V23D-2150
Stability and Occurrence of the Molecule-Containing SiO2 Clathrate Melanophlogite: Metastable Crystallization from a Colloid or Gel?
The mineral melanophlogite is the only known natural SiO2 clathrate. It has been found in a number of localities worldwide in different low-temperature geologic environments. Melanophlogite's thermodynamic stability is not known. Low-temperature hydrothermal laboratory experiments indicate that structure-directing agents and colloid formation are needed for crystallization. The formation of silica-rich colloids/gels and following crystal growth can be observed in glass-ampoule synthesis experiments. In order to better address these issues, the heat capacities of two different molecule-containing melanophlogites of approximate composition 46SiO2·1.80CH4·3.54N2·1.02CO2 from Mt. Hamilton, CA and 46SiO2·3.59CH4·3.10N2·1.31CO2 from Racalmuto, Sicily, along with a heated (molecule-free) sample of composition SiO2, were studied between 5 and 300 K using heat- pulse microcalorimetry. The molecule-free sample was obtained by heating a natural Racalmuto sample at 1173 K for 24 hr. It has a slightly larger low-temperature heat capacity and standard third-law entropy compared to other low-density SiO2 polymorphs such as various zeosils. The standard third-law entropy of the molecule-free sample is S° = 2216.3 J/(mol·K) for 46SiO2 and the natural Mt. Hamilton and Racalmuto samples give S° = 2805.7 J/(mol·K) and S° = 2956.8 J/(mol· K), respectively. The entropy and Gibbs free energy for molecule-free melanophlogite relative to quartz at 298 K are Δ Strans = 6.7 J/(mol·K) and Δ Gtrans = 7.5 kJ/mol, respectively and, thus, it does not have a thermodynamic field of stability in the SiO2 system. The difference in Cp values between molecule-containing and molecule-free melanophlogite is characterized by an increase in Cp from 0 K to approximately 70 K and then reaches a roughly constant value at 70 K < T < 250 K. The Δ Srxn at 298 K for 46SiO2(melan.) + xCH4(gas) + yCO2(gas) + zN2(gas) = 46SiO2·(xCH4)12·(yCO2, zN2)14 is estimated to be about -642 J/(mol·K) and -802 J/(mol·K) for the Mt. Hamilton and Racalmuto samples, respectively. The thermodynamic data, as well as published results on the occurrence of natural molecule-containing samples and associated phases such as cristobalite and chalcedony, suggest that melanophlogite crystallizes metastabily from gels. The occurrence of melanophlogite, and the lack of other SiO2clathrates/porosils in nature, are probably due to the essential role of molecular structure-directing agents. For melanophlogite they can be CO2, N2 and CH4, whereas the crystallization of other clathrate porosils requires more chemically and structurally complex molecules that are not naturally abundant.
V23D-2151
Molecular- and nm-scale Investigation of the Structure and Compositional Heterogeneity of Naturally Occurring Ferrihydrite
Ferrihydrite is a hydrated Fe(III) nano-oxide that forms in vast quantities in contaminated acid mine drainage environments. As a result of its high surface area, ferrihydrite is an important environmental sorbent, and plays an essential role in the geochemical cycling of pollutant metal(loid)s in these settings. Despite its environmental relevance, this nanomineral remains one of the least understood environmental solids in terms of its structure (bulk and surface), compositional variations, and the factors affecting its reactivity. Under natural aqueous conditions, ferrihydrite often precipitates in the presence of several inorganic compounds such as aluminum, silica, arsenic, etc., or in the presence of organic matter. These impurities can affect the molecular-level structure of naturally occurring ferrihydrite, thus modifying fundamental properties that are directly correlated with solid-phase stability and surface reactivity. Currently there exists a significant gap in our understanding of the structure of synthetic vs. natural ferrihydrites, due to the inherent difficulties associated to the investigation of these poorly crystalline nanophases. In this study, we combined synchrotron- and laboratory-based techniques to characterize naturally occurring ferrihydrite from an acid mine drainage system situated at the New Idria mercury mine in California. We used high-energy X-ray total scattering and pair distribution function analysis to elucidate quantitative structural details of these samples. We have additionally used scanning transmission X-ray microscopy high resolution imaging (30 nm) to evaluate the spatial relationship of major elements Si, Al, and C within ferrihydrite. Al, Si and C K-edge near- edge X-ray absorption fine structure spectroscopy and 27Al nuclear magnetic resonance spectroscopy were used to obtain short-range structural information. By combining these techniques we attain the highest level of resolution permitted by current analytical methods to study such naturally occurring nanomaterials, both at the molecular- and nm-scale. This work provides structural information at the short-, medium- and long- range, as well as evidence of compositional heterogeneity, and mineral/organic matter associations.
