TY - JOUR
T1 - The citric acid-MnIII,IVO2(birnessite) reaction. Electron transfer, complex formation, and autocatalytic feedback
AU - Wang, Yun
AU - Stone, Alan T.
N1 - Funding Information:
This work was supported by Grant Number R-82935601 from the U.S. Environmental Protection Agency’s Science to Achieve Results (STAR) program. Although the research described in this article has been funded wholly by this grant, it has not been subject to EPA review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. We thank Dr. Anne-Claire Gaillot (Department of Earth and Planetary Sciences, Johns Hopkins University) for her help with MnO 2 (birnessite) characterization. We thank Dr. Roland Glantz and Wei Long (Department of Geography and Environmental Engineering, Johns Hopkins University) for their help with MATLAB programming. We also thank Prof. James J. Morgan and two anonymous reviewers for thoughtful discussion and insightful comments.
Copyright:
Copyright 2011 Elsevier B.V., All rights reserved.
PY - 2006/9/1
Y1 - 2006/9/1
N2 - Citrate released by plants, bacteria, and fungi into soils is subject to abiotic oxidation by MnO2(birnessite), yielding 3-ketoglutarate, acetoacetate, and MnII. Citrate loss and generation of products as a function of time all yield S-shaped curves, indicating autocatalysis. Increasing the citrate concentration decreases the induction period. The maximum rate (rmax) along the reaction coordinate follows a Langmuir-Hinshelwood dependence on citrate concentration. Increases in pH decrease rmax and increase the induction time. Adding MnII, ZnII, orthophosphate, or pyrophosphate at the onset of reaction decreases rmax. MnII addition eliminates the induction period, while orthophosphate and pyrophosphate addition increase the induction period. These findings indicate that two parallel processes are responsible. The first, relatively slow process involves the oxidation of free citrate by surface-bound MnIII,IV, yielding MnII and citrate oxidation products. The second process, which is subject to strong positive feedback, involves electron transfer from MnII-citrate complexes to surface-bound MnIII,IV, generating MnIII-citrate and MnII. Subsequent intramolecular electron transfer converts MnIII-citrate into MnII and citrate oxidation products.
AB - Citrate released by plants, bacteria, and fungi into soils is subject to abiotic oxidation by MnO2(birnessite), yielding 3-ketoglutarate, acetoacetate, and MnII. Citrate loss and generation of products as a function of time all yield S-shaped curves, indicating autocatalysis. Increasing the citrate concentration decreases the induction period. The maximum rate (rmax) along the reaction coordinate follows a Langmuir-Hinshelwood dependence on citrate concentration. Increases in pH decrease rmax and increase the induction time. Adding MnII, ZnII, orthophosphate, or pyrophosphate at the onset of reaction decreases rmax. MnII addition eliminates the induction period, while orthophosphate and pyrophosphate addition increase the induction period. These findings indicate that two parallel processes are responsible. The first, relatively slow process involves the oxidation of free citrate by surface-bound MnIII,IV, yielding MnII and citrate oxidation products. The second process, which is subject to strong positive feedback, involves electron transfer from MnII-citrate complexes to surface-bound MnIII,IV, generating MnIII-citrate and MnII. Subsequent intramolecular electron transfer converts MnIII-citrate into MnII and citrate oxidation products.
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U2 - 10.1016/j.gca.2006.06.1551
DO - 10.1016/j.gca.2006.06.1551
M3 - Article
AN - SCOPUS:33747771000
VL - 70
SP - 4463
EP - 4476
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
SN - 0016-7037
IS - 17
ER -