Aluminum sulfate, with the chemical formula Al2(SO4)3, when used as a food additive, it has the European food additive number E520. It sometimes known as alum, but different with the later as the name “alum” is more correctly refer to double sulfate salt with the formula XAl(SO4)2·12H2O, where X is a monovalent cation such as potassium, sodium or ammonium.
Compared with its uses in food, aluminum sulfate is mainly used as a coagulant for drinking water and wastewater treatment. It also functions as a sizing agent for papermaking, blue and red color control of hydrangea in gardening, and as the raw material for the production of alums, such as ammonia alum, potassium alum, and sodium alum.
How is Aluminum sulfate Made?
The bauxite and cryolite are the main minerals for the production of aluminum sulfate. It is generally produced by the reaction between these two minerals with sulfuric acid. Here is the chemical reaction equation: Al2O3 + 3H2SO4 =Al2(SO4 )3 + 3H2O
With the appearance of white powder, shining plates or crystalline fragments or colorless solution, and with astringent taste.
It is hygroscopic, easy to absorb moisture and agglomerate when exposed in the air for a long time.
It exists in a number of different forms of hydrates, such as dihydrate, hexahydrate, decahydrate, tetradecahydrate, hexadecahydrate, and octadecahydrate.
|Other names||sulfuric acid, aluminum salt (3:2), aluminum trisulfate|
|PH||2.9 or above (5 % solution)|
|Solubility||Freely soluble in water, insoluble in ethanol.|
What’re the Uses of Aluminum sulfate?
Aluminum sulfate is an important aluminum salt. Its main purposes are used as a coagulant in the purification treatment of drinking water & industrial water, soil acidifier in gardening (e.g. change hydrangea color to blue), and as a sizing agent in papermaking.
Its uses in food is not common, when used as a food additive in Europe, it has the number E520 and categorized in Commission Regulation (EU) No 231/2012 in “ Additives other than colours and sweeteners” (1) and permitted to use in candied, crystallized or glacé fruit and vegetables, candied cherries and liquid egg white for egg foams (2).
Also, it is generally recognized as safe (GRAS) in food when used in accordance with good manufacturing practice by the FDA. (3).
Its food grade can be used as a firming agent, sequestrant, pH adjustment and leavening acid in food, such as in pickling, baked goods and cheese.
However, you should be careful when mixed with water, as it forms dilute sulfuric acid that may burn human skin and eyes.
Aluminum sulfate is commonly used as a traditional flocculant or coagulant in the purification of drinking water and wastewater, also in lakes and ponds to remove phosphorus for algae control.
It functions by reaction with water to dissociate the aluminum ion and generate aluminum hydroxide colloids as the following equation shows.
- Al2(SO4)3 = 2Al3+ + 3 (SO42-)
- Al3+ + 6H2O = 2Al(OH)3 + 6H+
When added to water, aluminum sulfate dissociates the aluminum cation and the sulfate anion. The aluminum ion is positively 3+ charged, which can easily neutralize the negative charges, such as negative fluorine, chlorine, and phosphorus. And it promotes the mutual connection and precipitation of colloidal particles through bonding, bridging, and crosslinking.
Meanwhile, it forms complexes and Al(OH)3 colloids. Due to the large surface area of Al(OH)3 colloids, which have a strong adsorption effect or a sticky characteristic that enables tiny impurities (e.g. bacteria, colloids and other suspended solids) that cannot be easily filtered to clump together into large particles, and then can be easily removed from the water by sedimentation. Finally, clarify the water.
It also can be used in swimming pools as a pool clarifier to decrease the turbidity of the water with the same principle.
What are flocculants?
The flocculation sedimentation method is a low-cost water treatment method. Substances that can cause flocculent precipitation of pollutants in wastewater are called flocculants.
A flocculant is mainly used to remove suspended solids & colloidal particles, and bacteria in wastewater, also reduce COD (chemical oxygen demand) and BOD (Biochemical oxygen demand). It has the functions of dephosphorization, decolorization, deodorization, and reducing eutrophication of water.
Commonly used flocculants can be divided into three categories: inorganic, organic and microbial flocculants. Inorganic flocculants are mainly based on aluminum salts (also polyaluminum chloride) and iron salts.
Aluminum salts are superior to iron salts in removing COD & BOD, enhancing cohesiveness, increasing sedimentation speed, and preventing water pH from going down in wastewater. Aluminum sulfate is one of the inorganic aluminum salt flocculants.
Hydrangea, with ornamental value and commercial value, has a variety of flower colors as its color is easily affected by external environmental factors such as cultivation conditions. The flower color control with aluminum sulfate is popular in the cultivation of potted and cut flowers of hydrangea.
The treatment of soil for hydrangea cultivation with different concentrations of aluminum sulfate can obtain various colors results from the combination of red and blue.
Aluminum sulfate promotes the absorption of Al3+ by the roots of the hydrangea, increases the accumulation of Al3+ in the petals of the hydrangea, thereby increasing the intensity and purity of the petals’ blue color.
What is Hydrangea and why does its color change?
Hydrangea macrophylla belongs to the genus of Hydrangea in the Saxifraga family. It is native to China and Japan and is widely used as garden ornamental plants and potted flowers.
The color of some hydrangea flowers is affected by the pH of the cultivation soil. It is blue in acidic soil but red in alkaline soil.
It is mainly because this kind of hydrangea species contains delphinium pigment, which makes the flower color blue after the combination of Al3+, or the flower color is red if without the addition of Al3+. This indicates that the Al3+ content in the soil plays a fundamental role in the color change of the hydrangea.
At the same time, the pH of the cultivation soil may also affect the purity of the petal color of the hydrangea.
The soil pH would be more acidic after the treatment of aluminum sulfate solution. And the blue purity and richness of the petals of hydrangea would be different if sprayed with its different concentrations solution.
As a soil acidifier and a fertilizer, aluminum sulfate sometimes is added to alkaline soils to lower the soil pH for acid-loving plants in gardening and lawning, for example, azaleas, camellias, hemlock, holly, cedar, blueberry, rhododendrons, and gardenias.
It alters the soil acidity by producing a diluted sulfuric acid solution when mixed with water. The reaction equation as follows: Al2(SO4 )3 + 6 H2O —- 2Al (OH)3 + 3H2SO4
Aluminum sulfate can promote the rapid setting and hardening of cement or concrete, and produce a certain strength in a short time by reacting with cement to produce a large amount of fine needle-like ettringite and intermediate product secondary gypsum, which leads to rapid setting of cement.
Sizing agent in Paper manufacturing
Aluminum sulfate can be used as a sizing agent in papermaking. Paper is composed of fibers (wood pulp or straw pulp), and unsized paper is not suitable for writing and printing. In addition, the strength of paper will decrease after absorbing water, which will affect its use.
Therefore, most paper needs to be coated with some water-resistant substances on the surface to give the paper the ability of delaying fluid penetration so that to achieve ink resistance (e.g. writing paper), oil resistance (e.g. food packaging paper) or other purposes, this process is called sizing, and the added substance is called a sizing agent.
Aluminum sulfate can also be used as a grease clarifier, petroleum deodorizing, decolorizing agent, mordant for improving adhesion of fabric dyes, leather tanning agent, medical astringent, deodorant, titanium dioxide filler and foam agent for fire extinguishing.
Now you may have a knowledge of the multi-functional ingredient– Aluminum sulfate (E520), from the following aspects:
- Production process
- Uses in food, water purification, gardening especially in cultivation of the blue color of hydrangea and more
Do you have experience in using it in gardening or finding it in your food? Let me know in the comments.