James P. McKinley

Lithoautotrophic Microbial Ecosystems in Deep Basalt Aquifers

Bacterial communities were detected in deep crystalline rock aquifers within the Columbia River Basalt Group (CRB). CRB ground waters contained up to 60 μM dissolved H2 and autotrophic microorganisms outnumbered heterotrophs. Stable carbon isotope measurements implied that autotrophic methanogenesis dominated this ecosystem and was coupled to the depletion of dissolved inorganic carbon. In laboratory experiments, H2, a potential energy source for bacteria, was produced by reactions between crushed basalt and anaerobic water. Microcosms containing only crushed basalt and ground water supported microbial growth. These results suggest that the CRB contains a lithoautotrophic microbial ecosystem that is independent of photosynthetic primary production.

Sorption of Cs+ to micaceous subsurface sediments from the Hanford site, USA

The sorption of Cs+ was investigated over a large concentration range (10−9−10−2 mol/L) on subsurface sediments from a United States nuclear materials site (Hanford) where high-level nuclear wastes (HLW) have been accidentally released to the vadose zone. The sediment sorbs large amounts of radiocesium, but expedited migration has been observed when HLW (a NaNO3 brine) is the carrier. Cs+ sorption was measured on homoionic sediments (Na+, K+, Ca2+) with electrolyte concentrations ranging from 0.01 to 1.0 mol/L. In Na+ electrolyte, concentrations were extended to near saturation with NaNO3(s) (7.0 mol/L). The sediment contained nonexpansible (biotite, muscovite) and expansible (vermiculite, smectite) phyllosilicates. The sorption data were interpreted according to the frayed edge-planar site conceptual model. A four-parameter, two-site (high- and low-affinity) numeric ion exchange model was effective in describing the sorption data. The high-affinity sites were ascribed to wedge zones on the micas where particle edges have partially expanded due to the removal of interlayer cations during weathering, and the low-affinity ones to planar sites on the expansible clays. The electrolyte cations competed with Cs+ for both high- and low-affinity sites according to the trend K+ >> Na+ ≥ Ca2+. At high salt concentration, Cs+ adsorption occurred only on high-affinity sites. Na+ was an effective competitor for the high-affinity sites at high salt concentrations. In select experiments, silver-thiourea (AgTU) was used as a blocking agent to further isolate and characterize the high-affinity sites, but the method was found to be problematic. Mica particles were handpicked from the sediment, contacted with Cs+(aq), and analyzed by electron microprobe to identify phases and features important to Cs+ sorption. The microprobe study implied that biotite was the primary contributor of high-affinity sites because of its weathered periphery. The poly-phase sediment exhibited close similarity in ion selectivity to illite, which has been well studied, although its proportion of high-affinity sites relative to the cation exchange capacity (CEC) was lower than that of illite. Important insights are provided on how Na+ in HLW and indigenous K+ displaced from the sediments may act to expedite the migration of strongly sorbing Cs+ in subsurface environments.

The influence of uranyl hydrolysis and multiple site-binding reactions on adsorption of U(VI) to montmorillonite

Adsorption of uranyl to SWy-1 montmorillonite was evaluated experimentally and results were modeled to identify likely surface complexation reactions responsible for removal of uranyl from solution. Uranyl was contacted with SWy-1 montmorillonite in a NaCIO4 electrolyte solution at three ionic strengths (I = 0.001, 0.01, 0.1), at pH 4 to 8.5, in a N2(g) atmosphere. At low ionic strength, adsorption decreased from 95% at pH 4 to 75% at pH 6.8. At higher ionic strength, adsorption increased with pH from initial values less than 75%; adsorption edges for all ionic strengths coalesced above a pH of 7. A site-binding model was applied that treated SWy-1 as an aggregate of fixed-charge sites and edge sites analogous to gibbsite and silica. The concentration of fixed-charge sites was estimated as the cation exchange capacity, and non-preference exchange was assumed in calculating the contribution of fixed-charge sites to total uranyl adsorption. The concentration of edge sites was estimated by image analysis of transmission electron photomicrographs. Adsorption constants for uranyl binding to gibbsite and silica were determined by fitting to experimental data, and these adsorption constants were then used to simulate SWy-1 adsorption results. The best simulations were obtained with an ionization model in which A1OH2+ was the dominant aluminol surface species throughout the experimental range in pH. The pH-dependent aqueous speciation of uranyl was an important factor determining the magnitude of uranyl adsorption. At low ionic strength and low pH, adsorption by fixed-charge sites was predominant. The decrease in adsorption with increasing pH was caused by the formation of monovalent aqueous uranyl species, which were weakly bound to fixed-charge sites. At higher ionic strengths, competition with Na+ decreased the adsorption of UO22+ to fixed-charge sites. At higher pH, the most significant adsorption reactions were the binding of UO22+ to AlOH and of (UO2)3(OH)5+ to SiOH edge sites. Near-saturation of AlOH sites by UO22+ allowed significant contributions of SiOH sites to uranyl adsorption.

