1. Introduction
Textile industries consume a huge quantity of water and chemicals in various pre-treatment processes before dyeing and the final finishing steps, which generates a massive amount of effluent [
1]. Minor fluctuations in the dyeing process conditions result in defective or uneven dyeing, which is not acceptable by the consumer. Therefore, color stripping (decolorization) is required to correct the defective dyed cotton fabric to maintain the product quality as per the consumer’s demand [
2]. Reactive dyes are strongly attached to the fabric through covalent bonding in the dyeing process, having chromophore groups such as azo, phthalocyanine and anthraquinone to impart specific color to the fabric, and reactive groups such as vinyl sulphone, monochlorotriazine, etc. [
3]. The strong chemical bonds between the dye and cellulosic fabric resist to react with oxidizing and reducing agents. A number of factors such as the type of dye, fabric material and dyeing method, and the process parameters such as pH, time, oxidant concentration, etc., determine the efficiency of the stripping process [
2]. Conventionally, sodium hydrosulphide (NaHS), along with auxiliary chemicals, is used for the color stripping of reactive dyes at 95 °C in a highly alkaline medium for 20–30 min, resulting in the generation of a highly polluted effluent [
4,
5]. Moreover, it is a highly toxic chemical and oxidizes into hydrogen sulfite, hydrogen sulfate and sulfur dioxide, which not only damages the environment, but also deteriorates the fabric quality. Thus, environmentally friendly technologies are required to reduce the harmful impacts of textile industries on the environment because of extreme environmental and social concerns [
6].
Currently, research is being conducted to discover and invent eco-friendly sustainable technologies, thus substituting harmful chemicals that are commonly used in conventional methods to reduce water consumption in the textile sector [
7,
8,
9,
10,
11]. Biological methods using enzymes and fungi have also been examined for the bleaching and color stripping of cotton fabric and are found not to be practically successful due to their low efficacy and time-consuming nature [
12].
In recent years, ozone has gained attention as a powerful oxidant and disinfectant. However, it cannot be stored in the laboratory due to its highly reactive and unstable nature; hence, it is generated electrically using the corona discharge method at runtime [
13]. Ozone gas is dissolved and dispersed in a liquid medium to react with the targeted compounds by diffusers and a venturi injection system [
14]. Ozone has also been applied in various textile wet processes because of its high oxidative capacity (2.07 V) as well as its working low temperature and pH [
15,
16]. Moreover, it requires less water without the use of chemicals for bleaching and stripping processes as compared to the conventional methods. It has been successfully used for textile effluent treatment without generating sludge, which is a by-product of conventional wastewater treatment methods whose disposal results in soil and ground water pollution [
17,
18,
19]. Many researchers have used ozone for the bleaching of various substrates such as cotton, silk, jute, etc., and discussed the role of various parameters on the efficacy of the ozonation process, but this technology has not been upgraded to the industrial scale because of its higher capital cost and non-selective reaction mechanism, resulting in the loss of strength [
20,
21]. Different studies have been conducted to improve the strength properties of ozone-treated cellulose fibres by adding various additives such as organic acids, alcohol and reducing agents [
16]. These additives enhance the ozone selectivity either by decreasing its decomposition or by scavenging the free radical species. However, the addition of additives will increase the cost of the process and pollution load as well [
22].
Moreover, it is also effective for reduction clearing and the wash off process for the removal of unfixed and hydrolyzed dyes [
23,
24,
25,
26]. It is also used to decolorize the residual dyeing effluent in the textile dyeing process. Ozone gas (O
3) was injected in the dyeing machine after the completion of the dyeing process for the removal of color and COD from the effluent [
27]. Recently, research has been focused on finding alternatives for water conservation by reusing the effluent rather than discharging it into water bodies. Senthilkumar and Muthukumar [
28] reused the dye bath after treating with ozone for three times without compromising the dyeing quality. Wash-off dyebath liquor, after treating it with the electrocoagulation process, was also reused eight times in the same process and the dyeing quality remained the same as indicated by the ∆E (total color difference between fabric samples dyed in fresh bath (reference) and dyebath treated with electrocoagulation process) values ranging between 0.38 and 0.85 as the ∆E value of less than one is commercially acceptable [
25]. Similarly, 20 different samples of raw cotton fabric were bleached with ozone using the same water bath, and the whiteness remained stable up to the 20th cycle [
9]. The same process was also successfully upgraded to the pilot scale for up to five cycles [
11].
