Biomedical subjects

Eric E Roden

Publications and source records attributed to Eric E Roden.

11 recordsLinked to original sources

PubMed2006

Effects of solid-to-solution ratio on uranium(VI) adsorption and its implications.

U(VI) adsorption onto goethite-coated sand was studied in batch experiments ata solid-to-solution ratio (SSR) ranging from 33.3 to 333 g/L. Batch kinetic experiments revealed that the presence of 10(-4) M phosphate increased both the initial rate and ultimate extent of U(VI) adsorption compared with phosphate-free systems. Our experimental U(VI) adsorption isotherms were independent of SSR in phosphate-free systems. However, the U(VI) adsorption isotherm became dependent on SSR in phosphate-containing systems (with a lower SSR resulting in stronger U(VI) adsorption). A surface complexation model (SCM) was used to conceptualize the interactions in systems containing U(VI), phosphate, and goethite contributing to this SSR effect. The SCM accounted for the effects of SSR on U(VI) adsorption reasonably well. This study implies that the extrapolation of batch-measured adsorption parameters of U(VI) (and potentially other radionuclides and metal(loid)s as well) to field conditions should be done with caution, especially in the presence of strongly interacting ligands.

Adsorption↗

PubMed2006

Anaerobic redox cycling of iron by freshwater sediment microorganisms.

The potential for microbially mediated anaerobic redox cycling of iron (Fe) was examined in a first-generation enrichment culture of freshwater wetland sediment microorganisms. Most probable number enumerations revealed the presence of significant populations of Fe(III)-reducing (approximately 10(8) cells ml(-1)) and Fe(II)-oxidizing, nitrate-reducing organisms (approximately 10(5) cells ml(-1)) in the freshwater sediment used to inoculate the enrichment cultures. Nitrate reduction commenced immediately following inoculation of acetate-containing (approximately 1 mM) medium with a small quantity (1% v/v) of wetland sediment, and resulted in the transient accumulation of NO(2)(-) and production of a mixture of gaseous end-products (N(2)O and N(2)) and NH(4)(+). Fe(III) oxide (high surface area goethite) reduction took place after NO(3)(-) was depleted and continued until all the acetate was utilized. Addition of NO(3)(-) after Fe(III) reduction ceased resulted in the immediate oxidation of Fe(II) coupled to reduction of NO(3)(-) to NH(4)(+). No significant NO(2)(-) accumulation was observed during nitrate-dependent Fe(II) oxidation. No Fe(II) oxidation occurred in pasteurized controls. Microbial community structure in the enrichment was monitored by denaturing gradient gel electrophoresis analysis of polymerase chain reaction-amplified 16S rDNA and reverse transcription polymerase chain reaction-amplified 16S rRNA, as well as by construction of 16S rDNA clone libraries for four different time points during the experiment. Strong similarities in dominant members of the microbial community were observed in the Fe(III) reduction and nitrate-dependent Fe(II) oxidation phases of the experiment, specifically the common presence of organisms closely related (>or= 95% sequence similarity) to the genera Geobacter and Dechloromonas. These results indicate that the wetland sediments contained organisms such as Geobacter sp. which are capable of both dissimilatory Fe(III) reduction and oxidation of Fe(II) with reduction of NO(3)(-) to NH(4)(+). Our findings suggest that microbially catalysed nitrate-dependent Fe(II) oxidation has the potential to contribute to a dynamic anaerobic Fe redox cycle in freshwater sediments.

Bacteria, Anaerobic↗

PubMed2005

Coupled Fe(II)-Fe(III) electron and atom exchange as a mechanism for Fe isotope fractionation during dissimilatory iron oxide reduction.

