Buffer-free ozone-ferrate(VI) systems for enhanced oxidation of electron-deficient contaminants: Synergistic enhancement effects, systematic toxicity assessment, and practical applications

The combination of ozone (O-3) and ferrate (Fe(VI)) oxidation technology demonstrates substantial potential for practical applications, though it has been underreported, resulting in gaps in comprehensive activity assessments and thorough exploration of its mechanisms. This study reveals that the previous use of a borate buffer solution obscured certain synergistic reactions between O-3 and Fe(VI), causing a reduction of activity by similar to 40 % when oxidizing the electron-deficient pollutant atrazine. Consequently, we reassessed the activity and mechanisms using a buffer-salt-free O-3/Fe(VI) system. Our findings showed that the hydroxyl radical ((OH)-O-center dot) served as the predominant active species, responsible for an impressive 95.9 % of the oxidation activity against electrondeficient pollutants. Additional experiments demonstrated that the rapid production of neglected and really important superoxide radicals (O-center dot(2)-) could facilitate the decomposition of O-3 to generate (OH)-O-center dot and accelerate the reduction of Fe(VI) to Fe(V), reactivating O-3 to produce (OH)-O-center dot anew. Intriguingly, as the reaction progressed, the initially depleted Fe(VI) was partially regenerated, stabilizing at over 50 %, highlighting the significant potential of this combined system. Moreover, this combined system could achieve a high mineralization efficiency of 80.4 % in treating actual coking wastewater, complemented by extensive toxicity assessments using Escherichia coli, wheat seeds, and zebrafish embryos, showcasing its robust application potential. This study revisits and amends previous research on the O-3/Fe(VI) system, providing new insights into its activity and synergistic mechanisms. Such a combined technology has potential for the treatment of difficult-to-degrade industrial wastewater.