Degradation of Remazol Brilliant Blue Using Plasma Electrolysis Method with NaCl and Fe2+ Ion Addition

Remazol Brilliant Blue is a dye waste mostly generated by the textile industry and can be very dangerous to the environment. Plasma electrolysis is a method that can produce hydroxyl radicals in large quantities in order to degrade the dye compounds. This study aims to test the ability of plasma electrolysis method to degrade Remazol Brilliant Blue wastewater using NaCl as electrolyte, with the addition of Fe 2+ ion and air injection. Before the degradation process was carried out, permanganometric test was performed to see the production of hydroxyl radicals with the variations of electrolyte concentration and voltage. The degradation process were investigated more specifically by looking at the effect of Fe 2+ ion concentration and the initial concentration of Remazol Brilliant Blue. Remazol Brilliant Blue degradation reached 98.5% in 30 minutes where the initial concentration of Remazol Brilliant Blue is 100 ppm, voltage of 750 V, NaCl concentration 0.03 M,with the addition of 40 ppm Fe 2+ ion and air injection. The results show that plasma electrolysis with NaCl as electrolyte using air injection has a good potential in degrading dye wastewater in the environment.


Introduction
The textile industry is one of Indonesia's most rapidlygrowing and prospective industries. The industry employs a huge number of people and contributes to the nation's income. However, the industry comes with several negative impacts. One of the most visible impact is the environmental pollution from liquid dye waste, which is discharged without being treated by SME (Small and Medium Enterprise). Dye waste is one of the heavy organic compounds which are difficult to degrade naturally by the microorganism. Therefore, research on methods to effectively and appropriately process dye wastewater for SMEs need to be done.
Plasma electrolysis method is a proven method that can degrade liquid waste compounds from the textile industry in terms of the percentage of degradation and processing time. Plasma electrolysis method produces reactive species of H 2 O 2 , •OH, H•, and e - [1]. These reactive species are produced due to the high potential difference of the electrodes. The degradation of textile waste can occur due to the presence of hydroxyl radicals and other reactive species that act as oxidizing agents in degrading waste.
The effectiveness of the plasma electrolysis method depends on the reactivity of the ion, electron, and radical species within the solution. Therefore, addition of electrolytes in certain amount of concentration are required to increase the solution's conductivity. However, in order for the technology to be appropriately used by the SMEs in the textile industry, a relatively cheap and easily obtainable electrolyte is needed. A potential electrolyte that can be used is NaCl, which can be obtained from table salt.
However, due to the extreme reactivity of hydroxyl radicals, the compounds can create bonds between hydroxyl radicals to form H 2 O 2 , which has a lower oxidizing ability than hydroxyl radicals [2]. Therefore, addition of Fe 2+ ions to prevent recombination of hydroxyl radicals to H 2 O 2 through the Fenton reaction [1] is required. In this research, air injection is done in the plasma electrolysis process to reduce energy consumption.
The dye waste model used in this research is Remazol Brilliant Blue, commonly found and widely used in the Indonesian textile industry. Remazol Brilliant Blue has a molecular formula C 22 H 16 N 2 Na 2 O 11 S 3 with a molecular weight of 626.53 g/mol. This research aims to evaluate the degraded liquid dye waste using the plasma electrolysis method and analyse the effects of voltage, NaCl concentration, addition of Fe 2+ , and initial concentration of waste on the degradation of Remazol Brilliant Blue.

Materials
This research used Remazol Brilliant Blue as waste, NaCl as the electrolyte solution, and Fe 2 SO 4 .7H 2 O as addition ions. The production of hydroxyl radicals were tested with the permanganometric titration using KMnO 4 , H 2 SO 4 , dan Na 2 C 2 O 4 .

Instrumentation
The main equipment in this research is a Plasma Electrolysis Reactor as seen in the instrument diagram in Figure 1. The electrical circuit consists of a transformer, a slide regulator, and a diode bridge. The reactor is equipped with an external cooled circulating system to control the temperature. A tungsten anode and a stainless-steel cathode is used. The anode is coated with quartz glass to keep the contact area constant and for air injection to reduce energy consumption.

