Effects Of Molecular Diffusion Research Articles

Non-monotonic effect of compaction on longitudinal dispersion coefficient of porous media

Utilizing the discrete element method and the pore network model, we numerically investigate the impact of compaction on the longitudinal dispersion coefficient of porous media. Notably, the dispersion coefficient exhibits a non-monotonic dependence on the degree of compaction, which is distinguished by the presence of three distinct regimes in the variation of dispersion coefficient. The non-monotonic variation of dispersion coefficient is attributed to the disparate effect of compaction on dispersion mechanisms. Specifically, the porous medium tightens with an increasing pressure load, reducing the effect of molecular diffusion that primarily governs at small Péclet numbers. On the other hand, heightened pressure loads enhance the heterogeneity of pore structures, resulting in increased disorder and a higher proportion of stagnant zones within porous media flow. These enhancements further strengthen mechanical dispersion and hold-up dispersion, respectively, both acting at higher Péclet numbers. It is crucial to highlight that hold-up dispersion is induced by the low-velocity regions in porous media flow, which differ fundamentally from zero-velocity regions (such as dead-ends or the interior of permeable grains) as described by the classical theory of dispersion. The competition between weakened molecular diffusion and enhanced hold-up dispersion and mechanical dispersion, together with the shift in the dominance of dispersion mechanisms across various Péclet numbers, results in multiple regimes in the variation of dispersion coefficients. Our study provides unique insights into structural design and modulation of the dispersion coefficient of porous materials.

A novel multiphase and multicomponent model for simulating molecular diffusion in shale oil reservoirs with complex fracture networks

Molecular diffusion is critical for enhanced oil recovery (EOR) in shale oil reservoirs with complex fracture networks. Understanding the influence of fractures on diffusive mass transfer is crucial for predicting oil recovery and remaining oil distribution. Diffusive mass transfer between fractures and matrix is critical in comprehensively and effectively simulating molecular diffusion. Resolution of matrix cells significantly affects diffusion accuracy at the fracture–matrix interface. Low resolution results in multiple fractures in the same matrix cell, leading to decreased precision in calculating mass transfer by conventional methods. To address this, a novel multiphase and multicomponent model is proposed. The new model integrating the consideration of fracture spacing modifies molecular diffusion transmissibility between fracture and matrix in an embedded discrete fracture model. The discretization employs the two-point flux approximation in the finite-volume method. Validation compares the coarser mesh to the finest grid as a reliable reference. Results show the proposed model accurately captures diffusive mass transfer in a coarser mesh. Modified models study molecular diffusion's effects on EOR in shale oil reservoirs with complex fracture networks by CO2 huff and puff. Results indicate that increasing injection rates cannot improve oil recovery under extremely low porosity and permeability. Molecular diffusion facilitates CO2 penetration into the formation. This expands the swept CO2 volume and increases both volume expansion and formation energy. In addition, the light and heavy components of the crude oil are diffused into the fractures and eventually produced, which reduces gas production in the case of diffusion.

Modified Reacting Solver: A Simplified Approach for Capturing the Molecular and Flow Diffusivities for the Nonpremixed MILD Flames.

The present study investigates the applicability of the inlet boundary species Lewis number (combined effect of molecular and flow diffusion) for the nonpremixed moderate and intense low oxygen dilution (MILD) flames. A modified reactive solver named modifiedReactingFoam is developed by including the enthalpy flux in the energy equation and using modified model constants in OpenFOAM. The present solver is tested on the delft-jet-in-hot-coflow burner operating under a moderate and intense low oxygen dilution combustion environment. Along with the flame with Reynolds number 4100, eight other jet-in-hot-coflow flames are simulated to test the capability of the present proposed solver. The main aim of the current work is to investigate the efficacy of the proposed solver in predicting the velocity field, temperatures, and flame lift-off height for the considered flames with a significant reduction in computational time. The predictions with the modified eddy dissipation concept model are improved. However, a significant deviation is still observed in the downstream direction of the burner. The numerical simulations are performed with methane Lewis numbers of 0.9-1.14 by keeping the respective constant Lewis numbers for the inlet boundary species. The modifiedReactingFoam predictions at a methane Lewis number of 1.12 are in very close agreement with the experimental results. The maximum deviation in lift-off heights is within ±3% of the experimental results. The present modified solver outperformed the other combustion models in the literature and reduced the computational time up to 10 times with a combination of DLBFoam compared to the inbuilt solver.

