No abstract available

[en] The development of effective remediation strategies requires a comprehensive understanding of contaminant behavior in the environment. At the Hanford Site, located in southeastern Washington State (United States of America), seasonal fluctuations of the nearby Columbia River cause flushing of the 300 Area uranium (U) plume in the lower vadose zone. The variation of water chemistry alternately promotes adsorption and desorption of U from sediment. Therefore, the following question arises: what is the mobility of U in groundwater that has only recently became associated with sediment relative to U that has been associated with the sediment for decades? Geochemical transformations, including surface complexation, precipitation, and/or physical processes will impact U speciation as the contact time with sediment increases. To investigate this question, dissolved ²³³uranyl nitrate [UO₂(NO₃)₂] was added to U-contaminated Hanford Site sediment and incubated for up to 1 year. Following 1-week, 1-month, and 1-year incubation periods, the extraction of U from the sediment was accomplished using either batch or continuous leach techniques, and multiple extractants. The elution of ²³³U during continuous leaching was influenced by the incubation period. The change in the ²³⁵U/²³³U ratio eluted was indicative of the extraction of different U phases, and was a function of the incubation period. Removal of ²³³U by batch extraction clearly showed the effects of the incubation period and extractant. The extractability of the ²³³U spike by some extractants is independent of incubation period (up to 1 year) suggesting it is present as either sorbed or surface precipitate phases. Model simulation of the data provides insight into the processes involved with the extraction of U from the sediment. (orig.)

[en] A series of laboratory experiments and computer simulations was conducted to assess the extent of uranium remobilization that is likely to occur at the end of the life cycle of an in situ sediment reduction process. The process is being tested for subsurface remediation of chromate- and chlorinated solvent-contaminated sediments at the Hanford Site in southeastern Washington. Uranium species that occur naturally in the +6 valence state ∼(VI) at 10 ppb in groundwater at Hanford will accumulate as U(N) through the reduction and subsequent precipitation conditions of the permeable barrier created by in situ redox manipulation. The precipitated uranium will W remobilized when the reductive capacity of the barrier is exhausted and the sediment is oxidized by the groundwater containing dissolved oxygen and other oxidants such as chromate. Although U(N) accumulates from years or decades of reduction/precipitation within the reduced zone, U(W) concentrations in solution are only somewhat elevated during aquifer oxidation because oxidation and dissolution reactions that release U(N) precipitate to solution are slow. The release rate of uranium into solution was found to be controlled mainly by the oxidation/dissolution rate of the U(IV) precipitate (half-life 200 hours) and partially by the fast oxidation of adsorbed Fe(II) (half- life 5 hours) and the slow oxidation of Fe(II)CO₃ (half-life 120 hours) in the reduced sediment. Simulations of uranium transport that incorporated these and other reactions under site-relevant conditions indicated that 35 ppb U(VI) is the maximum concentration likely to result from mobilization of the precipitated U(IV) species. Experiments also indicated that increasing the contact time between the U(IV) precipitates and the reduced sediment, which is likely to occur in the field, results in a slower U(IV) oxidation rate, which, in turn, would lower the maximum concentration of mobilized U(W). A six-month-long column experiment confirmed that uranium accumulated in reduced sediment was released slowly into solution with U(W) concentrations at only slightly greater than influent U(W) concentrations. This experiment also demonstrated that dissolved chromate, another oxidant likely to be present in some field systems, did not increase the release rate of uranium into solution

[en] Batch adsorption experiments were conducted with three zeolites (clinoptilolite, chabazite, and A-51) to determine their potential applicability as in-situ permeable barriers to ground water strontium migration in the 100-N Area of the Hanford Site. Each of the zeolites was an effective adsorbent for strontium, even in competition with calcium at concentrations typical of Hanford ground water, and the authors determined that clinoptilolite would be the most cost-effective. The strontium adsorption data for calcium-saturated clinoptilolite were fitted to a Langmuir isotherm, which is linear at solution concentrations of less than 10^-5 mol/L. In this region, the adsorption coefficient (K_d) was 956 L/kg. Because strontium concentrations in hanford ground water are typically 3 x 10^-6 mol/L, assuming linear adsorption (K_d = 956 L/kg) for modeling purposes is appropriate. These data were used to design an effective barrier and were incorporated into a transport model to assess its performance. Calculations indicated that a barrier 1.3 m thick would prevent strontium-90 migration to the Columbia river at 100-N Area for over 50 yr. Because of radioactive decay and adsorption, the maximum breakthrough of strontium-90, approximately 5% of the initial input, would occur at 100 yr. Preliminary experimental work was conducted to determine the adsorption kinetics of strontium on clinoptilolite. A comparison of the adsorption rate of strontium with its residence time (within the barrier) indicates that adsorption kinetics are sufficiently fast that the barrier performance will not be significantly affected

