Comparison of Organic Dyes Removal Mechanism in Electrocoagulation Process Using Iron and Aluminum Anodes A.R. Amani-Ghadim, a A. Olad, b S. Aber, a and H. Ashassi-Sorkhabi c a Environment Protection Research Laboratory, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran; soheil_aber@yahoo.com (for correspondence) b Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran c Electrochemistry Research Laboratory, Physical Chemistry Department, Faculty of Chemistry, University of Tabriz, Tabriz, Iran Published online 00 Month 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.11656 The removal of Reactive red 43, as a model pollutant, and some organic dyes with different structures by electrocoagula- tion process was investigated using iron (EC-Fe) and alumi- num (EC-Al) anodes to explain dyes removal mechanism differences in EC-Fe and EC-Al processes. First, the effects of current density, electrolysis time and pH on the removal of Re- active Red 43were studied. The maximum removal efficiency (RE) was obtained in relatively acidic medium (pH 5 4.25) in the first 12 minutes of EC-Al. While, in EC-Fe, a significant lag time between the starting of the process and initiation of con- siderable dye removal was observed. The color RE was supple- mented with determining the RE of chemical oxygen demand. The COD values were determined for decolorization of solu- tion containing 50 mg/L of RR43 and after 24 min of processes when the applied current density to monopolar batch electro- chemical reactor was 25 A/m 2 . The COD removal efficiencies for the mentioned conditions were 73.92% and 97.12% for EC-Al and EC-Fe processes, respectively. The kinetics studies were carried out for both EC-Al and EC-Fe processes. In the similar operational conditions, the removal of basic dyes was not considerable by EC-Al. However, the RE of anionic dyes was improved with the addition of the number of sulfonate substituted groups. The maximum RE of all cationic and ani- onic dyes (RE > 90%) was achieved using EC-Fe. The enmesh- ment of dyes on iron oxide/hydroxide precipitates and charge neutralization were the main removal mechanisms in EC-Fe and EC-Al processes, respectively. Ó 2012 American Institute of Chemical Engineers Environ Prog, 00: 000–000, 2012 Keywords: wastewater treatment, electrocoagulation, kinetics, removal mechanism, dye INTRODUCTION Wastewaters produced from textile and dyeing industries are characterized by low biodegradability, high chemical oxy- gen demand (COD), and biological oxygen demand (BOD) values due to the organic dyes existence [1, 2]. The presence of trace amount of dyes in these types of wastewaters is com- pletely visible and causes serious problems because majority of dyes and their break down products are toxic to human health [3]. Textile dyes have different chemical structures and characteristic properties. According to general property of dyes, important categories of dyes are as follows: acid (nega- tively charged dyes), basic (positively charged dyes), reactive (anionic dyes which react with fabrics covalently), mordant (contain a metallic ion to show color or stain selectivity), and disperse (non-ionic dyes which disperse in aqueous me- dium). The concerns about toxicity of the dyes and variety of their structures and characteristic properties have led to de- velopment of chemical and physicochemical treatment proc- esses, e.g., advanced oxidation processes [4, 5]. In the recent years, electrochemical techniques, especially electrocoagulation (EC), have been utilized for treating various types of wastewaters [6, 7]. EC is an efficient and cost-effective method for removing organic and inorganic pollutants which has some advantages such as easy automation, minimum amount addition of chemicals and simple equipments in com- parison with conventional coagulation. Moreover, the dose of coagulants depends on the applied current density and hydro- gen evolution from the cathode promotes the flocculation and destabilization of pollutants [5, 8]. Also, EC is found to be more effective than other electrochemical and advanced oxi- dation processes (e.g., UV/TiO 2 and electrofenton method) for removal of organic dyes by considering the terms of decolor- ization efficiency and electrical energy consumption [9]. In EC process, coagulants are generated by dissolving aluminum (which yields Al 31 [10]) or iron (which yields Fe 21 [11, 12]) from the anode. Simultaneously, hydroxyl ions and hydrogen gas are released from the cathode by the reduction of water molecules [10, 13]. The occurrence of electrochemi- cal reactions leads to the production of various ferrous, fer- ric, and Al(III) hydroxide and polyhydroxide species depend- ing on the pH of the aqueous medium [10]. EC is a complex process in which several mechanisms act in removing various pollutants. In the case of organic pollutants, two main mechanisms are supposed for removing dissolved organics matters: (i) charge neutralization with binding of cationic metal species (e.g., M(OH) 21 , Fe 2 (H 2 O) 6 (OH) 4 21 or Al 7 (OH) 17 41 , and so on) to anionic groups of organic molecules and resulting in reduced solubility and (ii) enmeshment of or- ganic pollutants on metal hydroxide precipitates [14, 15]. Although EC has been utilized for over a century, the available literature reveals little studies on the removal mech- Ó 2012 American Institute of Chemical Engineers Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep Month 2012 1