Colloid Transport and Colloid-Facilitated Transport in Groundwater Introduction DLVO Theory...
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Transcript of Colloid Transport and Colloid-Facilitated Transport in Groundwater Introduction DLVO Theory...
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 μMineral – detrital(as deposited) or autigenic
(from matrix)Layer silicatesSilica Rich ParticlesIron oxides
Organic – e.g. humic macromoleculesHumic macromoleculesBiocolloids – bacteria and viruses
Groundwater Transport in General
Usual conceptual model for groundwater transport as follows:Dissolved phaseAdsorbed 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
TransportThree phases
Dissolved phaseAdsorbed phase (onto soil/rock matrix)Adsorbed onto mobile particles
Colloid-Facilitated Groundwater Transport
Adsorbed
Solid matrix
Dissolved
mobilecolloid
DLVO TheoryDerjaguin, 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/lesho
uches2/leshouches2.htmlhttp://griergroup.uchicago.edu/~grier/comm
ent3b/
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
StabilizationTransportAggregation / Filtration / Straining
Detachment/Mobilization/Suspension
Colloids can detach from matrixBiogeochemical weatheringPrecipitation from solution (thermodyn’)Biocolloids or humics flushed from
shallow zonesIf cementing agents dissolveIf 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 surfacesHigh 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, geometryPhysicochemical straining – surface
chemical attraction to matrixCementation 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 TransportWhen a highly sorptive contaminant
(constituent) is adsorbed onto colloidsContaminant 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 interfaceCalled partitioning – same Kd.concept
Colloid Transport in the Unsaturated Zone
Ongoing Research
Film Straining of Colloidshttp://www.lbl.gov/~jwan/film_straining/film_str
aining.htmlhttp://www.lbl.gov/~jwan/particles_film/particle
s_film.html
Colloids Sorbing to the Air-Water Interface
http://www.lbl.gov/~jwan/colloid_partition/colloid_partitioning.html
ReferencesJohnson, 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 StudentGeorge Redden, INEEL