Dissolution Flux Research Articles

Stability analysis of dissolution-driven convection in porous media

We study the stability of dissolution-driven convection in the presence of a capillary transition zone and hydrodynamic dispersion in a saturated anisotropic porous medium, where the solute concentration is assumed to decay via a first-order chemical reaction. While the reaction enhances stability by consuming the solute, porous media anisotropy, hydrodynamic dispersion, and capillary transition zone destabilize the diffusive boundary layer that is unstably formed in a gravitational field. We perform linear stability analysis, based on the quasi-steady-state approximation, to assess critical times, critical wavenumbers, and neutral stability curves as a function of anisotropy ratio, dispersivity ratio, dispersion strength, material parameter, Bond number, Damköhler number, and Rayleigh number. The results show that the diffusive boundary layer becomes unstable in anisotropic porous media where both the capillary transition zone and dispersion are considered, even if the geochemical reaction is significantly large. Using direct numerical simulations, based on the finite difference method, we study the nonlinear dynamics of the system by examining dissolution flux, interaction of convective fingers, and flow topology. The results of nonlinear simulations confirm the predictions from the linear stability analysis and reveal that the fingering pattern is significantly influenced by combined effects of reaction, anisotropy, dispersion, and capillarity. Finally, we draw conclusions on implications of our results on carbon dioxide sequestration in deep saline aquifers.

Rate equations for sodium catalyzed amorphous silica dissolution

Newly measured amorphous silica dissolution rate data were combined with data from the literature to produce an equation that predicts the dissolution flux (J, mol/m2s) of amorphous silica as a function of temperature (T, K), sodium concentration (mNa+, molal), and hydrogen ion activity (aH+).J=14.6210-774002.303R1T+8.9510-777002.303R1TmNa+0.490aH+0.377The same data fit equally well to an equation that expresses the dissolution flux as a function of temperature, sodium concentration, and hydroxide ion activity (aOH−).J=14.6210-774002.303R1T+1.73×10610-777002.303R1TmNa+0.490aOH-0.377These equations have the same form as those for quartz (Rimstidt, 2015) and show that sodium ions have the same catalytic effect on amorphous silica dissolution as they do for quartz.

Face-Discriminating Dissolution Kinetics of Furosemide Single Crystals: In Situ Three-Dimensional Multi-Microscopy and Modeling

A versatile in situ multi-microscopy approach to study the dissolution kinetics of single crystals is described, using the loop diuretic drug furosemide as a testbed to demonstrate the utility of the approach. Using optical microscopy and scanning ion-conductance microscopy in combination, the dissolution rate of individual crystallographically independent crystal faces can be measured quantitatively while providing a direct visualization of the evolution of crystal morphology in real time in three dimensions. Finite element method models using experimental data enables quantitative analysis of dissolution fluxes for individual faces and determination of the limiting process—mass transport or interfacial kinetics—that regulates dissolution. A key feature of the approach is that isolated crystals (typically <60 μm largest characteristic dimension) in solution during dissolution experience high and well-defined diffusion rates. The ability to obtain this quantitative information for individual crystal faces...

Theoretical Analysis of Drug Dissolution: I. Solubility and Intrinsic Dissolution Rate

The first-principles approach presented in this work combines surface kinetics and convective diffusion modeling applied to compounds with pH-dependent solubility and in different dissolution media. This analysis is based on experimental data available for approximately 100 compounds of pharmaceutical interest. Overall, there is a linear relationship between the drug solubility and intrinsic dissolution rate expressed through the total kinetic coefficient of dissolution and dimensionless numbers defining the mass transfer regime. The contribution of surface kinetics appears to be significant constituting on average ∼20% resistance to the dissolution flux in the compendial rotating disk apparatus at 100 rpm. The surface kinetics contribution becomes more dominant under conditions of fast laminar or turbulent flows or in cases when the surface kinetic coefficient may decrease as a function of solution composition or pH. Limitations of the well-known convective diffusion equation for rotating disk by Levich are examined using direct computational modeling with simultaneous dissociation and acid–base reactions in which intrinsic dissolution rate is strongly dependent on pH profile and solution ionic strength. It is shown that concept of diffusion boundary layer does not strictly apply for reacting/interacting species and that thin-film diffusion models cannot be used quantitatively in general case.

