Aluminium sulfate is a salt with the
formula Al2 (SO4)3. It is soluble in water and is mainly used as a
coagulating agent (promoting particle collision by neutralizing charge) in the purification of drinking water and
wastewater treatment plant
Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environmen ...
s, and also in paper manufacturing.
The anhydrous form occurs naturally as a rare
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
millosevichite
Millosevichite is a rare sulfate mineral with the chemical formula Al2(SO4)3. Aluminium is often substituted by iron. It forms finely crystalline and often porous masses.
It was first described in 1913 for an occurrence in Grotta dell'Allume, Por ...
, found for example in volcanic environments and on burning coal-mining waste dumps. Aluminium sulfate is rarely, if ever, encountered as the anhydrous salt. It forms a number of different
hydrates, of which the hexadecahydrate Al
2(SO
4)
3·16H
2O and octadecahydrate Al
2(SO
4)
3·18H
2O are the most common. The heptadecahydrate, whose formula can be written as
2O)6">l(H2O)6sub>2(SO
4)
3·5H
2O, occurs naturally as the mineral
alunogen
Alunogen (from French ''alun'', “ alum”), also called feather alum and hair salt is a colourless to white (although often coloured by impurities, such as iron substituting for aluminium) fibrous to needle-like aluminium sulfate mineral. It has ...
.
Aluminium sulfate is sometimes called alum or papermaker's alum in certain industries. However, the name "
alum" is more commonly and properly used for any double sulfate salt with the generic formula , where ''X'' is a
monovalent cation such as
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosph ...
or
ammonium.
Production
In the laboratory
Aluminium sulfate may be made by adding
aluminium hydroxide
Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric, i.e., it has both basic an ...
, Al(OH)
3, to
sulfuric acid, H
2SO
4:
> + 3 H
2SO
4 → Al
2(SO
4)
3 + 6 H
2O
or by heating aluminium metal in a sulfuric acid solution:
: 2 Al + 3 H
2SO
4 → Al
2(SO
4)
3 + 3 H
2↑
From alum schists
The alum
schist
Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes ...
s employed in the manufacture of aluminium sulfate are mixtures of
iron pyrite
The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic luster and pale brass-yellow hue giv ...
,
aluminium silicate and various
bituminous
Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term ...
substances, and are found in upper
Bavaria
Bavaria ( ; ), officially the Free State of Bavaria (german: Freistaat Bayern, link=no ), is a state in the south-east of Germany. With an area of , Bavaria is the largest German state by land area, comprising roughly a fifth of the total lan ...
,
Bohemia,
Belgium
Belgium, ; french: Belgique ; german: Belgien officially the Kingdom of Belgium, is a country in Northwestern Europe. The country is bordered by the Netherlands to the north, Germany to the east, Luxembourg to the southeast, France to th ...
, and
Scotland
Scotland (, ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a Anglo-Scottish border, border with England to the southeast ...
. These are either roasted or exposed to the
weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...
action of the air. In the roasting process,
sulfuric acid is formed and acts on the clay to form aluminium sulfate, a similar condition of affairs being produced during weathering. The mass is now systematically extracted with water, and a solution of aluminium sulfate of
specific gravity
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...
1.16 is prepared. This solution is allowed to stand for some time (in order that any
calcium sulfate
Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris ...
and basic
iron(III) sulfate
Iron(III) sulfate (or ferric sulfate), is a family of inorganic compounds with the formula Fe2(SO4)3(H2O)n. A variety of hydrates are known, including the most commonly encountered form of "ferric sulfate". Solutions are used in dyeing as a morda ...
may separate), and is then evaporated until
iron(II) sulfate
Iron(II) sulfate (British English: iron(II) sulphate) or ferrous sulfate denotes a range of salts with the formula Fe SO4·''x''H2O. These compounds exist most commonly as the heptahydrate (''x'' = 7) but several values for x are know ...
crystallizes on cooling; it is then drawn off and evaporated until it attains a specific gravity of 1.40. It is now allowed to stand for some time, and decanted from any sediment.
From clays or bauxite
In the preparation of aluminium sulfate from
clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
s or from
bauxite
Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO ...
, the material is gently
calcined
Calcination refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), gener ...
, then mixed with sulfuric acid and water and heated gradually to boiling; if concentrated acid is used no external heat is generally required as the formation of aluminium sulfate is
exothermic. It is allowed to stand for some time, and the clear solution is drawn off.
From cryolite
When
cryolite
Cryolite ( Na3 Al F6, sodium hexafluoroaluminate) is an uncommon mineral identified with the once-large deposit at Ivittuut on the west coast of Greenland, mined commercially until 1987.
