Fundamentals of Adsorption onto Activated Carbon in Water and Wastewater Treatment - Activated Carbon for Water and Wastewater Treatment - page 17

Table 2.3 Comparison of physical and chemical adsorption.

Physisorption Chemisorption

Coverage Mono or multilayer Monolayer

Nature of adsorption Nondissociative and

reversible

Often dissociative, may be
irreversible

Specifity to adsorption sites Nonspecific Very specific

Temperature range Near or below the

condensation point of the
gas

unlimited

Temperature dependence of
uptake (with increasing T)

Decreases Increases

5-40 kJ mol 1 40-800 kJ mol 1 Adsorption enthalpy

Kinetics of adsorption Fast Very variable, often slow

Desorption Easy by reduced pressure or

increased temperature

Difficult - high temperature is
required to break bonds

Desorbed species Adsorbate unchanged Adsorbate may change

2.2.2
Factors Influencing Adsorption

Activated carbon adsorption is not a uniquely homogeneous process, but is rather
dependent on the various factors outlined below. These factors also play a signifi-
cant role in the integration of adsorption with biological processes (Chapters 3-11).

2.2.2.1
Surface Area of Adsorbent
The extent of adsorption is generally considered to be proportional to the specific
surface area. Specific surface area is that proportion of the total surface area which
is available for adsorption. The more finely divided and more porous adsorbents
would be expected to yield more adsorption per unit weight of adsorbent. The
surface can be characterized either as external when it involves bulges or cavities
with width greater than depth or internal when it involves pores and cavities that
have depth greater than width.

2.2.2.2
Physical and Chemical Characteristics of the Adsorbate
In general, the adsorbability of a compound increases with increasing molecular
weight and increasing number of functional groups such as double bonds or
halogens [4].
Larger molecules are adsorbed onto activated carbon better than smaller mole-
cules. The degree of solubility of the solute is also of primary concern for
adsorption. There is an inverse relationship between the extent of adsorption of a
particular solute and its solubility in the solvent from which the adsorption occurs.