Colloid release and transport processes in natural and model porous media
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ELSEVIER Colloids and Surfaces
A: Physicochemical and Engineering Aspects 107 ( 1996} 245 262
COLLOIDS AND A SURFACES
Colloid release and transport processes in natural and model porous media
Sujoy B. Roy, David A. Dzombak * Department o[ Civil and Environmental Engineering, Carnegie Mell(m University, Pittsburgh, PA 15213
Received 15 March 1995: accepted 21 July 1995
Colloid release was observed from packed columns for two natural porous media (sands) and one model system (glass beads with deposited latex colloids). Colloid release was found to occur in all cases when the ionic strength was reduced in columns that were in equilibrium with Na ions. Most of the released colloids from the natural porous media were smaller than 1 ~tm in size, and comprised pure and impure forms of silica (60 70% by mass) and clay minerals (20-30% by mass). For greater reductions in ionic strength, the total mass of released colloids increased, although the shape of the effluent mass concentration profile did not change. Release rate coefficients were obtained by fitting a colloid transport model (an advection-dispersion transport model with source/sink terms for colloid release and deposition) to the column effluent data. To fit the data for different ionic strengths, the total available mass of releasable colloids had to be adjusted, and fitted release rate coefficients were not sensitive to the ionic strength. In contrast, calculations based on Derjaguin-Landau-Verwey Overbeek (DLVO) theory indicate a strong dependence of release rate constants on ionic strength for homogeneous colloids. This discrepancy can be attributed to charge and size heterogeneity in the colloids, and to our inability to determine accurately interparticle forces at small separations. The trend of greater mass release for greater reductions in ionic strength could be explained qualitatively by computing interparticle interactions with a constant-charge boundary condition (albeit with a charge density much lower than that experimentally determined) which showed a decreasing energy barrier for particle detachment with decreasing ionic slrength.
Keywords: Colloid release; Colloids; Glass beads; Latex colloids; Natural sands; Packed columns; Porous media
Most natural porous media such as soils and aquifer materials contain some col loidal particles that are attached to the surfaces of larger fixed particles or are in a f locculated state. The mass fraction of colloid-size particles (generally defined as less than 2 gm in diameter) in a subsurface mineral grain assemblage can vary over a wide range (i.e. from less than 1% to tens of per cent)
* C,~rrcsponding author.
0927-7757/96/$15.00 (9 1996 Elsevier Science B.V. All rights reserved SSI)I 0927-7757( 95)03367-X
and is typically higher for surface soils than for aquifer materials. Col loidal particles in natural porous media are usually immobi le during normal electrolyte and water flow conditions. However, when the electrostatic repulsion between particles is increased, most commonly by lowering the ionic strength, they may disperse into the aqueous phase and be transported through the porous medium. An understanding of colloid release under changing ionic strength is of environmental interest because this is one mechanism by which suspended colloi- dal particles may be introduced in aquifers and
246 5:R Roy, D.A. Dzombak/Colloids SurJaces A: Physicochem. Eng. Aspects 107 (1996)245-262
potentially enhance the subsurface transport of contaminants sorbed on their surfaces [1,2].
The colloid release phenomenon has been studied in soil science and petroleum engineering where the primary concern is the decrease of permeability that results from clogging of pores from mobilized colloids in soils and oil-bearing rock formations [3-13]. More fundamental studies of colloid detachment (with a view to relating observed rates to interparticle forces) have been performed with model colloids and porous media [14-20] as well as flat plates [21-24].
The research presented here focused on measur- ing the rates of colloid release under changing ionic strengths from natural porous media of inter- est in contaminant transport, i.e. high permeability materials with low colloid contents. Release rates were also measured for a simpler system: latex colloids deposited in glass-bead-packed columns. The goals of this work were to gain insight into the influence of solution and surface chemistry on colloid release from porous media and to test the utility of available particle-particle interaction theories for predicting trends in colloid release.
