Charge transfer/mass transport competition in advanced hybrid electrocatalytic wastewater treatment: Development of a new current efficiency relation

The contribution of homogeneous versus heterogeneous catalytic oxidation has been studied for the first time in a hybrid anodic oxidation (AO)/electro-Fenton (EF) batch reactor at different initial organic load in terms of chemical oxygen demand (COD) values of 1.61, 12.1 and 23.3 g-O-2 L-1. The rate of three (electro)-chemical processes, e.g., (i) AO at the surface of boron-doped diamond (BDD) anode, (ii) Fenton oxidation in the bulk solution and (iii) mediated oxidation (MO) in the bulk solution, have been considered in a mathematical model. The mass transfer coefficient was depending on the concentration of the compound due to the unstationary behavior in the diffusion layer at the vicinity of the anode. After calibration and validation of the model with experimental data, it could be noticed that the competition between the mass transport and the charge transfer was evolving according to the COD load. At 1.61 g-O-2 L-1, the applied current density was higher than the limiting current density all along the treatment, meaning that the process was under mass transport control. In this case, the Fenton oxidation rate had the highest contribution in the oxidation efficiency while AO oxidation rate had the lowest efficiency. Increasing the initial COD concentration up to 23.3 g-O-2 L-1, made raising the influence of AO process that became predominant after a certain electrolysis time, before reaching the critical time (t(cr)). Finally, a new current efficiency expression has been proposed by taking into account the three reaction rates and the model could fit the experimental data, showcasing a promising approach. These results further emphasized the need to adapt the reactor design to favor either AO and/or Fenton oxidation according to the organic load of wastewater to be treated, i.e., as pre-treatment or polishing treatment.