Processes of colloid mobilization and transport in macroporous soil monoliths
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.Geoderma 93 1999 3359
Processes of colloid mobilization and transport inmacroporous soil monoliths
M. Lgdsmand a,), K.G. Villholth b, M. Ullum a, K.H. Jensen aa Institute of Hydrodynamics and Water Resources, Technical Uniersity of Denmark, 2800
Lyngby, Denmarkb VKI, Agern Alle 11, 2970 Hrsholm, Denmark
Received 21 September 1998; received in revised form 28 April 1999; accepted 29 April 1999
Transport of pesticides, PAH and other hydrophobic or surface-complexing contaminants insoils may be enhanced by colloid-facilitated transport. A prerequisite for colloid-facilitatedtransport is the release and transport of colloids. The mechanisms for colloid mobilization andtransport in a macroporous Alfisol have been evaluated by measuring the amount and type ofcolloids leached in two large soil monoliths during long duration simulated rain events. The soilwas irrigated with water having a chemical composition close to natural rainwater and atintensities as expected under natural conditions. The results showed that the colloids wereprimarily mobilized and transported in the macropores and that the source of colloids was notexhausted for extended rainfall duration. The first flush of water mobilized loosely bound colloidsthat had a high organic content relative to the bulk soil. After the initial release, the high ionicstrength in the percolating water limited the mobilization. For prolonged leaching, the diffusion ofcolloids from the macropore walls appeared to rate-limit the mobilization process. During the lateleaching phase, the rate of colloid mobilization was positively correlated with flow velocity.q 1999 Elsevier Science B.V. All rights reserved.
Keywords: colloidal materials; erosion; preferential flow; macropores; organic materials; diffusion
Colloids are defined as suspended particles with a small size. The maximumsize is limited by a tendency of larger particles to sediment and is generally
) Corresponding author. Present address: Department of Crop Physiology and Soil Science,Danish Institute of Agricultural Sciences, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele,Denmark. E-mail: email@example.com
0016-7061r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. .PII: S0016-7061 99 00041-5
( )M. Lgdsmand et al.rGeoderma 93 1999 335934
below a few micrometers. The minimum size, separating colloids from dissolved .matter, is about 10 nm Ryan and Elimelech, 1996 . Colloids may, due to their
small size and depending on the physico-chemical conditions, remain suspendedin electrolytic fluids. In addition, soil colloids are relatively reactive with respectto sorption of chemical species due to the large specific surface area and thehigh number of functional surface groups. Both the ability to be suspended andthe sorption capacity make the colloids potential carriers for pollutants inflowing water of rivers, oceans and soils. Soil colloids mainly consist of clayminerals, organic matter and oxidesrhydroxides. Colloid-facilitated transport in
.soils requires that three different processes take place: 1 the pollutant must . sorb to the colloid, 2 the colloid must be mobile mobilized either before or
. .after the sorption and 3 the colloidrpollutant complex must be transportedthrough the soil. Hence, pollutants that have a low solubility in water and a highpartition coefficient between soil and water, e.g., certain pesticides, heavymetals and PAHs, may be transported at a rate beyond what is expected frompartitioning only to the stationary soil matrix. A number of studies have dealt
.with the complex issue of colloid-facilitated transport. Vinten et al. 1983 .soils; clay; DDTr paraquat showed that the transport of pesticide was depen-dent on the ionic strength of the infiltrating water and the texture of the soil. Afew reports deal with experimental evidence of colloid-facilitated transport of
. chemical species in undisturbed soil media. De Jonge et al. 1997 structured.soil; natural colloids; prochloraz observed that about 20% of the leached
pesticide was sorbed to mobilized particles )0.02 mm. Seta and Karathanasis . .1996 structured soil; dispersed colloids; metolachlor found that the presenceof colloids enhanced the transport of pesticide by 22 to 70% depending on thecolloid type and mobility.
