Solid Mineral Phases Research Articles

A 300°C High Reliability HALT/HAST Screening/Sorting Procedure for Ceramic Capacitors

Abstract A highly accelerated life test (HALT) and highly accelerated stress test (HAST) procedure for ceramic capacitors developed by the author in the mid 1980's to early 1990's, and published in 1994, consists of a 400 Volt biased six (6) hour stress sort at 150°C (423K), a methanol current leakage test that located mechanical and structural cracks, a visual inspection at ten times (10×) magnification, and a capacitance and dissipation measurement before and after the test. In over thirty (30) years of use, there has never been a user reported in-circuit failure in industrial, military, and aerospace application at temperatures as high as 500°C (773K). However, reviewing user feedback, two concerns with the original sorting procedure are the stress is performed at 150°C (423K), and the lack of a more detailed ceramic capacitor electrical model. To address the first, the low aging temperature, the stress temperature was increased from 150°C to 300°C, in order to age ceramic solid state crystal mineral phases that may change with temperature. The test results for X7R and NP0/COG multilayer ceramic capacitors (MLCC) at 300°C, are compared to the test results using the original HALT/HAST procedure at 150°C. Differences between X7R/NP0/COG and porcelain capacitors are discussed when applicable. Further, a more detailed ceramic capacitor electrical model that represents the physical and electrical characteristics of the ceramic capacitors is presented, including the electrical current leakage effects with temperature, and the carbonized residue effects from the manufacturing process.

Simulation of Calcium Phosphate Species in Aqueous Solution: Force Field Derivation.

A new force field has been derived for the aqueous calcium phosphate system that aims to reproduce the key thermodynamic properties of the system, including free energies of hydration of the ions and the solubility of the solid mineral phases. Interactions of three phosphate anions (PO43-, HPO42-, and H2PO4-) with water were calibrated through comparison with the results obtained from ab initio molecular dynamics using both GGA and hybrid density functional theory with dispersion corrections. In the solid state, the force field has been evaluated by benchmarking against experiment and other existing models and is shown to reproduce the structural and mechanical properties well, despite the primary focus being on thermodynamics. To validate the force field, the thermodynamics of ion pairing for calcium phosphate species in water has been computed and shown to be in excellent agreement with experimental data.

Evaluation of aluminium mobilization from its soil mineral pools by simultaneous effect of Aspergillus strains' acidic and chelating exometabolites

This contribution investigates aluminium mobilization from main aluminium pools in soils, phyllosilicates and oxyhydroxides, by acidic and chelating exometabolites of common soil fungi Aspergillus niger and A. clavatus. Their exometabolites' acidity as well as their ability to extract aluminium from solid mineral phases differed significantly during incubation. While both strains are able to mobilize aluminium from boehmite and aluminium oxide mixture to some extent, A. clavatus struggles to mobilize any aluminium from gibbsite. Furthermore, passive and active fungal uptake of aluminium enhances its mobilization from boehmite, especially in later growth phase, with strong linear correlation between aluminium bioaccumulated fraction and increasing culture medium pH. We also provide data on concentrations of oxalate, citrate and gluconate which are synthesized by A. niger and contribute to aluminium mobilization. Compared to boehmite-free treatment, fungus reduces oxalate production significantly in boehmite presence to restrict aluminium extraction efficiency. However, in presence of high phyllosilicates' dosages, aluminium is released to an extent that acetate and citrate is overproduced by fungus. Our results also highlight fungal capability to significantly enhance iron and silicon mobility as these elements are extracted from mineral lattice of phyllosilicates by fungal exometabolites alongside aluminium.

