Removal of Organic Dyes from Industrial Effluents: An Overview of Physical and Biotechnological Applications - Green Chemistry for Dyes Removal from Wastewater - page 3

chemical properties of fresh water. Dyes in wastewater that can obstruct
light penetration and are highly visible, are stable to light irradiation and
heat and also toxic to microorganisms. The removal of dyes is a very com-
plex process due to their structure and synthetic origins [4].
Dyes that interfere directly or indirectly in the growth of aquatic organ-
isms are considered hazardous in terms of the environment. Nowadays a
growing awareness has emerged on the impact of these contaminants on
ground water, rivers, and lakes [5-8].
The utilization of wastewater for irrigation is an effective way to dis-
pose of wastewater [9]. Although various wastewater treatment methods
including physical, chemical, and physicochemical have been studied, in
recent years a wide range of studies have focused on biological methods
with some microorganisms such as fungi, bacteria and algae [10]. The
application of microorganisms for dye wastewater removal offers con-
siderable advantages which are the relatively low cost of the process, its
environmental friendliness, the production of less secondary sludge and
completely mineralized end products which are not toxic [11]. Numerous
researches on dye wastewater removal have been conducted which
have proven the potential of microorganisms such as Cunninghamella
elegans [12], Aspergillus nigerus [13], Bacillus cereus [14], Chlorella sp. [15]
and also Citrobacter sp . [16,17].
1.1.1 Dyes
A dye or a dyestuff is usually a colored organic compound or mixture that
may be used for imparting color to a substrate such as cloth, paper, plas-
tic or leather in a reasonably permanent fashion. The dye that is generally
described as a colored substance should have an affinity for the substrate or
should fix itself on the substrate to give it a permanent colored appearance,
but all the colored substances are not the dye [18,19]. Unlike many organic
compounds, the dyes which contain at least one chromophore group and also
a conjugated system and absorb light in the visible spectrum (400-700 nm)
and exhibit the resonance of electrons, possess special colors [20].
The relationships between wavelength of visible and color absorbed/
observed [21] are given on Table 1.1.
In general, a small amount of dyes in aqueous solution can produce a
vivid color because they have high molar extinction coefficients. Color
can be quantified by spectrophotometry (visible spectra), chromatography
(usually high performance liquid, HPLC) and high performance capillary
electrophoresis [19].