initial metal concentration and to a lesser extent
with pH of the solution [61]. A mixture of two surfactants,
i.e., SDS and nonyl phenyl ether (NPE) was reported to
have 99.5% removal of chromium ions. The addition of
salt (NaCl) to feed solution showed decrease in rejection
of chromium ion [62].
3.1.4. Removal of zinc ions
Zinc ions were removed efficiently using SDS as a
surfactant by various researchers [3,63-65] using
different operating conditions (Table 2). The adsorption
of zinc ions on SDS was found according to the Langmuir
adsorption isotherm. When the initial SDS concentration
was below the CMC, unexpectedly high rejection, i.e.,
97.5% was obtained due to concentration polarisation
occurring near the membrane-solution interface. MEUF
was not found applicable for intensively acidic water
conditions because of membrane performance which
was affected by intensively acidic water [3,63]. It was
found that nominal molecular weight limit, pressure and
their respective interaction present the influence on the
permeate flux and a negligible effect on the rejection
coefficient. The rejection up to 99.0% was achieved
when the S/M ratio was above 5 [64,65]. In one recent
study, experimental design and artificial neural network
(AAN) model were used for the modeling of zinc
removal using MEUF. For this purpose two surfactant
were (Oxyethylene lauryl ether (Brij35) and SDS) used,
observing an acceptable agreement between ANN
model and experimental data [66].
3.1.5. Removal of cadmium ions
Cadmium ions were first removed using 8-hydroxyquinoline as extractant, lauryl sulphate natrium as
surfactant and n-butanol as co-surfactant. It was
observed that the cadmium recovery depends on
different ranges of the trans-membrane pressure. The
addition of salt (NaCl) was observed to be less influential
while surfactant species, surfactant concentration and
pH value were more important to be considered [67-69].
Two kinds of nonionic surfactant, oxyethylene lauryl ether
(Brij35), Triton X-100 were reported to treat wastewater
containing cadmium ions. In view of cadmium rejection
36
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Table 6. Other removals by MEUF.
Pollutants
removed
Surfactant used
Parameters studied
Tertiary amine with two
Polyoxyethylene head
groups and an alkyl tail
of 18 carbons
Specific conditions
References
Effect of pressure
S/M ratio, surfactant
concentration
89.0%
MEUF using a twin-head
cationic surfactant
[85]
Potassium,
Phenol and
TBP
SDS
KDS
Effect of operating
parameters
96.0%of tertbutyl phenol
Recovery of surfactant
using a precipitation
process
[86]
Copper
Chromate
and TBP
SDS
CPC
Effect of
surfactant concentration
98.0%to 99.9%
MEUF in a spiral wound
ultrafiltration module and
comparison with stirred
cell flat sheet membrane
module
[87]
SDS
Effect of metal ion
and surfactant concentration
>90.0%
Inverted polarity MEUF
for the treatment of heavy
metal polluted wastewater
[88]
Copper
and
Cadmium
SDS
Effect of pH
100.0%
and 75.5%
respectively.
Comparison of separation
methods for the recovery of
surfactant
[89]
Methylene
blue
SDS
Effect of pressure and feed
SDS concentration
99.3%
MEUF in hollow fiber
membrane
[120]
Benzoic acid
Zinc
rate at mixing molar ratio of <0.5, the mixing effect of
Brij35 was better than Triton X-100 and when the ratio
was >0.5, these two effects were nearer to each other.
Considering in these respects of decreasing the amount
of surfactants residue in permeates and retaining higher
permeates flux, lesser membrane pollution resistance,
Triton X-100 was better than Brij35. The permeate flux
of MEUF with SDS was higher than that for MEUF with
mixed surfactants [33,38].
3.1.6. Removal of arsenic ions
Removal of arsenic ions from wastewater using
surfactant micelles and membrane materials of
regenerated cellulose (RC) and polyether sulfone (PES),
with nominal molecular weight cut-off (NMWCO) of 5 and
10 kD respectively was investigated [70]. The negatively
charged surface of regenerated cellulose membrane
provides better results for the arsenic removal than
PES membranes. The arsenic removal efficiency and
absolute permeate flux were found dependant on the
membrane materials, membrane NMWCO, and pH of the
feed solution. Almost 100% rejection was achieved for
the feed water having arsenic concentrations of 20 and
43 μg L-1 using 5 kD PES membranes at pH of 5.5 and
10 kD RC at pH of 8 [70]. CPC and a 5 kD PES membrane
were used to remove arsenic. It was reported that for the
addition of 10 mM CPC to the feed water gave rejection
from 78.1% to 100% for arsenic. An increase arsenic
concentration in a feed solution showed a decrease
in the permeate flux due to osmotic effect [71,72].
