Reactive High-Valent Iron Intermediates in Enhancing Treatment of Water by Ferrate

Abstract

Efforts are being made to tune the reactivity of the tetraoxy anion of iron in the +6 oxidation state (FeVIO4 2–), commonly called ferrate, to further enhance its applications in various environmental fields. This review critically examines the strategies to generate highly reactive high-valent iron intermediates, FeVO4 3– (FeV) and FeIVO4 4– or FeIVO3 2– (FeIV) species, from FeVIO4 2–, for the treatment of polluted water with greater efficiency. Approaches to produce FeV and FeIV species from FeVIO4 2– include additions of acid (e.g., HCl), metal ions (e.g., Fe­(III)), and reductants (R). Details on applying various inorganic reductants (R) to generate FeV and FeIV from FeVIO4 2– via initial single electron-transfer (SET) and oxygen-atom transfer (OAT) to oxidize recalcitrant pollutants are presented. The common constituents of urine (e.g., carbonate, ammonia, and creatinine) and different solids (e.g., silica and hydrochar) were found to accelerate the oxidation of pharmaceuticals by FeVIO4 2–, with potential mechanisms provided. The challenges of providing direct evidence of the formation of FeV/FeIV species are discussed. Kinetic modeling and density functional theory (DFT) calculations provide opportunities to distinguish between the two intermediates (i.e., FeIV and FeV) in order to enhance oxidation reactions utilizing FeVIO4 2–. Further mechanistic elucidation of activated ferrate systems is vital to achieve high efficiency in oxidizing emerging pollutants in various aqueous streams.

Peracetic Acid Enhances Micropollutant Degradation by Ferrate(VI) through Promotion of Electron Transfer Efficiency

“…Ferrate­(VI) [Fe­(VI), HFeO 4 – /FeO 4 2– ] − and peracetic acid [PAA, CH 3 C­(O)­OOH] , are two promising alternative oxidants that are attracting growing interest from environmental engineers and scientists due to their great capacity for pathogen inactivation − and micropollutant abatement. ,− Nonetheless, ferrate­(VI) and PAA both undergo self-decay under environmentally relevant conditions, and their reactivity toward certain contaminants is limited. ,− Thus, various methods for enhancing the oxidation ability of PAA and ferrate­(VI) have been developed in recent years. For example, UV, , Fe­(II), Fe­(III)-picolinic acid-, Co­(II), − Ru­(III), MoS 2 , and Co-based catalysts − have been developed to activate PAA.…”

Understanding Two Variation Patterns of Organic Contaminant Degradation with pH in the FeVI System under Acidic Conditions

“…This conclusion is consistent with Figure b, which indicates that • OH is still effective in degrading OCs with high IP values, while high-valent species primarily dominate in removing other OCs because of the low formation of • OH at pH 4.5. Moreover, PMSO possesses a high IP value, while it can be oxidized by • OH and high-valent Fe species, meaning that the steady-state concentration of high-valent Fe species during Fe VI oxidation at pH 3.0 might be higher than •OH . In conclusion, • OH and high-valent Fe species jointly contribute to degrading OCs in the Fe VI system, and their contribution depends on the IP value of the target OC and the steady-state concentration of reactive species.…”

“…Moreover, PMSO possesses a high IP value, while it can be oxidized by • OH and high-valent Fe species, meaning that the steady-state concentration of high-valent Fe species during Fe VI oxidation at pH 3.0 might be higher than •OH. 2 In conclusion, • OH and high-valent Fe species jointly contribute to degrading OCs in the Fe VI system, and their contribution depends on the IP value of the target OC and the steady-state concentration of reactive species. Figure 4 also shows that there is a poor relationship between the donating-electron ability of OCs and their removal efficiencies by Fe VI , which may be caused by the complex reactive species of the Fe VI system and their species-dependent activity in oxidizing various OCs.…”

“…For this reason, current studies mainly researched the oxidation kinetics, degradation mechanism, and environmental implication of Fe VI under neutral and alkaline conditions. 2,8,9 It should be noticed that the redox potential of Fe VI under acidic conditions (+2.20 V) is significantly superior to that under alkaline conditions (+0.70 V). 10 This implies that applying Fe VI at an acidic pH without accessional catalysis may be a feasible strategy to achieve rapid and efficient oxidation.…”

“…Fe VI oxidation is of great significance to environmental science and engineering technology owing to its potential applications in drinking and wastewater treatment, including oxidation, , disinfection, coagulation, and alleviation of membrane fouling . As a green oxidant, Fe VI can selectively degrade a mass of organic contaminants (OCs), especially those containing electron-donating moieties such as amines and phenols. , Unfortunately, the chemical instability of Fe VI in water, particularly at low pH values, may deteriorate the oxidant utilization efficiency, thus preventing Fe VI from being used in a wide pH range.…”