Decolorization of Azo Dyes using Contact Glow Discharge Electrolysis

Abstract

Contact Glow Discharged Electrolysis (CGDE) is a plasma electrolysis methode has proved to treat organic wastewater effectively. The objective of this study was to investigate decolorization process of the textile dyes Remazol Red RR, Remazol Yellow FG and Remazol Brilliant Violet with an air injection using CGDE method. Air injection was increased a percentage of decolorization Remazol Red RR up to 46.7% in 180 min. Proces efficiency for Remazol Red RR, Remazol Yellow FG and Remazol Brilliant Blue were : 11,520; 24,412.5 and 8,820 kJ/mmol, respectively

Introduction

During manufacturing and processing in the textile industry there are a large quantities of dyes used lost to the effluents (Akyol, et.al 2004). As a consequence, there is an increase of environmental contamination caused by the large amount of dyes involved in the textile process that are discharged in the liquid effluents (Torralba; 2009). The most commonly used synthetic dyes in textile, food, papermaking and cosmetic industries are azo dyes (Kamel, R.H, 2011). Azo dyes represent a major class of synthetic colorants that are both mutagenic and carcinogenic (Waghmode et al., 2012). Azo dye has a chromophore system of the azo group (-N=N-), which binds to the aromatic group. Release of residual azo dye into industrial effluents deteriorates the water quality because of their color which result in aesthetic problems and affects photosynthesis in aquatic plants (Camargo, et.al, 2013; Kamel, 2011, Lu 2010). Many azo dyes from wastewater and their breakdown products are toxic and/or mutagenic to various forms of life and may cause a significant impact on human health due to their mutagenic and carcinogenic effects (Kamel, 2011, Lu 2010).

Various methods have been suggested to handle the dye removal from water such as biodegradation, coagulation, adsorption, advanced oxidation processes (AOPs) and the membrane process. All these processes have some advantages or disadvantages over the other methods (Khataee, 2010). Coagulation-flocculation method requires a large quantity of chemical and produces an even greater volume of sludge. High concentrations of metals in the sludge limit the suitability for its disposal to land and the large quantities of bound water make dewatering and transport difficult and expensive (Keeley, et.al, 2014). Adsorption process using activated charcoal as adsorbent on the basis of different factors (temperature, contact duration, adsorbent dosage and agitator speed) indicated that maximum removal for colour (90%), BOD5 (81.0%) and COD (87.6%) was noted in adsorbent dose of 11 g/L (Aleem, et.al; 2016). Both of adsorption and coagulation process do not destroy the dyes, they rather result in phase transfer of the pollutant, and hence ultimate sludge disposal remains an unsolved problem (Kamel, 2011). Membrane process have major disvantages such as a high price, limited lifetime, and the high cost of periodic replacement (Sharma, 2015).

AOPs based on the generation of very reactive species such as hydroxyl radicals have been proposed to oxidize quickly and none selectively a broad range of organic pollutants (Khataee, et.al, 2010). In this method, the production of hydroxyl radical was generated from H2O2, ozone, oxidants in combination with using ultraviolet or photo-catalysis (Khrisnan, et.al; 2017). Ozon decolorized the colour of dye, especially fiber acid and reactive dye effectively. Hassaan, et.al (2017) menyatakan bahwa proses oksidasi zat pewarna direct blue 86 yang dilakukan pada pH 11, menggunakan ozon efektif mendegradasi zat pewarna (konsentrasi 100 ppm) hingga 98% pada menit ke-35. Independent of the initial dye concentration, ozonization process ineffective for the reduction in COD and TOC, usually not exceeding 50 and 40 %, respectively (Balasubramanyan, 2014). Degradasi zat warna azo oleh bakteri Aeromonas sp., Pseudomonas sp., dan Flavobacterium sp,. dalam kondisi anaerob lebih cepat dibandingkan aerob, namun demikian dihasilkan amina aromatik yang bersifat lebih toksik dibanding zat warna itu sendiri (Sastrawidana, 2012).

Contact glow discharged electrolysis is a kind of electrochemical process in which plasma is sustained by dc glow discharged between an electrode and the surface of the electrolyte (Gao, et.al., 2002). Principally, electrolysis plasma process similar with conventional electrolysis, but conducted in high voltage condition giving rise to plasma emission (glow discharged plasma) (Saksono, 2014). Under these conditions, gaseous H2O+ ions in the plasma are accelerated by a strong potential gradient toward the plasma liquid interphase. The ions gain enough energy to dissociate several water molecules by ionization and activation, generating a large number of hydroxyl radicals (OH). Hydroxyl radicals are active spesi that have a powerful oxidizing reagents with an oxidation potential of 2.33 V (Gao 2002). The life of hydroxyl radical very short (3,7 x 10-9 s) (Jiang et.al 2014). It is easily react with other compounds and with each other to form H2O2 because it has high reactivity. So its existence can be determined by measuring the concentration of H2O2 that is more stable than the OH , (Saksono, et,al 2014). The Fe2+ ion could improve the persent decolorization of dye, because it catalyze decomposition of H2O2 resulting in production of hydroxyl radicals (Jamroz, et.al, 2018).