[en] The time-variant chemical behavior of Co^IIEDTA (and other metal-EDTA complexes) was investigated in suspensions of iron oxide-coated sand to identify equilibrium and kinetic reactions that control the mobility of Me^II-EDTA complexes in subsurface environments. Batch experiments were conducted to evaluate the adsorption as a function of pH, concentration, and time and to quantify the rate-controlling step(s) of dissolution of the iron oxide by EDTA complexes. Ionic Co²⁺ exhibited typical cation-like adsorption, whereas Me^IIEDTA adsorption was ligand-like, increasing with decreasing pH. Adsorption isotherms for all reactive species exhibited Langmuir behavior, with site saturation occurring at molar values of <0.5% of Fe_tot. The adsorption of Me^IIEDTA enhanced the apparent solubility of the iron oxide phase, which destabilized the Co^IIEDTA complex, liberating Co²⁺ and Fe^IIIEDTA. The dissolution rate was an order of magnitude slower at pH 6.5 than at pH 4.5 and was influenced by the re-adsorption of solubilized Fe^IIIEDTA. Two multireaction kinetic models were developed that each included Langmuir adsorption for Co²⁺ and metal-EDTA species but differed in their depiction of the dissolution mechanism (i.e., ligand-versus proton-promoted dissolution). 45 refs., 8 figs., 6 tabs

[en] Highlights: • Ammonia transport can be predicted from gas movement and equilibrium partitioning. • Ammonia diffusion rate in unsaturated sediment is a function of water contents. • High pH induced by ammonia causes mineral dissolution and sequential precipitation. • Ammonia treatment effectively immobilized uranium from contaminated sediments. - Abstract: Use of gas-phase amendments for in situ remediation of inorganic contaminants in unsaturated sediments of the vadose zone may be advantageous, but there has been limited development and testing of gas remediation technologies. Treatment with ammonia gas has a potential for use in treating inorganic contaminants (such as uranium) because it induces a high pore-water pH, causing mineral dissolution and subsequent formation of stable precipitates that decrease the mobility of some contaminants. For field application of this treatment, further knowledge of ammonia transport in porous media and the geochemical reactions induced by ammonia treatment is needed. Laboratory studies were conducted to support calculations needed for field treatment design, to quantify advective and diffusive ammonia transport in unsaturated sediments, to evaluate inter-phase (gas/sediment/pore water) reactions, and to study reaction-induced pore-water chemistry changes as a function of ammonia delivery conditions, such as flow rate, gas concentration, and water content. Uranium-contaminated sediment was treated with ammonia gas to demonstrate U immobilization. Ammonia gas quickly partitions into sediment pore water and increases the pH up to 13.2. Injected ammonia gas advection front movement can be reasonably predicted by gas flow rate and equilibrium partitioning. The ammonia gas diffusion rate is a function of the water content in the sediment. Sodium, aluminum, and silica pore-water concentrations increase upon exposure to ammonia and then decline as aluminosilicates precipitate when the pH declines due to buffering. Up to 85% of the water-leachable U was immobilized by ammonia treatment