A volume equation of state that extends thermodynamic datasets, using the Bridgman Power Series, to very high pressures (20 GPa)

Thermodynamic datasets are essential for phase equilibrium computations, from which metamorphic pressure and temperature conditions of rocks can be estimated. In addition, they are used to model rock properties such as volume or density. The theoretical framework for thermodynamic volume modeling is imposed by the equation of state (EOS), which can be represented using many possible formulations. Until the 19909s, the Bridgman Power Series was a popular approach to model mineral volumes. Many experiments were fitted to this equation and input parameters for almost every mineral exist. Unfortunately, the equation has a drawback and does not allow the extrapolation of mineral volumes to pressures above 1 GPa. In this paper, an inverted EOS is introduced which uses the same input parameters as the Bridgman Power Series. Converting the fit parameters of the Bridgman Power Series to commonly used physical input parameters at reference conditions (κ00, κ′00, and α00, α′00) allows extrapolation of mineral volumes to very high pressure (20 GPa). The results from this study show in detail how the extended EOS improves the volumes of solid mineral phases such as coesite, jadeite and forsterite above 1 GPa, while retaining consistent volumes at low pressure temperature conditions. In this manner, the extended EOS also improves the phase equilibrium computations of the thermodynamic datasets built on the Bridgman Power Series. The proposed inverted EOS could be easily implemented by thermodynamic software packages with no modification of the dataset because the same input parameters are used.

Multiheme Cytochromes and the Bacterial Nanowires of Shewanella oneidensis MR-1: Regulation, Structure, and Extracellular Electron Transport Mechanisms

Dissimilatory metal-reducing bacteria can extract free energy from their environment by performing electron transfer to solid-phase minerals outside the cell. This extracellular electron transport (EET) has important implications in global elemental cycles as well as renewable energy technologies. Among the pathways for EET, bacterial nanowires have received significant attention in the past decade due to their unique ability to mediate long-range electron transport to electron acceptors microns away from the cell surface. Here we report a comprehensive characterization of the composition, structure, and regulatory network that underlies bacterial nanowires from the metal-reducing bacterium Shewanella oneidensis MR-1. Using fluorescent and atomic force techniques, we find that the Shewanella nanowires are extensions of the outer membrane and periplasm that contain multiple multiheme cytochromes. The localization of decaheme cytochromes MtrC and OmcA supports a multistep redox hopping mechanism, allowing long-range electron transport along a membrane network of heme cofactors that line the nanowires. The electron flux resulting from such a mechanism strongly depends on the cytochrome density and topology. Using correlated electron cryo-tomography and in vivo fluorescent microscopy, we are gaining new insight into the localization patterns of cytochromes along nanowires as well as the morphology and the formation mechanism of these structures. Finally, we report our progress on understanding the underlying regulatory network, by testing targeted mutations and analyzing the transcriptome of Shewanella chemostat cultures as they encounter electron acceptor limitation and form nanowires. The transcriptional response includes an increase in the expression of multiheme cytochromes, heme synthesis enzymes, and cytochrome maturation proteins. Our findings on the regulation, ultrastructure and electron transport mechanism help shape a biophysical understanding of these redox-functionalized membrane and vesicular extensions as a microbial strategy for electron transport and energy distribution.

Impacts of mineralogical compositions on different trapping mechanisms during long-term CO2 storage in deep saline aquifers

Deep saline aquifers in sedimentary basins are considered to have the greatest potential for CO2 geological storage in order to reduce carbon emissions. CO2 injected into a saline sandstone aquifer tends to migrate upwards toward the caprock because the density of the supercritical CO2 phase is lower than that of formation water. The accumulated CO2 in the upper portions of the reservoir gradually dissolves into brine, lowers pH and changes the aqueous complexation, whereby induces mineral alteration. In turn, the mineralogical composition could impose significant effects on the evolution of solution, further on the mineralized CO2. The high density of aqueous phase will then move downward due to gravity, give rise to “convective mixing,” which facilitate the transformation of CO2 from the supercritical phase to the aqueous phase and then to the solid phase. In order to determine the impacts of mineralogical compositions on trapping amounts in different mechanisms for CO2 geological storage, a 2D radial model was developed. The mineralogical composition for the base case was taken from a deep saline formation of the Ordos Basin, China. Three additional models with varying mineralogical compositions were carried out. Results indicate that the mineralogical composition had very obvious effects on different CO2 trapping mechanisms. Specific to our cases, the dissolution of chlorite provided Mg2+ and Fe2+ for the formation of secondary carbonate minerals (ankerite, siderite and magnesite). When chlorite was absent in the saline aquifer, the dominant secondary carbon sequestration mineral was dawsonite, and the amount of CO2 mineral trapping increased with an increase in the concentration of chlorite. After 3000 years, 69.08, 76.93, 83.52 and 87.24 % of the injected CO2 can be trapped in the solid (mineral) phase, 16.05, 11.86, 8.82 and 6.99 % in the aqueous phase, and 14.87, 11.21, 7.66 and 5.77 % in the gas phase for Case 1 through 4, respectively.

