Optimization of Sono-Electrocoagulation Process for the Removal of Dye Using Central Composite Design

Sono-electrocaogulation process was successfully applied for the removal of RR120 (Reactive Red 120) in the presence of activated carbon. For this purpose, the process variables were optimized using CCD (Central Composite Design). The operational parameters of the technology were the amount of activated charcoal (0.2-2.0g), amount of NaCl electrolyte (1-10g), sonication time (30-120 min) and RR120 dye concentration (40-120 mg/L), respectively. Consequently, the 100% dye removal was determined at the optimal conditions for the process obtained by CCD. In addition, the sonication time, amount of adsorbent and dye concentrations were found the significant process factors. Furthermore, the effects of other parameters like the electric current, current efficiency, amount of NaCl, temperature and formation of coagulants were also studied in the dye removal process, separately. The FTIR (Fourier Transfer Infrared) spectrums were monitored to identify the classes of functional groups present in the dye molecule before and after the treatment. Therefore, the sono-electrocaogulation process was proved an effective technique for purifying simulated wastewater containing RR120 dye.


INTRODUCTION
have been adopted to treat wastewater to reduce water pollution. Different treatment methods include chemical coagulation, adsorption, chemical-oxidation, photolysis, photo-catalysis, electro-photocatalysis [1][2]. Among different treatment processes, the EC (Electrocoagulation) process has also been used to treat wastewater.
Furthermore, It has been applied to remove oils, colorants, metals, halides, etc. [3][4]. In the recent years, the EC process has been studied by researchers for the treatment of simulated and real wastewater of colors [5][6]. The EC process requires proper conductivity of the system and less energy consumption [7]. The process has been proved to be a precised, critical and less timeconsuming technique [8][9]. Now-a-days, the applications of ultrasonic process are gaining importance by many researchers for the treatment of raw water [10][11]. The ultrasonic waves generated with ultrasonicator are transmitted in the liquid medium. As a result, air cavities are formed: cause molecular disintegrations. Additionally, they also generate ions and radicals in the solution [12][13]. Indeed, the sonic field increases the rate of formation of coagulants, during the EC method [14][15]. By employing the ultrasonic waves with the EC process, the OH • radicals are generated in wastewater which favor the subsequent oxidation of pollutants. Furthermore, many researchers have studied that the generation of hydroxyl radicals follow the first order kinetics [16]. Moreover, oxygen evolution occurs at the aluminium anode [17][18]. The mechanism of water sonolysis is represented as follows: Dye + OH •  Side products (3) Dye + HOO •  Side products (4) Water molecules undergo decomposition reaction in the presence of sonic field. It was firstly identified by Weiss, while, the synthesis of hydrogen peroxide was analyzed by Fitzgerald [19]. The generation of hydrogen peroxide, additionally, helps the EC process in contaminants removal from wastewater [20].
Pollutants + H 2 O 2  side products (5) The diffusion-controlled processes are involved for the interactions of electrons containing water molecules, hydrogen atoms and OH • radicals. However, the chemical species like hydrated electrons and hydrogen atoms have less reaction probability with the dye molecules. Because, oxygen molecule acts as scavenger of hydrogen radicals and hydrated electrons. Consequently, these species are transformed into peroxide and superoxide radicals.
Because, the reduction potential of RR120 dye azo group is more negative about -0.35 V(versus NHE) as compared to the reduction potentials of perhydroxyl (-0.33V) and superoxide (-0.037V) radicals . Therefore, perhydroxyl and superoxide radicals jointly couldn't reduce the azo group of RR120 at pH 7. Because, azo group is strong reducing agent. Hence, it does not undergo reduction process.
Furthermore, Reactive dyes are consumed at large industrial scale [21]. So, the removal of an anionic dye (RR120) has been studied. It has the potential risk to the bio-ecosystem because of its poor biodegradability [22][23]. Additionally, optimization of experimental variables on EC was analyzed by RSM (Response Surface Methodology) [24]. The purpose of RSM is to perform fewer experiment runs to optimize the operational parameters. Factors such as the amount of adsorbent (activated carbon) (g), electrolyte dosage of NaCl (g), sonication time (min) and dye concentration (mg/L) were selected as the operational parameters. Whereas, the removal of dye from simulated wastewater was analyzed as a response [25].

Apparatus and Instruments
The sono-electrocoagulation process was performed at laboratory scale. Lab setup is shown in Fig. 1

Reagents and Material
The physiochemical properties of RR120 are shown in Table 1.

Experimental Procedure
A stock solution of artificial RR120 dye was prepared.

2.2
Response Surface Methodology

Central Composite Design
The amount of adsorbent (Activated Carbon), electrolyte quantity (NaCl), sonication time and dye concentrations (RR120) were optimized for the removal of dye using CCD.
The % removal of dye was considered to be dependent on the amount of adsorbent (Activated Carbon), the electrolyte quantity (NaCl), sonication time and dye concentration (RR120).
The symbols X i X j are showing the independent variables,  0 is the intercept of the model. The linear coefficient is  i , and the quadratic coefficients are  ii ,  ij .

Statistics Modeling
The Minitab 17 Software was used to design the experimental runs. It was also employed for calculating ANOVA (Analysis of Variance), multiple regression analysis, full interaction plots and the main interaction effects. Furthermore, the Sigma Plot 12 software was used to draw the response surface plots in order to study the effects of two variables at a time on the response.

