Calvin C. Ainsworth's research while affiliated with Pacific Northwest National Laboratory and other places

An application of a fundamental thermochemical approach that relates the aqueous concentrations of elements to specific chemical reactions in waste/water systems is described. The approach involves the concept of waste as consisting of mixtures of discrete solids (pure solid phases and solid solution phases) that control aqueous concentrations of individual solutes through their solubilization or adsorption/desorption characteristics. Thermochemical data can then be used in conjunction with knowledge of the specific solid phases present in the wastes to calculate the aqueous concentrations of elements. This approach does not explicitly consider specific rates of kinetically controlled reactions but does incorporate empirical knowledge of the relative rates of several reaction types. Overall, the thermochemical approach is applicable to all waste types and environments, and is appropriate because (1) most waste solids that either are present or are expected to form during weathering have relatively rapid precipitation/dissolution kinetics, and (2) experimental results, enrichment of many constituents on particle surfaces, and the large specific surface areas of wastes suggest that many of the constituents present in the wastes are accessible to solution attack and are mobilized fairly rapidly. Supporting evidence for the successful application of this approach to predicting pore-water concentrations of several major and trace elements in utility wastes is discussed.

Column experiments combined with geochemical modeling, microscopic inspections, spectroscopic interrogations, and wet chemical extractions were used to study sediment-dependent Cr(VI) desorption, physical location, mineral association, and attenuation mechanism(s) in four freshly or naturally aged contaminated sediments exposed to concentrated Cr(VI) waste fluids. Results showed that majority of Cr(VI) mass was easily removed from the sediments (equilibrium site K(d) varied from 0 to 0.33 mL g(-1) and equilibrium site fraction was greater than 95%). Long tailings of time-dependent Cr(VI) concentrations above maximum contaminant level of 1.9 micromol L(-1) were also observed (kinetically controlled fraction K(d) and desorption reaction half-lives varied from 0 to 45 mL g(-1), and from 76.1 to 126 h, respectively). Microscopic and spectroscopic measurements confirmed that Cr was concentrated within fine-grained coatings in small areas mainly rich in phyllosilicates that contained both Cr(III) and Cr(VI). However, Cr(VI) reduction was neither significant nor complete. Under slightly alkaline and oxic conditions, contaminant Cr in the sediments occurred: (i) In the highly mobile pool (over 95% of total Cr); (ii) In the slow and time-dependent releasing pool, which served as long-term source of contamination; (iii) As reduced Cr(III) which most likely formed during Cr(VI) reaction with aqueous, sorbed, or structural Fe(II).

Quinoline is an N-containing heterocyclic contaminant associated with creosote wastes and may be biologically degraded under aerobic and anaerobic conditions. Quinoline-degrading bacteria were utilized to investigate the rate of quinoline desorption from smectite clay at low surface concentrations and its subsequent mineralization. Fluorescence spectroscopy and [sup 14]C-quinoline mineralization techniques were used to compare the observed desorption rates to mineralization rates. Microbial utilization was desorption-rate limiting. Desorption and microbial utilization may be modeled as pseudo-first-order reactions whose first-order rate constant (k[sub obs]) is independent of surface concentration. The difference between quinoline disappearance from solution and from the clay surface as observed by fluorescence spectroscopy was comparable to the difference between microbial utilization of the solution and the surface-bound species. The utilization of quinoline from solution was about 30 times more rapid than surface-bound quinoline. The surface-bound species appeared to be protected from microbial attack and, hence, desorption was required prior to utilization. The desorption process is suggested to be a chemical-reaction-controlled process rather than film diffusion controlled, as is typically suggested for an exchange process. 41 refs., 9 figs., 3 tabs.

In this project, the reduction rate of uranyl complexes with hydroxide, carbonate, EDTA, and Desferriferrioxamine B (DFB) by anthraquinone-2,6-disulfonate (AH2DS), a potential electron shuttle for microbial reduction of metal ions (Newman and Kolter 2000), is studied by stopped-flow kinetics techniques under anoxic atmosphere. The apparent reaction rates varied with ligand type, solution pH, and U(VI) concentration. For each ligand, a single largest kobs within the studied pH range was observed, suggesting the influence of pH-dependent speciation on the U(VI) reduction rate. The maximum reaction rate found in each case followed the order of OH- > CO32- > EDTA > DFB, consistent with the same trend of the thermodynamic stability of the uranyl complexes and ionic sizes of the ligands. Increasing the stability of uranyl complexes and ligand size decreased the maximum reduction rate. The pH-dependent rates were modeled using a second-order rate expression that was assumed to be dependent on a single U(VI) complex and AH2DS species. By quantitatively comparing the calculated and measured apparent rate constants as a function of pH, species AHDS3- was suggested as the primary reductant in all cases examined. Species UO2CO3(aq) , UO2HEDTA-, and (UO2)2(OH)22+ were suggested as the principal electron acceptors among the U(VI) species mixture in carbonate, EDTA, and hydroxyl systems, respectively.

Nine projects have been recently selected by the US Department of Energy (EM-22) to address groundwater contaminant migration at the Hanford Site. This paper summarizes the background and objectives of these projects. Five of the selected projects are targeted at hexavalent chromium contamination in Hanford 100 Area groundwater. These projects represent an integrated approach towards identifying the source of hexavalent chromium contamination in the Hanford 100-D Area and treating the groundwater contamination. Currently, there is no effective method to stop strontium-90 associated with the riparian zone sediments from leaching into the river. Phytoremediation may be a possible way to treat this contamination. Its use at the 100-N Area will be investigated. Another technology currently being tested for strontium-90 contamination at the 100-N Area involves injection (through wells) of a calcium-citrate-phosphate solution, which will precipitate apatite, a natural calcium-phosphate mineral. Apatite will adsorb the strontium-90, and then incorporate it as part of the apatite structure, isolating the strontium-90 contamination from entering the river. This EM-22 funded apatite project will develop a strategy for infiltrating the apatite solution from ground surface or a shallow trench to provide treatment over the upper portion of the contaminated zone, which is unsaturated during low river stage.

