Electro-osmotic dewatering

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... 

The methods of irreversible thermodynamics are useful in providing a quantitative approach to the phenomenon of electro-osmotic dewatering and its connection to other electrokinetic effects. The main ideas were developed by Overbeeki" and reviewed by DeGroot these ideas were applied by many workers to a number of problems, following the earlier papers of Overbeek and co-workers - on the treatment of electrokinetic phenomena in terms of irreversible thermodynamics. Recently we have shown that this approach can also be applied to EOD, as follows. 

Electro-Osmotic Dewatering of Ciays, Soils, and Snspensions... 

Components of Cnrrent and Flux During Electro-Osmotic Dewatering with or without Pressure... 

In order to obtain the electro-osmotic current in the absence of an applied field [Eq. (2)], one may use two identical inert electrodes (i.e., not sustaining electrode reactions such as corrosion, or oxidation or redox reactions), which must be coimected to each other externally to complete the circuit in order to allow the electro-osmotic current, driven by the zeta potential alone, to occur. Identical electrodes (ideally, gold electrodes but more practically, inert graphite or stainless steel electrodes) are required to eliminate (ideally, but more practically to minimize) the galvanic battery effects and the associated electrode reactions. The total water removal during combined pressure and electro-osmotic dewatering is thus given by... 

Although an irreversible thermodynamic approach provides a quantitative framework for the EOD phenomenon, it does not elucidate the mechanistic details of the physical and chemical events that take place in electro-osmotic dewatering in other words, it is phenomenological and not mechanistic in content. 

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. 

IV. THE ELECTROCHEMICAL APPROACH TO ELECTRO-OSMOTIC DEWATERING HELMHOLTZ-SMOLUCHOWSKI RELATION... 

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. 

V. ELECTRO-OSMOTIC DEWATERING SOME EXPERIMENTAL ASPECTS... 

Figure 7. A three-stage upper electrode-type electro-osmotic dewatering appara-...

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. ... 

In dc electro-osmotic dewatering, current interruption followed by short-circuiting removed 20-40% more water than continuous dc or interruption with an open circuit, for otherwise equal energy consumption. ... 

Electro-Osmotic Dewatering under Interrupted Direct Current... 

Electro-Osmotic Dewatering under Galvanic Conditions... 

In the field of environmental engineering, electro-osmotic dewatering can be used to remove organic contaminants or toxic heavy metals from... 

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