Surface-charge properties and UO22+ adsorption of a subsurface smectite

Abstract Surface charge and UO22+ adsorption were measured on a clay-sized, subsurface mineral isolate whose mineralogy was dominated by a ferrogenous beidellite. Experiments were performed in batch at 25°C with N2(g) atmosphere and sorbent suspensions (9.46 g clay/kg suspension) that had been adjusted in pH between 4 and 9. Surface charge was defined by measurements of adsorbed Na by isotopic exchange and of proton adsorption by potentiometric titration in NaClO4 (I = 0.1, 0.01, 0.001). Extraction of the clay with La(NO3)3 and aqueous-phase analyses were necessary to establish the contributions of Al and Si dissolution to the proton balance and the total adsorbed cation charge (i.e., Naads+ + 3Alads3+). The adsorption of UO22+ (7.5 × 10−6 mol L−1) was determined in Na+ (0.1, 0.01, 0.001 mol L−1) and Ca2+ (0.05 and 0.005 mol L−1) electrolytes. Adsorption of UO22+ showed contributions of ion exchange and edge complexation reactions in Na+ electrolyte, but by only edge complexation reactions in Ca2+ electrolyte. A multiple-site surface-complexation model containing fixed- X− and variable-charge sites (SiOH, AlOH) was fit to adsorbed cation charge data between pH 4 and 10, with the concentrations of AlOH, SiOH, and X− as the adjustable parameters. Surface acidity and ion-pair formation constants for gibbsite and silica were used to describe the ionization and electrolyte binding of the AlOH and SiOH sites. The model provided an excellent description of the surface-charge characteristics of the clay as measured by sodium isotopic exchange and potentiometric titration. A composite model was formulated to predict UO22+ adsorption by incorporating UO22+ aqueous speciation, competitive ion exchange with background electrolyte cations, and UO22+ complexation with AlOH and SiOH sites. UO22+ complexation with AlOH and SiOH was parameterized by UO22+ sorption on α-Al(OH)3(s) and α-SiO2(s), respectively. The composite model overpredicted UO22+ sorption across the entire pH range in both electrolytes. Acceptable predictions could be obtained if the UO22+ affinity for edge AlOH sites were adjusted 2.03 log units below that of gibbsite. Changes in chemical affinity arising from lattice substitutions and edge site morphology are, therefore, concluded to contribute significantly to adsorption, although the potential competitive effects of dissolved Al3+ and H4SiO4 could not be discounted. The adsorption of UO22+ on the subsurface smectite was similar to that of the reference montmorillonite, SWy-1, with the exception that Al dissolution contributed significantly to adsorbed cation charge.

Pore‐size constraints on the activity and survival of subsurface bacteria in a late cretaceous shale‐sandstone sequence, northwestern New Mexico

To investigate the distribution of microbial biomass and activities to gain insights into the physical controls on microbial activity and potential long‐term survival in the subsurface, 24 shale and sandstone cores were collected from a site in northwestern New Mexico. Bacterial biomass in the core samples ranged from below detection to 31.9 pmol total phospholipid fatty acid (PLFA) g‐1 of rock with no apparent relationship between lithology and PLFA abundance. No metabolic activities, as determined by anaerobic mineralization of [14C]acetate and [14C]glucose and 35SO4 2‐ reduction, were detected in core samples with pore throats <0.2 fan in diameter, smaller than the size of known bacteria. However, enrichments revealed the presence of sulfate‐re‐ducing bacteria, and 35SO4 2‐ reduction was detected upon extended (14 days) incubation in some small‐pore‐throat samples. In contrast, relatively rapid rates of metabolic activity were more common in core samples containing a significant fraction of pore throat...