The removal of arsenic using various surfactants
was also investigated in literature [43], which showed
Observed
rejection
that CPC is having greater removal efficiency than CTAB,
octa-decyl amine acetate (ODA) and benzalkonium
chloride, i.e., 96.0%. The benzalkonium chloride was
observed having the lowest removal efficiency, i.e.,
57.0% due to higher CMC than those of other surfactants.
So it is feasible to use CPC for arsenic removal,
sometimes CTAB can also be used [72-74].
3.1.7. Removal of nickel ions
The removal of nickel ions using SDS and linear alkyl
benzene sulfonate (LAS) was showed that for surfactant
concentrations beyond the CMC, nickel retention
with SDS was slightly higher than with LAS, i.e.,
at S/M=4.5, nickel retention was 70.0% and 55.0% for
SDS and LAS, respectively. The pH values between
4 and 8 did not affect nickel retention but enhanced
the SDS and LAS surfactant retentions [20,24].
Sodium lauryl ether sulphate (SLES) was also reported
to be used to remove nickel ion. The rejection of 98.6%
was obtained at pressure of 250 kPa. Under the effect
of increasing pressure it was observed that the rejection
of nickel and SLES increased, but the permeate flux
decreased. It has been determined that, the fouling
strongly depends on mechanisms controlled by the
formation of gel layer [75].
3.1.8. Removal of lead ions
The attempt for removing lead ions was made using
SDS surfactant [25,76]. A polysulfone (PS) membrane
of 10kD with varying pressure between 1 to 3 bars was
applied. The rejection higher than 95.0% was reported
at pH of higher than 1.8. An ionic exchange model
37
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Removal of heavy metals from wastewater
using micellar enhanced ultrafiltration technique: a review
had been used to study the interaction between the
lead cation and SDS micelles [76]. The lead removal
was investigated by using anionic surfactant SDS and
nonionic surfactants such as Triton-X 100 and nonyl
phenyl ethoxylate (NP12). Single surfactant (SDS)
was used, it gave high lead rejections but more SDS
in permeate while Triton-X 100 and NP12 showed
opposite results. At mole fraction of 1.37 mM for Triton-X
100 and NP12 with 12.3 mM of SDS, the lead rejection
was higher than 98.4% and also high rejections of SDS,
Triton-100 and NP12 (80.0%, >99.0%) were obtained.
It was also found out that the fouling resistances due to
mixed surfactants were higher than that of pure SDS but
lower than those of pure nonionic surfactants [25].
Researchers reported data of a metal-polluted
wastewater in order to evaluate the efficiency of MEUF
for the removal of Pb2+ from aqueous solutions by using
Fuzzy modeling and simulation. They found that in all the
cases the degree of agreement between experimental
values and numerical values were greater than 91%
[66].
3.1.9. Removal of other metal ions
Iron: This metal separation was reported by using
mixed micelles containing derivatives of salicylic acids
with an alkyl substituent. The metal ion accumulates in
the micelle aggregate after formation of complex with
the chelating ligand used. Then, it was successively
separated from the bulk solution by passing through
hydrophilic membrane of proper pore size. It was
observed that the rejection efficiency depends on the
chelating ligand and its complex formed. For carrying out
complete removal of metal, hydrophobocity of chelating
ligand should be properly modulated and surfactant
should be selected properly [12].
Uranyl: The removal of uranyl ions using chelating
ligands e.g. trioctylphosphine oxide, 4-aminosalicylic
acid was investigated. These ligand-doped micelles
attract the uranyl ions through the formation of tight
chelating bonds. The aggregates and metal complexes
formed were successively separated by ultrafiltration by
passing through hydrophilic membranes of suitable pore
size. The efficiency of the process depends upon the
affinity of the ligands towards the micelles [77,78].
