Generating High-Valent Iron-Oxo ≡Feⅵ=O Complexes by Calcium Sulfite Activation in Neutral Microenvironments for Enhanced Degradation of Cip

Abstract

Although alkaline sulfite activation of ferrate(VI) has advantages of fast response and high activity for degradation of organic contaminants, the main active species are still unclear, which is alone suitable for alkaline water purification. Based on this, our study constructed a novel advanced oxidation process of CaSO3 could activated ferrate(VI) continuously by Ca2+ buffering and investigated the mechanism under different pH values and CaSO3 dosages with ciprofloxacin (CIP) as a target organic pollutant. The results showed that Ca2+ stabilized the reaction process at a neutral/weakly alkaline microenvironment of pH 7-8, which could alleviate the hydrolysis of ≡FeIV=O by protons and iron hydroxyl groups. Besides, the removal of pollutants was favorable when sulfate was excessive and had a 3:1 ratio with ferrate(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 (EPR) spectroscopy techniques to revealed that the main active species in ≡FeIV=O−CaSO3 process were ≡FeIV=O/≡FeV=O. Furthermore, the reactivity of various sulfate species (such as SO32−, SO3•−, SO4•−, SO5•−) with ferrate(VI) was calculated using density functional theory (DFT), and found that Fe(VI)-SO32- reaction has a much lower energy barrier (-36.08 kcal/mol), indicating that SO32- can readily activate Fe(VI) and generate Fe(V) species to attack the atoms with high Fukui index (f -) in organic pollutants. The above results confirm the feasibility of ≡FeIV=O−CaSO3 process. Thus, this study can theoretically and practically prove that the main active species in ≡FeIV=O−CaSO3 process is ≡FeIV=O, rather than SO4•− or •OH.

Keywords: Active oxidants, Iron-oxo ≡FeIV=O, Neutral Microenvironment, CaSO3, DFT