Colloid Transport and Colloid-Facilitated Transport in Groundwater

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

2. 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

4. 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.

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

6. Colloid-Facilitated Groundwater Transport Solid matrix mobile colloid Adsorbed Dissolved

7. DLVO TheoryDerjaguin, Landau, Verwey, Overbeek • The stabilityof 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)

8. 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

9. 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/

10. 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!)

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

12. 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

13. 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

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

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

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

17. 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

18. 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

19. 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

20. 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

21. Colloid Transport in the Unsaturated ZoneOngoing 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

22. 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.

23. Acknowledgements • Jason Shira, MS Student • George Redden, INEEL