Gold: The MEUF of gold from dilute hydrochloric
acid media using a nonionic surfactant polyoxyetylene
nonyl phenyl ether (PONPE) was reported. The micelles
attract gold ions in aqueous solutions due to a high
affinity between the surfactant and the metal ions.
The rejection efficiency of gold increases with increasing
surfactant concentration, number of ethylene oxides
group in PONPE and the applied pressure as well as with
decreasing molecular weight cutoff of the membrane.
The results showed that polyoxyetylene nonyl phenyl
ether with 10 etylene oxide unit (PONPE10) have higher
selectivity to gold than those with charged surfactants,
i.e., CPC and SDS [79].
Cobalt: Humic substances (HS) can also be used
as complexing agents instead of synthetic chemicals.
These are sorts of natural organic matters and their
functional groups such as carboxyl and phenyl groups
which can bind with the cation to form complexes. The
effects of HS concentration and pH were studied using
cobalt metal. The results showed that as pH increased
from 4 to 8 the removal of cobalt also increased from
72.5% to 97.5% at the HS concentration of 3 g L-1 [80].
Platinum group: Application of MEUF technique
for the separation of platinum group metal ions was
investigated by using CPC as a surfactant. The result
showed that CPC retention depends on the composition
of feed stream (S/M ratio). The metal rejection was
greater than 90.0% [81].
Palladium: Removal of palladium using cationic
surfactant dodecyl trimethyl ammonium chloride (DTAC)
was investigated by Ghezzi et al. [82]. Addition of DTAC
at concentrations above the CMC resulted in palladium
removal exceeding 95.0% [82].
Americium: Removal of americium from nitric acid
solutions using anionic surfactant (SDS) and non-ionic
surfactant (polyethylene glycol ether) micelles was
reported. Almost 100% removal of americium was
achieved even in the presence of very low concentration
of SDS in the aqueous phase with pH > 2 while containing
americium < 10−3 mM. It was observed that micelle of
SDS fails to retain americium ions from aqueous phase
containing [NaNO3] > 0.5 M whereas, micelles of tergitol
could retain americium from aqueous solution containing
[NaNO3] up to 1 M [83].
Aluminium: The preconcentration of aluminium
using CTAB and lumogallion at pH 5-5.9 was reported.
The results showed that MEUF is very much useful in
pre-concentrating trace elements compared to that with
liquid- liquid extraction [84].
3.2. Removal of mixture of metal ions
The wastewater containing mixture of metals can also
be removed by MEUF technique. The use of ligand or
chelating agent in addition to the surfactant is helpful in
the removal of mixture of metals from the wastewater.
In most cases the agent used forms complex with
the metals to be removed and the competitor for the
attachment of metals to the micelles formed decreases.
This helps in the easy removal of metals. In some cases
it is also reported that mixture of surfactant used also
provides better percentage rejection of the metals from
the wastewater containing mixture of metals.
38
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Table 3 is prepared by compiling the data from the
literature from the best of the author’s knowledge for
the removal of different combination of metals by MEUF
technique. Table 3 consist of the list of the mixture of
metals removed in the literature with the surfactant used.
It also includes the parameters affecting the removal of
metals and the percentage rejection achieved. The data
provided in the Table is arranged on the yearly basis for
the progress achieved in the removal of mixture.
In 1994, firstly, a combination of metals was removed
by MEUF [16]. Mixture of nickel, copper, cobalt,
manganese and zinc was separated from wastewater
using Triton X-100. In this case chelating aggregates
in addition to Triton X-100 to assist micelles were
used for selective separation. The effective separation
was achieved at a pH of 6 [46]. Natural biodegradable
surfactant exhibiting emulsifying characteristics known
as Lecithin was also reported to be used to separate
the mixture containing cadmium, copper, nickel, zinc
metals. The binding of various Lecithins micelles to
cadmium, copper, nickel and zinc in a mixture and
individually was investigated and the affinity of the
metals towards micelles was obtained as shown in the
Table 3. In another studies it is reported that the
percentage rejection of >96.0% for the mixture was
achieved using anionic surfactants [85,86]. Mixture of
chromium, cobalt, nickel, and magnesium was separated
using SDS. The optimal ratio of S/M for a best removal
of metal ions was measured in between 5 and 8. The
affinity resulted in the order of Cr> Co> Ni> Mg [87].
