4. Entrapment of particles in the precipitate (sweep coagulation). When a high dose
of metallic salts is added to the solution, they form heavier amorphous gelatinous
flocs. These gelatinous flocs entrapped the particulate matters within themselves
or enmeshed them by their sticky surface and swept them out from the water
along with the settling flocs (Duan and Gregory 2003). This process of enmeshing
and sweeping out of colloidal particles from the suspension is generally termed as
sweep coagulation (Peavy et al. 1985). For this process, a high dose of metallic
salts and neutral pH are required. Besides these, the presence of various anions in
water and the higher concentration of colloidal particles enhance this coagulation
mechanism.
13.4 Dyes and Its Classification
Dyes are synthetic complex aromatic and heterocyclic molecules that comprise of
two groups, i.e. auxochrome (colour intensifier) and chromophores (colourimparting compounds) (Sonal et al. 2018; Patel and Vashi; 2015; Verma et al.
2012). These aromatic compounds are more stable and difficult to biodegrade. The
aromatic rings of dyes contain de-located electrons along with different functional
groups (Ding et al. 2010). Their auxochrome groups are electron donor or withdrawing groups and are responsible for dyeing capacity of any dye, while colourimparting group, chromogen-chromophores are electron acceptor. The chromogens
are composed of cyclic aromatic structures such as benzene rings, anthracene or
naphthalene which adhere to the chromophore radicals containing doubleconjugated bonds with de-located electrons. The chromophore groups are ethenyl
(–CC–), carbonyl (–C¼O), imino (–CN–), nitro (–NO 2 ), azo group (–N¼N–),
ethylene group (–C¼C–), methine group (–C¼H), carbon-nitrogen (¼C¼NH; –
CH¼N–), carbon-sulphur (¼C¼S; CS–S–C), nitro (–NO 2 ; –NO–OH), nitroso
(–N¼O; ¼N–OH) or quinoid groups, while auxochrome groups are amino (–NH 2 ),
carboxylic (–COOH), sulphonyl (–SO 3 H) and hydroxyl (–OH) group (Carmen and
Daniela 2012; Welham 2000).
The textile dyes are classified in various ways depending on different criteria of
origin, nature or reactivity (Fig. 13.3). On a general basis, the dyes are classified as
natural and synthetic textile dyes. Since the beginning of 2600 BC, natural textile
dyes extracted from vegetable and animal resources were used in textile processing
units in China (Carmen and Daniela 2012). During the fifteenth century, Phoenicians
used Tyrian purple produced from crushed sea snail’s species, and since 3000 BC, an
indigo dye extracted from the indigo plant was also used. Besides this, several other
naturally derived dyes such as saffron (natural yellow 6) extracted from the stigmata
of Crocus sativus and hematein (natural black 1) obtained from the heartwood of a
tree, etc. were discovered and commonly used (Carmen and Daniela 2012).
In 1856, the first synthetic dye named “mauve dye” (aniline), a brilliant fuchsia
colour, was first discovered by W.H. Perkin (UK), followed by some azo dyes
synthesized by diazotization reaction discovered in 1958 by P. Gries (Germany)
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S. Sonal and B. K. Mishra
of metallic salts is added to the solution, they form heavier amorphous gelatinous
flocs. These gelatinous flocs entrapped the particulate matters within themselves
or enmeshed them by their sticky surface and swept them out from the water
along with the settling flocs (Duan and Gregory 2003). This process of enmeshing
and sweeping out of colloidal particles from the suspension is generally termed as
sweep coagulation (Peavy et al. 1985). For this process, a high dose of metallic
salts and neutral pH are required. Besides these, the presence of various anions in
water and the higher concentration of colloidal particles enhance this coagulation
mechanism.
13.4 Dyes and Its Classification
Dyes are synthetic complex aromatic and heterocyclic molecules that comprise of
two groups, i.e. auxochrome (colour intensifier) and chromophores (colourimparting compounds) (Sonal et al. 2018; Patel and Vashi; 2015; Verma et al.
2012). These aromatic compounds are more stable and difficult to biodegrade. The
aromatic rings of dyes contain de-located electrons along with different functional
groups (Ding et al. 2010). Their auxochrome groups are electron donor or withdrawing groups and are responsible for dyeing capacity of any dye, while colourimparting group, chromogen-chromophores are electron acceptor. The chromogens
are composed of cyclic aromatic structures such as benzene rings, anthracene or
naphthalene which adhere to the chromophore radicals containing doubleconjugated bonds with de-located electrons. The chromophore groups are ethenyl
(–CC–), carbonyl (–C¼O), imino (–CN–), nitro (–NO 2 ), azo group (–N¼N–),
ethylene group (–C¼C–), methine group (–C¼H), carbon-nitrogen (¼C¼NH; –
CH¼N–), carbon-sulphur (¼C¼S; CS–S–C), nitro (–NO 2 ; –NO–OH), nitroso
(–N¼O; ¼N–OH) or quinoid groups, while auxochrome groups are amino (–NH 2 ),
carboxylic (–COOH), sulphonyl (–SO 3 H) and hydroxyl (–OH) group (Carmen and
Daniela 2012; Welham 2000).
The textile dyes are classified in various ways depending on different criteria of
origin, nature or reactivity (Fig. 13.3). On a general basis, the dyes are classified as
natural and synthetic textile dyes. Since the beginning of 2600 BC, natural textile
dyes extracted from vegetable and animal resources were used in textile processing
units in China (Carmen and Daniela 2012). During the fifteenth century, Phoenicians
used Tyrian purple produced from crushed sea snail’s species, and since 3000 BC, an
indigo dye extracted from the indigo plant was also used. Besides this, several other
naturally derived dyes such as saffron (natural yellow 6) extracted from the stigmata
of Crocus sativus and hematein (natural black 1) obtained from the heartwood of a
tree, etc. were discovered and commonly used (Carmen and Daniela 2012).
In 1856, the first synthetic dye named “mauve dye” (aniline), a brilliant fuchsia
colour, was first discovered by W.H. Perkin (UK), followed by some azo dyes
synthesized by diazotization reaction discovered in 1958 by P. Gries (Germany)
308
S. Sonal and B. K. Mishra