Michael J. Truex

Kinetics of U(VI) reduction by a dissimilatory Fe(III)‐reducing bacterium under non‐growth conditions

Dissimilatory metal-reducing microorganisms may be useful in processes designed for selective removal of uranium from aqueous streams. These bacteria can use U(VI) as an electron acceptor and thereby reduce soluble U(VI) to insoluble U(IV). While significant research has been devoted to demonstrating and describing the mechanism of dissimilatory metal reduction, the reaction kinetics necessary to apply this for remediation processes have not been adequately defined. In this study, pure culture Shewanella alga strain BrY reduced U(VI) under non-growth conditions in the presence of excess lactate as the electron donor. Initial U(VI) concentrations ranged from 13 to 1680 microM. A maximum specific U(VI) reduction rate of 2.37 micromole-U(VI)/(mg-biomass h) and Monod half-saturation coefficient of 132 microM-U(VI) were calculated from measured U(VI) reduction rates. U(VI) reduction activity was sustained at 60% of this rate for at least 80 h. The initial presence of oxygen at a concentration equal to atmospheric saturation at 22 degrees C delays but does not prevent U(VI) reduction. The rate of U(VI) reduction by BrY is comparable or better than rates reported for other metal reducing species. BrY reduces U(VI) at a rate that is 30% of its Fe(III) reduction rate.

The role of oxygen diffusion in passive bioremediation of petroleum contaminated soils

Abstract In passive bioremediation of petroleum hydrocarbon contaminated soils, oxygen diffusion is the primary mechanism for supplying the oxygen which is required for microbial hydrocarbon biodegradation processes. It is the objective of this research to theoretically evaluate whether passive bioremediation can be a feasible treatment alternative for petroleum contaminated soils. In this paper we derive equations for the steady-state oxygen concentration profiles which are expected to develop as a result of simultaneous oxygen diffusion and consumption in hydrocarbon contaminated soils. These equations are used to estimate the maximum oxygen penetration distance and the total cleanup time for several environmental scenarios such as surface and subsurface soil contamination as well as contaminated soil piles. It was found that oxygen is expected to penetrate most contaminated soils for up to several meters if hydrocarbon biodegradation rates are similar to those measured during bioventing respiration tests, i.e. approximately 2.5–10 ppm TPH day −1 . Both the depth of oxygen penetration and the total passive bioremediation cleanup time were found to be strongly dependent on the magnitude of the diffusion coefficient for oxygen in soil ( D s ). As expected, increased oxygen penetration distances and decreased cleanup times are associated with increased D s values. Since the magnitude of D s is inversely related to the soil moisture content, it is imperative to maintain moderately low soil moisture levels in order to maximize the effectiveness of passive bioremediation treatment. Passive bioremediation is expected to be a feasible and cost-effective treatment alternative for TPH contaminated soils in cases where the minimization of cleanup times is not a major remediation objective.

Rheological behavior of xanthan gum solution related to shear thinning fluid delivery for subsurface remediation

Xanthan gum solutions are shear thinning fluids which can be used as delivery media to improve the distribution of remedial amendments injected into heterogeneous subsurface environments. The rheological behavior of the shear thinning solution needs to be known to develop an appropriate design for field injection. In this study, the rheological properties of xanthan gum solutions were obtained under various chemical and environmental conditions relevant to delivery of remedial amendments to groundwater. Higher xanthan concentration raised the absolute solution viscosity and increased the degree of shear thinning. Addition of remedial amendments (e.g., phosphate, sodium lactate, ethyl lactate) caused the dynamic viscosity of xanthan solutions to decrease, but they maintained shear-thinning properties. Use of mono- and divalent salts (e.g., Na(+), Ca(2+)) to increase the solution ionic strength also decreased the dynamic viscosity of xanthan and the degree of shear thinning, although the effect reversed at high xanthan concentrations. A power law analysis showed that the consistency index is a linear function of the xanthan concentration. The degree of shear thinning, however, is best described using a logarithmic function. Mechanisms to describe the observed empiricism have been discussed. In the absence of sediments, xanthan solutions maintained their viscosity for months. However, the solutions lost their viscosity over a period of days to weeks when in contact with site sediment. Loss of viscosity is attributed to physical and biodegradation processes.