Mechanism of the EC Process
The mechanisms of the EC process are not so simple. It needs further study to illustrate the EC mechanisms for the removal of pollutants from wastewater [26]. The mechanism of the process is represented as follows:

Mechanism
At Anode: Bulk of solution: At Cathode : Over all reaction: The removal potency of the dye depends on the amount of Al(OH) 3 generated by the oxidation of the metal electrode. Consequently, the high removal efficiency was observed for the longer time period and the maximum electric potentials. The pH of the system also favors the synchronous formation of Al(OH) 3 . While, Al 3+ and OHions are generated by the electrode reactions as shown in Equation (7)(8)(9). After that, they form various aluminium species such as Al(OH) 2+ , , Al(OH) 4 and polymeric species such as Al 6 (OH) 3+ , Al 7 (OH)

Optimization of Sono-Electrocoagulation Process for the Removal of Dye Using Central Composite Design
system. The two major interface mechanisms were being considered in the recent years. The precipitation and adsorption processes, being proposed at separate pH data. Flocculation process occurs in a less acidic media and elucidated as precipitation. While, the adsorption process is followed at high pH. At high basic media pH  9, Al(OH) 4 -ions are additionally present in the system. Al(OH) 3 has reactive surface sites naturally which favor fast adsorption of solvent impurities and accumulation of colloidal particles [27]. Furthermore, Al precipitates are generated at high pH 7. showing that -OH groups have been increased in the sludge. Which is further confirmed from its high absorption in the spectrum.

RSM to Optimize the Operative Parameters
Optimization of the sono-electrocoagulation process OOP (Optimize Operating Parameters) was carried out.

Optimization of Sono-Electrocoagulation Process for the Removal of Dye Using Central Composite Design
While A, B, C and D are representing the coded variables such as the amount of adsorbent (Activated carbon), the quantity of electrolyte (NaCl), sonication time and RR120 dye concentrations, respectively. The optimum parameters were evaluated by the RSM graphs as delineated in Fig. 3(a-c).
The sono-electrocoagulation process is enhanced gradually by increasing the sonication time and amount of electrolyte. The regular residuals for the % dye removal and frequency are represented in Fig. 4(a).
The Fig. 4(a) is showing that the utmost residuals lie within the intervals between -5 and 5. A normal probability graph for the RR120 dye removal is shown in Fig. 4 Additionally, the CCD has the significant importance. It is cleared from Fig. 4(b) that there is no significant outlier relative to a normal distribution is noticed within the plots.
By the examination of variance, it is generally more viable (and straightforward) to perform this with the residuals.
The residuals and the observation order of the data are shown in Fig. 4 Table 3

Main Interaction Plots
The simplest graphical tool (Main Effects Plot) is to see, whether the analysis pattern is statistically vital or not.
In this plot the response means are plotted against levels of operational parameters. There is no main effect: When factors levels response overlap x-axis (response mean).
In plot Fig. 6 is showing that factors levels significantly moving the target mean values.

Full Interaction Plots
The  Fig. 7 for % color removal.

Effect of Activated Carbon on Sono-ECProcess
Activated carbon has been used to study its effect on the dye removal with sono-electrocoagulation process. Because, it is an excellent adsorbent [28]. Furthermore, on the surface of activated carbon, contaminants are adsorbed. It also reduces toxic nature of Cl 2 /HOCl/OClinto non-toxic Clions [29]. The Fig. 8(a-b) represent the efficacy of sono-electrocoagulation process regarding the amount of (AC) adsorbent. It is determined by the Response surface graphs (A) that the amount of adsorbent directly proportional to the dye removal potency. From the given Fig. 8(b) represents that mutually the amount of adsorbent and electrolyte considerably affect on the percent removal of the dye. Sonication time and amount of adsorbent also enhanced the percent removal of RR120 dye.

Evaluation of Current Efficiency and Energy Cost
The current efficiency was observed to be (108-154%), with artificial RR120 dye system. HOCl The

Influences of Current on Removal Potency
The electric current not only influences on the coagulants formation, additionally, the fizz formation rate, their size and also the flocs chain. To analyze the electric current effect, a series of experiments were performed with 0.2-1.41 A at neutral pH and 4.0 g optimum value of NaCl. The percent removal was improved from 20-60%. The results are showing that the higher current means large population of electrons are available to the electrochemical cell that enhanced the removal potency of dye. However, high generation of current could increase undesirable side-responses like scrounging loss reactions leading to the depletion of hypochlorite concentration [32]. At high current the quantity of metal hydroxide coagulants formation is enhanced. Hence, the rate of removal of pollutants from wastewater is also increased [33].

Impacts of Quantity of Electrolyte on Removal Efficiency
Conductivity of the wastewater plays an important role to decrease cell's voltage and the energy utilization. It was found by researchers [33][34] that the rate of dye removal enhanced by using NaCl, because of the formation of hypochlorite. The anodic oxidation of chloride ions resulted the chlorine evolutions as shown in Equations (16)(17). In water, chlorine rapidly disproportionates and yields hypochloric acid (HClO 4 ) and hypochlorite ion (OCl -), which can rapidly oxidize organic composites by electrochemical degradation as a result of indirect oxidation [35]. To study the effect of NaCl concentration on the removal proficiency of the RR120, its amount was varied from 2.0-10.0g. Fig. 10 illustrates that the removal efficiency is depending on the supporting electrolyte dose. The concentration of chloride plays an imperative role in the electro-oxidation process [34][35]. Moreover, it also decreases the passivity of the electrode surface due to its catalytic action [36].
The formation of H 2 O 2 , which is result of the cathodic reaction of atomic oxygen and has been proposed as a supportive compound for dye removal at current of high densities [38].

Temperature Effect on EC Process
The influence of temp eratur e o n the sonoelectrocoagulation process was studied at 30 -45 o C ± 0.1 o C. The results are shown in Fig. 11(a-b), they show that at higher temperatures the removal efficiency is

CONCLUSION
The present work highlighted the remediation of textile