Conceptual Model of Iodine Behavior in the Subsurface at the Hanford Site

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Plutonium (Pu) has been released to the environment worldwide, including approximately 1.85 × 1015 Bq (200 kg) of Pu from process waste solutions to unconfined soil structures at the Hanford Site in Washington State. The subsurface mobility of Pu is influenced by complex interactions with sediments, groundwater, and any co-contaminants within the waste stream. Previous investigations at Hanford have shown that Pu exists as discrete PuO2 particles forming before or after disposal, as secondary solid phases formed from waste interactions with sediments as adsorbed/incorporated species, and/or as dissolved species. In this research, new evidence is presented for the existence of PuO2, PuO2-Bi2O3 composites, and particles from burnt Pu metal in near-surface sediments where Pu-laden acidic process waste was disposed to sediments. Pu and americium (Am) L 3 X-ray absorption spectroscopy and density functional theory suggest that, in larger, more crystalline PuO2 particles, Am formed from radioactive decay is retained in the PuIVO2 structure as AmIV. The Pu and Am that were disposed of in an acidic waste stream have since migrated deeper into the subsurface with detection to at least 37 meters below ground surface. In contrast, Pu deposited near the ground surface from neutral pH waste is found to be homogeneously distributed and relatively immobile. Groundwater extractions performed on contaminated sediments indicate that both Pu and Am are recalcitrant, with Am being fractionally less extractable than Pu on a molar basis. These results suggest that the more mobile fraction of Am has migrated from the near-surface and may be present in the deeper sediments as a different phase than Pu. From these results, it is suggested that Pu and Am deposited from acidic wastes were initially mobile and became significantly less mobile as wastes were neutralized within the soil profile.

Section: Methodsmentioning

confidence: 99%

Influences on Subsurface Plutonium and Americium Migration

Emerson

Pearce

Delegard³

et al. 2021

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“…For the purpose of this work, two geochemical mechanisms for iodate and chromate removal should first be considered: mineral incorporation and adsorption. As previously reported, the incorporation of iodate and chromate into calcite occurs during calcite formation and growth; however, once calcite growth has ceased, aqueous iodate and chromate removal is controlled by adsorption. ,,, Adsorption of chromate however is highly susceptible to competing ions, and the adsorption behavior of both anions is highly dependent upon changes in aqueous conditions, e.g., pH, dissolved carbonate, and calcium content. In this work, calcite was observed to precipitate rapidly upon mixing of the CaCl 2 and (NH 4 ) 2 CO 3 solutions rather than grow slowly over the course of 21 days.…”

Section: Resultsmentioning

confidence: 82%

“…Of particular interest to the remediation of 129 I and Cr(VI) is the incorporation of iodate (IO 3 – ) and chromate (CrO 4 2– ) into calcite through carbonate (CO 3 2– ) substitution. Extensive research has already been published for calcite incorporation of iodate ,− and chromate, − all of which suggest successful incorporation of these anions into the calcite crystal structure. However, these studies have treated iodate and chromate incorporation independently and have not studied potential competitive effects that may exist over a range of concentration conditions.…”

Section: Introductionmentioning

confidence: 99%

Chromate Effect on Iodate Incorporation into Calcite

Saslow

Kerisit

Varga

et al. 2019

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Incorporation of iodate into calcite (CaCO3) may be used as an in situ treatment strategy for radioiodine in contaminated soils and groundwater, but the presence of other contaminants may inhibit its efficiency. To this end, the potential for chromate to interfere with iodate incorporation into CaCO3 was investigated as an example of how co-located contaminants may impact in situ remediation efficacy. Here, batch precipitation experiments were periodically sub-sampled over 21 days to determine the kinetic effects of chromate on iodate removal and incorporation into calcite. From these experiments, a decrease in iodate removal from >60 to <40% was observed upon chromate addition (1–112 ppm of chromate) and ≤11% of chromate was removed with a minor dependence upon the initial chromate concentration. Analysis of the solid phase using extended X-ray absorption fine structure (EXAFS) spectroscopy informed by ab initio molecular dynamics simulations revealed that the iodate incorporation mode remains unchanged by the presence of chromate. Iodate readily substitutes for carbonate (CO3 2–), and the calcite structure is charge-balanced primarily by substituting H+ for Ca2+. Furthermore, chromate incorporated as a nearest neighbor to iodate did not contribute to the EXAFS fit; therefore, iodate and chromate clustering is unlikely when co-incorporated into calcite.

“…The behavior of iodine in the environment is complex due to multiple physical and redox states, interactions with organic matter, and microbial transformations. 3 Iodine can form hypodiodous acid in water, and both species can react with natural organic matter to form organo-I compounds. 5 Hanford sediments generally have a very low natural organic matter content 6,7 and relatively low microorganism densities, except in the hyporheic zone along the Columbia River.…”

Section: Introductionmentioning

confidence: 99%

Experimental and Numerical Study of Radioiodine Sorption and Transport in Hanford Sediments

He,

Rockhold,

Fang

et al. 2024

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Radioiodine ( 129 I) poses a potential risk to human health and the environment at several U.S. Department of Energy sites, including the Hanford Site, located in southeastern Washington State. Experimental studies and numerical modeling were performed to provide a technical basis for field-scale modeling of iodine sorption and transport behavior. The experiments were carried out using six columns of repacked contaminated sediments from the Hanford Site. Although iodate has been determined to be the dominant iodine species at the Hanford Site, the sorption and transport behaviors of different iodine species were investigated in a series of column experiments by first leaching sediments with artificial groundwater (AGW) followed by AGW containing iodate (IO 3 − ), iodide (I − ), or organo-iodine (2-iodo-5-methoxyphenol, C 7 H 7 IO 2 ). Ferrihydrite amendments were added to the sediments for three of the columns to evaluate the impact of ferrihydrite on 129 I attenuation. The results showed that ferrihydrite enhanced the iodate sorption capacity of the sediment and retarded the transport but had little effect on iodide or organo-I, providing a technical basis for developing a ferrihydrite-based remedial strategy for iodate under oxidizing conditions. Data from the column transport experiments were modeled using the linear equilibrium Freundlich isotherm model, the kinetic Langmuir adsorption model, and a distributed rate model. Comparisons of the experimental data and modeling results indicated that sorption was best represented with the distributed rate model with rates and maximum sorption extents varying by iodine species and ferrihydrite treatment. However, the linear Freundlich isotherm (K d ) model was also found to fit the laboratory experimental data relatively well, suggesting that the K d model could also be used to represent iodine transport at the field scale.