Iron and Carbon Dynamics during Aging and Reductive Transformation of Biogenic Ferrihydrite

Natural organic matter is often associated with Fe­(III) oxyhydroxides, and may be stabilized as a result of coprecipitation or sorption to their surfaces. However, the significance of this association in relation to Fe and C dynamics and biogeochemical cycling, and the mechanisms responsible for organic matter stabilization as a result of interaction with minerals under various environmental conditions (e.g., pH, Eh, etc.) are not entirely understood. The preservation of mineral-bound OM may be affected by OM structure and mineral identity, and bond types between OM and minerals may be central to influencing the stability, transformation and composition of both organic and mineral components under changing environmental conditions. Here we use bulk and submicron-scale spectroscopic synchrotron methods to examine the in situ transformation of OM-bearing, biogenic ferrihydrite stalks (<i>Gallionella ferruginea-</i>like), which formed following injection of oxygenated groundwater into a saturated alluvial aquifer at the Rifle, CO field site. A progression from oxidizing to reducing conditions during an eight-month period triggered the aging and reductive transformation of <i>Gallionella</i>-like ferrihydrite stalks to Fe (hydroxy)­carbonates and Fe sulfides, as well as alteration of the composition and amount of OM. Spectromicroscopic measurements showed a gradual decrease in reduced carbon forms (aromatic/alkene, aliphatic C), a relative increase in amide/carboxyl functional groups and a significant increase in carbonate in the stalk structures, and the appearance of organic globules not associated with stalk structures. Biogenic stalks lost ∼30% of their initial organic carbon content. Conversely, a significant increase in bulk organic matter accompanied these transformations. The character of bulk OM changed in parallel with mineralogical transformations, showing an increase in aliphatic, aromatic and amide functional groups. These changes likely occurred as a result of an increase in microbial activity, or biomass production under anoxic conditions. By the end of this experiment, a substantial fraction of organic matter remained in identifiable Fe containing stalks, but carbon was also present in additional pools, for example, organic matter globules and iron carbonate minerals

Uranium Retention in a Bioreduced Region of an Alluvial Aquifer Induced by the Influx of Dissolved Oxygen

Reduced zones in the subsurface represent biogeochemically active hotspots enriched in buried organic matter and reduced metals. Within a shallow alluvial aquifer located near Rifle, CO, reduced zones control the fate and transport of uranium (U). Though an influx of dissolved oxygen (DO) would be expected to mobilize U, we report U immobilization. Groundwater U concentrations decreased following delivery of DO (21.6 mg O2/well/h). After 23 days of DO delivery, injection of oxygenated groundwater was paused and resulted in the rebound of groundwater U concentrations to preinjection levels. When DO delivery resumed (day 51), groundwater U concentrations again decreased. The injection was halted on day 82 again and resulted in a rebound of groundwater U concentrations. DO delivery rate was increased to 54 mg O2/well/h (day 95) whereby groundwater U concentrations increased. Planktonic cell abundance remained stable throughout the experiment, but virus-to-microbial cell ratio increased 1.8–3.4-fold with initial DO delivery, indicative of microbial activity in response to DO injection. Together, these results indicate that the redox-buffering capacity of reduced sediments can prevent U mobilization, but could be overcome as delivery rate or oxidant concentration increases, mobilizing U

Mineral Transformation and Biomass Accumulation Associated With Uranium Bioremediation at Rifle, Colorado

Injection of organic carbon into the subsurface as an electron donor for bioremediation of redox-sensitive contaminants like uranium often leads to mineral transformation and biomass accumulation, both of which can alter the flow field and potentially bioremediation efficacy. This work combines reactive transport modeling with a column experiment and field measurements to understand the biogeochemical processes and to quantify the biomass and mineral transformation/accumulation during a bioremediation experiment at a uranium contaminated site near Rifle, Colorado. We use the reactive transport model CrunchFlow to explicitly simulate microbial community dynamics of iron and sulfate reducers, and their impacts on reaction rates. The column experiment shows clear evidence of mineral precipitation, primarily in the form of calcite and iron monosulfide. At the field scale, reactive transport simulations suggest that the biogeochemical reactions occur mostly close to the injection wells where acetate concentrations are highest, with mineral precipitate and biomass accumulation reaching as high as 1.5% of the pore space. This work shows that reactive transport modeling coupled with field data can be an effective tool for quantitative estimation of mineral transformation and biomass accumulation, thus improving the design of bioremediation strategies

Ribosomal response to uranium additions in groundwater after 24h for 4 OTUs (TRF-212, 213, 214, 215 bp) from wells D02 (open square), D07 (closed square), sampled in 2008, D01 (open circle), D08 (closed circle), sampled in 2009 (note the difference in scales).

<p>Ribosomal response to uranium additions in groundwater after 24h for 4 OTUs (TRF-212, 213, 214, 215 bp) from wells D02 (open square), D07 (closed square), sampled in 2008, D01 (open circle), D08 (closed circle), sampled in 2009 (note the difference in scales).</p

Uranium Bioreduction Rates across Scales: Biogeochemical Hot Moments and Hot Spots during a Biostimulation Experiment at Rifle, Colorado

