Environ. Sci. Techno/. 1995,29,2963-2973
Colloid Deposition Dynamics in Flow Throut~hPorous Media: Role of Electrolyte Concentration D A Y L I N L I U , PHILIP R . JOHNSON, A N D MENACHEM ELIMELECH' Department of Civil & Environmental Engineering, University of California, Los Angeles, California 90095-1593
The role of solution ionic strength and counterion valence in controlling the dynamic (transient) behavior of colloid deposition in granular porous media has been investigated. Packed-bed column experiments were conducted using t w o different suspensions of positively charged colloidal latex particles and negatively charged glass bead collectors. Variations in the concentration of an indifferent, 1:l electrolyte solution are used to demonstrate an inverse logarithmic relationship between the average area of collector surface excluded ("blocked") by deposited colloidal particles and solution ionic strength. Both electrolyte concentration and type influence the dynamics of particle deposition in porous media. A transition from declining deposition rate ("blocking") to increasing deposition rate ("ripening") is observed when the concentration of an electrolyte containing divalent counterions is gradually increased. Results from experiments involving irreversible, monolayer particle deposition are used to determine values of the maximum attainable surface coverage for spherical collectors. These values are unique to the collector geometry, flow conditions, and particle sizes used in the experiments. Implications of the results to colloidal transport and mobilization in subsurface aquatic environments are discussed.
Introduction The fundamental relationship between colloidal transport and the dispersal of contaminants in the subsurface is becoming a major emphasis of environmental research (15). Not only is there an ongoing effort aimed at delineating the mechanisms that govern the transport and fate of pathogenic colloidal contaminants such as viruses (6, 7) and certain bacteria (8, 9), but it is now widely recognized that environmentally benign bacteria ( 1 0 , I I ) and colloidal minerals (2-4) can act as carriers for contaminants that would otherwise be immobile in the groundwater. Because colloidal particles of geologic and biologic origin are ubiquitous in natural aquatic environments (12,13),colloid* Author to whom all correspondence should be addressed; telephone: (310) 825-1774; fax: (310) 206-2222; e-mail address:
[email protected].
0013-935X/95/0929-2953$09.00/0
D 1995 American Chemical Society
facilitated contaminant transport is a potentially significant mechanism for the conveyance of avariety of low-solubility and surface-reactive compounds in the subsurface, includingradionuclides ( I , 14, 1 3 , certain metals (4, 16, 13,and hydrophobic organic compounds (10, 11, 18-20). This potentially major role played by colloids in groundwater contaminant transport underscores the necessity for gaining a thorough understanding of the mechanisms governing colloidal mobility in aqueous porous media. The extent of colloid transport in groundwater is largely determined by the rate at which colloidal particles deposit onto stationary mineral grain surfaces. Colloid deposition onto stationary surfaces is a dynamic phenomenon characterized by variable kinetics (21-23). A constant rate of particle deposition is only observed during the initial stage of deposition while collector grain surfaces are still devoid of retained particles. As colloidal particles accumulate on collector surfaces, the particle deposition process begins to exhibit dynamic or transient characteristics. The dynamic aspects of particle deposition have been experimentally observed as either an increasing or decreasing rate of deposition (21-27). Whether the rate of deposition rises or declines depends on a variety of factors, including solution chemistry and the chemical characteristics of colloid and collector grain surfaces (21-23, 27, 28). Under chemical conditions where repulsive forces between colloids are absent, the deposition rate tends to increase as particles accumulate on collector surfaces. This enhancement of deposition kinetics is attributed to the retained particles, which act as additional collectors, and is generally referred to as ripening (24, 27, 29). On the contrary, declining deposition rates are usually observed when intercolloidal forces are repulsive. The decline in deposition rate is attributed to the reduced availability of deposition sites on collector grains as deposited particles exclude the immediate vicinity of the collector surface from subsequent deposition of particles. This surface exclusion phenomenon has been termed blocking (22, 23, 30, 31). Under solution conditions that give rise to blocking, interparticle contact (attachment)is prohibited so that the maximum attainable density of retained particles on collector grain surfaces is a monolayer. In direct contrast to blocking, ripening is promoted by chemical conditions that favor interparticle contact and is therefore characterized by multilayer rather than monolayer coverage of collector surfaces. Most colloid and mineral grain surfaces in aquatic environments carry a net negative charge (12, 32, 331, resulting in conditions that are generally unfavorable for particle deposition. Colloid deposition onto mineral surfaces in natural and engineered systems under unfavorable chemical conditions is thought to be largely restricted to a minor amount of macroscopic surface patches having charge characteristics that favor deposition of colloids (25, 34). The oxides of Fe, Al, and Mn are a prominent source of patchwise surface charge heterogeneities in subsurface aquatic environments (9,25, 35-37). These oxides carry a positive surface charge at neutral pH and are generally present in minor amounts as patchy, uneven coatings on negativelycharged mineral grain surfaces (35)or as separate,
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accessory minerals interspersed within the primary mineral assemblage (37). Because the kinetics of particle deposition under unfavorable conditions are regulated by the availability of favorable patches on collector grain surfaces, the dynamic aspects of colloid deposition in porous media may be directly related to the rate at which retained particles accumulate on favorable surfaces. When blocking conditions prevail over ripening, a particle will generally occlude an area of collector surface from subsequent deposition by an amount that is larger than its projected area, due to the presence of long-range repulsive intercolloidal forces. The ratio between the blocked area and the projected area of a particle is generally referred to as the excluded area (23, 31,381. Among the chemical and physical factors affecting the excluded area are solution chemistry and ionic strength, surface chemistry, flow intensity, particle size, and collector geometry (23,31,39). A quantitative description of colloidal deposition dynamics in aqueous porous media requires an accurate determination of the excluded area for particle deposition onto spherical collector grain surfaces. At the present time, there are no systematic experimental studies available that have quantified variations in the excluded area for particles deposited onto granular porous media surfaces as a function of dissolved electrolyteconcentration. Objective and Scope. In this paper, we investigate the relationship between colloidal deposition dynamics and electrolyte concentration in flow through saturated porous media. Colloid deposition onto spherical collector grain surfaces is examined under a broad range of solution conditions by varying both solution ionic strength and counterion valence. We address the deposition process in heterogeneously charged granular media under so-called unfavorable conditions where colloid deposition is isolated to the minor fraction of collector surface area covered by favorable patches. This is accomplished by conducting experiments using colloidal particles and collector grains having opposite surface charge. Although this is not a sensu stricto depiction of actual deposition conditions found in groundwater where the majority of the collector surfaces have charge similar to the colloidal particles, it permits a thorough investigation of blocking as it pertains to deposition of colloids on favorable patches and allows a determination of the excluded area for various electrolyte concentrations. Deposition experiments were conducted using monodisperse aqueous suspensions of positively charged latex colloids and packed beds of negatively charged glass beads. Under these experimental conditions where particles and collectors are oppositely charged, particle deposition is essentially an irreversible process as long as chemical and hydrodynamic conditions remain unchanged (23, 31, 40, 41). Despite the obvious differences in charge between the experimental surfaces and the natural environment (Le., negatively charged colloids and positively charged favorable patches are characteristic of the subsurface environment when the concentration of dissolved organic matter is low), the experiments retain the proper sense of favorable deposition conditions due to the unlike charge of colloids and collectors. Implications of the results to colloidal mobility in subsurface aquatic environments are discussed.