A multi-mechanism numerical simulation model for CO2-EOR and storage in fractured shale oil reservoirs

Under the policy background and advocacy of carbon capture, utilization, and storage (CCUS), CO2-EOR has become a promising direction in the shale oil reservoir industry. The multi-scale pore structure distribution and fracture structure lead to complex multiphase flow, comprehensively considering multiple mechanisms is crucial for development and CO2 storage in fractured shale reservoirs. In this paper, a multi-mechanism coupled model is developed by MATLAB. Compared to the traditional Eclipse 300 and MATLAB Reservoir Simulation Toolbox (MRST), this model considers the impact of pore structure on fluid phase behavior by the modified Peng–Robinson equation of state (PR-EOS), and the effect simultaneously radiate to Maxwell–Stefan (M–S) diffusion, stress sensitivity, the nano-confinement (N-C) effect. Moreover, a modified embedded discrete fracture model (EDFM) is used to model the complex fractures, which optimizes connection types and half-transmissibility calculation approaches between non-neighboring connections (NNCs). The full implicit equation adopts the finite volume method (FVM) and Newton–Raphson iteration for discretization and solution. The model verification with the Eclipse 300 and MRST is satisfactory. The results show that the interaction between the mechanisms significantly affects the production performance and storage characteristics. The effect of molecular diffusion may be overestimated in oil-dominated (liquid-dominated) shale reservoirs. The well spacing and injection gas rate are the most crucial factors affecting the production by sensitivity analysis. Moreover, the potential gas invasion risk is mentioned. This model provides a reliable theoretical basis for CO2-EOR and sequestration in shale oil reservoirs.

Optimization of flip angle and radiofrequency pulse phase to maximize steady-state magnetization in three-dimensional missing pulse steady-state free precession.

Missing pulse (MP) steady-state free precession (SSFP) is a magnetic resonance imaging (MRI) pulse sequence that is highly tolerant to the magnetic field inhomogeneity. In this study, optimal flip angle and radiofrequency (RF) phase scheduling in three-dimensional (3D) MP-SSFP is introduced to maximize the steady-state magnetization while keeping broadband excitation to cover widely distributed frequencies generated by inhomogeneous magnetic fields. Numerical optimization based on extended phase graph (EPG) simulation was performed to maximize the MP-SSFP steady-state magnetization. To limit the specific absorption rate (SAR) associated with the broadband excitation in 3D MP-SSFP, SAR constraint was introduced in the numerical optimization. Optimized flip angle and RF phase settings were experimentally tested by introducing a linear inhomogeneous magnetic field in a range of 10-20 mT/m and using a phantom with known T1/T2 relaxation and diffusion parameters at 3 T. The experimental results were validated through comparisons with EPG simulation. Image contrasts and molecular diffusion effects were investigated in in vivo human brain imaging with 3D MP-SSFP with the optimal flip angle and RF phase settings. In the phantom measurements, the optimal flip angle and RF phase settings improved the MP-SSFP steady-state magnetization/signal-to-noise ratio by up to 41% under the fixed SAR conditions, which matched well with EPG simulation results. In vivo brain imaging with the optimal RF pulse settings provided T2-like image contrasts. Diffusion effects were relatively minor with the linear inhomogeneous field of 10-20 mT/m for white and gray matter, but cerebrospinal fluid showed conspicuous signal intensity attenuation as the linear inhomogeneous field increased. Numerical optimization achieved significant improvement in the steady-state magnetization in MP-SSFP compared with the RF pulse settings used in previous studies. The proposed flip angle and RF phase optimization is promising to improve 3D MP-SSFP image quality for MRI in inhomogeneous magnetic fields.

Single-Scan Ultraselective NMR Experiments with Preserved Sensitivity.

Selective NMR experiments provide rapid access to important structural information, and are essential to tackle the analysis of large molecules and complex mixtures. Single-scan ultraselective experiments are particularly useful, as they can rapidly select signals that overlap with other signals. Here, we describe a novel type of single-scan ultraselective NMR experiments that is robust against the effects of translational molecular diffusion, and thus make it possible to improve significantly the sensitivity of the experiment. This will largely broaden the applicability of this powerful class of experiments.