We aim to understand the scale-dependent evolution of uranium bioreduction during a field experiment at a former uranium mill site near Rifle, Colorado. Acetate was injected to stimulate Fe-reducing bacteria (FeRB) and to immobilize aqueous U­(VI) to insoluble U­(IV). Bicarbonate was coinjected in half of the domain to mobilize sorbed U­(VI). We used reactive transport modeling to integrate hydraulic and geochemical data and to quantify rates at the grid block (0.25 m) and experimental field scale (tens of meters). Although local rates varied by orders of magnitude in conjunction with biostimulation fronts propagating downstream, field-scale rates were dominated by those orders of magnitude higher rates at a few selected hot spots where Fe­(III), U­(VI), and FeRB were at their maxima in the vicinity of the injection wells. At particular locations, the hot moments with maximum rates negatively corresponded to their distance from the injection wells. Although bicarbonate injection enhanced local rates near the injection wells by a maximum of 39.4%, its effect at the field scale was limited to a maximum of 10.0%. We propose a rate-versus-measurement-length relationship (log <i>R</i>′ = −0.63 log <i>L</i> – 2.20, with <i>R</i>′ in μmol/mg cell protein/day and <i>L</i> in meters) for orders-of-magnitude estimation of uranium bioreduction rates across scales

Geophysical Imaging of Stimulated Microbial Biomineralization

Understanding how microorganisms influence the physical and chemical properties of the subsurface is hindered by our inability to observe microbial dynamics in real time and with high spatial resolution. Here, we investigate the use of noninvasive geophysical methods to monitor biomineralization at the laboratory scale during stimulated sulfate reduction under dynamic flow conditions. Alterations in sediment characteristics resulting from microbe-mediated sulfide mineral precipitation were concomitant with changes in complex resistivity and acoustic wave propagation signatures. The sequestration of zinc and iron in insoluble sulfides led to alterations in the ability of the pore fluid to conduct electrical charge and of the saturated sediments to dissipate acoustic energy. These changes resulted directly from the nucleation, growth, and development of nanoparticulate precipitates along grain surfaces and within the pore space. Scanning and transmission electron microscopy (SEM and TEM) confirmed the sulfides to be associated with cell surfaces, with precipitates ranging from aggregates of individual 3−5 nm nanocrystals to larger assemblages of up to 10−20 μm in diameter. Anomalies in the geophysical data reflected the distribution of mineral precipitates and biomass over space and time, with temporal variations in the signals corresponding to changes in the aggregation state of the nanocrystalline sulfides. These results suggest the potential for using geophysical techniques to image certain subsurface biogeochemical processes, such as those accompanying the bioremediation of metal-contaminated aquifers

Percent total peak area from community DNA of 16S rRNA genes TRFLP profiles of pre-filters 2008 well D04 during field acetate injection: TRF 212-white, 213-light grey, 215-black, other <i>Geobacter</i>-like TRF’s-dark grey stipple, all other TRF peaks-light checked.

<p>Percent total peak area from community DNA of 16S rRNA genes TRFLP profiles of pre-filters 2008 well D04 during field acetate injection: TRF 212-white, 213-light grey, 215-black, other <i>Geobacter</i>-like TRF’s-dark grey stipple, all other TRF peaks-light checked.</p

Electrode-Based Approach for Monitoring In Situ Microbial Activity During Subsurface Bioremediation

Current production by microorganisms colonizing subsurface electrodes and its relationship to substrate availability and microbial activity was evaluated in an aquifer undergoing bioremediation. Borehole graphite anodes were installed downgradient from a region of acetate injection designed to stimulate bioreduction of U(VI); cathodes consisted of graphite electrodes embedded at the ground surface. Significant increases in current density (≤50 mA/m2) tracked delivery of acetate to the electrodes, dropping rapidly when acetate inputs were discontinued. An upgradient control electrode not exposed to acetate produced low, steady currents (≤0.2 mA/m2). Elevated current was strongly correlated with uranium removal but minimal correlation existed with elevated Fe(II). Confocal laser scanning microscopy of electrodes revealed firmly attached biofilms, and analysis of 16S rRNA gene sequences indicated the electrode surfaces were dominated (67−80%) by Geobacter species. This is the first demonstration that electrodes can produce readily detectable currents despite long-range (6 m) separation of anode and cathode, and these results suggest that oxidation of acetate coupled to electron transfer to electrodes by Geobacter species was the primary source of current. Thus it is expected that current production may serve as an effective proxy for monitoring in situ microbial activity in a variety of subsurface anoxic environments

Phylogenetic tree of active TRF’s resulting from the uranium additions (star) are shown with the nearest cultured relatives.

<p>The reconstruction was done using maximum likelihood methods on 438 aligned bases. Bootstrap values >50 for 100 iterations are indicated.</p

Reactivity of Uranium and Ferrous Iron with Natural Iron Oxyhydroxides

Determining key reaction pathways involving uranium and iron oxyhydroxides under oxic and anoxic conditions is essential for understanding uranium mobility as well as other iron oxyhydroxide mediated processes, particularly near redox boundaries where redox conditions change rapidly in time and space. Here we examine the reactivity of a ferrihydrite-rich sediment from a surface seep adjacent to a redox boundary at the Rifle, Colorado field site. Iron­(II)–sediment incubation experiments indicate that the natural ferrihydrite fraction of the sediment is not susceptible to reductive transformation under conditions that trigger significant mineralogical transformations of synthetic ferrihydrite. No measurable Fe­(II)-promoted transformation was observed when the Rifle sediment was exposed to 30 mM Fe­(II) for up to 2 weeks. Incubation of the Rifle sediment with 3 mM Fe­(II) and 0.2 mM U­(VI) for 15 days shows no measurable incorporation of U­(VI) into the mineral structure or reduction of U­(VI) to U­(IV). Results indicate a significantly decreased reactivity of naturally occurring Fe oxyhydroxides as compared to synthetic minerals, likely due to the association of impurities (e.g., Si, organic matter), with implications for the mobility and bioavailability of uranium and other associated species in field environments