Ozone has been used for the decolorization of defective dyed fabric both at the laboratory and pilot scale and provides the desired results at a minimum environmental cost in contrast to the conventional chemical methods [
29,
30,
31,
32,
33]. However, more extensive research is required to study the ozone reactivity on various chromophore groups. Therefore, this study is aimed at developing a sustainable method for dyed cotton fabric stripping using ozone. To begin with, the ozonation process conditions (oxidant dose, pH, treatment time) were optimized to remove three dyes that have phthalocyanine (Reactive blue 21), diazo (Reactive black 5) and monoazo (Reactive yellow 84) chromophores. Subsequently, a novel approach was applied by reusing the same stripping water bath to strip dyed fabric samples for up to seven cycles. The dyed fabric samples were characterized in terms of color differences (L, ΔL) and bursting strength properties. The ozone stripping bath was also characterized after every reuse in terms of pH, electrical conductivity (EC), chemical oxygen demand (COD) and total dissolved solids (TDS) using standard methods.
2. Experimental
2.1. Apparatus Setup
The apparatus consists of three components including the oxygen concentrator (Mark 5 of Purezone
TM, USA), ozone generator (L10G of Faraday, USA) and reactor tube (60 cm height and 3.5 cm internal diameter) (
Scheme 1) [
34]. The ozone gas was generated from concentrated air (99% oxygen) using the corona discharge method, with the maximum capacity of 10 g/h. The ozone was introduced through a diffuser stone into the reactor to transfer it in the liquid medium for “cotton stripping”. The concentration of ozone gas (g/m
3) was measured by the ozone analyzer (UVP 200 Ozonova, Germany) at the inlet of the reactor and was controlled by regulating the knob present on the ozone generator. The ozone dose (g/h) at a constant flow rate of 2 L/min was measured by the following equation given below:
2.2. Dyeing Procedure
The bleached cotton knitted fabric samples were dyed with 2% o.m.f (of the material of fabric) of three types of reactive dyes in a dyeing machine at a liquor ratio of 1:10 at 60 °C for 45 min. The initial pH of the dyebath was adjusted to 6.5 and the temperature was increased at a rate of 1.5 °C/min. After 15 min, sodium carbonate (Na
2CO
3) was added, which increased the pH to 11, and the dyeing process was continued for a further 30 min. The fabric samples were removed and cooled down, and subsequently treated with 0.1 g/L acetic acid to adjust the pH. Two hot washes were applied at 60 °C, and one at 80 °C, followed by rinsing with tap water. The samples were dried in open air for ozone treatment. The details of the dyes are given in
Table 1.
2.3. Procedure for Ozone Treatment
A 4 g piece of dyed cotton fabric was treated with ozone in a bubble column reactor containing 50 mL water (1:10 liquor ratio), at room temperature. The effect of the process variables was assessed by varying the pH (3, 5, 7, 9), ozone dose (2 g/h, 5 g/h, 7 g/h, 10 g/h) and treatment time (30 min, 40 min, 50 min, 60 min). The ozone-stripped fabric samples were rinsed with tap water to remove ozonation by-products and unfixed dyes. The pH was adjusted by using acetic acid and sodium hydroxide (NaOH) for the acidic and alkaline media, respectively. The ozone-stripped fabric samples were rinsed with tap water to remove ozonation by-products and unfixed dyes. The quality of the stripped fabric samples was evaluated in terms of lightness (L) and bursting strength properties.
The optimized ozonation process was then used to decolorize multiple fabric samples dyed with reactive dyes by re-using the same stripping bath. The lightness values (L and ∆L) of the decolorized samples were measured after every reuse for up to seven cycles. The pH, EC, TDS and COD of the ozone stripping bath was also measured after every reuse and compared with the conventional chemical stripping method.