Geochemical effects of SO2 during CO2 storage in deep saline reservoir sandstones of Permian age (Rotliegend) – A modeling approach

Geochemical modeling was used to assess the impact of sulfur dioxide (SO2) in impure carbon dioxide (CO2) streams on fluid–rock interaction during storage in siliciclastic rocks of Permian age (Rotliegend). We focused on the impact of SO2 on the porosity evolution, as well as on the trapping of CO2 as solid mineral phase. Due to the lack of a validated approach to calculate SO2 solubility in highly saline brine at CO2 storage conditions, different calculation approaches were tested against experimental literature data. Our novel approach employs a pressure and temperature adjusted Henry's constant with salinity correction. Depending on the solubility calculation approach and on the consecutive reactions of dissolved SO2, different SO2 concentrations in the brine were calculated. Based on the results of these solubility calculations, a low and a high SO2 concentration case were defined for geochemical modeling to assess the impact of different SO2 concentrations on fluid–rock interactions in a CO2 storage reservoir. An exemplary Rotliegend sandstone composition was chosen reflecting compositions of potential target horizons in the North German Basin. Short-term dissolution of carbonates and, in the presence of SO2, subsequent precipitation of sulfur-bearing minerals results in a decrease in porosity. A precipitation of sulfates near the injection well may lower injectivity. With time, dissolution of alumosilicates further provides cations to the aqueous solution for a precipitation of secondary carbonates. The presence of SO2 modifies this long-term interplay between silicate and carbonate minerals. The higher the SO2 concentration in the brine, the lower the amounts of newly formed carbonates, i.e. the mineral trapping of CO2, and the lower the overall porosity decrease. Hence, the assessment of SO2 dissolution in the formation brine at subsurface conditions is crucial for predicting mineral reactions and porosity evolution during geological storage of impure CO2.

Magmatogene fluids of metal-bearing reefs in the Bushveld Complex, South Africa: Based on research data on fluid inclusions in quartz

Fluid inclusions in the Merensky Reef quartz and later pegmatite veins crosscutting the Platreef rocks of the Bushveld Complex are studied by a suite of advanced high-precision methods. Based on the conducted studies, we identify a few types of fluids, some having been separated during the crystallization of volatile matter-rich residual melt of original basic magma, while others are derivatives of later felsic (granite) melts that formed crosscutting veins in fully devitrified ultrabasic and basic rocks. The earliest fluid is captured by quartz in symplectitic intergrowths with intercumulus plagioclase from the Merensky Reef pyroxenite occurs as a homogenous dense dry reduced gas (CH4–N2 ± CO2) mixture separated from the aluminosilicate melt at 800–900°C and 3050 bar. The following heterophase highly concentrated fluids (60–80 wt % NaCl eq.) separated at over 550°C and below 3050 bar transport a large number of metals. Major saline components of such fluids included Na, K, Fe, Ca, and Mn chlorides, Ca and Na sulphates and carbonates. According to LA ICP-MS analysis data, inclusions of these fluids contain high concentrations of Fe, Cr, K, and Na at the level of a few wt % and also significant contents of Cu, Sn, Sb, Mo, Au, Ag, Bi, and Ni in a concentration range from a few to thousands of ppm. Relatively lower-temperature (much higher than 450°C) fluids accompanying the crystallization of crosscutting quartz–feldspar pegmatite veins at the Platreef are also highly concentrated (from 70–80% to 40–14 wt % NaCl eq.), oxidized and metal-bearing. High concentrations of metals such as Na, K, Ca, Mn, Fe, and Pb at the level of wt % and also Ni, Co, Cu, As, Mo, Sn, Sb, and Bi (1–500 ppm) in inclusions in quartz of later pegmatite veins suggest the possible participation of magmatogene fluids related to later felsic intrusions in the redistribution of primary magmatic concentrations of metals. The oxidation of reduced heterophase fluids may be the most important geochemical barrier invoking the crystallization of solid mineral phases from heterophase fluids.