Procedure
The research begins with the current and voltage characterization to determine where the plasma will form. The NaCl concentration was varied at 0.02 M; 0.03 M; and 0.04 M.
Measurement of hydroxyl radicals were analysed using a variety of process voltage and NaCl electrolyte concentrations. Measuring of the hydroxyl radicals formed only use an electrolyte solution to avoid mixing with the batik dye waste Remazol Brilliant Blue. The hydroxyl radicals are identified by measuring H 2 O 2 concentrations with the permanganometric titration method.
Next, the degradation of Remazol Brilliant Blue was done for 30 minutes and analysed with a UV-Vis spectrophotometer with a variation of initial waste concentrations and Fe 2+ concentrations. The amount of degradation was calculated with the following equation (equation 1): (1) C 0 = Initial concentration (ppm) C t = Final concentration (ppm) Figure 2 shows that a higher electrolyte concentration relates to a higher current. This is due to the higher number of ions in the solution becoming conductors of electricity. As the conductivity of the solution increases, it will makes the electrons to flow easier.

Fig. 2. Characterization of Volage-Current with Variation of NaCl Concentration
The higher the electric current, the easier it was for gas bubbles to form in the anode, decreasing the voltage needed to form plasma [3]. Thus, the higher the electrolyte concentration, the higher the I B and the lower the V B needed as shown in Table 1.  Figure 3 shows that hydroxyl radicals production was highest at NaCl concentration of 0.04 M. This is because conductivity is directly proportional with the concentration of electrolytes as the high number of ions become conductors.
With the increase of ions as the carriers of electrons, the possibility of electrons being excited become higher and plasma tends to form more quickly. This caused hydroxyl radicals to form more quickly [4]. Increase in conductivity caused an increase in current density and power dissipation around the anode, causing easier formation of plasma and gas sheath [5]. Figure 4 shows that at the same concentration of NaCl, radical hydroxyl production increased with an increase in operational voltage. This is because the formation of active species become faster with a higher operational voltage [6].
When the voltage increased, kinetic energy from the electrons passing through a magnetic field increases and creates heat. The heat accelerates the formation of gas sheath during the plasma electrolysis process. This caused water vapour to be excited and breaks down the water molecules, forming hydroxyl radicals.

Addition in Degradation of Remazol Brilliant Blue
The effect of adding Fe 2+ on the degradation of Remazol Brilliant Blue is shown in It can be observed that degradation without addition of Fe 2+ was not effective. This is caused by the unoptimized contact time between hydroxyl radicals and Remazol Brilliant Blue. Hydroxyl radicals, which are not stable, easily recombine with other hydroxyl radicals to form H 2 O 2 which can decrease the degradation effectiveness. The potential oxidation value of radical hydroxyl is 2.8 mV compared to 1.78 mV for H 2 O 2 [7]. Addition of Fe 2+ was done to increase the effectiveness of the degradation process. The Fe 2+ added to the plasma electrolysis process came from FeSO 4 . Addition of Fe 2+ to the system increased the effectiveness of the process because Fe 2+ can breakdown H 2 O 2 and form hydroxyl radicals through the Fenton reaction [1]. This is evidenced by the results shown in Table 2 where a 20 and 40 ppm addition of Fe 2+ significantly increased the degradation value.
A similar trend is shown on the degradation of Remazol Brilliant Blue with initial concentration of 150 ppm. From Table 3 it can be observed that without addition of Fe 2+ , the degradation process of Remazol Brilliant Blue was not effective, only reaching 55.26%. Meanwhile, the degradation process of Remazol Brilliant Blue with addition of 40 and 60 ppm of Fe 2+ increased significantly, reaching 96.15% and 98.31%, respectively.  [6]. This means that the higher number of Fe 2+ in the solution will cause more H 2 O 2 to turn into hydroxyl radicals. In turn, a higher number of hydroxyl radicals will increase the degradation rate of the dye in the liquid waste, increasing the effectivity.
However, the similar trend is not shown In Table 4 where it can be observed that addition of Fe 2+ does not have significant effects on the degradation of 250 ppm Remazol Brilliant Blue. This is caused by the high initial waste concentration and addition of Fe 2+ becomes insignificant as hydroxyl radicals can directly react with waste without recombining into H 2 O 2 . Hydroxyl radicals are powerful oxidators in degrading organic waste, but have low selectivity. This causes hydroxyl radicals to rapidly degrade waste in high concentrations of waste.