electrochemical treatment, membrane filtration, nanofiltration, biological degradation (aerobic, anaerobic) and ultrasonic mineralization. But among them, coagulation/flocculation gained more popularity because of its low cost, relatively simple
operation and potential decolourization of the wastewater without degradation and
by-product formation of dyes (Chen et al. 2010; Freitas et al. 2015). Coagulation/
flocculation also favours the removal of colour from the dye bath but not by partial
decomposition of dyes that results into formation of potentially more toxic and
harmful aromatic compounds (Golob et al. 2005).
However, selection of appropriate coagulant needs a detailed investigation,
including wastewater characteristics, the quantum of pollutant load and operational
conditions during the coagulation process. A comprehensive list of different coagulant’s performance for various dye molecules and the factors affecting the performance of selected coagulants has been described in the next sections.
13.6 Classified Coagulants and Its Application in Dye
Wastewater (Current Trends)
Different chemicals have been used to fasten the natural destabilization process of
colloidal particles along with some dissolved organic matter for accomplishing the
process of coagulation/flocculation. Earlier, the most commonly practised method of
coagulation was the addition of an adequate amount of electrolytes that sufficiently
compresses the electrical double layer resulting in the settling of the flocs (Peavy
et al. 1985). Later it has been noticed that the salts having trivalent ions of opposite
charges are found more effective than divalent metallic salts. This significant relation
between ionic charge and precipitating capacity of the metallic salts has been first
established by Schulze and verified by Hardy. Thus, on their name, this finding is
usually named as Schulze-Hardy rule. This rule states that “the coagulation of
colloidal particles is effected by that ion of electrolytes that has a charge opposite
in sign to that of the colloidal particles and the effect of such ion increases
remarkably with the number of charges over these ions” (Jarvis et al. 2012; Peavy
et al. 1985). Concomitantly, trivalent metallic salts such as alum, iron-based, titanium tetrachloride and zirconium oxychloride have been used for coagulation/
flocculation process for removal of different organic compound such as natural
organic matter (NOM), dyes, etc. (Jarvis et al. 2012; Hussain et al. 2014; Sonal
et al. 2018).
Accordingly, in dye wastewater, these coagulants have been extensively used to
remove the inorganic and organic components. In dye wastewater, various chemical
and natural coagulants have been used to remove the targeted dye molecules, which
are further classified in different groups, as shown in Fig. 13.4.
Some conventional alum and iron-based coagulants were practised widely, since
a long time back (Gao et al. 2007a, b). But as the industries grow, new dyes are
introduced, so these days some coagulant aid or synthetic/natural polymers have also
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S. Sonal and B. K. Mishra
operation and potential decolourization of the wastewater without degradation and
by-product formation of dyes (Chen et al. 2010; Freitas et al. 2015). Coagulation/
flocculation also favours the removal of colour from the dye bath but not by partial
decomposition of dyes that results into formation of potentially more toxic and
harmful aromatic compounds (Golob et al. 2005).
However, selection of appropriate coagulant needs a detailed investigation,
including wastewater characteristics, the quantum of pollutant load and operational
conditions during the coagulation process. A comprehensive list of different coagulant’s performance for various dye molecules and the factors affecting the performance of selected coagulants has been described in the next sections.
13.6 Classified Coagulants and Its Application in Dye
Wastewater (Current Trends)
Different chemicals have been used to fasten the natural destabilization process of
colloidal particles along with some dissolved organic matter for accomplishing the
process of coagulation/flocculation. Earlier, the most commonly practised method of
coagulation was the addition of an adequate amount of electrolytes that sufficiently
compresses the electrical double layer resulting in the settling of the flocs (Peavy
et al. 1985). Later it has been noticed that the salts having trivalent ions of opposite
charges are found more effective than divalent metallic salts. This significant relation
between ionic charge and precipitating capacity of the metallic salts has been first
established by Schulze and verified by Hardy. Thus, on their name, this finding is
usually named as Schulze-Hardy rule. This rule states that “the coagulation of
colloidal particles is effected by that ion of electrolytes that has a charge opposite
in sign to that of the colloidal particles and the effect of such ion increases
remarkably with the number of charges over these ions” (Jarvis et al. 2012; Peavy
et al. 1985). Concomitantly, trivalent metallic salts such as alum, iron-based, titanium tetrachloride and zirconium oxychloride have been used for coagulation/
flocculation process for removal of different organic compound such as natural
organic matter (NOM), dyes, etc. (Jarvis et al. 2012; Hussain et al. 2014; Sonal
et al. 2018).
Accordingly, in dye wastewater, these coagulants have been extensively used to
remove the inorganic and organic components. In dye wastewater, various chemical
and natural coagulants have been used to remove the targeted dye molecules, which
are further classified in different groups, as shown in Fig. 13.4.
Some conventional alum and iron-based coagulants were practised widely, since
a long time back (Gao et al. 2007a, b). But as the industries grow, new dyes are
introduced, so these days some coagulant aid or synthetic/natural polymers have also
312
S. Sonal and B. K. Mishra