Solute Mass Transport Research Articles

Simulating the in situ oxidative treatment of chlorinated ethylenes by potassium permanganate

Several laboratory and field studies have demonstrated the potential viability of oxidation schemes using MnO4− for the in situ treatment of source areas, which are contaminated by chlorinated ethylenes (PCE, TCE, and DCE). Chemically, the chlorinated ethylenes are oxidized to CO2, Cl−, and MnO2. The goal of this study was to develop a theoretical framework for the chemical and physical processes involved. To this end, a computer model was created to simulate the coupled processes of nonaqueous phase liquid (NAPL) dissolution, chemical reactions, and solute mass transport in the in situ chemical oxidization scheme. The model incorporates a kinetic description of reactions between the MnO4− and the chlorinated ethylenes and the rate of dissolution of the NAPL. A Strang operator‐splitting method, which coupled the different physical and chemical processes and an exponentially expressed solution of the kinetic equations, led to a significant speed up in the solution process. The products were calculated based on the stoichiometry of the reaction. We demonstrated the capabilities of this code using already published results of column, test cell, and field experiments. Generally, the simulated results matched well with experimental measurements. The computer model provides a useful tool for assisting in the design and the prediction of the oxidization processes under field conditions.

Electrocatalysis of O2 reduction at polyaniline+molybdenum-doped ruthenium selenide composite electrodes

Composite layers consisting of a polyaniline (PANI) matrix and a molybdenum-doped ruthenium selenide (MRS) dispersed phase have been tested as cathodes for O2 reduction in 0.5 M H2SO4. The investigation, carried out on rotating disc electrodes using steady-state techniques, shows that a 4-electron reduction of O2 is efficiently carried out on PANI+MRS composites. The onset potential is close to 0.5 V (SCE) and a current plateau is observed at E<0.1 V (SCE). The overall process is under mixed control. Besides the charge transfer and mass transport in solution, either O2 diffusion in the film or partial blockage of the electrode are possible limiting factors. The performance of PANI+MRS electrodes becomes better than that of Pt in 0.5 M H2SO4 in the presence of methanol. Diminished current densities are obtained at higher pH due to a decreased O2 solubility, the conductivity of the PANI matrix being always good at pH<3.

Mass Transport of Multicomponents Solute in Bentonite Clay

ABSTRACTA fundamental concept combining a molecular behavior and macro-continuum phenomenon is presented for a multicomponent solute diffusion problem in bentonite clay, which is a key component of the Engineering Barrier System (EBS) of high-level radioactive waste disposal (HLRW). Bentonite is a micro-inhomogeneous material. Properties of the saturated bentonite are characterized by the constituent clay mineral (montmorillonite) and water, namely montmorillonite hydrate. We analyze its molecular behavior by applying a molecular dynamics (MD) simulation to inquire into the physicochemical properties such as diffusivity of chemical species. For extending the microscopic characteristics of constituent materials to a macroscopic diffusion behavior of the micro-inhomogeneous material we apply a homogenization analysis (HA).

The mass transport of methanolic ferric chloride hexahydrate solution in poly(methyl methacrylate) and related optical properties

The mass transport of methanol mixed with ferric chloride hexahydrate (FeCl 3·6H 2O) in poly(methyl methacrylate) (PMMA) and the related optical properties have been investigated. The ratio of FeCl 3·6H 2O to methanol in the solvent mixture is X=0.013, 0.038 and 0.063 g/g. The mass transport is anomalous, which is a mixture of Case I with Case II transport. Both diffusion coefficient for Case I and velocity for Case II transport satisfy the Arrhenius equation. The activation energy of diffusion coefficient in Case I transport decreases with increasing X, but the trend for that of velocity in Case II transport is opposite. The mass transport is an endothermic process and satisfies the van't Hoff's plot. The heat of mixing increases with X. The transmittances of PMMA saturated with the solvent and solvent of methanol mixed with FeCl 3·6H 2O decrease with increasing X. The scattering intensity of the specimens after desorption is inversely proportional to the visible wavelength with an exponent, n, in the range from 1.02 to 3.25. The exponent is equal to 4 corresponding to the Rayleigh scattering. The wavelength dependence of scattering intensity is attributed to holes, the size of which is smaller than the visible wavelength, and to clouds. The holes and clouds in the solvent-treated specimen are observed by the scanning electron microscope and optical microscope. The deviation of the exponent n from 4 is due to the fractal dimension of hole and cloud. The FTIR spectra of specimens treated with solvent of different concentrations of FeCl 3·6H 2O are also studied.