A theoretical and numerical framework for modeling gypsum cavity dissolution

SummaryIn this paper, a large‐scale diffuse interface model is used to describe the evolution of a gypsum cavity formation induced by dissolution. The method is based upon the assumption of a pseudo‐component dissolving with a thermodynamic equilibrium boundary condition. A methodology is proposed based on numerical computations with fixed boundaries in order to choose suitable parameters for the diffuse interface model, and hence predict the correct dissolution fluxes and surface recession velocity. Additional simulations were performed to check which type of momentum balance equation should be used. The numerical results did not show a strong impact of this choice for the typical initial boundary value problems under consideration. Calculations with a variable density and Boussinesq approximation were also performed to evaluate the potential for natural convection. The results showed that the impact of density driven flows was negligible in the cases under investigation. The potential of the methodology is illustrated on two large‐scale configurations: one corresponding to a gypsum lens located strictly within a porous rock formation and the other to an isolated pillar in a flooded gypsum room and pillar quarry. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Two‐phase convective CO2 dissolution in saline aquifers

AbstractGeologic carbon storage in deep saline aquifers is a promising technology for reducing anthropogenic emissions into the atmosphere. Dissolution of injected CO2 into resident brines is one of the primary trapping mechanisms generally considered necessary to provide long‐term storage security. Given that diffusion of CO2 in brine is woefully slow, convective dissolution, driven by a small increase in brine density with CO2 saturation, is considered to be the primary mechanism of dissolution trapping. Previous studies of convective dissolution have typically only considered the convective process in the single‐phase region below the capillary transition zone and have either ignored the overlying two‐phase region where dissolution actually takes place or replaced it with a virtual region with reduced or enhanced constant permeability. Our objective is to improve estimates of the long‐term dissolution flux of CO2 into brine by including the capillary transition zone in two‐phase model simulations. In the fully two‐phase model, there is a capillary transition zone above the brine‐saturated region over which the brine saturation decreases with increasing elevation. Our two‐phase simulations show that the dissolution flux obtained by assuming a brine‐saturated, single‐phase porous region with a closed upper boundary is recovered in the limit of vanishing entry pressure and capillary transition zone. For typical finite entry pressures and capillary transition zone, however, convection currents penetrate into the two‐phase region. This removes the mass transfer limitation of the diffusive boundary layer and enhances the convective dissolution flux of CO2 more than 3 times above the rate assuming single‐phase conditions.

One-Dimensional Pit Experiments and Modeling to Determine Critical Factors for Pit Stability and Repassivation

Experiments on stainless steel artificial pit electrodes in sodium chloride were used to inform a diffusion model developed based on the mass transport behavior within a one-dimensional corroding pit. Measurable estimates of the dissolution flux as well as the potential describing the conditions of interest were obtained from experiment as the one-dimensional pit stability product under a salt film (i·x)saltfilm and the repassivation potential Erp, respectively. These parameter estimates were acquired as a function of pit depth and were related to the concentration of the metal cation at the corroding surface at each depth via a one-dimensional mass transport model. These results allowed for the construction of a quantitative framework relating the various electrochemical parameters describing the transition from pit stability to repassivation. Such an analysis permitted the straightforward estimate of the critical surface concentration associated with this transition, which resulted in a single conservative lower bound of 50% of the saturation concentration for the minimum aggressive chemistry to sustain stable pitting and prevent repassivation. Along with published data, these results were utilized to advance the idea that the critical pit solution chemistry is independent of bulk chloride concentration up to 4 M, a range frequently encountered in atmospheric conditions.

Amazon River dissolved load: temporal dynamics and annual budget from the Andes to the ocean.

