Diffusion-controlled Mass Transfer Research Articles

Synthesis, in situ spectroelectrochemical, in situ electrocolorimetric and electrocatalytic investigation of brown-manganese phthalocyanines

α- and β-substituted tetrakis(6-hydroxyhexylthiol) phthalocyaninato manganese (III) chloride complexes have been prepared via cyclotetramerization. Both complexes have been characterized by elemental analysis, FTIR, MS and UV–Vis spectral data. The voltammetric and in situ spectroelectrochemical studies reveal that both complexes exhibit an oxidation and three reduction processes having reversible, one-electron, and diffusion controlled mass transfer characteristics, which are assigned to Mn IIIPc 2−/Mn IVPc 2−, Mn IIIPc 2−/Mn IIPc 2−, Mn IIPc 2−/Mn IPc 2−, and Mn IPc 2−/Mn IPc 3− couples respectively. The existence of oxygen in solution significantly affects the in situ spectroelectrochemical behavior of the complexes due to the formation of μ-oxo MnPc species. An in situ electrocolorimetric method has been applied to investigate the colors of the electro-generated anionic and cationic forms of the complexes for the first time in this study. The complexes, coated on a glassy carbon electrode potentiostatically, show considerable high electrocatalytic activity to hydrogen evolution reactions in aqueous solution.

Preparation and characterization of poly(lactic-co-glycolic acid) microparticles containing DNA molecules encoding a malaria vaccine candidate

BACKGROUND: A novel ultrasonic atomization approach for the formulation of biodegradable poly(lactic-co-glycolic acid) (PLGA) microparticles of a malaria DNA vaccine is presented. A 40 kHz ultrasonic atomization device was used to create the microparticles from a feedstock containing 5 volumes of 0.5% w/v PLGA in acetone and 1 volume of condensed DNA which was fed at a flow rate of 18 ml h−1. The plasmid DNA vectors encoding a malaria protein were condensed with a cationic polymer before atomization. RESULTS: High levels of gene expression in vitro were observed in COS-7 cells transfected with condensed DNA at a nitrogen to phosphate (N/P) ratio of 10. At this N/P ratio, the condensed DNA exhibited a monodispersed nanoparticle size (Z-average diameter of 60.8 nm) and a highly positive zeta potential of 38.8 mV. The microparticle formulations of malaria DNA vaccine were quality assessed and it was shown that the microparticles displayed high encapsulation efficiencies between 82–96% and a narrow size distribution in the range of 0.8–1.9 µm. In vitro release profile revealed that approximately 82% of the DNA was released within 30 days via a predominantly diffusion controlled mass transfer system. CONCLUSIONS: This ultrasonic atomization technique showed excellent particle size reproducibility and displayed potential as an industrially viable approach for the formulation of controlled release particles. Copyright © 2009 Society of Chemical Industry

Standard‐free kinetic calibration for rapid on‐site analysis by solid‐phase microextraction

In this study, a new calibration method, standard-free kinetic calibration, is proposed for rapid on-site analysis by solid-phase microextraction (SPME), based on the diffusion-controlled mass transfer model and equilibrium extraction. With this calibration method, all analytes can be directly calibrated with only two samplings. The feasibility of this calibration method was validated in a standard aqueous solution flow-through system and a standard gas flow-through system. The distribution coefficients of five polycyclic aromatic hydrocarbons (PAHs), including naphthalene, acenaphthene, fluorene, anthracene, and pyrene, between water and the PDMS fiber coating were determined and the concentrations of the PAHs in the flow-through system were successfully calibrated with the proposed standard-free calibration method. The extracted amounts of BTEX (benzene, toluene, ethylbezene, o-xylene) at equilibrium were also successfully calibrated with this method with two rapid sampling periods at 5 and 10 s. Compared with the previous calibration methods for rapid on-site analysis by SPME, this method does not require a standard to calibrate the extraction, nor does it require additional equipment to control or measure the flow velocity of the sample matrix. In addition, all of the extracted analytes can be quantified without considering whether the system reached equilibrium. The newly proposed standard-free kinetic calibration approach enriched the calibration methods available for on-site analysis by SPME.

