Iron Redox Chemistry of Clays and Oxides: Environmental Applications
Iron (Fe) is the most abundant element in the earth, and fourth most abundant in the earth’s crust where it comprises about 5.1 wt% (1.9 atom%). This abundance, together with an aqueous standard reduction potential for the ferric/ferrous couple (+0.77 V) that is roughly midway between the limits for aqueous chemistry (−1.1 to +1.8V), gives Fe a central role in buffering (poising) the redox chemistry in natural systems. Nearly all the Fe occurs in the structures of minerals, but the greatest fraction of this Fe that is accessible to aqueous solution is associated with the clay-sized oxides, oxyhydroxides, and layer silicates, which are chemically dominant in soils and sediments because of their large specific surfaces. From an environmental perspective, therefore, the clay-sized Fe-bearing minerals represent an inorganic source or sink for electrons that can be manipulated to achieve environmental goals.
This chapter briefly discusses the mineral forms of Fe and its use as a reductant. The fundamental chemistry of Fe, including the structural, thermodynamic and kinetic factors affecting its reactivity are then discussed. The chapter concludes with a survey of the environmental applications of mineral forms of Fe, with a major emphasis on contaminant remediation technologies involving dithionite-generated Fe(II).
A little more than one-fifth of the volume of the earth’s crust is made of Fe-bearing primary minerals (Table 1), chiefly silicates such as olivine, pyroxene, amphibole, and biotite, but also oxides such as magnetite and ilmenite. The Fe content of the silicate minerals is commonly in the 10–20 wt% range
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.