MEUF of cobalt and nickel in the presence of a micellesolubilized hydrophobic ligand has been investigated,
using PONPE10, SDS and 2-Ethyl hexyl phosphoric
acid mono-2- ethyl hexyl ester (EHPNA) as a nonionic, anionic surfactant and an extractant respectively.
Both the metals were entrapped within the surfactant
micelles containing the extractant therefore effective
rejection was achieved [88]. Experiments for the
removal of cesium, strontium, manganese, cobalt,
copper, zinc and chromium mixture were performed
with polyamide, PES membranes with MWCO 10kD and
80kD respectively, pH 2 to 12 and surfactant to metals
ratio 0.5 to 2.7. It was shown that complete removal of
metal ions except for monovalent cesium, was reported
to be achieved [36]. The mixture of lead and arsenic
were separated from wastewater by using dodecyl
benzene sulfonic acid (DSA) as anionic surfactant and
dodecylamine as cationic surfactant. Concentrations of
lead and arsenic introduced in the water varied from 4.4 to
7.6 mg L-1, while DSA and dodecylamine concentrations
were equal to 10–5 M and 10–6 M, respectively which
were below their CMC. The rejection of >99.0% and
80.0% was achieved for lead and arsenic [89].
Certain experiments were performed for removal
of copper and calcium ions from aqueous solution
using MEUF. The permeate flux was determined using
gel layer controlling model. The extent of counter ion
binding of the single component as well as the mixture
was determined by application of localized adsorption
model [38]. In a mixture of divalent metal ion using SDS
as surfactant, the removal obtained was >95.0% at
surfactant to metal ratio >10. In the mixture, there was
slight difference in the removal efficiency of order of
cadmium>copper>cobalt>zinc. As S/M ratio increased,
the difference in removal efficiency diminished [90].
Simultaneous removal of nickel and cobalt from aqueous
feed using cross flow MEUF was investigated. A 20 kD
PS membrane was used in this experiment and the
rejection was obtained more than 99.0%. Presence of
salt in the aqueous feed results in drop in rejection from
99.0% to 88.0% [22].
The MEUF for removal of zinc and cadmium has
been reported to be carried out by two methods [91].
In first method, a chelating agent is added which shows
rejection of 90.1% for cadmium 87.1% for zinc. In second
method, an acidic agent i.e. sulphuric acid was added to
obtain rejection of 98.0% for cadmium and 96.1% for
zinc. The combination of cadmium, copper, cobalt, zinc
and lead when separated using SDS unexpectedly gave
higher percentage rejection with lower concentration of
surfactant at the beginning of the process [91]. Efficient
removal of cadmium and zinc had been performed
using a hollow fibre UF membrane. Adsorption of feed
as well as surfactant ions had been studied. It was
found that with cadmium and zinc feed concentration of
50 mg L-1 and SDS concentration of up to 2.15 g L-1,
the concentrations of heavy metal ions in permeate
stabilized at around 1–4 mg L-1 [92]. The performance
of a bio-surfactant (rhamnolipid) had been investigated
[93]. The effect of biosurfactant concentration was
observed on the rejection of metals. The optimum
conditions obtained during experimentation were;
pressure = 69 kPa, S/M ratio of approximately 2:1,
temperature = 25◦C, and pH = 6.9 at which >99.0%
of mixture rejection had been observed for the removal
of mixture of metals (copper, zinc, nickel, lead, cadmium)
[93].
In one study, MEUF was used to purify the
phosphorous rich wastewaters. Researchers found the
important operating parameters like effect of pH and
feed concentration affecting the simultaneous removal
of cadmium and copper from phosphorous rich synthetic
wastewaters. They found that phosphates can easily
separate from heavy metals because phosphorous was
not retained by the micelles and passed through the
ultrafiltration membrane [21].
