Aluminum

Typical wastewaters

• acid mine drainage dissolved Al³⁺ at extreme acidity, up to 90 mg/L; precipitates as Al(OH)₃ with iron when lime or limestone raises the pH to 6 to 7
• municipal wastewater dosed with aluminium salts for phosphorus removal AlPO₄ and Al(OH)₃ in the sludge, 1.5 to 3 times the theoretical 1 mol Al per mol P dosed
• alumina refining bauxite residue (red mud) leachate aluminate Al(OH)₄⁻ in pH 13 leachate; over 99 percent removed as a boehmite-like precipitate and dawsonite on HCl neutralisation, 86 percent with gypsum (calcite), over 99 percent with seawater (hydrotalcite) Ajka, Hungary spill leachate
• sulfuric acid anodising and caustic etching or chemical milling of aluminium Al³⁺ in spent sulfuric acid anodising electrolyte (5 to 10 g/L in service, bath dumped at 15 to 20 g/L) and sodium aluminate in caustic milling baths discarded above 70 g/L Al; sludge is Al(OH)₃ and NaAlO₂ STM BREF sections 4.11.3.1 and 2.5.23
• anodising plant mixed wastewater aluminium with iron, sulfate and COD in mixed anodic oxidation coating wastewater over 99 percent Al removal by PTFE membrane distillation
• aluminium forming (rolling, casting, quench cooling, cleaning and etching) aluminium in rolling emulsions, contact cooling water and cleaning or etching rinses BPT 49.55 mg/off-kg daily maximum for solution heat treatment contact cooling water, 0.525 mg/off-kg for rolling with neat oils
• secondary aluminium smelting (wet processing of dross and residues) aluminium with fluoride and ammonia in residue processing wastewater BPT 30-day average 1.0 kg/t product, fluoride 0.4 kg/t
• steel slag disposal drainage dissolved aluminium as aluminate in pH above 10 slag leachate rich in calcium and zinc 36 year dataset, northern England

1 · Identity

Symbol, number

Al, 13

Oxidation states in water

+3 only. Al³⁺ as the hexaaqua ion in acid water, hydrolysed stepwise to AlOH²⁺, Al(OH)₂⁺, solid Al(OH)₃ near neutral pH and the aluminate anion Al(OH)₄⁻ in alkaline water; polynuclear hydroxy cations in prehydrolysed coagulants; complexes with fluoride, sulfate, phosphate, silicate and natural organic matter.

Note

The element entry covers the ores, the smelter and the properties narrative already notes that the coagulants work by the same hydrolysis that makes acid soils toxic. This chapter is the water side: which aluminium species exists at which pH, how alum and PACl consume alkalinity, and what keeps residual aluminium below 0.1 mg/L.

2 · Occurrence in water

Natural sources

Weathering of aluminosilicates; dissolved aluminium is low near neutral pH because Al(OH)₃ caps it, and rises in acid waters (acid rain catchments, acid mine drainage, organic rich peat waters) where Al³⁺ and organic complexes are mobilised (WHO background document).

Anthropogenic sources

Carry over from aluminium coagulants is the main source in drinking water; acid mine drainage and metal pickling; alumina refinery red mud liquor and anodising rinse waters in the ledger's chemical and metals chapters (no concentration read for them here).

3 · Speciation

Aluminium in water is a solubility story governed by pH. Below about pH 5 the hexaaqua Al³⁺ ion and its first hydrolysis products dominate and aluminium is dissolved and toxic to fish and plants; between about pH 5.5 and 7 amorphous Al(OH)₃ precipitates and dissolved aluminium is at its minimum (WHO puts the minimum in pure water at pH 5.5 to 6.0); above pH 7 to 8 the aluminate anion Al(OH)₄⁻ redissolves it. Fluoride, sulfate, phosphate, silicate and humic substances form complexes that keep aluminium in solution beyond what the hydroxide alone allows. Coagulation exploits the middle of the curve: alum or PACl dosed at pH 5 to 7 hydrolyses within seconds to positively charged hydroxy polymers and then to Al(OH)₃ floc.

Solubility

Controlled by amorphous Al(OH)₃ in fresh floc and by gibbsite in aged deposits; dissolved aluminium is lowest at pH 5.5 to 6.0 in pure water (WHO) and about 6 to 6.5 in natural water with sulfate and organic matter, rising on both sides. No solubility products are quoted because the sources read do not print them.

Hydrolysis

Al³⁺ hydrolyses stepwise, releasing one proton per step, to AlOH²⁺, Al(OH)₂⁺, Al(OH)₃ and Al(OH)₄⁻; the first hydrolysis constant is near pK 5 (Stumm and Morgan chapter 6, not re-read), so aluminium salts are acidic and every mole of alum dosed consumes six equivalents of alkalinity. Partial neutralisation before dosing (basicity, OH to Al ratio) is what distinguishes PACl from alum.

