Diffusive Transport (diffusive + transport)

Distribution by Scientific Domains


Selected Abstracts


Effect of mass-transfer limitations on bioavailability of sorbed naphthalene in synthetic model soil matrices

ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 9 2000
Hendricus Mulder
Abstract External and internal mass-transfer resistances influencing the bioavailability of sorbed naphthalene in a synthetic model matrix for soil aggregates were investigated in batch experiments in mixed reactors. Amberlite® adsorption resins (XAD4 and XAD7) were used as the synthetic model for soil aggregates. The effect of hydrodynamic conditions in the slurry phase on the diffusive transport across a stagnant film surrounding the model particles was studied. In addition, a mechanistic model was developed based on mass balances, diffusion equations, a nonlinear sorption isotherm, and microbial degradation kinetics. Experimental results could be explained well with this model. In the absence of external transfer limitations, intraparticle effective diffusion coefficients of (3.55 ± 0.10) × 10,9 m2/s and (5.29 ± 0.86) × 10,10 m2/s were determined for naphthalene in Amberlite XAD4 and XAD7, respectively. [source]


Using PHREEQC to Simulate Solute Transport in Fractured Bedrock

GROUND WATER, Issue 4 2007
David S. Lipson
The geochemical computer model PHREEQC can simulate solute transport in fractured bedrock aquifers that can be conceptualized as dual-porosity flow systems subject to one-dimensional advective-dispersive transport in the bedrock fractures and diffusive transport in the bedrock matrix. This article demonstrates how the physical characteristics of such flow systems can be parameterized for use in PHREEQC, it provides a method for minimizing numerical dispersion in PHREEQC simulations, and it compares PHREEQC simulations with results of an analytical solution. The simulations assumed a dual-porosity conceptual model involving advective-reactive-dispersive transport in the mobile zone (bedrock fracture) and diffusive-reactive transport in the immobile zone (bedrock matrix). The results from the PHREEQC dual-porosity transport model that uses a finite-difference approach showed excellent agreement compared with an analytical solution. [source]


A Simple Model of Soil-Gas Concentrations Sparged into an Unlined Unsaturated Zone

GROUND WATER MONITORING & REMEDIATION, Issue 2 2003
David W. Ostendorf
We derive an analytical model of soil-gas contamination sparged into an imlined unsaturated zone. A nonaqueous phase liquid (NAPL) source lies in the capillary fringe, with an exponential sparge constant within the radius of influence and a constant ambient evaporation rate beyond. Advection, diffusion, and dispersion govern the conservative soil-gas response, expressed as a quasi-steady series solution with radial Bessel and hyperbolic vertical dependence. Simulations suggest that sparged contamination initially spreads beyond the radius of influence down a negative gradient. This gradient eventually reverses, leading to a subsequent influx of ambient contamination. Soil-gas concentrations accordingly reflect slowly varying source conditions as well as slowly varying diffusive transport through the radius of influence. The two time scales are independent: One depends on NAPL, airflow, and capillary fringe characteristics, the other on soil moisture, gaseous diffusivity, and unsaturated zone thickness. The influx of ambient contamination generates an asymptotic soil-gas concentration much less than the initial source concentration. The simple model is applied to a pilot-scale sparging study at Plattsburgh Air Force Base in upstate New York, with physically plausible results. [source]


Dynamic Hydrogels: Switching of 3D Microenvironments Using Two-Component Naturally Derived Extracellular Matrices (Adv. Mater.

ADVANCED MATERIALS, Issue 6 2010
6/2010)
The front cover image depicts a two-component extracellular matrix (ECM) in which one component acts as a stable structural element (which supports cell attachment and migration) and another component gels or dissolves reversibly (a modulatory component). Samuel K. Sia and co-workers show on p. 686 that by dynamically adding or removing crosslinks in the modulatory component, properties of the composite ECM, such as the ability of cells to migrate and the rate of diffusive transport, can be altered. [source]


Computational analysis of an instantaneous chemical reaction in a T-microreactor

AICHE JOURNAL, Issue 6 2010
Dieter Bothe
Abstract We extend and apply a method for the numerical computation of convective and diffusive mixing in liquid systems with very fast irreversible chemical reaction to the case of unequal diffusivities. This approach circumvents the solution of stiff differential equations and, hence, facilitates the direct numerical simulation of reactive flows with quasi-instantaneous reactions. The method is validated by means of a neutralization reaction which is studied in a T-shaped micromixer and compared with existing experimental LIF-data. Because of their large are-to-volume ratio, microreactors are well suited for fast chemical reactions which are seriously affected by the slow diffusive transport in aqueous media. Numerical computations for different reactor dimensions reveal the fact that, in a dimensionless setting, the obtained conversion is independent of the reactor size, if the flow conditions are the same. This corresponds to an increase of space-time-yield proportional to the square of the inverse scale factor. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]


