Estudo dos fenômenos de ativação de eletrodos, difusão e migração iônica em meio eletrolítico e sua representação matemática da demanda energética no processo de eletrocoagulação: o papel da condutividade elétrica na remoção de poluentes

Abstract

This work had as main objective the proposal to study the activation, diffusion and ion migration phenomena in order to model the total energy demand in the electrocoagulation process (EC), as well as to understand the importance of electrical conductivity in the control and enhancement of pollutant removal. A batch electrocoagulation reactor, composed of a pair of metallic electrodes of the same material (aluminum) with effective area of 0.1 m2 and a useful volume of 1.29 L, was built and operated in constant direct current mode. ZnSO4 electrolyte solutions were used to evaluate the effects of both electric current density and electrical conductivity on activation, diffusion and ion migration phenomena. Sets of EC experiments were planned and executed, and their results were compared with a theoretical model proposed based on the Nernst-Planck equation. Initial electrical conductivity (range 100 to 1000 µScm-1) and electric current density (range 2 to 8 Am-2) were the control variables in each experiment. Zn concentration, pH, electrical conductivity and total electric voltage measured by the power supply were the response variables measured during the experiments, and the behavior of these variables was properly correlated to the energy consumption of each phenomenon that governs the process (activation, diffusion and ion migration). All EC experiments showed a variable behavior over time for electrical conductivity, due to the input of Al ions through the anode as well as the drop in Zn concentration due to pollutant removal processes. The initial transient behavior of the total electric voltage during the EC experiments was well adjusted by the proposed mathematical model for electrode activation. The activation energy is governed by an equation that resembles the desorption process followed by diffusion, which changes parameters previously considered stationary by the classical Tafel equations, commonly used to describe activation potentials. The ionic concentration gradient, in turn, had much smaller effects on the total electric voltage when compared to the effects of electrode activation and ion migration, the latter being governed by Ohm's Law. Electrical conductivity has been shown to have a direct impact on all phenomena involved in EC. In addition, another set of experiments was carried out in order to evaluate the integrated performance of the EC in terms of the mass of pollutant removed per unit of energy consumed. In this set of experiments, ZnSO4 electrolytic solutions continued to be used, but a distinct batch reactor configuration based on aluminum electrodes was built (30 x 2 x 7 cm). Initial conditions of electrical conductivity (230 to 435 µS cm-1) and current density (4.76 to 21.45 A m-2) made it possible to compare performances in constant electric current mode (CEC mode) and constant electric voltage mode (CEV mode). The operation in CEC mode resulted in a higher integrated performance, with low energy consumption, despite the lower pollutants removal rates. The behavior of electrical conductivity over time and its impact on total electrical voltage reaffirmed its central role on driving the process.