Dawn M. Wellman

Uranium Geochemistry in Vadose Zone and Aquifer Sediments from the 300 Area Uranium Plume

This report documents research conducted by the RCS Project to update the record of decision for the 300-FF-5 Operable Unit on the Hanford Site.

Comparative Analysis of Soluble Phosphate Amendments for the Remediation of Heavy Metal Contaminants: Effect on Sediment Hydraulic Conductivity

Environmental Context. The contamination of surface and subsurface geologic media by heavy metals and radionuclides is a significant problem within the United States Department of Energy complex as a result of past nuclear operations. Water-soluble phosphate compounds provide a means to inject phosphorus into subsurface contaminant plumes, to precipitate metal ions from solution. However, phosphate phases can form within the sedimentary pore structure to block a fraction of the pore space and inhibit further remediation of the contaminant plume. A series of tests have been conducted to evaluate changes in sedimentary pore structure during the application of several proposed phosphate remediation amendments. Abstract. A series of conventional, saturated column experiments have been conducted to evaluate the effect of utilizing in situ, soluble, phosphate amendments for subsurface metal remediation on sediment hydraulic conductivity. Experiments have been conducted under mildly alkaline and calcareous conditions representative of conditions commonly encountered at sites across the arid western United States, which have been used in weapons and fuel production and display significant subsurface contamination. Results indicate that the displacement of a single pore volume of either sodium monophosphate or phytic acid amendments causes approximately a 30% decrease in the hydraulic conductivity of the sediment. Long-chain polyphosphate amendments afford no measurable reduction in hydraulic conductivity. These results demonstrate (1) the efficacy of long-chain polyphosphate amendments for subsurface metal sequestration; and (2) the necessity of conducting dynamic experiments to evaluate the effects of subsurface remediation.

Review: Technical and Policy Challenges in Deep Vadose Zone Remediation of Metals and Radionuclides

Contamination in deep vadose zone environments is isolated from exposure so direct contact is not a factor in its risk to human health and the environment. Instead, movement of contamination to the groundwater creates the potential for exposure and risk to receptors. Limiting flux from contaminated vadose zone is key for protection of groundwater resources, thus the deep vadose zone is not necessarily considered a resource requiring restoration. Contaminant discharge to the groundwater must be maintained low enough by natural attenuation (e.g., adsorption processes or radioactive decay) or through remedial actions (e.g., contaminant mass reduction or mobility reduction) to meet the groundwater concentration goals. This paper reviews the major processes for deep vadose zone metal and radionuclide remediation that form the practical constraints on remedial actions. Remediation of metal and radionuclide contamination in the deep vadose zone is complicated by heterogeneous contaminant distribution and the saturation-dependent preferential flow in heterogeneous sediments. Thus, efforts to remove contaminants have generally been unsuccessful although partial removal may reduce downward flux. Contaminant mobility may be reduced through abiotic and biotic reactions or through physical encapsulation. Hydraulic controls may limit aqueous transport. Delivering amendments to the contaminated zone and verifying performance are challenges for remediation.

Radioiodine Biogeochemistry and Prevalence in Groundwater

129I is commonly either the top or among the top risk drivers, along with 99Tc, at radiological waste disposal sites and contaminated groundwater sites where nuclear material fabrication or reprocessing has occurred. The risk stems largely from 129I having a high toxicity, a high bioaccumulation factor (90% of all the bodys iodine concentrates in the thyroid), a high inventory at source terms (due to its high fission yield), an extremely long half-life (16M years), and rapid mobility in the subsurface environment. Another important reason that 129I is a key risk driver is that there is uncertainty regarding its biogeochemical fate and transport in the environment. We typically can define 129I mass balance and flux at sites, but cannot predict accurately its response to changes in the environment. As a consequence of some of these characteristics, 129I has a very low drinking water standard, which is set at 1 pCi/L, the lowest of all radionuclides in the Federal Register. Recently, significant advancements have been made in detecting iodine species at ambient groundwater concentrations, defining the nature of the organic matter and iodine bond, and quantifying the role of naturally occurring sediment microbes to promote iodine oxidation and reduction. These recent studies have led to a more mechanistic understanding of radioiodine biogeochemistry. The objective of this review is to describe these advances and to provide a state of the science of radioiodine biogeochemistry relevant to its fate and transport in the terrestrial environment and provide information useful for making decisions regarding the stewardship and remediation of 129I contaminated sites. As part of this review, knowledge gaps were identified that would significantly advance the goals of basic and applied research programs for accelerating 129I environmental remediation and reducing uncertainty associated with disposal of 129I waste. Together the information gained from addressing these knowledge gaps will not alter the observation that 129I is primarily mobile, but it will likely permit demonstration that the entire 129I pool in the source term is not moving at the same rate and some may be tightly bound to the sediment, thereby smearing the modeled 129I peak and reducing maximum calculated risk.

