Catalytic/inhibitory effect of the joint presence of two dyes on its destruction by underwater plasma processes: a tool for optimization parameters of treatment
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The underwater plasma was used for the destruction of two-dye mixture. The Direct Blue azo dye and xanthene dye of Rhodamine 6G used as the model pollutants. The concentration of one of them is varied for modeling the processes in which one of the classes of the dye exceeded other dyes. The catalytic/inhibitory effect of the joint presence of two dyes is studied. The effect of initial temperature and the role of H2O2 in the mechanism of destruction were also investigated. It was established that the processes of decay of organic dyes proceed through the stages of the formation of intermediate products, which are catalysts or inhibitors of further degradation processes. An increase in the temperature of the solution has almost no effect on the degree of decolorization of the solution of the dye mixture. It is established that the destruction of Direct Blue 1 occurs under the action of hydrogen peroxide or HO2 radicals, while Rhodamine 6G is destroyed by interaction with hydroxide radicals. The comparison of energy yield values with published data showed that underwater plasma is an effective method for removing the organic dyes from wastewater.
KeywordsUnderwater plasma Destruction Dyes mixture Rhodamine 6G Direct Blue 1 Energy yield
Dyes are complex organic compounds that are used in various industries (textile, chemical, dyeing, electronics, etc.). The disposal of organic compounds from wastewater is one of the main issues of treatment. Recently, there has been significant interest in using advanced oxidative processes to solve this problem [1, 2, 3, 4, 5]. Of interest are developments based on the use of nonthermal plasma in contact with a liquid, as reflected in recent papers [6, 7, 8, 9].
There are many features of different types of electrical discharges. For example, the gliding arc discharge generates the nonthermal plasma in the gas phase with a high concentration of charged and neutral reactive species. On the other hand, the replacement of the plasma zone in a liquid volume and creating underwater discharge have some advantages in comparison with electrical discharges above liquids. It is a large size of the contact of plasma zone and liquids, convective flows, and cavitation effect.
There are a lot of data in which the destruction of single dyes by electrical discharges treatment is studied [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39]. One of the most popular objects of research is methylene blue dye, which is one of the thiazine dyes [10, 12, 16, 18, 21]. A large number of works are devoted to studying the effect of electric discharges on azo dyes due to the widespread use of this class of dyes [14, 15, 17, 19, 25, 26, 27, 30, 32, 34]. It should be noted that the growth of studies on the decomposition of anthraquinone dyes because of carcinogenic properties in water [20, 23, 24, 29, 31, 33, 35]. However, the wastewater of industrial factories contains more than one dye. It is the mixture of the organic and inorganic compounds. At present, there are some references regarding the application of nonthermal plasmas to a mixture of the organic dyes [39, 40, 41, 42]. In , it was found that the destruction of two dyes occurs faster in their mixture than in the solutions, which contain only one dye. It was assumed that the dyes in a mixture or its by-products could be a catalyst in the degradation process.
In this work, the object of research was a plasma treatment of a mixture of dyes Rhodamine 6G and Direct Blue 1. The choice of dyes was determined by their belonging to different classes of dyes. Rhodamine 6G is a representative of the class of xanthene dyes, which are used in both the textile and electronics industries. Direct Blue dye 1 is an azo dye. Its choice is explained by the fact that about 70% of the dyes used in the industry account for this class of dyes. The underwater diaphragm discharge was used as the nonthermal plasma. The main goal of this paper is to investigate the effect of a concentration of azo dye on decolorization efficiency and the rate of the destruction of dye’s mixture by underwater plasma treatment. Varying the concentration of azo dye concerning Rhodamine 6G models processes in which the content of azo dyes exceeds the concentration of other classes of dyes. Based on the early published data, we suggest that the presence of two or more organic dyes may induce the catalytic effect for fast degradation of both dyes. In the present work, we try to prove this assumption. The effect of the initial temperature of a solution was also studied.
2 Materials and methods
Aminoquinone was synthesized in laboratory at Department of Fine Organic Synthesis (Ivanovo State University of Chemistry and Technology).
2.2 Experimental setup
2.3 Methods of analysis
The mixtures of dyes were prepared by dissolving two dyes in deionized water. The concentration of Direct Blue 1 dye varied from 4 to 12 mg/L. The concentration of Rhodamine 6G was constant and equaled to 4 mg/L.
The whole absorption spectra in the range of wavelength 190–750 nm were registered by UV–Vis absorption spectroscopy (SF 104, Akvilon, Russia) before and after plasma treatment. The quartz cuvettes with a length of 10 mm were used.
The concentrations of oxidizers were determined using the following methods. The H2O2 is detected using the reaction of hydrogen peroxide with KI in the presence of NH4MoO4 as the catalyst in an acid medium. The total concentration of oxidizers was determined by using the reaction with potassium permanganate. The products of the degradation process were analyzed by high-performance liquid chromatography (Gilson 302) and gas chromatography mass spectrometry (Shimadzu GCMS QP2010 Ultra).
