Apparent Mass Transfer Coefficient Research Articles

Reduction in Apparent Permeability Owing to Surface Precipitation of Solutes by Drying Process and Its Effect on Geological Disposal

Disposal tunnels in geological repositories are ventilated continuously for over 50 years until their closure. Under these conditions, an unsaturated zone of mixed liquid and gas phases forms around the tunnels. Moreover, drying is assumed to progress from the host rock to the tunnels. To understand these drying processes, this study investigated the migration and precipitation of solutes via capillary forces during drying in packed columns using silica sand or glass beads as packed layers and X-ray CT analysis. In addition, the apparent permeability of a column packed with silica sand containing precipitation was examined using a flow experiment. The results indicate that the precipitation and accumulation of solutes were significant near the drying surfaces of the columns. The apparent mass transfer coefficient at a relatively early stage of the drying process indicates that the migration rate of solutes depends strongly on the capillary forces during the drying process. Furthermore, the apparent permeability of the columns with precipitation decreased significantly. These indicate that the precipitation and accumulation of solutes with drying in the groundwater reduce the porosity and permeability, and the advection of groundwater around the repository may be suppressed.

Vanadium-Doped Ti4O7 Porous Transport Layers for Efficient Electrochemical Oxidation of Industrial Wastewater Contaminants

Inexpensive electrode materials and effective cell designs are needed to advance electrochemical technologies for the oxidative treatment of wastewater. Novel vanadium-doped Ti4O7 porous transport layers (PTLs) used in a compact wastewater electrolyzer are developed and characterized and their performance for the electrochemical oxidation of synthetic wastewater is evaluated. An original analytical model predicting performance with the apparent mass transfer coefficient and cell potential is developed. The influence of operating parameters such as volumetric flow, current density, and PTL composition on performance is investigated. Decolorization and chemical oxygen demand (COD) removal of 100 mg L−1 of methyl orange (MO), an azo dye, in 1,500 mgNaCl L−1 is rapid with mass transfer coefficients as great as 377 ± 24 m s−1 for MO at 15 mA cm−2. After 2.5 Ah L−1 at 10 mA cm−2, >99 decolorization and >98% COD removal are achieved with a current efficiency of 19.2% and with specific and volumetric energy consumption of 120 and 84.1 kWh kg−1 for MO and COD, respectively, and 1.34 ± 0.09 and 6.45 ± 0.97 kWh m−3 order−1, respectively. A more energy-efficient electrochemical cell design for industrial wastewater treatment using less expensive high oxidation power (HOP) electrode materials is demonstrated with these results.

Modeling the drying kinetics of soybeans under intermittent operation in thin layer

The extraordinary expansion of soybeans production and trade volumes over the past few decades has undoubtedly generated global economic benefits.One factor responsible for the commercial success and extension of soybean shelf life is the drying process, which involves both heat and mass transfer. Alternatively, intermittent drying aims to improve the process in terms of yield, energy use and product quality. Thus, presenting a mathematical model that describes the soybean drying kinetics in intermittent thin-layer operation is a novelty in this study. The first order Runge-Kutta method was used to solve the proposed model, through which it was possible to obtain a detailed knowledge of the heat and mass transfer parameters involved in the drying process. It was possible to properly fit a mathematical model, based on mass and energy balances, which predicted the temperature and moisture profiles of the grain as a function of drying time, as well as the determination of physical parameters, such as global coefficients natural and forced convection heat transfer and the apparent global mass transfer coefficient.

Nitrogen Solubility and Gas Nitriding Kinetics in Fe–Cr–Mo–C Alloy Melts under Pressurized Atmosphere

