Page 48 - Partition & Adsorption of Organic Contaminants in Environmental Systems

Partition and Adsorption of Organic Contaminants in Environmental Systems. Cary T. Chiou
Copyright ¶ 2002 John Wiley & Sons, Inc.
ISBN: 0-471-23325-0

4 Fundamentals of the

Adsorption Theory

4.1 INTRODUCTION

Adsorption is a surface phenomenon that is characterized by the concentra-
tion of a chemical species (adsorbate) from its vapor phase or from a solution
onto or near the surfaces or pores of a solid (adsorbent). This surface excess
occurs in general when the attractive energy of a substance with the solid
surface (i.e., the adhesive work) is greater than the cohesive energy of the
substance itself (Manes, 1998). The adsorptive uptake is amplified if the solid
material has a high surface area. If the adsorption occurs by London–van
der Waals forces of the solid and adsorbate, it is called physical adsorption.If
the forces leading to adsorption are related to chemical bonding forces, the
adsorption is referred to as chemisorption. However, the distinction between
physical adsorption and chemisorption is not always sharp. For example, the
adsorption of polar vapors onto polar solids may fall under either classifi-
cation, depending on the adsorption energy. From a thermodynamic point of
view, the concentration of a substance from a dilute vapor phase or solution
onto a solid surface corresponds to a reduction in freedom of motion of mol-
ecules and thereby to a loss in system entropy. As such, the adsorption process
must be exothermic to the extent that the negative DH is greater in magnitude
than the associated negative T DS to maintain a favorable free-energy driving
force (i.e., for DG to be negative). For more detailed discussions on the ther-
modynamic aspect of the adsorption process, see Adamson (1967), Gregg and
Sing (1982), and Manes (1998).
When a vapor is adsorbed onto a previously unoccupied solid surface or its
pore space, the amount of the vapor adsorbed is proportional to the solid
mass. The vapor uptake also depends on temperature (T), the equilibrium
partial pressure of the vapor (P), and the nature of the solid and vapor. For a
vapor adsorbed on a solid at a fixed temperature, the adsorbed quantity
per unit mass of the solid (Q) is then only a function of P.The relation between
Q and P at a given temperature is called the adsorption isotherm. Q is fre-
quently presented as a function of the relative pressure, P/P°, where P is
normalized to the saturation vapor pressure (P°) of the adsorbate at temper-
ature T. The normalized isotherm is often more useful, as it enables one to
assess readily the net adsorption heats and other characteristics of vapors
over a range of temperatures. For adsorption of solutes from solution, one
39