Uranium Release from Acidic Weathered Hanford Sediments: Single

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Uranium Release from Acidic Weathered Hanford Sediments: Single-Pass Flow-Through and Column Experiments Guohui Wang, Wooyong Um, Zheming Wang, Estela Reinoso-Maset, Nancy M Washton, Karl T. Mueller, Nicolas Perdrial, Peggy A. O'Day, and Jon Chorover Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03475 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 9, 2017

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Environmental Science & Technology

Uranium Release from Acidic Weathered Hanford Sediments: Single-Pass Flow-Through and Column Experiments

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Guohui Wang1, Wooyong Um1,2,*, Zheming Wang1, Estela Reinoso-Maset3, Nancy M. Washton1, Karl T. Mueller1, Nicolas Perdrial4,5, Peggy A. O’Day3, and Jon Chorover4

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1

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3

Pacific Northwest National Laboratory, Richland, WA 99354, USA

Pohang University of Science and Technology (POSTECH), Pohang, South Korea

Sierra Nevada Research Institute and School of Natural Sciences, University of California Merced, Merced, CA 95343, USA 4

Department of Soil, Water and Environmental Science, University of Arizona, Tucson, AZ 85721, USA 5

Department of Geology, University of Vermont, Burlington, VT 05405, USA

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*Corresponding author: Wooyong Um

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Address: Pacific Northwest National Laboratory, P.O. Box 999, P7-54, 902 Battelle Boulevard, Richland, WA 99354

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E-mail: [email protected] ; [email protected]

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Phone: (509) 372-6227. Fax: (509) 371-7249

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* Current address: Division of Advanced Nuclear Engineering (DANE) / Division of Environmental Science and Engineering (DESE), Pohang University of Science and Technology (POSTECH), Pohang, South Korea

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E-mail: [email protected]

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Phone:+82-10-6765-2088

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To be submitted to Environmental Science & Technology

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1

ACS Paragon Plus Environment


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Abstract

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The reaction of acidic radioactive waste with sediments can induce mineral transformation

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reactions that, in turn, control contaminant fate. Here, sediment weathering by synthetic

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uranium-containing acid solutions was investigated using bench-scale experiments to simulate

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waste disposal conditions at Hanford’s cribs, USA. During acid weathering, the presence of

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phosphate exerted a strong influence over uranium mineralogy and a rapidly precipitated,

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crystalline uranium phosphate phase (meta-ankoleite [K(UO2)(PO4)·3H2O]) was identified using

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spectroscopic and diffraction-based techniques. In phosphate-free system, uranium oxyhydroxide

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minerals such as K-compreignacite [K2(UO2)6O4(OH)6·7H2O] were formed. Single-pass flow-

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through (SPFT) and column leaching experiments using synthetic Hanford pore water showed

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that uranium precipitated as meta-ankoleite during acid weathering was strongly retained in the

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sediments, with an average release rate of 2.67E-12 mol g-1 s-1. In the absence of phosphate,

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uranium release was controlled by dissolution of uranium oxyhydroxide (compreignacite-type)

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mineral with a release rate of 1.05-2.42E-10 mol g-1 s-1. The uranium mineralogy and release

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rates determined for both systems in this study support the development of accurate U-release

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models for prediction of contaminant transport. These results suggest that phosphate minerals

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may be a good candidate for uranium remediation approaches at contaminated sites.

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2


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Introduction Upon intentional or accidental introduction to the subsurface, radioactive waste interacts

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heterogeneously with host sediments by way of adsorption-desorption or precipitation-

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dissolution reactions.1-2 Waste solutions, which are often highly acidic or alkaline, may be far

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from equilibrium with receiving sediments, thereby leading to mineral transformation reactions

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that can affect the transport and fate of waste-derived contaminants.3-4 Considering the different

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geochemical properties between minerals with regard to their reactions with waste contaminants,

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resolving molecular-scale interactions between waste-derived contaminants and newly-

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developed geomedia constituents is needed to improve predictive modeling of reactive

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contaminant transport.

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One pertinent example of this need pertains to uranium (U) contamination of soil and

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groundwater at the Department of Energy’s Hanford site, WA, which is the largest and most

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complex environmental cleanup site in the USA and a major concern for public health. At

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Hanford, hundreds of millions of liters of uranium-containing acidic aqueous waste, generated

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during the bi-phosphate process for plutonium processing during World War II and the Cold War,

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were disposed of in a number of unlined underground, box-like excavation structures called cribs.

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Approximately 529 x 106 L of acidic waste was discharged into two cribs (denoted 216-U-8 and

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216-U-12) at 200 Area of Hanford site from 1952 to 1988. The waste contained about 34 Mg U,

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3400 Mg nitrate, and other fission products, e.g., Tc-99, H-3, Sr-90 and Cs- 137.5 Furthermore,

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these waste streams were known to contain variable concentrations of phosphate, which could

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have significant implications for U fate. The large volume of wastewater discharged to these two

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unlined cribs was sufficient to cause the contaminant-containing acidic aqueous waste to 3



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percolate into the sandy subsurface sediments and reach the underlying groundwater.5 Thus, the

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vadose zone beneath the cribs is known to contain subsurface contamination by radioactive and

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nonradioactive contaminants from U processing.5

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Uranium is hexavalent [U(VI)] and exists as the uranyl ion (UO22+) under oxic conditions.