Materials and Methods Particle Suspensions. Monodisperse suspensions of surfactant-free polystyrene latex particles containing amidine 2964
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("3)' surface functional groups (Interfacial Dynamics Corporation, Portland, OR) were used as model colloids in the deposition experiments. Two separate suspensions having mean particle diameters of 0.48 and 2.51 +m were utilized. Particle diameters were determined by the manufacturer from the measurement of 600 individual particles (of each suspension) by transmission electron microscopy. The titrated surface charge densities of the 0.48- and 2.51-pm particle suspensions, respectively, were 11.0 and 31.2 pClcm2 (as given by the manufacturer). Polystyrene particle density is reported as 1.055g/cm3.The particles were extensively dialyzed and ion-exchanged prior to use in order to remove any trace impurities that might have been introduced during their synthesis and preparation. Porous Media. A granular porous medium composed of uniform-size soda lime glass bead collectors (Class V, Ferro Corp., Jackson,MS) was used in the particle deposition experiments. The glass bead collectors have an average diameter of 0.46 mm. The glass beads have a reported chemical composition (by weight) of 72.0% SOz, 15.0% Na20, 7.0% CaO, 4.2% MgO, 0.4% Fez03,and 0.3% A120i. According to the manufacturer, the glass beads are 90% true spheres with less than 2%irregular-shaped beads. The glass beads were ultrasonicated in 0.01 M NaOH solution for 20 min, rinsed with deionized water, and then ultrasonicated for an additional 20 min in 1 M HNO? solution before a final thorough rinsing with deionized water. The beads were then dried in an oven at 60 "C. This cleaning procedure proved to be adequate for obtaining reproducible results in the deposition experiments. Solution Chemistry. Two types of electrolytes, namely, KCl and Na2S04, were used to investigate the effect of monovalent and divalent counterions on particle deposition dynamics. Electrolyte solutions were prepared using analytical reagent-grade KCI and Na2S04 salts (Fisher Scientific,Pittsburgh, PA) and deionized water (Nano Pure 11, Barnstead, Dubuque, IA). A wide range of electrolyte concentrations were used in the experiments so that the effect of interparticle double-layer repulsion on particle deposition dynamics could be systematically investigated. When necessary, solution pH was adjusted by adding small amounts of HCI. Electrophoretic Mobility Measurements. The electrophoretic mobilities of the polystyrene latex particles and glass bead fragments were measured using Lazer Zee Model 501 apparatus (Pen Kem Inc., Bedford Hills, NY). Particle suspensions of a few milligrams per liter having solution conditions similar to those employed in the deposition experiments were used in these measurements. In order to estimate the electrokinetic (5)potential of the glass bead collectors, the electrophoretic mobility of micrometer-size fragments of crushed glass beads was measured.
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tCP5 FIGURE 9. Determining the excluded area parameter for particle deposition onto spherical collectors at various electrolyte concentrations. The data sets correspond to the particle breakthrough curves displayed i n Figure 8. The jamming limit Om,, for each data set may be obtained as the vertical intercept value of a least squares fit of linear data near the origin.
Particle breakthrough curves of deposition experiments that approach the jamming limit for monolayer coverage were used to determine the excluded area for a wide range of ionic strengths. The breakthrough curves shown in Figure 8 provided the raw input necessary to obtain fractional surface coverage values according to eq 1. From the fractional surface coverage values, the series of plots displayed in Figure 9 were generated using eqs 4 and 5 . The values of Omax and p interpolated from the curves in Figure 9 are summarized in Table 1 for each particle suspension and ionic strength. An examination of the results shown in Table 1indicates a strong correlation between blocking and electrolyte concentration, whereby a reduction in ionic strength produces an increase in the excluded area. Rather large excluded areas are associated with deionized water, especially for the larger, non-Brownian particles where /?is 2970
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TABLE 1
Values for the Jamming Limit 8,,, and Excluded Area Parameter p at Various Ionic Strengths of an Indifferent Electrolyte (KCI) 2.51-pm latex particles
0.48-pm latex particles
ionic strength (M) B,,, 6x 10-5 10-4 10-3 10-2 10-1
p
0.071 14.08 0.092 10.87 0.173 5.78 0.273 3.66 0.322 3.11 0.592 1.69
ionic strength (M) Omax 6x
p
0.045 22.22 0.078 12.82 0.135 7.41 0.188 5.32 0.576 1.74
in excess of 22. Taken from a different perspective, deposition of the non-Brownian particles in a solution ionic strength of 6 x M (deionized water used in our