[en] Subsurface contaminants at Department of Energy (DOE) sites occur in both the vadose and groundwater saturated zones. Many of the groundwater plumes are already dispersed over large areas (square miles) and are located hundreds of feet below the ground. This type of dispersed, inaccessible contamination, which is more difficult than other types of contamination to treat using excavation or pump-and-treat methods, may only be treated successfully by the in situ manipulation of natural processes to change the mobility or form of the contaminants. An unconfined aquifer is usually an oxidizing environment, therefore, most of the contaminants that are mobile in the aquifer are those that are mobile under oxidizing conditions. If the redox potential of the aquifer is made reducing, then a variety of contaminants can be treated. The goal of In-Situ Redox Manipulation (ISRM) is to create a permeable treatment zone in the subsurface for remediation of redox sensitive contaminants in the groundwater. The permeable treatment zone is created by reducing the ferric iron to ferrous iron within the clay minerals of the aquifer sediments. This reduction can be accomplished with chemical reducing agents, such as sodium dithionite, or through the stimulation of naturally-occurring iron-reducing bacteria with nutrients (e.g. lactate). After the aquifer sediments are reduced, any reagent or reaction products introduced into the subsurface are removed. Redox sensitive contaminants that can be treated by this technology include chromate, uranium, technetium and some chlorinated solvents (e.g., carbon tetrachloride and trichloroethylene). Chromate is immobilized by reduction to highly insoluble chromium hydroxide or iron chromium hydroxide solid solution. This case is particularly favorable since chromium is not easily reoxidized under ambient environmental conditions. Uranium and technetium will also be reduced to less soluble forms, and chlorinated solvents will be destroyed

[en] An in situ redox manipulation (ISRM) method for creating a permeable treatment zone in the subsurface has been developed at the laboratory bench and intermediate scales and deployed at the field scale for reduction/immobilization of chromate contamination. At other sites, the same redox technology is currently being tested for dechlorination of TCE. The reduced zone is created by injected reagents that reduce iron naturally present in the aquifer sediments from Fe(III) to surface-bound and structural Fe(II) species. Standard ground water wells are used, allowing treatment of contaminants too deep below the ground surface for conventional trench-and-fill technologies. A proof-of-principle field experiment was conducted in September 1995 at a chromate (hexavalent chromium) contaminated ground water site on the Hanford Site in Washington. The test created a 15 m diameter cylindrical treatment zone. The three phases of the test consisted of (1) injection of 77,000 L of buffered sodium dithionite solution in 17.1 hours, (2) reaction for 18.5 hours, and (3) withdrawal of 375,000 L in 83 hours. The withdrawal phase recovered 87% to 90% of the reaction products. Analysis of post-experimental sediment cores indicated that 60% to 100% of the available reactive iron in the treated zone was reduced. The longevity of the reduced zone is estimated between seven and 12 years based on the post-experiment core samples. Three and half years after the field test, the treatment zone remains anoxic, and hexavalent chromium levels have been reduced from 0.060 mg/L to below detection limits (0.008 mg/L). Additionally, no significant permeability changes have been detected during any phase of the experiment

[en] This project was initiated to develop a strategy for infiltration of a Ca-citrate-PO₄ solution in order to precipitate apatite [Ca₆(PO₄)₁₀(OH)₂] in desired locations in the vadose zone for Sr-90 remediation. Laboratory experiments have demonstrated that infiltration of a Ca-citrate-PO₄ solution into sediments at low and high water saturation results in citrate biodegradation and formation of apatite. The citrate biodegradation rate was relatively uniform, in spite of the spatial variability of sediment microbial biomass, likely because of microbial transport processes that occur during solution infiltration. The precipitate was characterized as hydroxyapatite, and the Sr-90 substitution into apatite was shown to have an incorporation half-life of 5.5 to 16 months. One and two dimensional (1-D and 2-D) laboratory infiltration experiments quantified the spatial distribution of apatite that formed during solution infiltration. Slow infiltration in 2-D experiments at low water saturation show the apatite precipitate concentrated in the upper third of the infiltration zone. More rapid 1-D infiltration studies show the apatite precipitate concentrated at greater depth. (authors)

[en] Laboratory batch and column experiments were conducted with Hanford sediments to develop the capability to predict (1) the longevity of dithionite in these systems, (2) its efficiency as a reductant of structural iron, and (3) the longevity and reactivity of the reduced iron with soluble inorganic and organic species. After an initial induction period, the loss of dithionite by disproportionation and oxidation could be described by pseudo-first-order (PFO) kinetics. Other than the initial reaction with ferric iron, the primary factor promoting loss of dithionite in this system was disproportion nation via heterogeneous catalysis at mineral surfaces. The efficiency of the reduction of structural iron was nearly 100% for the first fourth of the ferric iron, but declined exponentially with higher degrees of reduction so that 75% of the ferric iron could be reduced. This decrease in reduction efficiency probably was related to differences in the accessibility of ferric iron in the mineral particles, with iron in clay-sized particles being the most accessible and that in silt- and sand-sized particles less accessible. Flow-through column studies showed that a reduced-sediment barrier created in this manner could maintain a reducing environment