Iodine-129 and Iodine-127 Speciation in Groundwater at the Hanford Site, U.S.: Iodate Incorporation into Calcite

The geochemical transport and fate of radioiodine depends largely on its chemical speciation that is greatly affected by environmental factors. This study reports, for the first time, the speciation of stable and radioactive iodine in the groundwater from the Hanford Site. Iodate was the dominant species and accounted for up to 84% of the total iodine present. The alkaline pH (pH ∼ 8) and predominantly oxidizing environment may have prevented reduction of the iodate. In addition, groundwater samples were found to have large amounts of calcite precipitate which were likely formed as a result of CO2 degassing during removal from the deep subsurface (>70m depth). Further analyses indicated that between 7 and 40% of the dissolved (127)I and (129)I that was originally in the groundwater had coprecipitated in the calcite. Iodate was the main species incorporated into calcite and this incorporation process could be impeded by elevating the pH and decreasing ionic strength in groundwater. This study provides critical information for predicting the long-term fate and transport of (129)I. Furthermore, the common sampling artifact resulting in the precipitation of calcite by degassing CO2, had the unintended consequence of providing insight into a potential solution for the in situ remediation of groundwater (129)I.

Sequestration and retention of uranium(VI) in the presence of hydroxylapatite under dynamic geochemical conditions

Environmental context. Contamination of surface and subsurface geologic media by heavy metals and radionuclides is a significant problem within the United State Department of Energy complex as a result of past nuclear operations. Numerous phosphate-based remediation strategies have been proposed to introduce hydroxylapatite directly or indirectly (i.e. through in situ precipitation) into subsurface regimes to act as an efficient sorbent for sequestration of metals and radionuclides such as uranium. Results presented here illustrate the importance of variable geochemical conditions on the mechanism of sequestration and long-term retention of uranium in the presence of hydroxylapatite. Abstract. Numerous solid- and aqueous-phase phosphate-based technologies for remediating heavy metals and radionuclides have the common premise of sequestration by hydroxylapatite. Complexation reactions and hydrolysis generally preclude actinides from incorporation into intracrystalline sites; rather, surface sorption and precipitation are significant mechanisms for the sequestration of actinides. The effect of pH, aqueous speciation, and the availability of reactive surface sites on minerals such as hydroxylapatite have a significant impact on the mechanism and degree of sequestration and retention of variably charged contaminants such as uranium. Yet, little attention has been given to the sequestration and retention of uranium by hydroxylapatite under dynamic geochemical conditions that may be encountered during remediation activities. We present the results of an investigation evaluating the removal of uranium by hydroxylapatite in systems near equilibrium with respect to hydroxylapatite, and the effect of dynamic aqueous geochemical conditions, such as those encountered during and subsequent to remediation activities, on the retention of uranium. Results presented here support previous investigations demonstrating the efficiency of hydroxylapatite for sequestration of uranium and illustrate the importance of geochemical conditions, including changes to surface properties and aqueous speciation, on the sequestration and retention of uranium.

Influence of acidic and alkaline waste solution properties on uranium migration in subsurface sediments

This study shows that acidic and alkaline wastes co-disposed with uranium into subsurface sediments have significant impact on changes in uranium retardation, concentration, and mass during downward migration. For uranium co-disposal with acidic wastes, significant rapid (i.e., hours) carbonate and slow (i.e., 100 s of hours) clay dissolution resulted, releasing significant sediment-associated uranium, but the extent of uranium release and mobility change was controlled by the acid mass added relative to the sediment proton adsorption capacity. Mineral dissolution in acidic solutions (pH2) resulted in a rapid (<10 h) increase in aqueous carbonate (with Ca(2+), Mg(2+)) and phosphate and a slow (100 s of hours) increase in silica, Al(3+), and K(+), likely from 2:1 clay dissolution. Infiltration of uranium with a strong acid resulted in significant shallow uranium mineral dissolution and deeper uranium precipitation (likely as phosphates and carbonates) with downward uranium migration of three times greater mass at a faster velocity relative to uranium infiltration in pH neutral groundwater. In contrast, mineral dissolution in an alkaline environment (pH13) resulted in a rapid (<10h) increase in carbonate, followed by a slow (10 s to 100 s of hours) increase in silica concentration, likely from montmorillonite, muscovite, and kaolinite dissolution. Infiltration of uranium with a strong base resulted in not only uranium-silicate precipitation (presumed Na-boltwoodite) but also desorption of natural uranium on the sediment due to the high ionic strength solution, or 60% greater mass with greater retardation compared with groundwater. Overall, these results show that acidic or alkaline co-contaminant disposal with uranium can result in complex depth- and time-dependent changes in uranium dissolution/precipitation reactions and uranium sorption, which alter the uranium migration mass, concentration, and velocity.