History
Cryolite was first described in 1798 by Danish vete ...
is used as the ore, it is mixed with
calcium carbonate and heated. By this means,
sodium aluminate
Sodium aluminate is an inorganic chemical that is used as an effective source of aluminium hydroxide for many industrial and technical applications. Pure sodium aluminate (anhydrous) is a white crystalline solid having a formula variously given as ...
is formed; it is then extracted with water and
precipitated
In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading ...
either by
sodium bicarbonate or by passing a current of
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
through the solution. The precipitate is then dissolved in sulfuric acid.
Uses
![Aluminum_Sulfate_Floc_in_Lake_Sediment](https://upload.wikimedia.org/wikipedia/commons/9/91/Aluminum_Sulfate_Floc_in_Lake_Sediment.png)
Aluminium sulfate is sometimes used in the human food industry as a firming agent, where it takes on
E number E520, and in animal feed as a
bactericide
A bactericide or bacteriocide, sometimes abbreviated Bcidal, is a substance which kills bacteria. Bactericides are disinfectants, antiseptics, or antibiotics.
However, material surfaces can also have bactericidal properties based solely on thei ...
. In the
USA
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
, the
FDA
The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...
lists it as "
generally recognized as safe" with no limit on concentration. Aluminium sulfate may be used as a
deodorant
A deodorant is a substance applied to the body to prevent or mask body odor due to bacterial breakdown of perspiration or vaginal secretions, for example in the armpits, groin, or feet. A subclass of deodorants, called antiperspirants, prevents ...
, an
astringent
An astringent (sometimes called adstringent) is a chemical that shrinks or constricts body tissues. The word derives from the Latin ''adstringere'', which means "to bind fast". Calamine lotion, witch hazel, and yerba mansa, a Californian pla ...
, or as a
styptic for superficial shaving wounds.
Aluminium sulfate is used as a
mordant
A mordant or dye fixative is a substance used to set (i.e. bind) dyes on fabrics by forming a coordination complex with the dye, which then attaches to the fabric (or tissue). It may be used for dyeing fabrics or for intensifying stains in ...
in
dyeing
Dyeing is the application of dyes or pigments on textile materials such as fibers, yarns, and fabrics with the goal of achieving color with desired color fastness. Dyeing is normally done in a special solution containing dyes and particular c ...
and
printing textiles.
It is a common
vaccine
A vaccine is a biological preparation that provides active acquired immunity to a particular infectious or malignant disease. The safety and effectiveness of vaccines has been widely studied and verified. adjuvant In pharmacology, an adjuvant is a drug or other substance, or a combination of substances, that is used to increase the efficacy or potency of certain drugs. Specifically, the term can refer to:
* Adjuvant therapy in cancer management
* Analgesi ...
and works "by facilitating the slow release of
antigen
In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune respons ...
from the vaccine depot formed at the site of
inoculation."
[
Aluminium sulfate is used in ]water purification
Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water that is fit for specific purposes. Most water is purified and disinfected for hu ...
and for chemical phosphorus removal Chemical phosphorus removal is a wastewater treatment method, where phosphorus is removed using salts of aluminum (e.g. alum or polyaluminum chloride), iron (e.g. ferric chloride), or calcium (e.g. lime). Phosphate forms precipitates with the metal ...
from wastewater
Wastewater is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of domestic, industrial ...
. It causes suspended impurities to coagulate into larger particles and then settle to the bottom of the container (or be filtered out) more easily. This process is called coagulation
Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechanis ...
or flocculation
Flocculation, in the field of chemistry, is a process by which colloidal particles come out of suspension to sediment under the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from ...
. Research suggests that in Australia, aluminium sulfate used in this way in drinking water treatment is the primary source of hydrogen sulfide gas in sanitary sewer
A sanitary sewer is an underground pipe or tunnel system for transporting sewage from houses and commercial buildings (but not stormwater) to a sewage treatment plant or disposal. Sanitary sewers are a type of gravity sewer and are part of an ...
systems. An improper and excess application incident in 1988 polluted the water supply of Camelford
Camelford ( kw, Reskammel) is a town and civil parish in north Cornwall, England, United Kingdom, situated in the River Camel valley northwest of Bodmin Moor. The town is approximately ten miles (16 km) north of Bodmin and is governed ...
in Cornwall
Cornwall (; kw, Kernow ) is a historic county and ceremonial county in South West England. It is recognised as one of the Celtic nations, and is the homeland of the Cornish people. Cornwall is bordered to the north and west by the Atlantic ...