The precise control of nitrogen content in high nitrogen martensitic stainless steel is the guarantee of its excellent performance, and is a crucial issue for its manufacturing process using pressurized gas nitriding. In this paper, the effects of nitrogen pressure, temperature and alloy composition on nitrogen solubility and pressurized gas nitriding kinetics of Fe–Cr–Mo–C alloy melts were investigated. In order to correct the deviation of nitrogen solubility from Sieverts’ law under high nitrogen pressure, the first- and second-order interaction parameters of nitrogen on itself were obtained; accordingly, the nitrogen solubility model under pressure was established and well verified. The nitrogen solubility decreased with increasing temperature and C content, and increased with increasing Cr and Mo contents. The nitrogen mass transfer in liquid phase was the nitriding rate-determining step under different nitrogen pressures. The apparent mass transfer coefficient of nitrogen was approximately 0.0218 to 0.0230 cm·s−1, and showed a weak dependence with pressure. The apparent mass transfer coefficient of nitrogen increased significantly with the increase of temperature and electromagnetic stirring, and was rarely affected by the content of Cr, Mo and C. Considering the influence of pressure change on nitriding rate in pressurization stage, a novel kinetic model for gas nitriding under high pressure was established and exhibited effectiveness in practice.

Theoretical and experimental investigations of time-dependent mass transfer mechanism of fine particulate matter under the sink effect

The mass transfer process of fine particulate matter (PM) under the sink effect is affected by many variables, including purification device performance, environmental conditions, and time. In this paper, the time-dependent mass transfer mechanism of PM was explored while all other variables were to remain the same. Firstly, the spatial and temporal profile characteristics of particulate matter concentrations (PMC) with different size fractions were investigated. Subsequently, the apparent mass transfer coefficient Dapp of PM was calculated based on Fick's second law, and the relevant parameters were determined by laboratory-scale experiments. According to the Dapp, the instantaneous mass transfer coefficient Dins of PM was further discussed and the prediction model of PMC was established. Finally, the stability and applicability of the prediction model were verified by experiments. The results showed that the farther the distance from the purification device, the smaller the particle size, then the smaller the decrease rate of PMC and the stronger the regeneration process. The value of Dapp could reflect the mass transfer process of PM at the macroscopic level. The value of Dins could better characterize the mass transfer process of PM at the level of detail, and it could also be used to predict and evaluate the dynamic mass transfer efficiency of PM. The established PMC prediction model had strong stability and good applicability. The research results of this paper can provide an important reference basis for practical engineering applications.

Selecting carrier material for efficient biomethanation of industrial biogas-CO2 in a trickle-bed reactor

Biogas upgrading via biomethanation of renewable H2 and the CO2 in biogas poses a potential key technology in the decarbonized energy system. This study uses a methanogenic trickle-bed reactor for upgrading raw biogas from an industrial anaerobic digester to natural gas-grade biomethane through ex situ biomethanation. The carrier material comprises a critical design parameter for trickle-bed reactors by supporting the catalytic methanogenic biofilm and thereby the interfacial area for H2 mass transfer. The present study applied an abiotic mass transfer-based screening of six different carrier types to identify a candidate material that can facilitate efficient H2 mass transfer in a biomethanation trickle-bed reactor. The screening included gas-liquid mass transfer experiments in an air-water system in addition to characterizations of the carriers’ surface area, dynamic liquid hold-up volumes, and external porosities. The abiotic experiments showed that the gas-liquid mass transfer coefficient is highly influenced by material type and cannot be predicted solely from differences in the carriers’ surface area. The abiotic characterizations led to the selection of crushed clay particles, whose application in biomethanation-based upgrading of raw biogas from an industrial anaerobic digester was examined. The clay particles facilitated an apparent volumetric mass transfer coefficient for H2 of 10 min−1, enabling CH4 production rates up to 6.1 L · L−1 · d−1 and more than 99 % H2 conversion over 30 days of continuous operation, where after accumulation of metabolic water in the packed bed impaired H2 mass transfer. Apart from the adverse effect of water accumulation, there was no indication that the maximum H2 mass transfer rate was reached, demonstrating the selected carrier material’s industrial potential for biomethanation-based biogas upgrading.

