Photocatalytic Processes for the Removal of Dye - Green Chemistry for Dyes Removal from Wastewater - page 141

encountered in the chemical or physical modification of these substances
to increase their efficiency [10,16,19-23].
The adsorption capacity of an adsorbent can be greatly influenced by
two well-known factors; high surface area, which assures a high amount
of adsorption sites for the adsorbates and abundant surface functional
groups, which can form complexes with the adsorbate molecules, enhanc-
ing the selectivity of the adsorbent [24]. Depending upon the modification
process and the activating agent applied, the aforementioned properties of
the adsorbent material can be systematically monitored.
There are two common types of activation methodologies basically
aimed at increasing the surface area of the material and/or conjugating
functional moieties to the surface of the adsorbent. Physical activation
involves a two-step process in which the carbonaceous precursor is pyro-
lyzed in an inert atmosphere at elevated temperatures in order to remove
the non-carbon elements and then the resultant char is exposed to a con-
trolled oxidizing atmosphere, usually steam or carbon dioxide. In this pro-
cess, the carbonaceous material reacts with the oxidizing agent and the
gaseous reaction volatiles are stripped off, building up a porous structure.
It is worth noting that some literature has reported the combination of
these two steps and the development of a single-stage activation process
with comparable surface areas. On the other hand, chemical activation of a
siliceous or carbonaceous substance includes a simultaneous pyrolysis and
activation using a dehydrating agent, such as zinc chloride and phosphoric
acid, at relatively high temperatures [25-28]. Chemical activation process
has several superior advantages over the physical activation technique
which includes lower reaction temperatures, less reaction time, higher
yield and higher surface area [29,30].
The mechanism of adsorption is usually governed chiefly by the binding
energy of the adsorbate to the adsorbent surface. Physisorption involves
the weak van der Waals interaction between the adsorbate and the surface
of the adsorbent. Since no chemical bond forms, the chemical nature and
the electronic orbital patterns of the adsorbent and adsorbate molecules
are not perturbed. In contrast, a chemical reaction between the adsorbate
molecules and the adsorption sites on the surface of the adsorbent drives
the chemisorption , so that the identity of the sorbed molecule might be
altered by the chemical bond [31-33].
This chapter will primarily provide an overview of the relevant literature
undertaken to modify the waste carbonaceous and siliceous materials for
use in dye adsorption purposes. It has been endeavored to compare differ-
ent activation conditions and their effects on the dye removal efficiency of
the resulting materials. Furthermore, the advantages and disadvantages of