Electrokinetic processes

In the following text, let us consider what happens if the charged particle or surface is under dynamic motion of some kind. Further, there are different systems under which electrokinetic phenomena are investigated. These systems are 

Electrophoresis This system refers to the movement of the colloidal particle under an applied electric field. 

Electroosmosis This system is one where a fluid passes next to a charged material. This is actually the complement of electrophoresis. The pressure needed to make the fluid flow is called the electroosmotic pressure. 

Streaming potent If fluid is made to flow past a charged surface, then an electric field is created, which is called streaming potential. This system is thus the opposite of electroosmosis. 

Sedimentation potential A potential is created when charged particles settle out of a suspension. This gives rise to sedimentation potential, 

Electric double layers are formed in heterogeneous electrochemical systems at interfaces between the electrolyte solution and other condncting or nonconducting phases this implies that charges of opposite sign accumnlate at the surfaces of the adjacent phases. When an electric held is present in the solntion phase which acts along snch an interface, forces arise that produce (when this is possible) a relative motion of the phases in opposite directions. The associated phenomena historically came to be known as electrokinetic phenomena or electrokinetic processes. These terms are not very fortunate, since a similar term, electrochemical kinetics, commonly has a different meaning (see Part 11). 

Four different electrokinetic processes are known. Two of them, electroosmosis and electrophoresis, were described in 1809 by Ferdinand Friedrich Renss, a professor at the University of Moscow. The schematic of a cell appropriate for realizing and studying electroosmosis is shown in Fig. 31.1a. An electrolyte solution in a U-shaped cell is divided in two parts by a porous diaphragm. Auxiliary electrodes are placed in each of the half-cells to set up an electric held in the solution. Under the inhuence of this held, the solution starts to how through the diaphragm in the direction of one of the electrodes. The how continues until a hydrostahc pressure differential (height of liquid column) has been built up between the two cell parts which is such as to compensate the electroosmotic force. 

Electrophoresis can be observed in solutions containing suspended matter (solid parhcles, liquid drops, gas bubbles) in a highly disperse state (Fig. 31.2a). Under the influence of an electric held, these particles start to be displaced in the direchon of one of the electrodes. Often, this movement is toward the negative electrode or cathode hence, electrophoresis has occasionally been called cataphoresis. 

In 1861, Georg Hermann Quincke described a phenomenon that is the converse of electroosmosis When an electrolyte solution is forced through a porous diaphragm by means of an external hydrostatic pressure P (Fig. 31.1ft), a potential difference called the streaming potential arises between indicator electrodes placed on different sides of the diaphragm. Exactly in the same sense, in 1880, Friedrich Ernst Dorn described a phenomenon that is the converse of electrophoresis During 

Fundamentals of Electrochemistry, Second Edition, By V. S. Bagotsky Copyright 2006 John Wiley Sons, Inc. 

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An electrokinetic process combined with in situ chemical degradation (Ho et al. 1999)... 

The electrokinetic processes can actually be observed only when one of the phases is highly disperse (i.e., with electrolyte in the fine capillaries of a porous solid in the cases of electroosmosis and streaming potentials), with finely divided particles in the cases of electrophoresis and sedimentation potentials (we are concerned here with degrees of dispersion where the particles retain the properties of an individual phase, not of particles molecularly dispersed, such as individual molecules or ions). These processes are of great importance in particular for colloidal systems. 

Theoretically, electrokinetic processes should also occur in nondisperse systems, but then additional factors arise (vortex formation in the liquid, settling of the particles, etc.) which produce a strong distortion. Hence, electrokinetic processes can be regarded as one of the aspects of the electrochemistry of disperse heterogeneous systems. 

Electrokinetic processes only develop in dilute electrolyte solutions. The second phase can be conducting or nonconducting. Processes involving insulators are of great importance, since they provide the only way of studying the structure and electrical properties of the surface layer of these materials when they are in contact with the solution. Hence, electrokinetic processes can also be discussed as one of the aspects of insulator electrochemistry. 

Transport processes of this type are called nonfaradaic transport. The nonfaradaic transport considered here is a steady-state process, in contrast to nonfaradaic currents mentioned previously that were due, for example, to charging of the electric double layer. Electrokinetic processes are of great practical significance, as discussed in Section 31.3. 

The electrokinetic processes have electrostatic origins they are linked to the charges present on both sides of the slip plane close to the phase boundary. The charge and potential distribution in the surface layer can be described by the relations and laws outlined in Chapter 10. 

As the particle moves relative to the electrolyte solution, the layer of water mol-ecnles that is directly adjacent to the particle surface is strongly bonnd and will be pnlled along. The thickness of this bonnd layer is approximately one or two diameters of a water molecule. We shall write x, for the x-coordinate of this layer s outer boundary, which is the slip plane. The electrostatic potential at this plane relative to the potential in the bulk solution is designated by the Greek letter and called the zeta potential or electrokinetic potential of the interface discussed. This potential is a very important parameter characterizing the electrokinetic processes in this system. 

