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Description of key information

Hydrolysis is an important factor influencing aluminium bioavailability in aquifers with pH, DOC and the presence of other ions involved in determining the specific degree of ligand formation and final in Aquo aluminium concentration present in the system. The calculation of a hydrolytic half life is not useful as regardless of pH, temperature or other parameters present, a specific equilibrium is ultimately reached between aquo to total aluminium concentrations.

Key value for chemical safety assessment

Additional information

The speciation of Aluminium is pH dependent. At low pH values, dissolved aluminum is present mainly in the aquo form (Al3+). Aluminum hydrolyses increasingly with increased pH and is poorly soluble in the neutral pH range between 6.0 and 8.0 due to the formation of less soluble hydroxide complexes (e.g., Al(OH)2+, Al(OH)2+). Its solubility is at a minimum near pH6.5 at 20°C and then increases as the anion, Al(OH)4–becomes more abundant.

In the presence of complexing ligands and under acidic (pH < 6) and alkaline (pH > 8) conditions, aluminium solubility is enhanced.


At 20°C and pH < 5.7, aluminum is present primarily in the formsAl3+ and Al(OH)2+. In the pH range 5.7 to 6.7, aluminum hydroxide species dominate, including Al(OH)2+ and Al(OH)2+, and then Al(OH)3. At a pH of approximately 6.5, Al(OH)3 tends to predominate over the other species. In this range, aluminium solubility is low. At pH > 6.7, Al(OH)4–becomes the dominant species.


Due to its size fluoride readily substitutes for OH- and has a strong affinity for Al under acidic conditions although this is limited by the relative low abundance of fluoride in the environment. Aluminium-hydroxide complexes predominate over aluminium-fluoride complexes under alkaline conditions.

Sulphate also complexes with aluminium under alkaline conditions (>pH 6). Nevertheless aluminium-sulphate complexes were found to represent only 5% of monomeric aluminium in lake studies. While all aluminium species described are present simultaneously at any pH value the relative proportion of these species will depend upon pH in aquatic systems.

Aluminium-organic complexes were a major component of monomeric Al over a wide range of pH from 4.3 to 7.0 and were found to positively correlate with increasing DOC concentrations as pH decreased.