Abstract

A kind of gelatin protein copper complex (GPCC for short) was synthesized by us. GPCC acting as catalyst was applied to catalyzed hydrogen peroxide for decolorization of C.I. Reactive Red 24 from simulated dyeing wastewater. The influence of catalyzed hydrogen peroxide by GPCC on the decolorization of C.I. Reactive Red 24 from simulated dyeing wastewater was discussed. The optimum decolorization technology condition was optimized. And the effect of salt on decolorization of C.I. Reactive Red 24 in this catalytic system was also investigated. The results showed that hydrogen peroxide can be catalyzed by GPCC. In the catalytic system, the structure of C.I. Reactive Red 24 can be degraded and a high decolorization rate can be obtained in a wide pH range. In addition, the decolorization percentage and decolorization rate will be further improved when salt is present in the catalytic system.

HIGHLIGHTS

  • Dyeing wastewater of reactive dyes causes serious pollution.

  • Decolorization of reactive dyeing wastewater needs to be studied.

  • We used a gelatin protein copper complex (GPCC for short) to act as a catalyst that was applied to catalyzed hydrogen peroxide for decolorization of C.I. Reactive Red 24.

  • The results show that a high decolorization rate can be obtained in the catalytic system.

INTRODUCTION

Reactive dyes are a class of widely used dyes owing to their complete chromatography and bright color, reactivity with a variety of fibers, and good color fastness. However, reactive dyes can be hydrolyzed, which leads to low dye utilization rate and large chromaticity pollution of the dyeing wastewater (Hu et al. 2016). So it is significant to study decolorization of reactive dyeing wastewater.

There are many ways to decolorize dyeing wastewater, such as adsorption, oxidation, biological methods and so on (Bilal et al. 2016; Meerbergen et al. 2017). In recent years, advanced oxidation processes (AOPs) have received intensive attention for wastewater treatment (Low et al. 2012; Lozovskyi et al. 2015; Li et al. 2018; Ding et al. 2019). Hydrogen peroxide is a widely used oxidizing bleaching agent, which can destroy the structure of pigment on the fabric and improve whiteness of the fabric (Yurdakul et al. 2019). But the decomposition activation energy of hydrogen peroxide is high, so it needs high temperature and high alkali to exert its good bleaching effect on fabric. Peroxide bleaching catalyst can be used to reduce the decomposition activation energy and bleaching temperature of hydrogen peroxide. When a catalyst (e.g. Fe2+ and Cu2+) is added to the hydrogen peroxide solution to form a Fenton reagent (H2O2/Fe2+), a catalytic system is constructed, enhancing the ability of hydrogen peroxide to destroy pigments and dyes. Fenton and Fenton-like technologies have been demonstrated to be effective in the treatment of various industrial wastewaters (Bedollaguzman et al. 2016; Quadrado & Fajardo 2017). In our previous research, a kind of gelatin protein copper complex (GPCC) was synthesized. GPCC acting as catalyst was applied to catalyzed hydrogen peroxide for bleaching cotton fabric. The results showed that hydrogen peroxide can be catalyzed and decomposed by GPCC. Then, the decomposed highly active components can effectively destroy the structure of the pigment on the fabric, improve the bleaching effect of hydrogen peroxide, and realize the low temperature bleaching of hydrogen peroxide (Wang & Tang 2017; Wang & Gao 2019). In this paper, the catalytic decolorization system of H2O2/GPCC/dye was constructed. GPCC was used to catalyze the decomposition of H2O2 to produce hydroxyl radicals and other highly active substances, which can destroy dye structure in dyeing wastewater. C.I. Reactive Red 24 is an azo reactive dye containing a monochlorotriazine active group. The chemical structural formula of C.I. Reactive Red 24 is shown in Figure 1. The dyestuff is often used for dyeing and printing to cellulose fiber, such as cotton. Azo dyes are an important kind dye, and occupy a very large proportion of dyes. C.I. Reactive Red 24 was chosen as the dye pollutant in the present study. The effect of the catalytic system on the decolorization of C.I. Reactive Red 24 was studied. The purpose of this study was to develop a new technology for decolorization of reactive dyes in order to solve the problem of severe chromaticity pollution in reactive dyeing wastewater.

Figure 1

The chemical structural formula of C.I. Reactive Red 24.