2.4. Fabric Evaluation
The lightness (L), lightness difference (ΔL) and whiteness of the stripped fabric samples were assessed using a spectrophotometer (SF 500+ Data Colour) [
35]. The ball burst method was used to measure the bursting strength of the fabric samples (ASTMD-3787-16).
2.5. Characteristics of Effluents
The pH of the stripping bath was measured using a pH meter (JENCO 6173) which is calibrated regularly at PH 4 and PH 7. The electrical conductivity (EC) was measured using a calibrated EC meter (JENWAY 470). The chemical oxygen demand (COD) and total dissolved solids (TDS) of ozone-stripping effluents were measured using standard methods for the analysis of water and wastewater [
36].
2.6. Statistical Analysis
The mean, variance and standard deviation values in triplicate measurements were calculated using descriptive stats. The multivariate test (ANOVA) was applied to check the impact of the input variables (pH, oxidant dose, treatment time) on the output (whiteness, lightness and bursting strength) of the process.
4. Discussion
The experimental variables such as pH, ozone dose and treatment time determine the color removal efficiency of ozone. The pH plays a crucial role in the ozonation process depending on the type of substrate as in high acidic pH; its mode of action is direct and selective, while above pH 4, it hydrolyzes into free radicals which are non-selective in behavior and reacts with a variety of compounds [
17]. In this study, the maximum color removal was achieved at pH 3, which is in agreement with the previous study [
33] which concluded that maximum decolorization (98%) was achieved at pH 3 with less mechanical damage. Similarly, the best color removal efficiency was achieved at the optimum ozone dose (7 g/h) and treatment time (40 min) and further increase did not improve the decolorization due to unavailability of the reaction sites.
Moreover, its efficiency varies with the type of dye as the chromophore and anchor groups of reactive dyes determine the oxidation capability of the dye. The results showed that ozone is more effective for reactive dyes that have vinyl sulphone anchor groups as compared to the monochlorotriazine group. Similarly, the difference in the chromophore group is also reflected in the results as CI Reactive blue 21 that has phthalocyanine chromophore is more sensitive toward ozone than CI Reactive black 5 that has diazo chromophore, which is also in agreement with the study conducted by Eren et al., 2016 [
40]. However, more extensive research is needed covering a larger number of the types of dyes to generalize the results.
Ozone reacts simultaneously with reactive dyes and water, thus making it possible to reuse the same ozone stripping bath for multiple times. Results showed that the ozone stripping bath can be reused up to four cycles without affecting the decolorization efficiency, which not only saves the water, but also reduces the cost of effluent treatment. Hence, the ozone stripping method with its repeated reuse of stripping bath provides an eco-friendly and sustainable alternative to conventional stripping methods for the textile industry.
5. Conclusions
This paper proposes an ozone stripping process for the decolorization of the reactive dyed cotton fabric with the multiple reuses of the ozone stripping bath. Three reactive dyes were selected that have phthalocyanine (Reactive blue 21), diazo (Reactive black 5) and monoazo (Reactive yellow 84) chromophores. The results indicate that the optimum experimental conditions including pH 3, ozone dose of 7 g/h and 40-minute treatment time gave the best decolorization results. The bursting strength of ozone-stripped fabric samples remained stable throughout the experiments irrespective of the process parameters. However, the stripping efficacy varies with the chromophore group of reactive dyes because each dye has its unique structure and properties. The results showed that the Reactive blue 21 that has phthalocyanine group was most easily removed as compared to Reactive black 5 and Reactive yellow 84 that have diazo and monazo chromophores, respectively. However, the ozonation process should be applied on a wide range of dyes to observe their behavior.
Moreover, the ozone stripping bath can be reused up to four times to decolorize dyed fabric without decreasing the lightness (L) value, as the L value dropped from 79.02 to 77.01, 72.32 and 62.81 at the fifth, sixth and seventh cycle. This decrease in lightness (L value) is mainly due to the build-up of removed chemicals and dyes from the stripped fabric. Hence, the ozonation process not only saves water, energy, chemicals and man-hours, but it also reduces the wastewater treatment expenditures and provides a sustainable solution to the textile industry.