V23D-2152
Nanoscale Zirconium-(oxyhydr)oxide in Contaminated Sediments From Hanford, WA - A New Host for Uranium
Zirconium-, uranium-, and copper-bearing wastes have leached from former disposal ponds into vadose zone sediments in the 300 Area at the Department of Energy's Hanford Site. Zirconium is enriched in the shallow portion of the vadose zone, and we have discovered an amorphous Zr-(oxyhydr)oxide that contains 16% of the total uranium budget (84.24 ppm) in one of the shallow samples. We have characterized the oxide using electron microprobe analysis (EMPA), a focused ion beam (FIB) instrument, and transmission electron microscopy (TEM). It occurs in fine-grained coatings found on lithic and mineral fragments in these sediments. The oxide is intimately intergrown with the phyllosilicates and other minerals of the coatings, and in places can be seen coating individual, nano-sized phyllosilicate mineral grains. Electron energy-loss spectroscopy (EELS) shows that the Zr-(oxyhydr)oxide has a P:Zr atomic ratio around 0.2, suggesting it is either intergrown with minor amounts of a Zr-phosphate or has adsorbed a significant amount of phosphate. This material has adsorbed or incorporated a substantial amount of uranium. Thus, understanding its nature is critical to predicting the long-term fate of U in the Hanford vadose zone. While the low-temperature uptake of U by Zr-(oxhydr)oxides and phosphates has been studied for several decades in laboratory settings, to our knowledge ours is the first report of such uptake in the field.
V23D-2153
Inhibition of nZVI reactivity by magnetite(Fe3O4)
Most studies of nano-sized zero-valent iron (nZVI) have been focused on the degradation of contaminants using mainly nZVI or modified nZVIs, which subsequently formed Fe(II)/Fe(III) oxide and hydroxide during the reaction. In this study, we demonstrated that the reactivity of nZVI can be inhibited by the iron oxide (magnetite) for the degradation of 1,1,1-trichloroethane (1,1,1-TCA). The reductive degradation of 1,1,1- TCA in nZVI and magnetite suspension was studied using a batch reactor under an anaerobic condition. No reductive degradation of 1,1,1-TCA in nZVI (0.01 g) + magnetite (0.5 g) suspension was observed, while a significant degradation (0.288 hr-1) was observed in nZVI (0.01 g) suspension. The inhibition effect on nZVI by magnetite occurred due mainly to the adsorption of nZVI onto magnetite surfaces by the strong magnetism. However, the nZVI and magnetite suspension showed the similar reactivity of nZVI (0.1g) suspension, as the amount of nZVI increased to 0.1 g. This is caused by the remnant nZVI unadsorbed on the magnetite surfaces after the full saturation of nZVI on the surfaces. TEM images showed the adsorption of nZVI onto the magnetite surfaces. The results obtained from this study could provide basic understanding to properly operate and apply nZVI to real contaminated sites containing high contents of iron-bearing soil minerals.
