Environ. Sei. Technol. 1994, 28, 737-745
Electrokinetically Enhanced Sedimentation of Colloidal Contaminants Julie E. Sauer and E. James Davis'
Department of Chemical Engineering, BF-IO, University of Washington, Seattle, Washington 98195 ~~~
Electrophoretic clarification of aqueous slurries has been carried out in the laboratory to elucidate the transport process and to explore the application of electrokinetically enhanced sedimentation to large-scale systems such as coal-washing ponds. Two colloidal systems were investigated: (i) a model system of monodisperse polystyrene latex (PSL) spheres and (ii) heavily contaminated polydisperse suspensions from a coal-washing pond used at a coal-fired power plant. The electrical power requirements and electrokinetic properties of the colloidal suspensions were measured. A light-scattering technique was used to follow the sedimentation process to determine sedimentation rates, which for the PSL system were found to agree with existing theories of electrophoretic mobility. Although flocculation of particles did not occur for the monodisperse PSL colloids, the coal/mineral particles flocculated after the electric field was applied for a time sufficient to initiate sedimentation. Once flocculation began, gravitational sedimentation dominated the process, and the electrical field could be turned off.
Introduction Electrokinetic techniques have been used in engineering applications for more than 50 years. In 1937, Casagrande (1) described the use of electroosmosis to remove water from soft soils and stabilize them during excavation. Recently, a strong interest in electrokinetic remediation of soils contaminated with heavy metal ions has developed. Two aspects of electrokinetics are particularly applicable to environmental engineering: (i) electroosmosis, which isthe movement of an electrolyte solution through aporous medium as a result of an applied electrical field, and (ii) electrophoresis, which is the movement of colloidal particles through stagnant water due to an electrical field. Both phenomena are a result of the surface charge that arises when a solid surface such as clay or silicaceousmatter is ionized in the presence of water. The electric potential at the shear plane between the fluid and the solid surface, which results from the surface charge, is called the potential, and the potential is frequently used to characterize the electrokinetic system. In aseries of studies executed by Sprute and Kelsh at the Bureau of Mines (2, 31, electrokinetic techniques were applied to the densification of mill tailings and hydraulic backfill and to the consolidation of mine slimes. Electrokinetic dewatering and densification of coalwaste were also explored by Sprute and his co-workers (4-6). Their investigations of the densification of colloidal waste in a coal mine sediment pond stimulated the work reported here. Additional investigations of electroosmotic dewatering include Lockhart's (7)application to fine tailings and clays (81,the study of kimberlite slimes from diamond mining by Wilmans and Van Deventer (9),and the dewatering of bentonite sludge by Yoshida et al. (IO). Most of these investigations have been feasibility studies, which do not
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0013-936X/94/0928-0737$04.50/0
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0 1994 Amerlcan Chemical Society
advance our understanding of the fundamental phenomena to improve the processes, but Van Diemen et a1. (11)and Grant et al. (12)endeavored to improve the effectiveness of electroosmotic dewatering by adding surfactants to mineral slurries to alter the natural potential of the mineral particles. The former investigators reported that electroosmotic dewatering of metal hydroxide sludges was more effective when either sodium dodecyl sulfate or cetyltrimethylammonium bromide (CTAB) were added, and Grant et al. found that the rate and extent of dewatering iron oxide fines improved when CTAB was added. The surfactant adsorbed on the particles and altered the surface charge. There has been a recent increase in interest in electroosmosis for the removal of heavy metals from contaminated soils, and reviews of some of these research efforts have been published by Cabrera-Guzman et al. (13) and Acar (14).Organic contaminants have also been removed from soils electrokinetically (15-18). Shapiro and Probstein (18) and Segall and Bruell(19) have addressed the complexity of the transport processes and the electrochemical processes associated with electroosmotic flow, but it is not yet possible to predict large-scale threedimensional behavior based on small-scale laboratory experiments. There is also a problem of predicting the rates of electrophoretic transport in suspensions, for particleparticle interactions complicate the hydrodynamics as well as influence the electrical field in the suspension. Shih et al. (20) explored the sedimentation of illite particles in toluene containing asphaltene and performed two experiments using electrical fields to enhance sedimentation. They developed a one-dimensional hydrodynamic model, but the effects of particle-particle interactions and the electrical double layer were incorporated as a parameter to be determined from the settling data. Such effects were considered to be a multiplicative correction to Smoluchowski's (21) theory. Smoluchowski described the electrophoretic mobility of the particle, Ue,by
