Diffusive Boundary Layer Thickness Research Articles

In situ measurement of estrogenic activity in various aquatic systems using organic diffusive gradients in thin-film coupled with ERE-CALUX bioassay

Organic-diffusive gradients in thin-film samplers (o-DGT), were developed and applied for accumulation of estrogen and estrogen-like compounds on a XAD18 resin and deployed in situ in the effluents of Beijing Gaobeidian Wastewater Treatment Plant (GWWTP) and Brussels North Wastewater Treatment Plant as well as in several aquatic systems in Belgium, including the Zenne River, the Belgian Oostende Harbor and the North Sea. Estrogenic compounds accumulate on the XAD18 resin and the estrogenic activity of the resin extract was measured with the Estrogen Responsive Elements-Chemically Activated LUciferase gene eXpression (ERE-CALUX) bioassay. With this result and by applying Fick's diffusion law, it is possible to calculate the estrogenic activity in the aquatic system, if the diffusion boundary layer (DBL) is known or negligible compared to the hydrogel diffusive layer thickness. The DBL thickness in our study varied from 0.010 to 0.023 cm and ignoring the DBL thickness would for instance, underestimate the estrogenic activity by 10–20%. Estrogenic activities in the secondary effluent of GWWTP were the highest (29 ± 4 ng E2-equivalents L−1), while the lowest level was found at the Belgian Oostende Harbor (0.05 ± 0.01 ng E2-equivalents L−1). Comparable estrogenic activities in water samples measured by o-DGT and grab sampling were obtained, confirming that o-DGT can be efficiently used in various aquatic systems. The advantage of our sampling and measuring method is that very low, time averaged estrogenic activities can be determined, with a minimum of sample treatment. The risk of sample contamination is very low as well as the cost of the whole analytical procedure.

Hydrodynamic exposure and time since application influence endothall amine potency against submersed aquatic plants

The amine formulation of endothall is a herbicide that may be applied to flowing irrigation canals to control submerged aquatic weeds and algae. We undertook a bioassay experiment to simulate the effects of water movement and increasing distance away from an application point on control of aquatic weeds. We exposed two submerged aquatic plants (Elodea canadensis and Vallisneria australis) to endothall amine solutions. This was done a) with solutions that were aged for 0, 12, 24 and 48 h prior to plant exposure, to simulate the effect of distance downstream of the application point, and b) in three hydrodynamic environments: quiescent, low-flow and high-flow. Endothall amine was less potent on both species as the solution aged, such that potency was generally reduced from 12 h after application. The concentration of endothall acid did not decrease in the exposure tanks during the experiment, suggesting that there is a change of the endothall amine molecule as it ages in water, which reduces the herbicide’s potency. Endothall amine efficacy was greater for both species when plants were exposed in moving water compared to quiescent water. We propose that the increased efficacy in moving water is caused by a reduction in the thickness of the aqueous diffusive boundary layer in the hydrodynamic environment, thus increasing the rate of endothall amine diffusion into the plant tissue. This information can be used to improve management of submersed aquatic weeds with endothall amine and to understand its potential impact on non-target aquatic plants as herbicide-treated water moves away from the area of application.

Controlling the diffusive boundary layer thickness above the sediment–water interface in a thermostated rotating‐disk reactor

AbstractThe diffusive boundary layer (DBL) is a thin layer of fluid at the interface with a solid surface in which frictional forces cause molecular diffusion to become the dominant mode of solute transport. The thickness of the DBL is a function of the nature and roughness of sediment substrates, as well as the bottom‐current speed. In low‐energy natural aquatic environments, such as abyssal plains or lakes, the thickness of the DBL can reach several millimeters and significantly impede the diffusive rate of solutes through the sediment–water interface (SWI). Thus, precisely reproducing the DBL in the laboratory is required to simulate benthic diffusive fluxes similar to those encountered in situ. Yet, an experimental apparatus allowing precise control of the DBL thickness at the SWI in the laboratory has not been described in the literature. Here, we present a simple temperature‐controlled rotating‐disk system, which is suitable for the use with natural sediments and that is capable of generating thick DBLs. Water overlying the rotating sediment can be sampled discretely or continuously to monitor the chemical reaction progress. We tested the validity of the reactor by dissolving a foraminiferal sand bed in natural seawater. We find that (1) measured dissolution fluxes agree with those predicted by theory and (2) the dissolution of calcite in these seafloor‐like hydrodynamic conditions is controlled by mass transfer across the DBL above the bed. Guidelines for best practices under various experimental conditions, possible future developments, and the theoretical equations to compute the DBL thickness and diffusive fluxes in this reactor are described.

