Application of electrochemically dissolved iron in the removal of tannic acid from water

Abstract

Effects of some experimental parameters (supporting electrolyte, initial pH and current density) on the performance of electrocoagulation process using iron electrodes were investigated. Results of experiments showed that dissolution of iron is purely electrochemical and fits well with Faraday's law and leads to Fe2+ which are chemically oxidized into Fe3+ in aerated conditions. In neutral and alkaline conditions, the reaction between hydroxyl ions generated at the cathode and dissolved iron ions forms insoluble hydroxo-iron species. Potentiodynamic polarization tests showed that the formation of passive film on iron anode limits the continuous electrochemical dissolution of iron. Corrosion and pitting potentials largely depend on the nature of supporting electrolyte. The dissolution of iron is facilitated by pitting corrosion with chloride ions, but it is inhibited in the presence of phosphate ions. However, fluctuations between pitting and passivity were observed in the presence of sulfate ions but this can be eliminated by the addition of small amount of chloride ions. Electrocoagulation using iron electrodes was applied to remove tannic acid (TA) from water. The obtained results have shown that the efficiency of this electrochemical technology depends largely on certain operating parameters including initial pH, current density and nature of supporting electrolyte. Almost complete COD removal can be achieved in batch electrochemical reactor using iron electrodes under optimized operating conditions after consumption of 1Ahdm−3. The elimination of TA from water involves a primary mechanism leading to the formation of stable black organometallic complexes and a second mechanism through sweep coagulation.