Innovative chemical processes for the treatment of water polluted by recalcitrant organic substances

Abstract

In the last years, many research groups have focused their attention on the innovative chemical processes adopted for the treatment of water effluents polluted by recalcitrant organic substances, i.e., substances resistant to biological treatment. The electrochemical oxidation is one of the most studied technologies because it presents high versatility and low cost, it is realized under mild conditions of pressure and temperature and generally it does not involve the use of toxic substances [1]. In this work, the comparative performance of different electrochemical approaches such as direct oxidation processes, oxidation by means of electrogenerated chlorine and electro-Fenton was investigated. The influence of numerous parameters, such as the nature of the electrodic material and of the organic pollutant, the pH, the flow dynamic regime, the current density, the pollutant concentration and the temperature, on the electrochemical incineration of some carboxylic acids and aliphatic chlorides, chosen as model organic compounds, was studied in detail. Two very different anodes were used: Ti/IrO2-Ta2O5, which presents a quite low oxygen overpotential, and boron-doped diamond (BDD), which certainly is one of the most promising materials for the electrochemical incineration [2,3]. Incineration of carboxylic acids was favored by high flow rate and low current density, i.e., when the oxidation process was mainly under kinetic control. Moreover, the process resulted to be favored by high initial concentrations of the organic substrate and low pH but it did not depend, under the adopted operative conditions, on the nature of the supporting electrolyte. The effect of sodium chloride on the electrochemical oxidation of oxalic acid was observed to depend on the nature of the anode as well as on the pH. The best results were achieved by using IrO2-Ta2O5 electrode, with addition of sodium chloride at acid pH. Furthermore, in the presence of high amounts of sodium chloride, a higher abatement of oxalic acid was obtained when high current densities and low flow rates were imposed. Comparing the performance of electro-Fenton process coupled with anodic oxidation at BDD anode and that of simple anodic oxidation, higher abatements of 1,2-dichloroethane and 1,1,2,2-tetrachloroethane were obtained in the first case thanks to the presence of Fe2+ in the solution. Higher applied currents led to a faster electrogeneration of H2O2 and regeneration of Fe2+, thus giving rise to a faster degradation of the starting compounds. The time course of the concentration of the main intermediates accumulated in the electrochemical cell during the treatments, particularly short-chain carboxylic acids and chlorinated ions, was also reported. [1] Jüttner K. et al., Electrochimica Acta, 45 (2000) 2575–2594. [2] Panizza M. and Cerisola G., Electrochimica Acta, 51 (2005) 191–199. [3] Martinez-Huitle C. A. and Ferro S., Chem. Soc. Rev., 35 (2006) 1324-1340.