Biomedical Engineering Reference
In-Depth Information
raise in dye concentration from 10-200 mg/L [27,70,71]. Conversely, the amount of dye
adsorbed declined with a rise in temperature from 301-333 K, implying an exothermic
nature of the adsorption process. The shape of isotherm curves resembled either L- or H-
type according to classification by Giles et al. [81]. These isotherm types, commonly
referred as Langmuir type, signified a high degree of adsorption at very low dye
concentration, and suggested the uptake of dyes by γ-PGA to be associated with chemical
forces rather than physical interaction [82,83].
An accurate mathematical description of equilibrium adsorption data is crucial for
reliable prediction of adsorption parameters and quantitative comparison of adsorption
behavior. The process variables and the underlying thermodynamic assumptions of the
equilibrium models often provide insight into the surface property and affinity of the
adsorbent as well as the adsorption mechanism. The equilibrium adsorption curves of Au-
O, Rh-B, Sa-O and BB-Y by γ-PGA were fitted with three most commonly used isotherm
models, namely, Freundlich [84], Langmuir [83] and Redlich-Peterson [85], represented
as
1
n
q=KC
(11)
e
F
e
qKC
q=
1+ K C
mee
(12)
e
ee
AC
q=
1+ K C
e
(13)
e
γ
Re
where C
e
is the dye concentration in solution at equilibrium, K
F
(mg/g) and n are Freundlich
constants denoting adsorption capacity and intensity of adsorption, respectively, whereas q
m
(mg/g) and K
e
(L/mg) represent the maximum adsorption capacity and energy of adsorption.
Non-linear regression analysis along with comparison of error parameters (r
2
and χ
2
) revealed
that the Redlich-Peterson model could describe the equilibrium data more closely than the
other two models (Table 7) [27,71]. The maximum adsorption capacity q
m
derived from the
Langmuir model was higher or comparable to those reported for nonconventional adsorbents
(Table 8) [86-111], demonstrating γ-PGA could be an effective adsorbent in scavenging
cationic dyes. The main characteristics of the Langmuir isotherm can be expressed by a
dimensionless separation factor or equilibrium parameter R
L
, which is defined as
R
L
=1/(1+K
e
C
o
), where K
e
(L/mg) is the Langmuir constant and C
o
(mg/L) is the initial dye
concentration. Hall et al. [112] proposed four idealized types of equilibrium behavior,
namely, irreversible (R
L
=0), unfavorable (R
L
>1), linear (R
L
=1) and favorable (0 < R
L
< 1).
The R
L
values for the dye concentration range 10-200 mg/L in Table 9 were 0-1, indicating a
favorable adsorption of cationic dyes by γ-PGA [27,70,71].