Microbial dissimilatory iron reduction (DIR) is an important pathway for carbon oxidation in anoxic sediments, and iron isotopes may distinguish between iron produced by DIR and other sources of aqueous Fe(II). Previous studies have shown that aqueous Fe(II) produced during the earliest stages of DIR has delta56Fe values that are 0.5-2.0%o lowerthan the initial Fe(III) substrate. The new experiments reported here suggest that this fractionation is controlled by coupled electron and Fe atom exchange between Fe(II) and Fe(III) at iron oxide surfaces. In hematite and goethite reduction experiments with Geobacter sulfurreducens, the 56Fe/54Fe isotopic fractionation between aqueous Fe(II) and the outermost layers of Fe(III) on the oxide surface is approximately -3%o and can be explained by equilibrium Fe isotope partitioning between reactive Fe(II) and Fe(III) pools that coexist during DIR. The results indicate that sorption of Fe(II) to Fe(III) substrates cannot account for production of low-delta56Fe values for aqueous Fe(II) during DIR.

Adsorption↗

PubMed2005

Chemical reduction of U(VI) by Fe(II) at the solid-water interface using natural and synthetic Fe(III) oxides.

Abiotic reduction of 0.1 mM U(VI) by Fe(II) in the presence of synthetic iron oxides (biogenic magnetite, goethite, and hematite) and natural Fe(III) oxide-containing solids was investigated in pH 6.8 artificial groundwater containing 10 mM NaHCO3. In most experiments, more than 95% of added U(VI) was sorbed to solids. U(VI) was rapidly and extensively (> or = 80%) reduced in the presence of synthetic Fe(III) oxides and highly Fe(II) oxide-enriched (18-35 wt % Fe) Atlantic coastal plain sediments. In contrast, long-term (20-60 d) U(VI) reduction was less than 30% in suspensions of six other natural solids with relatively low Fe(III) oxide content (1-5 wt % Fe). Fe(II) sorption site density was severalfold lower on these natural solids (0.2-1.1 Fe(II) nm(-2)) compared tothe synthetic Fe(lII) oxides (1.6-3.2 Fe(II) nm(-2)), which may explain the poor U(VI) reduction in the natural solid-containing systems. Addition of the reduced form of the electron shuttling compound anthrahydroquinone-2,6-disulfonate (AH2DS; final concentration 2.5 mM) to the natural solid suspensions enhanced the rate and extent of U(VI) reduction, suggesting that AH2DS reduced U(VI) at surface sites where reaction of U(VI) with sorbed Fe(II) was limited. This study demonstrates that abiotic, Fe(II)-driven U(VI) reduction is likely to be less efficient in natural soils and sediments than would be inferred from studies with synthetic Fe(III) oxides.

Adsorption↗

PubMed2005

Effect of watershed parameters on mercury distribution in different environmental compartments in the Mobile Alabama River Basin, USA.

Total mercury (THg) and mono-methylmercury (MeHg) levels in water, sediment, and largemouth bass (LMB) (Micropterus salmoides) were investigated at 52 sites draining contrasting land use/land cover and habitat types within the Mobile Alabama River Basin (MARB). Aqueous THg was positively associated with iron-rich suspended particles and highest in catchments impacted by agriculture. Sediment THg was positively associated with sediment organic mater and iron content, with the highest levels observed in smaller catchments influenced by wetlands, followed by those impacted by agriculture or mixed forest, agriculture, and wetlands. The lowest sediment THg levels were observed in main river channels, except for reaches impacted by coal mining. Sediment MeHg levels were a positive function of sediment THg and organic matter and aqueous nutrient levels. The highest levels occurred in agricultural catchments and those impacted by elevated sulfate levels associated with coal mining. Aqueous MeHg concentrations in main river channels were as high as those in smaller catchments impacted by agriculture or wetlands, suggesting these areas were sources to rivers. Elevated Hg levels in some LMB were observed across all types of land use and land cover, but factors such as shallow water depth, larger wetland catchment surface area, low aqueous potassium levels, and higher Chl a concentrations were associated with higher Hg burdens, particularly in the Coastal Plain province. It is suggested that the observed large variability in LMB Hg burdens is linked to fish displacement by anglers, differences in food web structure, and sediment biogeochemistry, with surficial sediment iron oxides buffering the flux of MeHg from sediments to deeper water pelagic food webs.

Alabama↗

PubMed2004

Conceptual and numerical model of uranium(VI) reductive immobilization in fractured subsurface sediments.

