Investigation of graphene/CNT hybrid
structural effect on
absorption<=
/b> of Mn2+ by
activated Carbon
Ahmad Ghozatloo1,* and Mojtaba
Shariaty-Niassar2
1
Research Institute of Petroleum Industry (RIPI), West
Blvd. Azadi Sport Complex, Tehran, Iran
2
Abstract
In this paper, an effort is made to improve the adsorption amount by
adding graphene/CNT hybrid in the structure of Activated Carbon (AC) for
removing manganese ions from aqueous solutions. The bonding G/CNT hybrid wi=
th
activated carbon creates a structure which is an excellent absorbent for
removing some heavy metal ions. Therefore, a comparison between the perform=
ance
increasing ion adsorption and AC results as the blank adsorbent was made. M=
oreover,
the influence of time, pH solution, initial concentration of Mn2+,
and temperature investigated as experimental conditions. Finally, the maxim=
um
adsorption was 89.38% and 96.18% for AC
and AC/G/CNT composite respectively at 30 min, initial concentration of 30
mg/L, pH=3D4.5 in 15 °C for both adsorbents. Finally, the results showed th=
at
under the same conditions, the presence of G/CNT hybrid in the structure of=
AC
increased the amount of adsorption by 7.5%
Keywords: Adsorption, Manganese,
Activated Carbon, graphene, CNT, hybrid
1. Introduction
Heavy meta=
l ions
and organics are two major classes of water pollutants. Heavy metal (a rang=
e of
toxic metals) ions as poisonous compounds are one of the main problems that
pollute the waters and plants. Discharging of pollutant water in plants wil=
l be
absorbed in soil and will not degrade and accumulate in living organisms,
causing various poisoning problems in animals and indirection for humans.
Therefore, elimination of these heavy metal ions from waste water is necess=
ary
and interest [1]. Moreover, the major techniques have been used to reduce or
decrease the heavy metal ions from wastewaters such as flotation, solvent
extraction, silica, adsorption processes, ion exchange, reverse osmosis, li=
me
chemical precipitation and coagulation [2]. Among these, adsorption is an
industrial and good method for removing heavy metals from aqueous solutions
[3]. Copper (Cu), mercury, cadmium, manganese (Mn2+), nickel,
chromium and lead are known to be greatly toxic effect on waters [4]. The i=
on
of Mn2+ can be found in the effluents of petrochemical plants is=
one
of dangers source of pollution. In addition, pore structure of some substan=
ces
which have active surface such as chitosan, anthracite, lignite, and specia=
lly
activated carbon (AC) are common and suitable for adoption Mn2+ =
and
widely used for high adsorption capacity [5]. The objective of this paper w=
as
to improve performance of AC by G/CNT structures. Because the multi-layer
configuration of graphene/CNT (G/CNT) hybrid structure is to have good
adsorption performance, so recently, graphene hybrid structure has been app=
lied
as the adsorbent for absorption of some pollutants from waste water,
because of its excellent capacitance. To improve interface and chemistry
surface behavior, wide researches have been conducted on the structure of G=
/CNT
hybrid structures [6]. In this paper, the effects of temperature, time, pH,=
and
initial Mn2+ concentration were investigated to remove the cations of Mn=
2+
from water by AC/G/CNT composite as adsorbent and comparison with AC.
2. Material and Methods
The chemic=
als
used for this paper were all Merck grade and used without further purificat=
ion.
Analytical grade of AC Powder was obtained from activated charcoal with the
purity of > 99.9%. The solutions were prepared in deionized water. The
Synthetic aqueous solutions of Mn2+ were prepared by dissolving
enough amount of Mn(NO3)2 =
in
pure water. In order to prepare the aqueous solutions with various
concentrations, first a stock solution was prepared with a concentration of
1000 mg/L of Mn2+. For this purpose, 0.65 g of Mn(NO3)2
was introduced in 1000 mL volumetric flask, added adequate water, and stirr=
er
the solution. Then the purposed
solutions were prepared with lower concentrations by dilution.
2.1. Preparation of activated carbon
First, 200=
g of
powdered glucose are placed in a tubular quartz reactor into the furnace un=
der
nitrogen atmosphere for 30 min. It is then gently warmed up to a temperatur=
e of
350 °C and remains for 2 hr. The glucose is carbonized under these conditio=
ns
and is colored as a black powder. The powder placed in a furnace and heated=
at
400 °C for 1 hr under neutral atmosphere. Next =
the
powder was extracted from the reactor and poured into 1 molar HCl at 90 °C for 30 min. The remaining mixture is fil=
tered
and washed with distilled water several times. Afterwards, the filter cake =
was
dried in oven 65 °C for 11 hr. Dried powder is a activated carbon that is
susceptible to participation in the carbon nanostructures.