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Interactions between U(VI) and mineral phases present in Hanford sediment, including

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sorption/desorption, complexation, and reduction reactions, have been well documented in the

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literatures.1-2, 6-12 In addition to these relatively direct U(VI)-mineral interactions, weathering of

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the underlying sediment minerals by the acidic or alkaline waste streams also affects the fate and

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transport of radionuclides in the subsurface. Previous studies have shown that leaking,

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hyperalkaline, caustic Hanford waste reacts with Hanford sediments, resulting in primary

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mineral dissolution and secondary mineral precipitation, and significantly altering sediment

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properties and radionuclide transport.3-4, 13-14 However, less is known about the geochemical

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reactions between acidic waste streams and the underlying sediments, and the subsequent

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contaminant release from waste-weathered sediment by natural pore water. Szecsody et al.15

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investigated mineral dissolution of several Hanford sediments at acidic conditions and their

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impacts on U transport. They found that mineral dissolutions led in rapid increases in aqueous

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carbonate and phosphate, resulting in significant shallow U mineral dissolution and deeper U

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precipitations (likely phosphates and carbonates) with downward U migration. Gateman et al.16

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examined the effects of low pH on uranium partitioning to Hanford sediments. They reported

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that much greater U partitioning was observed at conditions of pH 5 than at pH 2 and pH 8,

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which was likely due to the precipitation of uranyl phosphate in addition to adsorption

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mechanism. These studies mainly simulated the U fate and transport behavior during the acidic

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waste reservoir operating time. But unfortunately, both studies didn’t use waste simulants 4


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(acidified deionized or groundwater was used as the waste matrix), which could result in

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geochemical reaction bias from the real legacy sediment weathering history. Besides the above laboratory studies which addressed the vadose zone as a whole at

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Hanford site, extensive site-specific U speciation was investigated on the sediments below the

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inactive North Process Pond at Hanford site 300 Area which served as an acidic waste reservoir

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and neutralized with NaOH. Multi-scale electron microscopic and spectroscopic analyses on

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those samples focusing on U speciation and solid-phase host have been conducted by several

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researchers.17-21 They found that metatorbernite [Cu(UO2)2(PO4)2·8H2O] precipitated at the

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intermediate depths in the vadose zone, likely during pond operations when the U, Cu, and P

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present in this phase all originated from waste release, possibly at different times. However, the

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U(VI)-phosphates formation process discussed in these studies are speculative due to the lack of

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historical data, since the vadose zone pore water was not collected or analyzed during the

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lifetime of the pond operation. Contrast to 300 Area, less study on acidic waste contamination at

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200 Area has been conducted. Recently, Kanematsu et al.22 synthesized acidic solutions

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according to waste disposal documents on cribs 216-U-8 and 216-U-12, and studied uranium

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speciation in the presence and absence of dissolved silica and phosphate under variable pH

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conditions to assess thermodynamic and kinetic controls on the precipitation of uranyl solid

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phases. Among other observations, they reported strong effect of phosphate on uranium

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speciation in the tested systems; in the presence of phosphate (3 mM), meta-ankoleite initially

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precipitated as the primary phase at pH 3, 5, or 7 regardless of the presence of 1 mM dissolved

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silica.

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Overall, investigations on uranium speciation and transport in the Hanford vadose zone subjected to acidic weathering are not yet well documented. Specifically, the subsequent U 5



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release from those legacy weathered sediments is rarely reported. Understanding how acidic

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aqueous waste-sediment reaction affects U speciation and coordination, and how that, in turn,

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influences the subsequent U release, is key to predicting the environmental impact of these

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legacy sediments. Therefore, the objectives of this study were to: (i) determine the U phases

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present in acid-waste weathered Hanford sediments below the waste disposal cribs; (ii)

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understand how such sediments react with Hanford pore water; and (iii) estimate overall release

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rates of the U-containing phases to predict U release and subsurface transport. Legacy sediments

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from 216-U-8/U-12 crib sites were simulated in bench-scale experiments where uncontaminated

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Hanford sediment were reacted with synthetic acidic waste solutions containing U, in the

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presence and absence of dissolved phosphate (PO43-). The U speciation and the nature of U-

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containing mineral phases present in the weathered sediments were characterized using a

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combination of spectroscopic, microscopic, and diffraction-based techniques. Uranium release

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from the weathered sediments was investigated by leaching tests. Because of the strong effect of

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phosphate on uranium immobilization,23-27 weathering of sediment minerals by phosphate-

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containing acidic uranium waste would result in U sequestration in distinct solid-phase products

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with low aqueous solubility and slow release rates, thus limiting subsurface U transport.

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Materials and Methods

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Materials Preparation

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Acidic crib waste solution simulants. Based on the type and amount of waste discharged to the

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two cribs (216-U-8 and 216-U-12), several acidic synthetic crib wastes (ASCW) have been

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proposed.22 Solution chemistry indicates that the presence or absence of phosphate in the waste

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solution is a principal constraint on reaction pathways.22 In this study, two acidic waste simulants 6


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(pH 3) were adopted: ASCW3 containing no PO43- and ASCW6 containing PO43-. The two waste

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simulants were prepared using NO3- and were identical in chemical composition (U: 1 mM, NO3:

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~100 mM, Na: 0.75 mM, Cl: 2.08 mM, and K: 100 mM), except PO43- content (0 mM and 3.00

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mM of PO43- for ASCW3 and ASCW6, respectively). In ASCW6, rapid precipitation of a solid

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phase was observed after solution mixing, thus a parallel ASCW6 solution was prepared and the

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precipitate was filtered for solid characterization.

