Generating high-valent iron-oxo ≡Fe=O complexes by calcium sulfite activation in neutral microenvironments for enhanced degradation of CIP.

Although alkaline sulfite activation of ferrate (Fe(VI)) has advantages of fast response and high activity for degradation of organic contaminants, the specific production pathways of active species and the pH conditions still hinder its widespread application. Based on this, our study constructed a novel advanced oxidation process of calcium sulfite (CaSO) could activated Fe(VI) continuously by Ca buffering and investigated the mechanism under different pH values and CaSO dosages with ciprofloxacin as a target organic pollutant. The results showed that Ca stabilized the process at a neutral/weakly alkaline microenvironment of pH 7-8, which could alleviate the hydrolysis of ≡Fe=O by protons and iron hydroxyl groups. Besides, the removal of pollutants occurred efficiently when sulfate (SO) was excessive and had a 3:1 ratio of SO to Fe(VI), achieving more than 99% removal of electron-rich phenolic organic pollutants within 2 min. By adding different radical scavengers and combining electrochemical analysis methods and electron paramagnetic resonance spectroscopy techniques to revealed that the main active species in Fe(VI)/CaSO process were ≡Fe=O/≡Fe=O. Furthermore, the reactivity of various sulfate species (such as SO, SO, SO, SO) with Fe(VI) was calculated using density functional theory (DFT), and it was found that Fe(VI)-SO reaction has a much lower energy barrier (-36.08 kcal/mol), indicating that SO can readily activate Fe(VI) and generate ≡Fe=O to attack the atoms with high Fukui index (f ) in organic pollutants. The above results confirm the feasibility of Fe(VI)/CaSO process. Thus, this study can theoretically and practically prove that the main active species is ≡Fe=O, rather than SO or OH in Fe(VI)/CaSO process.