Single‐Scan Ultraselective NMR Experiments with Preserved Sensitivity

AbstractSelective NMR experiments provide rapid access to important structural information, and are essential to tackle the analysis of large molecules and complex mixtures. Single‐scan ultraselective experiments are particularly useful, as they can rapidly select signals that overlap with other signals. Here, we describe a novel type of single‐scan ultraselective NMR experiments that is robust against the effects of translational molecular diffusion, and thus make it possible to improve significantly the sensitivity of the experiment. This will largely broaden the applicability of this powerful class of experiments.

Numerical study on three-stage ignition of dimethyl ether by hot air under engine-relevant conditions

Non-premixed combustion often occurs in practical engines, and it is affected by the coupling effects of chemical kinetics and transport. This study aims to elucidate the individual effect of chemical kinetics, molecular diffusion, and convective transport on non-premixed combustion. To this end, three types of reactive systems are investigated by numerical simulations considering detailed chemistry and transport: (1) thermochemical system: 0D homogeneous autoignition, (2) thermochemical-diffusive system: 1D non-premixed ignition in a static diffusion layer, (3) thermochemical-diffusive-convective system: 1D non-premixed ignition in a counterflow and 2D lifted flame in a coflow. The simulations are carried out for diluted dimethyl ether and hot air under engine-relevant conditions with a pressure of 40 atm and hot air temperatures of 700∼1500 K. First, homogeneous ignition process of DME/air premixture is investigated. It is found that, apart from the low- and high-temperature chemistry which are essential in the typical two-stage ignition, the intermediate-temperature chemistry can also play an important role, especially for slow reaction process in fuel rich regions. Then, the effects of thermochemical conditions and molecular diffusion are assessed for non-premixed ignition process in the 1D diffusion layer. The results show that, the reaction front always initiates from local autoignition in most reactive regions; then it propagates either in sequential auto-ignition mode or in diffusion-driven mode as a deflagration wave. With various thermochemical conditions, the chemical kinetics behave differently and produce complex multibrachial (tetrabrachial, pentabrachial and hexbrachial) structures during the reaction front propagation. Decreasing the diffusion layer thickness generally delays the reaction front initiation but enhances its transition into a diffusion-driven flame. Finally, it is shown that 1D diffusion layer simulations can qualitatively reproduce the complex multibrachial structures in 1D counterflow and 2D coflow at certain conditions. A regime diagram is proposed to separate the effects of chemical kinetics, molecular diffusion, and convective transport.

Time fractional analysis of generalized Casson fluid flow by YAC operator with the effects of slip wall, and chemical reaction in a porous medium

The key aim of the present work is to apply the Yang-Abdel-Cattani operator of fractional order derivative with Rabotnov exponential kernel to the mass-heat transfer of the generalized Casson fluid with generalized Fick's and Fourier's laws. The flow is analyzed in a porous medium under the effects of chemical molecular diffusivity, heat generation and slip wall condition on the fluid velocity. The medium through which the fluid is flowing is subjected to the effects of Newtonian heating. The YAC operator of fractional order can describe the generalized memory effects more suitably than the other fractional operators. So for the sake of a better interpretation of the rheological behavior of the Casson fluid we have used the fractional operator of YAC with Rabotnov exponential kernel. The technique of the Laplace transform is used to acquire the exact analytical solution of the problem in the Mittag-Leffler form. Through MathCAD software physical behaviors of various parameters are investigated on the heat, mass and velocity of the fluid during the flow in a porous medium.

