Modeling U(VI) biomineralization in single- and dual- porosity porous media

Uranium extraction, processing and storage have resulted in a legacy of uranium-contaminated groundwater aquifers worldwide. An emerging remediation technology for such sites is the in situ immobilization of uranium via biostimulation of dissimilatory metal reducing bacteria (DMRB). While this approach has been successfully demonstrated in experimental studies, advances in understanding and optimization of the technique are needed. The motivation of this work was to understand better how dual-porosity (DP) porous media may affect immobilization efficiency via interactions with the dominant geochemical and microbial processes. A biogeochemical reactive transport model was developed for uranium immobilization by DMRB in both single and DP porous media. The impact that microbial residence location has on the success of biostimulated U(VI) immobilization in DP porous media was explored under various porosity and mass transfer conditions. Simulations suggest that DP media are likely to show delayed U(VI) immobilization relative to single porosity systems. U(VI) immobilization is predicted to be less when microbial activity is restricted to diffusion- dominant regions but not when restricted to advective- dominant regions. The results further highlight the importance of characterizing the bioresidency status of field sites if biogeochemical models are to predict accurately remediation schemes in physically heterogeneous media.

Modeling the effectiveness of U(VI) biomineralization in dual porosity porous media

David Andrew Barry

Recent attention has focused on the in situ immobilization of uranium by stimulation of dissimilatory metal-reducing bacteria (DMRB). The objective of this work was to investigate the effectiveness of this approach in heterogeneous and structured porous media, since such media may significantly affect the geochemical and microbial processes taking place in contaminated sites, impacting remediation efficiency during biostimulation. A biogeochemical reactive transport model was developed for uranium remediation by immobile-region-resident DMRB in two-region porous media. Simulations were used to investigate the parameter sensitivities of the system over wide ranging geochemical, microbial and groundwater transport conditions. The results suggest that optimal biomineralization is generally likely to occur when the regional mass transfer timescale is less than one thirtieth the value of the volumetric flux timescale, and/or the organic carbon fermentation timescale is less than one thirtieth the value of the advective timescale, and/or the mobile region porosity ranges between equal to and four times the immobile region porosity. Simulations including U(VI) surface complexation to Fe oxides additionally suggest that, while systems exhibiting U(VI) surface complexation may be successfully remediated, they are likely to display different degrees of remediation efficiency over varying microbial efficiency, mobile-immobile mass transfer, and porosity ratios. Such information may aid experimental and field designs, allowing for optimized remediation in dual-porosity (two-region) biostimulated DMRB U(VI) remediation schemes.

2011

Gaseous tracer diffusion from a point source as a site investigation method

David Werner

Transport modelling, risk assessment, and the evaluation of remediation strategies at sites contaminated with volatile organic compounds (VOCs) require the knowledge of gas-phase diffusion coefficients in unsaturated porous media. This thesis presents a new in situ method for the simultaneous measurement of effective and sorption-affected diffusion coefficients. The method relies on the injection of a gaseous compound and a conservative gaseous tracer using a soil gas probe. Concentrations are monitored at the tip of the probe during 8 hours and evaluated with an analytical equation for reactive transport in a homogeneous medium. Results are reported for volatile organic pollutants in a sand-filled lysimeter and at a field site with a sandy unsaturated zone. The determination of the effective diffusion coefficients showed good reproducibility and robustness with respect to analytical error, with an estimated overall error of 13%. The determination of the sorption-affected diffusion coefficients also showed good reproducibility. However, batch experiments suggest a systematic overestimation by 15-20%, because the tracers were not truly conservative. The in situ method avoids considerable uncertainties associated with estimating apparent diffusion coefficients from empirical relationships and avoids problems with establishing representative soil conditions in laboratory experiments. Another key issue for risk assessment at contaminated sites is the localization and quantification of nonaqueous phase liquids (NAPLs) such as gasoline, kerosene or heating oil in the subsurface. This thesis proposes the theory and practical application of a new diffusive partitioning tracer test (DPTT) for NAPL detection in the vadose zone. The general approach is the same as for the determination of apparent diffusion coefficients. A mixture of chlorofluorocarbons as gaseous tracers is injected into the vadose zone using a soil gas probe. While the tracers diffuse away, small volumes of gas are withdrawn from the injection point. The quantitative determination of the NAPL saturation is based on a comparison of the concentration decline of tracers with different air-NAPL partitioning coefficients. An alternative approach is the measurement of diffusive breakthrough curves in some distance from the injection point. The test has been evaluated in laboratory sand columns, at a field site with an artificial contamination embedded in a sandy unsaturated zone and in a sand-filled lysimeter, where the NAPL distribution was heterogeneous. NAPLs in saturations between 0 and 4 % of the total porosity have been reliably detected and quantified in a wide range of different water contents. NAPL saturations determined by DPTTs and by extraction from soil cores agree within a factor 2 or better. Results from the lysimeter study suggest that the NAPL saturation derived from the tracer concentrations at the injection point represent a small soil volume in the vicinity of the injection point, whereas the NAPL saturation determined from diffusive tracer breakthrough curves represent the soil between injection and monitoring point. These innovative investigation methods for the vadose zone are inexpensive, require little technical equipment on site, and allow the simultaneous determination of different parameters at one location from a few single measurements: as demonstrated in situ for two sandy soils, the effective and the apparent diffusion coefficient or the NAPL saturation, the effective diffusion coefficient and the soil gas concentration of several VOCs can be determined simultaneously by the proposed methods. This new approach has been tested in sandy, unsaturated porous media. The performance of the methods in heterogeneous environments or in porous media other than sand needs to be further investigated.

EPFL2002

Biogeochemical controls on the product of microbial U(VI) reduction

Daniel Scott Alessi, Rizlan Bernier-Latmani, Paul Pao-Yun Shao, Malgorzata Alicja Stylo

Biologically mediated immobilization of radio-nuclides in the subsurface is a promising strategy for the remediation of uranium-contaminated sites. During this process, soluble U(VI) is reduced by indigenous microorganisms to sparingly soluble U(TV). The crystalline U(IV) phase uraninite, or UO2, is the preferable end-product of bioremediation due to its relatively high stability and low solubility in comparison to biomass-associated nonuraninite U(IV) species that have been reported in laboratory and under field conditions. The goal of this study was to delineate the geochemical conditions that promote the formation of nonuraninite U(IV) versus uraninite and to decipher the mechanisms of its preferential formation. U(IV) products were prepared under varying geochemical conditions and characterized with X-ray absorption spectroscopy (XAS), scanning transmission X-ray microscopy (STXM), and various wet chemical methods. We report an increasing fraction of nonuraninite U(IV) species with decreasing initial U concentration. Additionally, the presence of several common groundwater solutes (sulfate, silicate, and phosphate) promote the formation of nonuraninite U(IV). Our experiments revealed that the presence of those solutes promotes the formation of bacterial extracellular polymeric substances (EPS) and increases bacterial viability, suggesting that the formation of nonuraninite U(IV) is due to a biological response to solute presence during U(VI) reduction. The results obtained from this laboratory-scale research provide insight into biogeochemical controls on the product(s) of uranium reduction during bioremediation of the subsurface.

Amer Chemical Soc2013

Gaseous tracer diffusion from a point source as a site investigation method

David Werner

EPFL2002