Electric field strength on MHD aluminum alloys (AA7075) nanofluid flow

This work concerns with a numerical study on heat transfer by MHD aluminum alloys nanofluid (AA7075-water) flow over a nonlinear wall for extensible compactness of electric field. The steady governing momentum and energy equations are transformed to similarity equations for certain families of the controlling parameters. The transformed equations of momentum and thermal transport are solved by applying Runge–Kutta Fehlberg method with shooting technique. The three classical form of nanoparticle shapes are registered into report, i.e., sphere, cylinder and lamina and the heat transfer by natural convection of AA7075-water was studied. It is observed that the thermal and diffusive boundary layer thickness of AA7075-water is weaker in the presence of electric field, E1 = 0.2 as compared to E1 = 0.0. In the presence of electric field, temperature and concentration of lamina shape AA7075 particles hit a dominant role on AA7075-water nanofluid flow regime.

Abiotic and biotic interactions in the diffusive boundary layer of kelp blades create a potential refuge from ocean acidification

Abstract Seaweeds are able to modify the chemical environment at their surface, in a micro‐zone called the diffusive boundary layer (DBL), via their metabolic processes controlled by light intensity. Depending on the thickness of the DBL, sessile invertebrates such as calcifying bryozoans or tube‐forming polychaetes living on the surface of the blades can be affected by the chemical variations occurring in this microlayer. Especially in the context of ocean acidification (OA), these microhabitats might be considered as a refuge from lower pH, because during the day photosynthesis temporarily raises the pH to values higher than in the mainstream seawater. We assessed the thickness and the characteristics of the DBL at two pH levels (today's average surface ocean pH 8.1 and a reduced pH predicted for the end of the century, pH 7.7) and seawater flows (slow, 0.5 and fast, >8 cm/s) on Ecklonia radiata (kelp) blades. Oxygen and pH profiles from the blade surface to the mainstream seawater were measured with O2 and pH microsensors for both bare blades and blades colonized by the bryozoan Membranipora membranacea. The DBL was thicker in slow flow compared with fast flow and the presence of bryozoans increased the DBL thickness and shaped the DBL gradient in dark conditions. Net production was increased in the low pH condition, increasing the amount of oxygen in the DBL in both bare and epiphytized blades. This increase drove the daily pH fluctuations at the blade surface, shifting them towards higher values compared with today's pH. The presence of bryozoans led to lower oxygen concentrations in the DBL and more complex pH fluctuations at the blade surface, particularly at pH 7.7. Overall, this study, based on microprofiles, shows that, in slow flow, DBL microenvironments at the surface of the kelps may constitute a refuge from OA with pH values higher than those of the mainstream seawater. For calcifying organisms, it could also represent training ground for harsh conditions, with broad daily pH and oxygen fluctuations. These chemical microenvironments, biologically shaped by the macrophytes, are of great interest for the resilience of coastal ecosystems in the context of global change. A plain language summary is available for this article.