Synthesis, electrochemistry and in situ spectroelectrochemistry of soluble lead phthalocyanines

The synthesis, characterization and voltammetric and spectroelectrochemical properties of newly synthesized lead phthalocyanines bearing tetra-(1,1-(dicarbpenthoxy)-2-(4-biphenyl)-ethyl), tetra-(1,1-(dicarbpenthoxy)-2-(1-naphthyl)-ethyl and tetra-((1,1,2-(tricarbopentoxyethyl)) substituents have been presented in this work for the first time. The characterization of the complexes was made by elemental analysis, 1H NMR, FT-IR, UV–vis and Maldi-TOF. The solution redox properties and spectroelectrochemical investigation of the complexes are studied using various electrochemical techniques in DCM on a platinum electrode. Cyclic voltammetry and differential pulse voltammetry studies show that the complexes give three one-electron ligand-based reductions and two one-electron oxidation couples having diffusion-controlled mass transfer character. Assignments of the redox couples were confirmed by spectroelectrochemical measurements. Spectroelectrochemical studies reveal that all complexes are demetallized during the spectroelectrochemical measurement under the applied potentials at the first reduction and oxidation potentials of the complexes. Different ring substituents of the complexes affect the easy demetallization processes of the complexes.

Voltammetry and Spectroelectrochemical Behavior of a Novel Octapropylporphyrazinato Lead(II) Complex

The synthesis, characterization, and voltammetric and spectroelectrochemical investigation of [2,3,7,8,12,13,17,18-octapropyl porphyrazinato] lead(II) are reported. The characterization of newly synthesized lead porphyrazine was made by elemental analysis, 1H NMR, FT-IR, UV−vis, and MALDI-TOF. The solution redox properties and spectroelectrochemical investigation of the novel lead(II) porphyrazine were studied using various electrochemical techniques in DCM on a platinum electrode. Cyclic voltammetry and differential pulse voltammetry studies show that the complex gives two one-electron ligand-based reductions and a one-electron oxidation wave having diffusion-controlled mass transfer character. Assignments of the redox couples were confirmed by spectroelectrochemical measurements. Spectroelectrochemical studies reveal that the complex is demetalized during the controlled potential coulometry measurement after the first reduction couple of the complex.

Mathematical modeling of emulsion liquid membrane pertraction of gold (III) from aqueous solutions

Modeling of emulsion liquid membrane pertraction of gold (III) ions from aqueous solutions using MIBK as carrier and LK-80 as emulsifier has been performed. Three models were considered: Advancing front model, diffusion controlled mass transfer inside and outside of globule model and finally, reaction and diffusion controlled mass transfer model. Results showed that the model considered as reaction and diffusion controlled mass transfer model is more compatible with experimental results and the compatibility of the model and the experimental results is more than 90%. This modeling showed that the pH of external phase is the most important parameter for the extraction. The deviation for “Advancing front model”, “diffusion controlled mass transfer model” and “reaction and diffusion controlled mass transfer model” were 82.83, 65.69 and 9.38%, respectively.

Crystallization differentiation of intrusive magmatic melt: Development of a convection-accumulation model

The paper summarizes the principal results obtained over the past three decades at the Vernadsky Institute and the Department of Geochemistry of the Moscow State University by the computer simulation of basaltic magma differentiation in magma chambers. The processes of diffusion-controlled mass transfer in a chamber are demonstrated to be principally limited by the heat resources of the cooling magma and cannot play any significant role during the large-scale partitioning of melt components. The leading mass-transfer mechanism is the settling of crystals from convecting magma in the form of suspension flows that are enriched and depleted in the solid phase. The physical prerequisite for the onset of this concentration convection is the existence of boundary layers, which are characterized by volume crystallization and a gradient distribution of the suspended phases. Considered in detail are the principles used in the development of algorithms with regard for the occurrence of a boundary layer and the “instantaneous” stirring of the crystallizing magma that does not hamper the settling of mineral grains forming the cumulus. The plausibility of the convection-accumulation model is illustrated by the example of the reconstructed inner structure of differentiated Siberian traps. In application to these bodies, it is demonstrated that the solutions of the forward and inverse simulation problems with the use of geochemical thermometry techniques are identical. This is a convincing argument for the predominance of convection-accumulation processes during the formation of thin tabular magmatic bodies. The further development of the computer model for the differentiation dynamics should involve the processes of compositional convection related to the migration and reactivity of the intercumulus melt.

