Oxidizing Capacity of Iron Electrocoagulation Systems for Refractory Organic Contaminant Transformation

Iron electrocoagulation (Fe EC) is normally considered as a separation process. Here, we found that Fe­(II)–O₂ interactions in Fe EC systems could produce reactive oxidants, mainly hydroxyl radicals (•OH), for refractory organic contaminant transformation. Production of reactive oxidants, probed by benzoate conversion to <i>p</i>-hydroxybenzoic acid (<i>p</i>-HBA), depended on dissolved oxygen (DO) concentration and Fe­(II) speciation. Measurable levels of DO were required for significant <i>p</i>-HBA production. Fe precipitates evolved from lepidocrocite to magnetite when DO decreased to below the detection limit. Both experiments and kinetic modeling suggest that the main Fe­(II) species contributing to reactive oxidants (mainly •OH) production changed from aqueous Fe­(II) initially to lepidocrocite-sorbed Fe­(II) with progressive precipitates formation. When DO was not measurable at high currents (≥50 mA or 100 mA/L), reactive oxidant production was ineffective because of pH rise and Fe­(II) preservation in magnetite, but it could be enhanced drastically by aeration. The reactive oxidants produced at 30 mA (or 60 mA/L) could degrade about 47% of 10 μM aniline and 34% of sulfanilamide within 6 h of Fe EC treatment. Our findings highlight the importance of reactive oxidants for refractory organic contaminants oxidation in Fe EC systems.