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The reduction and dissolution of Mn(III) and Mn(IV) oxides by organics

Stone, Alan Thomas (1983) The reduction and dissolution of Mn(III) and Mn(IV) oxides by organics. California Institute of Technology , Pasadena, CA. (Unpublished)

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Although it is known that manganese oxides are solubilized by reduction in anoxic waters, the chemical processes are poorly understood. A study of the reduction and dissolution of manganese oxide suspensions by twenty-seven organic substrates that have chemical structures similar to those of natural organics was undertaken to determine the rates and mechanisms of the solubilization reactions. Dissolution of suspensions by hydroquinone in the pH range 6.5 < pH < 8.5 is described by the following experimental rate law: d[Mn^(2+)]/dt = k_l{H^+}^(0. 46) [HQ]^(1.0) (Mn_T - [Mn^(2+)]) where [Mn^(2+)] is the amount of dissolved manganese, [HQ] is the hydroquinone concentration, and Mn_T is the initial amount of manganese oxide. The apparent activation energy of the reaction was found to be +37 kJ/mole. The Mn(III,IV) oxide suspension was prepared by oxidizing a Mn(OH)_2(s.) suspension with oxygen, and has a composition characterized by MnO_(1.66). Suspension particles were between 0.2 and 1.0 microns in diameter. Calcium and phosphate were found to inhibit the dissolution reaction, by adsorbing on the oxide surface. Dihydroxybenzenes and methoxyphenols dissolved the suspensions at appreciable rates. Of the aliphatic substrates examined, only ascorbate, oxalate, and pyruvate dissolved the oxide. Dissolution by marine fulvic acid was found to be photocatalyzed. A model was developed to explain the observed rate dependence and the relative reactivity of different organic substrates. The model assumes that complexes between substrate and surface sites form prior to electron transfer and dissolution. The pH dependence is not explained by this model; involvement of H^+ in the dissolution of reduced surface sites may be responsible for the observed fractional order with respect to H^+.

Item Type:Report or Paper (Technical Report)
Additional Information:© 1983 Alan Thomas Stone. All Rights Reserved. I wish to thank my advisor James J. Morgan for directing his attention towards my project and my welfare, and for allowing me freedom and flexibility in research. I would also like to thank those who encouraged me to enter this work: my mother, my father, Rena Zafiriou, and George Helz. A number of fellow students and associates contributed to this work through their advice, support, and friendship: Howard Liljestrand, Jim Young, Jim Hunt, Steve Johnston, Windsor Sung, Scott Boyce, Bruce Faust, Roger Bales, Connie Senior, and many others. Michael Barcelona and Simon Davies provided direction and much encouragement. Michael Hoffmann, Fred Anson, George Rossman, and John List kindly served on my examining committees. Michael Hoffmann's continual support and interest is greatly appreciated. Robert Koh helped with computational problems, both in person and indirectly through the program MAGIC. George Rossman, Roger Aines, Heinz Lowenstam, and Sten Samson assisted me in characterizing manganese oxides, and their help and enthusiasm was appreciated. Discussions with researchers from other institutions, especially Walter Schneider, David Waite, and William Sunda were quite helpful. The staff of Keck Labs went to great lengths to assist with whatever problems arose, especially Elaine Granger and Joan Matthews. I would like to thank my family for following my progress, and also Marty Gould, Dan Zwillinger, and the Alams (Alam, Natasha, and Mishi), close friends who always welcomed my company. Financial support from the Jessie Smith Noyes Foundation Fellowship, Union Oil of California, and the President's Fund is gratefully acknowledged.
Group:W. M. Keck Laboratory of Hydraulics and Water Resources
Record Number:CaltechKHR:AC-1-83
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ID Code:26019
Deposited By: Imported from CaltechKHR
Deposited On:07 Jan 2010
Last Modified:03 Oct 2019 03:11

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