V23D-2154
Effects of dissolved Ca2+, Mg2+, and Na+ ions on the supramolecular aggregation of natural organic matter in aqueous solutions
The complexation of natural organic matter (NOM) with metal ions, minerals and organic species in soil and water allows NOM to form water-soluble and water-insoluble aggregates of widely differing chemical and biological stabilities. Metal–NOM interaction induces strong correlations between the concentration of natural organic matter and the speciation, solubility and toxicity of many metals in the environment. In water purification and desalination, NOM is also implicated in fouling of nanofiltration and reverse osmosis membranes, either as the primary foulant or as a conditioning layer for microbial attachment ("biofouling"). In this work we investigated the effects of various metal ions on NOM aggregation in aqueous solutions, by a combination of dynamic light scattering (DLS), small angle neutron scattering (SANS) and large-scale molecular dynamics (MD) computer simulations. This allows a detailed molecular-scale statistical analysis of the size and the structural topology of metal–NOM aggregates. The DLS measurements show that Ca2+ ions present in a Suwannee River NOM (SRNOM) solution lead to the formation of a wide range of supramolecular structures with sizes between 100 and 1,000 nm. In contrast, Mg2+ and Na+ do not affect the aggregation of SRNOM as strongly. SANS data are inconclusive but indicate the presence of quite large (>50 nm) fractal particles formed presumably through a cluster-cluster aggregation. MD simulations confirm these observations and show that NOM can aggregate in aqueous solutions by two different mechanisms. On the one hand, NOM molecules can spontaneously aggregate by hydrogen bonding between their functional groups when only Na+ and Mg2+ are present as background cations. This promotes the formation of uniformly shaped NOM clusters. On the other hand, if Ca2+ ions are present in solution, they can more strongly bind two different NOM molecules by co-complexing the carboxylate groups, thus promoting the formation of longer linear and branched aggregate structures. When Ca2+ is added to the MD simulated solution, the aggregation also equilibrates noticeably faster (within 5 ns) and the observed aggregates are more stable than those with Mg2+ or Na+.
V23D-2155
Degradation of C60 Fullerol by White-Rot Basidiomycete Fungi: Implications for Environmental Release of Nanomaterials
Industrially produced carbon-based nanomaterials, including fullerenes and fullerols, will be introduced into the environment in increasing amounts over the next century. Oxygenated fullerenes are likely to be produced in the environment through both biotic and abiotic weathering, and yet the environmental fate of compounds like hydroxylated fullerenes are almost unknown. This study examines the ability of two white rot basidiomycete fungi (Phlebia tremellosa and Trametes versicolor) to metabolize and degrade 13C-labeled C60 fullerol. Both of these fungi were shown to degrade fullerol to CO2 both in the presence of wood tissue and without, and incorporate trace amounts of the carbon into fungal biomass. Absorbance data also indicate that a significant portion of the original fullerol was broken down into small molecular weight metabolites. Phlebia tremellosa proved to be, in general, more aggressive towards fullerol degradation than Trametes versicolor. These findings represent the report of fungal degradation of this important nanomaterial and also provide valuable information about the possible environmental fates of this compound.
V23D-2156
Oxide Particle Size, Loading, and Specific Surface Area Effects on Adsorption Isotherms
The specific surface area of mineral particles is usually determined by the BET gas adsorption isotherm method, and is a key property needed to normalize surface concentrations, mineral dissolution and growth rates, etc. per surface area, in studies of mineral surface reactions. However, aggregation and settling of particles could affect the available surface area for surface reactions, thus influencing the measured adsorption isotherms and surface reaction rates, with significant implications for the interpretation of the data. In this study, we have examined the effects of particle loading (mass of oxide per liter of solution) and size (micron- versus nanometer-sized) on adsorption isotherms of dipalmitoylphosphatidylcholine (DPPC), a common cell-membrane phospholipid (PL), at pH 7.2 and 55°C. The oxides included two nanoparticulate amorphous silicas, one nanoparticulate anatase (b-TiO2), and micron-sized quartz (a-SiO2), rutile (a-TiO2) and corundum (a-Al2O3). Phospholipids are amphiphilic molecules that comprise biological cell membranes, and self-assemble to vesicles in solution, serving as model proto-cells for studies relating to the origin of cellularity in the early evolution of life. The interactions and stability of PL vesicles and bilayers adsorbed at mineral