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Kinetic Behavior of Mn(III) Complexes of Pyrophosphate, EDTA, and Citrate

Klewicki, J. Kenneth and Morgan, James J. (1998) Kinetic Behavior of Mn(III) Complexes of Pyrophosphate, EDTA, and Citrate. Environmental Science and Technology, 32 (19). pp. 2916-2922. ISSN 0013-936X. http://resolver.caltech.edu/CaltechAUTHORS:20180313-085319215

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Abstract

Aqueous complexes of Mn^(3+) ion with the ligands pyrophosphate (P_2O_7^(4-)), EDTA (C_(10)H_(12)O_8N_2^(4-), ethylenediaminotetraacetate^(4-)), or citrate (CIT^(3-), C_6H_5O_7^(3-)) were prepared rapidly in solution by the general reaction:  Mn(VII) + 4Mn(II)L + L→ 5Mn(III)L, where L is the ligand (in known excess). Different initial pH values of the complex solutions were obtained by prior addition of required acid, base, or buffer. Disappearance of Mn(III)L complex with time was monitored by light absorbance at appropriate wavelengths. Rates of loss for each Mn(III)L complex are found to depend on pH and the ratio [ligand]_T/[Mn]_T. Relative chemical time scales for Mn(III)L disappearance at neutral pH and similar ligand-to-metal ratios were manganese(III) pyrophosphate > manganese(III) citrate >> manganese(III) EDTA. The kinetic observations are interpreted as reflecting ligand hydrolysis and disproportionation in the case of pyrophosphate (a nonredox active compound), inner-sphere electron transfer from citrate to Mn^(3+), and both outer-sphere and intramolecular electron transfer from EDTA to Mn^(3+). In the presence of O_2, manganese(II) citrate complex is reoxidized to manganese(III) citrate. The results suggest a potential for formation of Mn(III) complexes with ligands in great excess and kinetic stabilization under certain natural water conditions (e.g., where pronounced redox gradients are present or where appreciable concentrations of superoxide, peroxide, and hydroxyl radical are produced). Similar Mn(III) behavior to that reported here might be anticipated for other ligands such as catechol and humic mattter in natural waters.


Item Type:Article
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https://doi.org/10.1021/es980308eDOIArticle
https://pubs.acs.org/doi/abs/10.1021/es980308ePublisherArticle
Additional Information:© 1998 American Chemical Society. Received for review March 27, 1998. Revised manuscript received July 3, 1998. Accepted July 3, 1998. Publication Date (Web): August 15, 1998. The financial assistance of the Mellon Foundation, the U.S. National Science Foundation, and a General Motors Research Grant in support of this work is acknowledged. We thank Kenneth H. Nealson, George W. Luther III, and Alan T. Stone for directing our attention to possibilities for aqueous Mn(III) kinetics. Janet G. Hering and Michael R. Hoffmann proposed several valuable ideas. Constructive reviews by William Sunda and two anonymous referees proved very helpful.
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Andrew W. Mellon FoundationUNSPECIFIED
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General MotorsUNSPECIFIED
Record Number:CaltechAUTHORS:20180313-085319215
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20180313-085319215
Official Citation:Kinetic Behavior of Mn(III) Complexes of Pyrophosphate, EDTA, and Citrate. J. Kenneth Klewicki and and James J. Morgan. Environmental Science & Technology 1998 32 (19), 2916-2922 DOI: 10.1021/es980308e
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85269
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:13 Mar 2018 17:08
Last Modified:13 Mar 2018 17:08

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