Web Release Date: December 3,
Zinc Adsorption Effects on Arsenite Oxidation Kinetics at the Birnessite-Water Interface
and
Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 19717-1303, and U.S. Geological Survey, Water Resources Division, 345 Middlefield Road, MS 465, Menlo Park, California 94025
Received for review January 15, 2004
Revised manuscript received October 11, 2004
Accepted October 14, 2004
Abstract:
Arsenite is more toxic and mobile than As(V) in soil and
sediment environments, and thus it is advantageous to explore
factors that enhance oxidation of As(III) to As(V). Previous
studies showed that manganese oxides, such as birnessite
(
-MnO2), directly oxidized As(III). However, these studies
did not explore the role that cation adsorption has on
As(III) oxidation. Accordingly, the effects of adsorbed and
nonadsorbed Zn on arsenite (As(III)) oxidation kinetics
at the birnessite-water interface were investigated using
batch adsorption experiments (0.1 g L-1; pH 4.5 and 6.0;
I = 0.01 M NaCl). Divalent Zn adsorption on synthetic -MnO2
in the absence of As(III) increased with increasing pH
and caused positive shifts in electrophoretic mobility values
at pH 4-6, indirectly suggesting inner-sphere Zn adsorption
mechanisms. Arsenite was readily oxidized on birnessite
in the absence of Zn. The initial As(III) oxidation rate constant
decreased with increasing pH from 4.5 to 6.0 and initial
As(III) concentrations from 100 to 300 
M. Similar pH and
initial As(III) concentration effects were observed in
systems when Zn was present (i.e., presorbed Zn prior to
As(III) addition and simultaneously added Zn-As(III)
systems), but As(III) oxidation reactions were suppressed
compared to the respective control systems. The
suppression was more pronounced when Zn was presorbed
on the -MnO2 surfaces as opposed to added simultaneously
with As(III). This study provides further understanding
of As(III) oxidation reactions on manganese oxide surfaces
under environmentally applicable conditions where
metals compete for reactive sites.
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