Review: Technical and Policy Challenges in Deep Vadose Zone Remediation of Metals and Radionuclides

Contamination in deep vadose zone environments is isolated from exposure so direct contact is not a factor in its risk to human health and the environment. Instead, movement of contamination to the groundwater creates the potential for exposure and risk to receptors. Limiting flux from contaminated vadose zone is key for protection of groundwater resources, thus the deep vadose zone is not necessarily considered a resource requiring restoration. Contaminant discharge to the groundwater must be maintained low enough by natural attenuation (e.g., adsorption processes or radioactive decay) or through remedial actions (e.g., contaminant mass reduction or mobility reduction) to meet the groundwater concentration goals. This paper reviews the major processes for deep vadose zone metal and radionuclide remediation that form the practical constraints on remedial actions. Remediation of metal and radionuclide contamination in the deep vadose zone is complicated by heterogeneous contaminant distribution and the saturation-dependent preferential flow in heterogeneous sediments. Thus, efforts to remove contaminants have generally been unsuccessful although partial removal may reduce downward flux. Contaminant mobility may be reduced through abiotic and biotic reactions or through physical encapsulation. Hydraulic controls may limit aqueous transport. Delivering amendments to the contaminated zone and verifying performance are challenges for remediation.

Enhanced remedial amendment delivery to subsurface using shear thinning fluid and aqueous foam

A major issue with in situ subsurface remediation is the ability to achieve an even spatial distribution of remedial amendments to the contamination zones in an aquifer or vadose zone. Amendment delivery to the aquifer using shear thinning fluid and to the vadose zone using aqueous foam has the potential to enhance the distribution. 2-D saturated flow cell experiments were conducted to evaluate the enhanced fluid sweeping over heterogeneous system, improved contaminant removal, and extended amendment presence in low-permeability zones achieved by shear thinning fluid delivery. Unsaturated column and flow cell experiments were conducted to investigate the improvement on contaminant mobilization mitigation, amendment distribution, and lateral delivery implemented by foam delivery. It was demonstrated that the shear thinning fluid injection enhanced the fluid sweeping and increased the delivery of remedial amendment into low-perm zones. The presence of amendment distributed by the shear thinning fluid in the low-permeability zones was increased. Foam delivery was shown to mitigate the mobilization of highly mobile contaminant from sediments. It also achieved more uniform amendment distribution in a heterogeneous unsaturated system, and demonstrated remarkable increasing in lateral distribution of the injected liquid compared to direct liquid injection.

Assessing performance and closure for soil vapor extraction: Integrating vapor discharge and impact to groundwater quality

Soil vapor extraction (SVE) is typically effective for removal of volatile contaminants from higher-permeability portions of the vadose zone. However, contamination in lower-permeability zones can persist due to mass transfer processes that limit the removal effectiveness. After SVE has been operated for a period of time and the remaining contamination is primarily located in lower-permeability zones, the remedy performance needs to be evaluated to determine whether the SVE system should be optimized, terminated, or transitioned to another technology to replace or augment SVE. Numerical modeling of vapor-phase contaminant transport was used to investigate the correlation between measured vapor-phase mass discharge, MF(r), from a persistent, vadose-zone contaminant source and the resulting groundwater contaminant concentrations. This relationship was shown to be linear, and was used to directly assess SVE remediation progress over time and to determine the level of remediation in the vadose zone necessary to protect groundwater. Although site properties and source characteristics must be specified to establish a unique relation between MF(r) and the groundwater contaminant concentration, this correlation provides insight into SVE performance and support for decisions to optimize or terminate the SVE operation or to transition to another type of treatment.

