Revelation of ferrate(VI) unimolecular decay under alkaline conditions: Investigation of involvement of Fe(IV) and Fe(V) species

Abstract The kinetics and mechanisms of self-decay of ferrate(VI) (FeVIO42−, Fe(VI)) over the entire pH range from acidic to basic pH range need to be understood to assess the ability of Fe(VI) to oxidize pollutants at different pHs. Mechanism of self-decay of Fe(VI) has been extensively examined under acidic to neutral pH conditions. However, Fe(VI) self-decay at alkaline pH (e.g., pH 9.0 or higher) is poorly understood. This study performed kinetic and modeling studies of the Fe(VI) decay at pH 9.0 and 10.0. Our research reveals that the decay of Fe(VI) follows first-order kinetics (i.e., unimolecular decay) at pH 9.0 and 10.0 and the order changes to 3/2-order at pH 7.0 due to the different species of Fe(VI) (FeO42− versus HFeO4−). Results of unimolecular decay mechanism through water attack (WA) are supported by density functional theory (DFT) calculations, which indicate unfavorable dimerization of FeO42− through oxo-coupling (OC) under alkaline conditions. The WA on the monomeric FeO42− is proposed due to its lower activation barrier compared to OC. Kinetic simulation of Fe(VI) decay involving Fe(V) and Fe(IV) successfully predicts Fe(VI) disappearance and H2O2 generation (a product) under varied conditions. The decay of FeO42− is different from the second-order kinetics of protonated Fe(VI) species (H2FeO4 and HFeO4−). Our results will aid in comprehending oxidation power of Fe(VI) in degrading pollutants under alkaline conditions.