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Aluminium sulfate is a salt with the chemical formula . 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 plants, and also in paper manufacturing.
The anhydrous form occurs naturally as a rare mineral millosevichite, 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 , of which the hexadecahydrate and octadecahydrate are the most common. The heptadecahydrate, whose formula can be written as , occurs naturally as the mineral alunogen.
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 or ammonium.
It is a common vaccine adjuvant and works "by facilitating the slow release of antigen from the vaccine depot formed at the site of inoculation."
Aluminium sulfate is used in water purification and for chemical phosphorus removal from wastewater. 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 or flocculation. 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 systems. An improper and excess application incident in 1988 polluted the water supply of Camelford in Cornwall.
Aluminium sulfate has been used as a method of eutrophication remediation for shallow lakes. It works by reducing the phosphorus 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, 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 Hydrolysis 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. 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. Another use is a foaming agent in fire fighting foam.
It can also be very effective as a molluscicide, killing .
aluminium triacetate and aluminium sulfacetate can be prepared from aluminium sulfate, the product formed being determined by the amount of lead(II) acetate used:
Aluminium sulfate reacts with sodium bicarbonate to which foam stabilizer has been added, producing carbon dioxide for fire-extinguishing foams:
The carbon dioxide 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, smothering the fire. Chemical foam was unsuitable for use on 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 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.
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