Modeling of DNAPL-Dissolution, rate-limited sorption and biodegradation reactions in groundwater systems

This article presents an approach for modeling the dissolution process of single component dense non-aqueous phase liquids (DNAPL), such as tetrachloroethene and trichloroethene, in a biologically reactive porous medium. In the proposed approach, the overall transport processes are conceptualized as three distinct reactions. Firstly, the dissolution (or dissolving) process of a residual DNAPL source zone is conceptualized as a mass-transfer limited reaction. Secondly, the contaminants dissolved from the DNAPL source are allowed to partition between sediment and water phases through a rate-limited sorption reaction. Finally, the contaminants in the solid and liquid phases are allowed to degrade by a set of kinetic-limited biological reactions. Although all of these three reaction processes have been researched in the past, little progress has been made towards understanding the combined effects of these processes. This work provides a rigorous mathematical model for describing the coupled effects of these three fundamental reactive transport mechanisms. The model equations are then solved using the general-purpose reactive transport code RT3D (Clement, 1997).

Demonstration of combined zero-valent iron and electrical resistance heating for in situ trichloroethene remediation.

The effectiveness of in situ treatment using zero-valent iron (ZVI) for nonaqueous phase or significant sediment-associated contaminant mass can be limited by relatively low rates of mass transfer to bring contaminants in contact with the reactive media. For a field test in a trichloroethene (TCE) source area, combining moderate-temperature subsurface electrical resistance heating with in situ ZVI treatment was shown to accelerate TCE treatment by a factor of about 4 based on organic daughter products and a factor about 8 based on chloride concentrations. A mass-discharge-based analysis was used to evaluate reaction, dissolution, and volatilization processes at ambient groundwater temperature (~10 °C) and as temperature was increased up to about 50 °C. Increased reaction and contaminant dissolution were observed with increased temperature, but vapor- or aqueous-phase migration of TCE out of the treatment zone was minimal during the test because reactions maintained low aqueous-phase TCE concentrations.

Transport and biodegradation of quinoline in horizontally stratified porous media

Abstract An experimental study of the movement and biodegradation of quinoline was conducted in a saturated 2-layer system (1 m long) to identify processes that may result in increased microbial growth at hydraulic layer interfaces. The system contained two layers of contrasting hydraulic conductivity (1:12) and flow was parallel to layers. Tracer breakthrough, used to quantify interlayer mass transfer, showed that the transverse dispersivity was 0.3 cm near the interface and 0.036 cm within the low-conductivity (low-K) layer. Interlayer mass transfer resulted in arrival of substrate (quinoline) and oxygen 10s to 100s of hours sooner in the low-K layer near the interface compared to other locations within the low-K layer where substrates arrived via only advection. Early arrival of substrates near the interface resulted in biodegradation of quinoline for a longer period than within layers, yielding increased growth in a 1- to 3-cm-thick zone, as measyred by plate counts. Because biodegradation was oxygen limited in this system, microbial growth at all locations was small [log(maximum increase) ⩽ 1.0] and measured porous-medium hydraulic properties (dispersion, hydraulic gradient) were not affected by the biomass production. Although the thickness of the effected interface zone was small in this system, the effect on the overall transport of quinoline was significant; 19% of the growth (and corresponding degradation of substrates) in the low-K layer was in the relatively small interface zone. The effect of microbial biomass production at interfaces on overall solute movement is likely to be maximized in environments that have a high density of hydraulic or geochemical interfaces, particularly in settings where the interfaces serve as mixing zones between nutrient-limited waters.

Assessment of Carbon Tetrachloride Groundwater Transport in Support of the Hanford Carbon Tetrachloride Innovative Technology Demonstration Program

Groundwater modeling was performed in support of the Hanford Carbon Tetrachloride Innovative Treatment Remediation Demonstration (ITRD) Program. The ITRD program is facilitated by Sandia National Laboratory for the Department of Energy Office of Science and Technology. This report was prepared to document the results of the modeling effort and facilitate discussion of characterization and remediation options for the carbon tetrachloride plume among the ITRD participants. As a first step toward implementation of innovative technologies for remediation of the carbon tetrachloride (CT) plume underlying the 200-West Area, this modeling was performed to provide an indication of the potential impact of the CT source on the compliance boundary approximately 5000 m distant. The primary results of the modeling bracket the amount of CT source that will most likely result in compliance/non-compliance at the boundary and the relative influence of the various modeling parameters.