Electrochemical degradation of scarlet red dye from aqueous environment by titanium-based dimensionally stable anodes with SS electrodes

Textile effluents are toxic and carcinogenic materials that exist in the aquatic environment. In this study, the degradation efficiency of commercially available scarlet red dye investigated on TSA-SS Electro Fenton process (EFP) was reported. It is of great interest in the field of environmental engineering to remove dyes from aquatic environment. The influence of operating parameters such as pH (2–9), current density (0.1–0.5 mA/cm2), concentration of dye (0.1–0.5 g/L), H2O2 (0.1–0.5 g/L) concentration and Fe2+ concentration (0.01–0.03 g/L) were analyzed by batch system. The optimum degradation conditions were determined as pH—3, current density—0.4 mA/cm2, concentration of dye—0.4 g/L, H2O2 concentration—0.5 g/L and Fe2+ concentration—0.025 g/L. These results indicated that the degradation efficiency of scarlet red dye by EFP depends on solution pH and Fenton reagent concentration and a low pH value was favorable for the dye degradation. It has been demonstrated that more than 94% dye removal was obtained at 50 min. Electro Fenton process was also investigated by cyclic voltammetry technologies.


Introduction
The pot ability of water is affected by various factors which include wastewater from industries. Various industries such as textiles, rubber, plastics, paper, cosmetics and tanneries use various dyeing process. They release waste water which contains dye effluents in a major proportion. Among them, textile industries generate large volume of effluents which contains organic dye effluents. Various processes were adopted to treat wastewater containing organic contaminants, particularly electrocoagulation, ultra-filtration, reverse osmosis (RO) and activated carbon adsorption. These treatment processes were not much effective to improve the quality of water with tolerable levels of effluents. To attain this objective, a treatment method is applied which entail the application of advanced oxidation process (AOP) [1,2]. This method is recommended when wastewaters have high chemical stability, low biodegradability and this produces the complete mineralization of pollutants to CO 2 , H 2 O and inorganic compounds [3]. This AOP is attained when EFP is carried out. This EFP could be applied for various types of contaminants especially organic substances [4]. In EFP, H 2 O 2 serves as the source of OH radicals. So, with increase in H 2 O 2 concentration degradation takes place effectively. These hydroxyl radicals act as redox agent with high redox potential. But if H 2 O 2 concentration exceeds the limit, it will affect the process by production of per hydroxyl radicals [5] as shown in Eq. (1).
Also Fe 2? concentration helps in increase in efficiency by production of OH°radicals from H 2 O 2 . But after a particular concentration Fe 2? ions will have scavenging effect on OH°radicals [6] as shown in Eq. (2).
This will reduce the efficiency of the process. So an optimum of Fe 2? concentration had to be maintained for effective degradation of the solution. The pH was considered to be one of the important factors which help in effective degradation [7]. At higher pH, complex formation of Fe 2? ions will take place thereby reducing the process efficiency. In this study, the influence of various parameters had studied.

Physicochemical properties
In this work, commercially used dye scarlet red was purchased from Erode textile industry and the properties of scarlet red dye are given in Electrolysis was performed in a 1000-ml capacity cylindrical beaker with two electrodes. Titanium-based dimensionally stable anodes (DSA) and stainless steel (SS) were used as electrodes for anode and cathode, respectively. Area of each electrode was 50 cm 2 [8]. The synthetic dye solution of various concentrations was taken, and the pH was adjusted by adding 0.1 mM HCl solution. After that, various proportions of Fenton's reagents were added. The electrodes were partially immersed in a beaker that contains the synthetic solution 0.1 mM KCl chosen as the supporting electrolyte to maintain the current density [9]. The required electric power during the electrolysis process was provided by a laboratory DC power supply (Aplab 0.32 V, 0-10 A). The electrolytic cell was constantly stirred with a magnetic stirrer in order to avoid the concentration gradient.

Electrochemical measurements
Cyclic voltammetry (CV) measurements were carried out using a potentiostat/galvanostat EG&G Model 263A and a frequency response detector FRD 100 of Princeton Applied Research. The working electrode was the glassy carbon, the counter electrode was a platinum, and a reference electrode was silver electrode. All the experiments were carried out at room temperature.

Influence of pH
In this process, pH is an important parameter and it affects the hydroxyl radical mechanism of EFP. The pH was varied within the range of 2-9. The maximum degradation efficiency was obtained at pH 3 as illustrated in Fig. 1. Therefore, acidic pH favors the degradation when compared to the basic pH levels [10]. Because when the pH is increased Fe 2? ions form insoluble complexes which cannot activate H 2 O 2 to produce OH radicals. They finally precipitate as Fe (OH) 3 which cannot act as catalyst [11]. So degradation efficiency decreases with increase in pH. The previous results obtained [12] were well matched with the current data.  improvement as shown in Eq. (3) [13]. The Fe 2? concentration was varied between 0.01 and 0.03 g/L in the interval of 0.05 g/L.