The aim of the present study is to estimate the export fluxes of major dissolved species at the scale of the Amazon basin, to identify the main parameters controlling their spatial distribution and to identify the role of discharge variability in the variability of the total dissolved solid (TDS) flux through the hydrological cycle. Data are compiled from the monthly hydrochemistry and daily discharge database of the "Programa Climatologico y Hidrologico de la Cuenca Amazonica de Bolivia" (PHICAB) and the HYBAM observatories from 34 stations distributed over the Amazon basin (for the 1983-1992 and 2000-2012 periods, respectively). This paper consists of a first global observation of the fluxes and temporal dynamics of each geomorphological domain of the Amazon basin. Based on mean interannual monthly flux calculations, we estimated that the Amazon basin delivered approximately 272 × 10(6)tyear(-1) (263-278) of TDS during the 2003-2012 period, which represents approximately 7% of the continental inputs to the oceans. This flux is mainly made up by HCO3, Ca and SiO2, reflecting the preferential contributions of carbonate and silicate chemical weathering to the Amazon River Basin. The main tributaries contributing to the TDS flux are the Marañon and Ucayali Rivers (approximately 50% of the TDS production over 14% of the Amazon basin area) due to the weathering of carbonates and evaporites drained by their Andean tributaries. An Andes-sedimentary area-shield TDS flux (and specific flux) gradient is observed throughout the basin and is first explained by the TDS concentration contrast between these domains, rather than variability in runoff. This observation highlights that, under tropical context, the weathering flux repartition is primarily controlled by the geomorphological/geological setting and confirms that sedimentary areas are currently active in terms of the production of dissolved load. The log relationships of concentration vs discharge have been characterized over all the studied stations and for all elements. The analysis of the slope of the relationship within the selected contexts reveals that the variability in TDS flux is mainly controlled by the discharge variability throughout the hydrological year. At the outlet of the basin, a clockwise hysteresis is observed for TDS concentration and is mainly controlled by Ca and HCO3 hysteresis, highlighting the need for a sampling strategy with a monthly frequency to accurately determine the TDS fluxes of the basin. The evaporite dissolution flux tends to be constant, whereas dissolved load fluxes released from other sources (silicate weathering, carbonate weathering, biological and/or atmospheric inputs) are mainly driven by variability in discharge. These results suggest that past and further climate variability had or will have a direct impact on the variability of dissolved fluxes in the Amazon. Further studies need to be performed to better understand the processes controlling the dynamics of weathering fluxes and their applicability to present-day concentration-discharge relationships at longer timescales.

Intermediate-Scale Investigation of Enhanced-Solubilization Agents on the Dissolution and Removal of a Multicomponent Dense Nonaqueous Phase Liquid (DNAPL) Source

The presence of multicomponent nonaqueous phase liquid (NAPL) source zones in the subsurface can significantly complicate remediation efforts, transport predictions, and the development of accurate risk assessments. A series of flow-cell experiments was conducted to investigate the effectiveness of two different enhanced-solubilization agents for the removal of a multicomponent dense nonaqueous phase liquid (DNAPL) source zone from homogeneous porous media. The source zone consisted of an equal 1:1:1 mole mixture of cis-1,2-dichloroethene (DCE), trichloroethene (TCE), and tetrachloroethene (PCE) with NAPL saturation (Sn) targeted between 8 and 14 %. Solutions (5 wt%) of hydroxypropyl-β-cyclodextrin (HPCD) and sodium dodecyl sulfate (SDS) were flushed through the flow-cell system until nearly complete contaminant removal was achieved. Analysis of elution curves indicate that SDS was approximately 10 times more efficient at removing all three components from the system compared to HPCD. Although enhancement factor magnitudes vary for each specific contaminant component and enhanced-solubilization agent, the lowest-solubility contaminant component (i.e., PCE) consistently experienced the greatest relative solubility enhancement during flushing. SDS was generally superior when evaluated on a recovery basis; however, HPCD outperformed SDS for all contaminant components when compared based on moles-contaminant to moles-reagent removal efficiency analysis. Contaminant mass flux reduction analysis showed that enhanced-solubilization flushing (HPCD and SDS) resulted in general inefficient contaminant removal behavior. Raoult’s Law could be used to successfully predict aqueous contaminant concentrations from the multicomponent DNAPL source zone, indicating that dissolution processes were relatively ideal during both HPCD and SDS enhanced-solubilization flushing. These findings suggest that multicomponent NAPL source dissolution and removal depend upon the flushing agent itself and of the solubility and properties of the individual components of the NAPL mixture. The selection of a particular enhanced-flushing agent should be evaluated carefully prior to remediation as the dissolution, removal, and mass flux behavior of each component can vary significantly.

