Understanding molecular-level reactions between permanganate/ferrate and dissolved effluent organic matter from municipal secondary effluent

Abstract

Permanganate (Mn(VII)) and ferrate ((Fe(VI)) oxidations are well-recognized as advanced treatment processes to degrade organic components detected in secondary effluents from municipal wastewater treatment plant (WWTP). However, Mn(VII) or Fe(VI) reactions are sensitive to the water matrix components, especially the dissolved effluent organic matter (EfOM) contained in secondary effluent. Here, we found that Mn(VII) or Fe(VI) hardly mineralize EfOM, but do change its molecular-level composition. Tests with representative trace organic contaminants (TrOCs) spiked to EfOM-rich wastewater effluent showed that Mn(VII) and Fe(VI) preferentially oxidize phenolic TrOCs. UV–vis and fluorescence spectroscopy analyses suggested that molecular properties associated with optical characteristics of reaction solutions are altered by treatment, including decreases in aromaticity, molecular weight, and electron-donating capacity of EfOM. At the molecular-level, the observed phenomenon is ascribed to preferential oxidation of aromatic structures and electron-rich functional groups (e.g., phenolic structures and hydroxyl groups) within EfOM, as well as the transformation and decomposition of macromolecular sulfur- and nitrogen-containing compounds (most likely proteins or microbial byproduct-like material). These findings advance the application of Mn(VII) and Fe(VI) for optimization and proper control of EfOM of emerging concern within treatment trains being developed for advanced treatment facilities.