Effect of Oxygenation on Speciation, Behavior, and Fate of Chromium in Estuarine Sediments www.epa.gov/.../images/allied_tents.jpg www.epa.gov/.../newsletter/images/allied1988.jpg Amar R. Wadhawan and Edward J. Bouwer Department of Geography and Environmental Engineering Johns Hopkins University Sustainability: Reducing Toxic Pollutants Environmental Management for Enclosed Coastal Seas (EMECS9), 8/29/11
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Effect of Oxygenation on Speciation, Behavior, and Fate of ... · Toxicity in the Baltimore Harbor Sediments Cr speciation and biogeochemistry in anoxic estuarine sediments Experimental
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Effect of Oxygenation on Speciation, Behavior, and Fate of Chromium in
Influence of pH on Mn Speciation and Solubility in a Sulfidic Environment
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3 5 7 9 11
To
tal D
isso
lve
d [
i] (
mM
)
pH
HS-
Mn2+
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[i]
(mM
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pH
HS-
Mn2+
H2S
Mn(OH)4
Mn2(OH)3MnHS
MnOH
s2
MnS(grn)
MnS(pnk)
Calculations performed using Visual MINTEQ
Conclusions• CrVI formation occurs under aerobic conditions but not under anaerobic conditions. The CrVI formation
correlates negatively with the AVS concentration in the sediments and positively with porewater-Mn /AVS ratio.
• The rate of CrIII oxidation increases with increase in sediment loading for dmt-207 site which is low in AVSbut decrease with increase in sediment loading for 68-909 site which is high in AVS. This suggests that CrVI
formation is a function of the sediment oxidizing capacity but its persistence is a function of the sedimentreducing capacity.
• Once all CrVI is reduced to CrIII under anaerobic conditions, we observe CrVI reoccurrence upon sedimentoxygenation. This CrVI reoccurrence is a function of the aging time.
• No CrVI formation is observed below pH 7. The rate of CrVI formation increases with increase in pH abovepH 7. This CrVI formation is coupled with loss of dissolved Mn above pH 7.
• CrVI formation increases with increase in MnII addition. This coupled with the observations of initial lagtime, the loss of dissolved Mn upon aeration at pH ≥ 7, and no effect of microbial activity suggest that CrIII
oxidation in Baltimore Harbor sediments is controlled by heterogeneous autocatalytic oxidation ofdissolved Mn by O2 that results in the formation of MnIII,IV(hydr)oxides responsible for oxidizing CrIII insediments.
Conclusion• Oxygenation of anoxic estuarine sediment facilitates CrIII oxidation and CrVI
persistence in these sediments at pH ≥ 7.
• CrIII oxidizing potential of sediments, the lag time, and the variation in CrVI
formation rates among different sediment sites appear to correlate negatively with
[AVS] and positively with [PW-Mn]/[AVS].
• Loss of dissolved Mn from solution due to autocatalytic oxidation by O2 at pH ≥ 7
should result in the formation of reactive MnIII,IV(hydr)oxides responsible for
oxidizing CrIII in sediments upon oxygenation. However, CrVI persistence in
sediments depends on [AVS].
• CrVI reoccurrence is observed upon oxygenation of anoxic CrVI spiked sediments at
pH ≥ 7. The rate of CrVI reoccurrence decreases with increase in CrIII aging and
increase with increase in pH at pH ≥ 7.
Summary: CrIII Oxidation in SedimentsSpatial and temporal variation
Lag time and rates of CrVI formation among different sites correlate negatively with [AVS] and positively
with [PW-Mn]/[AVS]
Effect of [CrIII]0
Rates of CrIII increase with increase in [CrIII]0 upto 150 µM. At higher concentration surface Cr(OH)3 ,
CrxFe1-x(OH)3 precipitates seem to inhibit the reaction
Effect of sediment loading
Rates increase with increase in sediment loading for low AVS sites but decrease with increase in
sediment loading for high AVS sites. CrIII oxidation appears to be a function of dissolved [Mn] whereas
CrVI persistence appears to be a function of [AVS]
Effect of pH
CrVI formation observed at pH 7 and above and it correlates with loss of dissolved Mn suggesting that
autocatalytic Mn oxidation is potentially responsible for CrIII oxidation in sediments
Effect of MnII addition
Increase in [MnII] spike increases the reaction rate
CrVI reoccurrence is observed upon oxygenation of anoxic CrVI spiked sediments at pH ≥ 7. The rate of
CrVI decreases with increase in CrIII aging and increase with increase in pH at pH ≥ 7.