Study on the laminar combustion characteristics and kinetic of IC8H18/NH3 premixed flames

To promote the transition to a carbon neutral society, it is necessary to conduct a comprehensive research on the utilization of ammonia contained in gasoline engine. In the present study, the laminar burning velocity (SL), Markstein length (Lb) and flame instability of isooctane/ammonia (IC8H18/NH3)-air mixtures were analyzed at the initial temperatures (T) of 400 K and 470 K, initial pressure (P) of 1–4 atm, ammonia mole fraction (XNH3) of 0%-70% and an equivalence ratio (ϕ) ranging from 0.8 to 1.3. Also, the kinetic mechanism of IC8H18/NH3 as proposed by Cai et al. [Combust. Flame, 162, 2015] was further optimized and compared with the mechanism of CRECK and Cai. The results show that the difference between the Cai and present mechanisms is largely attributable to the variation in sensitivity of NH3 oxidation-related reaction to SL. In addition, it is widely known that SL is dependent on the initial temperature and pressure. In order to improve the applicability of IC8H18/NH3 fuel mixtures, their temperature (α) and pressure (β) coefficients were determined given varying ϕ and XNH3. Meanwhile, a mathematical correlation for SL was also conducted. The analysis of Lb was also conducted to obtain the results showing that Lb decreases with ϕ increasing, except for NH3-air flame. Moreover, as XNH3 increases, Lb varies between lean and rich flames. For lean-flames, NH3 addition leads to a significant decrease in Lb, while for rich-flames, NH3 addition causes a marginal increase in Lb. All these are attributed to the variation in Zel’dovich number (Ze), effective Lewis number (Leeff) and flame thickness (δ). Furthermore, the instability of flame suggests that the addition of ammonia enhances the stability of the flame due to the reduced molecular diffusion effect. Moreover, an increase in XNH3 contributes only to a slight decrease of critical Peclect number (Pecr), and a sharp rise in the critical flame radius (Rcr) results from the increase of flame thickness (δ).

Molecular Diffusion in Nanoreactors' Pore Channel System: Measurement Techniques, Structural Regulation, and Catalytic Effects.

Nanoreactors, as a new class of materials with highly enriched and ordered pore channel structures, can achieve special catalytic effects by precisely identifying and controlling the molecular diffusion behavior within the ordered pore channel system. Nanoreactors-driven molecular diffusion within the ordered pore channels can be highly dependent on the local microenvironment in the nanoreactors' pore channel system. Although the diffusion process of molecules within the ordered pore channels of nanoreactors is crucial for the regulation of catalytic behaviors, it has not yet been as clearly elucidated as it deserves to be in this study. In this review, fundamental theory and measurement techniques for molecular diffusion in the pore channel system of nanoreactors are presented, structural regulation strategies of pore channel parameters for controlling molecular diffusion are discussed, and the effects of molecular diffusion in the pore channel system on catalytic reactivity and selectivity are further analyzed. This article attempts to further develop the underlying theory of molecular diffusion within the theoretical framework of nanoreactor-driven catalysis, and the proposed perspectives may contribute to the rational design of advanced catalytic materials and the precise control of complex catalytic kinetics.

Life Cycle Assessment of Improved Oil Recovery While Helping to Achieve Net Zero Emissions from Shale Reservoirs

Summary Shale reservoirs will help to meet oil demand that is forecasted to continue increasing for several years. Oil recovery from shales is low and has been reported to range between 5% and 10%. The objective of this paper is to show how oil recovery from shale can be improved while simultaneously reducing CO2 emissions, thus contributing to the goal of a net-zero future. The proposed methodology shows how oil recovery from shales can be increased while simultaneously storing CO2 in undepleted (as opposed to depleted) shale oil reservoirs and consequently contributing to a future with net-zero emissions. The methodology is developed with the use of reservoir simulation and is achieved by performing the following procedure: (1) Start huff ’n’ puff CO2 injection 2 or 3 years after the well goes on oil production so the shale reservoir is essentially undepleted, and (2) store CO2 gradually in the shale reservoir during the huff periods, and continuously once the huff ’n’ puff project is finalized. The simulation model includes a history match period with actual production data from a pilot horizontal well and a forecast period with huff ’n’ puff CO2 injection. Two cases, one with diffusion and one without diffusion, are conducted for evaluating the molecular diffusion effect. The initial reservoir pressure is never exceeded during the life cycle of the project as a safeguard against the possible creation of new fractures or reactivation of faults. Life cycle assessment (LCA) indicates that the ratio of cumulative stored CO2 to cumulative equivalent CO2 emissions during the project is approximately 31.3%, helping us consequently in the goal to eventually achieve a future with net-zero emissions. A careful literature survey indicates that the methodology proposed in this paper that includes enhanced oil recovery (EOR) by huff ’n’ puff CO2 injection and the simultaneous storage of CO2 in the shale reservoir is novel and has not been considered previously in geoscience or petroleum engineering literature.