CGDE method has been proved effective to decolorization of the dye in water as reported by various studies. Saksono, et.al, 2018 stated that the use of air injection can make the process more efficient than without air injection. Remazol Brilliant Blue degradation for 30 minute process with microbubble addition and 40 mg/L of Fe2+ ion reached 99.63%. For this reason, this research was conducted.

Material:

The dyes used in the investigation were Remazol Red RR (RR), Remazol Yellow FG (RYFG) and Remazol Brilliant Violet (RBV). The dye solution were prepared at 100 mg/L and the concentration of sodium sulfate that used as an electrolyte solution was 0.02 M. Fe2+ ions of 20 mg/L was added to the solution. Total volume of solution was 2 L.

The CGDE reactor is made from transparent glass, it operated with batch system while the cooling water flows continously. The electrodes used are stainless steel with diameter 6.0 mm for cathode and tungsten with diameter 1.0 mm for anode. Anode was immersed 10 mm into the solution. During the process, the solution was magnetically stirred at a constant rate The flow rate of air injection was kept constant at 1.1 L/min to produce uniform size of bubbles and gas channels. The reactor operated at voltage 700 V. pH was measured using Agilent 3200P pH meter, electrical conductivity of the solution was measured by Agilent 3200P conductometer.

Effect of Air Injection

The experiment were done with and without air injection into the dye solution. The results as shown in Figure 2. Air injection was increased a percentage of decolorization RR up to 46.7% in 180 min. It was due to bubbling gas can create turbulent flow in reactor resulting in homogeneous pollutant distribution and alleviating mass transfer resistance (Jiang, et.al; 2014).

Injeksi gas pada daerah plasma juga bisa memfasilitasi produksi elektron dengan energi yang lebih tinggi yang nantinya akan menghasilkan spesi aktif radikal OH yang lebih banyak (Jiang, et al., 2014). Radikal OH yang telah terbentuk pada proses elektrolisis plasma memiliki sifat yang sangat reaktif terhadap semua spesi yang ada, sehingga dapat bereaksi dengan oksigen yang terkandung dalam udara yang diinjeksikan dan menghasilkan spesi reaktif lainnya. Gas O2 yang terdapat dalam udara juga dapat bereaksi dengan molekul H2O untuk menghasilkan O3 dan H2O2. The reaction between oxygen with hydroxyl radicals are as follow (Saksono, et.al, 2017).

From equation (1) to (4) it can be seen that the use of air injection will produce another number of reactive species besides OH, namely HO2 , O, and HNO3- radicals. Active spesi HO2 has reduction potensial 1, 06 V meanwhile O has reduction potensial 2,42 V (Augusto, et al., 2011, Ruma, 2015). From this reduction potensial, it can be seen that other reactive species formed as a result of air injection have also involved in the degradation process of pollutans. Jamroz, et.al (2017) stated that active spesies like OH and HO2 are likely to be responsible for the decolorization of organic dyes by the Fenton reactions in the plasma-reactor system using air injection.

During the process, the nitrogen content in the injectable air can form several nitrate products (NO, NO2-, NO3-), these are lead to formation of HNO2, HNO3 through reaction to OH and O. Ruma (2015) stated that only production of OH and H2O2 at no gas bubbling condition, so the removal rate of dye is lower compared to gas bubbling conditions.

Energy Consumption

The data obtained and presented in Table 1, illustrate the effect of air injection on the energy consumption. The result showed that air injection has decrease an energy consumption. When air was injected, it generated bubbles and gas channels inside liquid and in the region where electric field was dominant. These bubbles and gas channels reduced the required breakdown voltage as the dielectric strength of gas is much less then water. Therefore the required input breakdown voltage was higher in pure water medium compared to gas injected medium (Ahmed 2016).

Table 1 showed that an air injection decreased a consumption energy. It was clear that energy consumption decreased from 428.4 kJ to 415.8 kJ in minute 90 – 120. Ahmed, et.al (2016) stated that The injection of gas and generation of gas bubbles and gas channels leads to the reduction of breakdown voltage due to less dielectric constant of gas compared to liquid. Based on the decolorization data generated from the experiment, further studies were conducted in the same condition (with air injection) using RYFG and RBV dyes. The parameter test were : pH, conductivity, H2O2 remaining, persent decolorization and energy consumption. As it can be observed from Fig. 3, initial pH solution have a range 5.5 – 7.5. The pH condition of the solution supports the plasma electrolysis process because the Fenton oxidation reaction can decolorize azo dyes in neutral or acidic solutions effectively (Liu et.al, 2011).

