Optimization of parameters of electrocoagulation/ ﬂ otation process for removal of Acid Red 14 with mesh stainless steel electrodes

Dyes are persistent compounds that are not easily biodegraded and are considered as carcinogenic. Electro-coagulation and electro- ﬂ otation method, due to its adaptability and compatibility with the environment, is regarded as one of the appropriate methods for the treatment of industrial wastewater containing dye. In this study in which stainless steel mesh electrodes with a horizontal arrangement are used, the most important parameters affecting the performance of the simultaneous system of electro-coagulation and electro- ﬂ otation, including electrodes area, of distance between electrodes, electrical conductivity of the solution, type of electrolyte, and initial pH were examined. The effect of every one of these parameters in color removal ef ﬁ ciency of Acid Red 14 from arti ﬁ cial wastewater, energy consumption and anode was determined and their values were optimized. The area of the electrode equals 20.5 cm 2 , the distance between the electrodes is 0.5 cm, electrical conductivity 3,600 μ S/cm, and initial pH 7 were selected as the optimum values, and dye removal ef ﬁ ciency of 99% with initial concentration of 150 mg/L and electric current density 40 mA/cm 2 (0.8 A) were obtained under optimum conditions and within 20 minutes. The advantages of this method are low energy and material consumption, and low sludge production.


INTRODUCTION
Various amounts of different chemical dyes from different industrial applications including dyeing were utilized. Artificial dyes are regarded as pollutants of nature that enter into the environment with industrial waste and ultimately can cause contamination of natural ecosystems such as soil, surface waters, ground waters and living creatures. When colored wastewaters are discharged into the environment without treatment, they can affect aquatic ecosystems in different ways. The existence of a colored substance in water decreases the light penetration to the lower layers and so decreases the photosynthesis of plants which renders the water toxic resulting in the mortality of aquatic creatures and finally the rivers and streams that the wastewaters flow into, Chemical coagulation is efficient for sulfurous and disperse dyes. Acidic, direct, vat and reactive dyes coagulate with this method but do not settle, while cationic dyes do not even coagulate (Can et al. ).
The electrochemical method is a better treatment method with high efficiency for treating textile wastewaters which contain a high concentration of dye. This method has advantages over others for decolorization, such as the need for simple equipment, higher performance, and shorter retention time to remove contaminants, easier operation, and less need for chemicals (Yildiz ).
Electro-coagulation produces coagulating substances in situ using electrical decomposition of aluminum (Al) or iron (Fe) electrodes. Fe ions that are added electrically to water according to reaction (1), are much more active than Fe ions that are added chemically. Where there is wastewater between the positive anode and negative cathode, an electric field is established as a result of the solution's electrical conductivity.
By electrolysis of water, fine bubbles of oxygen and hydrogen are produced according to reaction (2) and move upward and form a layer in the surface. Bubbles bring the suspended particles and oil to the surface and a sludge layer is produced that is mechanically collected. Therefore, free atomic oxygen is formed in the anode diffusion layer and enters the wastewater by convection and oxidizes organic and inorganic substances. In a similar trend, a change occurs in motivated electrical hydrogen that leads to the rehabilitation reaction of wastewater contents. According to reaction (3), alkalinity is produced in the form of OH À at the cathode during the electrolysis. Gases producing oxygen and hydrogen (according to reaction 4) are very active, and when they attack the surface of the solids, change their buoyancy properties. These changes are called electrochemical effects which do not exist in other buoyancy (Matis & Peleka ). Chemical reactions occurring at the anode and cathode are shown below in reactions (1)-(4) (Khandegar & Saroha ): (1) 8H þ þ 8e ! 4H 2 (4) and in general (reactions (5) and (6)): Electro-coagulation has been utilized successfully in wastewater treatment of different industries such as plating

Materials and equipment
According to Figure   wastewater standards (APHA ) were conducted three times with repeatability and error percentage of 95% and 5%, respectively, and in situ (23 ± 2 W C).

Method
In order to determine the optimum effective area of parameters using single factor analysis (OFAT), synthetic wastewater was prepared with the desired characteristics and the magnetic stirrer was used to mix the solution in order to make the necessary mixing to the extent that does not create turbulence. By connecting the electrodes to the power supply, the electric current was kept constant at determined values and the required voltage was registered at the time of sampling and tests were conducted at laboratory temperature. According to the Beer Lambert law, with measuring the absorbance of samples at specified intervals at the maximum absorption wavelength of the dye (515 nm) in a spectrophotometer, dye concentration and removal rate were calculated based on Equation (7): where DR is dye removal efficiency, C 0 dye initial concentration, C the sample concentration in terms of mg/ L. It is worth mentioning that samples were centrifuged before measuring in a spectrophotometer in order to eliminate the error of existing flotation in solution.
In the electrochemical process, energy consumption is highly important due to the presence of electricity as a source of energy. This energy that can have a high effect on justifiability of the application process was calculated via Equation (8) in which SEC is specific energy consumption (kWh/kg dye removed ), U is voltage (V), I is electric current (A), t is electrolysis time (hr), V is wastewater volume (L), C 0 and C are initial and instant dye concentration, respectively (g/L) (Khandegar & Saroha ).
According to Equation (9), anode consumption (Kg), anode dissolution (Kg), dye removal using Equation (10)   In the present study, specific energy consumption (kWh/kg dye removed ) and anode consumption (Steel/Kg dye removed ) were examined as a criterion for better economic and environmental comparison. Figure 2

