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Title: Influence of Iron Redox Transformations on Plutonium Sorption to Sediments

Abstract

Plutonium subsurface mobility is primarily controlled by its oxidation state, which in turn is loosely coupled to the oxidation state of iron in the system. Experiments were conducted to examine the effect of sediment iron mineral composition and oxidation state on plutonium sorption and oxidation state. A pH 6.3 vadose zone sediment containing iron oxides and iron-containing phyllosilicates was treated with various complexants (ammonium oxalate) and reductants (dithionite-citrate-bicarbonate) to selectively leach and/or reduce iron oxide and phyllosilicate phases. Mössbauer spectroscopy was used to identify initial iron mineral composition of the sediment and monitor dissolution and reduction of iron oxides. Sorption of Pu(V) was monitored over one week for each of six treated sediment fractions. Plutonium oxidation state speciation in the aqueous and solid phases was monitored using solvent extraction, coprecipitation, and XANES. Mössbauer spectroscopy showed that the sediment contained 25-30% hematite, 60-65% Al-goethite, and <10%Fe(III) in phyllosilicate; there was no detectable Fe(II). Upon reduction with a strong chemical reductant (dithionite-citrate buffer, DCB), much of the hematite and goethite disappeared and the Fe in the phyllosilicate reduced to Fe(II). The rate of sorption was found to correlate with the 1 fraction of Fe(II) remaining within each treated sediment phase. Pu(V) wasmore » the only oxidation state measured in the aqueous phase, irrespective of treatment, whereas Pu(IV) and much smaller amounts of Pu(V) and Pu(VI) were measured in the solid phase. Surface-mediated reduction of Pu(V) to Pu(IV) occurred in treated and untreated sediment samples; Pu(V) remained on untreated sediment surface for two days before reducing to Pu(IV). Similar to the sorption kinetics, the reduction rate was correlated with sediment Fe(II) concentration. The correlation between Fe(II) concentrations and Pu(V) reduction demonstrates the potential impact of changing iron mineralogy on plutonium subsurface transport through redox transition areas. These findings should influence the conceptual models of long-term stewardship of Pu contaminated sites that have fluctuating redox conditions, such as vadose zones or riparian zones.« less

Authors:
; ; ; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
1000116
Report Number(s):
PNNL-SA-70518
30449; KP1704020
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Radiochimica Acta, 98(9-11):685-692
Country of Publication:
United States
Language:
English
Subject:
Environmental Molecular Sciences Laboratory

Citation Formats

Hixon, Amy E., Hu, Yung-Jin, Kaplan, Daniel I., Kukkadapu, Ravi K., Nitsche, Heino, Qafoku, Odeta, and Powell, Brian A. Influence of Iron Redox Transformations on Plutonium Sorption to Sediments. United States: N. p., 2010. Web. doi:10.1524/ract.2010.1769.
Hixon, Amy E., Hu, Yung-Jin, Kaplan, Daniel I., Kukkadapu, Ravi K., Nitsche, Heino, Qafoku, Odeta, & Powell, Brian A. Influence of Iron Redox Transformations on Plutonium Sorption to Sediments. United States. doi:10.1524/ract.2010.1769.
Hixon, Amy E., Hu, Yung-Jin, Kaplan, Daniel I., Kukkadapu, Ravi K., Nitsche, Heino, Qafoku, Odeta, and Powell, Brian A. Fri . "Influence of Iron Redox Transformations on Plutonium Sorption to Sediments". United States. doi:10.1524/ract.2010.1769.
@article{osti_1000116,
title = {Influence of Iron Redox Transformations on Plutonium Sorption to Sediments},
author = {Hixon, Amy E. and Hu, Yung-Jin and Kaplan, Daniel I. and Kukkadapu, Ravi K. and Nitsche, Heino and Qafoku, Odeta and Powell, Brian A.},
abstractNote = {Plutonium subsurface mobility is primarily controlled by its oxidation state, which in turn is loosely coupled to the oxidation state of iron in the system. Experiments were conducted to examine the effect of sediment iron mineral composition and oxidation state on plutonium sorption and oxidation state. A pH 6.3 vadose zone sediment containing iron oxides and iron-containing phyllosilicates was treated with various complexants (ammonium oxalate) and reductants (dithionite-citrate-bicarbonate) to selectively leach and/or reduce iron oxide and phyllosilicate phases. Mössbauer spectroscopy was used to identify initial iron mineral composition of the sediment and monitor dissolution and reduction of iron oxides. Sorption of Pu(V) was monitored over one week for each of six treated sediment fractions. Plutonium oxidation state speciation in the aqueous and solid phases was monitored using solvent extraction, coprecipitation, and XANES. Mössbauer spectroscopy showed that the sediment contained 25-30% hematite, 60-65% Al-goethite, and <10%Fe(III) in phyllosilicate; there was no detectable Fe(II). Upon reduction with a strong chemical reductant (dithionite-citrate buffer, DCB), much of the hematite and goethite disappeared and the Fe in the phyllosilicate reduced to Fe(II). The rate of sorption was found to correlate with the 1 fraction of Fe(II) remaining within each treated sediment phase. Pu(V) was the only oxidation state measured in the aqueous phase, irrespective of treatment, whereas Pu(IV) and much smaller amounts of Pu(V) and Pu(VI) were measured in the solid phase. Surface-mediated reduction of Pu(V) to Pu(IV) occurred in treated and untreated sediment samples; Pu(V) remained on untreated sediment surface for two days before reducing to Pu(IV). Similar to the sorption kinetics, the reduction rate was correlated with sediment Fe(II) concentration. The correlation between Fe(II) concentrations and Pu(V) reduction demonstrates the potential impact of changing iron mineralogy on plutonium subsurface transport through redox transition areas. These findings should influence the conceptual models of long-term stewardship of Pu contaminated sites that have fluctuating redox conditions, such as vadose zones or riparian zones.},
doi = {10.1524/ract.2010.1769},
journal = {Radiochimica Acta, 98(9-11):685-692},
number = ,
volume = ,
place = {United States},
year = {Fri Oct 01 00:00:00 EDT 2010},
month = {Fri Oct 01 00:00:00 EDT 2010}
}