Desorption kinetics of radiocesium from subsurface sediments at Hanford Site, USA

Abstract The desorption of 137 Cs + was investigated on sediments from the United States Hanford site. Pristine sediments and ones that were contaminated by the accidental release of alkaline 137 Cs + -containing high level nuclear wastes (HLW, 2 × 10 6 to 6 × 10 7 pCi 137 Cs + /g) were studied. The desorption of 137 Cs + was measured in Na + , K + , Rb + , and NH 4 + electrolytes of variable concentration and pH, and in presence of a strong Cs + -specific sorbent (self-assembled monolayer on a mesoporous support, SAMMS). 137 Cs + desorption from the HLW-contaminated Hanford sediments exhibited two distinct phases: an initial instantaneous release followed by a slow kinetic process. The extent of 137 Cs + desorption increased with increasing electrolyte concentration and followed a trend of Rb + ≥ K + > Na + at circumneutral pH. This trend followed the respective selectivities of these cations for the sediment. The extent and rate of 137 Cs + desorption was influenced by surface armoring, intraparticle diffusion, and the collapse of edge-interlayer sites in solutions containing K + , Rb + , or NH 4 + . Scanning electron microscopic analysis revealed HLW-induced precipitation of secondary aluminosilicates on the edges and basal planes of micaceous minerals that were primary Cs + sorbents. The removal of these precipitates by acidified ammonium oxalate extraction significantly increased the long-term desorption rate and extent. X-ray microprobe analyses of Cs + -sorbed micas showed that the 137 Cs + distributed not only on mica edges, but also within internal channels parallel to the basal plane, implying intraparticle diffusive migration of 137 Cs + . Controlled desorption experiments using Cs + -spiked pristine sediment indicated that the 137 Cs + diffusion rate was fast in Na + -electrolyte, but much slower in the presence of K + or Rb + , suggesting an effect of edge-interlayer collapse. An intraparticle diffusion model coupled with a two-site cation exchange model was used to interpret the experimental results. Model simulations suggested that about 40% of total sorbed 137 Cs + was exchangeable, including equilibrium and kinetic desorbable pools. At pH 3, this ratio increased to 60–80%. The remainder of the sorbed 137 Cs + was fixed or desorbed at much slower rate than our experiments could detect.

Influence of hydrolysis on the sorption of metal cations by smectites: Importance of edge coordination reactions

A comparison was made between the adsorptive behaviors of Cd2+ and UO22+ relative to two smectites that differed in their ratio of edge sites to fixed-charge sites. Adsorption varied with both pH and ionic strength, consistent with sorptive contributions of ion exchange and coordination reactions to edge hydroxyls. Both clay minerals exhibited a greater affinity for UO22+ than for Cd2+, and the clay with a higher proportion of edge sites retained both ions more strongly. A computational model including fixed-charge sites and edge hydroxyls resulted in a good prediction of overall UO22+ and Cd2+ uptake to both smectites.

Chromium Speciation and Mobility in a High Level Nuclear Waste Vadose Zone Plume

Radioactive core samples containing elevated concentrations of Cr from a high level nuclear waste plume in the Hanford vadose zone were studied to asses the future mobility of Cr. Cr(VI) is an important subsurface contaminant at the Hanford Site. The plume originated in 1969 by leakage of self-boiling supernate from a tank containing REDOX process waste. The supernate contained high concentrations of alkali (NaOH ≈ 5.25 mol/L), salt (NaNO3/NaNO2 >10 mol/L), aluminate [Al(OH)4− = 3.36 mol/L], Cr(VI) (0.413 mol/L), and 137Cs+ (6.51 × 10−5 mol/L). Water and acid extraction of the oxidized subsurface sediments indicated that a significant portion of the total Cr was associated with the solid phase. Mineralogic analyses, Cr valence speciation measurements by X-ray adsorption near edge structure (XANES) spectroscopy, and small column leaching studies were performed to identify the chemical retardation mechanism and leachability of Cr. While X-ray diffraction detected little mineralogic change to the sediments from waste reaction, scanning electron microscopy (SEM) showed that mineral particles within 5 m of the point of tank failure were coated with secondary, sodium aluminosilicate precipitates. The density of these precipitates decreased with distance from the source (e.g., beyond 10 m). The XANES and column studies demonstrated the reduction of 29–75% of the total Cr to insoluble Cr(III), and the apparent precipitation of up to 43% of the Cr(VI) as an unidentified, non-leachable phase. Both Cr(VI) reduction and Cr(VI) precipitation were greater in sediments closer to the leak source where significant mineral alteration was noted by SEM. These and other observations imply that basic mineral hydrolysis driven by large concentrations of OH− in the waste stream liberated Fe(II) from the otherwise oxidizing sediments that served as a reductant for CrO42−. The coarse-textured Hanford sediments contain silt-sized mineral phases (biotite, clinochlore, magnetite, and ilmenite) that are sources of Fe(II). Other dissolution products (e.g., Ba2+) or Al(OH)4− present in the waste stream may have induced Cr(VI) precipitation as pH moderated through mineral reaction. The results demonstrate that a minimum of 42% of the total Cr inventory in all of the samples was immobilized as Cr(III) and Cr(VI) precipitates that are unlikely to dissolve and migrate to groundwater under the low recharge conditions of the Hanford vadose zone.