Hyperalkaline and saline radioactive waste fluids with elevated temperatures from S-SX high-level waste tank farm at Hanford, WA, USA accidentally leaked into sediments beneath the tanks, initiating a series of geochemical processes and reactions whose significance and extent was unknown. Among the most important processes was the dissolution of soil minerals and precipitation of stable secondary phases. The objective of this investigation was to study the release of Fe into the aqueous phase upon dissolution of Fe-bearing soil minerals, and the subsequent formation of Fe-rich precipitates. Batch reactors were used to conduct experiments at 50°C using solutions similar in composition to the waste fluids. Results clearly showed that, similarly to Si and Al, Fe was released from the dissolution of soil minerals (most likely phyllosilicates such as biotite, smectite and chlorite). The extent of Fe release increased with base concentration and decreased with Al concentration in the contacting solution. The maximum apparent rate of Fe release (0.566×10−13molm−2s−1) was measured in the treatment with no Al and a concentration of 4.32molL−1 NaOH in the contact solution. Results from electron microscopy indicated that while Si and Al precipitated together to form feldspathoids in the groups of cancrinite and/or sodalite, Fe precipitation followed a different pathway leading to the formation of hematite and goethite. The newly formed Fe oxy-hydroxides may increase the sorption capacity of the sediments, promote surface mediated reactions such as precipitation and heterogeneous redox reactions, and affect the phase distribution of contaminants and radionuclides.

Hyperalkaline (pH ~14), high temperature (>100°C), high ionic strength (>5 mol L-1) waste fluids (WFs) contaminated with Cr(VI) have accidentally leaked from storage tanks at the Hanford Site in Washington into the underlying sediments. Previous laboratory studies conducted under such extreme conditions have shown that Cr(VI) was abiotically reduced to Cr(III) by aqueous Fe(II) released from dissolving soil minerals. However, these studies were conducted in the absence of other electron acceptors such as O2 that may be present, although in limited amounts, in inherently oxidized vadose zones, and may compete for Fe(II) electrons. In addition, Cr(VI) adsorption can become an important attenuation mechanism in WF-altered sediments because sorbents such as cancrinite, sodalite, and Fe oxides were formed in appreciable amounts in these geosystems. The objectives of this study were to estimate Cr(VI) attenuation via reduction in the presence of limited amounts of O2 and to determine the potential for Cr(VI) sorption in the vadose zone sediments of the hyperalkaline plume. Results from batch and column experiments conducted at 50°C with simulated WF and results of microprobe elemental mapping and micro-X-ray absorption near edge structure analyses performed on posttreatment sediments confirmed that the main attenuation mechanism was Cr(VI) reduction to less mobile Cr(III). Oxygen that was periodically introduced into the otherwise closed geosystem competed effectively with Cr(VI) for the available electrons only at low base concentrations, that is, 1 mol L-1. Localized reduced zones were created in the sediment when intensive dissolution of soil minerals occurred in the presence of high base concentrations, that is, 4 mol L-1, confirming that contaminant Cr mobility may be significantly reduced even in the presence of O2.

The effect of caustic NaNO3 solutions on the sorption of 137Cs to a Hanford site micaceous subsurface sediment was investigated as a function of base exposure time (up to 168 d), temperature (10°C or 50°C), and NaOH concentration (0.1 mol/L to 3 mol/L). At 10°C and 0.1 M NaOH, the slow evolution of [Al]aq was in stark contrast to the rapid increase and subsequent loss of [Al]aq observed at 50°C (regardless of base concentration). Exposure to 0.1 M NaOH at 10°C for up to 168 d exhibited little if any measurable effect on sediment mineralogy, Cs+ sorption, or Cs+ selectivity; sorption was well described with a two-site ion exchange model modified to include enthalpy effects. At 50°C, dissolution of phyllosilicate minerals increased with [OH]. A zeolite (tetranatrolite; Na2Al2Si3O10·2H2O) precipitated in 0.1 M NaOH after about 7 days, while an unnamed mineral phase (Na14Al12Si13O51·6H2O) precipitated after 4 and 2 days of exposure to 1 M and 3 M NaOH solutions, respectively. Short-term (16 h) Cs+ sorption isotherms (10−9–10−2 mol/L) were measured on sediment after exposure to 0.1 M NaOH for 56, 112, and 168 days at 50°C. There was a trend toward slightly lower conditional equilibrium exchange constants (Δlog NaCsKc ∼ 0.25) over the entire range of surface coverage, and a slight loss of high affinity sites (15%) after 168 days of pretreatment with 0.1 M base solution. Cs+ sorption to sediment over longer times was also measured at 50°C in the presence of NaOH (0.1 M, 1 M, and 3 M NaOH) at Cs+ concentrations selected to probe a range of adsorption densities. Model simulations of Cs+ sorption to the sediment in the presence of 0.1 M NaOH for 112 days slightly under-predicted sorption at the lower Cs+ adsorption densities. At the higher adsorption densities, model simulations under-predicted sorption by 57%. This under-prediction was surmised to be the result of tetranatrolite precipitation, and subsequent slow Na → Cs exchange. At higher OH concentrations, Cs+ sorption in the presence of base for 112 days was unexpectedly equal to, or greater than that expected for pristine sediment. The precipitation of secondary phases, coupled with the fairly unique mica distribution and quantity across all size-fractions in the Hanford sediment, appears to mitigate the impact of base dissolution on Cs+ sorption.