Figure 1

The chemical structural formula of C.I. Reactive Red 24.

EXPERIMENTAL

Reagents and instruments

C.I. Reactive Red 24 was supplied by Jinan Haoxing Chemical Co. Ltd (Jinan, China), which was used in this study without further purification. Gelatin protein copper complex (GPCC) was made by us. The 30% H2O2 and sodium chloride were of analytical grade and were purchased from Xi'an Chemical Reagent Co. Ltd (Xi'an, China). The HS high temperature computer program-controlled dyeing apparatus was made by Nantong Hongda Instrument Co. Ltd (Nantong, China). The STARTER 2100 pH-meter was made by OHOUS Instrument Co. Ltd (Shanghai, China). The UV-1900PC UV-Visible spectrophotometer was provided by AOE Instrument Co. Ltd (Shanghai, China).

Preparation of simulated dyeing wastewater

A certain amount of C.I. Reactive Red 24 was accurately weighed, and 2 g/L simulated waste dye solution was prepared.

Decolorization process of simulated dyeing wastewater

30% hydrogen peroxide 0–5 mL/L and catalyst GPCC 0–2 g/L were added to the simulated dyeing wastewater, in which the concentration of C.I. Reactive Red 24 was kept at 0.1 g/L. In addition, the pH value of the decolorization solution was adjusted between 4 and 10 with hydrochloric acid or sodium hydroxide. Decolorization was conducted on an HS type high temperature computer program-controlled dyeing apparatus at 70–90°C for a certain time.

Decolorization percentage test

The UV-1900PC UV-Visible spectrophotometer was used to measure the absorbance of the simulated dyeing wastewater contained C.I. Reactive Red 24 at the maximum absorption wavelength before and after decolorization, and the decolorization percentage R was calculated according to Equation (1):
formula
(1)
where Ai is the absorbance of the simulated dyeing wastewater after decolorization and diluted n times, A0 is the absorbance of the simulated dyeing wastewater before decolorization and diluted m times.

Determination of pH value change of decolorizing solution

The pH value of C.I. Reactive Red 24 solutions before and after decolorization were measured using a STARTER 2100 type pH-meter.

Determination of UV-Vis spectra performance change of decolorizing solution

The UV-1900PC UV-Visible spectrophotometer was used to measure the UV-Vis spectra performance change of the simulated dyeing wastewater contained C.I. Reactive Red 24 before and after decolorization.

RESULTS AND DISCUSSION

Effect of pH on decolorization

Since the pH value has significant influence on the catalytic reaction process, the catalytic reaction rate and the stability of the catalyst, the effect of pH value of the dye solution on the decolorization was discussed first. According to the decolorization procedure, the amount of catalyst GPCC was fixed at 1 g/L, the amount of 30% H2O2 was fixed at 4 mL/L, and the initial pH value of the decolorization solution was changed. Decolorization was conducted at 80 °C for 15 minutes. The result is seen Figure 2.

Figure 2

The effect of the initial pH on the decoloration percentage (▪) and on the final pH (▴) of C.I. Reactive Red 24 solutions.

Figure 2

The effect of the initial pH on the decoloration percentage (▪) and on the final pH (▴) of C.I. Reactive Red 24 solutions.

Figure 2 shows that the decolorization percentage increased with the increase in the initial pH. When the initial pH of the dye solution is 7, the decolorization percentage reaches 97.92%. After that a very slow increase in the decolorization percentage can be noticed, reaching nearly 100% at pH greater than 8. According to the different types of dyes, the actual pH value of dyeing wastewater is different. The pH value of acid dyeing wastewater is acidic or neutral, while that of reactive dye wastewater is usually alkaline. Generally, homogeneous Fenton catalytic decolorization must be carried out under strong acid conditions (the pH value is 3), and the decolorization conditions are harsh. There is only a low and narrow pH range for standard Fenton reagent to be effective (Ghanbarlou et al. 2020). For reactive dyeing wastewater, a large amount of acid must be used to adjust the pH value of the alkaline dye solution to strong acidity before decolorization using Fenton reagent, which increases the decolorization cost and decolorization process. In the H2O2/GPCC catalytic system, the decolorization effect is good in a wide pH range; it is especially suitable for decolorization of reactive dyeing wastewater. Therefore, this catalytic decolorization system has obvious advantages. For simplicity, the initial pH of decolorizing solution was not adjusted with hydrochloric acid or sodium hydroxide in the following experiments. The no-adjusted initial pH of decolorizing solution was 7.3. As can be also seen from Figure 2, when the initial pH value of the decolorizing solution was lower than 7, the final pH value of the decolorizing solution increased slightly. However, the final pH values decreased when the initial pH values of decolorizing solution were higher than 7. Moreover, the decreasing degree of pH value of the final dye solution increased with the increase of initial pH value. The results show that an H2O2/GPCC catalyst system can catalyze the oxidative degradation of dyes, but the pH value of decolorizing solution affects the reaction mechanism. Under alkaline conditions, the degradation of dyes may release acidic lower molecular weight species. Dulman V. et al. had done similar research (Dulman et al. 2012). The reaction mechanism of this catalytic system needs further study.

