CO2_2016 - page 49

47
Chimica Oggi - Chemistry Today
- vol. 34(2) March/April 2016
RESULTS AND DISCUSSION
Characterization of CMK-3
XRD.
XRD patterns of SBA-15 and CMK-3 (not shown) indicate
that these materials are hexagonally ordered mesostructure
which is in agreement with other authors (14).
N
2
Adsorption.
Typical irreversible type IV adsorption isotherms
(not shown) as defined by IUPAC (38) were observed for
siliceous SBA-15 and CMK-3 carbon, indicating mesoporous
characteristics with capillary condensation phenomenon. The
materials have narrow Gaussian pore size distribution centred
at 6.7 nm for siliceous SBA-15 and at 3.0 nm for CMK-3 carbon.
Textural properties of siliceous SBA-15 and CMK-3 carbon are
listed in Table 1. Surface area of CMK-3 (438 m
2
/g) is lower
than that of SBA-15 (825 m
2
/g), which could be explained
by the restriction of carbon deposition in the open pores of
SBA-15 due to the limited diffusion rate of the precursor, thus
some amorphous carbon is produced outside of the SBA-15,
resulting in a lower BET surface area (39).
Adsorption kinetics
Figure 1 depicts the influence of contact time between the
single metal solutions of Pb, Ni and Cd, and CMK-3 on the
efficiency of the adsorption process at 40 mg/l initial metal
ion concentration and 0,2 g/l CMK-3 concentration dose.
As shown in figure 1 the adsorption process on CMK-3 can
be considered very fast, attaining 90% adsorption efficiency
within 5 min for Pb (II) and Ni (II), and10 min for Cd (II); during
this period the removal of Pb (II), Ni (II) and Cd (II) was 199,4,
198 and 180 mg/g CMK-3, respectively. The equilibrium was
reached after 3h.
These results show the ability of CMK-3 to adsorb rapidly the
highly toxic Pb (II), Ni (II) and Cd (II) without any modification
of its surface.
In order to analyze the uptake rates of ions, a simple
kinetic analysis using the pseudo- second- order equation
was done. The pseudo-second order kinetic model is given
as (40):
t/q
t
= 1/k
2
q
e
2
+ t/q
e
(1)
where q
e
and q
t
are the amount of metal ions adsorbed on the
adsorbent in mg/g at equilibrium and at time t, respectively,
and k
is the rate constant of second-order adsorption in g mg
-1
min
-1
. The pseudo-second-order rate constant was determined
experimentally from the slope and the intercept of straight-line
plots of t/q
t
(min g mg
-1
) versus t (min), respectively, presented in
figure 2. The parameters of the pseudo-second order adsorption
kinetics model are listed in table 2. It can be inferred from this
table that the adsorption of Pb (II), Ni (II) and Cd (II) by CMK-3 is
well described by the pseudo second-order kinetic model which
implies that the adsorption of Pb (II), Ni (II) and Cd (II) onto CMK-
3 may occur via a chemical process involving valence forces
that share or exchange electrons (41-43).
Synthesis of CMK-3
CMK-3 was prepared according to the process described
in the literature (31). 1g of calcined SBA-15 was added to a
solution obtained by dissolving 1.25g of sucrose and 0.14 g of
H
2
SO
4
in 5 g of H
2
O. The mixture was placed in a drying oven
for 6h at 373K, and subsequently the oven temperature was
increased to 433K and maintained for 6h at this temperature.
The sample turned dark brown or black during the
treatment in the oven. The silica sample, containing partially
polymerized and carbonized sucrose at the present step, was
treated again at 373 and 433K using the same drying oven
after the addition of 0.8 g of sucrose, 0.09 g of H
2
SO
4
and 5
g of H
2
O. The carbonization was completed by pyrolysis with
heating to 1173 K under N
2
for 6h. The carbon-silica composite
obtained after pyrolysis was washed with hydrofluoric acid
(5 wt%) at room temperature, to remove the silica template.
The template-free carbon product thus obtained was filtered,
washed with ethanol and dried at 393K.
Characterization
X-Ray Diffraction (XRD): spectra were
performed on a D5000 Siemens powder
diffractometer (Cu K
α
) in the angle range of 0.5-
2° 2
Ө
with the steps of 0.01° and a step time of
1s and in the range of 10-60° 2
Ө
with the steps
of 0.04°.
The surface area and pore size analysis of
the samples were carried out by adsorption-
desorption of nitrogen on a Micromeritics
ASAP2010 instrument at 77K. Prior to N
2
adsorption, the samples were degassed under vacuum at
250°C for at least 2h.
The concentrations of metal ions in the solution before and
after the adsorption experiment were determined by an
atomic absorption spectrometer PERKIN-ELMER A analyst 300
model.
Batch adsorption experiments
Metal ion adsorption on CMK-3 was carried out by the solution
depletion method at 25°C at solution pH. Pb (II), Cd (II) and
Ni (II) were tested. CMK-3 (0,2g/l) was thoroughly mixed into
25 ml aqueous solution containing selected concentration
of metal ion. Several points of the adsorption isotherms were
obtained by using various initial concentrations (5-100 mg/l)
and 180 min of contact time.
The metal ion uptake capacity of CMK-3 was determined by
analyzing the metal concentration of the solution before and
after adsorption using equation:
q
ads
= (C
0
– C
t
) V / m
where C
0
and C
t
are the initial and final concentrations of
the metal ions in solution (mg/L), V is the volume of the metal
solution (L) and m is the dry weight of the adsorbent (mg).
The effects of the contact time, adsorbent dose, pH and
temperature on Pb (II) uptake from aqueous solution have
been studied for an initial concentration of Pb (II) of 40 mg/L.
The adsorption of Pb (II) when present in binary (Pb (II) +
Cd (II), Pb (II) + Ni (II)) or in ternary system (Pb (II) + Cd (II)
+ Ni (II)) was also investigated. The binary and ternary ions
adsorption experiments were conducted using the same
procedure and operating conditions used in the single
system adsorption. The metals ions were present in equal
initial concentrations.
Table 1.
Textural properties of purely siliceous SBA-15 and mesoporous carbon CMK-3,
(a): Unit cell Parameter a = 2 d100/√3 , (b): Pore wall thickness, t = a-pore diameter.
1...,39,40,41,42,43,44,45,46,47,48 50,51,52,53,54,55,56,57,58,59,...68
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