r
in which ue is the velocity of a particle in a stagnant fluid due to applied electrical field E, EDis the dielectric constant of the fluid, eo is the permittivity of free space, p is the viscosity of the fluid, and t is the t potential. Although Smoluchowski's equation is commonly used to relate the velocity and the electric field, its derivation involves a number of restrictions which make it invalid for many applications (22). It is restricted to thin electrical double layers ( ~ >> a l),modest f potentials (lfl I25 mV), and infinitely dilute suspensions (single particles). Here a is the particle radius, and K is the Debye-Huckel parameter, which is defined in terms of the concentration of ions, C i O and their valency, zi, by Environ. Sci. Technol., Vol. 28, No. 4, 1994
737
I
-
U “ I
where F is Faraday’s constant, R is the gas constant, and Tis the absolute temperature. The inverse of K , the Debye length, is a measure of the thickness of the electrical double layer. It is clear from eq 2 that the double layer thickness decreases as the concentration of ions increases. O’Brien and White (23)relaxed the first two restrictions associated with Smoluchowski’stheory, and they obtained numerical solutions for the flow and ion densities around a sphere. Ohshima et al. (24) extended that work to develop approximate analytical expressions for the mobility in dilute suspensions. Levine and Neale (25, 26) and Kozak and Davis (27,281 applied unit cell models to take into account particle-particle interactions, and the latter investigators obtained an analytical solution for the mobility for moderately thin double layers when there is a single anionic species and a single cationic species having the same absolute values of valence (a symmetric electrolyte), that is, Iz+I = (z-I = z. Marlow and Rowel1 (29) reported extensive measurements of the sedimentation potential gradient, the particle volume concentration, and the specific volume conductivity of the medium using 97bm glass microspheres in aqueous electrolyte solutions to explore deviations from Smoluchowski’s theory. They showed that Smoluchowski’sequation was followed below volume fractions of 0.018, but at higher volume fractions, the cell model theory of Levine et al. was required to explain deviations of -28 % from the linear approximation of Smoluchowski. Kozak and Davis obtained a solution for the electrophoretic mobility (U, = u,/E) of the form
numerical results of O’Brien and White for KC1 solutions in the limit as 4 0. Solutions of the governing equations for electrophoretic motion are not available for mixed electrolyte systems, and the effects of overlapping double layers are not accounted for in the analyses of Kozak and Davis. Furthermore, complications associated with flocculation of particles are not included, so we are not able to predict settling times and electrical power requirements from first principles. Thus, an experimental program was developed to explore electrophoretic densification of colloidal suspensions. To examine the effects of nonspherical particles and flocculation, we studied monodisperse suspensions of polystyrene latex particles (PSL), which were spherical and did not flocculate, and suspensions taken from a coalwashing pond at the WIDCO (Washington Irrigation and Development Co.) coal-processing plant near Centralia, WA, which were found to flocculate. Sedimentation Theory Kynch (30) developed a theory of sedimentation of uncharged particles which has been widely cited and applied to analyze sedimentation data, and at about the same time La Mer and Smellie (31-33) carried out extensive studies of flocculation, subsidence, and filtration of phosphate slimes, developing an empirical equation for the height of the subsidence level as a function of time. Marmur (34) extended Kynch’s approach to interpret “boundary anomalies” encountered in moving-boundary electrophoresis. For a nonflocculating monodisperse suspension of particles, all of which have the same density and electrokinetic properties, a material balance on a differential thickness of suspension located at a distance x above the bottom yields
(3)
where K is a function of the solids concentration or solids volume fraction, 4, of the suspension and of the diffusion coefficient,D-, of the anionic species (if the surface charge and { potential are positive) given by
Here u is the particle velocity relative to a stationary observer, and since there is an upward flow due to fluid displaced by the sedimenting particles, u is related to the electrophoretic velocity by u = (1- 4)ue
(7)
