Electrochemical Treatment of Textile Wastewater Containing Acid Red 14 by Aluminium Electrodes

 

1.    Introduction

Textile industry has a significant impact on the environment because large amount of water and chemicals is used in various processes of this industry such as sizing, scouring, washing, bleaching, dyeing, printing and finishing that leads to production of hazardous wastewater. This wastewater contains dying substances which remain visible even at low concentrations. Water clarity and dissolved oxygen decreases in the presence of even a small amount of dye. Azo dyes are considered to be carcinogenic that pollute groundwater and surface water (Khandegar and Saroha, 2013; Morshed et al., 2012; Merzouk et al., 2009).
Dye removal by different physical, chemical and biological methods or a combination of them could be possible. Physical methods such as adsorption, membrane filtration and ultrasonic waves, chemical methods like ion exchange, electrolysis, coagulation, flocculation, conventional and advanced oxidation and biological methods by algae, fungi and bacteria have been mentioned in the literature (Wang et al., 2014; Oliveira and Airoldi, 2014; Pajootan et al., 2012).
Recently, electrochemical method is considered as a convenient method for purification of industrial wastewater due to its versatility and adaptability to the environment. This method has advantages ,for decolorization, such as simple operation, high performance and short retention time for removal of pollutants, and requires less chemicals (Yildiz, 2008; Yuksel et al., 2011). Electrocoagulation is a method in which coagulants are dissolved into solution from anode electrode (Fe or Al) by applying electric current. Besides, by electrolysis of water, hydrogen bubbles are generated at cathode. This tiny bubbles move upward collide with flocs and form a sludge blanket on the surface. Furthermore, these bubbles are very active and can alter surface and buoyancy properties of the solids. These changes are known to be electrochemical effects that do not exist in the other flotation techniques (Matis and Peleka, 2010; Parsa et al., 2011).
Electrocoagulation has been used successfully for treatment of various industrial wastewaters such as plating (Adhoum et al., 2004), chemical and mechanical polishing (Drouiche al., 2007), textile (Khandegar and Saroha, 2013; Wei et al., 2012; Pajootan et al., 2012; Yuksel et al., 2011), olive oil (Tezcan et al., 2006), laundry (Wang et al., 2009), tannery (Feng et al., 2007), dairy (Şengil, 2006), pulp and paper (Khansorthong and Hunsom, 2009), and oil refinement (El-Naas et al., 2009).
For instance, Khandegar and Saroha (2013) treated textile wastewater by electrochemical method. Under Optimum conditions, with dye concentration of 10 mg/L Acid Red 131 and using aluminum electrodes, dye removal efficiency of 98% was obtained. Pajootan et al. (2012) studied removal of Acid Black 52 and Acid Yellow 220 from wastewater by electrocoagulation. With dye concentration of 200 mg/L and aluminum electrodes removal efficiency was 90 and 98%, respectively for the mentioned dyes.
The aim of this study was to assess the simultaneous performance of electrocoagulation and electroflotation techniques in an electrochemical system by using aluminium electrodes for removal of Acid Red 14 from aqueous solution. It was expected that the need for a gravity settling unit and as a consequence treatment cost would be reduced. In this study, the design of the reactor in a manner intended to take advantage of electrocoagulation and electroflotation methods simultaneously. Effect of four important parameters on the performance of electrochemical systems, including electrode surface area, interelectrode distance, electrical conductivity and current density was studied. The optimum values of these parameters were determined based on the amount of electrical energy and aluminium consumption and the best performance of coagulation and bubble generation.
 

2.        Materials and Methods

In this study, electrochemical process was developed at room temperature in a 5 L rectangular plexiglass cubic reactor which included two pure aluminium electrodes with monopolar and horizontal arrangment and a PM-3005D Megatek power supply. Considering that the generated hydrogen gas at cathode plays the main role of floating suspended particles, the cathode was placed above the anode. Before each experiment, the electrodes were sanded and then washed by diluted acidic solution and distilled water. The experiments were performed in batch mode.
An Anionic dye, Acid Red 14, as the main pollutant with structural formula of C₂₀H₁₂N₂Na₂O₇S₂, containing an azo group and molecular weight of 502.4 was used to prepare the synthetic wastewater. The concentration of dye in solution was measured by using a Hach DR-4000 spectrophotometer at the wavelength of maximum absorption of the dye (λ_max=515 nm). The other equipments used in this study included a Mettler PJ300 digital scale with accuracy of 0.001, Metrohm 691 pH meter, a Martini MI805 EC meter and an IKA RH-Bassic2 magnetic stirrer. NaCl (Merck) was used to obtain electrical conductivity in solution and synthetic wastewater was prepared with double distilled water. All measurements including dye concentration, EC and solids were according to water and wastewater standard methods (APHA, 2012).
Parameters including electrode surface area (24.86, 52.86, 80.86 cm²), interelectrode distance (1, 1.5, 2 cm), conductivity (800, 1600, 3000, 4000, 5000 µS/cm) and current density (10, 20, 30, 40, 50, 60 mA/cm²) were examined. Specific energy and aluminium consumption were calculated in terms of (kWh/kg dye _removed) and (kg Al/kg dye _removed), respectively. These two responses and TSS of the separated sludge were the basis for determination of parameters optimum value.
 