Effect of hydrogen peroxide dosage on decolorization

According to the decolorization process, the amount of catalyst GPCC was fixed at 1 g/L, the amount of 30% H2O2 was changed. The non-adjusted initial pH of decolorizing solution was 7.3. Decolorization was conducted at 80 °C for 15 minutes. The result is seen Figure 3.

Figure 3

Effect of 30% H2O2 dosage on decolorization percentage of C.I. Reactive Red 24.

Figure 3

Effect of 30% H2O2 dosage on decolorization percentage of C.I. Reactive Red 24.

It can be seen from Figure 3 that the decolorization percentage increased greatly with the increase of the dosage of hydrogen peroxide. When the concentration of H2O2 was 4 mL/L, the decolorization percentage was 99.91%. It can also be seen from Figure 3 that the decolorization percentage was only 9.63% when H2O2 was not added and only the catalyst GPCC was added in the dyeing wastewater. However the decolorization percentage can reach more than 90% in the H2O2/GPCC catalytic system. The reason for the improvement of the decolorization performance is that GPCC can catalyze H2O2 decomposes to generate highly active oxidizing components, such as hydroxyl radicals. The highly active oxidizing components increase as the concentration of H2O2 increases, and accelerate to destroy the chemical structure of dye, leading to increased decolorization percentage. From the experiment result, it can be seen that the concentration of 30% hydrogen peroxide was determined at 4 mL/L in the H2O2/GPCC catalytic system.

Effect of catalyst GPCC dosage on decolorization

According to the decolorization process, the amount of 30% H2O2 was fixed at 4 mL/L, the amount of catalyst GPCC was changed. The no-adjusted initial pH of decolorizing solution was 7.3. Decolorization was conducted at 80 °C for 15 minutes. The result is shown in Figure 4.

Figure 4

Effect of catalyst GPCC dosage on decolorization percentage of C.I. Reactive Red 24.

Figure 4

Effect of catalyst GPCC dosage on decolorization percentage of C.I. Reactive Red 24.

From Figure 4, it can be seen that the decolorization percentage increased highly with the increase in the amount of GPCC. When the dosage of GPCC was 1 g/L, the decolorization percentage was almost 100%. And it can also be seen from Figure 4 that the decolorization percentage was 20% without GPCC and only H2O2 in the dyeing wastewater. It indicated that the catalytic oxidation system composed of H2O2/GPCC can effectively decolorize and increasing the amount of GPCC can enhance to catalytic decomposition of H2O2, produce more highly active oxidizing components and destroy the structure of dyes in the dyeing wastewater. Thus the decolorization percentage is improved. From the experiment result, it can be seen that the dosage of GPCC was determined at 1 g/L in the H2O2/GPCC catalytic system.

Effect of temperature and time on decolorization

According to the decolorization process, the amount of 30% H2O2 was fixed at 4 mL/L, the amount of catalyst GPCC was fixed at 1 g/L. The no-adjusted initial pH of decolorizing solution was 7.3. The decolorization rates were measured at different temperatures. The results were shown in Figure 5.

Figure 5

Effect of temperature on decolorization rate of C.I. Reactive Red 24.

Figure 5

Effect of temperature on decolorization rate of C.I. Reactive Red 24.