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Removal of heavy metals from wastewater
using micellar enhanced ultrafiltration technique: a review
Researchers also found some optimal conditions by
using response surface methodology (RSM) approach to
treat six contaminated wastewaters from metal refining
industries using the rhamnolipid-enhanced ultrafiltration
process. The best operating conditions were a
transmembrane pressure of 69±2 kPa, biosurfactant-tometal molar ratios of approximately 2:1, a temperature
of 25±1ºC, and pH of 6.9±0.1. They found that resulting
heavy metal concentrations in the permeates were all
significantly reduced to be in accordance with the federal
Canadian regulations [93].
3.3. Removal of organics and metals
Table 4 is compiled by the data taken from various
literatures for the removal of the combination of
organics and metals by MEUF. The table also includes
the parameters affecting the removal of metals and the
percentage rejection achieved. The data provided in the
table is arranged on the yearly basis for the progress
achieved in the removal of mixture. MEUF has been
found as a promising method for the removal of lowlevels of heavy metal ions and organic compounds
simultaneously from industrial effluents. In the removal
of organics and metals simultaneously, the organic
solute tends to solubilise within the micelle formed and
the metal ions bind on outer surface of the micelle due to
electrostatic attraction. The solution is then treated with
UF membrane to carry out the required separation.
Simultaneous removal of dissolved organics and
metal cations from water using MEUF was first reported
in 1989 by Dunn et al. [94] where SDS was added to
the aqueous stream. Phenol was also removed along
with zinc ion removal (99.8%) in this study. The MEUF
technique had been successfully applied for the removal
of phenol and copper ions simultaneously [95]. Two
surfactants (SDS and Triton X-100) were found effective
than single surfactant which had low CMC. With a
surfactant concentration of 10 mM, the copper rejection
was negligible by using pure Triton X-100 and increased
with increasing SDS mole fraction with a value as high
as 85.0%, which concluded that the rejection of copper
was due to the electrostatic attraction between copper
and SDS. The rejection of phenol was obtained lower
(27.0%) than copper (85.0%).
The regenerated cellulose (RC) UF membrane was
used to carry out separation of mixture of ferric cyanide
and chromate ion from aqueous solution using ODA
as a surfactant [96]. In the ferric cyanide-chromateODA system, the removal of ferric cyanide observed
to be increased from 62.0% to 93.0%, while that of
chromate from 20.0% to 68.0% as the molar ratio of
ferric cyanide: chromate: ODA increased from 1:1:1 to
1:1:4, respectively. With the molar ratio of 1:1:6, the
removal of >99.9 and 98.0% was achieved for chromate
and ferric cyanide, respectively. Use of CPC for the
removal of mixture of ferric cyanide and nitrate ion
was investigated [97]. In the ferric cyanide-nitrate-CPC
system, the removal of ferric cyanide increased from
62.0% to 99.9%, while that of nitrate from <2% to 27% as
the molar ratio of ferric cyanide: nitrate: CPC increased
from 1:1:1 to 1:1:4, respectively. With the molar ratio of
1:1:10, the removals were >99.9% and 78.0% for ferric
cyanide and nitrate, respectively. Competitive binding
characteristics of chromate and ferric cyanide in MEUF
using CPC were investigated by Baek and Yang [98]. The
results showed that the removal of pollutants depend
on the binding of pollutants to surfactant micelles which
increases with the valence of anions. Thus, the removal
of ferric cyanide was higher than that of chromate at the
same molar ratio of surfactant due to the difference in
the valence of anion. The rejection was obtained up to
98.0%.
The separation of the mixed waste consists of
chlorinated aromatic hydrocarbons, nitrate, and
chromate was again investigated by Baek and Yang [99].
The co-presence of either of the ion did not observed
to affect the removal of others because chlorobenzene
was solubilized at the hydrophobic interior of the
micelles by hydrophobic interaction; while the nitrate
and chromate were bound to the outer shell of micelles
by electrostatic attraction. The rejection obtained was
>99.0%, 99.5% and 98.0% for chlorinated aromatic
hydrocarbons, chromate, nitrate ions respectively.
The simultaneous removal of trichloro-ethylene and
chromate using mixed surfactants (CPC, polyoxyethylene
(80) sorbitan monooleate (Tween 80)) was reported [47].