Influence of Fe 21 concentration
The optimum Fe 2? dosage with maximum degradation efficiency was found to be 0.025 g/L. After that point degradation efficiency starts to decrease. This was because of the scavenging effect of Fe 2? ions which convert OH radicals to OHions [14].

Influence of H 2 O 2 concentration
H 2 O 2 reagent provides the hydroxyl radicals for carrying out the mineralization mechanism that produces the sludge [8,15]. The concentration of hydroxyl radicals was dependent on the addition of H 2 O 2 concentration [16], and it influences the decomposition of complex organic compounds by Fenton's reaction. Due to the high cost of H 2 O 2 selection of an optimum H 2 O 2 concentration by Fenton's reagent is important for dye degradation [17]. The degradation increases with increase in H 2 O 2 concentration varied at a range of 0.1-0.5 g/L and was the maximum efficiency obtained at 0.5 g/L illustrated in Fig. 3. However, the increase of H 2 O 2 further results in the decrease in dye removal process because surplus H 2 O 2 molecules act as a scavenger of hydroxyl radical to formation of perhydroxyl radical with low oxidation potential [15].

Influence of dye concentration
Dye concentration was varied over the range of 0.1-0.5 g/ L. The optimum dye concentration was found to be 0.4 g/L for which maximum degradation was obtained. Figure 4 illustrates the influence of dye concentration in EFP process at lower concentration, the lesser degradation of dye was related to the low frequency of effective collisions between molecules and surplus Fe 2? molecules causing scavenging effect [18]. But also with higher concentration of dye, efficiency of the process decreases due to the intermediate components that react with hydroxyl radicals instead of dye components [19]. Also, the reactive species produced was not enough for effective degradation of dyes at higher concentration. So when the concentration of dye increases, the amount of Fenton's reagent added to the solution had to be increased. Figure 4 illustrates the

Influence of current density
In all oxidation mechanism of electrochemical method current density was considered to be the important parameter. The influence of current density had been examined in the range of 0.1-0.5 mA/cm 2 optimum of 0.4 mA/cm 2 , and the results are illustrated in Fig. 5. The data from the graph show that there is a gradual increase in the degradation rate with increase in current. The increase in current density enhances the oxidation power which in turn helps in the greater production of H 2 O 2 and leads to the generation of more amounts of hydroxyl radicals from Fenton's reagent [20]. At higher current density, the degradation efficiency was attained almost constant due to the formation of intermediates as complexes of Fe 2? with carboxylic acids that were more difficult to destroy during electrolysis [21].

Mechanism of degradation of azo dyes
In EFP the degradation of o'scarlet red dye was investigated in acidic pH of 3. Fe 2? ions were used as catalyst, and H 2 O 2 acts as a reagent. As the current density was intensified the degradation of dye also progressed gradually. The hydroxyl radicals produced by H 2 O 2 help in this process. As the color loss becomes visible it could be explained by reaction of OH radicals with azo bonds (-N=N-) [22]. They break the azo bond which causes the decolorization due to change in absorption properties. From the removal efficiency curves, we can infer that color intensity decreases as time increases during the reaction. The reaction between hydroxyl radicals and azo bonds produces hydroxylated derivatives [23]. The scarlet dye compound disappeared by producing intermediates like hydroxyl derivatives and other iron complexes. Figure 6 illustrates the cyclic voltammetry (CV) scan 0.4 g/L dye solution at pH 3.0; scan rate was 30 mV/s. It was clearly visible that the Oxygen Reduction Reaction (ORR) currents at the cathode potential higher than -0.9 V. As the operating cathode potential of H 2 O 2 -producing dye solution was changed from -0.9 to -0.7 V and cathode potentials appeared almost the same over the whole testing period, it was concluded that the presence of scarlet red dye had a slight impact on the ORR process

Discussion
Electro Fenton processes have been proved to be a feasible technology for the treatment of dyes from synthetic solutions. In this study, the degradation of scarlet red dye was investigated by EFP using TSA-SS electrodes. The influence of operating parameters such as pH (2-9), current density (0.1-0.5 mA/cm 2 ), concentration of dye (0.1-0.5 g/L), concentration of H 2 O 2 (0.1-0.5 g/L) and concentration of Fe 2? (0.01-0.03 g/L) was analyzed in a batch system. The optimum solution pH for EFP for the scarlet red dye was obtained at 3. The optimum concentration of H 2 O 2 , Fe 2? and dye concentration was 0.5, 0.025 and 0.4 g/L with the current density of 0.4 mA/cm 2 , respectively. At the optimal conditions, 94% of scarlet red was degraded from the solution at 50 min. The conducted cyclic voltammetry studies confirm that, an impact was occurred over the surface of TSA-SS Electro Fenton process by the presence of scarlet red dye.