Chemical mobilization of micron-sized particles in saturated porous media under steady flow conditions

Abstract

Particles in Saturated Porous MediaMany waterborne contaminants are particulate in nature or are associated with small particles or colloids. Thus, consideration of the mobility of colloids in porous media is necessary for an understanding of contaminant transport in groundwater. Immobilization of colloids in porous media has been studied extensively for water filtration applications, but the reverse process of mobilization is seldom considered. This study addresses the latter process. Theoretical treatment of the mobilization process in steady flow situations generally involves consideration of London-van der Waals attractive forces and electrical double-layer attractive or repulsive forces at the point of contact between media and particle surface. Laboratory sand columns were used to test the suitability of this approach with respect to observed mobilization under changing chemical conditions. Colloids deposited on the porous media surface were mobilized when exposed to lower ionic strength solution without disturbing the flow rate. Although results did not contradict theory qualitatively, further theoretical development is necessary to consider a broader range of surface forces likely to affect mobilization behavior. Flow rate dependence of mobilization is indicated and must be investigated further in order to understand its basis.

Colloid-Facilitated Transport of Radionuclides through the Vadose Zone

“…However, the results for flow 8 rate effect as reported below suggest that macropore flow might still occur in the columns packed with coarse Hanford sand due to initial non-uniform distribution of water contents between macropore and micropore, as well as coarse texture of the sediment in which flow was significantly subject to gravity effect. Physical and chemical perturbations resulted from the transient flow were assumed to be primarily responsible for the peak concentrations of the released colloids (McDowell-Boyer, 1992;El-Farhan et al, 2000;Blume et al, 2005). However, mechanisms dominating the transient flow effects may vary with flow stages and depend on many factors, such as, source colloid concentration, initial water content, chemical condition, sediment texture, flow velocity, pore straining, and preferential attachment of colloids to moving air-water interfaces .…”

“…However, mechanisms dominating the transient flow effects may vary with flow stages and depend on many factors, such as, source colloid concentration, initial water content, chemical condition, sediment texture, flow velocity, pore straining, and preferential attachment of colloids to moving air-water interfaces . As documented in the literature, in-situ colloid mobilization is driven by net interaction energy, which is defined by the sum of hydrodynamic forces, electrostatic double-layer (EDL) force, van der Waals force, and short-range solvation or steric force (McDowell-Boyer, 1992;Ruckenstein and Prieve, 1976;Israelachvili and McGuiggan, 1988). The colloid release pattern as shown in Figure 3 is presumably attributed to a coupling of two mechanisms.…”

A brief review of bacterial transport in natural porous media

“…Recent research has demonstrated that physical filtration is influenced by solute ionic strength (through its effect on the size of the diffuse double layer (Sharma et al 1985;Scholl et al 1990;McDowell-Boyer 1992;Tan et al 1994), pH (ionization of mineral grains and coatings (McEldowney and Fletcher 1988), and mineralogy (Scholl et al 1990). Shonnard et al (1994) found significant increases in adsorption of a trichloroethylene-WE-) degrading bacterium as a result of increased ionic strength of solution in coarse sands.…”

“…Available methods for incorporating the effect of ionic strength of solution (as in Wnek et al 1975) on the attachmentldetachment process are rarely used; these methods are not robust, as pointed out by Tobiason (1989) and by Elimelich and O'Melia (1990), who found large discrepancies between theoretical and model predictions. Experimental studies, on the other hand, have been used to examine the effect of ionic strength on the effective attachment coefficient (Fontes et al 1991;Hornberger et al 1992), on the effective detachment coefficient (McDowell-Boyer 1992), and on both (McCaulou et al 1994). Significant discrepancies are also usudly found between the theoretically estimated values of the parameters and those fitted to obtain best model fits (Hornberger et al 1992;McCaulou et al 1994).…”

“…textbook by de Marsily (1986)l and have been widely applied to microbial transport following the introduction of the idea by Harvey et al (1989) and its application by Harvey and Garabedian (1991). The consequent reversibility of physical filtration mass removal (via reduction in solute ionic strength; Scholl et al 1990;McDowell-Boyer 1992), as well as the reversibility incurred by changing the pore-water velocity direction , has generally been ignored in applications of filtration theory to microbial transport because the filtration models represent irreversible deposition only under conditions of uniform flow direction and fixed solution chemistry.…”