As can be seen from Figure 5, the decolorization percentage reached about 75% when decolorization took place at 70 °C for 20 min, and the decolorization percentage was close to 100% when decolorization took place at 80 °C for 15 min, and the decolorization percentage was close to 100% when decolorization took place at 90 °C for 6 min. It indicates that temperature has a great influence on the decolorization and the decolorization rate increased with the increase of temperature. The higher the temperature, the faster the catalytic decomposition rate of H2O2 by GPCC, the greater the amount of produced highly reactive oxidation particles, such as hydroxyl radicals, so the greater the amount of dyes destroyed in the dyeing wastewater, and the bigger the decolorization rate. It can also be seen from Figure 5 that if decolorization is carried out at 80 °C, the decolorization time is determined to be 15 minutes. If a lower decolorization temperature is selected, the decolorization time can be extended appropriately.

Effect of sodium chloride concentration on decolorization

Since a lot of electrolytes are usually added to the dye bath when dyeing with reactive dyes (Maiti et al. 2018), there are salts in the reactive dyeing wastewater. In order to investigate the effect of salt on decolorization, the following experiments were designed. According to the decolorization process, the amount of 30% H2O2 was fixed at 4 mL/L, the amount of catalyst GPCC was fixed at 1 g/L. Different concentrations of NaCl were added to the dyeing wastewater, respectively. Decolorization was conducted at 70 °C for 30 min. The effect of sodium chloride concentration on decolorization was measured. The results are shown in Figure 6.

Figure 6

Effect of NaCl concentration on decolorization percentage of C.I. Reactive Red 24.

Figure 6

Effect of NaCl concentration on decolorization percentage of C.I. Reactive Red 24.

As can be seen from Figure 6, decolorization percentage increased with the increase in NaCl concentration. When NaCl concentration was 8 g/L, the decolorization percentage reached about 99%. When NaCl concentration was more than 8 g/L, the decolorization percentage changed little. This may be due to the catalytic effect of the chloride ion itself on the reaction of H2O2, or the increase of the amount of dye molecules adsorbed on the GPCC surface in the presence of chloride ions. They are favorable for the highly active components produced by the catalytic decomposition of H2O2 to fully contact with the dye molecules, destroy the structure of the dye and improve the decolorization percentage. So it indicates that the salt in the reactive dyeing wastewater will not affect the decolorization in H2O2/GPCC catalytic system. On the contrary, salt existing in the reactive dyeing wastewater will promote decolorization. This is of great significance for the decolorization of reactive dyeing wastewater.

Comparison of decolorization rate with and without sodium chloride

According to the decolorization process, the amount of 30% H2O2 was fixed at 4 mL/L, the amount of catalyst GPCC was fixed at 1 g/L. And the decolorization rate was measured with and without sodium chloride (the concentration of sodium chloride was 4 g/L) in the dyeing wastewater at 70 °C. The results are shown in Figure 7.

Figure 7

Comparison of the decolorization rate with and without NaCl.

Figure 7

Comparison of the decolorization rate with and without NaCl.

It can be seen from Figure 7 that the decolorization rate in the presence of sodium chloride was significantly higher than that in the absence of a sodium chloride system. The decolorization percentage was more than 70% with sodium chloride and the decolorization percentage was about 55% without sodium chloride when decolorizing for 10 min at 70 °C. It took about 12 minutes with sodium chloride and it took about 25 minutes without sodium chloride when decolorization percentage reached to 90%. It was proved that the presence of sodium chloride in the reactive dyeing wastewater is beneficial to accelerate decolorization and shorten the decolorization time in the H2O2/GPCC catalytic system.

Effect of dye concentration on decolorization

According to the decolorization process, different concentrations of dye wastewater were decolorized, the amount of 30% H2O2 was fixed at 4 mL/L, the amount of catalyst GPCC was fixed at 1 g/L, and the amount of sodium chloride was fixed at 4 g/L, or no sodium chloride was added. Decolorizations were conducted at 80 °C for 15 min. The results are shown in Figure 8.

Figure 8

Effect of dye concentration and sodium chloride on decolorization.

Figure 8

Effect of dye concentration and sodium chloride on decolorization.

