CHEMICAL ENGINEERING TRANSACTIONS VOL. 81, 2020 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Petar S. Varbanov, Qiuwang Wang, Min Zeng, Panos Seferlis, Ting Ma, Jiří J. Klemeš Copyright © 2020, AIDIC Servizi S.r.l. ISBN 978-88-95608-79-2; ISSN 2283-9216 Treatment of Methyl Blue Dye Wastewater Solution by Three- Dimensional Electrolysis Ben Wanga, Yan Suna, Shengnan Yua, Qinghao Wub, Jinhua Yina,* aQingdao University of Science and Technology, No.53 Zhengzhou Rd, Qingdao, Shandong, P.R. China, 266042 bQingdao Q.K.L.Y.S&T Consulting Development Co.Ltd, No.51-2 Wuyang Rd, Qingdao, Shandong, P.R. China, 266042 yinjinhua@126.com Dye wastewater has complex structures and is difficult to be biodegraded. Three-dimensional electrolysis is advantageous in mass transfer and oxidation degradation ability. In this paper, a continuous flow three- dimensional electrolytic reactor with coconut shell activated carbon as the filling material is established to treat methyl blue dye wastewater. The influence on the dye wastewater treatment by the pH value of the system, the impressed voltage, the concentration of electrolyte, the aeration rate, the electrolysis time and the interactions of the five factors on COD degradation are investigated in the experimental part, and the optimal degradation condition is finally determined. Results show that it not only improved the degradation efficiency of dye wastewater, but also help to energy conservation, emission reduction and sustainable development. 1. Introduction The remediation of dye wastewater has been an urgent environment problem for every nation. Over the world, about 109 people are exposed to unsafe drinking water due to poor source water quality and lack of adequate water treatment (Han et al., 2009). The survey found that about 15 % of the total global production of dyes is lost during the dyeing process and released into the environment as textile effluent (Weber et al., 1993). Triphenylmethane dyes (TPM dyes) have the advantages of many systems and various, so it has been widely used. TPM dyes also have great harm to the environment and biology. In this paper, methyl blue was studied. There are various conventional methods used to treat methyl blue dye wastewater including biochemical method (Cui et al., 2016), photocatalytic method (Han et al., 2009), electrolytic method (Liu et al., 2020), hydrogel method (Chirag et al., 2020), adsorption method (Luo et al., 2019) and Novel bio-electro-Fenton Technology (Li et al., 2017). Recently, electrochemical methods have attracted more attention due to some advantages such as ease of operation, high efficiency and environmental compatibility (Pulkka et al., 2014). It has been found that the effect of three-dimensional electrolysis is about 15 % higher than that of two- dimensional electrolysis (Reza et al., 2020). Three-dimensional electrode water treatment technology can effectively increase the specific surface area of the working electrode and improve the mass transfer rate, effectively improving the degradation efficiency of organic matter (Gao et al., 2008). The selection of electrode materials determines the effect of wastewater treatment, so the appropriate electrode should be selected. Biocathode-electrocatalytic Reactor (BECR) successfully started up at 0.7 and 1 V and substantially improved MB and total organic carbon (TOC) removal compared with the electrocatalytic reactor with SS cathode (ECR- SS); less energy consumption, but still low efficiency of wastewater treatment (Mo et al., 2020). Fe/Cu, Fe/Al/Cu, Fe/Cu/C and Fe/Al/Cu/C internal electrolysis systems (IESS) were constructed and used to treat methylene blue dye (MB) wastewater. The degradation effect is good, but the energy saving is not realized (Liu et al., 2020). IrO2/Ti and RuO2/ Ti (DSA) can produce strong oxidizing groups and react with organics in wastewater (Song, 2008). DSA is used as anode material in this paper. The cathode material is graphite electrode, the price is low, and the COD removal rate is higher, so it is used as cathode material in this paper (Zhao, 2012). The continuous-flow three-dimensional electrode reactor could effectively remove the refractory organic pollutants in a secondary industrial effluent (Xu et al., 2008). However, as one of the most common types of industrial wastewater, the treatment of methyl blue dye wastewater by the continuous-flow three- DOI: 10.3303/CET2081023 Paper Received: 26/03/2020; Revised: 17/05/2020; Accepted: 21/05/2020 Please cite this article as: Wang B., Sun Y., Yu S., Wu Q., Yin J., 2020, Treatment of Methyl Blue Dye Wastewater Solution by Three- Dimensional Electrolysis, Chemical Engineering Transactions, 81, 133-138 DOI:10.3303/CET2081023 133 dimensional electrode reactor has not been studied. In this paper, a continuous flow three-dimensional electrolytic reactor with coconut shell activated carbon as the filling material was established to treat methyl blue dye wastewater, and the effect of COD degradation by changing the applied voltage, electrolyte concentration, electrolytic time, pH, and aeration conditions were analyzed by controlling single variable and orthogonal. This study also provides a basis for the continuous treatment of dye wastewater in the future. 