Internal loading: A new solution to an old problem in aquatic sciences

LAKES & RESERVOIRS: RESEARCH AND MANAGEMENT, Issue 1 2004
Lars Håkanson
Abstract Internal loading has long been regarded as an ,Achilles heel' in aquatic science and management. Internal loading is of fundamental importance in large and shallow lakes, where even low wind velocities can cause a considerable resuspension of matter deposited on the lake bed. The resuspended matter, and the chemical substances bound to the resuspended matter, will influence almost all processes in the aquatic ecosystem, such as water clarity and depth of the photic zone, and hence, primary and secondary production. If the sediments are contaminated, it will increase the concentrations of harmful substances in water and sediments and the potential ecosystem effects related to such concentrations. This paper presents an overview of the processes regulating bottom dynamic conditions in lakes (erosion, transport, accumulation), provides examples on the role of internal loading within the context of limnology and water management, and presents a new, general approach to quantify internal loading from sediments in lakes. The new approach has been critically tested, being a key factor behind the increase in predictive power of a new generation of lake models meant to be used for practical water management. Internal loading of any water pollutant depends on sedimentation. Sedimentation in this approach is presented as a function of two substance-specific variables, including the fall velocity of the carrier-particles and the particulate fraction (which, by definition, is the only fraction of a water pollutant that can settle out on the lake bed), and three generic variables, including mean depth, suspended particulate matter and ET-areas (areas of erosion and transport). On ET-areas there is, by definition, a discontinuous sedimentation of materials that settles according to Stokes' law. Basically, internal loading is the sum of advective (resuspension) and diffusive transport from the sediments. Resuspension from ET-areas is given as a function of the lake form (a new algorithm based on the volume development) and the age of ET-sediments. [source]


Variability in Transport Properties for Blackbutt Timber in New South Wales: Within and Between-Tree Variability

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 1-2 2006
S. J. Cabardo
Variability is a key issue in the processing of biological materials, in this case the hying of hardwood timber. This paper reports the measurements of variability of transport properties. which are relevant to the drying of blackbutt, Eucalyptus pilularis Sm, from northern New South Wales. Specifically, within-tree and between-tree variations are reported for two blackbutt regrowth logs. An analysis of variance showed that some timber properties were affected by the board positions within-trees and between-trees. Circumferential and radial efects were significant for the within- tree variability of most transport properties. Similarly, radial and circumferential effects were signlficant for most of the transport parameters between trees, but can be tentatively stated because only two regrowth logs were assessed. Timber boards with high initial moisture contents had higher rates of diffirsion and low basic densities using principal components analysis. A possible reason is that if there is less wood material per unit volume, these vacant spaces may be occupied by water, and there is also less resistance for diffusive transport of moisture. [source]


The effect of continuous culture on the growth and structure of tissue-engineered cartilage

BIOTECHNOLOGY PROGRESS, Issue 2 2009
Aasma A. Khan
Abstract The use of bioreactors for cartilage tissue engineering has become increasingly important as traditional batch-fed culture is not optimal for in vitro tissue growth. Most tissue engineering bioreactors rely on convection as the primary means to provide mass transfer; however, convective transport can also impart potentially unwanted and/or uncontrollable mechanical stimuli to the cells resident in the construct. The reliance on diffusive transport may not necessarily be ineffectual as previous studies have observed improved cartilaginous tissue growth when the constructs were cultured in elevated volumes of media. In this study, to approximate an infinite reservoir of media, we investigated the effect of continuous culture on cartilaginous tissue growth in vitro. Isolated bovine articular chondrocytes were seeded in high density, 3D culture on MillicellÔ filters. After two weeks of preculture, the constructs were cultivated with or without continuous media flow (5,10 ,L/min) for a period of one week. Tissue engineered cartilage constructs grown under continuous media flow significantly accumulated more collagen and proteoglycans (increased by 50,70%). These changes were similar in magnitude to the reported effect of through-thickness perfusion without the need for large volumetric flow rates (5,10,L/min as opposed to 240,800 ,L/min). Additionally, tissues grown in the reactor displayed some evidence of the stratified morphology of native cartilage as well as containing stores of intracellular glycogen. Future studies will investigate the effect of long-term continuous culture in terms of extracellular matrix accumulation and subsequent changes in mechanical function. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009 [source]