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Hanford Fine Sands (HS). Hanford Fine Sand (HS), collected at near surface burial ground

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excavation at 218-E-12B location and composed of the silty sand facies from the Hanford

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formation in the Hanford 200 area, was used to represent the sediments at the 216-U-8 and 216-

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U-12 crib sites. The HS, which contains 0.23 wt.% gravel, 72.61wt.% sand and 27.16 wt.% silt

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and clay 28, was sieved to remove the size fraction greater than 2 mm prior to experiments. The

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HS is typical of Hanford formation sediments and contains quartz, K/Na-feldspar and smectite,

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along with illite and chlorite, as the dominant mineral phases with a low calcium carbonate

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content (1.75 wt.%), as identified by mineralogical analysis in Serne et al.28

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Sediment weathering with U-containing acidic waste simulants. The HS was weathered in

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polyethylene bottles with a solution to solid ratio of 50 (6 L ASCW3 or ASCW6 solution + 120

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g dry HS) at room temperature (20 ºC). The bottles were shaken once a week and pH values of

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the supernatant solutions were monitored along the weathering process. After 3 or 15 months

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reaction, the supernatant solution was carefully removed, and the reacted sediment slurries were

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collected. Slurries were air-dried for further characterization and leaching experiments. Four

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weathered sediment samples were prepared and labeled as ASCW3-3, ASCW3-15, ASCW6-3,

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and ASCW6-15, respectively corresponding to the two solutions and the two weathering times 7



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(see Table 1). The initial solution pH was ~ 3.0 for ASCW3 and ASCW6, and was buffered to

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neutral (~ 6.5 - 7.4) after reacted with HS containing carbonate minerals. The evolution of the pH

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in both ASCW solutions during HS weathering as well as a sediment free control is provided in

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Figure S1 in the Supporting Information (SI).

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Characterization of solid samples. Detailed solids characterization, including mineral

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identification and U speciation, was conducted on the weathered sediments, along with the

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precipitate formed homogeneously from the ASCW6 solution, using X-ray diffraction (XRD),

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scanning electron microscope (SEM) with a Focused Ion Beam (FIB), transmission electron

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microscope (TEM), Energy-dispersive spectroscopy (EDS) mapping, extended X-ray absorption

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fine structure (EXAFS), and time-resolved laser-induced fluorescence (TRLIF) spectroscopy. In

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addition, U concentrations in the weathered sediments were determined by inductively coupled

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plasma-mass spectrometry (ICP-MS) analysis of HNO3/HF/HCl microwave-assisted digests. The

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specific surface area of the weathered sediments was measured using the BET (Brunauer-

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Emmett-Teller) method. Detailed characterization methods for XRD, SEM/TEM-EDS, EXAFS,

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TRLIF, BET, and microwave digestion are provided in SI.

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Single-Pass Flow-Through (SPFT) and column leaching experiments. Uranium release from

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the acid-waste weathered sediments was investigated using the synthesized Hanford site

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background pore water (BPW) with 0.3 mM HCO3- according to Um et al.8 A single-pass flow-

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through (SPFT) approach29 was used to investigate the dissolution of the uranyl minerals in the

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four weathered sediments, where the BPW was contacted with the acidic weathered sediments,

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resulting in dissolution of uranyl minerals and U release. In addition, two column leaching tests

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using two weathered HS samples (ASCW3-3 and ASCW6-3) were conducted to provide a more 8


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realistic simulation of the crib site leaching scenario. The column experiments used high solid:

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solution ratios (3.6 – 4.0 g/mL; porosity of ~0.38 – 0.4) and pore water flow rates (1 m/day; ~

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0.1 day solution resident time in the columns). The pH of the effluent was monitored and the

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uranium concentrations were measured by ICP-MS. The details of the SPFT and column

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experimental setup are provided in the SI.

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The normalized apparent steady-state release rate (r) of U-bearing minerals in weathered sediments under saturated conditions was determined (in unit of mol g-1 s-1) as: r = [

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m M

d

]/t

R

(1)

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where md is the dissolved mass of U (mol) within sampling intervals, calculated through

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multiplying the steady-state element concentration in the effluent solutions by the collected

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sample volumes, MR is the remaining mass (g) of U-minerals (meta-ankoleite or K-

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compreignacite) in the weathered sediments after each sampling event, and t is the length of each

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leaching interval in seconds. MR was calculated iteratively based on mass balance by subtracting

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the released U mass during n+1 sampling step from the remained U mas at n step (n = sampling

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times), and converted into mass of meta-ankoleite or K-compreignacite based on the

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stoichiometry of U in each of the U-bearing minerals. It should be noted that the calculation of

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MR was simplified, where only meta-ankoleite and K-compreignacite were assumed to be the

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dominant U-minerals respectively in ASCW6 and ASCW3 solution-reacted sediment samples,

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without considering other potential minor U-minerals in the weathered sediments.

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Results and Discussion

9



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Solid phase characterization

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Surface area and U concentration in weathered samples. The BET-determined

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specific surface area for air-dried weathered-HS varied between 6.04 and 9.19 m2/g (Table 1). A

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BET-surface area value of 13.7 m2/g on the untreated HS was reported by Um et al.30 Lower

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measured specific surface area in this study on weathered HS samples (ASCW solution pH 3)

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could be due to the dissolution of silty and clay particles present in HS. The U concentrations

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associated with the weathered HS, as determined by microwave digestion (Table 1), show a

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similar U loading (7.09 – 7.24 mg/g, corresponding to 0.71 - 0.72 wt.%) among all the weathered

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samples, which minimizes the effect of differences in initial U concentration on U leaching

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among different samples.

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X-ray diffraction. The rapidly precipitated phase observed in ASCW6 was identified as

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meta-ankoleite by powder XRD (Figure 1a). The measured U concentration of 431 mg/g of the

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precipitate from the ASCW6 solution (Table 1) is not far from a U concentration of 520 mg/g

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calculated for the theoretical meta-ankoleite composition, which further supports the XRD result.

