Chitosan and Chitosan Derivatives as Chelating Agents - Natural Polymers, Biopolymers, Biomaterials, and Their Composites, Blends, and IPNs - page 12

Pb, Hg, Cd and Cu due to cation-p interactions. The phenomenon may also reflect the
''principle of hard and soft acids and bases” introduced by Pearson (1963). Further-
more, highest sorption for mercury cation by C[4]BCP is in agreement with previous
studies (Yilmaz et al., 2006) where calixarenes bearing nitrile functionalities showed
much more affinity toward this cation.

adsorPtioN isotherms models
The most common sorption models used to fit the experimental data are the Langmuir
and Freundlich isotherm models (Tan et al., 2007). Readers interested in a detailed
discussion of sorption isotherms should refer to the comprehensive reference works
by Tien and McKay et al. (Tien, 1994; McKay et al., 2002).
The Langmuir isotherm model assumes that the adsorption occurs at specific ho-
mogeneous adsorption sites within the adsorbent. Furthermore, it assumes monolayer
adsorption and maximum adsorption occurs when adsorbed molecules on the surface
of the adsorbent form a saturated layer. All adsorption sites involved are energeti-
cally identical and the intermolecular force decreases as the distance from the adsorp-
tion surface increases (Unlu and Ersoz, 2006; Uysal and Ar, 2007; Vasconcelos et al.,
2008). The form for the liquid phase sorption system is as follows:

bQ C
bC

Q =

oe

(1)

e

1 +

e

where, C e is the equilibrium concentration of the adsorbate (mg/L), Q e is the amount
of adsorbate adsorbed per unit mass of adsorbent (mg/g), Q o is Langmuir constant
related to adsorption capacity (mg/g) and b (L/mg) is a constant related to the affinity
between the adsorbent and the adsorbate. The values of Q o and b can be determined by
plotting C e / Q e versus C e .
The linear form of Langmuir model is given as:

C
QQQ C

1 1

e

=+

(2)

e

e o o

The essential characteristics of the Langmuir isotherm can be expressed in terms
of a dimensionless equilibrium parameter (R L ), (Weber and Chakravorti, 1974) which
is defined as:

1

R =

(3) L

1 + bC

o

where, b is the Langmuir constant and C o is the highest ion concentration (mg/l). The
value of R L indicates the type of the isotherm to be either favorable (0<R L <1), unfavor-
able (R L >1), linear (R L = 1) or irreversible (R L = 0).
The widely used empirical Freundlich model expresses adsorption at multilayer
and on energetically heterogeneous surface multi-site adsorption isotherm for hetero-