The mobilization of colloids in soils and groundwater sediments due tochanges in flow or chemistry has been reported in several studies. Chemicalperturbation can affect the forces that keep the colloids bound to the grains and
thereby the mobilization of colloids Ryan and Gschwend, 1994a quartz sand;. .haematite ; Seaman et al., 1995 aquifer sand; natural colloids ; Kaplan et al.,
..1996 reconstructed soil; natural colloids . Diffusion from the detachment siteinto the flowing water may be the limiting factor for the rate of mobilization
when the chemical conditions are favourable for the release of colloids Ryan . and Gschwend, 1994a quartz sand; haematite ; Jacobsen et al., 1997 structured..soil; natural colloids . Dissolution of cementing agents may enhance the
. mobilization of colloids from grains. Ryan and Gschwend 1994b goethite-.coated aquifer sand; kaolinite found that the mobilization of kaolinite colloids
was increased when the goethite was dissolved due to changing redox condi-tions. Increasing the flow rate can cause enhanced mobilization Kaplan et al.,
. 1993 reconstructed soil; natural colloids ; Ryan and Gschwend, 1994a quartz. ..sand; haematite ; Govindaraju et al., 1995 sand; kaolinite . Kaplan et al.
.1993 found that the colloid concentration in the effluent from lysimeters
( )M. Lgdsmand et al.rGeoderma 93 1999 3359 35
depends on the flow velocity squared, suggesting mobilization by shear stress. .Pilgrim et al. 1978 reported that subsurface flows during storm events mobi-
lized large amounts of particles in the size range 48 mm. This was explainedby raindrop impact on the surface soil combined with macroporous flow paths.
Generally, the mobilization of colloids in homogenous sand is described inmany studies, but the mobilization of colloids in naturally structured soils is notwell described. Several studies have shown that colloidal particles applied
.externally can be transported in subsurface environments. Jacobsen et al. 1997 .undisturbed soil; illite and humus-coated illite showed that the mass recoveryof surface-applied colloids in leachate from subsoil columns was higher thanfrom topsoil columns and that the recovery increased with increasing flow rate.
. .McKay et al. 1993 fractured clay till; bacteriophages found a hundred-foldgreater retardation of conservative tracers compared to colloid tracers in a fieldexperiment and attributed it to the preferential diffusion of solutes into the
.matrix. Toran and Palumbo 1991 found that the retardation of colloids inpacked sand columns decreased when artificial macropores oriented in the flowdirection was embedded in the medium and that macropores with larger
.diameter created multiple peak breakthrough curves. Kretzschmar et al. 1995observed that the leaching of clay colloids passing through an intact saprolitewas dependent on the natural coating of the colloids with natural organic matter . .NOM . The untreated colloids with NOM resulted in blocking effects forfurther deposition due to a monolayer restriction for the continued attachment of
.colloids, while the treated colloids without NOM resulted in ripening due tomultiple layer attachment.
In the present study, the effect of macropores and low ionic strength .infiltration water corresponding to natural rain on colloid mobilization and
transport was investigated. The combination of continuous and hydraulicallyactive macropores and infiltration water with a low ionic strength can lead to anaccelerated removal of ions from the macropores and the surrounding matrix,resulting in destabilization of the aggregates at the macropore walls and therebyincreased mobilization and transport of colloids. Two undisturbed soil monolithsexcavated from a site in western Denmark and installed in the laboratory wereexposed to long duration rain events to investigate the processes of colloidmobilization under temporally changing chemical conditions and under varyingflow rates.
2. Materials and methods
2.1. Theoretical background
2.1.1. Chemical perturbationsThe chemistry of the pore water will affect the interacting forces acting on the
colloids: electrostatic forces, van der WaalsLondon forces and Born repulsion.
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The electrostatic forces are particularly sensitive to changes in the electrolyticproperties of the fluid. The electrostatic potential around a single spherical
.charged particle in an electrolytic fluid can be described by Kruyt, 1952 :
c sc eyk x 1 .e e,0
1 RT 1 3.09s s2) k 1000 I I2 N q .a eat Ts258C and s6.95=10y10C 2rJ m 2 . .
where c is the electrical potential at the surface of the colloid, c thee,0 eelectrical potential at the distance x from the colloid surface, 1rk the Debyelength, which is often interpreted as the thickness of the double layer of theparticle, the permittivity of the fluid, R the universal gas constant, T theabsolute temperature of the fluid, N Avogadros number, q the charge on thea eelectron, and I the ionic strength of the solution.
