Properties Use and Environmental Problems

Nearly half of the dyes being used in textile finishing processes are reactive dyes which are soluble in water (Schonberger 1996). The molecular structure is characterized by three functional parts:

• a hydrophilic group,

• a group which produces the colour,

• a group which reacts with the textile fiber.

Figure 9.6 demonstrates the fixation of a reactive dye at a textile fiber. HSO-is first separated from the reactive anchor group (step 1) before the vinyl sulfate group is hydrolyzed (step 2) and reacts directly with the fiber (step 3).

But a part of the vinyl sulfate groups formed is hydrolyzed and is not able to react with the fiber (step 4). It is lost for this dyeing process and remains as an impurity in the wastewater arising from the washing processes. The most important groups of reactive dyes are azo dyes. They are characterized by an azo group: -N=N-. Reactive black 5 (RB 5) is the most commonly used azo dye. It is a diazo dye with two azo groups. Figure 9.7 presents RB 5 in its vinyl sulfonic form, before fixation to the fiber and before hydrolysis. In this form with two vinyl sulfonic anchor groups, the RB 5 molecule can be fixed at one or two points on the fiber.

One vinyl sulfonic group can be hydrolyzed, resulting in a weaker fixation. No reaction with the fiber occurs if both groups are hydrolyzed.

Azo dyes occur in wastewater from textile works and also occur together with naphthalene sulfonic acids (NSA) in wastewater from the production of azo dyes. Let us discuss briefly some experiences showing the biodegradation of NSA.

reactive anchor group

| + OH" (NaOH^=>-Na* + OH") vinylsulfonic group R — S02 — CH = CH" + HSO4 + H20 step 1

competitive reaction hydrolysis fixation

fibre

Fig. 9.6 Formation of the vinyl sulfonic group in reactive dyes, fixation at textile fibers and hydrolysis.

Fig. 9.7 Reactive black 5 (RB 5) and metabolites of anaerobic treatment. (a) RB 5 with two vinyl sulfonic groups, ready for fixation on textile fibers. (b) Reduction of an azo dye. (c) Metabolites formed by anaerobic treatment: aminobenzene-2-hydroxy-ethylsulphonic acid (above; p-Base) and naphthalene metabolite (below; TAHNDS). (After Sosath 1999; Borchert 2002; Wiesmann 2002).

Fig. 9.7 Reactive black 5 (RB 5) and metabolites of anaerobic treatment. (a) RB 5 with two vinyl sulfonic groups, ready for fixation on textile fibers. (b) Reduction of an azo dye. (c) Metabolites formed by anaerobic treatment: aminobenzene-2-hydroxy-ethylsulphonic acid (above; p-Base) and naphthalene metabolite (below; TAHNDS). (After Sosath 1999; Borchert 2002; Wiesmann 2002).

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