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