Properties and Natural Occurrence

Aluminum was discovered in 1825 by the Danish chemist Oersted. It is a soft, ductile, malleable, silvery metal. Its atomic number is 13, and it has one stable isotope, 27Al. Aluminum belongs to group 3a of the periodic table, along with boron, indium, gallium, and thallium. It most commonly forms trivalent ionic (Al3+) compounds, but it has some covalent characteristics. Aluminum is the most common metal in the earth's crust and is the third most common element. It is too reactive to occur in nature as the free metal.

Aluminum occurs in natural systems as the triva-lent ion and in these it has no oxidation-reduction chemistry. In aqueous solution, the chemistry is complicated by the formation of several pH-depen-dent complex ions. These ions—Al(OH)2+, Al(OH)J, and Al(OH)-—compete with Al3+ and Al(OH)3 within aquatic systems. Aluminum is minimally soluble in water at approximately pH 6, when the Al(OH)J ion dominates, but solubility increases at lower and higher pH values. At pH 7 and higher, the most important ion is Al(OH)-, whereas at low pH values Al3+ dominates.

In contrast to its abundance in the earth's crust, most natural waters contain very little dissolved aluminum (often <10 mgl-1), reflecting the low solubility of minerals and the deposition of Al3+ in sediments as the hydroxide. Seawater contains only 1 mgT1 of aluminum, and much of this is thought to be bound within the skeletons of diatoms. Where natural waters have either been acidified by acid rain or treated with aluminum sulfate to produce drinking water, the levels of the metal are higher. Concentrations in acidified lakes and rivers (up to 700 mgl-1) commonly exceed levels toxic to fish. In acidic well water, concentrations >1mgl-1 may occur. Aluminum concentrations in tap water should not exceed 200 mgl-1a guideline specified by the World Health Organization (WHO) on esthetic grounds.

Air concentrations of aluminum range from less than 1 mg m~3 in rural environments to as high as 10 mgm-3 in urban, industrialized areas. The higher levels in the latter result from the dust-creating activities of urban man.

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