Ammonia harvesting via membrane gas extraction at moderately alkaline pH: A step toward net-profitable nitrogen recovery from domestic wastewater

Membrane gas extraction technique enables recovery of high-purity ammonia from waste streams through gas-phase mass transfer across hydrophobic membranes. This study demonstrates the potential of ammonia recovery with gas permeable membrane, operated at moderately alkaline feed pH, as a substitute for the current approach of biological nitrogen removal in domestic wastewater treatment process. The measured apparent ammonia mass transfer coefficient in 3-h operation at a feed pH of 9.2 was 0.0713 cm/min, which, despite being 1.53-fold smaller than that at a feed pH of 11, was excellent for application to domestic wastewater. Application of lower feed pH prevented inorganic fouling of membrane. A notable advantage of the moderately alkaline feed pH condition is that the consumption of alkaline agents added for deprotonating bicarbonate instead of ammonium can be minimized. Calculation shows an opportunity that the monetary value of recovered ammonia exceeds the cost of the base consumed when moderately alkaline feed pH is applied. This study substantially extends current knowledge of the membrane gas extraction technique for ammonia recovery by investigating the process kinetics, fouling propensity, and chemical requirements as functions of pH, thereby verifying the potential of this technique to contribute to improving the sustainability of the global nitrogen cycle.

Travel Time and Source Variation Explain the Molecular Transformation of Dissolved Organic Matter in an Alpine Stream Network

AbstractStreams and rivers are important components of the carbon cycle as they transport and transform dissolved organic matter (DOM). Using high‐resolution Fourier‐transform ion cyclotron resonance mass spectrometry, we studied the spatial distribution of DOM at the molecular level at more than 100 sites across a stream network during summer and winter baseflow. We developed a model approximating the time DOM spent in the fluvial network, a key constraint on the biogeochemical processing of DOM. Discharge‐weighted travel time explained the compositional changes of DOM, which differed markedly in summer and winter. We attribute these seasonal differences to variation in source material, putatively reflecting the dynamics of freshly produced DOM in summer and DOM with an imprint of leaf litter in winter. Hydrological mixing was an important driver of the spatial dynamics of DOM. From the convergence rate of DOM compound intensities to the network‐wide average, we inferred the spatial distribution of sources within the catchment. Finally, we estimated network‐wide apparent mass transfer coefficients (vf app) of individual DOM compounds, which describe the vertical velocity at which DOM compounds are removed by biotic and abiotic processes. We identified the oxidative state of carbon as an important factor explaining vf app, which we consequently attribute to biological uptake of thermodynamically favorable DOM compounds. This work contributes to our understanding of the spatial processes, temporal constraints, and chemical properties of DOM that regulate the transformation and diagenesis of DOM at the fluvial network scale.

Combining the Nonuniform Structure and Flow Maldistribution for the Accurate Prediction of the Process Performance of Monolithic Adsorbent Systems

Monolithic adsorbent systems offer the potential to achieve high throughput in fast-cycle adsorption processes. Modeling these systems is typically based on the “ideal monolith” assumption, neglecting channel nonuniformities. A new model that accounts for both structural nonuniformity and inlet flow maldistributions is presented and validated using literature data of breakthrough experiments and full-cycle process performance for carbon capture separation. A detailed comparison between predictions using an apparent mass transfer coefficient (MTC) obtained by fitting the breakthrough curve and the intrinsic MTC is presented. The process calculations based on the apparent MTC grossly underpredict the carbon dioxide recovery. A procedure is established to obtain the structural nonuniformity parameters and flow maldistribution effects from the breakthrough curve. With this model, the full-cycle predictions are within 1.20 and 1.25% of the experimental observations in terms of heavy product purity and recovery, respectively. The evidence reported in this article makes a strong case for independent measurement of the intrinsic MTCs in monolithic systems.

Enhancement of Ozone Mass Transfer by Stainless Steel Wire Mesh and Its Effect on Hydroxyl Radical Generation