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Ionic Strength (M) FIGURE 10. Excluded area parameter as a function of solution ionic strength, and determining the hard sphere jamming limits for the two particle suspensions. Values of /? obtained from data sets displayed in Figure 9 and summarized in Table 1. The asymptotic limit of the /? values above 1.83 are denoted by for each particle size. The reciprocal value of the asymptotic limit represents the hard sphere jamming limit 8, for deposition onto spherical collectors. Experimental conditions are as listed in the caption for Figure 8. The inset illustrates the relationship between the inverse Oebye parameter K-’ and ionic strength. The inverse Oebye parameter is a characteristic length for the diffuse double-layer thickness. (-e*-)
experiments) produces a completed monolayer at a surface coverage of only 4.5%. The values of B , below 1.83 noted in Table 1 are an indication of a deviation from monolayer coverage of collector surfaces due to reduction in particle stability and particle-particle attachment. Attachment of suspended particles to previously retained particles is initiated at moderately high ionic strengths with an indifferent electrolyte (KCl), occurring just above M for the submicrometer-size particles and around 10-2.5M for the larger particles. The cause of initiation of particle instability and possible multilayer coverage for the 2.51-pm particles at an ionic strength lower than that for the 0.48-pm particles has not yet been identified but may be attributed to loose particle capture by retained particles in secondary minima, as noted in coagulation experiments of large, non-Brownian particles (55). The excluded area exhibits an inverse logarithmic dependence on electrolyte concentration, as illustrated in Figure 10. The decline in p corresponds to so-called “hardening” of particle surfaces as increasing electrolyte concentration compresses the diffuse double-layer thickness. The asymptotic limit of the ,8 values above the limit of 1.83 may therefore be equated with the reciprocal of the hard sphere jamming limit for spherical collectors, 8,. As shown in Figure 10, both particle sizes have average 8, values that are less than 0.546, presumably because of hydrodynamic scattering associated with particle deposition
from flowing suspensions onto spherical surfaces (50,511. The hard sphere jamming limit value for non-Brownian particles is noticeably less than the same value for Brownian particles. This is because hydrodynamic scattering is a function of particle s u e (51)and also because submicronsize particles enjoya much greater accessibilityto spherical collector surfaces (231,as discussed in the previous section. It is important to remember that, unlike the hard sphere jamming limit value for flat surfaces which is invariant, these are surface-averaged jamming limit values that are specific to the flow rate maintained during experimentation. A lower flow rate may produce values closer to 0.546 due to a reduction in the fluid shear component of hydrodynamic scattering. The inset contained in Figure 10 demonstrates the relationship between the excluded area associated with deposition onto spherical collectorsand the actual thickness of the diffuse double layer surrounding charged particles. The inset shows changes in the inverse Debye parameter ( K - ] ) as a function of ionic strength, where the inverse Debye parameter represents the characteristic length of the diffuse double-layer thickness. The similarity in the shape of the inset curve with the main curves shown in Figure 10 reveals a strong correlation betweenp and K - ~ , This is a qualitative proof that the area excluded or blocked by a deposited particle is largely determined by the thickness of the electrical double layer and that changes in ionic strength will exert considerable influence over colloidal deposition dynamics in granular porous media. Implications to Colloid Mobility in the Subsurface Aquatic Environment. In subsurface aquatic environments where the concentration of dissolved natural organic matter is low, the transport of colloidal particles may be controlled primarily by the dynamics of particle deposition onto positively charged oxyhydroxide surface coatings. For a given subsurface stationary matrix, the capacity of the medium to accommodate deposited particles is a function of the excluded area associated with the retained colloidal particles. Colloidal mobility will be enhanced when the surface capacity of the porous medium is reduced, so that reductions in solution ionic strength can be expected to increase the mobility of colloids. Conversely,a rise in ionic strength will decrease particle mobility by reducing the excluded area of colloidal particles through compression of diffuse double layers. In the presence of moderately high levels of multivalent ions, such as Ca2’ and Mg2+, colloidal mobility may be greatly reduced due to specific adsorption and charge neutialization that promote colloidal instability and ripening. Enlarged excluded areas associated with expanding diffuse double layers may provide a partial