Electrochemical Aging and Halogen Oxides Formation on Multiwalled Carbon Nanotubes and Fe₃O₄@g-C₃N₄Coated Conductive Membranes

Electrochemical membrane filtration is widely reported to enhance water contaminants' degradation or rejection via anodic oxidation or cathodic repulsion. Despite their advances, electrochemical membranes or electrocatalysts often suffer from corrosion or passivation, especially under strong electrode potentials or reactions. Moreover, the formation of toxic byproducts, such as chlorinated organic compounds and oxyhalides (e.g., ClO₄^-) is another major concern. This study investigated the membrane aging processes of two types of conductive membranes, multiwalled carbon nanotubes (MWCNTs) and ferrite/graphitic carbon nitride hybrids (Fe₃O₄@g-C₃N₄) coated on ceramic membranes. Under high current densities (∼20 mA·cm^-2) with anodic potentials (∼10 V), MWCNTs and Fe₃O₄@g-C₃N₄catalysts underwent evident oxidation as indicated by an increase of the intensity ratio of the Raman spectral bands (I_D/I_G) and charge transfer resistance (R_ct) of two electrochemical membranes. Under variations of electrode potentials, chloride or bromide were shown to be oxidized to bromate (BrO₃^-) and chlorate (ClO₃^-) at levels of 1-10 mmol·L^-1. The formation of BrO₃^-and ClO₃^-was dependent on the solution pH, current densities (1-20 mA·cm^-2), and initial concentrations of Br or Cl ions. To warrant a safe and rational design and operation of electrochemically reactive membrane processes, membrane aging and toxic byproduct's formation deserve careful characterization under relevant water filtration environments.