Numerical study of different Pr number medium turbulent cross flow in staggered tube bundle

Tube bundle heat exchangers have been recognized as the most efficient means of thermal exchange. The staggered row method can enhance the performance of a heat exchanger. However, it results in a large pressure drop loss. As working media with high thermal conductivities, liquid metals can potentially replace conventional working media. In this study, for working fluids with low Prandtl number cross flowing through a staggered tube bundle, the turbulent Prandtl number model of cross flow was verified. The unsteady characteristics of the two working fluids traversing through the tube bundle were analyzed. The effects of molecular thermal diffusion on the total heat transfer capacity of the two working fluids during heat transfer were compared, along with the changes in the circumferential heat transfer characteristics of the tube bundles under different Reynolds numbers. The ζ factor was then proposed to analyze the non-uniformity in the heat transfer characteristics of a single tube. An empirical correlation describing the heat transfer non-uniformity at the fully developed region and inlet region and involving parameters such as working properties and flow state was proposed based on computational fluid dynamics data. Finally, the performance evaluation criteria(PEC) factor was used to analyze the comprehensive performance of the two heat transfer fluids. The results revealed that the comprehensive heat transfer performance of mercury was 60–70 times that of conventional working fluids.

The role of thermodiffusion in organic pollutant transport in landfill composite liner system

This paper presents a new analytical model for forecasting one-dimensional transient transport of organic pollutant in a typical landfill composite liner including a geomembrane (GMB), a geosynthetic clay liner (GCL) and a soil liner (SL). The model can consider the coupling effect of molecular diffusion, thermodiffusion, advection and linear adsorption processes. Zero concentration gradient condition is adopted at the bottom boundary. The separation of variables method and superposition principle are utilized to derive the solution. The solution is first validated against the results of experimental test and a numerical model. Then, the influence of thermodiffusion is investigated on pollutant migration in typical composite liner. The results reveal that thermodiffusion may significantly influence the pollutant migration. If the Soret coefficient exceeds 0.01 K−1, regardless of the combination conditions in terms of molecular diffusion, advection and adsorption, the influence of thermodiffusion generally cannot be ignored. The analytical model also can be utilized to design composite liner systems and to verify other numerical models.

Fundamental physics effects of background gas species and pressure on vapor plume structure and spatter entrainment in laser melting

First-principles modeling, based on the direct simulation Monte Carlo (DSMC) method, is applied to quantitatively explain the physics of stainless-steel (SS316L) vapor plumes and jets and powder particle entrainment under conditions relevant to laser welding and laser powder bed fusion additive manufacturing. The DSMC is a particle-based gas kinetic simulation approach, with few built-in assumptions, that resolves the Knudsen and diffusion layers in vapor microjets. The simulations are performed for different background gas species (argon, helium, and neon) at background gas pressures from 0.1 to 5 bar and molten pool temperatures from 3250 K to 4000 K. The simulations capture the evolution of the vapor jet structure from unsteady to steady-state vapor flow, the transition from subsonic flow at low surface temperatures to supersonic flow at surface temperatures close to boiling, as well as the effects of the background gas species and pressure. The simulations show that the vapor jet structure, as well as the magnitude and direction of the entrained gas close to the laser spot, greatly affects the process environment, creating particle spatter and denudation around the laser track. High pressure and lighter background gas can decrease the amount of spatter. At high pressure, the evaporation is reduced, which forms a low-velocity subsonic plume. For lighter gas species, the gas-entrainment velocity increases while the jet radius decreases which forces entrained particles to merge with the melt pool. The powder particle simulations reveal that the width of the denudation zone strongly depends on particle diameter and increases at low pressure and lighter background gas. This study emphasizes that the ambient gas species and the pressure in the build chamber should be treated as process parameters that are as important as controlling laser power and scan speed when trying to prevent defects. • Kinetic modeling reveals strong effect of molecular diffusion in AM vapor microjets. • The laser power and surface temperature stronlgy affect the transition from subsonic to supersonic vapor jet. • Ambient gas chamber species and pressure strongly affect the dynamics (gas, powder) near the laser spot. d. • Strong gas flow (helium) does not necessarily create more spatter as these merge with the melt pool and disappear. • The width of the vapor plume is more strongly dependent on the ambient gas pressure than on the laser beam size.