surfaces has relevance for understanding the potential role of minerals in promoting or inhibiting the evolution of cellularity. Amphipihile-mineral interactions are also relevant for bacterial cell surfaces in contact with soil minerals. We have previously established the effects of oxide mineral surface chemistry and PL head-group chemistry on adsorption and self-assembly (Oleson and Sahai, 2008a). In the present work, adsorption isotherms apparently showed less DPPC adsorption at higher particle loadings for micron-sized particles, indicating that a greater fraction of the suspended solid settles out, so that the specific surface area available for adsorption is less than that used for normalizing the adsorption isotherms. This effect is most apparent at higher solution concentrations of DPPC. The effect of particle aggregation on supression of isotherms is even more pronounced when comparing micron- versus nanometer-sized particles, with much lower apparent adsorption on the nanometer sized particles. Particle size analyses by HRTEM and Dynamic Light Scattering suggest that the amorphous silica and anatase nanoparticles aggregate into secondary particles of size from ~ 100-1000s of nanometers. There is considerably less aggregation of the micrometer-sized quartz and rutile. Furthermore, the extent of nanoparticle aggregation depends on the solution pH and ionic strength, which affect surface charge and van der Waals forces, as expected rom DLVO theory. Aggregation of the nanoparticles reduces the available surface area for adsorption than the specific surface area value used for the isotherms. Our results have implications beyond the adsorption of PLs. For any surface studies involving particle suspensions, our data indicate the critical need to determine (a) the particular mass loading for micron-sized particles that minimizes settling, (b) secondary particle size of aggregated nanoparticles, and (c) to develop relationships between particle size, shape, and effective specific surface area.
V23D-2157
Characteristics of Organobentonite and Study of Iodide Adsorption on Organobentonite using X-ray Absorption Spectroscopy
The adsorption of iodide on untreated bentonite and bentonites modified with organic cation (i.e., hexadecylpyridinium chloride monohydrate (HDP+)) was investigated, and the organobentonites were characterized using uptake measurements, micro X-ray diffraction (micro-XRD), and electrophoretic mobility measurements prior to reaction with KI solutions. Uptake measurements indicate that bentonite has a high affinity for HDP+. Increasing [HDP+](aq) results in an increase in HDP+ uptake on bentonite by up to 280% of the CEC equivalents of bentonite, and causes a concomitant increase in Na released as a result of the replacement of exchangeable inorganic cations in bentonite interlayers. Based micro-XRD, the d001 spacing of untreated bentonite was 1.22 nm whereas organobentonites modified with HDP+ at different equivalent amounts, corresponding to 100%, 200%, and 400% of the cation exchange capacity (CEC) of bentonite, showed d001 spacings of 1.96 nm, 3.77 nm, and 3.77 nm, respectively. Our micro-XRD study indicates that organobentonites significantly expanded in basal spacing and organic cations were substantially intercalated into the interlayer spaces of montmorillonite. The electrophoretic mobility indicates that the untreated bentonite had a negative surface charge over the entire pH range examined (pH 2-12) whereas the organobentonite at an equivalent amount corresponding to 200% of the CEC had a positive surface charge over this pH range. We found significant differences in adsorption capacities of iodide depending on the bentonite properties as follows: iodide adsorption capacities were 439 mmol/kg for the bentonite modified with HDP+ at an equivalent amount corresponding to 200% of the CEC of bentonite whereas no adsorption of iodide was observed for the untreated bentonite. The molecular environments of iodine adsorbed on organobentonites were further studied using I K-edge and LIII-edge x-ray absorption spectroscopy (XAS). The X-ray absorption near-edge structure (XANES) of iodine spectra from organobentonites was similar to that of KI reference solution. Quantitative analysis of EXAFS spectra of organobentonite samples indicates that iodine is bound to carbon and the coordination number and interatomic distances between I–C varied depending on the organic concentration on bentonite. Linear combination fitting of EXAFS data suggests the fraction of iodine reacted with the organic compound increased from 49% to 77% with increasing loading of the organic compound on organobentonites. In this study, we observed significant differences in the adsorption environments of iodide depending on the property of the bentonite, and suggest that these molecular-level differences result in an organobentonite that has potential as reactive barrier material around a nuclear waste repository containing radioactive iodide.
