Anodic Dissolution and Passivation of Zn Electrodes in Electrocoagulation: Effect of Urea Concentration and Chloride‐Induced Corrosion

This work highlights the phenomena that may occur on zinc electrodes used as sacrificial electrodes in a process for treating an effluent containing urea. Zinc ions (Zn²⁺) generated in situ promote coagulation, but in some cases, electrode passivation and localized corrosion can hinder dissolution and reduce treatment efficiency. For this reason, the effect of operational parameters such as current density, initial urea concentration, pH, and supporting electrolyte (NaCl) concentration on urea removal was studied in the first part. While the second part was dedicated to investigating the impact of urea and NaCl electrolyte concentrations on the electrochemical behavior of the zinc electrode, this was done by using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP), and cyclic voltammetry, while surface changes were analyzed via scanning electron microscopy coupled with energy-dispersive spectroscopy and x-ray diffraction. The obtained results show that the highest urea removal was obtained with the operating conditions: current density of 22 mA/cm², pH 10, 25 mmol/L NaCl, and an initial urea concentration of 20 mmol/L: 12 mmol/L of urea was removed, corresponding to 91 mg/L of dissolved zinc, a faradaic efficiency of 110%. With regard to the surface state of zinc, it was demonstrated that passivation through zinc oxide formation was confirmed by PDP analysis. The results revealed the presence of ZnO crystalline phases as well as surface deposits, both indicative of the development of an oxide layer, which limited further zinc dissolution and floc generation under specific EC operating conditions. Zinc corrosion behavior was strongly influenced by pH, chloride concentration, and urea levels, as evidenced by electrochemical diagnostics (polarization curves and impedance spectra). At alkaline pH and moderate chloride concentrations, enhanced zinc release was observed, while higher urea levels promoted surface degradation and oxide accumulation. These findings highlight the need to balance dissolution and passivation of electrodes to optimize EC performance for nitrogenous pollutant treatment.