2. Experiments COD was taken as the index in the experiment. Single variable experiment and orthogonal experiment considering interaction were carried out. According to the extremum of orthogonal experiment results, the influence degree of each factor is determined. 2.1 Experimental Set-up In this experiment, electrode material: graphite plate was used as cathode (Hamidi et al., 2004); DSA electrode was used as anode, and oconut shell activated carbon was used as a filling material. The cell volume was designed as: Xu et al. (2008) : "anode and cathode were 7 cm × 5 cm in size and were situated 3 cm from each other." Figure 1:Structure of Three-dimensional electrode reactor Paidar et al. (2002) studied the influence of the flow form of fluid in the three-dimensional electrolytic reactor on the degradation rate and current efficiency of pollutants in wastewater. The results showed that the current efficiency and pollutant removal rate of the flow plate electrolytic cell are higher, so a continuous flow three- dimensional electrolytic reactor with coconut shell activated carbon as the filling material was established in this paper. 2.2 Analytical method Potassium hydrogen phthalate was used as the reference material for COD standard curve calibration (Liang et al., 2017), and the catalyst was H2SO4 solution of Ag2SO4. According to the determination method of CODCr in national standard, K2Cr2O7 solution was used as the disinfectant, and the light transmittance data was measured by spectrophotometer at 445 nm. 3. Influence of single factor on the electrolysis process In this experiment, methyl blue simulated wastewater was used as the treatment object. A continuous flow three-dimensional electrolytic reactor with coconut shell activated carbon as the filling material was established to treat methyl blue dye wastewater. The Standard condition of the experiment was set as below: the initial concentration was set to 200 mg/L, the volume was 250 mL, aeration rate was 30 L/h, the electrolysis was 120 min, the Na2SO4 concentration was 0.1 mol/L, the pH value was 5, and the electrolytic voltage was set to 14 V. The effects of various factors on the degradation of methyl blue simulated wastewater by three-dimensional electrodes was investigated. 134 3.1 Effect of the applied voltage In this series of experiments, the applied voltage was taken as the operating variables and the other parameters were kept. The degradation rate of COD and current in the methyl blue wastewater were measured under different voltage conditions (Yan et al., 2011). Figure 2: Effects of the applied voltage on electrolytic process Figure 2 shows that as the applied voltage increase, the COD degradation degree and current in methyl blue simulate wastewater both increase. Before the applied voltage reach 14 V, the COD degradation rate increase faster and the current increase slowly. After the applied voltage of 14 V, the COD degradation rate increase slowly and the current increase faster. With the increasing of applied voltage, the current increase rapidly, while the removal rate of COD does not increase significantly, resulting in energy waste. Comprehensively, the optimal voltage is 14 V. 3.2 Effect of the electrolysis time In this part, the electrolysis time was taken as the operating variables in a set of experiments and the other parameters were kept. Samples were taken every 10 min to analyze the degradation of COD in methyl blue simulated wastewater at different times. Figure 3: Effects of the electrode time on electrolytic process Figure 3 shows that with the increasing of the electrolysis time, the degradation rate of COD of methyl blue dye wastewater increases. As the reaction time goes on, the slope of the curve of COD shows a downward trend, because the content of COD in the system is less and less. According to the slope of 80 ~ 120 min, we can know that it is an obvious increasing trend than 120 - 140 min. Although the growth trend slowed down between 80 min and 120 min, in order to achieve 83 % degradation rate, combined with the experimental results, we used 120 min. 135 3.3 Effect of the pH In this part, pH was taken as the operating variables and the other parameters were kept. The degradation experiments of methyl blue simulated wastewater were carried out under different pH conditions. Figure 4: Effect of the pH on the electrolytic process It can be seen from Figure 4 that the degradation rate of COD decreases with the increasing of pH value. When the pH is between 2 to 4, the degradation rate of COD is higher and does not change much, about 88 %. When the pH is between 4 to 7, the degradation rate of COD decreases sharply. With the decreasing of pH value, the conductivity of the methyl blue wastewater system would inevitably increase, which would lead to increasing reaction current and the consumption of electric energy. The pH value that the neutral electric energy is closer, the waste is less. Considering the treatment cost of the methyl blue wastewater, the pH is taken as 4. 