V23D-2158
Biogenic nanoparticulate UO2: Synthesis, characterization, and factors affecting surface reactivity
The surface reactivity of biogenic, nanoparticulate UO2 with respect to sorption of aqueous Zn(II) and particle annealing is different from that of bulk uraninite because of differences in particle size and the presence of surface-associated organic matter on the biogenic UO2. Synthesis of biogenic UO2 was accomplished by reduction of aqueous uranyl ions, UO22+ by Shewanella putrefaciens CN32, and the resulting nanoparticles were washed using one of two protocols: (1) 10 percent NaOH, followed by 4 mM KHCO3/KCl (particles referred to as NAUO2) to remove surface-associated organic matter and soluble uranyl species, or (2) 4 mM KHCO3-KCl (particles referred to as BIUO2) to remove only soluble uranyl species. A suite of bulk and surface characterization techniques was used to examine bulk and biogenic, nanoparticulate UO2 as a function of particle size and surface-associated organic matter. The N2-BET surface areas of the two biogenic UO2 samples following the washing procedures are 128.63 m2g-1 (NAUO2) and 92.56 m2g-1 (BIUO2), and the average particle sizes range from 5-10 nm based on TEM imaging. Electrophoretic mobility measurements indicate that the surface charge behavior of biogenic, nanoparticulate UO2 over the pH range 3-9 is the same as that of bulk uraninite (avg. diameter = 500 nm) (pHpzc = 5.6±0.03) and that there is no observed effect on surface charge caused by surface-associated organic matter for sample BIUO2. Both XPS and U LIII-edge XANES spectroscopy revealed that the uranium oxidation state of the biogenic, nanoparticulate samples is 4+, which is consistent with stoichiometric UO2. The EXAFS spectra for biogenic UO2 were best fit with half the number of second-shell uranium neighbors compared to bulk uraninite, and no oxygen neighbors were detected beyond the first shell around U(IV) in the biogenic UO2. At pH 7, sorption of Zn(II) onto both finely ground, bulk uraninite (500 nm average particle diameter) and biogenic, nanoparticulate UO2 is independent of electrolyte concentration, suggesting that Zn(II) sorption complexes are dominantly inner-sphere. The maximum surface area-normalized Zn(II) sorption loadings for the three substrates were 3.00±0.20 mmoles m-2 UO2 (bulk uraninite), 2.34±0.12 mmoles m-2 UO2 (NAUO2), and 2.57±0.10 mmoles m-2 UO2 (BIUO2), corresponding to 0.33, 0.26, and 0.27 effective monolayers of Zn(II) sorption complexes, respectively. Fits of Zn K-edge EXAFS spectra for biogenic, nanoparticulate UO2 indicate that Zn(II) sorption is dependent on the washing protocol. Zn-U pair correlations were observed at 2.8±0.1 Ĺ for NAUO2 and bulk uraninite; however, they were not observed for sample BIUO2. The derived Zn-U distance, coupled with an average Zn-O distance of 2.09±0.02 Ĺ, indicates that Zn(O,OH)6 sorbs as bidentate, edge-sharing complexes to UO8 polyhedra at the surface of NAUO2 nanoparticles and bulk uraninite. The absence of Zn-U pair correlations in sample BIUO2 suggests that Zn(II) binds preferentially to the organic matter coating rather than the UO2 surface. Surface-associated organic matter on the biogenic UO2 particles also inhibited particle annealing at 90oC under anaerobic conditions. These results suggest that surface-associated organic matter decreases the reactivity of biogenic, nanoparticulate UO2 surfaces relative to aqueous Zn(II) and possibly other environmental contaminants.