Complexation

Fluoride (AlF²⁺ and higher fluoro complexes), sulfate and phosphate as inorganic ligands; humic and fulvic acids as the organic ligands that hold aluminium in brown waters; silicate forms hydroxyaluminosilicates (WHO background document lists the ligands; constants not printed).

Precipitates

Al(OH)₃ (amorphous, then gibbsite), AlPO₄ (phosphorus removal), hydroxyaluminosilicates, basic aluminium sulfate in acid sulfate systems; Al(OH)₃ floc carries adsorbed arsenate, fluoride, phosphate and natural organic matter.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Total, dissolved and reactive aluminium differ by an order of magnitude in coagulated water; filter 0.45 µm in the field for dissolved aluminium and state the fraction. Colloidal Al(OH)₃ settles and adheres to bottle walls, so acidify total aluminium samples at once. Speciation (labile monomeric versus organically bound) needs field separation, not read in the sources this session.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

Little indication of acute oral toxicity; drinking water is usually under 5 percent of intake, food and antacids dominate. JECFA PTWI 1 mg/kg body weight (2007) for all aluminium compounds, expressed as a weekly value because of possible bioaccumulation. Epidemiological studies associating aluminium in drinking water above 100 µg/L with Alzheimer disease give relative risks below 2.0 and cannot be dismissed, but confounding and total intake were not controlled (WHO fact sheet, citing the 1997 EHC monograph).

Bioaccumulation

Absorption from water is usually less than 1 percent and depends on solubility, pH and speciation; JECFA kept a weekly rather than daily value because of the potential for accumulation (WHO).

Ecotoxicity

US EPA 2018 freshwater aquatic life criteria for aluminium vary with pH, total hardness and dissolved organic carbon: acute 1 to 4,800 µg/L and chronic 0.63 to 3,200 µg/L total recoverable aluminium, calculated per site; the 1988 fixed values were 750 µg/L acute and 87 µg/L chronic at pH 6.5 to 9.0. Acid water aluminium is the classic fish kill mechanism of acidified lakes.

Flags

• The occurrence figures for treated water are 1980s surveys quoted by the 2010 WHO background document; modern plants run lower.
• The seawater figure is a single PubChem abundance figure, not a range.
• The first hydrolysis constant (pK about 5), the PACl description and the membrane note are cited to textbook chapters from memory, not re-read this session.
• The alum alkalinity consumption (about 0.5 mg as CaCO₃ per mg alum 14-hydrate) is computed from the stoichiometry, not printed by a source read.
• The lime neutralisation equation for acid water is written here as the stoichiometry; the sources read describe neutralisation without printing it.
• The phosphorus precipitation ratio (1.5 to 3 times stoichiometric) is from Metcalf and Eddy chapter 6 by memory.
• Abu Dhabi values cover two media (marine 20 mg/L, sewer 100 mg/L) and differ five fold; other GCC states not read.
• The 2018 EPA aluminium criteria are site specific; the ranges quoted are the span of the calculator, not a single number.

Gaps

• No solubility product, hydrolysis constant table or complexation constants were read; Stumm and Morgan chapter 6 has them (from the chapter, not re-read).
• Alum sludge production and dewatering figures were not sourced.
• ISO 10566 is cited by number only; its catalogue page was not opened, so no url is given.
• The Standard Methods 3500-Al B detection limit was not read.
• Other GCC discharge standards (Saudi, Oman, Qatar) were not read.
• PACl basicity ranges and cold water performance are cited to MWH chapter 9 without re-reading; the Al₁₃ Keggin formula and the activated alumina ligand exchange are written from MWH chapters 9 and 16 on the same basis, from the chapter, not re-read, and no formation or surface constants were read.
• The caustic etch equation is standard amphoteric chemistry consistent with the book's own aluminium entry; no etch bath composition or spent liquor analysis was read.

