Water liquid water ensures the transmission of

 

 

Water
pollution is a source of concern in most of the countries such as India and
other developing nations. Speedy industrialization has provided much relief to
human beings; however, its unfavorable effects have emerged in the shape of
environmental deterioration. Industries discharging pollutants into the
environment include oil refineries, tanneries, textile plants, food,
pharmaceutical, paint, and coal processing industries. A large number of
organic substances are introduced into the water system from these sources as
industrial effluents and chemical spills. Their toxicity, steadiness to natural
disintegration and determination in the environment has been the cause of much worry
to societies and regulatory authorities around the planet (Bell and Buckley,
2003; McMullan et al, 2001).

 

 

1.1  DYES,
ENVIRONMENTAL CONCERN

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                    Decolourisation of
wastewater has become one of the major issues in wastewater pollution. This is
because many industries such as textiles, rubber, paper, plastics, leather,
cosmetics, food and mineral processing industries use dyes to colour their
products. Textile industries are one of the industries that consume large
volumes of water in the processing operations including pre-treatment, dyeing, printing
and finishing. In dying process, water vapour is used as heating agent for the
dye baths, while liquid water ensures the transmission of the dyes onto the fiber.
The release of the treatment baths which are highly poisonous and deeply
coloured after the dying procedure contributes to water pollution. One cannot
estimate the total amount of pollutants being generated each time the dyestuff
is discharged into the water stream. Mainly, the textile finishing industry has
an unambiguous water consumption (approximately 1L/kg of product), part of
which is due to dyeing and rinsing processes. Out of the total world production
of dyestuffs of around 10 million kg/ year, between 1 and 2 million kg of
active dye stuffs, either in the dissolved or suspended form in water, enter
the biosphere every year. These dyes are consistently left in the industrial
wastes. Since they have a synthetic origin and complex aromatic molecular
structures, which

make
them inert and difficult to biodegrade when discharged into waste streams,
people failed to notice their undesirable nature. Dyes, as they are intensively
coloured, cause special problems in effluent release and even small amount is
noticeable. The presence of very low concentrations of dyes in effluent is
highly visible and undesirable. The effect is aesthetically more offensive rather
than hazardous (Tariq et al 2008).

 

                      The wastewater from
textile mills causes serious impact on natural water bodies and land in the adjacent
area. High values of COD and BOD, presence of particulate matter and sediments,
chemicals which are dark in colour leading to turbidity in the effluents causes
depletion of dissolved oxygen, which has an unpleasant effect on the marine
ecological system. Since, dye can reduce light penetration; the efficiency of
photosynthesis in aquatic plants is decreased resulting in unfavorable impact
on their enlargement and progress (AL-Degs et al 2000). As dyes are designed to
be chemically and photolytically steady, they are highly persistent in natural
environments. Dyes also can cause severe injury to human beings, such as
dysfunction of kidney, reproductive systems, liver, brain and central nervous
system. Moreover, some dyes and their degradation products may be carcinogenic and
toxic and consequently, they are important sources of water pollutions. Hence,
decolorisation of dye house effluent through the removal of dye remains as a
major problem for environmental managers. Growing environmental concerns global,
along with more rigorous discharge standards have always been prompting the
textile industry to research and expand new, more effective decolorization and
treatment technologies (Tang and Chen 2004).

 

1.2 
DYES AND THEIR INTERMEDIATES

 

                        In textile industry,
dyes and their intermediates with high aromaticity and low biodegradability
have emerged as major environmental pollutants (Arslan et al, 2000; Sauer
et al, 2002) and nearly 10-15% of the dye is lost in the dyeing process
and released into the effluent which is an important source of environmental
contamination. Huge amount of water is used for dyeing and finishing of fabrics
in the textile industries (Sreedhar et al, 2006; Saggioro et al, 2011).

 

       The wastewater from textile mills has
caused a serious impact on natural water bodies and land in the surrounding
areas (Kanu and Achi, 2011). High values of COD, presence of particulate matter
and sediments, chemicals which are dark in colour leading to turbidity in the
effluents cause depletion of dissolved oxygen, which has an adverse effect on
the aquatic ecological system. Dyes are designed to be chemically and photocatalytically
stable and are highly persistent in natural environments. The improper handling
of hazardous chemicals in textile effluent also has some serious impact on the worker’s
health and safety putting them into the high risk bracket for contracting skin diseases
like chemical bums, irritation, ulcers and respiratory problems (Uzoekwe and Oghosanine,
2011; Neppolian et al, 2002 and Mathur et al, 2012).

 

1.3 
CLASSIFICATION OF DYES

 

                        The first organic dye
namely Mauvine was synthesized by William Henry Perkin in 1856, while many
natural dyes have been discovered. But, it was in the last century the
synthetic organic dyes became more popular owing to their variety, better
imparting property to the fabric materials. Over 700000 tons of approximately
10000 types of dyes and pigments are produced annually worldwide, among them
50-70% are azo dyes (Zollinger, 1987).   