V23D-2159
Soil structure, colloids, and chemical transport as affected by short-term reducing conditions: a laboratory study
Upland soils in the Midwestern US often undergo reducing conditions when soils are temporally flooded during the spring and remain water saturated for days or weeks. Short-term reducing conditions change the chemistry of the soil and may affect soil structure and solution chemical transport. The effects of short-term reducing conditions on chemical and physical properties of the soils, colloids, and associated chemical/nutrients transport are still not well understood and was the objective of our study. A biogeochemical reactor was built to achieve reducing conditions. Three cultivated and three uncultivated soils with different organic carbon contents were incubated in the reactor for 1 hour and 3 days under anaerobic conditions. Effects of the redox state on soil structure (pore size distribution) and drainable porosity, colloids mobility, and chemical transport were determined using high energy moisture characteristic and analytical methods. After each treatment, the soil solution was collected for redox potential (Eh), pH, and electrical conductivity (EC) measurements, and chemical analysis of metals (Ca, Mg, K), nutrients (N, P), and dissolved organic carbon. Strongly reducing conditions were achieved after 3 days of incubation and were followed by a decrease in soil porosity and an increase in pH, EC, clay dispersion, swelling, colloids mobility, and associated chemical transport. The trend for each soil depended on their initial structural stability and chemical properties. The structure of cultivated soils and the leaching of nutrients and carbon from uncultivated soils were more sensitive to the redox state. A strong correlation was found between changes in Eh and drainable porosity. The role of short-term reducing conditions on changes in redox sensitive elements, organic matter decomposition, pH, and EC and their influence on soil structure and soil particles or colloids/chemical transport for both soil groups are discussed in the paper. This study showed that short- term reducing conditions influence colloids/chemical transport and it should be taken into account for modeling because dissolved compounds and colloids facilitated nutrient and associated pollutant transport during coupled overland and soil-matrix flow conditions.
V23D-2160
Spectroscopic Insights Onto Iron/Organic Matter Association in Natural Waters
The understanding of iron speciation and its various biogeochemical interactions remains an important issue in environmental science. Indeed, available Fe is an essential element for numerous biota, including phytoplankton. In addition colloidal iron oxides are known to play an important role on the mobility and bioavailability of various contaminants, through their reactivity, their high surface area and ubiquity at the earth's surface. Recent modelling efforts have provided generic Fe binding parameters towards natural organic matter analogues, including oxy-hydroxides precipitation. Nevertheless, little knowledge is available at the molecular level on the associations between iron and organic matter. In this study, we joint used IR, EPR ans Mössbauer spectroscopies to charactrize the different forms of organic and mineral iron both onto natural and synthetic samples. Two main species were detected and analyzed in laboratory samples: Fe3+ complexed by carboxylic groups of organic matter that were predicted by modelling and unexpected oxyhydroxide nanoparticles. The spectroscopic signature of these species are compared to those obtain for organic an iron rich tropical rivers in order to evaluate iron speciation and fate in such environments.
V23D-2161
Carbonaceous Materials in Soil Particulates from Field Drainage Tiles
The leaching of particles from an Agrudalf to field drainage tiles has been monitored for 3 run-off seasons (Nov-March). The average yearly losses was determined at 2.1 kg particles (dw.) per ha. with fairly high variation among seasons, thought to reflect climatic variations. Time resolved monitoring of run-off and particulates during each run-off event, all revealed an offset of the peak values of the two parameters, with amount of particulates peaking first. This finding indicates the importance of preferential flow, directly connecting the near-surface soil with the tiles. Furthermore, monitoring the bulk element composition of the particulates from the time resolved sampling of each event, demonstrated a relatively higher content of carbon during the early part of the transport curve. This was particularly pronounced when the flow occurred following a frost period. The structure of the particulate carbonaceous material was investigated by TEM following preparation of maceral-like fractions. A turbostratic graphite-like stacking is found to dominate the carbonaceous matter, as it also dominates maceral from the Ap horizon. Compared with the total amounts of particles transported, the sink for both inorganic and carbonaceous particles in the A horizon seems infinite. The cause of this differential mobilization of particles remains unresolved.
