Enhanced Uranium Immobilization by Phosphate Amendment under Variable Geochemical and Flow Conditions

Insights from Reactive Transport Modeling

Hang Wen, Zezhen Pan, Daniel Giammar, Li Li

Research output: Contribution to journalArticle

6 Citations (Scopus)

Abstract

Phosphate amendment has shown promise for enhancing uranium immobilization. The mechanism of the enhancement, however, has remained unclear with contrasting observations under variable environmental conditions. A dual-domain reactive transport model is developed here with constraints from batch and column experimental data to understand the mechanisms and to explore the effectiveness of enhanced U(VI) immobilization under variable geochemical and flow conditions. Modeling results indicate that under low U(VI) conditions in natural waters, phosphate addition promotes U(VI) immobilization through the formation of U(VI)-phosphate ternary surface complexes and the precipitation of calcium phosphate, both decreasing the concentrations of mobile U-Ca-CO3 aqueous complexes. This contrasts with previous hypotheses attributing the immobilization enhancement to U(VI)-phosphate precipitation under experimental conditions with high U(VI). Sensitivity analysis shows that phosphate is effective under relatively low Ca (<0.1 mM) and total inorganic carbon (TIC) (<0.5 mM) conditions, where >60% of U(VI) still remains on sediments after 113 residence times of flushing with low phosphate solutions (<0.1 mM). Under high Ca or TIC conditions, a similar level of U(VI) immobilization can be achieved only when the phosphate concentration is higher than Ca or TIC concentrations. Compared to the strong geochemical effects, flow conditions have relatively limited impacts on U(VI) immobilization. These results explain contrasting field observations on the effectiveness of phosphate amendment and offer capabilities to extrapolate observations to other environmental conditions.

Original languageEnglish (US)
Pages (from-to)5841-5850
Number of pages10
JournalEnvironmental Science and Technology
Volume52
Issue number10
DOIs
StatePublished - May 15 2018

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Uranium
reactive transport
immobilization
uranium
Phosphates
phosphate
modeling
environmental conditions
Sensitivity analysis
flushing
Sediments
sensitivity analysis
residence time
calcium
Water

All Science Journal Classification (ASJC) codes

  • Chemistry(all)
  • Environmental Chemistry

Cite this

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title = "Enhanced Uranium Immobilization by Phosphate Amendment under Variable Geochemical and Flow Conditions: Insights from Reactive Transport Modeling",
abstract = "Phosphate amendment has shown promise for enhancing uranium immobilization. The mechanism of the enhancement, however, has remained unclear with contrasting observations under variable environmental conditions. A dual-domain reactive transport model is developed here with constraints from batch and column experimental data to understand the mechanisms and to explore the effectiveness of enhanced U(VI) immobilization under variable geochemical and flow conditions. Modeling results indicate that under low U(VI) conditions in natural waters, phosphate addition promotes U(VI) immobilization through the formation of U(VI)-phosphate ternary surface complexes and the precipitation of calcium phosphate, both decreasing the concentrations of mobile U-Ca-CO3 aqueous complexes. This contrasts with previous hypotheses attributing the immobilization enhancement to U(VI)-phosphate precipitation under experimental conditions with high U(VI). Sensitivity analysis shows that phosphate is effective under relatively low Ca (<0.1 mM) and total inorganic carbon (TIC) (<0.5 mM) conditions, where >60{\%} of U(VI) still remains on sediments after 113 residence times of flushing with low phosphate solutions (<0.1 mM). Under high Ca or TIC conditions, a similar level of U(VI) immobilization can be achieved only when the phosphate concentration is higher than Ca or TIC concentrations. Compared to the strong geochemical effects, flow conditions have relatively limited impacts on U(VI) immobilization. These results explain contrasting field observations on the effectiveness of phosphate amendment and offer capabilities to extrapolate observations to other environmental conditions.",
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Enhanced Uranium Immobilization by Phosphate Amendment under Variable Geochemical and Flow Conditions : Insights from Reactive Transport Modeling. / Wen, Hang; Pan, Zezhen; Giammar, Daniel; Li, Li.

In: Environmental Science and Technology, Vol. 52, No. 10, 15.05.2018, p. 5841-5850.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Enhanced Uranium Immobilization by Phosphate Amendment under Variable Geochemical and Flow Conditions

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AU - Wen, Hang

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AU - Giammar, Daniel

AU - Li, Li

PY - 2018/5/15

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AB - Phosphate amendment has shown promise for enhancing uranium immobilization. The mechanism of the enhancement, however, has remained unclear with contrasting observations under variable environmental conditions. A dual-domain reactive transport model is developed here with constraints from batch and column experimental data to understand the mechanisms and to explore the effectiveness of enhanced U(VI) immobilization under variable geochemical and flow conditions. Modeling results indicate that under low U(VI) conditions in natural waters, phosphate addition promotes U(VI) immobilization through the formation of U(VI)-phosphate ternary surface complexes and the precipitation of calcium phosphate, both decreasing the concentrations of mobile U-Ca-CO3 aqueous complexes. This contrasts with previous hypotheses attributing the immobilization enhancement to U(VI)-phosphate precipitation under experimental conditions with high U(VI). Sensitivity analysis shows that phosphate is effective under relatively low Ca (<0.1 mM) and total inorganic carbon (TIC) (<0.5 mM) conditions, where >60% of U(VI) still remains on sediments after 113 residence times of flushing with low phosphate solutions (<0.1 mM). Under high Ca or TIC conditions, a similar level of U(VI) immobilization can be achieved only when the phosphate concentration is higher than Ca or TIC concentrations. Compared to the strong geochemical effects, flow conditions have relatively limited impacts on U(VI) immobilization. These results explain contrasting field observations on the effectiveness of phosphate amendment and offer capabilities to extrapolate observations to other environmental conditions.

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