Transport and Reaction Processes in Bioremediation of Organic Contaminants. 2. Role of Aggregate Size in Soil Remediation in a Slurry Bioreactor

Transport processes in the soil matrix can control the rates of bioremediation of low-solubility contaminants. In this study, experiments were designed to test the hypothesis that diffusion of contaminants within soil aggregates of diameter 40 - 1000 micron was the limiting factor in bioremediation of creosote-contaminated soil. The concentrations of 6 different PAHs (acenaphthene, anthracene, chrysene, fluoranthene, phenanthrene, and pyrene) were monitored during the course of bioremediation of sonicated and non-sonicated soil by an active mixed culture in slurry bioreactors. Sonication of the soil to disperse soil aggregates increased the rate of microbial degradation up to 5 fold, however, there was no significant difference in the final residual concentrations between the two soil treatments. The aggregate size distribution after three weeks of treatment in a slurry bioreactor was comparable in both the sonicated and non-sonicated soils, which was consistent with the independence of the residual concentrations of PAHs on sonication treatment. The soil aggregates were modelled as porous materials, with pores filled with non-aqueous phase liquid and with films of non-aqueous phase liquid on particle surfaces. As the soil aggregates were dispersed, either by sonication or mixing, then the fraction of contaminants in exposed films increased. A diffusion-controlled mass transfer model was developed to represent release of PAHs from the soil, based on this physical model. The estimated diffusion coefficients of four of the PAHs (acenaphthene, phenanthrene, fluoranthene and pyrene ) in the residual creosote phase were in the range 4.4-4.8 x 10-14 cm2/s, while the diffusion coefficients for anthracene and chrysene were lower by a factor of 2. The similar values of diffusion coefficient between the components was consistent with release by diffusion through a viscous residual creosote. The magnitude of the diffusion coefficients was intermediate between the transport properties in high-viscosity oils, and polymers.

Multicomponent mass transfer across water films during hydrocarbon gas injection

Understanding the dynamics of pore-scale multicomponent gas and oil mass transfer across water films during hydrocarbon gas injection in petroleum reservoirs is important in the design of tertiary oil recovery schemes at the field scale. The water films prevent oil and gas coming into direct contact and, for miscible gas injection, delay the onset of miscibility. We use a pore-scale model to describe the diffusion-controlled mass transfer through the water films. The following different processes are found: (i) rapid oil swelling which results in short times needed to rupture the water films shielding the oil, (ii) slow oil swelling resulting in very long water film rupture times and (iii) both oil swelling and oil shrinking. The rate of oil recovery and the way it is recovered (either by rupturing the water film or being vaporized into the displacing gas) is critically dependent upon the oil and gas compositions, the oil droplet size and the water film thickness. We show cases at the pore scale where the time for gas and oil to be brought into direct contact and reach equilibrium in the presence of water films during miscible gas displacement is so long that water-blocking will adversely affect oil recovery at the field scale.

Electrochemical reduction of solutions of ReF6 in fused LiF-NaF-KF eutectic mixture

Electrolyte was prepared by introducing gaseous ReF6 into the molten LiF–NaF–KF eutectic at 600 °C. The electrochemical properties of the solutions were studied by voltammetric techniques. The reduction of ReF82− to Re occurred via a single irreversible step with diffusion controlled mass transfer. The diffusion coefficient of ReF82− was 8 × 10−10 m2 s−1 and the cathodic transfer coefficient was 0.13. Well-crystallized pure rhenium layers, up to 50 μm thick, were obtained on W, Ag, graphite and vitreous carbon substrates and were examined by SEM and X-ray diffraction techniques.