.
Aluminium sulfate has been used as a method of eutrophication
Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...
remediation for shallow lakes. It works by reducing the phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
load in the lakes.
When dissolved in a large amount of neutral or slightly alkaline water, aluminium sulfate produces a gelatinous precipitate of aluminium hydroxide
Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric, i.e., it has both basic an ...
, Al(OH)3. In dyeing and printing cloth, the gelatinous precipitate helps the dye adhere to the clothing fibers by rendering the pigment insoluble.
Aluminium sulfate is sometimes used to reduce the pH of garden soil, as it hydrolyzes
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolysi ...
to form the aluminium hydroxide precipitate and a dilute sulfuric acid solution. An example of what changing the pH level of soil can do to plants is visible when looking at ''Hydrangea macrophylla
''Hydrangea macrophylla'' is a species of flowering plant in the family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of ...
''. The gardener can add aluminium sulfate to the soil to reduce the pH which in turn will result in the flowers of the ''Hydrangea'' turning a different color (blue). The aluminium is what makes the flowers blue; at a higher pH, the aluminium is not available to the plant.
In the construction industry, it is used as waterproofing agent and accelerator in concrete
Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wid ...
. Another use is a foaming agent in fire fighting foam
Firefighting foam is a foam used for fire suppression. Its role is to cool the fire and to coat the fuel, preventing its contact with oxygen, thus achieving suppression of the combustion. Firefighting foam was invented by the Russian engineer a ...
.
It can also be very effective as a molluscicide
Molluscicides () – also known as snail baits, snail pellets, or slug pellets – are pesticides against molluscs, which are usually used in agriculture or gardening, in order to control gastropod pests specifically slugs and snails which damag ...
, killing spanish slugs.
Mordant
A mordant or dye fixative is a substance used to set (i.e. bind) dyes on fabrics by forming a coordination complex with the dye, which then attaches to the fabric (or tissue). It may be used for dyeing fabrics or for intensifying stains in ...
s aluminium triacetate
Aluminium triacetate, formally named aluminium acetate, is a chemical compound with composition . Under standard conditions it appears as a white, water-soluble solid that decomposes on heating at around 200 °C. The triacetate hydrolysis, ...
and aluminium sulfacetate
Aluminium sulfacetate is a mixture of aluminium salts dissolved in water with formula .
Uses
It is an evenly balanced mixture of aluminium sulfate and aluminium acetate. It can be used as a mordant, which is a substance used to set dyes on fabri ...
can be prepared from aluminium sulfate, the product formed being determined by the amount of lead(II) acetate
Lead(II) acetate (Pb(CH3COO)2), also known as lead acetate, lead diacetate, plumbous acetate, sugar of lead, lead sugar, salt of Saturn, or Goulard's powder, is a white crystalline chemical compound with a slightly sweet taste. Like many other l ...
used:
: + 3 → 2 + 3
: + 2 → + 2
Chemical reactions
The compound decomposes to γ-alumina and sulfur trioxide when heated between 580 and 900 °C. It combines with water forming hydrated salts of various compositions.
Aluminium sulfate reacts with sodium bicarbonate to which foam stabilizer has been added, producing carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
for fire-extinguishing foams:
: Al2(SO4)3 + 6 NaHCO3 → 3 Na2SO4 + 2 Al(OH)3 + 6 CO2
The carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
is trapped by the foam stabilizer and creates a thick foam which will float on top of hydrocarbon fuels and seal off access to atmospheric oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
, smothering the fire
Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products.
At a certain point in the combustion reaction, called the ignition point, flames a ...
. Chemical foam was unsuitable for use on polar solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for p ...
s such as alcohol, as the fuel would mix with and break down the foam blanket. The carbon dioxide generated also served to propel the foam out of the container, be it a portable fire extinguisher
A fire extinguisher is a handheld active fire protection device usually filled with a dry or wet chemical used to extinguish or control small fires, often in emergencies. It is not intended for use on an out-of-control fire, such as one which ha ...
or fixed installation using hoselines. Chemical foam is considered obsolete in the United States and has been replaced by synthetic mechanical foams, such as AFFF which have a longer shelf life, are more effective, and more versatile, although some countries such as Japan and India continue to use it.
References
Footnotes
Notations
*
*
External links
International Chemical Safety Card 1191
Aluminum and health
Government of Canada Fact Sheets and Frequently Asked Questions: Aluminum Salts
{{Authority control
Aluminium compounds
Sulfates
Molluscicides
Water treatment
Adjuvants
E-number additives