The declined flux was observed during filtration mainly
because of concentration polarization and high viscosity
of Tween 80 [47,100]. The removal of phenol, p-cresol,
xylenol and Cr3+ ions simultaneously was investigated
using mixed surfactants [101]. Rejection coefficients of
solutes were found increased with the increase of the
surfactant concentration. The presence of electrostatic
attraction between the chromium ions and the micells,
solubilisation of phenolic derivatives inside the micelles
caused almost 100% rejections at higher concentration
of surfactants. Separation of copper and beta naphthol
using SDS and an organic polyamide membrane was also
reported [102]. The rejection of copper was obtained in
the range of 92.0% to 99.0% and that for beta naphthol,
it was from 82.0% to 84.0%. Simultaneous removal of
dissolved organics namely di-butyl phosphate (DBP)
and tri-butyl phosphate (TBP) as well as uranyl ions
from aqueous solutions was investigated using SDS
as surfactants. The rejection obtained was greater than
90.0% for DBP, TBP as well as uranyl ions [103].
40
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A. A. Mungray, S.V. Kulkarni, A. K. Mungray
In one study, single and simultaneous removal of
Cd2+ and methylene blue (MB) by using MEUF was
investigated. SDS was used as a surfactant. Various
operating parameters (effect of pH, initial SDS level etc.)
were studied. The maximum removal efficiency obtained
for Cd2+ was 98.8% when initial concentrations of SDS
and MB were 1.0 CMC and 4 mg L−1, respectively.
And MB could reach more than 99.9% with initial SDS
concentration below 2.0 CMC [104].
Researchers also investigated the simultaneous
removal of cadmium ions and phenol by using MEUF
with the use of pure SDS and mixed surfactants (Triton
X-100/SDS). After experiments, they concluded that
with the increase of the molar ratios of Triton X-100
to SDS, the rejection of Cd2+ increased first and then
decreased, which implied that the nonionic surfactant
lowers the CMC more than the degree of counterion
binding. Additionally, the rejection of phenol increased
continuously when the molar ratios of Triton X-100 to
SDS was lower than 1.5 [105].
3.4. Combination
processes
of
MEUF
and
other
Table 5 indicates the MEUF combined with the other
processes to carry out separation. Table 5 consist of the
list of the separation processes used in addition to the
MEUF technique to achieve maximum separation and
also it includes the parameters affecting the removal of
metals and the percentage rejections achieved. The data
provided in the Table 5 is arranged on the yearly basis
for the progress achieved in the process modifications.
The investigations have shown that MEUF could be
used in combination with membrane bioreactor for
separating heavy metals. Here, lecithin was used as a
surfactant which gave rejections of 50.0% to 90.0% and
90.0% to 97.0% for aqueous and non aqueous wastes
respectively. The membrane bioreactors showed more
efficiency for removing metal ions so they were selected
ahead of conventional bioreactors for the combination
[106]. In further improvement, air stripping was reported
to be used in addition to MEUF to remove the polluted
contaminants from wastewater. In this study, batch and
continuous flow air stripping models were developed
based on air/water ratio, and surfactant concentration
whose predictions were used to validate the experimental
data [107].
The MEUF using ligands (ligand modified micellar
enhanced ultrafiltration (LM-MEUF)) and conventional
solvent extraction methods were economically
compared for the removal of copper [108]. It showed that
LM-MEUF process requires 17.0% higher capital and
43.0% higher operating cost for a 1×105 gallon per day
unit. This is due to the use of higher reagent and electrical
costs. The operating cost for LM-MEUF was observed
to be reduced by using a surfactant with a higher gel
point concentration and a lower CMC. A surfactant with
a lower CMC would cause higher micellar concentration
resulting in more ligand being solubilized in the surfactant
reducing ligand losses and it also results in smaller
concentration of surfactant going in the permeate side.
A hybrid process of combining electrolysis and MEUF
was successfully applied for the removal of heavy
metals like copper. This process required less amount
surfactant than conventional MEUF. The efficiency of
the hybrid system for removing metal ions increases
with increase in surfactant concentration, electrolytic
voltage and hydraulic detention time [109].
Removal of chromate using CPC by a combination
of MEUF and activated carbon fibre (ACF) was
investigated. In that, CPC removal efficiency was only
60.7% for the molar ratio of 1:5 which was increased
up to 98.0% when MEUF combined with ACF. The
chromate removal efficiency was also increased up
to 98.6%. In the same study, two ACF were used in
series, and observed removal efficiency was only
85.0% [110,111]. MEUF was combined with foam
fractionation unit for the removal of cadmium using SDS
as surfactant. Here, foam fractionation was used due to
its effectiveness in the recovery of SDS and cadmium
ions. The rejection of cadmium was found 99.4% [112].
