Electro-osmotic dewatering of clays

Vijh, A. K. Electro-Osmotic Dewatering of Clays, Soils, and Suspensions 32... 

The fundamental electrochemical approach of interest in this context is the electro-osmotic dewatering (EOD) of clays, the phenomenology... 

An electrochemical approach based on the considerations of double layers at the clay-water interface provides a more transparent approach for understanding the physical and chemical events occurring during electro-osmotic dewatering in particular, it shows more clearly how various parameters (applied voltage, pH profiles, zeta potential, electrode reactions, etc.) known to influence the EOD process come into play. The framework for the electrochemical interpretation of the EOD phenomenon is summarized in the next section. 

The theoretical approach generally used "in electro-osmotic dewatering is an electrochemical one in which the Helmholtz-Smoluchowski relation is used to relate the electro-osmotic convective liquid velocity to such parameters as the viscosity and permittivity of the solution, the zeta potential of the clay surface, and the strength of the applied field. Also, electrode kinetic effects are taken into account where the data point to the involvement of electrochemical reactions at the electrodes during the EOD process. " In combined pressure-electro-osmotic dewatering (CPEOD), the effect of pressure is interpreted in an empirical, ad-hoc manner without any attempt to develop a comprehensive theoretical framework that combines the two driving forces, namely, the pressure and the electric field. 

Figure 8. Diagram of an experimental setup for laboratory studies of electro-osmotic flushing technique to remove heavy metals from clays. It is basically an electro-osmotic dewatering setup with the addition of a continuous flow of purge solution at the anode and removal of contaminated water at the cathode.

A diagram of a typical experimental setup for laboratory studies on electro-osmotic dewatering is shown in Fig. 8, which is taken from Chen and the laboratory of Mujumdar and Weber at McGill University here EOD is being used to flush out heavy metals from the clay (see Section VI below). 

Salt or acid concentrations up to 10 M usually allow dewatering to proceed at lower voltages, compared with suspensions in pure water, presumably by lowering the resistance of the clay. At higher electrolyte concentrations (0.1 M), electro-osmotic dewatering became limited, and significant electrode polarization and electrode reactions were observed. ... 

The zeta potential is the potential difference between the plane of shear (or slipping plane) and the bulk solution. Erom Eq. (15), it is clear that for a given simation of water (electrolyte) in clay, the Ueo is proportional to the zeta potential and to the applied field strength. Also, in a real situation of EOD, it is necessary to use the length-averaged value of the zeta potential in order to take into account the effect of the axially variable zeta potential on the electro-osmotic velocity. Eurther, since both the zeta potential and the electric field depend on the ionic concentration and pH of the fluid (water in the present case), variations in electro-osmotic velocity can be expected as the dewatering experiment progresses in time. 

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