CO2_2016 - page 50

48
Chimica Oggi - Chemistry Today
- vol. 34(2) March/April 2016
isotherms can be expressed by eqs 2 and 3,
respectively:
C
e
/q
ads
= 1/(q
max
b) + (1/q
max
) C
e
(2)
ln q
ads
= (1/n) ln C
e
+ ln k
(3)
where q
ads
is the amount adsorbed (mg/g), C
e
is the equilibrium concentration (mg/l), q
max
is the maximal adsorption capacity, b is the
adsorption equilibrium constant (l.mg
-1
), and k
(mol g
-1
) and 1/n are the Freundlich constants
related to adsorption affinity of the adsorbent
and adsorption intensity, respectively.
The Langmuir constants, q
max
and b,
are determined from the intercept and
slope, respectively, of the linear plot of the
experimental data of C
e
/q
ads
(g/l) versus C
e
(mg/l) as illustrated in figure 3.
The Freundlich constants are determined from
the intercept and slope, respectively, of the
linear plot of the experimental data of ln q
ads
versus ln Ce as shown in figure 4.
The Freundlich constants and correlation
coefficient R
2
are listed in table 3. R
2
values
showed that the Freundlich model could
describe the adsorption of Pb (II), Ni (II) and Cd
(II) on CMK-3.
The adsorption capacity (mg/g) for Pb (II),
Ni (II) and Cd (II) followed the order Pb (II) >
Ni (II) > Cd (II) (table 4). From table 4, it can
be inferred that the adsorption capacity was
inversely proportional to the hydrated ionic
radius of the metals which is in agreement with
other authors (46-48). As can be seen from
table 5, the adsorption of Pb (II) onto CMK-3 is
found higher than that onto various adsorbents,
indicating that CMK-3 is a potential candidate
as adsorbent for the lead removal from
wastewater.
Adsorption of lead from binary or ternary metal
solutions
It is very important to evaluate the performance
of CMK-3 on adsorption of lead in multi-metal
reaction system since wastewaters usually
contain more than single metal. Multiple metals
are expected to cause interactive effects. Figure
5 showed the results of the adsorption of Pb(II) on
CMK-3 when present in binary (Pb (II) + Cd (II)
, Pb (II) + Ni (II)) or in ternary system ( Pb (II) + Cd
(II) +Ni (II)). The results showed that the adsorption
of Pb(II) decreased slightly in binary system,
which can be ascribed to the greater cumulative
occupancy of adsorption sites of CMK-3 by Pb(II)
with the smallest hydrated ionic radius (Table 4).
Interestingly, in the ternary system there was a substantial
improvement in the removal of Pb(II) in the presence of
Cd (II) and Ni(II)), which indicates that there is a synergistic
effect of nickel and cadmium ions in adsorption process of
lead in ternary system. In this work, the reason explaining this
enhancement is not clarified.
Adsorption isotherms
Initial concentrations C of metals in the solution were varied
between 5 and 100 mg/l.
The Langmuir (44) and Freundlich (45) adsorption isotherms
are generally applicable to describe the adsorption of heavy
metals on solid adsorbents.
The linear form of the Langmuir and Freundlich adsorption
Figure 1.
Adsorption Kinetics of Pb (II), Ni(II) and Cd (II) by 5 mg of CMK-3 at 25°C and
ion concentration 40 mg/L..
Figure 2.
Pseudo-second order kinetics for the adsorption of Pb(II), Ni(II) and Cd(II) ions
by 5mg of CMK-3 at 25°C and ion concentration 40 mg/L.
Table 2.
Parameters for adsorption of Pb(II), Ni(II) and Cd(II) onto CMK-3 derived from
pseudo-second-order kinetic models.
1...,40,41,42,43,44,45,46,47,48,49 51,52,53,54,55,56,57,58,59,60,...68
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