Sources

Identity

Name and symbol

Aluminum, Al

Atomic number

13 protons

Position

group 13 · period 3 · p-block · post-transition metal

CAS number

7429-90-5

Atomic structure

Atomic mass

26.981 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p¹
[Ne] 3s²³p¹

Electrons per shell

2, 8, 3

Valence electrons

3 outer shell

Physical properties

State at room temperature

Solid

Melting point

933.44 K (660.29 °C)

Boiling point

2,792 K (2,518.85 °C)

Density

2.7 g/cm3

Appearance

Silvery gray metallic

Thermal conductivity

237 W/(m·K)

Electrical resistivity

26.5 nΩ·m at 20 °C

Electrical conductivity

37.74 MS/m

Crystal structure

face-centered cubic

Molar heat capacity

24.2 J/(mol·K)

Chemical properties

Oxidation states

+3

Electronegativity

1.61 (Pauling Scale)

Ionisation energy

5.986 eV
1st 577.5, 2nd 1,816.7, 3rd 2,744.8 kJ/mol

Electron affinity

0.441 eV

Atomic radius

empirical 121, covalent 121, van der Waals 184 pm

Ionic radius

Al³⁺ 54 pm

Reactivity

A post-transition metal of the boron group that forms +3 compounds and has a great affinity for oxygen; a self-healing oxide film about 5 nm thick makes the metal passive to air, water and dilute acids despite its high intrinsic reactivity.

with water

Does not react under ordinary conditions because of the oxide film; once the film is stripped (hot concentrated hydrochloric acid, alkali, amalgamation with mercury) aluminium reduces water to hydrogen.

with oxygen, air

Forms its protective oxide layer at once in air and corrodes no further; the fine powder burns brilliantly and reacts explosively with liquid oxygen, and the heat of oxide formation drives the thermite reaction: $\ce{4 Al + 3 O2 -> 2 Al2O3}$

with acids

Passive to dilute acids and to oxidising acids such as nitric and concentrated sulfuric, which are stored in aluminium; dissolves in hot concentrated hydrochloric acid with evolution of hydrogen, and in aqua regia: $\ce{2 Al + 6 HCl -> 2 AlCl3 + 3 H2}$

with halogens

Reacts with the halogens on heating to the trihalides AlF3, AlCl3, AlBr3 and AlI3, Lewis acids used as Friedel-Crafts catalysts: $\ce{2 Al + 3 Cl2 -> 2 AlCl3}$

Typical compounds

• Al₂O₃ aluminium oxide alumina; corundum, ruby, sapphire; feed for the smelter
• AlCl₃ aluminium chloride Lewis acid catalyst for Friedel-Crafts reactions
• Al(OH)₃ aluminium hydroxide amphoteric; water clarification, antacid
• Al₂(SO₄)₃ aluminium sulfate two thirds of it goes to water treatment
• KAl(SO₄)₂.12H₂O potash alum soluble double sulfate; mordant in dyeing
• Na₃AlF₆ cryolite molten electrolyte of the Hall-Heroult process

Occurrence, production and use

Crustal abundance

8.23×104 milligrams per kilogram

Oceanic abundance

2×10-3 milligrams per liter

Occurrence and sources

The method of obtaining aluminum metal by the electrolysis of alumina dissolved in cryolite was discovered in 1886 by Hall in the U.S. and at about the same time by Heroult in France. Cryolite, a natural ore found in Greenland, is no longer widely used in commercial production, but has been replaced by an artificial mixture of sodium, aluminum, and calcium fluorides.

Aluminum can now be produced from clay, but the process is not economically feasible at present. Aluminum is the most abundant metal to be found in the earth's crust (8.1%), but is never found free in nature. In addition to the minerals mentioned above, it is also found in granite and in many other common minerals.

• dissolved in seawater about 0.002 mg/L; crustal estimate 82,300 mg/kg (Jefferson Lab figures via PubChem)
• bauxite (hydrated aluminium oxides) tropical laterites: Guinea, Australia, China, Brazil, Indonesia, India, Jamaica, Russia, Saudi Arabia, Vietnam; resources distributed Africa 32 percent, Oceania 23, South America and Caribbean 21, Asia 18
• clays, alunite, anorthosite, coal wastes, oil shales essentially inexhaustible subeconomic resources in most aluminium-producing countries; not competitive with bauxite
• cryolite, alum, feldspars and other aluminosilicates Greenland cryolite, now replaced by a synthetic fluoride bath; granite and most common rocks

Extraction, production

Bayer process: caustic digestion of bauxite to alumina

$\ce{2 Al(OH)3 (s) -> Al2O3 (s) + 3 H2O (g)}$

About 76 percent of bauxite consumed in the United States in 2024 was refined by the Bayer process to alumina or aluminium hydroxide; 69 percent of the alumina went to smelters and the rest to abrasives, ceramics, chemicals and refractories (USGS, printed pp. 42 to 43). Caustic soda for aluminium and metals was 6.0 percent of EU-27 and EFTA consumption in 2012 (CAK BREF, PDF p32). No equation is printed in the sources.

