Effects of Counterelectro-osmosis and Sodium Ion Exchange on
Permeability of Kaolinite A. S. MICHAELS AND C. S. LIN M a s s a c h u s e t t s I n s t i t u t e of Technology, C a m b r i d g e , Mass. *
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a
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UMEROUS attempts have been made to clarify the basic
mechanism of fluid flow through porous media consisting of colloidal size particles, in view of its importance in soil technology, ceramics, water and air pollution, and industrial processing of chemicals (10). The existence of electrical double layers a t the liquid-solid interfaces has been well established in colloidal systems containing highly polar liquid media; these double layers play an important role in phenomena such as flocculation and dispersion, streaming potential] and ion exchange (8). Changes of polarity of the suspending medium and concentration or type of ions in the medium have marked effects upon the double layer structure, and thus, as shown by much evidence, can alter the permeability of packed masses of colloidal particles to a very great extent (6, 9, 1 2 ) . This work represents a continuation of previous studies (9) aimed a t determining the degree t o which basic factors such as dispersion, electro-osmosis, and ion exchange influence the permeability of clay minerals. Earlier studies have shown that changes in degree of particle dispersion are predominantly responsible for the large differences of permeability of kaolinite to fluids of varying polarity (9). I n the kaolinite-water system, a small increase in permeability was observed in highly compacted clay beds as the polarity of the permeant was progressively decreased. This small increase in permeability may be explained in terms of simple “immobile film” concepts ( 7 , Q), counterelectro-osmosis (I%’), or the “electroviscous effect” developed in the recent work by Booth ( 1 ) and Elton (4). The last is essentially a modification of the second, in which the electrical resistance to flow is dealt with as a hypothetical increase in viscosity of the flowing fluid. The phenomenon of counterelectro-osmosis can be explained as follows: Because of the existence of the electrical double layer a t the solid-liquid interface] the movement of fluid past the solid surface causes a downstream transport of the ionic charges in the diffuse part of the double layer (8). This displacement develops a streaming potential which causes electro-osmosis in a direction opposite to the forward flow (see Figure 1). Counterelectroosmosis is believed by the authors to be a reasonable explanation for flow abnormalities, akhough direct experimental evidence of the phenomenon has been lacking. If the permeating fluid contains ions not initially present in the double layer, the additional complication of ion exchange may result. Ion exchange alters the structure of the double layer, changing the electrokinetic potential, and thus can change the degree of counterelectro-osmosis as well as cause dispersion or flocculation of the particles. Sodium kaolinite is more highly dispersed than hydrogen or calcium kaolinite in aqueous media. By its effect on particle dispersion, ion exchange (in the presence of simple electrolytes) might be the major factor influencing the clay permeability. By measuring the degree of particle flocculation in suspension by light extinction, Grace (6) has shown that improved dispersion is the main reason for the marked reduction of permeability by a variety of electrolytes. Whether counterelectro-osmosis contributed to the observed permeability changes was not established.
The purpose of this work has been to establish experimentally the existence (or lack thereof) of counterelectro-osmosis in the flow of aqueous solutions through packed kaolinite beds, and to determine to what extent, and by what mechanism, ion exchange affects kaolinite permeability. Kaolinite of high purity was selected for these studies. The source and properties of the kaolinite have been described in an earlier paper (9). THEORETICAL COUNTERELECTRO-OSMOSIS
When a liquid is mechanically forced through a porous medium, the displacement of counter ions in the double layer near the solid surface sets up a streaming potential determined by the following relation (8):
where the specific conductance of the clay bed, X, is the sum of the conductances of pore fluid and the solid surface. As the geometry of the pores is unknown, it is impossible to separate the bulk liquid conductance from the surface conductance, as can be done with a simple cylindrical capillary (8). NET DlREC7lOU OF
FLOW I
f
E.
JTREAUINC POTENTIAL
1
t CAPILLARY CROSS SECTION
b i -
II
I A
I
ANNULUS
YELOCIT Y PROFILE
Figure 1. Schematic diagram of counterelectro-osmotic flow in a fine capillary
The streaming potential causes electro-osmotic flow in a direction opposite to the forward flow. The relation between streaming potential and electro-osmotic flow is:
where U’osmis the linear electro-osmotic flow velocity in the pores, and L’is the actual length of the flow path through the porous medium. As the flow path is tortuous, the actual path length will exceed the bed thickness, L. Carman found, for granular
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beds, L’ = v 2 L ( 2 ) . For a porous bed composed of platy particles such as clay, the tortuosity factor depends upon the orientation of particles in the bed. If, however, the platelet particles are randomly packed, the average tortuosity factor will be nearly the same as that in the granular beds. Then Equation 2 may be written as:
The linear. electro-osmotic fluid velocity in the pores, U ‘ O B m , can be related to the superficial fluid velocity through the bed, U O B m , by the relation:
u,,,
=
U’,,,
e .-.1 +e
L - €