Laboratory Testing of Bulk Vitrified and Steam Reformed Low-Activity Waste Forms to Support A Preliminary Risk Assessment for an Integrated Disposal Facility

Laboratory testing was conducted on bulk vitrified and steam reformed waste forms to supply the input parameters needed for reactive chemical transport calculations with the Subsurface Transport Over Reactive Multiphases (STORM) code. This same code was used to conduct the 2001 ILAW performance assessment. The required input parameters for both waste forms are derived from a mechanistic model that describes the effect of solution chemistry on contaminant release rates. The single-pass flow-through test was the principal method used to obtain these input parameters, supplemented by product consistency test measurements and physical property measurements.

Synthesis and Characterization of Sodium meta-Autunite, Na[UO2PO4] - 3H2O

Abstract Long-chain sodium polyphosphate compounds have been recently proposed as a ‘time-released’ source of phosphate for precipitation of uranium-phosphate minerals. Elevated sodium concentrations presented by this technique promote the formation of sodium autunite relative to the more common calcium autunite mineral phase. In order to evaluate sodium autunite minerals as a long-term ‘sink’ for in-situ immobilization of uranium, it is necessary to quantify their longevity under environmentally relevant conditions. This paper describes a direct method for precipitating sodium autunite and provides a comparative analysis of the structural and chemical properties of direct versus indirectly precipitated sodium autunite. Extended X-ray absorption fine structure (EXAFS) spectroscopy, chemical digestion followed by inductively-coupled plasma-optical emission spectroscopy (ICP-OES) and inductively-coupled plasma-mass spectroscopy (ICP-MS) for elemental analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), multi point Brunauer–Emmett–Teller (BET) analyses and helium pyconometry were used to characterize the precipitate phases. Morphological differences are discussed in the context of conducting subsequent solubility and dissolution investigations. Research presented here is part of a larger effort to quantify the solubility and dissolution properties of uranium-phosphate minerals.

Experimental determination of the dissolution kinetics of zero-valent iron in the presence of organic complexants

Environmental context. Iron metal is being considered as a material to be used for the treatment of groundwater contaminated with toxic metals and organics. Although time-dependant information is available, predicting the long-term behaviour of this material has been complicated by the build-up of rust or other alteration phases on the surface of Fe metal. In addition to the build-up of rust, changes to important environmental factors also complicate these types of predictions. The research discussed in this paper uses a non-traditional experimental technique to isolate the impact of specific environmental factors (i.e. pH, temperature) and organic complexants on the dissolution of Fe metal. Abstract. The geochemical cycling of iron, the reactivity of iron minerals and, more recently, the reactivity of zero valent iron (α-Fe), have been the subject of numerous investigations for over more than three decades. These investigations provide a wealth of knowledge regarding the effect of pH, temperature, chelating agents etc. on the reactivity and mechanism(s) of dissolution for α-Fe and iron oxide/oxyhydroxide minerals. However, most investigations have been conducted under static conditions that promote the formation of a partially oxidised surface film (e.g. passivating layer). In the presence of a passivating layer, the proposed dissolution mechanisms are vastly different and are based on the composition of the partially oxidised surface film. The objective of this study was to quantify the dissolution of α-Fe under conditions that maintain the pO2 at a relatively constant level and minimise the formation of a passivating layer on the metal surface. Single-pass flow-through tests were conducted under conditions of relatively constant dissolved O2 [O2(aq)] over the pH(23°C) range from 7 to 12 and temperature range from 23 to 90°C in the presence of ethylenediamine tetraacetic acid (EDTA) and ethylenediamine di-O-hydroxyphenylacetic acid (EDDHA) to maintain dilute conditions and minimise the formation of a partially oxidised surface film and Fe-bearing secondary phase(s) during testing. Although more information is needed, these results suggest the adsorption of EDTA and EDDHA, or the diffusion of the oxidised Fe–organic complex from the surface of α-Fe, is the rate-limiting step in the dissolution reaction. Results also suggest that the rate of dissolution is independent of pH, has a non-linear dependence on the concentration of organic complexant, and the forward dissolution rate for α-Fe is as much as three orders of magnitude greater than when a passive film and corrosion products are present.