ab initio study of Mn-based systems for oxidative degradation

Organic contaminants can be removed from water/wastewater by oxidative degradation using oxidants such as manganese oxides and/or aqueous manganese ions. The Mn species show a wide range of activity, which is related to the oxidation state of Mn. Here, we use ab initio molecular dynamics simulations to address Mn oxidation states in these systems. We first develop a correlation between Mn partial atomic charge and the oxidation state based on results of 31 simulations on known Mn aqueous complexes. The results collapse to a master curve; the dependence of partial atomic charge on oxidation state weakens with increasing oxidation state, which concurs with a previously proposed feedback effect. This correlation is then used to address oxidation states in Mn systems used as oxidants. Simulations of MnO2 polymorphs immersed in water give average oxidation states (AOS) in excellent agreement with experimental results, in that beta-MnO2 has the highest AOS, alpha-MnO2 has an intermediate AOS, and delta-MnO2 has the lowest AOS. Furthermore, the oxidation state varies substantially with the atom's environment, and these structures include Mn(III) and Mn(V) species that are expected to be active. In regard to the MnO4-/HSO3-/O-2 system that has been shown to be a highly effective oxidant, we propose a novel Mn complex that could give rise to the oxidative activity, where Mn(III) is stabilized by sulfite and dissolved O-2 ligands. Our simulations also show that the O-2 would be activated to O-2(2-) in this complex under acidic conditions, and could lead to the formation of OH radicals that serve as oxidants.

Automated summary

In this study, ab initio molecular dynamics simulations were used to investigate the oxidation states of manganese in oxidant systems. The results show a correlation between manganese's partial atomic charge and its oxidation state, and the oxidation state of manganese is influenced by its environment. Additionally, a novel manganese complex that exhibits oxidative activity was proposed.