Coupled pyrite concentration and sulfur isotopic insight into the paleo sulfate–methane transition zone (SMTZ) in the northern South China Sea

The sulfate–methane transition zone (SMTZ) is an important diagenetic redox boundary within marine sediments where the anaerobic oxidation of methane (AOM), coupled with bacterial sulfate reduction, can promote sulfur isotopic enrichments in several solid phase minerals including pyrite (FeS2). Authigenic pyrite can form in concentrated abundances within the SMTZ and as such, can be used as a proxy to identify paleo-SMTZs. This study uses enrichments in 34S and anomalously high abundances of authigenic pyrites in 287 samples from the northern South China Sea (SCS) to determine the paleo-SMTZ. The pyrite samples were collected from sediment cores acquired at three sites, each of which are known to be located in natural gas hydrate-bearing regions. We assess the relative abundances of authigenic pyrites, the types of pyrite morphologies recovered in the cored sediments, and the sulfur isotopic values of recovered pyrite samples using two methods: (1) handpicked sample analysis using a binocular microscope, and (2) the chromium reduction method. Our results show that pyrite concentrations and sulfur isotopic compositions exhibit synchronous fluctuations, particularly from 6.8 m below seafloor (mbsf) to 8.4 mbsf at all three study sites. There is a significant increase in the occurrence of rod-like pyrite morphology within this key interval. We define the position of the paleo-SMTZ by the presence of anomalously high accumulations of pyrites at greater than 5.0wt.% using the handpicking method or greater than 0.5wt.% via the chromium reduction method, along with positive Δδ34S excursions greater than 10.0‰ VCDT. We discovered a regional paleo-SMTZ that is shallower than the modern SMTZ, suggesting a previous period of elevated methane flux from depth, possibly related to widespread gas hydrate dissociation.

Gold deposition caused by carbonation of biotite during late-stage fluid flow

Alteration reactions associated with gold mineralisation can be used to elucidate the nature of the fluid that transported gold into a deposit. At the Junction gold deposit, Kambalda, Western Australia, gold is hosted in a metamorphosed and hydrothermally altered dolerite. Mineralisation at the deposit scale is associated with zones of K, CO2 and S metasomatism, as is common in many greenstone hosted gold deposits. However, at the thin-section scale gold is not closely associated with sulphide minerals but within zones of carbonate metasomatism and K-loss where pre-existing biotite has reacted to produce chlorite, muscovite and Fe–Mg carbonates. Gold precipitation is intimately associated with biotite breakdown where calcite is locally absent. Quantified mineral modes from detailed microstructural mapping are used to balance reactions describing the breakdown of biotite in the presence and absence of calcite. Using the basic assumption that Al is immobile during metasomatism the reactions are successfully balanced, even in a manifestly open system. Modelling of fluid–rock reactions using HCh constrains the fluid composition (0.11<X(CO2)<0.13) and fluid–rock ratios (<12:1) that can produce the observed mineral assemblage. Additional modelling of solid solution mineral phases using thermocalc estimates alteration conditions of 390°C, 4kbar and also suggests a fluid X(CO2)~0.1. Both these models show that the observed muscovite and chlorite compositions can be produced primarily through the removal of K from the measured precursor biotite. We show that it is not possible to transport and deposit all the gold observed in the alteration zone with the low fluid–rock ratios obtained from modelling of silicate alteration and inferred gold concentrations in these fluids. We suggest that this is typical of greenstone hosted gold deposits and that mechanisms other than aqueous solution, which can transport higher gold concentrations, must be considered.

