Publications

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Arsenic removal technologies that are effective at the tens of ppb level include coagulation, followed by settling/microfiltration, ion exchange by mineral surfaces,and pressure-driven membrane processes (reverse osmosis, nanofiltration and ultrafiltration). This report describes the fundamental mechanisms of operation of the arsenic removal systems and addresses the critical issues of arsenic speciation, source water quality on the performance of the arsenic removal systems and costs associated with the different treatment technology categories.

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Groundwater plumes containing dissolved uranium at levels above natural background exist adjacent to uranium ore bodies, at uranium mines, milling locations, and at a number of explosive test facilities. Public health concerns require that some assessment of the potential for further plume movement in the future be made. Reaction-transport models, which might conceivably be used to predict plume movement, require extensive data inputs that are often uncertain. Many of the site-specific inputs are physical parameters that can vary spatially and with time. Limitations in data availability and accuracy means that reaction-transport predictions can rarely provide more than order-of-magnitude bounding estimates of contaminant movement in the subsurface. A more direct means for establishing the limits of contaminant transport is to examine actual plumes to determine if, collectively, they spread and attenuate in a reasonably consistent and characteristic fashion. Here a number of U plumes from ore bodies and contaminated sites were critically examined to identify characteristics of U plume movement. The magnitude of the original contaminant source, the geologic setting, and the hydrologic regime were rarely similar from site to site. Plumes also spanned a vast range of ages, and no complete set of time-series plume analyses based on the spatial extent of U contamination exist for a particular site. Despite the accumulated uncertainties and variabilities, the plume data set gave a clear and reasonably consistent picture of U plume behavior. Specifically, uranium plumes. © 2001 by AEHS.

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Conventional performance assessments assume that radioactive {sup 99}Tc travels as a non-sorbing component with an effective K{sub d} (distribution coefficient) of 0. This is because soil mineral surfaces commonly develop net negative surface charges and pertechnetate (TcO{sub 4}), with large ionic size and low electrical density, is not sorbed onto them. However, a variety of materials have been identified that retain Tc and may eventually lead to promising Tc getters. In assessing Tc getter performance it is important to evaluate the environment in which the getter is to function. In many contaminant plumes Tc will only leach slowly from the source of the contamination and significant dilution is likely. Thus, sub-ppb Tc concentrations are expected and normal groundwater constituents will dominate the aquifer chemistry. In this setting a variety of constituents were found to retard TcO{sub 4}: imogolite, boehmite, hydrotalcite, goethite, copper sulfide and oxide and coal. Near leaking tanks of high level nuclear waste, Tc may be present in mg/L level concentrations and groundwater chemistry will be dominated by constituents from the waste. Both bone char, and to a lesser degree, freshly precipitated Al hydroxides may be effective Tc scavengers in this environment. Thus, the search for Tc getters is far from hopeless, although much remains to be learned about the mechanisms by which these materials retain Tc.

{sup 137}Cs was dispersed globally by cold war activities and, more recently, by the Chernobyl accident. Engineered extraction of {sup 137}Cs from soils and groundwaters is exceedingly difficult. Because the half life of {sup 137}Cs is only 30.2 years, remediation might be more effective (and less costly) if {sup 137}Cs bioavailability could be demonstrably limited for even a few decades by use of a reactive barrier. Essentially permanent isolation must be demonstrated in those few settings where high nuclear level wastes contaminated the environment with {sup 135}Cs (half life 2.3x10{sup 6} years) in addition to {sup 137}Cs. Clays are potentially a low-cost barrier to Cs movement, though their long-term effectiveness remains untested. To identify optimal clays for Cs retention Cs resorption was measured for five common clays: Wyoming Montmorillonite (SWy-1), Georgia Kaolinites (KGa-1 and KGa-2), Fithian Illite (F-Ill), and K-Metabentonite (K-Mbt). Exchange sites were pre-saturated with 0.16 M CsCl for 14 days and readily exchangeable Cs was removed by a series of LiNO{sub 3} and LiCl washes. Washed clay were then placed into dialysis bags and the Cs release to the deionized water outside the bags measured. Release rates from 75 to 139 days for SWy-1, K-Mbt and F- 111 were similar; 0.017 to 0.021% sorbed Cs released per day. Both kaolinites released Cs more rapidly (0.12 to 0.05% of the sorbed Cs per day). In a second set of experiments, clays were doped for 110 days and subjected to an extreme and prolonged rinsing process. All the clays exhibited some capacity for irreversible Cs uptake so most soils have some limited ability to act as a natural barrier to Cs migration. However, the residual loading was greatest on K-Mbt ({approximately} 0.33 wt% Cs). Thus, this clay would be the optimal material for constructing artificial reactive barriers.

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Radionuclide transport in soils and groundwaters is routinely calculated in performance assessment (PA) codes using simplified conceptual models for radionuclide sorption, such as the K{sub D} approach for linear and reversible sorption. Model inaccuracies are typically addressed by adding layers of conservativeness (e.g., very low K{sub D}'s), and often result in failed transport predictions or substantial increases in site cleanup costs. Realistic assessments of radionuclide transport over a wide range of environmental conditions can proceed only from accurate, mechanistic models of the sorption process. They have focused on the sorption mechanisms and partition coefficients for Cs{sup +}, Sr{sup 2+} and Ba{sup 2+} (analogue for Ra{sup 2+}) onto iron oxides and clay minerals using an integrated approach that includes computer simulations, sorption/desorption measurements, and synchrotron analyses of metal sorbed substrates under geochemically realistic conditions. Sorption of Ba{sup 2+} and Sr{sup 2+} onto smectite is strong, pH-independent, and fully reversible, suggesting that cation exchange at the interlayer basal sites controls the sorption process. Sr{sup 2+} sorbs weakly onto geothite and quartz, and is pH-dependent. Sr{sup 2+} sorption onto a mixture of smectite and goethite, however, is pH- and concentration dependent. The adsorption capacity of montmorillonite is higher than that of goethite, which may be attributed to the high specific surface area and reaction site density of clays. The presence of goethite also appears to control the extent of metal desorption. In-situ, extended X-ray absorption fine structure (EXAFS) spectroscopic measurements for montmorillonite and goethite show that the first shell of adsorbed Ba{sup 2+} is coordinated by 6 oxygens. The second adsorption shell, however, varies with the mineral surface coverage of adsorbed Ba{sup 2+} and the mineral substrate. This suggests that Ba{sup 2+} adsorption on mineral surfaces involves more than one mechanism and that the stability of sorbed complexes will be affected by substrate composition. Molecular modeling of Ba{sup 2+} sorption on goethite and Cs{sup +} sorption on kaolinite surfaces were performed using molecular dynamics techniques with improved Lennard-Jones interatomic potentials under periodic boundary conditions. Ba{sup 2+} was observed to have a preference for inner sphere sorption onto goethite, with the (101) and (110) surfaces representing the controlling sorption surfaces for bulk K{sub D} measurements. Large-scale simulations of Cs{sup +} sorption on kaolinite (1000's of atoms) provide a statistical basis for the theoretical evaluation and prediction of Cs{sup +} K{sub D} values. Results suggest the formation of a strong inner sphere complex for Cs{sup +} on the kaolinite edge surfaces and a weakly bound outer sphere complex on the hydroxyl basal surface.