ABSTRACT In order to improve the mass transfer efficiency of ozone in water, stainless steel wire mesh (SSWM) corrugated structure was packed into a microbubble ozone reactor to enhance the mass transfer efficiency. The results showed that the SSWM/O3 system could effectively improve the mass transfer efficiency. When the concentration of ozone in the liquid phase reached a stable state, it was about 21 mg/L, which was about 14% higher than that of ozone alone; the apparent mass transfer coefficient (KLa) was 0.7255 min−1, which was about 51% higher than that by ozone alone systems. The hydroxyl radicals in the SSWM/O3 system were more generated than that of ozone alone. After 6 min of operation, the concentration of hydroxyl radicals increased by 60 µmol/L compared with that in ozone alone system. The Chemical Oxygen Demand (COD) removal efficiency of biologically treated leachate by SSWM/O3 system was about 10% higher than that of ozone alone system after 120 min of reaction. The effects of pressure, temperature, ozone inlet concentration, and flow rates on the ozone concentration in the liquid phase and the generation of hydroxyl radicals were also investigated. The results indicated that reactor pressure has little effect on ozone concentration in liquid phase, but increasing pressure helps to generate ·OH; ozone concentration and ·OH generation in liquid phase increase with the increase of inlet ozone concentration and flow rate; ozone concentration in the liquid phase decreases with the increase of temperature, but ·OH generation increases with the increase of temperature. Our results indicate that the system consisting of SSWM and microbubble column reactor is an efficient process for the intensification of ozone-based advanced oxidation processes.

Development of a Model to Determine the Baseline Mass Transfer Coefficient in Bioreactors (Aeration Tanks).

Over 60 percent of all wastewater treatment plants in the developed countries use the activated sludge process as their secondary treatment system. About 50 to 85 percent of the total energy consumed in a biological wastewater treatment plant is in aeration. The activated sludge process, the most common such process, is performed in large aeration basins to provide air for microorganisms to remove nutrients and pollutants through biodegradation. Designs for the air supply often fail to meet sustained peak organic loading, which may lead to unsatisfactory treatment performance and even plant failure. On the other hand, excessive safety factors used in design for the air supply may similarly lead to unsatisfactory treatment performance and energy wastage. Improper sizing of the aeration system is primarily due to inability to estimate the mass transfer coefficient (KL a) correctly for different tank depths, leading to improper blower design and to inappropriate operation, among other things. In general, the efficiency of porous fine-bubble diffuser systems varies from 10 to 30 percent or more, depending on tank depth. The term KL a is used in ASCE/EWRI Standard 2-06 to define the apparent volumetric mass transfer coefficient in a non-steady-state clean water test in an aeration tank. This parameter is a function of tank depth, due to the variation of oxygen gas depletion with depth. The objective of this paper is to introduce a baseline oxygen mass transfer coefficient (KL a0 ), a hypothetical parameter defined as the oxygen transfer rate coefficient at zero depth, and to develop new models relating KL a to the baseline KL a0 as a function of temperature, system characteristics (e.g., the gas flow rate, the diffuser depth Zd ), and oxygen solubility (Cs ). Results of this study on data extracted from the literature indicate that a uniform value of KL a0 that is independent of tank depth can be obtained experimentally. Using the baseline, a family of rating curves for KL a20 (the standardized KL a at 20 °C) can thus be constructed for various gas flow rates applied to various tank depths. The new model relating KL a to the baseline KL a0 is an exponential function, and (KL a0 )T is found to be inversely proportional to oxygen solubility in water (Cs )T to a high degree of correlation. Using a predetermined baseline KL a0 , the new model predicts oxygen transfer coefficients KL a20 for any tank depth to within 1-3% error compared to observed measurements, and similarly for the standard oxygen transfer efficiency. The discovery of a standard baseline (KL a0 )20 determined from shop tests is important for predicting the KL a20 value for any other aeration tank depth and gas flowrate, and this finding can be used in the development of energy optimization strategies for wastewater treatment plants. This work may also improve the accuracy of aeration models used for aeration system evaluations.