MASS TRANSFER EFFECTS ON SOUND PROPAGATION IN A DROPLET-GAS MIXTURE

The effect of mass transfer on the propagation of a plane acoustic wave in a stagnant two-phase medium is studied theoretically using a continuum formulation. It is shown that a diffusion-controlled mass transfer process can be a driving mechanism for acoustic-related oscillations if the undisturbed gas temperature is higher than the saturation temperature of the liquid. On the other hand, if the undisturbed gas-droplet mixture is at a saturated state, mass transfer is a damping phenomenon as velocity and thermal lags between the gas and the droplets. Besides, with the model used, the effect of mass transfer becomes significant and dominates the other exchange processes at low frequencies and/or for small diameters. However, these results must be taken with caution since the mass exchange term follows a quasi-steady law.

Monolayer penetration by a charged amphiphile: equilibrium and dynamics

The penetration of a soluble surfactant into an insoluble monolayer provides a means of understanding intermolecular interactions and their impact on equilibrium and dynamic surface pressures. In this paper, the adsorption of an ionic surfactant into an insoluble monolayer is studied theoretically and numerically. The equilibrium increase in surface pressure Δ π caused by the surfactant adsorption is derived for a Davies adsorption isotherm using a Gibbs adsorption equation properly constrained for the presence of the insoluble monolayer. The dynamic surface pressure is studied using this surface equation of state for Δ π assuming either diffusion controlled or mixed kinetic-diffusion controlled mass transfer. Several trends are predicted with variations of the surface coverage of the insoluble component, the concentration of soluble surfactant and the ionic strength of the surfactant subphase. Experiments in the literature have shown that, for an uncharged monolayer, Δ π at equilibrium is greater the higher is the surface coverage of the insoluble monolayer into which the ionic, soluble surfactant adsorbs. Our results show that this trend can be attributed to the role of the insoluble component in presenting an entropic barrier to adsorption, thereby reducing the repulsive surface charge density at a given net surfactant coverage, allowing more surfactant to adsorb. Greater surface pressures result. Signature trends in the timescales for diffusion controlled mass transfer of an ionic surfactant as a function of initial surface coverage of the insoluble monolayer are derived. This timescale is longer than predicted by simply accounting for the area blocked by the insoluble component using a Langmuir argument, and approaches the Langmuir argument with increasing ionic strength. For mixed kinetic-diffusion controlled mass transfer, because the insoluble component blocks interface, it reduces the amount of surfactant that can adsorb. This decreases the diffusion timescale, and allows adsorption-desorption kinetics to play a controlling role. Since electrostatic repulsion reduces the adsorption of the soluble component, kinetics also play a stronger role the lower is the ionic strength or the higher the surfactant valence. Considering a surfactant with fixed physicochemistry, a shift of controlling mechanism from diffusion control to kinetic control is demonstrated with increasing bulk concentration, surface charge density or surface coverage of insoluble component.

Molybdenum chemistry in molten LiCl–KCl eutectic: an electrochemical and absorption spectroscopy study of the concentration dependent stability of solutions of K 3MoCl 6

The chemical behaviour of LiCl–KCl–K 3MoCl 6 solutions between 400 and 500°C was investigated. They underwent slow decomposition, according to a concentration dependent disproportionation reaction yielding metallic molybdenum and a volatile black higher valency molybdenum compound, which corresponded with MoCl 5. Chronopotentiometric determinations showed a single 3-electron reduction, yielding Mo. The process was slow with diffusion-controlled mass transfer. The concentration of the Mo(III) species, presumed in the form of MoCl 6 3− ions, was determined using linear sweep voltametry and open circuit voltage measurements and the overall Mo(III) species concentration by chemical analysis. The results showed MoCl 6 3− to be the predominant species in the melt and the presence of a previously proposed dinuclear species, Mo 2Cl 9 3−, could not be confirmed electrochemically. This arose to quantification errors in the determination of the electroactive species MoCl 6 3− due to inherent problems in estimating the active surface area of the electrode. The electronic absorption spectrum of MoCl 6 3− in LiCl–KCl eutectic at 400°C is reported for the first time. Measurements were made above and below 0.045 mol dm −3 K 3MoCl 6, the concentration above which the disproportionation reaction was relatively slow. The results are discussed in detail and the spectroscopic parameters of the octahedral MoCl 6 3− complex were calculated as: Dq=1775 cm −1, B=511 cm −1 and C=1913 cm −1. Although an additional band, at 340 nm, appeared during disproportionation it could not be identified with any currently known polynuclear molybdenum chloro complex or oxychloro complex. It is probably associated with an intermediate in the disproportionation process. No spectral evidence was thus obtained in support of the formation of dinuclear molybdenum species.