For investigating an effect of the initial temperature of a solution on dye’s mixture decolorization in the absence of plasma, series experiments were carried out in the range of 10–80 °C. No changes in absorbance spectra are observed for 25 min. It means that dye’s mixture solution is thermally stable.
To determine the role of hydrogen peroxide in the destruction mechanism of dyes, the experiments were carried out without plasma treatment. 2 mL of H2O2 (30%) is added to the solutions of dye’s mixture and single dyes every 5 min. The changes of absorbance for both dyes are registered by photometric probes.
The acidity and conductivity of solutions were registered by pH meter I-160 (Akvilon, Russia) and conductivity meter InoLab (WTW, Germany) before and after plasma treatment.
3 Results and discussion
3.1 Decolorization efficiency (α)
The pH of treated solution did not change significantly (ΔpH = −0.5), whereas the conductivity of these solutions increased (up to 190 μS cm−1). This can be connected with appearance of anions in solutions. One can assume that an increase in the electrical conductivity of the solution may prevent the complete decomposition of the Direct Blue dye (Fig. 2).
The effect of the conductivity of the solution on the generation/accumulation of reactive species or the destruction of organic compounds in aqueous solutions under the action of non-equilibrium plasma is a very interesting and still open question. Thorough research is required to answer this question. In the works [25, 30], it was found that an increase in the electrical conductivity of the solution leads to a decrease in the destruction efficiency of the dye. To date, we can only assume that an increase in electrical conductivity leads to an increase in the channels of expenditure of reactive species (competing reactions).
3.2 Rate of destruction
The rates of destruction were estimated by numerical differentiation of the kinetic curves of decolorization. Our results showed that the rates changed with the time of treatment. The same behavior was found in our previous studies [39, 40, 41]. It means that the serial–parallel reactions pass in the solution and finally we have some effective process of destruction. As known, the ignition of electrical discharge inside or above the aqueous solution initiates the formation of reactive species in a solution. We suggested that the dyes react with reactive species at the initial stage of plasma action. And then, not only reactive species but intermediates take part in the destruction process. In this case, the values of the rate of destruction at the initial time of treatment were chosen for the comparison of the rate of destruction for both dyes at various experimental conditions.
3.3 Effect of initial temperature
The formation of hydrogen peroxide can occur as in the small volume of a solution near to plasma zone as well as in the bulk of a solution (diffuse zone). In this case, the temperature of solution effects on the rate of the interaction as reflected for Direct Blue 1, the increase in initial temperature of a solution leads to the growth in the rate of destruction (Fig. 6b).
3.4 A possible mechanism of the destruction
When the plasma is ignited above or inside aqueous solutions, the reactive species, such as hydrogen atoms, hydroxyl radicals, hydrated electrons, and oxygen, are formed in a liquid. The main oxidative agent is OH· radical, which leads to the destruction of organic dyes [48, 49, 50]. It was assumed that the hydroxyl radicals are responsible for the destruction of both dyes. However, analysis of kinetic curves for Rh6G and DB1 showed that the reactive species that are responsible for destruction might be different. According to data of kinetics of the decolorization, the process for Rhodamine 6G is faster than for Direct Blue 1. This means that the destruction process may occur with primary reactive species, such as OH· radicals for Rh6G. It is possible because of the high rate constant reaction of hydroxyl radical with Rh6G (1.1 × 1010 L mol−1 s−1) .
In the case of the azo dye, the addition of hydrogen peroxide without the plasma treatment of single-dye solution leads to 5% decomposition (curve 2a in Fig. 7). The combined effect of H2O2 and plasma causes 45% of DB1 destruction (curve 2b in Fig. 7). The greatest effect was registered in a mixture of dyes with the addition of hydrogen peroxide (curve 2c in Fig. 7).
We supposed that HO2· or H2O2 reactive species are responsible for the destruction of DB dye. The dimerization of hydroxyl radicals is not one of the ways for the production of hydrogen peroxide. Moreover, the results of chemical analysis showed that the concentration of total oxidizers is 0.3 mM/L, whereas the concentration of H2O2 is 0.225 mM/L and concentration of ozone did not exceed 0.05 mM/L. This means that about 0.025 mM/L of long-lived (stable) compounds with oxidizing properties is present in the solution.
3.5 Activation energy
Creating a plant for the treatment, the appropriate physical–chemical parameters are needed. In particular, the activation energy is one of a parameter that can predict the behavior of a reaction. In the case of single-step reactions, the value of Ea is determined easily. In the case of complex reactions, the activation energy value can estimate as an apparent Ea.