Rate equations for sodium catalyzed quartz dissolution

Quartz dissolution rate data were fit to an equation that predicts the dissolution flux (J, mol/m2sec) as a function of temperature (T, K), sodium concentration (mNa+, molal), and hydrogen ion activity (aH+). J=0.64610-748002.303R1T+6.6410-880002.303R1TmNa+0.331aH+0.441The same data fit equally well to an equation that expresses the rate as a function of temperature, sodium concentration, and hydroxide ion activity (aOH-). J=0.64610-748002.303R1T+1120010-716002.303R1TmNa+0.326aOH-0.441These equations are more convenient to use than those given by Bickmore et al. (2008) because rates can be predicted without the implementation of a surface speciation model. They predict that at 25°C quartz dissolves more than 200 times faster in seawater than in pure water.These two equations fit the data just as well as five other equations from Bickmore et al. (2008) that are based on surface species concentrations. All of these rate equations contain information about the reaction mechanism(s) for quartz dissolution but that information is ambiguous because the independent variables used to develop the equations are correlated. This means that rate equations alone cannot be used to infer the dissolution mechanism. Existing surface complexation, surface charge, terrace-ledge-kink, and Lewis acid-base models must be modified and amalgamated in order to develop a reliable model of the reaction mechanism(s).

Mcb (Mass and Charge Balance) Model Simulation of Corrosion of Co-Cr Alloy Stellite-6

Understanding of oxide formation and growth on an alloy is very important in predicting its corrosion behaviour. Several models for oxide growth as well as metal dissolution kinetics in the presence of an oxide layer have been developed. These models focus on describing the transport of charged species through the oxide film and the electrochemical reactions at the metal/oxide and oxide/solution interfaces, although the detailed description and formulation of these processes vary. However, these models do not consider the type of oxide that can form. Consequently, the models have limited capability of predicting changes in the corrosion rate with time as corrosion progresses, or the dependence of corrosion kinetics on the solution redox conditions. Recently, we have developed a corrosion model that can predict the rates of metal oxidation, oxide growth and dissolution simultaneously as a function of time. The model imposes reaction thermodynamics constraints, and mass and charge balance (MCB) requirements on corrosion reaction rates and hence is labelled the MCB model [1]. The mass and charge balance requirements dictate that the flux of metal cations created by oxidation of metal atoms at the m|ox interface (the oxidation flux) must be equal to the sum of the fluxes of the metal cations forming an oxide at the oxide/solution interface (the oxide formation flux) and the flux of metal cations dissolving into solution (the dissolution flux). The oxidation flux is calculated by using a modified Butler-Volmer equation with an effective overpotential that is defined as a function of the equilibrium potential of the metal oxidation and the potential drop across the oxide layer that is growing. Both the oxide formation dissolution fluxes have a first-order dependence on the oxidation flux. The first-order rate constant for the oxide formation follows an Arrhenius dependence with an activation energy that increases linearly with oxide thickness. The dissolution rate constant depends on surface hydration and the solution environment (pH and temperature), but is independent of the oxide thickness. Consequently, under constant solution conditions the rate constant for the oxide formation, kMO(t), changes with time as the oxide grows but the rate constant of dissolution, kdiss, is constant with time. Due to a mass balance constraint and competition between the two processes for the metal ions, the oxide formation and dissolution fluxes cannot vary independently. The fraction of the oxidation flux that leads to oxide formation or dissolution depends on their rate constants; fk-MO(t) = kMO(t)/(kMO(t) + kdiss) and fk-diss(t) = 1 - fk-MO(t), respectively. In this paper, we present MCB model simulation results of potentiostatic polarization experiments performed on Co-Cr alloy Stellite-6 [2]. The simulations results are compared with the experimental measurements of the corrosion current as a function of time and the final composition and structure of the oxide(s) that formed. In these simulations, the parameters such as rate constants, exchange current density and field strength (or specific potential drop) across an oxide layer were kept constant for a specific pH and temperature. The main rate parameter that varies with pH and temperature was . This rate constant ratio is higher under conditions which promote oxide formation over dissolution, such as high pHs where the solubility of metal cations is low. The MCB model with the same model parameters was then applied to different sets of experimental data which include measurements of corrosion potential as a function of time and determination of the amounts of dissolved metals in coupon corrosion tests conducted in sealed quartz vials. The excellent agreement between the model results and experimental data over a range of polarization potential, pH and temperature indicates that the MCB model is a valuable method for simulating time-dependent corrosion behaviour while an oxide film is changing.

