PNNL

Abstract

In the preceding chapter, the fundamental nature of sulfide mineral surfaces has been discussed, and the understanding we have of the ways in which the surface differs from a simple truncation of the bulk crystal structure reviewed. This naturally leads on to considering our understanding of sulfide surface chemistry, in the sense of how sulfide surfaces interact and react, particularly with gases and liquids. As noted elsewhere in this volume, research on sulfide mineral surfaces and surface reactivity is a relatively recent concern of mineralogists and geochemists, partly prompted by the availability of new imaging and spectroscopic methods, powerful computers and new computer algorithms. There has been a significantly longer history of sulfide mineral surface research associated with technologists working with, or within, the mining industry. Here, electrochemical methods, sometimes combined with analytical and spectroscopic techniques, have been used to probe surface chemistry. The motivation for this work has been to gain a better understanding of the controls of leaching reactions used to dissolve out metals from ores, or to understand the chemistry of the froth flotation systems used in concentrating the valuable (usually sulfide) minerals prior to metal extraction. The need for improved metal extraction technologies is still a major motivation for research on sulfide surfaces, but in the last couple of decades, new concerns have become important drivers for such work. In particular, much greater awareness of the negative environmental impact of acid and toxic metal-bearing waters derived from breakdown of sulfide minerals at former mining operations has prompted research on oxidation reactions, and on sorption of metals at sulfide surfaces. At the interface between fundamental geochemistry and industrial chemistry, the role of sulfide substrates in catalysis, and in the self-assembly and functionalization of organic molecules, has become an area of significant interest. Such work ranges in its application from the development of new industrial processes, to fundamental questions of the possible role of sulfide surfaces in catalyzing the formation of the complex organic molecules leading to the emergence of life on Earth. In this chapter, we aim to provide an overview of current understanding of sulfide surface chemistry. The size of this research fi eld is already such that it is impossible to discuss all of the published work, but key examples are considered and readers directed to the main literature sources. The chapter begins with some examples of reaction with gaseous species (O2, H2O, H2S, CH3OH) as these are the most accessible in terms of understanding reactivity at the molecular scale. The very important oxidation and related electron transfer reactions, in both air and aqueous solution, are then considered before considering examples of catalysis and functionalization/self-assembly and interaction with organic molecules. In the final section, sorption of metal ions onto sulfide mineral surfaces is discussed before a few words concerning the future outlook for research in this entire area.