 

All
aromatic compounds absorb electromagnetic energy but, only those that absorb
light with wavelengths in the visible range (~350-700nm) are coloured. Dyes are
composed of a group of atoms responsible for the dye color, called
chromophores, as well as an electron withdrawing or donating substituents that
cause or intensify the color of the chromophores, called auxochromes. It is
estimated that almost 10 million tons of dyes are produced annually in the
world.  . Usual chromophores are -C=C-,
-C=N-, -C=0-, -N=N-, NOi and quinoid rings, while the auxochromes are NH3,
COOH, SO3H and OH groups. Based on chemical structure or chromophore, 20-30
different groups of dyes can be discemed. Each different dye is given a C.I.
(Colour Index) generic name determined by its application characteristics and
its colour (Abrahart, 1977). The Colour Index discerns different application
classes which are as follows :

 

 

Figure: 1. Classification of Dyes based on their application

 

 

1.3.1    Direct
Dyes

 

                         Direct dyes have resemblance
to cellulosic fibers and they are soluble in water. These are comparatively
large molecules and dying with these dyes is simple. They are the second
largest dye class with respect to the amount of different dyes when classified
based on color index.

 

1.3.2    Acid
Dyes

 

                    The
largest class of dyes in the color index is referred to as acid dyes. They are
anionic compounds and are used for dyeing nitrogen containing fabrics like
wool, polyamide, silk and modified acryl.

 

1.3.3    Basic
Dyes

 

                   Basic dyes are cationic
compounds that are used for dyeing acid group containing fibers, usually
synthetic fibers like modified polyacryl. Most basic dyes are diarylmethane,
triarylmethane, anthraquinone or azo compounds.  Basic dyes stand for about 5% of all dyes
listed in the color index.

 

1.3.4    Reactive
Dyes

 

                           Reactive dyes are dyes with reactive groups
that form covalent bonds with NH2, OH, or SH groups in fibers like
cotton, wool, silk and nylon. The reactive group is often a heterocyclic
aromatic ring substituted with chloride or fluoride. Most reactive dyes are azo
or metal complex azo compounds.

 

1.3.5    Disperse
Dyes

 

                           Disperse dyes are hardly soluble
dyes that penetrate synthetic fibers like cellulose acetate, polyester,
polyamide, acryl, etc. This diffusion requires swelling of the fiber, either
due to high temperature or with the help of chemical softeners. Dying takes
place in dye baths with fine disperse solutions of these dyes. Disperse dyes
form the third largest group of dyes in the color index.

 

1.3.6
Vat Dyes

                

                     Vat dyes are water insoluble dyes that
are mainly used for dyeing cellulose fibers. The dyeing method is based on the
solubility of vat dyes in their reduced form. Vat dyes are reduced with sodium
dithionite to form a soluble form of dye. These reduced forms could be oxidized
after dyeing to bring back the dye to its insoluble form.

 

 

 

 

1.3.7
Sulfur Dyes

 

                       Sulfur dyes are complex
polymeric aromatics with heterocyclic sulfur containing rings. Even though it
represents about 15% of the global dye production, sulfur dyes are not so much
used. Dyeing with sulfur dyes involves reduction and oxidation, similar to that
of vat dyeing.

 

1.3.8
Azoic Dyes

 

                  These dyes possess an azo
group and are formed by the reaction of two separate components using which the
fibers are treated separately. Initially, the fiber is treated with primary
component which is nearly colorless and then treated with the second component
that has been diazotized already. These two components react immediately to
impart color to the fabric. Since these dyes contain azo groups, they are named
as azoic dyes.

 

1.3.9
Mineral Dyes

 

                          Some of the inorganic
metal oxides are colored and it is possible to form these oxides in the fiber
substance by first treating the textile material with a water-soluble salt of
the metal. Iron buff, manganese brown, chrome yellow, chrome orange etc. are
the examples for mineral dyes.

 

1.3.10
Oxidation Colors

 

                         There are special dye
stuffs which are ordinary organic compounds like aniline, amines and
parapheylene diamine etc. These organic compounds can be applied on cotton
fabric and subsequently oxidized in stages to a certain color. Normally these
dyes produce black shades, which have outstanding washing fastness. Since the
final shades are formed by oxidation, these are called oxidation colors.

 

1.4   AZO DYES

 

                       Azo dyes are
characterized by the presence of one or more azo groups (-N=N-). Azo dyes, the
largest and most versatile class of synthetic dyes have wide range of
applications in the textile, leather, paper, food, pharmaceutical and cosmetic
industries (Nachiyar and Rajakumar 2005). These dyes were initially made
available to the industries in large quantities in the early 20th century.
There is only one example for the presence of an azo group in a natural product (4,4?-dihydroxyazobenzene;
Gill and Strauch 1984).

