**1. Introduction**

In order to demonstrate the influence of the mentioned auxiliaries and byproducts on the dyeing processes carried out using treated water, this chapter is focused on the discoloration of aqueous solutions of two reactive dyes of wide application in the textile industry, Reactive Blue 160 (RB160) and Reactive Red 141 (RR141), through the application of ozone, with the purpose of reusing treated water in dyeing processes of cotton goods, taking advantage of the large quantities of inorganic salts contained in a spent dyebath.

The greatest water consumption in a textile plant occurs in the finishing areas, since the processes carried out there use substances in aqueous solution to modify the characteristics of the processed articles, being the main one of these characteristics the coloration. Reactive type dyes have been used since 1956 for the dyeing of articles that have a high resistance to deterioration by conventional washing, since

they are compounds that react to form covalent bond dye fiber [1, 2]. This type of resistance is known as washing fastness. However, reactive dye groups that do not react with the textile substrate are hydrolyzed during the dyeing process, which, in addition to drastically reducing the dye's performance during dyeing and increasing the amount of residual dye, precludes its reuse by reconstitution of the dyeing bath and causes them to be discarded directly to the effluent. A part of the dye (up to 50%) is hydrolyzed during dyeing. This dye is an important source of contamination and makes the reuse of water impossible [3].

Within the category of reactive dyes, the azoic type represents more than 95% of the existing structures in the most used color shades. In dyes of this kind, the color originates from nitrogen-nitrogen double bonds or azo bonds (─N═N─). In general terms, this is the chromophore group (color generator) most frequently used. The main field of application of reactive dyes is the dyeing of articles made of textile fibers derived from cellulose. Its application on woolen articles is more limited, since this last fiber presents good washing fastness when it is dyed with another type of higher-performance dyes, such as, for example, acid dyes [4].

For the elimination of dyes present in wastewater from the textile industry, various processes have been used, both physical and chemical, as well as biological processes. However, reactive dyes are toxic compounds that are difficult to eliminate by biodegradation or separation methods, such as membrane filtration, coagulationflocculation, or adsorption on activated carbon. The elimination of the color of a wastewater of this type can be carried out to comply with the environmental norms in the discharges, or else, in order to reuse the water. Both in the case of the treatment with activated carbon and in the coagulation-flocculation, results of color removal are obtained just over 50%, which are insufficient for any of the mentioned objectives [5].

Regarding the elimination of color to reuse water, very remarkable results have been achieved, close to 100%, through the use of nanofiltration membranes. However, this method does not solve the problem; it only separates the contaminant, without eliminating it. On the other hand, it presents technical difficulties, in terms of the difference in pressure due to the occlusion of the membranes, which causes its deterioration, in addition to the disposition of the separated contaminant. In the same way, chemical methods, particularly oxidation, have been used to eliminate the dyes present in wastewater in order to reuse it. The best results in terms of complete elimination of the dye until its mineralization have been obtained by photocatalysis, either homogeneous (photo-Fenton processes) or heterogeneous (using semiconductors, especially TiO2) [6].

The ozonation, on the other hand, has been used individually, combined with hydrogen peroxide (peroxonous), as well as in conjunction with the application of UV radiation or the presence of activated carbon, and various combinations of these methods. This type of combination, as well as photocatalytic and Fenton processes, is known as advanced oxidation processes (AOPs) [6].

Ozone is an oxidant that does not generate its own waste; this allows obtaining a treated water with suitable characteristics to be used in dyeing processes of textile materials. In addition, the use of ozone individually enables a better understanding of the oxidation process, by avoiding interferences due to the contributions of the processes involved in the combined methods [7].

The dyes were selected for study because they are among the highest consumption by the textile industry. On the other hand, these compounds belong to one of the groups of dyes that are used more frequently: RB160 and RR141 are monochlorotriazinic reactive dyes, considered as moderate reactive and dyed at temperatures of 80°C [8].

**37**

*Chemical and Tinctorial Aspects Related to the Reuse of Effluents Treated by Ozonation…*

In this study, the partial oxidation of the dyes is verified, since the complete mineralization of these products by ozonation is an extremely long process, which greatly increases the cost of a process of this type and, consequently, is not the most

