Aluminium sulphate

Adsorption / desorption

Description of key information

Adsorption of aluminium occurs only under pH conditions where it hydrolyzes to give various hydrolysis products. Progressive hydrolysis leads to the formation of colloidal aluminium hydroxide.
Aluminium sulphate has a propensity to leach through soil if water is applied, i.e. it does have mobility through soil, and providing sufficient water is present. As it moves downwards into layers where the water content is low, the leaching will stop. 
On this basis, it does not have a high potential for adsorption to soil if water is not present andonly part of the aluminium sulphate in the solid phase is adsorbed. 
  
On the other basis if water is present aluminium sulphate as aluminium hydroxide precipitate (Al(OH)3)  have a high potential for adsorption to soil. 
The estimated Soil Adsorption Coefficient was 75.41 L/kg measured by calculation from EPI SuiteTM v4.0..This is Exposure Assessment Tools and Models made from EPA (Environmental Protection Agency). 
 Koc Estimate from MCI:
  ---------------------
         First Order Molecular Connectivity Index  ........... :  7.431
         Non-Corrected Log Koc (0.5213 MCI + 0.60)  .......... :  4.4734
         Fragment Correction(s):
                  2   Miscellaneous  S(=O)  group  ..........  : -2.5960
         Corrected Log Koc  .................................. :  1.8774
 
                         Estimated Koc:  75.41  L/kg   <===========

Key value for chemical safety assessment

Koc at 20 °C:

75.41

Additional information

Adsorption of aluminium occurs only under pH conditions where it hydrolyzes togive various hydrolysis products.Progressive hydrolysis leads to the formation of colloidalaluminium hydroxide.

 

The rate at which soluble aluminium sulphate are gradually leached away is dependent upon the water supply. In humid regions, the upper layers of soil and rock are kept thoroughly leached, and as fast as they are formed the soluble products are removed in the drainage water. In semi-arid regions, the soils are not fully leached and soluble substances tend to accumulate.

 

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 solutionandreduce the pH of soil.

 

The anhydrous form occurs naturally as a rare mineral millosevichite, found e.g. 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 Al₂(SO₄)₃•16H₂O and octadecahydrate Al₂(SO₄)₃•18H₂O are the most common. The heptadecahydrate, whose formula can be written as [Al(H₂O)₆]₂(SO₄)₃•5H₂O, occurs naturally as the mineral alunogen.

 

 The above information indicates that aluminium sulphate has a propensity to leach through soil if water is applied, i.e. it does have mobility through soil, and providing sufficient water is present. As it moves downwards into layers where the water content is low, the leaching will stop.

 

On this basis, it does not have a high potential for adsorption to soil if water is not present andonly part of thealuminium sulphatein the solid phase is adsorbed.

 

On the other basis if water is present aluminium sulphate asaluminium hydroxide precipitate(Al(OH)3) have a high potential for adsorption to soil.

-          soil, the colloidal surface can adsorb large quantities ofaluminium

 

Selectivity of cation adsorption

The affinity of most cations for an adsorbing surface is greater for divalent than for monovalent ions, and for large cations than for small ones of the same charge because the larger the cation the less hydrated it is. The usual affinity is:

Al³⁺> Ba²⁺>Sr^{2 +}>Ca²⁺>Mg²⁺= Cs⁺>Rb⁺>K⁺= NH₄⁺>Na⁺

The soil cations that are readily adsorbed onto soil colloids can be divided ino two groups. Firstly there are the base catoins,which include the important plant nutrients Ca²⁺, Mg^{2+ ,}K⁺and Na⁺. Secondly there are acid cations,which include Al³⁺and H⁺. Related to this distinction in cations is the term base saturation, which is defined as the proportion of exchange sites occupied by base cations. A soil with a high base saturation (greater than 35%) is more fertile than a soil with a low base saturation.

Al³⁺, Ca²⁺and H⁺are the commonly adsorbed cations in humid regions. This reflects the long-term leaching loss of basic cations and their replacement by acidic cations. In contrast, Ca^2+,Mg²⁺, K⁺and Na⁺are the commonly adsorbed cations in arid regions.

 

Aluminium influences of soil acidity

As clay minerals weather and break down, the aluminium in the octahedral layer is released into the soil solution, where it either reacts with water or is adsorbed onto the exchange sites of negatively charged clay minerals. Al³⁺ions are adsorbed in preference to all the other major cations. The influence that aluminium has on soil acidy is itself dependant on the acidity of the soil. At Ph less 5, aluminium is soluble and exists as Al³⁺. When Al³⁺enters the soil solution it reacts with water (it is hydrolysed) to produce H⁺ions:

                   Al³⁺+ H₂O <===> AlOH²⁺+ H⁺

Thus the acidity of the soil increases (pH falls). In soils with a pH of between 5 and 6.5, aluminium also contributes H⁺oins to the soil solution but by different mechanisms, as aluminium can no longer exist as Al³⁺ions but is converted to aluminium hydroxyl ions:

Al³⁺+ OH^-<===> AlOH²⁺

AlOH²⁺+H^-<===> Al(OH)₂⁺ 

                                      ^{ALUMINIUM HYDROXY IONS}

These hydroxyl aluminium ions act as exchangeable cations just like Al³⁺, and are adsorbed by the clay minerals. They are in equilibrium with hydroxyl aluminium ions in the soil solution, where they produce H⁺ions by the following reactions:

              AlOH²⁺+H₂0 <===> Al(OH)₂⁺   + H⁺

 

              Al(OH)₂ ⁺+H₂0 <===> Al(OH)₃⁺   + H⁺

 

In soils where the pH is above 7 Ca²⁺and Mg²⁺dominated the exchange sites and most of the hydroxyl aluminium ions have been converted to gibbsite (Al(OH)₃), is insoluble and cannot be by the negative clay minerals as no charge. In a neutral soil the exchangeable cations that dominate the cation exchange sites are the base cations, whereas in an acidics soils aluminium and hydrogen ions dominate the exchange sites.

 

On this basis if water is present aluminium sulphate as aluminium hydroxide precipitate (Al(OH)3) have a high potential for adsorption to soil.

If released into water, aluminium sulphate is not expected to adsorb to suspended solids and sediment based upon the Koc. The Koc of aluminium sulphate can be estimated to be 75.41. This estimated Koc value suggests that Aluminium sulphate is expected to have very high mobility in soil.

These results suggest that Aluminium sulphate has high soil mobility and does not have a high potential for adsorption to soil.

The estimated Soil Adsorption Coefficient was 75.41 L/kg measured by calculation from EPI SuiteTM v4.0..This is Exposure Assessment Tools and Models made from EPA (Environmental Protection Agency).

 

Koc Estimate from MCI:

---------------------

First Order Molecular Connectivity Index ........... : 7.431

Non-Corrected Log Koc (0.5213 MCI + 0.60) .......... : 4.4734

Fragment Correction(s):

2 Miscellaneous S(=O) group .......... : -2.5960

Corrected Log Koc .................................. : 1.8774

Estimated Koc: 75.41 L/kg <===========