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Sistema de Información Científica
Red de Revistas Científicas de América Latina y el Caribe, España y Portugal
MOBILIZATION OF OXIDE-BOUND TOXIC METAL8 BY ORGANIC REDUCING
AGENTS AND COXPLEXING AGENTS
Alan T. Stone
Kathy Godtfredsen
Department of Geography and Environmental Engineering
The Johns Hopkins University
313 Arnes Hall, Baltimore, Maryland, 21218 USA. Fax 301-338-7092
Within oxid soils and aquifer sediments, manganese(II1,IV) oxides
and iron(II1) oxides are important sorptive phases of toxic
metals such as Co(I1,III)
,
Ni (11)
,
Cu(I1)
,
and Pb(I1, IV)
.
Changes
in ambient chemistry can, however, bring about the solubilization
and mobilization of these toxic metals. Our research examines
these processes, emphasizing how changes in the chemical nature
and quantity of natural organic compounds cause toxic metal
mobilization. To begin our work, toxic metals were equilibrated
with MnO2 (S) particles in suspension for 17 hours under
oxygenated conditions. Adsorption affinity decreased in the
followind order: Co,Cu>Ni. Low molecular weight organic
complexing agents and reducing agents were selected for study
that have either been identified in soils and sediments, or are
known to be produced by microrganisms. Above pH 6, 100 micromolar
oxalate acts a complexing agent, bringing about the release of
adsorbed toxic metals, by leaving the host Mn02 particles intact.
The total amount of toxic metals released in related to the
stability constant for metal-oxalate complex formation. Below pH
6, oxalate acts as a complexing agent and a reducing agent.
Reduction of Mn02 to Mn++ causes toxic metal release through two
indirect mechanisms: (1) Loss of host oxide surface area by
dissolution, and (2) production of Mn++, which displaces adsorbed
toxic metals at available surface sites. Other low molecular
weight compounds include pyruvate (acting only as a reducing
agent), and citrate (acting as both, complexing and a reducing
agent)
.
Natural organic matter (NOM) samples from southern U. S.
coastal swamps contain a complex mixture of low, medium, and high
molecular weight molecules that differ in the capacity to reduce
a complex metal. Addition of NOM brought about release of
adsorbed toxic metals and reductive dissolution of the host Mn02
phase. Our ultimate goal is to improve our understanding of metal
mobilization processes and improve predictions concerning
mobilization rates when ambient geochemical conditions are
changed. This should improve our ability to assess the impact of
existing environmental contamination, develop
methods of
remediation, and develop guidelines for future metal disposal
practices.
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