Vermetid gastropods modify physical and chemical conditions above coral–algal interactions

Interaction modifications can arise when a third species alters the physical and chemical environment within which two other species interact. On coral reefs, corals and algae commonly interact amid a suite of other species that may modify their interaction. Massive Porites coral and algal turfs often are covered by mucus nets cast by the vermetid gastropod, Ceraesignum maximum. Previously, vermetid mucus nets have been shown to have deleterious effects on corals. Here, we hypothesized that vermetids not only have direct effects on coral, but they also change the local physical and chemical environment establishing the potential for interaction modifications by intensifying the effects of algae on corals. To test this, we examined the effect of vermetids on physical and chemical aspects of the environments. We quantified light penetration, water flow, diffusive boundary layer (DBL) thickness, and oxygen concentrations in the presence and absence of vermetid nets. Vermetid nets did not affect light levels. Because we observed reduced water flow and increased DBL thickness in the presence of nets, we also expected to observe high oxygen concentration over coral surfaces. Instead, we observed no difference in oxygen concentrations in the presence of mucus nets. To explain the lower than expected oxygen concentrations, we hypothesize that nets decreased photosynthesis and/or increased respiration of corals and algae and their associated microbiota. This is the first study to explore mechanisms underlying the deleterious effects of vermetids on corals, and shows that vermetid mucus nets may modify coral-algal interactions by intensifying physical and chemical conditions.

Effects of bioirrigation on the spatial and temporal dynamics of oxygen above the sediment–water interface

AbstractBurrow ventilation by tube-dwelling benthic animals affects solute exchange between sediments and water by 2 means. Drawing of O2-rich water into the burrow increases O2 availability in the sediment and stimulates biogeochemical and microbial processes, whereas flushing of the burrow creates a 3-dimensional flow field above the burrow, which induces mixing. Previous studies have revealed the role of the diffusive boundary layer (DBL) thickness on the exchange of solutes between the sediment and overlying water. Mapping the O2 gradient within the DBL is a challenging task in the presence of benthic faunal activities. We used a novel lifetime-based laser induced fluorescence (τLIF) technique that enables unobstructed observations of spatial and temporal O2 dynamics above burrows inhabited by midge larvae (Chironomus plumosus). We observed instantaneous plumes of O2-depleted water released from the outlet of the burrows and drawdown of O2-rich water above the inlet caused by peristaltic pumping of C....

Local supersaturation and the growth of protective scales during CO2 corrosion of steel: Effect of pH and solution flow

By correlating in-situ synchrotron X-ray diffraction measurements with electrochemical measurements using a rotating disc electrode, we demonstrate the critical dependence on the local supersaturation of the kinetics of formation of a protective crystalline scale on the surface of carbon steel during CO2 corrosion in brine at elevated temperature. We show that the total current is the sum of a current due to dissolution of iron and a current due to growth of a crystalline layer. We show that the dissolution current and the surface supersaturation are controlled by the thickness of an initially-formed amorphous layer. As in earlier work at room temperature, we infer that the amorphous layer dissolves as a carbonato-iron complex with surface concentration of the dissolving species determined by the electrode potential, and speculate on the importance of the chemistry of this dissolution reaction in determining the corrosion result. We construct a simple transport-reaction model, which shows that the supersaturation is determined by the precipitation rate constant of colloidal FeCO3 and by the product of the current for Fe dissolution and the diffusion boundary layer thickness. Using this model, we show crystal growth rate varying quadratically with supersaturation at pH 6.8 and linearly at pH 7.3. The effects of electrode potential, surface roughness, microstructure and flow are simply to change supersaturation by changing the current density per unit projected area flowing through the amorphous initially formed layer. Variation of brine concentration has no effect. We illustrate the sensitivity to solution flow of the crystallinity of the final scale. We show that siderite is the first crystalline product and that chukanovite follows, with a delay time that decreases with increasing pH. The ratio of chukanovite to siderite is low at sufficiently high pH and increases with decreasing pH, possibly through a maximum. From the results, we advance ideas concerning the importance of local microenvironments and local fluctuations in mass-transport rate.