Analytical solution for solute transport in a triple liner under non-isothermal conditions

Analytical solutions for the transport of contaminant through a 1D triple-layer composite liner system consisting of a geomembrane (GMB), a geosynthetic clay liner (GCL) and a compacted clay liner (CCL) under non-isothermal conditions are derived using the generalized integral transform technique. The proposed analytical solutions account for the combined effects of molecular diffusion, sorption and thermodiffusion as well as the temperature-dependent distribution coefficient and effective diffusion coefficient. The proposed analytical solutions are successfully validated against the experimental results of thermodiffusion tests and verified against an analytical solution available in the literature and a numerical model based on commercial finite element software. Using the verified analytical solutions, simulations are performed with representative geometry and material properties for a GMB/GCL/CCL liner system. The results indicate that neglecting the effect of non-isothermal condition can underestimate the benzene outflow rate by over 30% when the temperature difference between the surface and bottom of the liner system exceeds 10 K. The temperature-dependent CCL effective diffusion coefficient and CCL distribution coefficient have a significant effect on benzene transport through the GMB/GCL/CCL liner system, whereas the liner thermal conductivity and the temperature-dependent GCL effective diffusion coefficient and GCL distribution coefficient have a negligible effect.

Uncertainty effect of CO2 molecular diffusion on oil recovery and gas storage in underground formations

CO2 molecular diffusion in underground reservoir fluids, quantified by diffusion (D) coefficient, is of great importance for CO2 storage and enhanced oil recovery (EOR) projects. The reported CO2 D coefficients values are uncertain due to difficulty of accurately measuring this parameter specifically at high pressure-high temperature conditions. Hence, this study aims to investigate the impact of the uncertainty in the value of CO2 D coefficient on both oil recovery and CO2 storage during intermittent CO2 assisted gravity drainage (CO2-GAGD) injection in oil reservoirs. The impact of the CO2 diffusion on oil compositional changes and fluid distribution in porous medium was also investigated here. To do so, using data from a Malaysian oil field, a tuned equation of state along with a composition simulation model were used to emphasis on the effect of diffusion during the intermittent CO2-GAGD process. The effects of reservoir permeability, reservoir porosity, injection rate, and soaking time on oil recovery and gas storage were tested across different values of CO2 D coefficient. Including the gas diffusion in the simulation resulted in enhancement of the gas penetration though the porous medium leading to the enhanced oil recovery especially at the final soaking-injection/production cycles of the intermitted CO2 injection. The lowest impact of the uncertainty in the CO2 D coefficient on the oil recovery and gas storage efficiency was found in the high permeability case. Additionally, decreasing the gas injection rate or increasing the soaking time magnified the impact of the uncertainty of the CO2 D coefficient. Furthermore, the CO2 storage efficiency was found to be highly sensitive to the changes in the D coefficient in the high porosity case. To verify the results, the obtained simulation results were compared and found to be consistent with previous findings reported in the literature. This study adds accuracy to predicting CO2 EOR and storage projects through understanding the uncertainty of molecular diffusivity.

Mixing in small scale fluidic systems swayed by rotationality effects

In the present endeavor, we discuss the enhancement strategy of important fluidic functionality, i.e., mixing in an on-chip device embedded in a rotating disk both qualitatively as well as quantitatively. Our analysis, on accounting for the effect of rotation in the framework, uses a set of mechanically consistent classical fluid dynamic equations in describing the mixing of the constituent fluids comprehensively. Motivated by the need of benchmarking our modeling framework, we perform experiments in the limiting case of pure diffusion and show that suggestions from the experimental part of this endeavor verify the numerical results quite effectively. The results indicate that the effect of molecular diffusion and rotation-induced forcing non-trivially modulates the underlying mixing in the portable fluidic device. Of particular interest, we show that, even for weak molecular diffusion between the chosen fluid pair, strong advective transport of species as rendered by a higher rotational effect results in an enhanced mixing, that too achievable at short distances from the channel entry. Finally, a phase diagram mapping the mixing efficiency in the flow-fluid properties plane is provided, expected to be a design guideline for the portable fluidic systems/devices, typically used for mixing and diagnosis of bio-fluids.

