Analysis of effective parameters on electro Fenton system with stainless steel and graphite electrodes for removing synthetic wastewater of acid orange 7

Synthetic dyes and specially azo dyes are common pollutants found in textile and dyeing industries effluent. azo dyes are the most important class of synthetic dyes and and represent about 70% of all world dyes consumption. Textile effluent can cause considerable pollution and rise high health risk factors due to loos of 20% of dyes in process and large scale of dyes used in these industries. The characteristics of the textile wastewater are high color intensity and visibility in very low concentrations, complex chemical structures, and light resistance and hard to biodegradability, variability in pH range and above of these they have high carcinogenic and mutagenic potential.
Generally, the physical, chemical and biological methods were used for treating textile wastewater can be mentioned as electrocoagulation, adsorption, Fenton, photo-Fenton and photo-catalytic process.
In recent years, advanced oxidation technologies have been described as efficient procedures to obtain high oxidation yields from several kinds of organic compounds. These methods mineralize and converse pollutants into CO2, H2O and inorganic ions, by the action of hydroxyl radical, which acts as a nonselective and strong oxidant of organics.
Electro-Fenton is a common advanced oxidation processes which contains electrochemical production of H2O2 and Fenton process that makes each process more efficient. Its advantages are low operation cost, high potential for complete destruction and removal of organic pollutants into harmless compounds such as CO2, water and mineral salts. Electro-Fenton Process involves the reaction of a homogeneous organic contaminants with strong oxidants, H2O2 that produced by injecting air into water near the carbon electrode cathode and the iron ion as catalyst produce hydroxyl radical which eventually led to the decomposition of organic compounds.
In this study, electrochemical process was developed at ambient temperature in a 500 mL rectangular plexiglass cubic reactor which includes two electrodes, an anode made of 304 stainless steel and a graphite cathode placed 3cm from each other and a PM-3005D power supply. Air was blowing in the cathodic zone by an RS Electrical 610 air generator pump and an IKA RH-Bassic 2 magnetic stirrer was used to mix and homogenize the sample. The other equipments used in this study include a Kern PLS 360-3 digital scale with 0.001 accuracy and Metrohm 691 pH meter. The amount of dye in solution is measured by using a Hach DR-4000 spectrophotometer at a wavelength of maximum absorption of acid orange 7 (485 nm) and the calibration curve of dye concentrations, respectively.
In this study, several parameters including current intensity (0.3, 0.6, 0.9 and 1.2 A), aeration rates (0, 3.5 and 7 L/min), electrodes area (30, 60, 90 cm2), initial pH (2, 3, 6.5 and 9) and energy consumption were examined.
In order to maintain the flow of electricity in the cells, Na2SO4 (Merck) 0.01 M was used. All experiments were performed according to the method of analysis of water and wastewater.
3.1. Effect of current intensity
The influence of current intensity has been investigated in the range of 0.3 to 1.2 A., when current intensity was 0.3, dye removal efficiency was 71% after 120 min reaction. Increase of current intensity to 1.2 A could enhance dye removal efficiency to 94%, caused by increasing production of ferrous ions and hydrogen peroxide that results to enhance the production of hydroxyl radical. When the current intensity was increased further, excessive hydroxyl radicals would be consumed via following side reactions which may reduce the dye removal efficiency.
Fe2 OH0 → Fe3 OH- (1)
H2O2 OH0 → HO20 H2O (2)
OH0 → O2(g) 2H 2e- (3)
Due to the dye removal efficiency at current intensities of 0.6 and 1.2 A were approximately equal, the current intensity of 0.6 A was selected as the optimum level with lower power consumption than other cases (0.24 KWh/ppm).
3.2. Effect of air flow
Increasing the air flow rate from 0 to 3.5 L/min resulted in an increase of the acid orange 7 removal efficiency from 80 to 90 percent at 150 min. The removal efficiency remained constant when the air rate was increased to 7 L/min. The experimental results indicated that increasing air flow leads to increase hydrogen peroxide and enhance dye removal efficiency by improving production of hydroxide radicals, But further increase in air flow would lead to reduce removal efficiency by consumption of hydroxide radical with exceed hydrogen peroxide (reaction 2).
3.3. Effect of electrode surface
The results showed that when the electrode surface were 30, 60 and 90 cm2, the degradation percent of acid orange 7 after 300 min were 68, 89 and 97 percent, respectively. However by increasing time reaction, dye removal reaches to constant value. It was well known that the amounts of electro-Fenton reagents would be increased by enhancing electrode surface and result in increasing dye degradation.
3.4. Effect of initial pH Due to the direct production of hydrogen peroxide in situ, the highest dye removal efficiency was obtained at pH=2 because in this pH, H2O2 is more stable and could be produced more efficiently. Anyway, increase the initial pH lead to reduce dye removal efficiency in the first 60 minutes. Dye removal efficiency is decreased by increasing the pH to the neutral and alkaline ranges because of the formation of ferric hydroxide species, reduction in the ferrous ions reproduction and reduction in hydrogen peroxide generation. The results show that the dye removal were 76, 64, 62, and 55 percent, with initial pH of 2, 3, 6.5 and 9 at 60 min electrolysis respectively. However with increasing time reaction, efficiency of dye removal improved at initial pH of 6.5 to 95 percent at 180 min electrolysis. So initial pH of 6.5 was selected as optimum condition for reducing chemical material for releasing wastewater into the environment.
This paper has considered the electro-Fenton treatment of an azo dye with producing in situ hydrogen peroxide by oxygen reduction on graphite cathode. The effects of current intensity, air flow rate, initial pH and electrode surface were investigated. The experimental results showed that electro-Fenton process is able to decompose organic compounds without producing sludge as well as the oxidizing agent (H2O2) that produce only oxygen and water, so this process can be used for treatment or pre-treatment of wastewater containing toxic and non-biodegradable materials, especially textile effluents. From the obtained results, after 300 min of electrolysis, 90 percent dye removal was achieved under optimum condition (current intensity= 0.6A, pH=6.5, no aeration, electrode surface= 60cm2 and energy consumption= 0.24KWh/ppm), which shows electro-Fenton is the proper way to degrade acid orange 7.