It can be seen from Figure 8 that the decolorization percentage gradually decreases with the increase of dye concentration in the dyeing wastewater, regardless of the presence of salt in the dyeing wastewater. It can also be seen that the decolorization percentage with salt is higher than that without salt in the same concentrations of dyeing wastewater. The degree of reduction of decolorization percentage with salt is less than that without salt with the increase of dye concentration. When the dye concentration in dyeing wastewater is low (e.g. 0.1 g/L), even if there is no salt, the decolorization percentage itself is very high, so the presence of salt does not significantly improve the decolorization percentage. However, when the dye concentration is high (e.g. 0.5 g/L) and there is no salt in the dyeing wastewater, the degree of reduction of decolorization percentage is obvious. Therefore, compared with salt-free conditions, the presence of salt in high concentration dyeing wastewater is more conducive to increase the decolorization percentage.

Changes of UV-Vis absorption spectra during decolorization

The UV-Vis absorption spectra of C.I. Reactive Red 24 simulated dye wastewater are shown in Figure 9 after treatment at 80 °C for 15 min in different systems of dye only, dye and GPCC, dye and hydrogen peroxide, and a combination of dye, GPCC and hydrogen peroxide, respectively. As Figure 9 shows, compared with the dye-only solution, in the dye and hydrogen peroxide solution, as well as in the dye and GPCC solution, the absorption peaks at 538 nm in visible light and at 318 nm near ultraviolet still existed, and the absorption peaks' intensity changed little. The results show that only hydrogen peroxide and only GPCC have poor catalytic degradation performance for dyes. However, the absorption peaks at 538 and 318 nm disappeared in the system of dye, GPCC and hydrogen peroxide after the dye solution was treated at 80 °C for 15 min. It indicates the cleavage of the azo chromophore during the catalytic decolorization process and proves that C.I. Reactive Red 24 is completely decolorized by the H2O2/GPCC system. It testifies that GPCC can be used as a catalyst to promote the decomposition of hydrogen peroxide into highly active components, effectively degrade dye molecules and achieve high decolorization. Similarly, some researchers had reported that some supported transition metal ion complexes can catalyze the decomposition of H2O2 to produce highly reactive hydroxyl radicals and degrade dye molecules (Gemeay et al. 2003; Gemeay et al. 2004; Aravindhan et al. 2006; Dulman et al. 2012). Therefore, in the H2O2/GPCC/dye catalytic system, H2O2 reacts with the GPCC according to Equations (2) and (3) to form highly reactive hydroxyl radicals. Hydroxyl radicals (.OH) degrade most of the organic pollutants rapidly and non-selectively (see Equations (4) and (5)). The proposed mechanism is as follows:
formula
(2)
formula
(3)
formula
(4)
formula
(5)
formula
(6)
Figure 9

The UV-Vis absorption spectra of C.I. Reactive Red 24 simulated dye wastewater after treated at 80 °C for 15 min in different systems.

Figure 9

The UV-Vis absorption spectra of C.I. Reactive Red 24 simulated dye wastewater after treated at 80 °C for 15 min in different systems.

CONCLUSIONS

From the above experimental results, we can draw the following conclusions. The optimal decolorization technology of C.I. Reactive Red 24 in the H2O2/GPCC catalytic system is as follows: for 0.1 g/L of C.I. Reactive Red 24, 4 mL/L of 30% H2O2 and 1 g/L of GPCC need to be added in the simulated dyeing wastewater. If a lower decolorization temperature is selected, the decolorization time can be extended appropriately. If the decolorization temperature is 80 °C, the decolorization time is 15 min. A good decolorization effect can be obtained for C.I. Reactive Red 24 in the H2O2/GPCC catalytic system. The H2O2/GPCC catalytic system has the advantages of being able to effectively oxidize and degrade dye molecules, and having a good decolorization effect in a wide pH range. Moreover, the presence of salt in reactive dyeing wastewater is beneficial to enhance the decolorization percentage and decolorization rate and shorten the decolorization time in the H2O2/GPCC catalytic system. The presence of salt in high concentration dyeing wastewater is more conducive to increase the decolorization percentage, due to the degree of reduction of the decolorization percentage in presence of salt being less than that in the absence of salt with the increase of dye concentration.

ACKNOWLEDGEMENTS

The authors acknowledge the support of the Industrial Research Program of the Technology Board of Shaanxi Province, China (No. 2014K08-08) for this work.

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.

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