Impact of TiO2 and TiO2/g-C3N4 Nanocomposite to Treat Industrial Wastewater

This work aims to implement the environmental application of nanotechnology in terms of semiconductor-based photocatalysis to treat real tannery effluent. In this context, g-C3N4 prepared by melamine pyrolysis was added with TiO2 at different weight percentage to form TiO2/g-C3N4 nanocomposites, these composites were characterized and based on the optical property, 80% TiO2/g-C3N4 nanocomposite with a band gap of 2.88 eV was chosen for the photocatalytic experiment. During the photocatalytic experiments, the performance of TiO2 and TiO2/g-C3N4 nanocomposite was compared and examined in terms of Phenol, Chemical oxygen demand (COD) and color reduction. It was observed that under optimum conditions (UV-irradiance time of 180 min, pH 5, with H2O2 concentration 0.8 g/L, flow rate of 3 l/h (litre per hour), and UV-light intensity 556.13 μE) TiO2/g-C3N4 composite exhibited higher photocatalytic efficiency than that of pure TiO2. These conditions assist 93.06% phenol, 85.62% COD, and 80.23% color reduction using TiO2/g-C3N4 nanocomposite while with TiO2 it was 72.39%, 68.95, and 65.80% respectively.Furthermore, the addition of H2O2 enhanced the photocatalytic degradation efficiency following a Langmuir-Hinshelwood model which authenticates pseudo-first-order reaction kinetics. The apparent rate constant and mass transfer coefficient for TiO2/g-C3N4/H2O2 was found to be 2.09 and 2.11 times higher than that of TiO2/H2O2.

Simulation of Hydrogen Sulfide Emission from Deep-Pit Manure Storage During Agitation

Abstract. Human and animal exposure to hydrogen sulfide (H2S) in animal barns has long been a serious issue due to the acute and chronic toxicity of H2S. The H2S concentration in the room air of deep-pit swine barns is usually within hundreds of parts per billion by volume; however, it can sharply increase to hundreds and even thousands of parts per million (ppm) during manure agitation and pump-out. To explore the sudden release and concentration distribution of H2S, this study collected and analyzed samples from varying depths of a normal non-foaming barn and a foaming barn and then mathematically simulated the H2S concentrations and emissions in the pit headspace and room air for both barns during pit agitation. Simulations were conducted for six ventilation scenarios, or six different combinations of pit fan and wall fan ventilation rates. The simulation results suggested that pit ventilation was more effective than wall ventilation in decreasing H2S concentration in room air where pigs may be housed during agitation. A minimal pit ventilation rate of 40 cfm per pig was necessary to lower the peak concentration in room air to less than the permissible exposure limit of 20 ppm. The simulation results also indicated that gas bubble release during agitation accounted for the main part (81%) of H2S emission in the foaming barn, and expedited molecular diffusion contributed the main part (70.2%) of H2S emission in the non-foaming barn. The disturbed air-manure interface during agitation induced a pH decrease and therefore increased the apparent overall mass transfer coefficient of H2S, resulting in a substantially increased mass transfer rate and concentration. The immediately dangerous to life or health (IDLH) concentration of 100 ppm may be reached during pit agitation if pit fan ventilation is not fully provided, and the duration of the exceedance could be more than 30 min. The results provide empirical data for future simulation of spatial and temporal H2S distribution and are beneficial for developing methods to control H2S below hazardous levels so that the health and safety of workers can be better secured. Keywords: Agricultural safety, Deep-pit storage, Hydrogen sulfide concentration, Sulfide distribution, Swine manure.

Ion-pair extraction-reaction of calcium using Y-shaped microfluidic junctions: An optimized separation approach

In this research, a continuous microsolvent extraction-reaction was developed for the efficient separation of calcium ion. This study gives a preliminary possible practical application of microfluidic devices in chemical exchange reaction for enrichment of 48Ca stable isotope. For this purpose, in the first stage, the hydrodynamic behavior of two immiscible liquids in a simple Y-shaped microfluidic junction is experimentally investigated, and then ion-pair extraction-reaction of Ca2+ using picric acid as a counter-ion and dicyclohexano-18-crown-6 (DC18C6) as a lipophilic ionophore is studied in this microfluidic and conventional batch method. The impact of main process parameters, including concentration of calcium chloride (0.001–0.1 M), picric acid (0.002–0.2 M), and DC18C6 (0.001–0.1 M) on the extraction-reaction efficiency and apparent overall volumetric mass transfer coefficient was comprehensively examined by utilizing response surface methodology (RSM), leading to an optimized condition for the separation of calcium. In addition, by changing the dimensions of microreactors, the influence of contact time and flow rates of both phases on the extraction-reaction efficiency was investigated. The results show that the efficiency varies significantly with changes in reagent concentration. Since the efficiency decreased with increasing the calcium chloride, the existence of a small amount of picric acid and DC18C6 in the organic phase facilitates the extraction-reaction. Moreover, the extraction-reaction is almost accomplished within ∼ 1.5 s with the efficiency of 98.55% by using the microfluidic device whereas the equilibrium condition is obtained in at least 30 min in a batch method. Furthermore, the higher contact time and flow rates result in the higher separation of the calcium ion.

