Electrochemical Transport and Transformations

The transport of charge from one location to another is a fundamental mechanism underlying many physical and chemical phenomena in natural and synthetic systems. The coupling of multiple physical and chemical processes in electrochemical transport makes it a fascinating and complicated diffusion event to understand (Rubinstein, 1990). 

In this chapter, we will review the fundamental principles of electrochemical transport as it appUes to a medium of saturated clay and show some viable results of electroremediation of clays based on the understanding of electrochemical transport and transformation in such media. 

As in many electrochemical systems, the flow of electric current through a network of a multiphase system occurs in different phases simultaneously in the bulk Uquid 

Electrochemical Remediation Technologies for Polluted Soils, Sediments and Groundwater, Edited by Krishna R. Reddy and Claudio Cameselle Copyright 2009 John Wiley Sons, Inc. 

The bulk transport of ions in electrochemical systems without the contribution of advection is described by Poisson-Nernst-Planck (PNP) equations (Rubinstein, 1990).The well-known Nernst-Planck equation describes the processes of the process that drives the ions from regions of higher concentration to regions of lower concentration, and electromigration (also referred to as migration), the process that launches the ions in the direction of the electric field (Bard and Faulkner, 1980). Since the ions themselves contribute to the local electric potential, Poisson s equation that relates the electrostatic potential to local ion concentrations is solved simultaneously to describe this effect. The electroneutrality assumption simplifies the mathematical treatise of bulk transport in most electrochemical systems. Nevertheless, this no charge density accumulation assumption does not hold true at the interphase regions of the electric double layer between the solid and the Uquid, hence the cause of most electrokinetic phenomena in clay-electrolyte systems. 

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The fundamental principles of electrochemical transport and transformations applied to electrokinetic processing of saturated clays were reviewed, and some new findings were also presented. 

The disproportionation reaction destroys the layered structure and the two-dimensional pathways for lithium-ion transport. For >0.3, delithiated Li, AV02 has a defect rock salt structure without any well-defined pathways for lithium-ion diffusion. It is, therefore, not surprising that the kinetics of lithium-ion transport and overall electrochemical performance of Li, tV02 electrodes are significantly reduced by the transformation from a layered to a defect rock salt structure [76], This transformation is clearly evident from the... 

It has been more than 60 years since Wagner s electrochemical tarnishing theory was developed. Finally, the two parallel suggestions of material transport and energy transformation in the theory are interconnected, and newly developed PEVD will make solid state ionic devices better to serve today s ever-growing energy and environmental demands. 

There are some exciting prospects—as yet unexplored—for the constructive interplay between electrochemical processes and membrane transport/ catalytic processes. This may lead to novel routes of organic chemical synthesis or removal of toxic substances from process streams. Many important enzymatic reactions involve oxidative or reductive transformations of the substrate. These usually require the participation of a cofactor which serves as the vehicle for transfer of electrons between the enzyme-bound substrate molecule and the participating oxidant or reductant in solution [71, 72], For such electron transfer to take place, both substrate molecule and cofactor must be localized in close proximity at the active site of the... 

We have seen that the rate of an electrochemical process is affected by the rates at which reactants can be supplied to the electrode and products can be dispersed from it. Often the overall process is governed completely by the rates of mass transport and homogeneous chemical reaction. One can usually write the set of coupled differential equations describing the transformations and movements of material, but often they can be solved in closed form with difficulty or not at all. Numerical methods are frequently applied to the solution of such equations (1). 

The implications of these electrolysis reactions are enormous in that they impact transport, transformation, and degradation processes that control the contaminant migration, removal, and degradation during electrochemical treatment. The different transport, transfer, and transformation processes induced by the applied electric field and how these processes are impacted by the electrolysis reactions at the electrodes are fundamental to the understanding of the electrochemical remediation technologies and are briefly presented in this section. 