Relationship between ash fusion temperatures of ashes from hard coal, brown coal, and biomass and mineral phases under different atmospheres: A combined FactSage™ computational and network theoretical approach

A broad set of ashes originating from national and international sources of hard coal, brown coal, and biomass is investigated under different ashing temperatures (815°C and 450/550°C) as well as oxidizing and reducing atmosphere conditions. In this computational study the mineral phase composition is estimated and the respective masses are not taken into account, since these approaches have been regularly less successful. The resulting mineral phases are assigned to main mineral groups and related to the experimentally determined ash fusion temperatures. On the one hand this relation is based on network theoretical considerations for the liquid slag and on the other it combines those with the fusion behavior of the residual solid mineral phases. Lowest ash fusion temperatures are addressed to the presence of low temperature melting mixtures of several mineral phases, which formed a very fragile network. Finally, the effects of different ashing temperatures and atmosphere conditions on the resulting mineral phases and respective ash fusion temperatures are studied.

Modelling CO2-brine-rock interactions in the Upper Paleozoic formations of Ordos Basin used for CO2 sequestration

Subsurface immobilization and conversion of CO2 into solid mineral phases in deep siliciclastic saline formations containing silicate minerals, commonly known as “mineral trapping”, is gaining research attention as a significant option to reduce CO2 emissions in the atmosphere. Although mineral trapping of CO2 is a long-term process, a combination of short-term results from both laboratory experiments and numerical simulations can lead to some general understanding of the required long-term CO2 sequestration mechanisms. This is a 100 year preliminary batch simulation study of four sandstone samples, under CO2 saturated water at 75 °C from the Upper Permian formations in the Ordos Basin, using the TOUGHREACT/ECO2N module to simulate the CO2-brine-rock interaction processes in deep siliciclastic multilayered saline aquifers. The samples approximately correspond to the four target saline formations selected by the Shenhua Group for a CO2 sequestration field demonstration project in the Ordos Basin, PR China. Preliminary simulation results show that the initial salinity of formation brine plays a significant role in determining the amount of CO2 that will be sequestered by solubility or mineral trapping in a deep saline aquifer. Minimal differences between experimental results and numerical calculation occur in low salinity waters, and significantly larger differences in high salinity waters, which is still under the maximum acceptable difference between experimental and computed data (10 %). The upper Liujiagou formation, with the highest level of salinity (ca. 88.7 g/L TDS) and lowest level of CO2 solubility, offers the highest mineral trapping capacity, with a maximum carbonate mineral storage of ca. 0.7 kg/m3 of bulk rock over a 100 year period. Regardless of the initial acidity or alkalinity of the aquifer brine, injection of CO2 will inflict a sudden drop in pH of the brine to acidity levels in a range of 3.0–4.6. The subsequent amount of dissolved and precipitated minerals, arising from the CO2-brine-rock interaction, is site specific and mainly dependent on initial aquifer mineralogy and brine composition.

Monitoring and assessment of surface water acidification following rewetting of oxidised acid sulfate soils

Large-scale exposure of acid sulfate soils during a hydrological drought in the Lower Lakes of South Australia resulted in acidification of surface water in several locations. Our aim was to describe the techniques used to monitor, assess and manage these acidification events using a field and laboratory dataset (n = 1,208) of acidic to circum-neutral pH water samples. The median pH of the acidified (pH < 6.5) samples was 3.8. Significant (p < 0.05) increases in soluble metals (Al, Co, Mn, Ni and Zn above guidelines for ecosystem protection), SO4 (from pyrite oxidation), Si (from aluminosilicate dissolution) and Ca (from carbonate dissolution and limestone addition), were observed under the acidic conditions. The log of the soluble metal concentrations, acidity and SO4/Cl ratio increased linearly with pH. The pH, alkalinity and acidity measurements were used to inform aerial limestone dosing events to neutralise acidic water. Field measurements correlated strongly with laboratory measurements for pH, alkalinity and conductivity (r (2) ≥ 0.97) but only moderately with acidity (r (2) = 0.54), which could be due to difficulties in determining the indicator-based field titration endpoint. Laboratory measured acidity correlated well with calculated acidity (r (2) = 0.87, acidity present as Al(III) >> H(+) ≈ Mn(II) > Fe(II/III)) but was about 20 % higher on average. Geochemical speciation calculations and XRD measurements indicated that solid phase minerals (schwertmannite and jarosite for Fe and jurbanite for Al) were likely controlling dissolved metal concentrations and influencing measured acidity between pH 2 and 5.