Hall-Heroult electrolysis of alumina dissolved in molten cryolite

$\ce{2 Al2O3 (l) + 3 C (s) -> 4 Al (l) + 3 CO2 (g)}$

Alumina is dissolved in a molten fluoride bath and reduced electrolytically to metal; aluminium fluoride and HF are key inputs and smelters recycle their fluorides (RSC; USGS fluorspar chapter). The reaction equation is not printed in the sources used.

Secondary aluminium from new and old scrap

About 3.6 million tonnes recovered from purchased scrap in the United States in 2024, 56 percent new and 44 percent old scrap; old scrap alone equalled 37 percent of apparent consumption (USGS, printed pp. 32 to 33).

Uses

Two important developments in the 1880s greatly increased the availability of aluminum. The first was the invention of a new process for obtaining aluminum from aluminum oxide. Charles Martin Hall, an American chemist, and Paul L. T. Héroult, a French chemist, each invented this process independently in 1886. The second was the invention of a new process that could cheaply obtain aluminum oxide from bauxite. Bauxite is an ore that contains a large amount of aluminum hydroxide (Al2O3·3H2O), along with other compounds. Karl Joseph Bayer, an Austrian chemist, developed this process in 1888. The Hall-Héroult and Bayer processes are still used today to produce nearly all of the world's aluminum.

With an easy way to extract aluminum from aluminum oxide and an easy way to extract large amounts of aluminum oxide from bauxite, the era of inexpensive aluminum had begun. In 1888, Hall formed the Pittsburgh Reduction Company, which is now known as the Aluminum Company of America, or Alcoa. When it opened, his company could produce about 25 kilograms of aluminum a day. By 1909, his company was producing about 41,000 kilograms of aluminum a day. As a result of this huge increase of supply, the price of aluminum fell rapidly to about $0.60 per kilogram.

Today, aluminum and aluminum alloys are used in a wide variety of products: cans, foils and kitchen utensils, as well as parts of airplanes, rockets and other items that require a strong, light material. Although it doesn't conduct electricity as well as copper, it is used in electrical transmission lines because of its light weight. It can be deposited on the surface of glass to make mirrors, where a thin layer of aluminum oxide quickly forms that acts as a protective coating. Aluminum oxide is also used to make synthetic rubies and sapphires for lasers.

It is extensively used for kitchen utensils, outside building decoration, and in thousands of industrial applications where a strong, light, easily constructed material is needed.

Although its electrical conductivity is only about 60% that of copper, it is used in electrical transmission lines because of its light weight. Pure aluminum is soft and lacks strength, but alloyed with small amounts of copper, magnesium, silicon, manganese, or other elements impart a variety of useful properties.

These alloys are of vital importance in the construction of modern aircraft and rockets. Aluminum, evaporated in a vacuum, forms a highly reflective coating for both visible light and radiant heat. These coatings soon form a thin layer of the protective oxide and do not deteriorate as do silver coatings. They are used to coat telescope mirrors and to make decorative paper, packages, and toys.

• Transport: alloys with copper, manganese, magnesium and silicon for aircraft, cars, trains and bicycles 36 percent of US aluminium consumption in 2024
• Packaging: cans, foil, beer kegs 23 percent of US consumption in 2024
• Construction and electrical: window frames and cladding; electrical transmission lines, cheaper than copper; vacuum-evaporated reflective coatings for mirrors and packaging building 14 percent, electrical 9 percent, consumer durables 8, machinery 8 of US consumption in 2024
• Chemicals: nonmetallurgical alumina for abrasives, ceramics, chemicals, proppants and refractories; aluminium sulfate and alum, the compounds of greatest importance after the oxide; aluminium fluoride from fluorspar or fluorosilicic acid
• Water treatment: alum, aluminium chloride, polyaluminium chloride and sulfate, and sodium aluminate as coagulants for colloids and suspended solids (CWW BREF, PDF p206 and p208)
• Textiles: alum as a dye mordant, a use as old as Greece and Rome

Safety, toxicity

GHS classification, signal word Danger

• H250 Catches fire spontaneously if exposed to air Pyrophoric liquids
• H261 In contact with water releases flammable gas Substances and mixtures which in contact with water, emit flammable gases
• H228 Flammable solid Flammable solids
• H370 Causes damage to organs Specific target organ toxicity, single exposure
• H372 Causes damage to organs through prolonged or repeated exposure Specific target organ toxicity, repeated exposure

Discovery and name

Discovered by

Hans Christian Ørsted

Discovered

1824

First isolated

not in sources

Named by

Humphry Davy

Origin of the name

from alumine, obsolete name for alumina

Pure aluminum, a silvery-white metal, possesses many desirable characteristics. It is light, it is nonmagnetic and nonsparking, stands second among metals in the scale of malleability, and sixth in ductility.