CHROMIUM OXIDATION AND REDUCTION BY HYDROGEN PEROXIDE IN DIVERSE SOILS AND SIMPLE AQUEOUS SYSTEMS
CHROMIUM OXIDATION AND REDUCTION BY HYDROGEN PEROXIDE IN DIVERSE SOILS AND SIMPLE AQUEOUS SYSTEMS
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Date
1999
Authors
Rock, Melanie Louise
Advisor
Helz, George R.
James, Bruce R.
James, Bruce R.
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DRUM DOI
Abstract
Hydrogen peroxide is being tested for in situ remediation of buried
contaminants - either as a direct chemical oxidant in Fenton-type reactions or as a
source of oxidizing equivalents in bioremediation. How it affects a common co-contaminant,
Cr, is explored here in four chemically diverse high-Cr soils.
Soils contaminated with high levels of soluble Cr(VI) from ore processing and
soils containing high levels of recently reduced Cr(III) from electroplating waste
showed marked increases in chromate after single applications of J-25 mM peroxide.
Cr(VI) in the leachates exceeded the drinking water standard (2μM) by 1-3 orders of
magnitude. Soluble Cr(III), in the form of dissolved organic complexes, contributed to
the likelihood of Cr(III) oxidation. Anaerobic soil conditions at a tannery site prevented oxidation of Cr(III). Naturally occurring Cr in serpentine soil also resisted
oxidation. Ambient soluble Cr(VI) in a contaminated aquifer disappeared from
peroxide leachates below pH 5, then reappeared as peroxide levels declined.
In solutions prepared under environmentally relevant conditions, aged 280
μM Cr(III) treated with 100 μM H2O2 showed increases in Cr(VI) over weeks with
maximum oxidation rates achieved in solutions prepared with 2:1 and 4:1 OH^-:Cr.
Although Cr(III) speciation differs in fresh and aged aqueous systems, a similar
mechanism involving the pre-equilibrium step: Cr(OH)/ + OH- .,. Cr(OH)/ may
account for Cr(III) oxidation in both systems. Under alkaline conditions, H2O2
enhanced the oxidative dissolution of Crn(OH)3n^0. The formation of peroxochromium
compounds in the presence of H2O2 and Cr(VI) may account for the disappearance and
reappearance of Cr(VI) in H2O2 treated soils; as does the possible formation and
subsequent reoxidation of Crm\(OH)3n-2^2+ oligomers.
Mobilization of hazardous Cr(VI) must be considered in plans to use H2O2 for
remediation of chemically complex wastes. Once Cr(III) is oxidized to Cr(VI) by H2O2
it may persist long after applied H2O2 treatments have disappeared. Further, hexavalent
Cr will behave as a catalyst toward H20 2 in soils, enhancing its oxidative capacity while
helping to dissipate high levels of applied H2O2.