Groundwater in a coral island

Freshwater aquifers in small coral islands usually occur as thin lenses which are significantly influenced by stresses such as groundwater pumping, sea tides, etc. Kavaratti, a coral island, is one of the 36 such islands in the Arabian Sea off the western coast of India. Detailed hydrogeological investigations were carried out to determine the quantity of freshwater that could be pumped in addition to the present usage. Salinity of groundwater must remain within permissible limits and a 2-D solute mass transport model with a vertical section of the island was constructed by the computer code SUTRA. The model was calibrated by obtaining a match of computed and observed values of the water table, tidal efficiency, and salinity of groundwater at the water table. The model analysis showed that the salinity of groundwater continues to increase unless groundwater pumping is kept below a certain rate. Groundwater potential can be augmented by reducing the subsurface outflow to the sea and by raising the water table by a subsurface dam.

Mass transport in polyelectrolyte solutions

The self-diffusion coefficients of the three components of a salt-free heavy-water solution of polymethacrylic acid, completely neutralized with tetra-methylammonium hydroxide, were measured over a broad concentration range. Three concentration regions were observed for the self-diffusion of both the polyions and the counterions. At polyion concentrations below 0.01 mol monomer kg-1, the dilute concentration regime for the polymer, the polyion self-diffusion coefficient approaches the self-diffusion coefficient of a freely diffusing rod upon dilution. At polyelectrolyte concentrations above 0.1 mol monomer kg-1, the self-diffusion coefficients of the solvent, the counterions and the polymer decreased with concentration, suggesting that this decrease is due to a topological constraint on the motions of the components. In the intermediate-concentration region, the self-diffusion coefficients of the polyions and the counterions are independent of the concentration. The polyion dynamic behaviour is, in the intermediate- and high-concentration regions, reasonably well described by that of a hard sphere, with a radius of 3.7 nm. A correct prediction for the solvent dynamics is given by the obstruction effect of this hard sphere on the solvent. The relative counterion self-diffusion coefficient is predicted almost quantitatively over the entire concentration range with the Poisson-Boltzmann-Smoluchowski model for the spherical cell, provided that the sphere radius and the number of charges are chosen appropriately (approximately the number of charges in a persistence length). Using this model, the dependence of the counterion self-diffusion coefficient on the ionic strength, polyion concentration and counterion radius is calculated quantitatively over a large concentration range.

Analysis of stability in a solute field under liquid phase epitaxy

We reported a critical gap length between two silicon substrates at which no epitaxial layer was grown on the upper silicon substrate [M. Saitou and S. Itomura, J. Mater. Sci.: Mater. Electron. 8, 321 (1997)]. In this study, using perturbation theory and a numerical solution, the abrupt change in the growth rate is investigated. We find that a nondimensional parameter GrSc∂c/∂y determines the instability in a solute field, and that under a critical value of this parameter no epitaxial growth on the substrate sets in because diffusion governs the mass transport of the silicon solute.

Mass transport of major solutes and sediments by the Volta river, Ghana, West Africa

(1998). Mass transport of major solutes and sediments by the Volta river, Ghana, West Africa. SIL Proceedings, 1922-2010: Vol. 26, No. 3, pp. 911-915.