Increased sediment oxygen flux in lakes and reservoirs: The impact of hypolimnetic oxygenation

AbstractHypolimnetic oxygenation is an increasingly common lake management strategy for mitigating hypoxia/anoxia and associated deleterious effects on water quality. A common effect of oxygenation is increased oxygen consumption in the hypolimnion and predicting the magnitude of this increase is the crux of effective oxygenation system design. Simultaneous measurements of sediment oxygen flux (JO2) and turbulence in the bottom boundary layer of two oxygenated lakes were used to investigate the impact of oxygenation on JO2. Oxygenation increased JO2 in both lakes by increasing the bulk oxygen concentration, which in turn steepens the diffusive gradient across the diffusive boundary layer. At high flow rates, the diffusive boundary layer thickness decreased as well. A transect along one of the lakes showed JO2 to be spatially quite variable, with near‐field and far‐field JO2 differing by a factor of 4. Using these in situ measurements, physical models of interfacial flux were compared to microprofile‐derived JO2 to determine which models adequately predict JO2 in oxygenated lakes. Models based on friction velocity, turbulence dissipation rate, and the integral scale of turbulence agreed with microprofile‐derived JO2 in both lakes. These models could potentially be used to predict oxygenation‐induced oxygen flux and improve oxygenation system design methods for a broad range of reservoir systems.

A Study on the Measurement Method for Benthic Nutrient Flux in Freshwater Sediments

Accurate measurement of benthic nutrient fluxes (BNF) is a prerequisite for evaluating the effect of sediments on nutrient cycle in the surface water. The intact sediment cores were collected in July 2015 at the midstream of Nakdong River. We identified pre-incubation time (6, 12, 24 hr), dissolved oxygen concentration (90, 70, 50% saturation), diffusive boundary layer thickness (0, 0.6-0.8, 1.2-1.4 mm), and incubation temperature (10, 17, 20, 25℃) as the most important control factors, and measured the BNF fluctuation with the variation of these factors using the laboratory sediment core incubation method. Since the chemical composition, redox condition, hydrodynamic regimes and microbial activities at the sediment-water interface were changed as a result of the alteration of control factors, sediment core incubation should be conducted under as close to the natural conditions of study site as possible, in order to produce the results similar to actual values. Relative percentage differences between two replicates were below 20% in most control factors, which showed satisfactory precision for strict compliance with the experimental conditions and procedures. In the further studies, we will compare the results of core incubation with those of in situ measurements to confirm the accuracy of the sediment core incubation method. Key words: Inorganic Nutrient, Benthic Flux, Sediment Core Incubation, Freshwater Sediments

Sediment Resuspension and Deposition on Seagrass Leaves Impedes Internal Plant Aeration and Promotes Phytotoxic H2S Intrusion.

HIGHLIGHTS: Sedimentation of fine sediment particles onto seagrass leaves severely hampers the plants' performance in both light and darkness, due to inadequate internal plant aeration and intrusion of phytotoxic H2S.Anthropogenic activities leading to sediment re-suspension can have adverse effects on adjacent seagrass meadows, owing to reduced light availability and the settling of suspended particles onto seagrass leaves potentially impeding gas exchange with the surrounding water. We used microsensors to determine O2 fluxes and diffusive boundary layer (DBL) thickness on leaves of the seagrass Zostera muelleri with and without fine sediment particles, and combined these laboratory measurements with in situ microsensor measurements of tissue O2 and H2S concentrations. Net photosynthesis rates in leaves with fine sediment particles were down to ~20% of controls without particles, and the compensation photon irradiance increased from a span of 20–53 to 109–145 μmol photons m−2 s−1. An ~2.5-fold thicker DBL around leaves with fine sediment particles impeded O2 influx into the leaves during darkness. In situ leaf meristematic O2 concentrations of plants exposed to fine sediment particles were lower than in control plants and exhibited long time periods of complete meristematic anoxia during night-time. Insufficient internal aeration resulted in H2S intrusion into the leaf meristematic tissues when exposed to sediment resuspension even at relatively high night-time water-column O2 concentrations. Fine sediment particles that settle on seagrass leaves thus negatively affect internal tissue aeration and thereby the plants' resilience against H2S intrusion.