It also follows from what was said that a zeta potential will be displayed only in dilute electrolyte solutions. This potential is very small in concentrated solutions where the diffuse edl part has collapsed against the metal surface. This is the explanation why electrokinetic processes develop only in dilute electrolyte solutions. 

The basic eqnations of electrokinetic processes establish the connection between zeta-potential valnes and the parameters of the electrokinetic processes the velocity... 

The equations of the electrokinetic processes were derived in 1903 by the Polish physicist Maryan Ritter von Smoluchowski on the basis of ideas concerning the function of EDL in these processes that had been developed by H. Helmholtz in 1879. These equations are often called the Helmholtz-Smoluchowski equations. 

Interrelations Between the Electrokinetic Processes Equation (31.4) for electroosmosis and Eq. (31.10) for the streaming potential, as well as the analogous equations for the other two electrokinetic processes, yield the relation... 

This equation is valid regardless of solution properties (the values of 8 and tj), surface properties (the value of 0, and the size of the disperse-phase elements. All parameters of this equation can be determined by independent measurements. The validity of Eq. (31.12) was demonstrated by such measurements. This result is an additional argument for the claim that all four of the electrokinetic processes actually obey the same laws and have the same physical origin. 

Electrokinetic processes are widely used in different fields of science and technology. We had already mentioned the use of electrokinetic processes for research into the electric properties of surface layers of insulating materials. Such measurements are used, in particular, when studying the surface properties of polymeric materials, their behavior in different media, and their interactions with other materials (e.g., with adsorbing surface-active substances). The results of this research are used in textile, cellulose and paper, and other industries. 

In situ soil remediation with physical methods includes the in situ heating (in situ thermal treatment), ground-freezing, hydraulic fracturing, immobilization/stabilization, flushing, chemical detoxification, vapor extraction, steam extraction, biodegradation/bioremediation, electroosmosis/ electrokinetic processes, etc. 

Electroosmosis refers to the movement of the liquid adjacent to a charged snrface, in contact with a polar liquid, under the influence of an electric field applied parallel to the solid-liquid interface. The bulk fluid of liquid originated by this electrokinetic process is termed electroosmotic flow. It may be prodnced either in open or in packed or in monolithic capillary columns, as well as in planar electrophoretic systems employing a variety of snpports, such as paper or hydrophilic polymers. The origin of electroosmosis is the electrical donble layer generated at the plane of share between the snrface of either the planar support or the inner wall of the capillary tube and the surronnding solntion, as a consequence of the nneven distribntion of ions within the solid/liquid interface. 

The electrokinetic process will be limited by the solnbUity of the contaminant and the desorption from the clay matrix that is contaminated. Heterogeneities or anomalies in the soil wiU rednce removal efficiencies. Extreme pHs at the electrodes and the may inhibit the system s effectiveness. Electrokinetic remediation is most efficient when the pore water has low salinity. The process requires sufficient pore water to transmit the electrical charge. Contaminant and noncontaminant concentrations effect the efficiency of the process. 

Lynntech, Inc. s (Lynntech s), electrokinetic remediation of contaminated soil technology is an in situ soil decontamination method that uses an electric current to transport soil contaminants. According to Lynntech, this technology uses both direct current (DC) and alternating current (AC) electrokinetic techniques (dielectrophoresis) to decontaminate soil containing heavy metals and organic contaminants. A non homogeneous electric field is applied between electrodes positioned in the soil. The field induces electrokinetic processes that cause the controlled, horizontal, and/or vertical removal of contaminants from soils of variable hydraulic permeabilities and moisture contents. 

The first three electrokinetic processes are our concern in this chapter, with the emphasis on electrophoresis. 

R. M. Pike, and Z. Szafran. Microscale Electrokinetic Processing of Soils, ... 

Sarahney, H., Wang, J. and Alshawabkeh, A. (2005) Electrokinetic process for removing Cu, Cr, and As from CCA-treated wood. Environmental Engineering Science, 22(5), 642-50. 

Sarahney et al. (2005) investigated the application of an ex situ electrokinetic process to remove arsenic, copper, and chromium from CCA-treated wood chips. According to Sarahney et al. (2005), oxalic acid and oxalic acid mixed with EDTA enhanced the treatment process for arsenic. The oxalic acid and EDTA mixture removed 88 % of the arsenic from the wood. 

Electrodialytic method Combines electrokinetic processes with electrodialysis membranes to remove contaminants from wet solids and aqueous solutions. 

Chew CF, Zhang TC. Abiotic degradation of nitrates using zero-valent iron and electrokinetic processes. Environ Eng Sci 1999 16 389-401. 

Fig. 15.31. One-dimensional laboratory test apparatus. (Reprinted from R. J. Gale, Soil Decontamination Using Electrokinetic Processing, in Environmental Oriented Electrochemistry, C. A. C. Sequeira, ed., Fig. 4, p. 362, copyright 1994. Reproduced with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)...

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