Page 107 - Adsorbents - fundamentals and applications

92 ACTIVATED CARBON

hydrophobicity, at even P/P 0 ≈ 1 (Kaneko et al., 1995; Kaneko et al., 2000).
The work on fluorination by Kaneko and co-workers was performed on active
carbon fibers (ACFs), but their observations are expected on activated carbon as
well. Incorporation of nitrogen atoms on ACFs, on the contrary, decreased the
hydrophobicity (Kaneko et al., 2000). A further understanding of the different
effects by different chemical modifications is clearly needed.

5.5. ADSORPTION FROM SOLUTION AND EFFECTS
OF SURFACE FUNCTIONALITIES
Adsorption from liquid solution is more complicated than that from the gas
phase. In a two-component liquid solution, both the solvent and the solute will
be adsorbed to different extents. Usually the adsorption of the solute is of interest.
The experimental procedure and expression for the amount adsorbed both differ
from those used for gas adsorption. The basis for expressing the amount adsorbed
from liquids is the concept of surface excess given by Gibbs in 1878. Gibbs
surface excess is the difference in the amount of a given component in the
surface layer (per unit surface area) over that in the bulk liquid.
The actual experiment involves measurements of the initial and final concen-
o
trations of the solution. Let n be the total number of moles in the initial solution,
o
o
o
and n = n + n , where subscripts 1 and 2 denote the solute and solvent, respec-
1 2
o
tively. Let x and x denote the initial and final mole fractions, respectively. Then
the Gibbs adsorption or the apparent adsorption is given, for 1 g of sorbent, by:
σ(n) o o s s s
n (5.1)
1
1
1 = n o (x − x 1 ) = n x 1 = n − (n + n )x 1
1
2
s
s
where n and n are the number of moles of component 1 and 2, respectively,
1 2
in the surface layer. For totally miscible solutions, two general types of adsorp-
tion isotherms of Gibbs adsorption, also referred to as the composite adsorption
isotherm, are shown in Figure 5.8.
s
For each composite isotherm, there are two real adsorption isotherms, n (x 1 )
1
s
and n (x 2 ), which can also be measured and are always positive. These two
2
general types have been further classified into five types by Nagy and Schay
(1960). The composite isotherm shown in Figure 5.8 clearly indicates the relative
strengths of adsorption: n-butyl amine > benzene > methyl acetate.
The composite isotherm can be changed by introduction of surface function-
alities such as oxygen. Oxidation will increase the preference for the more
polar component of the solution. This is shown by the example of adsorp-
tion of the ethanol–benzene solution, in Figure 5.9. The concentrations of the
four groups (phenolic−OH, carboxylic, lactone, and carbonyl) were measured
and were increased significantly by oxidation with nitric acid (Jankowska et al.,
1983). The total amount of oxygen-containing groups was increased from 2.05
to 6.1%. The preference for adsorption of ethanol over benzene is evident in
Figure 5.9. Degassing of the surface reduced the surface oxygen to 0.35%, and
resulted in a more preferential adsorption of benzene.