Prediction of aluminum, uranium, and co-contaminants precipitation and adsorption during titration of acidic sediments.

Batch and column recirculation titration tests were performed with contaminated acidic sediments. A generic geochemical model was developed combining precipitation, cation exchange, and surface complexation reactions to describe the observed pH and metal ion concentrations in experiments with or without the presence of CO2. Experimental results showed a slow pH increase due to strong buffering by Al hydrolysis and precipitation and CO2 uptake. The cation concentrations generally decreased at higher pH than those observed in previous tests without CO2. Using amorphous Al(OH)3 and basaluminite precipitation reactions and a cation exchange selectivity coefficient K(Na\Al) of 0.3, the model approximately described the observed (1) pH titration curve, (2) Ca, Mg, and Mn concentration by cation exchange, and (3) U concentrations by surface complexation with Fe hydroxides at pH < 5 and with liebigite (Ca2UO2(CO3)3·10H2O) precipitation at pH > 5. The model indicated that the formation of aqueous carbonate complexes and competition with carbonate for surface sites could inhibit U and Ni adsorption and precipitation. Our results suggested that the uncertainty in basaluminite solubility is an important source of prediction uncertainty and ignoring labile solid phase Al underestimates the base requirement in titration of acidic sediments.

Modeling uranium transport in acidic contaminated groundwater with base addition.

This study investigates reactive transport modeling in a column of uranium(VI)-contaminated sediments with base additions in the circulating influent. The groundwater and sediment exhibit oxic conditions with low pH, high concentrations of NO(3)(-), SO(4)(2-), U and various metal cations. Preliminary batch experiments indicate that additions of strong base induce rapid immobilization of U for this material. In the column experiment that is the focus of the present study, effluent groundwater was titrated with NaOH solution in an inflow reservoir before reinjection to gradually increase the solution pH in the column. An equilibrium hydrolysis, precipitation and ion exchange reaction model developed through simulation of the preliminary batch titration experiments predicted faster reduction of aqueous Al than observed in the column experiment. The model was therefore modified to consider reaction kinetics for the precipitation and dissolution processes which are the major mechanism for Al immobilization. The combined kinetic and equilibrium reaction model adequately described variations in pH, aqueous concentrations of metal cations (Al, Ca, Mg, Sr, Mn, Ni, Co), sulfate and U(VI). The experimental and modeling results indicate that U(VI) can be effectively sequestered with controlled base addition due to sorption by slowly precipitated Al with pH-dependent surface charge. The model may prove useful to predict field-scale U(VI) sequestration and remediation effectiveness.

Dissolution of uranium-bearing minerals and mobilization of uranium by organic ligands in a biologically reduced sediment.

The stability and mobility of uranium (U) is a concern following its reductive precipitation or immobilization by techniques such as bioremediation at contaminated sites. In this study, the influences of complexing organic ligands such as citrate and ethylenediaminetetraacetate (EDTA) on the mobilization of U were investigated in both batch and column flow systems using a contaminated and bioreduced sediment. Results indicate that both reduced U(IV) and oxidized U(VI) in the sediment can be effectively mobilized with the addition of EDTA or citrate under anaerobic conditions. The dissolution and mobilization of U appear to be correlated to the dissolution of iron (Fe)- or aluminum (Al)-bearing minerals, with EDTA being more effective (with R2≥0.89) than citrate (R2<0.60) in dissolving these minerals. The column flow experiments confirm that U, Fe, and Al can be mobilized by these ligands under anoxic conditions, although the cumulative amounts of U removal constituted ∼0.1% of total U present in this sediment following a limited period of leaching. This study concludes that the presence of complexing organic ligands may pose a long-term concern by slowly dissolving U-bearing minerals and mobilizing U even under a strict anaerobic environment.

Sequestering uranium and technetium through co-precipitation with aluminum in a contaminated acidic environment.

This research evaluated a method of controlled base addition for immobilizing uranium (U) and technetium (Tc) through coprecipitation with aluminum (Al) and other metal ions which coexist in a highly contaminated acidic environment. The batch and column experiments indicate that the addition of strong base (NaOH) provided a rapid yet effective means of sequestering U, Tc, and toxic metal ions such as nickel (Ni2+) and cobalt (Co2+) in the sediment and groundwater. Greater than 94% of soluble U (as UO2(2+)) and > 83% of Tc (as TcO4-) can be immobilized at pH above 4.5 by co-precipitation with Al-oxyhydroxides. The presence of sediment minerals appeared to facilitate co-precipitation of these contaminants at lower pH values than those in the absence of sediments. The immobilized U and Tc were found to be stable against dissolution in Ca(NO3)2 solution (up to 50 mM) because of the formation of strong surface complexes between U or Tc and Al-oxyhydroxides. This research concludes that as long as a relatively high pH (> 5) and a low carbonate concentration are maintained, both U and Tc can be effectively immobilized under given site-specific conditions.

Dissolution and mobilization of uranium in a reduced sediment by natural humic substances under anaerobic conditions.

