Non-conventional Adsorbents for Dye Removal - Green Chemistry for Dyes Removal from Wastewater - page 360

as they can be used to remove different types of pollutants [6]. In particu-
lar, adsorption/separation techniques are widely used to remove certain
classes of chemical (recalcitrant) pollutants from waters, especially those
that are hardly destroyed in conventional biological wastewater treatments.
Because of their great capacity to adsorb dyes, commercial activated car-
bon (CAC) and synthetic polymeric organic resins are the most effective
adsorbents. This capacity is mainly due to their structural characteristics
and their porous texture which gives them a large surface area, and their
chemical nature which can be easily modified by chemical treatment in
order to increase their properties. However, although these commercial
materials are preferred conventional adsorbents for dye removal, their
widespread industrial use is restricted due to high cost. As such, alterna-
tive non-conventional adsorbents were proposed, studied and employed as
inexpensive and efficient adsorbents [8].
For many adsorption processes, the separation is caused by a mass sepa-
rating agent. The mass separating agent is adsorbent. Consequently, the
performance of any adsorptive separation or purification process is directly
determined by the quality of the adsorbent. So, the first important step to
an efficient adsorption process is the search for a solid porous material with
high capacity. In principle, as adsorption is a surface phenomenon, any
porous solid having a large surface area may be an adsorbent [9]. However,
a suitable adsorption process of pollutants should also meet several other
requirements. Indeed the selection of an adsorbent is based on the follow-
ing criteria: low cost and readily available, granular type with a good par-
ticle size distribution, a well-developed (infra)structure, porous material
with a large total surface area, surface charge (acidity/basicity properties),
presence of adsorption sites or functional groups, high physical strength
(not disintegrating) in solution, suitable mechanical properties, efficient
for removal of a large range of dyes, high capacity, high rate of adsorp-
tion, high selectivity, a long life, and able to be regenerated if required [9].
By plotting solid phase concentration against liquid phase concentration
graphically, it is possible to depict an equilibrium adsorption isotherm.
An adsorption isotherm represents the relationship existing between the
amount of pollutant adsorbed and the pollutant concentration remaining
in solution. Equilibrium is established when the amount of pollutant being
adsorbed onto the material is equal to the amount being desorbed. Among
the numerous theories relating to adsorption equilibrium, the Langmuir
adsorption isotherm is the best known of all isotherms describing adsorp-
tion [10-13]. Using the Langmuir equation, it is possible to obtain an inter-
esting parameter widely used in the literature to promote a solid material
as adsorbent, i.e., the theoretical monolayer capacity or the maximum
adsorption capacity of an adsorbent (q max ). The equation of Langmuir is