Colloid Transport and Colloid-Facilitated Transport in Groundwater Introduction DLVO Theory Stabilization/Transport/Aggregation/Filtration Applications Special Case: CFT the Vadose Zone B.C. Williams, 2002

Colloids Defined Particles with diameters < 10 micron, < 0.45 μ Mineral – detrital(as deposited) or autigenic (from matrix) Layer silicates Silica Rich Particles Iron oxides Organic – e.g. humic macromolecules Humic macromolecules Biocolloids – bacteria and viruses

Groundwater Transport in General Usual conceptual model for groundwater transport as follows: Dissolved phase Adsorbed phase (onto soil/rock matrix) How a given chemical partitions into these two phases is represented by the partition coefficient, Kd.

Groundwater Transport Including Colloid-Facilitated Transport Three phases Dissolved phase Adsorbed phase (onto soil/rock matrix) Adsorbed onto mobile particles

Colloid-Facilitated Groundwater Transport Solid matrix mobile colloid Adsorbed Dissolved

DLVO Theory Derjaguin, Landau, Verwey, Overbeek The stability of a homogeneous colloidal suspension depends upon (stability=dispersed) Van der Waals attractive forces (promote aggregation) Electrostatic repulsive forces that drive particles apart If electrostatic dominates, particles are electrostatically stabilized (dispersed)

DLVO - stabilized Colloids are stabilized (in suspension) when: Double layers expand (by decreasing electrolyte concentration, decreasing ionic strength Net particle charge  0 Colloids coagulate/aggregate when: Double layer shrinks because of increasing ionic strength

Challenges to DLVO Hot controversy in literature on whether spheres of like charge always repel. Experimental evidence that colloidal electrostatic interactions include a long-ranged attractive component. http://griergroup.uchicago.edu/~grier/leshouches2/leshouches2.html http://griergroup.uchicago.edu/~grier/comment3b/

Stabilization – and sorbable species Sorbed species can influence surface charge, and therefore stability (end of DLVO discussion) Sorbed species can also be mobilized if the colloid is mobilized through the soil/rock matrix (colloid-facilitated transport!)

Colloid Transport in General (Saturated and Unsaturated GW) Detachment / Mobilization / Suspension Stabilization Transport Aggregation / Filtration / Straining

Detachment/Mobilization/ Suspension Colloids can detach from matrix Biogeochemical weathering Precipitation from solution (thermodyn’) Biocolloids or humics flushed from shallow zones If cementing agents dissolve If stable aggregates deflocculate

Transport More likely if colloid is neg’. charged, because most soil/rock matrices are neg’. Transport optimal if: Slow interpore transport rate – few collisions with side surfaces High velocities in preferential pathways In preferential pathways, may have faster travel times than ambient gw flows

Stabilization/Aggregation Aggregation occurs when double layer shrinks due to increasing ionic strength (slide #6)

Filtering / Straining Physical filtering – due to size, geometry Physicochemical straining – surface chemical attraction to matrix Cementation agents (iron oxides, carbonates, silica)

Applications Many engineering ramifications of passage versus filtration Colloid-facilitated transport – how a low-solubility (strongly-sorbed!) contaminant can travel miles from the source

Engineering Applications Wastewater – sand filters – removal is good, too-small particles clog Roads – clogging of drain filters  force buildup  failure Dams – matrix piping  erosion  26% of earth dam failures ref: Reddi, 1997

Engineering Applications, cont. Petroleum Extraction – permeability reduction termed “formation damage” Slurry Walls – very fines filtered by fines is considered good Lining of Lakes/Reservoirs – ditto ref: Reddi, 1997

Colloid-Facilitated Transport When a highly sorptive contaminant (constituent) is adsorbed onto colloids Contaminant of interest must have as high or higher affinity to sorb as other possible constituents Colloid may have “patches” of surface coatings (ferric, aluminum or manganese oxyhydroxides) that are best sites

Colloid Transport in the Unsaturated Zone Colloids may be strained, or retarded, if moisture content reduced so that water films have thickness less than colloid diameter Colloids may sorb to the air/water interface Called partitioning – same Kd.concept

Colloid Transport in the Unsaturated Zone Ongoing Research Film Straining of Colloids http://www.lbl.gov/~jwan/film_straining/film_straining.html http://www.lbl.gov/~jwan/particles_film/particles_film.html Colloids Sorbing to the Air-Water Interface http://www.lbl.gov/~jwan/colloid_partition/colloid_partitioning.html

References Johnson, P.R., Sun, N., and Elimelech, M., 1996. “Colloid Transport in Geochemically Heterogeneous Porous Media”, Environmental Science and Technology, 30, 3284-3293.  Reddi, L. N., 1997. Particle Transport in Soils: Review of Significant Processes in Infrastructure Systems. J. Infrastructure Systems. 3, 78-86.  McCarthy, J.F., Zachara, J.M., 1989. “Subsurface Transport of Contaminants”. Environmental Science and Technology, 23, 496-502. Wan, J. T.K. Tokunaga, 1998. "Measuring partition coefficients of colloids at air-water interfaces", Environ. Sci. Technol, 32, p3293-3298, Wan, J., Wilson, J.L., 1994. Colloid transport in unsaturated porous media. Water Resources Research. 30, 857-864.

Acknowledgements Jason Shira, MS Student George Redden, INEEL