Biomedical Engineering Reference
In-Depth Information
between HA and γ-PGA became stronger due to multiple physical interaction resulting in a
convex shape. The maximum adsorption capacity (q
m
, mg/g) derived by fitting the isotherm
data at pH 2.5 with a linear form of the classical Langmuir equation (1/q
e
=1/q
m
K
e
C
e
+1/q
m
)
[83] followed the order: Trp-p-2 (1428.57) > MeIQ (1250.00) > 4,8-DiMeIQx (666.67)
(Table 11). The difference in q
m
values between HAs may be due to variation in their
hydrophobic character as reported by Ferguson and Harris [122], who demonstrated the
adsorption of nine HAs on dietary fiber α-cellulose to increase with hydrophobicity. Also, the
number of bulky methyl groups in HA may be responsible for this effect. Accordingly, 4,8-
DiMeIQx containing three methyl groups showed the lowest q
m
value (666.67 mg/g),
followed by MeIQ (1250.00 mg/g) and Trp-p-2 (1428.57 mg/g). Thus, the interaction
between HAs and γ-PGA at pH 2.5 may largely depend on physical forces.
Table 11. Langmuir parameters for heterocyclic amines binding by
γ
-PGA at pH 2.5
a
HA
Langmuir parameters
r
2
q
m
(mg/g)
b (l/mg)
MeIQ
1250.00
0.0020
0.948
4,8-DiMeIQx
666.67
0.0004
0.933
Trp-p-2
1428.57
0.0005
0.952
Source
: Inbaraj et al. 2006b.
a
Experimental conditions: HA concentration range: 10-2000 mg/L; γ-PGA dose: 1 mg/mL.
On the other hand, the isotherm curves developed for HA concentrations 100-2000 mg/L
at pH 5.5 were characterized by two distinct portions (curve I and curve II) with a curve shift
appearing between the same C
o
values (600 and 800 mg/L) for all the three HAs (plots not
shown). This phenomenon suggested the presence of more than one adsorption site with
varied affinities and binding energies for HA molecules. Fitting curve I and II individually
with the Langmuir equation (C
e
/q
e
=C
e
/q
m
+1/K
e
q
m
) provided good correlation (r
2
> 0.98),
suggesting saturation of two binding sites with varying binding energies. Linear fitting of
curve I yielded the isotherm parameters for initial monolayer saturation, while curve II gave
relavant parameters for overall HA binding by γ-PGA (Table 12). Moreover, fitting with the
Scatchard equation (q
e
/C
e
=q
m
K
e
-q
e
K
e
) [123] showed a significant deviation from linearity,
but a good correlation (r
2
> 0.91) was observed when applied separately to curves I and II,
confirming the presence of at least two different types of binding (Table 12). The multisite
HA adsorption at pH 5.5 was also interpreted on the basis of a multimolecular adsorption by
using the BET equation [124] as given below:
⎛
⎞
⎛⎞
C
1a -1
C
e
=+
e
⎜
⎟
⎜⎟
(19)
(C - C )
aq
aq
C
⎝
⎠⎝⎠
s
e
m
m
s
where C
s
(mg/l) is the solute concentration at the saturation of all layers, q
m
(mg/g) is the
amount of HA required to form a unimolecular layer and a is a BET constant representing
average heat of adsorption in the first layer. Linear plots of C
e
/(C
s
- C
e
) versus C
e
/C
s
gave a
high correlation for 4,8-DiMeIQx and Trp-p-2 (r
2
> 0.96), but not for MeIQ (r
2
> 0.87). From
the fitted parameters, the q
m
value for the unimolecular layer was found to be ~2.5 times