Changes in dissolved iron deposition to the oceans driven by human activity: a 3-D global modelling study

Abstract. The global atmospheric iron (Fe) cycle is parameterized in the global 3-D chemical transport model TM4-ECPL to simulate the proton- and the organic ligand-promoted mineral-Fe dissolution as well as the aqueous-phase photochemical reactions between the oxidative states of Fe (III/II). Primary emissions of total (TFe) and dissolved (DFe) Fe associated with dust and combustion processes are also taken into account, with TFe mineral emissions calculated to amount to ~ 35 Tg-Fe yr−1 and TFe emissions from combustion sources of ~ 2 Tg-Fe yr−1. The model reasonably simulates the available Fe observations, supporting the reliability of the results of this study. Proton- and organic ligand-promoted Fe dissolution in present-day TM4-ECPL simulations is calculated to be ~ 0.175 Tg-Fe yr−1, approximately half of the calculated total primary DFe emissions from mineral and combustion sources in the model (~ 0.322 Tg-Fe yr−1). The atmospheric burden of DFe is calculated to be ~ 0.024 Tg-Fe. DFe deposition presents strong spatial and temporal variability with an annual flux of ~ 0.496 Tg-Fe yr−1, from which about 40 % (~ 0.191 Tg-Fe yr−1) is deposited over the ocean. The impact of air quality on Fe deposition is studied by performing sensitivity simulations using preindustrial (year 1850), present (year 2008) and future (year 2100) emission scenarios. These simulations indicate that about a 3 times increase in Fe dissolution may have occurred in the past 150 years due to increasing anthropogenic emissions and thus atmospheric acidity. Air-quality regulations of anthropogenic emissions are projected to decrease atmospheric acidity in the near future, reducing to about half the dust-Fe dissolution relative to the present day. The organic ligand contribution to Fe dissolution shows an inverse relationship to the atmospheric acidity, thus its importance has decreased since the preindustrial period but is projected to increase in the future. The calculated changes also show that the atmospheric DFe supply to the globe has more than doubled since the preindustrial period due to 8-fold increases in the primary non-dust emissions and about a 3-fold increase in the dust-Fe dissolution flux. However, in the future the DFe deposition flux is expected to decrease (by about 25 %) due to reductions in the primary non-dust emissions (about 15 %) and in the dust-Fe dissolution flux (about 55 %). The present level of atmospheric deposition of DFe over the global ocean is calculated to be about 3 times higher than for 1850 emissions, and about a 30 % decrease is projected for 2100 emissions. These changes are expected to impact most on the high-nutrient–low-chlorophyll oceanic regions.

