Fundamentals of Adsorption onto Activated Carbon in Water and Wastewater Treatment - Activated Carbon for Water and Wastewater Treatment - page 19

pH. This results from the neutralization of negative charges at the surface of the
carbon at low pH values. Neutralization of negative charges reduces the hindrance
to diffusion and leads to more active adsorption sites. The extent of this effect
varies with the type and activation technique of activated carbon. The differences
in pH values may also arise due to acidic or basic surface functional groups on
activated carbon. These groups could be freed by simple contact with distilled
water rather than the fixed surface functional groups. An inverse relationship has
previously been reported between adsorption capacity and surface acidity [12, 13].
2.2.2.4
Temperature
Adsorption involves specific relations between the properties of activated carbon and
the solute. Therefore, the quantitative effects of temperature are not the same with
all carbons and solutes. The extent of adsorption should increase with decreasing
temperature because the adsorption reactions are exothermic. However, increased
temperature also increases the rate of diffusion of the solute through the liquid to the
adsorption sites, which eventually leads to an increased adsorption. An important
difference in the adsorption of solutes versus gases is found in the role of
temperature. An increase in temperature increases the tendency of a gas to escape
from the interface and thus diminishes adsorption. However, in adsorption from
the liquid, the influence of temperature on solvent affinities is more dominant [10].
2.2.2.5
Porosity of the Adsorbent
The adsorption performance is dependent on the condition of internal surface
accessibility. A very important and decisive property of adsorbent materials is the
pore structure. The total number of pores and their shape and size determine
the adsorption capacity and even the rate of adsorption. The significance of
pores in adsorption processes largely depends on their sizes. Most of the solid
adsorbents possess a complex structure that consists of pores of different sizes
and shapes. Total porosity is usually classified into three groups. According
to the IUPAC recommendation, micropores are defined as pores of a width not
exceeding 2 nm, mesopores are pores of a width between 2 and 50 nm, and
macropores are pores of a width greater than 50 nm. The above classification is
widely accepted in adsorption literature. A further classification involves ultra-
micropores, which are pores of a width less than 0.7 nm [5].
The mechanism of adsorption on the surface of macropores does not differ from
that on flat surfaces. The specific surface area of macroporous adsorbents is very
small. Therefore, adsorption on this surface is usually neglected. Capillary adsorbate
condensation does not occur in macropores [14]. Macropores do not adsorb small
molecules by volume but by surface. For example, phenol adsorption on macro-
pores was reported to be less significant than that on meso- and micropores [15].
In the case of mesopores, the adsorbent surface area has a distinct physical
meaning. Mono- and multilayer adsorption takes place successively on the surface of
mesopores, and adsorption proceeds according to the mechanism of capillary