Colloid Transport and Colloid-Facilitated Transport in Groundwater

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Colloid Transport and Colloid-Facilitated
Transportin Groundwater

• Introduction
• DLVO Theory
• Stabilization/Transport/Aggregation/Filtration
• Applications

B.C. Williams, 2009
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Colloids Defined

• Particles with diameters lt 10 micron, lt 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

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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.

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Groundwater Transport Including
Colloid-Facilitated Transport

• Three phases
• Dissolved phase
• Adsorbed phase (onto soil/rock matrix)
• Adsorbed onto mobile particles

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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

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Strained versus Mechanically Filtered
dp/d50 ? .005
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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)

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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

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Theory (cont.)
Aqueous Phase Colloid Mass Balance Equation-
Bradford et al., 2003

• Where
• ?w 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

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Colloid-Facilitated Groundwater Transport
Solid matrix
mobile colloid
Adsorbed
Dissolved
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DLVO TheoryDerjaguin, Landau, Verwey, Overbeek

• The stability of a homogeneous colloidal
  suspension depends upon (stabilitydispersed)
• 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)

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Figures to Explain DLVO

• Colloid DLVO Theory
• http//www.malvern.com/LabEng/industry/colloids/dl
  vo_theory.htm
• Good figures including ionic strength definition
  http//wefcol.vub.ac.be/wefcol/lectures/hanoi/h1.p
  df

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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

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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

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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!)

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Colloid Transport in General(Saturated and
Unsaturated GW)

• Detachment / Mobilization / Suspension
• Transport
• Aggregation / Filtration / Straining

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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

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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

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Stabilization/Aggregation

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

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Filtering / Straining

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

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Applications

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

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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

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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

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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

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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

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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.