Mathematical models are useful to better understand the processes that occur under electric field and predict remedial performance in field application. Compared with laboratory studies, only few studies have been reported on the mathematical modeling of electrochemical processes and remediation. Generally, electrochemical remediation models should incorporate the contaminant transport, transfer, and transformation processes and dynamic changes in electrical conductivity, pH, and geochemical reactions. Recognizing this as a complex task, researchers have developed some simple models based on a set of simplified assumptions (Chapters 25 and 26). 

Much progress in the study of electrochemical properties of protein macromolecules has been achieved recently using another method, namely, the study of redox transformations of proteins, the carriers of electrons and enzymes, and their active groups at the electrode-electrolyte interface. This approach is intimately related to the use of enzymes to promote electrochemical reactions and pursues the purpose of elucidation of the mechanism of electron transport and the structural features of enzymes. 

The application of combinations of electrochemical methods with non-electro-chemical techniques, especially spectroelectrochemistiy (UV-VIS, FITR, ESR), the electrochemical quartz crystal microbalance (EQCM), radiotracer methods, probe beam deflection (PBD), various microscopies (STM, AFM, SECM), ellipsometiy, and in situ conductivity measurements, has enhanced our understanding of the nature of charge transport and charge transfer processes, stmcture-property relationships, and the mechanisms of chemical transformations that occur during charg-ing/discharging processes. 

These events are (a) transport of the reacting species from the bulk of solution to the position at which charge transfer takes place (b) the electrode process itself, i.e., the act of reaction or a reaction sequence leading to the change of the valency state of the metal, as indicated by the stoichiometry of the electrochemical reaction and (c) a sequence of events in the transformation... 

D Eha Camacho et al. (2011) proposed a novel concept using an assisted electrochemical reaction to produce atomic hydrogen from water electrolysis for different heterorganic compounds conversion. The electrochemical reactor is divided into two compartments by a palladium membrane in which atomic hydrogen is absorbed and permeated. Organic sulfur in the oil can be desulfurized and transformed to H2S in the electrochemical compartment. In addition, Lam et al. (2012) recently presented a review of electrochemical desulfurization technologies for fossil fuels. Various electrodes and electrolytes that have been used for desulfurization accomphshed by oxidation, reduction, or both were summarized by Lam et al. in their paper. Some electrochemical desulfurization processes for transportation fuels were chosen for listing in Table 14.2. 

EIS changed the ways electrochemists interpret the electrode-solution interface. With impedance analysis, a complete description of an electrochemical system can be achieved using equivalent circuits as the data contains aU necessary electrochemical information. The technique offers the most powerful analysis on the status of electrodes, monitors, and probes in many different processes that occur during electrochemical experiments, such as adsorption, charge and mass transport, and homogeneous reactions. EIS offers huge experimental efficiency, and the results that can be interpreted in terms of Linear Systems Theory, modeled as equivalent circuits, and checked for discrepancies by the Kramers-Kronig transformations [1]. 

CCL operation entails transport of gases, water, electrons, and protons, as well as interfacial transformation of species due to electrochemical reaction and evaporation. Effective parameters that steer the interplay of these processes are proton and electron conductivity diffusion coefficients of oxygen, water vapor, and residual gaseous components liquid water permeability as well as exchange current density and vaporization rate per unit volume. These parameters incorporate information about composition, pore size distribution, pore surface wettability, and liquid water saturation. This section introduces functional relationships between effective properties and structure. 

Only insoluble alkoxides can be obtained by this method because the soluble ones are normally reduced at the cathode, transforming the process into the electrochemical transport of the metal from anode to cathode. The products again are usually heavily polluted by halide admixtures and should be then washed repeatedly with alcohols to remove adsorbed conductive additives (Hubert-Pfalzgraf and Kessler, 1997). It has, however, been reported that application of amines (such as dipyridyl, phenantroline), giving rather stable insoluble complexes with Cd and Cu alkoxides, allows alkoxides free from halide admixtures to be isolated (Banait, 1986). 

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