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Rates of Abiotic Mn^(II) Oxidation by O₂: Influence of Various Multidentate Ligands at High pH

Morgan, James J. and Schlautman, Mark A. and Bilinski, Halka (2021) Rates of Abiotic Mn^(II) Oxidation by O₂: Influence of Various Multidentate Ligands at High pH. Environmental Science and Technology . ISSN 0013-936X. doi:10.1021/acs.est.1c01795. (In Press)

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Oxidation of manganous manganese (Mn^(II)) is an important process driving manganese cycles in natural aquatic systems and leading to the formation of solid-phase Mn^(III,IV) (hydr)oxide products. Previous research has shown that some simple ligands (e.g., phosphate, sulfate, chloride, fluoride) can bind with Mn^(II) to make it unreactive to oxidation by dissolved oxygen. However, there is little to no understanding of the role played by stronger, complex-forming ligands in Mn^(II) oxidation reactions. The objective of this study was to evaluate the rates of abiotic Mn^(II) oxidation by O₂ in the presence of low concentrations of several complex-forming model ligands (pyrophosphate, tripolyphosphate, ethylenediaminetetraacetic acid, oxalate) in bicarbonate-carbonate buffered laboratory solutions of pH 9.42, 9.65, and 10.19. The influence of increasing ligand concentrations on observed autocatalytic profiles of Mn^(II) oxidation was investigated, and initial oxidation rates were linked quantitatively to the initial Mn^(II) speciation in experimental solutions. Observed rates of Mn^(II) oxidation decreased with increasing ligand concentration for all four ligands tested. However, the profiles observed with time and the magnitudes of decrease in initial oxidation rates were different for the different ligands. Likely explanations for these observations include the denticity of the tested ligands, the relative strength of the ligands to complex Mn^(II) versus Mn^(III), and the ability of some ligands to enhance the reduction of Mn^(III) back to Mn^(II) on a time scale comparable to the forward homogeneous Mn^(II) oxidation reaction.

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Schlautman, Mark A.0000-0001-6522-4345
Alternate Title:Rates of Abiotic MnII Oxidation by O2: Influence of Various Multidentate Ligands at High pH
Additional Information:© 2021 The Authors. Published by American Chemical Society. ACS AuthorChoice - Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) Received: March 27, 2021; Revised: June 3, 2021; Accepted: June 22, 2021. We gratefully acknowledge the constructive comments and suggestions provided by the three anonymous reviewers. Caltech support for emeritus faculty (JJM) helped to make completion of this paper possible. Special thanks go to Alan Stone for his many helpful discussions, and to the many other colleagues, too numerous to mention, who have provided support and guidance. Elements of this paper were presented at the 2020 ACS Fall National Meeting in the Geochemistry Division symposium entitled Biotic/abiotic Redox Processes of Manganese in Natural and Engineering Systems, August 16–20. The authors declare no competing financial interest.
Subject Keywords:manganese, oxidation, ligand, dissolved oxygen, autocatalysis, redox reaction
Record Number:CaltechAUTHORS:20210715-165032904
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Official Citation:Rates of Abiotic MnII Oxidation by O2: Influence of Various Multidentate Ligands at High pH. James J Morgan, Mark A Schlautman, and Halka Bilinski. Environmental Science & Technology. Article ASAP; DOI: 10.1021/acs.est.1c01795
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:109849
Deposited By: Tony Diaz
Deposited On:15 Jul 2021 17:40
Last Modified:15 Jul 2021 17:40

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