Straining and Attachment of Colloids in Physically Heterogeneous Porous Media

Many soil column and batch experiments have been conducted to quantify fundamental properties and proColloid transport studies were conducted in water-saturated physicesses that control colloid transport and fate in the subcally heterogeneous systems to gain insight into the processes controlling transport in natural aquifer and vadose zone (variably saturated) surface, including sedimentation (Wan et al., 1995), hysystems. Stable monodispersed colloids (carboxyl latex microspheres) drodynamics (Wang et al., 1981; Tan et al., 1994), ionic and porous media (Ottawa quartz sands) that are negatively charged strength (Abu-Sharar et al., 1987), pH (Suarez et al., were employed in these studies. The physically heterogeneous systems 1984), chemical heterogeneity (Song and Elimelech, consisted of various combinations of a cylindrical sand lens embedded 1994; Song et al., 1994), hydrophobicity (Bales et al., in the center of a larger cylinder of matrix sand. Colloid migration was 1993), and surfactants (Ryan and Gschwend, 1994). The found to strongly depend on colloid size and physical heterogeneity. A primary mechanism of colloid mass removal by the soil decrease in the peak effluent concentration and an increase in the is typically ascribed to colloid attachment. Attachment colloid mass removal in the sand near the column inlet occurred when is the removal of colloids from solution via collision the median grain size of the matrix sand decreased or the size of the with and fixation to the solid phase, and is dependent colloid increased. These observations and numerical modeling of the transport data indicated that straining was sometimes an important on colloid–colloid, colloid–solvent, and colloid–porous mechanism of colloid retention. Experimental and simulation results media interactions. According to traditional clean-bed suggest that attachment was more important when the colloid size attachment theory (first-order attachment), colloid rewas small relative to the sand pore size. Transport differences between moval by a filter bed decreases exponentially with conservative tracers and colloids were attributed to flow bypassing depth. Colloid attachment theory also predicts an optiof finer-textured sands, colloid retention at interfaces of soil textural mum particle size for transport for a given aqueouscontrasts, and exclusion of colloids from smaller pore spaces. Colloid porous medium system (Yao et al., 1971; Rajagopalan retention in the heterogeneous systems was also influenced by spatial and Tien, 1976). Smaller particles are predicted to be variations in the pore water velocity. Parameters in straining and removed more efficiently by diffusive transport, and attachment models were successfully optimized to the colloid translarger particles by sedimentation and interception. port data. The straining model typically provided a better description of the effluent and retention data than the attachment model, espeExperimental observations of colloid transport are cially for larger colloids and finer-textured sands. Consistent with not always in agreement with colloid attachment theory previously reported findings, straining occurred when the ratio of the (Tufenkji et al., 2003). For example, researchers have colloid and median grain diameters was 0.5%. reported enhanced colloid retention at the soil surface (Camesano and Logan, 1998) and that the spatial distribution of retained colloids does not follow a simple I organic, and microbiological colloids exist exponential decrease with depth (Bolster et al., 1999; in natural and contaminated aquifer and vadose zone Redman et al., 2001; Bradford et al., 2002). Some of environments. These colloid particles can be released these discrepancies have been attributed to soil surface into soil solution and groundwater through a variety of roughness (Kretzschmar et al., 1997; Redman et al., hydrologic, geochemical, and microbiological processes 2001), charge heterogeneity (Johnson and Elimelech, (MacCarthy and Zachara, 1989; Ryan and Elimelech, 1995), and variability in colloid characteristics (Bolster 1996). Knowledge of the processes that control colloid et al., 1999). A time-dependent attachment rate has also transport and fate is required to assess the contaminabeen reported to occur as a result of differences in the tion potential and to protect drinking water supplies attachment behavior of colloids on clean porous media from pathogenic microorganisms (Bitton and Harvey, and on media already containing attached colloids 1992), to develop engineered bioaugmentation and bior(Johnson and Elimelech, 1995). Blocking and ripening emediation strategies (Wilson et al., 1986), and to devise refer to decreasing and increasing attachment with microbially enhanced oil recovery systems (MacLeod et time, respectively. al., 1988). Furthermore, the high surface area of colloids Some of the discrepancies between colloid transport facilitates the sorption of many organic and inorganic data and attachment theory may also be due to the fact contaminants. Colloids can then act as a mobile solid that attachment theory does not account for straining. phase that accelerates the transport of sorbed contamiStraining is the trapping of colloid particles in downnants (Kretzschmar et al., 1999; Ouyang et al., 1996). gradient pore throats that are too small to allow particle passage (McDowell-Boyer et al., 1986). The magnitude of colloid retention by straining depends on both colloid S.A. Bradford, M. Bettahar, J. Simunek, and M.Th. van Genuchten, George E. Brown, Jr. Salinity Laboratory, USDA, ARS, 450 West Big and porous medium properties. Straining occurs when Springs Road, Riverside CA 92507-4617. Received 1 May 2003. Special colloids are retained in dead-end pores that are smaller Section: Colloids and Colloid-Facilitated Transport of Contaminants than some critical size. Colloid transport may still occur in Soils. *Corresponding author (sbradford@ussl.ars.usda.gov). in pores that are larger than this critical size. SakthivadiPublished in Vadose Zone Journal 3:384–394 (2004).  Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: PV, pore volume.