1. 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

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 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)

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.

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 Zone Ongoing Research
Film Straining of Colloids
Colloids Sorbing to the Air-Water Interface

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

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