A new method for estimation of the overall mass transfer coefficient in pertraction

The pertraction of benzoic acid was studied in an agitated bulk liquid membrane system. The simultaneous effect of the operational parameters such as initial feed phase concentration (cf0), bulk liquid membrane volume (VLM) as well as a molecular descriptor (MD) which represent the organic solvent is discussed. The response surface methodology with a full factorial design and multiple linear regression analysis was applied for the description of the dependence of benzoic acid fluxes on selected parameters and descriptors.It was experimentally confirmed that the response surface methodology enables the determination of the parameter which can be considered as the apparent overall mass transfer coefficient dependent on the volume of the liquid membrane and the liquid membrane solvent in the studied system.It was additionally found that the extended linear regression models with two and/or three factor interaction terms describe over 99.5% of the total variance between the maximum output fluxes of benzoic acid and selected variables. To evaluate the prediction accuracy of the empirical regression models, the standard validation procedures were carried out. For example, the highest prediction accuracy (over 99.5%) was achieved in the case of the models involving three variables (cf0 and VLM parameters and 3D Wiener index (W3D)), and additionally, two factor interaction terms (cf0×VLM, cf0×W3D,VLM×W3D).Alternatively, similar models with comparable prediction accuracy but exploiting the squared Moriguchi octanol–water partition coefficient (MLOGP2) or reciprocal hyper-detour index (Rww) can be formulated. However, in this case, the models involves three factors interaction term (cf0×VLM×MD).

Electrically Enhanced Boron Removal from Silicon Using Slag

An approach to enhance silicon refining using slag has been developed. The enhancement of the process was carried out by applying electrical potential difference across the slag and the silicon phase. This resulted in a shift in the apparent equilibrium in favor of higher partition ratio for the impurities. The application of electrical potential difference also enhanced the mass transfer rate increasing the overall kinetics of the process. This has been demonstrated in laboratory experimentations for the removal of boron from silicon-boron melts using slag. A CaO-SiO2-Al2O3 slag was reacted with Si-B melt at 1823 K (1550 °C). Electrical potential differences were applied through graphite rods immersed in each of the liquid phase. The results showed that the apparent B-partition ratio and the apparent slag mass transfer coefficient were increased by a factor of 1.2 and 1.4, respectively, when a potential difference of 3 V was applied to the phases. The technique has the potential to be used for improving the existing slag refining process by increasing the overall kinetics and the slag capacity to absorb the impurities.

A new method for studying activity and reaction kinetics of photocatalytic water oxidation systems using a bubbling reactor

A novel method is proposed for studying kinetics and overall activity of water oxidation (WO) catalysts using a bubbling reactor, where oxygen concentration is measured simultaneously in the liquid and in the gaseous phase. Total oxygen evolution is obtained from direct integration. The actual rate of oxygen formation as a function of time, RO2(t) not accessible to direct measurement with batch reactors, is calculated from raw data through a simple but comprehensive mathematical model, taking into account mass transfer phenomena occurring in the system. Data concerning the activity of a nanostructured Co3O4 catalyst dispersed on a mesoporous silica (MSU-H), in the presence of tris(2,2′-bipyridyl)Ruthenium [Ru(bpy)3]2+ as sensitizer and Na2S2O8 as sacrificial reactant, are used to illustrate data processing. Behaviour of the system is complicated by the occurrence, besides WO reaction, of the degradation of the sensitizer. Increase of sweeping gas flow increases RO2(t), by decreasing diffusional limitations to the reactions in the system: conditions for minimizing those were established. Data reported show that the assumption generally made of equilibrium between gaseous and liquid phase through Henry’s law is incorrect, the more so the smaller the apparent mass transfer coefficient, kLa. An additional reason for removing oxygen from the liquid phase through bubbling is the occurrence of a parasitic reaction of molecular oxygen with the sensitizer. The method seems to yield reliable values of both kLa and the set of RO2(t) values: the former scales with the flow of sweeping gas, as expected; RO2(t) curves are in qualitative agreement with accepted reaction mechanisms. Results concerning RO2(t) lend support to our previous kinetic studies (M. Armandi et al., ACS Catal, 2013, 3, 1272) where the reaction rate was assumed as constant for the first ∼15min. Availability of RO2(t) data not too biased by diffusional limitations opens the way to realistic studies of the kinetic features of WO heterogeneous reactions, in the present case as well as in many others.