2.2. =
Synthesis of AC/G/CNT composite
To synthes=
is of
Graphene/CNT hybrid (G/CNT hybrid) a two-step method of chemical vapor
deposition (CVD) was used. Its mechanism includes copper sheets by FeMo nanoparticles (as a catalyst) in the space of
interlayer of copper sheets to formation of CNT/Graphene nanostructure as
illustrated in Fig. 1.
Figure 1: .
According =
to Fig.
1 in the section (a), metal NPs nucleated on the surface of copper sheets as
activated catalyst for growth of CNT. Section (b) is as first step of G/CNT
hybrid synthesis which CNT fully grown on copper sheets by FeMo
in 600 °C as low temperature process CVD catalyst and section (c) is as sec=
ond
step of G/CNT hybrid synthesis which uniform Graphene nanosheets deposited =
on
copper for G/CNT hybrid formation as higher temperature process. Finally, in
section (d), acid washing was done to remove FeMo,
copper sheets and other pollutants from G/CNT hybrid structure to obtain the
high carbon purity [7]. 0.3 g of G/CNT powder prepared in 200 ml of distill=
ed
water and exposed to ultrasounds at 100 W for 2 hr. Then the amount of 20 g=
of
AC powder is slowly added to the ultrasonic mixture for 3 hr. The mixture is
then placed in the 50 °C for a day and then the water evaporates. The remai=
ning
solid layer was entering in tube quartz, under nitrogen atmosphere and slow=
ly
heated to 350 °C and remains for 2 hr. The residual product in reactor us
AC/G/CNT composite. TEM image of the AC/G/CNT composite structure was shown=
in Fig.
(2).
Figure 2: .
2.3. =
Independent variables & experiment design
For econom=
ic
removal of Mn2+ from the wastewater, it is necessary to find the
optimum conditions of adsorption. Moreover, adsorption experimental paramet=
ers
such as Mn2+ ions concentration (30-130 mg/L), temperature (15-3=
5°C), time (15-45 min), and pH
(3.5-5.5) were studied in a batch mode. All experiments were conducted in 5=
00
mL Erlenmeyer flasks. The levels of variables are shown in Table 1.
Table 1: The levels of Independent Variables process.
|
Independent
Variables |
<=
span
dir=3DLTR>-1 level |
<=
span
dir=3DLTR>0 level |
<=
span
dir=3DLTR>+1 level |
|
Mn+2
conc. (mg/L) |
30 |
80 |
130 |
|
Temperature
(oC) |
15 |
25 |
35 |
|
Time
(min) |
15 |
30 |
45 |
|
pH |
3.5 |
4.5 |
5.5 |
According =
to Table
1, for design of experiments because of various variables (each at 3 varied
levels), Taguchi method was used. The software of Design Expert 8.3 was use=
d to
decrease the number of experiments to 9 based on Taguchi, and the results w=
ere
shown in Table 2 include of detail of conditions.
Table 2: Condition of designed experiments by Taguchi met=
hod.
2.4. =
Batch Adsorption process
The surfac=
es of
adsorbents are cleaned from pollutants, fats, and activates by alkali washi=
ng [8].
For this purpose, treatment of AC and AC/G/CNT hybrid as adsorbents were do=
ne
before using by alkali washing. Therefore, adsorbents separately were poured
into 5 wt.% of HCl, =
stirred
for 25 min, and washed with hot distilled water. Then the adsorbents washed
with 1 wt.% of NaOH =
solution
to eliminate the residual HCl [9]. The samples =
were
filtered and washed with pure water until the pH of the filtrating solution=
was
neutral. Next adsorbents were dried in oven (OVEN, XU 112, France) at 90 °C for 6 hr. Ground and sifted=
to
obtain fine powder. Finally, the adsorbents kept in desiccators for subsequ=
ent
uses.
The requir=
ed
amount of adsorbents was introduced in a stirred tank reactor containing 25=
0 mL
of the prepared solution of Mn2+ ions. The flasks were mechanica=
lly
agitating for the desired time, wished temperature in fixed 650 rpm. Soluti=
on
of hydrochloric acid/sodium hydroxide 0.1 M was also prepared for pH
adjustments. According to Table 2, after each condition, the adsorbent was
separated from the solution and filtered (0.45µm cellulose acetate paper). =
All
experiments were performed in duplicate. After filtration, Mn2+ =
ions
remaining in the solution were determined with a Perkin-Elmer 3100 atomic
absorption spectrophotometer. The amount of adsorbed Mn2+ ions w=
as
calculated using the qe =3D (C0=
–
Ce) × v/m equation. where qe =
is the
quantity of adsorbed Mn2+ ions at equilibrium (mg/g), C0
and Ce respectively are the initial and equilibrium concentratio=
ns
of Mn2+ ions (mg/L), V is the volume of metal solution (L), and =
m is
the weight of adsorbent used (g). Moreover, the removal percentage of Mn
3. Results and Discussion
3.1. =
Effect of pH
According =
to
surface chemistry, the amount of Adsorption depends on the surface behavior=
of
adsorbent, such as porosity and situation and distribution of active sites =
for
bonding with metal ions. These indexes generally depend on the pH of the
solution [10]. At this study the pH solution was varied at 3.5, 4.5 and 5.5.