Apparent activation energy values
Concentration of DB1 (mg/L)
Ea (kJ mol−1)
Decreasing activation energy values for Direct Blue 1 dye means that the substance/catalyst is present in the solution which eases the destruction process. It could be aminoquinone. And reverse, increasing Ea for Rhodamine 6G with growth concentration of blue dye means that the inhibitor appeared in the solution. This can be one of the products of DB1 destruction process, for example oxalic acid. To check these assumptions, the two series of experiments were carried out without plasma treatment. In the first experiment, synthesized aminoquinone is added to dyes mixture at a ratio of dyes of 1:1 at 20 °C. The obtained results showed that the decolorization efficiency of Direct Blue 1 dye is increased to 85% and the initial rate of destruction is increased by 1.4 times. This means that the by-product of Rhodamine 6G is a catalyst for the destruction process of DB1. In the other experiments, the oxalic acid was added to dyes mixture at the same conditions. The initial rate of destruction for Rh6G is decreased by 25%. And it means that oxalic acid is an inhibitor of the destruction process of Rh6G. The obtained results are consistent with previously published data [58, 60], which were found that oxalic acid is an inhibitor of the decomposition reaction of the dye Rhodamine B.
3.6 Energy yield
Energy yields of dyes by electric discharge treatment
Solution of mono dye
Acid Black 52
Acid Blue 25
Acid Orange 7 (Orange II)
Acid Red 4
Acid Red 27
Acid Red 88
Alizarin red S
Amaranth azo dye
Direct Blue 1
Naphthol Green B
Naphthol Blue Black
Reactive Blue 4
Reactive Blue 19
Sulfanilic acid azochromotrop
Mixture of dyes
Alizarin red S + Orange G
322 + 100
80 for both dyes
Rhodamine 6G + Direct Blue 1
4 + 8
0.4 + 0.4
4 + 4
4 + 6
4 + 8
4 + 10
4 + 12
Using the methylene blue dye as an example, we can trace that different values of G are obtained using the same dye concentration and even the same electrode configuration system (the same type of electric discharge). It should be noted that in the work , to accelerate the decomposition process, the experiments are carried out in an oxygen atmosphere, whereas the authors of  used the addition of iron(II) ions. According to the energy yield values, conducting the experiments in an atmosphere of O2 is more efficient.
An analysis of the data presented showed that the energy the yields are determined by the concentration of the dye, the degree of its decomposition, and the energy input. The combination of low-temperature plasma with the addition of hydrogen peroxide [12, 30, 31], iron(II) ions [12, 21, 25, 30, 35], bubbling with ozone [10, 14], or conducting experiments in an oxygen atmosphere [12, 16, 17, 33, 38], as well as the use of nanoparticles [14, 24, 32], accelerates the destruction processes and increases G.
The following point should be noted. When calculating the value of G, it is assumed that all the input energy is spent on the destruction processes. According to works [47, 61, 62], most of the input energy of 55–60% is consumed in heating the solution. Around 30% is distributed among radiant energy, generation of acoustic waves, and initiation of the chemical processes (23%) . As was mentioned above, the increase in temperature of a solution does not induce the degradation processes. This means that most part of input energy does not take part in the destruction processes. In [53, 59, 63, 64], it was found that destruction of dyes occurs by acoustic waves and radiant energy (UV and visible radiation) also. Hence, only 1/3 part of input energy is consumed in the destruction processes in a solution. In our case, this means that the G values should be higher and it could be improved by applying at treatment facilities of a textile factory.
The application of the underwater AC diaphragm discharge plasma for the destruction of a mixture of two dyes was found to be effective. The efficiency of the destruction process was proved by UV–Vis spectra and HPLC-GC/MS analysis, which indicated that the destruction process occurs via the formation of the by-products. Experimentally proven that the Rhodamine 6G degradation by-product (aminoquinone) is a catalyst for the Direct Blue 1 destruction process and oxalic acid is an inhibitor for the destruction of Rh6G. It is confirmed by the estimated energy activation of the destruction process for both dyes. The final compounds in a solution are oxalic acid and inorganic anions.
The results showed that the maximal decolorization efficiency (100%) is reached in the temperature range of 30–50 °C. This temperature range is an important parameter in the further application of this method to the real textile wastewater. It allows using the underwater diaphragm discharge with real wastewater without preliminary heat, since the real sewage of textile factories has a temperature around 30–40 °C. The presence of a catalyst in the system allows optimizing the conditions for cleaning. The formation of aminoquinone occurs for the first minutes of plasma treatment, and maximal concentration can reach for 10 min. The presence of catalyst and by-products can initiate the destruction process after plasma off (so-called post-effect). This allows reducing processing time and decreases energy costs. Based on these results, the next work is expected to develop the module prototype for cleaning textile wastewater in a sump of the textile factory.
The liquid chromatography and gas chromatography mass spectrometry analysis was carried out using by High Performance-Liquid Chromatograph Gilson 302 (France) and Shimadzu GCMS QP2010 Ultra (Shimadzu Europa GmbH) at the Interdepartmental Laboratory of Gas, Mass-Spectrometry, and EPR Spectrometry at the Ivanovo State University of Chemistry and Technology.
Compliance with ethical standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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