A mechanistic model for oxide growth and dissolution during corrosion of Cr-containing alloys.

We have developed a corrosion model that can predict metal oxide growth and dissolution rates as a function of time for a range of solution conditions. Our model considers electrochemical reactions at the metal/oxide and oxide/solution interfaces, and the metal cation flux from the metal to the solution phase through a growing oxide layer, and formulates the key processes using classical chemical reaction rate or flux equations. The model imposes mass and charge balance and hence, is labeled as the Mass Charge Balance (MCB) model. Mass and charge balance dictate that at any given time the oxidation (or metal cation) flux must be equal to the sum of the oxide growth flux and the dissolution flux. For each redox reaction leading to the formation of a specific oxide, the metal oxidation flux is formulated using a modified Butler-Volmer equation with an oxide-thickness-dependent effective overpotential. The oxide growth and dissolution fluxes have a first-order dependence on the metal cation flux. The rate constant for oxide formation also follows an Arrhenius dependence on the potential drop across the oxide layer and hence decreases exponentially with oxide thickness. This model is able to predict the time-dependent potentiostatic corrosion behaviour of both pure iron, and Co-Cr and Fe-Ni-Cr alloys.

Experimental and Modeling Studies on Mass Transport and Electrochemical Factors Influencing Stainless Steel Pitting and Repassivation

Studies on the critical conditions describing pitting stability and repassivation have focused on either determining the dissolution flux and solution chemistry or the potential. Experimental and modeling techniques toward building a quantitative relationship among the critical conditions were performed in this study. The artificial pit (lead-in-pencil) electrode constructed from stainless steel wires of small diameter was used as the tool to obtain measurable estimates of the critical dissolution flux and potential in aqueous sodium chloride (NaCl) solution. Both estimates were obtained using a single experimental technique. Experimental results so obtained were utilized to develop a one-dimensional diffusional model with representative boundary conditions. Results from these analyses indicated that the critical surface concentration required for stable pitting (or to prevent repassivation) may be more dilute than previously accepted in the literature.

Seasonal evolution of net and regenerated silica production around a natural Fe-fertilized area in the Southern Ocean estimated with Si isotopic approaches

Abstract. A massive diatom bloom is observed each year in the surface waters of the naturally Fe-fertilized Kerguelen Plateau (Southern Ocean). We measured biogenic silica production and dissolution fluxes (ρSi and ρDiss, respectively) in the mixed layer in the vicinity of the Kerguelen Plateau during austral spring 2011 (KEOPS-2 cruise). We compare results from a high-nutrient low-chlorophyll reference station and stations with different degrees of iron enrichment and bloom conditions. Above the plateau biogenic ρSi are among the highest reported so far in the Southern Ocean (up to 47.9 mmol m−2 d−1). Although significant (10.2 mmol m−2 d−1 on average), ρDiss were generally much lower than production rates. Uptake ratios (ρSi : ρC and ρSi : ρN) confirm that diatoms strongly dominate primary production in this area. At the bloom onset, decreasing dissolution-to-production ratios (D : P) indicate that the remineralization of silica could sustain most of the low silicon uptake and that the system progressively shifts toward a silica production regime which must be mainly supported by new source of silicic acid. Moreover, by comparing results from the two KEOPS expeditions (spring 2011 and summer 2005), we suggest that there is a seasonal evolution of the processes decoupling Si and N cycles in the area. Indeed, the consumption of H4SiO4 standing stocks occurs only during the growing stage of the bloom when strong net silica production is observed, contributing to higher H4SiO4 depletion relative to NO3−. Then, the decoupling of H4SiO4 and NO3− is mainly controlled by the more efficient nitrogen recycling relative to Si. Gross Si : N uptake ratios were higher in the Fe-rich regions compared to the high-nutrient low-chlorophyll (HNLC) area, likely due to different diatom communities. This suggests that the diatom responses to natural Fe fertilization are more complex than previously thought, and that natural iron fertilization over long timescales does not necessarily decrease Si : N uptake ratios as suggested by the silicic acid leakage hypothesis. Finally, we propose the first seasonal estimate of the Si biogeochemical budget above the Kerguelen Plateau based on direct measurements. This study points out that naturally iron-fertilized areas of the Southern Ocean could sustain very high regimes of biogenic silica production, similar to those observed in highly productive upwelling systems.