Steady-state voltammetry for the reduction of labile metal complexes in the absence and presence of different concentrations of supporting electrolyte

Abstract Steady-state voltammetry for the reduction of labile complexes of lead, cadmium and mercury, in the absence of deliberately added supporting electrolyte and with different concentrations of supporting electrolyte, has been studied at mercury (lead and cadmium) and platinum disc (mercury) microelectrodes. The effects of ionic strength and nature of an inert electrolyte on the migrational component, which usually adds to diffusion in the mass transport in solutions with low electrolyte, have been examined. The results obtained have demonstrated that the ratio of limiting current at different concentrations of supporting electrolyte to diffusive limiting current I lm / I ld , depended not only on the charge of the free ion, but also on the amount of labile complexes formed between the metal ion with ligands which may arise from the counter-ion accompanying the metal in its starting salt, from small amounts of supporting electrolyte and from the autoprotolysis of water, i.e., OH − . Moreover, the I lm / I ld ratio is affected by the background electrolyte, which is almost unavoidable in water, and by the hydrolysis of the metal ion which yields protons. A comprehensive theoretical equation, based on already known treatments, has been also derived to predict the limiting current ratios in the presence of all the effects cited above.

Transport of electrolytes through a weak acid nanofiltration membrane: Effects of flux and crossflow velocity interpreted using a fine-porous membrane model

A one-dimensional fine-porous membrane model has been exploited to determine the reflection coefficient, σ, and the internal solute mass transport coefficient, k m , for the nanofiltration of simple salts and dyes by two weak-acid membranes, HR and LR, differing in σ for a NaNO 3 solution. The rejection, r, was strongly dependent on the crossflow velocity, u, and the flux, J, in these experiments. The dependence of the external solute mass transport coefficient, k s , on u was also determined. The dominant parameter in determining the limiting, low J and high u, rejection for the systems is σ. It was influenced primarily by the charge of the coion for the simple sodium salts, NaNO 3 and Na 2SO 4. The k m remained constant within experimental error, for NaNO 3 and Na 2SO 4 using membrane HR and for the two dyes NaR4 and Na 2R1 using membrane LR even though the charge on the coions and their formula weights differed in each comparison. Transitions from regime I, i.e., decreasing r with increasing J at constant u, to regime II, i.e., increasing r with increasing J at constant u, behavior were observed at low u for a trivalent dye using membrane LR.

Study of calcium carbonate scales by electrochemical impedance spectroscopy

Calcareous deposits were investigated by electrochemical impedance spectroscopy for electrochemically accelerated scaling. Firstly, the oxygen reduction was studied in a solution of low ionic strength similar to the usual scaling water solutions. Its reaction mechanism can be described by an electron transfer limited by the mass transport in the solution bulk. Secondly, by measuring the impedance on a totally covered electrode at the end of scaling it was shown that dissolved oxygen reduction occurs, through the porous scale, in cavities under the blocks of calcite crystals. The resistance and capacitance of the scale layer was investigated by measuring the impedance at zero current. Impedance measurements give information on the compactness and the thickness of the deposit. Scanning electron microscopy revealed that the morphology of the scale deposit is dependent on mass transport in solution.

Effect of Easily Ionisable Elements on the Mass Transport Efficiency of Solutions and Slurries Used in Plasma Emission Spectrometry

The effect of easily ionised elements (EIEs) upon the mass transport of solutions and slurries used in plasma emission spectrometry has been directly measured. Solutions and slurries of 1000 µg ml -1 Mn 2+ and MnO 2 (equivalent to 1000 µg ml -1 Mn), respectively, were aspirated at a carrier gas flow rate of 1 l min -1 through a V-groove nebuliser, double-pass spray chamber and injector system in the presence and absence of equimolar amounts (0.05 m) of Li, Na, K and Cs. The aerosol exiting the 2 mm bore injector was directly collected and the analyte of interest (Mn) was determined using a titrimetric technique. The mass transport efficiency of an Mn only solution was found to be 1.33%. This efficiency decreased, when Li was added, to 90% of the solution only value, and down to 45% of this value when Cs was present. The mass transport decreased in the order Mn only >Li>Na>K>Cs. As equimolar amounts of the EIE were present, the effect is attributed to the total mass of the matrix present and not to the matrix being an EIE. The MnO 2 slurry showed the same effects in the presence and absence of EIEs. Larger solid particles (>3–4 µm) in the slurry resulted in a mass transport efficiency of 0.95% when no EIE was added. This decreased to 35% of the slurry only value when Cs was added. The mechanism for this decrease, together with the importance of matrix loading upon mass transport when determining EIE effects from intensities of concomitant analytes directly from plasma instruments, are discussed.