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The sharp peaks in the XRD (Figure 1a) indicate the formation of well-crystallized meta-

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ankoleite. This interpretation is supported by SEM images where clear and sharp mineral shapes

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were observed (Figure S2). Unreacted HS and HS samples weathered for 3 months in either

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ASCW6 or ASCW3 solutions were also analyzed by XRD, but no significant difference was

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observed between the weathered and unreacted samples (data not shown). Uranium was present

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in the weathered samples, as shown by microwave digest results (Table 1), but the amount (0.71

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- 0.72 wt. %) was too low to detect a specific U-containing mineral phase by XRD.

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FIB-SEM/TEM-EDS. The SEM-EDS analysis showed that the freshly precipitated

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meta-ankoleite from the ASCW6 solution was present as thin platy micrometer-sized particles

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(Figure S2) with a P:U atomic ratio of ~1. This ratio is further verified by wet chemistry analysis

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on ASCW6 solution (Table S1), where 0.99 mM U (99% of the U added) and 1.06 mM P were

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removed from the ASCW6 solution through meta-ankoleite precipitation. In the ASCW6-HS

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weathering system, meta-ankoleite appeared to precipitate on sediment grains shown in TEM

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images (Figure 1b), where the EDS mapping analysis illustrates that U is clearly associated with

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P, and K (indicating meta-ankoleite), but less with Si and Al which represent sediments in this

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study. Of particular interest, compared to the EDS spectra of the homogeneously precipitated

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meta-ankoleite from the ASCW6 solution where no Ca signal was observed (Figure S2), and

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considering the later introduce of Ca into the weathering solution from calcite dissolution, the

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clear association of Ca with U, P, and K in this system may imply a possible extra Ca-bearing

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layer “armoring” on the surface of the precipitated meta-ankoleite. A cross-section of the sample

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was cut using the FIB, and the FIB-TEM-EDS analysis showed that an amorphous phase,

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consisting of Ca, P, minor U, as well as Si, Al, and Na etc., precipitated on the sediments (Figure

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S3). In this amorphous layer, the possibility of Ca-P precipitates hosting minor U might not be

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ruled out due to the high amount of P (3.0 mM/L) and Ca (up to 2.7 mM/L; see Figure S4) in this

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weathering system. The significant differences of Ca aqueous concentrations (ca. 0.5 mM/L) in

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the ASCW3-HS (without PO43-) and ACSW6-HS (with PO43-) during the sediments weathering

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(Figure S4) supported this assumption, where the significant Ca mass loss from the ASCW6-HS

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could be explained by calcium phosphate precipitation. This finding is consistent with Mehta et

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al.,31 who found that, when simultaneously mixing of dissolved uranium, calcium, and phosphate

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at neutral pH, U was structurally incorporated into a newly formed amorphous calcium 11



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phosphate solid. However, it should be pointed out that the Ca-P phase is speculative, and further

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studies on reliable chemical compositions are needed.

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TRLIF spectroscopy. TRLIF analysis was conducted on the precipitated meta-ankoleite

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and the fluorescence spectrum was compared with that of samples ASCW3-3 and ACSW6-3.

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The emission fluorescence spectra for sample ASCW6-3 (Figure 2b) are the same as that for

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meta-ankoleite (Figure 2a) with five characteristic peaks at 503, 525, 549, 575 and 602 nm. The

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spectral similarity between the precipitated meta-ankoleite and ASCW6-3 indicates that the U

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solid phase is dominated by meta-ankoleite mixed with ASCW6-weathered sediments, in

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agreement with SEM/TEM-EDS analysis. The emission fluorescence spectrum for ASCW3-3

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does not show the five characteristic peaks for meta-ankoleite and instead shows three peaks

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shifted to longer wavelengths (Figure 2c), suggesting the presence of a different uranium phase.

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This can be explained by the difference in solution composition as ASCW3 did not contain

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added PO43-, thus it is not expected that uranyl phosphate phases, such as meta-ankoleite, would

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precipitate. Comparison of the fluorescence spectrum for ASCW3-3 with those for well-

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characterized standards 32-33 shows that the spectrum is most similar to that of a becquerelite

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reference spectrum [Ca(UO2)6O4(OH)6·8(H2O)], which is structurally similar to K- and Na-

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compreignacite (Figure 2c).34-35 The formation of compreignacite [K2(UO2)6O4(OH)6·7H2O] and

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becquerelite is thermodynamically favorable at near-neutral pH and in the presence of sufficient

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K and Ca, respectively.36 Due to the high concentration of K (100 mM) present in the ASCW

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solutions, the formation of K-compreignacite was expected, and was observed by Kanematsu et

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al.22 to precipitate from similar solutions at pH 5. Thus, additional EXAFS analyses were

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undertaken to characterize the dominant uranium phases in the sample ASCW3-3. 12


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EXAFS analysis. EXAFS spectra were collected for the sample ASCW3-3 (HS

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weathered in ASCW3 for 3 months) (Figure 3). The best linear combination fit of the spectrum

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of ASCW3-3 (for uranium mineral EXAFS standards, see Table S2, Figure S5) indicates that K-

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compreignacite is the dominant uranium phase (80%), but inclusion of the spectrum of a natural

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compreignacite in the fit improves the result and suggests substitution of other cations in the

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mineral structure (Table S3). Given both the TRLIF and EXAFS spectra for ASCW3-3, U is

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present in a compreignacite-type phase that is mostly K but could contain some Ca (as indicated

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by the TRLIF spectrum), which smears out the EXAFS features.22, 37 Substitution of different

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cations into the interlayers between U(VI)-OH polyhedral sheets in compreignacite-group

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minerals is common.37 Interestingly, despite the different U-bearing mineral phases formed in

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solutions with or without PO43-, the amounts of U removed from the initial waste solutions were

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similar (Table 1).