The oxidation of the dye implies that the treated water, at the end of the process, contains intermediate oxidation products. In the case of the reuse of water treated in dyeing processes, it is necessary to verify that such by-products do not interfere with the results obtained in terms of dyeing quality. For this purpose, dyeing tests were carried out in the laboratory with two of the most frequently used dye classes for dyeing cellulosic fibers: direct dyes and reactive dyes. These tests were evaluated by a standard procedure used in the production areas of the textile industry to evaluate dyeing results compared against a reference standard or sample to match. For the dyeing tests, a common trichromy of reactive dyes was used, that is, the cotton samples were dyed using Reactive Yellow 84 (RY84), Reactive Red 141 (RR141), and Reactive Black 5 (RB5), separately. While in the case of dyeing with direct dyes, a trichromy consisting of Direct Yellow 50 (DY50), Direct Red 23 (DR23), and Direct Blue 80 (DB80) was used. This trichromy of direct dyes was chosen because, like the three reactive dyes under study, they cover a wide range of tones and also cover the three categories of direct dye classification established by the Society of Dyers and Colourists (SDC), which is the broadest usage classification [8]. The results obtained show that the discoloration of the compounds studied is rapid even at very low concentrations of ozone, as is the case of the concentration used in this study. However, the decomposition of dyes is a much longer process and, in fact, is not carried out with ozone even using it for long periods of time at

The initial concentration of dye has a marked influence on the reaction rate, sharply changing the kinetics of discoloration within the range of concentrations studied. Likewise, the presence of chemical auxiliaries has a more pronounced influence at low concentrations, because at high concentrations the aggregation of

The ozone applied to the discoloration of the compounds studied does not negatively influence the concentration of sodium ion present in the solution but rather increases it slightly. This situation is important because the sodium ion is necessary

The influence of the by-products formed from each dye during the ozonation in the subsequent dyeing depends on the category of dye used to dye the goods, in this case of mercerized cotton. It is more pronounced for dyeing with reactive dyes. Through the treatment scheme used in this work, it is possible to eliminate the dyes contained in the wastewater for a very short time (10 min), using very low concentrations of ozone (2 mg/L). The kinetics of the process depend on the chemical constitution of the dye and on the residual water variables such as the initial concentration of dye and dyeing auxiliaries. The ozonation allows to reuse the treated water in dyeing processes with direct dyes and reactive dyes for up to four cycles, depending on the type of dye used. The by-products resulting from simple ozonation accumulate during the cycles of reuse without significantly affecting the quality of the resulting dyeing and can be converted into biodegradable substances by a longer ozonation. **Figure 1** shows the chemical structures of RB160

Within a limited range of cycles, the quality of the dyeing obtained using water treated by ozonation complies with the specifications of the method used in industry, called AATCC 173-2005 Calculation of Small Color Differences for Acceptability, established by the American Association of Textile Chemists and

*DOI: http://dx.doi.org/10.5772/intechopen.81468*

low concentrations.

Colorists [9].

the dyes increases, influencing the kinetics [1].

to carry out the process of dyeing the treated water.

and **Figure 2** shows the chemical structure of RR141.

frequent form of work in real water treatment systems.

### *Chemical and Tinctorial Aspects Related to the Reuse of Effluents Treated by Ozonation… DOI: http://dx.doi.org/10.5772/intechopen.81468*

In this study, the partial oxidation of the dyes is verified, since the complete mineralization of these products by ozonation is an extremely long process, which greatly increases the cost of a process of this type and, consequently, is not the most frequent form of work in real water treatment systems.

The oxidation of the dye implies that the treated water, at the end of the process, contains intermediate oxidation products. In the case of the reuse of water treated in dyeing processes, it is necessary to verify that such by-products do not interfere with the results obtained in terms of dyeing quality. For this purpose, dyeing tests were carried out in the laboratory with two of the most frequently used dye classes for dyeing cellulosic fibers: direct dyes and reactive dyes. These tests were evaluated by a standard procedure used in the production areas of the textile industry to evaluate dyeing results compared against a reference standard or sample to match.

For the dyeing tests, a common trichromy of reactive dyes was used, that is, the cotton samples were dyed using Reactive Yellow 84 (RY84), Reactive Red 141 (RR141), and Reactive Black 5 (RB5), separately. While in the case of dyeing with direct dyes, a trichromy consisting of Direct Yellow 50 (DY50), Direct Red 23 (DR23), and Direct Blue 80 (DB80) was used. This trichromy of direct dyes was chosen because, like the three reactive dyes under study, they cover a wide range of tones and also cover the three categories of direct dye classification established by the Society of Dyers and Colourists (SDC), which is the broadest usage classification [8].

The results obtained show that the discoloration of the compounds studied is rapid even at very low concentrations of ozone, as is the case of the concentration used in this study. However, the decomposition of dyes is a much longer process and, in fact, is not carried out with ozone even using it for long periods of time at low concentrations.

The initial concentration of dye has a marked influence on the reaction rate, sharply changing the kinetics of discoloration within the range of concentrations studied. Likewise, the presence of chemical auxiliaries has a more pronounced influence at low concentrations, because at high concentrations the aggregation of the dyes increases, influencing the kinetics [1].

