Multivariate optimization of the pulse electrochemical oxidation for treating recalcitrant dye wastewater

Electrochemical oxidation under pulse current mode was employed to treat bio-refractory dye wastewaters (i.e., Indigo Carmine [IC], Alizarin Red S [ARS], and Methyl Orange [MO]) with PbO2/Ti anode, and Box-Behnken designs (BBDs) were conducted to optimize the operating parameters for enhancing energy efficiencies. Pulse duty cycle, current density, and flow rate of effluent were determined to be the influencing factors for the degradation performance, and analyses of BBDs further revealed that the influencing degrees of these factors follow the order: current density > pulse duty cycle > flow rate for the degradation of IC and MO, but such conclusion is not applicable to that of ARS. the fused-rings structure of ARS is more difficult to be electro-oxidized than the ring-bridge-ring structures of IC and MO. The optimal solutions with the criteria of maximizing TOC degradation and minimizing energy consumption were obtained by the model fitting of BBDs, and with the optimized operating parameters the energy consumptions can be saved up to 35.5%, 40.1%, and 47.9% for IC, ARS, and MO, respectively, as compared with the traditional electrolysis process with galvanostatic mode. Such optimization method can improve the practicability of electrochemical oxidation process. Further investigation of quantitative relationships between the degradation efficiencies and the structures of organic pollutants is believed to help guide parameter optimizing, and such work is currently underway.