Transient modeling of spent nuclear fuel electrorefining with liquid metal electrode

During the molten salt electrorefining of spent nuclear fuel, multiple phases such as oxide, solid metal, liquid metal, and molten salt often co-exist. Computational modeling can be a useful tool for understanding the reaction mechanism across the multiple phases. The new model has been developed and applied to a lab-scale electrorefining with liquid metal anode and solid cathode LiCl-KCl molten salt. The benchmark study predicts anodic dissolution and cathodic deposition of U and Pu with minor disagreements. In particular, the on-set of Pu deposition on the surface of the solid cathode is well estimated, which is important for the quality of U ingot and the safeguards of process. The underestimation of U deposition (∼6%) and the overestimation of Pu dissolution (∼7%) at the end of simulation are explained by unconsidered reaction species such as Np and Am from the liquid Cd anode, which overestimates the dissolution of Pu from the anode compared to the measured data. The sensitivity study also reveals the transition behaviors of electrochemical reactions for U and Pu on the solid cathode are changed by diffusion boundary layer thickness, transfer coefficients, and the difference of electrochemical potentials more sensitively than those of the liquid metal anode. For this specific experiment case, the thinner diffusion boundary layer improves the prediction of cathodic reactions particularly at the end of electrorefining.

Diffusion or advection? Mass transfer and complex boundary layer landscapes of the brown alga Fucus vesiculosus.

The role of hyaline hairs on the thallus of brown algae in the genus Fucus is long debated and several functions have been proposed. We used a novel motorized set-up for two-dimensional and three-dimensional mapping with O2 microsensors to investigate the spatial heterogeneity of the diffusive boundary layer (DBL) and O2 flux around single and multiple tufts of hyaline hairs on the thallus of Fucus vesiculosus. Flow was a major determinant of DBL thickness, where higher flow decreased DBL thickness and increased O2 flux between the algal thallus and the surrounding seawater. However, the topography of the DBL varied and did not directly follow the contour of the underlying thallus. Areas around single tufts of hyaline hairs exhibited a more complex mass-transfer boundary layer, showing both increased and decreased thickness when compared with areas over smooth thallus surfaces. Over thallus areas with several hyaline hair tufts, the overall effect was an apparent increase in the boundary layer thickness. We also found indications for advective O2 transport driven by pressure gradients or vortex shedding downstream from dense tufts of hyaline hairs that could alleviate local mass-transfer resistances. Mass-transfer dynamics around hyaline hair tufts are thus more complex than hitherto assumed and may have important implications for algal physiology and plant-microbe interactions.

Scaling oxygen microprofiles at the sediment interface of deep stratified waters

AbstractDissolved oxygen microprofiles at the sediment‐water interface of Lake Geneva were measured concurrently with velocities 0.25 to 2 m above the sediment. The measurements and scaling analyses indicate dissolved oxygen fluctuations and turbulent fluxes in exceedance of molecular diffusion in the proximity of the sediment‐water interface. The measurements allowed the parameterization of the turbulent diffusion as a function of the dimensionless height above the sediment and the turbulence above the sediment‐water interface. Turbulent diffusion depended strongly on the friction velocity and differed from formulations reported in the literature that are based on concepts of turbulent and developed wall‐bounded flows. The dissolved oxygen microprofiles and proposed parameterization of turbulent diffusion enable a foundation for the similarity scaling of oxygen microprofiles in proximity to the sediment. The proposed scaling allows the estimation of diffusive boundary layer thickness, oxygen flux, and oxygen microprofile distribution in the near‐sediment boundary layer.