Experimental study of density-driven convection effects on CO2 dissolution rate in formation water for geological storage

Permanent storage of CO2 in saline aquifers is known as one of the best methods for carbon dioxide sequestration. Understanding the role of convection in CO2 dissolution of saline aquifers – which affects the long-term fate of the injected CO2 and the security of storage – is extremely important. In this paper, two types of experiments are performed; first, visualization experiment in Hele–Shaw cell; and second, quantitative experiments at elevated pressure. These experiments are used to investigate the effect of Density-Driven Convection (DDC) on CO2 dissolution rate in saline aquifers. For this aim; first, visualization test is performed in a Hele–Shaw cell to observe convective instabilities and its effect on dissolution rate. Afterward, quantitative tests are performed, in which four parameters are calculated: Total Dissolution, Diffusion Dissolution (DD), Convection Dissolution (CD), and CO2 Convective Flux (FCO2). Through employing these parameters, the behavior of convection and its effects on CO2 dissolution rate based on the scaled experiments is shown, and two scaling relationships one between Rayleigh and Sherwood number and the other one between CO2 Convective Flux and Rayleigh number are developed. Such scaling relationships provide an estimation of the maximum convective dissolution of CO2 in saline aquifers which would be useful for assessing suitable CO2 storage sites.

Motile Geobacter dechlorinators migrate into a model source zone of trichloroethene dense non-aqueous phase liquid: Experimental evaluation and modeling

Microbial migration towards a trichloroethene (TCE) dense non-aqueous phase liquid (DNAPL) could facilitate the bioaugmentation of TCE DNAPL source zones. This study characterized the motility of the Geobacter dechlorinators in a TCE to cis-dichloroethene dechlorinating KB-1™ subculture. No chemotaxis towards or away from TCE was found using an agarose in-plug bridge method. A second experiment placed an inoculated aqueous layer on top of a sterile sand layer and showed that Geobacter migrated several centimeters in the sand layer in just 7days. A random motility coefficient for Geobacter in water of 0.24±0.02cm2·day−1 was fitted. A third experiment used a diffusion-cell setup with a 5.5cm central sand layer separating a DNAPL from an aqueous top layer as a model source zone to examine the effect of random motility on TCE DNAPL dissolution. With top layer inoculation, Geobacter quickly colonized the sand layer, thereby enhancing the initial TCE DNAPL dissolution flux. After 19days, the DNAPL dissolution enhancement was only 24% lower than with an homogenous inoculation of the sand layer. A diffusion-motility model was developed to describe dechlorination and migration in the diffusion-cells. This model suggested that the fast colonization of the sand layer by Geobacter was due to the combination of random motility and growth on TCE.

Experimental results and integrated modeling of bacterial growth on an insoluble hydrophobic substrate (phenanthrene).

Metabolism of a low-solubility substrate is limited by dissolution and availability and can hardly be determined. We developed a numerical model for simultaneously calculating dissolution kinetics of such substrates and their metabolism and microbial growth (Monod kinetics with decay) and tested it with three aerobic phenanthrene (PHE) degraders: Novosphingobium pentaromativorans US6-1, Sphingomonas sp. EPA505, and Sphingobium yanoikuyae B1. PHE was present as microcrystals, providing non-limiting conditions for growth. Total PHE and protein concentration were tracked over 6-12 days. The model was fitted to the test results for the rates of dissolution, metabolism, and growth. The strains showed similar efficiency, with vmax values of 12-18 g dw g(-1) d(-1), yields of 0.21 g g(-1), maximum growth rates of 2.5-3.8 d(-1), and decay rates of 0.04-0.05 d(-1). Sensitivity analysis with the model shows that (i) retention in crystals or NAPLs or by sequestration competes with biodegradation, (ii) bacterial growth conditions (dissolution flux and resulting chemical activity of substrate) are more relevant for the final state of the system than the initial biomass, and (iii) the desorption flux regulates the turnover in the presence of solid-state, sequestered (aged), or NAPL substrate sources.

