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Manganese (Mn) is an essential element for life. Although its concentration is at (sub)nanomolar levels throughout the ocean, it affects the oxygen concentration of the ocean because it is central to the photosynthetic formation of dioxygen, O2, in photosystem center II. Mn inputs into the ocean are from atmospheric transport of particles and their dissolution to form dissolved Mn, and from the flux of dissolved Mn from rivers, sediments and hydrothermal vents. The main removal mechanism is transport of particulate Mn from dust and organic matter to the sediments. The environmental chemistry of manganese centers on its +2, +3 and +4 oxidation states. Most recent data show that Mn(II) is dissolved, that Mn(IV) is particulate MnO2, and that Mn(III) can be particulate or dissolved when bound to organic complexes [denoted as Mn(III)-L]. Mn(II) is oxidized primarily by microbial processes whereas MnO2 is reduced by abiotic and biotic processes. Photochemical processing aids redox cycling in surface waters. In suboxic zones, which are defined as areas with dissolved O2 concentrations below 3 M, both oxidation and reduction processes can occur but usually at different depths. In suboxic zones, dissolved Mn is also released from organic matter during its decomposition and from MnO2 reduction.
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Charge distributions and oxidation states in Mn-based oxidants from ab initio molecular dynamics simulation
We used ab initio molecular dynamics simulations to address Mn oxidation states in different Mn 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 b-MnO2 has the highest AOS, a-MnO2 has an intermediate AOS, and d-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−/O2 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 O2 ligands. Our simulations also show that the O2 would be activated to O22- in this complex under acidic conditions, and could lead to the formation of OH radicals that serve as oxidants.
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- Award ID(s):
- 1808406
- PAR ID:
- 10288081
- Date Published:
- Journal Name:
- Chemosphere
- Volume:
- In review
- ISSN:
- 0045-6535
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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