The simultaneous removal of nickel and zinc had been
performed by combining MEUF and ACF process. By
only using MEUF alone for the mixture, rejections were
observed to be 96.3% and 96.7% for nickel and zinc.
But, when MEUF was combined with ACF, percent
rejections were increased up to 99.3% and 99.9%
respectively [113]. It shows the effectiveness of the
combined process.
One study was based on the performance of MEUFACF hybrid processes for lead removal, in which SDS
was used as a surfactant. In this study, researchers
found the optimum conditions of the maximum removal
of lead. The optimum condition for average 95% lead
removal was molar ratio of lead to SDS of 1: 20, 1: 40
and 1: 100, respectively. Optimum molar ratio of lead to
SDS was found to be 1: 5 [114].
3.5. Other removals by MEUF
The other removals by MEUF containing pollutants are
compiled in Table 6. In the category of other removals,
some specific conditions are compiled which were
used for the separation by MEUF different than the
conventional ones. The data provided in the table is
arranged on the yearly basis for the progress achieved
in the process modifications.
41
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Removal of heavy metals from wastewater
using micellar enhanced ultrafiltration technique: a review
A twin head cationic surfactant having a tertiary
amine with two polyoxyethylene head groups and an
alkayl tail of 18 carbons (Rhodameen T-12) was used
for the removal of benzoic acid by MEUF technique.
The rejection was obtained almost 89.0% at the S/M
ratio of 1.2 [115]. Surfactant was recovered first time by
Wu et al. [116] by precipitation method from the MEUF
permeates in 1998. Here, monovalent potassium ion was
used to precipitate the dodecylsulfate anion from SDS.
The recovery process was quick and requires addition
of small amount of the electrolyte. The electrolyte
forms complexes with the surfactant ions, which can
be separated easily. When potassium dodecyl sulphate
(KDS) was used instead of SDS gave higher rejection of
TBP, i.e., 96.0%.
MEUF was applied in a spiral wound ultrafiltration
module for the removal of pollutants using anionic
and cationic surfactant by Roberts et al. [117].
Its performance was also compared by using stirred
cell flat sheet membrane module. The spiral wound
membrane module was observed to be effective in
separating dissolved tert-butylphenol, copper cation,
and chromate anion from aqueous streams than stirred
cell. The rejection obtained in the range of 98.0% to
99.9%. Inverted polarity for the removal of heavy metals
from wastewater was studied by Hankins et al. [118].
Greater than 90.0% removal of heavy metals was
found by using SDS as a surfactant. The aluminium
ion was used as a flocculent ion to remove zinc as
target metal. The flocculation of micelles had occurred
which helped to drag more zinc ions with the micelles.
The use of flocculation near micelles also reduced gel
layer formation and fouling on the membrane surface.
The separation of metal ions from simulated wastewater
by MEUF was studied by three methods namely
acidification followed by ultrafiltration, use of a chelating
agent followed by ultrafiltration, precipitation by ferric
and ferro-cyanide followed by centrifugation [119].
Out of these three methods, centrifugation showed
almost 100.0% recovery of SDS.
There are so many advantages of hollow fiber
membranes i.e., stable performance, a high density within
modules and low investment. Therefore, researchers
are more attracted with this type of membranes. In one
study of MEUF, methylene blue was removed by using
a polysulfone hollow fiber membrane and SDS as an
anionic surfactant. They found the rejection of MB and
SDS were 99.3 and 96.0%, respectively [120].
4. Discussion
For the removal of metals, MEUF is considered as a
better alternative to the typical available membrane
separation processes. The advantages of this method
over other methods are high removal efficiency,
low energy consumption and easy operation.
Various metals had been removed from wastewater
using MEUF process. The selection of surfactant for the
removal of metal is based on the charge present on the
target metal ion. The characteristic of membrane used
also has effect on the removal efficiency of the process.
