Transport of suspended participate matter is widely recognized to occur in subsurface environments. Field data indicate that viruses, bacteria, and clay minerals can migrate considerable distances and that small particles and macromolecules are implicated in the transport of organic contaminants and radio-nuclides. Furthermore, media permeability can be significantly altered by changes in aqueous chemistry through particle release and capture. Quantitative models for predicting particle transport are available within the water filtration literature that account for the mechanisms of particle-media collisions and the conditions for attachment. Predictions from the filtration models are used to analyze particle migration through porous media at typical groundwater flow velocities. As particles accumulate within media pores, available models become less predictive because of the coupling between particle retention and permeability reduction. An examination of filtration data reveals that retention of a relatively small solid volume within media pores can reduce media permeability by orders of magnitude. The fact that contaminants adsorbed to particles are mobile has important implications in understanding and predicting contaminant transport. The design of laboratory experiments and the collection of field samples often neglect contaminants transported by suspended colloids and particles.

Particle transport through porous media

A conceptual model for colloid transport is developed that accounts for colloid attachment, straining, and exclusion. Colloid attachment and detachment is modeled using first-order rate expressions, whereas straining is described using an irreversible first-order straining term that is depth dependent. Exclusion is modeled by adjusting transport parameters for colloid-accessible pore space. Fitting attachment and detachment model parameters to colloid transport data provided a reasonable description of effluent concentration curves, but the spatial distribution of retained colloids at the column inlet was severely underestimated for systems that exhibited significant colloid mass removal. A more physically realistic description of the colloid transport data was obtained by simulating both colloid attachment and straining. Fitted straining coefficients were found to systematically increase with increasing colloid size and decreasing median grain size. A correlation was developed to predict the straining co...

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Modeling colloid attachment, straining, and exclusion in saturated porous media.

[1] Saturated soil column experiments were conducted to explore the influence of colloid size and soil grain size distribution characteristics on the transport and fate of colloid particles in saturated porous media. Stable monodispersed colloids and porous media that are negatively charged were employed in these studies. Effluent colloid concentration curves and the final spatial distribution of retained colloids by the porous media were found to be highly dependent on the colloid size and soil grain size distribution. Relative peak effluent concentrations decreased and surface mass removal by the soil increased when the colloid size increased and the soil median grain size decreased. These observations were attributed to increased straining of the colloids; i.e., blocked pores act as dead ends for the colloids. When the colloid size is small relative to the soil pore sizes, straining becomes a less significant mechanism of colloid removal and attachment becomes more important. Mathematical modeling of the colloid transport experiments using traditional colloid attachment theory was conducted to highlight differences in colloid attachment and straining behavior and to identify parameter ranges that are applicable for attachment models. Simulated colloid effluent curves using fitted first-order attachment and detachment parameters were able to describe much of the effluent concentration data. The model was, however, less adequate at describing systems which exhibited a gradual approach to the peak effluent concentration and the spatial distribution of colloids when significant mass was retained in the soil. Current colloid xfiltration theory did not adequately predict the fitted first-order attachment coefficients, presumably due to straining in these systems. INDEX TERMS: 1831 Hydrology: Groundwater quality; 1832 Hydrology: Groundwater transport

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Physical factors affecting the transport and fate of colloids in saturated porous media

Cell seeding of three-dimensional polymer scaffolds is the first step of the cultivation of engineered tissues in bioreactors. Seeding requirements of large scaffolds to make implants for potential clinical use include: (a) high yield, to maximize the utilization of donor cells, (b) high kinetic rate, to minimize the time in suspension for anchorage-dependent and shear-sensitive cells, and (c) high and spatially uniform distribution of attached cells, for rapid and uniform tissue regeneration. Highly porous, fibrous polyglycolic acid scaffolds, 5-10 mm in diameter and 2-5 mm thick, were seeded with bovine articular chondrocytes in well-mixed spinner flasks. Essentially, all cells attached throughout the scaffold volume within 1 day. Mixing promoted the formation of 20-32-micron diameter cell aggregates that enhanced the kinetics of cell attachment without compromising the uniformity of cell distribution. The kinetics and possible mechanisms of cell seeding were related to the formation of cell aggregates by a simple mathematical model that can be used to optimize seeding conditions for cartilage tissue engineering.

Dynamic Cell Seeding of Polymer Scaffolds for Cartilage Tissue Engineering

http://dspace.nitrkl.ac.in:8080/dspace/bitstream/2080/134/1/tksen5.pdf

Review on subsurface colloids and colloid-associated contaminant transport in saturated porous media.

Flow of Suspensions through Porous Media—Application to Deep Filtration

A fluidized bed electrode system comprises an elec trode chamber having a porous base, a main electrode consisting of a bed of fine solid particles contained in such chamber and selected from the group consisting of metals, metal coated glass beads and metal coated plastic beads, a current feeder extending into the bed, means for leading air or inert gas through the porous base at a predetermined velocity and pressure to fluid ize the bed, and means for introducing a solution to be treated into the electrode chamber at a point above the porous base and for circulating such solution through the bed at a predetermined flow rate.

Applications of fluidized beds in electrochemistry

The general principles of fluidisation are reviewed, followed by a description of the principles of electrochemical mass transfer measurement, local porosity measurement by electrical conductivity, and measurement of porosity fluctuations. An original experimental layout is briefly described in which these three quantities are measured locally in a fluid bed with an aqueous alkaline potassium ferro-and ferri-cyanide solution. The results of these measurements are given in the form of mass transfer vs. fluid flow velocity curves. Curves for vertical probe measurements show a maximum and some curves obtained with a horizontal probe also feature a minimum at high velocities. An explanation is given for this. Except near the ends of the bed, porosity and porosity fluctuations do not vary with probe position along the bed centreline. If the mass transfer data are plotted against porosity it is seen that maximum transfer is associated with a porosity value of 0.6, and if the difference between the coefficients measured in the bed and in the liquid alone at the same interstitial velocity are plotted against porosity, the data for various particle sizes can be combined into a single curve. The downward limb of curves obtained with the probe vertical can be put in the general form jDe = 1.21 (Re) - 0.52 for 6 < Re < 200 and agrees well with the Jagannadha Raju correlation. The mass transfer data for the liquid alone also agree satisfactorily with published data. Solid concentration fluctuations increase steadily with fluid velocity, and therefore also with porosity. With a similar theory to the van der Waals gas density fluctuation theory it can be shown that the bed particles do not behave as independent entities but that they agglomerate and form groups, and that the smaller the particles, the bigger the groups are.

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P. Le. Goff

Papers

Flow of Suspensions through Porous Media—Application to Deep Filtration

Applications of fluidized beds in electrochemistry

Turbulent diffusion mass transfer in fluidised beds