The atmosphere and much of the rivers, lakes, and oceans of the Earth are oxygenated. Any pyrite that comes into contact with these environments becomes unstable and breaks down. The process is called oxidation. It is an exothermic process and, as described in Chapter 5, this process was thought to heat the Earth. It is the opposite of reduction, which we discussed with regard to the microbial formation of sulfide from sulfate in Chapter 6. The counterintuitive concept important here is that oxidation is a chemical process that does not necessarily need oxygen. This idea—that you can oxidize things in the absence of oxygen—is one that most natural scientists are aware of but that they need a couple of nudges occasionally to remind themselves about. This means that pyrite oxidizes not only in oxygenated environments—although that is what we are most familiar with—but also in oxygen-free environments. Among the products of pyrite oxidation are large quantities of acid. Although this happens naturally during rock weathering, the intervention of humankind has led to an enormous increase in the exposure of pyrite to the atmosphere. This has produced contamination of the atmosphere, groundwater, and watercourses on a regional scale. It has also increased the amount of uncontrolled coal burning in coal seams, coal mines, and coal waste tips worldwide, making whole towns uninhabitable and laying waste to large areas. In this chapter I consider in more detail what exactly the process of pyrite oxidation is and how it affects the Earth’s environment today, as well as the problems it stores up for humanity in the future. In chemical terms, oxidation does not mean just the addition of oxygen. Oxidation is a reaction that involves the removal of one or more electrons from a compound because of a chemical reaction. One of the most familiar oxidation reactions is combustion, where substances burn in air to produce heat. The way to put out such a fire is to restrict oxygen access using a chemical foam or fire blanket. Since this reaction with oxygen was the best known, the process was called originally called oxidation.
Keywords: Acidogene, Bingham Canyon copper pit (Utah), Gaia hypothesis, Rio Tinto, UK Central Electricity Generating Board (CEGB), US Environmental Protection Agency (EPA), acid Earth, anoxic pyrite oxidation pathway, ferrous iron
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