Colloid Transport and Colloid-Facilitated Transport in Groundwater

1. Colloid Transport and Colloid-Facilitated Transportin Groundwater Introduction DLVO Theory Stabilization/Transport/Aggregation/Filtration Applications B.C. Williams, 2009

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. Terminology – Mechanisms for Retention • Attachment – adhesion – sorption • Function of collision, collector efficiency, sticking efficiency • Mechanical filtration – complete retention of particles that are larger than all of the soil pores (formation of filter cake) • Straining – physical trapping in geometric corners • Particles can be smaller than smallest pore openings • Requires grain-grain contact • Only occurs in some fraction of soil pore space, transport occurs elsewhere

7. Strained versus Mechanically Filtered dp/d50 .005

8. Background • Clean-bed Filtration Theory • Depends on mechanism of attachment / detachment • Deviation from Clean Bed Filtration Theory • Unfavorable attachment condition; neg-neg • Fine sand and large colloids (dp/d50 .005)

9. Explanations for Deviation from CFT • Attachment w/ porous media charge variability – Johnson and Elimelech, 1995 • Attachment w/ heterogeneity in surface charge characteristics of colloids – Li et al, 2004 • Attachment w/ deposition of colloids in a secondary energy minimum – Tufenkji et al. 2003, Redman et al., 2004 • All of the above – Tufenkji and Elimelech, 2005 • Attachment w/ straining – Foppen et al, 2005, Bradford et al, 2006a, b

10. Theory (cont.) Aqueous Phase Colloid Mass Balance Equation- Bradford et al., 2003 Where: θw = volumetric water content [-] t = time [T] C = colloid concentration in the aqueous phase [N L-3] JT = total colloid flux [N L-2 T-1] EattSW = colloid attachment mass transfer between solid/water phases [N L-3 T-1] EstrSW = colloid straining mass transfer between solid/water phases [N L-3 T-1]

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

12. DLVO TheoryDerjaguin, Landau, Verwey, Overbeek • The stabilityof a homogeneous colloidal suspension depends upon (stability=dispersed) • Van der Waals attractive forces (promote aggregation) http://en.wikipedia.org/wiki/Van_der_Waals_force • Electrostatic repulsive forces that drive particles apart • If electrostatic dominates, particles are electrostatically stabilized (dispersed)

13. Figures to Explain DLVO • Colloid DLVO Theory – • http://www.malvern.com/LabEng/industry/colloids/dlvo_theory.htm • Good figures including ionic strength definition http://wefcol.vub.ac.be/wefcol/lectures/hanoi/h1.pdf

14. 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

15. 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://physics.nyu.edu/grierlab/gold13b/node1.html • http://physics.nyu.edu/grierlab/publications.html

16. 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!)

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

18. 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

19. 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

20. Stabilization/Aggregation • Aggregation occurs depending upon charge, and when double layer shrinks due to increasing ionic strength

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

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

23. 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

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

25. 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

26. 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

27. 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.