Page 49 - Partition & Adsorption of Organic Contaminants in Environmental Systems

40 FUNDAMENTALS OF THE ADSORPTION THEORY

constructs similar isotherm forms by relating Q with C e (the equilibrium
concentration) or with the relative concentration, C e /C s, where C s is the solu-
bility of the solute.
Except for rare cases where the microscopic structure of a solid surface is
nearly uniform, the surfaces of most solids are heterogeneous, with the result
that adsorption energies are variable. The adsorption sites are taken up
sequentially, starting from the highest-energy sites to the lowest-energy sites,
with increasing partial pressure or solute concentration. Thus the net (differ-
ential) molar heat of adsorption decreases with increasing adsorption and
vanishes when the vapor pressure or solute concentration reaches saturation.
Adsorption isotherms are typically nonlinear because of the energetic het-
erogeneity and the limited active sites or surfaces of the solid. Since a given
site or a surface of the solid cannot be shared by two or more different kinds
of adsorbates, the adsorption process is necessarily competitive, which is in
contrast to a partition process. The surface area or porosity of the solid is
usually the principal factor affecting the amount of vapor adsorption; there-
fore, a powerful adsorbent must have a large surface area. Adsorption of a
solute from solution is subject to competition by the solvent and other com-
ponents in the solution. Therefore, a powerful adsorbent for single vapors is
not necessarily a strong adsorbent for solutes from solution.
A number of adsorption isotherms have been recorded for vapors on a wide
variety of solids. Brunauer (1945) grouped the isotherms into five princi-
pal classes, types I to V, as illustrated in Figure. 4.1. Type I is characterized by
Langmuir-type adsorption (see below), which shows a monotonic approach
to a limiting value that corresponds theoretically to the completion of a surface
monolayer. Type II is perhaps most common for physical adsorption on rela-
tively open surfaces, in which adsorption proceeds progressively from sub-
monolayer to multilayer; the isotherm exhibits a distinct concave-downward
curvature at some low relative pressure (P/P°) and a sharply rising curve at
high P/P°. The point B at the knee of the curve signifies completion of an
adsorbed monolayer. It forms the basis of the Brunauer–Emmett–Teller
(BET) model for surface-area determination of a solid from the assumed
monolayer capacity, described below.
A type III isotherm signifies a relatively weak gas–solid interaction, as ex-
emplified by the adsorption of water and alkanes on nonporous low-polarity
solids such as polytetrafluroethylene (Teflon) (Graham, 1965; Whalen, 1968;
Gregg and Sing, 1982). In this case, the adsorbate does not effectively spread
on the solid surface. Type IV and V isotherms are characteristic of vapor
adsorption by capillary condensation into small adsorbent pores, in which
the adsorption reaches an asymptotic value as the saturation pressure is
approached. Adsorption of organic vapors on activated carbon is typically
type IV, whereas adsorption of water vapor on activated carbon is type V
(Manes, 1998), as shown later. The shape of the adsorption isotherm of a solute
from solution depends sensitively on the competitive adsorption of the solvent
and other components and may deviate greatly from that of its vapor on the
solid.