Supporting titanium dioxide photocatalysts on silica gel and hydrophobically modified silica gel

This report describes preparation and characterization of TiO 2 supported on silica gels and modified silica gels. The objective is manipulation of the photocatalytic TiO 2 phase on the support, separately from manipulation of the organic solute adsorption and mass transport characteristics of the support. If the two features can be controlled separately, these systems should become good models for elucidation of mechanistic behavior of supported photocatalysts. TiO 2 characteristics and adsorption of substrates are key variables. TiO 2 may be anchored on silica gel 100 by hydrolysis of Ti( OR) 4 ( R = isoprophyl) or heating mixtures of silica gel and TiO 2 crystals (p25). In the first case, TiO 2 penetrates pores of silica gel and forms a phase which is either amorphous or of particle size small enough to give broad lines in powder XRD at angles suggesting anatase. In the second case, anatase particles too large to penetrate pores are anchored on the outer surface. The former method produces a superior photocatalyst. The silica surface can be modified by esterification with octanol to produce a photocatalyst which adsorbs solutes from an aqueous medium in a manner describable by an octanol/water partition coefficient. This modification has little effect on photocatalytic reactivity. A larger particle size silica gel (silica gel 58) produces a photocatalyst with TiO 2 in the interior of the pores which is less active for oxidation of 2,4-dichlorophenol but produces long-lived trapped electron colors in the presence of oxidizable substrates (e.g. propanol). This indicates a way to explore the role of intraparticle diffusion in the larger particles which are attractive for large scale applications.

Modification of morphological stability by Soret diffusion

The solute mass transport that may lead to morphological instabilities in directional solidification is described within the framework of the nonlinear thermodynamics of irreversible processes. Specific calculations are carried out for tin-bismuth alloys for two models of the concentration dependence of the thermodiffusion flux. Depending on the sign of the thermodiffusion coefficient, morphological stability may be increased or decreased.

Mass transport in the crack solution as the rate-determining step in the crack growth mechanism during the corrosion fatigue of steel in saltwater solutions

The behavior of the effective diffusion coefficient Deff for the transport of solute species in corrosion fatigue (CF) cracks was investigated by using a triangular CF crack model. Analytical expressions are presented describing Deff as a function of a characteristic dimensionless parameter. Assuming relatively simple boundary conditions and considering the solute transport in the crack to be rate determining, the expected behavior of the crack growth rate as a function of ΔK and the frequency during constant load amplitude CF testing for steel in 3.5 Pct NaCl solutions could be calculated.

Computation of Transverse Mixing in Streams

The problem of solute-mass transport in natural streams is tackled via a governing equation that employs the cumulative discharge, instead of the transverse distance, as an independent variable thus eleminating the need to know the transverse-velocity field. The problem is solved by a numerical scheme, which utilizes exponential interpolation between adjacent nodes. The scheme can regress to a linear-interpolation scheme for Peclet number equal to zero or to an upwind scheme for very large values of the Peclet number. Comparison with field data showed that the problem can be solved satisfactorily based on the knowledge of cross-sectional-geometric data and total-river discharge only, provided that the riverine cross sections are not highly irregular. The computed transverse-dispersion coefficients agree well with values reported in the literature. An independent verification of the transverse-dispersion-coefficient values was obtained by application of a modified method of moments on the same field data.