V23D-2162
Time-dependent studies of metal retention to and desorption from nanoparticle aggregates
Nanoparticles play a potentially significant role in the mobility of aqueous metal species in a number of environmental systems through the sorption of metals onto their surfaces. Additionally, the natural and often rapid aggregation of nanosized particles in aqueous systems can lead to changes in their structure, available surface area, porosity, and reactivity that may modify the mechanisms and extents by which metal ions are retained. Such aggregation processes may subsequently facilitate the long-term sequestration of metals by inhibiting desorption from the nanoparticle aggregates, thus enhancing the removal of metals from solution into the solid phase. Real-time macroscopic adsorption/desorption batch experiments were conducted with synthetic iron oxyhydroxide nanoparticles to investigate changes in Cu(II) concentration under environmentally-relevant conditions. Nanoparticles were introduced into Cu(II)-bearing solutions as either monodispersed particles (initial size: ~5 nm) or multi-particle aggregates formed by increasing the pH, ionic strength, or temperature of the initial monodisperse suspensions. Ion-selective, conductivity, temperature and pH probes were used to track real time changes in the solution, specifically the uptake of Cu(II) and associated changes in pH and ionic strength. A desorption step was then performed by lowering the pH back below the macroscopic absorption edge previously determined for Cu(II) to the iron oxyhydroxide nanoparticles. Sufficient time was allowed after both the introduction of the nanoparticles and the desorption step for the system to reattain equilibrium. Results suggest that the various aggregation mechanisms influence the extent and timeframe of Cu(II) uptake and desorption differently depending on the degree of aggregation. Specifically, heating as a method of aggregation was the most effective at retaining metals in the solid phase and pH-based aggregation less so, while ionic strength-based aggregation had little effect relative to the monodisperse particles. These results agree with corresponding EXAFS studies of nanoparticle aggregates which indicate that the desorption step removes the weakly-held (i.e. surface-bound) metal fraction but retains strongly-held metals that appear to be more structurally incorporated within the nanoparticle aggregates. These findings have implications for the removal of hazardous metals from the aqueous phase and the design of remediation strategies targeting contaminated environments such as mine-impacted regions.
V23D-2163
Montmorillonite Dissolution in Simulated Lung Fluids
Because lung fluids" first interaction is with the surface of inhaled grains, the surface properties of inhaled mineral dusts may have a generally mitigating effect on cytotoxicity and carcinogenicity. Wendlandt et al. (Appl. Geochem. 22, 2007) investigated the surface properties of respirable-sized quartz grains in bentonites and recognized pervasive montmorillonite surface coatings on silica grains. The purpose of this study was to determine the dissolution rate and biodurability of montmorillonite in simulated lung fluids and to assess its potential to mitigate silica cytotoxicity. Modified batch reaction experiments were conducted on purified and size fractionated calcic (SAz-2; 0.4-5 μm) and sodic (DC-2; 0.4-2 μm) montmorillonites for 120 to 160 days of reaction time at 37°C in both simulated extracellular lung fluid (Lu) and simulated lysosomal fluid (Ly). Modified batch experiments simulated a flow-through setup and minimized sample handling difficulties. Reacted Lu and Ly fluid was analyzed for Mg, Al, and Si on an ICP-OE spectrometer. Steady state dissolution was reached 90-100 days after the start of the experiment and maintained for 40-60 days. Measured montmorillonite dissolution rates based on BET surface areas and Si steady state release range from 4.1x10-15 mol/m2/s at the slowest to 1.0x10-14 mol/m2/s at the fastest with relative uncertainties of less than 10%. Samples reacting in Ly (pH = 4.55) dissolved faster than those in Lu (pH = 7.40), and DC-2 dissolved faster than SAz-2. The measured range of biodurabilities was 1,300 to 3,400 years for a 1 μm grain assuming a spherical volume and a molar volume equal to that of illite. The difference in salinities of the two fluids was too slight to draw conclusions about the relationship of ionic strength to dissolution rate. Results indicate that montmorillonite dissolution is incongruent and edge controlled. Dissolution rates for DC- 2 and SAz-2 clays were comparable to those reported in the literature. Biodurability results fall well beyond the lifespan of humans confirming montmorillonite's potential to mitigate silica cytotoxicity.