Equation 6 can be written as and A2,o has the definition in which For a negative {potential, D- in eq 4 should be replaced by D+, the diffusion coefficient of the cationic species. The first term on the right-hand side of eq 3 is Smoluchowski’s result, and the second term takes into account particle-particle interactions, the opposing electrical field generated by distortion of the ion cloud due to the fluid motion around the particle (the so-called relaxation phenomenon), and other electrical double layer effects. Hydrodynamic interactions and double-layer effects reduce the mobility of the particle compared with Smoluchowski’s theory. We note that the corrections for particle interactions and retardation effects are additive factors rather than the multiplicative factors used by Levine and his co-workersand adapted by Shih et aZ. Kozak and Davis showed that eq 3 is in very good agreement with the 738
Envlron. Scl. Technol., Vol. 28, No. 4, 1094
Kynch showed that V(4) governs the behavior of the dispersion. If IV(4)I increases with 4, a discontinuity will form, for a concentration change in the denser region below will propagate upward through the less dense regions until a discontinuity occurs. For a monodisperse PSL suspension in an aqueous KC1 solution, one would expect a discontinuity to form. Using the results of Kozak and Davis, the electrophoretic velocity is given by
where the applied electrical field has been written in terms
of the current density, ilA, and the electrical conductivity of the suspension, k,. Here A is the cross-sectional area of the suspension, and i is the current. Kozak and Davis obtained the following expression for the ratio of the suspension conductivity to the conductivity, k,, of the particle-free electrolyte solution
For KC1, D+lD- = 1,and using eqs 10 and 11in eq 7, V(4) becomes
Differentiating, one obtains
Table 1. Characteristics of Polystyrene Latex Particles
*
1.06 2.2 mean diameter (Wm) f coefficient of variation (%) 8.4 f 0.1 solids in manufacturer's sample ( % ) 1055 density at 293 K (kgIm3) 8.50 x 104 surface charge density (pC/m2) -3.85 X 10-8 electrophoretic mobility (m2/V.s) -67 { potential in lo4 M KC1 (mV)
dependence is more complex. Michaels and Bolger (35) reviewed the early work on flocculated suspensions and investigated the sediment volumes of aqueous flocculated suspensions of kaolin. They correlated their data with equations based on a structural model based on the assumption that in a flocculated suspension the basic flow units are small clusters of particles or flocs. In the Smoluchowski limit, there is no size dependence on the electrophoretic mobility, but eq 3 and the analyses of O'Brien and White and of Levine and his co-workers indicate that the electrophoretic mobility is a function of Ka. We shall demonstrate that the electrophoretic sedimentation of the WIDCO suspensions results in greater dispersion rather than the formation of a sharply defined discontinuity. Colloid Properties
(13)
r
Here g(4) and KOhave the definitions
and
K O= K(4 = 0) = 2( 1+
The PSL particles with a nominal diameter of 1.0 pm were obtained from Interfacial Dynamics Corp. as a stabilized suspension having a solids content of 8.4 f 0.1% . The concentrated suspension was diluted with distilled and deionized water for the sedimentation experiments, and a potassium chloride solution was added to make the continuous phase 10-4 M in KC1. Table 1lists the properties of the PSL microspheres. The electrophoretic mobility of the particles was measured using a Rank Brothers Mark I1 microelectrophoresis apparatus. The potential of the PSL particles in 10"' M KC1 was found to be -67 mV using the measured electrophoretic mobility and the approximate theory of Ohshima et al. A low concentration of KC1 was used to maintain an expanded electrical double layer. For a concentration of lo4 M KCl, the conductivity of the electrolyte solution would be expected to be 0.001498 S/m. This yields a value of Ka = 17.4 for the 1.06-pm diameter PSL spheres. Based on Smoluchowski'sequation for the electrophoretic mobility, one obtains Ue = 4.97 x lo4 m2/V-s,but eq 3 yields Ue = 3.63 X lo4 m2/V.s for 4 = 0.03, the maximum solids fraction used, and U, = 3.69 X lo4 m2/V-sfor 4 = 0.005, the mimimum solids fraction. Thus, the retardation effects represent approximately 25 % of the Smoluchowskimobility for PSL in the KC1solution. The waste pond samples were obtained from three locations in the pond: (i) near the surface, (ii) from about 1 m below the surface, and (iii) near the bottom of the pond, which had a depth of 4-7 m. The colloidal particles in the upper meter did not settle gravitationally, and the solids concentration in the surface water was found to be 0.14wt % . The size distribution of particles in the surface water, measured with a Horiba Model CAPA-BOO centrifugal particle size analyzer, is presented in Figure 1. About 40% of the particles (by volume) were less than 1 pm in equivalent diameter, and the volume average diameter was 1.26 pm. The corresponding average electrophoretic mobility was -1.8 X 10-8m2/V.sand was found to be constant over a pH range of 4-9 for a constant total ionic strength. The pH was changed by the addition of
-)PD-K
4C R T
exp( 2RT) (15) Ka Thus from eq 13, V(4) is seen to depend on the electrokinetic parameters of the system as well as on the concentration of particles. It is not obvious from the complicated form of eq 13how (V(4)(varies as 4 increases, but in the limit of K