When the release of colloids from the grains is controlled by the electrolyticproperties of the fluid, the rate constant for detachment and attachment ofcolloids is proportional to an exponential function of the size of the energy
.barrier to detachment or attachment Ruckenstein and Prieve, 1976 :
Nf yf Nmax min ,1k Aexp y 3 .det /kTNf Nmaxk Aexp y 4 .att /kT
where k is the rate constant for detachment and k for attachment. f isdet att maxthe maximum of the potential energy between two colloids and f is themin,1primary minimum. This will cause the release of colloids across the energybarrier to be a first-order reversible heterogeneous reaction with the energybarriers serving as activation energy for the process. When the ionic strength is
.decreased, f is increased Ruckenstein and Prieve, 1976 and hence the ratemin,1of detachment is increased. If the process of colloid mobilization is controlledby chemical perturbations, there are two steps involved in the process. A processof detachment of colloids from the grains followed by a diffusion process fromthe grain surface into the pore stream. If the rate of detachment is higher thanthe rate of diffusion, e.g., at low ionic strength, the diffusion process will limitthe overall mobilization, and vice versa.
2.1.2. DiffusionFor the evaluation of diffusive transport of dissolved or suspended species
intrinsically present in macroporous soil, two different processes should be
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. .considered: 1 diffusion within the matrix and 2 diffusion from the walls ofthe macropores and into the main macropore stream. The matrix can be viewedas a semi-infinite medium and the macropore wall as the plane that limits it.When the initial concentration of the diffusing species throughout the matrix isC and the concentration at the macropore walls is zero, the accumulated0
. amount of the diffusing species M that is transported through the plane att. .xs0 per area at time t is Crank, 1975, p. 32 :
D tABM s2C 5 .(t 0 pwhere D is the diffusion coefficient of substance A in medium B. TheABconcentration in the macropore water is not zero due to upstream inflow of thesubstance, but compared to the much higher concentration in the matrix, hereconsidered the sole source, it can generally be neglected.
For the initial stages of a diffusion process out of a plane sheet of thickness lwith uniform initial concentration in the sheet and constant zero surfaceconcentration, a similar equation can be obtained for the accumulated loss of
.diffusing substance M out of the sheet Crank, 1975, p. 244 :t
D tABM s4M l 6 .(t pwhere M is the accumulated loss of diffusing substance out of the sheet for tapproaching infinity. This equation has been applied to indicate the diffusion-controlled process of non-equilibrium sorption or desorption of various sorbates
in soil in well-mixed batch or packed flow systems Pavlatov and Polyzopoulos,.1988; Kookana et al., 1992 . For a macroporous flow system, an analogy can be
made as a semi-stagnant sheet of water along the macropore wall develops .through which the substances need to diffuse. Jacobsen et al. 1997 observed
linearity of cumulative mass of colloids leached from undisturbed soil columnsvs. square root of time. Consequently, when a process is controlled by diffusion .either in the matrix or from the walls of the macropores , a plot of cumulativemass of diffusing substance vs. square root of time will produce a straight line .at least in the initial stages of the diffusion process .
The linear relation of cumulative mass vs. square root of time does not provethat diffusion controls the process, but if diffusion controls the process therelation will be linear. For a more stringent evaluation of substance mobility thatincludes radial diffusion from the macropore wall as well as the verticaladvective transport in the macropores, the soil is equivalated to a model of animpermeable matrix with equally sized, vertical cylindrical macropores. Themacropores are considered to be full-flowing and the flow is assumed to beequivalent to laminar Poiseuille flow throughout the tubes. The validity of this
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assumption requires that L the depth from the soil surface where the Poiseuillee.flow is fully developed is small. For steady-state conditions, the advectiondif-
.fusion equation can then be simplified to Clark, 1996 :ECA
E r /EC 1 ErAu sD 7 .z ABEz r Er
where z is the depth from the soil surface, r is the distance from the center ofthe tube, u is the advective velocity in the macropores and C is thez Aconcentration of diffusing substance A as a function of r and z. If the inletconcentration at the soil surface is zero and the concentration at the macropore
.surface C is constant the following solution is obtained Clark, 1996 :A,sC G zraA ,ave n 2s1y8 exp yl 8 .n2 /C l Re ScA ,s n
where C is the averaged concentration of substance A over the tube crossA,avesection at depth z, a is the radius of the tube, Re is Reynolds number, Sc isSchmidts number, G is an infinite series of constants and l are the eigenval-n n
..ues G and l are given by Clark 1996 . This model does not adequatelyn ndescribe the diffusion of species in a macropore as the flow in the actualmacropores probably does not fulfill the assumption of full-flowing macroporesat all times, but for evaluation of the trends it is useful. When Eq. 7 and Eq. 8
.apply, the outflow concentration C will be negatively correlated to theA,ave .flow velocity u as increased dilution of the diffusing substance occurs withz
increased flow. Hence, when e...