Kinetics of adsorption and redox processes on iron and manganese oxides: reactions of As(III) and Se(IV) at goethite and birnessite surfaces
Creators: name: Scott, Michael James
Publisher: name: California Institute of Technology
Year: 1991

description: Selenium and arsenic are naturally-occurring, non-metallic elements with complex chemical and biological behavior in aquatic environments. In this study, rates and mechanisms of adsorption, desorption, and electron transfer reactions involving selenium and arsenic oxyanions and two naturally occurring metal oxides, goethite (α-FeOOH) and birnessite (δ-MnO_2), have been investigated. Adsorption of Se(IV), As (III) , and As(V) on goethite and of Se(IV) and As(III) on birnessite occurs within a time scale of minutes. Equilibrium is achieved within a few hours. Adsorption behavior can be described accurately with a surface complexation model. 

Goethite does not oxidize Se(IV) or As(III) in solution at pH 4 and above. However, redox products (Mn(II), Se(V), As(V) are observed when Se(IV) or As(III) is added to aqueous suspensions of birnessite. In the arsenite-birnessite system, the rate of As(V) appearance in solution is equal to the rate of As(III) disappearance from solution while the appearance of Mn(II) in solution is slightly slower. In the selenite-birnessite system, uptake of Se(IV) occurs in minutes. Extent of adsorption decreases with increasing pH. The appearance of measurable Se(VI) occurs slowly (time scale of days to weeks) and is a function of adsorbed selenite. This indicates that the rate of selenite oxidation by birnessite is limited by the rate of electron transfer. Rate data from both arsenic and selenium redox systems are successfully described by a reversible four-step kinetic model that accounts for adsorption of the reduced species, electron-transfer, release of the oxidized species, and release of reduced Mn(II).

The data suggest that iron oxides provide an adsorptive sink for mobile Se and As oxyanions, while manganese oxides playa major role in accelerating the oxidation of Se(IV) and As (III). Results on the rates of key chemical processes affecting selenium and arsenic should be useful in understanding complex geochemical cycles and in finding solutions to problems in pollutant transport and accumulation in water-sediment systems.
description: © 1991 Environmental Quality Laboratory. California Institute of Technology. 

I wish to thank my advisor Professor James Morgan for his latitude in his guidance and support. Without it, this typical Scott project would have never matured into a typical Morgan thesis. I would also like to thank Professors Michael Hoffmann, Norman Brooks, Clair Patterson, and George Rossman for their scientific encouragement and for serving on my examination committees. Also, I would like to acknowledge Dr. Alan Stone of Johns Hopkins University for providing the computer code framework which made the kinetic modeling possible. 

I would like to gratefully acknowledge the financial support of the initial portion of my research by the Andrew W. Mellon Foundation. Also, the research on which this report is based was financed in part by the United States Department of the Interior, Geological Survey, through the State Water Resources Research Institute, Project No. 14-08-0001-G1550, and by the University of California Water Resources Center, Project UCAL-WRC-W-22605-89(03). Contents of this publication do not necessarily reflect the views and policies of the U.S. Department of Interior, nor does mention of trade names or commercial products constitute their endorsement or recommendation for used by the U.S. Government. 

I would like to thank the friendly and generous staff of Keck Laboratories -- Elaine Granger, Joan Matthews, Sandy Brooks, and Fran Matzen--and the Environmental Engineering librarians -- Rayma Harrison and Gunilla Hastrup -- for always making time for me and, especially, for Caitlin. I would like to express my appreciation for all their assistance and friendship in the classroom, laboratory, ball fields, and, in general, my life from 1985 to 1990 to Yigal Erel, Kevin Power, Nick Bauer, Julie Kern, David Wheeler, Bruce Daube, Jr., Jeff Collett, Bill Munger, Bob Arnold, Theresa Fall, Tom DiChristina, the Waterbugs, David James, Claudius Kormann, Kit Yin Ng, Liyuan Liang, Mark Schlautman, Sandy Elliot, Howell Yee, Natasha Kotronarou, Stan Grant, Jeremy Semrau, Annmarie Eldering, and Russell Mau. 

Finally, I would like to acknowledge the encouragement and support given to me by family and friends, without which I could have never reached this point in my life. And most of all, I would like to simply say to Wendy, "Thanks for your love -- it makes everything worthwhile." 

This report was submitted to the California Institute of Technology in May 1991 as a thesis in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Environmental Engineering Science.