which are sources of Ni, Fe, and rare earths. These form through extreme weathering of soils such that little remains but these highly insoluble elements; what we have learned about weathering, soil-forming processes, and the geochemistry of these metals will serve us well understand how these deposits form. Because of their importance to everything from flat panel computer displays and televisions to high performance magnets in wind turbines, electric cars, and speakers, we briefly examine rare earth ore deposits, which fall into many of the above categories.
Finally, we put our geochemical toolbox to use to understand how human activities can degrade environmental quality and how this can be addressed. Like ore deposits, this is an enormous topic and we have space to consider only a few examples. We begin with the problem of eutrophication and associated anoxia in fresh water lakes, using Lake Erie, one of the Great Lakes of North America, as an example. Eutrophication refers to situations where nutrient levels in water allow excessive growth of algae, usually cyanobacteria, which produce microcystin toxins. Lakes typically become temperature-stratified in summer such that oxygen in the deep water is not replenished. Bacteria consuming the remains of algae falling into the deep water can consume all available oxygen leading to anoxic conditions in the deep water and consequent fish kills. Persistent eutrophication in Lake Erie was successfully addressed by regulations in the 1970s that severely limited nutrients from sewage, industrial effluents and particulate phosphorus in agricultural runoff and the lake was restored to health. In the late 1990s eutrophication is summer began to occasionally reoccur due to dissolved phosphorus from agricultural runoff. Solving the problem will require further modification of farming practices.
Toxic metals are another important environmental problem. One source is mining of sulfide deposits, such as the several types described above. Sulfides exposed to water and atmospheric oxygen quickly weather to produce sulfuric acid, resulting in a problem known as acid mine drainage. Not only is the acidity a problem with pH values as low as 2, but under these conditions many otherwise insoluble toxic metals become soluble. The solution is certainly not to simply shut down mines as when pumps are shut off, water penetrates in mine shafts, pits, and tailings ponds and the problem worsens. Indeed, the bigger problem is old, abandoned mines as a number of strategies are deployed in modern mining operations to prevent the problem. Lead and mercury are highly toxic metals and anthropogenic release of these elements to the atmosphere has polluted the entire surface of the planet. Lead, however, is an example of an environmental success story largely due to the efforts of one geochemist, Claire Patterson. Regulations that eliminated Pb from gasoline and emissions from smelters have dramatically reduced the amount of Pb in the environment. Regulations have also starkly reduced emissions of Hg, at least in developed countries, and local sources of extreme pollution, such as in Minamata, Japan, where mercury poisoning killed over 1700 people and disabled many more, have been eliminated in most cases. Nevertheless, levels in the atmosphere, soils, plants, the ocean, and many fish species remain high and will decrease only slowly in the future, even if all emissions are eliminated. An understanding of the unique geochemistry of Hg will enable us to understand why.
Finally, we examine the problem of acid rain. This results from burning of fossil fuels, particularly coal, which oxidizes sulfur and nitrogen ultimately to sulfuric and nitric acid, although use of nitrogen fertilizers also contributes. This can lower pH in rain to values as low as 4. Depending on the nature of the soil and bedrock this may or may not be a problem, and the understanding of weathering reactions we gained in earlier chapters will help understand why. In areas where soils have developed through weathering of rocks with low acid neutralizing capacity, the low pH alone can have deleterious effects on trees, fish, and aquatic invertebrates, but that is not the main problem. Instead, the principal problems are loss of cations such as Ca2+ and aluminum toxicity. Aluminum is one of the most abundant elements in the Earth's crust, yet natural Al toxicity is rare. Once we understand the geochemistry of Al, we'll be able to understand why this is usually not an issue but can be when rain is acidic. Acid rain is another environmental success story, although a still unfolding one. Regulations have greatly reduced emissions in the developed world, but it will take decades before soils and stream chemistry returns to natural levels and for damaged ecosystems to heal.
REFERENCES AND SUGGESTIONS FOR FURTHER READING
1 Clarke, F. W. 1908. The Data of Geochemistry. US Geological Survey Bulletin 770. Washington, US Government Printing Office.
2 Vernadsky, V. I. 1926. Biosfera. Leningrad, Scientific Chemico-Technical Publishing.
3 Goldschmidt, V. M. 1937. The principals of distribution of chemical elements in mineral and rocks. Journal of the Chemical Society of London 1937: 655–73. doi: 10.1039/JR9370000655.
4 Gribbin, J. R. 1984. In Search of Schrödinger's Cat: Quantum Physics and Reality. New York, Bantam Books.
5 Lindley, D. 2001. Boltzmann's Atom: The Great Debate that Launched a Revolution in Physics, New York, The Free Press.
6 Morris, R. 2003. The Last Sourcers: The Path from Alchemy to the Periodic Table, Washington, DC, Joseph Henry Press.
7 Strathern, P. 2000. Mendeleyev's Dream: The Quest for the Elements, London, Berkley.
NOTES
1 ‡ Christian Friedrich Schönbein (1799–1868) was born in Metzingen in Swabia, Germany and served as professor at the University of Basel from 1835 until 1868. He is best known for his discovery of ozone.
2 * We will be using System International Units as much as practicable throughout the book. A list of these units and their abbreviations can be found in the Appendix.
3 † Randomness can affect the outcome of any experiment (though the effect might be slight). By definition, the effect of this randomness cannot be predicted. Where the effects of randomness are large, one performs a large collection, or ensemble, of experiments and then considers the average result.
4 † The motion of Mercury is an exception: its motion differs slightly from predictions based on Newtonian mechanics. But relativity theory does accurately predict its motion. This successful prediction was one of several that led physicists to accept Einstein's new theory.
5 ‡ Dmitri Ivanovich Mendeleyev was born in Tobolsk, Russia in 1834. He became professor of chemistry at St Petersburg in 1866. His periodic table was the sort of discovery that noble prizes are awarded for, but it came before the prize was established. He was honored, however, by having element number 101, Medelevium, named for him. Mendeleyev died in 1906.
6 † By convention, the mass number, which is the sum of protons and neutrons in the nucleus, of an isotope is written as a preceding superscript. However, for historical reasons, it is often pronounced “helium–4.” Note also that the atomic number or proton number can be readily deduced from the chemical symbol (atomic number of He is 2). The neutron number can be found by subtracting the proton number from the mass number. Thus, the symbol 4He gives a complete description of the nucleus of this atom.
7 ‡ The actual mass of an atom depends on the number of electrons and the nuclear binding energy as well as the number of protons and neutrons. However, the mass of the electron is over 1000 times less than the mass of the proton and neutron, which have nearly identical