Effects of Epiphytic Biofilm Activity on the Photosynthetic Activity, pH and Inorganic Carbon Microenvironment of Seagrass Leaves (Zostera marina L.)

Epiphytic biofilms on seagrass leaves can lead to extreme microenvironmental conditions for the encapsulated leaf limiting both its photosynthesis and respiration. Yet, little is known about how the biological activity of the biofilm itself changes the seagrass phyllosphere microenvironment and dynamics. We used microsensors to measure O2 concentrations and pH gradients and calculate fluxes of O2, CO2 and bicarbonate (HCO3−) around seagrass leaves (Z. marina L.) covered with artificial, inactive biofilms and natural epiphytic biofilms. A sterilized seawater-agar matrix was used to make an artificial “inactive” biofilm on seagrass leaves with the same thickness as the natural leaf epiphytic biofilm, which impeded turbulent exchange of gases but did not have microbial activity. We compared the concentration profiles and fluxes of O2 and inorganic carbon of the “active” and “inactive” biofilm to investigate the effect of microbial activity and molecular diffusion in seagrass leaf biofilms. In light, the O2 flux of leaves with inactive biofilm was only 31% of the leaves with active biofilm, indicating that the photosynthesis of the microbial community in the biofilm makes up the majority of O2 production in the leaf microenvironment. During darkness, the O2 concentration profiles and O2 fluxes were almost identical in the “active” and “inactive” biofilms. The pH profiles showed the same trend with an increase in pH of ~1.0 in the “active” biofilms and ~0.3 pH units in the “inactive” biofilms in the light, and both showing a decrease of ~0.3 pH units in darkness compared to the bulk seawater. Our measurements thus demonstrate strong photosynthesis in the epiphyte layer driving phyllosphere basification and inorganic carbon limitation. The calculated CO2 concentration on the leaf surface decreased to 0.09 μmol L-1 in the epiphytic biofilm in the light compared to leaf surface CO2 concentrations of 13.8 μmol L-1 on bare seagrass leaves, and the CO2 influxes were only 3.0% and 5.4% of O2 effluxes for leaves with “active” and “inactive” biofilm, respectively. Calculations also showed that HCO3− influxes in light accounted for 91-97% of the total inorganic carbon influx to the seagrass leaf, although the HCO3− utilization via CO2 concentration mechanisms is energy-consuming. Besides increasing mass transfer impedance, leaf epiphytic biofilm activity thus strongly affects the seagrass leaf microenvironment in the light by inducing higher O2 concentration and pH, increasing CO2 limitation and reducing the leaf photosynthetic efficiency.

A novel analytical model of solute transport in a layered aquifer system with mixing processes in the reservoirs.

Analytical models of solute transport have been widely used to aid the understanding of the physical and chemical processes undergone by substances introduced in a layered aquifer system. However, in previous studies, the advection component of transport was assumed to be one dimensional, while also ignoring the mixing processes that occur in the inlet and the outlet reservoirs. In this study, new sets of models describing those mixing processes are presented. Beyond that, these models were integrated into already existing models and the result is a novel analytical model of solute transport in aquifer-aquitard systems. The novel analytical solution was derived by the Laplace transform method and the finite-cosine Fourier transform method under the mobile-immobile (MIM) framework. The calculations take into account: the longitudinal and vertical dispersion, the molecular diffusion and the horizonal and vertical advection components of solute transport, as well as first-order chemical reaction, in both the aquifer and the aquitard. A finite-difference solution of the model is tested against experimental data in order to critique its reliability. Results indicate that the numerical and analytical solutions of the new model match well with experimental data. This new model outperforms the previous models in terms of interpreting experimental data. The mixing old and new water in the reservoirs during solute transport in aquifer-aquitard systems is important. Global sensitivity analysis demonstrates that the output concentration of solute in the aquifer-aquitard system is most sensitive to the volume of water in the inlet reservoir. The contribution of the molecular diffusion effect to the total mass flux of the tracer cross the aquifer-aquitard interface is much smaller than the contribution of the dispersive and advective effects.