Applicability of Anaerobic Nitrate-Dependent Fe(II) Oxidation to Microbial Enhanced Oil Recovery (MEOR)

Microbial processes that produce solid-phase minerals could be judiciously applied to modify rock porosity with subsequent alteration and improvement of floodwater sweep in petroleum reservoirs. However, there has been little investigation of the application of this to enhanced oil recovery (EOR). Here, we investigate a unique approach of altering reservoir petrology through the biogenesis of authigenic rock minerals. This process is mediated by anaerobic chemolithotrophic nitrate-dependent Fe(II)-oxidizing microorganisms that precipitate iron minerals from the metabolism of soluble ferrous iron (Fe(2+)) coupled to the reduction of nitrate. This mineral biogenesis can result in pore restriction and reduced pore throat diameter. Advantageously and unlike biomass plugs, these biominerals are not susceptible to pressure or thermal degradation. Furthermore, they do not require continual substrate addition for maintenance. Our studies demonstrate that the biogenesis of insoluble iron minerals in packed-bed columns results in effective hydrology alteration and homogenization of heterogeneous flowpaths upon stimulated microbial Fe(2+) biooxidation. We also demonstrate almost 100% improvement in oil recovery from hydrocarbon-saturated packed-bed columns as a result of this metabolism. These studies represent a novel departure from traditional microbial EOR approaches and indicate the potential for nitrate-dependent Fe(2+) biooxidation to improve volumetric sweep efficiency and enhance both the quality and quantity of oil recovered.

A study of the chemical, mineralogical and physicochemical properties of peloids prepared with two medicinal mineral waters from Lanjarón Spa (Granada, Spain)

We studied the interstitial liquid phase (Liqint), the crystallinity of solid phase minerals (kaolinite and saponite), the cation exchange capacity (CEC) and the exchangeable bases of peloids prepared with two medicinal mineral waters (MMWs) from the springs of Lanjarón Spa (Granada, Spain). The waters employed were: “Salud V” (chloride-, sodium- and calcium-rich, bicarbonated, previously unpublished) and “El Salado” (chloride- and sodium rich, bicarbonate, calcic and ferruginous). The solid phase was a mixture of kaolin and bentonite (9:1, w:w) and maturation times of the peloids were 1, 3 and 6months.The physicochemical properties of the Liqint of the peloid differed from those of the MMW: pH increased (from 6 to 8, approximately), Fe content decreased dramatically (from 12 to 0.03mgl−1), while potassium and bicarbonates also decreased and concentrations of other ions such as carbonates, sodium, chlorides and, especially, sulphates, increased. The effect of maturation time on Liqint properties depended on the MMW employed.Kaolinite crystallinity in the peloid decreased during maturation (Hinckley Index from 0.71 to 0.52), reaching similar values in the peloids from both MMWs after 6months. On the other hand, the crystal perfection of saponite increased, revealing the existence of a process of crystallochemical aggradation towards mica due to the presence of exchangeable ions in Liqint, particularly potassium.There was a change in CEC and the exchangeable bases from the initial mineral material to the solid phase of the peloid. In the initial material the base sequence was Na+>K+~Mg2+>Ca2+ while in the mineral phase of most of the peloids it was Ca2+>Na+>Mg2+>K+. The relationship between the exchangeable bases and the Liqint was demonstrated.We demonstrate that the peloid controls and modifies the properties of the starting materials: MMW and minerals. Therefore a new system of liqint - exchangeable phase - mineral phase interrelationships appears.