Direct Observation of Fluid Mass Transfer Resistance in Porous Media by NMR Spectroscopy

The kinetics of diffusion-controlled mass transfer between a fluid percolating through a bed of porous particles and the stagnant fluid in the pores of the particles (see drawing on the right) controls the rate of many important processes. The pulsed field gradient NMR method allows the rapid identification of the fraction of fluid exchanged between the stream and the pores during a given period of time. From this data the mass transfer kinetic coefficient, the porosity, the intraparticle diffusivity, and the tortuosity of the porous medium can be determined.

Dissolved Gas Transport in the Presence of a Trapped Gas Phase: Experimental Evaluation of a Two‐Dimensional Kinetic Model

AbstractQuantitative information on dissolved gas transport in ground water aquifers is needed for a variety of site characterization and remedial design applications. The objective of this study was to gain further understanding of dissolved gas transport in the presence of trapped gas in the pore space of an otherwise water saturated porous medium, using a combination of laboratory experiments and numerical modeling. Transport experiments were conducted in a large‐scale (4 × 2 × 0.2 m) laboratory physical aquifer model containing a homogeneous sandpack. Tracer (Br−) and dissolved gas (O2 or H2) plumes were created using a two‐well injection/extraction scheme and then were allowed to drift in a uniform flow field. Plume locations and shapes were monitored by measuring tracer and dissolved gas concentrations as a function of position within the sandpack and tune. In all experiments, partitioning of the dissolved gases between the mobile ground water and stationary trapped gas phases resulted in substantial retardation and tailing of the dissolved O2 and H2 plumes relative to the Br− plumes. Most observed plume features could be reproduced in simulations performed with a numerical model that combined the advection‐dispersion equation with diffusion controlled mass transfer of dissolved gas between the mobile aqueous and stationary trapped gas phases. Fitted values of the volumetric trapped gas content and mass transfer coefficient ranged from 0.04 to 0.08 and from 10−6to 10−5 sec−1, respectively. Sensitivity analyses were used to examine how systematic variations in these parameters would be expected to affect dissolved gas transport under a range of potential field conditions. The experimental and modeling results indicate that diffusion controlled mass transfer should be considered when predicting dissolved gas transport in ground water aquifers in the presence of trapped gas.

High-temperature deformation of a granitoid from the Zone of Erbendorf-Vohenstraß (ZEV)

Microfabrics and textures indicate that progressive mylonitization of granitoids from the vicinity of the KTB site occurred under high temperature and probably prograde conditions by pure shear deformation. Diffusion-controlled deformation mechanisms and diffusional mass transfer for all rock-forming minerals were prominent processes which also contributed to the development of compositional layering. Typical chessboard patterns in quartz suggest that even the latest increments of plastic deformation took place at high temperatures within the stability field of high quartz. Largely constant chemical bulk-rock compositions and lack of retrogressive reactions are evidence of a dry and closed system during mylonitization. Only a late post-mylonitic fluid influx is documented by secondary fluid inclusions along healed microcracks. Taking recent geochronological data into account, different geological scenarios are most suitable: (a) intrusion of the protolith between 525 and 475 Ma into Cadomian basement and subsequent mylonitization in a related extensional regime; and (b) Early Variscan mylonitization in a subduction zone environment between 475 and 380 Ma.