3.    Results & Discussion

3.1.  Effect of electrode surface area
Experiments were carried out at electrode surface area of 24.86, 52.86, and 80.86 cm². Other parameters were kept constant. By increasing electrode surface, generated oxygen bubbles were trapped under anode and continued to stick together and form large bubbles. When these large bubbles were released from under the anode, they collided with hydrogen bubbles on their path upward and form larger bubbles that were not capable to floate the existing flocs. By increasing the electrode surface, due to lower electrical resistance of the system, the voltage required to achieve a constant electric current was reduced. It have to be noted that the lower anode dissolution results less production of sludge, hence lower disposal management costs. So the electrode surface of 24.86 cm² with dye removal efficiency of 99% within less than 120 minutes, specific energy consumption of 193 kWh/kg dye _removed, anode dissolution of 3.908 kg Al/kg dye _removed and sludge TSS of 15050 mg/L was selected as the optimum value. Compared with conventional gravity settling tanks, this system has higher sludge TSS values.
 
3.2.  Effect of interelectrode distance
Experiments at interelectrode distances of 1, 1.5, 2 cm were carried out to assess the influence of this parameter. By increasing the distance between the electrodes, dye removal efficiency decreased due to the delay in forming coagulants and less mobility of the ions that were produced at the electrodes. By reducing the distance between the electrodes, the voltage required to achieve a constant electric current was reduced due to the reduction of electrical resistance. Aluminium dissolution with different interelectrode distances had close values; so interelectrode distance of 1 cm was selected as the optimum value for the following experiments.
 
3.3.  Effect of electrical conductivity
The electrical resistance decreases with increasing conductivity of the solution. Typically, the voltage required to achieve constant electric current decreases. Salts and ions are used to provide electrical conductivity. Deposition and corrosion caused by these ions on the electrodes make problems in the process, increase the electrical resistance and impose additional costs. It was observed that by enhancing the electrical conductivity and needing more time to completely separate the pollutant, the amount of aluminum in the separated sludge increased, taking into account turning of the sludge more into gray. The increasing of aluminum flocs that contain water, in the sludge causes the TSS to decrease. So the electrical conductivity of 1600 µS/cm with dye removal efficiency of 90% within less than 90 minutes, specific energy consumption of 130 kWh/kg dye _removed, anode dissolution of 2.615 kg Al/kg dye _removed and sludge TSS of 15050 mg/L was selected as the optimum value.
 
3.4.  Effect of current density
The rate of dye removal by increasing the amount of current density is greater. This phenomenon is because of the higher rate of production of coagulants and gases with increasing current density, which leads to faster coagulation, flocculation and separation of contaminants (Zodi et al., 2013). The lower current density leads to lower bubble generation. According to the observations, at low current density, due to the low volume of produced gases, the sludge was not floated well and after floatation it returned into the wastewater and the process of pollutant removal was more dependent on continuous separation of sludge. Thus at low current density, the efficiency of the process will be low in high pollutant concentration and load shock (Kobya et al., 2006). Optimum system performance was achieved at current density of 60 mA/cm².
 

4.      Conclusion

This paper has considered the electrochemical treatment of an azo dye (acid red 14) with electrocoagulation and electrofloatation simultaneous processes. The experimental results showed that electrocoagulation and electrofloatation system has good performance for rapid removal of dye, so this system can be used for treatment or pre-treatment of wastewater containing toxic and non-biodegradable materials, especially textile effluents. The process can easily be controlled and equipments are safe. Tiny bubbles of the same size are generated. There are few needs to add chemicals. Also, good efficiency in hydraulic, organic and toxic shocks, reduction in the number of process units and in consequence decrease in the required area for treatment plant, and lower operation cost are the other advantages of this technique.
The effects of electrode surface area, interelectrode distance, electrical conductivity and current density were investigated. From the obtained results, after 90 min of electrolysis, 90% dye removal was achieved under optimum condition of electrode surface area=24.86 cm², interelectrode distance=1 cm, electrical conductivity=1600 µS/cm and current density=60 mA/cm² with specific energy consumption=130 kWh/kg dye _removed, anode dissolution=2.615 kg Al/kg dye _removed and sludge TSS=15050 mg/L.