John Komlos

Biogenic U(IV) oxidation by dissolved oxygen and nitrate in sediment after prolonged U(VI)/Fe(III)/SO42- reduction.

Sediment column experiments were performed to quantify the effect of biogenic iron sulfide precipitates on the stability of bioreduced uranium during and after a simulated bioremediation scenario. In particular, this study examined the effect of different oxidants (dissolved oxygen and nitrate) on biogenic U(IV) oxidation in sediment that experienced significant sulfate reduction in addition to Fe(III) and U(VI) reduction. The experimental set-up included five replicate columns (each 5 cm in diameter, 15 cm long and packed with background sediment from a site contaminated with uranium) that were bioreduced for 70 days by injecting a nutrient media containing 3 mM acetate and 6 mM sulfate prior to oxidation. Upon oxidation, iron sulfide precipitates formed during bioreduction acted as a buffer to partially prevent biogenic U(IV) oxidation. The iron sulfides were more effective at protecting biogenic U(IV) from oxidation when dissolved oxygen was the oxidant compared to nitrate. A constant supply of 0.25 mM and 1.6 mM nitrate over a 50 day period resulted in uranium resolubilization of 11% and 60%, respectively, while less than 1% of the uranium was resolubilized in the column supplied 0.27 mM dissolved oxygen during the same time period. Over time, oxidation increased pore water channeling, which was more pronounced during oxidation with nitrate. Increased channeling with time of oxidation could affect the transport of an oxidant through the previously reduced zone, and hence the oxidation dynamics of the reduced species.

Microbial reduction of uranium under iron- and sulfate-reducing conditions: Effect of amended goethite on microbial community composition and dynamics

There is a growing need for a better understanding of the biogeochemical dynamics involved in microbial U(VI) reduction due to an increasing interest in using biostimulation via electron donor addition as a means to remediate uranium contaminated sites. U(VI) reduction has been observed to be maximized during iron-reducing conditions and to decrease upon commencement of sulfate-reducing conditions. There are many unknowns regarding the impact of iron/sulfate biogeochemistry on U(VI) reduction. This includes Fe(III) availability as well as the microbial community changes, including the activity of iron-reducers during the uranium biostimulation period even after sulfate reduction becomes dominant. Column experiments were conducted with Old Rifle site sediments containing Fe-oxides, Fe-clays, and sulfate rich groundwater. Half of the columns had sediment that was augmented with small amounts of Fe(III) in the form of (57)Fe-goethite, allowing for a detailed tracking of minute changes of this added phase to study the effects of increased Fe(III) levels on the overall biostimulation dynamics. Mössbauer spectroscopy showed that the added (57)Fe-goethite was bioreduced only during the first thirty days of biostimultuion, after which it remained constant. Augmentation with Fe(III) had a significant effect on the total flux of electrons towards different electron acceptors; it suppressed the degree of sulfate reduction, had no significant impact on Geobacter-type bacterial numbers but decreased the bacterial numbers of sulfate reducers and affected the overall microbial community composition. The addition of Fe(III) had no noticeable effect on the total uranium reduction.

Effect of sulfate on the simultaneous bioreduction of iron and uranium.

The biogeochemistry related to iron- and sulfate-reducing conditions influences the fate of contaminants such as petroleum hydrocarbons, trace metals, and radionuclides (i.e., uranium) released into the subsurface. An understanding of these processes is imperative to successfully predict the fate of contaminants during bioremediation scenarios. A series of flow-through sediment column experiments were performed to determine if the commencement of sulfate-reducing conditions would occur while bioavailable Fe(III) was present and to determine how the bioreduction of a contaminant (uranium) was affected by the switch from iron-dominated to sulfate-dominated reducing conditions. The results presented herein demonstrated that, under biostimulation, sulfate reduction can commence even though a significant pool of bioavailable Fe(III) is present. In addition, the rate of U(VI) reduction was not negatively affected by the commencement of sulfate-reducing conditions.

Effect of iron bioavailability on dissolved hydrogen concentrations during microbial iron reduction.

Dissolved hydrogen (H2) concentrations have been shown to correlate with specific terminal electron accepting processes (TEAPs) in aquifers. The research presented herein examined the effect of iron bioavailability on H2 concentrations during iron reduction in flow-through column experiments filled with soil obtained from the uncontaminated background area of the Field Research Center (FRC), Oak Ridge, TN and amended with acetate as the electron donor. The first column experiment measured H2 concentrations over 500 days of column operation that fluctuated within a substantial range around an average of 3.9 nM. Iron reduction was determined to be the dominant electron accepting process. AQDS (9,10-anthraquinone-2,6-disulfonic acid) was then used to determine if H2 concentrations during iron reduction were related to iron bioavailability. For this purpose, a 100-day flow-through column experiment was conducted that compared the effect of AQDS on iron reduction and subsequent H2 concentrations using two columns in parallel. Both columns were packed with FRC soil and inoculated with Geobacter sulfurreducens but only one was supplied with AQDS. The addition of AQDS increased the rate of iron reduction in the flow-through column and slightly decreased the steady-state H2 concentrations from an average of 4.0 nM for the column without AQDS to 2.0 nM for the column with AQDS. The results of this study therefore show that H2 can be used as an indicator to monitor rate and bioavailability changes during microbial iron reduction.

Feasibility Study of As-Received and Modified (Dried/Baked) Water Treatment Plant Residuals for Use in Storm-Water Control Measures

AbstractUse of water treatment plant residuals (WTRs) in storm-water control measures (SCMs) is a sustainable alternative to landfill disposal of WTRs. However, research is needed to determine how effective WTR-amended SCMs would be in field-scale applications and what modifications can be implemented to improve performance. The modifications examined in this study were oven-drying (105°C) and baking (1,000°C) of the WTRs. Results showed that both modifications increase the hydraulic conductivity by two orders of magnitude. Dried WTRs showed no loss of phosphate removal potential compared to the as-received WTRs. Baking the WTRs lowered the phosphate removal potential but prevented manganese resuspension. The as-received WTRs, as well as both modifications, removed copper, lead, and zinc from storm-water runoff to below detection. Taken together, these results suggest that amending SCMs with modified WTRs has the potential to enhance the water quality improvement processes of SCMs while maintaining the in...

Subsurface Biogeochemical Heterogeneity (Field-scale removal of U(VI) from groundwater in an alluvial aquifer by electron donor amendment)

Determine if biostimulation of alluvial aquifers by electron donor amendment can effectively remove U(VI) from groundwater at the field scale. Uranium contamination in groundwater is a significant problem at several DOE sites. In this project, the possibility of accelerating bioreduction of U(VI) to U(IV) as a means of decreasing U(VI) concentrations in groundwater is directly addressed by conducting a series of field-scale experiments. Scientific goals include demonstrating the quantitative linkage between microbial activity and U loss from groundwater and relating the dominant terminal electron accepting processes to the rate of U loss. The project is currently focused on understanding the mechanisms for unexpected long-term ({approx}2 years) removal of U after stopping electron donor amendment. Results obtained in the project successfully position DOE and others to apply biostimulation broadly to U contamination in alluvial aquifers.