 

                       Azo dyes are mostly used
for imparting yellow, orange and red colors (Christie 2001). To achieve the
target color, normally a mixture of red, yellow and blue dyes is used in dye bath.
These three dyes do not necessarily have the same chemical structure and they
may contain many different chromophores. The most important groups include azo,
anthraquinone and phthalocyanine dyes.

 

                     Azo
dyes are regarded as relatively persistent pollutants because they cannot be
readily degraded under aerobic conditions. The persistence of azo dyes was
mainly due to the presence of sulfo and azo groups, which are not naturally
occurring, making the dyes recalcitrant to oxidative biodegradation. The
appealing properties of these dyes include that they provide a wide range of
brilliant shades and provide high wet fastness.

 

                       Azo dyes are considered
as electron-deficient xenobiotic compounds because they possess the azo (N=N)
and sulfonic electron withdrawing groups, generating electron deficiency in the
molecule and making the compound less susceptible to oxidative catabolism by
bacteria. As a consequence, azo dyes tend to persist under aerobic environmental
conditions (Hsueh and Chen 2007).

 

                       Under anaerobic
conditions, azo dyes are easily converted to aromatic amines. Some azo dyes are
not normally cytotoxic, mutagenic, or carcinogenic but the amines formed by
anaerobic digestion may possess these characteristics. Although at least
certain azo dyes can be mineralized by anaerobic/aerobic treatment systems, but
this strategy have serious drawbacks. The fact that many of the amines formed
during the anaerobic reduction of the

azo
dyes (which are very often ortho-amino hydroxyl naphthalenes) are rather unsteady
under aerobic conditions and undergo auto-oxidation reactions, which is a
serious problem if a true mineralization of the azo dyes is the aim of the
treatment (Stolz 2001).

 

1.5  DYE
REMOVAL TECHNIQUES

 

                          The methods commonly
used for the removal of dyes from industrial effluents are physical and
chemical methods. These methods have their own limitations such as high cost,
low efficiency and inapplicability to a wide variety of dyes.

 

1.5.1    Physical
Methods

 

                         Physical methods mainly used for wastewater
treatment in textile industries include filtration and adsorption. Filtration
is an important physical method used in industrial wastewater treatment.  Methods such as ultrafiltration, nano-filtration
and reverse osmosis are used for water reuse and chemical recovery.   In textile industry, these methods can be
used for both filtering and recycling not only pigment-rich streams, but also
mercerizing and bleaching wastewaters. The main draw backs associated with membrane
technology include high investment costs, membrane fouling, and the production
of a concentrated dye bath which needs to be treated (Robinson et al 2001).

 

                 Adsorption methods for color
removal are based on the affinity of dyes towards adsorbents. Decolorization by
adsorption is influenced by some physical and chemical factors like dye – adsorbent
affinity, adsorbent surface area, particle size, temperature, pH and contact
time (Mattioli et al 2002). Activated carbon is the most common adsorbent and
reported to be effective in removing many dyes. However, its efficiency is
directly dependent upon the type of carbon material used and the wastewater
characteristics. Additionally, activated carbon is relatively expensive and has
to be regenerated offsite with losses of about 10% (Karcher et al 2001). A
number of low cost adsorbent materials like peat, bentonite clay and fly ash,
have

been
investigated for the removal of dyes. In these methods, the remains as such and
further degradation has to be carried out to destroy the toxic material (Anjaneyulu
et al 2005). This is the main disadvantage of the physical methods.

1.5.2   
Chemical Treatment

 

                          Chemical methods of treatment
rely upon the chemical interactions of the contaminants that have to be removed
from water. The addition of chemicals many either aid in the separation of
contaminants from water, or assist in the destruction or neutralization of
harmful effects associated with contaminants. Chemical treatment methods are
used as stand-alone technology or as an integral part of the treatment process
with physical methods.

 

                          The coagulation – flocculation
method is one of the most widely used processes in textile wastewater treatment
plants in many countries. It can be used either as a pretreatment,
post-treatment, or even as a main treatment. Coagulation – flocculation
methods were successfully applied for the removal of sulphur and disperse dyes,
whereas acid, direct, reactive and vat dyes showed very low coagulation – flocculation
capacity (Marmagne and Coste 1996). These methods demand large chemicals
inputs, and produce high volumes of polluted sludge, which must then be
disposed properly (Robinson et al 2001).

 

                        Chemical oxidation
methods typically involve the use of an oxidizing agents such as ozone (O3),
hydrogen peroxide (H2O2) and permanganate (MnO4)
to change the chemical composition of a compound or a group of compounds
(Metcalf and Eddy, 2003). Among these oxidants, ozone is most widely used
because of its high reactivity with many dyes and also shows good color removal
efficiency (Alaton et al 2002). In selective oxidation, ozone can be designed
in such a way that only –N=N-
bond scission occurs, and biodegradable compounds remain non-oxidized (Boncz 2002).
The lower color and COD removal obtained with conventional chemical oxidation
techniques can be overcome by employing advanced oxidation processes (AOP).

