Steven C. Smith

Biomineralization of Poorly Crystalline Fe(III) Oxides by Dissimilatory Metal Reducing Bacteria (DMRB)

Dissimilatory metal reducing bacteria (DMRB) catalyze the reduction of Fe(III) to Fe(II) in anoxic soils, sediments, and groundwater. Two-line ferrihydrite is a bioavailable Fe(III) oxide form that is exploited by DMRB as a terminal electron acceptor. A wide variety of biomineralization products result from the interaction of DMRB with 2-line ferrihydrite. Here we describe the state of knowledge on the biotransformation of synthetic 2-line ferrihydrite by laboratory cultures of DMRB using select published data and new experimental results. A facultative DMRB is emphasized ( Shewanella putrefaciens ) upon which most of this work has been performed. Key factors controlling the identity of the secondary mineral suite are evaluated including medium composition, electron donor and acceptor concentrations, ferrihydrite aging/recrystallization status, sorbed ions, and co-associated crystalline Fe(III) oxides. It is shown that crystalline ferric (goethite, hematite, lepidocrocite), ferrous (siderite, vivianite), and mixed valence (magnetite, green rust) iron solids are formed in anoxic, circumneutral DMRB incubations. Some products are well rationalized based on thermodynamic considerations, but others appear to result from kinetic pathways driven by ions that inhibit interfacial electron transfer or the precipitation of select phases. The primary factor controlling the nature of the secondary mineral suite appears to be the Fe(II) supply rate and magnitude, and its surface reaction with the residual oxide and other sorbed ions. The common observation of end-product mineral mixtures that are not at global equilibrium indicates that microenvironments surrounding respiring DMRB cells or the reaction-path trajectory (over Eh-pH space) may influence the identity of the final biomineralization suite.

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.

Interaction of Hydrophobic Organic Compounds with Mineral-Bound Humic Substances

The sorption of hydrophobic organic compounds (HOC) on mineral-associated peat humic acid (PHA) was evaluated under different pH and electrolyte regimes. Relative size distribution measurements indicated that PHA was [open quotes]coiled[close quotes] in solution at high ionic strength (I) and elongated at low I. The sorption of PHA to hematite and kaolinite varied with I and electrolyte cation, suggesting that the configuration of the humic acid in solution influenced its structure on the mineral surface. The sorption maxima for PHA on kaolinite indicated that PHA occupies twice the mineral surface area at low I (0.005) as that observed at high I (0.1). HOC sorption to mineral-bound PHA in Na[sup +] electrolyte was greater at lower I, indicating that humate structure was a plausible determinant of HOC sorption. Freundlich isotherms of dibenzothiophene on the PHA-coated kaolinite did not display unit slope, regardless of pH, I, or cation. Carbazole and anthracene displayed competitive behavior for sorption onto PHA-coated kaolinite. Collectively, the experimental observations indicate that hydrophobic adsorption rather than phase partitioning was the dominant mode of HOC binding. 70 refs., 8 figs., 1 tab.

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.

Solubilization of Fe(III) oxide-bound trace metals by a dissimilatory Fe(III)reducing bacterium

Trace metals associate with Fe(III) oxides as adsorbed or coprecipitated species, and consequently, the biogeochemical cycles of iron and the trace metals are closely linked. This communication investigated the solubilization of coprecipitated Co(III) and Ni(II) from goethite (α-FeOOH) during dissimilatory bacterial iron reduction to provide insights on biogeochemical factors controlling trace-element fluxes in anoxic environments. Suspensions of homogeneously substituted Co-FeOOH (50 mmol/L as Co0.01Fe0.99OOH; 57Co-labeled) in eight different buffer/media solutions were inoculated with a facultative, metal-reducing bacteria isolated from groundwater (Shewanella putrefacians CN32), and incubated under strictly anaerobic conditions for periods up to 32 days. Lactate (30 mmol/L) was provided as an electron donor. Growth and non-growth promoting conditions were established by adding or withholding PO4 and/or trace metals (60Co-labeled) from the incubation media. Anthraquinone disulfonate (AQDS; 100 μmol/L) was added to most suspensions as an electron shuttle to enhance bacterial reduction. Solutions were buffered at circumneutral pH with either PIPES or bicarbonate buffers. Solid and liquid samples were analyzed at intermediate and final time points for aqueous and sorbed/precipitated (by HCl extraction) Fe(II) and Co(II). The bioreduced solids were analyzed by X-ray diffraction and field-emission electron microscopy at experiment termination. Ni-FeOOH (Ni0.01Fe0.99OOH) was used for comparison in select experiments. Up to 45% of the metal containing FeOOH was bioreduced; growth-supporting conditions did not enhance reduction. The biogenic Fe(II) strongly associated with the residual Fe(III) oxide as an undefined sorbed phase at low fractional reduction in PIPES buffer, and as siderite (FeCO3) in bicarbonate buffer or as vivianite [Fe3(PO4)2 · 8H2O] when P was present. Cobalt(III) was reduced to Co(II) in proportion to its mole ratio in the solid. The release of bioreduced Co(II) to the aqueous phase showed complex dependency on the media and buffer composition and the fractional reduction of the Co-FeOOH. In most cases Co(II) was solubilized in preference to Fe(II), but in select cases it was not. These differences were rationalized in terms of competitive adsorption reactions on the residual Fe(III) oxide surface and coprecipitation in biogenic Fe(II) solids. The bioreduced Co-FeOOH surface showed unexpectedly high sorption selectivity for the biomobilized Co(II). The bioreductive solubilization of Ni(II) from Ni-FeOOH was comparable to Co-FeOOH. Our results indicate that Fe(III)-oxide-entrained trace metals can be mobilized during bacterial iron reduction leading to a net increase, in most cases, in aqueous metal concentrations. The enhancement in trace-metal aqueous concentration, e.g., in groundwater, may proportionally exceed that of Fe(II).