A conceptual model and numerical simulations of bacterial U(VI) reduction in fractured subsurface sediments were developed to assess the potential feasibility of biomineralization at the fracture/matrix interface as a mechanism for immobilization of uranium in structured subsurface media. The model envisions flow of anaerobic groundwater, with or without acetate as an electron donor for stimulation of U(VI) reduction by dissimilatory metal-reducing bacteria (DMRB), within mobile macropores along a one-dimensional flow path. As the groundwater moves along the flow path, U(VI) trapped in the immobile mesopore and micropore domains (the sediment matrix) becomes desorbed and transferred to the mobile macropores (fractures) via a first-order exchange mechanism. By allowing bacterial U(VI) reduction to occur in the mesopore domain (assumed to account for 12% of total sediment pore volume) according to experimentally-determined kinetic parameters and an assumed DMRB abundance of 10(7) cells per cm3 bulk sediment (equivalent to 4 mg of cells per dm3 bulk sediment), the concentration of U(VI) in the macropore domain was reduced ca. 10-fold compared to that predicted in the absence of mesopore DMRB activity after a 6-month simulation period. The results suggest that input of soluble electron donors over a period of years could lead to a major redistribution of uranium in fractured subsurface sediments, converting potentially mobile sorbed U(VI) to an insoluble reduced phase (i.e. uraninite) in the mesopore domain that is expected to be permanently immobile under sustained anaerobic conditions.

Adsorption↗

PubMed2004

Effects of phosphate on uranium(VI) adsorption to goethite-coated sand.

U(VI)-phosphate interactions are important in governing the subsurface mobility of U(VI) in both natural and contaminated environments. We studied U(VI) adsorption on goethite-coated sand (to mimic natural Fe-coated subsurface materials) as a function of pH in systems closed to the atmosphere, in both the presence and the absence of phosphate. Our results indicate that phosphate strongly affects U(VI) adsorption. The effect of phosphate on U(VI) adsorption was dependent on solution pH. At low pH, the adsorption of U(VI) increased in the presence of phosphate, and higher phosphate concentration caused a larger extent of increase in U(VI) adsorption. Phosphate was strongly bound by the goethite surface in the low pH range, and the increased adsorption of U(VI) at low pH was attributed to the formation of ternary surface complexes involving both U(VI) and phosphate. In the high pH range, the adsorption of U(VI) decreased in the presence of phosphate at low total Fe concentration, and higher phosphate concentration caused a larger extent of decrease in U(VI) adsorption. This decrease in U(VI) adsorption was attributed to the formation of soluble uranium-phosphate complexes. A surface complexation model (SCM) was proposed to describe the effect of phosphate on U(VI) adsorption to goethite. This proposed model was based on previous models that predict U(VI) adsorption to iron oxides in the absence of phosphate and previous models developed to predict phosphate adsorption on goethite. A postulated ternary surface complex of the form of (>FePO4UO2) was included in our model to account for the interactions between U(VI) and phosphate. The model we established can successfully predict U(VI) adsorption in the presence of phosphate under a range of conditions (i.e., pH, total phosphate concentration, and total Fe concentration).

Adsorption↗

PubMed2004

Microbial reduction of U(VI) at the solid-water interface.

Microbial (Geobacter sulfurreducens) reduction of 0.1 mM U(VI) in the presence of synthetic Fe(III) oxides and natural Fe(III) oxide-containing solids was investigated in pH 6.8 artificial groundwater containing 10 mM NaHCO3. In most experiments, more than 95% of added U(VI) was sorbed to solids, so that U(VI) reduction was governed by reactions at the solid-water interface. The rate and extent of reduction of U(VI) associated with surfaces of synthetic Fe(III) oxides (hydrous ferric oxide, goethite, and hematite) was comparable to that observed during reduction of aqueous U(VI). In contrast, microbial reduction of U(VI) sorbed to several different natural Fe(III) oxide-containing solids was slower and less extensive compared to synthetic Fe(III) oxide systems. Addition of the electron shuttling agent anthraquinone-2,6-disulfonate (AQDS; 0.1 mM) enhanced the rate and extent of both Fe(III) and U(VI) reduction. These findings suggest that AQDS facilitated electron transfer from G. sulfurreducens to U(VI) associated with surface sites atwhich direct enzymatic reduction was kinetically limited. Our results demonstrate that association of U(VI) with diverse surface sites in natural soils and sediments has the potential to limit the rate and extent of microbial U(VI) reduction and thereby modulate the effectiveness of in situ U(VI) bioremediation.

