Clays and Electrochemistry: An Introduction
Electrochemistry is a tool that has come of age in the field of clay chemistry. Electrochemistry has been used to monitor transport in natural clay films and in tailored clays films, to measure ion conductivities, to remediate heavy metal contaminated soils, and to remediate organic compounds contaminating soils and clays.
The purpose of this chapter is to outline some of the fundamental concepts used in electrochemistry. We begin with a survey of similar concepts of the electrified clay interface, followed by transport phenomena, and followed by development of the vocabulary used in transient electrochemical techniques. The final sections show how transient methods of electrochemical analysis are applied to representative problems at clay interfaces, and give specific laboratory practices. It is assumed that the reader has a basic knowledge of clay chemistry, so topics related to clay chemistry will be discussed only briefly.
At its simplest, an electrochemical experiment involves an electrified interface with its associated fluids and salts to conduct charge to a second electrified interface. The concepts do not differ from a clay chemist’s understanding of the surface potential of clays and the associated double layer swarm of ions found in a soil matrix.
The clay sheet found within the soil is made up of crystals containing tetrahedral silicates (Si-O) and/or octahedral aluminates (Al-O, Al-OH). Lateral sharing of oxygen atoms in tetrahedra (Figure 1) or octahedrons result in a sheet. Sharing of oxygen atoms between sheets results in a layer. Minerals may consist of one silicate and one aluminate sheet
Figures & Tables
Electrochemical Properties of Clays
The articles in this text have been assembled to give readers an idea of the large number of research possibilities associated with combining electrochemical methods with clay research. The first article is introductory in nature. It describes the common features of the diffuse double layer that intrigue clay chemists and electrochemists alike. Based on this common understanding it develops the flux equations that underpin all electrochemical experiments and ends by describing experimental details that the beginning electrochemists should be aware of when attempting to apply electrochemical methods to clays. The group of Fitch gives extensive review to applications of clay-modified electrode work in understanding flux of materials in clay films. At the workshop Dr. Janet Osteryoung presented the use of electrochemical methods, such as microelectrode cyclic voltammetry, in elucidating the charge of colloids. Dr. Osteryoung, at the time head of the NSF division of Chemistry, was unable to write an article, however the technique is extensively reviewed in the first article. The technique relies upon the difference in diffusion coefficient of a cation distributed between the diffuse double layer of a colloid and the bulk solution can be used to determine the distance between charges on the colloid. The first article also attempts to set the clay-modified electrode context for each of the other submissions in this volume: Yamagishi, Manias, Amonette, and Villemure. The use of clay-modified electrodes is detailed, particularly for the information it can give on diffusion in composite materials (see the article of Prof. Aki Yamagishi, now of the University of Tokyo). One of the problems with clay-modified electrodes has been in the structure of the films obtained and the thickness of those films. Yamagishi presents results using Langmuir Blodgett clay film formation using surfactants. Those clays films are then used in clay-modified electrodes to determine intrinsic electroactivity of very thin clay films. Along the same lines recent researchers are interested in the flux of both electroactive and electroinactive materials in nanocomposites of clays. One exciting area is that of polymer composites which are designed to give high sodium or potassium flux and good stability at relatively high temperatures. These membranes can be used in developing the next generation of fuel cells. The conductivity of such films exposes an interesting theoretical and experimental problem. E. Manias, A.Z. Panagiotopoulos, D.B. Zax and E.P. Giannelis of the Departments of Materials Science and Engineering, Chemical Engineering, and Chemistry and Chemical Biology, Cornell University review some of the theoretical work which attempts to elucidate the mechanisms by which high sodium flux occurs in these nanocomposites. The introductory article by Susan Macha, Scott Baker, and Alanah Fitch of Loyola University Chicago covers some of the experimental electrochemical techniques used to determine the conductivities of these clay nanocomposite films. Another exciting area of application of electrochemistry to problems in clay chemistry is in the area of the direct reduction and oxidation of redox metals in clay crystals. This work is of practical interest because of its potential application to remediation of contaminated military sites. It is known that clay interceptor beds that are reduced with dithionite serve to control contaminant plumes. James E. Amonette of Pacific Northwest Labs gives a review of the reactivity of iron in clay minerals. This review is followed by an article by Prof. Gilles Villemure of University of New Brunswick, Canada, which shows some of the most successful electrochemical experiments involving direct reduction of metals in clay lattices.