Aluminum - Al
Aluminum (Al) is a lightweight metal that forms a protective oxide layer in air. In water treatment, aluminum compounds such as alum are widely used as coagulants; the chemistry of dissolved aluminum depends strongly on pH and water composition.
Chemical properties · Uses and occurrence · Health effects · Environmental effects · Water and treatment
Chemical properties of aluminum
| Symbol / atomic number | Al / 13 |
|---|---|
| Group / period / block | 13 / 3 / p |
| Standard atomic weight | 26.9815384(3) |
| Electron configuration | [Ne] 3s2 3p1 |
| Common oxidation state | +3 |
| Density (20 °C) | 2.70 g cm−3 |
| Melting point | 660.323 °C |
| Boiling point | 2519 °C |
| Electronegativity (Pauling) | 1.61 |
| Non-bonded atomic radius | 0.184 nm |
| First ionisation energy | 577.5 kJ mol−1 |
| Stable isotope | 27Al |
| First isolated | Hans Christian Ørsted, 1825 |
| CAS registry number | 7429-90-5 |
Atomic weight follows CIAAW. Intervals reflect natural isotopic variation; parentheses indicate uncertainty in the final digits. Other properties follow RSC unless noted. Atomic radii depend on their definition.
Properties, uses and occurrence
Aluminum is a silvery, malleable metal. It is abundant in Earth's crust but is generally found in compounds such as bauxite, rather than as native metal. Its low density and useful mechanical properties make it common in transport, buildings, packaging and electrical applications. Aluminum alloys often contain silicon, magnesium or copper.
A thin oxide film forms on aluminum exposed to air and helps protect the surface from further corrosion. Industrial aluminum is produced from alumina by electrolysis in molten cryolite. The Hall–Héroult process was developed independently by Charles Martin Hall and Paul Héroult in 1886.
Health effects of aluminum
Aluminum has no known essential biological role. Exposure and effects depend on the chemical form, route and amount. The metal surface, insoluble mineral particles and soluble aluminum ions should not be treated as equivalent. In acidic conditions, more aluminum can dissolve from minerals; dissolved Al3+ can be toxic to plants and aquatic organisms.
Claims that aluminum causes Alzheimer's disease are not established. Occupational exposure to aluminum-containing dusts should be evaluated according to the material and applicable workplace controls.
Aluminum in the environment
Aluminum is widespread in rocks and soils. Its mobility and biological availability vary with pH and complexing substances. Acidification can increase dissolved aluminum and harm sensitive plants and aquatic life. Aluminum-containing treatment residuals should be managed according to their composition and local requirements.
Aluminum in water treatment
Aluminum sulfate (alum) and other aluminum coagulants are added to many water-treatment processes. Hydrolysis forms aluminum hydroxide floc that helps destabilise and capture suspended particles and some natural organic matter. The floc is then removed by clarification and filtration.
Coagulation performance and residual aluminum depend on coagulant type and dose, pH, temperature, alkalinity, mixing and source-water quality. Alum hydrolysis consumes alkalinity and can lower pH. Jar testing and monitoring help optimise treatment and manage residuals; there is no single dose or pH that fits every source water. Read more about aluminum in water.
When assessing aluminum in raw or treated water, distinguish dissolved from particulate aluminum and record the sample filtration and preservation method. Interpret results using the applicable local drinking-water requirements.
Aluminum page last reviewed: by Lenntech.
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