Environmental Engineering Reference
In-Depth Information
has huge limitations for removal of dyes in textile effluents, and is very
specific for dyes and other impurities present in wastewater, which reduces
its effectiveness [48,65].
The membrane processes have major disadvantages, such as a limited
lifetime and the high cost of periodic replacement. Liquid-phase adsorp-
tion is one of the most popular methods for the removal of pollutants from
wastewater and also an attractive alternative for the treatment of contami-
nated waters, especially if the sorbent is inexpensive and does not require
an additional pretreatment step before its application.
Adsorption, which is a well-known equilibrium separation process, has
been found to be superior to other techniques for water reuse in terms of
initial cost, flexibility and simplicity of design, ease of operation and insen-
sitivity to toxic pollutants. Decolorization, which is influenced by many
physicochemical factors such as dye/sorbent ratio, sorbent surface area,
particle size, temperature, pH, and contact time, is mainly a result of two
mechanisms: adsorption and ion exchange [66,57,50]. Also, adsorption
generally does not result in the formation of harmful substances.
1.3.2.1 Adsorption
The use of adsorption method for wastewater treatment has become more
popular in recent years owing to its efficiency in the removal of pollutants
too stable for biological methods. Adsorption is an economically feasible
process that can produce high quality water [67].
Because synthetic dyes cannot be efficiently removed from the wastewa-
ters by conventional methods, the adsorption of synthetic dyes on inexpen-
sive and efficient solid supports is considered as a simple and economical
method for their removal from wastewaters. The adsorption characteristics
of a wide variety of inorganic and organic supports have been measured
and their capacity to remove synthetic dyes has been evaluated [11].
Physical adsorption occurs reversibly via weak interactions, such as van
der Waals interactions, hydrogen bonding and dipole-dipole interaction,
between the adsorbate and adsorbent. Chemical adsorption, chemisorp-
tion, occurs irreversibly via strong interactions, such as covalent and ionic
bond formation, between adsorbate and adsorbent [68]. A summary of
commonly used adsorbents follows.
Activated carbon
is the most commonly used adsorbent for dye removal
by adsorption and is very effective for the adsorption of cationic dye, mor-
dant, and acid dyes and to a slightly lesser extent, dispersed, direct, vat,
pigment and reactive dyes [49, 69-73]. The performance is dependent
on the type of carbon used and the characteristics of the wastewater. Due
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