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SPFT and column leaching experiments

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SPFT leaching experiments. SPFT were performed with the four weathered HS samples

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(ASCW3-3, ASCW3-15, ASCW6-3, and ASCW6-15), where a constant effluent solution pH of

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~7.4 (Figure S6) was observed in all the samples. HS samples weathered with the same solution

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but different weathering times showed similar leaching behaviors, but ASCW3- and ASCW6-

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weathered HS exhibited different leaching trends (Figure 4a). For the HS weathered in ASCW3

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solution (without PO43-) for 3 months or 15 months, a constant U concentration of ca. 47 µM/L

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(= 11420 µg/L) was measured in the effluents after the first 20 mL (one cell solution volume).

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Thus, despite different weathering times, the steady-state rate of U release was similar for both

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flow-through cells. The nearly constant U release from these two ASCW3-weathered sediments 13



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are consistent with a leaching process that is controlled by steady-state dissolution of U-bearing

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solids (dominated by compreignacite in these samples). This behavior is contrast to previously

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reported U release from contaminated Hanford sediments, which was characterized by a rapid

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initial decrease in U solution concentration followed by a gradual decrease for the remainder of

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the experiment24. The two-stage leaching behavior was consistent with leaching kinetics from

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diffusion-limited intragrain regions within the lithic fragments1, 38-39. The behavior observed in

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our experiments implies that U release from weathered HS is not diffusion-limited and suggests

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that uranyl-bearing phases are widely distributed throughout the HS, rather than confined in

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lithic fragments. This likely resulted from the smaller HS grain radius (silty sand) and shorter

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weathering time (3-15 months) in this study compared to decades-weathered field samples.

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Different U release behavior was observed for HS weathered in ASCW6 solution (with

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PO43-) for 3 months or 15 months (sample ASCW6-3 and ASCW6-15) (Figure 4a). After a sharp

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decrease in U concentration during the initial 20 mL of effluent, U concentrations gradually

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increased over the course of the experiment until ~227 mL of leaching volume was reached. This

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was followed by a slow release to nearly constant concentration for the remainder of the

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experiment (Figure 4a). Given the constant temperature, pH, and flow conditions, a gradual

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increase in U leaching would only occur as a function of specific geochemical processes

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affecting the properties of the weathered sediments. Shi et al. 39 reported a similar leaching trend

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in their column experiments to investigate the inhibitory effect of secondary phosphate

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precipitation on U release from Hanford sediments. By comparing U leaching from Hanford 300

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area sediment samples treated with 1 and 50 mM PO43- solutions, they concluded that the

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inhibitory effect of PO43- decreased during U leaching if insufficient PO43- was provided. 14


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Similarly, uranium release from the ASCW6-weathered HS may have been inhibited by the

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observed amorphous layer (see FIB-SEM/TEM-EDS section). In addition, considering the high

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amount of phosphate (3 mM) applied in this weathering system, phosphate adsorption on solid

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phase might occur 40-41, which can also contribute to the inhibitory effect. The rapidly P decrease

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in the early stage of the column experiments (Figure 4c) might mainly result from the phosphate

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desorption, but the dissolution of the possible amorphous Ca-P could not be ruled out. Thus, U

317

leaching from meta-ankoleite would be strongly impacted by the above inhibition effects,

318

resulting in a low initial U concentration in the effluent. Following the infiltration of PO43--free

319

BPW leachate (pH 7.28), it is likely that the inhibitory effects from the armoring layer and

320

phosphate-adsorption gradually decreased during leaching through dissolution and desorption

321

processes, eventually, a constant U concentration in the effluent from the dissolution of meta-

322

ankoleite would be expected in the later stages of the leaching experiment. Although the

323

concentration of U in the effluent did not reach steady state during the leaching experiment, the

324

decrease in slope of the U release after 227 mL of cumulative leached volume suggests that the

325

system is approaching an apparent steady state. In contrast to the ASCW3-weathered samples,

326

HS weathering time in ASCW6 solutions affected U release behavior. Uranium effluent

327

concentrations from the HS sample weathered for 3 months (ASCW6-3) were 2-3 times higher

328

than effluent concentrations for the HS sample weathered for 15 months (ASCW6-15). The

329

difference in leaching behavior may be due to an increase in particle size and crystallinity (i.e.,

330

Ostwald ripening) of the precipitated meta-ankoleite with increased weathering time.

331 332

Effluent U(VI) concentration from HS weathered in ASCW6 solution (with PO43-) were significantly lower (2-3 orders of magnitude) than those from HS weathered in ASCW3 solution 15



333

(without PO43-) (Figure 4a). Cumulatively, more than 99% of the initial U mass remained in the

334

sample ASCW6-3 after greater than 400 mL of pore water leaching (~21 pore volumes), whereas

335

almost 25% of the U mass was removed from sample ASCW3-3 (Figure 4b). Clearly, the

336

different leaching behavior exhibited by these two weathered HS samples resulted from the

337

sequestration of U in distinct mineral phases that were formed during HS weathering by acidic

338

waste solutions with or without dissolved phosphate. The presence of phosphate exerts a strong

339

control over uranium mineralogy - the rapid precipitation of meta-ankoleite. Whereas in the

340

phosphate-free system, the main U-bearing mineral phases were compreignacite-type

341

oxyhydroxides. The insoluble U phosphate phase meta-ankoleite strongly retained U in the HS.