Jumping and overcoming diffusion limitation of nutrient uptake in the photosynthetic ciliate Mesodinium rubrum

AbstractFast, frequent, and spontaneous jumping by the photosynthetic ciliate Mesodinium rubrum is highly adaptive for overcoming diffusion limitation of nutrient uptake. Using high‐speed, high‐magnification, digital imaging, jumping behaviors of an Antarctic and temperate North American strain of M. rubrum were investigated. Both strains displayed multiple‐beat long jumps, wherein maximum jump speeds and total jump durations varied significantly with strain, temperature, and illumination. However, jump distances were surprisingly similar with means approximating six body lengths, which were just above the thickness of nutrient diffusive boundary layer surrounding the cell. Total jump durations scaled by diffusion time scale were < 1 for almost all observed jumps. Moreover, jump distances and square roots of pre‐jump residence times were linearly correlated. Thereby, jumping by M. rubrum is physically constrained by the small‐scale advection‐diffusion physics of the cell's immediately surrounding water. Additionally, to achieve similar jump distances as the temperate strain at warm temperature, the Antarctic strain lengthened each beat cycle of cilia to kinematically compensate low jump speed at cold temperature, but the ratio of power to recovery stroke duration (∼ 6 : 1) remained unchanged with temperature. As further shown by computational fluid dynamics simulations driven by empirical data, multiple‐beat long jumping allows both strains to completely detach the boundary layer and achieve similar Sherwood numbers significantly > 1, despite substantially different jumping kinematics. All these results support the notion that jumping is an essential behavior for M. rubrum to enhance nutrient uptake, and help to explain their high photosynthetic rates and ecological success.

Development and Calibration of an Organic-Diffusive Gradients in Thin Films Aquatic Passive Sampler for a Diverse Suite of Polar Organic Contaminants.

A unique configuration of the diffusive gradients in thin films sampler for polar organics (o-DGT) without a poly(ether sulfone) membrane was developed, calibrated, and field-evaluated. Diffusion coefficients (D) through agarose diffusive gels ranged from (1.02 to 4.74) × 10-6 cm2/s for 34 pharmaceuticals and pesticides at 5, 13, and 23 °C. Analyte-specific diffusion-temperature plots produced linear (r2 > 0.85) empirical relationships whereby D could be estimated at any environmentally relevant temperature (i.e., matched to in situ water conditions). Linear uptake for all analytes was observed in a static renewal calibration experiment over 25 days except for three macrolide antibiotics, which reached saturation at 300 ng (≈15 d). Experimental sampling rates ranged from 8.8 to 16.1 mL/d and were successfully estimated with measured and modeled D within 19% and 30% average relative error, respectively. Under slow flowing (2.4 cm/s) and static conditions, the in situ diffusive boundary layer (DBL) thickness ranged from 0.023 to 0.075 cm, resulting in a maximum contribution to mass transfer of <45%. Estimated water concentrations by o-DGT at a wastewater treatment plant agreed well with grab samples and appeared to be less influenced by the boundary layer compared to that of polar organic chemical integrative samplers (POCIS) deployed simultaneously. The o-DGT sampler is a promising monitoring tool that is largely insensitive to the DBL under typical flow conditions, facilitating the application of measured/modeled diffusion-based sampling rates. This significantly reduces the need for sampler calibration, making o-DGT more widely applicable, reliable, and cost-effective compared to current polar passive samplers.

Near-surface physics during convection affecting air-water gas transfer

The gas flux at the water surface is affected by physical processes including turbulence from wind shear, microscale wave breaking, large-scale breaking, and convection due to heat loss at the surface. The main route in the parameterizations of the gas flux has been to use the wind speed as a proxy for the gas flux velocity, indirectly taking into account the dependency of the wind shear and the wave processes. The interest in the contributions from convection processes has increased as the gas flux from inland waters (with typically lower wind and sheltered conditions) now is believed to play a substantial role in the air-water gas flux budget. The gas flux is enhanced by convection through the mixing of the mixed layer as well as by decreasing the diffusive boundary layer thickness. The direct numerical simulations performed in this study are shown to be a valuable tool to enhance the understanding of this flow configuration often present in nature.