Development and Testing of a Kinetic Model for Oxygen Transport in Porous Media in the Presence of Trapped Gas

AbstractThe ability to predict the transport of dissolved gases in the presence of small amounts of trapped gas in an otherwise water‐saturated porous medium is needed for a variety of applications. However, an existing model based on equilibrium partitioning of dissolved gas between aqueous and trapped gas phases does not accurately predict the shape of experimentally observed breakthrough and elution curves in column experiments. The objective of this study was to develop and test a kinetic model for dissolved gas transport that combines the advection‐dispersion equation with diffusion controlled mass transfer of dissolved gas between the aqueous and trapped gas phases. The model assumes one‐dimensional, steady‐state ground‐water flow, a single dissolved gas component, and a stationary trapped gas phase with constant volume. The model contains three independent parameters: the Peclet number, P, retardation factor, R, and dimensionless mass transfer parameter, ω. The model accurately described the shape of breakthrough and elution curves for dissolved oxygen in column experiments performed with a poorly graded sand and varying amount and composition of trapped gas. Estimated values of P for the bromide tracer increased from 5.92 to 174, corresponding to a decrease in dispersivity from 5.02 to 0.17 cm, as the trapped gas volume increased from 0 to 30% of the pore space. It is speculated that this effect is due to a narrower pore size distribution (and hence more uniform pore scale velocity distribution) caused by trapped gas bubbles selectively occupying the largest pores. Estimated values of R increased from 1 to 13.6 as the trapped gas volume increased and confirmed earlier observations that even small amounts of trapped gas can significantly reduce rates of dissolved oxygen transport. Estimated values of ω ranged from 0.3 to 12.1. Although it was not possible to independently measure mass transfer coefficients or interfacial areas, values computed from flow rates and estimated w values are consistent with values computed by assuming (1) that interfacial area is proportional to trapped gas volume, (2) that trapped gas bubbles are spheres with diameters the same size as soil particles, and (3) that mass transfer is limited by diffusion of dissolved oxygen through water films surrounding trapped gas bubbles.

A pore-scale investigation of mass transport from dissolving DNAPL droplets

Spheres and pendular rings of trichloroethylene and tetrachloroethylene are observed dissolving in an artificial porous medium consisting of a single layer of glass beads. Given the simple geometry of the droplets, pore-scale mass transfer coefficients are calculated. For dissolving spheres, mass transfer coefficients are found to be constant over time. However, coefficients for pendular rings are found to decrease as the rings shrink in size. This is consistent with the concept of diffusion-controlled mass transfer in low-flow zones. Sherwood numbers measured at Reynolds numbers between 0.0033 and 8.33 show a similar trend to empirical relationships established by previous researchers using other techniques. As in previous experiments, considerable scatter in mass transfer rates is observed at a given mean flux. It is hypothesized that this variation in local mass transfer rates is caused by variations in local pore velocity.

Bioavailability and Transformation of Highly Chlorinated Dibenzo-p-Dioxins and Dibenzofurans in Anaerobic Soils and Sediments

The long-term fate of environmentally relevant concentrations (ppb) of penta- to heptachlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) congeners was evaluated in both high and low organic carbon anaerobic sediments. The inocula were derived from historically contaminated environments and were amended with a mixture of aliphatic and aromatic acids to stimulate anaerobic microbial activity. The long-term (>2 years) removal patterns of sediment-sorbed PCDD/PCDF in both sediments could be explained by labile and resistant PCDD/PCDF desorption components, presumably due to intraparticle diffusion-controlled mass transfer limitations. Mass transfer limitations were based on incubation time-dependent decreased extraction efficiencies of PCDD/PCDF from autoclaved controls. Lesser chlorinated congeners were observed in both active and autoclaved sediments, accounting for up to 30% of the PCDD/F spiked. Whereas tetraCDD accumulated from hepta-, hexa-, and pentaCDD/F in active sediments, the extent of dechlorination in autoclaved sediments was limited to removal of one to two chlorines. These observations provide the first evidence for the contribution of biologically mediated reductive dechlorination of PCDD/PCDF to the overall fate of these contaminants in the environment and may lead the way to designing in situ transformation/biodegradation systems.

Critical size of crystals growing under diffusion conditions for loss of polyhedral stability

A new expression is proposed for calculating the critical size for which, when reached, the polyhedral stability of a crystal growing under diffusion controlled mass transfer disappears. The well-known relationships for critical size are obtained as special cases of the proposed expression, for infinitely dilute solutions. In addition, it is shown that when the solubility increases, the critical size of the crystal must also increase, all other parameters remaining identical. Similar is the dependence of the critical size on the concentration.