Page 126 - Industrial Wastewater Treatment, Recycling and Reuse

100 Industrial Wastewater Treatment, Recycling, and Reuse

1916), originally, it was a theoretical equilibrium isotherm relating to the
amount of gas adsorbed per unit mass of the adsorbent. It is based on a uni-
form, monolayer, and finite adsorption site. The Langmuir adsorption iso-
therm assumes that there is no interaction among molecules adsorbed on the
1
neighboring adsorption sites. The Langmuir constant, a L (mg g ) gives the
theoretical adsorption capacity, while C e and q e have their usual physical
1
meanings. b L (L g ) is another Langmuir constant whose value can be
determined from the plot of C e vs. C e /q e .
The Freundlich equation (Freundlich, 1906) is an empirical equation
based on adsorption on heterogeneous sites. It is also one of the most com-
mon two-parameter isotherms applied for wastewater treatment. It assumes
that stronger binding sites are occupied first, and the strength of adsorption
1
decreases with the degree of occupation. The constant, K (mg g )
1 1/n
(L mg ) in the Freundlich expression, is related to the capacity of the
adsorbent for the adsorbate; 1/n is a function of the strength of the adsorp-
tion, and it indicates the affinity between the adsorbent and adsorbate. A
value of 1/n below unity implies that the adsorption process is chemical;
if the value is above unity, adsorption is more of physical process; the more
heterogeneous the surface, the more the 1/n value approaches zero. The
values of k for several priority pollutants (e.g., nitrobenzene, styrene, chlo-
robenzene, bromoform) are in the range 60–360 mg/g and the values of
1/n¼0.12–0.98 (Munter, 2000).
The Dubunin-Radushkevich (Dubinin, 1960) is an empirical adsorp-
tion isotherm that is now mostly applied for metal ion adsorption from
an aqueous system. It is used to predict whether the adsorption is physi-
sorption or chemisorption from its mean free energy per molecule of
1
the adsorbate, E (kJ mol ). The energy of adsorption is computed by
the relationship
1
p
E ¼ ffiffiffiffiffiffiffiffiffi
2B D
The D-R equation takes into account the temperature effect as the fac-
1
tor. In the isotherm, q D (mol g ) is the theoretical saturation capacity, B D
2 2
(mol J ) is a constant related to the mean free energy per mole of the
1
adsorbate, and R is the molar gas constant (8.314 J K 1 mol ).
The Redlich-Peterson model is a three-parameter isotherm model
(Redlich and Peterson, 1959) compromising the features of both Langmuir
and Freundlich. It can be applied over a wide range of concentrations for
several solid-liquid adsorption equilibrium data.