Biological reduction and precipitation of uranium (U) has been proposed as a remedial option for immobilizing uranium at contaminated sites, but the long-term stability and mobility of uranium remain a concern because the uranium is neither removed nor destroyed. In this study, the dissolution and mobilization of reduced and oxidized forms of uranium [U(IV) and U(VI)] by natural humic substances were investigated in batch and column-flow systems using a bioreduced sediment containing both U(IV) and U(VI). The addition of humic substances significantly increased the dissolution of U(IV) under anaerobic conditions. Humic acid (HA) was found to be more effective than fulvic acid (FA) in dissolving U(IV) in 1 mM KCl or KHCO3 background solution. However, more U(VI) was dissolved in 1 mM KHCO3 than in 1 mM KCl background electrolyte. HA also was found to be more effective than FA in mobilizing uranium under reducing and column-flow conditions, although the cumulative amount of eluted U(VI) and U(IV) was relatively low (<60 microg) after leaching with approximately 97 pore volumes of the humic solution in 1 mM KHCO3. These observations suggestthat natural humic substances could potentially influence the long-term stability of bioreduced U(IV) even under strongly reducing environments.

Surface-enhanced Raman spectroscopy for uranium detection and analysis in environmental samples.

Techniques for rapid screening of uranium in environmental samples are needed, and this study entails the development of surface-enhanced Raman scattering (SERS) for analyzing uranium in aqueous media with improved sensitivity and reproducibility. A new SERS substrate based on (aminomethyl)phosphonic acid (APA)-modified gold nanoparticles was found to give greater than three orders of magnitude SERS enhancement compared with unmodified bare gold nanoparticles. Intensities of uranyl band at about 830 cm(-1) were proportional to the concentrations of uranium in solution, especially at relatively low concentrations (<10(-5) M). A detection limit of approximately 8x10(-7) M was achieved with a good reproducibility since the measurement was performed directly in dispersed aqueous suspension. Without pretreatment, the technique was successfully employed for detecting uranium in a highly contaminated groundwater with a low pH, high dissolved salts (e.g., nitrate, sulfate, calcium and aluminum) and total organic carbon.

Influence of bicarbonate, sulfate, and electron donors on biological reduction of uranium and microbial community composition.

A microcosm study was performed to investigate the effect of ethanol and acetate on uranium(VI) biological reduction and microbial community changes under various geochemical conditions. Each microcosm contained an uranium-contaminated sediment (up to 2.8 g U/kg) suspended in buffer with bicarbonate at concentrations of either 1 or 40 mM and sulfate at either 1.1 or 3.2 mM. Ethanol or acetate was used as an electron donor. Results indicate that ethanol yielded in significantly higher U(VI) reduction rates than acetate. A low bicarbonate concentration (1 mM) was favored for U(VI) bioreduction to occur in sediments, but high concentrations of bicarbonate (40 mM) and sulfate (3.2 mM) decreased the reduction rates of U(VI). Microbial communities were dominated by species from the Geothrix genus and Proteobacteria phylum in all microcosms. However, species in the Geobacteraceae family capable of reducing U(VI) were significantly enriched by ethanol and acetate in low-bicarbonate buffer. Ethanol increased the population of unclassified Desulfuromonales, while acetate increased the population of Desulfovibrio. Additionally, species in the Geobacteraceae family were not enriched in high-bicarbonate buffer, but the Geothrix and the unclassified Betaproteobacteria species were enriched. This study concludes that ethanol could be a better electron donor than acetate for reducing U(VI) under given experimental conditions, and electron donor and groundwater geochemistry alter microbial communities responsible for U(VI) reduction.

In situ bioreduction of uranium (VI) to submicromolar levels and reoxidation by dissolved oxygen.

Groundwater within Area 3 of the U.S. Department of Energy (DOE) Environmental Remediation Sciences Program (ERSP) Field Research Center at Oak Ridge, TN (ORFRC) contains up to 135 microM uranium as U(VI). Through a series of experiments at a pilot scale test facility, we explored the lower limits of groundwater U(VI) that can be achieved by in-situ biostimulation and the effects of dissolved oxygen on immobilized uranium. Weekly 2 day additions of ethanol over a 2-year period stimulated growth of denitrifying, Fe(III)-reducing, and sulfate-reducing bacteria, and immobilization of uranium as U(IV), with dissolved uranium concentrations decreasing to low levels. Following sulfite addition to remove dissolved oxygen, aqueous U(VI) concentrations fell below the U.S. Environmental Protection Agengy maximum contaminant limit (MCL) for drinking water (< 30/microg L(-1) or 0.126 microM). Under anaerobic conditions, these low concentrations were stable, even in the absence of added ethanol. However, when sulfite additions stopped, and dissolved oxygen (4.0-5.5 mg L(-1)) entered the injection well, spatially variable changes in aqueous U(VI) occurred over a 60 day period, with concentrations increasing rapidly from < 0.13 to 2.0 microM at a multilevel sampling (MLS) well located close to the injection well, but changing little at an MLS well located further away. Resumption of ethanol addition restored reduction of Fe(III), sulfate, and U(VI) within 36 h. After 2 years of ethanol addition, X-ray absorption near-edge structure spectroscopy (XANES) analyses indicated that U(IV) comprised 60-80% of the total uranium in sediment samples. Atthe completion of the project (day 1260), U concentrations in MLS wells were less than 0.1 microM. The microbial community at MLS wells with low U(VI) contained bacteria that are known to reduce uranium, including Desulfovibrio spp. and Geobacter spp., in both sediment and groundwater. The dominant Fe(III)-reducing species were Geothrix spp.