An investigation of the factors controlling the staining and destaining of electrophoresis gels.

A general analysis is developed for the effect of mass transport and kinetic parameters on the rate of staining and destaining of electrophoresis gels. The various contributions to these overall processes are discussed and, in particular, the ways in which the rate of solution flow, temperature and gel properties can influence staining procedures are highlighted. It is shown that for reproducible, rapid and uniform staining a rotating gel system, analogous to a rotating disc, provides the necessary controlled laminar flow. The theoretical equations derived are compared with experiments and it is shown that at high gel rotation speeds mass transport in solution does not have a major controlling influence on rates of staining and destaining. The temperature dependence of these rates also suggests that there is no significant control by the rate of interaction between stain and protein molecules. The major controlling factor under these conditions is then concluded to be the transport of stain in the gel itself, and the theoretical analysis and time-dependent experimental results allow a determination of the corresponding diffusion parameters.

Micro-rheology changes of Nafion® films with electrochemical mass-transport in hydroquinone solutions and in situ measurement using a quartz crystal analyzer

This paper shows that the electrochemical mass transport in film-coated electrodes involves the rheology change of the films and that these changes can be detected by measuring the resonant frequency and resonant resistance of a coated quartz crystal. We used a Nafion®-coated quartz crystal and measured the cyclic voltammograms in hydroquinone solution. The changes of the resonant frequency and resonant resistance showed that mass transport and viscoelastic changes had occurred in the Nafion films. The diagrammed resonant frequency and resonant resistance changes suggested that the Na ion de-doping from the films decreased the film mass and increased the film rigidity in the first oxidation process, the benzoquinone accumulation and anion doping increased the film mass in the following oxidation process, while the Na ion re-doped in the first reduction process, and benzoquinone was reduced from the films in the final reduction process. Study of the charge expended from the reaction suggested the participation of anion or water transport.

A mass conservative numerical solution of vertical water flow and mass transport equations in unsaturated porous media

The Galerkin finite element method is used to solve the problem of one-dimensional, vertical flow of water and mass transport of conservative-nonconservative solutes in unsaturated porous media. Numerical approximations based on different forms of the governing equation, although they are equivalent in continuous forms, can result in remarkably different solutions in an unsaturated flow problem. Solutions given by a simple Galerkin method based on the h-based Richards equation yield a large mass balance error and an underestimation of the infiltration depth. With the employment of the ROMV (restoration of main varaible) concept in the discretization step, the mass conservative numerical solution algorithm for water flow has been derived. The resulting computational schemes for water flow and mass transport are applied to sandy soil. The ROMV method shows good mass conservation in water flow analysis, whereas it seems to have a minor effect on mass transport. However, it may relax the time-step size restriction and so ensure an improved calculation output.

In vitro measurement and screening of monoclonal antibody affinity using fluoroscence photobleaching

Antibody screening is a routine in vitro assay in monoclonal antibody development and production. We have recently adapted the fluorescence photobleaching method to quantify antibody mass transport and binding parameters in bulk solution (Kaufman and Jain, 1990, 1991). The present study uses this in vitro method to screen a series of monoclonal antibodies (IgG) developed against the rabbit VX2 carcinoma tumor line. These experiments indicate that the three antibodies recognize distinct epitopes on the tumor, with equilibrium binding constantsof 1.3 ± 0.5, 5.1 ± 3.6 and 2.0 ± 1.1 × 10 7 M −1 for the antibodies RVC-184, RVC-626 and RVC-779, respectively. The antibody diffusion coefficient revealed no dependence upon protein concentration or antigen bead volume fraction within the ranges investigated. It was demonstrated experimentally that the interactions conformed to a reaction limited binding model of fluorescence recovery, that the system was at equilibrium, and that non-specific binding due to the fluorescein probe was not significant. Once the non-reactive fraction of antibody is determined, this photobleaching technique does not require perturbation or physical separation of the unbound species. As such, it has many potential applications including in vivo investigation of binding parameters.