In this section we describe the theoretical frame- work for the detachment of flocculated colloids and their transport in a porous medium. We then summarize findings from previous experimental work on colloid release from porous media. The emphasis in this discussion is on mineral colloids (which may or may not contain organic coatings), rather than on dissolved organic carbon. Dissolved organic carbon exists as polymeric units and its interaction with larger mineral particles is sig- nificantly different from that of clays and oxidic colloidal particles. Studies on the transport of dissolved carbon have been described elsewhere (see, for example, Refs. [25,26]). Field evidence of colloid movement in porous media (e.g. data from several sites presented by Backhus et al. ) is also not included in this discussion. Field data are of limited use compared to laboratory systems in trying to understand basic processes responsible for colloid generation/release and transport.
2.1. Basic theory
The interaction potential energy of two like- charged colloidal particles can be determined by summing the contribution of electrostatic repulsion and van der Waals attraction (i.e. the Derjaguin- Landau-Verwey-Overbeek (DLVO) theory), shown schematically in Fig. 1. For two particles that are flocculated at the primary minimum of potential energy, the DLVO theory predicts that the particles are at infinitesimal separation and that the attractive potential is infinite (the broken line in Fig. 1). In other words, two attached par- ticles would need an infinite amount of energy to detach. Experimentally, however, the detachment (or repeptization) of flocculated particles upon reduction in ionic strength has been observed in batch systems [28,29], often without any mechan- ical action. To account for the process of particle detachment, two simple modifications can be made to the DLVO theory. If the effect of short-range Born repulsion of electron clouds is included in the calculation of potential (see, for example, Ref. ), a finite energy minimum results (the solid line in Fig. 1). Similarly, if it is assumed that two surfaces cannot get any closer than the diame- ter of a hydrated counterion (a few AngstrOms), a finite minimum is obtained . With either of these modifications to the DLVO theory, particles flocculated in the potential energy minimum can be shown to have a finite energy barrier for detach- ment. Frens and Overbeek [ 31 ] compared particle detachment to a chemical reaction, and stated that it would occur when the energy barrier for detach- ment was not very high (analogous to an activation energy), and when there was a net reduction in particle potential energy upon detachment. This approach was found adequate to explain qualita- tively data on the repeptization of flocculated colloids.
Theoretical expressions for computing rate con- stants for colloidal particle detachment over a potential energy barrier have also been derived [30,32-34]. Particle detachment over an energy barrier occurs by means of diffusion and can be described either by the Smoluchowski equation or by the Fokker-Planck equation when the particle motion changes rapidly over the time scales of the
S.B. ROy, D, A. Dzombak/Colloids Surfaces A: Physicochem. Eng. As'pects 107 (1996) 245 262 247
. o -
I , I : !
Y i*mi. I' [ i Distance of closest
Fig. 1. Schematic of potential energy profile of the interaction of two colloid surfaces with inclusion of van der Waals attraction, electrical repulsion, and Born repulsion (solid line). The broken line excludes the effect of Born repulsion. Note that the distance of closest approach is often used as a fitting parameter and may not coincide with the minimum computed by the inclusion of the Born repulsion.
diffusion. One-dimensional solutions to these equa- tions generally result in release (or "escape") rate constants of the form
(~max --_ ~min~ K '=C exp\ kT ] (1)
where K' is a release rate constant, q~ma~ and 4min are the primary minimum and the maximum of the potential energy profile (~bma x-~bmi n is the energy barrier that a particle must overcome to escape from its deposited/flocculated state), and C is a function of the potential energy profile. Similar theoretical considerations can be used to obtain deposition rate constants (see, for example, Refs. [30,35,36]).
The exponential dependence of the theoretical release rate constants on energy barrier heights indicates the importance of the calculated profiles in the prediction of release (or deposition). The electrostatic component of the potential shown schematically in Fig. 1 changes significantly with changes in aqueous chemistry, and can be corn-
puted using solutions of the Poisson-Boltzmann equation. Most solutions for the interaction of spherical particles use a one-dimensional form of the Poisson Boltzmann equation and employ the Derjaguin approximation (where the interaction of two