An official website of the United States government
The kinetics of model contaminant 4-chloronitrobenzene (4-ClNB) reduction by Fe( ii ) in aqueous suspensions containing either or both goethite (α-FeOOH) nanoparticles and kaolinite (Al 2 Si 2 O 5 (OH) 4 ) were quantified to elucidate the effects of nonreactive clay minerals on the attenuation of nitroaromatic groundwater contaminants by iron oxide nanoparticles. Increasing the amount of kaolinite in the presence of goethite decreased the reduction rate of 4-ClNB and competitive Fe( ii ) adsorption on kaolinite occurred. Cryogenic transmission and scanning electron microscopy (cryo-TEM and cryo-SEM) images did not reveal significant loss of accessible reactive surface area as a result of heteroaggregation. Sequential-spike batch reactors revealed that in the presence of kaolinite, 4-ClNB reduction rate decreased by more than a factor of three with extended reaction as a result of kaolinite dissolution and subsequent incorporation of Al and Si in goethite or on the goethite surface. The reactive sites residing on the {110} faces were comparatively more reactive in the presence of a large loading of kaolinite, resulting in shorter and wider goethite particles after reaction. These results elucidate the mechanisms by which nonreactive clays affect the reactions of Fe( ii )/iron oxides in groundwater systems and indicate that nonreactive clays are not passive components.
more »
« less
Influence of pH on the balance between methanogenesis and iron reduction
Abstract Methanogenesis and iron reduction play major roles in determining global fluxes of greenhouse gases. Despite their importance, environmental factors that influence their interactions are poorly known. Here, we present evidence that pH significantly influences the balance between each reaction in anoxic environments that contain ferric (oxyhydr)oxide minerals. In sediment bioreactors that contained goethite as a source of ferric iron, both iron reduction and methanogenesis occurred but the balance between them varied significantly with pH. Compared to bioreactors receiving acidic media (pH 6), electron donor oxidation was 85% lower for iron reduction and 61% higher for methanogenesis in bioreactors receiving alkaline media (pH 7.5). Thus, methanogenesis displaced iron reduction considerably at alkaline pH. Geochemistry data collected from U.S. aquifers demonstrate that a similar pattern also exists on a broad spatial scale in natural settings. In contrast, in bioreactors that were not augmented with goethite, clay minerals served as the source of ferric iron and the balance between each reaction did not vary significantly with pH. We therefore conclude that pH can regulate the relative contributions of microbial iron reduction and methanogenesis to carbon fluxes from terrestrial environments. We further propose that the availability of ferric (oxyhydr)oxide minerals influences the extent to which the balance between each reaction is sensitive to pH. The results of this study advance our understanding of environmental controls on microbial methane generation and provide a basis for using pH and the occurrence of ferric minerals to refine predictions of greenhouse gas fluxes.
more »
« less
- Award ID(s):
- 1753436
- PAR ID:
- 10078722
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Geobiology
- Volume:
- 17
- Issue:
- 2
- ISSN:
- 1472-4677
- Page Range / eLocation ID:
- p. 185-198
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Integrating Tide‐Driven Wetland Soil Redox and Biogeochemical Interactions Into a Land Surface ModelAbstract Redox processes, aqueous and solid‐phase chemistry, and pH dynamics are key drivers of subsurface biogeochemical cycling and methanogenesis in terrestrial and wetland ecosystems but are typically not included in terrestrial carbon cycle models. These omissions may introduce errors when simulating systems where redox interactions and pH fluctuations are important, such as wetlands where saturation of soils can produce anoxic conditions and coastal systems where sulfate inputs from seawater can influence biogeochemistry. Integrating cycling of redox‐sensitive elements could therefore allow models to better represent key elements of carbon cycling and greenhouse gas production. We describe a model framework that couples the Energy Exascale Earth System Model (E3SM) Land Model (ELM) with PFLOTRAN biogeochemistry, allowing geochemical processes and redox interactions to be integrated with land surface model simulations. We implemented a reaction network including aerobic decomposition, fermentation, sulfate reduction, sulfide oxidation, methanogenesis, and methanotrophy as well as pH dynamics along with iron oxide and iron sulfide mineral precipitation and dissolution. We simulated biogeochemical cycling in tidal wetlands subject to either saltwater or freshwater inputs driven by tidal hydrological dynamics. In simulations with saltwater tidal inputs, sulfate reduction led to accumulation of sulfide, higher dissolved inorganic carbon concentrations, lower dissolved organic carbon concentrations, and lower methane emissions than simulations with freshwater tidal inputs. Model simulations compared well with measured porewater concentrations and surface gas emissions from coastal wetlands in the Northeastern United States. These results demonstrate how simulating geochemical reaction networks can improve land surface model simulations of subsurface biogeochemistry and carbon cycling.more » « less
