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Catalytic effects of the γ-FeOOH (lepidocrocite) surface on the oxygenation removal kinetics of Fe(II) and Mn(II)

Sung, Windsor (1980) Catalytic effects of the γ-FeOOH (lepidocrocite) surface on the oxygenation removal kinetics of Fe(II) and Mn(II). California Institute of Technology , Pasadena, CA. (Unpublished)

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Previous investigations of Fe(II) oxygenation had resulted in a wide range in the reported rate constant(s). While Fe(II) oxygenation rates are fast in simple laboratory systems (seconds to minutes when pH > 7), actual rates observed in natural waters can be orders of magnitude lower. Conversely, while Mn(II) oxygenation rates are slow in laboratory systems (days when pH < 9), much faster rates are observed in natural waters or implicated in model studies. The influences of ionic strength, temperature and anions on the Fe(II) homogeneous oxygenation rates were examined in this study. Other rate constants from the literature were successfully incorporated into this framework. Complexation by major anions (e.g., SO_4^(2-) and Cl^-) and ionic strength effects were sufficient to account for the retardation of Fe(II) oxygenation in seawater. Autocatalysis of Fe(II) oxygenation was observed for pH > 7. A general integrated autocatalytic rate expression suitable for Fe(II) or Mn(II) oxygenation was used to interpret laboratory-obtained kinetic data. Oxidation of Fe(II) in various laboratory systems with characteristics like those of natural water was shown to form the allotrope γ-FeOOH. The γ-FeOOH surface was shown to be an excellent catalyst for Fe(II) and Mn(II) oxygenation. The γ-FeOOH surface obtained by oxidizing milli-molar levels of Fe(II) in 0.7 M NaCl was studied in the following ways: surface charge characteristics by acid/base titration; adsorption of Mn(II) and surface oxidation of Mn(II). A rate law was formulated to account for the effects of pH and the amount of surface on the surface oxidation rate of Mn(II). The presence of milli-molar levels of γ-FeOOH was shown to reduce significantly the half-life of Mn(II) in 0.7 M NaCl from hundreds of hours to hours. The γ-FeOOH surface was shown to be as effective as colloidal MnO_2 in catalysing Mn(II) oxygenation.

Item Type:Report or Paper (Technical Report)
Additional Information:I wish to thank my advisor, James J. Morgan, for his patience. His timely criticisms were very helpful. Thanks are due to the following professors who have served on my committees: F. C. Anson, N. H. Brooks, R. C. Flagan, W. L. Johnson, W. J. North and G. R. Rossman. R. B. Burns at MIT has been a most stimulating professor during my undergraduate days and continues to be of much support. Technical help has been invaluable from the following: P. E. Duwez, S. Kotake and S. Samson for X-ray diffraction. R. Potter and G. R. Rossman for instructions on the infrared spectroscopy work. J. B. Earnest kindly did the BET surface area measurements for me in his spare time at JPL. Heartfelt thanks are due to Elaine Granger for typing my manuscript efficiently. Discussions with J. R. Hunt and J. R. Young have helped clear my thinking. I thank their patience for those uninvited walk-in discussions. Mr. and Mrs. K. Y. Ho and Mr. and Mrs. Z. K. Sung provided much needed moral support. Caltech has been an enjoyable institution to pursue academic work. Classical guitar lessons with D. Denning provided cultural breaks. Financial support from the Dupont and Union Oil companies is gratefully acknowledged.
Group:W. M. Keck Laboratory of Hydraulics and Water Resources
Record Number:CaltechKHR:AC-12-80
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ID Code:26002
Deposited By: Imported from CaltechKHR
Deposited On:10 Dec 2009
Last Modified:03 Oct 2019 03:10

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