342

In contrast, the rate of U release was much higher from the dissolution of more soluble U-

343

bearing oxyhydroxide minerals in the phosphate-free system. The application of polyphosphate

344

for the remediation of U has been tested for uranium-contaminated Hanford sediments and

345

results showed that the long-term decrease in U leaching rate and mass were significant (~54%

346

to 1 year) 24. Similar to the findings in this study, phosphate treatment decreases U leaching by

347

forming Ca-P precipitates coated on uranium mineral surface phases, U adsorption to

348

precipitates, or formation of uranium-phosphate precipitates.24, 26, 42

349

The apparent steady-state U release rate was calculated using the effluent concentration

350

data through Equation 1. For sample ASCW3-3, the average U concentration when U release had

351

plateaued, after a cumulative flow-through volume of ca. 48 mL (see Figure 4a), were selected to

352

represent the steady-state U concentration. The rate of U leaching dominated by K-

353

compreignacite dissolution at steady state was 1.05E-10 mol g-1 s-1 using BPW (HCO3- = 0.3

354

mM). For sample ASCW6-3, U release plateaued later after the inhibitory effect (see Figure 4a), 16


Page 16 of 30

Page 17 of 30


355

so the average U concentration after ca. 227 mL leaching volume was used to estimate the

356

release rate. Uranium release rate from the meta-ankoleite dominated ASCW6-3 sample was

357

2.67E-12 mol g-1 s-1. It should be noted that this is a pseudo release rate since U leaching had not

358

reached steady-state during the ASCW6-3 leaching experiment. Above all, it is clear that the U

359

release rate for weathered samples dominated by meta-ankoleite are about 2 orders of magnitude

360

lower than those dominated by compreignacite. As far as we know, there is no reported U release

361

rate for meta-ankoleite (K-autunite), although the dissolution rates (in terms of mol m-2 s-1) for

362

synthesized Na/Ca-autunite has been reported under different geochemical conditions43-46.

363

Unfortunately, it is hard to determine the correct dissolution rates in terms of mol m-2 s-1 in this

364

study and compare with those literature values. Because the surface area contributions of U

365

minerals in the bulk weathered sediments were not easy to determine due to possibly the

366

dissolution of silt/clay particles in the acidic ASCW solutions. As described before, lower BET

367

specific surface areas were observed in the ASCW-weathered HS samples comparing to the

368

untreated HS even with neo-precipitated U phases.

369

Column leaching test. Similar to the SPFT results, much less U was released from the

370

meta-ankoleite-dominated sample (ASCW6-3-column) compared to the compreignacite-

371

dominated sample (ASCW3-3-column). Cumulatively, more than 99% of the initial U mass

372

remained in the ASCW6-3-column after ca.176 mL of pore water leaching (= 263 pore volumes),

373

whereas almost 13% of the U mass had been leached from the ASCW3-3-column after ca.163

374

mL (= 256 pore volume) (Figure 4d). A steady-state U effluent concentration of ca. 58 µM/L

375

(=13843 µg/L) was observed for ASCW3-3-column after 51 mL of leaching (Figure 4c). Despite

376

the different reaction times (> 5 days vs ~0.1 day of solution residence time) in the column and 17



377

SPFT systems, U release from the ASCW3-weathered HS columns reached steady state rapidly.

378

The estimated rates for the two systems were similar, with the rate for the ASCW3-3 column

379

(2.42E-10 mol g-1 s-1) within a factor of ca. 2 of the SPFT rate. However, in ASCW6-3-column,

380

the U concentration continued to increase gradually during the entire test (ca.176 mL BPW), and

381

thus no reliable release rate can be determined; U leaching was far from steady-state, possibly

382

due to less effective removal of the armoring layer (compared to SPFT, where more vigorous

383

leaching (ca.431 mL) was applied). Interestingly, release of U from ASCW6-3 reacted sediments

384

in SPFT or column gives similar calculated release rates. Using average U concentrations for

385

BPW effluent volumes of 60 – 176 mL, pseudo release rates of 4.29E-13 mol g-1 s-1 or 6.7E-13

386

mol g-1 s-1 were obtained for SPFT or column experiments, respectively.

387

Environmental Implication. Results of this study show that uranium mineralogy below the two

388

studied cribs could be controlled by the presence of phosphate through rapid precipitation of the

389

U-bearing phosphate mineral meta-ankoleite. When the U- and phosphate-containing acidic

390

aqueous waste was historically disposed in the cribs and subsequently released into the

391

subsurface, the waste stream could have reacted and precipitated mainly as meta-ankoleite or

392

similar phosphate phases in these legacy contaminated Hanford sediments. Future U speciation

393

study on field sediments should pay attention on uranium phosphate phases based on this

394

simulation results. The meta-ankoleite that formed under simulated field conditions was well

395

crystalline, and uranyl phosphate phases in general have low thermodynamic solubility compared

396

with uranyl oxyhydroxides and silicate phases22, which could limit dissolved U(VI)

397

concentrations in groundwater. The U mineralogy and release rates determined in this study are

398

important for the development of accurate U-release models at these sites. Due to the slow U 18


Page 18 of 30

Page 19 of 30


399

release rate of meta-ankoleite and low meteoric water infiltration at Hanford site, limited U

400

solubilization and downward migration is expected beneath the 216-U-8 and 216-U-12 crib sites.