The ozone applied to the discoloration of the compounds studied does not negatively influence the concentration of sodium ion present in the solution but rather increases it slightly. This situation is important because the sodium ion is necessary to carry out the process of dyeing the treated water.

Within a limited range of cycles, the quality of the dyeing obtained using water treated by ozonation complies with the specifications of the method used in industry, called AATCC 173-2005 Calculation of Small Color Differences for Acceptability, established by the American Association of Textile Chemists and Colorists [9].

The influence of the by-products formed from each dye during the ozonation in the subsequent dyeing depends on the category of dye used to dye the goods, in this case of mercerized cotton. It is more pronounced for dyeing with reactive dyes.

Through the treatment scheme used in this work, it is possible to eliminate the dyes contained in the wastewater for a very short time (10 min), using very low concentrations of ozone (2 mg/L). The kinetics of the process depend on the chemical constitution of the dye and on the residual water variables such as the initial concentration of dye and dyeing auxiliaries. The ozonation allows to reuse the treated water in dyeing processes with direct dyes and reactive dyes for up to four cycles, depending on the type of dye used. The by-products resulting from simple ozonation accumulate during the cycles of reuse without significantly affecting the quality of the resulting dyeing and can be converted into biodegradable substances by a longer ozonation. **Figure 1** shows the chemical structures of RB160 and **Figure 2** shows the chemical structure of RR141.

*Textile Industry and Environment*

acid dyes [4].

objectives [5].

tion and makes the reuse of water impossible [3].

(using semiconductors, especially TiO2) [6].

processes involved in the combined methods [7].

they are compounds that react to form covalent bond dye fiber [1, 2]. This type of resistance is known as washing fastness. However, reactive dye groups that do not react with the textile substrate are hydrolyzed during the dyeing process, which, in addition to drastically reducing the dye's performance during dyeing and increasing the amount of residual dye, precludes its reuse by reconstitution of the dyeing bath and causes them to be discarded directly to the effluent. A part of the dye (up to 50%) is hydrolyzed during dyeing. This dye is an important source of contamina-

Within the category of reactive dyes, the azoic type represents more than 95% of the existing structures in the most used color shades. In dyes of this kind, the color originates from nitrogen-nitrogen double bonds or azo bonds (─N═N─). In general terms, this is the chromophore group (color generator) most frequently used. The main field of application of reactive dyes is the dyeing of articles made of textile fibers derived from cellulose. Its application on woolen articles is more limited, since this last fiber presents good washing fastness when it is dyed with another type of higher-performance dyes, such as, for example,

For the elimination of dyes present in wastewater from the textile industry, various processes have been used, both physical and chemical, as well as biological processes. However, reactive dyes are toxic compounds that are difficult to eliminate by biodegradation or separation methods, such as membrane filtration, coagulationflocculation, or adsorption on activated carbon. The elimination of the color of a wastewater of this type can be carried out to comply with the environmental norms in the discharges, or else, in order to reuse the water. Both in the case of the treatment with activated carbon and in the coagulation-flocculation, results of color removal are obtained just over 50%, which are insufficient for any of the mentioned

Regarding the elimination of color to reuse water, very remarkable results have been achieved, close to 100%, through the use of nanofiltration membranes. However, this method does not solve the problem; it only separates the contaminant, without eliminating it. On the other hand, it presents technical difficulties, in terms of the difference in pressure due to the occlusion of the membranes, which causes its deterioration, in addition to the disposition of the separated contaminant. In the same way, chemical methods, particularly oxidation, have been used to eliminate the dyes present in wastewater in order to reuse it. The best results in terms of complete elimination of the dye until its mineralization have been obtained by photocatalysis, either homogeneous (photo-Fenton processes) or heterogeneous

The ozonation, on the other hand, has been used individually, combined with hydrogen peroxide (peroxonous), as well as in conjunction with the application of UV radiation or the presence of activated carbon, and various combinations of these methods. This type of combination, as well as photocatalytic and Fenton

Ozone is an oxidant that does not generate its own waste; this allows obtaining a treated water with suitable characteristics to be used in dyeing processes of textile materials. In addition, the use of ozone individually enables a better understanding of the oxidation process, by avoiding interferences due to the contributions of the

The dyes were selected for study because they are among the highest consumption by the textile industry. On the other hand, these compounds belong to one of the groups of dyes that are used more frequently: RB160 and RR141 are monochlorotriazinic reactive dyes, considered as moderate reactive and dyed at temperatures

processes, is known as advanced oxidation processes (AOPs) [6].

**36**

of 80°C [8].

**Figure 1.** *Chemical structure of Reactive Blue 160 (RB160).*

**Figure 2.** *Chemical structure of Reactive Red 141 (RR141).*