Anthraquinones↗

PubMed2003

Microbial mercury transformation in anoxic freshwater sediments under iron-reducing and other electron-accepting conditions.

Potential rates of microbial methylation of inorganic mercury (added as HgCl2) and degradation of methyl mercury (MeHg) (added as CH3HgCl) were investigated in anoxic sediments from the Mobile Alabama River Basin (MARB) dominated by different terminal electron-accepting processes (TEAPs). Potential rates of methylation were comparable under methanogenic and sulfate-reducing conditions but suppressed under iron-reducing conditions, in slurries of freshwater wetland sediment In contrast, MeHg degradation rates were similar under all three TEAPs. Microbial Hg methylation and MeHg degradation were also investigated in surface sediment from three riverine sites, two of which had iron reduction and one sulfate reduction, as the dominant TEAP (as determined by 14C-acetate metabolism and other biogeochemical measurements). Methylation was active in sulfate-reducing sediments of a tributary creek and suppressed in iron-reducing, sandy sediments from the open river, whereas MeHg degradation was active at all three sites. Although iron-reducing conditions often suppressed methylation, some methylation activity was observed in two out of three replicates from iron-reducing sediments collected near a dam. Given that MeHg degradation was consistently observed under all TEAPs, our results suggest that the net flux of MeHg from iron-reducing surface sediments may be suppressed (due to inhibition of gross MeHg production) compared to sediments supporting other TEAPs.

Anaerobiosis↗

PubMed2003

Diversion of electron flow from methanogenesis to crystalline Fe(III) oxide reduction in carbon-limited cultures of wetland sediment microorganisms.

Electron flow in acetate-limited cultures of wetland sediment microorganisms was diverted from methane production to Fe(III) reduction in the presence of crystalline Fe(III) oxides at surface area loadings equivalent to that of amorphous Fe(III) oxide. The results indicate that inferences regarding the ability of microbial Fe(III) oxide reduction to compete with other terminal electron-accepting processes in anoxic soils and sediments should be based on estimates of bulk microbially available surface site abundance rather than assumed thermodynamic properties of the dominant oxide phase(s) in the soil or sediment.

Electron Transport↗

PubMed2002

Evidence for rapid microscale bacterial redox cycling of iron in circumneutral environments.

The potential for microscale bacterial Fe redox cycling was investigated in microcosms containing ferrihydrite-coated sand and a coculture of a lithotrophic Fe(II)-oxidizing bacterium (strain TW2) and a dissimilatory Fe(III)-reducing bacterium (Shewanella alga strain BrY). The Fe(II)-oxidizing organism was isolated from freshwater wetland surface sediments which are characterized by steep gradients of dissolved 02 and high concentrations of dissolved and solid-phase Fe(II) within mm of the sediment-water interface, and which support comparable numbers (10(5)-10(6) mL(-1)) of culturable Fe(II)-oxidizing and Fe(III)-reducing reducing. The coculture systems showed minimal Fe(III) oxide accumulation at the sand-water interface, despite intensive O2 input from the atmosphere and measurable dissolved O2 to a depth of 2 mm below the sand-water interface. In contrast, a distinct layer of oxide precipitates formed in systems containing Fe(IllI)-reducing bacteria alone. Examination of materials from the cocultures by fluorescence in situ hybridization indicated close physical juxtapositioning of Fe(II)-oxidizing and Fe(III)-reducing bacteria in the upper few mm of sand. Our results indicate that Fe(II)-oxidizing bacteria have the potential to enhance the coupling of Fe(II) oxidation and Fe(III) reduction at redox interfaces, thereby promoting rapid microscale cycling of Fe.

Aerobiosis↗