401

However, the experimental effluent U concentrations of hundreds of µg per liter in this study is

402

still above the EPA regulatory standard (30 µg/L; National Primary Drinking Water Regulations,

403

2000)46. In addition, the phosphate concentrations in the historically-disposed waste streams are

404

not well known, thus some discharges may have contained high PO43- while others might have

405

been very low. In the absence of dissolved phosphate, U precipitates in uranyl oxyhydroxide

406

minerals with relatively high solubility and fast release rates.37 Thus, the reality in terms of U

407

retention would be somewhere between the two endmembers explored in this study. The

408

weathered sediments below the two studied cribs may still represent a U-source term and require

409

remediation to prevent contamination of the underlying groundwater.

410

Acknowledgements

411

This research was funded by the U.S. Department of Energy (DOE) through the

412

Subsurface Biogeochemical Research (SBR) program under grant numbers SBR-DE-

413

SC0006781. Portions of this research were carried out at the Stanford Synchrotron Radiation

414

Lightsource, a national user facility operated by Stanford University on behalf of the U.S.

415

Department of Energy, Office of Basic Energy Sciences. A portion of this research was

416

supported by the Radioactive Waste Management Technology Program of the Korea Institute of

417

Energy Technology Evaluation and Planning (KETEP) granted financial resource from the

418

Ministry of Trade, Industry & Energy, Republic of Korea (No.20141720100610). Pacific

419

Northwest National Laboratory (PNNL) is operated for the DOE by Battelle Memorial Institute

420

under Contract DE-AC05-76RL01830. The authors appreciate Dr. Carolyn I Pearce for 19



421

technical review and discussion, and are grateful to Dr. Michele A. Conroy and Bruce Arey for

422

the SEM/TEM/EDS measurements. The XRD, SEM/TEM/EDS and TRLIF analyses were

423

performed in Environmental Molecular Sciences Laboratory (EMSL). EMSL located at PNNL

424

is a national scientific user facility sponsored by the DOE’s Office of Biological and

425

Environmental Research.

426

Supporting Information Available

427

More details for solid characterization methods, pH buffering during sediment weathering, and

428

column leaching experiment are available free of charge via the Internet at http://pubs.acs.org.

429 430

431

432

20


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433


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(23) Mehta V.S., Maillot F., Wang Z., Catalano J.G. and Giammar D.E. Effect of co-solutes on the products and solubility of uranium (VI) precipitated with phosphate. Chem. Geol. 2014. 364, 66– 75.

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(25) Wellman D.M, Glovack J.N., Parker K., Richards E.L., Pierce E.M. Sequestration and retention of uranium (VI) in the presence of hydroxylapatite under dynamic geochemical conditions. Envir. Chem. 2008, 5, 40– 50.

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(33) Wang Z., Zachara J.M., Liu C., Gassman P.L., Felmy A.R., Clark S.B. A cryogenic fluorescence spectroscopic study of uranyl carbonate, phosphate and oxyhydroxide minerals. Radiochim. Acta. 2008, 96, 591-598.

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(34) Burns P.C. The structure of compreignacite, K2[(UO2)3O2(OH)3]2(H2O)7. Can. Miner. 1998, 36, 1061-1067.

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(35) Burns P.C., Li Y. The structures of becquerelite and Sr-exchanged becquerelite. Am. Miner. 2002, 87, 550557.

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(36) Gorman-Lewis D., Burns P.C., Fein J.B. Review of uranyl mineral solubility measurements. J. Chem. Thermodynamics. 2008, 40, 335-352.

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(37) Reinoso-Maset E., Steefel C.I., Um W., Chorover J., O’Day P.A. Rates and mechanisms of uranyl oxyhydroxide mineral dissolution. Geochim. Cosmochim. Acta, 2017, 207, 298-321.

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(38) Liu C., Zachara J.M., Yantasee W., Majors P.D., McKinley J.P. Microscopic reactive diffusion of uranium in the contaminated sediments at Hanford, United States. Water Resour. Res. 2006, 42. 1-15.

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(39) Shi Z., Liu C., Zachara J.M., Wang Z., Deng B. Inhibition effect of secondary phosphate mineral precipitation on uranium release from contaminated sediments. Environ. Sci. Technol. 2009, 43, 83448349.

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(40) Stollenwerk K.G. Simulation of phosphate transport in sewage-contaminated groundwater, Cape Cod, Massachusetts. Appl. Geochem. 1996, 317-324.

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(41) Szecsody J.E., Rockhold M.L., Oostrom M., Moore R.C., Burns C.A., Williams M.D., Zhong L., Fruchter J.S., McKinley J.P., Vermeul V.R., Covert M.T., Wietsma T.W., Breshears A.T., Garcia B.J. Sequestration of Sr-90 subsurface contamination in the Hanford 100-N Area by surface infiltration of a Ca-citratephosphate solution. PNNL-18303. Pacific Northwest National Laboratory; Richland, WA, 2009.

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(42) Troyer L.D., Maillot F., Wang Z., Wang Z., Mehta V.S., Giammar D.E., Catalano J.G. Effect of phosphate on U(VI) sorption montmorillonite: Ternary complexation and precipitation barriers. Geochim. Cosmochim. Acta, 2016, 175, 86-99.

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(43) Wellman, D.M., Icenhower, J.P., Gamerdinger, A.P., Forrester, S.W. Effects of pH, temperature and aqueous organic material on the dissolution kinetics of metaautunite minerals, (Na, Ca)2 − 1[(UO2)(PO4)]2 · 3H2O. Am. Mineral. 2006, 91, 143.

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(44) Wellman, D.M., Gunderson, K.M., Icenhower, J.P., Forrester, S.W. Dissolution kinetics of synthetic and natural meta-autunite minerals, X3–n (n)+ [(UO2)(PO4)]2•xH2O, under acidic conditions. Geochem. Geophys. 2007, 8, 16.

