Electrochemical membrane usually enhances degradation of contaminants via anodic oxidation or production of reactive oxygen species (ROSs), reactive chlorine species (RCSs) and oxidants on the electrically charged membrane surface. This study investigated the membrane aging processes under the ROSs or RCSs generation of two types of conductive membranes, h-CNT and Fe3O4/g-C3N4 coated ceramic membranes. Moreover, the bromate (BrO3−) and chlorate (ClO3−) formation under different levels of the applied anodic or electrode potentials was determined using saline water that contained chlorine and bromine ions of environmentally relevant concentrations. Three influence factors, including the different applied current densities (the resulting electrode potentials), pH and initial electrolyte concentrations, were investigated to determine their effects on ROSs/RCSs generation and chromate/bromate formation on the hybridized CNT and Fe3O4/g-C3N4 modified ceramic membrane surface. In addition to the routine characterization of chemical states using X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and Raman, the surface catalyst’s electrochemical activity changes before and after the repeated filtration under electrochemical reactions were mapped using a hybrid scanning electrochemical microscope-atomic force microscopy (SECM-AFM), which acquires topographic and electrochemical information on the surface with high spatial resolution. Under high current densities and anodic potentials, CNT and Fe3O4/g-C3N4 catalysts underwent oxidation as indicated by the increase of –OH intensity and shifts. The initial potential of Cl– oxidation decreased from 1.3 V to 0.8 V with solution pH increase from 2 to 13. Moreover, our results indicate that under variations of electrode potentials, different speciation of chlorine or bromine may evolve and their oxidized forms, chlorate and bromate, may be thermodynamically or kinetically controlled or minimized. This work aimed to highlight the two fundamental issues of electrochemical membrane treatment, membrane aging and toxic byproduct’s formation such as chlorate, bromate and other potential halogenated organics, are worth further investigation to warrant a safe and rationale design and operation of electrochemically reactive membrane processes.

Electrochemical membrane aging and oxides of halogen formation

ACS Fall 2021

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