The membrane fouling is affected by the chemical
nature of the membrane materials. The rejection of
target ions does not dependent on the initial amount of
surfactant used but depends on its concentration near
the membrane surface. MEUF is successfully applied
for single metals with or without legands.
For the removal of mixture of metal ions, the addition
of mixed surfactant system, ligands and chelating agents
showed better rejection than using single surfactant.
The CMC of surfactant decreased when mixing with
other surfactant to show better result. The separation
of mixture of metals or other compounds using MEUF
depends on the valance of the species. The species
with greater valance attracts more towards micelles
than the species having lower valance to cause better
rejection of it.
At present MEUF is still at the laboratory stage. Some
authors used MEUF with real wastewaters [66,121,122].
Available research focused on the removal of metals
by MEUF mainly on type of surfactant used, surfactant
concentration, applied pressure, operating time and pH.
Formation of surfactant micelle and mechanisms for the
attraction between micelle and metal ions are ignored
during the research study in MEUF.
The MEUF process showed its use not only
in synthetic streams but also in combination with the
other processes to carry out removal in waste stream.
It showed its usefulness for both aqueous and nonaqueous streams when combined with the other
processes. Due to the requirement of less amount
of surfactant, the combination of MEUF with other
processes needs to pay attention. Very less amount of
information is available on processes for the recovery of
the surfactant from permeates, so it is also one of the
areas which can attract researchers.
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A. A. Mungray, S.V. Kulkarni, A. K. Mungray
5. Conclusions
1. The MEUF was observed as a successful technique
for the removal of heavy metal ions than the conventional
membrane separation processes. The application of
MEUF for removing heavy metal ions from wastewater
is gaining significant importance.
2. This method is having some advantages such as
simple operation, high removal efficiency and recycling
of the metal ions over the other process.
3. MEUF can be used to remove single metal ion
as well as mixture of metal ions with high removal
efficiency.
4. The mixed surfactant systems used showed
better results for the removal of metal and also having
economical advantage over the single surfactant used.
5. The mechanism of micelle formation and attraction
between metal ions and micelles needs to be studied in
detail.
6. Research was mainly focused on the simple
MEUF but for the economy as well as high rejection
purpose the MEUF in combination with other processes
also needs to be considered for the future research.
Abbreviations
ACF - Activated carbon fibre;
Brij35 - Oxyethylene lauryl ether;
C/M - Chelating agent to metal ion concentration ratio;
C12E8 - Octa-ethylene glycol mono-dodecyl ether;
CMC - Critical micellar concentration;
CPC - Cetyl pyridinium chloride;
CTAB - Cetyl tri-methyl ammonium bromide;
DBP - Di-butyl phosphate;
DDAB - Di-decyl di-methyl ammonium bromide;
DSA - Dodecyl benzene sulfonic acid;
DTAC - Dodecyl trimethyl ammonium chloride;
EDTA - Ethylene di-amine tetra acetic acid;
EHPNA - 2-Ethyl hexyl phosphoric acid mono-2- ethyl hexyl ester;
EPA - Environmental protection agency;
HS - Humic substances;
KDS - Potassium dodecyl sulphate;
LAS - Linear alkyl benzene sulfonate;
LM-MEUF - Ligand modified micellar enhanced ultrafiltration;
MEUF - Micellar enhanced ultrafiltration;
MWCO - Molecular weight cut off;
NMWCO - Nominal molecular weight cut off;
NPE - Nonyl phenyl ether;
NP12 - Nonyl phenyl ethoxylate;
NTA - Nitrilo tri-acetic acid;
ODA - Octa-decyl amine acetate;
PES - Polyether sulfone;
PONPE - Polyoxyetylene nonyl phenyl ether;
PONPE10 - Polyoxyetylene nonyl phenyl ether with 10 etylene oxide unit;
PP - Potassium permanganate;
PS - Polysulfone;
RC - S/M - Surfactant to metal ion concentration ratio;
SDS - Sodium dodecyl sulphate;
SLES - Sodium lauryl ether sulphate;
TBP - Tri-butyl phosphate;
Triton-X 100 - Poly-oxyethylene octyl phenyl ether;
Tween 80 - Polyoxyethylene (80) sorbitan monooleate.
43
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Removal of heavy metals from wastewater
using micellar enhanced ultrafiltration technique: a review
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