-
Abstract Water can be stored in nominally anhydrous minerals as substitutional hydroxyl, generating vast but commonly unrecognized H2O reservoirs in ostensibly dry regimes. Researchers have long known that hematite (α-Fe2O3) can accommodate small concentrations of hydroxyl through the substitution of Fe3+ by 3H+. Our study of natural hematite has demonstrated the occurrence of “hydrohematite” phases that are 10–20 mol% deficient in Fe and accordingly contain 3.6–7.8 mol% structural water. Intergrown with natural hydrohematite samples were superhydrous goethite-like phases exhibiting an Fe deficiency of 10–20 mol% relative to end-member goethite (α-FeOOH). We synthesized hydrohematite in alkaline solutions (pH 9–12) at low temperatures (T < 200 °C) using fresh ferrihydrite as the transient precursor, and we observed a nonclassical crystallization pathway involving vacancy inoculation by Fe as nanocrystals evolved. The high level of incorporation of H2O in iron (hydr)oxides dramatically alters their behaviors as catalysts and pigments, and the presence of hydrohematite in rocks may rule out high-T diagenesis. We propose that hydrohematite is common in low-T occurrences of Fe oxide on Earth, and by extension it may inventory large quantities of water in apparently arid planetary environments, such as the surface of Mars.more » « less
-
Abstract Light‐absorbing particles in atmospheric dust deposited on snow cover (dust‐on‐snow, DOS) diminish albedo and accelerate the timing and rate of snow melt. Identification of these particles and their effects is relevant to snow‐radiation modeling and water‐resource management. Laboratory‐measured reflectance of DOS samples from the San Juan Mountains (USA) were compared with DOS mass loading, particle sizes, iron mineralogy, carbonaceous matter type and content, and chemical compositions. Samples were collected each spring for water years 2011–2016, when individual dust layers had merged into one (all layers merged) at the snow surface. Average reflectance values of the six samples were 0.2153 (sd, 0.0331) across the visible wavelength region (0.4–0.7 μm) and 0.3570 (sd, 0.0498) over the full‐measurement range (0.4–2.50 μm). Reflectance values correlated inversely to concentrations of ferric oxide, organic carbon (1.4–10 wt.%), magnetite (0.05–0.13 wt.%), and silt (PM63‐3.9;median grain sizes averaged 21.4 μm) but lacked correspondence to total iron and PM10contents. Measurements of reflectance and Mössbauer spectra and magnetic properties indicated that microcrystalline hematite and nano‐size goethite were primarily responsible for diminished visible reflectance. Positive correlations between organic carbon and metals attributed to fossil‐fuel combustion, with observations from electron microscopy, indicated that some carbonaceous matter occurred as black carbon. Magnetite was a surrogate for related light‐absorbing minerals, dark rock particles, and contaminants. Similar analyses of DOS from other areas would help evaluate the influences of varied dust sources, wind‐storm patterns, and anthropogenic inputs on snow melt and water resources in and beyond the Colorado River Basin.more » « less
-
Semrau, Jeremy D. (Ed.)Streams impacted by historic mining activity are characterized by acidic pH, unique microbial communities, and abundant metal-oxide precipitation, all of which can influence groundwater-surface water exchange. We investigate how metal-oxide precipitates and hyporheic mixing mediate the composition of microbial communities in two streams receiving acid-rock and mine drainage near Silverton, Colorado, USA. A large, neutral pH hyporheic zone facilitated the precipitation of metal particles/colloids in hyporheic porewaters. A small, low pH hyporheic zone, limited by the presence of a low-permeability, iron-oxyhydroxide layer known as ferricrete, led to the formation of steep geochemical gradients and high dissolved-metal concentrations. To determine how these two hyporheic systems influence microbiome composition, we installed well clusters and deployed in situ microcosms in each stream to sample porewaters and sediments for 16S rRNA gene sequencing. Results indicated that distinct hydrogeochemical conditions were present above and below the ferricrete in the low pH system. A positive feedback loop may be present in the low pH stream where microbially-mediated precipitation of iron-oxides contribute to additional clogging of hyporheic pore spaces, separating abundant, iron-oxidizing bacteria (Gallionella spp.) above the ferricrete from rare, low-abundance bacteria below the ferricrete. Metal precipitates and colloids that formed in the neutral pH hyporheic zone were associated with a more diverse phylogenetic community of nonmotile, nutrient-cycling bacteria that may be transported through hyporheic pore spaces. In summary, biogeochemical conditions influence, and are influenced by, hyporheic mixing, which mediates the distribution of micro-organisms and thus the cycling of metals in streams receiving acid-rock and mine drainage.more » « less