Study effects of the kinetics of local composition diffusion on the melt pool coolability by developing a DBL-LIVE model

The LIVE-L4 test was conducted to investigate the transient and steady state behavior of the molten pool and the crust influenced by different heat generation rate. In previous work, a simple novel model of the LIVE code was developed to simulate the entire process of the LIVE-L4 test after the melt of KNO3NaNO3 poured into the test vessel. The LIVE code is a transient code and can be used as a fast computational program to simulate the LIVE tests. Calculation results indicated that the LIVE code could generally predict the main parameters of the melt and crust well during the LIVE-L4 test.However, the LIVE code could not predict some processes accurately, such as the early phase of the test after the melt poured into the test vessel, and heat flux distribution at small polar angle, due to no considering the composition change of local melt during the crust formation, which affected the properties of the local melt adjacent to the crust, especially the liquidus temperature of the local melt at the interface of melt and crust. Therefore, considering the effects of the crust growth rate and the diffusion of concentrated NaNO3 on the composition change of the local melt, the diffusion boundary layer (DBL) model was developed and combined with the LIVE code model to consider the composition change of the local melt during the crust formation, with the assumption of considering a quasi-stable plane front solidification of the melt. The DBL-LIVE code was used to calculate the characteristics of the melt and the crust during the heating phase 18 kW. The calculation results indicated that the DBL-LIVE code could predict the main parameters of the melt and crust much more accurately. It could also predict the transient and steady characteristics of the melt composition. Moreover, the thickness of the diffusion boundary layer has a marked influence on the composition distribution. Therefore, the sensitivity analysis of the DBL thickness was also conducted in this work.

Determination of mineral dissolution regimes using flow-through time-resolved analysis (FT-TRA) and numerical simulation

Flow-through time resolved analysis (FT-TRA) involves subjecting small mineral samples (<10mg) inserted in a miniature flow-through cell (50μL) to controlled flows of eluent analyzed on-line by ICP-MS. In this study, FT-TRA is used to empirically determine the dissolution regimes for the two well-studied minerals forsterite and calcite, representing minerals with relatively slow and fast dissolution kinetics. A proportional increase in steady-state effluent [Mg, Si] concentrations with increasing flow-through cell eluent residence times confirms a dominantly surface-controlled dissolution regime for a powdered forsterite sample at pH2.3, implying that transport limitations are negligible. In contrast, the relationship between flow rates and dissolution rates for single grain calcite samples at pH2.3–4 reveals that transport limitations affect the rate of calcite dissolution. To provide a quantitative and process-based assessment of the effect of diffusive transport limitations, simulations of the calcite experiments were performed with a high resolution, pore-scale model that considers the geometry of the calcite grain and the FT-TRA flow-through reactor. The pore-scale model reproduces the observed effluent [Ca] concentrations for all experimental conditions using a single set of surface kinetic parameters, by accounting for the formation of a diffusive boundary layer (DBL) that varies in thickness as a function of flow rates. These results demonstrate that combining FT-TRA with pore-scale modeling makes it possible to obtain unprecedented insights not achievable by either method separately, including quantification of DBL thicknesses and the determination of transport controls as a function of pH, flow velocity and residence times.

Scaling relationships for diffusive boundary layer thickness and diffusive flux based on in situ measurements in coastal seas

In situ measurements of the diffusive boundary layer (DBL) and bottom boundary layer (BBL) under different dynamic and oxygen environments in three coastal seas are analyzed. Previous scaling methods for the DBL thickness (δDBL) are summarized. Three methods that lead to consistent dimensions at both sides of the derived relationships have all been rooted in the Batchelor length scale. The method representing the Batchelor length scale as a function of flow speed (U) is found to be the most appropriate for scaling δDBL when the law of wall applies. Diffusive flux is controlled by the dynamic-forced δDBL and the difference in oxygen concentration over the DBL (ΔC). Values of ΔC could be scaled using the oxygen concentration of the BBL (CBBL) and the normalized benthic temperature. An effective method is developed for scaling the diffusive flux based on measurements of benthic temperature, salinity, U, CBBL, and the estimation of bottom roughness. The scaling of δDBL based mainly on U and the scaling of diffusive flux well fit data from the three sites, despite their distinct differences in dynamic and oxygen environments.