 

1.5.3    Advanced
Oxidation Processes

 

                       In advanced oxidation
process (AOP), oxidizing agents such as O3 and H2O2
are used with catalysts, either in the presence or absence of an irradiation
source (Anjaneyalu et al 2005). Recent advances in AOP, which effectively
generate free radical species, have shown that under appropriate conditions,
they produce complete color removal, detoxification and mineralization of
pollutants (Ince and Tezcanli 2001). The advantage of AOP over conventional
oxidation processes accrues from the reactivity of the free radical species
involved, especially by the hydroxyl radicals.

                       

                    One of the most common
methods to produce free radicals is ozonation, and laboratory level studies
show that ozonation is indeed an effective treatment technique for textile
dyebath effluents. Photochemical destruction of pollutants is another known AOP
that has been studied extensively. Increased temperature due to UV irradiation,
high energy consumption and a narrow range of usability are the main
disadvantages of this method. At present, many different combinations of these
AOP have been investigated for color removal and all such methods are capable
of producing free hydroxyl radicals. Sonochemical degradation of dyes is
considered as one of the important and efficient AOP that has been given much
attention in the

recent
past.

 

1.5.3.1           
Sonochemical Degradation of Dyes

 

                 Sonochemistry deals with the
chemical reactions driven by pressure waves inside or close to a cavitating
bubble produced inside of an organic or aqueous solution. Majority of chemical
effects like radical and excited species production, single electron transfer,
etc. and physico-chemical effects such as erosion, depassivation,
emulsification, etc. observed in a high intensity ultrasonic field are the
results of cavitation. There are two main theories to explain these effects.
The electrical theory, proposed by Margulis and Margulis (1999 and 2002),
states that the observed phenomena under ultrasonic fields are the results of
the electric double layer formation in any liquid around the surface of the
cavitation bubbles. On the other hand, the thermal theories proposed are: (1) the theory of a
“Hot-Spot” or adiabatic
heating of the gas during the collapse of a cavitation bubble, proposed by Noltingk
and Neppiras (1950); (2) the thermo-chemical theory, according to which the
heating of the vapour-gas mixture in a collapsing bubble produces the thermal
dissociation or ionization of water molecules and light emission, that is
originated from the recombination of radicals or ions (3) finally, the thermo-mechanic
theory proposed by Jarman (1960) postulates that the collapse of a cavitation
bubble is the cause of the increase in local temperatures, pressures and light
radiation.

 

                        Cavitation can be used
effectively for the destruction of the contaminants present in water using
localized high concentrations of the oxidizing species such as hydroxyl
radicals and hydrogen peroxide, higher magnitudes of localized temperatures and
pressures and the formation of the transient supercritical water. Moreover, the
type of the pollutants present in the effluent stream affects the rates of the
degradation process. Optimization of aqueous phase organic compound degradation
rates can be achieved by adjusting the energy density and the energy intensity.

 

                      The pollutants that are
successfully degraded using acoustic cavitation are p-nitrophenol, Rhodamine B,
1,1,1 trichloroethane, parathion,  pentachlorophenate,
phenol, CFC 11 and CFC 113, o-dichlorobenzene and dichloromethane, potassium
iodide, sodium cyanide and carbon tetrachloride (Adewuyi 2001). Apart from
destruction of chemicals in the wastewater stream, cavitation can be
effectively used for microbial disinfection and also for disintegration of the
microbial sludge. High power ultrasound, operated at low frequencies, can be
effectively used for the disruption of bacterial cells.

 

1.5.4    Biological
Treatment

 

                            Biological
treatment makes use of living microorganisms to degrade environmental
pollutants and to prevent pollution. The utilization of microbial biocatalysts
to reduce the dyes which are present in the effluent offer potential advantages
over physio-chemical processes. In particular, the ability of whole cells to
metabolize pollutants has been given significant attention. For the degradation
of dyes the use of whole cells rather than isolated enzymes is advantageous,
because costs associated with enzyme purification are negated and the cells can
also offer protection from the harsh process environment to the enzymes. Under
aerobic conditions, azo dyes are not readily metabolized. However, under
anaerobic conditions, many bacteria reduce the highly electrophilic azo bond
present in the dye molecule, using low specificity cytoplasmic azo reductases
to produce colorless aromatic amines. These amines are unwilling to further
anaerobic mineralization and can be toxic or mutagenic to animals. Fortunately,
once the xenobiotic azo component of the dye molecule has been removed, the resultant
amino compounds are good substrates for aerobic biodegradation.