Calcium carbonate dissolution in the upper 1000 m of the eastern North Atlantic

AbstractRecent analyses suggest that considerable CaCO3 dissolution may occur in the upper water column of the ocean (&lt; 1500 m). This study uses the distribution of particulate calcium from high‐resolution suspended matter sampling along the Climate Variability and Predictability/CO2 Repeat Hydrography A16N transect in 2003 to estimate CaCO3 dissolution in the top 1000 m of the North Atlantic. Dissolution rates were also approximated using changes in total alkalinity measurements along isopycnal surfaces. Water masses were found to be undersaturated with respect to aragonite at intermediate depths (400–1000 m) in the eastern tropical North Atlantic. The CaCO3 dissolution rate in this region is estimated to be 0.9 mmol CaCO3 m−2 d−1, indicating this region is a hotspot for upper water column CaCO3 dissolution compared to the Atlantic basin as a whole. Dissolution rates calculated from particulate calcium distributions outside of this region were significantly lower (0.2 mmol CaCO3 m−2 d−1) and are comparable to previous estimates of CaCO3 dissolution flux for the Atlantic Ocean. The magnitude of upper water column dissolution rates compared to measured surface ocean CaCO3 standing stocks suggests that biologically mediated CaCO3 dissolution may be occurring in the top 1000 m of the Atlantic.

Solutal-convection regimes in a two-dimensional porous medium

AbstractWe numerically characterize the temporal regimes for solutal convection from almost first contact to high dissolved solute concentration in a two-dimensional ideal porous layer for Rayleigh numbers $\mathcal{R}$ between $100$ and $5\times 10^4$. The lower boundary is impenetrable. The upper boundary is saturated with dissolved solute and either impermeable or partially permeable to fluid flow. In the impermeable case, initially there is pure diffusion of solute away from the upper boundary, followed by the birth and growth of convective fingers. Eventually fingers interact and merge, generating complex downwelling plumes. Once the inter-plume spacing is sufficient, small protoplumes reinitiate on the boundary layer and are swept into the primary plumes. The flow is now in a universal regime characterized by a constant (dimensionless) dissolution flux $F=0.017$ (the rate at which solute dissolves from the upper boundary). The horizontally averaged concentration profile stretches as a simple self-similar wedge beneath a diffusive horizontal boundary layer. Throughout, the plume width broadens proportionally to $\sqrt{t}$, where $t$ is (dimensionless) time. The above behaviour is parameter independent; the Rayleigh number only controls when transition occurs to a final $\mathcal{R}$-dependent shut-down regime. For the constant-flux and shut-down regimes, we rigourously derive upscaled equations connecting the horizontally averaged concentration, vertical advective flux and plume widths. These are partially complete; a universal expression for the plume width remains elusive. We complement these governing equations with phenomenological boundary conditions based on a marginally stable diffusive boundary layer at the top and zero advective flux at the bottom. Making appropriate approximations in each regime, we find good agreement between predictions from this model and simulated results for both solutal and thermal convection. In the partially permeable upper boundary case, fluid from the convecting layer can penetrate an overlying separate-phase-solute bearing layer where it immediately saturates. The regime diagram remains almost the same as for the impermeable case, but the dissolution flux is significantly augmented. Our work is motivated by dissolution of carbon dioxide relevant to geological storage, and we conclude with a simple flux parameterization for inclusion in gravity current models and suggest that the upscaled equations could lay the foundation for accurate inclusion of dissolution in reservoir simulators.