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

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.

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

David Andrew Barry

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.

2008

Modeling the role of partitioning substrates in DNAPL column studies

David Andrew Barry

SABRE (Source Area BioREmediation) is a public/private consortium of twelve companies, two government agencies, and three research institutions whose charter is to determine if enhanced anaerobic bioremediation can result in effective and quantifiable treatment of chlorinated solvent DNAPL source areas. The focus of this 4-year, $5.7 million dollar research and development project is a field site in the United Kingdom containing a DNAPL source area with groundwater concentrations exceeding several hundred mg/L TCE. Prior to field evaluation, a significant program of laboratory and modelling studies are being conducted to better understand and predict system performance. An extensive microcosm study with site soil and groundwater revealed emulsified vegetable (soybean) oil (EVO) to be particularly effective at promoting reductive dechlorination in the presence of high TCE concentrations. One-dimensional column studies examined the influence of EVO on bioenhanced mass transfer, reductive dechlorination, and related processes influencing the overall biodegradation efficiency. Monitoring of a large suite of relevant breakdown products and geochemical parameters in both time and space in the column studies provided a rich data set relevant to the field site. This paper describes a parallel and coordinated numerical modeling effort at University of Edinburgh intended to simulate the experiments for the purpose of interpreting and better understanding the underlying processes. The biogeochemical process model considers numerous processes including the rate-limited dissolution of DNAPL and EVO, and the fermentation and dechlorination processes that regulate H2 (direct electron donor) availability. The model was calibrated to experimental results in order to provide base case parameters associated with the column systems. These simulations provide insight into the influence of EVO on dechlorination behaviour and toxic inhibition: A thorough sensitivity analysis is being conducted in order to illuminate the relative influence of key parameters such as extent of EVO slow release, rate of EVO fermentation, and degree of bioaugmentation on the fate of chlorinated ethenes.

2007

Buffering soil-water acidity in chlorinated solvent bioremediation schemes

David Andrew Barry, Alessandro Brovelli, Christof Holliger, Elsa Marina Lacroix, Clare Robinson

Chlorinated solvents form a significant part of groundwater contamination worldwide. They are difficult to remove via physical means, and anaerobic source-zone remediation based on provision of fermentable e-donor is an attractive clean-up dechlorination option. However, organic acids and HCl lower the groundwater pH and thereby stall the microbial consortia responsible for the biodegradation process. Often, the soil’s natural buffering capacity will be exceeded, in which case a strategy of adding buffer to the groundwater is a priori beneficial to maintain dechlorination. Geochemical modelling was used to investigate the feasibility of adding naturally occurring buffering minerals to the groundwater for pH control. The simulations revealed that anorthite has the potential to be used as a sustainable pH buffering mineral.

International Union of Soil Sciences2010

Modeling the role of partitioning substrates in DNAPL column studies

David Andrew Barry

2007