547 548 549

(45) Gudavalli R.K.P., Katsenovich Y.P., Wellman D.W., Idarraga M., Lagos L.E., Tansel B. Comparison of kinetic rate law parameters for the dissolution of natural and synthetic autunite in the presence of aqueous biocarbonate ions. Chem. Geol. 2013, 351, 299-309.

550

(46) National Primary Drinking Water Regulations, 2000. Radionuclides, Final Rule. Fed. Regist. 65, 76707.

551 552

23



553 554

Page 24 of 30

Table 1. Summary of the acidic waste solution, weathered Hanford Fine Sands (HS), U loading, and BET specific surface area measured on sediments after weathering. Sample ID

PO43–

ASCW - HS weathering time [month]

U concentration in weathered samples [mg/g]

Specific surface area [m2/g]

ASCW3-3

-

3

7.24±0.01

6.04

ASCW3-15

-

15

ASCW6-3

+

3

ASCW6-15

+

15

Metaankoleite

+

Rapidly homogeneous precipitate from solution ASCW6 (with PO43–)

9.19 7.09±0.04

8.37 8.58

431±10

555 556 557 558

24


Page 25 of 30


559

Figure Captions

560 561 562

Figure 1. (a) X-ray diffractogram of the homogeneous precipitate filtered out of acidic waste solution ASCW6 compared with lines for reference meta-ankoleite; (b) TEM-EDS mapping of sample ASCW6-3 showing association of U with P, K, and Ca, but less with Si and Al.

563 564

Figure 2. TRLIF spectra of (a) meta-ankoleite, (b) ASCW6-3, and (c) ASCW3-3 with Uphosphate references.

565 566 567

Figure 3. Linear combination (LC) fits (blue dashed lines) to the U LIII-EXAFS spectra (black solid lines) of ASCW3-3 samples, and deconvolution of contribution from reference compound spectra (pointed and dash-pointed lines). Numerical values are given in Table S3.

568 569 570 571

Figure 4. SPFT (top: a, b) and column (bottom: c, d) leaching test results on the ASCW solutionweathered sediments; (left) Effluent uranium concentrations as a function of cumulative leached fluid volume, and (right) remaining uranium mass as a function of leached fluid volume. The black circle in (c) indicates P concentrations in the ASCW6-3-column effluent.

25



572

573 574 575 576

Figure 1. (a) X-ray diffractogram of the homogeneous precipitate filtered out of acidic waste solution ASCW6 compared with lines for reference meta-ankoleite; (b) TEM-EDS mapping of sample ASCW6-3 showing association of U with P, K, and Ca, but less with Si and Al.

577 578 579

26


Page 26 of 30

Page 27 of 30


(a) 6

8x10 Meta-AnKoleite λex=415 nm delay 207.5-707.5 µs gatewidth 100 µs

4

(b)

6

F12s22_pdw_01 F12s22_pdw_02 F12s22_pdw_03 F12s22_pdw_04 F12s22_pdw_05 F12s22_pdw_06

6

Intensity

Intensity

6x10

F12s11_pdw_01 F12s11_pdw_02 F12s11_pdw_03 F12s11_pdw_04 F12s11_pdw_05 F12s11_pdw_06

U(VI) in ASCW6-HS λex=415 nm delay 207.5-707.5 µs gatewidth 100 µs

4

2 2

400

450

500

550

600

650

400

450

Wavelength (nm)

580

500

550

600

650

Wavelength (nm)

Meta_ankoleite ASCW3-HS becquerelite_r2 Compregnacite Na_Compregnacite schoepite_r2 Cahill_Schoepite

(c)

Normalized Intensity

8x10

6

6

4

2

0 500

581 582 583

550

600

Wavelength (nm)

Figure 2. TRLIF fluorescence spectra of (a) meta-ankoleite, (b) ASCW6-3, and (c) ASCW3-3 with U-phosphate references.

584

27



585 586 587 588

Figure 3. Linear combination fits (blue dashed lines) to the U LIII-EXAFS spectra (black solid lines) of ASCW3-3 sample, and deconvolution of contribution from reference compound spectra (pointed and dash-pointed lines). Numerical values are given in Table S3.

589 590 591 592

28


Page 28 of 30

Page 29 of 30


100

(a) 10.00

1.00

0.10 ASCW3-3 ASCW6-3

ASCW3-15 ASCW6-15

Remaining U mass [%]

U concentration [µM/L]

100.00

0.01

90 85 80 ASCW3-3 ASCW6-3

75 70

0

100

200

300

400

500

0

Cumulative Flow-Through Volume [mL]

593 10.00

1000 800

1.00 600 ASCW3-3-column

0.10

400

ASCW6-3-column ASCW6-3-column-P

0.01

200

Remaining U mass [%]

1200

100

200

300

400

300

400

500

(d) 95 90 85 80 ASCW3-3-column

75

ASCW6-3-column

0 0

200

100

1400

(c)

100


P concentration [µM/L]

U concentration [µM/L]

100.00

594 595 596 597 598 599 600 601 602

(b)

95

70

500

0


100

200

300

400

500


Figure 4. SPFT (top: a, b) and column (bottom: c, d) leaching test results on the ASCW solutionweathered sediments; (left) Effluent uranium concentrations as a function of cumulative leached fluid volume, and (right) remaining uranium mass as a function of leached fluid volume. The black circle in (c) indicates P concentrations in the ASCW6-3-column effluent.

29



603

TOC

604

605 606

30


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Uranium Release from Acidic Weathered Hanford Sediments: Single

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