Distance between the electrodes
In order to determine the optimum distance between electrodes, the experiments were conducted in four electrodes of distances 0.25, 0.5, 1 and 2 cm based on the initial experiments, other parameters are considered constant. The obtained results are given in Figure 3.
As can be seen in Figure     As can be seen in Figure 4   electrodes. For example, 99% dye removal for NaCl, Na 2 SO 4, and MgSO 4 was observed after a duration of 20, 50 and 60 minutes, respectively.
The specific energy consumption using three different electrolytes in Figure 5(b) in terms of the time and Figure   5(c) independent of time is shown. As can be observed in the figure, energy consumption using NaCl is less than Na 2 SO 4 and MgSO 4 and is equal to 5, 13.5 and 21.5, respectively, for a dye removal efficiency of 99%. The required voltage for supplying fixed electrical current using NaCl is less and thus, total energy consumption is lower.
Electrical conductivity of the water sample can be obtained using Equation (11) as a good approximation: where EC is electrical conductivity (μS/cm), C i the ion concentration (mg/L) and f i factor of electrical conductivity for ions (Schröer & Weingärtner ). In the conducted experiments, the calculation of electrical conductivity created by different electrolytes is as according to Table 2. As can be seen, the higher voltage needed for supplying a fixed electric current using NaCl as electrolyte based on the conducted calculation is justifiable.
The electrical conductivity created by NaCl in solution is higher than the two other electrolytes at equal concentrations. Greater electrical conductivity means higher electrical conductivity of the solution, and for creating a fixed current conductivity, a higher voltage is needed. Therefore, for creating a fixed current conductivity, less NaCl is needed than other electrolytes and because NaCl is cheaper, the cost of treatment will be lower. In justifying higher electrical conductivity created by MgSO 4 to NaCl, it can be said that electric current is transferred both by cations and anions but the degree of conductivity is different. The created electrical conductivity by multivalent cations is higher than univalent cations. But this is not the case for anions (Gray ).
The amount of anode consumption using different electrolytes is shown in Figure 5(d). Anode dissolution rate using NaCl is lower due to the quicker trend of dye removal than other electrolytes, i.e., NaCl, Na 2 SO 4, and MgSO 4 was 5, 7 and 7.9 kg Fe/kg dye removed , respectively, obtained at dye removal efficiency of 99%.
Passing current in an aquatic environment like wastewater occurs due to the movement of ions towards the electrode with negative load. NaCl has a higher ionization speed and mobility due to the lower radiuses of Na and Cl than other ions like potassium, carbonate or nitrate and as a result, more current passes through wastewater and by increasing passing current, the speed of anode dissolution increases. On the other hand, producing acidic species such as HCl and ClO À enhances the desirability of revival conditions (Golder et al. ). Therefore, using NaCl as electrolyte has the advantage of lower price. Also, textile and dyeing industries use plenty of NaCl and wastewaters of these industries include ions of this salt. Cl À ions decrease the negative effect of other ions such as HCO 3 À and SO 4 2À .
The existence of carbonate causes the formation of intruder precipitation of Ca 2þ and Mg 2þ on electrodes. These precipitations increase the electrical resistance of system and energy consumption and disturb the treatment trend (Parsa et al. ). Chen () has suggested the existence of Cl À at the rate of 20% from all ions to ensure the natural electrical coagulation process.