Dielectric properties of a tilled sandy volcanic-vesuvian soil with moderate andic features

The apparent dielectric constant, ɛ, of many agricultural soils, measurable with the Time Domain Reflectometry (TDR) method, may be used to estimate soil water content, θ, with the Topp's empirical model. However, organic soils and those of volcanic origin do not obey this model, which has been termed “universal”. In particular, volcanic soils have singular physical properties in terms of bulk density, porosity, surface area, and variable charge essentially arising from the state of structural aggregation of particles of the solid mineral phase. After drying and/or intensive long-term tillage in arid and semi-arid climate, properties of these soils tend to change irreversibly. These modifications affect pore-sizes distribution and more stable larger size aggregates are formed. This in turn modifies porosity, surface area, variable charge, thus changing the dielectric characteristics and the relationship θ(ɛ) of the porous system. In this paper we compare the dielectric properties of two horizons of a volcanic-vesuvian soil profile. θ(ɛ) experimental relationships were measured in the laboratory on 16 undisturbed soil samples taken from Ap and Bw horizons of the soil profile. The subsoil Bw has kept its physical characteristics, typical of a volcanic soil having better expressed andic features. By contrast, topsoil Ap has been intensively cultivated and has altered its properties, partially losing its andic features. Physically based dielectric approaches explicitly taking into account the contribution of the changed properties are necessary for comparing the dielectric properties of the two soil horizons. The experimental θ(ɛ) were thus interpreted by using Maxwell–De Loor's approach and the so-called α-model which both consider soil as a multiphase system. The Topp's model was also used for evaluating its interpretative capability for the two soil horizons. By using the multiphase models in their three- and four-phases (including the effect of the bound-water) configurations on both the unaltered subsoil and the altered topsoil, it was possible separating the contribution of the different physical factors mainly involved in determining the singular dielectric properties of volcanic soils. In terms of model comparisons, we found no major differences between the three- and four-phase versions of the Maxwell–De Loor's and α-models, thus demonstrating the substantial irrelevance of the bound-water on the dielectric properties of the investigated volcanic soils. Besides, this confirms the strength of the completely physically based Maxwell–De Loor's model for interpreting soil dielectric properties without using any fitting parameters.

Pore-scale simulation of microbial growth using a genome-scale metabolic model: Implications for Darcy-scale reactive transport

Recent advances in microbiology have enabled the quantitative simulation of microbial metabolism and growth based on genome-scale characterization of metabolic pathways and fluxes. We have incorporated a genome-scale metabolic model of the iron-reducing bacteria Geobacter sulfurreducens into a pore-scale simulation of microbial growth based on coupling of iron reduction to oxidation of a soluble electron donor (acetate). In our model, fluid flow and solute transport is governed by a combination of the Navier–Stokes and advection–diffusion–reaction equations. Microbial growth occurs only on the surface of soil grains where solid-phase mineral iron oxides are available. Mass fluxes of chemical species associated with microbial growth are described by the genome-scale microbial model, implemented using a constraint-based metabolic model, and provide the Robin-type boundary condition for the advection–diffusion equation at soil grain surfaces.Conventional models of microbially-mediated subsurface reactions use a lumped reaction model that does not consider individual microbial reaction pathways, and describe reactions rates using empirically-derived rate formulations such as the Monod-type kinetics. We have used our pore-scale model to explore the relationship between genome-scale metabolic models and Monod-type formulations, and to assess the manifestation of pore-scale variability (microenvironments) in terms of apparent Darcy-scale microbial reaction rates. The genome-scale model predicted lower biomass yield, and different stoichiometry for iron consumption, in comparison to prior Monod formulations based on energetics considerations. We were able to fit an equivalent Monod model, by modifying the reaction stoichiometry and biomass yield coefficient, that could effectively match results of the genome-scale simulation of microbial behaviors under excess nutrient conditions, but predictions of the fitted Monod model deviated from those of the genome-scale model under conditions in which one or more nutrients were limiting.The fitted Monod kinetic model was also applied at the Darcy scale; that is, to simulate average reaction processes at the scale of the entire pore-scale model domain. As we expected, even under excess nutrient conditions for which the Monod and genome-scale models predicted equal reaction rates at the pore scale, the Monod model over-predicted the rates of biomass growth and iron and acetate utilization when applied at the Darcy scale. This discrepancy is caused by an inherent assumption of perfect mixing over the Darcy-scale domain, which is clearly violated in the pore-scale models. These results help to explain the need to modify the flux constraint parameters in order to match observations in previous applications of the genome-scale model at larger scales. These results also motivate further investigation of quantitative multi-scale relationships between fundamental behavior at the pore scale (where genome-scale models are appropriately applied) and observed behavior at larger scales (where predictions of reactive transport phenomena are needed).