V23D-2164
Kinetics of mercury-sulfide nanoparticle formation in the presence of dissolved organic matter
Sulfidogenic aquatic environments contaminated with mercury are likely to be supersaturated with respect to mercury-sulfides such as metacinnabar. In other metal-sulfide systems, supersaturation has been shown to produce nanoparticles, particularly where strong organic complexing agents are present. Current aqueous speciation models do not consider the possible existence of metal-sulfide nanoparticles, which can lead to misconceptions about the potential bioaccessibility of toxic metals such as mercury. In addition, little is known about the kinetics of nanoparticle formation, especially in the presence of heterogeneous ligands such as naturally derived humic substances. The speciation of inorganic mercury determines its propensity to be methylated to CH3Hg+. We examined the speciation of Hg(II) in the presence of dissolved organic carbon (DOC) and bisulfide (HS-) in batches of synthetic aqueous media. Our focus is on the kinetics of formation and properties of mercuric sulfide nanoparticles. Bicarbonate-buffered solutions containing 10-9 to 10-5 M of Hg(NO3)2 and 10 mg C/l (from one of three different DOC isolates) were allowed to react for 24 hours at pH 6-7, after which time boric acid-buffered (pH 9) HS- was added at a S:C molar ratio of 0.5. Over a period of one month, bulk samples were ultracentrifuged to separate particles nominally greater than 6 nm from the supernatant. Mercury and sulfide were measured in the supernatants using cold-vapor atomic fluorescence spectroscopy and differential pulse cathodic stripping voltammetry. The molecular speciation of Hg in quickly (one minute) frozen, freeze-dried samples was determined using Hg LIII-edge X-ray absorption spectroscopy (XAS). The nucleation, growth, and aggregation of nanophase-Hg was monitored in situ using small- and wide-angle X-ray scattering (SAXS/WAXS), UV-visible spectroscopy, and dynamic light scattering (DLS). XAS, SAXS, and DLS measurements suggest the formation of metacinnabar nanoparticles on the order of 10 nm over timescales of hours. Preliminary XAS results further suggest that during the first 30 hours of reaction time DOC competitively binds Hg(II) to both oxygen and sulfur functional groups in the presence of a 40-fold molar excess of HS- to Hg(II). If aqueous thermodynamic speciation models are to be used to better understand the impact of inorganic mercury loading on habitat ecology, we recommend that these models include the possible existence of nanoparticulate HgS(s). We infer that the kinetic efficacy of DOC for binding Hg(II) in the presence of bisulfide undermines the use of thermodynamic modeling over short timescales.
V23D-2165
Control of Montmorillonite Surface Coatings on Quartz Grains in Bentonite by Precursor Volcanic Glass
The pathogenic tendencies of respirable-sized quartz grains may be dependent on inherent characteristics of the quartz as well as external factors. Surface coatings on quartz are of particular interest as they modify both physical and chemical properties of quartz grain surfaces and sequester the grain from contact with reactive lung fluids. Wendlandt et al. (Appl. Geochem. 22, 2007) investigated the surface properties of respirable-sized quartz grains in bentonites and recognized pervasive montmorillonite surface coatings on the quartz that resisted removal by repeated vigorous washings and reaction with HCl. To understand the persistence of montmorillonite coatings on quartz grains of igneous origin, volcanic ash deposits of varying age and degree of alteration to montmorillonite were sampled in Utah, including the distal Lava Creek (c. 0.64 Ma) and Bishop Tuffs (c. 0.74 Ma), and SW Colorado (Conejos Fm, San Juan Volcanic Field) for comparison with commercial grade Cretaceous-age "western" and "southern" bentonites. Quartz grains, hand-picked from these samples, were analyzed using FE-SEM and HRTEM. Continuous coatings of volcanic glass occur on quartz grains from the distal volcanic ash samples. As glass alteration to montmorillonite becomes more extensive, quartz grain surfaces start to display patches of montmorillonite. These patches become continuous in extent on quartz grains from the bentonites. Late precipitation of opal- CT lepispheres is consistent with the alteration reaction for volcanic glass: Volcanic glass + H2O = montmorillonite + SiO2(am) + ions(aq). HRTEM of quartz grains reveals an amorphous surface layer, consistent with a volcanic glass coating. Our results indicate that persistent montmorillonite coatings on quartz grains in bentonites are related to precursor volcanic glass coatings on these grains. The absence of glass coatings on other mineral grains in bentonite (feldspar, biotite) may be a consequence of the presence of strong cleavage in these minerals and resultant grain disaggregation during eruption. The reaction of glass and montmorillonite coatings in simulated lysosomal lung fluid (pH=4.55) at 37°C is under investigation and the persistence of these coatings directly impacts human health risk from respirable quartz in bentonite.