Hen Feather: A Remarkable Adsorbent for Dye Removal - Green Chemistry for Dyes Removal from Wastewater - page 433

Equation 11.7. The negative values of Gibb's free energy (ΔG ) indicate
feasibility of the processes, while positive values suggest unfeasibility of the
adsorption process.
Similarly the idea about the exothermic and endothermic nature of
the ongoing adsorption reaction can be derived by determining the
sign of ΔH in Equation 11.8. If it is positive, the reaction is endother-
mic, and the heat is absorbed by the system due to greater enthalpy
than the reactants. On the other hand, if ΔH is negative, the reaction
is exothermic, that is the overall decrease in enthalpy is achieved by the
generation of heat. Similarly with the help of Equation 11.9, an increase
or decrease in randomness of the process can be postulated by getting
positive and negative values of change in entropy (ΔS ), respectively.
Positive value of entropy change (ΔS ) shows increased degree of ran-
domness at the solution-solid interface with some structural changes
in the adsorbent. It also corresponds to an increased degree of freedom
of the adsorbed species. The opposite trends of these parameters occur
when negative value of entropy change (ΔS ) is obtained during the
adsorption.

11.5.1.6 Favorability Determination

To define favorability of a chemical or physical process a dimensionless
constant called separation factor (r) has been introduced by Weber and
Chakravorti [82]. For the separation factor the calculation can done by
employing the following relation:

1

(11.10)

r

1 bC o

where b denotes the Langmuir constant (L.mol -1 ) derived from
Equation 11.2 and C o is the initial concentration (mol. L -1 ). Thus, Langmuir
constant 'b' is helpful in determining the favorability of the process. The
significance of separation factor lies in ascertaining the favorability of
adsorption and the shape of the adsorption isotherms [82]. The nature of
favorability is determined on the basis of 'r' values obtained. The param-
eters indicate the shape of the isotherm accordingly. If r is greater than
unity and equal to one, the process is said to be unfavorable and linear,
respectively. Irreversible condition arises when 'r' value is obtained to be
equal to zero. The only favorable condition is when 0 < r < 1. Thus, iso-
thermal models expand their applicability by determining the favorability
through separation factor.