The results showed that the pH of solution decreased along the process. pH of the dye solution of RR, RYFG and RBV were from 6.61 to 3.06, from 7.58 to 3.20 and from 5.76 to 4.71, respectively (Fig. 3). It was probably due to the formation of organic and inorganic acids as degradation products of dyes. The presence of nitrogen in the gas injection will produce some nitrate products (NO, NO2-, NO3-) which will form HNO2, HNO3 by reaction with OH and O radicals (Ruma, et.al, 2015). There were a significant drop of the solution pH in RR dye from 6.61 to 3.06 and RYFG from 7.58 to 3.20. It was probably due to the formation of organic and inorganic acids as degradation products of dyes. It was reported that low molecular weight organic acids, such as oxalic, acetic, formic, maleic, malonic, fumaric and succinic acid, are formed during degradation of dyes (Mitrovic, J, 2012). In neutral and weak acidic medium, the effect of these degradation products on the solution pH was more prominent. Meanwhile, in strong basic medium, weak organic acids were not formed in sufficient amounts for neutralization of the base (Mitrovic, J, et.al; 2012). Among the dye solutions, the pH of RBV solution has decreased slightly. It was due to the H2O2 remaining in the solution more less than another dyes, which in H2O2 will accumulate together with organic acids resulting in an acidic solution (Table 2).

Conductivity

Electrolyte solution plays an important role in the CGDE process, so sodium sulfate was added to the dye solution. Sodium sulfate is an inorganic sodium salt, it can disassociates in water to provide sodium ions and sulfate ions. These ions can conduct electricity through CGDE process. The conductivity of the dye solution were found to increase during the process (Fig. 4). At the end of the process, conductivity of the dye solution containing RR, RYFG and RBV were 4.32 μS cm–1, 4.83 μS cm–1, and 4.64 μS cm–1, respectively. This result is consistent with those of previous studies, in which the increase of conductivity with time can also be associated with the formation of the acid products in solution as well as with the formation of other mineralization products, such as NH4 +, NO3 –, NO2 – and SO4 2– (Mitrovic, J, et.al; 2012).

Decolorization of RR, RYFG and RBV

Figure 5 showed the graph of percentage decolorization dyes vs time (10 – 180 min). The result revealed that the rates of decolorization RBV was higher at the beginning (10-60 min) and then slowly increase from 90 to 180 minute. On the whole process, the removal color of RBV was maximum, followed by RR and RYFG. This result similar with previous study by khataee (2010). The chemical structure of dyes play role in the reactivity of these dye to the decolorization process through fenton-reaction.

Throughout the CGDE process, the present of Fe2+ ions in the solutions will capture H2O2 and produces hydroxyl radical. Furtherly, OH radicals may favor the oxidative cleavage of chromophores groups in the dye structure (-N=N-, or –C=C-), and through the sites near chromophores (-C-N=N). The reactivity depend on the structure of dye, especially the substituent around the chromophores groups. RBV have two hydroxyl group, one amide groups (-NHCOCH3), two sulfonic groups. Two hydroxyl groups and amide group directly connected to the aromatic rings (benzene and naphthalene) conjugated with azo bond which give an electron withdrawing inductive effect (-I effect) and an electron releasing group (+M resonance effect) to the structure. It can intensify the resonance and concequently rise the percentage of decolorization. RR has -OH and –OCH3 substituens connected with unsaturated systems in dye structure. Both of it influence the rate of decolorization through –I and +M effect. RYFG that contain two sulfonic substituents which give –I and an electron withdrawing group (-M resonance effect). The presence of the more powerful electron withdrawing sulfonic group on a molecule makes it only very slightly less sensitive to oxidation (Khatee, 2010). RYFG do not have substituent with –I and +M effect such us OH, OCH3 and –NHCOCH3 making it less degradable. RBV, RR, and RYFG have –SO2-CH2- group which is labile in the reaction environment. After the cleavage of azo bond, the second step mechanism for mineralizing azo dye molecules form the organic byproducts of aromatic ring opening reactions.

Conclusion

This research was successfully to decolorize dye solution of Remazol Red RR, Remazol Yellow FG and Remazol Brilliant Blue using CGDE methode. An air injection has increased persent decolorization of RR up to 46.7% and decreased energy consumption. Remazol brilliant violet was found to be more efficient process than Remazol Red RR and Remazol Yellow FG for decolorization with maximum decolorization of 95.70%, 70.36% and 51.26% respectively and process efficiency of 8,820 kJ/mmol, 11,520 kJ/mmol and 24,412.5 kJ/mmol respectively.