explanation for the previously noted but unexplained mobilization (release) of attached colloidal particles produced by reductions in ionic strength (56-59). As excluded area is increased by reducing the electrolyteconcentration, a fraction of particles deposited on small favorable regions (orpatches) of mineral grains may be forced out of the small surface patches as overcrowding occurs. Particles will detach from the collector grains until the surface density of patches is reduced to the new jamming limit associated with the reduced ionic strength. The fraction of retained particles that undergo detachment and re-entrainment into aqueous suspension will depend not only on the magnitude of change in the excluded area (which depends on the magnitude of change in ionic strength) but also on the original density of retained VOL. 29. NO. 12. 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
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particles prior to the perturbation in electrolyte concentration. For agiven amount of patchwise charge heterogeneity, the fraction of retained particles that is ejected from the collector surface will necessarily increase with decreasing electrolyte concentration and increasing surface coverage. This detachment phenomenon due to particle-particle repulsion may help to explain colloid release in soils commonly associated with infiltration of rainfall and other waters containing low electrolyte concentrations (60,61). Colloid mobility in subsurface aquatic environments is further complicated by the presence of dissolved organic matter. When adsorbed on mineral surfaces, dissolved organic matter may mask underlying surface charge heterogeneities, thereby altering the dynamics of colloid deposition and mobilization. The effects of dissolved organic matter, though beyond the scope of this paper, should be considered in future investigations of colloidal transport in heterogeneous porous media.
Conclusion Solution ionic strength influences the dynamics of colloidal deposition and transport in heterogeneous porous media by controlling the range and magnitude of interparticle forces. Low to moderate concentrations of indifferent electrolytes containing monovalent counterions promote interparticle repulsion and declining deposition rates as accumulated particles block collector surfaces from subsequent deposition. The presence of specifically interacting, divalent counterions at moderate concentrations promote interparticle attraction, and the presence of rising deposition rates as particles retained on stationary mineral grains act as additional collectors. The magnitude of the excluded area produced by particles deposited onto spherical collector surfaces under irreversible conditions exhibits an inverse logarithmic dependence on solution ionic strength. The surface area blocked by a particle deposited on a spherical collector increases as the ionic strength is reduced and is generally several times larger than the projected area of the particles. The excluded area parameter determined for spherical collectors is a composite, surface-averaged value that is unique to the given flow conditions, particle size, and solution chemistry. Unique values of the hard sphere maximum attainable surface coverage (jamming limit) for deposition onto spherical collectors are determined from the experimental data for the given flow conditions and particle sizes. These jamming limit values are considerably less than the value of 0.546 associated with irreversible,hard-sphere deposition onto a flat surface under stagnation-pointflow conditions. The difference is attributable to hydrodynamic scattering effects caused by fluid shear (“shadow”effect) and to the non-uniform particle deposition pattern onto spherical collectors. Variations in the magnitude of the excluded area may be, in part, responsible for the release (mobilization) of colloidal particles from stationary mineral grain surfaces that is commonly observed following a reduction in ionic strength. A reduction in ionic strength produces an expansion of the electricaldouble layer surrounding charged colloidal particles, thereby causing repulsion between particles deposited on favorable patches and reduction in the maximum attainable surface coverage corresponding to a monolayer of deposited particles. In order to alleviate overcrowding, a fraction of the retained particles must be 2972
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ejected from the favorable, macroscopic mineral patches until the patch surface density is reduced to the value corresponding to the new maximum attainable surface coverage.
Acknowledgments The authors acknowledge the support of the National Science Foundation under Research Grants BCS-9009233 and BCS-9308118.
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Received for review January 18, 1995. Revised manuscript received July 19, 1995. Accepted J u l y 28, 1995.@
ES950027W @Abstractpublished in Advance ACS Abstracts, October 15, 1995.
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