Observations pertaining to the origin and ecology of microorganisms recovered from the deep subsurface of Taylorsville Basin, Virginia

To understand the conditions under which microorganisms exist in deep hydrocarbon reservoirs, sidewall cores were collected from a natural gas‐bearing formation, 2800 m below the surface in Taylorsville Basin, Virginia. Data from chemical and microbial tracers and controls indicate that the interiors of some sidewall cores contained microorganisms indigenous to the rock formation. The cultured microorganisms were composed primarily of saline‐tolerant, thermophilic fermenting, Fe(III)‐reducing, and sulfate‐reducing bacteria (1 to 104 cells/g). The physiological capabilities of the cultured microorganisms are compatible with the temperature (76°C), pressure (32 MPa), and salinity (≈0.8 wt.% NaCl equivalent) in the sampled interval. The petrological data indicated that the strata contain intercrystalline pores of micrometer size, that occur between late diagenetic cement in siltstone and within cross‐cutting, mineralized fractures in shale. These pores made up only 0.04% of the rock volume, were mostly gas‐f...

Bacteria associated with deep, alkaline, anaerobic groundwaters in Southeast Washington.

The microbial diversity in two deep, confined aquifers, the Grande Ronde (1270 m) and the Priest Rapids (316 m), Hanford Reservation, Washington, USA, was investigated by sampling from artesian wells. These basaltic aquifers were alkaline (pH 8.5 to 10.5) and anaerobic (Eh −200 to −450 mV). The wells were allowed to free-flow until pH and Eh stabilized, then the microflora was sampled with water filtration and flow-through sandtrap methods. Direct microscopic counts showed 7.6 × 105 and 3.6 × 103 bacteria ml−1 in water from the Grande Ronde and Priest Rapids aquifers, respectively. The sand filter method yielded 5.7 × 108 and 1.1 × 105 cells g−1 wet weight of sand. The numbers of bacteria did not decrease as increasing volumes of water were flushed out. The heterotrophic diversity of these bacterial populations was assessed using enrichments for 20 functional groups. These groups were defined by their ability to grow in a matrix of five different electron acceptors (O2, Fe(III), NO3−, SO42−, HCO3−) and four groups of electron donors (fermentation products, monomers, polymers, aromatics) in a mineral salts medium at pH 9.5. Growth was assessed by protein production. Culture media were subsequently analyzed to determine substrate utilization patterns. Substrate utilization patterns proved to be more reliable indicators of the presence of a particular physiological group than was protein production. The sand-trap method obtained a greater diversity of bacteria than did water filtration, presumably by enriching the proportion of normally sessile bacteria relative to planktonic bacteria. Substrate utilization patterns were different for microflora from the two aquifers and corresponded to their different geochemistries. Activities in the filtered water enrichments more closely matched those predicted by aquifer geochemistry than did the sand-trap enrichments. The greatest activities were found in Fe(III)-reducing enrichments from both wells, SO4-reducing enrichments from the Grande Ronde aquifer, and methanogenic enrichments from the Priest Rapids aquifer. Organisms from these aquifers may be useful for high-pH bioremediation applications as well as production of biotechnological products. These organisms may also be useful for modeling potential reactions near buried concrete, as might be found in subsurface waste depositories.