Influence of humic substances on Co2+ sorption by a subsurface mineral separate and its mineralogic components

The sorption of Co2+ (10−6 mol/L) was measured on subsurface mineral materials in the absence and presence of a sorbed leonardite humic acid (LHA) to 1. (1) evaluate the sorptive role of mineral-bound humic substances, and 2. (2) establish approaches to model metal ion binding in composite materials. The subsurface materials were a < 2.0 μm size fraction of an ultisol saprolite (CP) and this same material treated with dithionite-citrate-bicarbonate (DCB) to remove Fe-oxides (DCP). Comparable experiments (with and without LHA) were also performed with mineral sorbents representing dominant phases in the CP separate (gibbsite, Al-goethite, and kaolinite) to evaluate their potential contributions to Co sorption. The mineral-bound LHA ranged in concentration between 0.1–0.4 mg-C/m2, representing approximately 0.7% of the subsurface isolate by mass. The sorption-desorption of LHA on the mineral surfaces, and its affinity for Co as a aqueous phase complexant were also determined. Batch measurements were employed (sorbents at 20–90 m2/L; LHA-DOC at ≈11 mg-C/L) over a range in pH and ionic strength (I) at I = 0.01 and 0.1 in NaClO4. The LHA strongly sorbed to the subsurface mineral isolates (CP and DCP), and to all the specimen sorbents except kaolinite. Maximum sorption of LHA occurred at lower pH (≈4.5). In solid-free suspensions, the affinity of LHA for Co increased with pH and decreasing I (Kd ranging 20–450 L/g). Mineral-bound LHA increased Co sorption on all the sorbents by factors of 10–60 %, with the greatest augmentation noted at pH values (4.5–6.5) where 1. (1) maximum LHA sorption occurred, and 2. (2) Co sorption to the mineral phase was weak and dominated by ion exchange. The LHA appeared simply to augment, rather than to change the intrinsic adsorption behavior of the mineral sorbents. Accordingly, predictions of the Kd for Co on the LHA-coated subsurface materials (DCP, CP) based on a linear additivity model agreed well with the experimental data, suggesting that the complex humic-mineral association acted as a noninteractive sorbent mixture at low aqueous Co concentrations.

The sorption of humic acids to mineral surfaces and their role in contaminant binding

Abstract Humic substances dissolved in groundwater may adsorb to certain mineral surfaces, rendering hydrophilic surfaces hydrophobic and making them sorbents for hydrophobic organic compounds (HOC). The sorption of humic and fulvic acids (International Humic Substances Society, IHSS, reference samples) on hematite and kaolinite was investigated to determine how natural organic coatings influence HOC sorption. The sorption behavior of the humic substances was consistent with a ligand-exchange mechanism, and the amount of sorption depended on the concentration of hydroxylated surface sites on the mineral and the properties of the humic substance. The sorption of the humic substances to two solids was proportional to their aromatic carbon content and inversely proportional to the O/C ratio. Increasing quantities of sorbed humic substances ( f oc , 0.01–0.5%) increased the sorption of carbazole, dibenzothiophene and anthracene. Peat humic acid, the most aromatic coating, showed the greatest sorption enhancement of HOC when sorbed to hematite. In addition, HOC sorption was greater on organic coatings formed at low ionic strength ( I = 0.005) as compared to higher ionic strength ( I = 0.1). We suggest that both the mineral surface and the ionic strength of the electrolyte affect the interfacial configuration of the sorbed humic substance, altering the size or accessibility of hydrophobic domains on the humic molecule to HOC.

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.

Nonlocal bacterial electron transfer to hematite surfaces

Mechanisms by which dissimilatory iron-reducing bacteria utilize iron and manganese oxide minerals as terminal electron acceptors for respiration are poorly understood. In the absence of exogenous electron shuttle compounds, extracellular electron transfer is generally thought to occur through the interfacial contact area between mineral surfaces and attached cells. Possible alternative reduction pathways have been proposed based on the discovery of a link between an excreted quinone and dissimilatory reduction. In this study, we utilize a novel experimental approach to demonstrate that Shewanella putrefaciens reduces the surface of crystalline iron oxides at spatial locations that are distinct from points of attachment.