Degradation of bromophenol blue molecule during argon plasma jet irradiation
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The aim of this paper is to study degradation of a bromophenol blue molecule (C19H10Br4O5S) using direct irradiation of cold atmospheric argon plasma jet. The pH of the bromophenol blue solution has been measured as well as its absorbance spectra and conductivity before and after the irradiation of non-thermal plasma jet in various time durations. The results indicated that the lengths of conjugated systems in the molecular structure of bromophenol blue decreased, and that the bromophenol blue solution was decolorized as a result of the decomposition of bromophenol blue. This result shows that non-thermal plasma jet irradiation is capable of decomposing, and can also be used for water purification.
KeywordsBromophenol blue Cold atmospheric plasma Conductivity Degradation Spectrophotometry pH
During the past few years, many studies have been carried out to find the effect of low-temperature plasma with a variety of remote and local plasmas in atmospheric pressure on biological and industrial fields such as different surface treatment [1, 2, 3, 4, 5, 6, 7, 8, 9], sterilization, and purification [10, 11, 12] processes using different electrical discharges including corona discharge, dielectric barrier discharge, micro-hollow cathode discharge, atmospheric pressure plasma jet, etc. One of the most important applications of plasma treatment is degradation of various molecules in water and other solutions. A number of papers have studied the generation of various plasma sources in water and also in contact with water [13, 14, 15]. Bruggeman et al. reviewed the atmospheric pressure non-thermal discharges in liquids and in contact with liquids . Sugiarto et al. have investigated degradation of organic dyes by the pulsed discharge plasma in contaminated water in three discharge modes including streamer, spark, and spark–streamer mixed modes . One of the most interesting topics, in recent years, has been studying degradation of different materials in aqueous solution such as degradation of methyl orange , methyl violet , phenol , methanol , diuron , 1-naphthylamine , pharmaceutical compound pentoxifylline , organophosphate pesticides , antibiotics , textile dyes , etc. In such studies, scientists are interested in investigating the effects of various parameters such as pH  and temperature  on the plasma-treatment process. The degradation process of some materials such as bromophenol blue during plasma irradiation has not been investigated; some studies concerned the degradation of bromophenol blue without using plasma irradiation [30, 31, 32, 33]. On the other hand, the degradation process of some materials such as methyl blue has been well investigated. Some research concerned degradation and decolorization of methyl blue using different plasma sources such as dielectric barrier discharge [34, 35], radio frequency plasma , microwave discharge plasma , and corona discharge . The aim of this paper is studying degradation of bromophenol blue molecules in water using capacitively coupled argon plasma jet irradiation. In the present study, atmospheric pressure plasma is irradiated to the solution of bromophenol blue to investigate the selected optical, chemical, and electrical properties of the solution after different treatment durations. The bromophenol blue solution was observed by spectrophotometry after 1, 5, and 10 min of irradiation, and they were compared with a solution without plasma treatment. Moreover, the pH and the conductivity behavior of the solution after different treatment durations of argon plasma were studied.
Materials and methods
Preparing bromophenol blue solution
Bromophenol blue (3,3′,5,5′-tetrabromophenolsulfonphthalein) powder from Merck company has been used to prepare the solution. Its formula is C19H10Br4O5S (CAS number: 115-39-9). Its molar mass equals 669.96 g/mol, and its bulk density equals 730 kg/m3. The bromophenol solution was prepared by dissolving its powder in distilled water with a concentration of 10 mg/L.
Developing plasma jet
The argon plasma jet was directly irradiated for 1–10 min onto 10 ml bromophenol blue solution in glass Petri dishes. The absorbance spectra of the bromophenol blue solution samples are obtained from spectrophotometer (Hach DR500), and the pH of the bromophenol blue solution samples is measured with pH meter (Metrohm 744), and the conductivity of samples is measured with conductometer (Metrohm 712). All measurements have been made in four various time durations: first, before irradiation of plasma jet; second, 1 min after irradiation of the plasma jet; third, 5 min after irradiation of the plasma jet; and finally, 10 min after irradiation of the plasma jet.
Results and discussion
This figure indicates the dependence of the absorbance spectrum of the bromophenol blue samples on the plasma jet irradiation duration. The peak wavelengths of all samples were observed around 590 nm.
The absorption and emission bands of material and as a consequence their colors depend on their molecular structure (energy levels). The colors of the solutions here are in a good agreement with their absorption spectra shown in Fig. 2. As is obvious from Fig. 2, before plasma treatment, the solution has a strong absorption in red region, medium absorption in green region and weak absorption in blue region; therefore, the main reflected/transmitted spectra must be in blue area, and it is in good agreement with the color of the solution before treatment. After 1 min of radiation, the solution has an equal absorption in whole regions, so it looks transparent. But after more irradiation of plasma, as it can be seen in the absorption spectra, due to the change in the molecular structure of the solution, it absorbs less light in red region in comparison with the solution with 1 min of plasma radiation, so it looks yellow (yellow color is the combination of blue absorption with red and green reflection/transmission).
The degradation process of bromophenol blue molecule by direct irradiation of non-thermal atmospheric argon plasma using a local plasma jet was experimentally studied. The absorbance spectra, the conductivity value, and the pH value of the bromophenol blue solutions have been measured before and after different plasma irradiation durations. The measured values were in good agreement and indicated that the lengths of conjugated systems in the molecular structure of bromophenol blue decreased, and that the bromophenol blue solution was decolorized as a result of plasma irradiation. Atmospheric cold plasmas have proved their potential to be effective in the treatment of aqueous solutions such as bromophenol blue.
Hereby, the authors would like to express their deepest appreciation to Ms. Bahareh Abbasi and Dr. Ramin Rahmani for their guidance and persistent helps in editing the manuscript, and also Mr. Shahriar Mirpour for his scientific discussion.
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