Degradation of perfluorooctane sulfonate via in situ electro-generated ferrate and permanganate oxidants in NOM-rich source waters

Abstract

A novel process involving the in situ electrochemical generation of ferrate and permanganate oxidants, in circumneutral conditions, from low concentration aqueous iron (Fe²⁺) and manganese (Mn²⁺), is investigated for the treatment of the ubiquitous and highly recalcitrant micro-pollutant, perfluorooctane sulfonate (PFOS). The present study investigated the efficacy of both electro-oxidation (EO), and the simultaneous EO and ferrate/permanganate generation and oxidation, of PFOS as a potential drinking water treatment technology. While permanganate was shown to have little effect on PFOS removal, significantly increased degradation was observed when EO was coupled with ferrate generation and oxidation, significantly exceeding that of solely EO. From an initial concentration of 0.80 μM, final PFOS concentrations of 0.53 (±0.004), 0.43 (±0.01) and 0.27 (±0.01) μM were yielded during 10, 40 and 80 mA cm⁻² electrolysis and an initial Fe²⁺ = 179 μM. In general, PFOS degradation rates increased with both increasing current density and initial Fe²⁺ concentration. Degradation was observed to follow mixed zero- and pseudo-first-order reaction kinetics for both the EO and simultaneous EO and ferrate oxidation. Finally, PFOS oxidation was not inhibited by the presence of low and high molecular weight organic scavenger species, and high concentrations of natural organic matter (NOM) improved PFOS removal due to hydrophobic interaction. Reduced ferrate species were also observed to increase NOM removal after electrolysis, by iron coagulant formation and subsequent flocculation.