A Study on the Optimization of the Mediated Electrochemical Incineration of Glucose

The electrochemical wastewater treatment for mineralization of polluting organics has been studied since the early ‘90’s [1-7]. An important result, common to most of the relevant literature, is certainly the remarkable dependence of the efficiency of the process on nature of the electrode material. The mineralization process takes place as an extreme case of anodic oxidation, and is concomitant with the oxygen evolution reaction. The formation of adsorbed hydroxyl radicals is a necessary condition for the oxidative attack of the organic substrate to take place and also for the oxygen evolution [4]. At high oxygen overvoltage anodes, like PbO2, Sb(V)- or fluoride-doped SnO2 , typically the anodic mineralization of organic substrates takes place with better faradaic yields. In the case of lead dioxide, however, problems of service life and of release of lead ions in the treated effluent, may represent serious drawbacks in a practical application. On the other hand, for tin dioxide-based electrodes, the short service life is an important inconvenience. While the improvement of these anodes is under investigation in different labs, it may be of interest to investigate the possibility to improve the efficiency of the electrochemical incineration at stable anodes, like galvanic platinum, iridium dioxide-based DSA’s®. This aim may be achieved using inorganic mediators of the oxidation of the organic substrate. In this respect active chlorine may be of particular interest, and papers already appeared on the electrochemical mineralization of bisphenol A [8] and phenol [9]. In the presence of chlorides the electrochemical treatment can be carried out at much lower potentials, compared with those required for the non-mediated (direct) anodic oxidation. Galvanic Pt and other DSA’s materials based on iridium and ruthenium oxides, can be used, the optimal choice depending on pH and other process variables. In a previous work [10] glucose has been studied as a model-substrate [7], considering that saccharides are important components in effluents from food industry (e.g.: olive mill wastewaters), and the efficiency of the mediated electrochemical incineration has been tested in the presence of different concentration of NaCl. It could be shown that at least four variables play an important role in the process: the current density j of the electrolytic process, the mediator (NaCl) concentration, cNaCl, the solution pH and temperature. The importance of the faradaic efficiency of the electrochemical incineration, requires an appropriate analysis of the dependence of global parameters like the electrochemical oxidation index (EOI), electrochemical oxygen demand (EOD) on the process variable. Due to the many variables to be taken into consideration an optimization study requires an adequate statistical approach. In the present work a detailed study has been carried out, of the dependence of instantaneous current efficiency, ICE, total current efficiency (TCE), EOI, EOD, on electrolysis parameters. The concentration of glucose was typically 10 g dm-3. 1 M sodium sulphate was used as supporting electrolyte. Different sodium chloride concentrations have been tested, between 1 and 10 g dm-3. On the basis of results on the influence of the alkalinity of the solution on the glucose incineration rate, most of the experiments have been carried out in alkaline media (0.1 M NaOH). An optimization of EOI, as a function of j and cNaCl has been attempted, making use of the commercially available optimization software MultiSymplex®. The mineralization rate of the organic substrate has been followed measuring the chemical oxygen demand (COD) in the solution at different electrolysis times. Under selected conditions, total organic carbon (TOC) has been measured as well. All experiments have been carried out at Ti/Pt electrodes. In agreement with previous measurements under different condiitons [10], the decrease of the solution temperature causes a considerable increase in the efficiency of the COD abatement. At a current density of 1200 A m-2, the time required for the complete abatement of COD is three time less when the temperature is 25°C, compared with the time required at 60°C and accordingly EOI parameter is three times less in the latter case. As previously mentioned, the optimization procedure by MultiSymplex® has been carried out on two variables only: namely j and cNaCl. Solutions containing 10 g dm-3 of glucose have been studied (COD ≈ 10,000 mg O2 dm-3), systematically choosing j-cNaCl couples, and measuring TCE for an abatement of COD to 3,000 mg O2 dm-3 (indicated as TCE3000). Disregarding the dispersion due to uncertainties in the COD measurements, the higher abatement rate can be achieved for the couple of variables: j = 925 A m-2; cNaCl = 7,4 g dm-3. The optimal efficiency of the process, in terms of energy consumption for and abatement of the initial COD to a value of 400 mg O2 dm-3, has also been studied. In this case the couple of coordinates corresponding to minimum energy consumption are: j = 1200 A m-2; cNaCl = 7,76 g dm-3. Apparently when the oxidative degradation is extended to lower COD values, the optimal performance requires higher current densi-ties and electrode potentials. References 1 S. Stucki, R. Koetz, B. Carcer, W. Suter, J. Appl. Electrochem. 21, 99(1991) 2 Ch.Comninellis and E. Plattner, Chimia 42, 250(1988) 3 Ch.Comninellis and C. Pulgarin, J. Appl. Electrochem. 23, 108(1993) 4 Ch. Comninellis, Electrochim. Acta 39, 1863(1994) 5 A. Boscolo, F. Gottardi, M. Tavan, R. Amadelli, A. De Battisti, A. Barbieri, G. Battaglin, J. Appl. Electrochem.. 24, 1052(1994) 6 Ch. Comninellis and A. Nerini, J. Appl. Electrochem. 25, 23(1995)