An Experimental Study of Copper Extraction Characteristics in a T‐Junction Microchannel

AbstractThe extraction performance in a T‐junction microchannel was investigated experimentally. CuSO4/H2SO4/AD‐100/260# solvent oil was chosen as the working system. The velocities of the aqueous and oil phases were varied from 0.0016 to 0.45 m s–1, with ReM varying between 9 and 200. The experimental results show that the extraction process of copper in the microchannel is controlled by both reaction intrinsic kinetics and mass transfer, depending on the characteristics of the reaction and the fluid hydrodynamics. The maximum extraction efficiency is about 0.96, and the apparent overall volumetric mass transfer coefficient (kappa) is in the range of 0.02–0.2 s–1. In addition, the effects of the Cu2+ concentration and the pH in the aqueous phase, the volumetric fraction of AD‐100 in the oil phase, and the temperature on the extraction performance were investigated in detail.

Bioturbation and Dissolved Organic Matter Enhance Contaminant Fluxes from Sediment Treated with Powdered and Granular Activated Carbon

Sediment amendment with activated carbon (AC) is a promising technique for in situ sediment remediation. To date it is not clear whether this technique sufficiently reduces sediment-to-water fluxes of sediment-bound hydrophobic organic chemicals (HOCs) in the presence of bioturbators. Here, we report polychlorobiphenyl (PCB) pore water concentrations, fluxes, mass transfer coefficients, and survival data of two benthic species, for four treatments: no AC addition (control), powdered AC addition, granular AC addition and addition and subsequent removal of GAC (sediment stripping). AC addition decreased mass fluxes but increased apparent mass transfer coefficients because of dissolved organic carbon (DOC) facilitated transport across the benthic boundary layer (BBL). In turn, DOC concentrations depended on bioturbator activity which was high for the PAC tolerant species Asellus aquaticus and low for AC sensitive species Lumbriculus variegatus. A dual BBL resistance model combining AC effects on gradients, DOC facilitated transport and biodiffusion was evaluated against the data and showed how the type of resistance differs with treatment and chemical hydrophobicity. Data and simulations illustrate the complex interplay between AC and contaminant toxicity to benthic organisms and how differences in species tolerance affect mass fluxes from sediment to the water column.

Micromodel Investigation of Transport Effect on the Kinetics of Reductive Dissolution of Hematite

Reductive dissolution of hematite in porous media was investigated using a micromodel (8.1 × 4.5 × 0.028 mm) with realistic pore network structures that include distinctive advection domain, macropores and micropores created in silicon substrate. The micromodel pore surface was sputter deposited with a thin layer (230 nm) of hematite. The hematite in the micromodel was reduced by injecting pH-varying solutions (pH 5.0, 6.0, 7.0) containing a reduced form of flavin mononucleotide (FMNH2, 100 μM), a biogenic soluble electron transfer mediator produced by Shewanella species. The reduction kinetics was determined by measuring effluent Fe(II) (aq) concentration and by spectroscopically monitoring the hematite dissolution front in the micromodel. Batch experiment was also performed to estimate the hematite reduction rate under the well-mixed condition. Results showed significant spatial variation in local redox reaction rate that was controlled by the coupled transport and reaction. The overall rate of the redox reaction in the micromodel required a three-domain numerical model to effectively describe reaction kinetics either with distinctive apparent rate parameters or mass transfer coefficients in different pore domains. Results from this study demonstrated the feasibility of a domain-based modeling approach for scaling reaction rates from batch to porous media systems where reactions may be significantly limited by transport.