Arsenic pollution and fractionation in sediments and mine waste samples from different mine sites

A characterization of arsenic pollution and its associations with solid mineral phases in sediments and spoil heap samples from four different abandoned mines in Spain is performed. Three of them were mercury mines located in the same mining district, in the province of Asturias, and the other one, devoted to arsenic mining, is in the province of León. A sequential extraction procedure, especially developed for arsenic, was applied for the study of arsenic partitioning. Very high total arsenic concentrations ranging 300–67,000mg·kg−1 were found. Arsenic fractionation in each mine is broadly in accordance with the mineralogy of the area and the extent of the mine workings. In almost all the studied samples, arsenic appeared predominantly associated with iron oxyhydroxides, especially in the amorphous form. Sediments from cinnabar roasted piles showed a higher arsenic mobility as a consequence of an intense ore treatment, posing an evident risk of arsenic spread to the surroundings. Samples belonging to waste piles where the mining activity was less intense presented a higher proportion of arsenic associated with structural minerals. Nevertheless, it represents a long-term source of arsenic to the environment.

Seasonal Variations of Arsenic in the Ganges and Brahmaputra River, Bangladesh

Dissolved arsenic concentrations in the Ganges, Brahmaputra Rivers and confluence of these two rivers show important seasonal variations and maximum arsenic concentrations are observed during the monsoon season (July–October). These seasonal variations of dissolved arsenic concentrations were closely related to intense river-water discharge during the monsoon season with high arsenic-rich suspended particulate matter (SPM) loads. These arsenic-rich SPM mainly are primarily originated from erosion of agricultural land in upstream region irrigated with arsenic contaminated shallow groundwater and to some extent weathering of bed rocks. Considerable amount of iron and manganese enriched SPM adsorbs arsenic and increased water temperature in the summer accelerates microbially-mediated reduction of arsenic (V) to more soluble arsenic (III). Additionally, dissolution of solid arsenic-bearing mineral phases also attributes to high arsenic concentrations in water and causes seasonal variations. It is realized that the SPM of these two major rivers primarily controls the arsenic inputs into the Ganges-Brahmaputra-Meghna delta system. The cycling of arsenic in this delta is related to the monsoon seasonal dynamics, land use patterns and biogeochemical processes.Keywords: Arsenic; Ganges; Brahmaputra; Seasonal variation; Bangladesh.© 2012 JSR Publications. ISSN: 2070-0237 (Print); 2070-0245 (Online). All rights reserved.doi: http://dx.doi.org/10.3329/jsr.v4i1.7820J. Sci. Res. 4 (1), 65-75 (2012)

Physicochemical Heterogeneity Controls on Uranium Bioreduction Rates at the Field Scale

It has been demonstrated in laboratory systems that U(VI) can be reduced to immobile U(IV) by bacteria in natural environments. The ultimate efficacy of bioreduction at the field scale, however, is often challenging to quantify and depends on site characteristics. In this work, uranium bioreduction rates at the field scale are quantified, for the first time, using an integrated approach. The approach combines field data, inverse and forward hydrological and reactive transport modeling, and quantification of reduction rates at different spatial scales. The approach is used to explore the impact of local scale (tens of centimeters) parameters and processes on field scale (tens of meters) system responses to biostimulation treatments and the controls of physicochemical heterogeneity on bioreduction rates. Using the biostimulation experiments at the Department of Energy Old Rifle site, our results show that the spatial distribution of hydraulic conductivity and solid phase mineral (Fe(III)) play a critical role in determining the field-scale bioreduction rates. Due to the dependence on Fe-reducing bacteria, field-scale U(VI) bioreduction rates were found to be largely controlled by the abundance of Fe(III) minerals at the vicinity of the injection wells and by the presence of preferential flow paths connecting injection wells to down gradient Fe(III) abundant areas.