Aluminium sulphate

Hydrolysis

Description of key information

A reaction time of about 1 hour is usually sufficient for hydrolysis of a high percentage of the Al 2 (SO 4 ) 3 solution, although longer periods of about 1 week may be required for substantially complete hydrolysis.
When dissolved in a large amount of neutral or slightly-alkaline water, aluminium sulfate  hydrolyzes to form the aluminium hydroxide precipitate (Al(OH)3)  and a dilute sulfuric acid solution and reduce the pH of  soil.
The different reactions involved in the formation of aluminum hydroxide in aqueous solution was described; the overall reaction can be represented by the following equation:
Al2(SO4)3+ 6H2O<=>2Al(OH)30+ 3H2SO4
Hydrolysis species are written without the water of hydration even though the water is present. The hydrolysis species of Al(III) are distributed as a function of pH; the most important forms of dissolved aluminium are Al3+, (Al(OH)2)+ and Al(OH)4-; each species predominates over a certain pH range: pH range                          predominant species
less than 4                                   Al3 +
5 - 6                                           (Al(OH)2)+
about 6                                       minimum solubility
more than 7                               (Al(OH)4 -

Key value for chemical safety assessment

Half-life for hydrolysis:

1 wk

at the temperature of:

25 °C

Additional information

Hydrolysis is a chemical reaction during which molecules of water (H2O) are split into hydrogen cations (H+, conventionally referred to as protons) and hydroxide anions (OH−) in the process of a chemical mechanism).

 

When released into water, the aluminum sulphate hydrolyses to form aluminum hydroxides.

Reactions between aluminum sulphate, water and associated “impurities” result in the formation of a floc, which separates from the water phase to form alum sludge. A small fraction of the aluminum can stay in the water in either colloidal or dissolved form.The different reactions involved in the formation of aluminum hydroxide in aqueous solution was described; the overall reaction can be represented by the following equation:

Al2(SO4)3+ 6H2O<=>2Al(OH)3⁰+ 3H2SO4

The aluminum hydroxide present in sludge is expected to remain mostly solid following release into surface water.Experiments were showed that less than 0.2% of the aluminum hydroxide present in sludge was released in supernatant water at a pH of 6 and less than 0.0013% was released at pH 7.65. In both cases, aluminum hydroxide was present mostly in particulate form. At these pH values, aluminum solubility is low and kinetics favour the formation of solid aluminum hydroxide.

 

When used to treat sewage water, alum will also react with phosphate, as shown in the following reaction:

Al2(SO4)3+ 2PO4^3– <=>AlPO4(s) + 3SO4^2–

 

This process has been used for many years to treat phosphorus in wastewaters, as well as to reduce phosphorus levels in runoff from land fertilized with poultry litter and restore phosphorus-enriched eutrophic lakes .

Aluminum is a strongly hydrolysing metal and is relatively insoluble in the neutral pH range (6.0–8.0) . In the presence of complexing ligands and under acidic (pH < 6) and alkaline (pH > 8) conditions, aluminum solubility is enhanced. At low pH values, dissolved aluminum is present mainly in the aquo form (Al3+).

 

Hydrolysis occurs as pH rises, resulting in a series of less soluble hydroxide complexes (e.g., Al(OH)2+, Al(OH)2+). Aluminum solubility is at a minimum near pH 6.5 at 20°C and then increases as the anion, Al(OH)4–, begins to form at higher pH.

 

Temperature has been shown to influence the solubility, hydrolysis and molecular weight distribution of aqueous aluminum species as well as the pH of solutions.There wasreported a higher degree of aluminum hydrolysis and greater polymerization to high molecular weight species in inorganic aluminum solutions stored for one month at 25°C compared with those stored for an equivalent period at 2°C. The researchers hypothesized that more advanced polymerization evident at the higher temperature resulted in more deprotonation and condensation reactions, possibly accounting for the observed lower pH of the 25°C test solutions (range 4.83 to 5.07 versus 5.64 to 5.78 in the solutions at 2°C).