3.4 Effect of the electrolyte concentration Methyl blue simulated wastewater containing different Na2SO4 concentrations was prepared, and electrolyte concentration was changed with other parameters remained. The effect of electrolyte concentration on the COD degradation and reactor current in methyl blue simulated wastewater was investigated. (a) (b) Figure 5: Effects of (a) the electrolyte concentration and (b) the aeration rate on electrolytic process As can be seen from Figure 5 (a), with the electrolyte concentration increasing, the COD degradation and current both show an upward trend. The COD degradation and current increase rapidly from 0.05 mol/L to 0.10 mol/L, while the degradation rate of COD and the current between 0.10 mol/L and 0.20 mol/L both increase slowly. When the concentration is larger than 0.2 mol/L, the COD degradation rate does not increase, 136 but the current still maintain the previous growth trend. Considering the waste of energy, the electrolyte concentration is taken as 0.2 mol/L. 3.5 Effect of the aeration rate In this part, the aeration rate was taken as the operating variables and the other parameters were kept. The degradation experiments of methyl blue simulated wastewater were carried out under different the aeration rate pH conditions. It can be seen from Figure 5 (b) that with the increasing of aeration, the COD degradation increase first and then decrease. When the aeration rate is 70 L/h, the COD degradation rate reached the maximum of 84 %. when the aeration is 50 L/h, the COD degradation rate reached 83 %. At 50 ~ 70 L/h, COD degradation rate has little change. Comprehensive consideration, aeration rate of 50 L/h is selected. 3.6 Investigation of the interaction of electrolytic factors Table 1: Orthogonal experiment of influence factors of electrolysis According to the range value in Table 1, the influence degree of each factor test is: electrolysis time > pH value > voltage > electrolyte concentration > aeration. Dye degradation increases with voltage, aeration, electrolyte concentration, time and acidity. When the electrolysis voltage is 17 V, pH is 3, aeration is 100 L/h, electrolyte concentration is 0.2 mol/L and time is 150 min, in methyl blue simulated wastewater has the highest degradation rate of COD of 88 %. Combined with the single variable experiment, considering the waste of energy, the optimal experimental electrolysis conditions for the treatment of methyl blue simulate wastewater by three-dimensional electrolysis are determined: pH is 4, voltage is 14 V, aeration rate is 50 L/h, electrolyte concentration of Na2SO4 is 0.15 mol/L, electrolysis time is 120 min. 4. Conclusions In this paper, a continuous flow three-dimensional electrolytic reactor was established to treat methyl blue dye wastewater with coconut shell activated carbon as the filler. This method is not only simple and easy to operate but also environment friendly, and has high degradation efficiency. Through the extremum of orthogonal experiment, the influence degree of each factor is determined. The regular is: electrolysis time > pH value > applied voltage > electrolyte concentration > aeration amount. Combined with the results of single- factor experiments, the optimal combination conditions for the three-dimensional electrolytic treatment of methyl blue dye wastewater are determined: The operating parameters for treating methyl blue dye wastewater are that electrolytic voltage is 14 V, electrolytic time is 120 min, pH is 4, electrolyte concentration Factor pH Voltage V Electrolyte concentration mol/L Time min Aeration L/h Degradation rate of COD % Group 1 3 8 0.05 60 0 69 Group 2 3 11 0.1 90 20 76 Group 3 3 14 0.15 120 50 85 Group 4 3 17 0.2 150 100 88 Group 5 4 8 0.15 120 20 83 Group 6 4 11 0.2 60 0 75 Group 7 4 14 0.05 90 100 76 Group 8 4 17 0.15 150 50 73 Group 9 5 8 0.2 90 50 73 Group 10 5 11 0.15 60 100 65 Group 11 5 14 0.1 150 0 84 Group 12 5 17 0.05 120 20 78 Group 13 6 8 0.1 120 100 75 Group 14 6 11 0.05 150 50 72 Group 15 6 14 0.2 60 20 69 Group 16 6 17 0.1 90 0 74 Average 1 79.5 70.0 73.7 69.0 75.5 — Average 2 77.7 72.0 77.0 74.7 76.5 — Average 3 75.0 75.5 76.7 78.2 75.7 — Average 4 72.5 76.2 76.2 79.7 76.0 — Range 7.00 6.5 3.25 12.75 1.00 — 137 is 0.15 mol/L and the aeration amount is 50 L/h. 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