It is perhaps difficult to imagine metals falling from the sky, but fall they do. Pollution of roadside soils and lake waters by the atmospheric deposition of lead and mercury are well known. Until a few decades ago, lead, derived from the combustion of leaded gasoline, was carried to the environment in combination with bromide, which was added to gasoline to make sure the lead cleared the exhaust system of your car. Pollution by the volatile metal, mercury, is more widespread, since mercury is a natural constituent of various coals and emitted in stack gases when they are burned. If you look carefully you can also measure copper, zinc, cadmium and other metals in rainfall samples.
Despite pollution controls, even today, there is more lead and mercury in the atmosphere from human activities than from all natural sources—dusts, volcanoes, forest fires— combined. Recently, working with several colleagues at Duke, we have found that the same is true for the metal vanadium, which is normally encountered as an alloy that makes steel harder.
Vanadium is found as a constituent of oil, particularly heavy oil, such as the bitumen that is mined from the tar sands of Canada. Vanadium is emitted to the atmosphere when these oils are burned or converted to petroleum coke, which is later burned in power plants. The proportion of oil that is classified as “heavy” oil has increased markedly in recent years, as the lighter grades of petroleum that are more easily refined have been depleted worldwide.
Emissions of vanadium to the atmosphere parallel the increased use of heavy oil and are now nearly twice the sum of all natural sources combined. We should expect high concentrations of vanadium in the atmosphere around Chicago, where heavy oil is refined by Koch Industries. Some of this will be found as particles, and some will be found as a dissolved ion in cloud waters. (If you have trouble envisioning a metal being found as a dissolved ion, think of the toxic concentrations of lead that were found in the drinking water in Flint, Michigan from the dissolution of lead pipes that carried it).
Luckily, the health impacts of vanadium do not rival those of lead and mercury, which have been known to be poisonous to humans since medieval times. But, we should recognize vanadium as an irritant in the lungs, potentially exacerbating asthma, emphysema, and COPD.
What impresses me most is the ability of a single species, namely Homo sapiens, to determine the chemical environment of all of Earth—that is, the evolutionary environment for all life on Earth. We’ve doubled the flux of nitrogen, phosphorus, lead and mercury to nature, and now have done the same for vanadium. Without greater cognizance of our emissions, we seem at risk of fouling (or worse) our own nest.
Gummow, B. 2011, Vanadium: Environmental pollution and health effects. Pp. 628-636. In J.O. Nriagu, ed., Encyclopedia of Environmental Health. Elsevier, Amsterdam
Schlesinger, W.H., E.M. Klein, and A. Vengosh. 2017. Global biogeochemical cycle of vanadium. Proceedings of the National Academy of Sciences doi: 10.1073/pnas.1715500114