Initial pH
Experiments at different initial pHs have been conducted in order to determine the optimum amount of this parameter in optimized conditions at the electrode surface, distances between electrodes, electrical conductivity and the type of electrolyte and the results obtained are presented in Figure 6.
At an initial pH of 3, dye process was completed within 5 minutes with high efficiency. Following this process becomes slow and because in order to measure the dye concentration, the samples first have to be treated then  . Therefore, no difference in significant efficiency in pH 5-9 according to Figure 6 can be explained by the above-mentioned reasons.
In order to determine the optimum conditions of initial pH and an economic comparison between them, the amount of specific energy and anode consumption are shown in Figure 6(a) and 6(b). As can be seen, for obtaining a dye removal of more than 90%, specific energy consumption increased from an initial pH of 3, decreased for an initial pH of 5, 7 and 9, and increased again for an initial pH of 11. Specific energy consumption was 6.1, 4.9, 5, 5.25, and 6.6 kWh/kg dye removed for 99% dye removal with these mentioned initial pH, respectively. A similar method was observed for anode consumption, with results of 4.4, 3.4, 3.6, 3.85 and 4.9 kFe/kg dye removed for 99% removal efficiency, respectively.
In Figure 6(d), separated sludge TSS is shown for different initial pH. As can be seen, the amounts related to TSS for initial pH of 3, 5, 7, 9 and 11 were 6,300, 21,400, 18,000, 143,000 and 8,900, respectively According to the results of Figure 6, the general performance of dye treatment system with initial pH ¼ 5 was better than at other pH. However, due to the proximity of values of the four parameters of removal efficiency, energy and anode consumption and sludge TSS between pH 5 and 7, and because there is no need to add chemicals for adjusting the solution pH containing Acid Red 14 which equals to 7, an initial pH ¼ 7 was selected as optimum for this study.
Tezcan Un & Aytac () did not observe much difference in the dye removal efficiency at different pH values.
Only in the first 10 minutes of electro-coagulation with an initial pH of 3 was dye removal higher than with an initial pH of 5 and 9, so by continuing the process within 90 min-  These compounds are lighter than the initial dye, each were created by separation of 1-2-sulfur trioxides, sodium, nitrogen, and or a naphthalene ring in the structure of the dye or separating of a mixture of these.
The second sample was taken within 20 minutes after the onset of the reaction for dye removal, efficiency of 99% had been recorded and LC-mass experiment was conducted. In Figure 7 and [M-C l0 H 6 -2Na-2SO 3 þ H], respectively. As can be seen in figure, after 20 min from the start of the reaction, there is no mother peak of Acid Red 14 and this is a confirmation of dye removal efficiency within 20 min in optimal conditions. Also, two peaks at 268.5 and 221.7 m/z are seen in this figure that exist in the previous figure with the difference that the former is increased and the latter is decreased.
The third sample for mass analysis was taken 120 min after the onset of reaction as observed in Figure 7(c). As can be seen in the figure, the predominant peak in this figure, just as in the two previous figures, is related to a compound with molecular weight of 221.7 m/z, and as mentioned before its molecule formula can be guessed as [M-C l0 H 6 -2Na-SO 3 -2N-H]. This compound, which is considered to be the structure of Acid Red 14, is the derivative of α-naphtole or 1-naphtole that existed in the analysis of the first sample in Figure 7(a). This compound is recognized as an environmentally hazardous compound and harmful for the aquatic environment but the degree of its toxicity is less than Acid Red 14. In this figure, in addition to a peak related to α-naphtole, there are also two other peaks.
One with a molecular mass of 172.8 m/z that was shown in

CONCLUSION
Produced oxygen is trapped under the anode and continuing production produces more bubbles and big bubbles. By increasing electrode surface, due to the decrease in electrical resistance of system, the required voltage for obtaining a fixed electric current declined and consequently, total energy consumption decreased. Therefore, from the three electrode surfaces, the intermediate surface (neither maximum nor minimum) was selected as optimum.
On increasing the distance between electrodes (from 0.5 to 2 cm), due to the delay in constituting coagulating materials with regard to the dependency of this action on mobilization and transfer of materials and produced ions in the electrodes, dye removal efficiency showed a decrease to time. Also, on decreasing the distance between electrodes, due to the decrease in electrical resistance of system, the required voltage for obtaining a fixed electric current declined and consequently, total energy consumption The performance of the system did not show much dependency on initial pH and environment. In acidic pH, the number of bubbles is increased but their size gets bigger which decreases their ability for flotation of tiny coagulations. In basic pH, the amount of bubbles is decreased but their size gets smaller. In pH 5-9 electro-coagulation and forming coagulations is done well, electro-flotation had good performance and high dye removal efficiency was obtained for the three pHs. Therefore, selection of pH ¼ 7 as an optimal pH was adopted due to economic and environmental parameters.
In conclusion, the results of this research with 99% of dye removal within 20 min, specific energy and anode consumption was 5 kWh/kg dye removed and 3.6 kg Fe/kg dye removed and sludge TSS 18,700 mg/L provide an electrocoagulation and electro-floatation method for dye-containing wastewater treatment with low material and energy consumption.
Using stainless steel mesh electrodes with a horizontal arrangement that emphasizes electro-coagulation properties by creating more surface for producing more bubbles of smaller sizes could be one reason for the higher removal efficiency with lower energy consumption and lower anode dissolution in comparison with the similar studies (particularly Hooshmandfar et al. ), as it simultaneously uses both properties of electro-coagulation and electro-floatation.
Also, the produced sludge causes a decrease in treatment costs and sludge disposal and its related problems. Consequently, using this method is considered to be a good option for constituting with usual methods of treatment like chemical coagulation and flocculation and/or pre-treatment before complementary treatment.