During adsorption process, pH of solution adjustment at fix purposed value =
and
measure with a pH meter using a combination glass electrode. Fig. 3 indicat=
es
the effect of pH solution on the removal of Mn2+ ions onto AC and
AC/G/CNT composite from aqueous solutions at 30 min.
Figure=
3: <=
/span>Effect of PH on adsorption.
The Mn
3.2. =
Effect of Time
Contact ti=
me is
a significant factor affecting removal of initial Mn2+ ions in t=
he
solution. Effect of time on the adsorption was shown in Table 2. The Results
reveal that increasing contact time caused increasing the adsorption of Mn<=
sup>2+
ions or different initial concentrations. So, if the time increases from 15=
to
30 min, the amount of absorption increases with more slopes, while with
increasing time from 30 to 45 min, the absorption rate increases with less
intensity. The Mn2+ removal is higher at the first 30 min. This =
is
due to larger surface area of the Adsorbents at the beginning for the
adsorption of Manganese. It has been matched with other results [11]. For
example, at fix Mn2+ concentration 80 mg/l, with the increasing =
time
from 15 to 30 min, the absorption rate was 3.6% while with the increasing t=
ime
from 30 to 45 min, this value was changed by only 2%. Therefore, based on
economic considerations and the rate of absorption changes, the best time f=
or
the absorption process is 30 min.
3.3. =
Effect of temperature and Mn2+ Concentration
Thermodyna=
mically
behavior of large amount adsorption process is exothermic and so temperature
has a negative effect on removing metal ions and other pollutants [12].
According to this, the mechanism of Mn2+ ions adsorption by AC a=
nd
AC\G\CNT was a function of temperature and has been considered this researc=
h.
Fig. 4 shows the effect of temperature on Mn2+ removing for both
adsorbents of AC and AC\G\CNT.
Figure 4: temperature
effect on Mn2+ removing for both adsorbents.
The result=
s of Fig.
4 were shown at initial concentration of 30 mg Mn2+ removing
decreased from 89.3% to 84.1% by AC and 96.18% to 91.16% by AC\G\CNT with
increasing of temperature from 15 to 35 °C.
This change for high concentration of Mn2+ is more evident at hi=
gher
temperatures. But the best initial concentration of Mn2+ is 80 m=
g/L.
At higher concentrations of this, between the adsorption of metal ions and =
the
surface of adsorbent, a competition phenomenon was occurred based on steric
hindrance and as a result, the amount of absorption decreases. The simultan=
eous
effect of three variables include initial Mn2+ concentration in =
the
range of 30, 80, and 130 mg/L for temperatures of 15, 25, and 35
Figure 5:
Based on F=
ig. 5,
it is observed that the effect of Temperature on amount of absorption is mo=
re
than time because by increasing temperature, this decreases with a constant
trend. However, when time increases, the amount of adsorption decreases, and
then it is flat that due to the saturation of the adsorbent surface. It is =
also
observed that the best absorption conditions occur at lower temperatures for
medium concentrations of Mn2+ at 30 min. At lower Manganese
concentration, the percentage removal of Mn2+ ions is high and by
increasing temperature the slop of adsorption decreases, this means is the
ratio of Mn2+ ions removing more decrease in high temperature.
Therefore, the technique of diluting wastewater and cooling during the remo=
ving
process can be used to absorb more metal Mn2+ ions.
4. =
Conclusions
Generally,=
at
fix conditions, the presence of G/CNT hybrid in the structure of AC 7.5%
increases the amount of adsorption. Moreover, the most important reason bei=
ng
to improve surface properties and create more porosity in the AC structure.=
In
addition, the inherent property of CNT in absorption of Mn2+ ions
based on ionic bonding under a wide range of graphene sheets has enhanced t=
he
absorption performance of AC. The best absorption conditions occurred at a =
15°C
and a concentration of 30 mg Mn2+ at pH of ab=
out
4.5. Increasing pH of the solution caused to the partial hydrolysis of Mn
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