The Mobilization of Aluminum into the Biosphere

Aluminum is currently the most widely used non-ferrous metal, and its extraction and purification from geological stores exceeds that of any other metal except iron (1, 2). In 2013, global primary aluminum production was ~52 million tons (104 billion pounds) or about 15 pounds for very person on the earth (1–4). The global outlook for aluminum demand from developing countries such as Brazil, China, India, and Indonesia is rapidly increasing, due to new applications for aluminum and aluminum alloys in infrastructural support, transportation including automobiles, aviation and aerospace applications, electrical transmission, and the generation of energy, including catalytic zeolites in the petroleum and petrochemical industries (5). Interestingly, the largest “machine” built by humankind is the domestic and international networks for the transmission of electricity. Although traditionallyused copper has a higher electrical conductivity, aluminum is only slightly less so, being lighter, more ductile, and less expensive; aluminum is now widely used for both high-voltage tower construction and the electrical transmission wires themselves (2–5). It has been estimated that within the next 10 years aluminum production will exceed that of the previous 150 years (1–3). This prolific de novo generation of aluminum combined with its highly efficient recycling means this metal is becoming increasingly present in our biosphere, defined as the sum of all ecosystems and living organisms on the earth. This short “Opinion” paper will overview and comment on the current massive mobilization of aluminum into the earth’s biosphere. ALUMINUM GEOLOGY, HISTORICAL AND INDUSTRIAL PERSPECTIVES Bound tightly by oxygen and silicon, aluminum oxides and silicates, commonly referred to as alumina and/or aluminosilicate, exist naturally in ores generically termed bauxite. Bauxite consists mainly of the hydrated aluminum oxide (Al2O3xH2O) minerals gibbsite, boehmite, and diaspore, and is the world’s main source of raw material for the production of aluminum. Aluminum is extremely abundant; and after oxygen and silicon is the third most abundant element in the earth’s crust and the most abundant metal (1–6). Bauxite ore often contains several varieties of iron oxides, mostly goethite, hematite, and the clay mineral kaolinite making them reddish in appearance; widely distributed in the tropics, rusty-red soil types called laterites are highly enriched in complex aluminumand iron-oxides. Interestingly, as the primordial earth cooled, the lighter, lowest-density elements rose to the surface crust, and aluminum, one of the lightest metals known, currently exists in relatively easy-to-access near-the-surface deposits (7). Hence, two geophysical features make bauxite relatively easy to acquire as (i) massive bauxite deposits lie very near the earth’s surface, conducive to strip mining, with little or no overburden to remove; and (ii) the aluminum content of bauxite is very high in the lithosphere, conducive to vast bauxite mining and smelting operations. It is not often appreciated that although aluminum averages 8% (w/v) of the entire earth’s crust, alumina-enriched bauxite ore deposits can often reach up to 50% (w/v); for example, the Gove and Weipa bauxite deposits of Northern Territory and Queensland, Australia contain ~50% available alumina, and are currently among the largest, most accessible, and highest grade bauxite mines in the world (4, 6). Remarkably, the largest aluminum mines in Australia can extract ~3,000 tons (6,000,000 pounds) of bauxite per hour, and these are the largest contributors to a global aluminum production, which is currently in excess of about ~6,000 tons (12 million pounds) of 99% pure aluminum produced every hour of every day (2–5). The virtual inexhaustible supply of alumina in the earth’s crust combined with the high recycling potential for aluminum (see below) guarantee to make aluminum an expanding presence and permanent fixture in our biosphere for the foreseeable future. Alumina, aluminosilicates, and bauxite are relatively inert, naturally occurring compounds, in contrast to aluminum’s extremely high reactivity in its pure elemental form (3). Aluminum was first produced experimentally in 1825 by the Danish chemist Hans Christian Oersted, and later the German, French, and Austrian chemists Friedrich Wöhler, Henri SainteClaire Deville, and Carl Joseph Bayer up-graded isolation efficiencies and purification technologies, (7, 8). Just ~75 years later, the inception and application of the Hall–Heroult–Bayer process, and later modifications and upgrades of this industrial technology, including implementation of the Soderberg and prebake technologies, has made aluminum mining, extraction, and purification a multibillion international industry. Global aluminum production since 1900 has increased an amazing ~13,000-fold (3, 5, 7). In the currently used Hall–Heroult–Bayer process molten