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Published March 3, 2022 | Published
Journal Article Open

Challenges of Measuring Soluble Mn(III) Species in Natural Samples

Abstract

Soluble Mn(III)–L complexes appear to constitute a substantial portion of manganese (Mn) in many environments and serve as critical high-potential species for biogeochemical processes. However, the inherent reactivity and lability of these complexes—the same chemical characteristics that make them uniquely important in biogeochemistry—also make them incredibly difficult to measure. Here we present experimental results demonstrating the limits of common analytical methods used to quantify these complexes. The leucoberbelin-blue method is extremely useful for detecting many high-valent Mn species, but it is incompatible with the subset of Mn(III) complexes that rapidly decompose under low-pH conditions—a methodological requirement for the assay. The Cd-porphyrin method works well for measuring Mn(II) species, but it does not work for measuring Mn(III) species, because additional chemistry occurs that is inconsistent with the proposed reaction mechanism. In both cases, the behavior of Mn(III) species in these methods ultimately stems from inter- and intramolecular redox chemistry that curtails the use of these approaches as a reflection of ligand-binding strength. With growing appreciation for the importance of high-valent Mn species and their cycling in the environment, these results underscore the need for additional method development to enable quantifying such species rapidly and accurately in nature.

Additional Information

© 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). Received: 7 November 2021 / Revised: 31 January 2022 / Accepted: 19 February 2022 / Published: 3 March 2022. (This article belongs to the Special Issue 25th Anniversary of Molecules - Featuring Element 25: Manganese and/or Other Metals). We wish to thank Jim Morgan for invaluable insight on the chemistry of manganese in natural waters and dedicate this work to his memory. This work was supported by the NSF GRFP (U.F.L.); NSF IOS grant 1833247 (W.F.); and NRF 2017R1A2B4005441 and 2019R1A2C2086249 funded by the Ministry of Science, ICT, and Future Planning of Korea (J.C.). Author Contributions. B.K., U.F.L., J.M., D.M., J.S.V., J.C. and W.F. contributed to the study conception and design. B.K. and U.F.L. generated data. U.F.L. wrote the manuscript with input from all other authors. All authors have read and agreed to the published version of the manuscript. Data Availability Statement. Data are reported in the figures. Raw data available upon request. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. The authors declare no conflict of interest.

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Created:
August 22, 2023
Modified:
November 16, 2023