Efficacy of Fered-Fenton advanced oxidation process for organic loading (COD) removal from saline tannery wastewater

The saline tannery wastewater also contains the high concentration of organic materials and various chemical compounds like lime, sodium sulfite, sulfate ammonium, sodium chloride, disinfections, vegetable tannins, salts of chromium and chloride. In recent years different applications of advanced oxidation processes (AOPs) are applied to remove organic loading (COD) from wastewater. Advanced oxidation processes produce Hydroxyl radicals that among them HO• is the most powerful one after Fluoride in oxidization of organic compounds. This radical is also favorable because of its high efficiency in oxidation beside non-selective reactivity that makes it able to take the electron from all organic compounds. EAOPs are based on the combination of Fenton reagents and electrochemical process. Compared to the conventional Fenton process, the electro-Fenton process has the benefit of allowing better control of the process, more efficacy in less hydraulic retention time, less excess sludge and avoiding the storing and transport of the H2O2. Electro-Fenton process has two different types. In the first one so-called Fered-Fenton, Fenton reagents are added to the reactor from outside and inert electrodes regenerate ferrous ions while in the second type, only hydrogen peroxide is added from outside and Fe2+ is provided from sacrificial cast iron anodes. The main purpose of this study was effective investigation on Fered-Fenton advanced oxidation process for removal of organic loading (COD) from saline tannery wastewater under optimized conditions including the effect of initial pH, [Fe2+]/[H2O2], H2O2 concentration, current and hydraulic retention time in the presence of high concentration of Cl ions. Pilot studies and investigations were carried out in laboratory scale. Saline tannery wastewater samples were taken from the wastewater treatment plant of Varamin, Iran. The examined Fered-Fenton reactor depicted on figure 1 consists of four graphite electrodes, two anodes and two cathodes, due to chemical inert and decomposable characteristic of graphite. The electrodes dimension were 140mm*60mm*1mm, and the distance between them was 1.5 cm. The reactor volume that is made of Plexiglas was 1 L, equipped with an electrical mixer and digital power to adjust amperage. Chemicals including ferrous sulfate, hydrogen peroxide, and concentrated sulfuric acid (Merk Chemical Co. Inc., Germany) and filter paper #42 (Whatman Co. Inc., United Kingdom) were analytical grade and used without further purification.. The pH values of wastewater samples were adjusted using sulfuric acid and soda before ferrous sulfate and hydrogen peroxide were added to the system. At first, ferrous sulfate was added and mixed vigorously, then hydrogen peroxide was injected into the reactor. After connecting electrodes to the power supply and putting them in the reactor, amperage and voltage were set. All tests were done according to the standard methods (APHA) and each part of experiments was repeated three times.
pH is one of the most impressive factors in Fenton reactions. Adjustment of pH is necessary for precipitation of Iron, Hydrogen peroxide decomposition, complex and recalcitrant Iron compounds and overall efficacy of the process. Based on the own results at the laboratory scale (Fered-Fenton reactor) observed the maximum efficiency of COD removal at pH between 2.5 and 3.5. Oxidation efficiency essentially reduces at pH values lower than 3 because of resulting stable complexes from Fe2+ and H2O2 that leads to deactivation of Fe2+ as a catalyst. While pH value increases from 3 to 5, the process efficiency declines continuously. At higher pH values than 5, Iron species begin to precipitate as ferric hydroxides and cause a reduction in efficiency of the process. During the process continuous detection of pH in order to keep optimized pH value constant is really essential. Generally, water hydrolysis and formation of Carboxylic acids make the pH decay in the system. Since high concentrations of Cl ions available in tannery wastewater, Hypochlorous acids (HClO) and Hydrochloric acid (HCl) are formed that they would also drop the pH. Hence, lack of continuous detection of pH results in the inadequacy of Fered-Fenton process for COD removal from saline tannery wastewater at pH optimized. In the Fenton and Electro-Fenton processes, the mass ratio of ferrous Iron and Hydrogen peroxide is very crucial in terms of overall cost and efficacy of the process in COD removal. Excess or shortage of any of these two reagents would make the process insufficient either by scavenging of Hydroxyl radicals or by stopping the process. In the absence of Fe2+ efficiency of the process is limited to only 14% that is because a few number of iron ions were available as the catalyst to generate enough OH•. The maximum efficiency of the system was achieved by [Fe2+]/[H2O2]=0.6. More addition of Fe2+ causes both OH• scavenging and more iron sludge. Hydrogen peroxide is the main source of Hydroxyl radicals in Fenton related processes. An inadequate dosage of H2O2 does not generate enough hydroxyl radicals to achieve complete mineralization. Although an increase in the amount of H2O2 raises the efficacy of the process, the overdose of it let the side reactions initiate i.e. all of the generated OH• would not be utilized to oxidize organic matters. New radical species generated from side reactions are not as strong as OH• so they reduce the efficacy of the process. Furthermore, COD is defined as the amount of a specified oxidant that reacts with the sample under controlled conditions. Therefore, the determination of Fenton reagent’s dosages should be made on the basis of initial COD for efficient treatment. By increasing the initial COD the required dosage of H2O2 would raise and consequently expand the amount of Fe2+. By considering the fact that samples are real saline tannery wastewater, all of the samples do not show the same initial COD and oscillate in a range of 900 to 1500 mg/lit. Further increase in the ratio of [H2O2]/[CODi] would lead to the adverse effect of scavenging reactions and finally to the ineffectiveness of the process. Therefore, the [H2O2]/[CODi]=2 would be considered as the optimum ratio. When no current was applied to the reactor Fenton process was predominant. With the increase in current, the electro-regeneration of Fe2+ from Fe3+ would be improved. At the current equal to 0.8 Ampere, the maximum COD removal occurred. Higher current has an inhibiting effect on redox of Fe3+ and also would decrease the efficiency of the process due to the decomposition of Hydrogen peroxide to oxygen and water. The hydraulic retention time (HRT) is an essential parameter in operation. Although the shorter hydraulic retention time ameliorates the operation, it corresponds to the higher treatment capacity. In the first 2 hours of the process, the highest COD removal (61%) was obtained due to high concentrations of the degradable organic matter and later it gradually slowed down. However increase in the hydraulic retention time would raise the chance of oxidation of organic matters. Energy consumption must be considered for long HRTs. Hence, in order to make a balance between COD removal, treatment capacity and energy consumption 120 minutes is the optimized HRT for Fered-Fenton process. Chlorine gas generated at anode due to high concentrations of Cl ions formed hypochlorous acids (HClO) and Hydrochloric acid (HCl) that could disinfect and diminish the disgusting odor of wastewater. Although the generated acids could oxidize organic matter, their oxidation power could not be compared with OH• oxidation power. Halide ions like Cl ions in saline tannery wastewater act as the significant scavengers of OH• but forming reactive halogen species (RHS). Chlorine was removed (40%) under optimum conditions of COD removal by Fered-Fenton process that could be continued as long as the current was applied to the system. According to the results and observations, it could be concluded that, although considerable amounts of Chloride as a key factor of salinity in tannery wastewater was eliminated, high concentrations of Cl ions in the tannery wastewater was restricting the effectiveness of COD removal by Fered-Fenton process.